Flow Analysis with Atomic Spectrometric Detectors, Volume 9

In the last twenty years the analytical community has regarded continuous flow systems as interesting, nice, innovative...

Author: A. Sanz-Medel

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In the last twenty years the analytical community has regarded continuous flow systems as interesting, nice, innovative approaches to link samples and standards to instruments in an automatic manner. At present, they can be regarded as useful tools for meeting a wide variety of real chemical information needs. The fact that an increasing number of papers and standard analytical methodologies relying on continuous flow systems include no such terms and continuous, flow injection, etc, in their titles and/or key words, is a good proof that these systems are accepted to be commonplace in Analytical Chemistry. Recently, S. Chalk (Talanta, 1998, 45, 591) has developed a flow injection analysis database (FAD) on the World Wide Web as a central resource for bibliographic information in this context. There are two general philosophies used in the analytical literature to describe the coupling of continuous flow systems to instruments. Continuous flow (FIA) researchers used to consider the detector as another module of the flow manifold. Spectroscopists consider continuous flow systems to be just another alternative to introducing samples into instruments. In fact both are correct because they are appropriate to describe the facts which, however, are partially distorted from both points of view. It is more relevant to discuss the degree of compatibility and adaptability between both partners, that is, the corresponding interface (see Valc~ircel et al., Fresenius J. Anal. Chem., 1998, 362, 58). The most simple interface between continuous flow systems and instruments is that established between these automatic systems and atomic spectroscopic instruments based on sample aspiration (FAAS, ICP-OES, ICP-MS) because of the high degree of compatibility between them. When the instruments use typical discrete sample introduction systems, as ETAAS, the interfaces are critical. This book edited by my admired friend and colleague Alfredo Sanz-Medel constitutes an excellent and timely approach to the symbiosis (not parasitism!) between continuous flow systems, with or without sample or reagent injection, and atomic spectrometric instruments. The synergistic effect of such combination is undeniable. The best proof of

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Notwithstanding the fact that Flow Injection Analysis (FIA) was born originally as a practical technique for automation of serial assays, it has evolved well beyond that original point. Today FIA has become a powerful analytical tool most adequate for performing advantageously the so called "preliminary steps" of analysis for any preparations before measurement, including sampling dissolutions, dilutions, matrix removal, preconcentrations, etc. On the other hand, the development of atomic spectrometry for inorganic analysis has been amazing in the last decades. It is undeniable that atomic spectrometric detectors dominate these days the inorganic elemental analysis. However, the above-mentioned preliminary steps, characteristic of almost any general analytical process, have not developed at the same pace as spectroscopic instrumentation. In fact, it can be said that they are still one of the pending problems of modem Analytical Chemistry in general and of Analytical Atomic Spectrometry in particular. Flow Analysis (FA) offers a most convenient and fast approach to enhance and automate such preliminary steps for atomic spectrometric detectors. Moreover, flow manifolds can ease the well-known problem of sample introduction/presentation to atomisers or even expand the classical scope of atomic/elemental information, characterising atomic spectrometry, into the realm of molecules and metal-compound analysis (e.g. by resorting to coupled separation techniques). All these facts could explain both the extraordinary interest for research and the great importance for practical problem-solving achieved nowadays by FA with atomic spectrometric detectors. In fact, research on FA to enhance or speed up atomic spectrometric methods is one of the most flourishing fields of the last decade of atomic spectrometry research. Moreover, FIA strategies with atomic spectrometric detectors are becoming of greater and greater practical use in routine laboratories, particularly after the introduction of Flow Injection--Atomic Absorption Spectrometry (FI-AAS) commercially available dedicated instruments and their widespread utilization worldwide. vii

viii

Preface

In brief, the fortunate marriage between flow analysis and atomic spectrometric detectors is now well established and in continuous expansion both in research and in routine laboratories. It is not surprising, therefore, that two books have already been published on this topic: "Flow Injection--Atomic Spectroscopy" edited by J.L. Burguera in 1989 and "Flow Injectiorr--Atomic Absorption Spectrometry", authored by Z. Fang and edited by Wiley in 1995. In any case, to the best of my knowledge, the former monograph can be considered now rather out-of-date because only publications up to 1986 were used by contributing authors. On the other hand, the monograph authored by Z. Fang is an excellent account of FI-AAS, but, as the title indicates, only atomic absorption techniques are covered. As atomic spectrometry, on the threshold of the year 2000, is much more than AAS (Plasma Emission and Plasma Mass Spectrometry are so important and popular nowadays!), I believed that a book covering all present atomic detectors and techniques where flow analysis has been or can be advantageously employed, was needed to complement the available bibliography in the field. Notwithstanding the popularity of FIA techniques, there are some problems and techniques in modem atomic spectrometry which might require a continuous aspiration of the sample in order to avoid background outbursts or even extinction of the discharge. What is more, conventional plasma emission or mass spectrometers use nebulisers with an independent propulsion system (e.g. a peristaltic pump) feeding the plasma in a continuous manner. Therefore, I preferred to use the more general concept of Flow Analysis with Atomic Spectrometric Detectors (FA-ASD) for the title of this monograph. The book has been conceived in three separate parts: Part I intends to give the fundamentals, instrumentation and potential of FIA as a most versatile sample presentation/introduction system for atomic spectrometry; Chapters 1 to 4 examine the basis of synergic combination of FI-AAS for chemical analysis (J. F. Tyson), the basic instrumentation and wealth of experimental designs available (R. Pereiro), the enormous analytical potential of flow techniques to expand the scope and possibilities of modem analytical atomic spectrometry (J. F. Tyson) and, finally, the important topic of FIA strategies available today for calibration in atomic spectrometry (M. De la Guardia). Part II aims at giving a modem account of fundamentals and possibilities offered by flow analysis to atomic spectrometry for on-line sample pretreatments, separations and preconcentrations in the most convenient, inexpensive and simple ways. The topics of this part are dealt with in the following five chapters: the use of FI systems for on-line dissolution of solid samples (J. L. Burguera), FI for on-line solid--liquid separation and

Preface

ix

preconcentration (Z. Fang), FI for on-line liquid-liquid separations and preconcentrations (M. Valc~ircel and M. Gallego), flow methods for gas-liquid separations (J. Dedina) and a final chapter devoted to continuous aspiration analysis, rather than FIA, particularly focused on atomic emission plasma detection (A. Sanz-Medel, E. SanchezUria, A. Men6ndez-Garcia). Part III includes the remaining five chapters, 10 to 14, aiming at applications of FAASD combinations to analytical problem-solving in the most varied fields and situations. Chapter 10 starts with applications in the field of environmental analysis (M. De la Guardia and A. Morales) and is followed by applications in clinical and biological analysis (R. Pereiro and A. Sanz-Medel). The next two chapters try to reflect the application of FA-ASD in one of the "hottest" areas of analytical research these days: "Trace Element Speciation". Thus, cryofocusing for on-line metal and metalloid speciation in environment (A. De Diego, C. P6cheyran, C. M. Tseng and O. E X. Donard) and chromatographic separations for trace element speciation in biological systems (A. Sanz-Medel) illustrate both the present importance of such analytical problems in Environment and Bioscience and the key role of FA-ASD in the solution of trace element speciation problems. The last chapter of the book introduces another research trend where FA-ASD combinations are of utmost importance; that is, the use of micro-column field sampling for enhanced trace analysis and trace element speciation (R. Ma and C. W. McLeod). Let me conclude by saying that this monograph can be a timely publication, integrating the most popular aspects of FIA, its new developments for sample on-line treatments and on-line non-chromatographic and chromatographic separations (all typical "flow analysis") in connection with all branches of analytical atomic spectrometry. After reading this book I hope that the reader will agree with me about how invaluable such flow techniques are today to enhance the intrinsic potential (atomic information) and to widen the scope (molecular information) of atomic spectrometry, particularly so for plasma detectors. Thus, I expect that academics, researchers and routine users of analytical atomic spectrometry will benefit altogether from this "Flow Analysis with Atomic Spectrometric Detectors" monograph.

Oviedo, 30 October 1998 Alfredo Sanz-Medel

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Foreword .

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vii

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List of contributors

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xvii

List of abbreviations .

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xix

Part I

Flow injection analysis: a versatile sample introduction system for atomic

spectrometry 1

FIA-atomie spectrometry: a synergic combination for chemical analysis Julian F. Tyson

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Introduction

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Basic concepts of flow injection analysis . . . . . . . . . . Basic concepts of atomic spectrometry . . . . . . . . . . .

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Flow injection and atomic spectrometry

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Basic instrumentation for FIA-atomic spectrometric detection Rosario Pereiro . . . . . . . . . . . . . . . . . . . . 2.1 Introduction: flow manifolds or what can be done with a peristaltic

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pump, an injection valve and some plastic tubes . . . . . . . . The flow manifold . . . . . . . . . . . . . . . . . The interfacing of flow manifolds with atomic detectors . . . . .

34 39 54

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References .

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FIA techniques and strategies expand the potential of atomic spectrometry Julian E Tyson

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Introduction

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3.2

Atomic spectrometer response characteristics . . . . . . . . . xi

64 64 66

xii

Contents 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10

4

Microsampling into nebulizers . . . . . . . . . . . Nebulization efficiency . . . . . . . . . . . . . . Dilution and calibration . . . . . . . . . . . . . . Study of interference effects . . . . . . . . . . . . Chemical pretreatment, matrix removal and preconcentration Detection limits in FI preconcentration procedures . . . . Summary . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

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69 78 80 82 82 92 95 96

FIA strategies for calibration and standardization in atomic spectrometry Miguel de la Guardia . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . 4.2 Calibration of atomic spectrometry instruments . . . . . . . . 4.3 Standardization strategies in flow analysis-atomic spectrometry. . 4.4 Concluding remarks . . . . . . . . . . . . . . . . . 4.5 References . . . . . . . . . . . . . . . . . . . .

98 98 99 100 129 130

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Part II Flow analysis: a convenient on-line sample pretreatment strategy for atomic spectrometry 5

6

Flow injection systems for on-line sample dissolution/decomposition Jos6 Luis Burguera and Marcela Burguera . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . 5.2 Microwave heating . . . . . . . . . . . . . . . . . 5.3 On-line microwave sample mineralization with discontinuous detection 5.4 On-line microwave sample mineralization with continuous detection . 5.5 Conclusions . . . . . . . . . . . . . . . . . . . 5.6 References . . . . . . . . . . . . . . . . . . . . Flow injection on-line solid-liquid separation and preconcentration atomic spectrometry Zhao-lun Fang and Guan-hong Tao . . . . . . . . . . . . . . 6.1 General . . . . . . . . . . . . . . . . . . . . . 6.2 General principles for the design of on-line solid-liquid separation and preconcentration systems for atomic spectrometry . . . . . . . 6.3 Preconcentration and separation by column sorption . . . . . . 6.4 On-line preconcentration by precipitation and coprecipitation . . . . 6.5 On-line solid-liquid separation by sorption on knotted reactors ( K R ) . 6.6 References . . . . . . . . . . . . . . . . . . . .

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135 137 138 142 161

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168 168 169 170

192 199 200

Contents 7

8

9

Flow-injection on-line liquid-liquid separation and preconcentration atomic spectrometry M. Valctircel and M. Gallego . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . 7.2 Main limitations of FI-liquid-liquid separations . . . . . . . . 7.3 Liquid-liquid extraction . . . . . . . . . . . . . . . 7.4 Dialysis . . . . . . . . . . . . . . . . . . . . . 7.5 Other systems involving organic solvents . . . . . . . . . . 7.6 References . . . . . . . . . . . . . . . . . . . .

203 203 205 206 229 233 234

Flow methods in gas--liquid separations J. D6dina . . . . . . . . . . . . . . . . . . . . . . 8.1 Scope of the chapter . . . . . . . . . . . . . . . . . 8.2 Hydride release . . . . . . . . . . . . . . . . . . 8.3 Hydride transport . . . . . . . . . . . . . . . . . . 8.4 Methods of hydride generation . . . . . . . . . . . . . 8.5 Experimental approaches to gas--liquid separation in flow methods . 8.6 Interferences to hydride generation . . . . . . . . . . . . 8.7 References . . . . . . . . . . . . . . . . . . . .

237 237 239 247 248 256 265 270

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Continuous aspiration and flow injection in atomic emission plasma spectrometry Alfredo Sanz-Medel, J. Enrique Sfinchez Uria and Alberto Men6ndez Garcia 9.1 Introduction . . . . . . . . . . . . . . . . . . . 9.2 Continuous aspiration and flow injection for hydride generation with ICP-OES detectors . . . . . . . . . . . . . . . . . 9.3 Continuous on-line volatile species generation to increase analytical sensitivity and selectivity of plasmas . . . . . . . . . . . 9.4 Flow generation of volatile compounds from organic solvents . . . 9.5 Continuous generation of volatile species from organised media. . . 9.6 Tandem on-line continuous separations for sample introduction into plasmas . . . . . . . . . . . . . . . . . . . . . 9.7 References . . . . . . . . . . . . . . . . . . . .

Part III 10

xiii

274 274 277 282 288 293 296 303

Applications of flow analysis with atomic spectrometric detectors

Applications in environmental analysis Angel Morales-Rubio and Miguel de la Guardia . . . . . . . . . . 10.1 Introduction and general strategies for environmental analysis with atomic detection . . . . . . . . . . . . . . . . . .

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xiv

Contents 10.2 10.3 10.4 10.5 10.6

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Water analysis . . . . . Soil and sediment analysis . Air analysis . . . . . Concluding remarks . . . References . . . . . .

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312 329 334 335 336

Applications of flow analysis with atomic spectrometric detectors in clinical and biological analysis Rosario Pereiro and Alfredo Sanz-Medel . . . . . . . . . . . . 11.1 Combining flow systems with atomic spectrometry for routine clinical and biological analysis . . . . . . . . . . . . . . . . 11.2 Flow manifolds for clinical and biological analysis using atomic detection . . . . . . . . . . . . . . . . . . . . 11.3 A powerful novel detector for clinical and biological analysis in flow systems: ICP-MS . . . . . . . . . . . . . . . . . 11.4 Selected applications to clinical and biological analytical problems. . 11.5 Final remarks . . . . . . . . . . . . . . . . . . . 11.6 References . . . . . . . . . . . . . . . . . . . . Cryofocusing for on-line metal and metalloid speciation in the environment A. de Diego, C. P6cheyran, C. M. Tseng and O. F. X. Donard . . . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . 12.2 Experimental set-up . . . . . . . . . . . . . . . . . 12.3 Interferences . . . . . . . . . . . . . . . . . . . 12.4 Applications . . . . . . . . . . . . . . . . . . . 12.5 Conclusions . . . . . . . . . . . . . . . . . . . 12.6 References . . . . . . . . . . . . . . . . . . . .

342 346 354 358 371 371

375 375 377 397 401 401 403

Chromatographic separations coupled to atomic detectors: trace element speciation in biological systems Alfredo Sanz-Medel . . . . . . . . . . . . . . . . . . . 13.1 Introduction . . . . . . . . . . . . . . . . . . . 13.2 The need for chemical speciation information in biological systems. . 13.3 Chemical speciation in biology and medicine: a challenge for modem analytical chemists . . . . . . . . . . . . . . . . . 13.4 Analytical approaches to chemical speciation in biological samples. 13.5 Hybrid techniques for speciation of biological systems . . . . . . 13.6 Trends in trace element speciation research in the biological and biomedical fields . . . . . . . . . . . . . . . . . .

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407 407 411 412 414 418 428

Contents

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xv

13.7 Concluding remarks . . . . . . . . . . . . . . . . . 13.8 References . . . . . . . . . . . . . . . . . . . .

434 436

Microcolumn field sampling and flow injection analysis: a strategy for enhanced trace analysis and element speciation Renli Ma, Glenn Woods and Cameron W. McLeod . . . . . . . . .

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14.1 Introduction

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14.2 Equipment and analytical strategies . . . . . . . . . . . .

441

14.3 Field investigations . . . . . . . . . . . . . . . . . 14.4 Conclusions and future developments . . . . . . . . . . . 14.5 References . . . . . . . . . . . . . . . . . . . .

444 456 456

Index .

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J. L. Burguera, IVAIQUIM (Venezuelan Andean Institute for Chemical Research), Faculty of Sciences, University of Los Andes, P.O. Box 542, M6rida, Venezuela. M. Burguera, IVAIQUIM (Venezuelan Andean Institute for Chemical Research), Faculty of Sciences, University of Los Andes, P.O. Box 542, M6rida, Venezuela. J. Dedina, Academy of Sciences of the Czech Republic, Institute of Analytical Chemistry, Laboratory of Trace Element Analysis, Videnskfi 1083, CZ-14220 Prague 4, Czech Republic. A. de Diego, Laboratoire de Chimie Bio-Inorganique et Environment, CNRS EP 132, H61ioparc 64000 Pau, France. O. F. X. Donard, Laboratoire de Chimie Bio-Inorganique et Environment, CNRS EP 132, H61ioparc 64000 Pau, France. Z. Fang, Northeastem University, Chemistry Department, Box 332, Flow Injection Research Center, 110006 Shenyang, China. M. Gallego, Department of Analytical Chemistry, University of C6rdoba, 14004 C6rdoba, Spain. M. de la Guardia, Department of Analytical Chemistry, University of Valencia, 50 Dr. Moliner St, 46100 Burjassot (Valencia), Spain. R. Ma, Centre of Analytical Sciences, Department of Earth Sciences, University of Sheffield, Sheffield $3 7HF, United Kingdom. C. W. MeLeod, Centre of Analytical Sciences, Department of Earth Sciences, University of Sheffield, Sheffield $3 7HF, United Kingdom. xvii

xviii

List of contributors

A. Men6ndez-Garcia, Department of Physical and Analytical Chemistry, University of Oviedo, 33006 Oviedo, Spain. A. Mor/fles-Rubio, Department of Analytical Chemistry, University of Valencia, 50 Dr. Moliner St, 46100 Burjassot (Valencia), Spain. C. M. P6cheyran, Laboratoire de Chimie Bio-Inorganique et Environment, CNRS EP 132, H61ioparc 64000 Pau, France. R. Pereiro, Department of Physical and Analytical Chemistry, University of Oviedo, 33006 Oviedo, Spain. J. E. Sfinchez-Uria, Department of Physical and Analytical Chemistry, University of

Oviedo, 33006 Oviedo, Spain. A. Sanz-Medel, Department of Physical and Analytical Chemistry, University of Oviedo, 33006 Oviedo, Spain. G. Tao, Northeastern University, Chemistry Department, Box 332, Flow Injection Research Center, 110006 Shenyang, China. C. M. Tseng, Laboratoire de Chimie Bio-Inorganique et Environment, CNRS EP 132, H61ioparc 64000 Pau, France. J. E Tyson, University of Massachusetts, Lederle Graduate Research Center, Box 34510, Amherst, MA 01003-4510, U.S.A. M. Valcfircei, Department of Analytical Chemistry, University of C6rdoba, 14004 C6rdoba, Spain.

AAS AES AFS ASD AS

El8 CCP CE CF CV CLLE CMP CVG DCP DD ECD ETAAS F FA FAAS FAES FAFS FI FIA FIAS GC

Atomic Absorption Spectrometry Atomic Emission Spectrometry Atomic Fluorescence Spectrometry Atomic Spectrometric Detector Atomic Spectrometry Octadecyl functional groups bonded silica gel Capacitive Coupled Plasma Capillary Electrophoresis Continuous Flow Cold Vapour Continuous Liquid-Liquid Extraction Capacitive Microwave Plasma Chemical Vapour Generation Direct Current Plasma Donnan Dialysis Electron Capture Detector Electrothermal Atomic Absorption Spectrometry Flame Flow Analysis Flame Atomic Absorption Spectrometry Flame Atomic Emission Spectrometry Flame Atomic Fluorescence Spectrometry Flow Injection Flow Injection Analysis Flow Injection Atomic Spectrometry Gas Chromatography xix

XX

GD HG HPLC IBMK ICP KR LC LD LL LLE MIP MS MW MWO OES PTFE QFAAS RSD SCF SCP SPE VG

List of abbreviations Glow Discharge Hydride Generation High Performance Liquid Chromatography Isobutyl methylketone Inductively Coupled Plasma Knotted Reactor Liquid Chromatography Limit of Detection Liquid--Liquid Liquid-Liquid Extraction Microwave Induced Plasma Mass Spectrometry Microwave Microwave Oven Optical Emission Spectrometry Poly(tetrafluoroethylene) Quartz Furnace Atomic Absorption Spectrometry Relative Standard Deviation Supercritical Fluid Chromatography Stabilized Capacitive Plasma Solid Phase Extraction Vapour Generation

spectrometry

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1.1

Introduction

A major driving force in research and development in chemical analysis is the need to make methods more cost-effective. A significant aspect of this concerns the reliability of procedures. Hertz has estimated I that in the United States alone, 250 million chemical measurements are made every day, 10% of which have to be repeated because of "suspected contamination, interference or poor result". Furthermore in industries in which product performance may be unambiguously correlated with chemical composition, the situation is even more alarming: "30% of the samples must be retested". Hertz has estimated that the cost of all these repeat analyses might be as much as $15 billion every year. Chemical analyses of this sort can be made more robust by the automation of as many of the operations as possible. 2'3 One basis for the automation of chemical reactions is the use of the hydrodynamics of fluid flow in narrow-bore, non-wettable conduits. Provided that the flow is controlled, a large number of chemical manipulations may be carried out in a contamination-free system with good precision. If the sample volume is also controlled, this concept is known as flow injection (FI) analysis. First described in the analytical chemistry literature in 1974-1975, 4 FI now has a literature of several thousand publications 5 and half a dozen books, and apparatus is commercially available from several manufacturers. In contrast to the technique of air-segmented continuous flow analysis (widely used in the form of Technicon's AutoAnalyzer instrumentation), FI lends

4

J.F. Tyson

itself to the implementation of a surprising number of chemical procedures, in addition to the mixing of homogeneous solutions. Procedures that have been adapted for FI include solid-phase extraction, liquid-liquid extraction, precipitation-collectiorv-redissolution, dialysis, and vapor generation and separation. Reactions have been promoted by the use of UV, ultrasonic and microwave radiation and even solid materials can be handled as slurries. The FI concept has proved to be an extraordinarily versatile and flexible concept. 6-9 In addition to making procedures more robust, automation also improves other performance parameters such as throughput. Unattended operation of equipment allows the working day to be extended, with the possibility of overnight operation, and makes laboratory personnel available for non-routine tasks or for those that still require supervision, such as the dissolution or digestion of samples in beakers on hot-plates or in closed vessels in microwave ovens. Sample preparation takes up the largest proportion of the time needed for chemical analysis; i~ thus there is considerable interest in the possibilities for developments of automated sample preparation and pretreatment procedures, developments in which flow injection techniques certainly have a role to play. In the area of trace element analysis, in addition to the needs for greater reliability and throughput, there are needs for improved accuracy and lower detection limits. To some extent, these performance parameters are inter-related. As detection limits are lowered, interferences from matrix components, which may be present in vast concentration excesses, become more serious. As will be discussed in several later chapters, the approach of separating the analyte species from the matrix species prior to presentation to the instrument is a major area in method development for many sample materials. Flow injection techniques have much to offer in this area and may also be used to improve the signal to noise ratio, leading to improved detection limits, by allowing the processing of larger amounts of sample than could otherwise be handled by the instrumentation. Another development in the area of trace element analysis is the characterization of materials for a range of chemical compounds containing an element of interest. This socalled "speciation" topic ~ is being actively researched in many groups around the world. A variety of approaches to the problems of speciation are being developed, the most versatile of which appear to be the various combinations of a high-efficiency separation technique (such as gas chromatography, liquid chromatography or capillary electrophoresis) with element specific detection. One way of viewing such sample treatment is as a branch of flow injection analysis, as the basic concepts of these separation techniques and those of flow injection are indistinguishable. Chromatography (and electrophoresis) with element specific detection is considered only a small part of this present text, an overview of importance and methodology is given in Chapter 13. There are, on the other

FIA-Atomic spectrometry: a synergic combination for chemical analysis

5

hand, a considerable number of reports in the recent literature of procedures, which are clearly within the broad definition of flow injection (to be discussed in the following section), for the determination of more than one chemical form of an element in a particular sample material. These will be discussed in several of the later chapters.

1.2

Basic concepts of flow injection analysis

As well as being a technique that will save an analytical laboratory lots of money, flow injection (FI) has been defined ~2 as "Information-gathering from a concentration gradient formed from an injected, well-defined zone of fluid, dispersed into a continuous unsegmented stream of a carrier." The extraordinary versatility embodied in this concept of the use of controlled fluid flow can be seen from the contents of later chapters in which a FI format of almost every possible chemical process, with the possible exception of fire assay, has been described. However, the academic definition given earlier does not allow the basic concept to be readily visualized and so here the basic principles will be introduced through the description of the basic FI experiment. The basic FI experiment is very simple. A discrete volume (about 50 txl) of liquid (in general this is the "sample") is injected into a continuously flowing (say 1 ml min -!) stream (which, in general, is the "reagent") in narrow-bore (0.5 mm) non-wettable tubing. The apparatus is shown schematically in Figure 1.1a, which illustrates the various conventions used for drawing FI equipment. The pump is represented by a rectangle, tubing by single lines (the zigzags indicate that the tubing is coiled), the arrow indicates the location of the injection valve and the circle is the detector. This type of system (called a manifold) would be used for microsample introduction to a flame or plasma spectrometer. This design of manifold is called a single-line manifold (as there is only one line of fluid being pumped). The equipment required for constructing FI systems will be discussed in more detail in Chapter 2. There is another basic type of manifold known as the multi-line manifold, of which the simplest case (the two-line manifold) is shown in Figure 1. lb. In this design of manifold, a reagent is added by merging at the confluence point. The injected sample is mixed with the carrier streams by a number of processes collectively known as "dispersion". These fluid flow processes are inherently precise phenomena and thus, if the pumping and injection processes are precise, the FI process is precise. By judicious choice of the various manifold components and operating conditions the extent of the various physical and chemical processes can be controlled. As the technique also uses a discrete volume of sample, the basic concepts of FI are often summarized as, (1) sample injection, (2) controlled dispersion and (3) reproducible timing.

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Almost every analytical chemistry method is made up of a sequence of procedures within which the sample material is chemically pretreated to convert it into a form which may be presented to the instrument chosen as the basis of the quantification stage. As a very wide variety of chemical manipulations can be carried out in a FI manifold, the scope of FI procedures is enormous. Although in the majority of procedures devised, the system uses the mixing of homogeneous liquid phases, many of the manifolds designed for use in conjunction with atomic spectrometric measurements make use of heterogeneous reactions and of procedures other than flow in open tubular reactors. These include liquidsolid reactions, in which a packed bed of solid-phase reagent is inserted into the manifold, and vapor generation reactions, in which a gas--liquid separator forms part of the manifold, as well as dialysis (sample components are transferred across a membrane), filtration (sample components are retained on a membrane) and liquid-liquid extraction. Many analytical instruments are designed to accept a sample solution and can be readily modified to handle a continuous liquid carrier stream. Thus FI methods have been developed for UV and visible absorption spectrophotometry, molecular luminescence (both photoluminescence and chemiluminescence), and a variety of electrochemical techniques (including potentiometry and voltammetry) as well as for the various atomic spectrometry techniques. In fact, the advent of FI procedures has given a considerable a I

I ir

MC

~

W

bMC W

Figure 1.1 Basic flow injection manifolds, (a) single line, (b) double line (the prototype for the multi-line manifold). C carrier stream, D detector, I injection valve, MC manifold components (a mixing coil in the simplest case, i.e. an open tubular reactor), P pump (usually peristaltic), R reagent, W waste.

FIA-Atomic spectrometry: a synergic combination for chemical analysis

7

boost to the use of chemiluminescence methods as the analytical signal depends on the kinetics of the mixing between sample and reagent solution. The FI format ensures high reproducibility. Flow injection has also been recognized as an ideal way to present samples to ion selective electrodes. The carrier stream provides a constant background electrolyte and gives a steady baseline and the reproducible transient nature of the sample zone removes the need to decide when the signal has reached steady state. Almost every existing laboratory-based analytical procedure can be adapted to a FI format with associated improvements in the method performance characteristics. As it is very simple and straightforward to change from one manifold design to another, it is worth considering the use of FI procedures even when the number of samples is limited. As will be illustrated later in Chapter 8, the basic characteristic of the FI procedure, namely that the chemistry in the manifold is under tight kinetic control, makes it possible to implement procedures in which differences in reaction rates are exploited. There is also a growing interest in the use of FI as the basis for the design of process analysis methods. The basic equipment needed is relatively low cost, mechanically simple and robust. The use of small amounts of reagent means long periods of unattended operation are possible and the transient nature of the signal means that, unlike the situation for continuous monitors, it is easy to discriminate between a real change in analyte concentration and a change in the baseline. Precise pumping and injection leads to precise fluid flow and this is the fundamental feature of FI that makes it useful as an analytical technique. Unlike the situation for manual procedures, every sample and standard is subjected, in a FI manifold, to identical treatment. This has the important consequence that it is not necessary to use reactions for which (a) the reactants are stable, (b) the products are stable or (c) the products are formed instantly. Thus FI greatly expands the range of chemistries which may be exploited for analytical purposes.

1.2.1 Basic principles of dispersion 1.2.1.1 Dispersion coefficient As a result of the dispersion processes caused by fluid flow in a FI manifold, the concentration-time profile of an injected sample of concentration Co at the detector is a single peak. Depending on the experimental conditions, the peak may be skewed (as shown in Figure 1.2) or may be a nearly symmetrical Gaussian shape. FI practice is to quantify the extent of dilution at the point of interest (usually the peak maximum) on the concentration profile of an injected solute in the absence of any chemical reaction, by a parameter known as the dispersion coefficient (for a definition see Table 1.1). This profile is shown in Figure 1.2.

J. F. Tyson

i i |

L i

t

P

v

time

Figure 1.2 Typical FI peak profile. The concentration at the peak maximum, C r occurs at time tr after injection. For an ideal detector, the instrument response as a function of time would be identical to this profile.

The ratio of the injected concentration, Co, of a given component to that corresponding to any point on the dispersed concentration gradient, C e is known as the dispersion coefficient, D e. Thus each peak is characterized by an infinite number of D e values ranging from infinity at the beginning of the peak, falling to a minimum at the peak maximum and rising to infinity again at the end of the peak (see Figure 1.2). The value of the dispersion coefficient at the peak maximum is normally used as a single parameter to characterize the extent of mixing in any particular FI system and is simply given the symbol D. Typically D is measured as the ratio of the detector response to the undiluted injected Table 1.1 The basic equations of flow injection, a distance from tube wall, Co concentration of injected species at time zero, C,, concentration of injected species at the peak maximum, d tube diameter, D dispersion coefficient, D,, molecular diffusion coefficient, l length, Q volumetric flow rate, r tube radius, s speed, s ...... maximum speed in tube center, s .... average linear velocity, V volume Parameter

Equation

1. Dispersion coefficient

D-Co/q,

......[1 - 4(a/d)2]

2. Parabolic velocity gradient

s=s

3. Linear velocity

s=l/t

4. Volume of a cylinder

V = 7rrZl

5. Average linear velocity

(it is useful to note that I mm 3 is equivalent to ! ixL) s,,,,= 1 . 6 7 Q / p r 2 = O . 5 s ....... (Q in mL/min and r in mm to give s in cm/s)

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9

solution to that at the maximum of the FI peak. A test compound to which the detector responds linearly is chosen. In practical terms, the dispersion coefficient is the ratio of concentrations, before and after the dispersion processes have occurred, in the element of fluid that yields the analytical read-out. Values of D may be conveniently classified as "reduced" (<1, i.e., on-line preconcentration), "limited" (1-3), "medium" (3-10) and "large" (> 10).

1.2.1.2 Laminar flow, dispersion and radial mixing As the basis of FI procedures is the use of fluid flow in closed conduits to perform chemical and physical manipulations on the sample, it is necessary to understand how it is possible to achieve mixing between fluids in a FI system. This is best done by considering the typical basic manifold systems shown in Figure 1.1. Conditions used in FI experiments produce what is known as laminar flow, i.e. on average the fluid molecules flow in stream-lines parallel to the walls of the tubing. The walls exert a viscous drag on the fluid and consequently there is a velocity gradient between the center stream line (which flows at twice the average linear velocity), and the wall (at which the velocity is zero). This velocity gradient is illustrated in Figure 1.3, which shows how the velocity varies with distance from the tube wall. The shape of this plot is a parabola whose equation is given, along with some other basic equations, in Table 1.1. When a sample solution is inserted into the flowing stream, the initially sharp boundaries between the sample and carrier are distorted by the fluid flow. In the narrow

Figure 1.3 Plot of parabolic velocity profile from wall (a=0) to tube center (a=r=d/2).

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bore tubing that is typically used in flow injection, the linear velocity can be surprisingly high (maybe as high as 1 m s-I). The shape of the sample zone after a few seconds of flow is perhaps best described as an elongated hollow needle. This is completely different from the situation in segmented flow analyzers. In addition to laminar flow, molecules undergo another type of movement in the tube, namely diffusion. The effect of laminar flow is to create concentration gradients within the tube which are both axial (along the length of the tube) and radial (across the tube). Species in solution will diffuse along concentration gradients. It is the radial concentration gradient that is important because molecules (or ions) of the sample on the leading boundary diffuse towards the walls and thus enter slower moving stream lines, whereas molecules of sample on the trailing boundary diffuse towards the middle of the tube and thus enter faster moving stream lines. Molecules of the carrier stream also experience concentration gradients and thus during the residence time in the tube an inter-diffusion of sample and reagent takes place. This is how sample and reagent mix in the single-line FI manifold. The processes are summarized in Figure 1.4 The change in speeds experienced by molecules on the two boundaries is not the same because the velocity gradient is parabolic. It can be seen from Figure 1.3 that movement from the walls by a certain distance speeds up a molecule's progress much more than movement by the same distance from the center slows down the progress of a molecule. The effect of diffusion on the sample is to slow down the front boundary and speed up the rear boundary. It is important to note that, under the conditions normally selected for FI experiments, by the time the dispersing sample zone passes through the detector, the two boundaries have overlapped and only one continuous profile is recorded. It is often said that diffusion in liquids is slow, and so it is in comparison with diffusion in gases, but it is certainly not negligible as it is the process which is responsible for the inter-mixing of the injected solution and the carrier stream in the basic FI manifold. Once molecules of the sample and of the carrier inter-diffuse, chemical reaction is

Figure 1.4 Laminar flow and diffusion. The initial sharp boundaries shown by the dotted lines are distorted by the velocity gradient produced by laminar flow to give the new boundaries shown by the solid lines, which represent the situation after a few ms. Molecules of sample diffuse into the carrier stream at positions a and, at position b, molecules of carrier diffuse into the sample.

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v

Figure 1.5 Geometryof a confluence point to promote rapid radial mixing. possible if the carrier contains an appropriate reagent. In the basic FI experiment it is the product of this reaction which is monitored at the detector. If no reaction occurs, as in microsample introduction to an atomic spectrometer, the laminar flow and diffusion processes in the manifold dilute the sample solution. This inter-diffusion is not the only way solutions are mixed in FI manifolds. In multiline manifolds there is a second mechanism for mixing, namely the confluence of merging streams. If the flowing streams are merged at the kind of Y-junction shown in Figure 1.1 b, radial mixing is slow as the two streams tend to flow side by side. In practice the streams should be brought together as shown in Figure 1.5. This junction produces turbulence at the confluence point which results in rapid radial mixing. For both single and multi-line manifolds, it is possible to produce additional secondary flow patterns to aid mixing by (a) coiling the tube tightly, (b) knotting or "knitting" the tube to produce a 3-dimensional disorientated reactor, (c) packing a wider tube with small glass spheres to give a packed-bed reactor and (d)to pack a narrow tube with a single line of glass beads to give a single bead-string reactor. The principle of all of these devices is to create a flow of fluid between the center and the walls of the tube (in addition to the diffusion process) so as to mix the sample and the carrier.

1.2.2 Factors controlling dispersion One of the basic features of FI is that the dispersion coefficient, D, is readily controlled experimentally and part of the power of the technique is that a manifold may be assembled with an appropriate value of D by selection of the appropriate design parameters on the basis of some simple guidelines. For flow injection atomic spectrometry, the most important experimental parameters are (1) volume injected and (2) tube dimensions.

1.2.2.1

Volumeinjected

For a single-line manifold as the volume is increased, the extent of the inter-diffusion of sample and carrier decreases as the residence time is the same. Thus the sample will be

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J.F. Tyson

less dilute at the peak maximum and the value of D will increase. Unless the flow cell used has a very large volume, an injected volume of 500 I~l typically gives a peak response close to the steady state value (i.e. a dispersion coefficient, D, of close to 1). For a doubleline manifold, the sample will always be diluted at the confluence point by the merging fluid and thus, although the same variation in peak height is seen with increasing volume injected as for the single-line manifold, the dispersion coefficient reaches a minimum set by the relative flow rates in the two lines. Usually changing the volume injected is the most powerful way of controlling D. 1.2.2.2

Tube dimensions

As the length of tubing between the injection valve and the detector is increased, the extent of mixing will increase and thus the dispersion will increase. It can easily be seen that in order to minimize unwanted dispersion, the lengths of tubing between components of a FI manifold should be kept as short as possible. The same trend is observed as tube diameter is changed, the wider the tubing, the bigger the dispersion. 1.2.2.3

Flow rate

As the flow rate is increased, the residence time in a single line is decreased and hence there is less contribution to dispersion from diffusion, thus the usual trend is to observe a decreasing value of D with increasing flow rate if all other parameters remain the same. For detectors whose response is independent of flow rate (such as that of a visible spectrophotometer) there may be little change in peak height as the flow rate is increased but a marked decrease in peak area. For a double-line manifold, the relative flow rates in the two lines has a direct effect on the value of D. For detectors whose response does depend on flow rate, such as a flame atomic absorption spectrometer or a plasma spectrometer, the situation is more complex. Usually an increase in D is observed as the flow rate of the fluid delivered to the nebulizer is decreased. The situation is discussed in more detail in Chapter 3. 1.2.2.4

Other parameters

Due to the secondary flow patterns produced, the use of tightly coiled or knotted tubes, packed beds and bead strings (described earlier) produces a decrease in D compared with that of an open tubular reactor. Dispersion coefficient may decrease with increasing temperature and may increase as the molecular diffusion coefficient of the species monitored increases. The effects described above may be summarized as a collection of "rules" concerning dispersion coefficient. These are: (a)

D increases with increasing tube length;

FIA-Atomic spectrometry: a synergic combination for chemical analysis

(b) (c)

(d) (e)

(f) (g) (h) (i)

13

D increases with increasing tube diameter; D increases with increasing average flow rate (this is a rather gross generalization as there are many manifolds in use for which D is largely independent of the average flow rate and there are situations in which D increases as the average flow rate decreases); D increases with increasing detector volume; D increases with increasing molecular diffusion coefficient (the effect is only observed for large differences in this parameter); D decreases with increasing volume injected (this is the single most powerful method of changing D); D decreases with the use of packed beds and bead strings; D decreases with increasing tortuosity of tubing (e.g., tight coiling, or knotting); D decreases with increasing temperature (again this is a rather gross simplification as the effect observed depends on the variation of diffusion coefficient and viscosity with temperature).

The only way at present to assemble a FI system with a desired D value is to use the guidelines above together with previous experience and measurements of the effect of changing some parameters on the observed value of D. This is particularly true for manifolds designed for gas diffusion, liquid-liquid extraction, liquid--solid extraction, dialysis or precipitation where the manifold may be designed for preconcentration. 1.2.3

Chemical reaction

All the discussion above concerning the effect of various experimental factors has been in relation to the processes of fluid flow and how these processes affect the nature of the concentration profile of the injected sample at the detector. However, if the detector monitors a reaction product between a sample component and a carrier stream component, then some additional properties of the system have to be considered if the effects of operating parameters on a product peak shape are to be accounted for. For example, if the reaction responsible for the product formation is slow, then a decrease in residence time (as would be produced by shortening the tube length) might well produce a decrease in the amount of product formed even though the dispersion of the system, as measured from the injection of a dye solution, had been decreased. In general, the FI system is designed so that the reagent concentration is in excess of the sample concentration across the entire dispersing zone and thus the product profile closely follows that of the sample. However, if the conditions are changed so that this is no longer the case, then the resulting product peak may have a shape that is very different from the sample profile. For example, as the volume injected into a single-line manifold

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J.F. Tyson

is increased, eventually the product peak splits into a doublet as there is no longer an excess of reagent in the center of the dispersed profile. To distinguish between various processes which contribute to the shape of the reaction product peak, those which operate on the injected dye solution are known as physical dispersion processes and effects such as doublet peak formation are ascribed to chemical dispersion processes.

1.2.3.1 Implicationsfor atomic spectrometry There are a number of uses of flow injection with atomic spectrometry in which on-line chemical reactions are carried out as part of the sample pretreatment such as for chemical vapor generation, solid phase extraction or microwave digestion, and thus consideration of the use of FI manifolds for chemical reaction is needed. The main consideration is to ensure that the manifold produces an appropriate degree of mixing between the sample and the reagent solutions so that the desired reaction proceeds. As mixing results in dilution, the sensitivity is decreased and thus it is necessary that the dispersion coefficient be optimized so that the desired interactions take place without unnecessary dilution. There are also situations where it is required to add a reagent so that a desirable reaction in the atomizer is promoted (for example, the addition of lanthanum as a releasing agent in FAAS). There is a general misconception, currently perpetrated in the FI literature and text books, to the effect that some designs of FI manifold for performing chemical reactions give rise to higher sensitivity than others. This is not true (and was recently experimentally proven). ~3 However it is important to realize to what extent sample and reagent solutions are diluted in a FI system so that the appropriate concentrations of reagent and sample can be achieved. Measurement of the dispersion coefficient clearly gives the sample dilution. In a single-line manifold, the reagent dilution may be calculated from the formula Dr=D~ (D - 1), where Dr is the reagent dispersion coefficient. ~4This formula, which is derived in Chapter 3, shows that low sample dilutions are accompanied by very high reagent dilutions. For example when D= I. 1, Dr- 1 l, i.e. a manifold which only diluted the sample concentration to 91% of the injected value would, at the peak maximum, dilute the reagent in the carrier stream to 9.1% of the concentration introduced into the manifold. In the double-line manifold, the reagent dilution depends on the relative flow rates of the streams arriving at the confluence point.

1.3

Basic concepts of atomic spectrometry

Detailed explanations of the basic concepts of atomic spectrometry are considered beyond the scope of the present text, though in order to appreciate the potential of FI techniques when coupled with atomic spectrometry it is useful to have some understanding of the

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15

scope and limitations of the present generation of atomic spectrometry instruments. Although the interaction of electromagnetic radiation of all energies with atoms can be exploited for the purposes of qualitative and quantitative analysis, the techniques which are the subject of this book are based on (a) the interrogation of gaseous atoms by UV and visible light and (b) the detection of gaseous ions after separation according to mass-tocharge ratio. In general the various atomic spectrometry techniques used for chemical analysis are classified on the basis of the nature of the spectroscopic interrogation method (absorbance, emission or fluorescence) and on the device used to convert the sample material into gaseous atoms (flame, graphite furnace or inductively coupled plasma). In terms of the use of instruments, it is flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS)- also known as electrothermal E T A A S - and inductively coupled plasma atomic emission spectrometry (ICP-AES) which are the most widely used. This latter technique is often referred to as ICP-OES, inductively coupled plasma optical emission spectrometry, in recognition of the fact that many elements are determined by virtue of their ionic emission from the plasma, rather than their atomic emission. There is a steady growth, however, in the use of inductively coupled plasma mass spectrometry (ICP-MS) and there are a small number of companies supplying instruments for flame atomic fluorescence spectrometry (FAFS). The atomic spectrometry research community also contains groups working with instrumentation that is not widely commercially available, such as laser excited atomic fluorescence in a graphite furnace or glow discharge atomic emission from a graphite furnace or atomic emission from a DC plasma. It should also be borne in mind that there is a considerable use of atomic spectrometry in the metallurgical industries in the form of arc and spark emission, though there is relatively little research and development activity compared with that associated with the flame, furnace and plasma instrumentation. For some elements it is possible to generate a volatile derivative outside the instrument and transport this into the atomizer. This procedure, which is quite widely used, is known as chemical vapor generation (CVG); the two main categories are hydride generation (HG) and cold vapor (CV) generation for the determination of mercury and cadmium. Detailed explanations of the basic concepts of all of these techniques may be found in several text-books, though the treatment by Ingle and Crouch is recommended. 15There are a number of texts which cover both basic concepts and provide good guides to the scope, limitations and use of atomic absorption spectrometry, ~6 hydride generation ~7 and plasma spectrometry. 18The continuing developments in all aspects of atomic spectrometry may be readily followed via the regular Atomic Spectrometry Updates review articles which appear throughout the year in the Journal of Analytical Atomic Spectrometry published by the Royal Society of Chemistry. ~9 Developments in atomic absorption spectrometry may be viewed largely as the results

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J.F. Tyson

of drives, (a) to achieve lower detection limits, (b) to achieve greater accuracy and (c) to achieve greater throughput, not just in terms of numbers of samples processed in a given time but also, more importantly, in the number of elements determined per sample. Even without the integration of FI techniques with atomic spectrometry instrumentation, many significant developments have been made in the performance of commercial instrumentation over the last 10-15 years.

1.3.1

Atomic absorption spectrometry

The absorption of radiation by free atoms follows the same general law as for all absorption processes given in the following equation

P=P,, exp( - kNL) where Po is the incident power per unit area, P is the transmitted power, L is the path length in the absorbing medium, N is the number of absorbing centers per unit volume and k is the absorption coefficient. The value of k depends on the frequency of the radiation and the transition probability for absorption. For this phenomenon to be useful, it must be possible to measure the absorption of light (i.e. P and Po) under conditions for which k is essentially constant. This is achieved by ensuring that the absorption profile of the atoms is broad compared with range of wavelengths measured at the detector. This is achieved by the use of a light source which emits narrow spectral lines characteristic of the atom to be detected. A consideration of the effects of temperature and pressure on the shape of spectral lines leads to the design of such a light source based on a low-pressure electrical discharge with a cathode configured as a hollow cylinder. The element of interest is a constituent of the cathode lining. Under these conditions absorbance, A, defined as log~o(Po/P) is a linear function of N, the number of absorbing atoms. As atoms may only absorb if they are in the lower of the two energy levels involved in the transition, A is directly proportional to Nj, the number of atoms in this lower state. At the temperatures of flame and fumace atomizers, Nj is the ground electronic state. To a reasonable approximation, these sources are in thermal equilibrium and thus ~ is directly proportional to N. Also to a reasonable approximation for flames and furnaces, Nj is equal to N. The other major feature of an atomic absorption spectrometer is a way of generating atoms from the wide range of sample materials for which information about the trace element composition is required. Although there has been a sustained research effort into methods for the direct handling of solid materials (with some success), the most widely used first step in the route to atomic vapor is to separate the sample constituents into species in aqueous solution. A number of options are then available. Volatile derivatives may be produced and blown out of solution. In this case, the atomizer is either a hot quartz

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17

tube or a graphite furnace on whose walls the analyte is trapped prior to atomization in a discrete step. The solution could be sprayed into a flame or a small sub-sample (about 20 I~1) could be placed in a graphite furnace atomizer. The atomizer must function so that the number density of atoms produced and interrogated by the optical components is directly proportional to the concentration of species containing the element(s) of interest in the solution and, ideally, is independent of the nature and concentration of matrix components. Atomic absorption spectrometry suffers from a number of interference effects which are almost always present, even when the matrix is distilled water. When other matrix components are present in high concentrations, quite severe interferences may be encountered. In general, these interferences can be divided into those related to the spectroscopy, those related to the chemistry of atom formation and those related to introduction of the sample solution into the atom source. Although the number of atoms in the ground state is approximately equal to the total number of atoms, this is not to say that the degree of excitation is insignificant. For example, at 2000 K about 4 of every 10 '~ atoms of copper are in the first excited state, however there may be as many as 10 ~z atoms per cm 3 per ppm in the solution introduced and it is easily possible to detect the radiation from these 400 atoms per cm 3 as they return to the ground state by the emission of light. Thus atom sources for atomic absorption spectrometry will emit radiation at the same nominal wavelength as is absorbed. This radiation will be measured by the detector and will cause a serious systematic error if not corrected. All atomic absorption spectrometers will modulate the light source and use a lock-in amplifier to discriminate against the dc radiation from the atom source. Should there be a very intense molecular emission from the atomizer, the detector electronics may become overloaded and precision and detection limits may suffer. In addition, there is also the possibility of attenuation of the source radiation by molecular absorption or scatter in the flame. As this generally occurs over a broader range of wavelengths, it is possible to correct for this background absorption by switching the instrument between two modes of operation, one of which measures total absorbance and the other which measures the molecular absorbance. Thus the difference between the two values gives the atomic absorbance. The production of atoms is the result of a complex series of chemical reactions and processes. Both chemical and thermal energy are used to break the bonds in the molecular precursors. Unfortunately many molecular precursors are very stable and the yield of free atoms may be rather low. These precursors include the oxides that result from the evaporation of dilute aqueous solutions, and so even in the absence of any other matrix components, the atom yield may be poor. Increasing the temperature will increase this yield, but simultaneously increases a loss mechanism by increasing the degree of

18

J.F. Tyson

ionization. As the loss of one (or more) electrons produces an ion species with a completely different electronic structure to that of the neutral atom, the absorption characteristics are completely different and the ion will not be detected at the wavelength of atomic absorption. 1.3.1.1

Flame atomization

The sources used are either the air-acetylene flame or the nitrous oxide acetylene flame, burning on a single slot burner several cm in length. The latter flame is significantly hotter and is needed to atomize some elements, such as aluminum, which form refractory oxides. To produce atoms from the sample, a solution is converted to a mist of fine droplets by a nebulizer. The device universally used is the concentric pneumatic nebulizer driven by the oxidant flow. The resulting spray may be further fragmented by an impact surface and the resulting aerosol is mixed with the fuel gas in a spray chamber and swept to the burner. About 10% of the solution is transported to the flame in a range of drop sizes from 1-10 I~m. The gases are also saturated with water vapor. During the rather limited residence time in the flame, the droplets are dried and the resulting salt particle vaporized. The resulting molecular species may be atomized by thermolysis or by chemical reaction with reducing species such as carbon and carbon monoxide. Gas phase species will diffuse laterally away from the center of the flame and the region of highest atom number density is a complex function of sample properties and flame conditions. The bumer is moveable with respect to the light beam so that the maximum absorbance signal can be found by a process of trial and error. Flame AAS is widely used for the determination of about 65 elements (metals and metalloids), with detection limits ranging from a few ppb to a few ppm. Instruments are single channel, i.e. only one element may be determined at a time, but there are spectrometers which will rapidly change operating conditions so that several elements can be determined in one sample in an automated sequential run. Instruments are reliable, robust and simple to use. Interference effects are well understood and characterized. These interferences include the spectroscopic effects described above of emission from the atomizer and molecular overlap or scatter of the source radiation. A major interference is stable compound formation. For some elements this intereference arises from the stability of the metal oxide, whereas for others, the interference is due to reaction with matrix components, particularly those which form oxyanions. Thus phosphate and aluminum can cause severe depressions in the extent of formation of atoms from elements in group 2 of the periodic table. These effects may be overcome to a certain extent by the use of the hotter nitrous oxide flame and/or by the addition of releasing agents. Releasing agents are elements which form even more stable compounds with the interferents than do the analytes. Lanthanum is a commonly used releasing agent, added in concentrations of

FIA-Atomic spectrometry: a synergic combination for chemical analysis

19

several thousand ppm. The hotter nitrous oxide actetylene flame causes an ionization interference as many elements are significantly ionized, though group 1 elements may suffer an ionization interference in the air-acetylene flame. Ionization interferences may be overcome by the addition of a high concentration of an easily ionized element, such as K, which increases the electron number density in the flame and suppresses the ionization of the analyte element(s) present in low concentrations. Factors which affect the solution uptake rate into the nebulizer and/or the characteristics of the aerosol formation processes will also act as interferences as these can affect the value of the number density of the atoms eventually formed. Thus the viscosity of the sample solution is an important factor to control as is the composition with respect to other solvents. Many organic solvents, with vapor pressures and surface tensions significantly different from that of water will act as interferences, though the presence of some organic solvents may enhance the signal compared with that obtained for an aqueous solution. This may be due to a combination of factors related to uptake rate, aerosol formation and desolvation and flame conditions (the solvent may burn, increasing the flame temperature). On the other hand some organic solvents, such as halogenated hydrocarbons cannot be tolerated, as they extinguish the flame. As will be discussed in Chapter 3, FI procedures can be used to overcome a number of these commonly encountered interferences in FAAS.

1.3.1.2

Graphite furnace atomization

The performance of AAS can be improved substantially in terms of the lowest concentration which can be detected by the use of the graphite fumace atomizer. A typical modem furnace is shown in Figure 1.6. The fumace is able to create a substantially greater number density of atoms compared with that in the flame and though the path length is

Figure 1.6 Cut-away drawing of a transversely heated graphite furnace atomizer with integral L'vov platform.

20

J.F. Tyson

shorter by a factor of 5 or so, the signal may be 2-3 orders of magnitude greater. As the limiting sources of noise are somewhat similar, the detection limits obtained with a graphite furnace atomizer are 2-3 orders of magnitude lower than for the same elements by FAAS. Samples are introduced as discrete volumes.of about 20 txl, usually solutions but the technique is well suited to the analysis of slurries. In fact, of the various atomizers, graphite furnaces are by far the most successful in terms of their capability to handle slurries. In some furnace designs, the sample is placed on a platform (the so-called "L'vov" platform) inside the furnace which is largely isolated from the rest of the tube so that while the furnace is heated by resistive heating on the passage of an electric current, the platform is heated predominantly by radiation from the hot tube walls. Thus the species which evaporate from the surface of the platform do so into an atmosphere which is hotter than the platform surface and so the formation of molecular species in the gas phase is decreased. This is a widely used procedure for overcoming some gas-phase stable compound formation interferences. Following drying and thermal pretreatment, the residual material is atomized as the furnace is rapidly heated to a temperature of between 1500 and 2600~ (depending on the element). A transient absorption signal is obtained with a basewidth of a few seconds. Peak area is measured. As the potential for interferences by matrix components is much more severe than in the case of flame AAS (the concentration of analyte being measured is much lower whereas the concentration of matrix components may be unchanged), there has been a major research effort devoted to the thermochemical separation of analyte and matrix species in a thermal pretreatment step in the furnace. In general, this procedure is known as "chemical modification" and in the most common form, a reagent is added which will form a thermally stable compound with the analyte so that more volatile matrix components may be removed from the furnace during this pretreatment stage. The modifier may be added prior to the sample and thermally treated or the sample and modifier may be mixed in solution and then introduced into the furnace. A popular modifier at present is palladium which appears to form thermally stable compounds with a number of metals. An analyte stabilizer, such as palladium, is often used in conjunction with an ashing aid. For example, the addition of magnesium nitrate helps to decompose organic species as oxygen is released when the dried salt is pyrolyzed. In some cases, oxygen gas may be bled into the furnace during the ashing stage to decompose large amounts of organic materials that would be present in samples such as sweet wines, soft drinks, blood, urine, and soil slurries. For some determinations, it is necessary to use a thermal pretreatment stage to convert different forms of the analyte element to a common form, otherwise the element may volatilize and atomize at different temperatures, giving rise to erratic signals. This may be rather difficult to do. For example in human urine, selenium

FIA-Atomic spectrometry: a synergic combination for chemical analysis

21

could occur as selenate, trimethylselenonium, and perhaps various selenoamino acids including selenomethionine, selenocysteine and selenopurine. The reliable determination of Se in human urine by a procedure in which diluted urine is introduced into the furnace has proved somewhat elusive. A digestion outside the instruments followed by a FI-based introduction procedure appears to offer improved performance. 2~ In addition to thermal pretreatment it is necessary to use reliable background correction with ETAAS. This topic has also been a major area of research and development, but is now an area of relatively little activity as the major problems are regarded as having been solved. Although there are three types of background correction procedure in use in commercial instruments, it is the methods based on (a) the use of a continuum source and (b) the use of the Zeeman effect which are widely implemented in commercial instruments. Although a detailed discussion of these is beyond the scope of this book, the basic principles need to be appreciated by every user of a graphite furnace AAS instrument. When a continuum light source is used in a conventional atomic absorption spectrometer, the absorption by atoms cannot be detected as the amount of light removed is only a very narrow slice of the relatively large range of wavelengths that pass through to the detector. On the other hand, a molecular species with a relatively broad absorption band will absorb a measurable amount of light and thus, when a continuum source is used, only molecular absorption is measured. Scatter from appropriately sized particles would also be measured with a continuum source. With the hollow cathode source, both molecular and atomic absorptions are measured and thus the difference represents the atomic absorption. When the atoms in the atom cell are placed in a strong magnetic field, the absorption spectrum splits into a number of components. This phenomenon is known as the Zeeman effect. The part of the profile which remains at the same wavelengths as for the situation when the magnetic field is off becomes polarized, i.e. will only absorb radiation polarized in a particular direction. If the light from the hollow cathode lamp is polarized at right angles to this direction, the atoms will not absorb any source radiation. The magnetic field needs to be strong enough to move the parts of the split profile that can absorb the radiation sufficiently far away in the spectrum that there is no source light at these wavelengths. Molecular species are unaffected by the magnetic field and will absorb the same amount of light with the field on as with the field off. When the field is off, both atomic and molecular absorption are measured and thus the difference between this absorbance and that with the field on gives the atomic absorption. As the concentration-time profiles of the atomic and molecular species are rapid transient events, the background correction system must be able to switch between the two types of measurement rapidly in order that the subtraction process gives an accurate

22

J.F. Tyson

atomic absorption profile. This is a useful feature for the detection of FI signals, which may also be rapid transient events. However, it may be difficult to persuade any given instrument to operate in the data collection and evaluation mode for a graphite furnace atomizer when a flame atomizer is used. In the flame mode, most instruments are expecting to deal with a steady state signal and may slow down the data collection rate. Sometimes modem instruments are too smart. 1.3.1.3

Chemical vapor generation and quartz tube atomization

There are many volatile metal and metalloid compounds known, but there are relatively few that (a) can be synthesized easily and rapidly from aqueous solution and (b) that are sufficiently volatile to be readily separated from the aqueous matrix at room temperature. However, for a few elements, there are a limited number of chemical reactions that have the appropriate characteristics that enable them to be used for the generation of volatile derivatives which can be separated easily from solution. The most widely exploited reaction is probably the reduction of mercury (in solution as Hg 2§ ions) to elemental mercury, which is sufficiently volatile at room temperature to be blown out of solution by a purge gas, such as argon or nitrogen. As mercury vapor is monatomic, no further atomization is needed and the vapor may be simply transported to a suitable cell and interrogated spectroscopically. There is a group of elements in the p-block of the periodic table which form volatile hydrides with appropriate reagents, for which the reaction characteristics are suitable as the basis of a analytical procedure. In this group are Ge, Sn, Pb (all in group IVA), As, Sb, Bi (all in group VA) and Se and Te (both in group VIA). The first reagent to be used to generate hydrides was the nascent hydrogen generated by the dissolution of a metal (e.g. zinc) in a mineral acid (such as hydrochloric acid), but the reagent which is almost always used nowadays is tetrahydroboratelII, BH4, commonly referred to as borohydride. Borohydride was introduced for this purpose in 1972, and this topic has been comprehensively reviewed in a recent book 17 and will be dealt with in detail in Chapter 8. For mercury and the elements arsenic, antimony, bismuth, selenium, tellurium and tin, chemical vapor generation is the most widely used procedure in their trace determination by atomic spectrometry. The hydride formation reaction (see Chapter 8) may be described as, initially the replacement of hydroxyl groups on the fully protonated analyte species with hydrogen, forming the analyte hydride and borane (BH3) followed by the hydrolysis of the borane to give boric acid and hydrogen. Reduction of oxo-anions by borohydride will only occur rapidly if the species is fully protonated and thus for the reaction to be useful in an analytical procedure, the acid

FIA-Atomic spectrometry: a synergic combination for chemical analysis

23

concentration must be high enough. For example arsenate, As(V), will only react in solutions which contain 1-2 M acid, whereas As(III) will react at pH 4 (10 -4 M acid). It is not possible to fully protonate selenate Se(VI) in aqueous acid solutions and thus it is not possible to produce hydrogen selenide from this species. A separate reduction stage is needed, often referred to as a "pre-reduction". For selenium, the usual pre-reducing agent is hot hydrochloric acid. For the elements typically determined by HG-AAS, the background absorption of the air-acetylene flame is very high and the associated flicker noise makes detection limits rather poor. A better flame, with improved background characteristics, is the so-called "hydrogen diffusion" flame, which is really a mixture of hydrogen and argon (or nitrogen) with air entrained at the burner head. This atomizer is typically used when measuring hydride forming elements by atomic fluorescence spectrometry. When atomic absorption is used as the measurement technique for HG, improved performance is obtained by using a heated quartz tube as the atom cell rather than a flame, though there are variations of this mode of atomization in which a flame is produced inside the tube. Although the tube can be heated by an air acetylene flame, better control over the temperature can be achieved by the use of an electrically heated mantle. The surface of the quartz atomization cell plays an active role in the decomposition of the hydride to release free atoms. The mechanism of the decomposition also requires a small amount of oxygen. This is not usually difficult to supply, as small amounts dissolved in the solution will be stripped and transported with the purge gases. However, for the determination of tin, an improved sensitivity may be obtained by the use of an Ar- 1% 02 gas stream. The surface activity of the tube needs to be restored periodically by etching in a solution of hydrofluoric acid. Details will be provided by the manufacturer of the particular unit used. CAUTION! HF is a potentially dangerous chemical and should be handled with the proper safety precautions. The activity of the quartz surface can be easily degraded if moisture or aerosol is transported from the gas-liquid separator (GLS). Aerosol can be particularly problematic, as the presence of salts in the tube causes a very severe depression in the atomization efficiency (and hence the signal). An important practical consequence is that the gas-liquid separator should not produce aerosol. This is not so much a problem with the devices used in continuous flow procedures as a large internal volume has no effect on the magnitude of the steady state signal. However in a FI system the peak shape is adversely affected by dispersion in the gas-liquid separator (and transfer tubing between the GLS and the atomizer) and so the device is typically designed with a small internal volume. To avoid the carry-over of aerosol and moisture, two additional devices are typically inserted into the transfer line. These are a filter to remove droplets and some sort of desiccant to remove the water vapor. Water vapor can be removed by the use of a special transfer tube device made of Nation | which dries the gas

24

J. F. Tyson s h e a t h gas + m o i s t u r e

ii

~

w e t gases f r o m gas-liquid separator

I

III

t

Nation membrane

f

d r y gases to a b s o r p t i o n cell

sheath gas

Figure 1.7 A Nation dryer tube. Water vapor is transported across the non-porous derivatized Teflon~ membrane and continuously removed by the dry purge gas. Well over 90% of the water vapor entering the dryer is removed with negligible loss of analyte. by the removal of water vapor through the Nation tube walls into a dry sheath gas. The device is shown schematically in Figure 1.7. The problems of water vapor and aerosol become worse when there is excessive foaming in the GLS. Foaming may be caused by matrix components which reduce surface tension and occurs when the borohydride concentration is high. A concentration of about 0.2% in 0.05% sodium hydroxide is suggested. This restriction on the borohydride concentration means that not all of the features of a batch procedure can be achieved in a FI system. For example, in the batch mode of generation it is possible to determine As(V) at the same sensitivity as As(Ill). In general, in a FI procedure a greater sensitivity is obtained for As(Ill) than for As(V); this is because the reaction conditions (borohydride concentration, residence time) do not produce complete reduction of As(V) to As(Ill). In general, there are three types of interference that have to be considered in hydride generation procedures. These are (1) interferences in the generation reaction (due to the nature of the analyte species or presence of matrix components which participate in competing processes), (2) interferences in the release of the hydride from solution and transport to the atom cell and (3) interferences in the atomization processes (not a problem in the case of the determination of inorganic mercury). In all but the simplest cases, such as the analysis of pristine waters for inorganic mercury, arsenic, selenium, etc. interferences have to be accounted for. One of the features that makes FI a suitable basis for vapor generation procedures is the superior tolerance to interference effects. The sensitivity of the procedure depends on the nature of the analyte species in solution. In general, the best sensitivity is obtained from simple inorganic analyte species in either the same oxidation state as in the hydride or in the nearest oxidation state. For example, for selenium the best sensitivity is obtained for selenite (SeO32-) in which selenium is in the + 4 oxidation state. No signal is obtained when selenium is present in the + 6 oxidation state. Acid solutions of selenide (the - 2 oxidation state) are not stable with respect to loss

FIA-Atomic spectrometry: a synergic combination for chemical analysis

25

of hydrogen selenide. Of course, many samples do not contain these desirable precursors; the element may be combined in an organic compound. For example in human urine, arsenic may be present as arsenobetaine and arsenocholine. These compounds arise from seafood in the diet and are quite harmless. As they do not form a volatile derivative with borohydride, they do not contribute to a measurement of arsenic based on hydride generation. This can be an advantage, as usually the purpose of the determination of arsenic in urine is to assess exposure to toxic forms of arsenic, which show up in the urine as species which will give a signal with borohydride. Some analyte species are capable of forming volatile derivatives with borohydride, but with reduced sensitivity compared with that obtained with the species used for calibration. For example, the methylated arsenic species which are formed in some soils and sediments by microbial action may be determined by HG, but appropriate calibration procedures are required. In general if HG is to be used for the determination of the total element, some sort of sample pretreatment is needed to convert the various forms of the analyte that may be present into the desired inorganic species in the appropriate oxidation state. Usually a twostage procedure is needed. The first stage would be the destruction of any organic compounds, a process which requires strongly oxidizing conditions, which would produce the analyte in the highest oxidation state possible. The second stage would be the reduction of the analyte to the desired state for hydride generation. The exceptions to this need for "pre-reduction" are tin and lead, which should be in the +4 state prior to the addition of borohydride. The presence of other sample components can cause severe depressive effects. There are several reasons for this. There can be sample components which compete with the analyte for the borohydride. These include other hydride forming elements and species which can be reduced by borohydride. In this latter category, transition metals can be particularly problematical as the reduction products are typically finely divided metal or metal borides (or both). This material can both adsorb hydrides and catalyze the decomposition to the element (and hydrogen). 1.3.2

Atomic emission spectrometry

The most widely used atom source for the determination of trace elements in solutions by emission spectrometry is the inductively coupled plasma (ICP). The basic principles of the source are illustrated in Figure 1.8. The plasma is sustained in argon gas by induction heating from a radio-frequency power supply operating at about 1-2 kW. The oscillating current in the load coil induces circular motion of charged species (electrons and ions) in the gas. These charged species collide with neutrals and a self-sustaining discharge is produced, provided that (a) argon gas is continually supplied, and (b) the plasma is kept away from the walls of the torch by a circular flow of coolant gas introduced tangentially

26

J.F. Tyson

lower down the body of the torch. Sample is introduced as a fine aerosol in a central vertical gas flow such that a hole is punched through the plasma producing a central axial channel. The plasma produces intense continuum emission due to normal "black body" behavior and also to recombination of energetic ions and electrons and thus the region which is interrogated for the detection of atomic emission is a few cm above the load coil in what is known as the normal analytical zone. Although the plasma is very hot, it does not have a substantial heat capacity and large amounts of water droplets and vapor will extinguish it. The sample introduction system is therefore designed to produce a finer spray than in the case of the combustion flame and only about 1-2% of the solution is transported into the plasma. The detail of the processes

H

H

angential coolant flow

uptlonal flow

Figure 1.8 Schematic cross-sectional diagram of an inductively coupled plasma torch and plasma. The orientation of the magnetic field H, as a result of a current I flowing in the load coil is shown. The gas flows are configured so that (a) the hot plasma is kept away from the walls of the torch (which would otherwise melt) and (b) the sample aerosol is injected up a thin central channel. Used with permission from ref. 15.

FIA-Atomic spectrometry: a synergic combination for chemical analysis

27

which occur as the droplets pass through the plasma have been poorly understood until recently. However a much clearer picture has emerged from the work of Olesik et al. 21'22 in which a single-stream, monodisperse droplet generator has been used for sample introduction. In general, the processes of evaporation, volatilization, atomization, and ionization occur sequentially, while the clouds of atoms and ions diffuse away from the originating salt particles. The plasma is optically thin and has very few analyte atoms outside the central channel. Thus, self-absorption is not significant and long linear calibration curves are obtained. Although very bright atomic and ionic emission is obtained, the detection limits for many elements are not much better than for flame AAS. The reason for this is that the predominant noise source is the flicker noise of the high background. The situation can be improved by about an order of magnitude by viewing the plasma axially (rather than radially). In this configuration, the signal to background noise ratio is improved, z3 Stable compound formation is not such a problem as for flames, as the conditions are such that virtually all chemical bonds are broken on passage through the plasma, though if the viewing height is too far above the load coil, molecular compounds may re-form. A major feature of ICP sources is that with an appropriate spectrometer, they can be used as the basis of simultaneous measurements of a large number of analytes. If an echelle spectrometer is used, the instrumentation does not require a very large amount of space and with an array detector, signals can be time-correlated so as to allow removal of flicker noise by the judicious choice of internal standards, z4 The major interference effects with ICP emission spectrometry are spectral interferences from matrix components. High concentrations of iron or uranium are particularly difficult to deal with as the emission spectra of these elements are very rich in lines and the chances of overlap with an analyte line are relatively high. Plasmas also suffer from effects due to elements which are easily ionized (the so-called easily ionized element or EIE effect), these elements can change the conditions of the plasma so that the best viewing height changes. In addition, any parameter which affects the sample uptake rate and nebulization process has the potential to act as an interferent, just as is the case for flame sources. 1.3.3

Inductively coupled plasma mass spectrometry

Although inductively coupled plasma mass spectrometry (ICP-MS) is a relatively new technique, it is rapidly becoming widely used because (a) it has excellent detection capabilities and, (b) with the quadrupole mass analyzers most commonly used, is capable of measuring almost every element almost simultaneously. In addition, it is possible to measure the signals from the various isotopes of the elements which opens up some interesting possibilities for the use of non-radioactive tracers and allows the method of

28

J.F. Tyson

isotope dilution to be used as a calibration procedure. At present, most inductively coupled plasmas in commercial atomic spectrometry instrumentation are used as devices to atomize and excite sample materials so that the resultant emission of light may be used as the basis of a quantitative measurement of the analyte. However, the plasma is an efficient ionization source, having the capability to ionize every element with a first ionization potential less than that of argon and, in fact, many emission spectrochemical determinations are made on the basis of measuring the light emitted from ions in the plasma, as the number densities of these are greater than the corresponding precursor atoms. Thus the plasma can be used as an ionization source in the mass spectrometric detection of analytes and no further ionization of the analyte species is needed. The interfacing of an inductively coupled plasma at atmospheric pressure with a mass spectrometer which operates at about 10 -5 Torr is a formidable task and much of the early instrumental research and development was concerned with the practicalities of making such an interface. The quadrupole mass filter devices which are commonly used in instruments allow ions of a particular mass to charge ratio (m/z) to pass through to the detector depending on the nature of the electrical potentials applied to the poles of the device. For a particular characteristic set of potentials, only ions of a given mass to charge ratio can reach the detector. A new set of potentials is required for ions of a different m/z, but as the electrical potentials can be changed very rapidly, it is possible to scan the entire mass spectral range in well under a second. Thus ICP-MS is a rapid sequential multielement technique. The low detection capability comes from the fact that the background signals between the peaks corresponding to the various analyte elements are very low, maybe only a few counts per second (cps). Typically the spectrometer may give between 106 and 10 7 cps per ppm of an analyte element introduced into the plasma. It is a reasonable approximation, especially at low count rates, to assume that the arrival of ions at the detector is a random process and thus the standard deviation of the signal is equal to the square root of the signal. Therefore, detection limits (defined as the concentration giving a signal equal to three times the standard deviation of the background) may be only a few parts per trillion (ppt) or lower! In common with analytical methods incorporating ETAAS, methods using ICP-MS suffer from the problems (a) of contamination of the sample with analyte due to impurities in the reagents used for sample preparation, (b) of carry-over in laboratory glass-ware and other items of equipment that come into contact with the sample solution and (c) of contamination from analyte elements in air-borne dust particles. Any laboratory using ICP-MS methodology needs to have clean sample preparation facilities and will need to operate the instrument in a clean environment. As has been discussed already in

FIA-Atomic spectrometry: a synergic combination for chemical analysis

29

connection with other atomic spectrometry techniques, FI can help to reduce carry-over and contamination from air-borne sources. In common with methods incorporating ICP-OES, ICP-MS methods suffer interferences from any sample component introduced into the plasma that alters the plasma conditions significantly. Such components include high concentrations of easily ionized elements, mineral acids and organic solvents. The approaches to dealing with interferences from these sources are much the same as for other matrix-derived interferences and include the use of matrix matched standards, the method of standard additions, the method of successive dilutions, and separation of the analyte from the matrix. All of these procedures can be implemented using FI techniques and are described in later chapters. In common with methods employing FAAS, plasma spectrometry methods suffer interferences arising from sample components which affect the processes of sample introduction and transport to the atomizer. One very useful feature of the rapid sequential nature of the mass spectrometer is that these kinds of interferences (and some of those which affect the ionization conditions in the plasma) can be compensated for by the use of an internal standard. It is very common in ICP-MS methods to use an internal standard. One way in which an internal standard can be added to samples and standards is to use FI with a merging stream or merging zones manifold. The major limitations of ICP-MS are (a) the relatively fragile nature of the devices which extract ions from the plasma, which limit the total dissolved solids to about 0.1%, (b) the formation of polyatomic ion species which give rise to spectral interferences, and (c) the non-specific matrix interference due to coulombic repulsion effects in the ion beam in the spectrometer. Recently the situation regarding interferences in ICP-MS has been reviewed] 5 Flow injection techniques featured prominently among the procedures used to overcome the various interferences discussed. To some extent FI can be used to help offset the first limitation by virtue of the microsampling nature of the technique. And just as for FAAS, FI techniques allow the introduction of small volumes of sample in ICP-MS that would not be possible under conditions of continuous aspiration. In fact, this was the subject of the first papers in the research literature describing the use of FI techniques with ICP-MS. The problem of spectral overlap can be very serious. For example, it is difficult to determine low concentrations of arsenic in the presence of chloride because the arsenic peak at 75 m/z is obscured by a peak due to ArC1 (made up of argon-40 and chlorine-35); a similar problem is found in the determination of iron at mass 56. There is an enormous background signal from ArO. The oxygen comes from the substantial amounts of water introduced by the conventional sample introduction by nebulizer and spray chamber. A major aspect of research into the development of ICP-MS instrumentation has been (and still is) concerned with overcoming these polyatomic interferences. The number density of

30

J.F. Tyson

some of these polyatomic species can be reduced by the addition of molecular gases, such as nitrogen, to the plasma. The interferences can be spectrally separated if a mass spectrometer with much higher resolving power is used (this usually means a doublefocusing instrument containing electric and magnetic sectors; such instruments are considerably more expensive than quadrupole instruments). Alternatively if the interference arises from some matrix component, it may be possible to separate the analyte from the matrix. There are many examples of the use of FI techniques to implement analyte and matrix separation. For example, hydride generation would be useful for the separation of arsenic from a chloride containing sample. The coulombic repulsion effects arise because the ion lens system discriminates in favor of positively charged species over negatively charges species (which would be mainly electrons) and thus the particle beam in the spectrometer is positively charged. Lighter ions in a beam from a sample may be repelled out of the main trajectory to an extent which results in a decrease in the signal intensity compared with that obtained from a standard. Effects such as these are known as "space charge" effects and are worst when determining light elements in the presence of a heavy element matrix. Again, one way of overcoming such effects is to separate the analyte and matrix and clearly FI procedures can be used for such separations. Although the first ICP-MS instruments to be commercially available did not have the capability to process transient signals, the situation has changed because the signals produced from various other sample introduction devices are also transients. These altemative other devices are based on laser and spark ablation, and electrothermal vaporization. There is also a growing interest in the use of the ICP-MS as a multi-element detector for liquid chromatography, which of course, also produces transient signals. The transient signal handling capability of ICP-MS instrumentation is probably further advanced than that of ICP-OES instrumentation and it is important that users of ICP-MS instrumentation know how to optimize data collection and evaluation for rapid sequential multi-element determinations based on transient signals.

1.4

Flow injection and atomic spectrometry

In general, analytical methods involving an atomic spectrometry instrument suffer from interference effects. Usually these effects arise from matrix components or components of the sample as it is constituted after preparation and pretreatment. These interferences are, for the most part, well understood and documented but, of course, have to be considered when evaluating the performance of any atomic spectrometry method. A large part of the success of the FI atomic spectrometry (FIAS) combination is the ability to overcome interference effects, firstly by the exploitation of some inherent feature

FIA-Atomic spectrometry: a synergic combination for chemical analysis

31

of the FI format, such as the use of a continuous carrier stream or the micro-sample volume. Secondly, interferences may be alleviated by the implementation of some pretreatment chemistry in the FI format. A large number of the limitations from which atomic spectrometry procedures suffer could be overcome by the separation of the analyte species from the unwanted matrix species, i.e. by converting every sample into a mixture of analyte(s) and a standard background matrix, chosen so as not to cause any interference in the atom formation process and/or subsequent interaction with radiation in the atom cell. Often such separation procedures also result in increased analyte mass flux into the atom source with consequent improvements in sensitivity and detection limits. All of these features of FIAS will be discussed in detail in later chapters and it will be seen that FI techniques can be used to enhance the performance of every type of atomic spectrometry. Some of these techniques are independent of the nature of the spectroscopic technique used. For example, a procedure that separates trace elements from a large volume of a highly saline medium and releases them into a smaller volume of dilute nitric acid can be used in conjunction with any type of spectrometer. The first papers on the use of FI sample introduction for atomic spectrometry (AS) appeared in 1979. Since then there has been a steady increase in the research activity and the relevant literature now stands at many hundreds of publications and is growing at the rate of almost 100 papers per year. Over this 20-year period, a considerable number of review articles and even two books 26'27dealing with aspects of the FIAS combination have appeared. A compilation of these review articles is given at the start of Chapter 3. Taking a very broad overview of the current status of FIAS, the situation may be summarized as follows. FI procedures are used in conjunction with atomic spectrometry for the following purposes. (1) Automated micro-sample introduction system. The use of discrete sample volume injection provides improved tolerance to high dissolved solids, organic solvents and variable viscosity. FI also provides on-line dilution and a suitable means of handling slurried samples. (2) Automated preconcentration with direct coupling to the spectrometer. Retention of the analyte on a solid phase extractant followed by dissolution in a clean matrix (such as dilute nitric acid) is the most widely implemented procedure. This also provides separation of the analyte from potentially interfering matrix components. (3) Automated implementation of chemical vapor generation procedures (hydride generation for arsenic, selenium etc. and cold vapor mercury). There are significant advantages to the use of a FI procedure instead of a batch procedure. For example, the interferences by first row transition metals (such as copper and nickel) are considerably decreased. In addition to coupling with the normal quartz tube atomizer,

32

J.F. Tyson this FI method may be readily coupled with a graphite fumace atomizer.

There are a number of procedures which have been described in the literature but which have yet to be widely implemented, and thus future uses of FI with atomic spectrometry could include, (1) Automated variable dilution factors. This would allow an off-range sample to be diluted by a known factor and re-run against the calibration already stored in memory. (2) Automated single standard calibration. The availability of a range of controlled dilution factors would allow a range of calibration standards to be produced from a single stock standard. (3) Implementation of the standard additions method. Several manifold designs have been described for the addition of several standards to a sample. These manifolds could also be used for the addition of internal standards. (4) Extension of the sample pretreatment chemistries that can be automated and coupled to the spectrometer. In addition to solid phase extraction and chemical vapor generation, procedures could include liquid-liquid extraction, precipitation, dialysis and even distillation. (5) Sample dissolution and digestion with direct coupling to the instrument. The most likely procedure is the handling of sample slurries in a pressurized flow-through system with microwave heating. All of these topics are discussed in detail in the later chapters of this book. As previously indicated in Section 1.2, the basic concepts of flow injection are (a) discrete analyte amount, (b) controlled dispersion and (c) reproducible timing. These latter two characteristics arise from the inherently reproducible nature of fluid flow processes in narrow bore conduits under conditions of predominantly laminar flow. When coupled with the concept of discrete sampling, a rapid analytical procedure emerges in which samples are serially processed. By the incorporation of appropriate devices into the flow lines and through control over the various fluid flows, a wide range of chemical processes may be incorporated into the flow system. As the chemical species from the processed sample emerge in a continuous fluid stream, the system may be readily interfaced with an atomic spectrometer for quantification of the target analytes. Modulation of the fluid flow allows unwanted species to be diverted to waste. A wide variety of such FI systems can be constructed to automate the chemical processing of samples so that an interference-free measurement with appropriate signal to noise ratio may be made. No matter how complicated the sample handling may be, once a FI system has been designed to carry

FIA-Atomic spectrometry: a synergic combination for chemical analysis

33

out the procedure, then a large part of the analysis has been reduced to the simple operation of introducing the sample.

1.5 R e f e r e n c e s 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

20 21 22 23 24 25 26 27

Hertz, H. S., Anal. Chem., 1988, 60, 75A. Stockwell, P. B. and Corns, W. T. Automatic Chemical Analysis, Taylor and Francis, London, 1996. Valcarcel, M. and Luque de Castro, M. D. Automatic Methods of Analysis, Elsevier, Amsterdam, 1988. Ruzicka, J. and Hansen, E. H.,Anal. Chim. Acta., 1975, 78, 145. Chalk, S. J., Talanta, 1998, 45, 591, http://schalk.as.unf.edu/fad.html Tyson, J. F., Mikrochem. J., 1992, 45, 143. Ruzicka, J. and Hansen, E. H., Flow Injection Analysis, Wiley, New York, 2nd edn., 1988. Valcarcel, M. and Luque de Castro, M. D., Flow Injection Analysis, Principles and Applications, Ellis Horwood, Chichester, 1987. Karlberg, B. and Pacey, G. E., Flow Injection Analysis, A Practical Guide, Elsevier, Amsterdam, 1989. Kingston, H. M. http://nexus.chemistry.duq.edu/sampleprep/mission.html Lobinski, R., Appl. Spectrosc., 1997, 51,260A. Ref. 7, page 380. Chalk, S. J. and Tyson, J. F., AnaL Chem., 1994, 66, 660. Tyson, J. F.,Anal. Chim. Acta., 1986, 179, 131. Ingle, J. D. and Crouch, S. R., Spectrochemical Analysis, Prentice Hall, Englewood Cliffs, 1988. Haswell, S. J., Ed., Atomic Absorption Spectrometry, Theory, Design and Applications, Elsevier, Amsterdam, 1991. Dedina, J. and Tsalev, D. L. Hydride Generation Atomic Absorption Spectrometry, Wiley, Chichester, 1995. Montaser, A. and Golightly, D. W., Eds., Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH, New York, 2nd ed., 1992. http://www.rsc.~176176 Tyson, J. F., Sundin, N. G., Hanna, C. P. and Mclntosh, S. A., Spectrochim. Acta Part B, 1997, 52B, 1773. Olesik, J. W. and Hobbs, S. E.,Anal. Chem., 1994, 66, 3371. Olesik, J., Appl. Spectrosc., 1997, 51, 158A. lvaldi, J. C. and Tyson, J. F., Spectrochim. ,4cta Part B. 1995, 50, 1207. lvaldi, J. C. and Tyson, J. F., Spectrochim. Acta Part B, 1996, 51, 1443. Evans, E. H. and Giglio, J. J., J. Anal. At. Spectrom., 1993, 8, 1. Burguera, J. L., Ed., Flow Injection Atomic Spectroscopy, Marcel Dekker, NY, 1989. Fang, Z., Flow Injection Atomic Absorption Spectrometry, Wiley, Chichester, 1995.

2.1 Introduction: flow manifolds or what can be done with a peristaltic p u m p , an injection valve and s o m e plastic tubes Flow analysers based on the use of a stream of liquid or, much less often a gas, have aroused a great interest in the analytical community. There are two types of continuous analysers, namely:

9 Gas segmented- flow analysers, originally developed as early as in the 1950s by Technicon under the name of "Autoanalysers", which were based on a liquid flow segmented by air bubbles aimed at preserving the integrity of the transported samples. These instruments were mainly used in clinical laboratories for the analysis of a few, clearly defined, parameters in a large number of similar samples. 9 Unsegmented-flow analysers, which can be classified according to whether samples are injected (flow injection analysis, FIA) or continuously introduced into the system (continuous flow). Nowadays these flow analysers, which have been used since the 1970s, '-6 are much more popular than the gas segmented-flow ones and constitute the main subject of this book. The basic concepts of flow injection analysis have been thoroughly reviewed in Chapter 1 in connection with atomic spectrometric detection. General diagrams of the basic instrumentation for gas-segmented flow, continuous flow, and flow injection analysers are shown in Figure 2.1. As can be seen for the FIA mode, 34

Basic instrumentation for FIA-atomic spectrometric detection

air

U==~

,.

de-aeration

.il 0

reagent ~ propulsion system

DETECTOR time

I propulsion system

DETECTOR

sample

ttme

signal

_

ECTOR

DE~W

-

time

propulsion system Figure 2.1 Schematic diagrams of basic manifolds for flow analysers: (a) segmented flow; (b) continuous flow; (c) flow injection analysis. the carrier solution and the reagents are delivered continuously by a propulsion system through a long, narrow tube. A well-defined sample volume is introduced with a sampleloop into the carrier stream with the help of an injection valve. After the reactions occurring in the manifold, the products of the reaction monitored at the detector give rise to a transient response. In this introduction a brief overview of the advantages of using flowing systems for sample introduction, the scope of pretreatment processes which can be carried out with flowing systems and also the requirements of the detector to be used, will be addressed. As pointed out in Chapter 1, the use of flow systems to carry out sample pretreatment

R. Pereiro

36 Table 2.1

Advantages of sample presentation via flowing systems (in terms of instrumentation, sample manipulation and analytical performance) ii

i

ill

i

i

[" - Flexibility and versatility. INSTRUMENTATION

SAMPLE

MANIPULATION

L - Ease o f use and possibility o f automation.

F - High sample throughput. Microsample analysis. L - Less contamination o f the sample.

|i [

Fl[ L-

| | [ [

1-

| ......... A N A L Y T I C A L RESULTS

High precision. On-line sensitivity enhancement and elimination of interferences. Reliable indirect analyses.

L processes brings several advantages as compared to the manual batch analogues and Table 2.1 shows, in a schematic manner, most of these advantages. A property of the instrumentation for flow systems which deserves to be highlighted is the high degree of flexibility and versatility for solution handling allowed by its modular character. In fact, in the case of FIA just having a propulsion unit (e.g. a peristaltic pump), a low pressure injection valve, PTFE tubing (between 0.5 and 1.0 mm of inner diameter) and some connections of polypropylene or PTFE, the analyst can tailor the FIA system to his own needs (Figure 2.2); besides, units for on-line separation/preconcentration based on different principles (e.g. gas-liquid, liquid-liquid, solid-liquid separation methods) can easily be incorporated in the manifold and several approaches for sample digestion (e.g. ultrasound, UV radiation, microwave heating, etc.) can also be embodied in the system. Another property of flowing sample introduction systems to be stressed in terms of instrumentation is related to the possibility of automation. The fully automation of an analyser for sample introduction requires a series of characteristics which can easily be achieved with flow manifolds: v (a) introduction of standard calibration solutions and samples in an automatic fashion with the aid of an autosampler and/or the use of electrically actuated injection valves; (b) accurate control for the automatic functioning of peristaltic pumps; (c) straightforward automation of the transport-reaction system, which

Basic instrunaentation for FIA-atomic spectrometric detection

37

is an inherent property of sample introduction flow systems; and, (d) a computer furnished with a passive interface for data acquisition and treatment and an active interface for controlling operational modules. Flow systems are mainly used for liquid samples, and their complexity can range from simple to very complex manifolds to deal with ultratrace amounts of the target analyte in complex matrices which often require on-line separation/preconcentration steps. However, both gas and solid samples can be introduced into the liquid flow manifold if special adaptations are made. Gas samples simply require impermeable tubing. In the case of solid samples, these can be either introduced in the system and leached with the help of auxiliary energy (e.g. ultrasounds) or introduced as slurries. Nevertheless, the weighing operation can only be done automatically by a robotic station. Table 2.2 collects a bunch of processes, all related to the sample pretreatment stage, that are inside the scope of analytical processes which can be tackled with flowing systems. As can be seen, these processes extend from simple dilutions to sample digestion and even in situ uptake of samples from the industrial process/environment/patient and their on-line pretreatment. Throughout this chapter and all this book, the instrumentation available nowadays for these purposes will be reviewed. Concerning the requirements of the detector, it is important to point out that apart from the desirable basic properties of any detector for chemical analysis (e.g. linear response over a wide analyte concentration range, no matrix response, high sensitivity, good repeatability and reproducibility, etc.) when interfacing such detector to a FIA system transient signals are produced. Therefore, other detector characteristics such as fast response, small dead volume, low memory effect, etc., will become of particularly marked importance. Table 2.2

Some analytical processes enhanced by the use of flowing systems in atomic spectrometry On-line serial dilutions / calibration.

\~ Filtration. \x Buffers / reagents addition. ~ k Interferences removal. \,, xx, Preconcentration (solid phase extraction, liquid phase extraction, coprecipitation, etc). ~k Analyte vapour generation (covalent hydrides, Nk cold atomic vapour, etc). Nk Sample digestion, x "In situ" uptake of samples from the industrial process/ \ environment/patient, etc, and on-line pretreatment, . '~\

38

R. Pereiro

In this chapter, particular attention is first paid to the instrumentation characteristics of flow manifolds. Thus, initially the elemental components are reviewed with selected examples of integrated flow systems and different degrees of complexity. A second main section deals with the coupling of flow manifolds to atomic detectors dealing with both, continuous and discontinuous atomic spectrometric detectors.

Figure 2.2 Basic components of a flow manifold for sample introduction. (a) Peristaltic pump; (b) injection valve (for FIA analysis); (c) inert tubing; (d) multiway mixing connections.

Basic instrumentation for FIA-atomic spectrometric detection

2.2

39

The flow manifold

In this section the more common designs of the basic units used in flow manifolds are first reviewed and then selected examples of assembled flow systems will be explained. In most cases, along with schematic diagrams of home-made manifolds examples of commercially available instrumentation will be included.

2.2.1

Elemental components

The three basic components of a flow manifold are the sampling system, the propulsion unit and the connecting tubes. Besides, the ever growing demand for sensitive and selective methods of analysis is giving rise to the insertion in the basic flow manifold of special on-line devices to allow for separation/preconcentration or for sample digestion. 2.2.1.1 Sampling systems To introduce samples in the flow injection manifolds an injection port, which in most cases consists either of a septum or a valve, is used. Liquid samples can be introduced in the injection port manually, pumped from a sample cup of an automatic sampler or in situ drawn from the sample source (industrial process, environment, patient, etc.). Most companies of analytical atomic spectrometric instrumentation market automatic samplers as accessories. Usually, these autosamplers are made of inert or corrosion-resistant materials, they have a self-contained micro-processor easily programmed and computer-interfaced through serial or parallel communication, offer random access capabilities and have integrated peristaltic pumps. The latter allows also for flowing rinse thus providing efficient cleaning of the sipper probe for trace level determinations. The main purpose of the injection ports, inserted in the flow system, is to precisely introduce a volume of sample as a plug in the continuously moving carrier stream in such a way that the movement of the stream is not disturbed. In the early stages of development of FIA, the liquid samples were manually introduced with a syringe through a rubber septum. 8'9 However, the system was far from ideal since lack of reproducibility in the manual injection, not compatibility of the septum with the samples and carrier solutions, components leaching from the septum material and leakage after repeated injections on the same spot, are problems which may occur with this type of sample introduction systems. Since then, the number of modifications in sample introduction ports to improve the performance of the FIA methods has become important, e.g. stopcock, ~~ sliding commutator, l~ rotary injection valves, 12-14 and different valveless injection systems 15-18 were among the systems proposed. Figure 2.2b shows the picture of a commercially

40

R. Pereiro

Table 2.3

Examples of injection ports used for sample introduction in FIA systems

Brief description

References

Septum injection device Stopcock injection valve Double proportional sliding valve injector-commutator Rotary six-way sliding valve Two four-wayslider valves with two pneumatic actuators Pneumatically operated two-layerrotary valve for diversion of streams Hydrodynamic injector with part of the carrier stream tubing as a sample loop Timed solenoid valve-injector

8, 9 10 11 12 13 14 15, 16 17, 18

available sliding rotary valve while Table 2.3 collects a compilation of representative liquid sample injection systems. The set-up of a manifold for hydrodynamic injection (valveless system) is shown in Figure 2.3.15 The sample solution is aspirated by pump 1, operating at a pumping rate of y ml/min, and a fixed volume of sample from reservoir S passes into conduit L. Subsequently pump 2 is activated, pumping at rates x= z ml/min, and the sample is flushed through the reactor to the detector. The operation of the two pumps is controlled by a timer. In Figure 2.4 the schematics of a commercially available six-way sliding rotary valve and of a double proportional injector are presented. The sliding rotary valve (Figure 2.4a) has an external loop. In the "load" position the loop is filled with the sample while in the "injection" position the loop content is dragged through the manifold by the carrier solution. ~2 To introduce a different sample volume the external loop has to be changed. This type of valve is by far the most frequently used. Figure 2.4b shows the diagram of the double proportional injector. '~ In the "loading"

LJ 'w Figure 2.3 Schematic diagram of a manifold for hydrodynamic injection. 1 and 2, pumps; S, sample reservoir; L, conduit; R, reactor; D, detector; T, timer. The time sequence of events are as depicted (reproduced from Reference 15 with permission from Elsevier Scientific Publishing).

Basic instrumentation for FIA-atomic spectrometric detection

41

position the sample is aspirated to fill the sample loop, which exactly defines the sample volume; the excess of sample goes to waste. Simultaneously, the reagent is pumped to fill the reagent loop; its excess, slightly diluted by the reagent carrier stream, is accumulated in the reagent recovery vessel. In this position, the sample and reagent carrier streams are pumped to merge and then going to the manifold. In the "injection" position, the selected volumes of sample and reagent are pushed by the carrier streams, merging simultaneously and then going to the manifold. The sample aspiration tube aspirates water and the pumped reagent is directed back to the reagent reservoir.

(a) LOAD

INJECT

carrier

carrier _

tector

tector

(b) LOAD

M

INJECT

C

Figure 2.4 Schematicdiagrams of two examples of injection valves for sample introduction in flow injection analysis. (a) Six-way rotary valve; (b) double proportional injector. S and R are sample and reagent; Ls and LR are the sample and reagent loops, Cs an CR are the sample and reagent carrier streams, D is the reagent outflow to a recovery vessel, W is the waste, A is the confluence point, M represents the manifold (reproduced from Reference 11 with permission from Elsevier Scientific Publishing).

42

R. Pereiro

The use of timed solenoid injectors which can deliver variable volumes of samples has been also developed; ~7'18 in essence, they use temporal, as opposed to spatial control of injections. Common insertion ports for the introduction of gaseous samples are gas-tight rotary valves and also what is called "exponential dilutors" 19.20 consisting of a magnetically stirred chamber of known volume, where the sample is diluted in the carrier gas. Injections are made through a septum by means of a gas-tight syringe. A set of two valves (or stopcocks) permits the chamber to be isolated from the carrier gas flow during injections. After the sample is stirred for some seconds in the isolated chamber, the valves are opened giving rise to a signal peak with an exponential-decay as a function of time. The concentration output of the exponential-dilution chamber can be written as: C, = Co exp( - Ut/V) where C, is the analyte concentration at time t; Co is the analyte concentration at the start of the dilution; U is the flow rate of the carrier gas; t is time and V is the volume of the exponential-dilutor chamber. Using the above relationship it is possible to obtain calibration graphs with single sample injections by following, for a single plug of a standard, the analyte signal profile versus the time elapsed after the valve was turned and considering the exponential dilution in concentration that takes place in the flask. 2~

2.2.1.2

Propulsionsystems

The propulsion system has the function of providing a continuous and reproducible flow rate of the solutions passing through the manifold. The ideal solution delivery system should provide a pulsed-free flow which rate can be easily selected. Besides, in some cases, more than one solution has to be propelled and, therefore, multichannel capabilities are advisable. Three main types of propulsion systems have been described for the combination of flowing systems with atomic detectors: (a)

(b)

the propulsion-less manifold, which relies on the negative pressure generated by the nebulizer to draw the liquid solution to the instrument. This type of system has been mainly used with FAAS and the control of the carrier flow rate was possible by adjusting the airflow regulating valve of the nebulizer 2''22 the use of a gas-pressurized carrier reservoir to propel solutions through the manifold has also been described. 12 The gases commonly used are air or nitrogen. To keep a constant flow rate the pressure has to be maintained by means of a precision regulator. Although this system gives rise to a pulsed-free flow, drawbacks such as the consumption of gases or the difficulties to achieve constant flow rates have greatly limited its use

Basic instrumentation for FIA-atomic spectrometric detection

(c)

43

Pumps are frequently employed. The use of positive displacement pumps has been described as propulsion systems in flow manifolds. However, they introduce spikes in the analytical system and when two or more reagents are delivered in parallel with these pumps, these pressure spikes cause a non-uniform mixing of reagents reducing the accuracy and precision of the analysis. Although several depulsing systems have been described, z3 this type of pump does not enjoy great popularity.

By far, the propulsion system most used is the peristaltic pump (Figure 2.2a). These pumps allow the use of more than one channel and to achieve a constant and accurate flow rate, which can be easily modified according to the analyst needs. ~!' ~4.~5.~7.~8The pumping rate provided depends on the rotation rate and the inner diameter of the pumping tubes. Nowadays, a wide variety of flexible pumping tubes made of different materials (tygon, PVC, PVC derivatives, silicone rubber, etc.) to be selected upon the type of solution to be pumped (e.g. acids, organic solvents, etc.) and with different inner diameters are available. Peristaltic pumps tend to give slight pulses which can be almost eliminated by an adequate adjustment of the clamps pressing the peristaltic tubes or by the use of coils located in the manifold right next to the propulsion unit.

2.2.1.3

Connecting tubes

Chemically inert and flexible tubing, in most cases made of polyethylene or PTFE, with inner diameters in the range from 0.3 to 1.5 mm, constitutes the arteries which allow the connection of the different units of the flow manifold (Figure 2.2c). In some parts of the system, the tubing is bent with a coiled-shape to allow for an efficient mixture of reagents coming from different channels. ~~' 15.16.18 Multiway connectors, usually made of polypropylene or PTFE, are also frequently used to mix flows containing different solutions dragged by the propulsion unit (Figure 2.2d).

2.2.1.4

Two-phasesseparation units

A most interesting advantage of using flowing systems is the ease of incorporating two phase separation units (for analyte separation/preconcentration) in an on-line mode with the detector. 24 The basic instrumentation to achieve gas-liquid, liquid--liquid and solidliquid separation in a flowing system will be reviewed below as an introduction to the application of such manifolds dealt with in detail in Part II of this book. Gas-liquid separation. It is well known that mass transfer between a liquid phase initially containing the analyte, and a gas phase which becomes enriched in the volatized analyte offers important advantages when using atomic detection (i.e. almost 100% of the analyte in that gaseous form is introduced in the atomic detector as compared to 1-5% of

44

R. Pereiro

analyte introduced by liquid nebulization). As a result, sensitivity improvements between one and two orders of magnitude can be achieved (see Chapter 8). The co-ocurrence of a gas and a liquid phase in a hydrodynamic system gives rise to problems which call for ingenious technical solutions. 25-3~Many different designs have been evaluated as gas-liquid separators (GLS). The most common devices consist of chambers typically made of glass in which the reaction products are separated by employing an inert purge gas. The systems that employ these GLS exhibit varying degrees of sensitivity, with no apparent agreement being reached as to the optimum GLS design. In any case, it is generally agreed, that separators to be used in continuous separation systems should meet two basic requirements: (a) they should work smoothly and regularly in order to avoid irreproducibilities and signal fluctuations; (b) they should induce minimal dispersion or dilution of the vaporized analyte, i.e. they should have as low dead volumes as possible, but not so low as to give rise to incomplete gas-liquid separation. Some designs described for GLS are depicted in Figure 2.5. The separator model "a" is the biggest and is filled with glass beads which allow a smooth separation of gases from liquids; however, its dead volume gives rise to an important dispersion of the volatile analyte in the gas phase, particularly when small volumes of analyte are introduced (e.g. FIA mode). The separators "b", "c", and "d'" of Figure 2.5 can be used both in FIA mode or in continuous sample introduction. The use of gas permeable membranes, either flat 29 or concentric hollow cylinders, 3~"e" and "f" in Figure 2.6, has also been described to separate gases from liquids in continuous flow systems. Higher increases in sensitivity can be achieved by further concentration of the analytes once released to the gas-phase. The amalgamation of atomic mercury with noble metals and posterior release by heating is a well-established method (commercial equipment from different companies is available) to further preconcentrate vapour mercury. The trap usually consists of gold-coated silica powder, gold-coated sand, or amalgams such as gold-platinum gauze. 3~ Also, approaches to trap the hydrides and other volatile analyte species involve the use of cryogenic traps 32 or the retention of volatile hydrides on a slightly heated surface (usually graphite coated with noble metals) followed in both cases by release by further heating. The latter system has found an important field in ETAAS. As we will see in Chapter 8, the graphite fumace is used as both the hydride trapping medium and the atomization cell. Here, the hydride is purged from the generator and then is trapped in the preheated furnace, at a temperature in the range 300-600~ The trapped analyte is subsequently atomized at temperatures generally over 2200~ This methodology has been shown to enhance the sensitivity significantly and to eliminate the possible influence of the hydride generation kinetics on the signal shape. The nature of the graphite tube is expected to affect greatly the efficiency of the hydride adsorption and it has been

Basic instrumentation for FIA-atomic spectrometric detection

45

shown that the coating of the graphite tube with Pd, Zr, Ag or Pd-Ir mixtures, improves the sensitivity and precision significantly. 33 Liquid-liquid separation. As described in more detail in Chapter 7 by Gallego and Valcarcel, a continuous liquid-liquid extractor consists basically of three units: a solvent segmentor, an extraction coil and a phases separator. The extractor receives the streams of

~---~- ,o,ca

Figure 2.5 Diagrams of selected devices for on-line gas-liquid separation (not drawn at the same scale). (a) From Reference 25 with permission of The Society for Applied Spectroscopy; (b) from Reference 31 with permission of the Royal Society of Chemistry; (c) from Reference 27 with permission of the American Chemical Society; (d) from Reference 28 with permission of Elsevier; (e) from Reference 29 with permission of the Royal Society of Chemistry; (f)from Reference 30 with permission of the Royal Society of Chemistry.

46

R. Pereiro

two non-miscible phases and gives rise to a segmented flow (solvent segmentor), then, mass transfer takes place through the multiple interfaces established throughout an extraction coil and, finally, two separate streams of the two phases emerge from a liquidliquid phases separator. 34-37

Figure 2.6 Multipurpose integrated flow sample introduction systems. (a) FIAS 400 from Perkin Elmer; (b) PrepLab from VG elemental (with permission of Perkin Elmer and VG Elemental respectively).

Basic instrumentation for FIA-atomic spectrometric detection

47

The liquid-liquid phases separator is probably the most important unit in the system since it must work highly efficiently and rapidly. In addition, it should not add dilution to the analyte and its operation should be smooth and consistent throughout. There is a large variety of phase separators (see Chapter 7), but probably the most popular design is based on the use of hydrophobic membranes which are only permeable to the organic solvent. In general terms, the use of liquid-liquid separation manifolds is subjected to some serious drawbacks including lack of compatibility of the organic effluents with plasmas, the limited preconcentration achieved and the frequent running problems of the present set-ups. Detailed information on this topic can be found in Chapters 7 and 9 of this book, more specifically oriented to liquid-liquid separations. Solid-liquid separation. Solid--liquid separation units may be classified in three categories according to the separation principles and media used for retention. (a)

(b)

(c)

Sorption on a solid phase, generally consisting of a minicolumn, inserted in the flowing system 5' 12,38--4~filled with an ion-exchanger, a selective reagent bound to a solid or an adsorbent packing. Examples of a flow manifold involving sorption processes will be given in Section 2.2.2.2. Precipitation and coprecipitation, based on the combination in a flow of precipitation of the analyte, its filtration and final dissolution processes. The filter is the key part of the system since it is intended to retain quantitatively the precipitate formed in the coil and it should be of a material as resistant as possible to the solutions to be circulated inside. To carry out the filtration process, the use of planar or cylinder filters (stainless steel, PTFE, paper, etc.), columns (glass beads, polystyrene granules), etc., has been described. 42'43 In comparison with precipitation methods, coprecipitation is less demanding on the solubility of the precipitate formed. As will be detailed in Chapter 6, in 1991, Fang et al. presented an interesting approach in which the analyte was coprecipitated with the iron(II)-hexamethylenedithiocarbamate complex and collected in a "knotted reactor" made of ethyl vinyl acetate tubing (the knotted reactor provides relatively large collection capacities with low flow impedance). The precipitate was dissolved in isobutyl methyl ketone. 44 In recent years, similar approaches have been exploited using this or other collectors in combination with different atomic detectors. 45"46 Anodic stripping voltametry based on the use of flow electrolytic cells to eliminate non-depositing sample matrix components and to preconcentrate analytes deposited at a working electrode. These analytes are then released for adequate atomic detection.a7.48

48

R. Pereiro

2.2.2 Integrated flow systems: selected examples The versatility of flow manifolds today allows a great number of applications. In fact, several companies are marketing sample introduction flow systems allowing for an automated preparation of the most varied sample matrices. Two examples of such integrated systems are collected in Figure 2.6: the first is equipped with two peristaltic pumps, a 5-port FI valve, carrier gas control, quartz cell and electrical heating. It can be used for FI/hydride generation or FI-flame Atomic Absorption, on-line dilution, on-line analyte preconcentration, and on-line reagent addition (Figure 2.6a). The commercial system of Figure 2.6b has been designed mainly for Inductively Coupled Plasma-Mass Spectrometric systems and allows for microsampling, hydride generation, solid-phase chelation, on-line standard additions, on-line addition of internal standard and on-line dilution. Representative examples of manifolds allowing for several sample pretreatment operations are reviewed below. However, it is important to emphasise, once again, that the modular character of these manifolds allows them to adapt to almost every particular sample pretreatment needed.

2.2.2. l Basic systems Many types of manifolds can be mounted using just the components depicted in Figure 2.2. In order to illustrate this versatility, Figure 2.7 presents five diagrams of different configurations of basic flow manifolds for FIA. The scheme shown in Figure 2.7a is the simplest one (single-line manifold) allowing for the introduction of small volumes of samples in a reproducible mode into the detector. Besides the obvious advantages of using this type of simple system (e.g. speediness, analysis of small sample volumes, etc.), it has been proved that its use in atomic spectrometry allows a straightforward analysis of samples with high viscosity or with high dissolved solids content since blockage of the nebuliser or the burner is minimised. 49'50 The addition of a second channel (Figure 2.7b) allows the carrying-out of some manipulations or pretreatment of the sample, such as dilution and addition of reagents or masking agents (e.g. addition or caesium or lanthanum in flame atomic absorption or emission spectrometry or on-line isotopic dilution in inductively coupled plasma-mass spectrometry). The set-up depicted in Figure 2.7c, called "merging zones", allows important savings in reagent consumption 5~ and is very useful for on-line standard additions. Figures 2.7d and e collect the schematics of two specific operation-oriented systems. The manifold shown in Figure 2.7d has been described for achieving a variety of dilution factors for a single injection without the need for controlled timing of any operation. 52The manifold consists of tubing of different lengths and then the residence times of the

49

Basic instrumentation for FIA-atomic spectrometric detection

sub-sampled zones in each branch is different. On recombination multiple peaks are formed. In general, for n unequal branches, n peaks will be produced. Figure 2.7d illustrates a three-branch manifold which gives rise to three partially overlapping peaks and five measurement points, three maxima and two minima. A similar system, involving sample zone splitting, has been described for the quasi-simultaneous determination of three analytes using a sequential atomic emission detector. 53 As before, the process involves the splitting of the injected sample zone into three different coils of different coil lengths: the three sample portions reach the detector at different times and this is now used to determine the ions in turn by moving the filter on the detector from one analyte to another at the correct timing. More than 1000-fold dilution with a precision of better than 2% was achieved with a manifold based on the injection of low- and submicrolitre sample volumes and their dispersion in a mixing coil. Precise control of volumes was possible using computercontrolled stepper motordriven peristaltic pumps and small-bore pump tubes. 54 Figure 2.7e shows a basic system based on sequential injections which has been described for gradient exploitation: 55 several plugs of a solution are simultaneously intercalated into the carrier stream, and then overlap strongly as a consequence of IV

,v

R

5]

p

P

(c) IV C2

P

~

~

I~

0

P

IV

IV

IV

IV

P

Figure 2.7 Schematics for selected examples of basic flow operation systems using FI systems. P, peristaltic pump; IV, injection valve; C, carrier; R, reagent; D, detector. (a) Single line; (b) multi line; (c) merging zones; (d) three-branch on-line sample dilution system; (e) sequential injection flow system.

50

R. Pereiro

dispersion. Provided that plugs of the same solution are used, their asynchronous merging results in a final dispersed zone characterized by several sites without appreciable concentration gradients, corresponding to the maximum and minimum values of the concentration/time profile.

2.2.2.2 Systems with separation/preconcentration capabilities From Chapters 6 to 9 of this book, the use of separation/preconcentration manifolds is thoroughly reviewed. Therefore, in this section just a couple of illustrative examples of manifolds, one involving gas-liquid separation and the other for solid-liquid separations are presented to show general concepts in this type of instrumentation. The conversion of the analyte to a volatile derivative by a chemical reaction to enhance transport to the atomic detector has been traditionally exploited for the determination of mercury, by formation of "cold vapour", and for elements forming covalent volatile hydrides, like As, Sb, Sn, Bi, Pb, etc. 56 More recently, the use of this strategy has been expanded to other elements and methodologies such as the determination of Cd by cold vapour~7 or the formation of volatile tetraethyl derivatives of the analyte. 58 Also, the generation of chlorine, bromine, iodine and their respective hydrides as a means of halide introduction in the detector has been reported 27 and will be dealt with more extensively in Chapter 9. Despite its well-known advantages, batch volatile species generation procedures have a number of pitfalls, such as: (a) throughputs are relatively modest; (b) the precisions are poor due to inadequate control of reaction conditions; (c) the hydride generation process generates hydrogen released in a sudden burst, which affects the flame or plasma based detectors; (d) interferences occasionally pose serious problems (kinetic discrimination); (e) use of quite large amounts of sample, etc. Figure 2.8 depicts a classical FI manifold for mercury, or cadmium cold vapour, and for

Stripping gas TO D E T E C T O R

Sample

Carrier /

~

Gas Liquid Separator

Acid to waste

Reducing Agent Pump

Figure 2.8 Basic diagram of an on-line system for hydride generation.

Basic instrumentation for FIA-atomic spectrometric detection

51

hydride generation. In some cases an additional channel is also included for the introduction of other reagents favouring the conversion of the analyte to the corresponding volatile hydride (e.g. iodide to help pre-reduction of As(V) to As(Ill)). Most frequently, the stripping gas is introduced after the mixing point of the carrier and reagents or directly through the separation unit. The liquid phase, after passing through the GLS, is sent to waste either directly (e.g. by overflow in a communicating-vessel system) or by aspiration with the peristaltic pump. Besides the typical factors affecting the sensitivity (e.g. concentration of reagents and type of detector), operation parameters such as the flow rate of the sample, the ratio of flow rates between the different liquid channels, the flow rate of the stripping gas and the design of the gas liquid separator have to be considered for the continuous systems. Concerning solid--liquid separations instrumentation, Figure 2.9 shows the basic schematics of two classic solid-phase extraction approaches for elemental analysis: in Figure 2.9a the analyte ions are collected directly 37'38 by immobilised counter ions or immobilised chelate functions (sorption of ions)while in Figure 2.9b the ions are sorbed on a solid support (e.g. activated carbon, octadecyl functional groups bonded silica, etc.), as hydrophobic chelates 39"4~formed previously in the liquid stream (sorption of metal chelates). The characteristic features of these on-line preconcentration systems demand some

(a)

carriert

sample

eluent solid active phase

pump

sample eluent

carrier carrier

I

pump

adsorbent

~

to detector

reagent

Figure 2.9 Basic manifolds for on-line preconcentration on a solid phase. (a) System for sorption of ions; (b) system for flow metal-chelate formation and subsequent sorption.

52

R. Pereiro

properties of the packing material. Such features may be only of minor importance in batch or traditional column procedures but they are critical in flow operation (i.e. in flow systems the column material has to be reusable, the kinetic processes or reactions rapid and the flow uniform). Special requirements for such packing materials should at least include the following: (a) high mechanical resistance, in order to withstand high flow rates through the column and assure acceptable column lifetimes; (b) favourable kinetic properties for retention and elution; (c) low degree of swelling and shrinking when being transformed from one form to another, or by changing solvent or solvent conditions (the swelling/shrinking of solid supports will give rise to backpressures and non uniform flow pattems). In retention/elution processes parameters of paramount importance are the type and pH of the carrier solution, the type and concentration of the eluent and the flow rates of both. The higher the flow rates the higher the sample throughput, but it has to be low enough to allow an efficient retention/release of the analyte and to avoid backpressure problems. Occasionally, the complexity of the sample matrix or the sensitivity requirements may demand the use of more than one separation/preconcentration technique to accomplish adequate selectivity and/or sensitivity. Coupling two separation techniques in a tandem mode for sample preparation may be an effective solution for dealing with such complex samples. For example, a manifold of such characteristics has been described in Chapter 9 by combining a continuous liquid-liquid extraction module (to get rid of interferences and for analyte preconcentration) with an on-line generation of gaseous hydride unit addressed to further increase the sensitivity for the determination of As by ICP--OES. 59 Other on-line approaches involve the elimination of interferences, prior to the hydride generation using a cation exchange column, such as the system described for the determination of As, Sb and Se in a cobalt matrix 6~ or the preconcentration of Se and Bi on an anion exchanger prior to the on-line hydride generation. 6t

2.2.2.3 On-lineflow digestion of samples Considering that most atomic detectors are designed for introduction of liquid samples, the possibility of carrying out the decomposition/dissolution of solid samples on-line with the detector is attracting a great deal of interest (see Chapter 5). Besides, in order to achieve on-line volatile species generation from different analyte compounds, some organospecies need first to be decomposed, this approach being particularly interesting in metal speciation for species decomposition at the interface between the exit of a liquid chromatographic column and the atomic detector (see Chapter 13). For all these types of purposes, different principles, such as on-line chemical oxidation at room temperature, photo-oxidation and microwave heating have been investigated and three illustrative examples of instrumentation used will be given in this section. 62-67

Basic instrumentation for FIA---atomic spectrometric detection

53

Figure 2.10a depicts the schematics of an on-line photo-oxidation system for the determination of organoarsenic compounds by AAS with continuous arsine generation. 63 The photo-reactor unit is made from the mercury lamp of a portable ultraviolet lightsource (1.5 mm o.d., length 20 cm). The lamp is wrapped with 5 m of 0.56 mm i.d. PTFE tubing. The unit is enclosed in aluminium foil to increase the light intensity reaching the coil and to prevent eye exposure to ultraviolet radiation. The set-up of a manifold for on-line microwave-assisted digestion of solid samples for their analysis by AAS is given in Figure 2.10b: the direct injection in a water carrier flow of slurries of the sample in concentrated nitric acid, the merging of these slurries with 30% (v/v) H202 and the microwave-assisted digestion in a PTFE coil of 0.1 m permit a fast and quantitative extraction of Cu and Mn from different solid matrices, such as vegetables, powdered dietary products and sewage sludges. 64 The development of an interface (icecooled bath), in which digested samples are cooled and degassified, previous to their _

i

~

,0"8

mL/min

9

i' .....i--,,,

ICE COOLED

I

[, I[ CARRIER (WAT]SR)

[ PUMP

,

BATH

INJECTOR

MICROWAVE OVEN

INTRR.PHASB

ATOMIC S PECTRO1VIETER

Figure 2.10 Approaches for on-line flow sample digestion. (a) UV radiation with a high intensity mercury lamp (from Reference 63 with permission of Elsevier Science Ltd); (b) microwave-assisted digestion (from Reference 64 with permission of Elsevier Science Ltd).

54

R. Pereiro

introduction into the nebuliser of the instrument, allows a full automation of the digestion and measurement steps. Other approaches proposed to remove the fumes produced during acid decomposition of organic material involves the use of diffusion cells connected to a vacuum pump and back pressure regulators. Finally, it is interesting to highlight that the in vivo sample uptake and on-line measurement of Cu and Z n 66 and C o 67 in whole blood by microwave-assisted mineralization and flow injection introduction into the atomic detector (FAAS 66 or ETAAS 67) has been described (see Chapter 5). In this case, samples were pumped directly from the vein of a patient's forearm to a timed injector in a flow system thus avoiding the manual handling of blood (therefore, sample contamination and also analyst risks of being infected by contagious diseases are minimised).

2.3

The interfacing of flow manifolds with atomic detectors

Nowadays, the great advantages of using flow manifolds as sample presentation systems for atomic detectors have been demonstrated for a variety of techniques including different atomic sources and/or detectors characterised by continuous operation (e.g. FAAS, ICPOES, MIP-OES, SCP-OES, GD-OES, ICP-MS, etc.) and characteristic examples of the instrumentation required will be shown. The intrinsic ETAAS discontinuous operation explains that such couplings are dealt with in a second specific section. 2.3.1

On-line coupling to continuous atomic spectrometric detectors

The interfacing of flow manifolds to continuous atomic spectrometric detectors for direct analysis of samples in liquid form requires a nebuliser and spray chamber to produce a well-defined reproducible aerosol whose small droplets are sent to the high temperature atomic cell. Various types of nebulisers are available in FAAS, being the concentric pneumatic nebuliser most frequently used. 68"69 A theoretical modelling approach for nebuliser behaviour in FIA-FAAS has been given by Appleton and T y s o n 68 and the reading of Chapter 1 is recommended in this respect. The suitability of the response kinetics of the FAAS as detector, as well as the contribution of individual components to over-all dispersion of an injected sample has been investigated by Fang et al. 69 Dispersion effects in the nebuliser-burner system, which influence the response kinetics of the detector, generally showed a very limited contribution to the overall dispersion of the injected samples. A paper published in 1979 about FIA coupled to FAAS early reported the possible beneficial effects of introducing a compensation fluid, via a T piece, in an interface just prior to the nebuliser. TM The compensation fluid is not pumped, and then the fluid will only

Basic instrumentation for FIA-atomic spectrometric detection

55

flow when the flow delivered via the pump (the carrier) is less than that required by the suction developed by the nebuliser. Flow injection with air-compensation 7~ is usually observed to give rise to improved precision over that obtained without a compensation flow, particularly at very low flow rates. The following guidelines for optimum performance in terms of sensitivity and precision when using a barrel nebuliser and FAAS detection were given by Brown and Ruzicka: 7' (a) the flow rate of the carrier stream, pumped into the nebuliser, should always be greater than the natural aspiration rate of the nebuliser; (b) the sensitivity of measurement is a function of the flow-rate of the carrier stream entering the nebuliser and the natural aspiration rate of the latter; (c) to achieve an optimum flow through the nebuliser, an additional stream may be added to augment the flow of the carrier stream without a significant decrease in sensitivity, due to dilution. In the case of plasma-based atomic sources the more commonly employed is, by far, the ICP which was first used for OES and, more recently, for MS detection. A variety of nebulisers have been described for the ICP work, 72 and some representative examples are collected in Table 2.4, 73-89 including low-flow nebulisers (sample flow rate in the interval between 1 to 100 txl/min). As expected, limits of detection comparing FIA to continuous mode, using conventional cross-flow, concentric or Babington-type pneumatic nebulisers as well as ultrasonic nebulisers, have been reported to be generally poorer for the FIA mode. 73-75 Thermospray probe interfaces were also investigated for the coupling of FIA to ICPs. 8~ In this case, the expansion chamber, into which the thermospray aerosols were injected, was wrapped with heating tape. Further solvent removal makes a thermospray a very Table 2.4

Examples of types of nebuliser used for sample introduction in ICP

Principle

Reference

Cross-flow pneumatic nebuliser Pneumatic concentric nebuliser Babington pneumatic nebuliser High pressure concentric pneumatic nebuliser Glass-frit pneumatic nebuliser Grid pneumatic nebuliser Ultrasonic nebuliser Thermospray nebuliser Jet impact nebuliser High pressure hydraulic nebuliser Electrospray Microconcentric nebuliser Oscillating capillary nebuliser Direct injection nebuliser Direct injection high efficiency nebuliser

73 74 75 76 77 78 79 80, 81 82 83 84 85 86 87, 88 89

56

R. Pereiro

stable nebuliser for ICP, which is suitable for reproducible analysis of microlitre volumes (via FI techniques). Microconcentric nebulisers, inserted directly into the tip of a conventional sample introduction tube on an ICP torch, have also been investigated. 85 This type of nebuliser provides a very low dead volume interface yielding very little postcolumn band broadening. Lafreniere, Rice and Fassel have designed a direct injection nebuliser (DIN) which is ideally 100% efficient at transporting introduced analyte into the ICP. 87 This DIN is essentially a microconcentric pneumatic nebuliser placed at the base of the plasma and has been also evaluated as a FIA interface to the plasma. The relative detection limits found for 30 I~1 sample injection volumes were generally comparable to those obtained for FIA introduction of 200 ~1, or continuous sample introduction, into a conventional crossflow nebuliser and FIA introduction of 500 Ixl, or continuous sample introduction, into an ultrasonic nebuliser. Concerning spray chambers, the most frequently used in ICP is the Scott-type. 9~A new type of spray chamber combining gravitational, centrifugal, turbulent and impact loss mechanisms in one apparatus to efficiently remove large droplets, increase transport efficiency and reduce memory effects was designed and evaluated by Wu and Hieftje. 9~ Compared with the commonly used Scott-type spray chamber, this vertical rotary has at least 30% higher sample-utilisation efficiency, 2-3 times shorter sample clean-out time, half the cost, and simplified construction. Moreover, it offers somewhat better detection limits and precision. This low-volume spray chamber was investigated for FI-plasma optical emission spectrometry 92 and the same sensitivity of detection was achieved by continuous nebulization than injecting volumes of 200 ~1. Concerning the use of plasma sources other than the ICP, it is important to highlight the low-power MIP. This source provides high sensitivity for a wide range of elements, including non-metals, and is inexpensive and simple to operate. The major limitation of using a MIP as an excitation source is its low tolerance to the introduction of liquid samples and also its susceptibility to interferences caused by easily ionized elements. However, a relatively powerful MIP excitation source called Microwave Plasma Torch (MPT) has been developed which, unlike more conventional MIP supporting torch/ cavities structures, tolerates aqueous aerosols and molecular gases introduced into the discharge. 93 A study exploring the potential of the FI mode for sample presentation to the MPT--OES with an ultrasonic nebuliser 94 found a reduction of memory effects without loss of sensitivity or precision. Furthermore, by appropriate choice of sample dispersion, a significant reduction of the Na and K interferences were observed. Besides, a FI system incorporating a microcolumn to determine Cu in synthetic seawater was also described in that work. As commented on earlier, the introduction of analytes as gaseous derivatives offers

Basic instrumentation for FIA-atomic spectrometric detection

57

special advantages in atomic detection. The earliest flow hydride generation systems connected on-line to ICP-OES and AAS detectors were reported by Thompson et al. in 197895 and by Astrom in 1982, 96 respectively. Flow systems for volatile analyte generation have been coupled also to other atomic excitation sources for optical emission, 97-1~176 atomic fluorescence ~~ and mass spectrometric ~~ detection. Using quadrupole-based MS detectors, the elimination of matrix interferences is of particular interest and flow systems are routinely used to overcome these problems. As a typical example, selenium is one of the most difficult elements to be determined by ICP-quadrupole MS because of different nature isobaric interferences on several of its most abundant isotopes. By using on-line flow hydride generation coupled to ICP-MS the sensitive analysis of Se in biological or environmental matrices is now easily carried out. !~ Nowadays, the detection systems used in a routine basis for hydride generation and cold vapour determinations are AAS and AFS. These detectors have much less running costs than the ICP-OES and offer similar or even better detection limits. Plasmas are also adequate sources to accept volatile species and Table 2.5 shows a comparison of detection limits in ng/ml, obtained for continuous hydride generation methods using different optical emission and mass spectrometric sources. Owing to their high electronic temperatures, conventional designs of MIPs and SCPs using He as plasma gas offer very good sensitivity for the non-metals (e.g. halogens) provided that the samples are introduced as a gas phase. The continuous and FIA determination of halides by MIP and SCP following OES detection with on-line previous chemical volatilization (by halide chemical oxidation to the corresponding halogen with K M n O 4 or H202 in acid media), has been successfully investigated in our laboratory. 28 Figure 2.11 shows the experimental set-up for the analysis of chloride, bromide and iodide. Sample solutions were prepared in 7.5 M HzSO 4 when analyzing bromide or chloride, while the iodide samples were prepared in aqueous medium. For on-line oxidation of samples containing chloride or bromide a solution of 0.05 M K M n O 4 in concentrated H2SO4 was pumped to the sample stream. For iodide the dissolved samples were mixed on-line with two solutions: one of 3 M HzSO 4 and the second one containing 3 M H202. Halide detection limits achieved were in the low ng/ml level. 2.3.2

Semi on-line coupling to electrothermal atomic absorption spectrometry

The intrinsic discontinuous nature of the ETAAS technique has limited the interest of interfacing basic continuous flow manifolds to this detector. However, several flow approaches offer special attractiveness for their combination with ETAAS, particularly:

(a)

separation and preconcentration by on-line column sorption, coprecipitation and solvent extraction

OO

Table 2.5 Comparison in terms of detection limits (ng/ml) of continuous hydride generation systems connected on-line to atomic detectors (calculated as three times the standard deviation of the background noise) Element As

Sb

Se

Wavelength

ICP-OES(97)

APMIP-OES(98)*

LPMIP-OES(99)t

193.7 nm 228.8 nm

1

4.8 3.2

1.2 0.7

206.8 nm 231.1 nm 252.8 nm

0.4

196.0 nm 203.9 nm

0.5

GD-OES(100)~ 20

ICP-MS(102) 0.13 (isotope 75)

0.9 5.9

0.21 (isotope 78)

4.1

0.11 (isotope 121)

* APMIP-OES: Atmospheric Pressure Microwave Induced Plasma-Optical Emission Spectrometry. t LPMIP-OES: Low Pressure Microwave Induced Plasma-Optical Emission Spectrometry. ++GD--OES: Glow Discharge-Optical Emission Spectrometry.

O

Basic instrumentation for FIA-atomic spectrometric detection o) (_'ONTIN[J()UN

|,'|.(}W

Peristaltic

Data

pump

acquisition

Plasma

4 ~ i':-:~'_..-f~.......................

i

h9 Y I . O W I N J E C T i O I ~

"!

i

2

-- =

['~P'

[

'

,pt;'al

'

Power supply

~

.... ~ . . . . .

o

"

...............................

system

Figure 2.11 Schematic diagram of the flow halogen introduction system and the plasma - OES setup. 1, oxidant; 2, sample; 3, H2SO4 (channel 3 was used only for iodide analysis) (from Reference 28 with permission of Elsevier). (b)

(c)

formation of volatile derivatives of the analyte and their preconcentration on a graphite tube slurry sampling.

Although such couplings will be dealt with in more detail in the following chapters, the special features of the required set-ups and instrumentation will be stressed here.

Sorption, coprecipitation and solvent extraction. FIA preconcentration and matrix separation methodologies are often discontinuous processes appropriate for the discrete, non-continuous nature, of ETAAS. The compatibility of organic solvents with ETAAS has prompted a number of interesting applications 4~ 104-~07 based on the sorption of metal chelates on non-ionic sorbents and their later elution with organic solvents such as ethanol, methanol or acetonitrile (see chapter 6 for more details and particular examples). Similarly, coprecipitation systems have also been synchronously coupled to ETAAS for the determination of trace amounts of heavy metals. 45 The analytes can be coprecipitated with voluminous agents, such as the iron (II)-hexamethylenedithiocarbamate complex, on the walls of a knotted reactor without using a filter. The precipitate is later dissolved in a few microlitres of isobutyl methyl ketone, stored in a PTFE tube and delivered finally into the graphite tube atomiser (Chapter 6). Also, solvent extraction flow manifolds were developed for ETAAS. 36 In this case, a

60

R. Pereiro

chelate containing the analyte is first formed and then extracted into an organic solvent. The organic phase is separated from the aqueous phase and stored in a collector tube, from which 50 Ixl organic concentrate was introduced into the graphite tube by an air flow. The ETAAS determination can be performed in parallel with the extraction process, as can be seen in Section 7.3 by Valcarcel and Gallego.

Formation of volatile derivatives of the analyte and preconcentration on the graphite tube. Continuously heated graphite furnaces, wherein the volatile analyte hydride enters a hot furnace (1800-2300~ and is atomised during its transit time through the device, have had limited use as atomisers. However, in situ trapping techniques, which couple hydride generation with the graphite furnace analyte collection, are arising a growing interest in recent years because a clean, rapid separation/preconcentration of the analyte from the matrix is attained in a simple way. In situ trapping of previously volatilised species and final ETAAS detection has been successfully applied to the determination of As, Bi, Ge, Hg, In, Pb, Sb, Se, Sn, Te 33 and Cd~08. ~09in a great variety of matrices. Slurry sampling. Solid samples may be placed directly in the graphite furnace being in most cases efficiently atomised (powder atomisation). However, the coupling of slurry sample introduction systems to ETAAS has proved to be more interesting and versatile. In fact, commercial accessories are available today for slurry-ETAAS technique work. ~~ Once the slurry is formed (by grounding, sieving and mixing with a liquid phase by agitation) and stabilised (e.g. with surfactants), an aliquot is introduced into the flow manifold and then on-line mixed downstream with differently needed solutions such as the carrier, digesting reagents, chemical modifiers, etc. ~ The prepared sample is then loaded into a cup of the ETAAS autosampler, where the analyte concentration is eventually measured in the usual way. As an example, Figure 2.12 shows the diagram of a manifold for the direct determination of aluminium in milk desserts. ~2 The method uses a flow injection system connected semi-on-line to an ETAAS system through an autosampler, allowing for preparation of the slurries, addition of a chemical modifier and dilution-homogenisation of the slurry in a mixing chamber. Initially, a manually homogenized milk dessert or the standard is aspirated through loop Ls, while the chemical modifier [0.1 mol 1-~ Mg(NO3)2+0.01 mol 1-~ Pd(NO3)2] and the carrier (0.2% HNO3) were recycled outside the measurement line of the injector commutator. As the commutator was switched to its alternative position, 200 txl of sample or standard are inserted into the carrier stream and then merged with the chemical modifier at point "X"; the mixed solution was passed through the mixing chamber for dilution and homogeneization of the slurry. After mixing,

Basic instrumentation for FIA-atomic spectrometric detection [ i

ML

w~-

IC

Figure 2.12 FIA manifold for used for the determination of A1 in milky desserts: M, chemical modifier; C, carrier stream; Ls, sample loop (200 Ixl); IC, injector-commutator; MC, mixing chamber; P, peristaltic pump; W, waste (from Reference 112 with permission of the Royal Society of Chemistry). the solution was loaded into an autosampler cup for 120 s. Calibration is performed with aqueous standards.

2.4 I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

References Hansen, E. H., Anal. Chim. Acta, 1995, 308, 3. Tyson, J. F., Spectrochim. Acta Rev., 1991, 14, 169. Burguera, J. L. and Burguera, M., J. Anal. At. Spectrom., 1995, 10, 473. Burguera, J. L. and Burguera, M., J. Anal At. Spectrom., 1997, 12, 643. Fang, Z.-L., Spectrochim. Acta Rev., 1991, 14, 235. Fang, Z., Xu, S. and Tao, G., J. Anal. At. Spectrom., 1996, 11, 1. Luque de Castro, M. D. and Vfilcarcel, M., In: "Automation in the Laboratory", Ed. Hurst, W. J., 1995, VCH, New York. Ruzicka, J. and Hansen, E. H., Anal. Chim. Acta, 1975, 78, 145. Alexander, P. W., Finlayson, R. J., Smythe, L.E. and Thalib, A., Analyst, 1982, 107, 1335. Rocks, B. F., Sherwood, R. A., Turner, Z. J. and Riley, C., Ann. Clin. Biochem., 1983, 20, 72. Bergamin, F. H., Zagatto, E. A. G., Krug, F. J. and Reis, B. F.,Anal. Chim. Acta, 1978, 101, 17. Olsen, S., Pessenda, L. C. R., Ruzicka, J. and Hansen, E. H., The Analyst, 1983, 108, 905. Stewart, K. K., Beecher, G. R. and Hare, P. E., Anal. Biochem, 1976, 70, 167. Jorgensen, S. S., Petersen, K. M. and Hansen, L.A.,Anal. Chim. Acta, 1985, 169, 51. Ruzicka, J. and Hansen, E. H., Anal Chim. Acta, 1983, 145, 1. Wada, H., Hiraoka, S., Yuchi, A. and Nakagawa, Anal. Chim. Acta, 1986, 179, 181. Rothwell, S. D. and Woolf, A. A., Talanta, 1985, 21, 431. Burguera, J. L., Burguera, M., Rivas, C., de la Guardia, M. and Salvador, A., Anal. Chim. Acta, 1990, 234, 253. Lovelock, J. E., Anal Chem., 1961, 33, 162.

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20 Camufia-Aguilar, J. F., Pereiro-Garcia, R., Sanchez Uria, J. E. and Sanz-Medel, A., Spectrochim. Acta, 1994, 49B, 545. 21 Burguera, J. L., Burguera, M., Gallignani, M. and Alarcon, O. M., Clin. Chem., 1983, 29, 568. 22 Attiyat, A. S. and Christian, G. D., Anal. Chem., 1984, 56, 439. 23 Stewart, K. K.,Anal. Chem., 1977, 49, 2125. 24 Vaicarcel M. and Luque de Castro, M. D., "Non-chromatographic Continuous Separation Techniques", 1991, The Royal Society of Chemistry, Cambridge. 25 Pyen, G. S., Long, S. and Browner, R. F., Appl. Spectrosc., 1986, 40, 276. 26 Stroh A. and V611kopf, U.,J. Anal. At. Spectrom., 1993, 8, 35. 27 Chan, C. C. Y., Anal. Chem., 1985, 57, 1482. 28 Camufia, J. F., Montes, M., Pereiro, R., Sanz-Medcl, A., Katschthaler, C., Gross, R. and Knapp, G., Talanta, 1997, 44, 535. 29 Wang, X. and Barnes, R. M.,J. Anal. At. Spectrom., 1988, 3, 1091. 30 Barnes, R. M. and Wang, X., J. Anal. At. Spectrom., 1988, 3, 1083. 31 Hanna, C. E, Haigh, E E., Tyson, J. F. and Mclntosh, S., J. Anal. At. Spectrom., 1993, 8, 585. 32 Burguera, M., Burguera, J. L., Brunetto, M. R., de la Guardia, M. and Salvador, A., Anal. Chim. Acta, 199 l, 261, 105. 33 Matusiewicz, H. and Sturgeon, R. E., Spectrochim. Acta, 1996, 51B, 377. 34 Nord, L. and Karlberg, B., Anal. Chim. Acta, 1980, 118, 285. 35 Gallego, M. and Valcarcel, M.,Anal. Chim. Acta, 1985, 169, 161. 36 Tao, G. and Fang, Z., Spectrochim. Acta, 1995, 50B, 1747. 37 Perez, C., Mendndez, A., Sfinchez Uria, E. and Sanz-Medel, A., Fresenius J. Anal. Chem., 1995, 353, 128. 38 Pereiro, R., Lopez-Garcia, A., Diaz-Garcia, M. E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1990, 5, 15, 39 Cox, A. G. and McLeod, C. W., Anal. Chim. Acta. 1987, 200, 35. 40 Sperling, M., Yin, X. and Welz, B., J. Anal. At. Spectrom., i 99 l, 6, 295. 41 Petit de Pefia, Y., Gallego, M. and Valcarccl, M., J. Anal. At. Spectrom., 1994, 9, 691. 42 Martinez, R., Gallego, M. and Valcarcel, M.,Analyst, 1987, 112, 1233. 43 Santelli, R. E., Gallego, M. and Vaicarcel, M., Anal. Chem., 1989, 61, 1427. 44 Fang, Z., Sperling, M. and Welz, B.,J. Anal. At. Spectrom., 1991, 6, 301. 45 Fang, Z. and Dong, L., d. Anal. At. Spectrom., 1992, 7, 439. 46 Chen, H., Xu, S. and Fang, Z., Anal. Chim. Acta, 1994, 298, 167. 47 Pretty, J. R., Blubaugh, E. A., Evans, E. H., Caruso, J. A. and Davidson, T. M.,J. Anal. At. Spectrom., 1992, 7, 1131. 48 Pretty, J. R., Blubaugh, E. A., Caruso, J. A. and Davidson, T. M., Anal Chem., 1994, 66, 1540. 49 McLeod, C. W., Worsfold, P. J. and Cox, A. G., Analyst, 1984, 109, 327. 50 Fang, Z., Welz, Z. and Schlemmer, G., J. Anal. At. Spectrom., 1989, 4, 91. 51 Zagatto, E. A. G., Krug, F. J., Bergamin, F. H., J6gersen, S. S. and Reis, B. F.,Anal. Chim. Acta, 1979, 104, 279. 52 Tyson, J. E and Bysouth, S. R., J. Anal. At. Spectrom., 1988, 3, 21 I. 53 Doku, G. N. and Gadzekpo, V. P, Y., Talanta, 1996, 43, 735. 54 Fang, Z., Welz, B. and Sperling, M., Anal. Chem., 1993, 65, 1682. 55 Zagatto, E. A. G., Gin6, M. F., Fernandes, E. A. N., Reis, B. F. and Krug, F. J., Anal. Chim. Acta, 1985, 173, 289. 56 Nakahara, T., Spectrochim. Acta Rev., 1991, 14, 95. 57 Sanz-Medel, A., Vald6s-Hevia, M. C., Bordel, N. and Fernfindez de la Campa, R., Anal. Chem., 1995, 67, 2216. 58 Vald6s-Hevia, M. C., Fernfindez de la Campa, R and Sanz-Medel, A., J. Anal. At. Spectrom., 1994, 9, 231. 59 Men6ndez, A., Sanchez-Uria, E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1989, 4, 585. 60 Wang, H., Chen, Y. and Wang, J.,Anal. Proc., 1994, 31,357. 61 Zhang, S. C., Xu, S. K. and Fang, Z. L., Quire. Anal., 1989, 8, 159. 62 Hanna, C. P., Tyson, J. F. and Mclntosh, S., Anal. Chem., 1993, 65, 653. 63 Attayah, R. H. and Kalman, D. A., Talanta, 1991, 38, 167. 64 de la Guardia, M., Carbonell, V., Morales-Rubio, A. and Salvador, A., Talanta, 1993, 40, 1609. 65 Burguera, M. and Burguera, J. L., Anal. Chim. Acta, 1988, 214, 421.

Basic instrumentation for FIA-atomic spectrometric detection

63

66 Burguera, J. L., Burguera, M. and Brunetto, M. R.,At. Spectrosc., 1993, 14, 90. 67 Burguera, M., Burguera, J. L., Rodon, C., Rivas, C., Carero, P., Gallignani, M. and Brunetto, M. R., J. Anal, At. Spectrom., 1995, 10, 343. 68 Appleton, J. M. H. and Tyson, J. F., J. Anal. At. Spectrom., 1986, 1, 63. 69 Fang, Z., Welz, B. and Sperling, M., J. Anal. At. Spectrom., 1991, 6, 179. 70 Yoza, N., Aoyagi, Y., Ohashi, S. and Tateda, A., Anal. Chim. Acta, 1979, 111, 173. 71 Brown, M. W. and Ruzicka, J., Analyst, 1984, 109, 1091. 72 Todoli, J. L., Mori, J., Hernandis, V., Canals, A., in "Sistemas de introduccion de muestras liquidas en espectrometria at6mica", 1996, Secretariado de Publicaciones, Alicante University, Spain. 73 Novak, J. W., Lillie, D. E. and Browner, R. F., Anal, Chem., 1980, 52, 576, 74 Meinhard, J. E., ICP Inf. Newsl., 1976, 2, 163. 75 Garbarino, J. R. and Taylor, H.,Appl. Spectrosc., 1979, 33, 393. 76 Todoli, J. K., Mufioz, M., Valiente, M., Hernandis, V. and Canals, A., Appl. Spectrosc., 1994, 48, 573. 77 Ibrahim, M., Nisamaneepong, W. and Caruso, J. A., J. Chrom. Sci., 1985, 23, 144. 78 Brotherton, T., Barnes, B., Vela, M. and Caruso, J. A., J. Anal. At. Spectrom., 1987, 2, 389. 79 Vermeiren, K., Vandecasteele, C. and Dams, R., Analyst, 1990, 115, 17. 80 Koropchak, J. A. and Winn, D. H., Anal. Chem., 1986, 58, 2561. 81 Bordera, L., Todoli, J. L., Mora, J., Canals, A. and Hernandis, V., Anal. Chem., 1997, 69, 3578. 82 Doherty, M. P. and Hieftje, G. M., Appl. Spectrosc., 1984, 38, 405. 83 Jakubowski, K., Feldmann, I., Stuever, D. and Berndt, H., Spectrochim. Acta, 1992, 47B, 119. 84 G6tz, R., Elgersma, J. W., Kraak, J.C. and Poppe, H., Spectrochim. Acta, 1994, 49B, 761. 85 Lawrence, K. E., Rice, G. W. and Fassel, V. A., Anal. Chem., 1984, 56, 289. 86 B'Hymer, C. and Caruso, J. A., Winter Conference on Plasma Spectrochemistry, M6, 1998, Scottsdale, Arizona, USA. 87 LaFreniere, K. E., Rice, G. W. and Fassel, V. A., Spectrochim. Acta, 1985, 40, 1495. 88 Crain, J. S. and Kiely, J. T., J. Anal. At. Spectrom., 1996, 11,525. 89 McLean, J.A. and Montaser, A., Winter Conference on Plasma Spectrochemistry, M7, 1998, Scottsdale, Arizona, USA. 90 Scott, R. H., Fassel V. A., Kniseley, R. N. and Nixon, D. E., Anal. Chem., 1974, 46, 75. 91 Wu, M. and Hieftje, G. M., Appl. Spectrosc., 1992, 46, 1912. 92 Wu, M., Madrid, Y., Auxier, J. A. and Hieftje, G. M.,Anal. Chim. Acta, 1994, 286, 155. 93 Jin, Q., Zhu, C., Borer, M. W. and Hieftje, G. M., Spectrochim. Acta, 1991, 46B, 417. 94 Madrid, Y., Wu, M., Jin, Q. and Hieftje, G. M., Anal. Chim. Acta, 1993,277, 1. 95 Thompson, M., Pahlavanpour, B., Walton, S. J. and Kirkbright, G. F.,Analyst, 1978, 103, 568. 96 Astrom, O., Anal. Chem., 1982, 54, 190. 97 de Oliveira, E., McLaren, J. W. and Berman, S. S., Anal. Chem., 1983, 55, 2047. 98 Pereiro, R., Wu, M., Broekaert, J. A. C. and Hieftje, G. M., Spectrochim. Acta, 1994, 49B, 59. 99 Lunzer, F., Pereiro, R., Bordel-Garcia, N. and Sanz-Medel, A., J. Anal. At. Spectrom., 1995, 10, 311. 100 Broekaert, J. A. C., Pereiro, R., Starn, T. K. and Hieftje, G. M., Spectrochim. Acta, 1993, 48B, 1207. 101 Corns, W. T., Stockwell, P. B., Ebdon, E. and Hill, S. J.,J. Anal. At. Spectrom., 1993, 8, 71. 102 Vijayalakshmi, S., Prabhu, R. K., Mahalingam, R. R. and Mathews, C. K.,At. Spectrosc., 1992, 13, 26. 103 Tao, H., Lam, J. W. H. and McLaren, J. W., J. Anal. At. Spectrom., 1993, 8, 1067. 104 Fang, Z., Sperling, M. and Welz, B., J. Anal. At. Spectrom., 1990, 5, 539. 105 Sperling, M., Yin, X. and Welz, B., J. Anal. At. Spectrom., 199 !, 6, 615. 106 Porta, V., Abollino, O., Mentasti, E. and Sarzanini, J. Anal. At. Spectrom., 1991, 6, 119. 107 Ma, R., Van Mol, V. and Adams, F., Anal. Chim. Acta, 1994, 293, 251. 108 Goenaga-Infante, H., Fern~indez-Sfinchez, M. L. and Sanz-Medel, A., J. Anal. At. Spectrom., 1996, 11, 571. 109 Goenaga-Infante, H., Fern~indez-S~mchez, M. L. and Sanz-Medel, A., J. Anal. At. Spectrom., 1997, 12, 1333. 110 Miller-Ihli, N. J., J. Anal At. Spectrom., 1989, 4, 295. 111 Valc/trcel, M. and Gallego, M., Talanta, 1997, 44, 1509. 112 Arruda, M.A.Z., Gallego, M. and Valc~ircel, M.,J. Anal, At. Spectrom., 1995, 10, 55.

3.1

Introduction

As was briefly mentioned at the conclusion of Chapter 1, the first publications describing the use of flow injection techniques for the introduction of samples into atomic spectrometers (atomic absorption spectrometers) appeared in the literature in 1979. These early papers described the use of FI as a microsampling system, but it was soon recognized that the flow injection atomic spectrometry (FIAS) combination had a number of features that gave rise to improved analytical methodology. Over the last twenty years or so there has been a sustained research activity in this area with an attendant burgeoning literature. This literature has been comprehensively reviewed on an annual basis as part of a much larger review process which started as a single volume Annual Reports in Analytical Atomic Spectrometry, but which now appear as a series of Atomic Spectrometry Updates in the Journal of Analytical Atomic Spectrometry. These updates essentially cover the 4000 or so annual publications in two ways: classification according to technique, and classification according to matrix. Material related to flow injection can be readily identified in the technique-based reviews, as the topic is highlighted in the sections dealing with sample introduction. Over the years there have been a number of review articles, and even two books, which have surveyed the status of FIAS and thus the development of the techniques and applications of FIAS can be traced via the pages of these publications. A list of some 21 of these is provided in Table 3.1. In this chapter, an overview of the ways in which FI procedures have been used to 64

FIA techniques and strategies expand the potential of atomic spectrometry Table 3.1 Date 1985 1985 1986 1986

1986 1987 1987 1988

Reviewarticles and books concerned with flow injection atomic spectrometry Title Flow injection analysis techniques for atomic absorption spectrometry. Flow injection techniques for flame atomic absorption spectrophotometry. Flow injection analysis - a survey of its potential for spectroscopy. Combination of flow injection techniques with atomic spectrometry ir~ agricultural and environmental analysis. Sample preparation and presentation in inductively coupled plasma spectrometry. Flow injection analysis: a novel tool for plasma spectroscopy. Flow injection techniques in inductively coupled plasma spectrometry. Flow injection calibration techniques.

1988

Atomic spectrometry and flow-injection analysis: a synergic combination.

1989

Flow injection atomic spectrometry.

1990

Flow injection analysis and chromatography: Twins or siblings? Atomic spectrometric detectors for flow injection analysis. Flow injection analysis

1990 1990 1991 1991

1991

Flow injection on-line column preconcentration in atomic spectrometry. Inductively coupled plasma mass spectrometry in hyphenation: a mulielemental analysis technique with almost unlimited potential. Flow injection atomic spectrometry.

1992

Putting the chemistry back into analytical chemistry.

1995

Flow injection atomic absorption spectrometry. Developments and trends in flow injection atomic absorption spectrometry. Flow injection analysis state of the art applied to atomic spectroscopy. Flow injection automation in atomic spectrometry.

1996 1996 1997

65

Comments First comprehensive review. Section on ICP-ES included. Brief survey.

Ref. 1 2

Survey of both AAS and ICE Covers FAAS, chemical vapor generation and ICE

4

Discussion of limitations of ICP techniques and advantages of FI. Basic principles and all aspects of sample pretreatment. All aspects of sample introduction ICPMS as well as ICP--OES. Examples taken mainly from atomic spectrometry. Survey with emphasis on reduced uptake rate, air compensation, peak area measurement. Book with chapters contributed by many leading workers. General survey with some examples of sample pretreatment for AAS. Survey of all atomic spectrometry techniques. Chapter in book: emphasis on coupled continuous separation methods. Discussion of quantitative performance parameters and practical aspects. Survey includes both flow injection and chromatography.

5

Comprehensive review of all aspects, including real sample analyses. Review of unique features of FI for coupling reaction chemistry with instrumentation with emphasis on atomic spectrometries. Single author book. Comprehensive and beautifully illustrated. Journal article companion to literature of Reference 18. Special journal issue devoted to FIAS. Contributions from 20 research groups. Comprehensive reviews covering all atomic spectrometries.

6 7 8 9 10 11 12 13 14 15

16 17

18 19 20 2l

66

J.F. Tyson

enhance the performance of atomic spectrometry is given. The topics may be divided into two broad categories. Those features of FI which affect the basic performance of the spectrometer and those FI procedures for interfacing various sample pretreatment procedures with the spectrometer. These latter topics will be dealt with in considerable detail in subsequent chapters: sample decomposition and digestion in Chapter 5, solidphase extraction in Chapter 6, liquid-liquid extraction in Chapter 7 and vapor generation procedures in Chapter 8. In this chapter the former topics will be covered in some detail, but first some basic ideas about the dynamic response of atomic spectrometers need to be discussed.

3.2

Atomic spectrometer response characteristics

In comparison with other instruments which are used as detectors in flow analysis systems, atomic spectrometers are less than ideal. Leaving aside the graphite furnace with its intermittent operation, it must be borne in mind that flame and plasma instruments perform a complex series of physical and chemical transformations on the solution entering the instrument in order to produce a cloud of atomic vapor. Thus a step change in concentration from C! to C2 immediately prior to the nebulizer does not result in a step change in instrument response. The signal changes relatively slowly to reach a new steady state value after a response time, t, characteristic of the particular instrument. Thus if a rectangular concentration pulse (C, to C2 to C~) is introduced, whose width is less than 2t,., the instrument response function will be a continuously changing function (similar to that shown in Figure 1.2, if C2>C~) and the peak height will be less than the steady state response to a solution of concentration C2. For instruments with short tr values, this situation would be interpreted as on-line dilution. However, for the atomic spectrometer with relatively long tr, the dilution is more apparent than real, as the main reason for the long tr value is the flow of aerosol droplets through the spray chamber. The droplets that are eventually carried to the flame or plasma are the result of various fragmentation and evaporation processes, with little or no opportunity for the redistribution of solutes among the droplets. Thus the signal appears to be that of a diluted solution, as the cloud of droplets entering the atomizer consists of a certain fraction whose concentration is C~ and a complementary fraction whose concentration is C2. The signal obtained is indistinguishable from that produced by a cloud of droplets, each of which contain the same diluted solution. This can have some important consequences if the dispersion coefficient of the system is measured from the response of the spectrometer as it typically would be. The value of D measured would overestimate the extent of real dilution in the manifold and thus the extent of mixing between the analyte and any reagent will be overestimated. Such reagents

FIA techniques and strategies expand the potential of atomic spectrometry

67

might be complexing agents or buffers in the case of solid phase extraction procedures, or releasing or protecting agents or ionization buffers in the case of modifications to the flame or plasma chemistry. The large t,. values for typical spray chambers mitigates against the use of simple interfaces between a chromatographic separation by high performance liquid chromatography (HPLC) and a flame or plasma instrument as element-specific detector because of the loss of chromatographic resolution caused by the extra-column broadening in the detector. This topic is dealt with in more detail in Chapter 13. Useful results can be obtained by simply connecting the eluent from the chromatograph to the standard nebulizer, but there are alternative devices (some of which work at low flow r a t e s - below 100 I~1 min -~) which offer improved performance. For a limited range of elements, notably As and Se, interfacing via chemical vapor generation is an option, though it must be remembered that not every arsenic- or selenium-containing compound yields a volatile compound on reaction with borohydride in aqueous acid solution. Chemical vapor generation is discussed on more detail in Chapter 8. The transient nature of the signal has some implications for the data collection procedure; a basic requirement is that the collection rate must be fast enough so that the peak maximum is accurately detected. For atomic absorption instruments, this may not be a problem as the instrument will most likely have been designed to work with a graphite furnace atomizer; it may be difficult, though, to access this mode of operation if a flame or quartz tube atomizer is being used. Most recent-generation plasma mass spectrometer instruments have signal collection and processing capabilities for monitoring transient signals, though many of the first two generations of such instruments do not. Most plasma emission instruments do not have this capability, though it is probably a matter of the software for data processing as the instruments have signal collection facilities that are more than adequate for monitoring transient signals. The transient signal handling capability of ICP-MS instrumentation is probably further advanced than that of ICP-OES instrumentation and it is important that users of ICP-MS instrumentation are aware of how to optimize data collection and evaluation for rapid sequential multi-element determinations based on transient signals. This is discussed further in Section 3.3.2. However, the situation is likely to change for the better in the near future, as several other sample introduction procedures, such as electrothermal vaporization and laser ablation, also produce transient signals.

3.2.1

Single well-stirred tank model for dispersion

It is useful to be able to predict the influence of various parameters, notably volume injected and flow rate, on the flow injection signal. One way to do this is to use a model for the transient behavior of the flow injection system. Although a number of models have

68

J.F. Tyson

been proposed, a simple model can be used for the behavior of FI peaks with atomic spectrometry detectors requiring only a one-parameter fit. In this model all the dispersion is assumed to be due to the passage of the injected volume, Vi, through a single well-stirred tank of volume, V (the parameter to be fitted). At time t=0, the sample volume starts to enter the tank and thus for a volumetric flow rate of Q the maximum peak height is reached at time tp-Vi/Q. From time t=0 to t-tp, the concentration, C, rises exponentially according to Equation (3. l) C=C0[1 - exp( - QtlV)]

(3.1)

where Co is the concentration injected, reaching a maximum value of Cp given by Ct,= Co[ 1 - exp( - Vi/V)]

(3.2)

For t> t,, the concentration falls exponentially according to

C=Cp e x p [ - Q(t- tp)/V]

(3.3)

Thus the dispersion coefficient D at the peak maximum (defined as Co/C.) is given by D= [1 - exp( - Vi/V)]-'

(3.4)

If a sample of known volume is injected, then the measured value of the dispersion coefficient can be used to calculate V. The derivation of these equations can be found in references 22 and 23. In addition to the prediction of how peak height is affected by experimental parameters (this model predicts that peak height is independent of flow rate), the corresponding equations for peak width 24 and peak area 25 have also been derived. The peak width, At, is given by At=(WQ) ln[(Co/C')- 1] [exp(Vi/V)- 1]

(3.5)

where C' is the concentration at which the width of the peak is measured. For the situation where Co/C' is ~> 1, so that ln[(Co/C')- l] can be approximated as ln(Co/C'), the peak width at some fixed height above the baseline is directly related to the logarithm of the concentration. In this mode, the FI system gives a response analogous to that of an ionsensitive electrode and the working range is considerable increased at the high concentration end. 26'27 The peak area is given by CoVi/Q and is independent of the volume of the tank. The model predicts that the peak area increases in inverse proportion to the flow rate and thus at low flow rates, large values of the area will be obtained. The model does not account for the fact that the response of an atomic spectrometer is also a function of flow rate, though perhaps this would not be so difficult to do if the experimentally derived relationship were used. 23

FIA techniques and strategies expand the potential of atomic spectrometry

69

The single well-stirred tank model predicts that the peak height will be given by Equation (3.2). If the exponential term is expanded as a power series, and squared and higher terms neglected, this expression simplifies to Cp=CoVi/V and so, to a first approximation, the sensitivity is directly proportional to the sample volume injected and thus if the volume is decreased by a given factor, the sensitivity is also decreased by the same amount. While this results in an increase in the detection limit, it provides an increase in the working range and it allows a greater sample throughput to be achieved as peak width js also a function of sample volume. If the basewidth is taken to be the width at some finite height above the baseline, it is possible to estimate the throughput for given values of V, Q and Vi from equation (3.5) To a first approximation, this may be taken as the time to peak maximum, Vi/Q, plus the time taken from the peak maximum to "baseline" given by (V/Q) In Cp/C'.

3.3

Microsampling into nebulizers

One of the basic features of FIAS is the handling of discrete small sample volumes under controlled conditions. While this is the essence of the FI experiment involving on-line chemical reaction, it also has some useful features when the system is employed simply to transport the sample to a nebulizer. There are a number of advantages to the introduction of micro-volumes of sample, many of which have already been exploited in the various "discrete" nebulization or "onedrop" procedures which have been described in the literature for a number of years. 28 Flow injection introduction has several advantages over these procedures. The nebulizer and spray chamber are continuously washed with carrier stream and this ensures that there is no carry-over from one sample to the next. It also maintains constant wetting of the surfaces involved in spray production and droplet removal and keeps the spray chamber and interior burner surfaces at a constant temperature. If air is drawn in through the nebulizer, the interior surfaces dry out and the temperature rises, this gives rise to irreproducible transport efficiency and hence poor signal precision. Discrete procedures also suffer from memory effects in which components of one sample left on the spray chamber walls can be remobilized by the aerosol from the next sample. Discrete procedures are operator intensive, whereas a flow injection system can be completely automated. Most of the examples used in this chapter relate to flame atom sources, though many of the features of FI introduction to flames are also applicable to plasma atomizers. Many plasma source instruments deliver the liquid to the nebulizer by means of a peristaltic pump rather than allow free aspiration under the action of nebulizer suction. Some

70

J.F. Tyson

elements are prone to absorption onto the peristaltic pump tubing commonly used, which can cause inaccuracies and memory effects. This absorption can be avoided by the introduction of sample via an injection valve located downstream of the pump. For instruments which cannot handle a transient signal, the injected volume could be sufficiently large to produce fiat-topped peaks and thus the FI system could allow operation in a pseudo-steady state mode.

3.3.1

Limited sample volume

With the possible exception of certain areas of clinical analysis, there are not many analytical applications where there is a serious shortage of sample material. However, for these applications, FI introduction allows the amount of sample needed for a measurement to be decreased. For conventional introduction, enough sample is needed to allow the signal to rise to steady state before an integration of signal intensity lasting several seconds may be made. For FI introduction useful signals may be obtained from an injection volume, Vi, of as little as 35 IxL. Both peak height and peak area may be used as quantitative parameters and clearly as the dispersion coefficient of the system increases from unity, the peak height signal becomes less than the steady state signal. This decrease in sensitivity may not be a problem in practice unless it is required to make measurements of very low concentrations. The effect on detection limit of changing to FI introduction system is discussed in more detail later in this chapter. Depending on the design of nebulizer and spray chamber (and of course, on the FI manifold) it may be possible to achieve for flame AAS, a peak height equal to the steady state signal for a volume injected of around 300-500 IxL. It should be borne in mind that an additional volume of solution is required to flush out the loop (three-times the loop volume is suggested) and any connecting lines between the autosampler and the valve. Thus the volume of solution needed in practice to make a FIAS measurement will be greater than just the loop volume. The use of sequential injection 29 or controlled dispersion techniques 3~ may be able to reduce the amount of sample needed. It is a feature of FI systems that both sample and reagent volumes are reduced compared with those needed for batch procedures with continuous nebulization. This can lead to significant savings in the costs of the disposal of waste solutions.

3.3.2

Multi-element determinations by FI-ICP-MS

The consequences of using FI sample introduction with ICP-MS in terms of the impact of the limited sample volume have been comprehensively discussed by Denoyer and Lu. 31

FIA techniques and strategies expand the potential of atomic spectrometry

71

3.3.2.1 Effect of injection volume Flow injection microsampling is often used with ICP-MS to limit the amount of total dissolved solids entering the mass spectrometer. This approach can improve long-term signal stability by controlling deposition of sample material on the ICP-MS interface cones. 32 In FI-ICP-MS microsampling, the peak area (integrated ion signal) for an analyte is directly proportional to the volume of sample injected. The signal peak height also increases with injection volume but reaches a plateau at about 500 txL, when the signal is equal to that obtained with continuous sample introduction. Therefore, an injection volume of around 500 txL offers a good balance between achieving high signal intensity, while maintaining long-term stability by injecting small sample volumes into the ICPMS. The effect on detection limits is shown in Figure 3.1 Obviously, detection limits are worse for small sample volumes but improve rapidly with increasing sample volume. Detection limits equivalent to those obtained for steady state integrations between 1 and 10 s can be achieved for flow injection with volumes between 50 and 500 I~L. An

Figure 3.1 Relativedetection limit for Rh as a function of volume injected. Note the similarity of the plot to the relationship shown in Figure 3.9. The figure also shows the detection limits obtained for normal nebulization for three integration times. Used with permission from ref. 31.

72

J.F. Tyson

important difference between FI introduction and continuous aspiration is that the time available to make the measurement is fixed in FI for a given injection volume. Therefore, the comparison between FI and continuous aspiration will depend on the number of elements to be determined. Although earlier flow injection applications usually involved the determination of single analytes, simultaneous or fast-sequential techniques such as ICP-OES, ICP-MS and, now, simultaneous AAS can be used to monitor multiple analytes within a single flow injection peak. Recently, more and more publications have become available in the literature which describe multi-element flow injection methods. This time-dependence of the signal has some important consequences with regard to how an ICP-MS instrument is used, particularly for multi-element determinations. With multi-element techniques, transient flow injection signals have structure not only in the time domain, but also in the spectral domain. It is therefore important to know how to optimize multi-element data gathering and manipulation, when data is collected sequentially.

3.3.2.2 Effect of number of elements determined As the spectrometer is rapidly scanning across the mass range, only a fraction of the total time available can be devoted to the measurement of a given analyte. Therefore the more analytes that are measured, the less time spent on each. Consequently, the signal per element is inversely proportional to the number of elements, n, measured. This is shown in Figure 3.2 for Rh. However, the detection limits measured for any single element should degrade only by the square root of n. So for example, when 64 elements are determined instead of only two, the detection limits for Rh are not 32 times worse, but only about five to six times worse. This is an advantage because increased elemental coverage does not result in as severe a trade-off in detection limits as might be expected. This is also shown in Figure 3.2. Increasing the volume injected improves detection limits by extending the measurement time available (see Figure 3.1). This would be a good approach to use when wide elemental coverage and low detection limits are more important analytical objectives than minimizing sample consumption and/or maximizing throughput. 3.3.2.3 Effect of scan mode and dwell time The mass spectrometer can be scanned in two modes known as (a) ramp scanning and (b) peak hopping. In each mode the characteristics of the voltage signals applied to the quadrupoles are kept constant for a period, known as the dwell time, while signal is accumulated. In the peak hopping mode, the electronics are allowed to settle for a few ms (the settling time) prior to the signal collection. In the ramp scanning mode there is no need for any settling time except at the start of each scan following rapid sweepback. Both

FIA techniques and strategies expand the potential of atomic spectrometry . . . . . . . . . . . . . . . . . .

0.8

0.005

ca e~

0.6

-

o.oo4

.E

-

O.OO3

13

-

O.OO2

~

0

0.4

O.Z -

........

Figure 3.2 Plot of relative signal for Rh and limit of detection (measured and calculated) as a function of the number of elements measured. Used with permission from ref. 31. theory and experiment indicate that for a high-speed quadrupole system with settling times of 1-5 ms, peak hopping with dwell times of 40-50 ms would give the higher signal and therefore would be the scan mode of choice for FI-ICP-MS. However, it is important that enough data points per peak (about 20) are collected so as to characterize the peak shape accurately and thus dwell times may have to be reduced if a large number of elements are being measured.

3.3.2.4 Comparison of peak height and peak area measurements, and the effect of signal smoothing The precisions achieved using peak height and peak area are shown in Figure 3.3 for the measurement of three elements at concentrations of 10 ppb (100-1000 times greater than the detection limits) in ten replicate 500-~1 injections. Peak area precision is clearly better than that of peak height. There are a number of smoothing routines that can be used to improve signal to noise ratio. The most widely used are based on some version of the "moving average" method. In the Savitsky--Golay procedure 33 each point is replaced in succession by the average of a user-determined number of points. In the 9-point procedure used here, points 1-9 are replaced by their average, points 2-10 are replaced by their average, points 3-11 are replaced by their average and so on. The signal evaluation procedure, such as the algorithm for determining the peak height is then implemented on the new set of points. As can be seen from the data in Figure 3.3, smoothing improves the

74

J.F. Tyson

Figure 3.3 Relative standard deviation of signals for three elements. A 9-point Savitsky-Golay smoothing routine was used. Both peak height and peak area data are shown. Ten replicate measurements of 500-~1 of a 10 ng/ml multi-element standard were made. Used with permission from ref. 31. precision of peak height measurements in FI-ICP-MS. However, smoothing does not significantly improve the precision of the peak area measurements, since integrating the area is itself an averaging process. Similar behavior is observed with detection limits. As can be seen from Figure 3.4, smoothing significantly improves peak height detection limits, but has no effect on peak area values which are much better than those based on peak height without smoothing, but only slightly better than those based on peak height with smoothing. It is important to remember that for peak height, the data is limited to that gathered during a short time interval at the peak maximum. While this may not be such an important concern for single element determinations, it is especially important when multiple elements are measured and the measurement time per element is at a premium. One consequence of the sequential nature of the data collection is that in multi-element determinations it is easy to miss the peak maximum especially if (a) only a limited number of points per peak are collected and (b) the peaks are narrow. Thus the preferred method of signal evaluation is calculation of peak area. However, there is an optimum with regard to the number of points per peak as the dwell time is decreased as the number of points

FIA techniques and strategies expand the potential of atomic spectrometry

75

Figure 3.4 Detection limits based on peak height and peak area with and without 9-point SavitskyGolay smoothing. Used with permission from ref. 31. per peak increases. As a general rule, no more than 50 points should be used to characterize a FI peak in ICP-MS. For signals with mainly random noise, which is the case for ICP-MS with signal intensities less than 10 000 cps, an improvement in peak area signal to noise ratio may be obtained by integrating over a window which is less than the full width of the peak. 3a This process is known as gated integration and can lead to improvements in detection limits for FI-ICP-MS of up to a factor of 2 at the optimum window. 3~

3.3.3

High dissolved solids

Continuous nebulization of solutions with a high salt content will eventually lead to blockage of the nebulizer (flame and plasma instrumentation) or burner slot (flame AAS) or injector tip (plasma instrumentation) or sampling cones (plasma mass spectrometry). Such a high dissolved solids content may be an inherent property of the sample (as for example with sea water or plating bath solutions) or may have arisen as a result of sample preparation (as for example with a fusion in a molten salt). Even "discrete nebulization" can be problematical if the dissolved solid content is really high. Attempts to introduce 30% m/V sodium chloride solutions were described as "disastrous". 35 In addition to introducing a limited mass of material, a FI procedure has another key

76

J.F. Tyson

feature, namely the continuous washing of the nebulizer and spray chamber with the carrier fluid. This not only prevents the build-up of salt deposits (allowing prolonged use before any cleaning is needed), but also maintains constant operating conditions in the sample introduction system. If a flame AAS instrument is allowed to draw air in through the nebulizer, the spray chamber dries out and the burner temperature rises. This gives rise to an enhanced sensitivity for a single shot discrete introduction, but is not a reproducible phenomenon on a usable timescale, and leads to poor precision. Under normal conditions, the immediate nebulization of a concentrated acid solution following a sample or standard can remobilize material from the impact surfaces and spray chamber walls, giving rise to spurious, transient blanks or carry-over. Neither of these problems arises when FI introduction is used. There is also a problem with plasma instrumentation, which arises when the acid concentration is changed, that is related to the equilibration of the composition of the liquid film on the spray chamber interior with the vapor-phase composition. 36

3.3.4

Slurry nebulization

Some sample materials particularly those with silicate or aluminosilicate matrix components, do not dissolve in the common mineral acids, even under conditions of increased pressure and temperature. To achieve complete dissolution, hydrofluoric acid is necessary. The aggressive nature of this acid means that safety precautions must be strictly observed and, of course, as the acid attacks silica, normal glass laboratory-ware cannot be used. Beakers and flasks made from Teflon | or polypropylene are required. In addition, unless the resulting solution is treated to remove the free fluoride ions, the acid solution will attack the silica-based spray chambers, injector tubes and torches commonly used in plasma spectrometry, and components made from alternative materials (such as alumina for the injector tubes) are needed. It is possible to complex the fluoride by the addition of a suitable reagent such as boric acid (to form the highly stable fluoroborate ion), but this both increases the amount of dissolved solids and adds analyte elements as contaminants. However, many of these difficulties could be avoided if this dissolution step could be omitted and the solid sample handled directly. The general topic of solid sampling is outside the scope of this book (as it includes laser ablation, spark ablation, electrothermal vaporization), however FI may be used for the introduction of sample slurries. There is a sustained, though relatively low level, research activity in this topic, from which it appears that by far the most successful approach is to introduce the slurry into a graphite furnace atomizer. However, there are a number of papers describing the use of a single line FI manifold to transport a slurry sample to a flame atomic absorption spectrometer. Although there are few, if any references, to the use of FI for slurry nebulization into plasma sources,

FIA techniques and strategies expand the potential of atomic spectrometry

77

there are numerous reports of the successful use of slurry nebulization into plasmas for emission and mass spectrometries and thus it is expected that FI would be a useful procedure to adopt for this purpose. For example, Vinas and coworkers 37 determined calcium, magnesium, iron, zinc and manganese in vegetable material by the injection of an ash slurry in a 10% glycerol-1% hydrochloric acid solution. Precisions of less than 2% relative standard deviation were obtained and the FI manifold was used for on-line dilution in the case of the determination of calcium and magnesium, as the concentrations of these two elements were outside the working range of the instrument. Although, it might appear that slurry sampling is only applicable if the sample contains a predominantly organic matrix which is also amenable to fine grinding, there have been reports of the use of the techniques with sludges, sediments and other predominantly mineral sample materials. The pretreatment usually involves the use of moderately concentrated mineral acids, including HF, for samples with a high silica content, with prolonged stirring and/or grinding and thus there is a good possibility that the analyte elements are at least partially leached from the solid matrix prior to introduction to the spectrometer. None-the-less, such preparation procedures are preferable to time-consuming fusion techniques, which require considerable operator skill, specialized apparatus, high purity fluxes and produce solutions with high dissolved solids. So far, fusion is one of the few sample pretreatment procedures that has not (yet) been converted to a FI format. Procedures for the on-line decomposition and dissolution of samples are discussed in Chapter 5.

3.3.5

Organic solvents

As with solutions containing high dissolved solids, the continuous nebulization of many organic solvents can be problematic, either because of the flame (or plasma) instability produced or the toxic combustion products produced. It is possible to introduce small volumes of solvents, such as chloroform or carbon tetrachloride which under continuous nebulization conditions would extinguish the flame. 3s These solvents are preferred from the chemical point of view as many separation procedures in which the analyte derivative is extracted into a chlorinated solvent are highly efficient. However, the use of such solvents is decreasing because of the difficulties associated with handling laboratory waste. There are of course instances where the sample itself is an organic solvent (such as an oil or gasoline) and the use of a single line FI introduction system can simplify the overall procedure considerably. With regard to preconcentration and matrix separation procedures, there is more interest in solid phase extraction procedures than in liquid-liquid extraction procedures. However, there have been a number of methods described in which a solid-phase procedure has used an organic solvent to remobilize the analyte derivative from the solid phase. There are

78

J.F. Tyson

several reports of the derivatization of metals as the dithiocarbamate complexes with retention on a hydrophobic surface (including the interior walls of a knotted tubular reactor) followed by elution in methanol, acetonitrile or isobutylmethyl ketone. These solvents work well with flame sources as the enhanced nebulization efficiency and increased flame temperature compared, with the situation for an aqueous sample, give rise to an enhanced signal. The solvents can also be introduced successfully into a graphite furnace atomizer, but the situation with plasma sources is less satisfactory. However, the addition of oxygen (though an additional gas line, merged with the aerosol carrier gas line) can improve the source stability considerably as can the use of a cooled spray chamber (to reduce the solvent vapor loading).

3.3.6

Viscosity effects

Under the action of nebulizer suction, the rate of uptake of a solution depends on its viscosity and thus the analyses of sample solutions which differ significantly in viscosity from that of the standards will be inaccurate. The use of FI, particularly with small sample volumes (30-50 I~1), considerably reduces viscosity effects and can eliminate the need for matrix matching.

3.3. 7 High pressure and other nebulization devices In addition to the basic concentric and cross-flow devices used with flame and plasma instruments, there are a variety of other aerosol generation devices used for sample introduction. The reason for using these devices is to achieve improved performance over that available from the conventional devices, usually in terms of detection limit. These devices include the ultrasonic nebulizer, the thermospray nebulizer, the direct injection nebulizer, the frit or grid nebulizer, the oscillating capillary nebulizer, and the hydraulic high pressure nebulizer. Details of the principles and performance of most of these devices can be found in an appropriate text. 39As they are all designed to operate with a continuous stream of liquid they can all be used for FI introduction. At least two of these devices, the thermospray 4~ and the hydraulic high pressure nebulizer 41 require delivery of the liquid from a high pressure pump and thus the introduction of the sample by means of an HPLC valve is often used to avoid contact between the pump head components and the possibly corrosive sample solutions. These devices are thus based on FI for sample introduction.

3.4

Nebulization efficiency

Some of the first papers on FI introduction for flame AAS suggested that because the nebulization efficiency of concentric pneumatic nebulizers is improved under starvation conditions, improved sensitivity could be obtained from a FI introduction system in which

FIA techniques and strategies expand the potential of atomic spectrometry

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the peristaltic pump was used to reduce the flow rate and thus it was possible to obtain increases in detection limits. There was also debate over whether peak height or peak area should be used as the measurement mode. The claims concerning detection limit were looked at very carefully 3"4 and were found to be without foundation. The best detection limits for analyte species in solution are obtained from an introduction procedure which gives the maximum signal to noise ratio (SNR). In FI terms this means that the dispersion coefficient should be 1. However, although the flow rate should be high enough to give a signal close to the maximum peak height, it should be borne in mind that, in general, conditions which produce the maximum signal will not simultaneously produce the maximum SNR. This is because the largest signal will be produced by an aerosol with relatively large droplets, which can be produced by pumping at higher flow rates, as this increases the mass flux into the atomizer. However, droplet size distribution under these conditions will be relatively broad and this will give rise to greater noise due to variations in the appearance time of analyte atoms in the flame. The smaller droplets evaporate first and the larger droplets evaporate last giving an uneven release of analyte into the flame gases over time compared with the situation where the droplets all have approximately the same size. In addition to the magnitude of the signal and the noise, the extent of stable compound formation is also governed by the aerosol characteristics. The interferences are reduced with smaller drop size distributions. These features of nebulizer performance means that the manufacturer's recommended operating conditions (or the factory-set conditions) are likely to be those which produce a compromise between signal, noise and extent of interference. Thus it is likely that by suitably varying the uptake rate, conditions can be found for which the signal is increased over that obtained for the "conventional" operating conditions. This does not mean however, that the detection limit will be improved. If the flow ~rate to the nebulizer is controlled by a peristaltic pump, then there is an additional source of noise, namely the contribution from the small changes in flow rate produced by the pressure pulsations as the pump rollers engage and lift off the tubing on the pump platen. At slow pump-head rotation speeds, these pulsations can be a major source of noise. The fluctuations can be decreased by the use of pulse dampers or by the use of fluid compensation. Both of these procedures involve the insertion of a T-piece into the flow line after the peristaltic pump. In the case of the pulse damper, a vertically mounted sealed side-arm is inserted between the pump and the valve. The air trapped in this side-arm alternately compresses and expands out of phase with the pressure pulsations in the line, helping to smooth out the resulting fluid flow. In the case of fluid compensation, a T-piece is inserted in the line just prior to the nebulizer through which either an aqueous solution (matched to the carrier stream composition) or air is aspirated due to the excess of nebulizer suction over the line pressure at the junction.

80

J.F. Tyson

Having said that, not all atomic spectrometers have identical nebulizer and spray chamber performance and there can be quite large variations between different manufacturers' designs, especially in the case of flame AAS. Also, there have been some significant improvements in nebulizer and spray chamber performance in recent years, so that the behavior of an older instrument from manufacturer A may be significantly different from that of a newer instrument from manufacturer B or even the same manufacturer. It is possible, therefore, that for a particular instrument under FI operating conditions, improvements in detection limits can be obtained compared with those obtained with the "conventional" introduction procedure. Such improvements are likely to be relatively minor and it may well not be worth the time and effort needed to obtain such a marginal improvement, compared with the improved performance that is available from implementing a preconcentration procedure, such as solid phase extraction (see Chapter 6). Stable compound formation is reduced under conditions of increased nebulization efficiency and so certain kinds of depressive effect, such as that of phosphate on calcium, can be removed or decreased by suitably adjusting the flow rate to the nebulizer. However, an equally effective strategy is to reoptimize fuel-oxidant ratio and burner height for minimum interference rather than maximum sensitivity and it should be borne in mind that many of these types of interference disappear when the nitrous oxide acetylene flame is used. Because FI introduction has increased tolerance to high dissolved solids over conventional nebulization, improved detection limits in solid samples will be obtained as a higher concentration of the material in solution can be used.

3.5

Dilution and calibration

Flow injection introduction is normally accompanied by a sample dilution as the usual situation is that the dispersion coefficient of the manifold is less than unity. This is always the case when the manifold is used to perform some chemistry (other than preconcentration) as reagent addition cannot be achieved without dilution. The factors which control the extent of dilution were discussed in Section 1.3. In principle, an infinite number of dilution factors is obtained from each injection, as the signal can be measured at any point on the dispersed sample zone (see Section 1.1). This approach has been used for dilution and calibration and is referred to as "electronic dilution" as the procedure requires that the signal be measured at a known and reproducible time after the valve has been actuated. In addition, there are a number of procedures in which the measurement at the peak maximum is retained as this is a readily identified point, and some parameter varied so as to achieve the desired dilution. If the manifold is to be used for calibration purposes, it is necessary that several dilutions of known value may be obtained so that a series of

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calibrations standards may be prepared from a single concentrated stock standard and introduced in rapid succession. The possibilities for FI dilution and calibration are discussed in detail in Chapter 4. 3.5.1

Variable volume injected

As it is not practical to change the loop between injections, variation of the volume injected is achieved by controlled timing of operations. The most commonly used approach is to control the time in which the valve is in the inject position. If the valve is returned to the "fill" position before the contents of the loop have been completely flushed into the manifold, a volume of sample solution less than the loop volume is injected. This procedure is known as "time" injection (the term is usually considered to be restricted to the case where the dispersing rear boundary of the sample has not emerged into the manifold). Under these circumstances, the front and rear boundaries have the same residence time. The alternative procedure, in which the entire contents of the loop are flushed into the manifold, is known as "slug" injection. With slug injection the rear boundary has a longer residence time than the front boundary and the peaks exhibit greater tailing. Time injection can be achieved by a simple stream-switching valve whose position alternates between one allowing carrier solution to pass and one allowing sample solution to pass. The precision of the volume injected is governed by the precision of both the flow rate and the timing of the valve switching. Recently a method was described 42 in which the amount of solution introduced into the sample loop was carefully controlled by the computer driven stepper motor peristaltic pump of a FIAS 200 unit. Dilution factors ofup to 1330 were achieved for sub-lxl sample volumes. If the dissolved solid content and/or analyte content of a sample is completely unknown, it is possible to avoid overloading the spectrometer by gradually increasing the volume injected until a usable signal is obtained. In practical terms, the useful range of sample volumes that can be introduced by slug injection from a rotary valve with an external sample loop is from about 50 Ixl to 1000 ILl. Although there is, of course no theoretical upper limit, in practice there is little point in increasing the volume beyond the value at which a steady state signal is obtained unless the object of the exercise is to preconcentrate the analyte by retaining the analyte from a relatively large volume on a solid phase reagent with subsequent elution into a relatively small volume. 3.5.2

Other methods

As will be discussed in Chapter 4, there are a large number of possibilities for the use of FI techniques for the controlled dilution of stock solutions for calibration purposes. In

82

J. E Tyson

C

W

Figure 3.5 Manifold for investigating interference effects. C, carrier; A, analyte; I injection of interferent; D, detector; and w, waste. addition, there are numerous ways in which the method of standard additions has been implemented in FI manifolds and there are even one or two papers describing the method of infinite dilution. 42

3.6

Study of interference effects

In developing an analytical method in which an atomic spectrometry technique is used for quantification, it is often necessary to investigate whether any of the other components of the sample interfere with the measurement of the analyte(s). A merging streams manifold may be used in a rapid screening procedure as shown in Figure 3.5. 38 The sample (this time, a known concentration of the analyte) is pumped continuously. This stream is merged with a carrier stream into which is injected solutions of the potential interferents. Some of the well-known interference effects in FAAS are shown in Figure 3.6.

3.7

Chemical pretreatment, matrix removal and preconcentration

3. 7.1

Reagent addition

Many flame and plasma procedures require the addition of components to the sample as part of the strategy for overcoming interferences (such as releasing agents or ionization suppressors) or as part of the calibration procedure (such as internal standards). Some cautionary words about the use of a single line manifold are needed. With this manifold, the reagent would be in the carrier stream and mixing is required so that at the peak maximum, the required reagent concentration is achieved (for a releasing agent to be effective, a substantial concentration excess is needed). Mixing means dilution and therefore a decrease in the sensitivity, not usually a desirable feature, and so manifolds for FIAS are often designed to minimize the on-line dilution. The interdependence of sample

FIA techniques and strategies expand the potential of atomic spectrometry

/

Figure 3.6 Plot of absorbance against time for the injection of some potential interferents in the determination of calcium by FAAS. 1, 100 ppm Ca; 2, 1000 ppm A1; 3, 1000 ppm phosphate; 4, ethanol; 5, IBMK; 6, 1000 ppm K; and 7, 10% La. Used with permission from ref. 38. and reagent dilutions was discussed in Chapter 1, and the role of the spray chamber was discussed in Section 3.2. The nebulizer and spray chamber produce an aerosol for transport to the atomizer in which some droplets contain sample and some contain reagent. The resultant evaporation and vaporization processes in the atomizer are unlikely to produce a distribution of species in the atom source that is identical to that which would be obtained if the reagent and sample were genuinely mixed in the solution being nebulized, and thus the effectiveness of the reagent may be decreased in comparison with what was anticipated.

3.7.2

Solid phase extraction (SPE)

The use of a solid reagent phase to separate analyte species from potentially interfering matrix components is a well-established sample pretreatment strategy. The procedure is

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based on the selective retention characteristics of the chosen solid phase reagent. The general procedure is that a known volume of sample solution (whose pH etc. have been adjusted as necessary) is passed through a small column of the solid reagent. The species of interest is retained, the unwanted sample components may be discarded (pumped to waste), the column is washed to remove unwanted sample solution in the interstices between the reagent particles, and the target species eluted by the passage of a solution of a compound which displaces it from the solid reagent. Usually the analyte species (rather than matrix species) is/are retained on the solid phase and so preconcentration is also possible as the analyte may be eluted into a smaller volume of solution than that from which it was extracted. The integration of solid phase reagents into FI manifolds has proved to be a popular research activity and a number of different manifold designs and a very large number of chemistries have been described in the literature. At present about 10--20 papers a year are published describing FI-SPE just for FAAS. The best manifold design is one in which the unwanted sample components are not introduced into the spectrometer and in which the retained analyte species may be removed by back-flushing. Both of these features may be obtained if the solid phase reagent is mounted in the external loop of the injection valve. One possible configuration with a Perkin-Elmer valve is shown in Figure 3.7. In this manifold, the sample volume is controlled by the time during which the pump delivers sample solution, rather than by the use of an injection valve. Note that a simple 4-port injection valve cannot be used for this purpose as it is not possible to flush the delivery line with the next sample. An extra port is needed on the valve. A

B

extractant

.l. waste

AA

extractant

elUen' AA

Figure 3.7 FI-SPE manifold. The valve acts as an interface between the separation chemistry and the preconcentration and transport to the instrument. In position A, the analyte is loaded onto the extractant which is located in the loop of the injection valve. In position B, the analyte is eluted from the column and transported to the instrument.

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In general, three types of SPE have been used. These are (a) ion-exchange, (b) chelating ion-exchange and (c) sorbent extraction. It is not possible to devise a general manifold system that will work with all elements and matrices, and the successful design of a SPE system will involve a knowledge of the chemistry of the analyte(s) and matrix components. Clearly cation exchange materials are highly selective for cations over anionic or uncharged species, but are limited in their ability to select one metal over another. In general, the best selectivity is obtained from chelating ion-exchange materials such as immobilized 8-hydroxyquinoline or materials with iminodiacetate functionalities. Sorbent extraction is based on the formation of an uncharged metal-complex which is retained on a reversed-phase liquid chromatography phase such as octadecylsilyl silica (often simply referred to as C- 18) or other hydrophobic phases (such as the interior surface of the tube wall). All extractions are pH sensitive and any procedure will require the use of a buffer solution to adjust the pH to a value in the required range. Control of pH confers some degree of selectivity on the process. There is some interest at present in the use of biological materials as solid phase extractants, and the use of algae and yeasts (metals are retained on the cell walls) have been reported. Examples of the applications of these materials are given in Table 3.2. Solid phase extraction has been used with both flame, quartz tube and graphite furnace atomizers for atomic absorption spectrometry and in both ICP--OES and ICP-MS applications. In principle, FI-SPE is a very attractive method for the automated preconcentration and separation of analyte from matrix for determination by any atomic spectrometric technique, but to be able to use the procedure satisfactorily requires some understanding of the chemical principles involved in the reactions being used. Most solid phase reagents are not inherently very selective in their retention (apart from the gross difference basis already described above), but can be made conditionally selective. That is selectivity can be conferred by adjustment of the reaction conditions. The usual conditions involved are (1) pH and (2) the presence of additional reagents. For example, cation exchangers will not retain metals in concentrated acid solution (as all the exchange sites are overwhelmed by the high concentration of H +), this is why acids are typically used to elute metals retained by solid phase extractants. As many sample pretreatment procedures result in a strongly acidified sample solution, it may be necessary to adjust the pH of the sample prior to passing through the resin. Simply injecting into or merging with a buffer stream may not be adequate as the acidity of the sample may overwhelm the capacity of the buffer. Thus it is necessary to bear in mind some elementary solution chemistry related to pH control and buffer capacity. Hydrated metal cations will not be retained by C-18 material (apart from a small amount of retention due to the inevitable, but unwanted, residual hydroxyl groups on the silica surface) and so they must be converted to uncharged non-polar species if they are to be squeezed out of the aqueous solution into the octadecyl hydrocarbon fur

86 Table 3.2

J.F. Tyson Applications of FI solid phase extraction for atomic spectrometry

Extractant

Analyte(s)

Matrix

Eluent

Technique

Ref

oxine on CPG C- 18 + DDC 8531 fiber cysteine on CPG

AI, Ga, In Cd, Cu Au Cu, Cd, Zn, Pb, Hg, Co Cd, Cu, Pb

waters biologicals ores synthetic

acid methanol thiourea acid

FAAS FAAS FAAS FAAS

43 44 45 46

soil, ash sediment synthetic ocean water, tobacco leaf sediment

methanol

FAAS

47

sodium hydroxide

FAAS

48

acid

FAAS

49

sediment

various

FAAS

50

Rh(lll), Co, Cu, Hg(l), Hg(ll) Pb

synthetic

acid

FAAS

51

synthetic

acids

FAAS

52

Cd, Cu, Pb, Ni

river, sea, estuarine water open ocean water biologicals

ethanol

ETAAS

53

acids

ETAAS

54

ammonia

ETAAS + ICPOES

55

Antarctic sea water

acetonitrile

ETAAS

56

lake, sea and drinking water synthetic aqueous synthetic aqueous synthetic aqueous

ethanol

ETAAS

57

nitric acid

ICP-OES

58

nitric acid

ICP-OES

59

ammonia+ potassium hydroxide conc nitric acid nitric acid

ICP-OES

60

ICP-MS ICP-MS

61 62

nitric acid

ICP-MS

63

ammonia+ ethanol

ICP-AFS

64

C- 18 + DDDC alumina + nitroso-Rsalt

Co

yeast on CPG

Cd, Zn, Cu. Pb, Fe Cu, Cr(IIl) Ag,

algae on CPG

Cr (Vl) dialkydithiocarbamate on CPG cross-linked chelating agents C-18+DDC

oxine on silica polystyrenedivinylbenzene + C-18 pyrrolidine- 1-yl dithioformates C-18+DDC Chelex- 100 Muromac A- 1 activated alumina

Fe, Cd, Zn, Cu, Ni, Mn, Pb Pd, Pt, Rh as bis(carboxymethyl) dithiocarbamates Cu, Pb, Cd, Ni, Co, Fe As(Ill) and total As heavy metals Cr, Ti, V, Fe, A! As, B, Cr, Mo, P, Se, V

Anion exchange activated alumina

Re As, Cr, Se, V

activated alumina

U

anion exchange

W. Mo

sea water marine biological river, sea, mineral water synthetic aqueous

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attached to the surface of the silica. Thus the sample needs to be merged with a reagent stream (say a solution of diethyldithiocarbamate (DDC)) so that an appropriate complex can form. This reaction will be pH dependent and so the reagent may well be made up in a buffer solution and again the buffer capacity of this and the sample acidity are relevant parameters. It is possible to achieve a considerable degree of selectivity by an appropriate choice of solid phase reagent and reaction conditions, but if the sample contains high concentrations of an element it is possible that there will be an interference even if the element is only weakly bound. Solid phase extraction is probably best suited for samples in which the total metal content is low (apart from those which are not retained) and which have moderate to low acidity. For many SPE procedures, the kinetics of the reactions involved are not a limiting factor and high flow rates can be used when loading the analyte(s) onto the extractant. The elution flow rate is not a critical parameter in terms of the efficiency of removing material. The slower the flow rate, the higher the peak concentration, but if peak area is used as the analytical parameter, there is a limited dependence of the signal on elution flow rate. However, if the eluted material is delivered directly to a flame or plasma spectrometer then the flow rate should be compatible with optimum performance of the nebulizer and thus the selection of elution flow rate becomes more complicated. As will be discussed in more detail in Chapter 6, flow injection techniques are well suited to the implementation of SPE procedures with electrothermal AAS. The intermittent operation of the spectrometer can be synchronized with the intermittent operation of the FI system, so that essentially the preconcentration may be performed without any addition to the overall analysis time. The miniaturized nature of the FI system makes the eluent volumes compatible with the capacity of the furnace and the nature of the operation of the furnace means that there are few limitations on the chemical composition of the eluent. In principle, if sample volume is not a limiting factor, the detection limit could be reduced to any required value simply by increasing the sample volume. In practice this approach is limited by the increase in noise which arises from the increased blank which is obtained due to the presence of contaminants in the reagents used. This is discussed further in Section 3.8. There is a need for procedures that will separate and preconcentrate a number of analytes prior to detection by plasma spectrometry. This presents a more serious challenge for solid phase extraction procedures, as the requirement is now that the chemical basis of the retention of the analytes is applicable to a number of elements. It is likely that retention by immobilized chelating agents will be the most suitable materials to use as these have the ability to chelate a number of elements with a wide range of binding constants so that matrix elements such as sodium and calcium are effectively not complexed. Materials such

J. E Tyson

88

as immobilized 8-hydroxyquinoline are potential candidates for method development, though this material has limited availability from commercial suppliers.

3. 7.3

Vaporgeneration

Flow injection is a particularly useful procedure for the implementation of chemical vapor generation (CVG) methods. These procedures are based on the generation of a volatile chemical derivative of the analyte species, removal from solution by a gas--liquid separation device and transport to the atomizer. In terms of the numbers of analyses performed by CVG, the determination of mercury by the generation of the monatomic elemental vapor is by far the most widely used procedure. However, considerable use is also made of the generation of the volatile hydrides of arsenic and selenium. As this topic of FI--CVG is of such importance, it is dealt with more fully separately in Chapter 8. An important aspect of the FI-CVG procedure is that it permits the exploitation of a difference in reaction rates to minimize certain interferences. In the generation of volatile hydride species by reaction of the analyte with sodium tetrahydroborate(III) (also known as sodium borohydride), transition metals interfere as they react with this reagent to produce the free metal and/or various metal borides. This both consumes reagent and provides, in the finely divided metal, a solid surface which efficiently traps the volatile analyte derivative. In a FI hydride generation procedure a double line manifold is used in which the sample, in an acid carrier, is merged with a stream of borohydride at the confluence point. A short distance downstream argon gas is merged in the first stages of the gas-liquid separation process. By keeping the time between the confluence point and the addition of argon short, the fast hydride generation reaction is favored over the slower metal precipitation reaction. While most of the work reported so far has been performed in conjunction with a heated quartz tube atomizer, it has been found possible to trap the volatile analyte hydride on the interior of a graphite furnace thereby allowing a preconcentration prior to atomization. Detection limits down to parts per trillion concentrations may be achieved. This procedure is similar to FI-SPE in that in principle the detection limit can be as low as desired in the absence of any blank signal. However, in practice, the presence of analyte contamination in the reagents sets a lower limit to the detection limit that can be achieved. The issue of detection limits for this procedure and for solid phase extraction is discussed is more detail in Section 3.8. 3. 7.4

Liquid-liquid extraction

There is a considerable literature devoted to the use of liquid-liquid extraction (LLE) procedures for the preconcentration and separation of analytes from potentially interfering matrices, mainly for determination by FAAS. 65 Such procedures find little use in the

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laboratory these days because of the difficulty of automating the use of a separatory funnel procedure, the need to handle and dispose of organic solvent waste and the slow throughput. It is possible to perform a liquid-liquid extraction in a FI mode. A merging streams manifold would be used in which the sample is merged with a stream of a derivatizing agent (to form an extractable compound) which in turn would merge with a stream of extractant. The immiscible liquids break up into alternate segments whose dimensions are a function of the relative flow rates (among other parameters) and during transport through an open tubular reactor, mass transport between the aqueous phase and the organic phase take place. The mechanism of transport is fascinating and involves the formation of a thin film of organic solvent on the wall of the hydrophobic tubing. After sufficient residence time in the reactor, the phases are separated and the organic phase is directed towards the spectrometer. Phase separation may be achieved in a number of ways including gravity. However, the most effective is probably the use of a microporous hydrophobic membrane in either a tubular or sheet configuration. For use in conjunction with FAAS, there is a problem of flow rate optimization as the best ftow rate for achieving efficient extraction with preconcentration will be too low (well below 1 ml/min) for efficient introduction to the instrument. This may be overcome if sample volume permits by relocation of the injection valve in the manifold as shown in Figure 3.8. With this configuration, the flow rate to the spectrometer can be independent of the flow rate of the organic extractant and the valve can be thought of as an interface between the two flow systems (a liquid-liquid extraction system and an organic solvent sample delivery system). Although there is a sustained interest in the development of liquid--liquid extraction procedures for atomic spectrometry, it is at a low level in comparison with the interest in the development of SPE procedures. In general, the latter methods have a number of features which make them more attractive than LLE, including greater convenience and ease of use (delivering certain organic solvents with a peristaltic pump can be a non-trivial exercise). However, there are some possibilities for the development of indirect determinations of species which can form extractable compounds with an easily determined metal that would be difficult to put into a SPE format. This topic is discussed in more detail in Chapters 7 and 9.

3. 7.5 Precipitation Although the use of precipitation techniques is well established in analytical chemistry for both separation and preconcentration, it is a little used procedure in the area of analytical atomic spectrometry with the possible exception of the fields of X-ray emission and fluorescence analyses. The reasons for this are probably much the same as those for the

J. F. Tyson

90 I C

W

R

Figure 3.8 FI-LLE manifolds. In the upper manifold, the eluent from the phase separator is fed directly to the spectrometer. The relocation of the valve as shown in the lower manifold, allows independent optimization of the flow rates for extraction and delivery to the instrument. C, cartier; R, reagent forming an extractable complex; E, extractant; PS, phase separator; W, waste; I, injection; and D, detector. relative unpopularity of LLE procedures, namely the considerable degree of operator skill required, coupled with difficulties of automation and the limited throughput. Provided the amount of material is small, it is possible to produce and transport precipitated material in a flow injection manifold. A number of different research groups have shown that the concept is viable at least in terms of the chemistry involved. Precipitated material has been trapped on filter media in much the same way as analytes are trapped on a solid phase extractant (see Figure 3.7). Indeed, for the retention of insoluble derivatives on C-18 material it is difficult to see the difference between SPE and precipitation other than that in the former a "filter column" is used instead of a planar filter medium. It has been shown possible to retain precipitated material on the walls of a tubular reactor. Normally the adherence of the precipitate to the reactor walls is an unwanted phenomenon, but in the best tradition of analytical chemistry, Fang and co-workers 66 turned this disadvantage into an advantage by using a knotted tubular reactor to promote the contact between the solid material and the tube wall. Following collection, the

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precipitate was dissolved in an organic solvent chosen to produce enhanced sensitivity when introduced into a flame atom source. Selenium has been determined in water samples by co-precipitation with lanthanum hydroxide. 67 Lanthanum nitrate was added to the samples which were introduced into the system using time-based injection and merged with a stream of ammonia. The precipitate was collected on the inside walls of a knotted tubular reactor and subsequently eluted with hydrochloric acid. The eluent stream was merged with borohydride and the hydrogen selenide stripped from solution by an argon stream and transported to a heated quartz tube atomizer for determination by AAS. For a sample volume of 6.7 mL, an enrichment factor of 24, a detection limit of 1 ppt and a throughput of 33 samples/h were obtained.

3.7.6 Dialysis The selective transport of small species across a semi-permeable membrane is a powerful technique for separating analyte species from large potentially interfering molecules. The technique is widely used with clinical samples to obtain a sub-sample of the analytes free from protein and other large bio-polymers. In its simplest form, that of molecular filtration, the procedure produces an inherent dilution of the analyte but is easy to implement in an on-line, flow-through format. The procedure is relatively little used because in many trace element determinations the analyte is associated (at least partially) with species that are excluded by the dialysis membrane. Thus some sort of sample preparation is used in which such compounds are destroyed or the metals released. For many clinical applications, the concentrations of the analytes may be so low that the instrument of choice is the graphite furnace atomizer which is capable of a thermal pretreatment of the sample in the atomizer. If the process is made more selective by the use of a charged membrane then it is possible to use dialysis for the concentration of analyte elements. For example, a cationselective membrane will allow the accumulation of a cationic analyte from dilute solution as cations from a highly concentrated acceptor solution diffuse out into the analyte solution. This is not a particularly attractive procedure as the resulting solution for analysis contains a relatively high concentration of other ions which could interfere in the subsequent determination. Both of these types of dialysis have been used with atomic spectrometry. For the determination of calcium and potassium in wines by FAAS and sodium and potassium by flame atomic emission spectrometry, a FI manifold incorporating a fiat membrane dialyser was used to provide on-line dilution so that the working range could be extended up to 1500 ppm. 68 For the determination of lead in drinking water, preconcentration by a factor of about 100 was achieved with a Nation tubular cation-exchange membrane in the loop of the injection valve. 69 A 1 m length of tubing containing a mixture of electrolytes

92

J.F. Tyson

(strontium and aluminum nitrates) was immersed in the stirred sample solution for 10 min. During this period, cations diffused across the membrane into the sample and to preserve electroneutrality cations from the sample diffused into the interior of the sample tube. It was possible to determine lead reliably at the EPA limit of 5 ppb with FAAS.

3.8

Detection limits in FI preconcentration procedures

For both solid phase extraction and chemical vapor generation with collection on the interior of a graphite tube, the signal is directly related to the sample volume processed. A feature of the procedures is that they allow the amount of analyte present in a relatively large volume of sample to be eventually atomized as though it was contained in a relatively small sample volume. There is a tendency to think that as sensitivity can be increased to as large a value as desired by increasing the sample volume, it is possible to decrease the detection limit to any desired value by this means also. Many recent publications in this area make this claim. In a small number of papers, data is included in support of this statement, but in many papers the authors simply speculate that this improvement would occur. The impression is often given in discussions of this relationship that (a) detection limit is linearly related to sample volume (as values are given for only two volumes) TM and (b) the detection limit achievable may be as low as desired; it is simply a matter of making the sample volume as large as necessary. The first of these statements is not true. The relationship between detection limit and sample volume is one of inverse proportion 7~ and an inversely proportional relationship is not linear, though there may be regions in which the relationship approximates to linear. The function relating the two variables is one half of a rectangular hyperbola (that is, a hyperbola whose asymptotes are at right-angles to each other). The second of the statements is only true when there is a contribution to the measured signal from analyte present in the reagents which is either zero or independent of sample volume used. In FI-HG-ETAAS and FI-SPE, the signal due to contamination of the reagents by the analyte in fact increases as the sample volume increases. In the case of HG, the quartz probe, which delivers the hydride to the furnace, needs to be positioned inside the furnace for a longer period of time. In the case of SPE, the reagent solutions are passed through the column for a longer period of time. In both cases, analyte species in the reagent are retained, giving rise to an increased blank signal. This increased blank signal has increased noise associated with it and this offsets the improvement in detection limit which would be obtained as a result of the increased sensitivity. A simple theory for the effect of sample volume may be derived on the basis that, for many trace element determinations, the general relationship between the standard deviation in the concentration domain, Sc, and the analyte concentration, C (made up of

FIA techniques and strategies expand the potential of atomic spectrometry

93

the concentration in the sample and the concentration added due to contamination of reagents, and carry over from previous samples), may be modeled as a simple linear function. 72

sc=so+kC

(3.6)

where So is the standard deviation of the field blank (a sample with zero analyte concentration) and k is a constant. When this expression is converted to standard deviation in the signal domain and the condition that the blank signal is proportional to the sample volume taken into account. The-expression for the detection limit Ca1 becomes

Cat= 3 N//2sA.o/SVi+ 3 ~/2kCh

(3.7)

where SA.Ois the standard deviation of the signal for the field blank, S is the sensitivity, Vi is the sample volume and Cb is the concentration of analyte added to the sample from the reagents needed to process a sample volume of Vi. In deriving this expression it has been assumed that the process of subtracting the blank signal is subject to random error which has a Gaussian distribution and thus the V'2 appears in each term on the right-hand side of the equation. It can be seen that this equation predicts that there is an inversely proportional relationship between detection limit and sample volume and that for large volumes, which approach infinity, there is a limiting value of the detection limit given by 3X/2kCb. This situation may be contrasted with that for which the blank value is independent of sample volume, as would be the case for the batch HG with trapping on the interior of a graphite furnace, for which the corresponding equation would be

Cat= [(3SA.o]S)+ 3kCb]/Vi

(3.8)

In this case, the detection limit is inversely proportional to sample volume and the infinitevolume value is zero. The curve shown in Figure 3.9 has been experimentally determined for the case of FIHG-AAS of arsenic 73 and it was found that as the sample volume was increased, the detection limit improved significantly from 0.3 lug 1-1 to around 0.05 Ixg 1-1 up to a volume of about 500 Ixl. Between 500 txl and 1000 txl, a further improvement, to around 0.02 lug 1-~, was obtained; but for volumes larger than 1000 I~1, no further significant improvement was obtained. Good agreement between the predicted and experimentally determined variation in detection limit with sample volume was obtained and the underlying inverse proportionality of the relationship between detection limit and sample volume was confirmed. In order to obtain the lowest possible detection limit, it is necessary to reduce as far as possible the contribution to the analyte signal from the reagents used. Instrumental

94

J. F. Tyson 0.3 o~q

Qv-4

.a-a

I

0.2 I o o 8w~q

.,.,

0.1 v

I

9

I

2(XX) sample volume

(microliters)

Figure 3.9 Plot of detection limit as a function of sample volume for hydride generation with trapping on the interior of a graphite furnace. modifications which increased the sensitivity would also be beneficial, as would control over the features which influence the signal variation in the absence of analyte. For absorption spectrometries, these latter features are well known, and discussions may be found in standard texts (e.g. reference 74). It is possible that there are contributions to the blank signal from sources other than reagent contamination, such as carry-over from one sample to the next, and therefore, that the blank is made up of contributions which are dependent on sample volume and contributions which are independent of sample volume. For this latter situation, an equation for the variation of detection limit with experimental parameters may also be derived on the basis of a Gaussian distribution of errors, but is much more complicated showing a minimum in the relationship between detection limit and sample volume. From a practical viewpoint, it can be concluded that for the determination of As by FI-HG-ETAAS, there is little point in using sample volumes in excess of 1000 ILl and that a considerable increase in throughput could be obtained with little loss in detection limit by using a 500 I~l sample volume. Even if the blank is independent of sample volume (or zero), the nature of the rectangular hyperbola relationship needs to be kept in mind. As the sample volume is increased, there will eventually come a point beyond which the improvement in detection limit obtainable is not significant or is only made with considerable costs in terms of throughput. Only limited data are available for FI solid-phase extraction experiments, 75'76 but the same underlying model appears to hold. The sample volume that can usefully be preconcentrated will depend on the magnitude of the contribution to the blank from the relevant reagents, but could be as large as 10-20 mL. In field sampling protocols,

FIA techniques and strategies expand the potential of atomic spectrometry

95

discussed later in Chapter 14, the sample may have very little (or no) reagent added and thus relatively large volumes could be sampled before the situation of "diminishing returns" applies.

3.9

Summary

The controlled introduction of microsamples into an atomic spectrometer via an FI manifold extends the performance characteristics of the spectrometer so that samples which would be diffii~ult or impossible to aspirate directly into the instrument can be analyzed. Thus samples with high dissolved solid contents (or slurries), organic solvent content, variable viscosity and so on can be introduced over periods of time long enough for realistic use in routine analytical situations. Exploitation of the inherent dilution feature of FI allows numerous possibilities for handling samples whose analyte concentrations would be too high for conventional introduction. Dilution may also be exploited as a means of overcoming matrix interference. If the extent of dilution is both controlled and variable, calibration may be performed via a FI manifold. Many sample pretreatment procedures for preconcentration and separation of analyte species from potentially interfering matrix species may be implemented by FI with direct coupling to the instrument. Many methods involving solid phase extraction and chemical vapor generation have been developed. There is a growing interest in the development of multielement FI procedures, particularly for ICP-MS. The transient nature of the FI signal places some constraints on the instrument operating parameters. Simple models for both the kinetic behavior and the factors controlling detection limit for SPE and CVG can be used to account for some of the fundamental characteristics of FIAS systems. The provision of information about the different chemical forms of a element in a complex matrix is increasingly required. This is a formidable task. Most approaches to speciation involve the combination of separation and element specific detection. As has been discussed in the earlier sections of this chapter, flow injection techniques can be used to implement various chemical separation procedures and thus can be used in speciation studies. The information available may not be very detailed. For example, the use of hydride generation would distinguish those species which can react with borohydride to give a volatile derivative and those forms of the element which do not. This would distinguish between selenite and trimethylselenonium ion. The innocuous forms of arsenic in seafood (arsenobetaine and arsenocholine) are positively charged in aqueous solution whereas the toxic forms of inorganic and methyl arsenic are negatively charged (or neutral, depending on the p H - it is possible, though, to protonate the dimethyl form if the pH is low enough). Passage of the sample through a cation exchange resin would prevent the non-toxic forms from contributing to any arsenic signal subsequently measured.

96

J.F. Tyson

Though the procedure already mentioned in connection with certain selenium species could also be implemented here, namely that in a reaction with borohydride solution, arsenobetaine and arsenocholine do not give rise to volatile derivatives. There are several inorganic metal species which may also be readily separated on the basis of charge, including CrlII (which is typically cationic) and CrVI (which is typically anionic). There is a continued interest in the determination of these two species in environmental and biological samples and several papers describing FIAS procedures appear every year. This is an area that is likely to see continued development, as there are other analytical problems, such as the distinction between inorganic and methyl mercury, that are amenable to solution using FI techniques involving simple non-chromatographic separations as shown in Chapter 12. 3.10

References

I Tyson, J. F.,Analyst, 1985, II0, 419. 2 Tyson, J. F., Trends Anal. Chem., 1985, 4, 124. 3 Ruzicka, J. Fresenius'Z. Anal. Chem., 1986, 324, 745. 4 Fang, Z., Xu, S., Wang, X. and Zhang, S., Anal. Chim. Acta., 1986, 169, 325. 5 Barnes, R. M., Spectroscopy, 1986, I, 24. 6 Christian, G. D., Ruzicka, J., Spectrochim. Acta, 1987, 42B, 157. 7 McLeod, C. W., J. Anal. At. Spectrom., 1987, 2, 549. 8 Tyson, J. F., Fresenius' Z. Anal. Chem., 1988, 329, 663. 9 Tyson, J. F., Anal. Chim. Acta, 1988, 214, 57. 10 Burguera, J. L., Ed., Flow Injection Atomic Spectrometry, Dekker, New York, 1989. !1 Ruzicka, J. and Christian, G. D.,Analyst, 1990, 115, 475. 12 Tyson, J. F., Anal. Chim. Acta, 1990, 234, 3. 13 Valcarcel, M. In: Sample Introduction in Atomic Spectroscopy, Elsevier, Amsterdam, 1990, 289. 14 Fang, Z., Spectrochim. Acta Rev., 1991, 14, 235. 15 Beauchemin, D., Trends Anal. Chem., 1991, 10, 71. 16 Tyson, J. F., Spectrochim. Acta Rev., 1991, 14, 169. 17 Tyson, J. F., Microchem. J., 1992, 45, 143. 18 Fang, Z., Flow Injection Atomic Absorption Spectrometry, Wiley, Chichester, 1995. 19 Fang, Z., Xu, S. and Tao, G., ,1. Anal. At. Spectrom., 1996, 11, I. 20 Mclntosh, S. and Brindle, I., Eds., Spectrochim. Acta Part B, 1996, 51, 1733. 21 Burguera, J. L. and Burguera, M., J. Anal. At. Spectrom., 1997, 12, 643. 22 Tyson, J. F. and ldris, A. B.,Analyst, 1981, 106, 1125. 23 Appleton, J. M. H. and Tyson, J. F., J. Anal. At. Spectrom., 1986, I, 63. 24 Tyson, J. F., Anal. Chim. Acta, 1986, 179, 13 I. 25 Tyson, J. F.,Anal. Chim. Acta, 1988, 214, 57. 26 Tyson, J. F., Analyst, 1984, 109, 319. 27 Tyson, J. F.,Anal. Chim. Acta, 1986, 180, 51. 28 Cresser, M. S., Progr. Anal. At. Spectrom., 1981,4, 219. 29 Ruzicka, J. and Marshall, G. D., Anal Chim. Acta, 1990, 23% 329. 30 Sherwood, R. A. and Rocks, B. F. In: Flow Injection Atomic Spectroscopy, Marcel Dekker, New York, 1989. 31 Denoyer, E. R. and Lu, Q., Atomic Spectroscopy, 1993, 14(6), 162. 32 Stroh, A., V611kopf, U. and Denoyer, E. R., ,1. Anal. Atom. Spectrom., 1992, 7, 1201. 33 Savitzky, A. and Golay, M. J. E.,Anal. Chem., 1964, 36(8), 1627. 34 Voigtman, E., Appl. Spec., 199 i, 45, 237.

FIA techniques and strategies expand the potential of atomic spectrometry 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 5i 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76

97

Fang, Z., Welz, B. and Schlemmer, G., J. Anal. At. Spectrom., 1989, 4, 91. Olesik, J. W., ICP Information Newsletter, 1998, 23, 48. Vinas, F., Campillo, N., Lopez Garcia, I. and Hemandez Cordoba, M., Anal. Chim. Aeta, 1993, 283, 393. Tyson, J. E, Adeeyinwo, C. E., Appleton, J. M. H., Bysouth, S. R., Idris, A. B. and Sarkissian, L. L., Analyst, 1985, I 10, 487. Sneddon, J., Ed., Sample Introduction in Atomic Spectroscopy, Elsevier, Amsterdam, 1990. Koropchak, J. and Weber, M., Crit. Rev. Anal. Chem., 1992, 23, 113. Schaldach, G. and Bemdt, H., Fresenius' J. Anal. Chem., 1994, 350, 481. Fang, Z., Weiz, B. and Sperling, M., Anal. Chem., 1993, 65, 1682. Mohammad, B., Ure, A. M. and Littlejohn, D., ,/. Anal. At. Spectrom., 1993, 8, 325. Xu, S. K.; Sperling, M. and Welz, B., J. Anal. Chem., 1992, 344, 535. Qi, w., Wu, X., Zhou, C., Wu, H. and Gao, Y., Anal. Chim. Acta, 1992, 270, 205. Elmahadi, H. A. M. and Greenway, G. M., J. Anal. At. Spectrom., 1993, 8, 1011. Ma, R., Van Mol, W. and Adams, E, Anal. Chim. Acta, 1994, 285, 33. Trojanowicz, M. and Pyrzynska, K., Anal. Chim. Acta, 1994, 287, 252. Maquiera, A., Elmahadi, H. A. M. and Puchades, R., Anal. Chem., 1994, 66, 1462. Elmahadi, H. A. M. and Greenway, G. M., J. Anal. At. Spectrom., 1993, 8, ! 011. Townshend, A. and Habib, K. A. J., Microchem. J., 1922, 45, 210. Purohit, R. and Devi, S., Anal. Chim. Acta, 1992, 259, 53. Welz, B., Yin, X. and Sperling, M.,Anal. Chim. Acta, 1992, 261,477. Azeredo, L. C., Sturgeon, R. E. and Curtius, A. J., Spectrochim. Acta Part B, 1993, 48, 91. Lee, M. L., Tolg, G., Beinrohr, E. and Tschopel, P., Anal. Chim. Acta, 1993, 272~ 193. Porta, V., Abollino, O., Mentasti, E. and Sarzanini, C., J. Anal. At. Spectrom., 1991, 6, 119. Sperling, M., Yin, X. and Welz, B., Spectrochim. Acta Part B, 1991, 46, 1789. Hartenstein, S. D., Ruzicka, J. and Christian, G. D., Anal. Chem., 1985, 57, 21. Irata, S., Umezaki, Y. and lkeda, M., Anal. Chem., 1986, 58, 2602. Cook, I. G., McLeod, C. W. and Worsfold, P. J., Anal. Proc., 1986, 23, 5. Shabani, M. B. and Masuda, A.,Anal. Chim. Acta, 1992, 261,321. Ebdon, L., Fisher, A. S. and Worsfold, P. J., J. Anal. At. Spectrom., 1994, 9, 119. Dadfarnia, S. and McLeod, C. W., Appl. Spec., 1994, 48, 1331. Greenfield, S., Durrani, T. M., Kaya, S. and Tyson, J. F.,Analyst, 1990, 115, 531. Cresser, M. S., Solvent Extraction in Flame Spectroscopic Analysis, Butterworths, London, 1978. Fang, Z., Sperling, M. and Welz, B., J. Anal. At. Spectrom., 1991, 6, 301. Tuo, G. and Hansen, E. H.,Analyst, 1994, 119, 333. Lima, J. L. F. C., Rangel, A. O. S. S. and Roque da Salva, M. M. S.,Atomic Spectrosc., 1991, 12, 204. Koropchak, J. A. and Allen, L., Anal. Chem., 1988, 61, 1410. Infante, H. G., Femandez-Sanchez, M. L. and Sanz-Medel, A., J. Anal. Atom. Spectrom., 1996, 11,571. Sinemus, H. W., Stabel, H. H., Raziuk, B. and Kleiner, J., Spectrochim. Acta Part B, 1993, 48, 643. Analytical Methods Committee, Analyst, 1987, 112, 199. Tyson, J. F., Ellis, R. I., Mclntosh, S.A., and Hanna, C. P., J. Anal. At. Spectrom., 1998, 13, 17. Ingle, J. D. and Crouch, S. R., Spectrochemical Analysis, Prentice-Hall International, London, 1988, p. 150. Tyson, J. E and Kradtap, S., unpublished work. Carrero, P. E. and Tyson, J. F., Analyst, 1997, 22, 915.

4.1

Introduction

Calibration and standardization are two terms indistinctly employed in practice, but they are in fact different and complementary, being calibration related to the whole correct operation of an instrument or apparatus, and standardization related to the characterization of the response of an instrument against standards of known concentration. Calibration involves both non-analytical and analytical aspects, and affects both the qualitative and quantitative information provided by the instrument. Standardization is the operation which makes it possible to establish an unequivocal relationship between the instrumental signal and the analyte concentration in order to allow the process of extracting quantitative information from the measurements. Taking into account the aforementioned definitions, well justified by Valcarcel and Rios, 1 it is evident that the contribution of flow analysis to improve the performance of atomic spectrometry is especially interesting in the field of standardization. Flow analysis can provide a faster and reliable method of obtaining relationships between absorbance, emission signals or counts (at a specific mass number) and the concentration of the elements to be determined; it is less useful, however, for apparatus calibration. Through this chapter the main strategies for flow analysis calibration and standardization in atomic spectrometry will be considered for all the most common atomic spectrometry techniques, including FAAS, ETAAS, ICP-OES and ICP-MS. However, in order to avoid redundancies, the information of this chapter has been organized from a 98

FIA strategies for calibration and standardization in atomic spectrometry

99

standardization strategy viewpoint as: (a) extemal calibration, (b) standard addition or (c) internal standard, instead of using the atomic detector technique to classify the different approaches published in the literature of recent years.

4.2

Calibration of atomic spectrometry instruments

In order to consider the parameters to be calibrated in atomic spectrometry instruments, a clear difference between photon counting atomic spectrometry and ion counting atomic spectrometry, 2 which affects the main parameter to be measured and correlated with the analyte concentration, should be established. In classical atomic spectrometry the main parameter to be calibrated is the wavelength, and this is the case in FAAS, ETAAS, in all the different possibilities of plasma-OES (e.g. ICP, DCP, MIP or GD) or in HG-AAS and AFS. For the modem ion counting techniques, based on elemental mass spectrometry coupled to any atomization, or more properly, ionization device, the main parameter is the mass number. So, in ICP-MS, MIP-MS, dc-glow discharge-MS, rf--glow discharge-MS, spark source-MS, laser ablation/ionization-MS, ion beanv-MS, electrospray-MS, capacitive microwave plasma (CMP)-MS, etc., the mass number instead of the wavelength is the parameter to be calibrated. 3 Other parameters affecting the efficiency of the atomization system are the temperature of the heating system, the flow of gases and the power supplied, and can be directly controlled in ETAAS, FAAS and other techniques. They must be carefully checked in order to have an appropriate comparison of data, specially when data found for different instruments of the same type have to be normalized and compared. Table 4.1 summarizes the main instrumental parameters to be calibrated in several instruments of common use in Atomic Spectrometry indicating their special importance for the different techniques considered. However, taking into account the great number of variables which can affect the atomic signals (absorbance, fluorescence, emission or counts at a specific mass number), it is common practice in atomic spectrometry to do customarily analytical measurements of Table 4.1

Parameters to be calibrated in atomic spectrometry instruments

Parameter

Techniques

Wavelength Mass number Temperature Power supplied Gas flow Sample flow

FAAS, ETAAS, OES, HG-AAS, AFS ICP-MS ETAAS OES, OES-MS FAAS, ETAAS, OES, AFS, HG--AAS,ICP-MS FAAS, OES, AFS, ICP-MS

100

M. de la Guardia

both samples and appropriate standards. Because of that, in general, the routine quality control of atomic spectrometers involves the verification of the wavelength, or mass number, adjustment. This verification is commonly carried out by the introduction of appropriate standard solutions of a series of elements in the continuous form (e.g. FAAS, OES, HG-AAS, AFS and ICP-MS) or alternatively by discrete injection (e.g. ETAAS). For calibration, pure solutions of a single element (as for example in FAAS, HG-AAS, ETAAS or AFS), or mixtures of different elements suitable to provide a series of well defined optical emission lines in OES or (ions or different mass in ICP-MS) can be used. The aforementioned controls can be carried out by using the flow injection approach and this involves of course, the use of lower amounts of reagents than the corresponding batch procedures, providing an excellent laboratory productivity through the reduction of the time required to perform the analytical controls and the characteristic high sampling frequency provided by flow injection.

4.3

Standardization strategies in flow analysis-atomic spectrometry

As indicated before, standardization, rather than calibration, is the keyword in the practice of atomic spectrometry with flow analysis. To do that, the main strategies are to a great extent independent of the specific technique employed, except for ICP-MS where internal standard and isotope dilution strategies, coming from the intrinsic specificity of this technique of counting ions, provide new and exciting possibilities which can be fueled by the use of FA-ICP-MS. Table 4.2 summarizes the different strategies proposed so far in the scientific literature, going from simple external calibration to the use of matched standards and the standard addition method, (which are general strategies for all the considered techniques) and also including the aforementioned procedures developed for ICP-MS. Through the next pages the advantages provided by the use of flow analysis in all such standardization processes will be discussed in detail. 4.3.1

4.3.1.1

External standardization Use of a series of standards

The possible non-linearity in response, more common than thought at first sight in atomic absorption spectrometry, is the reason for traditional calibration in atomic spectrometry performed by using a series of standard solutions. Standards could be pure aqueous solutions, obtained directly by successive dilutions of a stock one, or, in the case of complex matrices, matched solutions prepared with some of the major components of the matrix, in order to avoid physical and other interferences.

> Table 4.2

Standardization strategies in flow-analysis-atomic spectrometry

EXTERNAL CALIBRATION (With simple or matched standards)

ON-LINE STANDARD ADDITION METHOD

Use of a series of standards

Peak height Peak area Peak width

Use of a single standard

Variation of sample volume Variation of the mixing coil dimensions Use of mixing chambers Merging flow Time-based electronic dilution Zone sampling Partial overlapping zones Split flow dilution Cascade dilution Gradient ratio Flow rate variation Use of solenoid valves

Merging zones Interpolative method Partial overlapping Time-based electronic dilution Zone sampling Flow rate variation

O~ ~..,o

o" ga ~..,o

~,,,d ~

INTERNAL STANDARD METHOD ISOTOPIC DILUTION

0 t~

q

O

102

M. de la Guardia

The use of a series of standards is a tedious time- and reagent-consuming practice which is only justified in FAAS, HG-AAS, AFS and ETAAS, but also in these cases the use of flow analysis could improve the standardization step by reducing drastically the volume of standard solutions required. Additionally, the use of transient signal-time dependent measurements allows us a drastic reduction of the volume of standards consumed, from 1-2 ml to less than 500 Ixl. This also reduces the amount of waste while providing a good sampling frequency. However, it must be strongly recommended to take advantage of all the potentialities of flow analysis, thus carrying out the standardization using a single standard solution whenever possible. On the other hand, the use of transient signals also opens new possibilities for measurements and in this sense both the peak height or peak area values could be used, being specially interesting to use the latter mode for improving the sensitivity, while peak width mode can be selected for extending the dynamic range of the calibration plot. Tyson proposed in 1984 the use of the peak width for standardization under low dispersion conditions, 4 based on his previous experience on the study of the dilution of an injected sample volume inside a well-stirred tank. As shown in Chapter 3, if a volume of sample, V,, is passed through a nebulizing system of a volume V at a flow rate Q, the width of the transient peak obtained, At, at concentration C can be described as:

At=(V/Q) ln((Co/C) - 1) - (V/Q) ln(D - 1)

(4.1)

where Co is the original concentration injected and D the dispersion coefficient. By plotting At against ln((Co/C) - 1) a linear calibration curve can be obtained if C--Co. When Co ~ C, a linear relationship could be found between At and In C. In the above-mentioned conditions it is clear that the use of peak width is appropriate for evaluation of highly concentrated samples providing off-scale signals or a loss of linearity when peak height measurement mode is employed. 5

4.3.1.2

Use of a single standard

It is well known that the dispersion of the injected sample into the carrier stream is one of the fundamental aspects of the FIA methodology. This dispersion, which is described by the dispersion coefficient, is equivalent to a continuous dilution of the injected sample volume and provides the way to have a kinetic discrimination of the behaviour of the sample components. However, dispersion is also the reason for which, when low injection volumes are used, the FIA approach provides lower sensitivity than the traditional batch measurements. One common practice in the work of analytical chemists is to try to obtain advantages

FIA strategies for calibration and standardization in atomic spectrometry

103

from apparently adverse situations. In the case of the FIA dispersion, it provides an excellent way to obtain a reproducible dilution of concentrated samples to different levels and, taken into account that the use of different dilutions of a concentrated stock standard solution is the base of the preparation of an external calibration using a series of standards, it is clear that the controlled dispersion of a single standard solution, in the experimental manifold, provides the way to carry out standardization in a continuous mode. The topics of on-line dilution and standardization using concentrated solutions have been reviewed by Tyson,6"7 Fang8 and Trojanowicz and Olbrych-Sleszynska. 9 Fang, in an excellent book recently published about Flow Injection Atomic Absorption Spectrometry, which contains two specific chapters devoted to dilution and calibration, ~~ has classified the FIA dilution systems as a function of two basic mechanisms: (a) sample dispersion and (b) flow manipulation; both mechanisms are very useful for extemal standardization in atomic spectrometry by using a single standard. The "practical dilution limit" depends more on the system design employed than on the mechanism considered; thus, for sample dispersion, 30 times maximum dilution has been reported using a mixing coil or a multi-line network, 50 times for a zone penetration, 100 times for a mixing chamber or a zone sampling, 1500 times for a microsampling device and a maximum 30 000 times dilution for a combined system of microsampling plus zone penetration. On the other hand, using the flow manipulation a 10 times maximum dilution has been reported using merging-flow, 30 times for split-flow and 500 times for a cascade device. Additionally, Costa Lima ~ has proposed a novel mechanism, based on the on-line dialysis, for the on-line dilution of wine samples in order to do the determination of Ca, Mg, Na and K by FAAS and flame emission spectrometry. An important aspect of the aforementioned strategies for on-line dilution is that peak height measurement is employed mainly for readout, except for zone penetration, where peak maxima and minima are used, and microsampling, for which peak area measurements have also been proposed. However, maximum information obtained from the standard dispersion mode in an on-line dilution system comes from gradient readout, characteristic of the mixing coil and mixing chamber approaches. In the following paragraphs, the fundamentals and some details on the use of dispersion strategies for standardization with a single solution in atomic spectrometry will be discussed. 4.3.1.2.1

Standardization by variation of sample volume

A simple strategy, based on the consideration of dispersion coefficients as dilution factors, consists of injecting different volumes of a standard solution in the same manifold, thus providing serial dilutions of a single standard.

104

M. de la Guardia

Changing the sample loop, in a fixed configuration manifold, a linear dilution of the standard solution injected can be obtained, thus providing both the enlargement of the dynamic range, when reduced volumes are injected, and also a simple way for standardization. 12.13 However there is a serious limitation of the application of different injected sample volumes by using normal variable volume injection valves, for which the minimum volume to be employed is between 15 and 20 Ixl. Because of that, to obtain a higher dilution the use of fixed volume valves is required, thus reducing drastically the flexibility of the system. The relationship between the dispersion coefficient, D, and the sample volume, Vs, in a flow system can be expressed as (see Chapter 3): D=Co/C=

1 - exp( - V~k) - 1

k being a constant determined by the experimental conditions. ~4 In expression (4.2), for D values above 2, an almost linear relationship between Vs and C could be obtained, indicating that large dilution factors could be found simply by decreasing the sample volume appropriately. As indicated above, the minimum volume of the loop of typical injection valves seems to be the upper limit for obtaining a fast and flexible way for standardization using different volumes of the same standard. However, as suggested by Sherwood et al., 15 different dispersion coefficients can be achieved by carrying out a partial emptying out of a fixed sample loop. The technique called "controlled dispersion" consists, basically, of using a peristaltic pump, which transports the carrier solution at a low speed, and an injection valve, which is commuted to the injection position during short and well-controlled periods of time, thus providing the introduction of reproducible small portions of the sample loop. In 1990, Burguera et al. 16 developed a time-based injector consisting of a two-way solenoid valve and a simple timer circuit, which can switch the current on and off at desired fixed intervals. The system, depicted in Figure 4.1, permits the delivery of precise and variable standard volumes of 4, 14, 22, 34, 48 and 62 txl with an average relative standard deviation of 0.6% using solenoid inactive times of 4.5, 4.0, 3.5, 3.0, 2.5 and 2.0 s followed by activation periods of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 s respectively using a carrier flow of 2.5 ml min-x.~7 A detailed study of the operation conditions and performance of the aforementioned system, carried out by de la Guardia et al., 18 evidenced the flexibility of this device to provide different sample volumes as a function of the pulse position, and also depending on the aspiration flow. In this sense, Table 4.3 depicts the variation of the sample volume

FIA strategies for calibration and standardization in atomic spectrometry

105

Figure 4.1 Flow maniflod employed for time-based standardization. Inset: Schematic drawing of the pulse generation and injection unit: T, tranformer; S, switch; MP, metal plate; R, rotor; M, motor rotor unit; SI, sample introduction; CI, carrier stream; SL, solenoid valve; EM, electromagnet. (Reprinted with permission from Burguera et al.~7). injected with the aspiration flow and the time during which the sample tube remains open. The time-based solenoid valve injector has been employed for real analysis of blood sera, in the determination of Na and K by flame emission, of Ca and Mg by FAAS, 17 of Fe, Ca and Mg by FAAS and of Na and K by flame emission, in a series of ceramic materials. ~8An additional advantage of this system is that it can provide a sampling rate of the order of 400 measurements per hour, 17 thus being a highly productive device. Fang et al. ~9 achieved the reproducible introduction of microlitre volumes by using a

106

M. de la Guardia

Table 4.3 Variation of the sample volume injected using a time-based solenoid injector as a function of the aspiration flow and the sampling time Pulse 1 2 3 4 5 6

Time (s) 0.25+0.03 0.63• 1.15• 1.44• 1.90• 2.63•

Variationfunction

r

Q

Variationfunction

r

V(ixl)=3.36Q+ 0.25 V(ixl)=9.32Q+0.91 v(l~l)=17.71Q+ 1.32 V(I~I)=22.96Q+0.4 v(ixl)=30. ! IQ+3.32 V(I~I)=40.20Q+1.13

0.998 0.994 0.996 0.9996 0.998 0.998

2.7 3.7 4.7 5.5 7.5 9.4

V(~l)=39.8t+1.6 v(ixl)=58.2t- 1.11 v(ixl)=77.9t- 2.6 V(txi)=90.6t- 4.5 V(~l)=119.0t+0.5 V(~l)=145 t - 3.8

0.993 0.9995 0.9992 0.9992 0.999 0.999

The duration of each pulse was determined by carrying out 25 independent measurements(values are indicated as the average+its standard deviation of all measurements). Q: aspiration flow in ml min', t: time in seconds, r: regression coefficient (from la Guardia et al.'S).

computer-controlled stepper-motor-driven peristaltic pump and thin-bore neoprene pump tubes. In this system, sample volumes as low as 0.7 I~l were pumped into a rotary injection valve, partially filling a sample loop of approximately 100 p~l. The injected volume was then dispersed into the carrier. The manifold developed by Fang is equipped with two peristaltic pumps and includes a mixing coil of PTFE with 160 cm length and 1.3 mm internal diameter. It can provide dilution factors ranging from 10 to 1000. Computer controlled stepper-motor-driven peristaltic pumps were employed by Fang ~~ for standardization, based on a serial dilution of a single standard by varying its injected volume, based on the fact that at high dispersion factors, of the order of ten, the dispersion coefficient is inversely proportional to the injected sample volume. In this approach the volume of standard introduced was determined by the filling time, at a well-defined sample filling flow-rate. With the above mentioned system considerable on-line dilution levels can be obtained with a sampling frequency of the order of 60-100 h-~, using long reactors to increase the dispersion and computer-controlled pumps equipped with adequate tubes. 2~ It must be noted that the aforementioned system has serious problems derived from the pulsation of the peristaltic pump. In fact, to inject a discrete small volume of a standard solution, the injection valve must be commuted from the filling position to the injection position during a small period of time (of the order of 1 or 2 s). During this time the pump does not turn completely, due to the low speed required, and thus the volume really injected, during the time at which the injection valve remains open, depends on the exact position of the pump at the beginning of the injection. This problem, which has been identified by Fang e t al., 19 is difficult to solve via software. However, Lopez-Garcia e t al. 2~ have demonstrated that small sample volumes could be injected reproducibly when the injection time employed is a whole multiple of the period of the pulsation caused by the rollers of the pump.

FIA strategies for calibration and standardization in atomic spectrometry

107

The strategy of Lopez-Garcia et al., 21 based on the use of programmed pump rates, combines favourably the features of the approaches reported by Fang '9'2~ and Sherwood 15 providing an extremely versatile tool for standardization in flow analysis. The main conditions to be taken into account are the following: (a)

(b)

(c)

(d)

The pump rate is modified by applying a linearly increasing voltage between an initial potential E0 and the maximum available potential of 5 V, thus modifying the pump speed from an initial value w0 (in revolutions min-~) to the maximum value of 47.9 rev min -!. To do that a slow voltage ramp of 100 mV s-~ is employed in order to reduce the inertia effects. The duration of the gradient depends on the initial pump rate and the change on the flow delivered by the pump is compensated by using a T piece, located before the nebulizer, from which a diluent flow is aspirated. The fact that the pump is running at a low rate permits a small volume of standard to be measured into the flow system and the increase of the pump rate by means of the increase of the voltage, after the injection step, increases the sampling frequency. The injection is carried out by means of a laboratory-made rotary valve provided with an electromagnetic system in order to allow its remote operation, being the total internal volume of the injection system 115 Izl. The switching of the valve from the loading to the injection position and vice versa is carried out with a 5 V voltage pulse of 5 ms provided by means of a simple keystroke. The acquisition of data is made by using a PCLab 818PG card together with a laboratory-made software programmed to obtain 10 measurements per sec through the non-damped analogue output of the spectrometer while the voltage ramp is applied to the pump. The peristaltic pump is equipped with ordinary PVC flexible tubes lubricated with silicone oil. To do a rapid refilling of the sample loop a 2 mm i.d. tube is employed but, to transport the carrier solutions, any tube, from 0.25 to 1.42 mm i.d., can be used.

As indicated before, the way to obtain reproducible small injection volumes, of the order of a few microlitres, is that the product of the pump rate and the injection time be a whole multiple of the period of the rollers' pulse. The period of the pulse can be predicted from the number of rollers (n) and the radius of the pump head (R), taking into account that the distance between two successive points on which a roller is pressing the tube is given by 2 zrR/n and the linear speed of the pump head is 2 zrRwo/60, the pump rate in rev min-~ being w0. The time elapsed from a point of maximum pressure to the following, the pulse period (~-), can be obtained by dividing the distance between these points by the linear speed, thus

108

M. de la Guardia

obtaining the equation which relates the pulse period with the number of rollers and the pump rate

r=60/nwo

(4.3)

When the time employed for injection (ti) is equal to Nr, N being an integer, the following relationship can be found taken into consideration (Equation (4.3)),

ti= Nr= N60/nwo

(4.4)

N=woti/6

(4.5)

and thus

when a ten rollers pump is employed. In the aforementioned conditions, Lopez-Garcia et al. 2~ showed that using w0=2 rev min-i and ti= 3 s, to obtain an N value of 1, 9 Ixl were injected with a RSD of +0.7%. From Equation (4.5) it can be predicted that the lower the N value, the higher is the dispersion achieved, thus for an N value of 1, 1.5 Ixl can be injected when a 0.38 mm internal diameter pump tube is used, but a volume as small as 0.65 Ixl can be introduced by using a tube of 0.25 mm. However, the use of such small pump rates adversely affects the sampling frequency and thus, the use of a gradient increase of the pump rate after injection is absolutely necessary in order to obtain sampling frequencies between 40 and 120 h -I for on-line dilution levels of the order of 2000 times and relative standard deviations of + 1.15% and + 1.6% for integrated absorbances and peak height values, respectively. For standardization the following expression

A =KAN~C ~

(4.6)

A=Ao+KaC

(4.7)

and

were obtained for integrated absorbances (A) and normalized signals to N= 1 (~,), being a and fl statistically equals 1 and KA=0.001 +0.0002. For peak height absorbance (H) the relationship between the experimental data and analyte concentration must take into consideration the pump rate

H= KHNsCw'~

(4.8)

being 6 and y statistically equal to 1, but y can depend on the exact geometry of the manifold and must be recalculated if the pump tube is changed.

FIA strategies for calibration and standardization in atomic spectrometry 4.3.1.2.2

109

Standardization by variation of the mixing coil dimensions

Under fixed injection volume and flow-rate conditions, the dispersion of the sample bolus increases with the length and inner diameter of the mixing coils, thus different dilution factors can be achieved using increasing length mixing coils. However it seems neither practical, nor reproducible to change the mixing coils of the FIA manifold to do the standardization with a single standard. Thus, Tyson et al. developed in 1985 an on-line dilution and calibration system with six parallel PTFE transport lines of different volumes, having their ends connected to two six-way valves. 22 In this system the injected standard is switched sequentially down into the set of parallel coils. Coil lengths of 110, 250, 425, 560, 1115 and 2000 mm produced dispersion coefficients of 2.52, 3.49, 4.43, 5.24, 7.53 and 14.9 respectively, for an injection volume of 65 ixl.22 This approach is simple and does not require sophisticated instrumentation, the drawback being that it provides dilution factors below 20, while maintaining sampling frequencies of no less than 100 h-!. On the other hand, a relatively long time of 90 sec is required for peaks to return to baseline under the highest dilution factor. The same system has been slightly modified, decreasing the sample volume to 12.5 I~1 and the internal diameter of the coils to 0.7-1.1 mm, thus improving the injection time 23 and providing an excellent way to carry out both the standardization of the atomic spectrometry procedure with a single standard and the on-line dilution for off-range samples (by injecting these samples into the most appropriate line). 4.3.1.2.3

Standardization by using mixing chambers

As mentioned earlier, the reduction of the injection volume and the increase of the inner diameter and the length of the transport lines provides the simplest way to do the on-line dilution of a concentrated standard and to obtain, by controlling appropriately the dispersion coefficient, a series of measurements corresponding to different concentrations of the element to be determined. The aforementioned methodologies require, in general, the use of discrete values, as for example different injections of the same solution, in order to achieve the standardization of the system. Thus, they are time consuming and the information corresponding to the intermediate situations (not employed to construct the calibration line) is lost; that sacrifices the tremendous possibilities offered by FIA in order to describe, in a continuous way, the behaviour of the injected solutions. In fact, the use of the FIA peak profile, instead of the use of the peak height or peak area values, provides an excellent alternative way to do the standardization with a single solution. The study of the sample dispersion through a narrow tube under laminar flow conditions is one of the constants of the early FIA work 24 and provides a number of guidelines for the design of manifolds 25 but in atomic spectrometry determinations, additional physical

1 10

M. de la Guardia

and chemical processes occur in the detector converting the analyte solution flowing into the nebulizer into a population of free atoms or ions. So, the resultant signal-time relationship is, in a good approximation, exponential and, because of that, atomic instruments behave as a well-stirred mixing chamber in which the variation of concentration of the injected solution as a function of time can be described as follows (see previous chapter): C= C0(1 -exp(-Qt/V))

(4.9)

Thus, according to Beer's law, the shape of the absorbance-time curve will be: A =Ao(1 - exp( - Qt/V))

(4.10)

being Co the injected concentration and Ao the maximum absorbance obtainable, which can be measured during the process, corresponding to the steady-state absorbance. Based on these principles, atomic instruments employing a nebulization system can be described as equipped with a hypothetical single mixing chamber located immediately prior to the detector. 26 Extremely important conclusions can be drawn from using the exponential concentration gradient obtained by the dispersion of a discrete volume of a concentrated standard through a well-stirred mixing chamber z7 which provides a high dilution capacity. In this sense it must be taken into consideration that the standard injected is more intensively dispersed in a mixing chamber than in a mixing coil with identical volume. Figure 4.2 shows, as an example, the absorbance-time profile found for the injection of a concentrated standard of sodium and the absorbance peaks corresponding to various other standards. The first part of the absorbance profile cannot be used for calibration purposes, but the absorbance values corresponding to different times in the tailing part of the graph can provide a continuous description of the absorbance versus concentration, based on Equation (4.9) and the experimental values of the absorbance-time variation. However, a series of practical problems, related to the exact determination of the experimental parameters,disturb the use of the theoretical dilution profiles and then empirical approaches can be used more safely. 29 The empirical relationship between the absorbance at each time volume of the dilution profile, using the most concentrated standard, and the experimental values found by discrete injection (in the same manifold) of standards with a known concentration, provides a dilution equation

C=AB'

(4.11)

FIA strategies for calibration and standardization in atomic spectrometry

111

Figure 4.2 Diagrams corresponding to the calibration line and the absorbance-time profile found for a single concentrated standard of Na injected in a manifold equipped with a mixing chamber of 2245 ill (Reproduced with permission of Springer-Verlag from the paper of de la Guardia et al.28).

in which A and B are empirical coefficients, where A is directly proportional to the concentration of the standard injected and B depends on the manifold volume and flow rate employed, t being the time in seconds. Using Equation (4.11) the continuous, dilution absorbance-time profile can be employed, as a calibration graph, to relate the absorbance values to the concentration. For the determination of Na, K and Ca in waters a general concentration-time relationship of C= 1.05"0.92' can be employed to describe continuously the dispersion of a concentrated single standard through the manifold. 28 This approach has been applied to the determination of Cr in steels, 26 Na, K, Ca and Mg in waters, 28 A1, Fe, Ca, Mg, Na and K in ceramic materials, ~2Mg in waters and geological samples, 3~ Mg in mineral water. 31 In some cases in order to obtain a high dilution of the injected samples and standards it is necessary to increase the volume of the mixing chamber or to couple on-line two of these devices. 12 Figure 4.3 shows, as an example, different configurations used to achieve different online dilution levels in flame absorption spectrometry and it is interesting to note that both the volume of the chamber and the position at which different volume mixing chambers are located influence the dispersion (see Figure 4.4 for the Mg peak profiles). An important development in this field was incorporated by Beinrohr et al. in 19913~ using a variable volume dilution system adapted from a 10 ml polyethylene syringe. This

M. de la Guardia

112

F~S 0.25 m

p

,

,

I IRi L_..J

L.._I I

~-~-I

-

"

F~S

1 t__J I

R

] I

I

-q

~ - or'~'_

!

I

I

Figure 4.3 Manifolds employed to carry out the on-line dilution of ceramic samples prior to their analysis by FAAS. P, peristaltic pump; I, injection valves; C, three way connector; M1 and M2, mixing chambers of 1 and 3 ml volume respectively; S, magnetic stirrer and R, recorder. The inset depicts the geometry of the mixing chambers employed (from Carbonell et al., ~2 reproduced with permision of the Royal Society of Chemistry).

simple and inexpensive device can provide dilution factors by 2-3 orders of magnitude and is extremely flexible. The main disadvantage of the use of mixed chambers for standardization in atomic spectrometry is the significant loss in sample throughput and, in this sense a sampling frequency of 60 h-1 has been reported for the injection of 10 lxl into a 1 ml mixing chamber, providing a dilution factor of 100 but for 10 ml mixing chambers the frequency decreases to 6 h-~ but it provides a dilution of 1000 times. 3j The sampling frequency can be improved by calibration with a single standard for all the elements considered, also determining the dilution equation of the system for a single one of the elements considered. 28 So, based on these conditions an average frequency of

FIA strategies for calibration and standardization in atomic spectrometry

113

mint

B

.

.

.

.

.

.

.

.

.

Figure 4.4 Effect of the dilution levels on the Mg peak shape for A 4, B 8, C 16 and D 32 mg 1Mg concentrations injected in a manifold including: A, 1 ml volume mixing chamber; B, a 3 ml mixing chamber; C, a 1 ml plus 3 ml on-line mixing chambers and D 3 ml plus 1 ml on-line mixing chambers (reproduced with the permission of the Royal Society of Chemistry from the paper of Carbonell et al.12). 180 h-1 can be obtained for the determination of four elements using an injection volume of 100 ixl and a mixing chamber of 2245 txl. Lopez-Garcia et al. 32 proposed the use of the variable volume mixing chamber 3t as an on-line discrete dilution device, instead of as a continuous flow dilution device. The flow injection manifold developed included a peristaltic pump, two interconnected injection valves and two 10 ml polyethylene syringes. Figure 4.5 depicts the schematic diagram developed by Lopez-Garcia in which the sample injected by means of a valve (V2) is transported by a carrier to a variable volume mixing chamber (S1). A plug of air, introduced through a valve (V1) is employed to displace the diluted solution from the chamber, while facilitating the mixing process, and a constant signal is obtained for the absorbance-time relationship. The plateau height or the integrated absorbance can be used as the analytical signal. At the beginning of the experiment, the manifold was filled with distilled water (the carrier solution) and valves switched to the load position. The upper outlet of the dilution chamber was closed by means of V3. The first syringe (S1) was loaded with air using valve V 1 and the injection loop was loaded with the sample using valve V2. Once the first

M. de la Guardia

114

lAir

L

V3

Figure 4.5 Schematic diagram of the on-line dilution system employed by Lopez-Garcia et al. for extending the calibration range of flame atomic absorption spectrometry. V 1 and V2 injection valves, V3 two-way valve, S1 and $2 10 ml polyethylene syringes (SI used to inject air and $2 used as a mixing chamber), L injection loop, C 30 cm coil length and T air compensation T piece (reproduced from Lopez-Garcia et al. n with permission of the Royal Society of Chemistry). syringe was completely filled with air, syringes V1 and V2 were simultaneously switched to the inject position. In this way the air was propelled by the carrier stream and the sample bolus was introduced to the mixing chamber. The entry of air into the chamber produces an effective mixing of the sample with the carrier by introducing bubbles and displaces the solution towards the spectrometer. After the measurement step, when the signal of the recorder fell to zero, V3 was switched to the 1-2 position to permit the air to be drained from the manifold, V1 and V2 were switched to the load position and the manifold filled with the carrier and the air syringe refilled with air. The sample loop was refilled, V3 was switched to the 2-3 position and a new injection was carried out. In the aforementioned system the time needed to perform the measurements depends -1 only on the volume of the chamber and the flow rate and a sampling frequency of 15 h can be obtained for 200 times dilution factors. However this approach does not allow the standardization with a single standard. Recently E1 Azouzi e t al. 33 have developed a similar approach, based on the use of syringes to do the on-line dispersion of concentrated samples and the calibration with a single standard. The method uses the syringes as variable volume mixing chambers in order to obtain a continuous dilution of a concentrated standard to be used as a calibration

FIA strategies for calibration and standardization in atomic spectrometry

Valve I

I

I

-2'

_,

,, Pump

Valve II

c h a m b e r II

Dilution chamber

I

115

FAAS

l

Carrier

MANIFOLD

Figure 4.6 Manifold employed for the 130000 on-line dilution of Cu samples and comparison of the recordings obtained using the entire peak profiles and those found stopping the carrier flow rate to clean and refill the syringe (from El Azouzi et al. 33 reproduced with permission of Elsevier). line through the procedure described before but with the possibility to empty the syringe out rapidly to improve the sampling frequency. So, it provides a good compromise between the strategy proposed by Beinrohr e t a l . 31 and that developed by Lopez-Garcia e t al. 32

Figure 4.6 depicts the manifold employed and the typical recording obtained by a fast emptying out of the manifold. As can be seen in this figure, reproducible measurements for 100 000 ppm of Cu can be achieved at a dilution level of 130 000 times with a relative standard deviation of the order of 3%, reducing the time required for each measurement from 386 to 27 s, thus improving tremendously the sampling frequency.

116 4.3.1.2.4

M. de la Guardia Standardization by merging flow

Merging the sample carrier with another carrier flow provides a linear dilution of the sample plug which depends not only on the sample injected volume and the manifold volume but also on the proportion between the two carrier flow-rates. Zagatto e t a l . 34 have reported a 40-fold dilution of an Mg solution by injecting 10 Ixl of a standard and pumping the carrier at 12 ml min-1. In this system a T piece located before the nebulizer of a FAAS instrument allowed that half the total flow was drawn off in order to adapt the flow-rate to the nebulizer uptake. A sampling frequency of 300 h-1 was achieved with a 0.5% relative standard deviation. The dilution factors obtained in the merging-flow mode depend on both flow-merging and dispersion. In the case of the system employed by Zagatto e t al. 34 the first mechanism provided a 13 times dilution and the second one a 3.1 dilution factor and that proves that the merging-flow is, in general, employed in combination with other strategies. Some of the previously reported procedures of calibration, based on the variation of the sample volume ~2'i6-~8 or in the use of mixing chambers 12"28'33 are really combined strategies also involving merging-flow, especially those including a flow compensation. 2~'32A good example of the practical importance of this latter strategy is the method developed by Mauri e t al. 35 for FAAS determination of Au in jewellery samples, where the on-line dilution of samples increases the dynamic range. However, for the determination of Au in real samples containing more than 75% (w/w) of metal a bracketing standardization must be carried out in order to obtain acceptable reproducible results (as those found by the "fire assay" reference procedure).

4.3.1.2.5

Standardization by time-based electronic dilution

The so-called "electronic dilution" method, consisting of timing the readout delay following sample dispersion, was developed by Olsen e t al. 36 in 1982 for molecular spectrometry. It was adapted by Araujo e t al. 37 for single standard addition calibration in FAAS and employed by Gan e t al. 38 for single-standard calibration of a FAAS. In the electronic dilution, the dilution factor is varied by changing the readout time while the dispersion coefficient-time relationship of the falling gradient of a peak profile (obtained with a single standard solution) can be employed for standardization, taking into account the relation between the dispersion coefficient (D) and concentration indicated in Equation (4.2). This procedure is the basis of all the gradient techniques for standardization with a single standard. However the precision of the time readouts is generally lower than that obtained for the peak height.

FIA strategies for calibration and standardization in atomic spectrometry 4.3.1.2.6

117

Standardization by zone sampling

The zone sampling strategy developed by Reis et al. 39 for extending the calibration range in FAAS is a powerful alternative to the gradient calibration by time-based electronic dilution. In this strategy a portion of the dispersed sample is re-sampled from a section of the gradient and dispersed again, thus providing dilution factors of over 100 times with a good precision. This technique generally uses an injection valve with two parallel low volume loops. In the first loop the sample is loaded and, after injection and dispersion in a mixing coil, it is transported into the second loop. This latter loop holds only a fraction of the dispersed sample zone which is injected in a second carrier flow and then transported to the detector. The degree of dilution in zone sampling depends on the dimensions of the mixing coils and also on the position of the re-sampled zone; so the dilution factor can be modified by conveniently timing the actuation of the injection valve or by changing the flow-rate of the first carrier flow. The aforementioned strategy is suitable for increasing the dynamic range of atomic spectrometry techniques, but also allows standardization with a single standard. 4.3.1.2.7

Standardization by partial overlapping zones

Another alternative to the time-based gradient exploitation in standardization and dynamic range enlargement in atomic spectrometry is that proposed by Zagatto et al., 4~ previously described as "sequential injection" and finally called "partial overlapping zones". 41 This strategy is based on the previous studies of Mindegaard 42 and Hansen et al. 43 o n the reproducible partial overlapping of injected zones. The zone penetration is provided by the partial overlap of two or more sequentially injected sample zones which penetrate each other. It offers a series of minima between neighbouring zones where the injected samples are diluted to different degrees by the carrier diluent, all these points being easily located. The sample dispersion in a zone penetration device can be combined with changes in the sample injected and coil volumes. In general 2 n - 1 readout points (between maxima and minima) can be obtained with n sample zones injected. So, injecting a series of solutions with different volumes and allowing to penetrate, it is possible to have readouts at different dilution levels, thus improving the dynamic range of responses. The multiple injections, in which the technique is based, could be carried out by using multiple loop valves 4~44 or network manifolds. 45 For multiple-loop valves the dilution factors of the readout points are controlled by the volume of the sample loops as well as by the volume of the intermediate coils and that of the final mixing coil. In order to obtain the fractional overlapping zones, by using

118

M. de la Guardia

networks, a single injected solution is split into two or more branches of a branched-flow line. The ratio of the split-flow depends on the characteristics of each individual branch. The split fractions merge again at the outlet of the network to create a series of overlapping peaks. Xu and Fang 2~ have combined the partial overlapping zones strategy with the microsampling approach in order to improve the capacity of this system to provide dilution factors of 3000-27 000 and, as for example, using a micro-sampling dilution system, capable of producing 1500 times dilutions of the peak maxima. The incorporation of the partial overlapping zones methodology provides a 20-fold extra dilution effect at the minima between two peaks, thus achieving approximately 30 000 times dilution. The manifold developed by Xu and Fang includes a single loop used to inject two sample zones via a computer controlled stepper-motor-driven peristaltic pump to partially fill the loop. The two sample zones were introduced sequentially into the loop, under a fixed carrier flow-rate, and the interval between the two injections determined the extent of separation or penetration of the two zones and, consequently, the dilution factor at the peak minima. This method was applied to the determination of Mg in brines at concentrations of several grams per litre, achieving a sampling frequency of 45 h-l and a relative standard deviation of 1.8%. 4.3.1.2.8 Standardization by split flow dilution A simple way to reduce the amount of sample or standard injected, which takes part in the dilution process, is to split the sample carrier flow to obtain higher dilution factors by the merging-flow approach, without decreasing the injected sample volume. To do that, a fraction of the sample flow is drawn to waste by means of a T-piece, following some preliminary sample dispersion, and the remaining fraction is merged with a diluent flow. Mindel and Karlberg 46 developed this system to expand the calibration range indicating that the splitting process can be repeated several times in order to achieve high dilution factors. These factors are related to the proportion of sample flow drawn-off and made up downstream and could be modified also by varying the diluent or the drawn-off flowrate. However, the split-flow strategy is limited by the long- and short-term fluctuations of two larger flows which are transferred to a low flow-rate stream being the difference of the two larger flows. This magnifies the imprecision related to the flow-rates and, because of that, the drawn-off proportions must be no more than 80%. 4.3.1.2.9 Standardization by cascade dilution Whitman and Christian 47 developed a special split-flow dilution system involving withdrawal of the sample fraction to be further diluted, instead of the fraction to be discarded (as carried out in the split-flow dilution). This procedure was proposed for

FIA strategies for calibration and standardization in atomic spectrometry

119

molecular spectrophotometry and amperometry but, obviously, it is clearly useful in atomic spectrometry. In the cascade dilution method, the withdrawn fraction is delivered through a new pump channel and merged with a diluent stream, thus avoiding the weaknesses of the original split-flow method, and providing high dilution factors by using low draw-off fractions. After two stage dilutions, a dispersion coefficient of the order of 500 can be obtained for the injection of 20 p.1 with a relative standard deviation of 3% and a sampling frequency of 100 h-~. 4.3.1.2.10

Standardization by gradient ratio method

The ratio between the gradient dilution profiles found in FAAS for both samples and standards, is an excellent standardization strategy, which could solve the main problems found in FAAS due to the response curvature and interferences. This method was proposed by Sperling eta/., 48 based on the fact that interferences can be reduced by simply diluting the samples and consist of the collection of the response data of a standard solution and those of a sample at a defined delay time, for identical dispersion coefficients, and the use of the quotient of these signals in the falling gradient of the peaks at delay times taken every 20 ms. These data are curve-fitted, using an exponential function algorithm, being the ratios at lower absorbances corrected by extrapolation by means of the information contained in the whole falling gradient through a weighted regression. The control of the peak drift was carried out by using the Fan and Fang 49 method for compensating the peak shift, by means of the evaluation of the gradient ratios of sample and standard on the rising and falling gradients for five delay times and the positioning of the sample peak with respect to the standard peak. The evaluation of the aforementioned strategy demonstrated that both the compensation of response curvature and the elimination of phosphate interference, can be carried out for Ca determination by FAAS. 48 4.3.1.2.11

Gradient standardization by flow rate variation

Lopez-Garcia et al. 5~ have proposed a procedure to do the standardization in FAAS by using a single standard and a gradient technique. In this study, a stock standard solution is pumped at a linearly increasing flow rate, from zero up to the nebulizer uptake rate, and a T piece, with a tip immersed in distilled water, is employed to compensate the carrier flow in order to feed the nebulizer at a constant flow. Basically the system operates by modifying linearly the pumping rate (Qpt) at which a solution, with an analyte concentration Cp is pumped through the manifold, from a flowrate 0 (for t=0) to the flow rate equal to the nebulizer uptake rate (Qr) for t=5. At the same time a solution of concentration Ca is aspirated through the T piece at a flow-rate equal to Qo,. Thus, it can be written

120

M. de la Guardia

Qr=Qpt+Qat

(4.12)

and assuming that the proportionality constant of the detector is the same for the solutions pumped and aspirated, the absorbance measured at time t can be described as:

A,=K(Qp,CpQ,+Q,,,Ca)QT

(4.13)

K being the proportionality constant. Combining equations (4.12) and (4.13) and taking into account that Qp, is directly proportional to the time elapsed from the starting to pump, the absorbance can be defined as a function of time as:

A,=K(C,+(Cp- C,,)t/T)

(4.14)

The approach of L6pez-Garcia et al.5~is extremely flexible and thus, aspirating pure water for compensation C,,=Oand the calibration line can be obtained in few seconds by pumping a concentrated standard being

A,=KC,,t/T

(4.15)

and Cp, being the instantaneous concentration of the solution being pumped, it follows:

Cr,=Cpt/T

(4.16)

A,=KCp,

(4.17)

it can be written

which corresponds to the calibration line. To do the determination of the analyte, using this approach, the unknown solution is aspirated through the T piece with the pump stopped. For extending the calibration range, the concentrated samples can be aspirated through the T piece, using a Cp concentration of zero. Thus,

At=KC,(1 - t/T)

(4.18)

which is only valid for the portion of the absorbance-time plot which is linear. In this case, a curve is obtained at the start of the experiment and only when the aspirated solution is sufficiently diluted a straight line with a slope-KC,,T-~ is obtained. To do the determination of the concentration of the aspirated solution the following equation is available:

FIA strategies for calibration and standardization in atomic spectrometry

Islope of dilution curve I_ Ca slope of calibration Cp

121

(4.19)

Additionally, the aforementioned strategy could be useful to carry out determinations by the standard additions method, but this aspect will be considered in the next sections. A recent modification of the aforementioned procedure has been developed by the same group of research 5~ based on the use of two variable-speed peristaltic pumps. One of the pumps transports a standard solution and the other a diluent solution, both solutions being merged before the detector. The flow-rate of the first pump is increased linearly with time and that of the second pump is decreased linearly at the same rate. So, an on-line concentration gradient is obtained whereas the flow-rate through the detector remains constant. The absorbance-time profiles obtained in this way are used for calibration carrying out the standardization of the system with 145 txl of a simple standard in only 50 s. The use of two programmed flow-rates has been taken from the normal practice in FIA-titrimetry and there are some detailed studies of Fuhrmann and Spohn 52"53 about the generation of linear concentration gradients by means of computer controlled micropumps in order to implement triangle-programmed flow titrations and the calculation of calibration graphs. Using two common peristaltic pumps, and applying a controlled voltage variation from zero to 5 volts (which corresponds to a maximum speed of 47.9 rpm), an absorbance-time profile, which is very close to a calibration graph found in the conventional way, can be obtained for a single gradient, or a double gradient (if the dispersion effects are not very severe) f r o m a standard solution propelled by one of the pumps merged with a diluent solution. For the analysis of unknown samples, they are continuously pumped through the manifold by one of the pumps, the other pump remaining stopped. If the unknown solution is too concentrated to provide a response within the linear range of the detector, the manifold permits the on-line dilution of the sample before the measurement by pumping the sample continuously by means of one of the pumps delivering a constant flow-rate below the total flow while the other pump delivers a flow-rate equal to the difference between the total flow and that employed for the sample. The dilution degree can be established from the quotient between the maximum voltage which can be applied and that employed to transport the concentrated sample. The problems related to the inner volume and the inertial effects can be solved using three different procedures for the treatment of data based on: (a) the treatment of signaltime pairs by displacement on the x-axis the signal-time profile obtained when performing a single gradient, (b) the use of a double gradient and the measurement of the width of the triangle-shaped profile and (c) the use of the area values limited by the signal-time profile.

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The three procedures mentioned provided good analytical results in spectrophotometric measurements but that based on the area values was the most advantageous (because it is unaffected by deformations in the signal-time profiles due to dispersion effects). The aforementioned strategy has recently been applied to intelligent data treatment in analytical atomic spectrometry. 54 4.3.1.2.12

Standardization by using solenoid valves

The Piracicaba group has been working, during the last years, on the improvement of traditional systems of introduction and handling of samples and reagents inside the flow systems by resorting to fully electronical operation of the FIA manifolds. To do that, they have applied time-based processes 55 and solenoid valves previously reported by MalcolmLaves and Pasquini 56'57 t o control carrier, reagents and sample flows and so they developed highly flexible devices. 58 A multipurpose flow injection system, including a sliding injector and a series of threeway solenoid valves, has been developed for both programmable dilutions and standard additions techniques. The sliding injector is responsible for the commutation of solids of several channels by moving the central part of the injector and the solenoid valves for discrete commutations. The basic system includes the injector commutator and one module, composed of two solenoid valves, Y-shaped connections and helical reaction coils, but additional modules can be added. In the aforementioned system the dilution of a high concentrated standard or a concentrate sample can be carried out in two ways. In points close to the injector the degree of dilution is proportional to the flow rates of the confluent streams but in other points the injected solution is also dispersed through the transport inside the reaction coils. In both cases, typical relative standard deviation values lower than 1% were obtained. The synchronization of this flow system and the ICP-OES detection provides the simultaneous determinations of several elements in real samples by using the concepts of zone sampling previously reported. 39 This strategy, based on the use of solenoid valves, also opens new possibilities for trace elements determination using the standard additions method. 4.3.2

On-line standard additions

The standard additions method provides an excellent way to verify the presence or absence of matrix interferences in atomic spectrometry analysis, and to correct this type of problem. The possibilities offered by flow injection procedures in order to do any kind of sample on-line and standard manipulations provides interesting solutions in order to add sample to standard or standard to sample in a faster way than that provided by the classical batch

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procedures. Tyson has reviewed and discussed the various ways proposed so far for standard addition methods applied in flow injection-atomic spectrometry. 59 Many of the "single standard" calibration strategies discussed in the previous sections are useful also to propose matched standards (by adding on-line the main components of the sample matrix) or to carry out on-line standard additions. In the following sections we will discuss in detail some of the most relevant procedures proposed in the literature. 4.3.2.1

M e r g i n g zones standard additions

A series of standards can be injected through a flow injection manifold to merge with a sample in an analogous way to that employed in off-line standard additions but in a faster and simpler mode which requires less handling and reagents consumption. There are two single requirements of the "merging zones" procedures, namely the synchronization of the merging zones and the maintenance of the flow rate. Zagatto et al. 6~ and Gin6 et al. 61 have employed the merging zones approach in ICPOES generalized standard additions procedures. This procedure can be also employed with the zone sampling strategy 39 in order to work with a single standard solution which is diluted at different degrees by using an injection valve with two parallel loops, as commented on in Section 3.1.2.6. To avoid the problems of synchronization of the merging zones Marshall and Van Staden 62 have proposed a strategy based on the injection of standards in a carrier stream which is merged with a sample stream, thus providing the dispersion of standards in a sample bolus which reached the steady-state. The signals obtained for the injected standards, from the base-line created by the sample measurement, are characteristic of the calibration line in the presence of the sample matrix. This procedure has been employed for the arsenic determination in industrial effluents by HG-AAS. 4.3.2.2

Interpolative standard additions method

In 1981 Julian Tyson proposed, for the first time, a simple calibration method for FAAS based on the injection of discrete volumes of standards on an unknown sample stream. 63 This method, provided positive peaks when the concentration of the element to be determined in the injected standard solution was higher than that present in the sample but, on the contrary, negative peaks were obtained for injected standard solutions less concentrated than samples. In this way a series of positive and negative peaks on the addition of a series of well-known standard solutions are obtained and the concentration of the sample is determined by interpolation in the regression line ~A versus concentration of injected standard. This method is extremely simple, not requiring the use of multiple line manifolds, and avoids the problems of synchronization caused by the merging zones approach. Additionally the interpolative method is less dependent on the linearity of responses than

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M. de la Guardia

the extrapolative method. In fact, the main advantage of the strategy developed by Tyson is that it solves the tremendous mathematical doubts of data obtained by extrapolation. Tyson et al. 26"64 applied the aforementioned procedure to the determination of Cr in steels by FAAS and it has also been employed using ICP-OES. 65'66 Israel and Barnes 65'67 modified the method of Tyson by employing the standard to sample single standard addition 65 and also, using the sample to standard additions calibration. 67 On comparing the single standard addition method with that of multiple standard additions it must be taken into consideration that the first one is faster than the second. However, it is also conditioned by the dynamic range of the technique employed (e.g. the single standard addition, using the injection of a blank solution and that of a concentrated standard into the sample carriers or, altematively, that of a blank and the sample on a standard carrier, is more appropriated for ICP-OES than for FAAS or ETAAS). The use of the standard to sample additions method requires the use of a high volume of sample as compared with the sample to standard additions but the first method involves a lower dilution of samples, being more appropriate for the determination of sample components at low concentration levels. As an example of the on-line standard-to-sample additions method, Figure 4.7 depicts the calibration line obtained for Cu determination in olive oil samples. 68 L6pez-Garcia et al. 69 extended to slurries the sample to standard single addition method for FAAS trace element determinations in iron oxide pigments. This approach combines both the standard additions and the use of matched standards, and consists of injecting a slurry of the sample and a slurry of a standard of known analyte concentration into a dissolved standard carrier stream. Diane Beauchemin has adapted a single line flow injection manifold to do the standard addition on-line in ICP-MS. 7~ In order to avoid clogging in ICP-MS when the sample contains more than 0.2% of dissolved solids, 71 she proposed to inject a discrete volume of the sample in two different carriers, a blank and a standard with a greater concentration than the sample, as well as the injection of the standard in the blank carrier. This method does not require the use of matrix-matched blanks, minimizes the sample consumption compared to the reversed FI method and only reduces the analytical sensitivity by a factor of 0.5 as compared to continuous nebulization. A complete replicate multi-elemental analysis can be accomplished in 200 s using 100 ~tl injections. 4.3.2.3

Partial o v e r l a p p i n g z o n e s s t a n d a r d additions

The partial overlapping strategy (see Section 3.1.2.7) has been adapted by Fang et al. 72 to allow standard additions techniques using a single injection and exploiting the gradient of concentration provided at the penetrating zones of carrier, sample and standard. To do that,

FIA strategies for calibration and standardization in atomic spectrometry

125

0.02

...

!

|

|

Figure 4.7 Determination of Cu in olive oil by the on-line standard to sample additions method. (a) FI recordings obtained by injection of a Conostan standard in a carrier stream of the sample and (b) plot of AA versus Cu concentration obtained from the FI recordings (from Carbonell e t al. 68 reproduced with permission of Royal Society of Chemistry).

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M. de la Guardia

the sample is introduced between a carrier water and an appropriate standard solution carrier and the sample volume must be chosen to provide a dispersion coefficient close to unity. The sample is diluted in the rising part with water and in the falling part with the standard, thus producing a series of different sample to standard ratios on the falling slope, which could be correlated with the delay time after sample injection. To determine the sample concentration a pair of signals, obtained at delay times tr and tr corresponding to symmetrical positions with respect to the maximum signal obtained for the sample in both the rising part and the falling part of its transient signal, are used. For both time values, the signal peak height values obtained are A,. and Ar respectively, but being A r a big value obtained by dispersion of the sample inside the standard carrier solution. Thus, it can be written: Cs= K A / ( A r - A,.) (4.20) assuming a linear concentration-signal relationship in the interval of absorbance values considered and K being a constant which depends on the concentration of the standard carrier, (C,-,), and the dispersion coefficient, (D), of the sample K= C,( 1 - d)/d

(4.21)

d= I/D

(4.22)

being

This procedure has been applied to the determination of Ca in soil extracts by flame photometry. 72 4.3.2.4

Time-based electronic dilution standard additions

Araujo et al. 37 employed the time-based electronic dilution procedure described in section 4.3.1.2.5 to accomplish the standard additions calibration by merging an injected standard solution with the sample stream. By changing the readout time in the falling part of the peak obtained, different signal values can be found corresponding to decreasing concentration additions of the standard to the sample carrier as a function of the dispersion of the standard injected. The concentration values of the standard added at the different time delays can be established previously by injecting the standard in a blank carrier and determining the concentration values corresponding to different readout times. This calibration method has been applied to the flame atomic absorption determination of Ca, K and Na in samples containing ethanol obtaining errors lower than 3%.

FIA strategies for calibration and standardization in atomic spectrometry

4.3.2.5

127

Zone sampling standard additions

The zone sampling concept developed by the group of Piracicaba was also applied to accomplish standard addition calibration 6~ and recently it has been improved by incorporating the use of solenoid valves connected to a sliding injector. 58 This standard addition technique using solenoid valves uses aliquots of a standard solution, sequentially delivered from a dispersed zone, which are added to the sample via a merging zones configuration. The number of additions depends on the actuation time of the valves, the volume injected, the dimension of the dispersion zone and the flow rate values. The aforementioned strategy has been applied to the ICP--OES determination of Zn and Mn in plant digests. 58

4.3.2.6 Standard additions byflow rate variation L6pez-Garcia et al., 5~ employing a single line manifold with a T piece for flow compensation, have also performed on-line standard additions calibration. Basically the system is that described in section 3.1.2.11 but in this case both Ca and Cp are different from zero. In the standard addition Ca is the sample concentration which is aspirated through the T piece and Cp is the concentration of the standard merged with the sample through the pump. The linear absorbance-time plot must follow equation (4.13) and thus, the concentration of the unknown sample, Ca, can be calculated either by comparison of the slope of the graph with that of a calibration graph following: slope of standard additions_ Cp slope of calibration C p - Ca B

_

_

or by using the intercept on the time axis, t,,d. In this latter case equation (4.13) is expressed as

Ca= Cpt~j/(t,,d - T) T being the time necessary to reach the nebulizer uptake rate. This procedure has been successfully applied to determine Cu in the presence of different matrices. 5~

4.3.3

Internal standard method

The use of an internal standard is a common practice in multidetection techniques, to account for instability problems during the measurement of both standards and samples. This inestability can affect short-term precision and accuracy.74"75

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M. de la Guardia

Flow injection offers a direct way to perform internal standardization in atomic spectrometry, by the merging zones approach, which reduces the amount of solutions employed, the time required and the cost of analysis (the price of the internal standard increases the cost of the whole analysis). On-line internal standardization has been employed in ICP-OES TM and in ICP-MS.

4.3.4 Isotopic dilution ICP-MS offers, as compared with other spectrochemical techniques, the unique capability of rapid determinations of the isotopic composition and the isotope ratios of elemental components in liquid or dissolved samples with a precision of 0.1-1%.v7. 78This fact opens the possibility for convenient isotope dilution analysis by ICP-MS. Isotopic dilution can be applied only to elements which have at least two natural isotopes. A spike, enriched in one of the isotopes of the element to be measured, is added to the sample in a known concentration thus modifying the isotopic ratio. From the measurement of the relative abundance of each one of the two considered isotopes in both, the unspiked sample and the spiked one, the concentration of the element can be calculated according to the formula:

c=(Av- RB~,)w~p (RB.,.-A,)ws

(4.25)

C being the sought concentration, A and B the relative abundances, as a percentage, of isotopes of A and B mass in the sample, A,. and B.~,or in the spike, Asp and Bsp, w~the weight of sample in g, and w v, the weight of the spike in I~g, R being the measured isotope ratio. The measurement of isotope ratios, rather than absolute intensities, compensates for matrix effects and instrumental drift, thus providing highly accurate and precise results. TM In conventional "batch" isotope dilution, the process involves preparation of both unspiked and spiked samples, calibration of the spike solution and addition of a known amount of the isotope spike to the sample. The use of flow injection ICP-MS provides an excellent way to carry out on-line isotopic dilutions requiring only one sample preparation which can be merged on-line with a spike. The on-line isotope dilution can be carried out by merging two carrier streams or by the simultaneous injection of two solutions into the carrier stream and any FI system can be adapted to do that procedure. The problem is only being the synchronization of both the FI and the ICP-MS systems. Viczifin et al. 79'8~ have found that using a total measuring time in the ICP of at least one or more periods of the injection cycle the best reproducibility can be achieved. The advantages of on-line isotope dilution include: (a) the use of the same solution for

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129

concentration and isotope ratio determination without any extra sample treatment, (b) the fact that only the sample volume consumed for measurement is mixed with the spike, the rest of the sample remaining intact, (c) the possibility of realizing in the same manifold on-line sample and isotope dilution, as required. The aforementioned procedure has been evaluated for example using Pb as a test element 79 and applied to avoid analytical errors on the determination of Mo. 81 Rottmann and Heumann 82 have employed the on-line isotope dilution with HPLC-ICP-MS for the determination of Cu and Mo species bound to the dissolved organic matter by using a single manifold for the post-column continuous addition of a spike and standard injection for calibration of the spike flow. The method provides an excellent precission and accuracy, assuming the internal standardization and elimination of matrix effects and measurement drift and the most important thing is the possibility offered to also quantify species with unknown structure and composition. However, flow injection on-line isotope dilution offers some disadvantages related to the dilution of the sample caused by the merging with spike and to the fact that the spike is not mixed with the sample prior to the sample pretreatment. Thus, the method cannot compensate for analyte losses during the preparation step as for isotope dilution carried out off-line.

4.4

Concluding remarks

Having delineated the specific advantages provided by flow analysis to solve problems related to the extremely short dynamic ranges in atomic absorption spectrometry, by means of the on-line dilution, and the tremendous possibilities offered by flow analysis to carry out standard additions or internal standardization, it is worth noting that the basic advantage provided by flow analysis to atomic spectrometry is to transform first-order instruments like spectrometers, only capable of generating multiple measurements at one time or for one sample, to second-order instruments able to generate a second-order tensor (a matrix) of data per sample. Flow analysis-atomic spectrometry instruments are able to provide data on different wavelengths or mass numbers as a function of time for the same sample or standard injected and that provides, according to the Booksh and Kowalski classification of the calibration paradigms, 83 some of the most powerful analytical tools, comparable to hyphenated GC/MS or two dimensions NMR techniques. The aforementioned advantage has not been appropriately stressed so far, but it must be taken into consideration because second-order instrumentation allows accurate results for analysis even in the presence of any component in the sample not included in the calibration model. Moreover, it could be achieved with just one calibration solution.

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M. de la Guardia

Additionally the second-order instruments are unaffected by multiple spectral interferences which have no real detrimental effect on second-order calibration aside from decreased signal averaging. Standardization is theoretically still possible by second-order standard addition method when matrix effects are present in the samples providing different instrument response to a standard solution of an analyte than that for the same analyte in the presence of other matrix compounds. So, although some of the problems related to the development of appropriate statistics for second-order calibration are not fully investigated so far, it is clear that in the near future the use of flow analysis-atomic spectrometry could solve important problems related to spectral interferences in optical emission or matrix effects plaguing electrothermal atomization, simply by treating the signal as a function of time or introducing simple non-chromatographic separation methods based on flow injection principles.

4.5

References

I Valcarcel, M. and Rios, A., The Analyst, 1995, 120, 2291. 2 Sanz-Medel, A., Espectrometria atomica analitica: Presente y futuro. In: Fronteras de la Quimica Analitica, Barcelona, October, 1996. 3 Blades, M. W., Appl. Spectrosc., 1994, 48, I1, 12A. 4 Tyson, J. F., Analyst, 1984, 109, 319. 5 Bysouth, S. R. and Tyson, J. F., Anal. Chim. Acta, 1986, 179, 481. 6 Tyson, J. F., Fresenius d. Anal. Chem., 1988, 329, 663. 7 Tyson, J. F., Spectrochim. Acta, 1991, 14, 169. 8 Fang, Z. In: Flow Injection Atomic Spectroscopy, J. L. Burguera (ed.), Marcel Dekker, 1989, NY. 9 Trojanowicz, M. and Oibrych-Sleszynska, E., Chem, Anal., 1992, 37, i l l . 10 Fang, Z., Flow injection Atomic Absorption Spectrometry, J. Wiley, Chichester, (UK), 1995. I I Costa Lima, J. L. F., Rangel, A. O. S. S. and Roque da Silva, M. M., Connaiss Vigne Vin, 1990, 24, 167. 12 Carbonell, V., Sanz, A., Salvador, A. and de ia Guardia, M., J. Anal. At. Spectrom., 1991, 6, 223. 13 Burguera, M., Burguera, J. L., Granado, D. D. and Aiarcon, O. M.,Acta Cient. Venez., 1988, 39, 323. 14 Ruzicka, J. and Hansen, E. H., Flow Injection Analysis 2nd edn, John Wiley, New York, 1988. 15 Sherwood, R. A., Rocks, B. F. and Riley, C., The Analyst, 1985, 110, 493. 16 Burguera, J. L., Burguera, M., Rivas, C., de la Guardia, M. and Salvador, A., Anal. Chim. Acta, 1990, 234, 253. ! 7 Burguera, M., Burguera, J. L., Rivas, C., de la Guardia, M., Salvador, A. and Carboneil, V., d. Flow Injection Anal., 1990, 7, 11. 18 de la Guardia, M., Morales-Rubio, A., Carbonell, V., Salvador, A., Burguera, J. L. and Burguera, M., Fresenius J. Anal. Chem., 1993, 345, 579. 19 Fang, Z., Welz, B. and Sperling, M., Anal. Chem., 1993, 65, 1682. 20 Xu, S. and Fang, Z., Microchem. J., 1994, 50, 145. 21 Lopez-Garcia, I., Vifias, P. and Hernandez-Cordoba, M., J. Anal. At. Spectrom., 1994, 9, 1167. 22 Tyson, J. F., Adeeyinwo, C. E., Appleton, J. M. H., Bysouth, S. R., Idris, A. B. and Sarkissian, L. L., The Analyst, 1985, 110, 487. 23 Tyson, J. F., Mariara, J. R. and Appleton, J. M. H., d. Anal. At. Spectrom., 1986, 1,273. 24 Vanderslice, J. T., Stewart, K. K., Rosenfeid, A. G. and Higgs, D. J., Talanta, 1981, 28, 11. 25 Ruzicka, J. and Hansen, E. H., Anal. Chim. Acta, 1978, 99, 37. 26 Tyson, J. F., Appleton, J. M. H. and Idris, A. B.,Anal. Chim. Acta, 1983, 145, 159. 27 Tyson, J. E and Appleton, J. M. H., Talanta, 1989, 31, 9.

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28 de la Guardia, M., Carbonell, V., Momles-Rubio, A. and Salvador, A., Fresenius J. Anal. Chem., 1989, 335, 975. 29 OIsen, S., Ruzicka, J. and Hansen, E. H., Anal. Chim. Acta, 1982, 136, 101. 30 Hon, X. D., Xu, P. F. and Sun, Z. H., Guang puxue Yu Guangpy Fenxi, 1990, 10, 38. 31 Beinrohr, E., Cisemi, P. and Tyson, J. F., J. Anal. At. Spectrom., 199 l, 6, 307 32 Lopez-Garcia, I., Arroyo-Cortez, J. and Hemandez-Cordoba, J. Anal. At. Spectrom., 1992, 7, 1291. 33 El Azouzi, H., Perez-Jordan, Y., Salvador, A. and de la Guardia, M., Spectrochim. Acta B, 1996, in press. 34 Zagatto, A. E. G., Krug, E J., Bergamin, E H., Jorgensen, S. S. and Reis, B. F.,Anal. Chim. Acta, 1979, 104, 279. 35 Mauri, A. R., Huerta, E. and de la Guardia, M., Fresenius J. Anal. Chem., 1990, 338, 699. 36 Olsen, S., Ruzicka, J. and Hansen, E. H., Anal. Chim. Acta, 1982, 136, 113. 37 Amujo, M. C. V., Pasquini, C., Bruns, R. E. and Zagatto, E.A.G.,Anal. Chim. Acta, 1984, 171,337. 38 Gan, X. X., Xie, E H., Ma, C., Bai, Y. C., Yang, J. N. and Ma, C. I., Fenxi Huaxue, 1992, 20, 1269. 39 Reis, B. E, Jacintho, A. O., Morlatti, J., Krug, F. J., Zagatto, E. A. G., Bergamin, E. H. and Pessenda, L. C. R., Anal. Chim. Acta, 1981, 123, 24. 40 Zagatto, E. A. G., Gin6, M. K, Femandes, E. A. N., Reis, B. E and Krug, E J., Anal. Chim. Acta, 1985, 173, 209. 41 Zagatto, E. A. G., Krug, F. J., Bergamin, F. H. and Jorgensen, S. S., in the book Flow Injection Atomic Spectroscopy (J. Burguera Ed), M Dekker NY, 1989. 42 Mindegoord, J. Anal. At. Spectrom., 1979, 104, 185. 43 Hansen, E. H., Ruzicka, J., Krug, E J. and Zagatto, E. A. G., Anal. Chim. Acta, 1983, 148, 111. 44 Fang, Z. I., Sperling, M. and Welz, B., Anal. Chim. Acta, 1992, 269, 9. 45 Tyson, E J. and Bysouth, S. R., J. Anal. At. Spectrom., 1988, 3, 211. 46 Mindel, B. and Karlberg, B., Lab. Pract., 1981, 30, 719. 47 Whitman, D. A. and Christian, G. D., Talanta, 1989, 30, 205. 48 Sperling, M., Fang, Z. and Welz, B., Anal. Chem., 1991, 63, 151. 49 Fan, S. H. and Fang, Z. I., Anal. Chim. Acta, 1990, 241, 15. 50 Lopez-Garcia, I., Vifias, P. and Hemandez-Cordoba, M., J. Anal. At. Spectrom., 1994, 9, 553. 51 Lopez-Garcia, I., Vifias, P. and Hemandez-Cordoba, M., Anal. Chim. Acta, 1996, 327, 83. 52 Fuhrmann, B. and Spohn, U., Anal. Chim. Acta, 1993, 282, 391. 53 Fuhrmann, B. and Spohn, U., J. Automat. Chem., 1993, 15, 209. 54 L6pez-Garcia, I., Sanchez, M., Hemandez-Cordoba, M. and Martinez-Ortiz, F., paper presented at the 8th JAI, Barcelona, 22-25 October 1996. 55 Krug, F. J., Bergamin, F. H. and Zagatto, E. A. G., Anal. Chim. Acta, 1986, 179, 103. 56 Malcolm-Lawes, D. J. and Pasquini, C., J. Autom. Chem., 1988, 10, 25. 57 Malcolm-Lawes, D. J., Meyers, J. and Pasquini, C., Lab. Microcomputer, 1988, 7, 89. 58 Reis, B. E, Gin6, M. E, Krug, F. J. and Bergamin, E H., J. Anal. At. Spectrom., 1992, 7, 865. 59 Tyson, F. J., Spectrochim. Acta Rev., 1991, 14, 169. 60 Zagatto, E. A. G., Jacintho, A. O., Krug, F. J. and Reis, B. F., Anal. Chim. Acta, 1983, 145, 169. 61 Gin6, M. F., Reis, B. F., Zagatto, E. A. G., Krug, F. J. and Jacintho, A. O., Anal. Chim. Acta, 1983, 155, 131. 62 Marshall, G. D. and Van Staden, E J.,J. Anal. At. Spectrom., 1990, 5, 675. 63 Tyson, F. J., Anal. Proc., 1981, 18, 542. 64 Tyson, F. J. and Idris, A. B., Analyst, 1984, 109, 23. 65 Israel, Y. and Barnes, R. M., Anal. Chem., 1984, 56, 1188. 66 Fang, Z., Xu, S., Wang, X. and Zhang, S., Anal. Chim. Acta, 1986, 179, 325. 67 Israel, Y. and Barnes, R. M.,Analyst, 1989, 114, 843. 68 Carbonell, V., Mauri, A., Salvador, A. and de la Guardia, M., J. Anal. At. Spectrom., 1991, 6, 581. 69 L6pez-Garcia, I., Ortiz, F. and Hemandez-Cordoba, M., Analyst, 1991, 116, 831. 70 Beauchemin, D., Anal. Chem., 1995, 67, 1553. 71 Beauchemin, D., TrAC, 1991, 10, 71. 72 Fang, Z., Harris, J. M., Ruzcika, J. and Hansen, E. H., Anal. Chem., 1985, 57, 1457. 73 Gin6, M. E, Krug, E J., Bergamin, H., Reis, B. E, Zagatto, E. A. G. and Bruns, R. E., J. Anal. At. Spectrom., 1988, 3, 673.

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74 Belchamber, R. M. and Horlick, G., Spectrochim. Acta Part B, 1982, 37, 17. 75 Mennet, J. M. and Ivaldi, J. C., J. Anal. At. Spectrom., 1993, 8, 795. 76 Jacintho, Ao, Zagatto, E. A. G., Bergamin, F. A., Krug, E J., Reis, B. F., Bruns, R. E. and Kowalski, B. R., Anal. Chim. Acta, 1981, 130, 243. 77 Douglas, D. J. and Houk, R. S., Prog. Anal. Spectrosc., 1985, 8, I. 78 Heumann, K. G., Mass. Spectrom. Rev., 1992, 11, 4. 79 Beauchemin, D., McLaren, J. W., Mykytiuk, A. E and Beman, S. S., Anal. Chem., 1987, 59, 778. 80 Viczi/m, M., Lfisztity, A., Wang, X. and Barnes, R., J. Anal. At. Spectrom., 1990, 5, 125. 81 Marchante-Gay6n, J. M., Garcia-Alonso, J. J. and Sanz-Medel, A. In: Plasma Source-Mass Spectrometry. Developments and Applications (G. Holland and S. D. Tanner ed.), The Royal Society of Chemistry, Cambridge, ! 997 82 Rottmann, L. and Heumann, K. G., Fresenius J. Anal. Chem., 1994, 350, 221. 83 Booksh, K. S. and Kowalski, B. R., Anal. Chem., 1994, 66, 782A.

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74 Belchamber, R. M. and Horlick, G., Spectrochim. Acta Part B, 1982, 37, 17. This Page Intentionally Left Blank 75 Mennet, J. M. and Ivaldi, J. C., J. Anal. At. Spectrom., 1993, 8, 795. 76 Jacintho, Ao, Zagatto, E. A. G., Bergamin, F. A., Krug, E J., Reis, B. F., Bruns, R. E. and Kowalski, B. R., Anal. Chim. Acta, 1981, 130, 243. 77 Douglas, D. J. and Houk, R. S., Prog. Anal. Spectrosc., 1985, 8, I. 78 Heumann, K. G., Mass. Spectrom. Rev., 1992, 11, 4. 79 Beauchemin, D., McLaren, J. W., Mykytiuk, A. E and Beman, S. S., Anal. Chem., 1987, 59, 778. 80 Viczi/m, M., Lfisztity, A., Wang, X. and Barnes, R., J. Anal. At. Spectrom., 1990, 5, 125. 81 Marchante-Gay6n, J. M., Garcia-Alonso, J. J. and Sanz-Medel, A. In: Plasma Source-Mass Spectrometry. Developments and Applications (G. Holland and S. D. Tanner ed.), The Royal Society of Chemistry, Cambridge, ! 997 82 Rottmann, L. and Heumann, K. G., Fresenius J. Anal. Chem., 1994, 350, 221. 83 Booksh, K. S. and Kowalski, B. R., Anal. Chem., 1994, 66, 782A.

5.1

Introduction

The knowledge given by the analytical chemistry is obtained during the analytical process, which comprises three stages: preliminary operations, measurement and data acquisition and report. The preliminary operations: sampling and sample pre-treatment, refer to the series of operations related to the preparatory and sample treatment techniques used to render an appropriate aliquot for the analytical measurement. The selection of the procedure to follow at the pretreatment stage depends on the aim of the analysis, the type of matrix, the analyte and its expected concentration in the sample and the analytical technique to be used. The initial process, therefore, frequently includes dilution, decomposition, preconcentration, separation, or other procedures. When trace elements are to be measured in a sample, its preparation procedure must be rigidly checked: indeed, the time and effort devoted to the collection, processing, and storage of a sample may be completely negated by various errors introduced at the moment of their preparation for the actual measurement. Improper handling of the sample during any of these processes may easily introduce errors into the trace levels to be measured. Thanks to the development in the field of chemometrics the signal acquisition and data treatment reached a high level of automation. ~ Sampling and sample preparation techniques, however, have not kept step with this progress, although much development has been noticed lately. 2-6 With careful programming, the automation of the analytical process leads to greater accuracy and far greater versatility in the overall treatment of the 135

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results. The development of procedures of preparatory techniques dealing with solid, liquid and gaseous samples has been a long-term research theme. 7-~2 The limiting step or Achilles heel of modem analytical methodology is clearly sample preparation which, in comparison to the financial and research investment that has gone into instrumental developments, still remains the poor relative of analytical chemistry. Sample preparation frequently requires the use of digestion chambers and reasonably large sample masses and reagent amounts. The batch methodology which is usually applied for transformation of the samples into an appropriate form that can be introduced in a specific instrument, is the most time-consuming step in the analytical process and often requires unpleasant and tedious procedures. The analysis of a sample can only be simpler, quicker, accurate and precise if the sample preparation is automated. Particularly, flow injection (FI) is a technique that has introduced total automation of the sample handling steps without or with minimal intervention by the analyst. 4-~~ Concerning the on-line coupling of a FI manifold to an instrument, the FI system can be regarded as an interface between samples or standards and the instrument, providing useful analytical information about the qualitative and quantitative composition of samples. The analytical techniques most commonly coupled to FI samples work-up devices for the determination of metallic species in samples of different nature, are FAAS, HG-AAS or ETAAS as well as ICPOES, ICP-MS or AFS. These techniques are routinely used in most analytical laboratories. The main advantages of FI when interfaced with atomic spectrometers are the development of automated systems for the on-line chemical modification and manipulation of the analyte prior to its introduction .into the spectrometer. The basic objectives pursued in partly or completely automating the sample dissolution/decomposition procedure are" (a) to process a large number of samples and therefore to increase sample throughput, (b) to reduce human participation and so to avoid errors and cut costs, (c) to lower consumption of sample and/or reagent(s), (d) to develop procedures which are simple, relatively safe to use and applicable to samples of different nature and (e) to decrease the blank values and to reduce the contamination risk. In a typical digestion process, there are some important points which have to be taken into account: the efficiency of solubilization of inorganic material, the efficiency of oxidation of organic constituents, the efficiency of recovery of the analyte from the system, control over carryover or cross-contamination between subsequent samples, accuracy and precision and ease of use. A closed look at the recent literature points out the microwave (MW) heating as most reliable to improve solid samples decomposition processes and speed-up or favor some chemical reactions, ~-~8 although UV-radiation ~9 and ultrasonic agitation 2~ are useful altematives. The aim of this chapter is to describe in detail how FI-MW systems for on-line

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dissolution/decomposition of solid samples have been used since early publications as an efficient way to offer more reliable and rapid forms for analyte detection. The principles, basic instrumentation, applications and current research are provided.

5.2

Microwave heating

Most of the solid samples which require elemental analysis are not soluble in water or inorganic solvents because of the nature of the inorganic substances or carbohydrates, proteins, lipids and other constituents of the geological, agricultural, metallurgical or biological materials. Even liquid samples need some treatment, although under milder conditions. Conventional methods for sample mineralization, such as dry and wet ashing procedures, involve heating of the sample for extended periods of time in open flasks over flames or hot plates or in PTFE-lined stainless steel closed vessels. The time-consuming procedures, the errors due to losses of volatile elements, the use of strong mineral acids and the failure to completely dissolve residues, are some of the problems that occur with these methods. One of the most revolutionary aspects and modern alternative in sample preparation is the use of MW heating as a source of intense energy for rapid mineralization of liquid and solid samples when combined with a suitable chemical reaction. T M The reduction on the digestion time and the higher reaction speed claimed by the users of MW devices with analytical purposes, have two possible explanations: the energy transfer is improved by MW and the MW field has a specific chemical influence on organic molecules in acidic media. 13 MW is an electromagnetic energy with a frequency in the range 300-300 000 MHz corresponding to wavelengths between 1 m and 1 mm. Experimentally, it is produced in a magnetron by altemating electric and magnetic fields. ~1'~3 Microwave energy found widespread use in the development of domestic ovens which defrost, heat and cook food very fast. Commercial ovens use an energy of 2450 MHz and generate a power between 60 and 900 W. Polar molecules align with the electric field which is changing its direction 2.45x l 0 9 times s -1. So, the heating in microwave ovens is accomplished by the mechanical stress induced by aligning and realigning the polar molecules, a process which produces heating through frictional effects with the mechanical agitation and rupture of the surface layers of the solid material, exposing fresh surfaces to acid attack. Molecular motion is governed by the migration of ions and rotation of dipoles. In other words, the heating will be affected by the nature of the molecules present, the concentration and mobility of the ions (size and charge), type of solvent and physical properties (viscosity, density, etc.) of the solution. It is expected therefore that higher temperatures will be gained in acidic or any ion-rich solution than in pure water, while non-polar solvents will be transparent to microwave radiation. The dielectric liquids heated in contact with the

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dielectric particles will generate the heating of the surface molecules of a sample. This can create large thermal convection currents which agitate and destroy the surface layers. Thus the heating in a microwave oven is not produced by an external source of heat but by interaction between the microwave radiation and the sample molecules. The rapid uptake of radiation throughout the sample solution eliminates the heat conduction stage and the sample/acid mixture acts as a lossy dielectric in which an internal heating is produced as a consequence of the mechanical stress induced by the alignment of the polarized molecules. However, this technique had a late acceptance in the chemical laboratories, probably due to various factors: the lack of communication between users and manufacturers, the little knowledge about MW fundamentals and properties of the processed material as well as the understanding of the difference between the mechanism of energy transfer under the MW field and the conventional heat transfer by conduction, radiation and convection. ~2'~3 A physical model studied the MW energy transfer and correctly predicted the heating temperature of reagents and the time necessary to heat a reagent up to a given temperature in the presence of a MW field. 21"22 In fact, different reagents do not absorb the same amount of energy under the same operating conditions because of their different heat capacities and densities. Early works in acid dissolution with microwave radiation as a heat source was accomplished in-batch by using closed- and open-chemically inert digestion vessels. Actually, commercially available microwave dissolution systems include dissolution vessels with pressure-relief valves, turnable Teflon units and a capping station. '!'23'24 The digestion of samples for their analysis by FI-AS methods could be carried out both in batch 24-27and in flow systems. ~4-~8The microwave oven (MWO) is incorporated into the FI manifold to allow the automatic mixing of the sample with the adequate acids, to pump the mixture through the oven and finally collect the digest in open vessels for subsequent analysis by any analytical technique (off-line detection) or directly couple the oven outlet to the instrument (on-line measurements). According to the location of the MWO in the system, there are two types of manifolds described in the literature so far: before and after the injection unit. ~5 The next two sections of this chapter will describe the recent applications of such systems.

5.3 On-line microwave sample mineralization with discontinuous detection For most of the AS techniques, the complete digestion of the sample should be unnecessary provided that reproducible recoveries are obtained for the element under determination. Throughout this chapter the terms digestion and mineralization will be used to describe the application of microwave energy in promoting the dissolution and

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decomposition of inorganic and organic materials. It should be pointed out that the different authors have used either of these terms and therefore it was necessary to choose arbitrarily either term in this chapter. Sometimes, the digest is needed for the analysis of a number of analytes by different techniques; coupling the pretreatment system to a single instrument is not advisable since most of the information is lost. With this purpose, a robotic MW digestion system was used for the dissolution of titanium dioxide containing samples. 28 The experimental arrangement performs several operations: weighs out samples, adds reagents, carries out the digestion, dilutes and transfers solutions and finally cleans the digestion vessels. The dissolution process was followed by the determination of up to 40 major and minor elements by different analytical techniques. In order to accommodate powdered samples that require much longer digestion time, the flow may be interrupted for a period of time while the sample is in the operating oven, resulting in stopped-flow digestion. 29-3~ Such a prototype system was developed by Karanassios et al. for the mineralization of botanical and biological reference materials with the subsequent collection of the digest for its analysis by ICP-OES. 29 A sample plug was pumped into the center of the coil located inside the oven leaving about 50 cm of air on both ends of the tubing. During digestion, the sample slowly rotates inside the coiled tube either clockwise or anticlokwise due to differences developed on either side of the sample plug, which never extended outside the microwave cavity. This rotation served as a stirring mechanism and also helps to reduce the effects of non-uniform heating due to "hot spots". The oven was modified by placing an electric fan on its side to vent hot air during operation and to help cool the tube at the end of a digestion. The major feature of Karanassios's work was that it minimized the risk of sample cross contamination when digesting multiple samples simultaneously, it offered additional safety protection to the oven cavity and personnel by limiting exposure to hazardous acid fumes and reduced reagent handling with quantitative digestion times of 2 min. Elemental recoveries for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Pb and Zn in the digest were about 100%. Problems were encountered however during short digestion times for the recovery of A1, but these were overcome by prolonged irradiation to increase the recovery close to 100%. The efficiency of the digestion procedure depended upon the residence time of the sample plug in the microwave cavity and nitric acid or its mixtures composition. The system does however have shortcomings such as: aging of the digestion tube, the accumulation of undigested material (which may give rise to memory effects), the presence of undigested residues of the organic material (thus, an extra time-consuming and labor-intensive filtration step is needed) and the necessity to cool the digested samples prior to analysis due to the absence of a cooling device. These problems were overcome by cleaning the digestion tube regularly and the periodic replacement of corroded tube, T pieces and valve joints.

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Gluodenis and Tyson also described a FI system which incorporated an on-line stoppedflow microwave-heated reactor for the dissolution of slurries prepared from biological (horse kidney), food (cocoa powder) and environmental (coal) samples. 3~ This work introduced further improvements to a previously developed system where a resistively heated oven was used as irradiation heating source for the dissolution of cocoa powder slurries and subsequent determination of Cu and Fe by flame atomic absorption spectrometry. 3t In this instance, the authors chose microwave energy to control the pressure generated inside the reactor and the energy influx into the sample due to a more specific and directly focused heating into the components of the digestion matrix. In this system, slurry samples were injected into the manifold and transported into a glass reactor mounted inside the oven. Nitric acid was flushed into the reactor, which was then sealed, heated using an appropriate microwave program, pressurized (with the reaction column headspace being vented via a series of PTFE valves) and cooled. After venting, the reactor's contents were flushed out into a calibration flask and diluted to volume in order to be analyzed by both AAS and ICP-OES. A comparison of the results showed no significant difference (95% confidence limit) for the trace element (Fe, Zn, Cd and Mg) contents of horse kidney certified material and cocoa powder (for which the results were compared with those of two other digestion procedures). The determination of calcium in both sample types was found to be problematic because blank values were erratic and digestion of an aqueous calcium standard in the manifold yielded inconsistent results; this behavior could be the result of interaction between the calcium in the sample and the borosilicate glass column occurring at elevated temperature and pressure. Another limitation of the proposed system was encountered in the digestion of bituminous coal due to an incomplete digestion of the sample which was prepared and digested in the same manner as the horse kidney and cocoa powder. It was concluded that the acid leach was unsuccessful, with varying concentrations of the trace metals remaining bound to the undigested silicate material. This limitation of the system might possibly be overcome through the use of an alternative material for construction of the digestion column and a more appropriate acidic digestion mixture. Particularly, the use of PTFE tubing is not possible since the high pressures achieved would result in rupture. Another limitation of the Gluodenis and Tyson methodology is that the manifold requires a certain degree of routine maintenance in order to function efficiently and occasionally the magnetron must be shut down to avoid over-pressurization. Results also illustrate some advantages of this method: it eliminates the need for merging acid streams, the column provides suitable headspace for the expansion of nitrogen oxides and the digestion column and related valves are mounted on a polystyrene backing so that it is free-standing inside the microwave cavity. The commercial success of ICP-MS in analytical instrumentation has had a significant

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impact on chemical analysis worldwide and its coupling to automated, computercontrolled devices for on-line samples processing is shown to enhance the powerful capabilities of the technique. A FI design already reported by Haswell and Barclay 32 was successfully applied as an alternative sample preparation for environmental lead determination using isotope dilution ICP-MS. 33 The sample, (leaves, air filters, urine, sludge, dust and paint) introduced as a slurry is microwaved in a long digestion coil (30 m) in the presence of an acidic mixture (HC1On/HF/HNO3) and the digest collected. The computer controlled commercial system, Spectra Prep (from CEM Corp., Matthews, N.C.) has been modified to include a waveguide cavity for digestion and filters for removing undissolved particles. The system requires two autosamplers, one for sampling and one for collection and also the use of internal standards for accurate analysis of powdered solid samples. Moreover, both, the continuous flow MW device and the ICP-MS are amenable to direct coupling since they both utilize autosamplers with similar flow rates and sample volumes. Another simple approach was used for the digestion of food samples for the sequential determination of aluminum by ETAAS. 34 Sample and nitric acid, simultaneously injected through an injector conmutator were merging-zone mixed before entering a reaction coil wrapped around an Erlenmeyer flask filled with water (dummy load to prevent damage of the magnetron). The digests were collected in the autosampler cups. The performance of the method was evaluated by determining A1 in shellfish, which typically contains abundant organic matter. The recovery for aluminum was only about 90%, indicating incomplete matrix decomposition and which entailed using a correction factor in order to determine the real amounts of analyte in the real samples. However, other authors 35 centered their interest on the total destruction of the silicate matrix and all mineral species comprising sediment samples, in order to ensure the measurement of the total trace element content. Although a strong acidic mixture (HNO3, HC1 and HF) was used and clear digests were obtained, a fine residue was observed in the bottom of the autosampler collection tubes on standing. The fact that aluminum was not fully recovered from standard sediments (only 68% of the certified value), suggests that the residue is due to undissolved alumina. As these FI-MWO systems were not directly coupled to the instruments, in neither case were there problems associated with pressure changes; no special devices were necessary for degassing and the methods permit the sequential and economical treatment of different types of samples. To avoid the digest manipulation prior to the analyte measurement, totally on-line systems are preferred. There was an extremely rapid expansion in the field of on-line sample digestion, as proven by the recent literature. The procedures embodied in these publications were readily and further automated. This involves: (a) the partial automation of the first stage of the analytical process, which is the accurate measurement

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of the sample mass (slurry preparation) and (b) automation of the development of the analytical reaction (digestion) and the transport of the reacting plug to the detector without human intervention. The use of such FI procedures are described in the following section.

5.4 On-line microwave sample mineralization with continuous detection The choice of the analytical technique to be coupled to any treatment system depends on the concentration of the element sought and the sensitivity required. FAAS and ICPO E S 36-44 a s well as volatile compounds formation (mercury vapors and hydrides) followed by AAS detection, 4s-52 naturally working with a continuous supply of sample are well suited for on-line connections. The relative lack of sensitivity of FAAS, restricts its use to Ca, Mg, Fe, Cu, Zn and sometimes Mn; advantages like speed, simplicity, low cost and ease of operation of the FAA spectrometers are ideal and necessary in many situations. ETAAS however is preferred when lower concentrations are to be determined. The late development of on-line systems might be attributed to the difficulty in coupling a discrete sampling technique like ETAAS to a continuously flowing system. Once efficient interfaces have been designed, the two techniques could be efficiently and harmoniously combined. 5-s-59 This coupling converts the experimental arrangements into automatic approaches from sampling through the measurement and data processing. The first applications were directed to liquid samples such as blood, urine and waters, but, the recent literature shows increasing tendencies in solving problems related to solids decomposition. The degree of dissolution in the MW cavity is governed by three variables: MW power, tubing dimensions (length and diameter) inside the MWO and the acid slurry strength. At certain power level an increase in gas formation is observed, the sample plug undergoing fragmentation, which on cooling recombines to produce the original sample with minimal dispersion. The presence of a gas phase however, has been found to be essential for the digestion of samples; although the mechanism is not yet clear, it appears that the presence of gas, liquid and solid phases in narrow bore tubing under the influence of heat from MW power is significant and is recommended to form the basis of a more fundamental study. 5.4.1

F A A S a n d I C P detection

The earliest work reported in this field was by Burguera et al. who described a simple configuration for the analysis of metals (Fe, Cu and Zn) in blood serum, plasma or whole blood by FAAS. 36 The sample and acid mixture were simultaneously injected and merged synchronously to mix downstream in a Pyrex coil located inside the microwave oven. The

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pumping rate and the length of the reaction coil were responsible for the residence time of the sample in the oven, while the boiling of the mixture and so the disruption of the flow profile was simply controlled by optimizing the oven power and exposition time. The great advantage of this technique over the previous batch methods was that it minimized the problems of acid fumes generated during the digestion of biological samples in the vessels used in such analyses. Solid samples call for more sophisticated systems to rapidly destroy organic matrices. A first attempt aimed at simplifying the digest manipulation was reported in 1988. 37 Lyophilized, finally ground and weighed samples of liver and kidney were placed in test tubes together with mineral acids and the contents shaken before exposing to MW radiation to avoid violent reaction with abundant foaming formation. The tubes were located in a covered Pyrex jar inside a domestic MWO operated over certain time at a given power; the acid fumes evolved were removed via a water aspirator jet. The whole arrangement was connected to a vacuum-based suction system which selectively introduced each digest in a closed FI system, from which, an aliquot was selected and injected into a carrier and transported to the FAAS detector. Although this was not a truly on-line approach, it had the great advantage of showing the possibility of mineralization of solids followed by the on-line manipulation of the digests. Although, good precision (ca. 2-6% RSD) and good recoveries (97-103% for Zn and 96-98% for Cd) were achieved with a reduced risk of sample contamination, the operating procedure was rather complicated due to an increased number of steps. In this work, the importance of the effects of sample composition, amount of sample, volume of acid, microwave power and irradiation time in order to reduce the presence of residues in the transmision tubes which could retain some of the analyte, was recognised. Complete automation of the analytical process, from laboratory sampling to instrumental measurement was possible with the introduction of the samples as stabilized slurries. 38-44 In order to produce an uniform slurry, homogenization and size fractionation are required. Slurries are usually prepared by accurately weighing an appropriate mass of the finally ground and sieved (< 180 txm) solid and adding dilute HNO3 to obtain a percentage slurry in the range 0.005-0.5%. 32 The selection of this percentage is dictated by the elemental concentration in the particular sample, but the lower limit is chosen to give an acceptable minimum mass (no less than 25 mg) while the upper limit (no more than 1%) is chosen to avoid blockage in the small diameter tubing used throughout the system. It is not just the mass of the sample taken that influences precision, but also the wettability or dispersion of the solid as a slurry that affects the results. In general, the particle size of the dispersed solid sample must be less than half of the inside diameter of the tubes employed to transport the samples in order to avoid clogging of the system and the use of a stirring to ensure the homogeneity of the solid in the acid during the period

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of analysis. The slurries are maintained stable by magnetic 32'38-41 or ultrasonic stirring. 37 The performance of these kind of systems can be improved if a more stable suspension of the sample powder is produced. This can be readily achieved by altering the surface tension of the slurry solution through the addition of modifiers such as surfactants. 42 In this way, the agglomeration of larger particles of the sample and interactive electrostatic forces in the system are minimized with an improvement of precision and accuracy of the analysis. Due to the fact that some particulates visible by eye may always be present, some authors placed a back-flush filter in the system to protect the back-pressure regulators from possible blockage. 32 The geometry of the tubing inside the MWO is also important. Knotting of the tubing was found not to be advantageous because it led to local heating of the solid material and eventual blockage of the tubing, as it become physically stuck in the tight curves of the knot. The best geometry was found to be when it was wound around the carousel designed conventionally for holding the digestion bombs :9"32 or wrapped around an Erlenmeyer flask filled with water. 34 The length of the mineralization tubing varies considerably depending upon sample type and bigger internal diameter was found to give a longer residence time in the cavity. 32 Various devices including several injection valves, 3~ back-flush filters, 32 back pressure regulators, 29'32"33 antifreeze cooling baths or a gas diffusion cell 55-5s have been tested to avoid erratic flows and dispersion effects. Other configurations include ice chambers 39-41 or an antifreeze bath cooled by Peltier devices. 32,33 Carbonell et al. have initiated the determination of metallic elements in solid samples using the slurry approach coupled with microwave oven digestion in a FI system for FAAS determination of lead 38 and copper and manganese. 39 Samples of different nature (artichoke, diet chocolate, sewage sludge, tomato leaves), real and certified, were slurried in a mixture of nitric acid and hydrogen peroxide by magnetic stirring in a beaker located on a hot-plate, followed by continuous pumping around an open recirculating system, part of which (120 cm PTFE tubing) was located in a domestic MWO. No modifications to the oven were necessary since the tubing was passed through the existing vent holes. An icebath, which was part of the recirculating loop outside the oven, was used to de-gas the sample. When the sample had been completely digested, a sub-sample trapped in the valve loop was injected into a single line manifold for delivery to the FAA spectrometer. A double valve configuration also allowed the injection of standards during the sample preparation period. The accurate determination of lead in sewage sludge was employed as a test system for the proposed on-line sample digestion manifold. 38 Good precision (ca. 2% RSD), sensitivity (4.0• 10-~ txg g-~) and detection limit (0.2 Ixg g-t in the diluted sample or 0.034 mg of Pb per gram of dried sample, for a sample mass of 150 mg) were obtained with reduced sample handling. The maximum sampling rate was limited by the

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digestion time and varied between 12 and 60 samples per hour. Also the limit of detection, for a confidence level of 95%, corresponded to 0.07 mg 1-! for copper and 0.05 mg 1-I for manganese in the diluted sample or 3.5 Ixg g-~ of copper and 2.5 txg g-~ of manganese in the solid sample, for a sample mass of 200 mg dispersed in a volume of 10 m l . 39 The system was further modified by the incorporation of two rotary valves in order to inject simultaneously, acidified slurry sample and concentrated hydrogen peroxide. 4~ A relatively short digestion coil (0.1 m) was incorporated in the system for the accurate determination of Cu and Mn in plant, dietary and sludge samples by flame atomic absorption spectrometric analysis. A special interface which helps degassing the digestion was designed to couple the FI manifold with the FAAS nebulizer. The development of this interface made possible the on-line degasification and cooling of digested samples providing a sample frequency of 180 injections per hour. The effect of some experimental parameters: digestion mixture composition, digestion time, reaction coil length, aspiration and carrier flow rates, injection volume and of the amount of dispersed solids was evaluated in detail. Later, Haswell and Barclay 32 described a FI system for on-line microwave digestion of slurries samples for the determination of Mg, Ca, Zn and Fe in organically based reference samples (Figure 5.1). The slurries were agitated in batch by a magnetic stirrer for approximately 30 s, and an aliquot of approximately 2-3 ml was taken using a syringe to fill the 1 ml sample loop. The digestion conditions or degree of dissolution in the microwave cavity were governed by four variables, namely, microwave power, acid slurry strength, time of irradiation and geometry of tubing in the microwave oven cavity. In order to minimize heating of the oven components by continuous use, 90% of power was selected. Also, an acid strength above 5% nitric acid was used to improve the digestion of samples and therefore recovery values. The sample irradiation time was optimized basically based on the internal diameter and length of tubing; 0.8 mm i.d. tubing was found to give a longer residence time in the cavity than 0.3 and 0.5 mm i.d. tubing with no blocking effects or loss in sensitivity. Total digestion of samples was possible by using tubing lengths greater than 20 m. The most interesting features of this work have been: the use of a more efficient cooling device (a 5 m cooling loop in an antifreeze bath cooled by six Peltier devices) and out gassing and bubble control devices. A back-flush filter was fitted in the place of the injection loop of an injection valve to protect the back-pressure regulator (coupled directly to the nebulizer of the spectrometer) from possible blockage, and to prepare for the likelihood of sample residual material. The use of a pressure regulated device offers an efficient control on bubble formation which occurs due to boiling and the release of gaseous components from the decomposition of a sample matrix. Elemental recoveries were typically found to be in the range 94-107% with a sample preparation and analysis time for ten replicate samples of approximately 0.5 h for

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Figure 5.1 Schematic diagram of the on-line microwave digestion FI system developed by Haswell and Barclay (reproduced with permission of the Royal Society of Chemistry32). the method described compared with 2 h for the same number of samples prepared by the microwave bomb digestion technique. Results are also shown for the optimization of an automatic FI system that combines MW digestion with FAAS and ICP-OES detection for the determination of heavy metals (Zn, Cu, Pb, Cd, Ni and Cr) in slurried sewage sludge. 4~ The slurry, prepared by suspending 100-150 mg of solid in nitric acid, flows through a 3 m long PTFE capillary tube placed inside a conventional MWO operated at 662 W for no less than 5 min. The synergistic combination of the decomposition and measurement techniques made it possible to measure the heavy metals in one single injection, thus reducing the overall time of the analysis, besides taking advantage of increased sensitivity and extended dynamic range provided by ICP-OES. Both units were linked by means of an injection valve. During the first step (digestion), about 0.5 ml of the sample is circulating in a closed flow

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MW heating system with a volume of about 6 ml, and in the second step (measurement), the whole previous circuit was used as an injection loop, making it possible to carry out three replicate determinations for each element from a single initial injection. The experimental data (% recovery vs. digestion time) have been fitted to a mathematical model in order to quantify the influence of each of the variables studied. In comparison with the conventional methods of sewage sludge analysis, the proposed procedure is less time consuming, while being equally precise and accurate. A robotic station controlled by a robot was used to fully automate a method for the determination of some metals (Fe, Cu and Zn) in soil samples by FAAS. 43A focused MW digester was included as a module of the robotic station; the digestion step was shortened from 2 h to 3 min, when compared to the traditional digestion by stirring. After digestion, the sample was transported to the spectrometer either after on-line dilution by the robot or by aspiration to feed a valve-loop where dilution is adjusted as required by the concentration of each element in the samples. The sampling frequency with the available equipment was 5 samples h-~. This performance can be improved by using more than one digester, which is one of the most advantageous features of the proposed method. The benefits of automated systems have also been demonstrated in an on-line microwave-assisted mineralization manifold designed for in vivo sample uptake of whole blood samples (Figure 5.2). The samples were drawn and at the same time pumped directly from the patient's forearm to a timed injector, which is automatically controlled to inject the sample-acidic solution-anticoagulant mixture into the carrier stream. For the determination of Cu and Zn by FI-FAAS, 44 the carrier stream of an acidic solution flows through the microwave oven and a gas diffusion cell into the nebulizer of the spectrometer. In either case, the precision of the measurements varied from 2 to 6% RSD and the analytical recoveries of inorganic zinc and copper that were added to pooled whole blood samples averaged 98.2 and 97.3%, respectively. 5.4.2

Volatile compounds formation

Hydride generation in FI systems proved to have many advantages over the conventional approaches, such as smaller sample size, higher sample throughput, better tolerance to interferences, improved absolute detection limits, lower consumption of reagents and ease of automation. 45 However, transforming the elements into an appropriate oxidation state for hydride generation as well as the pretreatment aimed to destroy the organic matter are rate-limiting steps in this type of analysis. On-line MW digestions are ideally suited for FI-HG systems and have been extensively used to shorten the pre-detection steps. On-line MW sample pretreatment found widespread application in the determination of hydride forming elements (As, Sb, Se, Sn, Pb) probably due to the well-known problems related to their volatility and adsorption losses during extensive pretreatment procedures. 45-48MW

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.

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Figure 5.2 Schematic diagram of the FI-microwave oven-ETAAS manifold for in vivo sample uptake of blood samples. It-17, tubing length; VI-V4, valves of the time-based solenoid injector; RI, mixing tube; R2, sample/reagent entrapment tubing; I, valve injector; SAA, sampling arm assembly (reproduced with permission of the Royal Society of ChemistrySS). energy accelerates the analyte conversion from its form bound to the organic matrix to a free inorganic oxidation state. The general procedure consists of mixing the liquid samples with an appropriate reagent (acid) which is carried to the MWO and the resulting digest mixed with sodium borohydride to form the respective hydride which is swept into the quartz cell of an AAS detector. A bromination mixture (bromate/bromide) and persulfate in acidic media proved effective for the determination of arsenic, antimony, tin and lead. 45 An increase in sensitivity by a factor of 2 or 3 was noticed for tin and antimony; the recovery for lead however, from (1 +2) diluted urine samples was only 52-84% as compared to standard solutions. There was a necessity to calibrate by standard additions, in order to account for the variations in the foaming pattern and pH adjustment of individual urine samples. The detection limits were: 0.5 lug 1-1 for arsenic, 0.07 lug 1-~ for bismuth and 0.1 lug 1-! for tin. Recoveries of tin (76-100%) and of lead were low (52-84%) due to problems with pH control in narrow intervals. Hence, the studies on lead were not carried on further and tin was only determined using the standard addition method. Due to the previously given reasons, the precision for the determination of tin in real samples increased up to 10%

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RSD, which is higher when compared with the values obtained for the determination of the other species (typically below 4% RSD). A fully automatic procedure based on an on-line MW acid digestion and pre-reduction of As(V) for HG-AAS detection, was developed for measuring inorganic arsenic levels in blood samples. 46 The merging zone technique was used to save digestion-prereduction reagent (HNO3/HC104/L-cysteine), while the efficiency of the digestion process was increased by the use of a flow restrictor, installed downstream, which built-up a pressure of about 6 Bar. Complete organic matter destruction was indicated by the absence of foam formation during the hydride generation in the presence of NaBH4. L-cysteine was found to be superior to potassium iodide as pre-reductor because milder acidic conditions are required and the analytical response in blood samples and standards are essentially identical, making possible the use of simple standard calibration in the range 0-30 I~g 1-!. A sampling frecuency of 7-10 h - ' , a limit of detection (30" of the blank) of 0.25 Ixg 1-! for a 500 Ixl sample volume and a recovery of 94--98% for 10 Ixg 1-i As(V) added to a blood sample were accomplished. Another FI-HG-AAS with on-line MW digestion was used to determine lead in wine, beverages and fruit slurries. 47 The plumbanne was generated from H N O 3 / H 2 0 2 medium using sodium borohydride as reducing agent. As no matrix interferences were found; the method enabled the determination of lead in untreated samples using an aqueous calibration graph. It was also found that MWs increased the selectivity and the hydride generation efficiency by ca. 50%. The detection limit was 10 txg 1-! in wine and other beverages and 1.0 ng in fruit; for fruit, the plumbanne was generated from slurries of the fresh samples. A similar approach has been developed for the determination of total arsenic in urine and geothermal water samples. 48 An appropriate volume of sample (0.1-2.0 ml) mixed with acids (H2SO 4 and HNO3) and an oxidizing agent (K2S208) is subjected to MW heating. The cooled digest was mixed with a prereductant (KI and ascorbic acid) and sodium borohydride to form volatile arsine which was detected by FAAS in a quartz cell. The method allowed the determination of total arsenic with a detection limit of 0.2 ng of arsenic, a precision (RSD) of 2.5-4.5% and recovery values in the range 10 1-102%. It is well known that mercury enters the environment through a number of natural and industrial processes and it is found as several chemical forms which exhibit different toxicity. Despite the need for speciation, most of the analytical work, done on mercury determination following MW heating of the samples refers to total mercury. 49-52 Such is the case that total mercury was determined in water and urine 49 whole blood 5~and slurried solid samples 5~'52 by FI cold vapor (CV) AAS after on-line MW treatment. Although developed in different laboratories, the manifolds are very similar: the sample plug mixed with an oxidizing agent is carried to the MWO which is part of the FI system; mercury

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vapor is generated with sodium tetrahydroborate 49'5~ or tin chloride 5~'52 and the analyte detected by C V - A A S 49" 50 or CV-AFS. 51'52 Tsalev et al. have initiated the on-line microwave sample pre-treatment for the determination of Hg in urine and environmental waters by CV-AAS. 45"49 The system is based on a focused microwave oven and a manifold with two coils (Figure 5.3). A reaction coil and a ballast-load coil are placed and oriented vertically within the cavity and chimney of the oven in a manner that provides a reliable long-term operation and increased reaction times (46 s) and irradiation times (e.g., 6.3 s at a carrier flow rate of 8.5 ml min-1). Samples are mixed in batch with an appropriate oxidation reagent and loaded on an autosampler. Thereafter, all further operations of sample uptake, digestion, hydride or vapor generation, amalgamation preconcentration (optional for Hg determination), all measurements, calibration and data processing are performed automatically within 90--402 s (depending on the sample volume and the instrument program used for a given determination). Particularly, mercury was determined directly without amalgamation and with amalgamation in a gold gauze device. When the amalgamation accessory Iniect S C

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Figure 5.3 Schematic diagram of the manifold and instrumental set-up for on-line microwave digestion and mercury determination using CVAAS. P1 and P2, peristaltic pumps of FIAS-2000 system; P3, lsmatec peristaltic pump; L 1, sample coil; L2, reaction coil; L3, 'dummy load' coil; L4, MHS reaction coil; MWD, microwave digestor; MHS, mercury/hydride system manifold; GLS, gasliquid separator; F, filter; AA, amalgamation accessory; S, sample; C, carrier; R, reductant; W, waste; and QAC, quartz absorption cell (reproduced with permission of the Royal Society of Chemistry45).

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was used, a PTFE filter holder with a PTFE micropore membrane filter was placed between the gas-liquid separator and the quartz absorption cell to avoid the entrance of aerosol droplets in the amalgamation accessory. 49 In either case, much attention has been given to the optimization of the manifold (reaction time, irradiation time, microwave oven power setting, post-microwave temperature, digestion mixture and composition of reagent. Some problems were encountered with peak broadening and loss of peak height sensitivity due to dispersion but these were compensated for by the introduction of hot samples, thus improving sensitivity. However, overheating of samples (i.e. irradiating at higher power settings) caused problems with the decomposition of some less stable hydrides and aerosol formation with a reduction in precision. The limits of detection were 0.01 and 0.2 ~g 1-1 for mercury, with and without amalgamation, respectively. In the work described by Guo and Baasner 5~a method for the determination of mercury in whole blood by FI-CVAAS was described. After dilution of the sample with Triton X100 (1 : 1), to improve the fluidity of the sample solution, and addition of an oxidant, all further treatment and measurements were performed automatically, on-line. Diluted whole blood samples containing oxidation reagent were injected into the carrier stream via the FI valve. The carrier stream was then mixed with acid in the manifold, and the mixture flowed through the digestion coil located inside the microwave oven and was heated by microwave energy. On leaving the microwave oven the heated mixture cools in the cooling coil and is then mixed with a KMnO 4 and thereafter with a NaBH 4 solution to further decompose organic mercury compounds in the sample and to reduce Hg(II) to Hg(0), respectively. The digestion and cooling coils were made in a knitted form in order to reduce dispersion. The cooling coil was sealed inside a plastic tube through which cooling water was circulated using one of the pumps. As previously reported by Tsalev e t a/., 45"49 here also were problems such as aerosol formation and vapor evolution due to the violent reaction between the heated sample, carrier and the reductant within the manifold and the gas-liquid separator. One of the problems with the measurements was the production of foam, which can enter the quartz cell and interrupt the measurement of Hg by the CV technique. Therefore, the addition of an antifoam reagent to the reductant solution was necessary. Five mercury-containing samples were used for the recovery test, among them sterile bovine, human and lyophilized human reference whole blood samples. The accurate determination of mercury was possible with a precision of 6-7% RSD, a detection limit of 0.1 p~g 1-1 in the diluted samples and a sample throughput of 45 measurements/h. In order to protect the magnetron, a constant load for the oven was provided by a continuous flow of cooling water (3.7 ml min -1) which absorbed ca. 10-20% of the incident power. The same authors have published technical note reports on the evaluation of a PerkinElmer Flow Injection Mercury System (FIMS) for the determination of mercury in blood 51

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and in sewage sludge, sediment and soil samples. 52 The FIMS consists of two peristaltic pumps, one to propel the carrier and reagents and the other to fill the loop of the sampling valve, an autosampler, a focused MWO, a cooling device and the common generation system connected to a quartz cell located in the optical axis of the specrometer. Comparison with an off-line digestion procedure and with a direct procedure using amalgamation confirmed the need to digest the blood samples in order to obtain accurate results. The comparison showed that on-line digestion was superior to the off-line method with regard to sample throughput, sample consumption and total measurement time. The detection limits were as low as 5 i~g 1-1 for sample volumes of 500 txl and RSD of 2% (n= 3) at 10 Ixg 1-! Hg level. A stabilizing solution of potassium dicromate in nitric acid was used in both cases, but sodium borohydride and tin chloride were used as reducing agents for blood and the digested solid samples, respectively. Morales-Rubio e t al. 53 have described a 2-line system for the determination of mercury in environmental materials (sewage sludge, polluted farmland soil and lake sediment) using on-line microwave digestion and atomic fluorescence spectrometry. The acidified slurry samples (400 txl) were injected into the carrier stream, passed through the PTFE digestion coil (4 m) located inside the microwave oven (at 10% power level) for a time of irradiation of 50 s and after, through an ice/water bath, to avoid possible mercury loss in the degassing trap (located between the ice bath and the T-connector for the reductant stream). The reductant stream merged with the carrier and the mixture passed through a reduction coil before entry to the gas liquid separator and atomic fluorescence detection. Much attention was given to optimize the variation of some key system parameters in real samples, such as: the influence of digestion time, microwave oven power settings, reaction coil tubing length, carrier flow rate, sample volume and nitric acid strength. This system allowed the accurate determination of mercury in certified environmental reference materials with a precision ranging between 0.5 and 1.5% RSD, a limit of detection of 0.09 ng g-~ and a throughput of 15 samples per hour. An on-line MW digestion system was also described by Lamble and Hill 54 for the determination of total mercury in environmental solid samples. The slurried samples were injected into a carrier of hydrochloric acid and brominated mixture before passing through the coil situated in the MW cavity. After mixing with hydroxylammonium chloride to remove excess of bromine, mercury was determined in an on-line generation system. Each sample was processed in 50 s with a detection limit of 13 ng 1-~. The performance of the proposed procedure was comparable with a batch method that uses a mixture of hydrogen peroxide and nitric and sulfuric acids for digestion in an open focused MWO. The fully enclosed systems used so far for volatile compounds forming elements reduce potential losses by volatilization during the delay between sample digestion and analysis, which is particularly important at the levels these elements are generally found in the

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biological and environmental samples studied. Safety is also improved by avoiding the open manipulation of hazardous oxidizing agents. 5.4.3

E T A A S detection

On-line MW sample treatment followed by ETAAS detection of the analyte has scarcely been used. The very precise timing which is characteristic of the FI systems, guarantee the perfect synchronization of a continous flow with the discontinous furnace program, this being the success of the analytical applications of such hybrid combinations. The first on-line systems have been developed by Burguera and Burguera. 55-59 They produced studies involving the determination of lead in biological materials 55 and of titanium dioxide in soaps. 56 These publications illustrate the rapid advances in technology for FI microwave systems. For the determination of lead in biological tissues, acidic slurry samples were homogeneously dispersed in a Triton X-100 solution with an ultrasonic device. Mineralization of samples was accomplished with an acidic solution in a PTFE coil located inside the MWO. The dissolution and mineralization of the soap samples also occur inside the microwave oven in a stream of an admixture of sulfuric and nitric acids. The confinement of the sample plug, within carbon tetrachloride plugs, minimized the sample dispersion, and a gas diffusion cell device allowed an efficient degassification of the mineralized samples (Figure 5.4). In both cases fully automated computer monitored manifolds were described; the design and operation of the automated on-line methodology followed by on-line analysis by ETAAS were discussed. Precise volumes of mineralized samples, collected in a capillary of a sampling arm assembly, were introduced by means of positive displacement with air through a time-based solenoid injector into the graphite furnace. After solving the initial problems of solid sample introduction into the FI system, the optimization of the manifolds was carried out in three stages; (a) the digestion/ dissolution conditions, (b) FI parameters, and (c) detector conditions. Each of the three steps are described in detail and an in-depth profile of optimized experimental conditions are given. The furnace and the spectrometer autosampler used for the matrix modifier introduction were programmed to coordinate with the operations performed in the flow

Figure 5.4 Sample confinement principle used to control sample dispersion (reproduced with permission of VCH Publishers, Inc.56).

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J. L. Burguera and M. Burguera

system. In either case, the precision of measurements was between 1.8 and 3.5% RSD with detection limits of 0.8 pg Pb (0.1 p~g 1-~) and 80 pg Ti (4 ng ml -~) Although the accuracy for SRM biological tissues was good, the lead content in pine needles and olea Europaea was slightly lower than certified values, suggesting that more drastic mineralization conditions should be used when botanical materials are under evaluation. A continuous procedure has been used for the microwave-assisted mineralization of adipose tissue prior to the automated determination of iron and zinc by ETAAS. 57 Mineralization of samples was accomplished with a mixture of sulfuric and nitric acids and by the alternative exposition of the sampling unit in the irradiated zone of a focused MWO. Additional flows of Triton X-100 and Pd-Mg matrix modifier were introduced to avoid detrimental accumulation of solids on the wall of the tubing and to minimize matrix interference effects, respectively. On the other hand, the introduction of the additional stream of the surfactant lowered the temperature of the mineralized sample; thus, the use of an additional cooling device was not necessary. The mineralization coil inside the oven was wound along a PTFE arm in such a manner that only 0.1 m of the tubing was coiled vertically along the PTFE arm with the stream entering at the lower end. The oven was alternatively turned on and off ten times for periods of 1.0 and 5.0 s to ensure the mineralization and homogeneity of dissolved samples in the acidic solution, which flowed in a closed-flow system. As in previous works, 55'56 a gas diffusion cell permitted degassification of the mineralized sample. Selected aliquots of the digests were introduced via a sampling arm assembly, by means of air dispositive displacement, into the graphite tube atomizer. The effects of acid type and concentration, sample amount, microwave oven conditions, matrix chemical modification and addition of Triton X-100 were studied to allow accurate and precise (3.2 and 2.3% RSD for Fe and Zn, respectively) results. When the amount of sample exceeded 40 mg, longer times were required to complete the sample dissolution process and difficulty was observed in the flowing pattern of the acid solution under study. The irradiation time varied inversely proportional with the mineralization coil length and carrier flow rate. More rapid dissolution was obtained in the order sulfuric >nitric > hydrochloric acid. The detection limits were of 20 and 30 pg Fe and Zn, respectively. There are some practical problems to overcome when working with adipose tissue. It is difficult and cumbersome to place and to weigh wet and lyophilized samples in the sampling unit, which consisted of a PTFE home-made-like pipette tip. An on-line automated MW-assisted mineralization manifold was designed for in-vivo uptake of whole blood samples for the determination of cobalt (Figure 5.2). Volumes of 20 Izl of the mineralized samples were introduced by means of positive displacement with air through a time-based solenoid injector into the graphite tube atomizer. 58 The spectrometer autosampler, used for the introduction of the chemical modifier and the furnace program were re-programmed to synchronize with the operation of the flow

Flow injection systems for on-line sample dissolution/decomposition

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system. Cobalt in whole blood was accurately determined with a precision of 2.6-3.1% RSD and the detection limit was of 0.3 Ixg 1-~. The following general prevailing observations were made: (a) the residence time of the sample-reagent mixture in the oven must be optimal to achieve satisfactory mineralization, and therefore, to obtain accurate results; (b) higher carrier flow rates provided more precise results; (c) longer tubing length tended to provide less precise results; and (d) too short a coil length produced less accurate results. It is well known that sulfur anions, have a depressing effect in the iron ETAAS response due to background absorption. A FI-MW-ETAAS system was recently used to determine iron in geothermal fluids containing high amounts of dissolved sulfate and sulfide ions. 59 The sulfate interference was minimized by using lutetium as a chemical modifier. The presence of sulfide ions still deteriorates the precision, therefore are precipitated with hydrogen peroxide in a knotted reactor exposed to MW radiation to aid the precipitation process. The knotted reactor promoted radial mixing of sample and reagent providing reproducible conditions for the precipitation and also acted as a collector of the precipitate. The sulfate and iron-containing sample is flowing downstream; a portion of it is sequestered in a sampling arm for the sequential deposition of fixed aliquots onto the graphite platform by means of positive air displacement. Meanwhile, the sulfur precipitated is dissolved on-line with carbon tetrachloride and the liquid diverted to waste. This sequence was synchronized with the spectrometer computer which was preprogrammed to introduce aliquots of Lu-modifier previous to each sample deposition. For the less sensitive iron line (296.7 nm) the linear range was from 12 to 280 txg 1-~, with a characteristic mass of 104.8 pg and a detection limit of 72 pg (3.6 Ixg 1-I). The precision for ten consecutive measurements was around 3% and the accuracy was checked by recovery tests and comparison with an independent analytical method. 5.4.4

Speciation studies

Most of the speciation MW-aided on-line studies are seldom limited to aqueous samples, probably because of the problems related to changes between species during the treatment of solids. Although some tentative attempts appeared in the literature, dealing with this subject, 6~ more work has to be done to ensure the quality of the results and the accuracy of the treatment procedures. The speciation capabilities of on-line FI-MW systems coupled to different separation techniques have been exploited for the determination of some species of toxicological importance. 61"62 Lbpez-Gonz/dvez et al. 63 have described an on-line HPLC-microwave oxidation HGAAS coupled four-channel system for the selective determination of arsenite, arsenate, dimethylarsinate (DMA), monomethylarsonate (MMA), arsenobetaine (AsB) and arsenocholine (AsC) in environmental samples (mineral water, sewage-water, harbor sea-water,

J. L. Burguera and M. Burguera

156

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Figure 5.5 HPLC-microwave oxidation-HGAAS manifold for arsenic speciation developed by Lbpez-Gonzhlvez et al. 63 (reproduced with permission of the Royal Society of Chemistry63). synthetic fish extract and sediment extract) (Figure 5.5). Samples and multi-standard solutions were injected in an anionic cartridge leading to a 100 txl sample loop, placed before an HPLC anionic column. Arsenite, arsenate, MMA and DMA were quantitatively retained in the anionic cartridge while AsB and AsC passed through it into the sample loop and are separated into the chromatographic column. The column effluent mixed with the oxidizing solution flowed to the digestion coil in the microwave oven for decomposition. The solution from the microwave oven was cooled in an ice-bath and a T-junction was used to acidify the sample. In a second T-junction, the digested sample was mixed with the reductant; the resulting solution, containing volatile arsine, flowed to the gas-liquid separator where the liquid phase was drained off and the gas phase entered the quartz atomization cell. The anionic species can be separated and determined by removing the anionic cartridge from the system and introducing water instead of the oxidizing agent. After optimization of the chromatographic and microwave-oxidation parameters, the RSD were 3-5% with detection limits between 0.3 and 0.9 ng for all species (for 100 lxl sample volume) and conversion efficiency close to 100% in either case. The novel coupling of anionic cartridges to HPLC column was of great interest and made possible the determination of the six arsenic species without the chromatographic overlapping of arsenite and AsB. A similar procedure for arsenic speciation was developed by Le e t a l . 64 except that they used a dual system to avoid removal of the cartridge during the procedure. Comparable resolution was obtained by using HPLC-separation-MW-digestion with HGAAS or ICPMS detection. Complete separation of five arsenicals was achieved on a reversed phase C,8 column by using sodium heptanesulfonate as ion pair reagent and the detection limits were for example 10 Ixg l-~ for arsenite, DMA and AsB, 15 Ixg l- ' for MMA and 20 Ixg l- • for arsenate. However, instead of oxidation, the prereduction of all arsenic species to arsenite with Lcysteine, after HPLC separation of an enzymatic extract from solid biological samples, has the benefit of obtaining the same response on HG, apart from the fact that the formation

Flow injection systems for on-line sample dissolution/decomposition

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of the hydride from As(Ill) is more efficient than from As(V). 65 A 9 m knitted coil was used inside the MWO to reduce dispersion. A power of 50 W asured recoveries of 103% for 30 ~g 1-~ AsB, suggesting that the digestion was complete in 23 s. Although no species-specific certified SRM was available, the system has been validated using samples for which the total arsenic content is known, using a mass balance approach. Simple system modifications allow the determination of total arsenic or of total reducible arsenic species in the sample. The determination of total and toxic arsenic in urine with and without persulphate, respectively, was acomplished by quantitatively converting all the arsenic species to arsenate in the M W O . 66 Persulfate in basic media was used to completely convert arsenicals to arsenate. The microwave oxidation system developed is a good alternative to photo-oxidation and thermo-oxidation for the decomposition of the organoarsenicals AsB and AsC. An automatic and on-line cryogenic trapping system and MW heating was used for the determination and speciation of arsenic in aqueous samples by FI-HG-AAS using sodium tetrahydroborate as reductor. 67 The separation of the species was based on a pH selective procedure: arsine from As(III) alone was generated in the presence of citric acid while nitric acid was used to generate the corresponding arsines from total inorganic arsenic, MMA and DMA. They were cryogenically trapped in a PTFE coil knotted and sealed inside another wider diameter tube located in a MWO and through which liquid nitrogen was suctioned by negative pressure. Based on their different dielectric constants, the arsines were selectively liberated by using a heating cycle of MW radiation with volatilization times of 90, 180 and 235 s for dimethylarsine, monomethylarsine and arsine, respectively. The detection limits were in the range 20 to 60 ng As for a 10 ml sample volume, with the possibility to improve it by increasing the sample size or running several consecutive reactions. In a recent study, a method was optimized to determine arsenobetaine (AB) in canned seafood products by coupling HPLC, MW-assisted oxidation and HGAAS. 6s The real samples were cut into pieces and frozen at - 2 0 ~ freeze-dried, crushed and homogenized in a water-cooled mill to obtain a fine powder. About 2 g of the lyophilized sample was mixed with methanol-water and the extract evaporated to dryness, redissolved in HC1 and the pH adjusted to less than 2. The resulting solution was passed through a strong cation exchanger; the ABsorbed was eluted with phosphate buffer at pH 5.0 and mixed on-line with persulfate (prepared in NaOH) before entering the 1.6 m long PTFE tubing located in the MWO operated at 1100 W for 4 min. The thermo-oxidized effluent, cooled in an ice-bath was also on-line introduced in a continuous HGAAS system. An additional oven load of 400 ml water was necessary to avoid overheating of the system. Quantifications were made by the method of standard additions using arsenite standards

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for calibration. A clean-up procedure made possible the isolation of AB from other arsenical species. The detection limit was 0.68-27.20 ng As g-t (fresh mass), the relative standard deviation ranged from 0.4 to 6% and the recovery was 104 + 5%. In the case of selenium, the hydride generation process require the reduction of Se(VI) or any of its organic forms to Se(IV) which is able to react with NaBH4 and form Sell2. Some on-line speciation procedures used hydrochloric acid and the MW radiation to aid the reduction step 69--76 The sample was first analyzed for Se(IV), then heated with HC1 to reduce Se(VI) and analyzed for total Se using HG-AS techniques 69-75 or FI cathodic stripping voltammetry 76 as detectors. These methodologies were applied to the speciation of inorganic selenium in certified aqueous samples, 69'70 citric fruit juices, 71 spiked tap water, 72"74'75 urine 73 or food samples. 76 Although these approaches enabled any selenium(IV) to be determined directly, most of these procedures still relied upon a difference calculation to provide the concentration of any Se(VI) present in the sample (Figure 5.6). A brominated mixture proved equally efficient for on-line pretreatment of selenium compounds (selenite, selenate, selenomethionine, selenoethionine and selenocysteine) for the final detection of total selenium. 73 The system allow speciation of inorganic selenium by simple on/off operation of the focused MWO connected to the flow system. The fast conversion of selenium compounds studied into volatile species could be considered as a sort of specific "general" detector for selenium compounds which, can be extremely useful in combination with chromatographic separations. Indeed, such systems have been further developed to chromatographically separate the inorganic selenium species 7~75 or some organoarsenicals. 72'74 Both inorganic species are retained in the ion-exchange column and then are selectively eluted with potassium sulfate TM or sequentially eluted with formic (for Se(IV)) and

Ar

Figure 5.6 On-lineHPLC MW oxidation and MW reduction for the speciation of Se(VI), Se(IV), and trimethylselenium with HG-AAS detection (reproduced with permission of Springer-Verlag72).

Flow injection systems for on-line sample dissolution/decomposition

159

hydrochloric (for Se(VI) acids). 75 Each effluent is acidified before flowing through the MWO where Se(VI) is reduced to Se(IV), suitable to undergo hydride generation. AFS used as detector 7~75 offers excellent sensitivity for selenium. The limits of detection were 0.2 and 0.3 ng for Se(IV) and Se(VI) respectively, with a precision (RSD) in the order of 1.5 to 2.0% for both species. An anion exchange column was used to separate first trimethylselenium (TMSe) and then Se(IV) and Se(VI), followed by the MW-induced thermooxidation of the former in the presence of persulfate and on-line determination of the element by HG-AAS. Thermoreduction of Se(VI) to Se(IV) was accomplished by heating with HC1 in a domestic MWO. The method has been applied to tap water with recoveries closed to 100% and absolute detection limits of 1.1, 1.4 and 2.2 ng for TMSe, Se(IV) and Se(VI), respectively. 72 An HPLC-MW digestion-HG system coupled on-line with three atomic detectors (AAS, ICP-OES and ICP-MS) has been designed and investigated for selenium speciation and determination. TM Total inorganic selenium, selenomethionine and selenoDL-ethionine are separated by reversed-phase chromatography prior to on-line MW digestion of selenocompounds. The digestion is accomplished with a bromination mixture (HBr/KBrO3) to continuously form Se(IV) which is transformed into hydrogen selenide with a merging flow of sodium borohydride. This system, allows, in a single injection, reliable speciation of selenoaminoacids in urine, versus total inorganic selenium. Further speciation of the overlapped inorganic Se(IV) and Se(VI) peaks is accomplished by a second injection of the urine sample to determine only Se(IV) by avoiding MW heating. Best detection limits were obtained with ICP-MS as detector. Burguera e t al. 71 have recently described a FI system for the selective determination of Se(IV) and Se(VI) in citric fruit juices and geothermal waters by HGAAS with microwave energy aided heating prereduction of Se(VI) to Se(IV) (Figure 5.7), The samples and the prereductant solutions which circulated in a closed-flow circuit were injected by means of a time-based injector. 77 This mixture was displaced by a carrier of an acidic solution through a PTFE coil located inside the focused MWO and mixed downstream with the reductant solution to generate the hydride. The separation of the two phases was completed in the gas liquid separator and the resulting vapor was carried into the quartz atomization cell. In this work, the introduction and mixing of sample and prereductant solution are performed automatically in three sequences: a filling, injection and measurement operations. The continuous flow of a reductant solution ensured a stable baseline and eliminated the problems of varying blank levels. The effect of reagent concentrations, sample volume, tubing length, carrier gas flow rate, microwave settings and Se(IV)--Se(VI) weight ratio was carefully studied in order to allow the selective determination of both species in real samples. The detection limits were of 1.0 txg 1-1 for

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Se(IV) and 1.5 Ixg 1-~ for Se(VI). The sample throughput was about 50 measurements h-1 with no dead sample volume, a precision of ca. 2.0-2.5% RSD and recoveries of 96-98% for Se(IV) and 94-98% for Se(VI). This on-line system reduced sampling handling and exposure to the environment, it reduced to a minimum the sample and reagents waste and made complete automation of the different selenium species determination possible. There was also published an interesting approach for speciation of mercury in solid samples which is based on continuous MW-assisted pervaporation followed by AFS detection. TM Pervaporation consists of combining evaporation of the analyte or its volatile reaction product from the sample matrix (liquid or solid) with gas-diffusion through a membrane to a static or flowing acceptor solution. The sample (diatomaceous earth spiked with the mercury compound) was transferred to the donor chamber of the pervaporation cell which, after being tightly closed, was located in the MWO and then connected to the loop of a selection valve. This allowed the carrier gas to either pass through the pervaporation unit or via a by-pass to the detector so the baseline did not change. The reagents for vapor generation (HC1 and SnCI2) were injected directly in the donor chamber via a GC septum and allowed to react for 1 min. Then, the MWO was switched on for a pre-set time after which the cell was allowed to cool down. The selection valve was switched to divert the carrier gas through the acceptor chamber of the pervaporation cell

Flow injection systems for on-line sample dissolution/decomposition

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taking the mercury to the detector. Moderate MW heating was found to increase sensitivity by easing the diffusion of the mercury vapor through the membrane. The procedure allowed speciation of inorganic/organic mercury due to the fact that SnC12 only reduces inorganic mercury. For total mercury determination, a strongly oxidizing agent like Br-/BrO/ along with H2SO4/CuSO4/K2S208 was studied with good recovery values. 5.5

Conclusions

This chapter has attempted to give a comprehensive review of the historical development and current status of flow injection systems for on-line dissolution/decomposition of samples with emphasis on their performance in terms of reliability, sample and reagent consumption, degree of flexibility and automation capabilities. Table 5.1 summarizes the literature concerning on-line microwave oven sample pretreatment of samples. Most of the method's validation relied on the analysis of certified standard reference materials. Other methods include the analysis of the sample material by another, comparative technique and recovery studies, particularly in the cases where certified standard reference materials are not available. Optimal conditions for microwave digestion in FI systems depend on the sample weight and composition, volume of digestion reagents, reaction temperature, pressure, irradiation time, etc. However, the flexibility of the new generation of computerized instruments will permit the rapid development of software which selects the optimized procedure as a function of sample matrix and the analytical method to be followed. Although mixtures of acids remain the most popular by a large margin for specific purposes, attention must be given to large-scale evolution of acid fumes which may cause pressure build up with serious deteriorations in the flowing pattern of the streams. One of the most important aspects to consider when developing new microwave enhanced methodologies is safety, therefore, a careful selection of the chemistries employed and the use of joints and plastic/glass fittings in the microwave field should be avoided as these are often a weak point where bursting can take place. 79It is clear from this chapter that on-line treatment of samples has a tremendous potential, providing efficient methods of dissolution and decomposition of inorganic and organic materials. The main advantages over conventional wet- and dry-ashing and other in batch procedures are speed, sample and reagents economy and significantly improved contamination control due to the simplicity of the procedures in closed systems. However, it is unlikely that there will be significant lowering of detection limits, particularly in view of the fact that the sample must be mineralized in extra volumes of acidic components. Future studies should involve simultaneous multi-element, selective microwave solvent extraction, the removal of some of the matrix components before preconcentration procedures and selective determination of different species, which may also include on-line automated sample treatment with

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--._

. 0,,,,,

0

r~

~~'~ -

~ .=-~

° ,,,,, 0

z

_

~

8,-,

~-~

.... ¢~

~

__ 0

0

.-~ 0

r,t l 0 0 .,.., r~ 0

0 r,.

'e 0 °,..~ m

o r~ . ,...,

0 . ,,,,,

8 ,,,,~

~.~ ._o . n

~.~

".E

Table 5.1

Continued Analyte

Sample

Validation

Technique

Water and urine

Hg

SRM

CVAAS

Whole blood

Hg

SRM,C

CVAAS

Reasons for FI format

A, with amalgamation A, OL-FTMW

Sewage sludge, polluted farm land and lake sediment Dogfish muscle, marine harbour sediment Sewage sludge

Hg

SRM

AFS

Hg

SRM,C

CVAFS

Nebulization of digested slurry A

Hg

SRM,

Biological (whole blood, bovine muscle, pig kidney, bovine liver) and botanical (Olea Europaea, pine needles) Soap

Pb

SRM

Pervap., AFS ETAAS

Pervaporation aided by MWO A

Ti

C

ETAAS

Adipose tissue

Fe,Zn

R,C

ETAAS

Manifold

OL-3-LMFMO OL-4-LMFMO OL-2-LM

Detection limit

Precision (% RSD)

Sample throughput (h -I)

Ref. ,.q m

O

60--100 pg

<4

40

49

0.1 ixggl -t

6-7

45

50

o p,,,,~ ~

0.09 ng g-i

0.5--1.5

15

53

4-lines, OL-FMO OL-DLM

13ngl -I

1.5

NS

54

NS

<3

NS

78

OL-2-LM

0.8 pg

2.5-3.0

10

55

0

t~

O ! i,,,d 9

Whole blood

Co

SRM

ETAAS

Water (mineral, sewage, harbour sea), synthetic fish extract and sediment extract

Organo-arsenicals

R

HPLCHGAAS

HPLCHGAAS HGAAS

A, use of confinement principle A

OL-3-LMFMO with recirculation OL-3-LMFMO with recirculation 4-LM

80 pg

1.8-3.5

12

56

20,30 pg

3.2, 2.3

!0

57 t~ 0,,~.

6.1 pg

2.1-3. I

0.3-0.9 ng, CD

3-5

A,S

OL-4-LM with FTMWD for cationic species OL-3-LM

0.5-6.0

0.5-3

NS

65

t~ 0 N

A,S

4-LM

3.0-7.0

90

66

O

OL-FMO selective volatilization OL-I-LM

4--6 ixg 1-I CD 20-60 ng 1-~

1.5

NS

67

0.68-27.2 ngg -t

0.4-6

NS

68

A,in vivo sample uptake S

Dogfish, Lobster hepatopancreas Urine

As species

SRM

As species

R

Waters

As species

R

HGAAS cryotrapping

A,S

Dogfish muscle, Canned seafood

AsB

SRM, C

HPLCHGAAS

A,S

20

58

6-10, CD

63

O 0..t.

O

_~.

(table continued on p. 164)

k~

o~

T a b l e 5.1

Continued

Sample

Certified waters Citric fruits, geothermal Waters Tap water

Analyte

Se(IV) Se(VI) Se(IV), Se(VI) Se(IV), Se(VI), TMSe

Validation

Technique

Reasons for FI format

Manifold

Detection limit

Precision (% RSD)

SRM

HPLCHGAAS HGAAS

A,S

OL-FMO reduction 4-LM

0.2, 0.3 ng ml1.0,1.5 p,g I-' 1.4, 2.2, !. I ng

R

A,S

R

HPLCHGAAS

A,S

Tap water, urine

Inorganic Se, Selenoaminoacids

R

HPLC-HGwith three detectors

A,S

Milk, sausage, pig kidney

Se(IV) Se(VI)

SRM

Cathodic stripping voltametry A,S

OL-3-LM, FMO reduction

OL-I-LM, FMOoxidation OL-3-LM FMOD

NS

Sample throughput (h-')

Ref.

1.5-2.0

NS

70

2.0-2.5

50

71

< 10

NS

72

<5

NS

74

20

76

0"~ 6.8 FAAS 30 ICP-OES O. 16 ICPMS (for Se(IV) in I.t.g 1-i 2

Abbreviations: 1. Validation: SRM standard reference material, C comparison with other method, R recovery test, IS internal standard 2. Technique: ICP-OES inductively coupled plasma atomic emission spectrometry, FAAS flame atomic absorption spectrometry, CVAAS could vapor atomic absorption spectrometry, HGAAS hydride generation atomic absorption spectrometry, AFS atomic fluorescence spectrometry, ETAAS electrothermal atomic absorption spectrometry, HPLC high performance liquid chromatography. 3. Reasons for Fl format: LSV limited sample volume, FTMWD flow through microwave digestion, A automation, OL on-line, S speciation. 4. Manifold: I-LM single line manifold, 2-LM doble line manifold, 3- or 4- line manifold. FMO focused microwave oven. 5. Detection limit: STD sample type dependent, ED element dependent, CD compound dependent. 6. Detection limit, precision and sample throughput: NS, not stated in the paper.

Flow injection systems for on-line sample dissolution/decomposition

165

ultraviolet-digestion, ~9 ultrasonic irradiation, 2~ and electrochemical dissolution processes, t5 Thus, coupled ion-selective membranes, 8~ capillary zone electrophoresis, 82'83 electrodyalisis and liquid-chromatography 84 techniques with FI-AS systems will not be uncommon to the scientific community. The successful interfacing of a robotics microwave digestion system for sample dissolution 28'43 has already been shown to have significant advantages in terms of efficiency and it is likely that in the next few years we will see a major role of robot sampling systems interfaced with on-line FI sample dissolution manifolds.

5.6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

References

Adams, M. J., Chemometrics in Analytical Spectroscopy, RSC, London, 1995. Hurst, W. J. (Ed.), Automation in the Laboratory, VCH, New York, 1995. Stockwell, P. B. and Corns, W. T., Automatic Chemical Analysis, Taylor & Francis, London, 1996. Fang, Z., Flow Injection Atomic Absorption Spectrometry, Wiley, Chichester, 1995. Burguera, J. L. (Ed.), Flow Injection Atomic Spectroscopy, Marcel Dekker, New York, 1989. Ruzicka, J. and Hansen, E. H., Flow Injection Analysis, John Wiley, New York, 2nd edn, 1988. Burguera, J. L. and Burguera, M.,J. Anal. At. Spectrom., 1995, 10, 473. Burguera, J. L. and Burguera, M.,J. Anal. At. Spectrom., 1997, 12, 643. Fang, Z., Xu, S. K. and Tao, G., J. Anal. At. Spectrom., 1996, 11, 1. Tyson, J. F., Spectrochim. Acta Rev., 1991, 14, 169. Kingston, H. M. and Jassie, L. B. (Eds.), Introduction to Microwave Sample Preparation. Theory and Practice, American Chemical Society, Washington, 1988. Kingston, H. M. and Haswell, J. S. Microwave Enhanced Chemistry, ACS, London, 1997. Zlotorzynski, A., Crit. Rev. AnaL Chem., 1995, 25, 43. Burguera, M. and Burguera, J. L., Quim. Anal., 1996, 15, 112. Zi, Z. L., Rios, A. and Valcarcel, M., Crit. Rev. Anal. Chem., 1996, 26, 239. Smith, F. E. and Arsenault, E. A., Talanta, 1996, 43,~ 1207. Matusiewicz, H. and Sturgeon, R. E., Spectrochim. Acta, 1996, 51B, 377. Burguera, M. and Burguera, J. L., Anal. Chim. Acta, 1998, 366, 63. Golimowski, J. and Golimowska, K., Anal. Chim. Acta, 1996, 325, 111. S~inchez, J., Garcia, R. and Mill~in, B., Analusis, 1994, 22, 222. Feinberg, M. H., Ireland-Ripert, J. and Mourel, R. M., Anal. Chim. Acta, 1993, 272, 83. Suard, C., Mourel, R. M., Cerdan, B., Bart, G. and Feinberg, M. H.,Anal. Chim. Acta, 1996, 318, 261. Feinberg, M. H.,Analusis, 1991, 19, 47. Matusiewicz, H. and Sturgeon, R. E., Prog. Analyt. Spectrosc., 1989, 12, 21. de la Guardia, M., Salvador, A., Burguera, J. L. and Burguera, M., Jr. Flow Injection Anal., 1988, 5, 121. Rondon, C., Burguera, J. L., Burguera, M., Brunetto, M. R., Gallignani, M. and Petit de Pefia, Y., Fresenius J. Anal. Chem., 1995, 353, 133. Burguera, J. L., Burguera, M., Matousek de Abel de la Cruz, A., Afiez, N. and Alarcbn, O. M., At. Spectros., 1992, 13, 67. Norris, J. D., Preston, B. and Ross, L. M., Analyst, 1992, 117, 3. Karanassios, V., Li, F. H., Liu, B. and Salin, E. D., J. Anal. At. Spectrom., 1991, 6, 457. Gluodenis, T. J. and Tyson, J. F., J. Anal. At. Spectrom., 1993, 8, 697. Gluodenis, T. J. and Tyson, J. F., J. Anal. At. Spectrom., 1992, 7, 301. Haswell, S. J. and Barclay, D.,Analyst, 1992, 117, 117. Beary, E. S., Paulsen, P. J., Jassie, L. B. and Fassett, J. D., Anal Chem., 1997, 69, 758. Arruda, M. A. Z., Gallego, M. and Valcarcel, M., J. Anal At. Spectrom., 1995, 10, 501. Sturgeon, R. E., Willie, S. N., Mathven, B. A., Lam, J. W. H. and Matusiewicz, H., J. Anal. At. Spectrom., 1995, 10, 981.

166

J. L. B u r g u e r a a n d M. B u r g u e r a

36 Burguera, M., Burguera, J. L. and Alarcbn, O. M.,Anal. Chim. Acta, 1986, 179, 351. 37 Burguera, M., Burguera, J. L. and Aiarcbn, O. M., Anal. Chim. Acta, 1988, 214, 421. 38 Carbonell, M., de la Guardia, M., Salvador, A., Burguera, J. L. and Burguera, M., Anal Chim. Acta, 1990, 238, 417. 39 Carbonell, V., Morales-Rubio, A., Salvador, A., de la Guardia, M., Burguera, J. L. and Burguera, M., J. Anal. At. Spectrom., 1992, 7, 1085. 40 de la Guardia, M., Carbonell, V., Morales-Rubio, A. and Salvador, A., Talanta, 1993, 40, 1609. 41 Bordera, L., Hernandis, V. and Canals, A., Fresenius ,1. Anal. Chem., 1996, 355, 112. 42 Rond6n, C. E., Burguera, M., Burguera, J. L., Brunetto, M. R. and Carrero, P., J. Trace Elements Med. Biol., 1995, 9, 49. 43 Torres, P., Ballesteros, E. and Luque de Castro, M. D.,Anal. Chim. Acta, 1995, 308, 371. 44 Burguera, J. L., Burguera, M. and Brunetto, M. R., At. Spectrosc., 1993, 14, 90. 45 Tsalev, D. L., Sperling, M. and Welz, B.,Analyst, 1992, 117, 1735, 1729 46 Welz, B., He, Y. and Sperling, M., Talanta, 1993, 40, 1917. 47 Cabrera, C., Madrid, Y. and C~imara, C., J. Anal. At. Stectrom., 1994, 9, 1423 48 Burguera, J. L., Burguera, M. and Danet, A. F., Review Roum. Chim., 1998, in press. 49 Welz, B., Tsalev, D. L. and Sperling, M.,Anal. Chim. Acta, 1992, 261, 91. 50 Guo, T. and Baasner, J., Talanta, 1993, 40, 1927. 51 Guo, T. and Baasner, J., J. Autom. Chem., 1996, 18, 2 i 7. 52 Guo, T. and Baasner, J.,J. Autom. Chem., 1996, 18, 221. 53 Morales-Rubio, A., Mena, M. L. and McLeod, C. W., Anal. Chim. Acta, 1995,308, 364. 54 Lamble, K. J. and Hill, S. J., J. Anal. At. Spectrom., 1996, I 1, 1099. 55 Burguera, J. L. and Burguera, M., J. Anal. At. Spectrom., 1993, 8, 235. 56 Burguera, M. and Burguera, J. L., Laboratoly Robotics and Automation (LRA), 1993, 5, 277. 57 Burguera, J. L., Burguera, M., Carrero, P., Rivas, C., Gallignani, M. and Brunetto, M. R., Anal. Chim. Acta, 1995, 308, 349. 58 Burguera, M., Burguera, J. L., Rond6n, C., Rivas, C., Carrero, P., Gallignani, M. and Brunetto, M. R., J. Anal. At. Spectrom., 1995, 10, 343. 59 Burguera, J. L., Burguera, M. and Rond6n, C., VII International Conference on Flow Analysis, Piracicaba, Brazil, August, 1997. 60 Rond6n, C., Burguera, J. L., Burguera, M., Brunetto, M. R., Gallignani, M. and Petit de Pefia, Y., Fresenius J. Anal Chem., 1995, 353, 133. 61 Burguera, M. and Burguera, J. L., Talanta, ! 997, 44, i 581. 62 Trojanowicz, M., Ann. Assoc. Bras. Quim., 1996, 45, 158. 63 L6pez-Gonz~ilvez, M. A., G6mez, M. M., C~imara, C. and Palacios, M. A., ,I. Anal. At. Spectrom., 1994, 9, 291. 64 Le, X.-C., Cullen, W. R. and Reimer, K. J., Talanta, 1994, 41,495. 65 Lamble, K. J. and Hill, S. J., Anal. Chim. Acta, ! 996, 334, 261. 66 L6pez-Gonz~ilvez, M. A., G6mez, M. M., C~imara, C. and Palacios, M. A., Mikrochim. Acta, 1995, 120, 301. 67 Burguera, J .L., Burguera, M., Rivas, C. and Carrero, P., Talanta, 1997, 45, 531. 68 V61ez, D., Yb~ifiez, N. and Montoro, R., ,I. Anal. At. Spectrom., 1997, 12, 91. 69 Pitts, L., Worsfold, P. J. and Hill, S. J.,Analyst, 1994, 119, 2785. 70 Pitts, L., Fisher, A., Worsfold, P. J. and Hill, S. J.,J. Anal. At. Spectrom., 1995, 10 519. 71 Burguera, J. L., Carrero, P., Burguera, M., Rond6n, C., Brunetto, M. R. and Gallignani, M., Specrochim. Acta, 1996, 51B, 1837. 72 Cobo-Fern~indez, M. G., Palacios, M. A., Chakraborti, D., Quevauviller, P. and C~imara, C., Fresenius ,I. Anal. Chem., 1995, 351,438. 73 Gonz~ilez LaFuente, J. M., Fernfindez S~inchez, M. L., Marchante-Gay6n, M., S~inchez Uria, J. E. and SanzMedel, A., Spectrochim. Acta, 1996, 51B, 1849. 74 Gonz~ilez LaFuente, J. M., Fern~indez S~inchez, M. L. and Sanz-Medel, A., J. Anal. At. Spectrom., 1996, 11, 1163. 75 Bryce, D. W., Izquierdo, A. and Luque de Castro, M. D., ,1. Anal. At. Spectrom., 1995, 10, 1059. 76 Bryce, D. W., Izquierdo, A. and Luque de Castro, M. D., Analyst, 1995, 120, 2171.

Flow injection s y s t e m s for on-line s a m p l e d i s s o l u t i o n / d e c o m p o s i t i o n

167

77 Carrero, R, Burguera, J. L., Burguera, M. and Rivas, C., Talanta, 1993, 40, 1967. 78 Bryce, D. W., Izquierdo, A. and Luque de Castro, M. D., Anal. Chim. Acta, 1996, 324, 69. 79 Williams, K., Applying Flow Methodology to Microwave Enhanced Reactions, Ch. 6, Elsevier, Amsterdam, 1997. 80 Futher, F., Ross, B. and Cammann, K., Anal. Chim. Acta, 1995, 313, 83. 81 Wong, P. S. H. and Cooks, R. G.,Anal. Chim. Acta, 1995, 310, 387. 82 Li, K. and Li, S. F. Y., Analyst, 1995, 120, 361. 83 Blanchflower, W. J. and Kennedy, D. G.,Analyst, 1995, 120, 1129. 84 Groenewegen, M. G. M., van de Merbel, N. V., Slobodnik, S., Lingeman, H. and Brinkman, U. A. Th., Analyst, 1994, 119, 1753.

spectrometry

6.1

General

On-line separation and preconcentration methods for atomic spectrometry based on solidliquid phase transfer form an important sub-discipline of flow injection atomic spectrometry, one which has experienced perhaps the fastest development in this field. This is a consequence of the dramatic enhancements in performance of atomic spectrometric methods achieved by such combinations involving relatively simple equipment and expertise. Flow injection on-line solid-liquid separation and preconcentration methods for atomic spectrometry may be classified into the following categories according to the separation principles and media used for retention of analyte or interferent species:

Sorption * packed columns talon-exchanger packing madsorbent packing mother sorbents * knotted reactors 168

Flow injection on-line solid-liquid separation

169

Precipitation and coprecipitation mon-line filters mknotted reactors.

6.2 General principles for the design of on-line solid-liquid separation and preconcentration systems for atomic spectrometry The principles for the design of solid--liquid preconcentration systems, based on different mechanisms (sorption and precipitation) and used for different detection systems (FAAS, AFS, ICP--OES, ETAAS and VG-AAS), may differ in some respects. However, some general concepts may still be identified. These will be discussed in this section, while the more specific principles will be treated in the related sections

6.2.1

Time-based and volume-based sample loading

All on-line solid-liquid separation and preconcentration systems basically involve two main stages of operation, i.e., retaining the analyte or interferent on a solid support, be it a column packing, a filter or knotted reactor (referred to as sample loading), and subsequently eluting the retained species (referred to as elution or dissolution). The sample-loading mode is an important feature of a solid-liquid separation system. For pure separation purposes, as in the case of interference removal, where sensitivity is not of much concern, usually injection of a defined sample volume from a sample loop measuring a fraction of a milliliter will be 9the method of choice, both because of its simplicity and reliability. With volume-based sample loading, the amount of sample processed is determined by the sample loop, and the filled sample is subsequently evacuated from the loop by a suitable carrier. In FI preconcentration systems, the sample volume processed directly affects the achievable enrichment factor, in which case the efficiency of sample introduction into the separation media becomes more important. With such systems samples may still be introduced on a volume basis as performed for pure separation systems but can also be loaded on a time basis. With time-based loading, the amount of sample processed is determined by the sampling flow-rate and sampling time. Volume-based sampling usually consumes more time to load the same volume of sample when compared to the time-based alternative owing to the need of first filling the sample loop and suffers from stronger dispersion effects during transport, particularly when large sample loops measuring a few milliliters capacity are used. The dispersion may be minimized by using sample loops with a knotted configuration or by introducing small air segments at the two ends of the sample zone. However, in any case large-volume sample loops are not convenient to use, and create extra impedance in the separation flow lines.

Z.-1. Fang and G.-h. Tao

170 6.2.2

Kinetic considerations

For all on-line solid-liquid separation and preconcentration systems, the sample-loading sequence is always associated with retention of a chemical species, often containing the analyte on a solid surface. For both time-based and volume-based sample loading using a defined manifold design, the enhancement factor (EF) is directly related to the amount of sample loaded on the collection media, and the loading flow-rate is usually the main factor which determines the sample throughput, i.e. efficiency of the procedure. High loading rates are of course preferable for achieving high EF; however, the maximum loading _. flowrates are limited by kinetic features of the reaction system and retention or collection media. The on-line reaction time in the liquid phase and between the liquid and solid phases is often in the range from a fraction of a second to a few seconds, making the kinetic requirements on solid supports such as column packings much more important than in batch procedures. Excessively high loading rates result in incomplete retention and low mass transfer of the analytes even before the saturation capacity of the retention media has been reached, owing to insufficient contact time, although higher EF may be achieved. With a fixed loading period in time-based sampling, the amount of analyte retained on the retention media achieves a maximum with an increase in sample loading rate (and sample volume), beyond which the retained mass decreases owing to insufficient contact time and reduced phase transfer. Although in FI preconcentration procedures complete retention of the analyte is not a prerequisite, systems with excessively low phase transfer are more sensitive to interferences if they do occur. On the other hand, quite often interferences can be significantly reduced under low retention efficiencies through kinetically discriminating the retention process of the analyte from that Of the interferent. For most on-line collection media, a recommended sample loading rate which may be used as a starting point for further optimization is 4-5 ml/min.

6.3 6.3.1

Preconcentration and separation by column sorption General

Separation and preconcentration using ion-exchange and adsorption columns have been used extensively in the batch mode to improve the sensitivity and selectivity of atomic spectrometric methods for many years. The use of an on-line ion-exchange preconcentration column in a FAAS system based on FI principles was first reported by Olsen et al. 1 The manifold used for this early stage application is quite simple (Figure 6.1), but sufficient to demonstrate its potentials. The system was composed of two injection valves and a micro-column connected in series with the spectrometer detector. The column was packed with a chelating ion-exchanger. A defined volume of sample was loaded into the

Flow injection on-line solid-liquid separation $

E

!

!

171

Golumn P Figure 6.1 FI manifold of a single-channel on-line sorption column preconcentration system for FAAS. S, sample injector valve; E, eluent injector valve; C, carrier; P, pump. sample loop of the sample injection valve, and upon injection this sample volume was transported by a carrier stream through the micro-column where the trace analyte was retained, while the sample matrix passed into the spectrometer, and thence to waste. The retained analyte was then eluted into the spectrometer when a volume of eluent was injected and carried through the column by the second valve. The fast development in this field following this contribution is well documented in several reviews. 2'3 The advantage of on-line sorption column systems is that they are relatively easy to operate, and the equipment is generally more robust, particularly when compared to on-line liquid-liquid extraction systems. However, the flow-impedance of packed columns (particularly with fine particle packings) may create problems in flow stability, unless good quality pumps are used with good care in operational details. The slow kinetic properties of some sorbent materials may also degrade the enrichment performance of on-line sorption column preconcentration systems. The slow kinetics may induce a further limitation of such systems, which is their generally lower tolerance to interfering matrices when compared to on-line liquid-liquid extraction or coprecipitation systems, unless very selective sorption systems are used. Fang 2 has provided some guidelines for system design using a spectrometer with pneumatic aspiration, employing time-based sample loading on columns using peristaltic pumps, a revision of the main points are given here. (a)

(b) (c) (d)

Avoid suction of samples through the column, instead, a good quality peristaltic pump should be used to propel the sample through the pump onto the downstream column. Use the shortest possible pump-tube lengths to change samples rapidly, to decrease sample consumption, and to reduce sorption of analyte on pump tube walls. Avoid discharge of sample waste (effluent) with high matrix concentrations into the spray chamber. Column washing and equilibration can be achieved by pumping sample through the column within a few seconds at the beginning of the loading stage. Introduction of other washing and equilibration agents are usually not required.

Z.-1. Fang and G.-h. Tao

172 (e)

(f) 6.3.2

Reversal of the flow direction through the column during elution in respect of sample loading is important in improving precision by avoiding progressive tightening of column packings and increase of flow impedance. The thinnest and shortest conduits that are practical should be used to transport the eluate to the spectrometer to reduce dispersion.

Column packings

With on-line column separation and preconcentration systems the speed of the FI procedure has to be made compatible with that of the atomic spectrometric determination, in order to justify their feasibility in routine practice. A column must also be stable enough to be used for at least hundreds of sorption/elution cycles. Therefore, the use of column packings exhibiting good chemical and physical stability as well as fast kinetics in relatively selective sorption of the analyte and its release in a suitable eluent is extremely important for achieving good performance. The properties of various sorbents used in on-line preconcentration systems have been reviewed by Fang, 2' 3 and will be mentioned here only briefly. Chelex-100, which has been used successfully in many batch procedures owing to its selectivity to trace metals, and used in earlier applications for on-line preconcentration, was shown to be a non-ideal sorbent for the latter purpose, both owing to its strong swelling and shrinking properties and its relatively slow kinetic properties. Muromac A-1, a Japanese product also with the same imminodiacetic functional group as Chelex-100 exhibited significantly better properties in on-line preconcentration both with AA and ICP detectors. 4-6 The CPG-8Q chelating ion-exchanger, with quinolin-8-01 functional groups azo-immobilized on porous glass is characterized by excellent mechanical and kinetic properties, but relatively low exchange capacity, which limited its use to samples with relatively simple matrices such as natural waters. 3 Ion-exchangers without chelating functional groups are usually not selective enough for on-line preconcentration purposes, although some cationic analytes may be transformed into anionic species (e.g. chloro-complexes) and retained on strongly basic ion-exchangers in the presence of other matrix cationic constituents. Some anionic species may also be preconcentrated using anion exchangers. 7' 8 Sorption of metal complexes of low polarity from the aqueous phase by reversed phase adsorption using non-polar or medium polar sorbents, such as C18, in on-line preconcentration atomic spectrometric systems has demonstrated some advantages over ion-exchange systems. 9 The sorbent was used to retain organic metal complexes with dithiocarbamates (DDC, 9' Io APDCII), and ammonium diethyldithiophosphate (DDPA). 12 The sorbed species were conveniently and readily eluted by ethanol or methanol, and interferences from common matrix elements such as calcium and magnesium are much

Flow injection on-line solid-liquid separation

173

less pronounced than using most of the chelating ion-exchangers. Since the alkaline earth metals are not retained, this is particularly advantageous in ICP-OES applications where strong emission from calcium and magnesium lines can cause strong spectral interferences. The relatively small particle size of CI8 sorbents creates high flow impedance, and while peristaltic pumps are used for sample loading their use is limited to short columns. The advantages of polymeric-based supports over silica-based supports for C~8 have been demonstrated by Lancaster et al. 13 Activated carbon has been used as a sorbent to be pre-loaded with a complexing agent such as oxine followed by metal retention, 14 or to retain metallic complexes directly. ~5'16 Activated alumina has also been employed to sorb trace analytes or their complexes. 17"18 Polymeric adsorbents of the XAD series have been used occasionally in on-line preconcentration systems. For example, polyacrylate adsorbent XAD-8 was used to collect the chloro-complex of gold, and subsequently eluted by ethanol. ~9 Despite the seemingly weaker mechanical properties of fibrous ion-exchange materials, several successful applications on the use of such materials have been reported, including the use of chelating ion-exchangers based on cellulose with EDTrA (ethylenediaminetriacetic acid), HYPHAN [ 1-(2-hydroxyphenylazo)-2-naphthole], and TETPA (triethylenetetraminepentaacetic acid) functional groups, 2~ and rhodamine chelating fiber ionexchanger. 22 Recently, algae and yeast immobilized on controlled pore glass 23-27 have also been employed as sorbents to retain trace metals in on-line column preconcentration systems. However, the selectivity of these materials does not seem to be very remarkable. 6.3.3

On-line sorption c o l u m n p r e c o n c e n t r a t i o n systems f o r F A A S a n d I C P - O E S

The earliest version of such systems was that reported by Olsen et al. ~ for FAAS, which was a single-channel system with volume-based sampling and elution using separate injectors. The disadvantage of the system is the discharge of waste into the spray chamber and the uni-directional flow through the columns. However, its advantage lies in its simplicity. A more efficient system for achieving optimum performance, embracing all the important principles mentioned in the preceding guidelines is shown in Figure 6.2. 28 The system is characterized by time-based sample loading on a micro-column with effluent discharge into waste, reversed-flow elution without washing sequence, and short transport conduits. The system was later slightly modified by Xu et al. 29 to produce higher enrichment factors by introducing small air segments before and after the eluent, i.e., at the sample/eluent interface, which restricted mutual dispersion between the two zones. The dual-column system shown in Figure 6.3 operated in parallel on an eight-channel valve was proposed by Fang et al. for FAAS 3~to further improve the efficiency of on-line

Z.-1. Fang and G.-h. Tao

174

c

Ca)

-.

.

,..,

.

.c

(b)

Figure 6.2 FI manifold of a high efficiency on-line sorption column preconcentration system for FAAS with time-based sample loading and countercurrent elution: (a) sample loading sequence; (b) elution sequence; P1, P2, peristaltic pumps; E, eluent; S, sample; B, buffer/reagent; C, packed conical column; V, injector valve; W, waste. column preconcentration, when high enrichment factors were pursued. Later the system was also used for ICP-OES by Hartenstein et a/. 31'32 with minor modifications. In this system two columns were loaded simultaneously to save time in the loading of large sample volumes, and elution was performed sequentially using double pump operation. An alternative to using double pumps is to use a single pump with an additional two-way valve. 33 A more recent approach for achieving high enrichment factors in FAAS employing dual columns packed with activated carbon fiber was to load the two columns in parallel, but eluting the columns connected in series 34 (Figure 6.4). In contrast to the system in Figure 6.3, where two elutions were made in each cycle, only a single elution peak with a larger enrichment factor, double that using a single column was obtained. For on-line column preconcentration in FAAS and ICP--OES, column capacities of 50--200 Ixl are often used. With such columns sample loading rates in the range of 2-10 ml/min, depending on the kinetic properties and particle size of the packing, and dimensions of the column, are usually employed. The optimum elution flow-rate for achieving high enrichment factors is a compromise between the free uptake of the nebulizing system and the release kinetics of the elution, and usually falls in the range 2-4 ml/min for FAAS, but may be lower for ICP systems, where sample uptake rates are lower.

175

Flow injection on-line solid-liquid separation

P2

____ml/min

,

I

I............................~

W

E1 "------------------".

81 E2

S2

.....................

....................

/.,.///.r

....................

r , / , , / . ~ ,x x ~ x ~ \

W

P1

(figure continued on p. 176)

Figure 6.3 Dual-column FI on-line preconcentration system for FAAS. FI manifold: (a) sample loading sequence; (b) elution sequence for column C1. P 1, P2, peristaltic pumps; S 1, $2; samples; El, E2; eluent; R, buffer solution; V, 8-channel multifunctional valve; C1, C2; packed sorption columns; W, waste flows. The upper waste line in (a) is shown as the lower waste line in (b), owing to the circular arrangement of the eight channels on the valve rotor. Water is introduced into the spectrometer by nebulizer suction. (c) Operational sequence of the pumps and valves; T, valve switching point.

176

Z.-1. Fang and G.-h. Tao

[

i

:

Go

Go l i

Figure 6.3

....

Continued

Typical applications of on-line column preconcentration FAAS systems and ICP-OES systems are shown in Tables 6.1 and 6.2 respectively, together with their characteristic performance data. The term concentration efficiency (CE) proposed by Fang et al., 33 defined as enrichment factor achieved per minute, is used to compare the efficiency of different systems.

6.3.4 On-line sorption column separation and preconcentration systems for VG--AAS and VG--ICP-OES 6.3.4.1 FI-HGAAS and HG-ICP--OES with on-line separation of interferents using ionexchange columns In FI-VG, interferences from transition metals are usually significantly reduced compared to batch procedures; nevertheless, often the tolerance level is still insufficient to meet the requirements of samples with high transition metal contents. Interferences may be removed conveniently by on-line separation of interferents using an on-line column. Micro-columns packed with cation exchangers may be used to retain transition metal interferents while anionic analytes such as selenium and arsenic pass freely through the columns into the vapor generation system. The columns are periodically regenerated. In their FI-VG-AAS systems, Offley et a l . 57' 58 and Tyson et al. 59 incorporated the column in an injector valve as a special form of sample loop, while Marshall and van Staden 6~ connected a column in series with the sample injector downstream before the reaction coil. Although the first system, shown schematically in Figure 6.5 is more complicated, the retained interferents are subsequently eluted to waste without entering the reactor which was not possible with the second. The systems were used successfully for the determination of arsenic and selenium in copper and nickel-based alloys. A similar device with on-line removal of interfering matrix elements was also employed for the determination of arsenic and selenium in copper and cobalt-based alloys by ICPO E S . 61.62

Flow injection on-line solid-liquid separation

177

~

C1

(figure continued on p. 178)

Figure 6.4 Manifold of Dual-column in parallel FI on-line preconcentration system: (a) sample loading; (b) elution; (c) principle diagram of the system. C1, C2, packed sorption columns; S, sample; W, waste; P 1, P2, peristaltic pumps; V, injector valve; E, eluent. The upper waste line in (a) is shown as the lower waste line in (b).

178

Figure 6.4 6.3.4.2

Z.-1. Fang and G.-h. Tao

Continued On-line sorption column preconcentration systems for VG-AAS

Application of on-line column separation and preconcentration systems for vapor generation (VG)-AAS are not as extensive and straightforward as for nebulization FAAS. However, direct connection of an on-line column system used for FAAS to an F I - V G AAS manifold can be achieved readily at least for preconcentration systems involving an acid elution. The acid conditions and flow-rates for elution from the sorbent columns are often similar to, but somewhat larger than those required for the ensuing hydride generation reaction. The required mass for achieving optimum sensitivity is often larger than FAAS systems owing to the relatively large volume of the atomizer and gas-liquid separator. Therefore usually columns larger than 200 Ixl are used for the separation. A dual-column on-line preconcentration VG-AAS system used for the determination of selenium, bismuth and mercury is shown in Figure 6.6. 7 The performance of some typical column preconcentration systems for hydride generation and cold vapour AAS are shown in Table 6.3. 6.3.4.3

On-line sorption column matrix modification for HGAAS

The direct determination of tin by FI-HGAAS poses some difficulties in producing a robust method owing to the well-known critical acidity requirements for generation of stannane, which is even further enhanced under non-equilibrated FI conditions. Fang et al. 67 attempted to improve the robustness of the procedure by modifying the acid matrix of the sample (normally 1-2 mol/l HCI medium) to meet the requirements of the hydride generation reaction (0.01-0.05 mol/1 HNO3 medium) using an on-line ion-exchange

Table 6.1

Typical applications of FI on-line column preconcentration for FAAS

Sample matrix

Analyte/sorbed species .

Hair, tea, peach leaves Estuary, sea waters Rocks Biological, Environmental Water Ores Minerals, milk Solid-waste, sewage, sludge, sediments Waters

.

Co nitroso-R-TBA ion pair Cr(llI), Cr(VI) Pd Zn, Cd, Cu, Pb, -DDP Pb, speciation Pd, Pt, lr Pb-APDC Cd, Cu, Fe, Pb, Zn

Waters

Pt(IV) chlorocomplex Pb

Sea water Standard solutions

Co Cu, Zn, Cd, Pb, Co, Hg

Waters Biological tissues, urine, bovine liver Ores & rocks Biologicals

Cr(IlI), Cr(Vl) Cd, Cu-DDC

Natural, sea water Waters

Column packing .

.

.

.

.

Eluent .

EF*

CE** (/min)

RSD(%)

Detection limit (p.g/i)

Ref.

.

Cj~

Ethanol

40

30

2.4

3

35

CPG-8Q, Muromac A- 1 Activated carbon Ct8

2M HCI/2M HNO 3

23, 61

7,19

5

6,2

36

1M HNO3 Methanol

150 35

38 42

3.9 !-3

0.3 0.5, 0.8, 1.4, 10

37 38--40

Cellex P, etc. a-aminopyridine resin Activated carbon CPG-cyanobacteria/ yeast

1M HNO3/ethanol 0.5M HCI, 0.5M HCLO4, Mg(CIO4)2 IBMK Zn, 0.1M HNO3; Cd, Pb, 0.5 M HNO3; Fe(lll), IM HNO3/0.1M HC1 2M ammonia

100-150 30-50

10-15 20-35

0.17 (Pb) 0.017, 0.009, 0.1 l

41 42

2.

50 21

21 8

5.7 2.9, 1.3, 3.8 3.0 2.6

0.01 0.i (Zn); 0.2(Cd); 8.0 (Pb); 0.7(Cu); 0.6 (Fe(Ill))

43 21,22

0

600

110

9

0.02

44

Activated alumina

/

p..., o

0

,=

Silica gel coated with Aliquat 336/Nitroso-R Modified alumina CPG-cysteine

0.1M HCI

37-100

6

6--12

4-10

45

0.1M NaOH 1M HNO3

300 62-875

40 31291(Hg)

2.3 I. l-! .4

46 47

Activated alumina Cts

1M HNO3, 0.5M NH 3 Methanol

25 126, Cd; 1 14, Cu

23 73, 64

1.1-1.3 1.4-2.3

0.44 0.1 (Cu); 0.2(Zn); 0.8(Co); 30(Hg); 0.1 (Cd); 2.1(Pb) 1.0(Cr(III)); 0.8(Cr(VI)) 0.15, Cd; 0.2, Cu

48 29

0

,.Q

~..~~

0

Au-CI complex Cd, Fe(III), Cr(III), Cu, Mn(lI),Pb, Zn Cu, Cd, Pb

Cts Muromac A-1

Ethanol 2M HNO 3

35 46-96

35 10-21

3-5 0.7-1.47

4

CPG-8Q

2M HCI

25-31

50-62

1.2-1.8

28

Pb, Cd, Cu, Zn

Chelex- 100

2M HCl(dual column)

50-105

50-105

* Enrichment factor. ** Concentration efficiency: EF achieved per minute.

0

19

0.07(Cu); 0.03(Zn); 0.5(Pb);

0.05(Cd)

30

oo

Table 6.2

Typical applications of FI on-line column preconcentration for ICP-AES

Sample matrix High purity reagent Geologicals Natural, sea waters Waters, rain Natural waters Waste water Tap, drinking waters Tap, runoff waters Waters

CE**

Analyte/sorbed species

Column packing

Eluent

EF*

(/min) RSD(%)

La, Ce, Nd

CPG-bacteria

NaOH

36-51

REEs, Yttrium Cr(IIl), Ni

HEH resin IDAEC resin

Cd, Cu, Pb, Zn

CPPI chelating resin

3.5M HNO3 g-15 2M 250 HNO3/HCI 2M HNO3 35-55

12-17 9(La); 0.21(Ce); 0.54(Nd) 2-4 0.05--0.5tzg/g 50 0.05(Cr); 0.14(Ni)

CL-P507 resin Amberlite A-26/mercaptoacetoxycellulose 22 elements PDTC, CPPI chelating resins Ba, Be, Cd, Co, Cu, Mn, Chelex 100 Ni, Pb Cr(Ill), Cr(VI) Acidic alumina La Au

Reference steel PO3 4Standard Bi, Ce, Co, Ni, V solutions Biologicals Cu, Cd

20

0.05(Cu); 00.8(Cu); 0.6(Pb); 0. l(Zn) 0.7 0.8--1.7

I M HCI 25-30 2M NH4OH 40--50 0.05M KCN

60 5

2M HNO~

65(Sc)

3

2M HNO 3

10-30

3-10

20 fold better

5--15

0.2

IM NH4OH 10-30

Acidic alumina CPG-8Q

0.5M NaOH !.3-3.3 36 2M HCI 24--42 60

Muromac A- 1

8M HNO3, 1M HCi

20, 25

7-10

Detection limit (~g/l) Ref. 1.5

27

5-10 0.8-0.9

49 50

3-5

51

1.7 1.2--3.5

52 53

3-6

54

6

31,32

N !

tm

2.2, Cr(lll); 1.2 Cr(Vl) 0.6 mg/i 1.6 0.1(Be); 3(Ce); 0.6(Co); 2.8-7.7 3(Ni); 0.7(V) 1.5 (Cu); 0.05(Cd) 3.5(Cu); 2.2(Cd)

17 55 56 4,5

# o

Flow injection on-line solid-liquid separation

181

Figure 6.5 Manifold for on-line removal of interferences. AAS, atomic absorption spectrometer with quartz tube atomizer; A 1, A2, acid streams; Ar, argon stripping gas; C, micro-column; P 1, P2, peristaltic pumps; R, sodium tetrahydroborate reductant; S. sample; GLS, gas--liquid separator; V 1, V2, injector valves; W, waste. column. The analyte was initially transformed into its chlorostannate complex in 2 mol/l HC1 and the sample in this medium was loaded on the column packed with a strongly basic anion exchanger to retain the complex. The sorbed complex was then eluted by 0.1 mol/1 HNO3 and merged downstream with the reductant which partially neutralized the acid to produce the required acidity for stannane generation. With this approach, the reaction medium was optimized with simultaneous preconcentration of the analyte.

6.3.5

On-line sorption column preconcentration systems for ETAAS

FI on-line column separation have been developed into an effective means of matrix modification and analyte preconcentration for ETAAS, owing to its capability in providing efficient and automated micro-scale separations. The selectivity and/or sensitivity of ETAAS procedures can be significantly enhanced by often completely separating the interfering sample matrices and/or by preconcentration with an efficiency compatible with ETAAS operations. In comparison with FAAS or ICP-OES, ETAAS discontinuous operation poses some specific requirements on the design and operation of FI on-line separation systems summarized by Fang 68 as follows:

182

Z.-1. Fang and G.-h. Tao

Figure 6.6 FI manifold for on-line ion-exchange preconcentration VG-AAS system (dual-column system): P, pump; S, sample; B, buffer; R, tetrahydroborate reductant; E, HCI eluent; V 1, 8-channel valve; V2 2-way valve; CA, CB, ion-exchange columns; SP, gas--liquid separator; W, waste. Ar, argon stripping gas. Lower figure, operational sequence; T l, T2, actuation points for V1 and V2, respectively.

(a)

(b)

Being normally operated in the batch mode, the preconcentration of analyte for the graphite furnace atomizer cannot be operated in a continuous flow mode. FI operations are carried out either in parallel with the fumace temperature program or sequentially (i.e., the FI system remains in standby mode during the furnace program). The maximum sample volume which can be conveniently introduced into a graphite furnace is 70-100 p~l. In order to meet this requirement various approaches for introducing concentrates obtained by FI pretreatment have been proposed, including multiple injections with intermediate drying; ~ zone-sampling, ~~air flow transporta-

o Table 6.3

Characteristic data on performance o f typical on-line column preconcentration vapor generation AAS systems

b..~.

5. t~

Matrix

Analytical species sorbed

Column Packing

Eluent

EF

CE(/min)

RSD(%)

DL(ng/1)

Ref.

Waters Sea, Waters

Se(IV), Se(VI) Hg-DDC/APDC

Alumina C,8

2M NH 3 Ethanol

-

3.3

Sb(III) Se(W)

CPG-8Q D-201 anionic

4M HCI 1M HCI

20 18

1.0 1.1

6 16(DDC), 67(APDC) 1.5 2

63 64

Waters Waters, soil extract Sea, tap, waste waters Waters Canned foods

45-fold improved 20 15 20

20

1.2

2

66

30 4

25 9 5

1.0 2.2

1 80

7 67

~..J.

Hg(II) Bi(III) Sn--CI complex

D-201/Dowex I-8X anionic

2M HCI in 0.5% thiourea 2M HCI H20, 0.1M HCI

65 65, 66 i,,,,. 9

~,,.o

~..J.

o

184

(c)

(d)

Z.-1. Fang and G.-h. Tao tion of eluates, 69 and deposition of concentrate aerosol by high pressure nebulization. 7~ In contrast to FAAS applications, column washing following sample loading is usually indispensable for most ETAAS applications owing to interferences from the residual sample matrix, unless the interfering matrix can be conveniently removed by ashing. Automated control of the more complicated operations is therefore preferable. The relatively long measurement cycle of ETAAS (as compared to other atomic spectrometric methods) makes the FI on-line preconcentration system more readily adaptable to the efficiency of the spectrometric procedure. Thus sample loading and elution rates often can be made lower to achieve better separation effects and improve the overall performance.

Hitherto, mainly two types of manifold designs have been proposed for on-line column preconcentration in ETAAS systems, characterized by the different location of the sorption column. The system proposed by Porta et al. ~ is representative of the type in which the micro-column is mounted in the tip of the autosampler sampling probe (see Figure 6.7); and that reported by Fang et al. ~~ is representative of the type with the column loaded within the injection loop (see Figure 6.8). The advantage of the column-in-tip design is that almost no analyte dispersion occurs during transport of eluate to the fumace. However, flow-reversal in the column is not possible, except by suction. This may deteriorate the precision during long term operations with some packings. This can be avoided using the column-in-loop design shown in Figure 6.8. A drawback of the latter system is the additional dispersion of the eiuate zone when delivered throughthe long transport line to the atomizer. However, this has been overcome by more recent modifications, using small-bored transport tubings of 0.2-0.3 mm i.d., and/or air-flow transportation of the eluate z o n e . 69 6.3.5.1

Column-in-tip preconcentration systems

The column-in-tip system proposed by Porta et al. ~ shown in Figure 6.7, used a 2 . 1 x 2 0 m m micro-column packed with C~a, XAD-2, or XAD-7 to retain APDC complexes of Cd, Pb, Cu, Ni, Co and Fe from sea water samples delivered by a peristaltic pump to the column by time-based loading at 3 ml/min. The sorbed species were subsequently eluted into the furnace by two 80-1~l volumes (with intermediate evaporation of the eluate in between two elutions) of acetonitrile aspirated from a sample cup by the spectrometer autosampler pump. The pump was also used to deliver a 1.4 ml volume of washing buffer through the column before elution. A column-in-tip preconcentration system similar to that reported by Porta et al. ~z was used by Yang et al. TM with a 2.4 x 15 mm micro-column packed with a chelating resin with

Flow injection on-line solid--liquid separation

185

carboxymethylated poly(ethylenimine)isocyanate functional groups (CPPI) for the determination of copper in snow water. Azeredo e t al. 71 developed a column-in-tip preconcentration system using peristaltic pump propelled air flows to deliver the samples, washing solutions and eluents through the column. Dispersion between the solutions was minimized owing to the air barrier.

SA C

Pl

9/

/ P2 E

B

Figure 6.7 FI manifold of column-in-tip sorption preconcentration system. (a) Load sequence; (b) elute sequence. P1, peristaltic pump; P2, autosampler pump; S. sample; R. complexing reagent; E, eluent; B, buffered rinsing solution; V, valve; W, waste; SA, sampling arm; C, micro-column; I, injection tip; GT, graphite tube.

186 6.3.5.2

Z.-1. Fang and G.-h. Tao Column-in-loop preconcentration systems

The operational sequences of the sorbent extraction column preconcentration system in Figure 6.8 is shown in Table 6.4. The procedure consisted of 5 steps, including (a) loading the complexed analyte of the sample on a 15-1xl capacity miniature conical column, packed with CI8; (b) washing the column with deionized water; (c) slowly eluting the retained analyte with ethanol into a 0.35 mm i.d., 60-70 txl PTFE collector tube, storing

W (figure continued on p. 187)

Figure 6.8 FI manifold and operational sequences of column-in-loop sorbent extraction preconcentration for ETAAS (see Table 6.4 for function of sequences). P1, P2, peristaltic pumps; C, micro-column packed with C~8; V, multifunctional valve; L, eluate collector tube; W, waste; GF, graphite tube.

Flow injection on-line solid-liquid separation

187

the central zone of the eluate; (d) introducing the collected eluate into the furnace using a stream of low air flow; and (e) completing the cycle by propelling the residual eluate to waste. Employing such systems, with modifications in the mode of rinsing, sodium chloride matrix background in sea water samples were eliminated almost completely during the preconcentration. 72'73 Welz et al. 69 further modified the system by evacuating the column using air flows between sample loading, washing and elution, and succeeded to quantitatively elute the retained analyte in 80 ~1 eluent. A zone-sampling approach is therefore no longer required.

....

W W

Pl On W (d)

W W

L

W (figure continued on p. 188)

Figure 6.8

Continued

Z.-I. Fang and G.-h. Tao

188

W

~e'~71

............I ~ ~ . ~

~

W Figure 6.8 Continued

6.3.5.3 Performanceof sorption preconcentration ETAAS systems Performance data for some typical ETAAS systems with FI on-line column preconcentration are shown in Table 6.5. The CE values given refer to those of the FI preconcentration system excluding the ETAAS procedure.

6.3. 6

On-line sorption column separation and preconcentration systems for ICP-MS

Despite its reputation for being a relatively spectral interference-free technique, inductively coupled plasma mass spectrometry (ICP-MS) still suffers from spectral interferences, particularly from polyatoms, and also from matrix interferences. The direct analysis of samples with high levels of dissolved solids can also be problematic owing to the rapid blockage of sampling interface cones and/or torch injector. Recently, FI on-line column separation has been shown to provide an efficient and automated means of matrix elimination for I C P - M S . 75-87 Manifolds used in ICP-MS are very similar to those used for FAAS and ICP-OES. Both time-based 79-82'85,86 and volume-based 75-78'83,84 sample loading modes can be found in the literature. Elements of interest are retained on the incorporated micro-column and isolated from the matrix. Subsequently, the retained analytes are eluted directly into the nebulizer. In addition to the required matrix separation, analyte preconcentration can be also achieved. Application of FI on-line column separation and preconcentration systems for ICP-MS are summarized in Table 6.6.

~..~o

Table 6.4

Sequence of operation for FI on-line sorbent extraction preconcentration for ETAAS

5. ~.~~

Sequence

Pump

Medium pumped

Flow-rate (ml/min)

60

1 1 2 2

30

1

30

1

Air Ethanol Sample Sample DDC solution Water Air Ethanol

0.8 0.5 5.0 2.1 0.4 1.6 0.8 0.15

Duration (s)

o Function

o

Displace previous eluate Elution of residual analyte from previous run Sample change Sample loading on column

b,,,~ 9

!

10 20

Total

Column rinsing Evacuation of eluate collector Sample elution and eluate collection

t~ o

~,,io

150 l,,Jo

o

oo

Table 6.5

Performance of typical on-line sorption preconcentration systems for ETAAS Sorbent

Eluent

EF

CE (/min)

DL

System design

Pb-DDC Cd, Cu, Ni, Pb-DDC As(Ill), As(Ill) +As(V)-DDC Cd, Pb-DDC

Col. -in-loop Col.-in-loop

C z8 C~s

Ethanol Ethanol

26 i 7-27

10.4 6.2-9.9

Col. -in-loop

C~8

Ethanol

7.6

Col.-in-loop

C~8

Methanol

Cd, Cu, Pb-DDPA Cd, Pb, Cu, Ni, Co, Fe-APDC Fe, Cd, Zn, Cu, Ni, Mn, Pb

Col.-in-loop Col.-in-tip

Ct8 Cjs

Ethanol Acectonitrile

Col.-in-tip

2M HCI+0.8M HNO 3

Cu

Col.-in-tip

silica immobilized 8-HQ CPPI resin

Cd, 62; Pb, 64 12-13 Cd, 20; Co, 225 50-250

32

11

Analyte species sorbed

RSD(%)

Ref.

3 Cd, 0.8; Cu, 20; Ni, 40; Pb 7 110-150

1.9 2.0-3.3

10 73

5.5

74

o'o

18

Cd, 0.7; Pb, 4.5

3

69

13-14 Cd, 10; Co, 26

Cd, 3; Pb, 40 Cd, 0.4; Cu, 10

2 3-5

12 11

C~ "r

10-20

Co, 1; Ni, 9; Pb, 4;Cd, 0.06; Cu, 1; Ni, 2; Pb, 1; Mn, 0.4; Zn, 0.3 4

6

72

N

o

0.05M HNO3

!

o

71

Table 6.6

Application o f FI on-line column separation and preconcentration systems for I C P - M S DL (~g/l)

Matrix

Analytical species sorbed

Column Packing

Eluent

EF

CE (/min)

RSD(%)

Biologicals

V, Mn, Cu, Zn, Cd, Pb

IDA resin

3M HNO 3

-

-

3--4

Brines

Cu, Mo, Ni, U, Zn, In

1M HNO 3

-

-

-

Biologicals

As, Cr, Se, V

1M HNO 3

100

15

3-4

Sea water Water Mineral and water Iron metal

Mn, Co, Cu, Zn, Pb U Au

Xylenol orange/ Metpac CC- 1 Activated alumina IDA resin Alumina Sulfydryl cotton

2M HNO 3 2M HNO3 10 mM KCN

40 160

10 15

2.3-6.9 4.5 3.6

-

Nb, Ta, W, Zr, Hf

Dowex 1X8-100

-

0.7-3.0

Sediments

Mn, Co, Cu, Zn, Cd

CPG-8HQ

0.7M HNO3, 0.5M Hcl, 0.05M H202 2M HNOs

-

<3

Waters Waters

Pt Mn, Co, Cu, Ce, Cd

Alumina IDA resin

2M NH4OH I M HNO 3

600 3

8(Nb); 5(Ta); 14(W); 12(Zr); 10(Hf) ng/g 0.53(Mn); 0.01(Co); 0.30(Cu); 0.38(Zn); 0.02(Cd) 2 ng/l 2.5(Cd); 2.6(Co); 3.9(Cu); 1.8(Ce); 4.5(Cd)

0.6(V); 6.0(Mn); 0.45(Cu); 9.9(Zn); 6.0(Cd); 1.2(Pb)

o Ref. 75

t~ t~

o 76

o !

1.2(V); 6.0(Cr); 9.0(As); 65(Se)

77

4 ng/l 0.19 ng/l

78 79 80

o p.io

,.Q

50 0.3

4 1.1-4.5

83 84

t~

85 86

O

192

6.4 6.4.1

Z.-I. Fang and G.-h. Tao

On-line preconcentration by precipitation and coprecipitation General

Preconcentration of trace analytes by precipitation, and particularly by coprecipitation, using batch operations have been used for many years in combination with atomic spectrometric methods, yet without ever having been accepted broadly in routine practice. Despite the capability of effectively separating interfering matrices, and achieving high enrichment factors, coprecipitation procedures operated in a manual batch mode are usually tedious. They also require large sample volumes, and precipitates are easily contaminated. The recent development of automated FI on-line precipitation and coprecipitation has changed the situation considerably, and a number of efficient and relatively robust preconcentration methods for use with atomic spectrometry are now available. 6.4.2

F l preconcentration systems with on-line precipitation

The earliest FI on-line precipitation systems with FAAS detection were reported by Valcarcel's group s8 employing on-line stainless steel filters used in HPLC. A typical system is shown schematically in Figure 6.9. The system design is similar in principle to that of the column system in Figure 6.1, with the on-line filter replacing the column. The precipitate was formed in the reaction coil, retained in the filter and subsequently dissolved by a suitable dissolution agent. High enrichment factors of several hundreds were reported with such systems, using larg e sample volumes and long loading periods, but with relatively low sample throughput. The relatively low efficiency of on-line precipitation systems is probably related to the relatively low liquid--solid phase transfer associated with the solubility of the precipitate and slow kinetics of the precipitation reaction when dealing with trace analytes at the Ixg/1 level. Not very many precipitation reactions are available which are selective, sensitive and fast enough to be used for preconcentration purposes under on-line precipitation conditions.

Figure 6.9 FI manifold of FI on-line precipitation-dissolution system for FAAS. RC, reaction coil; V, injector valve; D, dissolution solution; R, precipitating solution; S, sample; W, waste; F, stainless steel filter, P, peristaltic pumps.

Flow injection on-line solid--liquid separation

6.4.3

193

Fl preconcentration systems with on-line coprecipitation

6.4.3.1 General In comparison with on-line preconcentration methods based on precipitation, coprecipitation methods are less demanding on the solubility of the precipitate formed. In addition to direct precipitation, trace analytes may be also precipitated by adsorption, occlusion, and formation of isomorphic crystal through the use of an appropriate precipitate carrier. Therefore coprecipitation is suitable for more general use with less interference from matrix constituents. However, the on-line manipulation of a relatively large amount of precipitate in a continuous flow system poses some difficulties in producing a robust system. Fang et al. s9 circumvented the problems associated with the use of on-line filters by the implementation of knotted reactors (KR) as precipitate collectors. The KR provides relatively large collection capacities with low flow impedance. Precipitates are retained on the PTFE KR tube walls both through adsorption and centrifugal force created by the continuous change of flow direction in the reactor. More recently, on-line coprecipitation using on-line filters using silk fabric 9~ cellulose acetate membranes, and filter papers ~ were also reported. Recent developments show that FI on-line coprecipitation can be applied successfully to FAAS, HGAAS, ETAAS and ICP-OES. 6.4.3.2

On-line coprecipitation FAAS systems

The FI on-line coprecipitation system with a KR as collector used for FAAS s9 is shown in Figure 6.10. This configuration is similar in many respects to on-line preconcentration systems based on sorption columns shown in Figure 6.2. The main differences are the substitution of the column by a KR and locating the reagent merging point within the KR loop. Such measures were taken because merging reagents upstream of the valve as in column preconcentration systems results in precipitation in the valve channels and in a part of the connection conduits upstream of the valve. These channels are not rinsed by eluents during precipitate dissolution, and the deposits gradually accumulate, causing blockages after a few determinations. With the system in Figure 6.10, in the loading (precipitation) sequence of the preconcentration, samples were introduced in the time-based mode and merged with the precipitant. The precipitate was retained in the KR while the rest of the reaction mixture flowed to waste. The collected precipitate was then dissolved by a dissolution agent in the next sequence and delivered to the spectrometer. The majority of applications in FI on-line coprecipitation using KR made use of precipitates formed from metallo-organic complexes such as hexamethylene dithiocarbamates (HMDTC) and APDC, and organic solvents were used as dissolution agents. Solvent resistant pumping devices (including solvent displacement bottles) are therefore required for operation.

194

Z.-1. Fang and G.-h. Tao

>

MC

I

), KR

Figure 6.10 FI on-line coprecipitation preconcentration FAAS system using a knotted reactor precipitate collector: (a) precipitation (loading) sequence; (b) dissolution sequence. P 1, P2, pumps; V, multifunctional valve; S, sample; R, precipitation reagent; DS, dissolution agent; KR, knotted reactor; W, waste.

Some guidelines for the design of KR coprecipitation systems, particularly for FAAS were given by Fang. 68 The main points are summarized below. Ca)

The amount of precipitate formed should not exceed that required for coprecipitating the analyte. Larger amounts decrease the capacity for preconcentration, and reduce the enrichment factor.

Flow injection on-line solid-liquid separation (b) (c) (d)

195

The dissolution reagent should be strong enough to dissolve the precipitate rapidly at room temperature. One to two meters KRs produced from 0.5 mm i.d. PTFE tubing with about 3-4 ml/ min sample loading rates are generally recommended for collection of precipitates. Sample loading periods of 20-40 s are recommended for optimized efficiencies.

While hitherto most applications of on-line preconcentration by coprecipitation for FAAS were based on the use of KRs as precipitate collectors, Lin et al. 9~ reported the use of an on-line silk fabric filter with a 2-mm diameter filtering area. The fabric was fixed inside a 100 I~l demountable Perspex glass column jacket. The system was used for the determination of strontium with lead sulfate as carrier, and EDTA as eluent. The performance data of typical FI on-line coprecipitation systems are given in Table 6.7. 6.4.3.3

On-line coprecipitation HGAAS systems

Tao and Hansen 97 were the first to adapt on-line preconcentration by coprecipitation using a KR to a FI hydride generation AAS system for the determination of selenium(IV). Employing lanthanum hydroxide as carrier for the preconcentration of selenium, the system was also the first to report the use of an inorganic coprecipitation carrier for collection in a PTFE KR. The system is shown in Figure 6.1 l, and is almost a direct connection of the system in Figure 6.10 to a FI-HGAAS system. The precipitate is readily soluble in hydrochloric acid which is also the required medium for the ensuing hydride generation reaction. This facilitated the direct combination of the on-line coprecipitationdissolution with the hydride generation process. At a sample consumption of 6.7 ml, a concentration efficiency of 13 was achieved with a precision of 0.7% RSD. Satisfactory results were obtained for the analysis of Se(IV) in tap- and well-water samples. More recently, a similar system was extended to the determination of selenium with on-line addition of lanthanum hydroxide, 98 and the determination of arsenic(III), using lanthanum hydroxide or hafnium hydroxide as coprecipitant. 99 6.4.3.4

On-line coprecipitation ETAAS systems

The FI on-line coprecipitation system shown in Figure 6.10 was adapted by Fang and Dong ~~176 to be used for ETAAS in the determination of trace heavy metals in whole blood digests. Cadmium and nickel in the digest were coprecipitated with the iron(II)-HMDTC complex on the walls of a KR. The manifold is similar to that used for FAAS except the precipitate was dissolved in 60 p~l of IBMK, and stored in a 30 cm long 0.5 i.d. PTFE collector tube connected to the valve exit before introduction into the graphite tube. Complete dissolution of precipitate could not be achieved using the 60 Ixl solvent for the ETAA determination, and further rinsing of the KR with IBMK was required after deposition of the concentrate to remove the residual (about 5%) retained analyte. The

o~

Table 6.7

Characteristic data on performance of typical FI on-line coprecipitation systems for FAAS

Analyte

Coprecipitant

Dissolution agent

EF

CE (/min)

RSD(%)

DL(I~g/I)

Ref.

Cr Cu, Ni Sr

La(OH)3 Fe(OH)3 PbSO4

22 61

10 20-30

2.7 Cu, 62; Ni, 3.2 3.5

0.8

92 93 90

Ag Co, Ni, Cd

Fe(II)-DDC Fe(II)-APDC

0.5M HCi 2M HCI Hot 5% EDTA/ 1%KH(C6H4)/ I%NaOH IBMK IBMK

26 30-42

27 42-50

2.1 2.4

0.5 Co, 1; Ni, 3; Cd, 3 Cd, 0.15; Co, 1.3, Ni, 1.5 2

Cd, Co, Ni

Pb

Fe(II)-HMDTC

Fe(II)--HMDTC

IBMK

IBMK

43--52

66

52-62

99

1.5-2.7

2.7

N

0.9

94 95

! ~

~rJ O~

~

~r

96 o 89

197

Flow injection on-line solid-liquid separation

m

KR :: . J , , . . "

: ' ~

"

"

~ .... ~

V

B~

'

I

t

I

......-~

Figure 6.11 FI on-line coprecipitation preconcentration HGAAS system using a knotted reactor precipitate collector: (a) precipitation (sample loading) sequence; (b) dissolution sequence. P 1, P2, pumps; V, multifunctional valve; S, sample counting 0.5% La; R, tetrahydroborate reductant; B, NH4OH-NH4CI buffer precipitant (pH 9); C, reaction coil; KR, knotted reactor; GLS, gas-liquid separator; W, waste.

198

Z.-1. Fang and G.-h. Tao

HMDTC reagent was also used as washing solution to remove the sample matrix after sample loading by extending its propulsion for 10 s after the termination of the sample flow. These operations required a total of seven steps for each preconcentration cycle. Enrichment factors of 16 and 8 were obtained for cadmium and nickel respectively. The same authors i~ extended the coprecipitation preconcentration ETAAS system to the determination of cadmium in blood, using dithizone as a coprecipitating agent. In this application, it was necessary to combine a KR collector with a miniature membrane filter to retain the precipitate completely and improve reproducibility. More recently, Chen et al. ~~ used the coprecipitation preconcentration system for the determination of molybdenum with Fe(II)-APDC as coprecipitation agent. It was observed that washing the collected precipitate caused losses of the precipitate when the flow-rate of the washing solution was significantly lower than that during sample loading. A reduction of centrifugal force which is responsible for keeping the precipitate particles adhered to the walls of the KR was assumed to be the reason for this phenomenon. In order to avoid this, the flow-rate of the washing water flow was made identical to that used for sample loading. An enrichment factor of 22 was achieved with a precision of 3.1% RSD. The detection limit (3o') was 0.03 i~g/l. The system was successfully applied to the determination of molybdenum in water, human hair and high-purity reagents. 6.4.3.5

On-line coprecipitation I C P - - O E S system

The multi-element treatment capability of coprecipitation procedures has been exploited in the combination of on-line coprecipitation with ICP-OES by Zhuang et al. ~~ for the simultaneous multi-element determination of Cd, Cu, Fe, Ni, Pb, and Zn in rain water using Co-APDC as carrier. The optimized precipitate collector consisted of a 1.5-m 0.5mm i.d. PTFE KR and a PTFE membrane filter with 0.078 cm 2 area. The precipitate could not be dissolved using organic solvents as in FAAS, owing to difficulties in maintaining the plasma, therefore 7 M HNO 3+ 19% m/v H202 was used as eluent to circumvent the problem. Enrichment factors ranging from 10 to 50 (depending on the element) for 100-s time-based sample loading. RSD's in the range 1.9-5% were obtained with a sample throughput of 20/h. Park and Pak 9~ reported the determination of Pb, Cd, and Cu using a sequential ICP spectrometer combined to an on-line coprecipitation system, employing indium hydroxide as carrier and 3 M HNO 3 as dissolution agent. On-line filtration media produced from cellulose acetate membranes and filter papers were used to collect the precipitate (dimensions of filter housing not described). Difficulties were encountered with an increase in the amount of precipitate formed owing to buildup of back pressure, and enrichment factors did not exceed 15 at a sampling frequency of 10/h.

Flow injection on-line solid-liquid separation

6.5

6.5.1

199

On-line solid-liquid separation by sorption on knotted reactors

On-line KR sorption preconcentration f o r F A A S

Following the use of knotted reactors for precipitate collection, Fang et al. ~~ recently reported the phenomenon that PTFE knotted reactors are capable of sufficiently strongly retaining some soluble metallo-organic complexes such as those formed with dithicarbamates (PDC and DDC) presumably through molecular sorption, so that they can be used as collection media for on-line preconcentration of these species. The system design is almost identical to that used for on-line coprecipitation shown in Figure 6.7. The main advantage of such systems is the relatively low flow impedance of the KR compared to packed columns and the absence of other limitations associated with column packings, such as mechanical stability and packing tightening under uni-directional flows. For the retention of cadmium-DDC complex using a 3-m long KR, 80% was retained at a loading flow-rate of 5.2 ml/min and 90% retention at 3.2 ml/min flow-rate. The CE (61/min) achieved was better than that obtained for a DDC sorbent extraction system using C18 packed columns. This approach was further extended by Chen et al. ~~ to the preconcentration of copper in sea water and rice samples 6.s

On-line K R sorption preconcentration f o r E T A A S

More recently, the on-line KR sorption preconcentration system was extended to ETAAS for the determination of lead in sea-water by Sperling et a/. 1~ and antimony(III) in water samples by Yan et al. 1~ In their systems, the metallo-organic complexes formed with DDC or PDC were retained on the inner wall of the KR during the sample loading step. The non-adsorbed constituents of matrix in the KR was removed by a subsequent rinsing step. It was observed that the adsorbed complex could be easily removed with a diluted nitric acid or even deionized water as a rinsing solution. To prevent the loss of the analyte, appropriate amounts of complexing agent were added to the rinsing solution. The residual rinsing solution in the KR and in the eluate delivery tube was excluded by an air flow before elution. An eluent loop was used to reproducibly introduce a certain amount of eluent which was driven by an air flow to the graphite fumace. These measures effectively minimize the eluent volume needed for quantitative elution and reduce the dispersion during eluate delivery and introduction to graphite furnace. Chemical modification was thus no longer needed due to the complete removal of the sample matrix. Most recently, by using a more selective complexing agent, diethyldithiophosphate, Yah et al. !~ applied the preconcentration system to the determination of lead in biological and environmental samples. An enrichment factor of 125 and a detection limit (30-) of 4.8 ng/1 along with a

Z.-1. Fang and G.-h. Tao

200

sampling frequency of 31/h were achieved. The precision for eleven replicate measurements at 0.5 Ixg/1 was 2.1% RSD.

6.6

References

I OIsen, S., Dan. Kemi, 1983, 64, 68. 2 Fang, Z.-I., Flow injection separation and preconcentration, VCH Publishers, Weinheim, 1993.

3 Fang, Z.-I., Spectrochim. Acta Rev., 1991, 14, 235. 4 Kumamaru, T., Murakami, K., Nakata, F., Sunahara, H., Kiboku, M. and Matsuo, H., Anal. Sci., 1987, 3, 161. 5 Kumamaru, T., Matsuo, H., Okamoto, K. and Ikeda, M.,Anal. Chim. Acta, 1986, 181,271. 6 Hirata, S., Honda, K. and Kumamaru, T., Anal. Chim. Acta, 1989, 221, 65. 7 Zhang, S.-c., Xu, S.-k. and Fang, Z.-I., Quim. Anal., 1989, 8, 191. 8 Patel, B., Haswell, S. J. and Grzeskowiak, R., J. Anal. At. Spectrom., 1989, 4, 195. 9 Ruzicka, J. and Arndal, A., Anal. Chim. Acta, 1989, 216, 243. 10 Fang, Z.-I., Sperling, M. and Weiz, B., J. Anal. At. Spectrom., 1990, 5, 639. 11 Porta, V., Abollino, O., Mentasti, E. and Sarzanini, C., J. Anal. At. Spectrom., 1991, 6, 119. 12 Ma, R.-I., van Mol, W. and Adams, E, Anal. Chim. Acta, 1994, 293, 251. 13 Lancaster, H. L., Marshall, G. D., Gonzalo, E. R., Ruzicka, J. and Christian, G. D., Analyst, 1994, 119, 1459. 14 Murakami, K., Okamoto, Y. and Kumamaru, T., J. Flow Injection Anal., 1992, 9, i 95. 15 Santelli, R. E., Gallego, M. and Valcarcel, M., Talanta, 1994, 41,817. 16 Petit de Pena, Y., Gallego, M. and Valcarcel, M., J. Anal. At. Spectrom., 1994, 9, 691. 17 Cox, A. G., Cook, l. G. and McLeod, C. W.,Analyst, 1985, 110,331. 18 Cantarero, A., Gomez, M. M., Camara, C. and Palacios, M. A., Anal. Chim. Acta, 1994, 296, 205. 19 Xu, S.-k., Sun, L.-j. and Fang, Z.-I., Anal. Chim. Acta, 1991, 245, 7. 20 Knapp, G., Muller, K., Strunz, M. and Wegscheider, W., J. Anal. Atom. Spectrom., 1987, 2, 611. 21 Burba, P. and Blodorn, W., Vom Wasser, 1992, 79, 9. 22 Guo, X.-w., Yuan, Y. and Su, Z.-x., Yankuang Ceshi, 1993, 12, 241. 23 EImahadi, H. A. M. and Greenway, G. M., 3'. Anal. At. Spectrom., 1991, 6, 643. 24 Elmahadi, H. A. M. and Greenway, G. M., J. AnaL.At. Spectrom., 1994, 9, 547. 25 Maquieira, A., Elmahadi, H. A. M. and Puchades, R., Anal. Chem., 1994, 66, 1462. 26 Maquieira, A., EImahadi, H. A. M. and Puchades, R., Anal. Chem., 1994, 66, 3632. 27 Maquieira, A., EImahadi, H. A. M. and Puchades, R., J.Anal. At. Spectrom., 1996, I1, 99. 28 Fang, Z.-l. and Welz, B., ,/. Anal. At. Spectrom., 1989, 4, 543. 29 Xu, S.-k., Sperling, M. and Welz, B., Fresenius'J. Anal. Chem. 1992, 344, _535. 30 Fang, Z.-I., Ruzicka, J. and Hansen, E. H., Anal. Chim. Acta, 1984, 164, 23. 31 Hartenstein, S. D., Christian, G. D. and J. Ruzicka, Can. ,1. Spectrosc., 1985, 30, 144. 32 Hartenstein, S. D., Christian, G. D. and J. Ruzicka, Anal. Chem., 1985, 57, 21. 33 Fang, Z.-I., Xu, S.-k. and Zhang, S.-c., Anal. Chim. Acta, 1987, 200, 35. 34 Lin S.-I., Zheng, C.-s. and Yun G.-z., Talanta, 1995, 42, 92 l. 35 Liu, X.-z. and Fang, Z.-l., Anal. Chim. Acta, 1995, 316, 329. 36 Pasullean, B., Davidson, C. M. and Littlejohn, D.,J. Anal. At. Spectrom., 1995, 10, 241. 37 Lin, S.-i., Zheng, C.-s. and Yun, G.-z., Talanta, 1995, 42, 921. 38 Ma, R., Van-Mol, W. and Adams, F., Anal. Chim. Acta, ! 995, 309, 395. 39 Ma, R., Van-Mol, W. and Adams, F., Anal. Chim. Acta, 1994, 293, 251. 40 Ma, R., Van-Mol, W. and Adams, F., Anal. Chim. Acta, 1994, 285, 33. 41 Naghmush, A. M., Pryzynsk K. and Trojanowicz, M., Talanta, 1995, 42, 85 I. 42 Di, P. and Davey, D. E., Talanta, 1995, 42, 685. 43 Petit-de-Pena, Y., Gallego, M. and Valcarcel, M., Talanta, 1995, 42, 211. 44 Cantarero, A., Gomez, M. M., Camara, C. and Palacios, M. A., Anal. Chim. Acta, 1994, 296, 205. 45 Rodriguez, D., Fernandez, P., Perez-Conde, C., Gutierrez, A. and Camara, C., Fresenius ',I. Anal. Chem., 1994, 349, 442.

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202 97 98 99 100 101 102 103 104 105 106 107 108

Z.-I. Fang and G.-h. Tao Tao, G.-h. and Hansen, E. H.,Analyst, 1994, 119, 333. Nielsen, S., Sloth, J. J. and Hansen, E. H.,Analyst, 1996, 121, 31. Nielsen, S., Sloth, J. J. and Hansen, E. H., Talanta, 1996, 43, 867. Fang, Z.-I. and Dong, L.-p., J. Anal. At. Spectrom., 1992, 7, 439. Dong, L.-p. and Fang, Z.-I., Fenxi Shiyanshi, 1992, 11/6, 9. Chen, H.-w., Xu, S.-k. and Fang, Z.-I., J. Anal. At. Spectrom., 1995, 10, 533. Zhuang, Z.-x., Wang, X.-r., Yang, P.-y., Yang,-C. and Huang,-B.-I., J. Anal. At. Spectrom., 1994, 9, 779. Fang, Z.-I., Xu, S.-k., Dong, L.-p. and Li, W.-q., Talanta, 1994, 41, 2165. Chen, H.-w., Xu, S.-k. and Fang, Z.-I., Anal. Chim. Acta, 1994, 298, 167. Sperling, M., Yan, X.-p. and Welz, B., Spectrochim. Acta, Part B, 1996, 51, 1891. Yan, X.-p., van Mol, W. and Adams, F., Analyst, 1996, 121, 1061. Yan, X.-p. and Adams, F.,J. Anal. At. Spectrom., 1997, 12, 459.

spectrometry

7.1

Introduction

Mass transfer between two liquid phases in a continuous flow-injection manifold connected on-line to an atomic spectrometer as detector is the subject matter of this chapter. This type of separation aims, among others, at the following: (a) increasing sensitivity through preconcentration; (b) improving selectivity through interference removal; (c) facilitating the atomic spectrometric detection of both metal and non-metal analytes; (d) enabling automatic handling of samples, solvents and reagents; (e) facilitating the automatic implementation of both chemical reactions and continuous separations; (f) minimizing sample, solvent and reagent consumption; and (g) reducing hazards to operators and the environment. These aims are shared by the other nonchromatographic continuous separation techniques 1'2 coupled on-line to atomic spectrometers described in Part II of this book. Figure 7.1 shows a schematic diagram of the general manifold for implementing liquidliquid separations in combination with AS detection. Four types of liquid (sample, standards, reagents and solvents) are sequentially introduced into the liquid-liquid continuous separator. The stream containing the species to be detected can be connected on-line to the device that feeds the AS instrument or collected in an autosampler cup from which an aliquot can be inserted into the ETAAS discrete-introduction AS detector. This 203

204

M. Valc~ircel and M. Gallego

Figure 7.1 Generaldiagram of Fl-liquid-liquid-AS approaches. Arrowheads mark the places were the injection valve can be placed. latter option can be considered a mixed on-line/off-line approach. This chapter discusses the coupling of liquid-liquid separations to the principal atomic spectrometric techniques (FAAS, FAES, ETAAS, AFS, ICP-OES, ICP-MS) in a systematic manner as opposed to Fang's excellent book on the topic, 3 which deals exclusively with atomic absorption techniques. There are only two variants of liquid--liquid continuous separations: dialysis and liquid-liquid extraction. The main difference between them lies in the miscibility of the two liquids involved, which in turn dictates the way they are brought into contact for mass transfer in practice. In dialysis processes, the two (usually aqueous) phases are miscible, so an isolation barrier (usually a membrane) is mandatory. The membrane acts as an active or passive filter which allows analytes, reaction products or interferences to be continuously separated from the liquid sample matrix. The dialyser is thus a continuous device in which the two phases involved enter and leave. Continuous liquid-liquid extraction involves mass transfer between two (almost) immiscible phases, viz. the aqueous and the organic phase. Mass transfer between them does not ordinarily take place in a static, well defined interface as in dialysis. Rather, both phases are brought into contact to form a stream of regular, alternate segments that are subsequently separated. The dynamic extraction interface here comprises both the zone of contact between the segments and the film area of the phase that wets tubing walls. A continuous liquid-liquid extractor consists of three essential elements, namely: a segmenter (viz. a point of confluence for the two phases), an extraction coil and a phase separator. All three work in a sequential fashion. In this context, the word "extraction" is used to describe a variety of separation alternatives and can thus be misleading. Liquid-

Flow-injection on-line liquid-liquid separation

205

liquid extraction is the best way to define mass transfer between two liquids (sample and solvent); on the other hand, "solvent extraction" is not appropriated as it also includes leaching (extraction of analytes with a solvent or supercritical fluid). Basically, mass transfer between the two phases results from different physico-chemical processes in both techniques. In dialysis, the driving separation force is differential ion or molecule mobility across membranes; in liquid-liquid extraction, it is solubility differences. Occasionally, dialysis and liquid--liquid extraction can be integrated in a single module (e.g. liquid supported membranes in continuous sandwich configurations). Separation takes place in a dialysis framework but the underlying principle of the mass transfer is a two fold liquid-liquid extraction between the aqueous sample donor stream, the organic solvent immobilized on the membrane and the aqueous acceptor stream. The scope of this chapter is restricted to liquid-liquid flow systems involving an injection valve because Chapter 9 in part II of this book is devoted to flow systems based on continuous aspiration and deals with liquid-liquid separations as well. The injection valves used in the continuous liquid-liquid manifolds discussed in this chapter can be used for a variety of purpose including injection of sample, reagents and reaction products into the aqueous and organic streams, and also stopping the flow. They can be placed before or after the separation module, and also inside it (see Figure 7.1). There are relatively few publications on this specific topic in the analytical literature owing to the basic and practical advantages provided by other types of continuous separation techniques (sorption, ion-exchange, precipitation) and continuous aspiration systems. Also, most such publications are not recent, so applications of this type are clearly on the decline.

7.2

Main limitations of Fl-liquid-liquid separations

Correctly assessing the potential of FI-AS coupled systems involving liquid-liquid separations entails discussing its merits and pitfalls in relation to the other alternatives (sorption, precipitation) dealt with in the preceeding chapter, particularly as regards the following features:

Separation efficiency. Dialysis analyte recoveries rarely exceed 15-20% and continuous liquid-liquid extraction recoveries typically range from 70% to 90%. By contrast, continuous solid-phase extraction (SPE) and precipitation frequently reach 90-95% analyte recoveries. Enrichment factor. The extent to which continuous separation techniques enhance sensitivity in an indirect manner depends strongly on the volume ratio for the two liquids involved. Because both liquids are in motion, in conventional dialysis and liquid-liquid extraction, affordable phase ratios (and hence enrichment factors) are limited to about

206

M. Valc~rcel and M. Gallego

10-20, which are quite modest relative to other preconcentration approaches. There are several practical ways to partly circumvent such a serious constraint that are described later on in this chapter. In FAAS, this drawback is partly offset by the increased sensitivity resulting from the organic solvent used. Ruggedness. Based on the critical influence of experimental factors such as flow-rates, chemical parameters, etc., FI-liquid--liquid separations must be less robust than other continuous separation alternative. Operational skills. Ensuring correct performance and reliable analytical results requires sound operator experience in each particular system. These limitations have hindered extensive development of F I L L separation--AS methodologies, the earliest of which were reported more than 15 years ago by Nord and Kalberg in connection with FI-FAAS. 4' 5

7.3

Liquid-liquid extraction

Liquid-liquid extraction is a convenient way of preparing samples for AS. There are many reported examples in which batch (manual) extraction of neutral metal chelates or ion pairs has efficiently helped improve the quality of the analytical information obtained about solid or liquid samples; the principal objectives here are interference removal and preconcentration. 6-~5 Liquid-liquid extraction is also highly suitable as a manual pretreatment procedure for hydride generation-AAS '6-'9 and speciation. ~6'17,19--21 Recent innovations in this context include the use Of a robotic station 22 and liquid-supercritical fluid extraction. 23'24 Nevertheless, the implementation of liquid--liquid extraction in a continuous fashion enables on-line coupling to AS instruments and contributes advantages inherent in automatic methods of analysis. 25 A detailed technical description of continuous liquid-liquid extraction processes can be found in several books. !'2'26'27 In this section, different operational aspects of an extractor coupled to AS instruments are discussed. The most outstanding procedures described so far in this context are dealt with here according to the particular AS technique used. 7.3.1

Technical features of a continuous liquid-liquid extractor

A continuous liquid--liquid extractor (CLLE) is a device that receives streams of the two immiscible phases and produces single segmented flow within which mass transfer takes place; the segmented flow finally emerges from it as two separate streams containing one phase each. With few exceptions, conventional CLLEs consist of three basic components that are

Flow-injection on-line liquid--liquid separation

207

11l Figure 7.2 Basic components of a conventional continuous liquid-liquid extractor based on phase segmentation. For details, see text and Figures 3 and 4.

connected on-line in the following sequence (see Figure 7.2): a solvent segmenter, an extraction coil and a phase separator. Operationally, all three rely on two fundamental principles, viz. the density difference between the two immiscible phases and, more important, selective wetting of the inner component surfaces by both organic (wet PTFE and other plastic surfaces) and aqueous phases (wet glass surfaces), which, in turn, are repelled by glass and PTFE surfaces respectively. The solvent segmenter (SS) is a device that receives the streams of the two immiscible liquid phases, which converge on a minichamber or confluence point, and establishes an emerging segmented flow consisting of alternate, regular plugs of the two phases that is driven to the extraction coil. It should operate smoothly and allow selection of the length of the segments of both phases via the incoming flow-rates in such a way that the enrichment factor can be slightly modified at will. Although there are many different possible SS designs, the most simple configurations (T, Y or W confluence points) are the most frequently used. The functioning of a T-type SS device is depicted in Figure 7.3, where a distinction is made according to the material of its inner components, which determines the profile of the outcoming segmented flow via selective wetting/repulsion of organic and aqueous phases. The extraction coil is usually a helically (10-20 mm) coiled tube 2-3 m long and 0.5-1.0 mm ID that receives the segmented flow from the solvent segmenter and drives it to the phase separator. Mass transfer takes place in the segmented flow through the moving interfaces as well as in that formed as a result of the flow of static liquid wetting the inner surface of the tube. When the analyte (or its reaction product) is to be transferred from the

208

M. Valcb.rcel and M. Gallego

Figure 7.3 Operational diagram of a T-type solvent segmenter and its relationship to the extraction coil. The profile of delivered segmented flow (a, b) depends on the materials of its components. A, aqueous phase; O, organic phase; SF, segmented flow.

aqueous phase to the organic phase, the situation depicted in Figure 7.3(a) (PTFE coil) is the most favourable; on the other hand, tubing of glass or other hydrophilic material is to be preferred for transfer in the other direction (Figure 7.3(b)). The extraction time is a function of the flow-rate of the segmented flow and the coil dimensions. The phase separator receives the segmented liquid-liquid flow from the extraction coil and separates the segments in such a way that two independent streams of the two phases are obtained at its exit. Ordinarily, the phase in which the determination is to be performed (generally the organic one) is almost completely free from the other. It is usually the critical point of liquid-liquid extractors. There is a great variety of continuous phase separator designs, all of which are based to a variable extent on two principles, namely:

Flow-injection on-line liquid--liquid separation

209

t

+ Figure 7.4 Most usual configurations of continuous phase separators: (a) gravity type; (b) T-type; (c) sandwich membrane type. SF, incoming segmented flow; LP, lighter phase; HP, heavier phase; O, organic phase; A, aqueous phase; S, PTFE strip. For details, see text. density differences between the phases and selective wetting of inner surfaces. The most common types are depicted in Figure 7.4. Those based exclusively on density differences between the phases (Figure 7.4(a)) consist of an open or closed minichamber into which the segmented flow penetrates to form a single interface in the centre. The flows of the lighter and heavier phase leave the separator from its top and bottom, respectively. The main drawbacks of such a simpleapproach are poor reproducibility and high contributions of extracted species. The T-type phase separator (Figure 7.4(b)) relies on both gravitational and selective wetting effects. The segmented flow in a PTFE tube enters a T-configuration glass tubing in the centre of which a PTFE strip is located. The lighter, organic phase, leaves from the top, whereas the heavier, aqueous phase containing some organic phase, leaves from the bottom. However, the best results in terms of reproducibility and separation efficiency are accomplished by using sandwich-type membrane separators (Figure 7.4(c)). They are based on the principle that hydrophobic microporous (PTFE) membranes are penetrable by most organic solvents but repel aqueous phases. The PTFE membrane is placed between two grooved plastic (or metal) blocks. The segmented stream crosses the lower part of the minichamber. The lighter, organic phase, passes across the membrane and the two phases are separated in essentially the same way as in the abovedescribed separators. The membrane separator has important advantages, namely: high phase separation efficiency and phase ratio, applicability to a large variety of organic solvents, increased reproducibility and relatively easy manipulation. However, it has one serious practical drawback, namely the short lifetime of the membrane (from a few hours to several days). The phase separator is by far the most widely studied component of the continuous liquid--liquid extractor because of its decisive influence on performance. In 1993, Ling and

210

M. Valc~,rcel and M. Gallego

Hwang 28reported a glass made T-type separator with a dual chamber. Similar models have also been developed recently. 29'3~

7.3.2 CLLE-AS configurations The outcoming phase containing the extracted species (generally neutral chelates or ion pairs) is connected on-line or on-line/off-line to the atomic spectrometric detector in continuous liquid--liquid extractor (CLLE)-atomic spectrometric (AS) coupled systems. The CLLE is placed in a flow injection manifold furnished with conventional components such as perisl~altic pumps, tubing, connectors, and injection and switching valves, a detailed description of which is beyond of the scope of this chapter. Nevertheless, it is interesting to emphasize two peculiar technical aspects of these manifolds, viz. the fact that an even flow of an organic solvent is established and the need to control waste from the CLLE in order to ensure correct operation. Pumping organic solvents through ordinary pump tubing causes serious fluctuations in the flow-rate as a consequence of internal corrosion; the use of flexible tubing resistant to organic solvents, which is expensive and less durable, is the most immediate alternative. The best way to circumvent this problem is by using displacement flasks, which are stoppered glass bottles that allow entry and exit of water and organic solvent, respectively. The water flow established by the peristaltic pump enters through the bottom or top of the flask (depending on the relative densities of the two phases involved), and the organic stream leaves through the top or bottom. Smooth operation of a phase separator is accomplished by controlling the outcoming waste via a peristaltic pump and/or by using a long tube as restrictor. CLLE-AS associations can be classified according to two different criteria. In terms of injection valves, CLLE-AS coupled systems can be completely continuous or use one or several valves. Injection valves can be placed before or after the continuous extractor. In terms of the link between the continuous flow configuration and the atomic spectrometer, there are several choices that are schematically depicted in Figure 7.5, namely: (a)

(b) (c)

On-line connection between the CLLE and AS. FI and AS are very easy to couple via a continuous sample insertion instrument of the type used in FAAS, FAES, AFS, ICP-OES or ICP-MS for a technical details, readers are referred to Part I of this book. Mixed on-line/off-line connection when a typically discrete sample insertion system such as those used in ETAAS is employed (see Chapter 6); and Off-line connection, i.e. a manual link between the CLLE and the atomic instrument.

All these technical alternatives are discussed in subsequent sections of this chapter devoted to CLLE coupling with different atomic spectrometers.

Flow-injection on-line liquid-liquid separation

211

Figure 7.5 Main ways of linking an FI-configuration furnished with a continuous liquid-liquid extractor to an atomic spectrometer. For details, see text.

7.3.3 Chemical systems involved The CLLE and atomic spectrometer used must be made chemically compatible owing to the intrinsic nature of metal ions, which are the types of specie-analytes most frequently determined in AS but are not directly involved in the liquid-liquid extraction. A derivatization reaction must thus be used at some time in the continuous process in order to convert the analytes into extractable species. Generally, an aqueous stream of reagent is mixed with another aqueous stream of sample or, alternatively, an aqueous carrier containing an injected sample plug, a mixing coil being used before the aqueous stream reaches the solvent segmenter. The formation of neutral chelates with typical soluble ligands such as ammonium pyrrolidinedithiocarbamate (APDC) is the most frequent chemical reaction used when the analytes are metal ions (direct determinations). The scope of application of CLLE-AS combinations can be expanded by using metal ions as reagents to determine other inorganic or organic species by formation of extractable chelates or ion-pairs. These are called "indirect determinations" because the species directly responsible for the AS signal is not the analyte but the reagent. In most applications reported in this context, isobutyl methyl ketone (IBMK) provides the best results as organic solvent. Carbon tetrachloride and chloroform are generally unsuitable here owing to their poor combustion properties and high toxicity, when using flames and their lowering of the temperature of the plasma in ICP--OES or ICP-MS.

7.3.4

CLLE-AS coupling

7.3.4.1 FI configurations Figure 7.6 depicts the most relevant FI configurations involving liquid-liquid extraction coupled on-line to FAAS. They differ in the way the sample is inserted and in the location of the injection valve.

212

M. Valc~ircel and M. Gallego

!

W

"1

"1

I

"

] I

H

-----~ w s

o~

Figure 7.6 Three main types of FI manifolds including a continuous liquid-liquid extractor coupled on-line to a conventional flame atomic absorption spectrometer (FAAS) based on sample injection (a) and sample aspiration (b, c). The injection valve can be located before (a), after (b) and in (c) the continuous liquid-liquid extractor. DF, displacement flask; RC, reaction coil; IV, injection valve; W, waste; SS, solvent segmenter; EC, extraction coil; PS, phase separator.

Flow-injection on-line liquid-liquid separation

213

Figure 7.6(a) shows a CLLE-FAAS coupling based on sample injection before the extractor. The sample plug is injected into an aqueous buffer carrier and mixed with a stream of reagent (ligands, counter ions). Neutral extractable species are formed in the reactor coil that are driven to the extractor. The organic phase stream, obtained by using a displacement flask, also reaches the extractor, where mass transfer takes place. The outgoing organic stream, containing derivatized analytes, is connected directly on-line to the flame atomic absorption spectrometer. This configuration has two main practical drawbacks. Thus, the sample volume introduced is low so preconcentration of the analytes is almost impossible; this arrangement is suitable for interference removal in samples with high analyte contents. The other drawback arises from the difference of roughly one order of magnitude between the minimum uptake sample flow of the instrument and the maximum delivery rate of the organic stream leaving the extractor. The precision and sensitivity clearly suffer as a result of the starvation conditions created in the nebulizing system. This shortcoming can be circumverted by inserting a stream of the same or a different solvent into the outgoing organic stream via a T-piece; this creates dilution effects that decrease the sensitivity and precision markedly. It is thus necessary to save an appropriate volume of separated extractant for incorporation into a stream compatible with the FAAS sample introduction system. Figure 7.6(b) shows a CLLE-FAAS coupling based on sample aspiration in which the injection valve is located after the continuous extractor. The sample stream (the volume of which is controlled via the flow-rate and aspiration time) is continuously introduced and mixed with a reagent stream. After the reaction coil, the aqueous stream containing the derivatized analytes reaches the solvent segmenter of the extractor, which also receives the organic solvent stream. The outgoing organic extractant fills the loop of a conventional injection valve and is sent to waste. A water or air carrier at an appropriate flow-rate transports the extractant plug to the nebulizer of the FAAS instrument on switching the injection valve. The shortcomings of the other configuration (Figure 7.6(a)) are minimal because it affords high sample volumes and there is no divorce between the flow-rate of the outgoing stream from the FI manifold and the incoming flow-rate of the atomic instrument. This configuration was that originally developed by Nord and Karlberg. 4' 5 A multipurpose injection valve can also be placed before or in the liquid extractor, as shown in Figure 7.6(c). In the extraction step (that shown in the Figure), the sample/ reagent stream is mixed with the organic solvent in the solvent segmenter. The segmented stream is passed through the valves and mass transfer takes place in the coil and the segmented flow is driven to the phase separator. When the extracted organic phase (lighter than water) fills the upper part of the separator, the valve is actuated (second step). A water stream at an appropriate flow-rate propels the organic phase to the FAAS instrument; during this step, pump 1 is running and pump 2 stopped. 28

214

M. Valcfircel and M. Gallego

The assemblies of Figures 7.6(b) and 7.6(c) allow the discrete introduction of plugs of organic solvents. Under these conditions, the flame properties are altered; background correction is desirable, though not indispensable. The low enrichment factors achieved in CLLE-FAAS are due to its low flexibility as regards changing the aqueous sample/organic solvent ratio, which is directly related to the flow-rate ratio. Furthermore, ensuring smooth operation of this system in routine analyses is rather difficult owing to its little ruggedness. These are the sources of the small number of reported applications relative to other FI continuous separation alternatives. This is quite true of direct determinations of metal traces; however, the indirect determination of non-metal species opens up interesting analytical prospects. These are the topics dealt with below.

7.3.4.2

Direct determinations of metal traces

Books, i-3'26'27 reviews 3~ and an on-line literature survey provide an indication of the main

direct applications of CLLE-FAAS configurations, which in fact have been rather scarce in the last few years despite the promising prospects envisaged in early reviews. 32-35Table 7.1 shows the most outstanding direct determinations of FI-FAAS coupled systems including a liquid--liquid extractor. The majority of these manifolds are based on that depicted in Figure 7.6(b), which almost invariably uses an air or water stream to transport a plug of organic extractant after it leaves the phase separator. 36 Traces of copper, 5'28"36-39 zinc, 5"41"42'44'47 gold, 28"37"46 lead, 5"33 iron, 36 manganese, 4~ magnesium,44 cobalt,43.47 nickel 5.43,47 and indium 45 have been determined in this way in a great variety of real samples, the majority of which required a sample pretreatment (e.g. dissolution, solid-liquid extraction), so they are not fully automated methods except in the case of water samples that are directly introduced into the FI manifold. In many instances, limits of detection in the nanogram-per-millilitre range are achieved. Throughput is quite high in general, but particularly when a mixed organic solvent mixture (toluene/xylene) is used 38 instead of IBMK. The time needed for sample pretreatment was not taken into account, which is quite usual with the FI method. In some instances, the ligand is manually added to the sample before it is aspirated into the FI manifold 4~ without exploiting the advantages of implementing this step in a continuous, automatic way. The precision is quite acceptable; RSD values are further improved by using more efficient phase separators. A continuous water/oil/water (W/O/W) emulsion/demulsion process was used to implement atypical continuous liquid--liquid extraction coupled on-line to FAAS for the direct determination of metal ions. 44 Figure 7.7 shows a schematic diagram of the

Table 7.1

Flow injection liquid-liquid extraction methodologies coupled on-line to FAAS instruments. (I) Direct determinations

Analyte

Sample

Cu, Zn, Pb, Ni

Ligandreagent

Organic solvent

Detection limit (txg/mi)

Frequency (h-t)

RSD (%)

Reference

PDC

IBMK

~ 1 x 10- 3

40

1-6

5

DDTC SCN- (Cu) CI- (Au) 5-7 dibromo-oxine 5-7 dibromo-oxine

IBMK IBMK

36 37,28

toluene/xylene (1:4) xylene

~4.3 1.8 (Cu) 2.5--4.9 (Au) 0.5%

PDC

IBMK

m

Fe, Cu Cu, Au

gelatins ores

Cu Cu

tap water rain and tap waters hair, waters, pharmaceuticals synthetic environmental and biological

Mn Zn Zn Pb Co, Ni Mg, Zn In Au Cu, Cd, Co, Ni, Zn

steels reagents and alloys geological samples industrial streams geological

SCN-

IPDC

IBMK

IBMK IBMK

continuous emulsion-demulsion process (double phase separator) bis(2-ethylhexyl)IBMK phosphate

1 x 10-3 (Cu) 1.8 x 10- 3 (Au) 7x 10 -3

20 180 80

40

0.2 0.01

41 42

60

1.4 3.9-6.0

! p.Jo

b..,~

38 39

0.076

60

o

o o .-.:. t~

33 43

0.02 0.01 (Co) 0.02 (Ni) 0.02 (Mg) 0.10 (Zn) 0.03

60

1.5

45

0.005

24

2-7

46

<0.001

off-line

44

15

o

supported liquid membranes 47

to

216

M. Valcarcel and M. Gallego

Figure 7.7 Continuousemulsion/demulsion processes for extracting metal ions coupled on-line to FAAS. MC, mixing coil; IV, injection valve; NV, needle valve; AP, aqueous phase; O,oil phase; W, waste. For details, see text. manifold employed for the preconcentration-determination of magnesium and zinc in duralumin alloys and zinc in commercial reagents. The formation of W/O emulsions takes place in MCI as a result of a stream of HC1 (inner aqueous phase) and a stream of the "oil phase" [SPAN-80 as surfactant, palmitic acid or di(2-ethylhexyl)phosphate as extractant in kerosene as solvent] being merged. The W/O emulsion is merged with an outer aqueous phase stream (water containing the injected sample plug) at the extractor's confluence point. The W/O/W emulsion is formed in the extraction coil; a T-type phase separator allows sending the outer aqueous phases (AP1) to waste. The remaining W/O emulsion, containing the analytes, is passed through a dry bath at 120~ after mixing with a stream of 2-ethyl-hexanol (demulsifier). A second T-type separator sends the organic phase to waste; the outgoing aqueous stream (AP2), containing the analyte, is transferred to the FAAS with the aid of an air stream. The preconcentration factors thus achieved are 2.4 for Mg and 1.8 for Zn; additional figures of merit of this methodology are given in Table 7.1. Continuous liquid-liquid extraction for the direct determination of metal traces can also be implemented with supported liquid membranes (SLM) with none of the typical extractor elements (segmenter, coil or phase separator). Figure 7.8 depicts the two main technical alternatives, viz. column and sandwich configurations. The classical approach (Figure 7.8(a)) to continuous extraction using SLM employs a sandwich arrangement similar to that of the phase separator. 47 The supported liquid membrane is a porous PTFE membrane previously soaked in di-n-hexyl ether in the presence or absence of a positively charged organic counter ion (e.g. Aliquat-336). The donor stream is the result of continuous mixing of the sample containing the metal ion and ligand buffer streams. Neutral (copper-oxine) or anionic (Co(SCN)]-) complexes are transferred through the membrane, as such and as ion-pairs, respectively. The acceptor stream must contain a ligand forming strong (charged) complexes in such a way that the metal ions retained as chelates or ion-pairs on the SLM pass into this phase. The system operates in two steps.

217

Flow-injection on-line liquid-liquid separation

I'. ACCEPTOR v ~[ "

-

_

Figure 7.8 Continuous liquid-liquid extraction configurations coupled to FAAS based on supported liquid membranes (SLM): (a) sandwich approach; (b) "in situ" SLM formation. RC, reaction coil; MC, mixing coil; W, waste; IV, injection valve; DF, displacement flask; O, organic phase. For details, see text. In the first, pump 1 works while pump 2 is at rest; formation of chelates or ion-pairs takes place in the reaction coil and mass transfer through the SLM occurs; a volume of a few microlitres of acceptor stream is stopped in the separator to effect preconcentration. In the second step, pump 1 is stopped while pump 2 works; the plug containing the extracted metal ions is transferred directly to the FAAS instrument or 2 ml of acceptor solution to the autosampler carousel of ETAAS. 47 The figures of merit of this approach are shown in Table 7.1.

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M. Valc~ircel and M. Gallego

SLM technology can also be approached by "in situ" formation of supported films of the organic extractant solution on a polymeric support packed in a minicolum, as is shown in Figure 7.8(b). 46 First, the SLM is formed as a thin layer in the Polysorb MP-1 support minicolumn located in the loop of the injection valve IV2 by injecting (via IV1) 10 Ixl of organic phase (1% tridecylamine in 1:1 heptane/tri-n-butylphosphate), which is passed through the minicolumn. Second, a time-based sample volume is introduced and merged with a buffer stream, the analyte [dicyanoaurate (I) ion] being retained in the SLM; during this step, pump 2 is stopped. Then (third step), pump 2 is started and IV2 is actuated to pass upstream a methanol stream through the minicolumn, the eluted analyte being transferred to the atomic absorption instrument. The excellent figures of merit of this approach are shown in Table 7.1. 7.3.4.3

Indirect determinations

FAAS indirect determinations are based on the use of metal ions as reagents forming chelates and/or ion pairs with inorganic and organic anions, and neutral species, which are the analytes. The product formed must be separated from the sample matrix in order to remove excess reagent added; solvent extraction and precipitation are by far the most frequently used separation techniques in batch-manual procedures, 48 which are tedious, time-consuming and a source of systematic and random errors. The automation of such procedures via FI(CLLE)-FAAS couplings opens up interesting prospects in this context as it minimizes or avoids the main drawbacks of the manual alternatives. 49 The FI-FAAS manifolds generally used to implement these indirect determinations are similar to that depicted in Figure 7.6(b). A sehematic depiction is provided in Figure 7.9. They are based on the injection of an extracted plug coming from a sandwich membrane separator into a water or air carrier. In order to optimize performance, the continuous system is completed by an additional line connecting the output of the sample loop valve to a peristaltic pump (via a displacement bottle) in order to carefully control the operation of the phase separator. Therefore, two displacement bottles are included in the manifold to control the input and output organic streams. The most relevant indirect FI-FAAS determinations based on liquid-liquid extraction are summarized in Table 7.2. The majority of the methods are based on the formation of ion pairs between the charged analyte (A) and cationic or anionic metal chelates which act as extractant reagents; in some instances, the ligand is added to the organic phase because it is insoluble in water. 5~ When an inorganic ligand such as SCN- ,54,58,59 i-,59 or C104 (Reference 60) is used, it is previously mixed with the metal ion to form the reagent complex stream, which is then merged with the treated sample stream containing the analyte (A). There are two exceptions to this general behaviour; the corresponding methods are based on the direct formation of a neutral metal chelate with a ligand that is

Flow-injection on-line liquid-liquid separation

219

WATER W

I Figure 7.9 General diagram of CLLE-FI-FAAS couplings for indirect determinations. RC, reaction coil; DF, displacement flask; IV, injection valve; W, waste. For details, see text. either the analyte 56 or its reaction product, 57 formed in the reaction coil (Figure 7.9) by mixing the sample stream containing amphetamines with a continuously stirred reagent carrier containing the metal ion, carbon disulphide and ammonia. Both are carbamate-type ligands (DTC) that form chelates with copper(II), 56 or copper(II), zinc(II) and Ni(II). 57 IBMK is the solvent most frequently used in this context by virtue of its favourable properties both for the continuous extraction and the FAAS determination. Copper is the most widely used element determined by FAAS in a direct fashion, followed by cobalt, 54 zinc and nickel, 57 chromium, 58 and bismuth and iron. 59 Except in a few cases, 56"58,59 limits of detection are below 1 ~g/ml for a great variety of analytes; sensitivity is thus quite favourable in automatic indirect FAAS determinations. Selectivity is also quite acceptable, as revealed by a systematic study of the indirect determination of cocaine, in presence of common alkaloids, with various counter-metal ion complexes such as BiI4, Co(SCN)4z-, Ni(SCN)4z-, Fe(SCN) 3- and Reinecke salt. 59 The best overall results were achieved with the ferrothiocyanate complex, but the sensitivity was better with Dragendorf reagent (BiI4) (see Table 7.2). The precision, as RSD, was quite favourable (less than 5%) and so was the throughput which always exceeded 25 samples/hour. These automatic indirect FAAS determinations have been applied to a variety of real

i,J

Table 7.2

Flow injection liquid--liquid extraction methodologies coupled on-line to FAAS instruments. (II) Indirect determinations

Analyte (A)

Sample

Species extracted

Organic solvent

TagDetection Sampling element limit (l~g/ml) RSD (%) frequency Reference

Cu Perchlorate urine [Cu(1)-6methyipicolineaidehyde azine]-A IBMK* Cu Nitrate [Cu(I)-neocuproine]-A IBMK* Nitrite (separated) Cu Nitrate and nitrite meat products [Cu(l)-neocuproine]-A IBMK* (discriminated) Cu Nitrate and nitrite pond waters [Cu(l)-neocuproine]-A IBMK* (discriminated) Co Cationic surfactants waters [Co(SCN~- I-A2 IBMK Cu Anionic surfactants wastewaters [Cu-ferroine]-A IBMK Cu Ziram (fungicide) Cu(II)-A IBMK M :Zn, Amphetamines pharmaceuticals DTC-M(II) IBMK Ni, Cu and urine Cr Amylocaine and synthetic Reineckates 1,2-dibromhexine mixtures chioroethane Bi Cocaine [Bil4-]-A 1,2-diFe [Fe(SCN)36- ]-A 3 chloroethane Cu Bromazepam plasma and [Cu(flO4)2]-m 3 IBMK pharmaceuticals * Ligand of the extractant species in the organic phase

0.07 0.04 (NO3) 0.40 (NO2-) 0.04 (NO3) 0.04 (NO~-) 0.03 (NO;)

3.7 4.7 3.7 4.1

0.7 (NO;) (NO2-) (NO3) (NO2-)

45 35

50 51

20

52

25

53

0.1-0.2 0.05 0.12 i~mol 0.005-0.040

1.2 0.8 2.6 1.8-2.5

35 40 30

54 55 56 57

2.1-2.8

1.0-3.4

30

58

0.2 5 0.1

2.6 3.2 1.7

30

59

40

60

O

o

Flow-injection on-line liquid-liquid separation

221

samples. A preliminary preparation step is almost invariably needed. Discriminate determinations based on selective chemical reactions are also feasible. Such is the case with the nitrate and nitrite mixture, 52'53 the discrimination method for which is based on two sequential insertions of sample aliquots; one is an aliquot previously treated with Ce(IV) to oxidize NOZ to NO3-; and the other is an aliquot mixed with sulfamic acid to transform NO~- into N2. The signal for the second sample aspiration gives the NO3 concentration, that of NO~- being obtained by difference. Chapter 9 can also be consulted for this topic of indirect determinations.

7.3.5 CLLE-ETAAS coupling The synergistic effects of combined analytical systems frequently arise from mutual offset of the individual drawbacks of the instruments or apparatuses coupled by favourable features of the other partner. Such is the case with the CLLE-ETAAS coupling, which is schematically depicted in Figure 7.10. The intrinsically discrete nature of ETAAS insertion mechanism makes on-line coupling to continuous flow systems theoretically incompatible; several innovative on-line and mixed on-line/off-line assemblies for circumventing this shortcoming have been developed, however. Unformity of extracted species in the output stream of the CLLE is one clear advantage over continuous alternatives (sorption, precipitation); this effectively raises the precision of the coupled system as it offsets one pitfall of ETAAS, viz. the fact that the graphite furnace only allows introduction of a few microlitres of extracted sample. In addition, there is no need for quantitative phase separations; thus, a simple T-type separator that requires no maintenance can be used instead of membrane separators. The strong negative influence of matrix effects in ETAAS is clearly offset by CLLE systems, which allow both efficient interference removal and automatic addition of modifiers in a highly convenient way. The well-known high sensitivity of ETAAS-based methods compensates for the low enrichment factors provided by CLLE owing to its inflexibility as regards the aqueous/organic flow-rate ratio. It is also interesting to note that the signal enhancement produced by organic solvents in CLLE-FAAS is not obtained in C L L E ETAAS; however, the high sensitivity of ETAAS clearly offsets this drawback. On the other hand, the low ruggedness of CLLE can be improved to some extent in certain configurations. Continuous liquid extraction can be performed either in parallel with the furnace temperature program (e.g. two samples are simultaneously processed in the CLLE and graphite furnace) or sequentially (e.g. the CLLE remains at rest during ETAAS operation). Use of a dual-integrated computer controlled system is mandatory in the parallel operation mode, but the throughput is increased by a factor of at least two. Fang's book 3 and a recent review by himself 6~provide an excellent overview of the topic

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C L L A

'

| ~

FACTORS -LOWROBUSTNESS

Figure 7.10 Manual offset of the advantages (+) and disadvantages ( - ) provided by each partner in the CLLE-ETAAS couplings.

in the context of FI-AAS. In this chapter, a more comprehensive approach has been adopted to demonstrate new possibilities of the CLLE-ETAAS coupling. There are several technical ways to establish a link between liquid-liquid extraction and ETAAS (Figure 7.10). The traditional mechanical-shaking liquid-liquid extraction is a typical off-line approach; an aliquot of the dried organic phase is manually transferred to the graphite furnace, either directly or indirectly (through the autosampler). 62 A continuous liquid-liquid extractor can be combined with an ETAAS instrument in three main ways (Figure 7.11 ), namely:

(1) The extractant outcoming stream is collected into an autosampler cup in a typical offline approach, similarly to manual extractions.

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@

I

I"

I Figure 7.11 General ways of connecting continuous liquid--liquid extraction to an ETAAS instrument furnished with an autosampler: (a) off-line coupling; (b) mixed on-line/off-line coupling; (c) on-line coupling. PAl, programmable arm of the autosampler; PA2, external programmable arm; GT, graphite tube. For details, see text.

(2) The extractant stream is connected on-line to an external programmable arm (PA2) as a part of a laboratory-made automatic device for off-line introduction of an aliquot of the extracted plug into a cup (which acts as an interface) of the autosampler, which has its own programmable arm (PAl) for aliquot insertion into the graphite furnace. This mixed on-line/off-line configuration requires that the automatic external device work in a harmonious manner with the autosampler and ETAAS instruments. Thus, a microcomputer for controlling operational timing is mandatory if this type of hybrid system furnished with two programmable arms is to be used in routine analyses. This alternative has not yet been fully explored. (3) The programmable arm (PAl) built in to commercially available ETAAS instruments, or a laboratory-made arm, is connected on-line to the outgoing extractant stream in such a way that an aliquot of a few microlitres is introduced batchwise into the graphite furnace.

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7.3.5.1

Off-line CLLE-ETAAS configurations

The most simple technical way of connecting a continuous liquid-liquid extractor and the graphite furnace of an ETAAS instrument is the off-line approach depicted in Figure 7.11 (a). This assembly was reported in 1984. 63,64 It allowed the determination of cadmium, copper, lead, cobalt, nickel, iron and zinc by dual continuous-sequential liquid-liquid extraction. 63 A schematic diagram of this configuration is depicted in Figure 7.12(a). The first step involves the extraction of trace of metals forming dithiocarbamate complexes into a Freon 113 stream. In the second, the organic stream containing the metal chelates is mixed with a back-extraction stream (H3 O+, Hg2+); mercury ion forms highly stable DDTC complexes and displaces the metals ions to the aqueous phase in the second liquidliquid extractor. The organic phase is discarded and the aqueous phase collected in a small sample cup. An aliquot of this aqueous phase is manually transferred to the graphite furnace. It is interesting to note that the phase separator in the first extractor is membranetype in order to assure quantitative phase transfer, whereas that in the second is a more simple, T-type phase separator because quantitative phase recovery is not required. Analyte recoveries ranging from 80-95% are somewhat lower for cobalt. Metal concentration factors of 25-30 are thus obtained except for cobalt, which is only concentrated 15-fold.

7.3.5.2

On-line CLLE-ETAAS configurations

Backstrom and Danielsson 6~7 converted the original off-line CLLE-ETAAS couplings into on-line systems and used them as dual extraction system 6-v65 depicted in Figure

W

>2 ?i 'i

I|

',

I ""[ I" w

~W i

Figure 7.12 Off-line (a) and on-line (b) coupling of a dual continuous extraction system to an ETAAS instrument for the determination of transition metal ions. CLLE1, continuous liquid-liquid extractor furnished with a membrane phase separator; CLLE2, continuous liquid--liquid extractor for back-extraction based on a T-type phase separator. IV, injection valve; W, waste; GT, graphite tube.

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7.12(b). The FI-CLLE system and the furnace programme operate in parallel. The interface between the two coupled systems is a 6-port injection valve, the 23-txl loop of which is filled with the outgoing aqueous stream from the second separator. The loop contents are delivered directly into the graphite tube through the programmable arm by using an air stream generated at a low flow-rate by an additional peristaltic pump. Preconcentration factors are 50 to 100 and the sampling frequency 30 h-~. The RSD is between 1.5% and 2.7%, and limits of detection about 10 ng/litre. Extraction yields range from 80% to 107% for cadmium, cobalt, copper, iron, nickel and lead. Zinc cannot be determined in this way because of the high blanks produced by the pump tubing as a result of using no displacement flask to propel the organic solvents. 66 The same Swedish group 68 adapted the CLLE-ETAAS on-line coupling for extraction in a single step and insertion of the organic phase containing the neutral chelates. This approach has been successfully applied to the determination of "quickly reacting aluminium" in natural waters. The sample stream is merged with an oxine stream and then with a toluene stream. After mass transfer in the coil, a modified gravitational phase separation allows the organic phase containing the Al(Ox)3 chelate and excess oxine to fill the loop of the injection valve as shown in Figure 7.12(b). The limit of detection thus achieved for A1 is 0.5 ng/ml and the RSD 3.6%. A similar approach was used by the same group 69 to systematically evaluate the liophilicity of metal complexes via the aluminium partition coefficient in n-octanol/aqueous systems. By using a configuration similar to that of Figure 7.6(c) including a batch gravitational phase separator, 28 Lin et al. TM developed a CLLE-ETAAS coupled system with an interface similar to that developed by Danielsson et al. 65-69 Traces of gold and cadmium in geological samples were determined with a sampling frequency of 20-30 per hour and RSD values ranging from 4.1% to 4.9%. With a slight variation of the manifold depicted in Figure 7.6(c), Tao and Fang 7~ accomplished the determination of nickel in biological samples with excellent figures of merit, namely: LD 4 ng/L, RSD 1.5-2.2% and a throughput 20 samples/h. A new PTFE-stainless steel gravitational phase separator was employed to increase robustness. 7.3.6

C L L E - I C P coupling

Continuous liquid--liquid extraction is highly convenient as a non-chromatographic separation technique for coupling to ICP-OES because it minimizes or avoids the main drawbacks of such powerful techniques including poor sensitivity (especially with conventional nebulization devices), spectral interferences caused by matrix effects, nebulizer blockage due to high solid contents, etc. Automatic matrix isolation and preconcentration by CLLE are the two main aims of this coupling. On the other hand, sample flow intakes of conventional ICP-OES instruments are considerably lower than

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those of FAAS, also compatibility problems between CLLE and FAAS are also minimized or avoided. A continuous liquid--liquid extractor can be coupled to an ICP--OES instrument by using off-line, on-line or tandem on-line approaches, which are schematically depicted in Figure 7.13 and described in the following sections.

7.3.6.1

Off-line CLLE-ICP configurations

The off-line liquid-liquid extraction-ICP coupling can be approached in two main ways. Thus, the separation can be implemented in the batch mode, with mechanical shaking and manual transfer of the organic phase to the ICP introduction device; such has been the case with the determination of cobalt, copper, nickel and cadmium in biological materials. 72 Figure 7.13(a) illustrates the other off-line alternative, which involves manual transfer of the collected outgoing organic phase from a continuous liquid-liquid extractor, as in the determination of beryllium in Mg/AI and copper alloys. 73An aliquot of the dissolved alloy is mixed with the ligand (acetylacetone), buffer (acetate), masking agent (EDTA) and extractant (C14C). The separator comprises an inner micro-porous PTFE tube through which the mixed phase flows and an outer PTFE tube that removes the separated organic phase. Aliquots of this phase are acidified with HNO3 and subjected to a dual evaporation

|

:

Figure 7.13 Schematic diagram of the main technical ways of connecting a continuous liquidliquid extractor and an ICP--OES instrument: (a) off-line; (b) on-line; (c) tandem on-line.

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re-evaporation process; the final solution is manually transferred to the ICP instrument. The LD thus achieved is 1.2 ng of Be, and the RSD 1.3%. A critical comparison between continuous liquid-liquid extractors coupled on-line and off-line to ICP for the determination of metals at the picogram-per-millilitre level based on complex formation was made in 1989 by Backstrom et al.. TM 7.3.6.2

On-line CLLE-ICP configurations

As is shown in Figures 7.13(b) and 7.14, there are two main ways of coupling a continuous liquid-liquid extractor and an ICP instrument on-line. Direct on-line coupling of a CLLE and ICP (Figure 7.14(a)) entails continuous transfer of the outgoing organic stream to the nebulizer, the nature of which strongly influences the design and performance characteristics of the coupled system. Copper in biological tissues can be determined with an LD of 1.5 ng/ml by batchwise formation of its macrocyclic dioxotetra-amine chelate. 75 A stream of the solution is pumped to the liquid--liquid extractor, which also receives a stream of chloroform. After mass transfer in the coil, the phase separator allows the organic extract to be pumped to the ICP conventional nebulizer. The LD for copper (0.1 ng/ml) can be improved by continuously extracting the metal with 0.02% dithizone in C14C; the extracted phase is directly inserted into an ultrasonic nebulizer.76 Trace of metals (Cd, Co, Cu, Fe, Mo, Ni and V) have thus been determined in

Figure 7.14 On-line coupling of a continuous liquid-liquid extractor and the nebulizer (N) of an ICP--OES instrument: (a) direct coupling; (b) coupling based on a continuous interface. IV, injection valve; W, waste. For details, see text.

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natural waters with limits of detection of 0.1-2.2 ng/ml by batchwise formation of carbamate complexes and continuous liquid-liquid extraction with IBMK; the organic phase is directly transferred to an ICP recycling nebulizer system. 77 The indirect determination of fluoride in waters based on the formation of the lanthanum-alizarin complexone-fluoride complex can be accomplished by using a CLLEICP-OES direct coupling and hexanol containing N,N-diethylaniline as extractant. TM The top or waste-water sample is injected into a water carrier that is merged sequentially with streams of acetate buffer and reagent (lanthanum-alizarin complexone); the resulting stream is merged with the organic phase in the solvent segmenter. Formation and extraction of the ternary complex takes place in the coil, after which a membrane separator effects phase separation. In order to ensure that the organic stream is completely free of aqueous phase, it is passed through another phase separator. The clean outgoing organic stream is directly connected to the concentric-glass nebulizer via a Scott-type double-pass spray chamber prior to the ICP torch; the emission intensity of La at 333.75 nm is then measured. The sampling rate thus achieved is 36 h-I. The RSD is 2.2% for 1 I~g/ml F-~. Compared to the batch method, 79 the LD (30 ng/ml) is 50 times higher; however, the continuous method is more rapid, precise and selective. The other way of coupling a CLLE and an ICP instrument on-line is by using an interface such as that of Figure 7.14(b) which consists of an injection valve and an additional peristaltic pump, and is similar to that used in the CLLE-FAAS coupling (see Figure 7.6(b)). The outgoing organic phase fills the loop of the valve and is injected into a stream of air or clean organic phase that is directly connected to the ICP nebulizer. 8~This is a convenient way of circumventing the problems arising from incompatibility between the two flow-rates used. This type of system was used to determine cadmium 8~and Cu, Co, Zn and Cd in biological materials. 8'

7.3.6.3

Tandem on-line CLLE-ICP configurations

The sensitivity of ICP determinations of elements forming volatile species (e.g. hydrides or Hg vapour) is markedly improved when the elements are introduced as gases rather than dissolved in liquids; this is a result of the efficiency of analyte transport to the ICP torch being dramatically increased. However, the generation of such volatile species is subject to interferences. Sequential liquid-liquid-extraction/formation of volatile species/gasliquid separation is an excellent example of how continuous flow techniques can dramatically improve the ICP determination of traces of this type of metal through interference removal in the liquid-liquid extraction and the continuous formation of volatile species in organic media (for hydrides) and the use of a stream containing a reductant such as NaBH4 or SnC12 (for mercury). This triply coupled system was called "tandem on-line" by Sanz Medel's group. A detailed description of these systems can be

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found in Chapter 9 of this book. They have been successfully used for the sensitive, selective determination of arsenic, 82-84 mercury, 85-87 antimony 88'89 and iodide. 9~ In some cases, they have also been used for speciation purposes. 85'89

7.4

Dialysis

Dialysis is a barrier separation technique in addition to osmosis and ultrafiltration, among others. Mass transfer takes place through a semipermeable membrane located between two liquid (donor and aceptor) phases that need not be immiscible. Separations are kinetically controlled and based on the occurrence of concentration gradients between the two liquid phases isolated by a selective membrane that discriminates solutes according to pore size and/or electrical charge. The coupling of continuous dialysis to AS instruments has received little attention probably because of the limited capabilities of a continuous dialyser to implement preconcentration as compared to other non-chromatographic continuous separation techniques. ~

7.4.1 Typesof dialysis Dialysis processes can be classified according to various criteria including the dynamic state of the two phases involved and the nature of the membrane, among others. The two phases can be quiescent or in motion. If both phases are quiescent, then the process is allowed to develop until the mass transfer equilibrium is reached. If one or both phases are in motion, equilibrium is never reached and operational time must be controlled if the system is used in analytical methodologies. There are several possibilities that depend on the particular type of dialyser: (a)

(b)

Flowing systems in which both phases reach a tubular or sandwich type dialyser (Figure 7.15(1)), either downstrean or upstream; both phases can be in motion or one remain quiescent during the separation process. Batch approaches, in which one or two fixed volumes of the two phases are efficiently agitated. A quite usual alternative involves the use of a stirred volume of one of the phases that is crossed by a flow of the other in a tube acting as a dialysis membrane (Figure 7.15(2))

According to the nature of the membrane, which determines the nature of the process that takes place across it, dialysis processes may be passive or active (Donnan dialysis). In passive dialysis, a neutral polymeric membrane of controlled porosity (passive membrane, PM) acts as a highly efficient molecular or ionic "filter", separations are thus based on size (molecular weight) differences. The main drawback of this approach is the low solute diffusion rates involved, which decreased the efficiency (1-7%) of dynamic

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Q AS

V

DS i:i:i:i:i:i:i:i:i:! DS ~-.-.:~..::~.~:..--,~:::.-..x~ :::::::::::::::::::::::::::::::::

SB

Figure 7.15 Most usual types of continuous dialysers coupled to atomic instruments for analytical purposes. (1) Continuous flow dialysers in a sandwich (la) or tubular (lb) configuration; (2) Batchflow dialysers mainly used in Donnan processes. DS, donor (sample) stream; AS, acceptor stream ("dialyser"); PM, passive membrane; AM, active (Donnan) membrane; C, downstream acceptor stream; CC, upstream acceptor stream; SB, magnetic stir-bar; IV, injection valve. For details, see text. approaches (see Figure 7.15(1)). For this reason, this approach cannot be used for preconcentration. Its main analytical applications are automatic on-line dilution and interference removal (e.g. of suspended solids in the liquid sample). In active (Donnan) dialysis, a porous ion-exchange barrier or membrane (active membrane, AM) separates a high-ionic strength aqueous phase from a low-ionic strength aqueous phase (Figure 7.15(2)). Ions of appropriate charge for the membrane are transported to the more concentrated phase (acceptor) from the more diluted phase (donor). Since the membrane is impermeable to co-ions, ions from the dilute solution must diffuse to the more concentrated phase in order to maintain electroneutrality. Tubular active membranes are particularly advantageous here since they have high surface area-tointernal volume ratios and can be readily coupled on-line to various types of detector. It is interesting to note that the configuration depicted in Figure 7.15(2) is not typically online because the main process takes place in a batch mode; the term "batch-flow" dialyser

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could be more appropriate to describe it. Preconcentration of metal traces is quite feasible. 7.4.2

Passive continuous dialyser-AS coupling

The main analytical objective of the passive dialysis-AS coupling is to implement robust on-line sample dilution for analytes whose concentration in the sample falls outside the calibration range of the atomic detector. The earliest attempt in this context was made by Lima et al. 9~ in the FAAS or FAES determination of calcium, magnesium, sodium and potassium in wines. The dialysis unit was of the sandwich type and furnished with a microporous PTFE membrane. The sample was injected into a water carrier that was merged with a 0.2 M HC1 stream to remove interferences. The resulting stream acted as donor stream in the dialyser. The acceptor stream, propelled at a flow-rate compatible with the atomic detector, was water. The limits of determination achieved were 7 (Ca), 5 (Mg and Na) and 9 i~g/ml (K). A high throughput (120-150 samples/h) and a relatively low RSD (N4%) were achieved. The same research group reported 92 the determination of the same analytes (K and Na by FAES; Mg and Ca by FAAS) in soil extracts by using a stream splitting and dialysis unit for the determination of soluble Na and K, and an additional sandwich dialyser for the determination of available K, Ca and Mg. The throughput was 60-150 samples/hour and the RSD 4%; FI-FAAS and FI-FAES results were consistent with those provided by a reference method. Recently, this group 93 developed a tandem on-line double dialysis system for the determination of Na and K in vegetables following dissolution. After injection of a sample aliquot (~ 300 I~l) into a water stream, the first dialysis unit enabled the direct determination of K by FAES in the acceptor water stream. The outgoing donor stream was connected to the second dialyser, the acceptor stream from which was transferred on-line to the flame emission instrument for Na determination. An additional goal of the continuous passive dialyser-AS coupling is automatic adaptation to analyses of samples with highly variable analyte contents, which require an automatically variable dilution factor. Such is the case with the determination of copper in industrial effluents. Van Staden and Hattingh 94 developed a multipurpose sandwich analyser consisting of a single donor sample channel and a multi-acceptor channel with multi-outlets. The analyte in the sample stream was subjected to automatic dilution the degree of which (dilution factor) depended on the specific draining point. The diluted analyte in the dialysate from a chosen draining point was channelled via a three way valve to a common sampling valve loop. A carrier stream transported the injected plug to the FAAS detector. Inserting a detector into each outgoing dialysate stream allowed the system to be used for multideterminations. One of the main drawbacks of passive dialysers is that sensitivity cannot be improved

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owing to the low efficiency of mass transfer through the membrane (PM). There are several ways to accomplish preconcentration in this type of system. By stopping the flow of the acceptor stream during mass transfer, the analytes are concentrated in the acceptor channel of the dialyser; after this step, the flow is restored and the plug driven to the detector. This approach has not been used in combination with atomic spectroscopy, but only with molecular spectroscopies and electrochemical detection. 95 One other way of implementing preconcentration in this context is by combining a typical continuous separation technique with the dialyser in a tandem configuration. Such is the case with the manifold developed by Van Staden and Hattingh 96 for the determination of copper in extracts of vitamin and mineral food supplements for dogs and cats. The continuous system coupled to a FAAS detector was quite complicated (three peristaltic pumps, four valves, a sandwich dialyser and an ion-exchange minicolumn) and required computer control for appropriate performance.

7.4.3 Active (Donnan) dialyser-AS coupling The so called "FI-Donnan dialysis" (FI-DD) is based on the use of a tubular ion-exchange membrane (e.g. Nation) that is immersed (as a coil) in a sample donor solution. A highionic content acceptor stream is passed through it that is on-line connected to an AS instrument as shown in Figure 7.15(2). Transfer of the target analytes takes place through the tubing walls as a consequence of the different ionic strength of the donor and acceptor solutions. The operation usually requires stopping the acceptor flow and vigorous stirring of the sample donor solution. The efficiency of the process depends on several experimental variables such as stop-time, temperature, pH, ionic strengths, ionic composition of both solutions, etc. The main analytical advantages of this approach were demonstrated by Koropchak and Dabek-Zlotorzynska97 in 1987 in the determination of Ca, Cu, Co and K by FAAS; signal enhancement factors up to 22 were obtained within 5 min. The typical phosphate interference in the determination of Ca was strongly reduced: PO34-/Ca 2+ ratios up to 80 were tolerated. This mixed batch-flow approach, thus affords preconcentration. An almost identical system was used for the FAAS determination of lead in drinking waters; 98 enrichment factors of up 100 to were achieved within 5 min of operation. A sensitive FAAS determination of gold traces (35 ng/ml as LD) was accomplished by using an off-line approach based on the batch collection of a 5 ml volume that was recirculated in a tubular Nation membrane inserted in the donor sample solution, which contained AuBr4- and ethylenediamine. An aliquot of the acceptor stream was manually introduced into the FAAS instrument. 99 In 1988, Koropchak and Dabek-Zlotorzynska ~~176 reported the first FI-DD--ICP-OES coupling for preconcentration and interference removal. A systematic study of the influence of the key experimental variables was made. The promising results obtained in

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terms of enrichment factors and independence of sample matrix over a wide range of concentrations opened up interesting prospects. In 1993 the same research group reported a more detailed and practical study on the FI-DD-ICP-OES on-line coupling. ~~ By using acceptor streams of either Sr2+ or Mg 2§ the system provided enrichment factors up to 200 (with a preconcentration time of 8 min) afforded nanogram-per-millilitre level LD and significantly reduced spectral and matrix interferences. Increasing dialysis time and/or the temperature of the donor sample stream considerably improved enrichment factors; thus, a signal enhancement factor of 650 for Ag § (LD 11 pg/ml) was achieved by using a 30 min dialysis time and longer lengths of the cation-exchange tubing at a 42~ final solution temperature.

7.5

Other systems involving organic solvents

The use of organic solvents in continuous flow systems that involve no mass transfer between two immiscible (extraction) or miscible (dialysis) liquid phases is quite interesting as it improves on several key analytical aspects. The earlier attempt in this context was reported by Fukamachi and Ishibashi in 1980 ~~ in the form of a single line FI manifold connected on-line to a FAAS instrument. An aqueous sample volume was injected into an organic carrier that directly fed the detector's introduction device. Signal enhancement factors up to 4.8 were thus achieved for several metal ions by using either IBMK or n-butylacetate. Similar performance was obtained by Kumamaru e t al. 1~ in 1987, using ICP-OES as detector but an off-line approach. It is well known that continuous hydride generation performance can be dramatically improved in organic media. The tandem on-line connection of a continuous organic stream from a liquid-liquid extractor to a hydride generator prior to the gas-separator is an interesting approach developed by Sanz Medel's group 82-85'88-9~that is described in detail in Chapter 9 of this book. In this context, the effect of several organic solvents on the Se response has also been evaluated by using FI with hydride generation and ICP-MS detection. Results show that the presence of alcohols leads to an important reduction of polyatomic interferences as well as an enhancement factor of the Se signal of 5-fold. Linearity ranging from 10 pg up to 1 txg was established, being the most sensitive method for Se determination published to date. !~ Recently, an interesting approach was reported under the name "near melting point flow injection", 1~ that is quite useful for one particular purpose: the direct determination of Cs impurities in Na samples by leaching the solid sample heated near the melting point with a stream of 95% ligroin/5% 2-butoxyethanol as solvent in a continuous fashion, i.e. under non-equilibrium conditions. The solid-liquid extraction becomes highly efficient just before complete melting, when heating up the sample increases the volume ratio of the

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liquid intragranular phase to the solid phase to about 3%. The outcoming organic reagent is connected on-line to an FAAS instrument. The limit of detection is 5 I~g/kg, which is 100 times better than that obtained with conventional FAAS methodology. 7.6

References

1 Valcfircel, M. and Luque de Castro, M. D., Non Chromatographic Continuous Separation Techniques, Royal Society of Chemistry, Cambridge (UK), 1991. 2 Fang, Z., Flow Injection Separation and Preconcentration, VCH Publishers, New York, 1993. 3 Fang, Z., Flow Injection Atomic Absorption Spectrometry, Wiley, New York, 1995. 4 Nord, L. and Karlberg, B.,Anal. Chim. Acta, 1981, 125, 199. 5 Nord, L. and Karlberg, B.,Anal. Chim. Acta, 1983, 145, 151. 6 Burgucra, J. L., Burguera, M., La Cruz, O. L. and Naranjo, O. R., Anal. Chim. Acta, 1986, 186, 273. 7 Liang, L., D'Haese, P. C., Lamberts, L. V., Van de Vyver, F. L. and De Bror M. E., Anal. Chem., 1991, 63, 423. 8 Hu, Q. L. and Luo, S. M., Fenxi Huaxue, 1993, 21, 1240. 9 Gu, Z. K., Lu, Z. E., Lu, Y. H., Tong, J. Y. and Hong, L., Lihua-Jianyan, Huaxue Fence, 1993, 29, 291. 10 Katoh, T., Akiyama, M., Ohtsuka, H., Nakamura, S., Haraguchi, K. and Akatsuka, K., J. Anal. At. Spectrom., 1996, I 1, 69. 11 Alonso, E. V., Garcia de Torres, A. and Cano, J. M., Talanta, 1996, 43, 493. 12 Brenner, !. B., Zhu, J. and Zander, A., Fresenius J. Anal. Chem., 1996, 355, 774. 13 Adachi, A., Ogawa, K., Tsushi, Y., Nagao, N. and Kobayashi, T., Water Res., 1997, 31, 1247. 14 De la Calle-Guntinas, M. B. and Adams, F. C., J. Chromatogr., 1997, 764, 169. 15 Burrini, C. and Cagnini, A., Talanta, 1997, 44, 1219. 16 De la Calle-Guntinas, M. B., Madrid, Y. and Cfimara, C., Mikrochim. Acta, 1992, 109, 149. 17 Hasegawa, H., Sohrin, Y., Matsui, M., Hojo, M. and Kawashima, M., Anal. Chem., 1994, 66, 3247. 18 Martinez, L., Baucells, M., Pelfort, E. and Roura, M., Fresenius J. Anal. Chem., 1996, 354, 126. 19 Chatterjee, A., Das, D., Mandal, B. K., Chowdhury, T. R., Samanta, G. and Chakraborti, D., Analyst, 1995, 120, 643. 20 Szpunar, J., Schmitt, V. O., Lobinski, R. and Monod, J. L., J. Anal. At. Spectrom., 1996, 11, 193. 21 Dirkx, W. M. R., De la Calle, M. B., Ceulemans, M. and Adams, F. C. J., Chromatogr. A, 1994, 683, 51. 22 Torres, P., Ballesteros, E. and Luque de Castro, M. D., Anal. Chim. Acta, 1995, 308, 371. 23 Laintz, K. E. and Tachikawa, E., Anal. Chem., ! 994, 66, 2190. 24 Meguro, Y., Iso, S., Takeishi, H. and Yoshida, Z., Radiochim. Acta, 1996, 75, 185. 25 Valc~ircel, M. and Luque de Castro, M. D., Automatic Methods of Analysis, Elsevier, Amsterdam, 1989. 26 Valc~ircel, M. and Luque de Castro M. D., Flow Injection Analysis. Principles and Applications, Ellis Horwood, Chichester, 1987. 27 Ruzicka, J. and Hansen, E., Flow Injection Analysis, 2nd edition., New York, 1988. 28 Lin, S. I. and Hwang, H., Talanta, 1993, 40, 1077. 29 Qui, H. and Shuai, Q., Fenxi Huaxue, 1997, 25, 72. 30 Lin, S. L., Shuai, Q., Qui, H. and Tang, Z. Y., Spectrochim. Acta Part B, 1996, 51B, 1769. 31 Carbonell, V., Salvador, A. and De la Guardia, M., Fresenius J. Anal. Chem., 1992, 342, 529. 32 Kuban, V., Kumarek, J. and Cajkova, D., Chem. Listy., 1990, 84, 376. 33 Fanz, Z., Zhu, Z., Zhang, S., Xu, S., Gu, L. and Sun, L., Anal. Chim. Acta, 1988, 214, 41. 34 Tyson, J. F., Adeeyinwo, C. E., Appleton, J. M. H., Bysouth, S. R., Idris, A. B. and Sarkissian, L. L., Analyst, 1985, 110, 487. 35 Tyson, J. F., Adeeyinwo, C. E. and Bysouth, S. R.,J. Anal. At. Spectrom., 1989, 4, 191. 36 Chen, S. Y., Zhang, M., Lin, S. Q. and Cheng, L., Guanpgpuxue-Yu-Guangpu-Fenxi, 1995, 15, 85. 37 Huang, H. P. and Lin, S. L., Fenxi Shiyanshi, 1994, 13, 73. 38 Memon, M. A., Wang, X. R. and Huang, B. L., At. Spectrosc., 1993, 14, 99. 39 Memon, M. A., Zhuang, Z. X. and Fang, Z. L.,At. Spectrosc., 1993, 14, 50. 40 Chi, X. Z. and Zhou, J., Guanpgpuxue-Yu-Guangpu-Fenxi, 1994, 14, 91. 41 Sweileh, J. A. and Cantwell, F. F., Anal. Chim., 1985, 57, 420.

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42 Ogata, K., Tanabe, S. and lmanari, T., Chem. Pharm. Bull., 1983, 31, 1419. 43 Gong, Y., Zhou, J. M., Tang, Z. Y., Zhang, S. C. and Jin, Z. X., Lihua Jianyan Huaxue Fence, 1997, 33, 59. 44 Yokoyama, T., Watarai, T., Uehara, T., Mizuoka, K. I., Kohara, K. Kido, M. and Zenki, M., Fresenius J. Anal. Chem., 1997, 357, 860. 45 Coello, J., Danielsson, L. G. and Hernfindez-Cassou, S., Anal. Chim. Acta, 1987, 201,325. 46 Taylor, M. J. C., Barnes, D. E. and Marshall, G. D., Anal. Chim. Acta, 1992, 265, 71. 47 Papantoni, M., Diane, N. K., Ndung'u, K., Joensson, J. A. and Mathiasson, L., Analyst, 1995, 120, 1471. 48 Hassan, S. S. S., Organic Analysis Using Atomic Absorption Spectrometry, Ellis Horwood, Chichester, 1984. 49 Valcfircel, M., Gallego M. and Montero, R., J. Pharm. Biomed. Anal., 1990, 8, 655. 50 Gallego, M. and Valcfircel, M.,Anal. Chim. Acta, 1985, 169, 161. 51 Gallego, M., Silva, M. and Valcfircel, M., Fresenius J. Anal. Chem., 1986, 323, 50. 52 Silva, M., Gallego, M. and Valcfircel, M., Anal. Chim. Acta, 1986, 179, 341. 53 Qiu, H. O., Shuai, Q., Tang, Z. Y. and Lin, S. L., Fenzi Huaxue, 1997, 25, 72. 54 Martinez-Jim6nez, P., Gallego, M. and Valcfircel, M., Anal. Chim. Acta, 1988, 215, 233. 55 Gallego, M., Silva, M. and Valcfircel, M., Anal. Chem., 1986, 58, 2265. 56 Jim6nez de Bias, O., Pereda de Paz, J. L. and Hernfindez-M6ndez, J., J. Anal. At. Spectrom., 1990, 5, 693. 57 Montero, R., Gallego, M. and Vaicfircel, M., Anal. Chim. Acta, 1991,252, 83. 58 Eisman, M., Gallego, M. and Valcfircel, M., J. Pharm. Biomed. Anal., 1993, 11, 301. 59 Eisman, M., Gallego, M. and Valcfircel, M., Anal. Chem., 1992, 64, 1509. 60 Santelli, R. E., Gallego, M. and Valcfircel, M., Talanta, 199 l, 38, 1241. 61 Fang, Z. L. and Tao, G. H., Fresenius J. Anal. Chem., 1996, 355, 576. 62 Collado, G., Bosch-Ojeda, C., Garcia de Torres, A. and Cano-Pav6n, J. M., Analusis, 1995, 23, 224. 63 Backstrom, K., Danielsson, L. G. and Nord, L.,Analyst, 1984, 109, 323. 64 Bengtsson, M. and Johansson, G.,Anal. Chim. Acta, 1984, 158, 147. 65 Backstrom, K. and Danielsson, L. G., Anal. Chem., 1988, 60, 1354. 66 Backstrom, K. and Danielsson, L. G., Anal. Chim. Acta, 1990, 232, 301. 67 Backstrom, K. and Danielsson, L. G., Mar. Chem., 1990, 29, 33. 68 Danielsson, L. G. and Sparen, A., Anal. Chim. Acta, 1995, 306, 173. 69 Danielsson, L. G., Zhang, Y. H. and Sparen, A., Analyst, 1995, 120, 2513. 70 Lin, S., Zhao, Q. and Yu, G., Fenxi Kexue Xuebao, 1994, 10, 24. 71 Tao, G. and Fang, Z., Spectrochim. Acta Part B, 1995, 50B, 1747. 72 AIonso, E. V., Garcia de Torres, A. and Cano-Pav6n, J. M., Talanta, 1996, 43, 493. 73 Yamamoto, M., Obata, Y, Nitta, Y. Nakata, F. and Kumamaru, T., J. Anal. At. Spectrom., 1988, 3, 441. 74 Backstrom, K., Gustavsson, A. and Hietala, P., Spectrochim. Acta Part B, 1989, 44B, 1041. 75 Kumamaru, T., Nitta, Y., Matsuo, H. and Kimura, E., Bull. Chem. Soc. Jpn., 1987, 60, 1930. 76 Wang, X. and Barnes, R. M., Fenxi Shiyanshi, 1991, 10, 7. 77 Cheng, X. H., Zhou, J. M., Liu, H. D., Tang, Z. Y., Shuai, Q. and Jin. Z. X., Fenxi Shiyanshi, 1996, 15, 66. 78 Manzoori, J. L. and Miyazaki, A., Anal. Chem., 1990, 62, 2457. 79 Miyazaki, A. and Bansho, K., Anal. Chim. Acta, 1987, 198, 297. 80 Vereda-Alonso, E., Siles-Cordero, M. T., Garcia de Torres, A. and Cano-Pav6n, J. M., Fresenius J. Anal. Chem., 1995,351,802. 81 Siles-Cordero, M. T., Vereda-Alonso, E., Garcia de Tones, A. and Cano-Pav6n, J. M., J. Anal. At. Spectrom., 1996, 11, 107. 82 Men6ndez-Garcia, A., Sfinchez-Uria, J. E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1989, 4, 581. 83 Men6ndez-Garcia, A., Sfinchez-Uria, J. E. and Sanz-Medel. A., Anal. Chim. Acta, 1990, 234, 133. 84 Cervera, M. L., Montero, R., Sfinchez-Uria, J. E., Men6ndez-Garcia, A. and Sanz-Medel, A., At. Spectrom., 1995, 4, 139. 85 Men6ndez-Garcia, A., Fernfindez-Sfinchez, M. L., Sfinchez-Uria, J. E. and Sanz-Medel, A., Mikrochim. Acta, 1996, 133, 157. 86 Cafiada-Rudner, P., Cano-Pav6n, J. M., Garcia de Torres, A. and Sfinchez-Rojas, E, Fresenius J. Anal. Chem., 1995, 352, 615. 87 Cafiada-Rudner, P., Garcia de Torres, A. and Cano-Pav6n, J. M., J. Anal. At. Spectrom., 1993, 8, 705.

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88 S~inchez-Uria, J. E., Men6ndez-Garcia, A., Canto-Fonseca, J., Fern~indez-S~inchez, M. L. and Sanz-Medel, A., Lab. Rob. Autom., 1993, 5, 263. 89 Men6ndez-Garcia, A., P6rez-Rodriguez, M. C., S~inchez-Uria, J. E. and Sanz-Medel, A., Fresenius J. Anal, Chem. 1995, 353, 128. 90 Men6ndez-Garcia, A., S~inchez-Uria, J. E., Sanz-Medel, A., Quintero-Ortega, M. C. and Cotrino Bautista, J., Mikrochim. Acta, 1992, 106, 277. 91 Lima, J. L. F. C., Rangel, A. O. S. S. and Roque da Silva, M. M. S.,At. Spectrosc., 1991, 12, 204. 92 Ferreira, A. R. M., Rangel, A. O. S. S. and Lima, J. L. F. C., Commun. Soil Sci. Plant. Anal., 1995, 26, 183. 93 Lima, J. L. F. C., Rangel, A. O. S. S. and Souto, M. R. S., Anal. Sci., 1996, 12, 81. 94 Van Staden, J. F. and Hattingh, C. J., d. Anal. At. Spectrom., 1995 10, 727. 95 Chang, Q. and Meyerhoff, M. E.,Anal. Chim Acta, 1986, 186, 81. 96 Van Staden, J. F. and Hattingh, C. J., Anal. Chim. Acta, 1995, 308, 214. 97 Koropchak, J. A. and Dabek-Zlotorzynska, E., Appl. Spectrosc., 1987, 41,1231. 98 Koropchak, J. A. and Allen, L., Anal. Chem., 1989, 61, 1410. 99 Pyrzynska, K., Talanta, 1994, 31, 381. 100 Koropchak, J. A. and Dabek-Zlotorzynska, E., Anal. Chem., 1988, 60, 328. 101 Kasthurikrishnan, N. and Koropchak, J. A., Anal. Chem., 1993, 65, 857. 102 Fukamachi, M. and lshibashi, M.,Anal. Chim. Acta, 1980, 119, 383. ! 03 Kumumaru, T., Okamoto, Y., Yamamoto, Y., Nakata, F., Nitta, Y. and Matsuo, H., Fresenius d. Anal Chem., 1987, 327, 777. 104 Mufioz-Olivas, R., Qu6tel, C. R. and Donard, O. F. X.,J. Anal. At. Spectrom., 1995, 10, 865. 105 Walker, S. and Aleboyeh, A., Fresenius d. Anal. Chem., 1996, 355, 687.

8.1

Scope of the chapter

In principle, a sample can be introduced to an atomizer/detector in a condensed phase or in gaseous phase. Gaseous phase sample introduction techniques are usually based on volatile compound generation. It is a selective conversion of the analyte from the liquid sample to the gaseous phase via an appropriate chemical reaction resulting in a volatile compound of the analyte. The popularity of volatile compound generation arises for several reasons. One is its relative simplicity and low cost of the apparatus. However, the main reason lies with the principle of the method. It involves analyte preconcentration and separation from the sample matrix. This results in a superior sensitivity, and mainly, in a striking suppression of interferences during atomization. The process of determination of elements forming volatile compounds by any atomic spectrometric method involves three independent steps: (1) sample preparation, (2) volatile compound generation, and (3) atomization/detection. The first step involves transformation of the original sample into solution. Its purpose is to convert sample into solution without analyte losses and contaminations. Biological or organic matrix should be completely decomposed. In reality, sample preparation is a crucial step, being prone to major volatilization losses, incomplete decomposition of biological matrix (increased risk of matrix interferences!- see Section 8.6.2.2), significant reagent blanks, incomplete conversion of analyte to the optimum valency (compound interferences!- see Section 8.6.2.1), etc. The second step, volatile compound generation consists of the release of the volatile compound from the sample solution, i.e. conversion of an analyte in sample to the compound and its transfer to the gaseous phase, and 237

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transport of the released compound by a flow of the purge gas to an atomizer/detector. Whereas the last step of determination of elements forming volatile compounds (performed in atomizers placed within the spectrometer employed) is specific for the atomic spectroscopy method to be employed, the first two steps are common to all detection methods. This chapter is devoted to the fundamentals of gas-liquid separation which is a critical part of the generation step in the vast majority of existing approaches to the volatile compound generation. In principle, the gas-liquid separation can be carried out either in the batch mode or in a flow mode. All aspects of the gas--liquid separation for flow methods will be discussed with the emphasis on comparison with the batch mode. The aim is not to review the literature comprehensively but to illustrate the important features of the gas-liquid separation. The far most popular volatile compounds generated are covalent binary hydrides and therefore they will be taken in this chapter as the model for all volatile compounds. In addition to binary hydrides, alkyl substituted hydrides are also generated as detailed in Chapters 12 and 13. Besides hydrides, the other analytically useful volatile compounds are alkyl derivatives, chelates, oxides, halides and oxides. '-3 Atomic vapours of mercury 2'4 and cadmium, ~7 are also successfully generated. The interested reader can also find relevant information in the next chapter. Hydride generation (HG) has been employed for over 100 years for determination of arsenic in methods known as the Marsh reaction or the Gutzeit test. The introduction of AAS to laboratories throughout the world was an incentive for a massive application of HG in AAS and also in other methods of analytical atomic spectrometry. It was introduced around 1970 to overcome problems associated with flame AAS determinations of arsenic and selenium. In the following years, the unquestionable advantages of the method led to its application to virtually all elements forming volatile hydrides, namely arsenic, antimony, bismuth, germanium, lead, selenium, tellurium and tin. HG of indium and thallium was even described. See Table 8.1 for the list of analytically important hydride forming elements, optimum conditions for their conversion to corresponding binary hydrides and basic physical constants of the hydrides and elements. Theory, instrumentation, methodology and analytical applications of HG for atomic spectroscopy methods are exhaustively and critically covered until 1992-1993 in recent monograph.' For additional recent reviews see References 8-14. HG consists of: hydride release from the sample solution, i.e. conversion of an analyte in a sample to hydride and its transfer to the gaseous phase, and transport of the released hydride by a flow of the purge gas to an atomizer/detector. As obvious from the treatment below, the subject of the present chapter, i.e. gas-liquid separation, overlaps both hydride release and transport which therefore will be treated in detail in Sections 8.2 and 8.3,

239

Flow methods in gas--liquid separations

Table 8.1 Volatile hydride forming elements (reproduced from Reference 1 by permission of John Wiley & Sons Limited) Hydride

Analyte

Optimum valency

OptimumHCI concentration Formula Name

Melting point (~

Boiling Solubility point in water (~ (mg/ml)

antimony arsenic bismuth germanium lead selenium

Sb(lll) As(Ill) Bi(lll) Ge(IV) Pb(II) Se(IV)

pH9-9M pH6-9M pH6-9M pH I-pH7 pHl-pH2 a 0.2M-9M

SbH3 AsH3 Bill3 GeH4 PbH4 Sell2

-

- 18 - 62 17 - 88 - 13 - 41

4.1 0.7 insoluble 38

tellurium

Te(IV)

0.2M-7M

Tell2

- 51

- 4

tin

Sn(II), Sn(IV)

0.1M-I M

SnH4

- 146

- 52

very soluble -

stibine arsine bismuthine germane plumbane selenium hydride tellurium hydride stannane

88 117 67 165 135 66

a Reaction modifiers required. respectively. Sections 8.4 and 8.5, respectively, are devoted to individual methods of HG and to gas-liquid separation in flow modes of HG. The subsequent section treats interferences to HG.

8.2

Hydride release

Several "wet chemistry" reactions have been used to convert analyte present in the solution to hydride. Most often, either a metal/acid or tetrahydroborate reduction is employed for HG (see below). There are also reports on electrolytic cells employed for hydride formation (Section 8.2.8). Before discussing HG reactions, requirements to the sample preparation step specific for HG will be shortly summarized.

8.2.1

Samplepreparation

With the exception of selective HG (see next section) analyte should finish up in the valency optimum for HG (Table 8.1). In the case of antimony, arsenic, selenium and tellurium which are often present in the higher valency a pre-reduction is required since the analytes in higher valency either are not converted to hydrides at all (selenium, tellurium) or the conversion may be less efficient (antimony, arsenic). Numerous procedures for sample preparation, often specific for given matrix/analyte combination, and for the pre-reduction can be found in the literature. Regarding the pre-reduction, there is a large variety of described procedures, ~ although the results of existing reports are not always consistent. In essence, the procedures for pre-reduction of As(V) and Sb(V) are

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rather similar. Most of the pre-reduction procedures employ iodides and/or ascorbic acid or, in recent years, L-cysteine. ~5-32 Also the pre-reduction procedures for Se(IV) and Te(VI) are mutually similar but different from those used for As(V) and Sb(V). It should be noted that selenium and tellurium species are reduced to their elemental forms (and therefore lost for HG!) by most common pre-reductants (iodides, ascorbic acid) for As(V) and Sb(V). A typical procedure for Se(IV) or Te(VI) consists in heating sample solutions with HC1 (1 + 1) for 25--30 min on a boiling water bath at 95-100~ 8.2.2

Selective hydride g e n e r a t i o n

Various analyte forms may be converted to hydrides with different efficiency depending on acidity but also on other reaction conditions which are discussed in Sections 8.2.6 and 8.5. This is usually applied to speciation of oxidation states of inorganic antimony, arsenic, selenium and tellurium in water solutions: Sb(III) and Sb(V), As(III) and As(V), Se(IV) and Se(VI), Te(IV) and Te(VI). The typical procedure is based on the determination of the lower valency in one sample aliquot and of the sum of both the forms in the second aliquot. Then the content of the higher valency is calculated by difference. The lower valency is determined by selecting conditions of HG for complete reduction of the form to hydride and, simultaneously, preventing HG from the higher valency. The sum of both the forms may be determined either after pre-reduction of the higher form or, in the case of antimony and arsenic, by selecting conditions of HG for complete reduction of both the form to hydride. This scheme can be extended to selective generation of alkyl substituted hydrides (see Reference 1 for detailed information on the optimum conditions for HG of individual analyte forms). 8.2.3

Metal~acid reduction

The earlier technique for conversion of analyte present in the solution to hydride, essentially the Marsh reaction or the Gutzeit test, used a metal/acid system, most often Zn/ HCI, to form atomic hydrogen for analyte reduction to hydride: Zn + 2 H § ---. Zn2§ + 2H

(8.1)

A m§ + (m + n)H---* A H . + m H +

(8.2)

where m and n, respectively, is valency of the analyte A in the sample solution and in hydride. Zinc metal in the form of granules, tablets, dust or slurry have been employed. As detailed in previous reviews, 33'34 other metal/acid reactions have also been investigated: either mixtures of Mg and TIC13 or an aqueous slurry of A1 reacted with hydrochloric and sulfuric acid. The generation of arsine by the analyte reduction by metallic aluminum in

Flow methods in gas--liquid separations

241

a basic medium have recently been investigated as an alternative avoiding the use of acidic solutions. 35.36 8.2.4

Tetrahydroborate decomposition in acidic media

The hydride formation by the tetrahydroborate reduction (BH4/acid reaction) of analyte is usually performed in acidic (most often HCI) medium. Tetrahydroborate decomposes very fast in acidic media to form boric acid and hydrogen. Between pH 3.8 and 14, the decomposition reaction is of a second order (with the rate directly proportional to tetrahydroborate and hydrogen ion concentration) having the rate constant (30~ 1.22 x 108 m o l - i lmin-1.37.38 Evidence has been presented that at pH> 7 the decomposition is a two-step reaction. 37'39In the first, slow, step, an activated complex is formed from tetrahydroborate anion and hydrogen cation which decomposes to form hydrogen and aquated BH3. In the second, fast, step BH3 reacts with water to form boric acid and hydrogen. 4~ At pH<3.8 the decomposition reaction seems too fast for kinetic measurements. It has only been found that tetrahydroborate is completely decomposed within 1 millisecond. 37 Because of the extremely high reaction rate the mechanism of the decomposition at highly acidic media (pH 1 and lower) has not yet been determined. Consequently, the above discussed two-step reaction scheme does not have to be valid at conditions typically employed for HG. It should be mentioned that Arenas et al. 4~ assumed, without presenting any supporting evidence, diborane as the product of the acid tetrahydroborate decomposition: _

2BH4 + 2H +---, BzH6 + 2142

(8.3)

However, the only experimental evidence indicating other products of the acid tetrahydroborate decomposition than boric acid and hydrogen is by Vien and Fry42 who identified traces of volatile boranes as byproducts of HG when using the BHn/acid reaction. 8.2.5

Mechanism o f the hydride formation by the tetrahydroborate reduction

Even less about the mechanism of tetrahydroborate decomposition is known about the mechanism of analyte reduction to hydride by the BH4/acid reaction. In the first review on HG published in 1979, Robbins and Caruso 33 postulated a nascent hydrogen mechanism analogous to that for metal/acid system: (a) atomic hydrogen is formed in the summary reaction scheme of tetrahydroborate decomposition (8.4) and (b) analyte is reduced to hydride according to reaction (8.2). BH4 + 3H20 + H § - - ' * H3BO 3+ 8H

(8.4)

The nascent hydrogen mechanism is usually accepted although it has never been proved.

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J. D6dina

A modification of the nascent hydrogen mechanism has been postulated by Broekaert and Boumans: 43 tetrahydroborate decomposition to borane and atomic hydrogen B H 4 + H + ---* BH3 + 2 H

(8.5)

followed by analyte reduction to hydride according to reaction (8.2). However, there is significant evidence against the nascent hydrogen mechanism owing to results yielded by alkaline mode HG and by investigation of the effect of cysteine on HG. A modification of the BH4/acid reaction has been employed for selenium hydride,44-48 arsine, 49-5j tellurium hydride, 5~ germane 52 and stannane 53 generation: alkaline sample solution containing tetrahydroborate is mixed with acid (instead of mixing acidic sample with alkaline tetrahydroborate solution in the typical arrangement of the BHa/acid reaction). As suggested by Bye,45 who studied the generation of selenium hydride in the alkaline mode, selenite is reduced by tetrahydroborate (which is a strong reductant in alkaline solution) to selenide which is released as hydride after acidification of the mixture. Nevertheless, the experimental observations presented 45 might be compatible also with the nascent hydrogen mechanism assuming that analyte is converted to hydride after acidification of the alkaline sample solution containing tetrahydroborate. A direct proof of the presence of selenide in alkaline solutions which would verify the non-nascent mechanism is difficult. However, the accuracy of the mechanism suggested by Bye45 was indicated by Qiu 52 who showed that stannane can be generated from the alkaline medium. Consequently, a similar non-nascent mechanism of the hydridization reaction could take place even in the typical, "acid", arrangement of the BH4/acid reaction. For stannane, Qiu et al. 53 inferred from a series of experiments that its generation "might be an induced reaction, induced or catalysed by the neutralization of H § and O H - " and that a "nonnascent" mechanism of the hydride formation takes place. Following the original communication of Brindle et al., 54 a number of papers have appeared describing the beneficial effect of cysteine to generation of hydrides of arsenic, antimony and tin. In summary, cysteine additions to the sample solution facilitate the hydride-forming reaction, reduce liquid phase interferences and pre-reduce As(V) and Sb(V) forms to the tervalent ones. Brindle and Le 55 found evidence in ultraviolet and nuclear magnetic resonance spectra for a complex between tetrahydroborate and cysteine. They suggested the following reaction of the thiol group of cysteine (Cys-SH) and tetrahydroborate: B H 4 + H S - C y s ---, BH3 - S - C y s - + 1-12

(8.6)

The resulting complex might be the intermediate that is more efficient in the hydride-

Flow methods in gas-liquid separations

243

forming reaction. 55 This would be a strong indication against the nascent hydrogen mechanism. However, Le et al. 28 presented an alternative explanation of the significant effect of cysteine additions to the sample solution on the arsine generation. They proposed that arsenic acid reacts with cysteine to form As(III)-cysteine complex: As(OI-D3 + 3HS - Cys ~ A s ( S - Cys)3 + 3 H 2 0

(8.7)

Howard and Salou 19 suggested that the As(III)-cysteine complex can be readily reduced to arsine by tetrahydroborate. Naturally, this explanation of the cysteine effect is compatible both with-the nascent hydrogen mechanism as well as with a non-nascent mechanism of the hydride formation. However, Howard and Salou 19 seem to prefer a non-nascent mechanism since they state that the complex is less sterically hindered with respect to tetrahydroborate attack than the parent compound. Braman and Foreback 56 pointed out that pH requirements of the reduction reaction indicate that the various arsenic acids must be in the completely undissociated form before they can be reduced to the corresponding arsines. It should be pointed out that this is in outright disagreement to the above discussed hypothesis on the mechanism of the alkaline mode HG. Howard 8 extended the requirement, that the target species must not be present in solution as a negatively charged species for the reduction reaction to proceed rapidly, from arsenic also to other hydride-forming elements. He suggested explicitly a nonnascent mechanism of the arsine formation: A s ( O H ) 3+ 3 B H 4 + 3 H + ~ AsH3 + 3BH3 + 3 H 2 0

(8.8)

The borane generated hydrolyses giving boric acid and gaseous hydrogen. 8 However, no convincing evidence has been presented to support this reaction scheme similarly as for the previous ones. In conclusion, the actual mechanism of the reduction reaction still remains to be clarified, however, the mechanism is not of a primary importance for the analytical praxis since there are always several orders of magnitude excess of tetrahydroborate over analyte. 8.2. 6

E x p e r i m e n t a l a p p r o a c h e s to the tetrahydroborate reduction

Sodium salt of tetrahydroborate is usually employed. Initial applications used NaBH4 pellets but aqueous solution stabilized by or sodium or (rarer) potassium hydroxide is currently the most popular and most convenient agent. Tetrahydroborate concentration varies by several orders of magnitude depending on analyte element and on the type of hydride generator used. It should be underlined that even alkaline NaBH4 solutions slowly

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decompose. The decomposition is probably surface catalysed and is slower in more concentrated hydroxide solutions. Filtration, or preferably, membrane ultrafiltration is therefore advisable. Large excess of NaOH could be detrimental, since it has to be neutralized as well as for higher blanks entailed. A total of 0.05-2% m/v of N a O H is typically employed for stabilization. Although reductant solutions are prepared fresh daily in many laboratories it is not generally necessary. The stability is generally better in filtered and more alkaline solutions. The lifetime of the reductant could be extended considerably when stored under refrigeration or even frozen. Technically, the reducing reaction is performed almost exclusively by mixing an excess of acidic sample volume with tetrahydroborate so that the reacted out mixture is still acidic. The "inverse" approach described above, when alkaline sample solution containing tetrahydroborate is mixed with acid, has been employed in several applications. 44-53There are also reports on using BH4 bound to a stationary phase in a chromatographic column through which the acid sample is passed. 57's8 As mentioned in Section 8.2.1, various chemical forms of individual analyte elements can be converted to hydrides. Table 8.1 presents the most convenient chemical forms of individual analyte elements to be converted to binary hydrides and a rough estimate of optimum HCI concentration for hydride release. The change of the reaction mixture acidity during the reaction should be taken into account. However, optimum conditions for hydride release involving the effect of various acids as reaction media, their concentration, sometimes presence of reaction controllers (such as peroxodisulfate for lead) or buffers for tin or germanium, depend besides analyte identity and chemical form also on the method of riG employed. They should therefore be optimized for the given combination of analyte and sample matrix and for the actual experimental set-up. Starting parameters can be found in Table 8.1 or in the reviews quoted at the end of Section 8.1. Additions of surfactant-based "ordered media" to the reaction mixture have also been investigated in order to enhance HG by organization of reactants at molecular level. 13'59'6~ This might be a promising means to improve hydride release especially in case of difficult matrices or unstable hydrides. There is always excess, usually of many orders of magnitude, of the reducing agent over the analyte under typical conditions of HG. Resulting hydrogen obviously drives the hydride from the reaction mixture to the gaseous phase since the concentration of hydride in the reaction mixture is typically much lower than the hydride solubility (Table 8.1). This is the additional function of the reducing agent. In some designs of hydride generators (see Sections 8.4.2 and 8.5) the purge gas is mixed with or bubbled through the reaction mixture. This supports the release of reduced hydride from the solution and can partially reduce the demand of the reducing agent. Under the optimum conditions and in the absence of a matrix, the hydride release efficiency approaches unity.

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8.2. 7 A model for hydride release in the case o f tetrahydroborate reduction A simplified model of hydride release has been presented TM based on the following assumptions: (a)

The decrease of tetrahydroborate concentration, cB, in the reaction mixture after acidification is a first order process with the rate constant k, (dimension is time-!) which does not change over the course of hydride release: c8= (cB)0 e x p ( - k,t)

(b)

(8.9)

Hydride release from the solution is a second order process with rate constant kz having dimension (concentration time)-': analyte(l) + BH4 ---, hydride(g)

(8.10)

The process involves analyte reduction tothe hydride as well as the liberation of hydride to the gaseous phase out of the reaction mixture. Consequently, the magnitude of the rate constant is also influenced by the purge gas flow rate, by chemical composition of the sample solution and by design of the employed hydride generator. The analyte concentration in the reaction mixture, c, follows the second order kinetics: dc dt

~=-k2cc

8

(8.11)

Solving this equation becomes evident that after complete tetrahydroborate decomposition (t>> 1/k,) the analyte concentration is given by the following formula:

C=Co exp

[ k2] -(CB)o~

(8.12)

where Co is analyte concentration in the sample, Now it is convenient to introduce hydride release efficiency, fir, which is the fraction of analyte released in the form of hydride from the solution. It characterizes the efficiency of the gas-liquid separation:

Nreleased ~r'-~ coVs

(8.13)

where Nreleased is the total number of analyte atoms released from the liquid sample in the form of hydride and Vs is the volume of the sample. Hydride release efficiency can be

J. D6dina

246 expressed from Equation (8.12) as:

fir = 1 - exp

[

(8.14)

Evidently, complete hydride release is reached when

kl

(CB)o>~k-z

(8.15)

It should be underlined that this model is valid regardless of the actual mechanisms of tetrahydroborate decomposition and of the reduction reaction. The model was formulated for an ideal case when tetrahydroborate is perfectly mixed with the acid and analyte before the chemical reactions start. ! It is assumed the model describes hydride release in real situations satisfactorily, however, actual non-idealities of the process are reflected in values of the rate constants k~ and k2 which thus also depend on actual experimental set-up. The model implies that a complete hydride release (fir = 1) is a matter only of a sufficient tetrahydroborate supply and sufficient time for tetrahydroborate decomposition. This agrees reasonably with experimental observations for analyte concentrations down to the range of pg ml-~.! Amount of tetrahydroborate demanded for complete hydride release is therefore controlled only by sample volume and b y t h e ratio of rate constants k2/kt. The ratio (dimension volume mass-~) for given analyte is characteristic for composition of sample solution and is influenced by the experimental set-up of the hydride generator. Vice versa, the amount of tetrahydroborate required for complete release (or for any chosen hydride release efficiency) can be easily estimated from Equation (8.7) if the magnitude of the ratio is known. Obviously, the knowledge of the ratio is extremely useful for optimization of hydride release parameters. The magnitude of the ratio may be determined from the influence of the mass of tetrahydroborate added either on the fraction of analyte remaining in a reaction mixture or on the observed absorbance. I

8.2.8

Electrochemical reduction of analyte to hydride

Electrochemical investigated for electrolytically electrochemical

HG, which is an alternative to the BH4-/acid reduction, was recently generation of arsine, stibine and selenium hydride. 29'62-~7 The analyte is converted to hydride at the cathode. The marked advantage of HG is that beside acid there is no other reagent required. Analyte

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concentration in blanks can be therefore made very low. This offers potential to achieve excellent detection limits. 66

8.3

Hydride transport

The hydride released from sample solution is transported by a flow of the purge gas to an atomizer/detector. The efficiency of this process, /3t, is the fraction of released hydride transported:

No Nreleased

/3,=--

(8.16)

where No is the total number of analyte atoms supplied in the form of hydride to the atomizer/detector. Purge gas may also assist in hydride release by stripping the hydride out of the sample solution. There are quite a number of gases satisfying the essential requirement on the purge gas: that it must be inert towards the hydride. The purge gas is chosen with respect to optimum economy and atomizer/detector performance. Only in the case of cryogenic collection (see Section 8.4.1) is it not convenient to employ a gas having the boiling point above the trapping bath temperature. The most popular purge gas is argon; nitrogen is often utilized, too. Helium and hydrogen have also been used. Principally, hydrogen released during HG can be used as the purge gas as well. Lower dilution of analyte thus contributes to enhanced sensitivity. Losses of hydride on glass or plastic surfaces during its transport can be serious. Losses or delay of transported hydrides are probably due to their decomposition or sorption. Water traces which are typically present in hydride generators can also enhance the losses. For example, solubility of selenium hydride in solutions at pH around 2 to 3 is very high due to its acid dissociation. These effects can reduce hydride transport efficiency, i.e. the observed sensitivity, but mainly deteriorate reproducibility of measurements. An efficient way to prevent loss of hydrides on glass surfaces is their silanization to block the surface active sites. 1 In conclusion, hydride transport losses can be made negligible in an optimized experimental setup. The optimization includes the use of a reasonably high gas flow rate, minimization of surfaces coming into contact with transported hydride and, if necessary, silanization of glass surfaces. The tubing serving for the transport of hydrides to the atomizer/detector should be as short as possible to avoid the transport losses. The interaction of hydride with the tubing surface is obviously reduced in narrower tubings, however, too narrow tubing increases the

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risk of overpressure problems (see Section 8.5.5). The flow of gases transports together with generated hydride also a certain amount of spray of the reaction mixture stripped out of the reaction vessel. Condensation of the reagent mist and/or water vapour in the transfer line should be avoided because of the above-mentioned losses or delay of hydrides. Various desiccants have thus been employed such as sulfuric acid, calcium chloride, silica gel, calcium sulfate (Drierite), magnesium perchlorate, or a glass or quartz tube cooled to temperatures below zero down to - 80~ A different possibility is to carry out the drying of gases by a hygroscopic membrane dryer tube. 5~ Tao and Miyazaki 68 even reported on a similar membrane-based device able to remove, besides water vapour, also hydrogen. Such devices might be a very useful component of hydride generators; however, they should be extensively tested to avoid (possible) hydride losses in the membrane. Another means to avoid carryover of liquid drops to the transfer line is to employ membrane gasliquid separators (see Section 8.5.7) or to fit a membrane, permeable for gases but trapping liquid spray, to the gas outlet from the hydride generator (see Section 8.5.6).

8.4

Methods of hydride generation

As stated above, HG consists of hydride release from the sample solution, i.e. conversion of an analyte in sample to hydride and its transfer to the gaseous phase, and transport of the released hydride by a flow of the purge gas to an atomizer/detector. Both these steps are characterised by corresponding efficiencies defined above. The efficiency of HG,/3~., is defined as the fraction of analyte transported in the form of hydride to the atomizer/ detector.

fls=~

No CoVs

(8.17)

~g=~r~t

(8.18)

Obviously:

There are two basic modes of HG, direct transfer mode and collection mode. In the direct transfer mode, hydride released from a sample solution is directly transported to the atomizer/detector. In the collection mode, the hydride is trapped in a collection device until the evolution is completed and then is transported all at once. In principle, every collection generator is a direct transfer mode generator supplying hydride not to an atomizer/detector but to a collection device from which it is subsequently discharged. Even in the case of collection mode, it is convenient to describe the process only by the hydride release efficiency, which is always related to the release of hydride from the

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249

reaction mixture, and the hydride transport efficiency. The latter thus includes efficiencies of transport to the collection device, collection and transport to the atomizer/detector. Every hydride generator can be characterized by two parameters- outlet functions: (a) a hydride supply rate, S, which is the number of analyte atoms delivered to the atomizer/ detector in the form of hydride per unit time and (b) a total gas flow rate, F, comprising the purge gas flow, F0, and the flow of hydrogen released from decomposed reducing agent. It should be mentioned that sensitivity generally increases with increasing hydride supply rate and with decreasing total gas flow rate since it controls the dilution of hydride supplied to the atomizer/detector. Generally, both outlet generator functions are timedependent and their explicit description of the outlet generator functions is rather complex.' Direct transfer mode is performed either in the batch arrangement or in a flow mode. There are two approaches to the flow mode, i.e. continuous flow (CF) and flow injection (FI). Assuming constant tetrahydroborate flow rate, f8 (always expressed as the mass flow) to direct-transfer generators, the total gas flow rate is constant: ~

F--Fo+fBVhydrogen

(8.19)

where Vhydrogen is the hydrogen volume liberated per mass unit of tetrahydroborate (2540 ml g - ' at 20~ The hydride supply rate produced by direct transfer mode generators is always controlled by (a) release rate, p(t), (which is the number of hydride atoms delivered to the dead volume of the hydride generator, Vdead , per unit time) and (b) dispersion of hydride in the dead volume of the generator. (Dispersion in the transport tubing between generator and atomizer/detector is assumed to be negligible, l) The dispersion can be approximated by a first-order process with a rate constant (dimension is time -~) F

km=Vdea d

(8.20)

(It should be noted that the reliance time of gases in the dead volume is the reciprocal of the rate constant.) The hydride supply rate can be thus expressed as'

S(t)=fltk m

p(t') exp{km(t' - t)} dt'

(8.21)

Obviously, the hydride supply rate increases with increasing p(t) and km and vice versa. The expressions describing hydride supply rate by individual direct transfer mode generators are treated in corresponding sections.

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A number of workers have reported on the batch generation. At the moment, it is losing its popularity in favour of the flow systems since the simplest equipment is the only significant advantage of batch mode. CF and FI modes are very convenient methods. Presently, they are the most popular methods of HG. They are simple to operate, easy to automate, accurate and reasonably sensitive. The only disadvantage of FI mode compared to CF is lower sensitivity and slightly more complicated apparatus. However, it can be converted to CF mode simply by increasing sample coil volume and it has lower sample consumption and higher sample throughput. Individual HG methods will be detailed in subsequent sections. 8.4.1

Collection methods

Collection methods were employed much more frequently in early years of the application of HG when relatively slow approaches to hydride release were employed. Obviously, it was advantageous to collect the released hydride and then to sweep it to the atomizer/ detector in the shortest possible time. It should be underlined that when using the present BHa/acid system as the reducing agent the need to collect released hydride is much less pressing so that the simpler direct transfer methods are more often utilized now. A collection generator consists of a "reaction vessel", which is a batch or CF generator (in principle, a FI generator can be employed as well) and of a collection device. After hydride evolution from the "reaction vessel" has been finished the collected hydride is either released from the collection device and transported to the atomizer/detector on line or it can be stored and atomized off line. Among collection methods, ~ only cryogenic trapping and in situ trapping in the graphite furnace are used more often than occasionally. They employ collection devices through which hydrogen evolved in the reaction vessel passes freely and is not collected. The main reason for using them is the significant enhancement of sensitivity over the direct transfer methods since cryogenic trapping and in situ trapping make possible an efficient collection of analyte from large sample volumes. 8.4.2

Batch mode

A batch generator is a vessel made of glass or plastic serving both as the reactor as well as the separator of gases from reaction mixture. A batch of acidified sample solution is placed inside and then the tetrahydroborate solution is introduced. The purge gas flow is introduced either to the dead volume of the generator or below the liquid level. 1 Released hydride is thus supported by the purge gas flow to the atomizer/detector together with the hydrogen formed from reducing agent decomposition. After finishing the hydride evolution the reacted mixture has to be disposed to waste, generator rinsed and a new batch of sample is added.

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251

The hydride supply rate is a peak shaped function. Its maximum depends, apart from the hydride transport efficiency and the analyte concentration in the sample, only on: ~ (1) (2) (3) (4)

the ratio of rate constants k2/ki, the ratio of generator and sample volumes, purge gas flow rate, F,,, tetrahydroborate flow rate, fB.

This makes possible a simple evaluation of the influence of size and other experimental parameters of batch hydride generators on the performance of the analysis. As mentioned above, sensitivity generally increases with increasing hydride supply rate and with decreasing total gas flow rate. The maximum of the hydride supply rate can be simply increased by increasing the tetrahydroborate flow and/or by decreasing the dead volume of the generator, however, the flow rate of released hydrogen increases with tetrahydroborate flow rate enhancing the total gas flow rate and reducing or even eliminating the gain of sensitivity. Besides, increasing the tetrahydroborate flow rate and/or decreasing the dead volume of the generator is limited by the risk of frothing and/or penetrating of spray of the reacting mixture to the atomizer/detector. Generally, the hydride supply rate and, consequently, the analytical signal, is always dependent on the rate constant ratio kz/k l which, in turn, may be expected to be sensitive to small changes in the sample composition. 8.4.3

Flow methods

A general scheme of CF hydride generator is in Figure 8.1. A constant flow of sample solution is mixed with a constant flow of tetrahydroborate solution and of the purge gas. Liquid and gas are then separated in a gas-liquid separator (GLS) yielding two outlet flows- the gaseous hydride with hydrogen and purge gas flowing to the atomizer/detector and the drained liquid effluent. The use of the CF HG system had been described as early as in 1973. 69 Several commercial systems are available; laboratory-made ones are often employed as well. Some CF systems employ a separate delivery of sample and acid to be mixed in a mixing coil as shown in Figure 8.1, however, in many cases the sample solution is acidified off line. The acid channel and the mixing coil are not thus required. A general scheme of FI hydride generator is in Figure 8.2. The experimental arrangement is similar to the CF method with the difference that a constant flow of acid carrier, usually diluted hydrochloric acid, is used instead of the sample flow. A volume of the sample is injected, most often by a special valve (see Reference 70 for detailed treatment) and subsequently dispersed into the carrier stream. The remainder of the experimental set-up is identical as in the case of the CF mode. The first FI-HG system was

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252

t

Figure 8.1 A scheme of the continuous flow HG system (reproduced from Reference 1 by permission of John Wiley & Sons Limited)

I WASTE~~

SOLUTION 1 Figure 8.2 A scheme of the flow injection HG system FI valve positions: fill-injection loop is filled with sample solution; inject-sample is injected into the carrier stream (reproduced from Reference 1 by permission of John Wiley & Sons Limited)

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253

described by Astr6m 7t in 1982. Until 1990, publications in this field remained scarce; however, since then a number of papers dealing with this novel and certainly a very advantageous technique have been published. Either laboratory made or the commercially available FI-HG systems are employed. All aspects of the FI generation are treated in detail by Fang. 7~ Most of flow systems use a peristaltic pump to drive reactant solutions. Typical reagent flow rates are 1 to 10 ml rain- i. The acid sample/carrier flow merge the reductant solution flow upstream a reaction coil which serves to provide sufficient time for the hydride release. The purge gas is introduced either upstream the reaction coil or downstream the coil. In the latter case, it can be introduced to GLS either directly through a particular inlet (as shown in schemes in Figures 8.1 and 8.2) or through the same inlet as the reaction mixture. The reasons for introducing purge gas upstream the reaction coil are that it: (a) facilitates the release of hydride from the mixture in the reaction coil and, consequently, reduces the required amount of tetrahydroborate and (b) reduces gas pressure pulsations downstream the point where acid sample/carrier flow merge the reductant solution and, consequently, reduces noise of observed signals. Since too high purge gas flow rate introduced upstream the reaction coil can be inconvenient, the purge gas flow is divided as shown in Figures 8.1 and 8.2 to both points in many designs. Flow rate of the purge gas has to be chosen with regard to an optimum function of the atomizer/detector. Typical gas flow rate introduced either upstream and/or downstream the reaction coil is between 50 and 500 ml min-1. From the reaction coil the mixture enters a GLS, where the gaseous phase is separated from the reacted liquid and flows, supported by the purge gas, to the hydride atomizer/detector. A number of modifications of the described schemes of the CF and FI generators have been used to comply with demands of the actual analyte and sample matrix. They are covered, together with details of individual components of the generators, in the recent monographs, l' 7o Our attention will be focused to those components of the experimental schemes (Figures 8.1 and 8.2) controlling the gas-liquid separation, i.e. to the reaction coil and, mainly, to the gas-liquid separators. Before doing that it is useful to evaluate quantitatively influence of pertinent experimental parameters on the hydride supply rate for both flow methods. The hydride supply rate is transient for the FI generators. This is due to the dispersion of the injected sample volume, Vs, in the carrier stream. It may be assumed that the dispersion upstream GLS takes place only before the point in the manifold where the flow of acid (with dispersed sample) merges with the flow of tetrahydroborate solution since here is the flow segmented by hydrogen bubbles formed by tetrahydroborate decomposition. However, any dispersion occurring after this point does not violate the validity of the following treatment. The extent to which dispersion occurs is quantified in the FI literature 7~by the dispersion coefficient:

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J. D6dina

D=~

Co

(8.22)

Cmax

where Cmax is the "peak" analyte concentration in the acid stream. The supply of analyte to the merge point is expressed through the supply function of liquid analyte s(t). It takes the shape of a nonsymmetrical peak with a steep leading edge and a tailed falling o n e see Reference 70. Obviously:

s(t)=c(t)f~

(8.23)

where c(t) and f~, respectively, is the analyte concentration in the acid stream at the merge point and the flow rate of the carrier/sample stream. The release rate (which here is the number of hydride atoms delivered to the dead volume of GLS, Vdead,per unit time) is

p(t)= fl, s(t)

(8.24)

The shape of the hydride supply rate function can be thus obtained by solving Equation (8.21). It appears that the dead volume of GLS deforms the peak of hydride supply rate, S(t): it makes it broader and lower compared to the peak of the supply function of liquid analyte, s(t). The extent of deformation increases with the residence time of gases in the dead volume of GLS, i.e. with increasing the dead volume and with decreasing the gas flow rate there. The deformation is negligible if the width of function s(t) is large enough in comparison with the residence time in the dead volume. Thus the shape of the hydride supply rate, S(t), should be close to the shape of the supply function of liquid analyte, s(t). Quantitatively, it takes place ~ if:

Vdead <~ ~

(8.25)

It should be noted that dead volume in the atomizer/detector deforms the signal analogously. If the condition (8.25) is fulfilled the solution of formula (8.21) yields

S(t)=~r~Y(t)

(8.26)

The height of the hydride supply rate function, Smax, is Smax--~r~tf~.Cma x

(8.27)

(See References 1 and 61 for a detailed treatment of the dependence of the hydride supply rate on time and on pertinent experimental parameters.) The above treatment provides a simple lead on how to estimate the extent of the sample dispersion in the carrier stream

Flow methods in gas-liquid separations

255

(quantified by the dispersion coefficient D) independently of the dispersion of gaseous hydride in the dead volumes. Since the dispersion in the dead volumes decreases with increasing the purge gas flow rate, under sufficiently high gas flow rate the signal shape does not change further with increasing the gas flow rate. Thus the signal shape is actually the shape of the function s(t). The difference between the actual signal shape and the signal shape under normally employed purge gas flow rate shows the extent of dispersion in the dead volumes. The inverse ratio of the actual signal height to that for sample volume high enough to obtain steady state signal (CF conditions approached) is thus equal to the dispersion coefficient D. In the case of the CF arrangement the hydride supply rate is constant: l S= flr fl, LCo

(8.28)

Here, the dead volume of GLS influences only the rising and falling edges of the analytical signal but not the steady state plateau of the signal. The sensitivity in CF arrangement is thus D-times higher than in the case of the FI generation. If the condition (8.25) is not fulfilled the FI signal height is further reduced by the dispersion in the dead volume of GLS so that the ratio between sensitivity in CF and FI arrangement is larger. Obviously, to increase sensitivity of the FI mode, volume of the sample injected can be increased. For sufficiently high injected volumes, depending on the actual generator design, steady state signal can be reached. The FI mode is thus converted to its limiting c a s e - CF mode. As discussed in Section 8.2.7, the hydride release efficiency becomes unity at a sufficiently high tetrahydroborate inflow. Equations (8.27) and (8.28) thus show that the hydride supply rate functions depend, besides hydride transport efficiency and the dispersion coefficient (only in the case of FI), only on the sample flow rate and on the analyte concentration in the sample. For tetrahydroborate flow rate not high enough to reach the complete hydride release the hydride supply rate functions are also proportional to hydride release efficiency which is dependent on the rate constant ratio kz/k~ - see Equation (8.14). In this case, the sensitivity depends on the kinetics of hydride release and/ or tetrahydroborate decomposition in the same way as in the case of the batch method. Since the rate constant ratio may be expected to be sensitive to small changes in the sample composition the sufficient tetrahydroborate excess is beneficial for overall reproducibility and accuracy of the determination. Since the observed sensitivity increases with increasing hydride supply rate and with decreasing total gas flow rate, a simple optimization of design and parameters of CF hydride generators in order to maximize sensitivity can be made. The hydride supply rate can be simply enhanced by increasing the sample flow rate, however, as discussed above, the tetrahydroborate flow rate must be increased proportionally to keep the hydride release

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efficiency close to unity. However, the increase in the tetrahydroborate flow rate enhances the hydrogen flow rate arising from the reagent decomposition which contributes to the total gas flow rate. Additionally, substantially increased fraction of hydrogen in the atomizer atmosphere can slightly reduce sensitivity in quartz atomizers for AAS ~ and can bring problems with plasma torches. It should be noted that CF and batch modes, when optimized, reach the same hydride concentration in the outlet gas and, consequently, the same sensitivity. ~ However, in the case of CF generation, the sensitivity optimization requires to employ a tetrahydroborate flow rate not high enough to reach the complete hydride release (see above). Thus, if the advantage of the continuous generation- the independence the signal on the kinetics is to be preserved, a lower sensitivity has to be accepted.

8.5 Experimental approaches to gas-liquid separation in flow methods The component of the experimental set-up of a flow method where the separation of gas from the liquid reaction mixture takes place is GLS. However, the section of the experimental set-up (Figures 8.1 and 8.2) between the point where the sample/carrier solution is mixed with the tetrahydroborate solution stream and GLS cannot be disregarded since the separation of gaseous phase starts already there. Consequently, to optimize gas--liquid separation, all relevant components of the experimental set-up, i.e., the mixing devices, reaction coil and GLS, must be taken into account (see following sections).

8.5.1

Mixing devices

The signal reproducibility may be critically reduced by flow fluctuations in the manifold. It is therefore quite important to minimize the pressure fluctuations in the manifold which are due to large amounts of hydrogen liberated in the narrow tube system. The simple Tor Y-pieces with i.d. around 1 mm are typically employed as connectors to merge sample/ carrier and tetrahydroborate solutions and also to introduce the purge gas to the reaction mixture stream. To keep the purge gas flow rate independent of the pressure fluctuations, Fang 72 recommends the use of a throttle in the form of a small copper disc with a tiny aperture of about 0.2 mm fixed in the outlet of the pressure gauge of the gas cylinder. A backing pressure of more than 300 kPa could be produced when the gas flow rate was as low as 150 ml min-~ and the flow rate was not affected by small back pressure variations in the flow system, thus ensuring a uniform flow. 72 Though the "successive" mixing of sample/carrier, tetrahydroborate and purge gas

Flow methods in gas-liquid separations

257

streams using T- or Y-pieces is typically employed, a four-way mixing manifold to mix sample/carrier, tetrahydroborate and purge gas in the same point was also described. 73 The advantage is that the stream of the reacting mixture in the reaction coil is more homogeneous compared to using the typical simple T- or Y-piece. 73 More smooth reaction, and consequently more homogeneous stream in the coil can be also achieved by infusing the tetrahydroborate solution through a fine capillary axially oriented in the sample/carrier stream TM or by making the inner diameter of the connector smaller than that of both the reagent and sample tubing. 75However, sample/carrier and tetrahydroborate streams can be also mixed directly in GLS. 15"25'76-81 In some designs, both streams are mixed in a way to achieve a fine spray of the reaction mixture to the dead volume of GLS. 79's~Similarly, in some approaches to using the spray chamber of an ICP instead of a GLS (see below), sample is nebulized to the spray chamber and mixed there with separately introduced tetrahydroborate. 82-84 It is also of primary importance to keep the tetrahydroborate flow as smooth as possible. Hydrogen bubble formation in the reducing agent delivery tubing must be prevented, e.g. by using a more stable solution. Where possible, the use of inert materials for tetrahydroborate transport tubing is recommended (glass is better than plastics, worn out plastics tubing should be replaced by a new one). 8.5.2

Reaction coils

Plastic tubings with i.d. around 1 mm are usually employed as reaction coils but some authors employ tubings with i.d. up to 3 mm. Narrower tubing may increase back pressure and pressure fluctuations downstream the pump, broader tubing increases the internal volume of the apparatus which, in turn, increases analysis time and consumption of sample and reagents. Length (and/or diameter) of the reaction coil should be sufficient to provide sufficient time for optimum release of the analyte hydride. Consequently, it depends (a) on the release rate of the analyte hydride which is controlled, as discussed above, by analyte identity, by sample acidity and composition, by the ratio of tetrahydroborate and sample flow rates and by the flow rate of purge gas introduced upstream the reaction coil; and (b) on the total flow rate of the reaction mixture through the reaction coil and on the i.d. of the reaction coil tubing. In principle, too short a reaction coil might cause incomplete hydride release but too long a coil increases the risk of interferences in liquid phase (as discussed in Section 8.6.2.3) or losses of analyte by another mechanism. 85"86 As stated by Fang, TM the length of reaction coil usually does not affect the sensitivity significantly above 30-50 cm up to a few metres for purge gas introduction downstream from the coil (using about 1 mm i.d. conduit, reagent flow rates in the range of ml/min and tetrahydroborate concentration up to 1%). An exception is As(V) which requires tube

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length in the upper part of the above range. 2s' 70,86-89The observation on the relatively large length of reaction coil required for a complete hydride release in the case of As(V) seems to be in conflict with the extremely fast decomposition rate of tetrahydroborate in acidic media (see Section 8.2.4) since tetrahydroborate should be completely decomposed very shortly downstream of the merge point. The plausible explanation is that the mixing of tetrahydroborate solution with the acidic sample/carrier in the very narrow tubing is highly inefficient because of gas zones formed. The fate of tetrahydroborate in the reaction coil would thus be critically dependent on experimental parameters influencing the mixing of solutions and formation of hydrogen bubbles which are responsible for segmentation of the reaction mixture in the reaction coil. Such parameters are: flow rates of reagents, tetrahydroborate concentration, sample/carrier acidity and tubing diameter. Further, the critical parameter is the design of the mixing piece discussed above. This might be the reason that the data on the optimum length of the reaction coil found in different laboratories can differ substantially. Many authors report that at least some reaction coil length is required for maximum release efficiency of various hydride forming elements although there is not much agreement in the optimum length. 25"38'70,80,85,86, 90,91 However, some authors found that the reaction coil is unnecessary for reaching maximum sensitivity. ~5,28,3~,76, 79, 82--84, 92-99 However, an optimization of the coil length is advisable for the given experimental parameters and for the given GLS namely in the case of unsatisfactory function of the generator. It should be reiterated that the reaction coil length is a critical parameter in the case of liquid phase interferences as discussed in Section 8.6.2.3.

8.5.3 Typesof gas-liquM separators Numerous designs of GLS have been employed for flow generation method generators. Generally, these can be classified into three types: (a) hydrostatic, (b) with forced outlet and (c) membrane ones; additionally, (d) the spray chamber of flame burner or ICP torch is used as GLS in some applications. All these types of GLS are performing in the continuous mode and they will be treated in subsequent sections. However, gas-liquid separation can also be performed in a batch mode even within flow hydride generators. The simplest implementation of this approach is to slightly modify a conventional GLS (see below) to make possible switching off the outlet of the reacted mixture from GLS for a given time interval. It is necessary that GLS has sufficient dead volume to accommodate the mixture volume introduced to GLS within the time interval. After that, the outlet is released to dispose the waste and the cycle can be repeated. Hydride losses, unavoidable in GLS with force outlet (see below), are reduced with this approach. The disadvantages are: (a) more complicated set-up even though it is not difficult to automate such a sequence and (b) the influence the decreasing dead volume in

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GLS has on the signal shape. The latter can be eliminated by setting the dead volume so high that the decrease is not significant but this is not useful for FI since the dispersion is magnified accordingly. In conclusion, this approach conforms best with the arrangement when a CF generator supplies hydride to a collection device. 29'1~176176However, the influence the decreasing dead volume has on the signal shape is not necessarily detrimental so that the batch mode gas-liquid separation might be beneficial also for FI or CF modes of hydride supply to the atomizer/detector.

8.5.4

Spray chambers

Spray chambers of ICP torches 16'31,32,60,75,82-86, 94-96, 98, 99, 104are used much more often than flame burner ones. 97 Technically, tetrahydroborate is usually mixed with sample solution and subsequently nebulized 16'31'32'6~176 or sample is nebulized and mixed with tetrahydroborate which is introduced to the spray chamber separately. 82-84 Alternatively tetrahydroborate can be mixed the with sample solution and subsequently introduced to the spray chamber through a custom-made inlet 98'99 or through a modified nebulizer. 75'86 The obvious disadvantage for FI is the dispersion in the chamber which deforms the analytical signal making it lower and broader as discussed in Section 8.4.3. Consequently, the gas-liquid separation in the spray chamber is employed much often in the CF m o d e 32'6~ than in the FI mode. 31 The advantage of employing spray chambers is the simplicity of the experimental set-up.

8.5.5

Hydrostatic separators

Hydrostatic GLS are actually syphons, usually U-shaped, with the liquid column to balance small pressure variations in the chamber where the separation is achieved. While the released gases are discharged through the "gas" outlet, the reaction mixture overflows to waste. Figure 8.3a shows the model employed in the first paper dealing with the CF HG. 1~ Thompson et al.[ 106] modified it by inserting a separate inlet of the purge gas to the bulb. These approaches have been both commercialized and employed either with slight modificationsS3, 67,71,79,91,107-113 or without any change in a number of applications until today. Another popular design, also a part of a commercially available CF generator TM based on a model described by Sturman, 115 is given in Figure 8.3b. Also different designs either commercialized 1~6or not TM have been shown to perform satisfactorily. The liquid level in hydrostatic GLS can be controlled by employing a pump to draw the reacted liquid from the "waste" arm of syphon by a pump through a tubing whose tip is easy to adjust to a chosen height. ~3 In some modifications of the hydrostatic GLS, the purge gas is introduced under the liquid level in GLS, most conveniently through a

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I

+ Figure 8.3 Hydrostatic gas-liquid separators (reproduced from Reference 105 by permission of Perkin Elmer Corporation (a) and from US Patent 4,559,808 (Dec. 24, 1985) by permission of Varian Australia Pty Ltd. (b))

Flow methods in gas-liquid separations

261

medium porosity dispersion tube. The fine bubbles of the purge gas formed assist in an efficient gas/liquid separation. 76 The disadvantage of hydrostatic GLS is that they cannot work with an overpressure exceeding that given by the height of water column in the "waste" arm of the syphon. Nevertheless, a modification of the hydrostatic GLS shown in Figure 8.3b makes it possible to manage overpressure at least 600 mm Hg. ~t2 The other disadvantage of hydrostatic GLS is their relatively large dead volume which reduces the peak height in the FI mode. However, this argument is not proper in the case of large gas flow rates. (See Section 8.4.3 for details). Hydrostatic GLS are very popular because of their inherent simplicity but they have not been employed often for FI mode generators. 25'36'36'71,91,92,!17 8.5.6

Separators with forced outlet

Similarly as with hydrostatic GLS, the separation here takes place in a chamber but it is tightly closed and the waste must therefore be disposed by using a forced outlet (by a peristaltic pump). Figure 8.4a shows the first model 118 of GLS with forced outlet. It is actually a Buchner fritted glass funnel with its inferior end serving for introduction of the purge gas flow. Brindle et al. ~5 modified it by introducing carrier/sample and tetrahydroborate streams separately to the chamber. A unique design of GLS was described ~9 having inlets of the reaction mixture and purge gas and outlet of the gaseous phase to the atomizer/detector in the chamber with a cotton gauze at the bottom. The liquid passes through the gauze to the waste and no separate pump is required. The first design of GLS with forced outlet which has found widespread application since it is utilized in a commercially available FI generator 89 is shown in Figure 8.4b. Total inner volume of this GLS, made of glass, is around 3 ml which makes it suitable for FI generation mode. The performance was further improved by packing GLS half-full with glass beads of approximately 3 mm diameter, which decreases aerosol formation and foaming. TM Recently, this GLS evolved into also commercialized GLS fabricated from transparent plastic (Figure 8.4c). Several variations more or less similar to the models shown in F" ,,re 8.4 have been described in the literature. 3~49, 74,81, 93, 102, 121-125 Dead volume of GLS with forced outlet can be made smaller compared to hydrostatic GLS which is especially desirable for the FI mode. They also handle overpressure better. This makes it possible to insert a gas-permeable microporous membrane at the gas outlet29, 74,80,89,120 t o reduce condensation of the reagent mist and/or water vapour in the transfer line. The disadvantage of GLS with forced outlet is that they require a separate pump to draw the reacted liquid to the waste. Since there is no liquid column to balance small pressure variations inside, the pressure can fluctuate or drift appreciably. (The pressure drift may occur when using the gas-permeable microporous membrane at the gas outlet because of changing permeability of membranes.) The other disadvantage of GLS

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with forced outlet is that, to handle frothing and/or variations in the flow rate of liquids, the draw pumping rate must be higher than the liquid inflow to GLS. Consequently, certain losses of hydride depending on the gas flow rate, liquid flow rate and pumping rate have to be expected.

m

IE (c)

GASTO ATOMIZER LIQUID + GASIN~

LIQUIDBY f PUMPTO / WASTE Figure 8.4 Gas-liquid separators with forced outlet (reproduced from Reference 118 by permission of Marcel Dekker, Inc. (a), from Reference 89 by permission of Perkin Elmer Corporation (b) and from Reference 120 by permission of Perkin Elmer Corporation (c))

Flow methods in gas-liquid separations 8.5. 7

263

M e m b r a n e separators

Instead of performing gas--liquid separation in a chamber, as in previous types of GLS, membrane GLS achieve the separation in a channel with the reaction mixture flowing through so that it actually is the (extension of) reaction coil. The wall of the channel is fully or partially constructed of a microporous membrane, permeable only for gases. Gases diffuse through the membrane to an adjacent channel from where they are driven by a stream of purge gas to the atomizer/detector. The liquid reaction mixture remains in the reaction coil and flows to the waste. In principle, there are two basic designs of membrane GLS: (a) tubular 73'126-129 and (b) sandwich 29'65'13~ shown schematically in Figure 8.5. Membrane GLS were introduced recently; t26"~3~ the membrane is made either of polytetrafluoroethylene,29, 63.65,73,80, !12.126.127, 130-137 o f polypropylene ~28'138 or of siliconrubber. 129, 139, 140 The marked advantage of membrane GLS is a complete removal of a fine spray ~4~ formed in other types of GLS. However, there is a risk of hydride losses due to an incomplete separation from the reaction mixture especially under non-optimized conditions. A separation efficiency less than 100% was indicated for arsine separation in a tubular PTFE membrane GLS and was reported in one of first applications of membrane GLS. 127 Creed et al. 129 reported recently that their tubular silicon rubber membrane GLS was unable to quantitatively remove all the gas from the reaction mixture so that an excess of hydrogen formed by tetrahydroborate decomposition diluted hydride (arsine) and reduced the generation efficiency. Van Elteren et al. ~2 compared applicability of the hydrostatic GLS with the tubular PTFE membrane GLS for arsine generation. They found quantitative evolution and stripping of hydrogen formed from decomposition of the tetrahydroborate solution in the hydrostatic GLS for broad range of the reducing solution flow rate, but only 79% of hydrogen evolved from the reaction mixture in the case of the membrane GLS having tube length 1.2 m. Changing the tube length to 0.5 and 0.2 m, respectively, gave 76 and 72% of hydrogen evolved. Although there is a preference for longer tubings, the use of length much over 1.2 m was not advisable as this resulted in generation of much water vapour and in many water droplets in the jacket. Using 74As radiotracer, the authors 1~2 found a quantitative generation of arsine in both GLS under atmospheric conditions. This means that AsH 3 diffuses through the tubing more effectively than H2, possibly as a result of a higher solubility of AsH 3 in polymers. For overpressure conditions, the membrane GLS yielded poor HG efficiency-even 50 mm Hg overpressure reduced the efficiency to below 80%. No data are available regarding the behaviour of other hydrides. ~2 However, the incomplete separation of hydrogen mentioned above indicates that there is a risk of low HG efficiency for other hydrides even at atmospheric

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conditions. Ding and Sturgeon 65 found overall efficiency for stibine electrochemical generation and in situ trapping of 92"4-2%. This suggests that the employed sandwich type membrane GLS (of the design shown in Figure 8.5a) made of PTFE microporous membrane Versapor-1200R works efficiently. In contrast, Welz and Schubert-Jacobs 89 found sensitivity only about 50 to 60% of that obtained with GLS with forced outlet (of the design shown in Figure 8.4b). In addition, the used tubular membrane GLS (10 to

/M F

F

,

G

t

B

o

T

~

,

I

~

W

,

C Figure 8.5 Membrane gas--liquid separators: (a) sandwich-type and (b) tubular-type. AAS, outlet to atomize/detector; A, B, plastic blocks; C, purge gas; F, threaded fittings; G, engraved grooves; M, microporous membrane; 0, outer tube of non-porous material; R, inlet of the reaction mixture; T, inner tube of microporous membrane; W, waste (reproduced from Reference 70 by permission of John Wiley & Sons Limited)

Flow methods in gas--liquid separations

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50 cm long PTFE microporous tubing) always required 10 to 20 determinations before a stable sensitivity was attained and its lifetime was typically in the order of 100 determinations. Problems with condensation of water in the tubing were also reported. ~2 However, the development of membrane GLS certainly has not yet been finished and an improved design and/or a new advanced material for the membrane may eliminate the above mentioned disadvantages.

8.5.8 Selective hydride generation In general, the selective HG (see Section 8.2.2) can be performed successfully in the batch arrangement as well as in a flow mode since the optimum conditions for HG of individual analyte forms are controlled mainly by the composition of the reaction mixture. The only advantage of flow methods in this respect is the influence of the length of reaction coil on the hydride release efficiency in the case of As(V) (see Section 8.5.2). This can be used to control the efficiency of arsine release from As(V) instrumentally. Yet, for reasons discussed in Section 8.4, the flow modes are much more popular for the selective HG also for other analyte forms.

8.5.9 Electrochemical hydride generation The experimental set-up for the electrochemical reduction of analyte to hydride is analogous to that used for the tetrahydroborate reduction. Recent reports on this technique, employ either CF or FI mode arrangement of HG. 29"62-67 The only difference in the experimental scheme compared to Figures 8.1 and 8.2 is that an electrolytic flow cell replaces tetrahydroborate channel and the reaction coil. Continuous flows of catholyte and anolyte, respectively, are pumped through cathode and anode channels of an electrolytic flow cell. Diluted hydrochloric or sulfuric acid can be used as the catholyte and anolyte. The catholyte stream is the sample solution in the case of CF mode or it is the carrier to which the sample is injected in the case of FI mode. The separation of hydrides and hydrogen, formed at the cathode in the electrolytic cell, from waste aqueous solution is facilitated in a subsequent GLS as discussed above. The estimated efficiencies of HG for selenium, antimony and arsenic are between 60 and 95%.

8.6

Interferences to hydride generation

They may principally occur either in the liquid phase during hydride formation and its transfer from solution (liquid phase interferences) or they can affect the analyte in the gaseous phase during passage of hydride from the sample solution to the atomizer/detector (transport interferences). Liquid phase interferences are changes in the rate of hydride

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release from the liquid phase (release kinetics interference) and/or by a decreased efficiency of hydride release (release efficiency interference) caused by interfering species present in sample solutions. A release kinetics interference is actually a change of the rate constant k2 (see Section 8.2.7) of the hydride release. Transport interferences are obviously caused by volatile species, most often by hydrides but also by other compounds, or by liquid spray, produced in the hydride generator. These interferences can take place on the surface or in the dead volume of the generator and in the connective tubing causing delay (transport kinetics interference) and/or loss (transport efficiency interference) of hydride. All the interferences could have either a direct effect (if observed only simultaneously with generation of the interferent) or a memory effect (if they persist after the cessation of the interferent generation). With the only exception of compound interferences (see Section 8.6.2.1), species responsible for the interferences are usually present in great excess to the analyte. Then the extent of the interference should not depend on the analyte concentration but only on the interferent concentration. ~A consequence of the independence of interference magnitude on analyte concentration is that the method of standard additions may be, in principle, employed to alleviate the interferences; however, it is usable only when the interference does not reduce the observed signal too much. The other generally applicable method is to dilute the sample. However, sensitivity is reduced accordingly when employing the direct transfer HG methods. This approach is obviously possible only for samples with higher analyte content or when employing collection methods of HG. The extent of feasible dilution is thus limited mainly by the analyte blank in reagents. The risk of an interference is naturally enhanced if the efficiency of the process where the interference takes place is low. For example, a relatively slight change in sample matrix composition will probably change the hydride release efficiency, i.e. cause the interference, when the generation conditions are poorly optimized so that the hydride release is less than 100% even in the absence of interferents. Therefore, optimum conditions for hydride release and transport as discussed above, should be observed. Influence of interferences on the observed signal shape are discussed in the next section. General features and mechanism of interferences are treated in the following.

8. 6.1

Influence of interferences on the signal shape

The CF mode represents the simplest situation since release kinetics interferences are eliminated here and transport kinetics interference influences only the time required to reach the steady state, but not the signal plateau height. Efficiency interferences influence the signal plateau height. However, if the rate constant k2 is reduced to such an extent that the hydride release from the liquid phase is not finished during the time the reaction mixture has at disposal in the generator, the release efficiency interference appears. The

Flow methods in gas-liquid separations

267

appearance of the release efficiency interference in such a case can be prevented, or at least attenuated, by increasing either the tetrahydroborate supply (see Equation (8.12)), and/or the lengths of the reaction coil. In the FI generation mode, the release kinetics interference is eliminated for the same reasons as for the CF mode. The transport kinetics interference can, however, change the shape of the observed signal and, consequently, its height. Since the kinetics transport interference rather delay than enhance hydride transport the signal peaks will be broadened and reduced in height. The peak area which is given by the total mass of hydride introduced to the atomizer/detector should not be influenced when the total gas flow rate is constant since the integrated absorbance does not depend on hydride supply rate but only on the hydride mass delivered to the atomizer/detector. Any efficiency interference should reduce the peak area and height in the same extent since the peak shape should not be influenced. It should be underlined that the peak shape of transient signals produced by FI mode can be changed even when only the efficiency interferences take place. The reason is that, as shown above, the interference magnitude is not influenced by the analyte concentration but by the interferent concentration. Obviously, the changing of the interferent concentration within the time interval critical for forming the signal shape is to be expected because of the sample plug dispersion in the manifold (see Section 8.4.3). The same applies for the transport interference since the gaseous interferent concentration in the generator dead volume changes within the peak interval. In contrast to the kinetics transport interference, the efficiency interferences cannot broaden the observed signal. Signals produced by batch generation 'mode are influenced by release kinetics interference since the corresponding shape of the hydride supply rate function depends on the release rate (see Section 8.4.2). The shape of the observed signal is obviously influenced also by transport kinetics interference. Kinetics interferences should not influence the peak area since they do not influence hydride mass delivered to the atomizer/ detector. However, if there are significant changes in total gas flow rate within the peak interval, as can happen in the batch mode even the peak area can be influenced by a kinetics interference. Efficiency interferences reduce the peak area but the peak shape is probably to be influenced as well for the same reason as discussed above for the FI mode: the interferent concentration in the reaction mixture as well as in the gaseous phase may change considerably within the time interval critical for forming the signal shape. Analogously as in the case of FI mode, efficiency interferences cannot broaden the observed signal. It can be concluded that flow methods of HG are substantially more resistant to kinetics interferences than the batch mode. Due to the peak shaped signals produced, FI mode could be expected to be somewhat less resistant than CF.

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8.6.2 Liquid phase interferences There are numerous interferences in the liquid phase but all fall into two basic groups: compound interferences and matrix interferences.

8.6.2.1

Compound interferences

If the analyte in the sample is partially in a different form than in the employed standard solution, release of hydride may be different from the sample than from the standard, even if the standard is added to the sample. Thus, the compound interferences arise. These take place if organic bonds of analyte were not completely decomposed during sample pretreatment or if the analyte is in a valency which is converted to hydride with lower efficiency. To prevent compound interferences, adequate precautions (appropriate digestion method and/or pre-reduction) have usually to be taken in the sample preparation step. The choice of HG method could affect compound interferences only when pentavalent forms of arsenic or antimony are to be converted to hydrides without prereduction. The compound interference is thus actually the release kinetics one and flow methods are more convenient assuming sufficiently long reaction coil and/or sufficient tetrahydroborate supply (see Section 8.6.1).

8.6.2.2

Matrix interferences

They take place when matrix components affect the hydride release. There is a large variety of sources of the interference: every process preventing analyte to be released in the form of hydride from the reaction mixture. Matrix interferences due to heterogeneous phase components present in the reaction mixture (solid particles or organic suspension) can be quite serious either due to sorption of analyte ions or capture of the formed hydride. Dissolved organic compounds should be also regarded as potential interferents. However, the most frequently encountered type of matrix interferences are interferences of dissolved inorganic compounds (inorganic interferences). The most serious inorganic interferents are ions of transition and noble metals, e.g. Ni, Cu, Co, Fe, Ag, Au, Pd, Pt, Rh. The probable mechanism of the interference is a capture of hydride by species formed by the reaction of interferent with tetrahydroborate. A number of other species can interfere depending on the element determined and on the employed HG set-up. Detailed information can be found in Reference 1. Interferences of other species are typically much less pronounced than interferences of transition and noble metals. An interference mechanism analogous to that in the case of ions of transition and noble metals should be expected.

8.6.2.3

Control of matrix interferences

Detailed description of the means of interference control as described in the literature for individual analytes and various interfering compounds is outside the scope of this chapter.

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See Reference 1 for detailed treatment. In this section, only general features of matrix interference control will be discussed. Sample dilution and the method of standard additions were discussed at the beginning of Section 8.6. The most straightforward way to prevent matrix interferences is to remove the interferent. An appropriate sample preparation should be taken to minimize content of solid particles or organic suspension in the sample. In the case of inorganic interferences, removal of interferents requires a separation. Although generally inconvenient since it complicates the analysis and brings a risk of contamination it has been employed in numerous applications. Interferent separation is typically achieved when analyte preconcentration aimed to improve detection limit is used. Some popular approaches to analyte preconcentration/separation are: (a) solvent extraction, (b) ion-exchange separation, (c) co-precipitation with Fe(OH)3, La(OH)3 , Mg(OH)2-Ca(OH)2, hydrated Mn(IV) oxide etc., (d) distillation and (e) selective precipitation of the interferent. Many separation procedures specific for given interferent/analyte combinations can be found in the literature and are dealt with in Chapters 6 and 7. The other way to control inorganic interferences is the proper choice of chemical and instrumental conditions of the HG step. This can be done efficiently on the basis of the knowledge of the interference mechanisms: the extent of interferences is reduced by preventing precipitation of interferents or by minimizing the contact of formed hydride with the precipitated form of the interferent. The precipitation can be reduced employing masking or releasing agents. Masking agents bound the interferent in a compound (a kind of masking some interferents is for example to increase hydrochloric acid concentration) whereas releasing agents react with tetrahydroborate preferentially. In both cases the interfering precipitate formation is delayed or prevented. A number of masking and releasing agents has been described. Alternatively, tetrahydroborate supply rate to hydride generator can be decreased. The precipitation of the interferent is slowed down more than the hydride release so that the time of contact of formed hydride with the precipitate is cut down. The manipulations with hydrochloric acid concentration or with tetrahydroborate supply rate have been successfully used in selenium hydride and arsine generation, however, they should work for other analytes as well. The other manner to decrease the time of contact of formed hydride with the precipitate is to make the gas-liquid separation faster. In principle, it can be achieved either chemically (e.g. by iodide addition to tetrahydroborate solution or L-cysteine presence in the reaction mixture) or instrumentally by optimizing the HG set-up. It is the potential of the instrumental designs to speed up gas-liquid separation which makes the flow mode of HG the method of choice for controlling the matrix interferences in liquid phase. The interferences are typically much more pronounced in the batch arrangement since the inherently long time of contact of formed hydride with interfering

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precipitates. In contrast, flow methods provide much more freedom to minimize the contact time by shortening or even removing the reaction coil and/or employing GLS with minimum residence time of reaction mixture inside (see Sections 8.5.2 to 8.5.7 for detailed discussion). FI and CF arrangements are analogous in terms of inherent interference suppression since both can employ the same reaction coil length and the same GLS. However, memory interferences, which may be observed in some cases due to deposition and accumulation of interferent within the conduits should be less pronounced in the FI arrangement since the sample containing the interferent is not introduced continuously.

8.6.3

Transport interferences

An increased risk of transport interferences has to be expected if the hydride is delayed or even partially lost by an interaction with the apparatus surface. There are not many identified cases of transport interferences in the literature. The interaction of hydride with the surface manifests itself more often as a memory effect, i.e. as a change in sensitivity with a dependence on changing surface properties of the apparatus rather than as an interference. The obvious way to reduce risk of transport interferences is to make the transport of hydride as fast and efficient as possible. Flow mode hydride generators are again superior here since their miniaturization is simpler than in the case of batch generators.

Acknowledgment This work was supported by grants of GACR no. 203/95/0496 and no. 203/98/0754.

8.7

References

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61 D6dina, J., Prog. Analyt. Spectrosc., 1988, 11,251. 62 Lin, Y. H., Wang, X. R., Yuan, D. X., Yang, P. Y., Huang, B. L. and Zhuang, Z. X., J. Anal. At. Spectrom., 1992, 7, 287. 63 Brockmann, A., Nonn, C. and Goiloch, A., J. Anal. At. Spectrom., 1993, 8, 397. 64 Hueber, D. M. and Winefordner, J. D., Anal. Chim. Acta, 1995, 316, 129. 65 Ding, W. W. and Sturgeon, R. E., J. Anal. At. Spectrom., 1996, I 1,225. 66 Ding, W. W. and Sturgeon, R. E., J. Anal. At. Spectrom., 1996, 11,421. 67 Schickling, C., Yang, J. F. and Broekaert, J. A. C., J. Anal. At. Spectrom., 1996, 11,739. 68 Tao, H. and Miyazaki, A.,Anal. Sci., 1991, 7, 55. 69 Kan, K. T., Anal, Lett., 1973, 6, 603. 70 Fang, Z. L., Flow Injection Atomic Absorption Spectrometry, Wiley, Chichester, 1995. 71 Astrom, O., Anal. Chem., 1982, 54, 190. 72 Fang, Z. L. in: Flow Injection Atomic Spectroscopy, J. L. Burguera (Ed.), Marcel Dekker, New York, 1989, pp 134-158. 73 Magnuson, M. L., Creed, J. T. and Brockhoff, C. A., J. Anal. At. Spectrom., ! 996, 1 I, 893. 74 Buckley, W. T., Budac, J. J., Godfrey, D. V. and Koenig, Anal. Chem., 1992, 64, 724. 75 Zhang, L. S. and Combs, S. M.,J. Anal. At. Spectrom., 1996, 11, 1043. 76 Le, X. C., Cullen, W. R., Reimer, K. J. and Brindle, 1. D., Anal. Chim. Acta, 1992, 258, 307. 77 Le, X. C., Cullen, W. R. and Reimer, K. J., Appl. Organomet. Chem., 1992, 6, 161. 78 D6dina, J., Fresenius. J. Anal. Chem., 1986, 323, 771. 79 Zhang, L., Shan, X. and Ni, Z., Fresenius. J. Anal. Chem., 1988, 332, 764. 80 Ding, W. W. and Sturgeon, R. E., Anal. Chem., 1997, 69, 527. 81 Hwang, J. D., Guenther, G. D. and Diomiguardi, J. P., Anal. Chem., 1989, 61,285. 82 Huang, B. L., Can. J. Appl. Spectrosc., 1994, 39, 117. 83 Huang, B., Zhang, Z. and Xianjin, Z., Spectrochim. Acta B, 1987, 42, 129. 84 Huang, B., Zeng, X., Zhang, Z. and Liu, J., Spectrochim. Acta B, 1988, 43, 381. 85 Noelte, J., At. Spectrosc., 1991, 12, 199. 86 Zhang, L. S. and Combs, S. M., J. Anal. At. Spectrom., 1996, I 1, 1049. 87 Anderson, R. K., Thompson, M. and Cuibard, E., Analyst, ! 986, I I 1, I 143. 88 Hanna, C. P., Tyson, J. F. and Offley, S. G., Spectrochim. Acta B, 1992, 47, 1065. 89 Welz, B. and Schubert-Jacobs, M.,At. Spectrosc., 1991, 12, 91. 90 An, Y., Willie, S. N. and Sturgeon, R. E., Spectrochim. Acta B, 1992, 47, 1403. 91 Chan, C. C. and Sadana, R. S., Anal. Chim. Acta, 1992, 270, 23 I. 92 Fernandez, M. G. C., Palacios, M. A. and Camara, C., Anal. Chim. Acta, 1993, 283, 386. 93 Burguera, M., Burguera, J. L., Rivas, C., Carrero, P., Brunetto, R. and Ga, Anal. Chim. Acta, 1995, 308, 339. 04 Watling, R. J.,Analyst, 1988, 113, 345. -~. Heitkemper, D. T. and Caruso, J. A., Appl. Spectrosc., 1990, 44, 228. 96 Schramel, P. and Xu, L. Q., Fresenius. J. Anal. Chem., 1991,340, 41. 97 Yu, X. A., Dong, G. X. and Li, C. X., Talanta, 1984, 31,367. 98 Hutton, R. C. and Preston, B.,Analyst, 1983, 108, 1409. 99 Qin, F., Gu, G. and Xie, C., Appl. Spectrosc., 1991, 45, 287. 100 Narasaki, H. and Ikeda, M., Anal. Chem., 1984, 56, 2059. 101 D6dina, J., Matousek, T. and Frech, W., Spectrochim. Acta B, 1996, 51, 1107. 102 Do~ekal, B., D6dina, J. and Krivan, V., Spectrochim. Acta B, 1997, 52, 787. 103 Pinillos, S. C., Asensio, J. S. and Bernal, J. G., Anal. Chim. Acta, 1995, 300, 32 !. 104 Valdes-Hevia y Temprano, M. C., Fernandez de ia Campa, M. R. and Sanz-Medel, A., J. Anal. At. Spectrom., ! 993, 8, 821. 105 Vijan, P. N. and Wood, G. R., At. Abs. Newslett., 1974, 13, 33. 106 Thompson, M., Pahlavanpour, B., Walton, S. J. and Kirkbright, G. F., Analyst, 1978, 103, 568. 107 Lafuente, J. M. G., Sanchez, M. L. F. and Sanz-Medel, A., J. Anal. At. Spectrom., 1996, 1I, 1163. 108 Brindle, I. D. and Lugovska, E., Spectrochim. Acta B, 1997, 52, 163. 109 Pierce, F. D., Lamoreaux, H. R., Brown, H. R. and Fraser, R. S., Appl. Spectrosc., 1976, 30, 38. 110 Broekaert, J. A. C., Pereiro, R., Starn, T. K. and Hieftje, G. M., Spectrochim. Acta B, 1993, 48, 1207.

Flow methods in gas--liquid separations 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140

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Garcia, A. M., Sanchez-Uria, J. E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1989, 4, 58 I. Van Eiteren, J. T., Das, H. A. and Bax, D., J. Rad. Nucl. Chem., 1993, 174, 133. Ornemark, U., Pettersson, J. and Olin, A., Talanta, 1992, 39, 1089. Varian VGA76 vapor generation accessory, Varian Techtron Pty Ltd, Pub. No. 8510057700 February 1989, Mulgrave, Victoria, Australia, 1989. Sturman, B. T., Appl. Spectrosc., 1985, 39, 48. Rayman, M. P., Aboushakra, F. R. and Ward, N. I., J. Anal At. Spectrom., 1996, I 1, 61. Yamamoto, M., Yasuda, M. and Yamamoto, Y., Anal. Chem., 1985, 57, ! 382. Schmidt, F. J., Royer, J. L. and Muir, S. M.,AnaL Lett., 1975, 8, 123. Marshall, G. D. and Vanstaden, J. F.,J. Anal At. Spectrom., 1990, 5, 675. Davidowski, L., At. Spectrosc., 1994, 15, 140. Nygaard, D. D. and Lowry, J. H., Anal Chem., 1982, 54, 803. Anderson, S. T. G., Robert, R. V. D. and Farrer, H. N., J. Anal. At. Spectrom., 1994, 9, 1107. Varga, I., Ach-Models Chemistry, 1994, 131,661. Laborda, F., Gomez, M. T., Jimenez, M. S., Mir, J. M. and Castillo, J. R.,,/. Anal At. Spectrom., 1996, 11, 1121. Yang, H. J. and Jiang, S. J.,J. Anal. At. Spectrom., 1995, 10, 963. Yamamoto, M., Takada, K., Kumamaru, T., Yasuda, M., Yokoyama, S. and Yamamoto, Y., Anal. Chem., 1987, 59, 2446. Barnes, R. M. and Wang, X., J. Anal. At. Spectrom., 1988, 3, 1083. Story, W. C., Caruso, J. A., Heitkemper, D. T. and Perkins, L., J. Chromatog. Sci., 1992, 30, 427. Creed, J. T., Magnuson, M. L., Brockhoff, C. A., Chamberlain, I. and Sivaganesan, M., J. Anal At. Spectrom., 1996, 11,505. Wang, X. and Barnes, R. M., J. Anal At. Spectrom., 1988, 3, 1091. Pacey, G. E., Straka, M. R. and Gord, J. R., Anal. Chem., 1986, 58, 502. Ozakl, E. A. and de Oliveira, E., J. Anal. At. Spectrom., 1993, 8, 367. Canham, J. S. and Pacey, G. E., Anal. Lett., 1988, 21, 1619. Wang, X., Viczian, M., Lasztity, A. and Barnes, R. M., J. Anal At. Spectrom., 1988, 3, 821. Lee, Y. K., Lee, D. S., Yoon, B. M. and Hwang, H., Bull. Kor. Chem. Soc, 1991, 12, 290. Chan, W. F. and Hon, P. K.,Analyst, 1990, 115, 567. Nakata, F., Sunahara, H., Fujimoto, H., Yamamoto, M. and Kumamaru, T., J. Anal. At. Spectrom., 1988, 3, 579. Marawi, I., Wang, J. S. and Caruso, J.A.,Anal. Chim. Acta, 1994, 291, 127. Cave, M. R. and Green, K. A., J. Anal. At. Spectrom., 1989, 4, 223. Branch, S., Corns, W. T., Ebdon, L., Hill, S. J. and O'Neill, P.,J. Anal. At. Spectrom., 1991, 6, 155.

Alberto Men6ndez Garcfa

9.1

Introduction

In the last half of this century we have witnessed impressive developments in analytical atomic spectrometry instrumentation, spanning from initial atomic absorption spectrometry (AAS) instruments to multielemental plasma optical emission (OES) spectrometers and lately, to the successful introduction and worldwide acceptance of mass analysers (MS) coupled with an Inductively Coupled Plasma (ICP) as the ionisation source. Sample introduction for atomic detectors, however, has not probably progressed at a similar pace. The simplicity and excellent performance of conventional pneumatic nebulisers and their overwhelming use in AAS and ICP--OES could be blamed for the comparatively scarce real breakthroughs attained on more efficient sample introduction systems for atomic spectrometry. This fact has been stressed by reknowned scientists of the field who labelled sample introduction as the "Achilles heel ''~ or even the "bottle neck ''2 of atomic spectrometry techniques. Conventional pneumatic nebulisers, extensively used for elemental analysis routine work, allow an efficiency of the sample solution transport to the atomiser of 5-12% in AAS and such efficiency falls down dramatically to 1-3% in ICPOES. Thus, in conventional ICP-OES analysis 97-99% of the total sample volume aspirated runs down the drain. It is not surprising therefore that ICP-OES users, after having completed tedious, hard and labour-taking procedures in order to get their samples 274

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into solution, may feel rather frustrated about conventional liquid sample introduction techniques of plasma spectrometry. That explains why this longstanding problem of more efficient sample introduction into the atomiser, still takes a substantial part of the research time of many atomic spectroscopists today. The great variety of designs proposed so far to introduce the sample into an ICP-OES, collected in Figure 9.1, shows the many research efforts in that direction along the years and also illustrate that the "definitive" solution to the problem is probably still to come. It should be noted here, as stated by Z. Fang, 3 that use and exploitation of flow injection analysis (FIA) techniques into this field has contributed significantly to alleviate sample introduction and sample presentation problems of atomic spectrometry.

9.1.1

Conventional continuous aspiration and flow injection in plasma

emission

"Continuous" sample introduction techniques, characterised by the presence of a flow of the sample that facilitates its continuous introduction into the atomisation cell, constitute the "conventional" way of sample introduction in atomic spectrometry. Traditionally, samples are adequately dissolved and the liquid continuously fed into the atomiser, via pneumatic nebulisation to accomplish a steady-state analytical signal. Thus, coming from AAS experience, continuous aspiration techniques providing a constant steady-state signal have been the familiar systems for ICP-OES users. However, Greenfield at the beginning of 19814 published a tutorial paper on the application of flow injection (FI) techniques to ICP-OES analysis and then the application of a "FIAtron" SHS-200 flow injection system for the analysis of Portland cement. 5 In 1981 Zagatto et al. 6 demonstrated the viability of FI in ICP-OES emphasising two distinct situations: first, when the sample dispersion tends to zero and the inherent sensitivity of the spectrometer is preserved; second, when sample dispersion is medium in a typical FI system. This latter situation allows the incorporation into ICP-OES technology of several processes, such as controlled sample dispersion, preconcentration by solvent extraction or by ion-exchange, standard additions, etc., processes all of them easily performed by FI systems. Chapter 3 summarizes many illustrations of such applications. As for flame-AAS detection, 7 in continuous flow conventional nebulization ICP-OES, kinetic features of the detector are not of primary importance because the signal is measured in the steady state (e.g. the volume of the sample does not influence the emission signal). However, by using FI for ICP-OES liquid samples manipulation a well defined volume of injected sample is transported reproducibly to the plasma, and so the particular volume of sample injected must be considered to understand adequately the observed "dispersion", D (D= Co/C, being Co the original concentration of analyte before dispersion and C its concentration in the plug after the dispersion and therefore the actual

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Pneumatic Nebulization .,.ata~ r

-

m t

trie G l a s s N e b u l i z e r C r o s s F l o w N e b u l i z e r

-

DRAIN

Babin~ton Nebulizer

Glass-frit Nebulizer

Ultrasonic Nebulization

0

Electrothermal Vaporization

i

Graphite Furnace

E l e c t r o t h e r m a l V a p o r i z a t i o n Device

Hydride generation

i

?i Art/SIMIrk

Figure 9.1

i?;2,;2 ~

Examples of different sample introduction systems in the ICE

Direct Insertion System

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\

C \

Figure 9.2 The influence of injected volume on sample concentration (dispersion) at peak maxima.

t: time, C: concentration (reproduced by permission of John Wiley & Sons). concentration from which the eventual signal is obtained). The expected influence of sample volume on the transient emission signal (le=k. C) observed should be of the wellknown type represented in Figure 9.2. 7 As shown by the figure, only after a minimum volume of injected sample can the steady state signal emission be achieved. Other experimental parameters influencing the dispersion of the injected bolus which need to be optimised in the system should include the nebuliser spray chamber volumes, the flow rates of carrier liquid and plasma gases and geometrical dimensions and designs of transport conduits (see Chapter 1, by J. Tyson).

9.2 Continuous aspiration and flow injection for hydride generation with ICP-OES detectors Most flow injection methods with spectrophotometric detection 8 are based on monitoring the extent to which an on-line chemical reaction, to derivatize the analyte, has proceeded as the dispersed sample zone passes the detector. The coloured reaction product formed in the downstream solution is usually detected by molecular absorptiometry as a "FIAgram". For measurements based on peak height of this FIA-gram the mixing analyte/reagent should be secured in such a way as to obtain the desired degree of reaction at the peak maximum (e.g. by securing sufficient concentration excess of reagent over analyte). A volatile species generation process, so common nowadays in atomic spectrometry in order to obtain a transport efficiency of the analyte from the sample solution to the atomization cell close to 100% (see Chapter 8 by J. Dedina), is at first sight one particular

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case of such derivatization reactions for which FI manifolds are so useful. Therefore, one would expect that principles governing reaction products formation, in a kinetically controlled manner in the manifold, would be similar to those amply described for downstream molecular absorptiometric detection, s Of course, with atomic detection finish of the volatile species formed there is also a gas-liquid separation complicating the final amount of reaction product which reaches the detector (and so determining the final emission signal). Thus, apart from the conventional manifold design for chemical derivatization we should consider and optimise: (a) the release of chemical species from the solution; (b) the physical separation of gases from the liquid solution (optimization of gas-liquid separators becomes of primary importance); (c) the final transport of separated gases to the atomiser with minimum decomposition of the generated volatile species. Using plasmas as detectors a further consideration arises. The use of reagents such as NaBH4 or NaB(C2H5)4, in adequate concentrations for derivatization reactions from aqueous solutions, will bring about on acidification a large amount of molecular hydrogen (or etane) which could easily extinguish the plasma. This is a well-known effect in the Inductively Coupled Plasma when samples and reagents are introduced in a classical flow injection mode. 9 Of course, this drawback is even more important using Microwave Induced Plasmas (MIP) for final detection of the volatilized analyte. ~~The low power of this last, low cost, excitation plasma source (its operating power is ten times lower than that of an ICP) limits seriously the capacity of MIP to vaporize and to atomize solid/liquid samples and also explains its critical sensitivity to changes in the impedance of the discharge (e.g. to small amounts of molecular gases introduced suddenly into the plasma). In other words, for a reliable determination of the analyte using plasma emission detectors a smooth running of the discharge must be secured during measurement time. Outbursts of molecular gases (produced in the flow system on generating the desired analyte volatile compound to be eventually detected) should be avoided. Different flow manifolds have been proposed in ICP-OES for smooth running of the discharges. Different designs of continuous flow exist and Figure 9.3 illustrates a typical continuous flow volatile compound generation/analyte introduction into the plasma which should provide a stable background signal over which the transient FI peak of the analyte signal could be accurately measured.

9.2.1 Batch generation of hydrides As shown in the preceding chapter, ten elements (As, Sb, Bi, Ge, Sn, Pb, Se,Te, T1, Cd) can form volatile hydrides and so be analysed today at ultratrace levels by resorting to hydride generation with atomic spectrometric detection. The main problem presented by initial "batch" systems connected to an ICP-OES, lies in the massive production of hydrogen which may extinguish the plasma when entering

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Plasma

Gas meter

Spectrometer

1 Output Reductant ~,,..! O Sample

~[ O

I

_5 v Volatilized analyte

RF gencrator

Gas-liquid separator

J~ ""A

waste

Figure 9.3 A typical sample and reagent continuous flow hydride generation. the ICP torch, along with the metal hydride. In order to avoid such difficulties, the first devices for coupling the hydride generation with the ICP-OES used a trap of liquid nitrogen between the hydride reactor and the torch; this trap allowed by a double-way valve venting out the hydrogen generated while the hydride was collected in the trap. Then the liquid nitrogen was removed and the hydride could reach the ICP-OES generating a transient signal. This system never enjoyed popularity.

9.2.2

Flow Injection for ICP--OES with volatile species generation

The use of FI for hydride generation, as batch generation, gives rise to a transient signal. However, in this case the amount of hydrogen generated from samples with the volatile hydride is small (microvolumes can be injected) and so the plasma remains stable during the determination without resorting to complicated devices for securing the plasma stability (as needed for batch mode generation). Figure 9.4a shows the main components for FI hydride generation with continuous aspiration of reducing reagent and on-line detection by ICP. A peristaltic pump, P, supplies a continuous flow of the carrier, the acid or other reagents and the reductant solutions. Sample is injected through the injection valve, S and in the subsequent coil sample it is

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PUMP

ICP

Ar

Z?

N a B E t 4 or N a B H 4 CO1

Reagent

Connecting tubing

. .

I

S

Buffer carrier

Reaction coil Sample Gas-liquid separator

Drain

PUMP ICP

Sample (700 ~tl)

Bul'ti~r

Z?

carrier S

Connecting tubing

Water

I

N a R E t 4 (800 ktl)

Gas-liquid separator

Drain

Figure 9.4 Flow injection strategies: (a) schematic diagram of the flow injection system for hydride generation; (b) schematic diagram of a typical "merging zones" FIA system for ICP-OES. conditioned to the adequate pH. The generation of the hydride takes place in the second reaction coil. A phase separator drives the gaseous phase into plasma torch. It is clear that this arrangement of sample presentation to the plasma ensures a smooth running of the discharge; when the burst of the hydride enters the plasma a continuous flow of H2 (coming from the NaBH4 continuous descomposition) has already mixed with Ar plasma gas originating a mixed gases stable discharge hardly disturbed by the small outburst of gases coming from sample injection in the flow. In other words, the plasma

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characteristics are only slightly changed during transient analytical signal production processes using these FIA manifolds. As an example, Pyen et al. ~'~2 injected microvolumes of sample solution in a continuous flow of reagent using a three-channel system. In this way a precise and accurate simultaneous determination of As, Sb and Se hydride generation was achieved (although FIA detection limits for each element were slightly poorer than by continuous merging flows operation). Figure 9.4b shows a typical "merging zones" FIA arrangement, where both sample and reducing agent are adequately merged. This system is extremely useful when the reagent is expensive or scarce. Thus, some authors 13 prefer to use the "merging zones" technique, by injecting equivalent volumes of sample and reductant solutions into a continuous flow of water; in this way, the amounts of sample and borohydride solutions are minimized and a high sample throughput obtained. ~4 In any case, volatile species generation characteristics for ICP work are different if ethylides are used instead of hydrides in order to increase the sensitivity of atomic emission signals. Continuous operation of the plasma aspirating the reducing agent (Figure 9.4a) is adequate for both flow ethylation or hydridation. Background emission of the plasma increases with increasing concentration of reducing reagent solution but reflected RF power does not increases too much (up to 12 watts for 0.5% NaBH4 with 1 M HC1 acid solution) to seriously disturb the discharge. Ethylation in normal conditions (e.g. 0.2-1% NaBEt4 at pH around 3) does not perturb visually the plasma, while hydridation does produce visual discharge instability due to hydrogen entrance in the discharge at the acidity normally employed (e.g. 1 M HC1). The use of"merging zones" mode (Figure 9.4b) is more critical as a high concentration of reductant (e.g. 5% NaBH4) is accepted in continuous mode while it extinguishes the plasma in the FIA arrangement (500 i~l of each, sample and NaBH4). In any case, it seems that at reducing agent concentrations below 2.5% the "merging zones" approach can be used without serious perturbations of the plasma discharge. Of course, the background and reflected power changes observed during sample measurement are always much higher using NaBH4, with HC1 1M, than using NaBEt4 at normal ethylation acidity conditions. Therefore, an increase of using ethylation rather than hydridation can be envisaged for the near future not only in ICP-OES but also in ICP-MS work. 9.2.3

Continuous flow hydride generation~nebulization with I C P - O E S detection

Hydride generation (HG) in a continuous flow (Figure 9.3) is probably the more common method for coupling hydride generation to ICP-OES, since proposed in 1978 for the improved determination of As, Bi, Sb, Se, and Te by Thompson et al. ~5 However, a combination of HG and conventional nebulization in a single manifold is possible. Huang et al. 16 proposed such a useful system by using a modified concentric

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Ar Nebulizer ~ ~ , ~

End cap

To ICP

1I~ Wash ,~......~~ Hydridegenerator

Sample NaBH4

Spraychamber Drain

Figure 9.5 Schematic of the nebulizer-hydride generator system (reproduced from Reference 16 by permission of Elsevier Science). nebulizer (Figure 9.5). The sample is introduced by a peristaltic pump via pneumatic nebulization; the same peristaltic pump drives by another channel a NaBH4 solution that flows over the hydride generation device (a sort of spoon made in glass) where the hydride formation takes place when the spray of acid sample and reductant mix in the glass device. The nebulization chamber acts as a gas-liquid separator. This Huang's design, shown in Figure 9.5, allows analytes of the sample to reach the plasma in two different ways: as a volatile hydride in the case of the elements forming hydrides and as a liquid aerosol for non-hydride-forming elements. Of course, very important improvements in the detection limits (LD) of elements forming hydrides are observed, without degradation of LDs attained for non-hydride forming elements. The introduction system was later modified ~7 in order to attain a smoother reaction of hydride formation. This system was also used to generate continuously hydrides from organic solvents. ~7

9.3 Continuous on-line volatile species generation to increase analytical sensitivity and selectivity of plasmas In the previous section we have outlined the different approaches to sample presentation/ introduction into the ICP-OES with special reference to generation of hydrides as an advantageous alternative to pneumatic nebulization. Cold vapour generation of mercury 18 and cadmium ~9 are similarly exploited to obtain liquid to gas phase conversion of the analyte at room temperature. However, other types of derivatization reactions of the analyte to form chemical vapours from a solution at room temperature are available.

Continuous aspiration and flow injection

9.3.1

283

Alternative metal volatile species-forming reactions

Acctylacctonates 2~ and other volatile compounds as SiF4,22 methyl borate 23 or nickel carbony124 have been utiliscd in on-line designs coupled with the atomic detector to enhance analyte transport to the atomiscr. In many of those instances, however, more or less complicated devices arc required to raise the temperature in the reactor to promote volatile species generation. A much more general and versatile approach is the recent exploitation of the formation of cthylatcd derivatives by using sodium borotctraethylide, NaBEt4. The metal-ethylderivatives formed are stable and quite volatile, and so ethylation with this reagent can compete with hydride generation. Particularly interesting is the possibility of its extension to enhance detectability of different elements of non-hydridegenerating elements (e.g. Zn, Cu, T1, etc) able to form ethylderivativcs. 25 Since 1986, when lead and organolead were spcciated based on their respective cthyldcrivatives, 26 several papers, making use of such derivatization reactions, have appeared in an attempt to improve the efficiency of analyte transport to the atomizer. D'Ulivo et al. 27 report the use of such alkylation reactions for the atomic fluorescence spectrometric (AFS) determination of very low amounts of Cd. Liang et al. 28 also used this approach for mercury speciation by GC-CV-AFS, while other authors used it for the determination of Pb 29 or Se. 3~An interesting review of this subject has been published. 31 Recent work in our research group has also used ethylation reactions for Pb determination investigating continuous alkyl lead vapour generation in the presence of "organized media" (micclles and vesicles) with final detection by ICP-OES, 32 FIA Bi determinations in urine 33 using "merging zones" or Cd analysis in environmental and food samples after continuous alkylation with sodium tetracthylboratc. 34 Further details about the application of such analyte volatilization reactions to toxic organometallic compounds speciation are given in section 12.2.1 of Chapter 12 of this book. Of course, principles, techniques and potential applications of these alternative volatile species forming techniques for metals and scmimetals should be similar to those of hydrides, discussed in detail in the preceeding section. Therefore, in the following we will rather concentrate in a more elusive area of atomic spectrometry: the direct determination of non-metals by atomic emission spectrometry with different plasmas (e.g. ICP-OES, MIP--OES, GI)-OES) via volatile species generation in a continuous flow. 9.3.2 Direct determination of non-metals by I C P - O E S after continuous generation o f volatile species at room temperature

Atomic spectroscopic methods have been scarcely used for the direct determination of non-metals and this is particularly so for ICP-OES which has been used only for a few

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non-metal direct determinations (e.g. phosphorus 35'36 or sulphur. 37-39) When sulphur is present in the sample as sulphide, the determination of sulphur is possible by generating on-line SH2 volatile species to feed the ICP-OES. For instance, ICP-OES was proposed as detector for sulphide determination in groundwater samples after the formation of H2S by continuous reaction of the sample with a non-oxidizing acid, HC1, after continuous mixing of both solutions forced by a peristaltic pump. A gas--liquid separator allowed final feeding of H2S formed to the ICP, although atomic emission of sulphur was measured in the vacuum UV.4~41 Men6ndez Garcia et al. 42 proposed the direct determination of low levels of sulphur in cast steel and low alloy steel by ICP-OES. Sulphides of steel sample were released by sample dissolution in a non-oxidizing 6 M HC1 acid solution collecting the sulphide in a 1 M NaOH solution. Continuous merging of this sulphide-containing solution and a 6 M HCI solution, in a T-piece, as shown by Figure 9.6, produced continuously hydrogen sulphide gas. This gas, separated from the liquid phase in a gas-liquid separator by a smooth stream of argon, led the formed H2S to the torch for sulphur monitorization at 182.04 nm in an Ar purged spectrometer. Calibration was carried out with certified cast steel. SO2 determinations should also be possible with this methodology in the ICP-OES. When sulphur is found in the sample as S(VI) the determination of the element by ICP-

Peristaltic Pump

waste

Figure 9.6 Experimentalset-up for H2S generation coupled to the ICE

Continuous aspiration and flow injection

285

OES after H2S generation is possible, but much more complicated because the required reduction is usually very difficult. 43'44

9.3.3 Direct determination of non-metals by MIP-OES after continuous chemical generation of volatile species at room temperature Argon or Helium Microwave Induced Plasmas, MIPs, are more adequate for non-metal determinations than ICPs as they are able to excite efficiently high level-lying energy levels. Moreover, for non-metalic elements as S, C1, Br, etc., non-resonant lines in the visible and near-IR regions are also observed. 45 This capacity of MIP for efficient excitation of non-metals is attributed to the presence within the discharge of electrons of very high kinetic energy (the electronic temperature of an MIP of Helium will be around l0 000~ in contrast to the low kinetic temperature of the gas of the plasma, about 700~ These plasmas, usually operated at powers of 100-200 w and formed in quartz or ceramic tubes of 2-3 mm of inner diameter, lack the power of ICPs for volatilisation and atomisation of liquid samples. Therefore extinction of the plasma by liquids is unavoidable and the sample must be introduced into the MIP as a gas or a vapour; in other words, the use of reactions of volatile species formation to lead the analyte into the plasma becomes very useful in MIP determinations. Only high power MIPs, approaching in this way ICP performance for volatilisation and atomisation, allow direct liquid sample introduction as demonstrated by Carnahan 46 who used conventional nebulization of liquids in a He-MIP to determine halogenides. For some time now MIP-OES was proposed as an atomic detector for gas chromatography, 47 but halogen direct determinations have been developed more recently. Non-metal determinations after some type of vapour generation at room temperature, using continuous flow systems, were initially very scarce. Sulphide in alkaline solutions was determined 48 with a similar device to the one shown in Figure 9.6 except that here a flask containing concentrated H 2 S O 4 as desiccant must be included between the gas-liquid separator and the He-MIP-OES-surfatron discharge in order to eliminate the water vapour accompanying to H2S generated. Comparative analytical performance characteristics obtained for the determination of sulphur, introduced as continuously generated SH2, using MIP-OES and ICP-OES are summarized in Table 9.1. According to Table 9.1, MIP-OES provides detection limits around 40 times better than the ICP for the analysis of sulphur. As the MIP operates normally at a power of 100-200 w, (ICP-OES around 1250 w), the flow of plasma gas is much lower for the MIP than in the ICP and the determination by MIP does not need the optical path to be purged. MIP-OES clearly offers a more convenient sulphur determination. 49 Nakahara e t al. 5~ also described a method for the determination of low sulphur levels by continuous generation of H2S and SO2 (by acidification of solutions of sulphides and

286

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Table 9.1 Comparative analytical characteristics of sulphur determinations by plasma optical emission after SH2 continuous generation (reproduced from Reference 48)

Wavelength (nm) LD (ng ml-t) Linear range (l~g ml- ~) Precision (RSD, %) Interferences Need of a previous purge Plasma gas

MIP-OES

ICP--OES

560.61 0.5 5. LD- 100" 8 N.D.b No He

182.04 20 5. LD-500" 3 N.D.b Yes Ar

~'maximum concentraci6n level tested. b N.D. =Not detected LD: limit of detection

sulphites, respectively) using a MIP-OES as detector. The DLs found at 180.73 nm emission line, were 0.13 ng ml-~ and 1.28 ng ml-~ for the HzS and SO2, respectively, and the method was applied to the determination of sulphides in residual waters and of sulfite in table wines. Helium offers very good possibilities for non-metal excitation because of its high ionization potential (24.6 eV), its comparatively simple background spectrum, its high energy metastable levels (19.73 and 20.53 eV) and a high optical transparency at short wavelenghts. The performance of both Ar and He as support gas of MIP has been compared. 5~ First studies using He-MIP-OES at atmospheric pressure for the determination of halides (after chemical generation at room temperature of the respective volatile halogen) used batch procedures. 52 Later on, systems of continuous flow developed. Nakahara et al. published iodide 53'54 and bromide 55"56 determinations by using a He-MIP-OES with continuous generation of the halogen and a TMo~o Beenakker cavity. However, other cavities may be favourable to generate a MIP as is the "surfatron" proposed by Moissan. 57 Sanz-Medel et al. 58"5~ also used an Ar-MIP generated by surface waves, surfatron, for iodide and bromide determination. The halides were oxidised in a continuous flow to iodine and bromine vapours before introduction on-line into the plasma. 58-6~The method was successfully applied to low level iodide determinations in table salt 58 and low levels of bromide in drug preparations. 59 The decisive influence of the size and geometry of the gas-liquid separator and the concentration of iodine on the final stabilization of the analytical signal was demonstrated. 6~ Oxidation of halides to their respective halogens may be also carried out "on-line" by using solid oxidants as PbO2 or MnO2, packed in a mini-column inserted in a continuous flow system. When acidic solutions containing the halides flow through the mini-column, halides are continuously oxidized to their respective volatile halogens; the vapour is then

Continuous aspiration and flow injection

287

continuously separated from the liquid phase to feed the MIP-OES. Chloride, bromide and iodide in aqueous solution were so sequentially analysed by MIP-OES 61 with 50 ng ml-~ (for C1 at 837.6 nm), 50 ng ml-~ (Br at 827.26 nm) and 200 ng mL-~ (I at 804.37 nm) detection limits. After careful comparison of both liquid and solid systems of on-line oxidation in our laboratory,62 it appears that solid-liquid PbO2 oxidation is more troublesome (mini-columns of PbO2 providing reproducible and reliable results are difficult to prepare). The MIP--OES has also been used as atomic detector of semimetal elements forming volatile hydrides, after their adequate trapping. The pioneering paper from Lichte and Skogerboe 63 on arsenic determination, via arsine generation, must be cited as an example. Venting of hydrogen formed with the volatile hydride is possible in this way. The contribution of Barnett et al. 64 using a solid minigenerator of hydride and operating with a flow injection system minimizes the quantity of hydrogen so providing quite good plasma stability. A review has been published recently on volatile hydrides generation and trapping in a flow system before final atomic spectrometric detection 65 which may be useful for MIP-OES spectroscopists.

9.3.4 Determination of non-metals by glow-discharge (GD)-OES after volatile species generation The possibility and potential of radiofrequency glow discharges (rf-GD)-OES to determine gaseous non-metal elements in a flow has been investigated in our laboratory 68 because these low pressure plasmas exhibit detection limits for halogens, (as vapours) in the pg/g region. The sample introduction system used was an "exponential dilutor" previously used in our MIP-OES work. 66 Figure 9.7 shows the schematic of this device which proved very useful to introduce volatilized samples (heating up to 200~ is possible) into reduced pressure plasmas in a way that resembles the flow injection of a vapour sample into a carrier gas stream. The instantaneous concentration of analyte reaching the plasma after a time t of injection is: 67

C= Co e- (r/vo), (Co being initial concentration in the flask, F carrier gas flow and Vo total volume of the exponential dilutor flask), rf-GD--OES detection limits obtained in this way for C1, C, Br, and S are lower than those obtained using more conventional plasmas. 68 The recent use of these glow discharges for GC detection 69 or as ionization sources for traces of organic compounds determination in air by GD-MS TM using flow systems should be mentioned. Present research points to these low pressure plasmas as a source for detection of non-metals vapours, an alternative to the He--ICP or He-MIP better known plasmas.

288

9.4

A. Sanz-Medel et al.

Flow generation of volatile compounds from organic solvents

Generation of hydrides and other volatile species from an aqueous medium is found to be strongly interfered by the majority of the transition elements. 7~-73 Furthermore, other hydride forming elements can consume reductant and so reduce analyte hydride formation. Generation of hydrides from an organic solvent could provide several advantages: Ca) (b)

Metals present in hydrophobic samples (e.g. polymers) could be directly solubilized in organic solvents TM and directly analysed. A previous liquid-liquid extraction in an organic solvent can be used to separate/ eliminate interferences and so improve selectivity. Then the corresponding organic extract could feed the nebuliser of the plasma source. Three-way stopcock Carrier gas w

Electrical heating tape

~

detector

Magnetic stirrer

Figure 9.7 Diagramof the exponential dilutor system.

Continuous aspiration and flow injection

(c)

289

Such liquid-liquid extraction into an organic solvent could also enhance the sensitivity by preconcentration of analyte into the organic phase.

An efficient generation of analyte hydrides from an organic solvent should be aimed at. To get it reducting agents, e.g. NaBH4 or SnCI2, are prepared by dissolving their corresponding salts in an organic solvent, as dimethylformamide (DMF), compatible with the solvent used for the previous liquid-liquid extraction. Likewise, compatible acid media should be secured using organic acids, as acetic or formic (or perhaps mixtures of mineral acids with organic acids, HCI-DMF or H2SO4--DMF. 75) 9.4.1

Generation o f volatile hydrides

In this way, the generation of volatile hydrides from organic phases, after liquid-liquid extraction, could be formulated according to the following scheme: M "+(aq) + nLH---} (ML,,)~org)+ n H (aq) +

(ML,)(org)+nH ++ NaBH4 (in DMF) ~ M H n T + H2(exc)T + H3B03 Aznarez's group 76-78 pioneered these types of studies of volatile hydride generation from an organic medium, in batch systems, as a way of a more efficient sample introduction to flame AAS. They determined in this way Pb in steels (after its extraction with pyrrolidine-l-carbodithioate in CHC13 and then H4Pb generation in this phase by using NaBH4 and 1% sufuric acid solutions in DMF), 76 Sb in steels 77 and Sn in tinned foods (after the extraction of its complex with the ammonium salt of N-nitrosophenylhydroxammic acid in CHC13 and H4Sn generation). TM They used flame AAS detection, but their work opened the way for the use of other atomic detectors, including plasma sources. Similar batch procedures were used later on for determinations of lead in petrol 79 and organotin compounds in N-N dimethylformamide, s~ In 1989 Cacho and Nerin 75 described the generation of a volatile cadmium species from an organic medium of DMF, in the presence of diethyldithiocarbamate (a catalyst according to the authors) using NaBH4 solution in DMF for reduction and AAS measurements. 75 Mercury ultratrace determinations by CV-AAS, from an organic medium were proposed by Poluktov and Vitkum much earlier, 81 even if the work of Ott and Hatch s2 usually stands for the first analytical application of cold mercury generation-AAS determinations. Some hydride forming elements, e.g. as As and Sb, cannot be easily determined by HG-FAAS from an organic medium, unless efficient background correction is used (they possess their resonance lines close to the vacuum UV and the organic vapour molecular bands may produce serious spectral overlapping, s3' 84) ICP-OES detection can be advantageously used for hydride-forming elements after

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Table 9.2 Detection limits by hydride generation-ICP--OES of non-aqueous test solutions (reproduced from Reference 17, by permission of Elsevier Science) Detection limit (~g/i)* Solvent-acid combination

As

Sb

Bi

Se

Te

Pb

Cloroform-acetic Xylene-acetic Ethyl acetate-acetic IBMK-acetic IBMK-formic lsoamyl alcohol-acetic CCl4-acetic acid Aqueous solution (5 M HCi)

1.5 1.2 3.1 1.0 0.8 4.6 3.9 1.3

4.3 3.7 6.7 4.3 4.3 7.5 6.2 2.1

!.0 i.6 !.4 0.9 0.5 2.8 3.0 0.3

2400 > 3000 ! 80 650 220 9.6 230 0.6

620 > 3000 230 700 186 > 3000 320 1.4

880 1500 280 550 430 > 3000 490 -

* 20-as given by software of the spectrometer

their volatile species generation from an organic phase in a continuous mode. B. Huang t7 first realized this ICP-OES application for the determination of As, Sb, Bi, Se, Te and Pb using previous on-line hydride continuous generation from several organic solvents. For comparison, Table 9.2 resumes the DLs published by these authors ~7for different hydride forming elements by HG-ICP-OES from various organic solvents, as compared with those detection limits obtained with generation from aqueous phase. In general terms, it seems that some detectability is lost in organic solvents as compared with HC1 media. The stability of the ICP-OES discharge can be affected by the arrival of the solvent volatile organic vapour accompaying the volatile hydride. Therefore, the organic solvent must be carefully selected bearing in mind two main considerations: first, it should be well tolerated by the ICP--OES, and secondly the selected organic solvent must be adequate to achieve the quantitative extraction of the analyte. Solvents with a low vapour pressure are usually better tolerated by the ICP discharge. Regarding complete extraction of the charged analyte, formation of an extractable coordination complex (of the MLn type) or an ion-pair (containing the element generating the hydride) are typical. 85 From exhaustive studies on the effect of organic solvents introduced into the ICP-OES, Browner 86 and Miyazaki 87 concluded that the organic solvents more adequate for ICPOES work (i.e. those to be used when a volatile hydride is going to be generated from an organic phase) are those exhibiting lower vapour pressure as shown in Table 9.3. Methanol, ethanol and benzene are badly tolerated by the plasma. Xylene is a very good choice, both for extraction and also well-tolerated by the ICP discharge. Organic solvents with vapour pressure higher than that of xylene seem to be worse tolerated by the plasma. Soon after the seminal paper of Huang 17 on the extraction of arsenic using AsI 3 extraction in IBMK, A. Men6ndez et al. 88 proposed the extraction/determination of arsenic

291

Continuous aspiration and flow injection

Table 9.3 Tolerance of ICP--OES technique to the introduction of organic vapours into the plasma Boiling Temperature in ~

Solvent Methanol Ethanol Butanol Hexanol Benzene Toluene Xylene Nitrobenzene IBMK Chloroform Water

Vapour Pressure (20~ in mm Hg

64.7 78.3 117.5 157.9 80.1 110.8 140.6 210.9 115.8 61.2 100

105 120 5 1 76 21 4 1 5 105 18

Ease of Introduction v.d. v.d. e. v.e. v.d. e. e. v.e. e. e. v.e.

v.d. very difficult, e. easy, v.e. very easy

in an organic phase of xylene by continuous flow hydride generation and ICP-OES final detection with a great enhancement in selectivity (none of seventeen ions of transition metals assayed interfere) and excellent sensitivity (0.7 ng m1-1 of As detection limit). Similarly, Cano Pav6n et al. 89 determined cadmium in biological samples after its extraction as a complex with 1,5-bis 1-(2-pyridyl) ethylene thiocarbonohydrazide at pH 4.7, in IBMK. A preconcentration factor around 25 was reached and a 0.15 ng mL -1 detection limit for Cd reported. Other metal liquid-liquid extraction determinations in a batch mode and ICP-OES detection using several hydrazide derivatives reagents, have been proposed by the same authors obtaining both, sensitivity and selectivity improvements.90.91 The same group proposed later the on-line preconcentration of Cd 92 and Co, Cu and Ni 93 in a continuous liquid-liquid extraction, by using 1,5-bis(di-2 pyridyl)methylene thiocarbohydrazide in IBMK with final determination of such elements by ICP-OES. 9. 4.2

Generation of alternative volatile s p e c i e s

Nebulization of volatile acetylacetonates of several metallic ions in an organic solvent has also been proposed for improving the sensitivity of analytical determinations by ICP-OES (in this way the vapour reaching the plasma is enriched in the analyte). In this manner Black and Browner 94 extracted several fl-dicetonates of different cations in xylene and so Fe, Cu, Zn, A1, Mn and Cr determinations from organic solvents were reported. 94 Table 9.4 resumes the analytical performance characteristics of these peculiar determinations for bovine liver and human blood serum. In a similar vein, enhancements of the ICP-OES sensitivity of molibdenum were found by Sanz-Medel et al. 95 when the metal was dissolved in several organic phases (e.g. xylene, butanol, chloroform, etc.) as the volatile molibdenum hexacarbonile. Nickel is

292

A. Sanz-Medel et al.

known to form gaseous carbonyl with carbon monoxide at near-ambient conditions of temperature and pressure. Vijan 24 was the first to utilize this reaction for analytical purposes. Nowadays, the reaction is performed by bubbling carbon monoxide gas through slightly alkaline reaction mixture with elemental nickel freshly precipitated by tetrahydroborate. Lee96 used the cryogenic trapping of nickel carbonyl generated in the batch mode. Sturgeon et al. 97 applied in situ trapping of the nickel carbonyl also generated in a batch arrangement. Alary et al. 9s employed batch generation even in the direct introduction mode and claimed that although the procedure was less sensitive than the cryogenic trapping, it is simpler and well adapted to routine determination of nickel in a wide range of samples. Drews et al. 99 utilized the flow arrangement for nickel carbonyl generation in the direct introduction mode. The same group subsequently incorporated cryogenic Table 9.4 Determination o f some metals by I C P - O E S continuous nebulization o f acid digests and directly extracted metal trifluoroacetylacetonates a (reproduced from Reference 94 by permission o f A m e r i c a n C h e m i c a l Society)

A. NBS Bovine Liver (SRM 1577)

Metal Fe Cu Zn AI Mn Cr

NBS value, b txg. g-~

Free metal in acid digest, ixg- g-t

268+8 193• 10 130• 13 c 10.3• 1.0 9.08 • 0.012

266+5 187• 130• 8.2• 9.5• 0.092 • 0.009

Recovery %

Metal Trifluoroacetylacetonate in xylene

Recovery %

99 97 100 d 92 104

259+ 12 180• 120• 8.0• 9.2• 0.072 • 0.008

97 93 92 d 89 82

Values reported as mean:i: SD (12 determinations) b NBS certificate of analysis Not given d Not determined

B. Human Blood Serum concns, found, Ixg mi- J mean+SD a Metal

Acid digest

Extracted tfac in Xylene

Recovery %

Fe Zn Cu Cr

0.65+0.04 0.60+0.05 0.86+0.05 nd b

0.60+0.05 0.58+0.06 0.76+0.06 0.009 c

88 93 94

a Six determinations b Below LD c Average of two values

Continuous aspiration and flow injection

293

trapping of the carbonyl to increase the detection power of the method. 1~176 Erber and Cammann ~~ adopted the flow mode of nickel carbonyl generation with subsequent collection by in situ trapping. The "true" flow mode with gas liquid separator and forced outlet was used. Sample and tetraethylborate streams were merged together and then mixed with the carbon monoxide stream by simple T-connections. A reaction coil was required to provide sufficient reaction time but there was a decrease of the signal for too large coils.

9.5

Continuous generation of volatile species from organised media

"Organised" media as micelles and vesicles exhibit unique properties in solution that can also modify chemical reactions. Such surfactant aggregates can compartmentalize and preconcentrate reactants in a selective manner. Thus, it seemed likely that their use as a peculiar reaction medium could produce improvements in volatile species generation to increase the efficiency or the kinetics of analyte transport from a liquid sample to a plasma. In principle, further improvements of the analytical sensitivity and selectivity achievable using volatile species generation/atomic detection could be obtained by adding surfactants, 1~ because: (1) ordered media may concentrate reactants at a molecular level and, consequently, modify favourably the thermodynamic and kinetic reaction constants of volatile species generation. Accordingly, we could observe substantial changes of the experimental analytical sensitivity selecting an adequate organised medium; (2) ordered media may solubilize, in a selective manner, analytes and reactants in the aggregates. Thus, other competing reactions (interferences) detected in the bulk aqueous phase may be modified by the special microenvironment existing in/on these aggregates. In this vein, during the last five years we have identified at least three different possible effects when a certain "organised medium" is added, as reaction medium, for volatile species generation reactions: (a) kinetic effects, that is, a variation of reaction rates of volatile species generation, (b) efficiency enhancements of some unstable species generation/volatilization and (c) improvements of selectivity observed of typical generation/volatilization methods. 9.5.1

Improving kinetics

Mercury cold vapour generation-ICP-OES was studied in various organised media 1~ including cationic (surfactants, hexadecyltrimethylammonium bromide, (CTAB)); anionic surfactants (sodium laurylsuphate, (SDS)); non-ionic surfactants (Triton X-100); zwitterionic surfactants (zwitteragent 3,16 (ZW-3,16)); cationic vesicles of didodecyldimethylammonium bromide, (DDAB); anionic vesicles of dihexadecylphosphate (DHP). Among

294

A. Sanz-Medel et al.

all of them, only cationic micelles of CTAB increased the signal by 20-30% as compared to conventional CV-ICP--OES. When cationic vesicles were added it was observed that the Hg signal intensity doubled, if the DDAB surfactant was simultaneously sonicated with the Hg 2§ sample. Conversely, no positive effects were noticed when Hg 2§ was added to a DDAB solution previously sonicated. These results indicate that Hg 2§ must be trapped into the vesicles in order to observe signal enhancements by the ordered medium. The effect of different types of surfactants on the atomic absorption signal of mercury by cold vapour generation--AAS was also investigated by Gutierrez et al. i~ They reported enhancements of 50% in the signal when the Hg vapour was generated in a continuous mode from a sodium dioctylsulphosuccinate solution, which gave a signal 50% higher than that for conventional Hg cold vapour generation. According to the authors, the system follows the known Hartley's rule: the ionic heads of the anionic surfactant and the Hg 2§ ion are of opposite charge, so the analyte tends to associate with the micelle surface via electrostatic attraction and the micelle surface becomes analyte enriched. The study of the effect of different organised media on arsine generation 1~ demonstrated that cationic vesicles of DDAB provided a more adequate reaction medium than normal acid media, because sensitivity of HG-AAS determination of arsenic increased by a factor of two as compared to conventional aqueous medium (detection limits were 0.6 ng ml-' of As for DDAB and 1.3 ng ml-' in conventional aqueous media). Besides, significant improvements in the precision of the HG-ICP--OES emission signals were observed by using DDAB vesicles for continuous flow arsine generation. A similar methodological approach to that used for arsine generation, was employed to study the effect of organised media on plumbane generation; the positively charged surfactants, CTAB and DDAB, produced an important increase in the lead signal when the plumbane is generated from a medium containing K2S208 and nitric acid. ~~ Ellis and Tyson 1~ investigated the use of didodecyldimethylammonium in the determination of arsenic by flow injection hydride generation atomic absorption spectrometry. Calibrations in the presence and absence of the surfactant in the sample solution were not significantly different, either for the case where vesicles were formed in the presence of the analyte or where they were preformed in the surfactant solution and then added to the analyte. They concluded that the role of surfactants in the hydride generation process using FIA techniques is not yet unambigously established. Recent work in our research group and elsewhere points to a catalytic action of surfactant aggregates, a sort of "micellar phase transfer catalysis" (MPTC), where an efficient transfer of all reactants into the aggregates, and their molecular ordering there, would account for the observed enhancements of the hydride generation reaction rates. ~~ Considering the kinetic character of such derivatization reactions, the final inprovement/ decrease of signals observed by continuous flow (steady state signals) hydride generation

Continuous aspiration and flow injection

295

techniques could be quite different to those observed by using a typical FIA experiment ~~ where non-steady-state atomic signals are measured.

9.5.2

Improving efficiency of reactions

Cadmium hydride, CdH2, seems to be very unstable at temperatures above liquid nitrogen and, therefore, it is difficult to envisage its use to increase the sensitivity of Cd determinations in atomic spectrometry. However, the special microenvironment provided by organised media could favour a higher formation efficiency or perhaps increase the stability of such volatile cadmium species at room temperature. In fact, the presence of micelles and vesicles has proved to be critical on the formation of volatile species of cadmium by reduction of the cation with NaBH4 ~~ ~0 in flowing systems. These CdH2 formation reactions I~~are most adequate for in situ trapping of the hydride in a graphite furnacel~, !~2 allowing the determination of extremely low levels of this toxic metal.

9.5.3

Improving analytical selectivity

As indicated earlier, in the new microenvironment created by organised media, interfering reactions may change. That means that the analytical selectivity could be improved by using a suitably organised medium. In fact, this effect has already been demonstrated in several cases of volatile species generation. For instance, results obtained in the determination of lead using peroxodisulphate-nitric acid mixture for plumbane generation i~ showed how certain elements (As, Se, Hg, Sn, etc.) do not interfere in the micellar medium while they do in the absence of CTAB. Similar favourable effects on selectivity of organised media have been observed for Hg determination by CV-ICP-OES in vesicles of DDAB 1~ and also for bismuth generation carried out in a micellar medium of non-ionic micelles of Triton X-100. This generation proved to be much more selective than a conventional hydride generation. 113 On the other hand, arsine generation performed from DDAB vesicles also presented a lower level of interferences than arsine generation from aqueous media. ~~ Madrid et al. TM studied the effect of interferences of transition metals and hydride-forming elements in cold vapour generation of mercury and also observed that the selectivity in the medium created by the organised assemblies was much better than in conventional aqueous media. In brief, by selecting micelles or vesicles oppositely or identically charged to the analyte or interferent, respectively, the beneficial surfactant effects can be extended to get a reduction of interferences. Finally, alkylation reactions for volatile species generation could also be manipulated by adding organised media. Results using NaBEt4 in the presence of surfactants are scarce, but it appears that surfactant addition could be benefitial for those systems where, as for SeEt4 and SnEt4, the alkylation kinetics are rather slow. ~~

296

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9.6 Tandem on-line continuous separations for sample introduction into plasmas As we have seen in Chapter 8, it is well established that the sensitivity of analytical determinations of about ten ions by atomic spectroscopic methods can be dramatically enhanced by generation of volatile hydrides at room temperature. However, the generation of volatile hydrides is subject to several interferences, particularly from some transition metal ions affecting the efficiency of hydride generation. 73' 115 Approaches for improving the selectivity exist (such as standard addition techniques or discontinuous separation/ preconcentration of the analyte from the rest of the matrix) but they are time consuming. Another possibility to overcome problems of lack of sensitivity/selectivity in plasma spectroscopy consists of using continuous separation methods of enrichment or preconcentration of the analyte in the sample before its determination by atomic spectroscopy (e.g. liquid--liquid extractions as shown in Chapter 7 or solid-liquid separations dealt with in Chapter 6). These sample treatments on-line with plasmas allow the implementation, as in AAS, 3 of many analytical determinations at ultratrace levels by ICP-OES and offer the most remarkable improvements in sensitivity, selectivity, precision, saving of reagents and minimal participation of the operator, when compared with conventional batch operation. In all solid-liquid separations a discontinuous operation mode is mandatory (preconcentration time) for adsorption/desorption or precipitation/redissolution. Conversely, using continuous liquid-liquid extraction, which allows a continuous separation/preconcentration, real-time detection is affordable. The sample is continuously pumped into the separation interface and the extracted analyte continuously nebulised into the atomiser. Thus, a convenient strategy can be the separation of the analyte from the interfering elements by means of a continuous liquid--liquid extraction followed by the generation of the volatile hydride directly from the organic phase and final ICP--OES detection. This can considerably improve the sensitivity of ICPOES determinations of toxic elements in environmental and biological materials. 1~6 A limitation of the technique can be the degree of compatibility of the plasma with the organic solvent used for the liquid--liquid extraction. As was discussed in Section 9.4.1, if the hydrides are generated from an organic phase, a certain amount of the organic solvent (depending of the volatility of the solvent) reaches the plasma. In this instance a large additional amount of energy of the ICP will be necessary to continously break up the molecules of organic solvent and so a lesser amount of thermal energy will be available for realizing the needed phenomena of volatilization, atomisation and excitation of the samples. When the total mass of solvent vapour reaching the torch of the ICP is too high, the plasma becomes unstable and eventually it may be extinguished. There are several reports in the literature on hydride generation from organic solvents

Continuous aspiration and flow injection

297

with AAS detection. 76-78'117-119 In all cases, however, the hydrides are generated directly in the organic phase, containing the analytes, before sending them to a quartz cell heated electrically. The organic solvent vapour, accompanying the hydride, originates strong background absorption difficult to be adequately corrected with a common continuum source AAS corrector. As noted in Section 9.4.1, the first report dealing with ICP-OES detection combined with continuous hydride generation from an organic phase is the paper of Huang et al., ~7 who used the hydride generator shown in Figure 9.6. Then, the group of Sanz-Mede188 proposed the continuous merging of two flows, one of the extracted analyte in xylene and the other of NaBH4 in dimethylformamide for the continuous generation of arsine which fed the ICP torch. 9. 6.1

The tandem on-line c o n c e p t

In a later paper, the same group 12~ proposed a methodology with a higher degree of automation by resorting to the "tandem on-line" concept. Having in mind the convenience of increasing the sample frequency in real situations and the convenience of automating the separation, the authors combined in a single on-line configuration two continuous separation units, based on two different separation mechanisms: a continuous liquidliquid extraction in the first instance and then an on-line coupling gas-liquid separation; the first unit allows important improvements in both, selectivity and sensitivity (if a preconcentration is carried out), while the second separation step provides an improvement in the sensitivity (i.e. the effiency of analyte transport to the ICP is around 100% as hydride). This idea of "tandem on-line" was first applied to As determinations in different real samples by coupling on-line the two continuous separation devices (liquid-liquid and gasliquid) to an ICP.12~Food samples (including tomatoes, beer, liver paste and mussels) after adequate dry ashing decomposition were analysed successfully. 121 Later on this tandem technique was also applied successfully to the control of Sb (at trace levels) in industrial electrolytic solutions of zinc sulfate, as used for the manufacture of the metal by electrodeposition in a local factory. ~22 The tandem on-line continuous separation device is depicted in Figure 9.8. A peristaltic pump, drives the flow of the sample (i.e. As(Ill) solution in a 9 M HC1 solution or Sb(III) in a 3 M H2SO 4 solution), or the blank, and the flow of extracting reagent, (a) 0.1 M KI solution in HC1 medium) at a confluence point; the emerging aqueous phase is allowed to react in the mixing coil (b), where the extractable compound is formed; then it merges with a continuous flow of xylene, provided by the pump (via a displacement bottle) in the solvent segmentor (c) (in Figure 9.8). The solvent segmentor provides alternate regular segments of both phases which enter the extraction coil where the mass transfer from the aqueous phase to the xylene takes place. Then, both aqueous and organic phases, reach a

298

A. Sanz-Medel et al.

membrane phase separator (made in PTFE following other designs 123) which splits the segmented flow into two streams: the aqueous phase to waste and the organic phase which goes to the second separation unit. There, it is mixed with the flows of solutions of NaBH4 in dimethylformamide and glacial acetic acid and so the hydride is generated directly from the organic phase (e.g. H3As). Then a smooth gas/liquid separation of the volatile species is achieved in a "U"-type gas-liquid separator before transport of such species into the plasma drawn by a stream of Ar (see Figure 9.8). Xylene tumed out to be a convenient solvent because it provided a good, fast and clear extraction, a good stability of the plasma and a high efficiency for hydrides generation from this organic phase in a continuous manner. This tandem on-line device allows the use of RF power and flows of reagents lower than those used by Huang, 17 apart from the liquid-liquid separation which determined that transition metals (interfering classical hydride generation from an aqueous medium) did not significantly interfere on the Sb or As determinations by tandem on-line

IARiO"I I I /

I NaBH 4

I

ACID

meter

,

v

PTFE

ICP

u Organic phase

PUMP SAMPLE

.

as-liquid separator

c

CARRIER b WATER

e waste ~waste Aqueous phase

Figure 9.8 Schematic diagram for continuous liquid-liquid extraction coupled on-line with a hydride generator gas-liquid separator system and the ICP. (a) T-piece; (b) mixing coil; (c) segmentator; (d) displacement bottle; (e) extraction coil; (f) membrane phase separator; (g) T-piece, (h) interface vessel gas-liquid separator (reproduced from Reference 120 by permission of Elsevier Science).

C o n t i n u o u s aspiration and flow injection

299

Table 9.5 Optimum values for As (228.82 nm) and Sb (206.81 nm) and some analytical figures of merit by tandem on-line continuous separation and ICP--OES determination As

Sb

Condition

Value

Value

RF Forward Power Viewing height (above coil) Inner argon flow rate Outer argon flow rate Auxiliary flow rate Integration time

1200 w 11 mm 1 litres min- ~ 18 litres min- ~ 2 litres min-~ 5s

Internal diameter of coil Mixing coil length Extraction coil length Aqueous phase flow rate (sample+0.1 M KI) Xylene flow rate

0.7 mm 60 cm 100 cm 2.60 ml min-~ (1.30+ 1.30 ) ml min -~ 0.40 ml min-~

NaBH4 concentration NaBH 4 solution flow rate Glacial acetic flow rate Sample organic flow rate Solvent extraction system

1% (w/v) in DMF 1 ml minI ml min0.40 ml min0.1 M KI + 7.2M HCl/xylene

Precision (RSD%) at 100 ng ml-~ LD (ng ml-I)

4.5 2

3 2

Linear range (lxg ml - ~) Interferences Sample frecuency

5. LD- 100 None 30 per hour

5- LD- 100 None 28 per hour

Plasma operation 1250 w 11 mm 0.76 litres min18 litres min2 litres min5s

Continuous flow analysis 0.7 mm 100 cm 250 cm 2.60 ml min(1.30+ 1.30) ml min -~ 1.15 ml min-t

Hydride generation 2.5% (w/v) in DMF 1 ml min1 ml min1.15 ml min -I 0.1 M KI+3M H2SO4/xylene

Analytical figures of merit

ICP__OES. 12~ O p t i m u m conditions for both d e t e r m i n a t i o n s (As and Sb) using r a d i o f r e q u e n c y p o w e r values about 1200 w and solutions o f N a B H 4 o f 2 % (w/v) can be seen in Table 9.5, w h e r e the analytical figures o f merit are also s u m m a r i z e d . P e r h a p s , the m o r e salient aspect o f this t a n d e m on-line device is its h i g h level o f a u t o m a t i o n : f r o m the peristaltic p u m p aspirating the s a m p l e solutions up to the final d e t e c t i o n o f the desired e l e m e n t in the ICP, the two sucessive s e p a r a t i o n steps o c c u r w i t h o u t any analyst contribution. O n the other hand, the limit o f detection and p r e c i s i o n attainable are also g o o d and are a s s o c i a t e d to e x c e l l e n t selectivity.

300 9. 6.2

A. Sanz-Medel et al. Indirect determinations

The tandem on-line concept also allows the straightforward extension of the scope of atomic spectroscopy to traditionally elusive fields such as the determination of non-metals and organic compounds. Non-metallic elements indirect analysis by atomic methodologies has been previously adressed by several investigators TM generally resorting to off-line separations before AAS determinations. The idea behind the use of continuous tandem online determinations of anions involves the formation of an ion-pair of the desired non-metal A with an adequate metal, M, able to form volatile hydrides (or Hg for cold vapour generation) via an organic reagent allowing for an appropriate complexation. Thus, the general reaction should be: n M + R + m A ~ (M,,R+A,~, )Aqueous--'* (MnRAm)organic

R being an appropriate organic reagent. If the ion-pair is extracted into an organic solvent well tolerated by the ICP, as xylene, the coupling on-line of the two separation units (the continuous liquid-liquid separator-driving the ion pair to the xylene, and the gas-liquid separator, providing the separation of the volatile form of the analyte from the organic phase where it is generated) offers a rapid and convenient automatic sample pretreatment to the ICP--OES, which can be used to carry out indirect determination of anions or other non-metalic compounds in a faster, more reliable and easier way than that allowed by conventional batch procedures. 124 Using tandem on-line continuous separations, trace amounts of iodide could be determined by ICP-OES in a sensitive and selective way. 125To do so the sample solution of iodide (or a blank) and the reagent solution (2,2'-dipyridil 0.1% (w/v) + 10 Ixg ml- ~Hg (II)) are pumped continuously to a T-piece and the ternary complex, formed in the aqueous phase, is then continuosly extracted into xylene in the extraction coil (Figure 9.8e). The alternative segments of the aqueous and organic phases reach the membrane phase separator (Figure 9.8f), where a clear continuous separation of phases occurs. Once separated the xylene phase extract, cold vapour of mercury is continuosly generated, by continuous mixing of this phase with glacial acetic acid and a 5% (w/v) of NaBH4 solution in dimethylformamide. The CV of mercury is separated from the liquid xylene in the beads of the separator and driven into the ICP by a continuous stream of Ar. The emission of mercury was monitored at the 253.65 nm emission line under the optimum operating conditions for Hg ~ detection and a detection limit of 20 ng ml -~ of iodide (300 times better than that obtained by pneumatic nebulization) was obtained. The precision, at 500 ng ml-~ level, was of 5%, the sample frequency was about 20 samples per hour and the level of interferences was very low, as none of the more typical ions accompanying iodide in common real samples produced interferences.

Continuous aspiration and flow injection

301

This methodology can be extended to other organic compounds which have been analysed by automatic indirect FAAS determinations (see Chapter 7) if they form extractable complexes and contain volatile species-forming metals. For instance, cocaine is a psychotropic drug that can be determined at very low levels indirectly by ICP-OES with a tandem on-line separation system. ~26 Here, BiI4 (Dragendorff reagent) is selected as the counter ion of the protonated cocaine, Coc.H+; the ion-pair (Coc.H+)-BiI4 continuously formed at pH = 1 (the pH is a critical parameter because this acidic medium is necessary for the protonation of-NH2 groups of the cocaine molecule) in the aqueous phase is then extracted in a continuous manner into CHCI 3 along the extraction coil (Figure 9.8e). After liquid-liquid separation, the chloroformic extract is mixed continuously with NaBH4 in dimethylformamide solution and glacial acetic acid to form Bill3. Then, the hydride is drawn to the ICP and there Bi is continuously monitored (and so the amount of drug indirectly determined from the Bi content of the extract). Although this methodology was applied to the determination of normal levels of cocaine in confiscated samples, the very low DL attained (2 ng ml-1) and the high selectivity of the method (other similar drugs as heroin, morphine, codeine, quinine, atropine, strychnine or methadone did not cause interference) would allow the successful application of this methodology to the determination of the drug at very low levels in urine or serum, avoiding having to resort to more laborious techniques as GC-ECD or HPLC (involving several time-consuming extractions and purifications of the cocaine extracts prior to injection into the chromatograph). 9.6.3

Non-chromatographicspeciation

Finally, the tandem on-line continuous separation concept has proven to be most appropriate to carry out simple non-chromatographic speciation of trace elements in an easy and inexpensive way. In fact, Sb(III) and Sb(V) ~27 and Hg(II) and CH3Hg+, ~28 have been separated and analysed following similar ideas to those delineated above using an experimental set-up similar to the scheme shown in Figure 9.8. For Sb(III)/Sb(V) speciation, a pyrrolidine-1-dithiocarboxylic acid ammonium salt (APDC) solution of 0.1% (w/v) was used as reagent for Sb(III) complexation because only the lower oxidation state of antimony is complexed. The Sb(III)-APDC complex, continuously formed at pH= 1, was extracted in a continuous mode into IBMK, and once separated, the organic phase H3Sb was formed as before (Figure 9.8), with NaBH4 on-line, and carried to the plasma for Sb monitoring at its 217.58 nm emission line. Sb(V) remains in the aqueous phase (coming from the first liquid-liquid separation step) and so was adequately collected. After its adequate pre-reduction with KI, stibine generation in a continuous mode was accomplished to determine total Sb in this remaining solution by ICP-OES. The detection limit attained for Sb(III) and Sb(V) determinations, under optimum experimental

302

A. Sanz-Medel et al.

conditions, were 3 ng ml-1 and 8 ng ml-~ respectively. Sb(III)/Sb(V) speciation in sea water samples, spiked with different amounts of Sb(III) and Sb(V), in waste water or polluted sea water without previous off-line preconcentrations could be achieved. 127 Inorganic mercury and methylmercury can be also speciated by tandem on-line continuous separation--ICP-OES. In this case, methylmercury was selectively extracted as CH3Hg § Br- complex into xylene in a continuous way and determined on-line by ICPOES after continuous volatile mercury generation from the xylene extracts. 128 Inorganic mercury (Hg 2+) was determined, after collection of the aqueous waste from the first step, as described for Sb(V). The LD's obtained for methylmercury and inorganic mercury were 4 ng ml-1 and 2 ng ml-~ respectively. It was demonstrated that the main ions contained in sea water did not cause interference in this speciation. The method was applied to mercury speciation in samples of sea water, spiked with different amounts of inorganic mercury and methylmercury with very good results. ~28 By use of a methodology based on the tandem on-line concept, Cano Pav6n et al. also reported an on-line preconcentration and determination of mercury by ICP-OES using flow injection as sample presentation to the plasma instead of continuous aspiration. ~29The operation of its FIA system, shown in Figure 9.9, is as follows: samples or standard solutions are pumped into the system through channel 1 of the peristaltic pump and mixed with the buffer solution (channel 2). This aqueous phase merges with a continuous flow of DPTH into IBMK in the solvent segmentator S, which provides alternate regular P (mL.min -t)

SnCl 2 (DMF

0.5

, IBMK/DMF

3.3

DP'm (mMKI

0.5

Buffer

0.4

~

W2

C

Ar+Hg [ (into ICP torch)

w, Sample

1.4

,

PS

R

W3

Figure 9.9 Schematic diagram of FI manifold: P, peristaltic pump; S, T-solvent segmentator; E, extraction coil; PS, phase extractor; L, injection loop; R, restrictor; M, mixing coil; C, gas--liquid separator, W 1, W2, W3, waste (reproduced from Reference 129 by permission of Springer Verlag).

Continuous aspiration and flow injection

303

segments of both phases prior to entering the extraction coil E where the formation and extraction of the complex takes place. The aqueous and organic phases reach the membrane phase separator PS, which splits the flow into two streams: the aqueous phase, going to waste WI, and the organic phase filling the sample loop L with its excess being discarded by W2. When the valve V is turned in to inject position, the sample is injected into the carrier stream (continuously pumped with the ICP peristaltic pump) and mixed with the SnCI2 solution (pumped through channel 4) in the mixing coil M, where direct generation of mercury vapour takes place. The gas generated and the solvent are then passed into the gas-liquid separator which provides separation of gases from liquid. The liquid is drained and the generated mercury vapour is swept into the ICP torch by a stream of argon. This FIA system allows the determination of mercury, at trace levels, in biological materials (human hair, pig kidney and dogfish muscle), with a high sensitivity and selectivity. The LD attained was 4 ng. ml-1, and the precision, at 10-900 n g - m l level, was about 1%, with a sampling rate of about 20 per hour In conclusion, we can summarize by saying that the tandem on-line continuous separation concept and similar devices, coupled on-line to an ICP-OES as detector, can provide an effective alternative to carrying out many analytical separations on-line with sensitive determinations. This concept is not restricted to metallic ions in different matrices. As we have seen before, convenient indirect analysis of anions (e.g. iodide) or drugs (e.g. cocaine) and analytical non-chromatographic speciation of inorganic or organic species are possible. The corresponding methods of continuous aspiration - ICP--OES detection exhibit a high level of possible automation, are comparatively interference-free and low, txg 1-l, detection limits with reasonable precision at such levels are attained. Therefore the exploitation of such devices coupled on-line with other plasmas, as M I P OES or GD-OES and ICP-MS, can be anticipated as a useful flow strategy for sample presentation to such analytical plasmas.

9.7

References

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304 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

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103 Aizpun Feha/mdez, B., Fernfindez de la Campa, M. R. and Sanz-Medel, A., J. Anal. At. Spectrom., 1993, 8, 1097. 104 Guti6rrez, J. M., Madrid, Y. and Cfimara, C., Spectrochim. Acta, 1993, 48B, 1551. 105 Aizpun Fern/mdez, B., Vald6s Hevia y Temprano, M. C., Fern/mdez de la Campa, M. R., Sanz-Medel, A. and Neil, P., Talanta, 1992, 39, 1517. 106 Vald6s Hevia y Temprano, M. C., Aizpun Fernfindez, B., Fernfindez de la Campa, M. R. and Sanz-Medel, A.,Anal. Chim. Acta, 1993, 283, 175. 107 Ellis, R. I. and Tyson, J. E, Spectrochim. Acta, 1996, 51B, 1859. 108 Sanz-Medel, A., Fernfindez de la Campa, M. R., Blanco, E. and Fernfindez Sfinchez, M. L., Spectrochim. Acta, 1998, Part B, in press. 109 Sanz-Medel, A., Vald6s Hevia y Temprano, M. C., Bordel Garcia, N. and Fernfindez de la Campa, M. R., Anal. Proc., ! 995, 32, 42. 110 Sanz-Medel, A., Vald6s Hevia y Temprano, M. C., Bordel Garcia, N. and Fernfindez de la Campa, M. R., Anal. Chem., 1995, 67, 2216. 111 Goenaga, H., Fernfindez Sfinchez, M. L. and Sanz-Medel, A., J. Anal. At. Spectrom., 1997, 12, 1333. 112 Matusiewicz, H, Kopras, M. and Sturgeon, R. E.,Analyst, 1997, 122, 331. i 13 Viilanueva Tagle, M., Fern/mdez de la Campa, M. R. and Sanz-Medel, A., Anal. Quim. Int., 1996, 92, 213. 114 Madrid, Y., Guti6rrez, J. M. and Camara, C., Spectrochim. Acta, 1994, 49B, 163. 1 ! 5 Nakahara, T., Anal. Chim. Acta, ! 981,131, 73. 116 Sanz- Medel, A., Men6ndez, A., Fernfindez, M. L., L6pez, A. and Pereiro R. In: Element Concentration Cadasters in Ecosystems, Ed. by Lieth and Markert B., Osnabrfik, 1990, pp 145. 117 Rezende, M. C. R., Campos, R. C. and Curtius, A. J.,J. Anal. At. Spectrom., 1993, 8, 247. 118 Aznarez, J., Vidal, J. C. and Carnicer, R.,J. Anal. At. Spectrom., 1987, 2, 55. 119 Castillo, J. R., Martinez, C., Chamorro, P. and Mir, J. M., Mikrochim. Acta, 1986, Ill, 95. 120 Men6ndez Garcia, A., Sfinchez Uria, J. E. and Sanz-Medel, A., Anal. Chim. Acta, 1990, 234, 133. 121 Cervera, M. L., Montoro, R., Sfinchez Uria, J. E., Men6ndez Garcia, A. and Sanz-Medel, A., At. Spectroscopy, ! 995, 9, 139. 122 Sfinchez Uria, J. E., Men6ndez Garcia, A., Fernfindez Sanchez, M. L., Canto, J and Sanz-Medel, A., Labor. Robot. Autom., 1993, 5, 263. 123 Gallego, M. and Valcfircel, M.,Anal. Chim. Acta, 1985, 169, 151. 124 Hassan, S. S. M. In: Organic Analysis Using Atomic Absorption Spectrometry, Ed. Ellis Horwood, Chichester, 1984. ! 25 Men6ndez Garcia, A., Fernfindez Sfinchez, M. L., Sfinchez Uria, J. E. and Sanz-Medei, A., Mikrochim. Acta, 1992, 106, 277. 126 Men6ndez Garcia, A., Sfinchez Uria, J. E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1996, 11,561. 127 Men6ndez Garcia, A., P6rez Rodriguez, M. C., S~nchez Uria, J. E. and Sanz-Medel, A., Fresenius J. Anal. Chem., 1995,353, 128. 128 Men6ndez Garcia, A., Fernfindez Sfinchez, M. L., Sfinchez Uria, J. E. and Sanz-Medel, A., Mikrochim. Acta, 1996, 122, 157. 129 Cafiada Rudner, P., Cano Pav6n, J. M., Garcia de Torres, A. and Sfinchez Rojas, F., Fresenius J. Anal. Chem., 1995, 352, 615.

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10.1 Introduction and general strategies for environmental analysis with atomic detection The different strategies developed using flow analysis atomic spectrometry to solve environmental analytical problems depend on the nature of samples analyzed as well as on the species to be determined and their concentration levels. This chapter reviews and discusses the literature published till now on such analytical strategies. In fact, more than 200 papers, published in 53 journals during the period 1983-1994 have been evaluated and more than 100 more recent publications, appeared in 1995-1996, have been thoroughly revised. For a systematic organization, the literature published about flow analysis atomic spectrometry, applied to the determination of cations, anions and organic molecules in environmental samples, has been divided into three main topics: water, soil and sediments, and air and gaseous emissions (including fly ashes). Biota analysis was not included here because this topic is treated in more detail in Chapter 11 devoted to Applications in Clinical and Biological analysis. Through the different sections considered, the literature has been classified on the basis of elements determined and, in order to make available a complete picture of the different techniques and procedures developed, the sections have been organized considering the sample matrix. In this way, the reader can quickly find both information regarding the desired analyte and sample matrix. It is interesting to note that the main part of the developed methodologies has been focused on elemental analysis, as expected for atomic spectrometry determinations, but 309

A. Morales-Rubio and M. de la Guardia

310

the easy automation of complex procedures provided by FIA has encouraged the development of indirect determinations of anions and organic molecules of great interest in environmental analysis. The environmental applications of flow analysis have been reviewed in different chapters of the available literature on flow injection, ~-3 specially relevant being the chapters of the AAS books of Burguera 4 and Fang 5 and the monograph of Fang about flow injection separation and preconcentration. 6 Additionally, some of the main topics involved in the environmental applications of flow analysis atomic spectrometry have been reviewed by Kuban et al., 7 Tsalev, 8 Fang et al. 9 and Zagatto et al., i~ which have considered the applications of flow injection in environmental analysis, while Luque de Castro et al.~, ~2and Campanella et al. t3 have focused on specific topics such as, speciation ~2'13and hyphenation of flow injection systems with high discrimination power instrumentation. ~ A general review of the literature published on flow injection atomic spectrometry from 1978 until today, reveals that the number of scientific papers published on this topic has grown in an exponential way almost parallel to the development of general flow injection literature (see Figure 10.1). Regarding journals in which more papers about FI-AS applications in environmental analysis have been published, three scientific journals stand up: Analytica Chimica Acta,

e-

9

9 0 AA

9

A

9. 9 . ~_. A . 9 9

1.980

1.985

1.990

1.995

Figure 10.1 Evolutionof the scientific literature about flow injection in general and flow injection atomic spectrometry.

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/

/

\ \/

311

~2'j/j-~,.-~'////-/'~.~..~./~____ .... Anal. Sci. 1"/,

! ""-

,

/

Figure 10.2 Distributionof the journal sources of the literature published on flow injection atomic spectrometry analysis of environmental samples through the period 1983-1996. (Data have been obtained from the Analytical Abstract data-base period 1983-1994, and from journals published during the period 1995-1996). Journal of Analytical Atomic Spectrometry and Fresenius' Journal of Analytical Chemistry. They cover the 19%, 18% and 12% respectively of the whole literature published on this topic, as can be seen in Figure 10.2. So, it can be concluded that flow injection is nowadays a research topic well installed in the basic literature on both atomic spectrometry and general analysis. Moreover, atomic spectrometric detectors have demonstrated their impressive usefulness in flow injection strategies for environmental analysis. Conceming main general strategies developed in flow analysis atomic spectrometry for improving environmental determinations we would stress: (a) the use of on-line dilution procedures for fast and precise determination of major and minor components by means of the appropriate design of manifolds, (b) the development of on-line preconcentration procedures, such as on-line solid phase extraction and on-line precipitation, whose basic principles have been discussed in detail in Chapter 6, or on-line solvent extraction ~4 considered in Chapter 7, (c) hyphenation between separation techniques and atomic spectrometry detectors with interfaces which include a derivatization step, particularly useful for speciation studies in the environment, (d) automatization of indirect procedures for anion and organic molecule determinations. Also noteworthy are the tremendous advances provided by the consideration of the whole analytical process steps integrated, as a whole, in a FIA manifold (from the in the field-sampling to the measurement step through complex sample pretreatments). (e) The recent development of on-line microwave-assisted treatments and on-line derivatization procedures for environmental samples represents a remarkable scientific advancement, as pointed out in other chapters. It is also expected that in future years analytical flow injection atomic spectrometry

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Figure 10.3 Distribution of the preconcentration studies by means of flow injection atomic spectrometry as a function of the elements determined in environmental samples. procedures will incorporate on-line the appropriate waste treatments in order to provide more environmentally friendly methodologies. 15 Figure 10.3 shows that preconcentration studies using flow injection atomic spectrometry have been applied to many types of analytes being Cu, Pb, Cd, Cr and Ni the most studied elements, although the vast majority of trace elements of interest in environmental analysis have been covered at a lesser extent. On the other hand, trace element speciation studies in environment are nowadays at a crucial stage of progress. FIA speciation strategies for As, Cr, Hg, Sn, Se and at a lesser extent Sb and Pb have been widely described in environmental issues ~2"13 and this topic will be dealt with at length in Chapter 12.

10.2

Water analysis

Water is probably the main type of environmental sample. Waters are currently analyzed for both environmental and health purposes. Flow injection atomic spectrometry methods for Cu, Pb and Hg are the more numerous, followed by As, Cd, Cr and Se, thus confirming the applicability of this approach for trace metal analysis. The inherent sensitivity of atomic procedures has been most frequently enhanced by flow analysis with on-line trace enrichment and/or matrix removal. It is interesting to note the dramatic influence of FIAS in the hydride generation atomic spectrometry field (both with absorbance and emission measurements) and in cold vapour atomic absorption spectrometry, as it is the explosion, in recent years, of speciation studies. To classify the literature concerning flow injection atomic spectrometric applications to

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water analysis, four sections have been considered: (a) single element determination, (b) multielemental determination, (c) determination of anions; and (d) determination of organic molecules. The following section, devoted to single element determinations, includes papers in which specific strategies were developed for both single total element determination and analysis of different species of a given element. Although this last topic will be extensively treated in Chapter 12, the approach followed there concentrates on just cryogenic trapping-gaseous products analysis. Papers in which the same strategy has been applied to the determination of several elements in the same sample (with the same sample preparation procedure) will be discussed in a separate section. Special attention has been paid to indirect determinations, particularly, anion determinations and surfactant and pesticide determinations by resorting to flow injection atomic spectrometry.

10.2.1

Single element determinations

Let us revise the analysis of each element in waters by flow injection-atomic spectrometry. Aluminium

Several procedures have been developed for the flow injection-atomic spectrometric determination of A1 in waters based on on-line preconcentration 16-22 of the element with FAAS, 16-18ETAAS, 19'2~ICP-OES 16'21 or ICP-MS 2z for detection. Solid phase extraction is the most common preconcentration technique with different column packings as SCX (propilbencenesulphonyl), ~8 100-200 mesh Chelex 100 resin at pH 6.5,17 immobilized reagents on AG 1 x 8 ion exchange resin 22 and oxine immobilized on controlled-pore glass. 2~ The formation of A1 complexes with 2-(N-morpholine)ethanosulfonic acid ~6 or oxine 19"21 followed by immobilization on Amberlite IRA-400,16 or Amberlite XAD-2, 2~ have also been employed. Antimony

Formation of volatile SbH3 followed by its determination is a common practice in flow injection-atomic spectrometry, 23-26using both ICP---OES23 or AAS 24-26as the measurement technique. Menendez Garcia et al. 23 developed a continuous tandem on-line separation device in connection with ICP-OES for separation and analytical speciation of Sb(III) and Sb(V) in sea waters. As shown in Chapter 8 the detection limits were 3 and 8 ng/ml for Sb(III) and

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A. Morales-Rubio and M. de la Guardia

Sb(V) respectively. Fukuda et al. 24 reported a detection limit for Sb in water of 0.4 ng/ml after its preconcentration on a column filled with silica gel loaded with 2-mercapto-N(2-naphthyl)acetamide, carrying out the elution with 6 M HNO3 and the determination with hydride generation-AAS. Ding and Sturgeon 25 developed a continuous flow electrochemical hydride generation system for the determination of total antimonium in river and sea waters. Both Sb(III) and Sb(V), were derivatized as hydrides with 92% efficiency and the hydrides were trapped in a Pd-coated graphite furnace prior to atomization. The RSD obtained was lower than 6% at a concentration level of 0.2 ng/ml. Yan e t al. 26 coupled a FI on-line sorption preconcentration system and ETAAS for the selective determination of trace amounts of Sb(III) in sea-water samples. The determination was achieved by selective complexation of Sb(III) with ammonium pyrrolidine dithiocarbamate, sorption of the complex onto the inner walls of a knotted PTFE reactor, and quantitative elution with ethanol. The relative standard deviation found, at a sampling frequency of 42/h, was 2.9%. Arsenic

The on-line generation of AsH3 by NaBH4 is the normal way for As determination in waters by AAS, 27'28 DCP--OES 29 and ICP--OES. 3~ However, in the last years alternative procedures for hydride generation based on the use of aluminium reactors 3~ and electrochemical systems have been proposed. 32 Inorganic arsenic (III) and arsenic (V) species were determined by Burguera et al. 31 using an arsine generator of aluminium and cold trapping arsine collection. Water samples were mixed with 5 M NaOH before passage through an electrically heated pyrex column containing a rolled foil of metallic A1 to reduce As(Ill) to AsH 3. The detection limit obtained was 0.25 ng/ml Schauml6ffel and Neidhart 32 described a flow injection system with electrochemical hydride generation and AAS detection for As(III)/As(V) determination in well water. The detection limits obtained were 0.4 ng/ml for As(Ill) and 0.5 ng/ml for total inorganic arsenic using a sample volume of 1 ml. Sperling et al. 33 determined sequentially arsenic (III) and total arsenic in sea, lake and drinking waters by ETAAS, while speciation of As(Ill) and As(V) in tap, ground, mineral and artificial waters was reported by Torralba et al. 27 using HG-AAS. The on-line coupling between chromatographic techniques and atomic spectrometry has been improved by the use of flow injection strategies, 34-36 also incorporating new procedures using microwave ovens. 34 Stummeyer et al. 35 combined anion-exchange HPLC with HG-AAS for the routine speciation of arsenite, arsenate, monomethylarsenic acid and dimethylarsinic acid at trace levels in natural waters. Magnuson et al. 36 employed ion

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chromatography to speciate the same four environmentally more toxic forms of arsenic. Hydride generation was used to convert the species to their respective hydrides that were detected with ICP-MS. Lopez-Gonzalvez et al. 34 developed an on-line microwave oxidation method for the determination of organoarsenic compounds by HPLC-HG-AAS. Samples were passed through a Hamilton PRP-X column and mixed with K2S208 in 2.5% NaOH inside a 700W microwave oven. After cooling, samples were treated with 2 M HCI and 3% NaBH4 and the generated hydride was determined by AAS. Nielsen et al. z8 described a time-based FI procedure for the determination of ultra-trace amounts of inorganic As(Ill) which combines HG-AAS with on-line preconcentration of the analyte by inorganic coprecipitation-dissolution in a filterless knotted reactor. At a sampling frequency of 33/h, the limit of detection achieved was 0.003 ng/ml and the precision obtained was 1% at the 0.1 ng/ml level. Beryllium, bismuth and boron

Do-Nascimento and Schwedt 37 determined beryllium in tap water by GF-AAS. Samples were treated with succinic acid buffer (pH 4.5) and Chrome azurol S, and passed through a polyethylene powder column. The Be-complex was eluted with ethanol. The detection limit obtained was 0.8 ng/ml. Also, Bi has been determined in water by flow injection hydride generation and atomic absorption spectrometry, 38'39 together with Te and Ge 38 or with Se. 39 Takahashi 4~determined boron in river waters by ICP-OES. Boron was preconcentrated from samples into an Amberlite XE-243 resin column and eluted with 0.5 M HC1. Kempster et al. 41 incorporated in the FI-ICP-OES system a mini-chamber containing a tangentially attached aerosol exit tube that induced spiral motion in the aerosol within the chamber. Precision obtained for boron determination in waters was 5.4% at a sample throughput of 320/h. Cadmium

In general this element has been determined together with Cu, Ni, Pb and Z n 42-48 and so the main strategies will be commented on in the multielemental determination section. However, there are a few papers focused specifically on Cd determination. Valdes et al. 49 determined cadmium in sea water by ICP-OES. Samples in 0.2 M HC1 were merged with a 1.2% tetraethylborate in 1% NaOH solution and cadmium diethyl species determined. The detection limit obtained was 0.4 ng/ml and precision 1.4%. Bermejo-Barrera et al. 5~ determined cadmium in sea waters by Cold Vapour/trapping and atomization in a graphite furnace. By using an Ir-coated graphite tubes the detection limit obtained was 4 ng/1. A similar approach was reported recently by A. Sanz-Medel et

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based on vesicular hydride generation-in s i t u trapping-ETAAS Cd determination, at 0.1 ng/1 levels, in river water.

al. 5~

Calcium

Reis e t al. 52 proposed a monosegmented system for calcium determination in natural waters by FAAS. In the method proposed by Kempster et al. 53 the FI system incorporated a centrifugal spray chamber to introduce a 10% HNO3 carrier stream into the plasma torch. In another approach, 54 samples were injected and on-line mixed with a 0.05 M HNO3 solution containing 50 mg/1 ammonium oxalate monohydrate. After calcium oxalate precipitation, a stream of 5% HCI was used to dissolve it. As many studies devoted to Ca determination also include the determination of Mg and alkaline elements or that of a series of divalent elements, 5s-57 these studies will be discussed in the following section. Carbon

Emteryd et al. 58 determined organic carbon in natural waters by ICP-OES after evaporation of inorganic carbon. This procedure enabled the separation of inorganic carbon from the organic carbon and determination of the latter at 193.09 nm. Between 15 and 500 mg/1 of organic carbon could be determined with a detection limit of 5 mg/1 in the presence of 500 mg/l of carbonate. Chromium

Chromium, both total chromium and ionic chromium species (Cr(llI) and Cr(VI)) is an ideal system to improve the analytical features of the atomic spectrometry by means of the on-line coupling of separation steps using flow injection analysis and, because of that, there are many papers focused on Cr determination. 59-74 Activated alumina, 59-62 coated alumina with sodium dodecyl sulphate 63 or ionic exchange columns 64-65 have been employed for on-line preconcentration of Cr(III) and/or Cr(VI) species. On the other hand, C~8 columns have been employed 66'67 in combination with reagents such as sodium diethyldithiocarbamate, which form a complex with Cr(III) or tetrabuthyl ammonium bromide used as ion-pair forming agent. 68 For a fast and very sensitive determination of chromium in water samples the tandem on-line between chromatography and ICP-MS has been proposed. 68-71 Other interesting alternatives proposed in the literature for chromium preconcentration and final determination by flow analysis atomic spectrometry are based on on-line stripping voltammetry,72 microwave-assisted sorbent extraction 73 and on-line coprecipitation. TM Pretty et al. 72 determined chromium in waters by using an on-line anodic-stripping voltammetry flow cell with detection by ICP-MS, with recoveries of the order of 100%. Kubrakova et al. 73 described a rapid and sensitive method for the differential determination

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of Cr(III) and Cr(VI) in waters, using sorption on a polymeric Detata sorbent containing aminocarboxylic groups and using microwave heating energy to promote the sorption. The detection limit by ETAAS achieved was 30 ng/1. Zou et al. TM developed a FI on-line coprecipitation FAAS system for the determination of Cr in waters. Cr(III) was preconcentrated and separated from potential interferents by on-line coprecipitation with lanthanum hydroxide. The precipitate was collected on the walls of a knotted reactor without filtration, dissolved with HC1 and introduced into the nebulizer system. Speciation of Cr(IIl) and Cr(VI) was achieved. The method provided a sampling rate of 30/h and the relative standard deviation was 2.7%. Cobalt

The low concentration of Co in waters demands, for a sensitive and accurate determination, the use of on-line preconcentration strategies. 75-77 Ion exchange columns, 75 imidodiacetic acid ethylcellulose resins 44 and modified columns like alumina with 1-nitroso-2-naphthol-3,6-disulfonate 77 or diethylditiocarbamate and C~8 columns, 76 have been proposed for preconcentration of Co and its determination by F A A S , 75' 77 ETAAS76 o r I C P - - O E S . 44 Copper

Copper is one of the elements most investigated using flow injection-atomic spectrometry procedures in the last years. 78-94 However, most studies also involve the determination of other metal ions like Cd, Zn, Co, Ni, Fe or Pb. Copper has been on-line preconcentrated by solid phase extraction using silicaimmobilized oxine, TM or cellulose bonded oxine, 79 dowex l-x4, 8~ spheron oxin 1000 8~ or catechol violet Amberlite XAD-2; 82 but there are also many procedures based on liquidliquid extraction using 5,7-dibromo-8-hydroxiquinoline in xylene, 83 or in a mixture of toluene and xylene, 84 ammonium pyrrolidine-l-carbodithioate in IBMK 85 or dithizone in CC14 .86

Recently a series of methods has been developed based on the on-line precipitation and adsorption on a support followed by elution with IBMK and for that the use of ammonium pyrrolidine dithiocarbamate and adsorption on activated carbon 87 or the precipitation of this compound and its retention on a knotted reactor 88 or a filter membrane has been proposed. 89 Hiraide et al. 9~ have monitored copper in water by coprecipitation with indium hydroxide followed by flotation in a flow system for ICP--OES detection. Recoveries ranged between 91-98%. Madrid et al. 91 evaluated FI techniques for MPT (microwave plasma torch) OES determining copper in synthetic sea water. Samples were passed through a C~8 bonded silica microcolumn and copper chelated with sodium diethyldithiocarbamate. After elution

318

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with ethanol aliquots of samples were introduced into the MIE The detection limit obtained was 0.16 ng/ml and recoveries achieved were from 94% to 97%. Germanium and gold

The determination of trace and ultra trace amounts of germanium in tap water was carried out by preconcentration in a graphite fumace using FI hydride generation, as' 95 Tao and Fang developed a method in which samples in HC1 were merged with NaBH4, and hydride generated was transferred to a Pd-coated graphite fumace and atomized at 2500~ The method was linear up to 0.5 ng/ml, the detection limit obtained was 0.004 ng/ml and precision was 2.0% at a concentration level of 0.3 ng/ml. 95 Gomez and McLeod 96 determined gold from refinery effluent waters by ICP-OES. Samples were passed through an Amberlyst A-26 PTFE column (a) or a mercaptoacetoxycellulose PTFE column (b) for enrichment of gold. Detection limits obtained were 1.7 ng/ml for the first column and 0.8 ng/ml for the second. Iron and lanthanum

Iron in waters has been determined by resorting to atom trapping 92 or on-line solid phase extraction, 93 having also proposed a procedure for on-line precipitation and redissolution as suggested also for Ca and Ag determination. 57 Krekler et al. 97 proposed the simultaneous determination of iron(II) and iron(III) in water by solvent extraction in FI-AAS. Samples containing 0.05% ferrozine and 12% sodium acetate in 1 mM HC1 (pH 4.5) were passed through a Cls-silica microcolumn. Fe(III) was not retained, while Fe(II)-ferrozine complex was retained and eluted with methanol. The precision obtained was 5% at a sampling frequency of 30/h. Fan and Fang 98 determined lanthanum in tap and polluted waters by ICP-OES. Samples preconcentrated on a CL-PS07 resin column were eluted with 1 M HC1. The detection limit obtained was 0.7 ng/ml and recoveries ranged between 90 and 110%. Lead and manganese

Among the numerous studies devoted to Pb determination by using flow injection atomic spectrometry42, 44,45,89,94,99-112two main strategies are apparent: (a) the simplest one based on liquid on-line preconcentration of Pb of the water by means of on-line solid phase extraction and (b) a GC-AAS methodology, developed for the speciation of alkyl-lead compounds. For on-line preconcentration of Pb many methods have been proposed: a simple precipitation with NH3 followed by retention on stainless-steel filter and redissolution, 99 precipitation with ammonium pirrolydin-dithiocarbamate, filtration with a membrane and dissolution with IBMK, 89 solid phase extraction on basic alumina after formation of leadtartrate, ~~176retention on 2-methylquinolin-8-01 packed column, 1~ immobilized

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8-hydroxyquinoline column ~~ the use of a cation exchange tubing allowing Donnan dialysis, i~ the use of the C~8 columns after formation of Pb-diethyldithiocarbamate ~~ or Pb-pyrrolidine-dithiocarbamate, ~~ or the simple retention using sulphydryl cotton ~~ or fibrous alumina. ~~ Gallego et al. ~~ have employed fullerenes as sorbent material for lead preconcentration from water and determination by FAAS. Samples were merged with ammonium pyrrolidinedithiocarbamate, the chelate adsorbed on different microcolumns and then eluted with IBMK. Willie ~~ determined lead in natural water certified reference material by ETAAS. Samples were treated with 0.1% sodium tetraethylborate to produce tetraethyllead which was collected into the graphite fumace at 300~ The detection limit obtained was 2 pg/ml. Naghmush et al. ~~ determined lead in waters by FI preconcentration and described its speciation using a functionalized cellulose sorbent with FAAS detection. Tetraalkyl-lead determination in waters by GC-AAS has been described by several authors: the organic extract was into a column coated with immobilized methylsilicone and organo-lead compounds determined by AAS at 217.6 nm. TM The method provided a precision of 10% and a detection limit of 4 ng/l. Bergmann and Neidhart also developed a simple and reliable method for the determination of eleven organolead compounds in water samples by GC-AAS after in situ buthylation with ammonium tetrabuthylborate. ~2 Methods developed for flow-injection atomic spectrometric determination of Mn frequently involve the determination of other elements, such as Cu, Cd, Fe o r Ni 43'92,94 and are based on solid phase extraction 43 or liquid-liquid extraction. For instance, manganese was determined in surface waters by Chi and Zhou ll3 by FAAS. Samples mixed with acetic acid/sodium acetate (pH 6) and ethanolic 1.2% ammonium pyrrolidinedithiocarbamate were merged with IBMK into an extraction system and the organic extract containing Mn-complex was measured automatically by AAS. The detection limit obtained was 0.076 mg/1 while recoveries ranged from 94 to 102%. Mercury

There are many specific studies devoted to the flow analysis-atomic spectrometric determination of Hg. 1~4-130 Usually Hg is determined by cold vapour atomic absorption after sample treatment with SnC12 in acidic TM or alkaline medium 1~5 or using NaBH4, ll4'll6 the studies carried out by Munaf et al. ~5-~8 being specially relevant in this field, who evaluated different digestion methods for the determination of Hg in waste waters tt8 and studied matrix effects on this determination 1~5 for both, total mercury determination and its speciation (inorganic mercury, methylmercury and ethylmercury) in waste waters by microcolumn liquid chromatography and CV-AAS. ~7

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On-line preconcentration studies have been carried out in order to increase the analytical signals particularly in CV-AAS TM and ICP-MS; 119 additionally, the use of a gold trap to amalgamate and preconcentrate Hg after vapour generation provided excellent limits of detection ~19'12~ Among the on-line preconcentration strategies developed, immobilized reagents such as quinolin-8-01 on porous glass TM or Sep-pak C~8 modified with 2-mercaptoethano1121 have been employed; Hg species have been complexed with diethyldithiocarbamate 122 or pyrrolidinedithiocarbamate 123 and thus retained on C!8 columns or directly removed from waters by using sulphydryl cotton in the column. TM Madrid et al. ~25 have employed baker's yeast biomass for separation of methylmercury, which is immediately bonded to the yeast cells, from Hg(II). MIP--OES has been employed for the determination of Hg vapour obtained from waste water 126 and also for Hg speciation by GC-MIP, directly or after preconcentration using sulphydryl cotton. 127 In recent years a series of studies were focused on AFS12~ 124.128for both total Hg determination and also for speciation of methylmercury and ethylmercury by capillary gas chromatography-AFS. ICP-MS provides detection limits of the order of 200 pg/l for the determination of Hg in waters 119and has been also applied as detector in HPLC for speciation studies. 129 On-line microwave assisted sample treatments have been employed for the determination of Hg in lake, rain and river waters using a bromination reagent in 0.3% HC194 and also to speciate inorganic mercury and methylmercury in drinking and waste waters. ~3~ Molybdenum and nickel

The determination of Mo in waters has been carried out directly by ICP-MS using an online standard addition method TM and also by flame atomic absorption ~32 or by ICP__OES133. 134 after on-line preconcentration. Ni determinations in waters by flow injection atomic spectrometry are commonly based on an on-line preconcentration followed by FAAS, 93' 124ETAAS45.135.136or by ICP--OES; 46 a method, developed by Cserfalvi and Mezei, 82 determined Ni and other elements by electrolyte cathode discharge atomic emission spectrometry in a flow using a 20 Kv direct current arc. On-line ETAAS has also been employed with preconcentration by sorbent extraction, with dialkyldithio phosphates. The detection limit obtained was 0.07 ng/ml with good precision (+ 2-3%). 135 Finally, a sensitive method for the on-line preconcentration of nickel in a certified reference water material, using the coupling of a flow system with a transversely heated ETAAS, was based on the volatilization of Ni as nickel tetracarbonyl and its trapping on the graphite tube surface. 136 A detection limit of 0.18 ng/ml with a relative standard deviation below 3% were reported.

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321

Palladium, phosphorus and platinum The determination of Pd at ppb level in water samples demands an on-line preconcentration in order to improve the sensitivity of classical FAAS and ETAAS. Lin et al. 137 determined palladium in waters by FAAS combined on-line with FI preconcentration using a micro-column packed with activated carbon fibre. The detection limit obtained was 0.3 ng/ml and the relative standard deviation was 3.9%. A fully automated on-line column separation and preconcentration system for the determination of trace and ultratrace levels of Pd in waters consisted on preconcentration of the metal in a microcolumn loaded with the selective complexing agent N,N-diethyl-N'-benzoylthiourea. The Pd-complex was eluted with ethanol and directly determined by ETAAS (limit of detection was 25 ng/ml). 138 Phosphorus determination in waters by ICP-OES using flow injection provided an enhancement in sampling throughput to 40 samples/h 139or 80/h ~4~with detection limits of the order of 0.5 ppm. The determination of Pt in waters necessarily involves an on-line preconcentration step and the research group of Madrid reported detection limit values of 10 ng/ml by F I FAAS 141 and 4 ng/l by FI-ICP-MS. 142 Selenium Se is commonly determined in waters by on-line hydride generation, 39' 143--150but in recent years new approaches have appeared in the literature 15~'~52 Hydride generation of Se through reaction with NaBH4 is most suitable for specific determination of Se(IV) 39' 143 and also for speciation of Se(IV), Se(VI) and other selenium compounds based on the selective elution of previously preconcentrated species on a Dowex 1 • 8 column, 144 the on-line pre-reduction of Se(VI) to Se(IV) carried out in convective heating systems ~45or with a microwave-assisted treatment. ~46Microwaves have also been employed for post-column derivatization in HPLC followed by HG-AFS 147 or HG-AAS. 148 On-line preconcentration procedures for FI-AAS determination of Se in waters have also been reported using ion exchangers 39' 144,153or coprecipitation with La(OH)3 .143' 149

Silver and strontium The use of on-line precipitation of AgC1 followed by dissolution with NH3, 57 the use of alumina columns, 154 exchange columns ~55 or sulphydryl cotton microcolumns, have been reported for silver in order to lower FAAS limits of detection to 1 to 5 ng/ml. Lin et al. 156 determined strontium in waters at low concentration levels by FAAS using on-line continuous coprecipitation preconcentration based on the formation of lead sulphate followed by elution with hot EDTA solution. At a sampling rate of 20-30/h the relative standard deviation achieved was 3.5% (detection limit obtained was 0.9 ng/ml).

322

A. Morales-Rubio and M. de la Guardia

S u l f u r a n d tellurium

Gerbersmann et al. ~57 determined 13 volatile sulfur compounds in waters. The analytes were stripped from samples by a flow of He and preconcentrated in a capillary cryogenic trap. Determination was carried out by GC-MIP--OES. The detection limit values obtained ranged from 0.4 (ethanethiol) to 0.9 ng/1 (dimethyldisulfide) and recoveries achieved in fresh and sea waters were around 82 to 103%. Tellurium has been determined in industrial waste waters by Klinkenberg et al. ~58 using isotopic dilution ICP-MS. Samples were treated with NaOH and EDTA to convert organic Te in Te(IV) and Te(VI). After 1 h equilibrium at room temperature, samples were measured by ICP-MS with the continuous addition of a Te solution enriched in Te 125. Tellurium can be concentrated in situ on a Pd-coated graphite furnace after hydride generation and determined by in situ trapping-ETAAS. 38 ~n

Tin determination in waters is a good example for the coupling on-line of chromatographic procedures, like GC !~' 15~-~60or HPLC 161'162with atomic detectors. In Chapter 12 we will see more examples of this application where samples are usually treated with an organic solvent for species extraction and are then injected in a GC column coated with immobilized methylsilicone. Organotin compounds are then determined by AAS, MIP--OES, ICP-MS, etc. Ebdon et al. ~6~ reported the determination of tin species in sea water, separated by HPLC on a column of Partisil 10 SCX, coupled on-line with HG-AAS using previous decomposition of species with UV radiation. Buthyltin and phenyltin were determined in waters by Ceulemans et al. 159 Water samples were treated with sodium acetate (pH 5) and sodium tetraethyl borate in hexane and shacken for 5 min. Aliquots of the hexane phase were injected on the GC-MIP-OES instrument. The detection limit obtained was 0.1 ng/l with recoveries from 85 to 97%. Another selected example is the application to harbour waters by HPLC-ICP-MS coupled through ultrasonic nebulization. The detection limits obtained, at a sampling frequency of 10/h, were in the range of 2.8--16 pg for various organotin species. ~62 Most recently, De Smaele et al. 16~ described the development of a GC-ICP-MS system for this determination of organotin compounds in harbour waters. Detection limits obtained for alkyltin compounds were better than any of the reported methods and ranged between 15 and 35 fg of Sn. Titanium, u r a n i u m a n d v a n a d i u m

Titanium and vanadium in natural water reference materials were determined by ICP-MS with on-line preconcentration, using a miniature chelated ion-exchanger column based on

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323

quinolin-8-ol-cellulose. The detection limit found was 0.01 ng/ml and the RSD was lower than 5 % . 163 Aldstadt et al. 164 determined very recently uranium in certified groundwater samples by on-line preconcentration through a column separation module, coupled with the ICP-MS. The detection limit found was 0.3 ng/1 at a sampling rate of 10/h. Vanadium has been determined in waters by using an on-line anodic-stripping voltammetry flow cell before detection by ICP-MS. 72 Samples were introduced into a flow cell incorporating a reticulated vitreous carbon working electrode. The stripped metal was determined by ICP-MS with recoveries of the order of 67%; ICP-MS with on-line preconcentration as for Ti(IV) has also been reported and very low detection limits of around 0.001 ng/ml achieved. ~63 Zinc

The flow injection-atomic spectrometric determination of Zn alone, or together with other elements, is commonly carried out in environmental samples by means of on-line preconcentration through the use of immobilized reagents ~65or chelating resins. 42'47,92 10.2.2

Multielemental determinations

The simultaneous or sequential determination of several elements in the same water sample has been attempted mainly using multielemental atomic detectors, although AAS can be also considered. This topic has been divided according to the detection technique employed. Papers summarized in this section are given in Table 10.1 and are particularly interesting because the proposed strategies could be extremely useful for pollution screening environmental studies. 10.2.2.1

Multielemental detectors

M I P - O E S as detector

Ng and Shen 166 employed a MAK nebulizer to inject liquid aerosols into a low power MIP-OES in order to determine directly a series of elements. The detection limit obtained in synthetic sea water containing 3% of HNO3 for Cr, Mn, In, V, Pb, Sr and Zr were 62, 18, 18, 91, 139, 13 and 3945 ppb respectively. I C P - O E S as detector

The flow injection ICP-OES technique is the most widely employed methodology for simultaneous multielemental determinations in sea water, 44'167'168 r a i n 94'169 and fresh waters. 167,168, 170, 171 Different approaches have been developed for the determination of elements; such as

to

Table 10.1 Examples of multielemental determinations for water analysis Elements

Sample

Detection

166 44 168

ICP-OES

Liquid aerosols injected by a MAK nebulizer On-line preconcentration step with imidodiacetic acid ethylcellulose Samples were splited into two streams one pumped directly to the nebulizer and the other mixed with Hcl and NaBH4 solutions and hydrides generated were swept into the plasma together with sample Electrolyte cathode discharge Coprecipitation with cobalt On-line preconcentration with functionalized XAD-2 resin with 1-(2-thiazolylazo)-2-naphthol Direct analysis

ICP-OES

Direct analysis of acidified samples with 70 mM HNO3

171

Sea

ICP-OES ICP-MS

46 176

Sea Ground Sea

ICP-MS

On-line preconcentration with silice immobilized 8-quinolinol Preconcentration on-line by adsorption on a microcolumn of silica immobilized 8-hydroxyquinoline Sorption on activated carbon

ICP-MS

Cellulose inmobilized ethylenediamineacetic acid

178

Sea

ICP-MS

175

Sea

ICP-MS

Sea

ICP-MS

On-line preconcentration involving the solid phase chelation on an iminodiacetate column On-line preconcentration involving the solid phase chelation on an iminodiacetate column On-line elimination of interferences by a sorption column

Reference

ICP-MS

Estuarine

ETAAS

Sea

ETAAS

Cr, Mn, In, V, Pb, Sr, Zr Bi, Cd, Cr, Cu, Fe, Mn, Pb, Sb, Se

Sea Sea Sea River

MIP-OES ICP-OES ICP-OES

Cu, Mn, N i, Pb Cd, Cu, Fe, Ni, Pb, Zn Cd, Cu, Fe, Mn, Ni, Zn

Rain Rain Sea Fresh Estuarine Fresh Surface

ICP-OES ICP-OES ICP-OES

AI, Ca, Fe, Mg, P, Si Al, Ba, Ca, Cd, Cu, Fe, K, Li, Mg, Mn, Na, P, Pb, St, Zn m

La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Cd, Co, Cu, Ni, Pb, U, Y Cd, Co, Cu, Mn, Ni, Pb, U, Zn Co, Cu, Mn, Ni, Pb, U, Zn, Ca, Cr, Fe, V V, Cr, Ni, Co, Cu, Mo, Pt, Hg, Bi Ni, Co, Mn, Cu, Zn, Mo, Cd, Pb, U Cd, Cu, Ni, Pb Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn

Ref

Comments

Isotope dilution. Metals were preconcentrated from samples by absorption on silica inmobilized quinoli-8-ol Acidified samples mixed with sodium diethyldithiocarbamate and complex formed absorbed on a RPC18 microcolumn Preconcentration in a quinolin-8-ol inmobilized on Porasil and elution with HCI/HNO3

94 169 167 170

o

c:r o

177

178 174 172 45 179

(table continued on p. 325)

f~ b.,~ ~

Table 10.1 Continued Sample

Elements

Detection

Cd, Cu, Fe, Mn, Ni, Pb, Zn

Sea

ETAAS

Cd, Co, Cu, Fe, Ni, Pb

Antartic sea

ETAAS

Cd, Co, Cu, Fe, Ni, Pb

Sea

ETAAS

Cd, Cu, Pb, Zn

Sea

FAAS

Cd, Cu, Mn Cd, Co, Cu, Hg, Pb, Zn Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn Heavy metals Ag, Cd, Co, Cu, Fe, Ni Ag, Cu, Fe, Ni

River -

FAAS FAAS FAAS

Natural, Sea River Tap

FAAS FAAS FAAS

Trace elements

Tap

FAAS

Cu, Fe, Mn, Zn

FAAS

Ag, Ca, Fe

River Estuarine -

FAAS

Ca, K, Mg, Na

Natural

FAAS

Comments Preconcentration of metals as their chelates on a column of silica inmobilized quinolin-8-ol and elution with HC1/HNO3 Samples were mixed with ammonium pyrrolidinecarbodithioate and complex adsorbed on silica gel C~8 microcolumn Samples were mixed with ammonium pyrrolidinecarbodithioate and extracted with 1,1,2-trichloro- 1,2,2-trifluoro ethane Column packed with Chelex-100 or quinolin-8-ol-azo inmobilized on porous glass Retained metals eluted with HNO 3 Chelating resin containing glutaraldehyde immobilized cysteine Chelating resin containing pbly(aminophosphoric) acid

Ref 182 181 180 42

> ~..,l o t') m,o O

43 185 186

~..,o

188 189 93

O

0....~ 9

Solid phase extraction on high surface titanium dioxide Solid phase extraction on dithiocarbamate chitin On-line preconcentration of elements previous its chelation by 8-quinolinol followed by retention on poly(chlorotrifluoroethylene) column On-line preconcentration of elements previous its chelation by 8-quinolinol followed by retention on nucleosil 100 C~8 Water cooled dual silica tube and a double-slotted quartz tube as trapping techniques Precipitation as AgC1/CaCO3/Fe(OH)3 in a PTFE tube conected to a Tygon tube filled with glass beads and dissolving precipitates with NH3/HCI/HNO3 On-line dilution steps in order to extend the dynamic range

184 92

~..io

57

55 187

ta~

326

A. Morales-Rubio and M. de la Guardia

direct analysis, 16s"170,171 on-line incorporation of preconcentration steps, 44'46,167 electrolyte cathode discharge, 94 or coprecipitation with cobalt. 169 For on-line simultaneous preconcentration of several elements in ICP-OES imidodiacetic acid ethylcellulose, 44 silica immobilized 8-quinolinol, 46 or functionalized XAD-2 resin with 1-(2-thiazolylazo)-2-naphtho1167 were employed.

ICP-MS as detector In all the studies carried out by ICP-MS for multielement determination of trace level elements a previous preconcentration step has been employed because of the low level of elements determined and in order to separate and eliminate the matrix and so to avoid interferences. ~n-lvs These procedures have been employed in the analysis of reference samples, NASS-2172 and NASS-3173 waters; sea 174-178and ground water, 177being employed for different element determination using different strategies, as can be seen in Table 10.1. 10.2.2.2 Atomic absorption for sequential determinations Although AAS both using flame or electrothermal atomization, is a typical unielemental technique, the absence of serious interferences in FIA determinations carried out in water samples permits the sequential determination of several elements in a similar sample and using the same manifold and the same sample pretreatment. Flow injection graphite furnace atomic absorption spectrometry has been employed for multielemental determination in estuarine, 45"48 sea 45' 179-182 and river45 waters. Preconcentration and separation of elements has been carried out to eliminate interference matrix in all cases. Flow injection flame atomic absorption spectrometry has been employed for multielemental determinations in sea, 42'183 tap, 93'184 lake, 183 river 43'92'183 and waters in general.47' 5~-57.~85--~89 The FI-hydride generation atomic absorption spectrometry is the most employed methodology for the determination of hydride forming elements. 39"190 On-line hydride generation has been also employed in electrothermal AAS. 3s In situ preconcentration of Bi and Ge 38 has been carried out from natural water samples on a graphite furnace coated with palladium. Filtered samples were treated adequately and hydride generated were stripped by Ar and fed into the graphite tube for detection. Other elements, like As, Bi, Sb, Se, Sn and Fe have been determined simultaneously in waters by HG-AAS, ~9~ and As, Bi, Sb, Se and Sn have been determined in reference waters by HG-ICP-OES. 19~

10.2.3

Indirect determination of anions

Anions can be determined by atomic spectrometry via direct measurement of their elemental components, as for example chromium in chromates, phosphorus in phosphates

Applications in environmental analysis

327

or sulphur in sulphates. However, the most common practice is the indirect determination through previous stoichiometric separation (e.g. precipitation) with a cation which can then be easily determined by atomic spectrometry. The main drawback of indirect procedures for anion determinations by atomic spectrometry is the lack of selectivity of these kind of reactions ~gz which in general can involve several anions. The possibilities offered by flow injection to couple on-line adequate separation procedures opens new perspectives to obtain indirect fast and accurate determinations. In this section indirect methods proposed so far for chloride, cyanide, fluoride, iodate, phosphate and sulphate have been summarized. Direct methods of anions, such as those related to arseniate or chromate using direct determinations of the element, were considered in Section 10.2.1 devoted to single element determinations.

Chloride, fluoride, cyanide and iodide Schulze et al. 193 determined chloride in waters by indirect FAAS. Degassed waters were injected into a stream of 2 M HNO3 and mixed with a solution of AgNO3. The precipitate was filtered off and the filtrate analyzed by FAAS at 328.1 nm. A sampling rate of 40 samples/h was obtained. Indirect ICP-OES determination of fluoride in water samples by FI solvent extraction has been proposed. 194The method involves formation of a lanthanum alizarin complexone fluoride complex and its subsequent extraction in hexanol containing NN'-diethylaniline. Fluoride was determined indirectly by introduction of the separated organic layer into the plasma and measurement of the emission intensity of La at 333.75 nm. Sampling rate achieved was 36/h and the variation coefficient for a solution containing 1 mg/1 was 2.2%. Cyanide in waters was determined by indirect FAAS. ~95 Cyanide was injected into a column packed with CuS and the eluate, containing the analyte as a cuprosocyanide complex; C u ( C N ) 3 2 - , w a s measured at 324.7 nm by FAAS (detection limit, 1 ppm of CN-). Nakahara and W a s a 196 determined iodide indirectly in sea water by continuous flow CV-MIP-OES. SnC12 in HCI and Hg(II) in HNO3 were added to the water sample solution containing I - , and the decrease of the mercury signal was measured at 253.7 nm. The detection limit obtained was 0.74 ng/ml and recoveries were higher than 93%.

Phosphate and sulphate Phosphate in lake and waste waters was determined by ion exchange HPLC-FAAS. 197 Samples were passed through a Dowex l-X8 column and phosphate eluted with 0.4 M HC1. Eluate was mixed with a stream of Ca solution and P as phosphate present measured by its depression of the Ca AAS signal at 422.7 nm.

328

A. Morales-Rubio and M. de la Guardia

Sulphate can be determined indirectly also by FAAS using Pb; 198 or Ba~ 199 for this determination. Zorro et al. 198 mixed the samples with Pb 2§ (pH 4.2-5.2), formed PbSO4 was filtered offand the residual Pb 2§ determined at 283.3 nm. The detection limit obtained was 1 mg/l and precision at a sampling rate of 60/h was about 3%. The group of C6rdoba also tried 10 mM HC1 and Ba 2+ in 40% ethanol stream ~99and then the detection limit was 5 mg/1 of sulphate.

10.2.4 Determination of organic compounds Classical literature on indirect atomic spectrometry 192 evidenced the applicability of atomic spectrometry to be extended to the determination of molecules. Methods for the determination of organic molecules with atomic detectors are based on indirect methodologies using separations (e.g. based on precipitation or ion-pair extractions) of compounds, with well established stoichiometries or, in some cases on the emission of some heteroatoms like phosphorus] ~176 Obviously, in environmental analysis, there is a great demand for methods to determine pesticides, surfactants and other organic pollutants. Thus, in the last years, we have witnessed the development of some flow injection procedures to obtain, in an easy way, fully automated measurements of organic molecules by atomic absorption and atomic emission. Some examples are given in the following paragraphs. Surfactants

Gallego et a l l ~ determined anionic surfactants in waste waters by an indirect FAAS detection method. The method involves the extraction in IBMK of the ion pair surfactant1,10-phenanthroline-copper. Determinations were made by measuring the copper level in the separated organic layer. Limits of detection of 45 ng/ml were reported and precisions were down 1%. Cationic surfactants in natural, tap and waste waters were also determined 2~ by an indirect FAAS. The method involves the extraction into IBMK of the ion-pair surfactanttetrathiocyanatecobaltate. As before, determinations were carried out by reading the AAS cobalt signal of the organic phase. Pesticides

Jimenez de Bias et a l l ~ determined diethyldithiocarbamate compounds by FAAS with continuous extraction and this method was applied to the determination of the fungicide "ziram". The method involves the formation of the dithiocarbamate ion-copper complex and its extraction in IBMK. Measurement of the copper signal at 324.7 nm provides the ziram level. Eleven organophosphorus pesticides in waters by GCMIP-OES were analysed by

Applications in environmental analysis

329

The detection limit obtained in this method was 0.1 ng/ml and precision Rinkema et al. 2~176 achieved ranged between 4 and 22%.

10.3

Soil and sediment analysis

Soil and sediments are solid samples with a variable content of organic matter but mainly constituted by inorganic compounds. So, the digestion of this type of samples, in order to obtain appropriate solutions for their analysis by flow injection-atomic spectrometry, can be carried out in a simple way by classical acid treatments. After dissolution, flow injection is a very useful approach for on-line preconcentration of the trace elements to be determined. However, in recent years, a change of mentality from total digestion procedures to more soft treatments TM has contributed to the development of slurry introduction procedures with on-line microwave-assisted sample treatments, for selective leaching of sought elements without increasing the presence of dissolved matrix components in the sample solutions. On the other hand, the combination of classical sequential extraction methodologies and tandem on-line separation methods with atomic spectrometry offers tremendous possibilities for speciation studies in soil and sediment analysis. As in waters, application studies on flow-injection atomic spectrometry are organized here considering the studies involving single element determinations and those determining several elements in the same sample with the same sample treatment. Individual elements are considered in the first case for convenience, while Table 10.2 resumes the papers and analytical characteristics of multielemental analysis. 10.3.1

Single element determination

Aluminium

Mitrovic et al. 2~ developed an HPLC-ICP--OES procedure for the speciation of A1 over a wide pH range, from acidic to alkaline regions, in soil extracts. Aluminium species were separated from polymeric, neutral and negatively charged species on a cation-exchange fast protein LC column and the separated species were determined off-line by ICP-OES. The limit of detection found was 120 ng/ml of A1. Antimony and arsenic

The flow injection-atomic spectrometric determination of Sb in soil and sediments has been commonly carried out by hydride generation using FAAS 2~ or ETAAS 2~ or HG-in situ trapping-AAS 2~ for determination. These methods involve in general a previous digestion of dry samples followed by a reduction of Sb(V) to Sb(III). The sample digestion step involves treatments with HNO3/HC1Oa/HF 2~ or HNO3/

Table 2

Multielemental determinations in soil and sediment samples by FI atomic spectrometry Elements

La, Ce, Pr, Nd, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu Cd, Cu, Pb Cd, Cu, Pb

Sample

Detection

Comments

Ref

Geological

ICP-OES

On-line preconcentration on conically shaped ion exchange columns

245

Limestone

ICP-OES

On-line preconcentration on a aminocarboxylic sorbent microcolumn

246

o

Sediment, Sludges Sediment, Soil

ICP-OES FAAS

210 211

7~

Cd, Fe, Pb, Zn

Sludges

FAAS

Cu, Mn, Pb, Zn Ag, Cr, Cu, Zn

Sludges Sludges

FAAS FAAS

Cu, Mn Cu, Mn, Pb Ca, Fe, Mg As, Se As, Bi, Sb

Sludges Sludges Sediment, Soil Sewage sludges Soils

FAAS FAAS FAAS HG-AAS HG-ETAAS

-

Complete digestion by acid mixtures and mixed with ammonium diethyldithiophosphate. Complexes formed were preconcentrated on a C18 bonded silica gel column Complete digestion by acid mixtures and preconcentration of elements Direct injection of acid slurries by on-line microwave leaching Complete digestion by acid mixtures and preconcentration on inmovilized selenestrum capricornotum algae Direct injection of acid slurries Direct injection of acid slurries after microwave assisted treatment Direct injection of sonicated slurries Acid sample solution were merged with a stream of NaBH4 in NaOH for generation of hydrides

t'i)

p,,,, o

o 212 213 214 215 216 224 209 207

t~

~,,~o

Applications in environmental analysis

331

H2SO4/HC104 .2~ Furthermore, on-line separation microwave-assisted digestion of sewage and sediment was developed by Lopez-Gonzalez et al. 34 for speciation of different compounds of arsenic and has been commented on in more detail in Chapter 5. The reduction of As(V) to As(III) by KI-ascorbic acid, followed by formation of ASH3, is the most common approach in flow injection-atomic spectrometry using arsine generation.34, 208,209 Cadmium, copper and germanium

Cadmium has been determined in sediments 2~~ and sewage sludges 21~ by FAAS 211'212 or ICP-OES, 2~~usually along with other trace metals as Cu and Pb 2~~ or Fe and Zn. 2~z Copper in sediments and sludges has been determined along with Cr, Mn, Pb, Ni, Ag or Zn by FAAS 211'2t3-216or by ICP--OES. 21~ Ma et al. 2~7 determined germanium in soil samples using a HG-in situ trapping-ETAAS. A detection limit of 1.3 pg, with a sample throughput of 30/h, and recoveries between 99.5 and 100% were obtained. Gold, lead and magnesium

Gold has been determined in soil by FAAS. 218 Samples were ignited for 2 h at 700~ and dissolved with aqua regia. Solutions were centrifuged for 3 min and supernatant passed through a column packed with VS II anion exchange fibre. Au was eluted at 50~ with 0.5 M HC1 and determined by AAS at 242.8 nm. MIP-OES has also been used with the additional determination of silver. 219 Lead has been determined in sediments and sludges by FAAS, 211-213'216'22~ICP-OES 2~~ or HG-in situ trapping-ETAAS. TM The group of Sanz-Medel determined lead in sediments by continuous hydride generation using CrzO72/lactic acid mixture and 5% NaBH4 in 0.1% NaOH obtaining a detection limit of 2 ng/ml by ICP-OES. 222'223 Sooksamiti et al. 22~ has recently reported the determination of lead in digested soil samples by in-valve solid-phase extraction-FI-FAAS. Lead in solution was first loaded in a mini-column packed with a crown ether resin. Retained lead was successfully eluted with ammonium oxalate (detection limit 80 ng of the metal) and a sampling rate of 25/h was reported. Magnesium determination in soils is usually attempted by F A A S . 224' 225 Mercury

Flow injection-cold vapour (CV) atomic spectrometry is the most employed approach for the determination of Hg in sediments, soils and sludges. 226-234 Hg vapour generation can be accomplished by using the stand-alone flow injection mercury system, in which a continuous flow of HC1 acid carrier stream transports the

332

A. Morales-Rubio and M. de la Guardia

sample to the mixing manifold where it is merged with SnC12.226 In some cases mercury determination in soils involves permeation of Hg vapour through a PTFE membrane 227 or the use of a new injection technique by using tubing clamp valves. 22s Rokkjaer et al. 229 evaluated the interferences produced by volatile nitrogen oxides in the Hg determination and they concluded that interferences could be avoided by purging the sample with Ar. On-line microwave-assisted treatment procedures, developed to reduce handling steps, have been applied to quantitative extraction of Hg from solid samples and sampling throughputs of 15/h with limit of detection of 0.09 ng/g by using AFS detection were reported. 23~Other techniques employed a bromide-bromate oxidation reaction during the microwave digestion procedure in order to oxidize organomercury compounds to inorganic mercury. TM Virtually all atomic detectors have been employed for Hg determination, e.g. C V ETAAS, 232 CV-ICP-OES, 233 CV-MIP-OES, TM etc. The determination of methylmercury in sediments incorporating supercritical fluid extraction and gas chromatography coupled with MIP-OES is interesting: TM the target compound is derivatized to butylmethylmercury with butylmagnesium chloride to be amenable to GC. The detection limit for the methylmercury determination in sediments was 0.1 ng/g. M o l y b d e n u m and nickel

As detailed earlier, Guo et al. 133 determined molybdenum in waters and soils by on-line preconcentration using an ion exchange column packed with Dowex-1 resin. The determination of Ni in rocks 235 can be carried out by FAAS after precipitation of nickel as 1-nitroso-1-naphthol complex which is retained in a filter; elution of the complex with ethanol and FAAS detection allowed a detection limit of 5 ng/ml at a sample throughput of 15-20/h. Selenium a n d tin

The most common technique employed for the flow injection-atomic spectrometric determination of Se in soils and sediments is again hydride generation (with NaBH4) and final AAS detection. 2~ 236, 237 Elemental selenium and pyrite-selenium were determined in sediments by Velinsky and Cutte1236 using HG-AAS. Sediments were dried and treated with 1M NaSO2 to solubilize elemental selenium. MIP--OES, however, has also been used: Ng et al. 237 determined Se in soils at the 196.0 nm line. Acidified sample solutions were mixed with NaBH4, passed through a gasliquid separator and the hydride was transferred directly to the plasma. Detection limits of 40 ~g 1-~ were reported. Zhang et al. 238 determined tin in standard reference river sediment materials by trapping

Applications in environmental analysis

333

the hydrides on a Pd-coated L'vov platform at low temperature before ETAAS measurement. The detection limit obtained was 7 ng/l, while Mclntosh et a/. 239 determined tin in river sediments by HG-AAS. The detection limit achieved was 0.05 Ixg 1-~ and the precision around 1-3%. Speciation of buthyltin compounds in sediments has been very popular these years; e.g. Cai et al. 24~using GC-AAS, Astruc et al. 241 by ETAAS with an HPLC separation system, while Dirkx et a l l 42 determined organotin compounds in river sediments by GC-FAAS. Tin compounds in waters and a harbour sediment reference material (PACS-1) by HPLC and ultrasonic nebulization-ICP-MS has also been reported. ~62The detection limit achieved, at a throughput of 10 samples/h, were between 2.8 and 16 pg for various tin species. As a latest illustrative example (Chapter 12 details this topic at length) Chau e t al. 243 determined ten organotin compounds in sediments by GC-MIP-OES with excellent results for bibuthyl and tributhyltin. Uranium and zinc

Silva et a l l 44 determined uranium in reference standard soil materials by ICP-OES at 409 nm. Uranium was leached from soils by HC1 and supernatant solutions aspirated inside the spectrometer. Results agree well with reported values. The flow injection atomic spectrometric determination of Zn in soils and sediments has been carried out usually by FAAS, 165"212-214together with other elements, like Cu, Cr, Fe, Mn, Pb or Ag. 212~!4 When necessary, preconcentration can be used; Ma et al. 165 employed dialkyldithiophosphates as complexing agent for flow injection on-line sorbent extraction of Zn in calcareous loam soil (CRM 141) and river sediment (CRM 320) certified reference materials with FAAS detection. 10.3.2

Multielement determinations

Table 10.2 summarizes the studies published on multielemental determination in soil and sediment samples by FI-Atomic Spectrometry using classical multielement techniques, like ICP-OES, MIP-OES or ICP-MS or altematively by sequential analysis of aliquots of a similar sample by FAAS, HG-AAS or ETAAS. Simultaneous determination of several elements in geological 245 and limestone 246 samples has been carried out by ICP-OES. Due to the low level of elements, preconcentration was made by both on-line microcolumn sorption on aminocarboxylic sorbent 246 and on-line conically shaped ion exchange columns. 245 Flow injection flame atomic absorption spectrometry has also been employed for sequential multielemental determinations in sludges, 212-2~6soils 211,224 and sediments. 21i, 214 Two ways have been employed for the analysis of these materials: (a) the complete

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digestion of solid samples by acid mixtures 211'212'214before FI-AAS measurement or, (b) the use of acid slurries of these materials which are directly injected, e.g. Cu and Mn determination, 2~5 previously sonicated, 224 microwave-assisted 2~6 or on-line microwaveleached. 2~3In some cases a preconcentration step has been coupled on-line with the FAAS determination.2 ~~-2~2.214 Hydride generation AAS has been used for As and Se determination in sewage sludge samples. 2~ Furthermore, Sb, As and Bi have been determined in soils by F I - H G ETAAS. 2~ MIP-OES has been employed for Au, Ag, Pt, Rh and Pd determination in sediments using an ultrasonic nebulizer and a desolvation system. 2~9

10.4

Air analysis

Published applications of flow injection atomic spectrometry to air analysis are less numerous than those developed for water and soil analysis. The main trends in flow injection applied to trace metal determinations in air (including gaseous compounds, aerosols, particulates and fly ashes) consist of the hyphenation between chromatographic methods and atomic spectrometry (see Chapter 12), the on-line preconcentration by means of on-line complexation and sorbent extraction, or by on-line gold amalgamation, and speciation studies in aerosols. We will revise such applications, element by element, in the following paragraphs. Arsenic

The collecting conditions for determination of arsenic in the working atmospheres of semiconductor manufacturing process has been studied using HG-AAS. Linearity obtained was from 8.7 to 420 i~g/m3 and recovery achieved w a s 9 5 % . 247 Arsenic was determined by Nerin et al. 248 in fly ashes from a lignite thermal power plant by HGAAS using slurry sampling. The detection limit reported was 2.8 ng. Foster and Howe 249 determined arsenic in workplace air by HG-AAS. Air was sampled on a cellulose ester membrane filter conditioned with Na2CO 3 in aqueous 5% glycerol. The detection limit achieved was 0.3 ng/ml of arsenic with a 3% of variation coefficient. Cadmium, copper and chromium

Ma et al. TM determined Cd and Cu in coal fly ash by FAAS following digestion of samples in a PTFE bomb with HF and HNO3. After cooling, HC104 was added to the slurry and heated for 8 h at 150~ Silicates were removed by addition of HF and HNO3 and evaporation to dryness and the remaining residue was dissolved in HNO3 and injected into a FI system. It was merged with ammonium diethyldithiophosphate and the complexes formed were sorbed onto a C~8 bonded silica gel column. Elution was carried out with

Applications in environmental analysis

335

methanol. Detection limits achieved for Cd and Cu were 0.8 ng/ml and 1.4 ng/ml respectively. Lead was also determined with a limit of detection of 10 ng/ml. Lintschinger et al. 25~determined simultaneously Cr(III) and Cr(VI) in aqueous extracts of coal fly ash standard reference material (NIST SRM 1633a) by reversed-phase ion-pair HPLC, employing FAAS and ICP-MS detection. The relative standard deviation values obtained were lower than 2% for both detection systems at a throughput of 20/h is reported. Lead and mercury

Lead was determined by HG--AAS in air samples by Tavares et al. TM Air samples were passed through cellulose filters, and filters treated with HNO3 for 10 min at 70-80~ The metal has also been determined in aerosols by FAAS and ETAAS. 252 Particulates were collected on glass-fibre filters and digested by refluxing for 2 h with HNO3/HC1. The most commonly employed methodology for ultratrace Hg determination using flow injection-atomic spectrometry is the on-line cold vapour generation followed by enrichment through gold amalgamation and atomic absorption determination. 253 Snell et al. TM recently reported an on-line amalgamation trap for the collection of mercury species in natural gas, which were separated by capillary GC for final detection by MIP-OES. Using the amalgamation trap, mercury can be selectively collected from the GC column eluate (and subsequently passed to the plasma in a flow of pure He), being determined without carbon compound interferences. The limit of detection achieved for dimethylmercury was 0.24 ng/ml, and for derivatized (butylated) monomethyl and inorganic mercury 0.56 ng/ml. Sulphur compounds

Swan and Ivey255 analysed atmospheric sulphur gases by capillary gas chromatography with atomic emision detection. Air was sampled through tubes with Au-coated glass wool. Then the sulphur compounds were thermally desorbed at 300~ separated in a silica column and detected by atomic emision. Detection limits obtained for (CH3)2S and CS2 were 20 pg of sulphur.

10.5

Concluding remarks

There are a great number of papers publishing evidence that flow injection with atomic spectrometry constitutes a synergic combination which can improve the selectivity and sensitivity of the determinations of trace elements in environmental samples. Nowadays it is well demonstrated that flow injection is the best alternative to obtain simultaneously the preconcentration of trace elements and also matrix removal, thus improving the sensitivity and selectivity of more classical atomic spectrometry

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procedures. Additionally, the use of flow injection to link chromatography with atomic spectrometry offers interesting alternatives, particularly for speciation in liquid or dissolved samples. The development of soft digestion procedures, based on specific leaching of trace element contaminants in a fast way, offers encouraging possibilities for the whole automatization of the analytical procedures for environmental analysis by incorporating on-line treatments via microwave-assisted digestion (see Chapter 5 by J. L. Burguera). However, additional efforts must be carried out in order to secure the validation of the developed methodologies. Therefore it is necessary now to increase the intercalibration studies, to search for new reference methodologies free from handling errors and to provide new reference environmental materials with certified concentration levels of elements and species (if possible). That and the continuous effort of analytical laboratories in order to improve and test carefully the analytical figures of merit of new procedures could advance this field from the present laboratory research stage into a new stage in which they can solve the real problems of environmental monitoring (with more efficiency, faster, at a lower cost, at much lower concentration level, etc.) which present society and technology are demanding these days. 10.6

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A. Morales-Rubio and M. de la Guardia Porta, V., Sarzanini, C., Aboilino, O., Mentasti, E. and Carlini, E., J. Anal. At. Spectrom., 1992, 7, 19. Li, Z., Mclntosh, S. and Slavin, W., Anal. Proc., 1992, 29, 438. Zhuang, Z, Wang, X, Yang, P, Yang, C. and Huang, B., J. Anal. At. Spectrom., 1994, 9, 779. Gine, M. F., Bergamin, H., Reis, B. F. and Tuon, R. L., Anal. Chim. Acta, 1990, 234, 207. Musil, J., Chem. Listy, 1990, 84, 1308. Beauchemin, D. and Berman, S. S.,,4nal. Chem., 1989, 61, 1857. Ebdon, L., Fisher, A., Handley, H. and Jones, P., J. Anal. At. Spectrom., 1993, 8, 979. Piantz, M. R., Fritz, J. S., Smith, F. G. and Houk, R. S.,Anal. Chem., 1989, 61,149. Taylor, D. B., Kingston, H. M., Nogay, D. J., Koller, D. and Hutton, R., ,1. Anal. At. Spectrom., 1996, 11, 187. Halicz, L., Gaurieli, I. and Dorfman, E., J. Anal. At. Spectrom., 1996, I I, 811. Murty, D. S. R. and Chakrapani, G., J. Anal. At. Spectrom., 1996, 11,815. Neims, S. M., Greenway, G. M. and Koller, D., J. Anal. At. Spectrom., 1996, I 1,907. Nakashima, S., Sturgeon, R. E., Willie, S. N. and Berman, S. S., Fresenius '-Z. Anal. Chem., 1988, 330, 592. Backstrom, K. and Danielsson, L. G., Mar. Chem., 1990, 29, 33. Porta, V., Abollino, O., Mentasti, E. and Sarzanini, C., J. Anal. At. Spectrom., 199 I, 6, 119. Azeredo, L. C., Sturgeon, R. E. and Curtius, A. J., Spectrochim. Acta Part-B, 1993, 48B, 91. Burba, P., Rocha, J. C. and Schulte, A., Fresenius '-J. Anal. Chem., 1993,346, 414. Bemdt, H., Mueller, A. and Schaldach, G., Fresenius '-J. Anal. Chem., 1993, 346, 711. Elmahadi, H. A. M. and Greenway, G. M., ,1. Anal. At. Spectrom., 1993, 8, 101 I. Yebra Biurrum, M. C., Bermejo Barrera, A., Bermejo Barrera, M. P. and Barciela Alonso, M. C., Anal. Chim. Acta, 1995, 303, 341. Lopez Garcia, I., Vifias, P., Campillo, N. and Hernandez Cordoba, M., Fresenius J. Anal. Chem., 1996, 355, 57. Vassileva, E., Proinova, i. and Hadjiivanov, K.,,4nalyst, 1996, 121,607. Hase, A., Kawabata, T. and Terada, K., Anal. Sci., 1990, 6, 747. Welz, B. and Schubert-Jacobs, M.,,4t. Spectrosc., 1991, 12, 91. Davidowski, L.,At. Spectrosc., 1994, 15, 140. Hassan, S. S. M., Organic Analysis Using Atomic Absorption Spectrometry, Ellis Horwood Lim., Chichester, 1984. Schulze, G., Elsholz, O., Hielscher, R., Rauth, A., Recknagel, S. and Thiele, A., Fresenius '-Z. Anal. Chem., 1989, 334, 9. Manzoori, J. L. and Miyazaki, A., Anal. Chem., 1990, 62, 2457. Haj-Hussein, A. T., Christian, G. D. and Ruzicka, J., Anal. Chem., 1986, 58, 38. Nakahara, T. and Wasa, T., Microchem. J., 1990, 41,148. Gunz, D. and Schnell, E., Mikrochim. ,4cta, 1984, !11, 409. Zorro, J., Gallego, M. and Valcarcel, M., Microchem. J., 1989, 39, 71. Gallego, M. and Valcarcel, M., Mikrochim. Acta, 1991,111, 163. Rinkema, F. D, Louter, A. J. H. and Brinkman, U. A. T., J. Chromatogr. ,4, 1994, 678, 289. Gallego, M., Silva, M. and Valcarcel, M., Anal. Chem., 1986, 58, 2265.291 Martinez-Jimenez, P., Gallego, M. and Valc~ircel, M., Anal. Chim. ,4cta, 1988, 215, 233. Jimenez de Bias, O., Pereda de Paz, J. L. and Hernandez Mendez, J., J. Anal. At. Spectrom., 1990, 5, 693. de la Guardia, M. and Morales-Rubio, A., Trends in Anal. Chem., 1996, 15, 311. Mitrovic, B., Milacic, R. and Pihlar, B.,,4nalyst, 1996, 121,627. Donaldson, E. M., Talanta, 1990, 37, 955. Wang, Z. and Wang, Y., Fenxi-Huaxue, 1992, 20, 670. Wang, X. and Fang Z., Fenxi-Huaxue, 1988, 16, 912. Saraswati, R., Vetter, T. W. and Watters, R. L. Jr., Analyst, 1995, 120, 95. Verrept, P., Galbacs, G., Moens, L., Dams, R. and Kurfuerst, U., Spectrochim. Acta Part-B, 1993, 48, 671. Ma, R., Van-Mol, W. and Adams, F., Anal. Chim. Acta, 1994, 285, 33. Maquieira, A, EImahadi, H. A. M. and Puchades, R., Anal. Chem., ! 994, 66, 3632. De la Guardia, M., Carbonell, V., Morales-Rubio, A. and Salvador, A., Talanta, 1993, 40, 1609. Elmahadi, H. A. M. and Greenway, G. M., ,I. Anal. At. Spectrom., 1994, 9, 547.

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215 Carbonell, V., De la Guardia, M., Salvador, A., Burguera, J. L. and Burguera, M., Microchem. J., 1989, 40, 233. 216 Martinez-Avila, R., Carbonell, V., De-la-Guardia, M. and Salvador, A., J. Assoc. Off. Anal. Chem., 1990, 73, 389. 217 Ma, Y. P, Hart, Y. Q, and Ariguli, R., Fenxi-Huaxue, 1994, 22, 586. 218 Bao, C., Sun, Q. Z., Zhang, K. and Fan, Q. Z., Fenxi-Huaxue, 1993, 21, 1363. 219 Yu, A. M., Zhang, H. Q., Jin, Q. H. and Liu, X. J., Fenxi-Huaxue, 1993, 21, 1320. 220 Sooksamiti, P., Geckeis, H. and Grudpan, K., Analyst, 1996, 121, 1413. 221 Haug, H. O., Spectrochim. Acta Part B, 1996, 51, 1425. 222 Vald6s-Hevia, M. C., Fernandez de la Campa, M. R. and Sanz-Medel, A., Anal. Chim. Acta, 1995, 309, 369. 223 Valdes Hevia y Temprano, M. C., Fernandez de la Campa, M. R. and Sanz-Medel, A., J. Anal. At. Spectrom., 1993, 8, 82 I. 224 Lopez-Garcia, I., Arroyo-Cortez, J. and Hemandez-Cordoba, M., Talanta, 1993, 40, 1677. 225 Oguma, K., Nara, T. and Kuroda, R., Bunseki-Kagaku, 1986, 35, 690. 226 Kingston, K. J. and McIntosh, S. A., At. Spectrosc., 1995, 16, 115. 227 Zhang, S., Fang, Z. and Sun, J., Guangpuxue-Yu-Guangpu-Fenxi, 1987, 7, 57. 228 EIsholz, O., Fresenius J. Anal. Chem., 1996, 355, 295. 229 Rokkjaer, I., Hoyer, B. and Jensen, N., Talanta, 1993, 40, 729. 230 Morales-Rubio, A., Mena, M. L. and McLeod, C. W., Anal. Chim. Acta, 1995, 308, 364. 231 Lamble, K. J. and Hill, S. J., J. Anal. At. Spectrom., 1996, l 1, 1099. 232 Ma, Y. E, Gulikezi and Fang, X. H., Guangpuxue-Yu-Guangpu-Fenxi, 1993, 13, 107. 233 Anderson, K. A., Isaacs, B., Tracy, M. and Moeller, G., J. AOAC-Int., 1994, 77, 473. 234 Emteborg, H., Bj6rklund, E., Odman, E, Karlsson, L., Mathiasson, L., Frech, W. and Baxter, D. C., Analyst, 1996, 121, 19. 235 Mendes, E C. S, Santelli, R. E, Gallego, M. and Valcarcel, M., J. Anal. At. Spectrom., 1994, 9, 663. 236 Velinsky, D. J. and Cutter, G. A., Anal. Chim. Acta, 1990, 235, 419. 237 Ng, K. C., Xu, X. and Brechmann, M. J., Spectrosc. Lett., 1989, 22, 1251. 238 Zhang, L., McIntosh, S., Carnrick, G. R. and Slavin, W., Spectrochim. Acta Part-B, 1992, 47B, 701. 239 Mclntosh, S., Li, Z., Carnrick, G. R. and Slavin, W., Spectrochim. Acta Part-B, 1992, 47B, 897. 240 Cai, Y., Rapsomanikis, S. and Andreae, M. O., J. Anal. At. Spectrom., 1993, 8, 119. 241 Astruc, A, Dauchy, X., Pannier, E, Potin-Gautier, M. and Astruc, M., Analusis, 1994, 22, 257. 242 Dirkx, W. M. R., de la Calle, M. B., Ceulemans, M. and Adams, F. C., J. Chromatogr. A, 1994, 683, 5 I. 243 Chau, Y. K., Yang, E and Maguire, R. J., Anal. Chim. Acta, 1996, 320, 165. 244 Silva, A. E, Bajic, S. J. and Zamzow, D., Anal. Chem., 1993, 65, 3174. 245 Zhong, Z. Y. and Wan, Y. N.,At. Spectrosc., 1995, 16, 243. 246 Grebneva, O. N., Kuz'min, N. M., Tsysin, G. I. and Zolotov, Y. A., Spectrochim. Acta Part B, 1996, 51, 1417. 247 Koyama, M., Chaki, S., Yamamoto, M. and Kumamaru, T., Bunseki,Kagaku, 1991, 40, 149. 248 Nerin, C., Zufiaurre, R. and Cacho, J., Mikrochim.Acta, 1992, 108, 241. 249 Foster, R. D. and Howe, A. M., J. Anal. At. Spectrom., 1994, 9, 273. 250 Lintschinger, J., Kalcher, K., G6ssler, W., K61bl, G. and Novic, M., Fresenius'J. Anal. Chem., 1995, 351, 604. 251 Tavares, H. M. E, Vasconcelos, M. T. S. D., Machado, A. A. S. C. and Silva, E A. P., Analyst (London), 1993, 118, 1433. 252 British-Standards-Institution, British Standard, BS 1747: Part 13:1994 [ISO 9855: 1993], 15 Feb 1994, p12. 253 Friese, K. H., Roschig, M., Wuenscher, G. and Matschiner, H., Fresenius '-J. Anal. Chem., 1990, 337, 860. 254 Snell, J. E, Frech, W. and Thomassen, Y.,Analyst, 1996, 121, 1055. 255 Swan, H. B. and Ivey, J. E, J. High Resolut. Chromatogr., 1994, 17, 814.

biological analysis

11.1 Combining flow systems with atomic spectrometry for routine clinical and biological analysis Total element information, as directly provided by atomic techniques, is today of paramount importance in clinical and biological analysis. ~-4 The determination of electrolyte elements and of those considered essential trace and toxic metals is often imperative to help the clinician in the diagnosis of diseases, to detect intoxication cases and also in therapeutic treatment control (Figure 11.1). At present, the analysis of metals in the clinical and biological fields is predominantly carried out by atomic spectroscopic methods. Fortunately, appropriate atomic spectrometric techniques are available nowadays in the laboratory for the analysis of body fluids and biological tissues to help the clinician to know metal concentration levels, going from thousands of mg 1-~ (ppm) (as is the case for sodium in blood serum), to a few txg 1(ppb) (e.g. aluminium in renal failure patients). In this context, the FAAS remains the technique of choice for routine analyses of the major metals calcium and magnesium and for the trace elements zinc, iron and copper while FAES is used for sodium and potassium. For the analysis of the metals present at lower concentrations, ETAAS can still be considered as the technique more frequently chosen although it is not free from shortcomings such as lengthy analysis time, 342

Applications of flow analysis with atomic spectrometric detectors

343

~ i e m e n t ~

m,

*Diagnosis and treatment progress of defficiency-based diseases. *Detrimental effects and mechanisms of toxic metals. *Therapeutic metal-drug monitoring. *Biochemical trace-elements research.

Figure 11.1 Information on metals required in the clinical and biological fields and areas of impact of such information. single-element analysis capability and matrix interferences. The higher versatility of the ICP--OES, allowing for simultaneous multielemental analysis, has not eventually beaten the ETAAS in the daily round, probably because individual elements are usually requested in routine analysis and also because the ICP--OES offers poorer detection limits with higher running costs, as compared to ETAAS. However, the ICP-OES remains as a useful tool for research development especially in the area of toxic metals. Finally, the advent of the ICP-MS, offering extremely low detection limits and both, multielemental and isotopic information, is opening new avenues for the research addressed to unveil biochemical processes, to establish metabolic models and to investigate heavy metal poisoning)" 6 Furthermore, in the last years it has been recognized that the identification, confirmation and determination of those molecules or species formed by trace and ultratrace metals in biological samples can be essential to understanding important biochemical problems and clinical treatments (Figure 11.1). Having in mind the limitations of atomic methods to distinguish individual physico-chemical forms of the metal (speciation), different analytical strategies have been successfully assayed, broadly based on applying differentprinciple-based separations coupled to a final metal-selective atomic detection step. 7-~~ Considering that Chapter 13 of this book is specifically addressed to trace metal speciation in biological systems, the coverage of the topic here has been purposely reduced to a few

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examples. Also, the possible use of chromatographic techniques with atomic detectors, being the most powerful hybrid methods for metal speciation, will be dealt with in Chapter 13 and not be discussed in this chapter. The coupling of continuous flow non-chromatographic systems to atomic detectors for clinical analysis has been reviewed earlier. ~~-~3The advantages brought about by using this sample introduction tool for atomic spectroscopic detectors shine considerably when considering both routine clinical analysis and research addressed to tackle the elemental analysis of particular biological materials. Among these advantages, besides the high sample throughput and ease of automation of crucial interest for routine analysis, it is important to emphasize the following aspects for biomedical work: (a)

(b)

(c)

(d)

Decrease in the required sample volume (especially when using FI for sample introduction). This feature is critical for most of the analysis carried out in clinical and biological chemistry and in particular in the analysis of body fluids such as saliva ~4 or cerebrospinal fluid ~5 and tissues like nails and hair 16'~7 for which the amount of sample imposes serious restrictions to the selected method of analysis. Less contamination risks (of special interest when very low concentrations are being analysed). The use of highly sensitive methods to analyse elements present at very low concentrations is always accompanied by serious difficulties to obtain reliable results. Aluminium determination in serum is a typical example: it is well known that accumulation of AI in the body plays an important role in some clinical disorders identified in patients undergoing regular dialysis. As for other trace metals, it is necessary to establish "basal" serum levels for aluminium, 18that is, the values' range of the element's normal concentrations in healthy individuals, to diagnose deficiencies or toxic overloading, as detailed in Chapter 13. Contamination and other methodological problems in sample collection, sample storage and preparation of the sample for measurement could be the source of the formerly observed high values for basal levels. I~ Sensitivity enhancement and elimination of interferences (carried out in a faster mode using low sample volumes). As shown in Section 11.2.2 of this chapter, the use of preconcentration methods, such as those based on sorption on minicolumns, and of vapour analyte generation techniques (e.g. mercury cold vapour or volatile hydride generation) greatly benefit from the use of continuous flow manifolds. Indirect analysis of organic pharmaceutical products is straightforward. It is possible to dramatically increase the analysis reproducibility, as compared with the batch analogues, and in some cases such analyses can only be performed when using flow systems (see Section 11.4.5).

As shown in Figure 11.2, the bunch of improved sample manipulation processes

Applications of flow analysis with atomic spectrometric detectors

345

covered via flow operation procedures, eventually coupled to atomic detection in the clinical and biological fields, extends over a wide range. These processes can be classified according to a hierarchy going from simple operations such as on-line sample dilution or standard additions, 19 to preconcentration/separation processes, 2~ sample digestion 21 and even to in vivo sample uptake. 22Although a higher degree of flow manifold complexity is expected on descending the pyramid of Figure 11.2, this approach allows the opening of new paths both in terms of automation to facilitate routine analysis and of availability of new analytical tools to solve present clinical and biological challenges. Finally, for the organization of this chapter, three consecutive main sections have been considered:

~[MPLE OPERATIONS Dilution. Standardadditions. -

-

J

*Sensitivity enhancement: -

-

preeoneentration. vapour analyte generation.

* Interferences removal.

Sample digestion: -

-

Addition of oxidation reagents. Microwave heating.

In-vivo uptake of specimens from patient and on-line pretreatment of the sampl Figure 11.2 Schematic diagram of sample preparation operations for clinical and biological analysis which can be carried out using flow systems and atomic spectrometric detectors.

R. Pereiro and A. Sanz-Medel

346

9 the first refers to flow manifolds proposed. Thus, here special attention is paid to the latest developments/new strategies allowing the extension of the practical scope of flow analysis-atomic detectors for clinical and biological analysis 9 the second section will be devoted to the use of the ICP-MS for biological analysis, using flow manifolds as sample introduction systems 9 the last section remarks on the applied nature of this chapter. Specific applications are described according to the sample matrix: (a) body fluids, including blood, serum, plasma and urine and a second category to describe applications to not so commonly analysed fluids (e.g. saliva or cerebrospinal fluid for which the sample volume could impose serious restrictions to the technique of choice); (b) tissues and biological materials: two consecutive sections of this chapter are addressed to review first the analysis of soft tissues and then the analysis of hair and nails; (c) pharmaceutical preparations, including solutions for hemodialysis.

11.2 Flow manifolds for clinical and biological analysis using atomic detection 11.2.1

Introduction

In recent years continuous progress has been made in the development and application of new strategies for analysis based on flow sample pretreatment and atomic detection which will allow less participation by the human operator in the analytical process. The reduction of human participation in sample handling is of particular interest in the clinical laboratory: regardless of the analytical system used, the manual handling of clinical samples such as blood is not only time consuming and prone to contamination, it is also dangerous because the analyst is at great risk of being infected by contagious diseases. These advances have reached its cutting edge in the analysis of Cu and Zn 23 by FAAS and Co by ETAAS 22 using an on-line continuous flow system directly from the patient's forearm to the detector. Figure 5.2 in Chapter 5 of this book shows a scheme of the system proposed. The blood sample is drawn and pumped directly from the vein on the forearm of a patient to an on-line sample pretreatment manifold, which includes a microwave oven, and then the sample is automatically introduced into the atomic detector. Special attention will be paid in the following to recent advances in on-line sample pretreatment processes, and particularly stressed are the topics: 9 on-line enhancement of sensitivity and interference removal 9 introduction of samples as slurries 9 flow microwave digestion procedures.

Applications of flow analysis with atomic spectrometric detectors

347

These topics have been dealt with earlier in this book from a general view; however, in this chapter their particular application to the clinical and biological field is reviewed.

11.2.2 Flow manifolds for elimination of interferences and~or enhancement of sensitivity A prerequisite for preconcentration of trace metals in biological materials is that the method should be able to tolerate complicated sample matrices and high concentrations of some commonly occurring elements. One of the more significant and recent advances in sample introduction in atomic spectrometry is related to on-line sample preconcentration/ matrix separation using a two-phase system. The use of gas-liquid systems (in particular hydride and cold vapour generation), liquid-solid methods (e.g. retention on a minicolumn, coprecipitation or anodic stripping voltammetry) and liquid-liquid extraction manifolds, coupled on-line to atomic detectors for medical and biological purposes, will be revised. As discussed by J. Dedina in Chapter 8, the formation of analyte vapours and, in particular, hydride generation (HG) and cold vapour (CV) techniques allow for great improvements in sensitivity as the vapour generated is 100% transferred into the detector as compared to the 1-5% introduced when using conventional nebulization systems of liquid samples. These techniques have greatly benefited from the use of continuous and FI systems 24 and manufacturers are selling simple and automated HG and CV flow equipment in combination with AAS or AFS for detection. The clinical and toxicological relevance of mercury and of most of the oligoelements forming hydrides alike make these techniques of particular interest here. The use of flow systems in combination with HG or CV allows for selectivity improvements possibly in the chemical vapour generation process or in the eventual detection step. For example, in the determination of lead by HGAAS it has been found that the use of the nitroso-R-salt increases noticeably the sensitivity. Unfortunately transition elements, such a copper, give rise to severe negative interference effects; however, when using a FI method the tolerance to copper was improved fourfold as compared to a batch procedure and the analysis of lead in blood, hair and soft tissues was satisfactorily carried out with this procedure. 25 The elimination of matrix interferences in the detection step is of particular interest for ICP-MS and continuous or FI systems are routinely used to overcome these problems (e.g. selenium is one of the more difficult elements to determine by ICP-MS because of isobaric interferences on several of its most abundant isotopes. Using flow HG-ICP-MS the sensitive analysis of selenium in biological matrices is now routinely carried out). 26 Flow systems also allow for straightforward on-line sample digestion prior to the vapour generation step. For example, using 0.5 ml sample loops, the analysis of total

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R. Pereiro and A. Sanz-Medel

mercury in urine was carried o u t 27 by on-line oxidation with potassium persulfate oxidizing agent in sulfuric acid media prior to the mixing with tin chloride reducing solution for CV generation (Figure 11.3). Also to decompose organomercury compounds at room temperature it has been proposed that the on-line addition of K M n O 4 to urine previously treated with bromate-bromide reagent and samples are on-line measured by FI-CVAAS by reduction to cold vapour with sodium borohydride. 28 The use of microwaves for similar purposes will be dealt with in Section 11.2.4 of this chapter. Speciation can also be undertaken using flow systems and HG or CV generation. As an example, a method has been developed for the determination of inorganic mercury and methylmercury in biological tissues using FI-CVAAS. 29 Total mercury analysis was achieved by using a flow-through photo-oxidation reactor and sodium borohydride as the reducing agent while inorganic mercury was determined using tin chloride without photooxidation. Methylmercury concentrations were determined by difference. Higher increases in sensitivity of CV and HG methods can be attained by further concentration on a solid surface of the atomic mercury or the volatile hydrides released to the gas phase. 3~ In this context, the determination of mercury with an absolute detection limit of 0.13 ng was achieved using an on-line Au-Pt grid for mercury amalgamation prior to AAS detection. 3~ Also hydride generation in a continuous mode with subsequent trapping and atomization in a coated graphite atomizer has been successfully described for

30

10

C SA ~3.7 0A

5.7

Ar Figure 11.3 Manifold design for the determination of dissolved mercury in urine. C is the aqueous carrier solution, SA is concentrated sulfuric acid, OA is 5% (m/v) potassium persulfate oxidizing agent, and RA is 10% (m/v) tin(II) chloride reducing agent in 10% (v/v) hydrochloric acid. W is waste. Numbers refer to flow rates in ml min-~. S refers to sample introduction (0.5 ml) and GLS to gas-liquid separators (reproduced from Reference 27 with permission of the American Chemical Society).

Applications of flow analysis with atomic spectrometric detectors

349

the analysis of lead, 32 selenium, 33 tellurium 33 and cadmium 34 in different types of biological samples. Preconcentration by retention in a solid phase (Chapter 6) has grown dramatically in the last few years as a consequence of the easiness of on-line coupling of atomic detectors with flow systems; cheap and simple minicolumns containing the solid phase and allowing great preconcentration factors are responsible for this success. Elimination of interferences in these columns can be achieved either by retention of the analyte, while the matrix components are not, or by matrix retention. Table 11.1 collects, following a chronological order, examples of analyte preconcentration and/or matrix separation systems using minicolumns coupled on-line to atomic detectors, proposed for the analysis of body fluids and biological tissues. 35--53 Other examples employing on-line minicolumns for the analysis of pharmaceutical products or hemodialysis fluids are presented in Section 11.4.5. of this chapter. The methods involving preconcentration, compiled in Table 11.1, can be divided in two general groups: (a)

free analyte ions are collected by adequate chelating reagents immobilised on a solid phase35-37, 39-42,44.47

(b)

metal chelates or ion pairs are previously formed in a flow and then sorbed on solid non-ionic sorbents. 38'43, 45, 46, 48-51

Besides, aiming most to the removal of anionic interferences than to the preconcentration of the analytes, the use of minicolumns with ion-exchange resins to retain anions, which are connected on-line to the sensitive ICP-MS has been described for the determination of metals in biological samples. 52'53 Coprecipitation is also an important and effective approach for improving the sensitivity and specificity of instrumental analytical methods used for the determination of trace elements (see Chapter 6). On-line coprecipitation using a filterless flow injection preconcentration system and either FAAS 54'55 or ETAAS 56 detection has been successfully demonstrated for the analysis of Pb, 54 Co, 55 Cd, 55'56 and Ni 55'56 in blood, urine and tissues. For example, in the case of cadmium and nickel the sample digests were coprecipitated with the iron(II)-hexamethylenedithiocarbamate complex at the walls of a knotted reactor. The precipitate was dissolved in 60 txl of isobutyl methylketone (IBMK) and detection limits of 0.003 ppb for Cd and 0.002 ppb for Ni were observed with recoveries of Cd and Ni in the blood digests of 103 and 106% respectively. The use of a flow-cell based on the principle of anodic stripping voltammetry (ASV), on-line with an ICP-OES or ICP-MS, has been investigated to allow improved analysis of selected species (those which can be deposited at a working electrode and later released for detection to the ICP). This cell eliminates sample matrix components that are not electroactive and do not deposit during passage of the sample through the cell and is also

ta~

Table 11 1 Selected examples o f on-line preconcentration/separation methods using minicolumns for the analysis o f body fluids and biological tissues 9

Analyte

Sample

Detector

AI

Blood serum

FAAS

Cd, Cu, Mn Ni, Zn Cd V, Cr, Ni, Co Cu, Pt, Mo, Hg, Bi Cd, Co, Cu, Pb Cu, Fe, Zn, Cr Ni, Mn, V Cu, Mn, AI, Fe Cd Zn, Cu, Ni, Mn Fe, V, Cr Pd, Pt, Rh

Biological tissues

ETAAS and ICP-OES FAAS ICP-MS

Urine Urine Urine Hair and biological tissues Blood serum Hair and biological tissues Biological materials

ICP-OES ICP--OES ICP-OES ICP--OES

V, Mn, Cu, Zn, Cd, Pb Cd, Cu, Pb

Biological tissues

ETAAS and ICP-OES ICP-MS

Biological tissues

ETAAS

Pb

Biological tissues

FAAS

As, Cr, Se, V Cd

Biological Tissues Hair

ICP-MS FAAS

Co

Hair and biological tissues Biological tissues

FAAS

Pb Pb Ge, As, Se B, AI, Cr, Mn Fe, Cu, Zn, Pb, S

Hair and biological tissues Urine and blood Plant tissues

FAAS FAAS ICP-MS ICP-MS

Reagent/Eluent Chromeazurol S immobilized on an exchange resin. Elution with 2 M HCI. Minicolumns packed either with Chelex 100 or oxine-cellulose. Elution with 2 M HNO3. Minicolumn packed with alumina. Eiution with 8% v/v HNO3 Analytes are complexed with bis(carboxymethyl)dithiocarbamate and adsorbed on a non-ionic resin. Elution with 0.1 M NH4OH Iminodiacetic acid/ethylcellulose minicolumn. Elution with 2 M HNO3. Minicolumn of EDTrA-cellulose. Elution with a mixed solution containing 2 M HNO3 +2 M HCI. 8-hydroxyquinoline-5-sulphonic acid immobilized on active carbon-silica gel. Elution with 2 M HCI. 8-hydroxyquinoline(oxine)-cellulose Elution with I M HNO3 or 0.5 M HCI. Formation of bis(carboxylmetyl)dithiocarbamate chelates and retention on XAD4. Elution with diluted NH4OH. A resin containing iminodiacetate functional groups is used. Analytes are eluted with 1 M nitric acid. On-line reaction with ammonium diethyldithiophosphate and retention on C~8. Elution with ethanol. On-line formation of the dithizonate complex and retention on activated carbon. Elution with IBMK. Minicolumn packed with alumina. Elution with 1 M HNO3. The on-line formed chelate between Cd and diethyldithiocarbamate is sorbed on the inner walls of a PTFE knotted reactor. Elution with IBMK. Ion pair adsorption of the cobalt-nitroso-R salt complex in the presence of tetrabutylammonium bromide on C~8. Elution with ethanol. Formation of diethyidithiocarbamate chelate of Pb in the flow system and retention in C~8. Elution with IBMK. Ion-pair (iodo-plumbate-(tertiary-amine)) sorbent extraction on CI8. Elution with ethanol. Anion-exchange resin for removal of interferences (CI, S, etc.). Removal of anions with AG I-X8 resin. Sulphate was eluted afterwards and on-line determined.

t~

Ref. 35 36 37 38

40 41 42

.7:1 t~ t~ ~" o

43 44 N ~ 45 r 46 47 48 49 50 51 52 53

Applications of flow analysis with atomic spectrometric detectors

35 1

suitable for signal enhancement of such analytes as they may be preconcentrated at the electrode (e.g. the analysis of copper and cadmium in urine has been successfully carried out using this methodology and ICP-MS detection). 57 Moreover, as As(Ill) and Se(IV) are electrochemically active in most electrolytes, while As(V) and Se(VI) are not, on-line ASV was used to achieve valence speciation of both analytes in urine with ICP-MS detection. 58 On-line liquid-liquid extraction procedures have also been applied for the determination of metals in biological matrices by atomic methods. However they are much less used than the solid-liquid altematives probably due to two limitations frequently found in these systems: high preconconcentration factors cannot be easily obtained and, secondly, the online set-up is more prone to operational problems, as pointed out in Chapter 7. Examples of applications in biological materials of this type of systems include the determination of Hg 59 and other heavy metals 6~by ICP-OES and of Ni by ETAAS. 61 Occasionally, the complexity of the sample matrix or the sensitivity requirements make it necessary to use more than one separation/preconcentration method as carried out in tandem on-line continuous separation/preconcentration techniques, which have proved to be particularly useful for the determination, and even speciation, of toxic elements such as As, Sb or Hg (see Chapter 9).

11.2.3 Slurry sampling Solid samples can be directly introduced into flow systems as slurries. Among the advantages of direct solid analysis over conventional sample dissolution procedures, such as acid digestion or alkaline fusions, it is important to highlight the speed of the analysis, the lower cost of the determination, reduced risk of sample contamination and the decreased danger of hazards associated with the use of acids and fusions. Slurry techniques in atomic spectrometry are midway between solid and solution analysis and can be particularly useful for total trace element analysis of biological tissues. The preparation of the slurries consists of adding an aqueous solution to the solid material previously grounded and sieved (when necessary), weighed and placed into a flask; the slurry or suspension is formed there and should be stable during the time required for analysis. Approaches such as addition of stabilizing agents 62 like surfactants or glycerol, magnetic stirring and vortex mixing, 63 ultrasonic agitation, 64 or gas mixing 65 of the slurries have been described in order to maintain a stable homogeneous suspension of the solid sample into the solution. 66 Once the slurry is formed and stabilized, a volume is introduced into the flow system and on-line mixed downstream with different solutions such as the carrier, digesting reagents, chemical modifiers, standards, e t c . 67 Unlike nebulization techniques such as FAAS or ICP--OES, ETAAS does not suffer significantly from particle size effects because it offers longer residence times in the

352

R. Pereiro and A. Sanz-Medel

atomiser. As a result, samples may be placed directly in the graphite fumace and, in most instances can be completely atomised. The coupling of slurry sample introduction systems to ETAAS has proved to be most efficient and commercial accessories consisting of ultrasonic, pneumatic movable, slurry autosamplers 68 are available. Examples of successful applications of slurry techniques to ETAAS include the analysis of different sample types such as human hair, 69 bovine liver, 63'7~ mussels, 69 oysters 65 and vegetables.65, 69--72 Slurry sample introduction into different atomisers for biological analysis using flow systems and including an on-line microwave digestion step is a modem and interesting trend which will be dealt with in next section. 11.2.4

Flow microwave digestion systems

The development of flow microwave systems for sample digestion connected on-line to the atomic detector represents a further step towards automation. On-line microwave digestion in a flow was a priori expected to be associated with serious problems derived from the vigorous conditions (e.g. excess of acids), high temperature and pressure, long digestion times, etc., which should probably be required to achieve complete decomposition. Besides, the evolution of gases during digestion is an additional problem to be faced. However, these problems seem to be creatively solved as more and more interesting applications have appeared during the last decade, as shown in Chapter 5 of this book, specifically oriented to the details of the use of on-line microwave dissolution/ decomposition systems. Table 11.2 collects most applications described so far for on-line flow microwave digestion systems for the analysis of body fluids and tissues following a chronological order.E1-23, 73--90 For the analysis of some viscous body fluids, such as whole blood, the dilution of the sample with Triton X-100 is recommended to avoid the clogging of the tubing by the sample. The analysis of tissues generally involve the introduction of the weighted solid in the form of a slurry. These slurries are usually prepared by pretreatment with an acid or a mixture of reagents and final dispersion by stirring in a vessel or an ultrasonic device. Then, the slurries are fed by a peristaltic pump into the flow system where a coil, introduced in a microwave device (both continuous and stopped-flow digestion systems have been proposed), helps with sample digestion in the flow. The fumes produced during acid decomposition of organic materials can be removed either by using an ice bath, or a back pressure regulator, or a diffusion cell connected to a vacuum pump. The output solution is continuously fed into detectors such as F A A S , 21'23'73'77'78'82 HGAAS o r C V A A S , 74--76' 79, 81, 83, 85 ICP_OES84 or ETAAS 22'80,86.87 for analysis. Just as an illustrative example, Figure 5.1 of Burguera's chapter shows the instrumental set-up for an on-line flow injection-microwave digestion system described for the analysis

Table 11.2

Examples o f on-line digestion procedures with microwave heating for body fluids and tissues

Analyte

Sample

Detection technique

Cu, Fe, Zn

Whole blood

FAAS

Cd, Zn

Biological tissues

FAAS

As, Bi, Hg, Pb Sb, Se, Sn, Te As, Bi, Hg, Pb, Sn Hg Cu, Mn Ca, Fe, Mg, Zn Hg Pb

Urine, water

HGAAS CVAAS HGAAS CVAAS CVAAS FAAS FAAS CVAAS ETAAS

Urine, water

As

Urine Plants Biological tissues Whole blood Whole blood and biological tissues Whole blood

HGAAS

Zn, Cu

Whole blood

FAAS

Cu, Mn

Plant tissues

FAAS

As

Urine

CVAAS

Ca, Cu, Fe, Mg, Mn, Zn As Fe, Zn

Whole blood, urine, milk Urine Adipose tissue

ICP-OES HGAAS ETAAS

Co AI Se

Whole blood Biological tissues Biological tissues

ETAAS ETAAS ETAAS

Hg

Biological tissues

CVAFS

Comments Samples diluted in Triton X-100 (anticlogging agent) and solution reagent (diluted HCI + HNO3) are injected in parallel into the flow system. Batch digested samples are pumped to a closed flow-circuit from which aliquots are injected for detection. Samples and reagents are pre-mixed in the autosampler vessels to avoid the need of acid-resistant valves. Optimization of the digestion mixture was needed for each analyte. Bromate--bromide and peroxodisulfate were the best oxidation systems. Recoveries of inorganic and four organomercury species were studied. Slurries containing H202"1- HNO3 are introduced Samples were introduced as slurries in 5% v/v HNO 3. A digestion coil in a knitted form was used to reduce the dispersion. Samples dispersed in 0.4% v/v Triton X-100 are mixed with a HCI-HNO3 plug prior the microwave oven digestion. Samples acid digestion and pre-reduction are carried out in a microwave oven digestion. In vivo sample uptake. Samples were drawn directly from a patient's forearm to a timed injector. EDTA was used as anticoagulant. Sample slurries in concentrated nitric acid are mixed, by merging zones with 30% H202 prior to a microwave oven treatment. Decomposition of all organoasenicals to arsenate by microwave digestion Stopped-flow digestion system. Different chamber designs are assayed. Samples are on-line oxidized with persulphate in basic media. H2SO4+ HNO 3 was used for mineralization. An additional flow of Triton X-100 was used to avoid deposition of solids on the wall tubing. In vivo sample uptake from the vein of the forearm to a timed injector. Sample slurries in 0.2% v/v HNO 3 merged with 3M HNO 3 for digestion. Sample slurries prepared in 0.2% v/v HNO3 are digested with 6M HNO 3. Merging zones technique is applied. Slurried samples are digested with a potassium bromide-bromate solution in acid medium.

Ref. 21 73 74 75 76 77 78 79 80

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81 23

O

82 83 O

84 85 86 22 87 88 89

t~ t~ O

354

R. Pereiro and A. Sanz-Medel

of animal and botanical tissues by FAAS. TM Typically, samples are prepared as slurries in 5% v/v nitric acid and sample volumes of 1 ml are introduced into the system. A coil of 20 m length and 0.8 mm i.d. of PTFE tubing was used as digestion loop in the microwave oven and the flow rate was around 5 ml min -I. Various samples were examined and elemental recoveries for Ca, Fe, Mg and Zn were typically found to be in the range 94-107% with precision better than + 4.5% relative standard deviation. The throughput of samples in the system was found to be 1-2 min per sample. Before concluding this section it is important to point out that, although notorious progress has taken place in this field of on-line flow microwave digestion of biological material (allowing to envisage their great throughput potential for easy-to-handle samples), the available instrumentation so far, even commercial models, does not yet provide reliable performance. In a routine basis this technique needs further instrumental developments to solve frequent breakdowns and malfunctions which, however, are expected to be overcome in the near future.

11.3 A powerful novel detector for clinical and biological analysis in flow systems: ICP-MS The growth of interest experienced by ICP-MS as an analytical technique to solve clinical and biological problems has been spectacular in the 90s. As can be seen from Figure 11.4, the ICP-MS has become the technique more frequently used after the ETAAS in the scientific papers related to the analysis of biological materials which were published in recent years. The high sensitivity provided by this multielemental technique offers special attraction for its use in the analysis of low level difficult elements, like the uranium group metals, and also as detector in speciation studies. Besides, the ability of the ICP-MS to measure accurately different isotopes opens new avenues in nutrition, e.g. to investigate the uptake of trace elements or their target organs and for identification of the source of environmental exposure to toxic elements. However, the technique is not free from serious problems, apart from its well known high price of acquisition and maintenance. In the analysis of biological material particular operational shortcomings occur such as spectral overlaps from polyatomic ions (formed from biological matrix elements like Na, Ca and C1, unavoidable in body fluids and biological tissues), suppression or occasional enhancement of the analyte ion signal with high levels of salts or heavy matrix ions, carbon deposits, deposition of other materials on the sampling orifice at the interface, influence of sample viscosity, contamination problems, etc. Several strategies for the presentation of the sample to the plasma have been adapted to minimise problems caused by matrix effects, including matrix matching, standard addition

Applications of flow analysis with atomic spectrometric detectors

355

Figure 11.4 Trends in the development of atomic spectroscopic techniques for the analysis of biological materials as number of publications collected from the Atomic Spectrometry Updates. techniques, internal standardization, isotope dilution and eventual physical separation of the analyte from the matrix. Of course, ICP-MS has also benefited from the use of on-line flow strategies to alleviate matrix effects in clinical and biological analysis. 9~176 The introduction of 100 Ixl sample volumes proved to be very efficient to avoid problems of clogging the interface from the high salt matrix content in organomercury extracts, where Hg was determined by isotopic-dilution ICP-MS. 9~FI also proved it could enable the successful analysis of solutions with high salt content, viscosity and high acid strength without blockage or distortion of the sample interface in the determination of zinc isotope ratios in microliter samples of blood. 9~ Besides, the rapid sample throughput possible with FI allows a notorious increase in the sampling rate compared with conventional nebulization techniques of particular importance for the cost-effective operation of ICP-MS. On-line sample treatment methods of the types described in Part II for matrix removal, analyte preconcentration and even element speciation, including the use of minicolumns,a4, 46, 52, 53, 96, 97 anodic stripping voltammetry 57'58,99 and hydride generation 34' 10~-~05 techniques have also been successfully assayed using ICP-MS detection and representative examples are collected in Table 11.3.

Table 11.3

o~

Examples o f clinical and biological analysis using flow systems and I C P - M S detection

Elements/ species analysed Hg (organomercury) Zn isotope ratios

Sample

V, Cr, Ni, Co Cu, Pt, Mo, Hg, Bi V, Mn, Cu Zn, Cd, Pb As, Cr, Se V A1

Ref. 90

Hair

Organomercury was extracted with toluene and back extracted into a cysteine acetate solution. Problems of clogging the interface were avoided with FlA. Problems of high salt content, viscosity and high acid concentration are alleviated by FI sample introduction. Memory and matrix effects were decreased by adding a surfactant (Triton X-100) to the carrier stream. Aliquots (50 ILl) of hair dissolved in nitric acid were introduced in the FIA system.

Urine Urine

A simple and rapid flow-injection method is proposed. On-line standard additions method.

94 19

O

Urine

The on-line sample dilution and on-line addition of internal standard and calibrant solution is investigated. The reagent bis(carboxymethyl)dithiocarbamate is used to complex the analytes. The chelates are adsorbed on acidic conditions and eluted in basic conditions. A resin containing iminodiacetate functional groups is used to preconcentrate the analyte. Analytes are afterwards eluted with 3 M nitric acid. The pH of the sample is adjusted to 9 and the solution passed through an activated alumina column. The retained analytes are eluted using 1 M nitric acid. AI is retained at pH 7.0 on Chromotrope 2B immobilized on AG I-X8 ion exchange resin. 1 M hydrochloric acid was selected for elution. Anion-exchange resin for removal of interferences (CI, S, etc) On-line removal of anions with AG I-X8 resin. Sulphate was eluted afterwards with 1 M nitric acid and on-line determined. Vesicular hydride generation - in situ trapping. Isotopic dilution On-line anodic stripping voltammetry (ASV) is used to eliminate the effect of matrix components which are not electroactive and to preconcentrate the analytes. On-line ASV for speciation of As and Se oxidation states. On-line ASV for Cr(VI) and V(V) speciation. Special emphasis is given to isolation from matrix interferences An automated on-line FI-ASV system combined with isotopic dilution is used

95

o

Biological tissues Blood Urine

AI, As, Cd, Hg Pb, Se, TI, Zn Cu, Cd, Pb Be, Mn, Ni, Cu, Se, Cd, Pt, Hg, Pb Pb

Comments

92 93

~..,t ~

Urine Biological tissues Biological tissues Hemodialysis fluids

Ge, As, Se B, A1, Cr, Mn Fe, Cu, Zn, Pb, S Cd Mo Cu, Cd

Urine Biological tissues Urine

As(III), Se(IV) Cr(VI), V(V)

Urine Urine

Cu, Cd, Pb

Urine and Biological tissues

Urine and Blood Plant tissues

96 44

rao

46

N

97 o

52 53 34 98 57 58 99 100

>. m-,o

,.J~

O

O

Table 11.3

Elements/ species analysed

Sample

Se

Biological tissues Blood, serum, biological tissues Blood serum

As

Urine

Se, As

Biological tissues

As

Urine

Se

Se

:m

Continued

O

Comments Continuous flow hydride generation (HG). HG is compared to pneumatic nebulization. Continuous flow HG system. Special care was taken to minimize interferences from HCI and maximizing analytical sensitivity. Continuous flow HG system. Special attention is paid to the choice of the digestion procedure for organic selenium compounds. FI-HG system. The use of L-cysteine as pre-reductant in mild nitric conditions is investigated to avoid the use of hydrochloric acid. Continuous flow HG system. Evaluation of sample digestion, chemical and isobaric interferences is carried out. Graphite furnace hydride preconcentration and subsequent detection by ICP-MS.

Ref. 101

~.,..

,.,., o

102

::r O

103

i,,,,,.~

104 105

O

~..i~

O O'J

t~ "..d

358

R. Pereiro and A. Sanz-Medel

11.4 Selected applications to clinical and biological analytical problems The use of FI-AAS for clinical applications including the analysis of blood, serum, tissues and pharmaceuticals has been thoroughly reviewed in 1995 by Fang. ~3The scope of these applications is extended and widened here to other atomic detectors, particularly spectrochemical plasmas. 11.4.1

Blood, serum, plasma and urine in humans

The majority of measurements carried out in the clinical laboratory are performed in blood specimens (whole blood, serum or plasma). The analysis of urine is also important since urine samples are readily obtained non-invasively, providing information about exposure to many chemical species and different body's metabolic malfunctions. Also, urine analysis is a convenient approach to assess some well known occupational exposure problems. Representative examples of the determination of electrolyte, essential trace, and toxic metals as well as anions and drugs in such real samples is reviewed below. 11.4.1.1

"Electrolyte" metals

Sodium, potassium, calcium and magnesium are essential macroelements in living tissues. Sodium and potassium are among the most frequently requested tests in routine clinical analysis. An averaged-sized district general hospital in the UK serving a population of approximately 200 000 people would carry out 150-200 analyses for sodium and potassium each working day. ~~ Calcium and magnesium have also been measured in body fluids for many years. Calcium performs essential functions in bone structure, muscle contraction, membrane permeability, blood clotting and enzyme catalysis while magnesium is an activator of many enzyme systems, especially those which utilize ATP; Mg metabolism is closely related to that of other elements such as Ca, Na and K and is essential for neuromuscular action and muscle contraction. The analysis of these four metals by atomic techniques is mostly carried out by FAAS and FAES. However, these analyses are prone to chemical interferences and besides, important dilutions of the sample are usually needed in order to get the analyte concentration within the linear range of the method. Burguera et al. circumvented such problems by introducing small volumes of the undiluted sample (5 or 20 Izl) in a FI mode, to analyse sodium, ~~ potassium ~~ and calcium. ~~ The system was pumpless being the flow rate determined by the aspiration of the nebulizer. To avoid clogging of the rotary valves commonly used for FI sample introduction, the serum samples were injected with a Hamilton microsyringe. A similar approach, but using a pumped system, was proposed

Applications of flow analysis with atomic spectrometric detectors

359

by Rocks et al. ~09 and better reproducibility was observed, as compared to the pumpless counterpart. A variable volume injector for the FI-FAAS determination of electrolytes in serum was also proposed by Burguera et al.. ~~ The determination of sodium and potassium as well as lithium in a single run by F I FAES has been described using a sequential detector. ~ In this case, the injected sample was split into three coils of different lengths, thus allowing three sample portions to reach the detector at different times. The simultaneous analysis of Na, K, Ca and Mg in blood serum by ICP-OES using the FI introduction mode was reported. ~2'~3 It is noteworthy that the multielemental capabilities of the ICP-OES are exploited, as expected, for the simultaneous analysis of the four electrolyte ions plus essential trace elements such as copper, iron or zinc. 11.4.1.2

E s s e n t i a l trace metals

Among the essential trace elements, the determinations of iron, copper, and zinc by continuous flow systems using atomic detectors are commonplace and will be dealt with in the following. Iron is involved in many biochemical functions including oxygen transport from the lungs to tissues by hemoglobin, oxygen storage in myoglobing, while divalent Fe is a cofactor in heme enzymes; iron deficiency is characterized by anaemia, stunted growth, fatigue and lowered resistance to infection. Copper is essential to iron utilization and functions in enzymes for energy production, connective tissue formation and pigmentation and its deficiency results in anaemia, abnormal bones, poor growth and cardiovascular failure. On the other hand, zinc is, after K, Ca, and Mg, the metal with the highest intracellular concentration and skin disorders and skeletal defects are among the manifestations of Zn deficiency. ~'2 A simple procedure for the determination of zinc in human serum by FI-FAAS with a fully automated, microcomputer controlled system, has been developed using a pumpless system (the negative pressure created by the nebuliser is used for aspiration of the sample injected through a three-way valve). The limit of detection reported was 0.14 ppm with a precision around 5%. TM When using multielement detectors such as the ICP-OES, most frequently the simultaneous determination of those three essential elements in human serum by FI coupled to ICP-OES is accomplished. 1~2'~3 Matrix interferences were detected injecting undiluted serum, but this effect was minimized at low injection volumes and/or by using a relatively high radiofrequency power. 112 On-line digestion procedures using microwave heating were proposed for the determination of those three trace elements in whole blood and urine using FAAS 2~'23 or

360

R. Pereiro and A. Sanz-Medel

ICP--OES 84 detection. As commented on in Section 11.2.1, the so far ultimate development of blood analysis is the system developed by Burguera et al. 23 based on the in vivo sample uptake from the forearm of the patient and the on-line mineralization in a microwave system, allowing for complete automation of the analysis. The system permits accurate determination of zinc and copper in whole blood at a sampling rate of about 30 samples per hour. 11.4.1.3

Toxic metals and metals used in therapeutic treatments

In this section, the analyses of three toxic ultratrace elements, namely mercury, arsenic, and lead is collected as representative examples. Also here are included those elements used in specific therapeutic treatments which have a narrow gap between therapeutic and toxicity levels in their blood concentration, like lithium (employed as antidepressant) and gold (gold-based drugs used in arthritis). Examples of the FI sample introduction methodology for these therapeutically used metals are the simultaneous analysis of lithium, sodium and potassium in blood serum by FAES TM and lithium, gold and trace essential elements by ICP--OES. 1~5 In any case, readers interested in a broader coverage may consult the April issues of J. Anal. At. Spectrom., where ASU reviews (Atomic Spectrometry Updates) are dedicated to clinical and biological materials since 1986. Mercury is considered a dangerous environmental poison and it is well known today that inorganic mercury is converted into the more toxic methylmercury by aquatic organisms. In most cases, the analysis of total mercury is carried out by cold vapour generation coupled to an atomic detector. The determination of total mercury using CV usually requires a previous oxidation step to decompose possible organic mercury (e.g. methylmercury), since it has been reported that sodium borohydride does not reduce all organic mercury compounds to elemental mercury. On-line procedures for the determination of total mercury in blood and urine have been described using the cold vapour methodology27.74. 76,79 for example the analysis of total Hg in urine has been described by on-line oxidation with concentrated sulfuric acid and potassium persulfate prior to the CV generation step 27 while other procedures involve the use of on-line digestion procedures with microwave heating. 74'76,79 The chemical toxicity of arsenic has been well studied because of the extensive use of arsenic compounds in the past. Generally inorganic As is more toxic than organic As and trivalent arsenic is more toxic than the pentavalent form, owing to the slower excretion and greater retention of As(Ill) in the tissues. The determination of arsenic, as As(Ill), is mostly carried out by hydride generation coupled to atomic detectors. 81'83'85'1~ Total content of arsenic in blood or urine can be determined with HG by digestion of the sample (for example by adding nitric acid and heating in a microwave oven) followed by a prereduction to form As(Ill), using agents such as L-cysteine, and final generation of the

Applications of flow analysis with atomic spectrometric detectors

361

arsine with sodium borohydride. The FI-HGAAS technique has been proposed to differentiate toxic species of As [arsenite (As(III)), arsenate (As(V)), monomethylarsonic acid (MMA) and dimethylarsinie acid (DMA)] from less toxic organoarsenic compounds usually present in seafood (e.g. arsenobetaine). The method is based on the fact that the direct analysis of urine by HGAAS gives total concentration due to As(III), As(V), MMA and DMA (because arsenobetaine is not reducible upon treatment with borohydride). However, all arsenicals are completely decomposed to inorganic arsenic via a microwave oven digestion and this can be measured by HGAAS. This method has been applied to the determination of urinary arsenic and proved useful for the assessment of occupational exposure to arsenic without interference from excess organoarsenicals due to the consumption of seafood. 83 Based on a similar principle the speciation of the toxic and non-toxic arsenic fractions in urine has been successfully carried out using a microwave oven on-line connected to the HGAAS system. 85 Lead toxicity was already recorded by ancient Greek and Arab physicians. Lead accumulates in bones and it may remain immobilized for years in the skeleton; however, any metabolic disturbance resulting in osteolysis will liberate Pb from its skeletal storage. Considering that normal concentrations of lead in biological fluids are low it is necessary to resort usually to preconcentration techniques or to sensitive detectors such as the ETAAS or the ICP-MS. The determination of lead as well as other toxic elements such as Cd and Co in urine by ICP-OES has been reported after on-line preconcentration on a minicolumn filled with iminodiacetic acid/ethylcellulose and elution with 2 M nitric acid. 39Analysis of lead in urine by FI-ICP-MS has also been successfully evaluated. 95 The flow injection system used allows on-line sample dilution and on-line addition of internal standard and calibrant solution. The relative merits of external calibration, standard additions and isotope dilution for the calibration of the transient lead signals were compared. It appeared that from the viewpoint of accuracy, precision and flexibility, the standard additions method was preferable. The analysis of lead in whole blood by on-line digestion with microwave heating and determination by ETAAS has been also reported. 8~ 1 1.4.1.4

Anions and drugs

The determination of anions and a great variety of organic compounds of clinical importance, which cannot be determined directly by atomic spectrometry, can be successfully carried out via the measurement of appropriate metal tags (indirect analysis). The use of flow systems for indirect analysis with atomic detectors was addressed by Valcarcel and Gallego in Chapter 7 and offers remarkable analytical advantages for anions and drugs, as compared to their batch operation counterparts, generally much more timeconsuming, cumbersome and imprecise.

R. Pereiro and A. Sanz-Medel

362

For instance, the determination of perchlorate in serum and urine samples has been successfully performed by continuous liquid-liquid extraction of the copper (I)/ 6-methylpicolinealdehyde azine/perchlorate ion-pair to an organic phase in a FI manifold. The measurement of the copper atomic absorption signal from the organic phase allows the determination of 0.1-5 ppm of perchlorate. The sampling frequency is about 45 h - i . !!6

The analysis of drugs such as amphetamines in serum, 117 or bromazepam ~8 and sulphonamines 119 in urine have also been carried out using flow procedures and FAAS detection. For the analysis of bromazepam and amphetamines the indirect methodology is based on the formation of either salt precipitates !~7 or ion pairs ~8 containing a metal. As before, these analyte-metal derivatives are on-line extracted into an organic solvent and the metal content, determined by FAAS, can be directly related to the concentration of the drug. For the analysis of sulphonamides ~9 the methodology proposed is based on the formation of a precipitate by injecting a cation (copper or silver ions) into a carrier containing the sample and the precipitate is retained in a stainless-steel filter. The signal of the residual metal in the stream is then measured and sulphonamides can be determined in the range 2.5--35 ppm with a sampling frequency of 100-150 h-~. Unfortunately, the literature on application of flow indirect methods for the analysis of body fluids is rather scarce. These methods have been particularly exploited for the determination of organic compounds in pharmaceutical products and a more systematic approach, addressed to gather most of the flow indirect procedures described for such samples, is given in Section 11.4.5.

11.4.2

Saliva, cerebrospinal fluid and other human fluids

The determination of metals in saliva is becoming increasingly important for the assessment of metal contamination in the environment and at the workplace. For example, a method has been developed for the determination of total mercury in saliva samples ~4 based on FI--CVAAS. The method uses a brominating reagent, followed by on-line addition of K M n O 4 at room temperature to convert organically bound mercury to inorganic mercury ions. Complete recoveries of five mercury compounds from saliva were attained under these conditions by FI--CVAAS detection. The analysis of electrolyte and essential metals in other human fluids of clinical interest such as cerebrospinal fluid, vitreous humour, etc, has been also reported using flow injection procedures. Cerebrospinal fluid (CSF) is a liquid that fills the ventricles (cavities) of the brain and the spinal cord and acts as a lubricant and a mechanical barrier against shock. During certain diseases that affect the meninges or the central nervous system, or both, the CSF may change significantly in physical characteristics, cytological

Applications of flow analysis with atomic spectrometric detectors

363

constituents and chemical content. Careful examination of the CSF in such situations may be useful in differential diagnosis. A method has been proposed for the determination of Na, K, Ca, Mg, Fe, Cu and Zn in CSF by FI-FAAS. ~5 Standards are prepared in solutions containing physiological concentrations of various chemical species and sample sizes between 5 and 100 pA depending upon the analyte were injected in the flow system. Burguera et al. also applied FI-FAAS to the determination of Cu, Fe and Zn in human vitreous humour. Standards were matched for the matrix by addition of physiological concentrations of selected constituents and for viscosity by addition of glycerol. ~2~The determination of copper, zinc and iron in parotid saliva has also been successfully accomplished by FI-FAAS. TM Standards are prepared in solutions containing physiological concentrations of Na, K, and albumin. The procedure is simple, quick, reliable and reproducible (volumes of sample between 20 and 100 ~1, depending upon the analyte, were injected). 11.4.3

Soft tissues

Traditional methods for the analysis of soft tissues require a previous step for destruction of the organic matter and dissolution of the sample. For example, the analysis of Cd, Cu, Mn and Zn in bovine liver, after decomposition of the samples in closed quartz vessels with a high pressure asher, has been reported by on-line preconcentration on a minicolumn packed with Chelex 100 or oxine bound to cellulose and ICP-OES detection. 36 Also the analysis of lead in bovine liver was reported using a flow injection system with on-line preconcentration by coprecipitation with the iron(II)-hexamethylenedithiocarbamate complex, redissolution in isobutyl methyl ketone and determination by FAAS (being the sample previously digested with nitric acid in sealed teflon digestion vessels). 54 Cadmium in bovine liver has been determined by volatile species generation on-line coupled to CVAAS following a detection procedure previously described. ~22 In this application, samples were dissolved in nitric and perchloric acid. ~23 For the simultaneous determination of As, Sb and Se in bovine liver, the samples were digested in open vessels at room temperature with concentrated nitric acid, followed by digestion with a mixture of nitric, perchloric and sulfuric acids on a hot plate and the analysis was carried out by continuous hydride generation and ICP-OES detection. ~24 Four digestion procedures for the determination of selenium in bovine liver by HGAAS have been tested: nitric acidbomb digestion, nitric-perchloric-sulphuric acid digestion, nitric acid-magnesium nitrate digestion and nitric-perchloric acid digestion. ~25All four procedures provided results in good agreement, provided that the standard additions method was used. ICP-MS was used for the determination of organomercury in marine biological materials. 9~ The organomercury was extracted as the chloride from the material with toluene and back extracted into an aqueous medium of cysteine acetate. Since the final

364

R. Pereiro and A. Sanz-Medel

extracts contained more than 4% sodium, isotopic dilution and flow injection analysis were used in order to minimise the effect of concomitant elements and to avoid clogging the interface, respectively. The analysis of previously digested fish tissue (mussels, oysters, lobster, dogfish, cod, etc.) has also been frequently reported 42'45-47,50.55.60,100, 126using different flow approaches and a variety of atomic detectors. For example, the determination of Cd, Cu and Pb in mussels and cod by sorbent extraction, using ammonium diethyldithiophosphate as complexing agent and an extraction column packed with octadecyl functional groups bonded silica, with final detection by ETAAS has been reported. 45 Cd, Co and Ni in oysters and lobster were successfully analysed by FAAS following an on-line preconcentration procedure by co-precipitation without filtration. 55The analysis of Zn, Cu, Ni, V, Cr, Fe and Mn in mussels was carried out by on-line preconcentration on 8-hydroxyquinoline-cellulose minicolumn coupled to an ICP-OES 42 and the analysis of As, Cr and Se and V in dogfish and lobster by ICP-MS was reported using an activated alumina (acidic form) column for on-line elimination of interferences and preconcentration. 46 The introduction of soft tissues as slurries into the detector is an alternative approach to avoid the tedious and cumbersome digestion step. For example, the determination of A1, Mo, Mn, Zn, Cu and Fe in bovine liver by ETAAS introducing samples as slurries has been thoroughly investigated. 68 The slurry was prepared with 10 mg of the homogeneous powdered material and 5 ml of 5% HNO3 containing 0.04% Triton X-100. Ultrasonic agitation of the slurry was used to avoid settling of the particles. Sample volumes of 20 1~1 were then injected into the graphite furnace atomiser. The calibration was carried out using aqueous standards and average accuracies in the analysis in the order of 100:t: 12% were observed. The analysis of other tissues such as oysters, mussels, etc., by slurry sampling coupled to ETAAS has also been reported. 63'65.69 The use of on-line microwave digestion procedures to analyse slurried soft tissues in flow systems has also been described. 8~ For example, the analysis of lead in bovine muscle, bovine liver and pig kidney by ETAAS has been demonstrated by dispersing the sample in a 0.4% Triton X-100 solution with an ultrasonic device. 8~The mineralization of these materials were accomplished with a HCI-HNO3 plug downstream in a PTFE coil located inside the microwave oven and to achieve efficient degasification of the mineralized samples a gas diffusion cell device was used. The analysis of Se and A1 by ETAAS in shellfish tissue (mussels, clamps, cockles and oysters) constitute another example of on-line microwave digestion. 87"88 In these cases the use of air streams to transfer the sample to the microwave oven and an open system to collect the digested sample minimizes the problems posed by digestion fumes. These on-line microwave

Applications of flow analysis with atomic spectrometric detectors

365

systems proved to be useful for the accurate and precise determination of Fe and Zn even in human adipose tissue. 86In this case the mineralization was accomplished with a mixture of sulfuric and nitric acid and by the alternative exposition of the sampling unit to a conveniently microwave irradiated zone. 11.4.4

H a i r a n d nails

Hair and nails can be considered as the easiest to get sample solid specimens from the body; besides, their conservation is also easier as compared with other biological tissues and fluids: they are painlessly removed and does not require specialized equipment and refrigerated storage facilities. Although the relationship between concentration of trace elements in hair and internal organs is still controversial, the usefulness of these analyses has been proved in the evaluation of exposure to toxic elements t27 or as an assessment of the nutritional status 128 provided that samples are carefully cleaned to eliminate contamination from the environment (dirt, dust, sweat, cosmetics, pharmaceutical preparations, etc.). The use of flow-systems in combination with atomic detectors is of particular interest for the analysis of hair and nails considering the low amount of sample usually available as well as the frequent need of analyte preconcentration. For example, FI-HGAAS has been used to evaluate the possibility of using hair and nail arsenic as a useful indicator of body arsenic burden. Six districts in West Bengal, India, had drinking water geologically contaminated with As; thus, symptoms of As toxicity were common in the people living in these areas. Studies on "the biggest arsenic calamity in the world" were reported by Das et al. ~29 Hair and nail arsenic analysis was used in the study of affected people. Samples were dissolved in H N O 3 by pressure digestion and As determined by FI-HGAAS. The results demonstrated that if external contamination is negligible, the content of arsenic in hair and nails is directly related to the arsenic body burden. The determination of mercury in hair has been used since the 1960s to assess the level of impregnation of the organism in order to show both acute and chronic exposure to mercury. Relative to other body organs and fluids, scalp hair contains elevated concentrations of trace elements (about 300 times more Hg than in blood) which makes analysis easier. The determination of total mercury in this type of samples can be performed by gold amalgamation CVAAS after dissolving the samples by HNO3 digestion. 3~ The observed linear working range, using this preconcentration method, extended from 0.5 to 12.5 ng of total Hg. Other examples of flow procedures used to introduce such type of samples include the determination of Mo by coprecipitation in a flow with Fe(II)-pyrrolidinedithiocarbamate on a knotted reactor, subsequent dissolution in IBMK and ETAAS detection. ~3~the use of minicolumns to preconcentrate the sample in the analysis of several metals by ICP-OES

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366

by complexation on the commercial material "EDTrA-cellulose", 4~ on 8-hydroxyquinoline(oxine) bound to cellulose, 42 or by ion-pair sorbent extraction on nonpolar solid phases.49. 51 Also, the determination of tin TM and germanium 132by analyte preconcentration in a graphite furnace, using FI-HG, has been proposed with final ETAAS detection. The distribution of elements in hair and nails is not uniform and variations along the length of the hair could be used to detect changes in exposure to metals or changes in lifestyle or diet. A very interesting example was presented by Yoshinaga et al. 93 measuring the Hg and T1 concentrations along the length of single strands of hair. In that way it was possible to reconstruct the recent exposure and toxic metal uptake history of the subjects. Short lengths (mm) of a hair strand were dissolved at room temperature in a small volume of HNO3, and diluted adequately. 50 Ixl aliquots of such samples were introduced by flow injection into an ICP-MS. Hair from a Russian scientist visiting Japan showed a low Hg concentration at the distal end which corresponded to his life in Russia when his fish consumption was minimal. The much higher concentration at the follicle end corresponded to his more recent life in Japan, where he often ate fish. Hair samples from a patient poisoned by T1 similarly reflected his T1 exposure history. 93

11.4.5

Pharmaceutical preparations and solutions for hemodialysis

11.4.5.1

Determination of organic compounds

The selectivity of indirect methods referred to before is rarely adequate for the determination of analytes in samples with complicated and variable matrices (e.g. serum); however, such limitations are frequently not so serious in the analysis of pharmaceuticals. Table 11.4 shows a list, in alphabetic order, of organic compounds analysed by indirect methods in pharmaceutical products by FAAS. As can be seen, most of the methods consist intrinsically of three consecutive steps which are carried out on-line in a flow manifold: (a)

(b)

(c)

Chemical reaction between the analyte and a metal tag species, involving the formation of a precipitate, 1~9'137, 141.142 a complex, 1~7 an ion-pair 1~8'133,136 o r the dissolution of a metal packed on a column. 134'135,138-140.143-145 Separation of the tagged analyte species by solid to liquid transfer, 134"135,138-140,143-145 solvent extraction 1~7'118'133'!36 or precipitate retention; 119'137'142 in the case of precipitation a subsequent step of dissolution may follow. TM Determination of the metal tag by atomic techniques. In some cases the excess tagelement is quantified, ~9'~37'142 while in others the reacted tagged analyte is measured.i ~7,118,133--136,138-141,143-145

Although more scarcely, ICP-OES has been also used as detector in these indirect

Table 11.4

Indirect analysis in pharmaceuticals by non-chromatographic on-line flow procedures and FAAS detection

Analyte

Element measured

Alkaloids

Bi or Co

Amphetamines*

Cu, Ni or Zn

Bromazepam**

Cu

Chloramphenicol

Cd or Zn

Chlordiazepoxide

Cd or Zn

Cocaine Cocaine, papaverine and strychnine Glycine

Bi or Fe Bi

Glycine Isoniazid

Cu

Mn

Levamisole

Hg

Local anesthetics (lidocaine, tetracaine and procaine hydrochlorides) Methadone

Co

Ondansetron

Pb

Cu

Cd or Zn

Ref.

Flow procedure Formation of ion pairs between the alkaloids and inorganic complexes, BiI4 and Co(SCN)24-, followed by liquid-liquid extraction and measurement of bismuth and cobalt in the organic phase. Reaction of methamphetamine and amphetamine with carbon disulfide to yield dithiocarbamic acids and reaction of these acids with Cu(lI), Ni(II) and Zn(II) to form a salt or complex which is extracted into IMBK. Extraction of the analyte as the ion-pair (bromazepam)3Cu(CIO4)2 into IMBK which is separated and introduced into the spectrometer. Reduction of the analyte to the amino derivative in a column of Cd of Zn granules and measure of the released cadmium or zinc ions. Reduction with Cd or Zn columns and measurement of the released ions. The method is selective in the presence of other 1,4-benzodiazepines. Liquid-liquid extraction by ion pairs formation to 1,2-dichloroethane. Precipitation of ion-pairs between the analyte (continuously flowing) and injected BiI4-. The analyte concentration is proportional to the decrease in the absorbance of Bi. Release of immobilized Cu(II) from an on-line column made of copper carbonate physically entrapped in a polyester during polimerization. Samples reacted with powdered CuCO3 and the glycine-Cu complex eluted. Determination of Mn(II) released from MnO2 by the analyte. The reagent was entrapped in polymeric material packed in an on-line reaction column. On-line precipitation of the HgI4 levamisole paired-ion compound. The precipitate is collected on a filter and dissolved in a flow of ethanol. On-line precipitation system based on the measurement of the residual cobalt signal after retaining the precipitate formed between cobalt and the analytes. Reduction of the keto group of the analyte in a cadmium or zinc minicolumn and determination of the metal ion formed. Reaction of the drug in an oxidative solid-phase reactor made by lead dioxide physically entrapped by polymerization and measurement of the released Pb(II).

133

> ~..~. ~...i.

117 O

118

~t~ o

134

m

135

'~ ~.,,,

136 137

~" ~r

138

o ~,,d.

139 140 O

141 142

b,,..

t:x, 143

O

144

(table continued on p. 368)

o~

OO

Table 11.4

Continued

Analyte

Element measured

Salicylic Acid

Cu

Sulphonamides*

Ag or Cu

* Urine was also analysed by this method. ** The method was also applied to spiked plasma.

~.,Lo

Flow procedure Reaction of the drug with copper carbonate entrapped in a polymeric material in a solid phase reactor; the released cupric ions are monitored. Continuous precipitation of the analyte with Cu(ll) or Ag(l) injected into a sample stream. The signal of the residual metal in the stream passing through a filter is measured.

Ref.

O

145 119 N

Applications of flow analysis with atomic spectrometric detectors

369

determinations of organic compounds. For example the determination of cocaine in illicit drugs was carried out by ICP--OES 146using a continuous tandem separation arrangement (see Chapter 9). Also, the determination of quinine in pharmaceuticals by precipitation with tetraphenylborate was proposed using ICP--OES detection. 147 The precipitate was collected on a minicolumn filled with porous glass beads and the borate in the filtrate is monitored continuously. The fall in borate concentration was related to the concentration of quinine in the sample.

11.4.5.2 Determination of metal contamination in solutions for hemodialys• The potential toxicity of aluminium in man is nowadays well documented. Certain clinical abnormalities or disorders (including dialysis dementia, dialysis osteodystrophy and microcytic anaemia) that have been identified in renal-failure patients undergoing regular hemodialysis are associated with aluminium loading in the human body. For this reason, the solutions used for dialytic treatment should be checked for very low levels of AI. However, the determination of A1 in concentrates for hemodialysis constitutes a formidable challenge since a low concentration of the metal must be determined in a dramatically high concentration of inorganic salts (e.g. from 1 x 105 to 1.5 x 105 ppm of chloride and from 5 • 103 to 8 x 104 ppm of acetate). This problem has been successfully overcome by the use of on-line minicolumns with appropriate solid supports able to preconcentrate the analyte and to remove the matrix.97. 148--150ion_exchangers,148 chelating resins such as Chelex 100149 o r Chromotrope 2B immobilized on AG l - X 8 97 and sorbent extraction procedures for analyte derivatives tS~ have been effectively tested for this purpose. The eluted analyte has been eventually detected by flame FAAS, 97"148.149ETAAS,~50 ICP--OES~48' 149o r ICP-MS. 97 Figure 11.5 shows one of such flow manifolds which has been successfully tested for the analysis of A1 in dialysis fluids and concentrates in our laboratory. In this case, the setup consisted of a peristaltic pump, a septum for sample injection, a mixing coil, two rotary valves (one for eluent injection, the other placed after the minicolumn to minimise clogging of the nebuliser), a minicolumn containing Chelex 100 resin, and the detector (FAAS or ICP--OES), As shown in Figure 11.5a, the sample injected through septum B is mixed with the buffer carrier along the mixing coil and pumped with valve C in the "load" position, through the minicolumn where A1 is retained. During this part of the cycle the second injection valve E was in the load position to send the matrix salts to waste and water was continuously pumped to the detector. When the preconcentration cycle was completed, valve E was at once returned to the "injection" position. The releasing solution (100 txl) was then intercalated in the flow stream by turning valve C to the "inject" position, hence eluting A1 directly into the nebuliser of the spectrometer. This configuration not only prevents clogging of the plasma, but also allows careful control of

R. Pereiro and A. Sanz-Medel

370

A.

1

l

Figure 11.5 Flow diagram of the system used for the determination of AI. (a) preconcentration step; and (b) elution step. A, peristaltic pump. B, septum. C and E, injection valves. D, minicolumn. Reproduced from ref. 149 with permission of The Royal Society of Chemistry. optimum experimental conditions for the determination of AI (a standard solution of the metal can be pumped instead of water through the secondary line for detector control purposes). Using this system, detection limits of 15 ppb and 3 ppb were obtained, respectively, by FAAS and ICP-OES detection when preconcentrating sample volumes of 1 ml.

Two methods based on flow injection--atomic fluorescence spectrometry have been developed in order to speciate selenium, as Se(IV) and Se(VI), in hemodialysis fluids. ~51 The maximum legal Se (total) concentration in such samples is 90 ppb. Both methods used hydride generation from Se(IV) solutions as the derivatization step for final AFS detection. However, two sample plugs are injected simultaneously in series in the first method, so that the first plug passes straight to the detector to determine Se(IV). The second plug

Applications of flow analysis with atomic spectrometric detectors

371

passes through a focused microwave device where Se(VI) is reduced to Se(IV) prior to its conversion into the hydride and final analysis. The Se(VI) content is then given as the difference between the two results. In an alternative method a minicolumn is used to retain both Se species. Se(IV) and Se(VI) are then eluted sequentially with formic and hydrochloric acids, respectively, and then analysed by AFS. Both methods showed very good sensitivity (limits of detection of 0.004 ppb) and a linear range up to 50 ppb. TM

11.5

Final remarks

There is a vast literature on the use of flow techniques coupled to atomic detectors for clinical and biological applications. However, the implementation of such strategies in routine clinical analysis is taking place at a rather slow pace. Up to now, clinical chemists appear to have ignored the enormous potential of flow techniques for atomic spectrometric determinations. This might be due to the fact that clinical chemists would prefer simple equipment oriented specifically to a particular application (while many analytical chemists in universities would prefer more sophisticated, but also more versatile instrumentation). However, this gap between routine clinical practice and research laboratories practice is being gradually reduced as continuous or FI analysers become commercially available and are implemented in clinical laboratories with a high sample throughput. The simplification of the sample preparation steps, the possibility of a rapid determination of very low levels of toxic metals by sample preconcentration, the reduction of contamination risks, and the development of fully automated flow manifolds constitute objective advantages of flow systems that will convince clinical analysts to adopt flow systems to enhance the analytical capabilities of atomic spectrometric techniques of any kind (FAAS, ETAAS, ICP-OES, etc.) in their use to determine electrolyte, essential and toxic bioelements.

11.6 References 1 Biochemistry of the essential ultratrace elements, Ed. Frieden, E., 1984, Plenum Press, New York. 2 Trace elements in human and animal nutrition, Ed. Mertz, W., 5th Edn, 1987, Academic Press Inc., London. 3 Paschal, D. C., Spectrochim. Acta, 1989, 44B, 1229. 4 Subramaniam, K. S., Spectrochim. Acta, 1996, 51B, 291. 5 Barnes, R. M., Anal Chim. Acta, 1993, 283, 115. 6 Modem methods for trace element determination, Vandecasteele, C. and Block, C. B., 1993, John Wiley and Sons, Chichester. 7 Species identification for trace inorganic elements in biological materials, Topics in current chemistry, 1987, Vol. 141, Springer-Verlag, Berlin. 8 Trace element speciation, Ed. Batley, G. E., 1989, CRC Press, Boca Raton, FL. 9 Improvement of speciation analysis in environmental matrices, BCR Workshop, Arcachon (1990), Mikrochim. Acta, 1992, 109. 10 Sanz-Medel, A., Analyst, 1995, 120, 799.

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11 Burguera, J. L. and Burguera, M., J. Trace Elem. Electrolytes Health Dis., 1993, 7, 9. 12 Applications in clinical chemistry, Sherwood, R. A. and Rocks, B. F. In: Flow injection atomic spectroscopy, Ed. Burguera, J. L., 1989, Marcel Dekker, Inc., New York. 13 Clinical and pharmaceutical applications. In: Flow injection atomic absorption spectrometry, Fang, Z., 1995, John Wiley and Sons, Chichester. 14 Guo, T., Baasner, J., Gradl, M. and Kistner, A., Anal. Chim. Acta, 1996, 320, 171. 15 Burguera, J. L., Burguera, M. and Alarcon, O. M., J. Anal. At. Spectrom., 1986, 1, 79. 16 Yoshinaga, J., Shibata, Y. and Morita, M., Clin. Chem., 1993, 39, 1650. 17 Burguera, J. L., Burguera, M., Rondon, C. E., Rivas, C., Burguera, J. A. and Alarcon, O. M., J. Trace Elem. Electrolytes Health Dis., 1987, 1, 21. 18 Versiek, J. and Cornelis, R.,Anal. Chim. Acta, 1980, 116, 217. 19 Wiederin, D. R., Smyczek, R. E. and Houk, S. R., Anal. Chem., 1991, 63, 1626. 20 Non-chromatographic continuous separation techniques, Valcarcel, M. and Luque de Castro, M. D., The Royal Society of Chemistry, 1991, Cambridge. 21 Burguera, M., Burguera, J. L. and Alarcon, O. M.,Anal. Chim. Acta., 1986, 179, 351 22 Burguera, M., Burguera, J. L., Rondon, C., Rivas, C., Carero, P., Gallignani, M. and Brunetto, M. R., J. Anal. At. Spectrom., 1995, 10, 343. 23 Burguera, J. L. Burguera, M. and Brunetto, M. R., At. Spectrosc., 1993, 14, 90. 24 Nakahara, T., Spectrochim. Acta Rev., 1991, 14, 95. 25 Liu, X., Xu, S. and Fang, Z., At. Spectrosc., 1994, 6, 229. Pb. 26 Tao, H., Lam, J. W. H. and McLaren, J. W., J. Anal. At. Spectrom., 1993, 8, 1067. 27 Hanna, C. P., Tyson, J. F. and Mclntosh, S., Anal. Chem., 1993, 65, 653. 28 Guo, T. and Baasner, J., Anal. Chim. Acta, 1993, 278, 189. 29 Atallah, R. H. and Kalman, D. A., J. of Anal. Toxicol., 1993, 17, 87. 30 Matusiewicz, H. and Sturgeon, R. W., Spectrochim. Acta, 1996, 51B, 377. 3 ! Bruhn, C. G., Rodriguez, A. A., Barrios, C., Jaramillo, V. H., Becerra, J., Gonzalez, U., Gras, N. T., Reyes, O. and Salud, S., J. Anal At. Spectrom., 1994, 9, 535. 32 Xiu-ping, Y. and Zhe-ming, N., ,I. Anal. At. Spectrom., 1991, 6, 483. 33 Zhe-ming, N., Bin, H. and Heng-bin, H., J. Anal. At. Spectrom., 1993, 8, 995. 34 Goenaga, H., Fem~indez-S~inchez, M. L. and Sanz-Medei, A., J. Anal. At. Spectrom., 1998, 13, 899. 35 Hernandez, P., Hernandez, L. and Lozada, L., Fresenius, ,I. Anal. Chem., 1986, 325, 300. 36 Knapp, G., Mueller, K., Strunz, M. and Wegscheider, W., J. Anal. At. Spectrom., 1987, 2, 611. 37 Karakaya, A. and Taylor, R.,J. Anal. At. Spectrom., 1989, 4, 261. 38 Plantz, M. R., Fritz, J. S., Smith, F. G. and Houk, R. S., Anal. Chem., 1991,284, 18 I. 39 Caroli, S., Alimonti, A., Petrucci, F. and Horvath, Zs., Anal. Chim. Acta, 1991, 248, 241. 40 Schramel, P., Xu, L.-G., Knapp, G. and Michaelis, M., Mikrochim. Acta, 1992, 106, 191. 41 Peng., X., Jiang, Z. and Zen., Y., Anal Chim. Acta, 1993, 283, 887. 42 Schramel, P., Xu, L.-G., Knapp, G. and Michaelis, M., Fresenius, ,I. Anal. Chem., 1993, 343, 600 43 Lee, M. L., Toelg, G., Beinrohr, E. and Tschoepel, P., Anal. Chim. Acta, 1993, 272, 193. 44 Ebdon, L., Fisher, A. S., Worsfold, P. J., Crews, H. and Baxter, M, J. Anal. At. Spectrom., 1993, 8, 691. 45 Ma, R., Van Mol, W. and Adams, F., Anal. Chim. Acta, 1994, 293, 251. 46 Ebdon, L., Fisher, A. S. and Worsfold, P. J., J. Anal. At. Spectrom., 1994, 9, 6 ! 1. 47 Petit de Pefia, Y., Gallego, M. and Vaicarcel, J. Anal. At. Spectrom., 1994, 9, 691. 48 Fang, Z., Xu, S., Dong, L. and Li, W., Talanta, 1994, 41, 2165. 49 Liu, X. and Fang, Z.,Anal. Chim. Acta, 1995, 316, 329. 50 Lima, R., Leandro, K. L. and Santelli, R. E., Talanta, 1996, 43, 977. 51 Tao, G. and Fang, Z., At. Spectrosc., 1996, 17, 22. 52 Ko, F.-H. and Yang, M.-H., J. Anal. At. Spectrom., 1996, 1 I, 413. 53 Menegario, A. A. and Gine, M. F.,J. Anal. At. Spectrom., 1997, 12, 671. 54 Fang, Z., Sperling, M. and Welz, B., J. Anal. At. Spectrom., ! 991, 6, 301. 55 Welz, B., Xu, S. and Sperling, M.,Appl. Spectrosc., 1991, 45, 1433. 56 Fang, Z. and Dong, L., J. Anal. At. Spectrom., 1992, 7, 439 57 Pretty, J. R., Blubaugh, E. A., Evans, E. H., Caruso, J. A. and Davidson, T. M., J. Anal. At. Spectrom., 1992, 7,1131.

A p p l i c a t i o n s o f flow a n a l y s i s w i t h a t o m i c s p e c t r o m e t r i c d e t e c t o r s

373

58 Pretty, J. R., Blubaugh, E. A. and Caruso, J. A., Anal Chem., 1993, 65, 3396. 59 Cafiada Rudner, P., Garcia de Torres, A. and Cano Pavon, J. M., J. Anal. At. Spectrom., 1993, 8, 705. 60 Siles Cordero, M. T., Vereda Alonso, E. I., Garcia de Tones, A. and Cano Pavon, J. M., J. Anal At. Spectrom., 1996, 11, 107. 61 Tao, G. and Fang, Z., Spectrochim. Acta, 1995, 50B, 1747. 62 Hoenig, M. and Van Hoeyweghen, P., Anal Chem., 1986, 58, 2614. 63 Lynch, S. and Littlejohn, D., J. Anal At. Spectrom., 1989, 4, 157. 64 Miller-Ihli, N. J., At. Spectrosc., 1992, 13, 1. 65 L6pez-Garcia, I., Sanchez-Merlos, M. and Hernfindez-C6rdoba, J. Anal At. Spectrom., 1997, 12, 777. 66 Bendicho, C., Margaretha and de Loos-Vollebregt, T. C., J. Anal At. Spectrom., 1991, 6, 353. 67 Valcarcel, M. and Gallego, M., Talanta, 1997, 44, 1509. 68 Miller-lhli, N. J., J. Anal At. Spectrom., 1989, 4, 295. 69 Ebdon, L., Fisher, A. S., Parry, H. G. M. and Brown, A. A., J. Anal At. Spectrom., 1990, 5, 321. 70 Miller-lhli, N. J., J. Anal At. Speetrom., 1988, 3, 73. 71 Arruza, M. A., Gallego, M. and Valc~ircel, M., J. Anal. At. Spectrom., 1994, 9, 657. 72 Vifias, P., Campiilo N., Lopez-Garcia, I. and Hernandez-Cordoba, M., Talanta, 1995, 42, 527. 73 Burguera, M., Burguera, J. L. and Alarcon, O. M., Anal Chim. Acta, 1988, 214, 421. 74 Tsalev, D. L., Sperling, M. and Welz, B., Analyst, 1992, 117, 1729. 75 Tsalev, D. L., Sperling, M. and Welz, B.,Analyst, 1992, 117, 1735. 76 Welz, B., Tsalev, D. L. and Sperling, M., Anal Chim. Acta, 1992, 261, 91. 77 Carbonell, V., Morales-Rubio, A., Salvador, A., de la Guardia, M., Burguera, J. L. and Burguera, M., J. Anal At. Spectrom., 1992, 7, 1085. 78 Haswell, S. J. and Barclay, D., Analyst, 1992, 117, 117. 79 Guo, T. and Baasner, J., Talanta, 1993, 40, 1927. 80 Burguera, J. L. and Burguera, M., J. Anal At. Spectrom., 1993, 8, 235. 81 Welz, B., He, Y. and Sperling, M., Talanta, ! 993, 40, 1917. 82 de la Guardia, M., Carbonell, V., Morales-Rubio, A. and Salvador, A., Talanta, 1993, 40, 1609. 83 Le, X.-C., Cullen, W. R. and Reimer, K. J., Talanta, 1993, 40, 185. 84 Martines Stewart, L. J. and Barnes, R. M., Analyst, 1994, 119, 1003. 85 Lopez-Gonzalvez, M. A., Gomez, M., Camara, C and Palacios, M. A., Mikrochim. Acta, 1995, 120, 301. 86 Burguera, J. L., Burguera, M., Carrero, P., Rivas, C., Gallignani, M. and Brunetto, M. R., Anal Chim. Acta, 1995, 308, 349. 87 Arruda, M. A. Z., Gallego, M. and Valcarcel, M.,J. Anal. At. Spectrom., 1995, 10, 501. 88 Arruda, M. A. Z., Gallego, M. and Valcarcel, M., J. Anal At. Spectrom., 1996, 11, 169. 89 Lamble, K. J. and Hill, S. J., J. Anal. At. Spectrom., 1996, 11, 1099. 90 Beauchemin, D., Siu, K. W. M. and Berman, S. S., Anal Chem., 1988, 60, 2587. 91 Dean, J. R., Ebdon, L., Crews, H. M. and Massey, R. C., J. Anal At. Spectrom., 1988, 3, 349. 92 Lorber. A., Karpas, Z. and Halicz, L., Anal. Chim. Acta, 1996, 334, 295. 93 Yoshinaga, J., Shibata, Y. and Morita, M., Clin. Chem., 1993, 39, 1650. 94 Lu, P.-L., Huang, K.-S. and Jiang, S. J.,AnaL Chim. Acta, 1993, 284, 181. 95 Goossens, J., Moens, L. and Dams, R.,Anal. Chim. Acta, 1994, 293, 171 96 Plantz, M. R., Fritz, J. S., Smith, F. G. and Houk, R. S.,Anal. Chem., 1989, 61,149. 97 Martin-Esteban, A., Fernandez, P., Perez-Conde, C., Gutierrez, A. and Camara, C., Anal Chim. Acta, 1995, 304, 121. 98 Garcia-Alonso, J. I., Gutierrez-Camblor, M., Montes. Bay6n, M., Marchante-Gay6b, J. M. and Sanz-Medel, A., J. Mass Spectrom., 1997, 32, 556. 99 Pretty, J. R., Blubaugh, E. A., Caruso, J. A. and Davidson, T. M., Anal. Chem., 1994, 66, 1540. 100 Hwang, T.-J. and Jiang, S.-J., J. Anal. At. Spectrom., 1996, 11,353. 101 Janghorbani, M. and Ting, B. T. G.,Anal. Chem., 1989, 61,701. 102 Rayman, M. P., Abou-Shakra, F. R. and Ward, N. I., J. Anal. At. Spectrom., 1996, 11, 61. 103 Huang, M.-E, Jiang, S.-J. and Hwang, C.-J., J. Anal. At. Spectrom., 1995, 10, 31. 104 Tao, H., Lam, J. W. H. and McLaren, J. W., J. Anal. At. Spectrom., 1993, 8, 1067. 105 Marawi, I., Wang, J. and Caruso, J. A.,Anal. Chim. Acta, 1994, 291. 127. 106 Worth, H. G. J.,Analyst, 1988, 113, 373.

374 107 108 109 110 111 112 113 I 14 115 116 1! 7 118 ! 19 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151

R. Pereiro and A. Sanz-Medel Burguera, J. L., Burguera, M. and Gallignani, M., An. Acad. Bras. Cienc., 1983, 55, 209. Burguera, J. L., Burguera, M., Gailignani, M. and Alarc6n, O. M., Clin. Chem., 1983, 29, 568. Rocks, B. F. Sherwood, R. A. and Riley, C., Ann. Clin. Biochem., 1984, 21, 51. Burguera, J. L., Burguera, M., Rivas, C., de la Guardia, M., Salvador, A. and Carbonell, V., J. Flow Injection Anal., 1990, 7, 11. Doku, G. N. and Gadzekpo, V. P. Y., Talanta, 1996, 43, 735. McLeod, C. W., Worsfold, P. J. and Cox, A. G., Analyst, 1984, 109, 327. Alexander P. W., Finlayson, R. J., Smythe, L. E. and Thalib, A.,Analyst, 1982, 107, 1335. Simonsen, K. W., Nielsen, B., Jensen, A. and Andersen, J. R., ,/. Anal. At. Spectrom., 1986, 1,453. Lim, H. B., Han, M. S. and Lee, K. J., Anal Chim. Acta., 1996, 320, 185. Gallego, M. and Valcarcel, M.,AnaL Chim. Acta, 1985, 169, 161. Montero, R., Gallego, M. and Valcarcel, M., Anal Chim. Acta, 1991,252, 83. Santelli, R. E., Gallego, M. and Valcarcel, M., Talanta, 1994, 41,817. Montero, R., Gallego, M. and Valcarcel, M., J. Anal At. Spectrom., 1988, 3, 725. Burguera, J. L., Burguera, M., Rivas, C., Alarcon, O. M. and Ibarra de Diaz, N., Acta Cient. Venez., 1988, 39, 222. Burguera, M., Burguera, J. K., Rivas, C. and Alarcon, O. M.,At. Spectrosc., 1986, 7, 79. Sanz-Medel, A., Vald6s-Hevia, M. C., Bordel, N., Fern~indez de la Campa, M. R., Anal. Chem., 1995, 67, 2216. Xiao, G. and Ming, G. X., J. Anal. At. Spectrom., 1995, 10, 987. Oliveira, E., McLaren, J. W. and Berman, S. S., Anal Chem., 1983, 55, 2047. Petterson, J., Hansson, L. and Olin, A., Talanta, 1986, 33, 249. Zhang, L. S. and Combs, S. M., J. Anal. At. Spectrom., 1996, I 1, 1049. Wang, X., Zhuang, Z., Zhu, E., Wang, C., Wan, T. and Yu, L., Microchem. J., 1995, 51, 5. Contiero, E. and Folin, M., Biol. Trace. Elem. Res., 1994, 49, 151. Das, D., Chatterjee, A., Mandal, B. K., Samanta, G., Chakraborti, D. and Chanda, B., Analyst, 1995, 120, 917. Chen, H., Xu, S. and Fang, Z., J. Anal. At. Spectrom., 1995, 10, 533. Tao, G. and Fang, Z., Talanta, 1995, 42, 375. Tao, G. and Fang, Z., J. Anal. At. Spectrom., 1993, 8, 577. Eisman, M., Gallego, M. and Valcarcel, M.,J. Anal. At. Spectrom., 1992, 7, 1295. Montero, R., Gallego, M. and Valcarcel, M., Talanta, 1990, 37, 1129. Montero, R., Gallego, M. and Valcarcel, M.,Analyst, 1990, 115, 943. Eisman, M., Gallego, M. and Valcarcel M., Anal Chem., 1992, 64, 1509. Eisman, M., Gailego, M. and Valcarcel. M.,J. Anal At. Spectrom., 1993, 8, II 17. Garcia Mateo, J. V. and Martinez Calatayud, J., Anal. Chim. Acta, 1993, 274, 275. Martinez Calatayud, H. and Garcia Mateo, J. V.,Analyst, 1991, 116, 327. Lahuerta-Zamora, L., Garcia-Mateo, J. V. and Martinez Calatayud, J., Anal. Chim. Acta, 1992, 265, 81. Laredo Ortiz, S., Garcia Mateo, J. and Martinez Calatayud, J., Mikrochem. J., 1993, 48, 112. Montero, R., Gallego, M. and Valcarcel, M.,Anal. Chim. Acta, 1988, 215, 241. Montero, R., Gallego, M. and Valcarcel, M.,Anal. Chim. Acta, 1990, 234, 433. Lahuerta Zamora, L. and Martinez Calatayud, Anal Chim. dcta, 1995, 300, 143. Rivas, G. A. and Martinez Calatayud, J., Talanta, 1995, 42, 1285. Menendez Garcia, A., Sanchez-Uria, E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1996, 11,561. Chen, W., Jinag, Z.-C., Kong, L.-Y. and Chen. H., Fenxi Shiyanshi, 1995, 14, 60. Pereiro Garcia, M. R., Diaz Garcia, M. E. and Sanz-Medel, A., J. Anal. At. Spectrosc., 1987, 2, 699. Pereiro Garcia, M. R., Lopez Garcia, A., Diaz Garcia, M. E. and Sanz-Medel, A., J. Anal At. Spectrosc., 1990, 5, 15. Aceto, M., Abollino, O., Sarzanini, C., Mentasti, E. and Mariconti, F., At. Spectrosc., 1994, 15, 237. Bryce, D. W., lzquierdo, A. and Luque de Castro, M. D.,J. Anal At. Spectrom., 1995, 10, 1059.

12.1

Introduction

The determination of total concentrations of metals is not sufficient nowadays for most of the environmental applications, l The physico-chemical behaviour, toxicological risk and bioavailability of metals and metalloids are highly dependent on its chemical formulation. A good understanding of the general cycles of metals in the environment asks for the determination of each chemical form the metal presents in the different environmental compartments. Metal and metalloid species in the environment occur either as a result of direct anthropogenic inputs or via natural biogeochemical processes. Organometallic compounds are usually much more toxic than the corresponding inorganic species. 2 Methyl- and butyltins, methylmercury and several alkyl-lead compounds have been of major concern during the last 25 years due to their high toxicity and the relative ease to enter the trophic chain. The toxic effect of inorganic forms of arsenic, As(Ill) and As(V), and selenium, Se(IV) and Se(VI), however, highly decreases when combined with organic groups. High concentrations of organometallic compounds in environmental samples are usually related to anthropogenic inputs. 3 Methylmercury has been largely used as a fungicide during a long period of time. After several collective poisonings via ingestion of methylmercury polluted food, this application was completely banned in most countries. 4 375

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The combustion of gasoline has increased the concentration of alkyl-leads in air samples from urban areas. 5 Accumulation of organotin compounds in harbour and marine sediments has also been observed after a massive utilisation of organotin-based antifouling paints in ships. 6 Further to direct anthropogenic inputs, mobility of metal can take place via natural pathways. The existence of natural methylation pathways in the environment is, nowadays, generally accepted for metals such as mercury, tin, arsenic, selenium, tellurium, germanium and, perhaps, antimony. 3 Methylation of inorganic metal species mostly occurs in top marine or estuary sediments with high biological activity, although abiotic ways of alkylation have also been recognised. 7 Despite the fact that natural methylation processes generally give rise to low concentrations of organometallic compounds in environmental samples, the ability of some of these species to accumulate in specific compartments such as fish tissue results in organic metal/inorganic metal ratios as high as 90% in the case of mercury. 4 The study of the fate of metals in the environment requires, therefore, information which can only be provided by species determination approaches. The success of such analysis applied to environmental samples relies in a careful combination of three different steps: (a)

(b) (c)

preconcentration of the sample in order to gain enough sensitivity to match the low concentration of metal species found in natural environmental samples efficient separation of the analytes selective detection and quantification.

These requirements are usually not achieved by most of the present commercial instruments. To overcome this problem, the hyphenation of different techniques has been attempted. This approach has allowed to answer many of these analytical requirements. Different preconcentration techniques connected to either off-line or on-line hyphenated systems are of general use. 8 Most successful techniques for metal speciation analysis combine separation by chromatography and single- or multi-element detection by atomic spectroscopy. 9 The quality of the hyphenation is directly related to the quality of the interface. Gas chromatography usually offers good results since the introduction of the sample to the detector in a gaseous state is not problematic. Hyphenated techniques like GC-FPD (flame photometric detector), GC-OES, GC-QFAAS or GC-MS provide excellent separation skills and very low detection limits. 3 Liquid chromatography separation techniques are very powerful and enlarge the molecular size spectrum of organometallic compounds to be investigated. I~ Interfacing between LC and candidate detectors such as FAAS or ETAAS is however complex, and results in poor detection limits not directly applicable to

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environmental issues. The relative introduction of the hyphenation between HPLC and ICP/MS is an exception due to the high sensitivity of the multi-element ICP/MS detector. Most techniques, however, require off-line preconcentration methods. The conversion of the analytes to gaseous forms is not new and provides enormous flexibility to the development of flow analysis systems (see Chapter 8 by J. Dedina). This approach allows later on-line cryofocusing of the analytes prior to their desorption in an spectroscopic detector. This strategy permits the use of a large range of sample volume and makes this technique directly applicable to environmental species determination. 3 Systems based on on-line derivatization of the analytes in a liquid medium (by hydride generation (HG) or ethylation (Eth)), preconcentration by cryogenic trapping (CT), separation by GC and detection by QFAAS provide low detection limits suitable for a selected range of organometallic compounds in the environment. This simple, fast and versatile technique has been successfully applied to the speciation analysis of any hydride or ethyl-derivative forming element such as tin, lead, mercury, selenium, tellurium, germanium, arsenic, bismuth or antimony. 7 The technique is limited, however, to low boiling point organometallic species and shows limited resolution capabilities. In order to illustrate the potential of such hyphenation, the results obtained on an automated system on-line integrating all the fundamental steps of this analytical strategy will be presented. General working conditions including chemical derivatization reactions, cryofocusing and separation, and detection will be illustrated. Limiting conditions will also be highlighted. Finally, specific applications of this hyphenated system in metal speciation analysis will be summarised.

12.2 Experimental set-up The technique discussed here combines volatilization of the analytes, cryogenic trapping on small packed gas chromatography columns prior to detection by atomic absorption spectroscopy. This method was first used for species determination of selenium some 25 years a g o . II Since then, the introduction of new derivatizating reagents like sodium tetraethylborate (NaBEt4) and the evolution of detectors (AFS, ICP/MS) have allowed new geochemical pathways for metal and metalloids to unravel. The on-line arrangement of the different fundamental and separated analytical steps includes volatilization of the analytes in an aqueous sample, direct preconcentration by cryofocusing, separation based on differences in the boiling point of the compounds upon gentle warming of the column, atomization and detection in an atomic spectrometric detector. All these steps need to be carefully controlled to obtain optimal sensitivity. This high sensitivity results from the 100% transmission of the analytes to the detector. This approach also separates the analytes from the matrix. The selectivity of atomic absorption

A. de Diego et al.

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minimises spectroscopic interferences. The most severe limitation of the technique is related to its low resolution capacity, with an average number of theoretical plates of about 1300,s, 12 which may allow coelution of species with large boiling points. Finally, this technique is also limited to derivatized species with boiling points below 250~ A fully automated on-line HG/Eth-qET--GC-QFAAS hyphenated system has been recently reported in the literature. 13 It is schematically presented in Figure 12.1. The system includes a peristaltic pump to quantitatively introduce the derivatizating reagent (NaBH4 or NaBEt4) to the reaction flask, two electronic Teflon valves to allow the stripping of the analytes from the aqueous sample, their cryocondensing on the small

v

Figure 12.1 Automated on-line HG/Eth-CT-GC-QFAAS hyphenated system for organometallic speciation analysis.

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packed column, their release upon warming of the column, and detection in a quartz cell aligned in the beam of an atomic absorption spectrophotometer. Atomisation of the analytes is provided by maintaining the quartz cell at 850~ and by the addition of auxiliary gas for enhanced atomization. The signal is continuously monitored and processed by a chromatographic software. 14

12.2.1

Liquid-to-gas conversion of the analytes

This first step of the analytical procedure converts the analytes of interest into a gaseous state. The analytes can therefore be handled prior to detection for direct preconcentration by cryofocusing. This part of the analytical system is performed on-line but improved technical control of cryotrapping technology allows to look for possible novel extension of the method. Derivatization and cryotrapping can be performed directly in-field off-line the instrumentation. This approach brings a new concept of sample handling and detector interface for the future. 12.2.1.1

The reaction vessel

The system classically employed by most authors makes use of the static derivatization procedure. The liquid sample is stored and derivatized in the reaction flask. Volatilization of the target analytes takes place in a reaction vessel, usually constructed in round-bottom borosilicate glass. 3 Its volume may range from a few milliliters to even up to one liter. It must be designed to avoid any trapping of the analytes. Once derivatized into a gaseous form, analytes are stripped from the reaction vessel to the cryogenic column. The use of a bubbler with a wide contact surface made in fritted glass provides a more efficient removal of the analytes from the reaction medium. The design described here integrates quantitative volatilization and efficient gas-liquid separation which results in low signalto-noise ratio. 3 Careful design of the reactor allows optimization of the stripping of the analytes by a stream of helium. ~5'16 The derivatization and stripping stage are facilitated and more reproducibly performed with an homogeneous mixing of the solution by a magnetic stirrer. The design is also important to limit problems associated with excessively foaming samples during the derivatization stage. This is the case with samples containing a high level of organic matter and using solubilizing solution such as tetramethyl ammonium hydroxide (e.g. extracts from bio-tissues). When the system design is not sufficient to prevent foaming to reach the column, the addition of antifoaming agent may facilitate this derivatization and stripping stage. The time length of this first analytical step is highly variable, ranging from 2 to 8 minutes, and mostly depends on the type of derivatization reaction used and the sample matrix. These procedures are used for static derivatization vessels. They can certainly be improved in the future by using on-line flow injection systems. This approach may

380

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improve the reproducibility of this important analytical step and allow greater flexibility in the sample volume to be analysed. For optimal system design, connections between the reaction vessel and the cryofocusing column must be made in inert material and present the smallest possible dead volume. These details are important to minimize memory effects, losses of analytes and later tailing of chromatographic peaks. When properly designed, these details contribute significantly to the overall sensitivity and performance of the instrument. 12.2.1.2

Derivatization reactions

During the derivatization step, the analytes present in an aqueous phase under mild acidic conditions are converted into volatile species by reaction with an alkylating agent, generally sodium borohydride (NaBH4) or sodium tetraethylborate (NaBEt4). The counterions (hydroxide, halogen, acetate, oxide, etc.) of the inorganic or organic metallic compound are substituted by a hydrogen or an ethyl group. An organometallic compound with low boiling point, e.g., with good volatility at ambient temperature, is obtained. The continuous stripping of the solution with a gentle flow of helium allows complete removal of the analyte from the liquid sample, and their condensation and preconcentration on the top of the cryotrap immersed in liquid N2. Chemical conditions of the reaction need to be carefully controlled in order to obtain the highest yield of removal of the analytes from the sample. Several parameters are of paramount importance. The most important ones are as follows: (a) (b)

the concentration and the volume of reagent introduced in the reaction vessel, the pH of the derivatization reaction.

These chemical conditions should be carefully controlled to obtain quantitative conversion of the target analytes, preserve the cationic moieties of the species of interest and minimize potential interferences likely to take place during the reaction. These possible interferences will be discussed in a later section. The best volatilization conditions of the analytes vary both with respect to the sample matrix and to the type of derivatizating agent used. Under appropriate conditions, a volatilization yield of 100% can be achieved routinely (see Chapter 8). Derivatization with sodium borohydride rapidly generates organometallic hydride derivatives of low boiling points. '7 During the reaction, a large excess of hydrogen is produced which helps the purge of the volatile species from the vessel to the cryogenic trap. In routine analysis, NaBH4 is relatively cheap and easy to handle. Some hydrides, however, are not stable. In such cases, the use of NaBEt4 as ethylation agent is a good alternative. The ethylated volatile compounds formed present slightly higher boiling points and molecular weights than their hydride equivalents. In general, ethylation

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381

reactions are less sensitive to interferences than hydride generation, especially when complex samples such as sediments are analysed. ~8 Good peak resolution and sensitivity are also obtained when NaBEt4 is used as derivatization agent, due to its high reactivity and the stability of the volatile analytes formed. However, NaBEt4 is considerably more expensive than NaBH4, is air-sensitive and it must be handled under air-free atmosphere to avoid decomposition of the reagent. It also often presents trace metal impurities which may interfere in the analysis. 19'2~Further, gas generation during ethylation reactions shows slow kinetics. As a result, longer stripping time compared to hydritization reaction is required, which may result in water built up in the cryotrap, limiting the desorption stage. Both derivatization reactions are complementary with respect to speciation analysis. Further than species determination, they should also allow the use of these systems for improved sensitivity total determination of metals which are able to react and form volatile species. Both reactions allow to cover a large array of analytical capabilities. They are presented and summarized in Figure 12.2. Determination of trace metal through gaseous analyte handling is an old and efficient procedure. Introducing a cryotrapping stage certainly facilitates the sample introduction procedure with 100% efficiency, resulting in a tremendous overall increase in sensitivity for on-line flow analysis. Finding new derivatizating reagents to achieve the volatilization of a larger amount of metalloids is an important task for the future. A new reagent such as LiBEt3H has been reported to

mb[

.,

X

X

X

X

X

X

X X X

X .....

Figure 12.2 Chemical forms of different elements susceptible to analysis by CT-GC-AAS after derivatization by hydride generation or ethylation.

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A. de Diego et al.

quantitatively convert inorganic mercury and methylmercury into volatile forms as the hydride. 2~'22 Other simple and efficient derivatization reagents in water will certainly appear in the future. 12.2.1.2.1

Hydride generation (HG)

Hydride generation was first used by Holak for total determination of arsenic by atomic absorption spectroscopy. 23It is now well established for elements such as arsenic, bismuth, antimony, selenium, tin, germanium and tellurium. 24 Some of the early applications for speciation analysis were conducted for the determination of methylated forms of selenium in water samples. ~1 The technique also gives access to the determination of redox inorganic species such as arsenic (III and V), 25 antimony (III and V) 26 and selenium (IV and VI). 27 The determination of organometallic forms including monomethylarsonate, dimethylarsinate, dimethylselenide, dimethyldiselenide and diethylselenide is also possible. 28 Speciation analysis of organo-germanium species after reaction with NaBH4 was reported by Hambrick et al. 29 Methyl and butyl tins are also rapidly converted into their gaseous derivatives by hydride generation. 3~Despite the fact that hydride generation was initially considered not to be suitable for mercury speciation analysis due to possible reduction and conversion of the analytes to Hg ~ this technique has recently been demonstrated to be fully applicable for mercury speciation purposes. Mercury hydrides can be readily formed before total reduction to Hg ~ and their half-life prior to total reduction allows them to be used in an analytical procedure. 21'3t'32 Immediate cryotrapping considerably facilitates this procedure. 33 This technique has recently been successfully applied to the simultaneous determination of inorganic mercury and methylmercury in alkaline extracts of bio-tissues using an HG--CT-GC--QFAAS system. 33 The general reaction of hydride generation can be written as follows for alkylated tin compounds: RnSn ~4-'') (liquid) + NaBH 4~ RnSnn~4_,l) (gas) + H2 (gas)

(12.1)

R: methyl ethyl, or butyl group n: 1 , 2 o r 3 Important production of hydrogen during the reaction improves the purging efficiency of the volatile hydrides from the vessel. It may also generate over-pressure problems in the system. Many elements may form hydrides which will also be stripped simultaneously. This fact can be an advantage for simultaneous detection, but may also generate later interferences. A careful selection of the experimental conditions (pH and volume of reagent) results in selective conversion to hydrides, minimizing the number of species

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383

trapped in the column, and in reduction of potential interferences in the later detection step. The optimal conditions for hydride generation with regards to a large array of species have been reported in the literature for several inorganic and organometallic compounds and are summarized in Table 12.1. In general, the hydritization yield of the reaction is optimal for a pH slightly below the pK a of the species to be converted. 43 It is worth noting that most of the compounds suitable to be analysed by atomic absorption after hydride generation are naturally present in the environment. 7 12.2.1.2.2 Ethylation reactions (EthG) As pointed out in Chapter 9, the use of NaBEt4 as derivatization agent in speciation analysis was not proposed until 1986, when Rapsomanikis et al. developed a procedure for lead speciation analysis. 44 Derivatization by ethylation had already been used in 1961 to determine total concentrations of lead and mercury. 45 The potential of ethylation for speciation analysis was first proposed with a cryofocusing system for the determination of methylated lead species in aqueous solutions. Its potential for Hg species determination was also later reported. 39 These applications were fully developed for the determination of inorganic mercury and methylmercury by Bloom 4~and Fisher et al. ~9 The method was later extended by Ashby and Craig, 18 and Cai et a/. 37'38 to the analytical determination of organotin compounds in sediments. Finally, a new application using a cryofocusing system has recently been proposed for the quantitative determination of methylmercury and inorganic mercury in sediments by conversion of the analytes to their ethylated derivatives after microwave assisted leaching of the sample. ~3 Methylmercury and inorganic mercury react in the presence of NaBEt4 according to the next expressions: MeHg + (liquid) + NaBEt4 ~ MeHgEt (gas) + BEt3 + Na +

(12.2)

Hg 2+ (liquid)+ 2NaBEt4---. HgEt2 (gas)+ 2BEt3 + 2Na +

(12.3)

BEt3 stands for a very unstable reaction product which rapidly decomposes. Optimal pH for the reaction is higher than for the analogous in hydride generation 7 and most usually ranges between 4 and 5. Lower amounts of reagent are required in this case, due to the higher reactivity of NaBEt4 compared to NaBH4. Little gas displacement is generated during the reaction. Complete stripping of the volatile derivatives requires, therefore, a longer stripping time. Operational working conditions for speciation analysis of several metals using ethylation reactions are summarised in Table 12.1. Ethylation is a good alternative when the corresponding hydrides are not stable and provides, in general, improved sensitivity, better peak resolution and appreciable reduction of interferences. '8 Different chromatograms obtained for tin speciation after derivatization by hydride generation or ethylation are provided in Figure 12.3.

ta4 OO

Table 12.1 Derivatizing conditions reported in the literature for several inorganic and organometallic compounds

4~

Species

Reagent

Derivatization conditions

Sample pretreatment

Reference

Inorganic Sn MexSn{4-

NaBH 4

1 ml of 4% aqueous NaBH4

1 ml of HAc 2 mol dm-3

[34]

2 x 1 ml of 4% aqueous NaBH4

4 ml ofTris-HCl 2 mol dm-3; pH~6.5

[25]

0.2 mi of HNO3 5 mol dm-3; pH ~ 2

[35]

NaBH4

1 ml of 4% aqueous NaBH4 in NaOH 0.02 mol dm-3 2 x 1.5 ml of 4% aqueous NaBH4

2 ml of HNO 3 5 mol dm-3; pH ~2

[30]

NaBH4

2 • 2.5 ml of 6% aqueous NaBH4

2 ml of HNO 3 5 mol dm-3; pH~ 1.6

[36]

~>

NaBEh NaBH4

0.13 ml of I% aqueous NaBEt4 6 ml of 20% aqueous NaBH4 in NaOH 0.06 mol dm -3 per 100 ml of sample

[37, 381 [29]

o~

NaBH4 NaBH4

2 ml of 2% aqueous NaBH4 4 x 2 ml of 2% aqueous NaBH4

pH~4.1 5 ml of Tris-HCl 1.9 mol dm-3+ 10 ml of 30% NaCI + 1 ml of EDTA 0.2 mol dm-3 per 100 ml of sample 1-3 ml of 5% potassium biphtalate; pH ~ 1.6 5 ml of 10% oxalic acid; pH ~ 1.-1.5

NaBEt4 NaBEt4 NaBH4 LiB(C2Hs)3H NaBH4 NaBEt4 NaBH4

3 ml of 0.43% aqueous NaBEt4 0.05 ml of 1% aqueous NaBEh 1 mi of 0.4% aqueous NaBH4 0.1% solution of LiB(C2Hs)3H in THF 10 ml of 4% aqueous NaBH4 0.05 ml of 1% aqueous NaBEt4 0.8 ml of 6% aqueous NaBH4

pH-4.1 Acetate buffer solution; pH~4.9 pH~4 pH~4 0.5 ml of acetate buffer 1 mol dm-3; pH-4.9 2 ml of acetate buffer; pH~4.5 HCI 0.01 mol dm- 3; pH~ 2

[39] [40]

NaBEt4 NaBH4 NaBEt4

10 ml of 0.01% aqueous NaBEt4 5 ml of 4% aqueous NaBH4 0.1 ml of 0.3% aqueous NaBEt4

0.5 ml of acetate buffer 5 mol dm-3; pH~4.5 0.15 ml of HCl 12 mol dm-3; p H - 1.5 Acetate buffer; pH ~5

[13] [33] [42]

MexSnO- x)+ Inorganic Sn MexSnt4 n-BuxSn (4- ~)+ Et3Sn + BuxSnC4-x)+ Inorganic Ge MexGe(4- x)+ As "l As v Monomethylarsine Dimethylarsine Trimethylarsine MexPb(4- x)§ MeHg § Hg2+ MeHg § Hg 2+, MeHg + Hg 2+, MeHg § Hg2§ MeHg + Me2Hg, Et2Hg MeHg + Hg 2+, MeHg § Me3Sn+, Me2Sn2+, MeSn 3+ Hg2+, MeHg § Me3Pb+, Me2Pb2+

b...L.

o [281 [28]

[21] [21] [41] [19] [32]

Cryofocusing for on-line metal and metalloid speciation in the environment

385

r

,a,

o Jl

~

'~

m

Figure 12.3 Chromatograms obtained in the analysis of a 50 ml aqueous solution containing MeSn3+ (11.7 ng as Sn), EtzSn2+ (12.2 ng as Sn)and Bu2Sn2+ (11.1 ng as Sn) with MeaSn as internal standard by CT-GC-QFAAS after derivatization by (a) hydride generation and (b) ethylation.

12.2.2

Cryofocusing and separation

Following the volatilization of the analytes, the cryofocusing and separation steps represent an important part of the analytical scheme. Gaseous handling of the analytes allows to preconcentrate them on-line, similarly to flow injection technique using liquid micro-columns. Cryofocusing is a simple and efficient way to obtain preconcentration factors of 50--100 fold. 46 This step is fundamental in the analytical chain since it offers excellent on-line sensitivity and provides low detection limits. Large sample volumes can also be processed, improving the detection capabilities of the system. In general, a significant amount of water is simultaneously volatilized together with the derivatization of the analytes. To remove this water, a large array of desiccant systems has been tried. Calcium carbonate and other drying agents usually result in partial trapping or decomposition of the analytes. A Naphion dryer permeable tube can be successfully used. The best water removal systems are based on cold traps. A cold trap made of a mixture of dry ice and acetone ( - 2 0 ~ placed just before the cryotrap removes most of the water. Recently, a cryogenically based system has allowed accurate cold temperature control and has proven the removal of most of the water without trapping the analytes of interest. 47 In most cases, however, if the reaction vessel has been designed properly in order to

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minimise foam carry-over, the water trap is not needed. Trapping water will not affect the detection step if desorption cycles are efficient. The design of the cryo- and separation trap and the quality of the packing phase are critical for optimal selectivity and sensitivity. Cryofocusing of the analytes is most classically performed in a U-shaped glass column (30-45 cm length, 6 to 8 mm i.d.) packed with chromatographic material and immersed in a bath of liquid nitrogen. The packing material is secured with silanized glass wool plugs. The quality of the stationary phase, the chromatographic support and its mesh size are critical. They should offer inert support for the preconcentration step, have a mesh size that allow sufficient gas stream to pass through during the warming stage, and contribute to the separation of the compounds upon warming of the column. Mesh sizes ranging from 80 to 120 with 5% to 20% loading of non-polar silicon-based stationary phases, such as OV 1, OV 3, OV 101, SE 50, SE 54 and SP 2100, have been used for the determination of methylated tin, alkyl lead and mercury species. 25"28"30"34"35"39"48 Lighter loadings, from 3% to 5%, are necessary for methylated arsenic 49 or selenium 5~and species with higher boiling points such as butyltin compounds. 36 A correct passivation of the column is of outstanding importance since analytes may decompose on active surface sites. This problem is easily overcome by extensive silanization of the whole column. Poor silanization leads to loss of analytes, poor reproducibility, appearance of memory effects, alteration in the retention times, peak tailing and shorten of the average life time of the column. Slow silanization by passing a silanizing agent such as hexamethyldisilazane through the column with a flow of helium during gradual warming of the column provides high desorption efficiency, as concluded in our laboratory (unpublished data). This cryogenic trap is also used as a limited chromatographic separation column after it is removed from the bath and electrically heated by a Nichrome resistance wire connected to a power supply. Despite its low resolution ability (about 1300 theoretical plates), 8"~2 such a simple chromatographic step is enough to separate most of the organometallic compounds of environmental interest. 46 Separation of the trapped species does not rely mainly on the chromatographic properties of the chromatographic material, but on differential sublimation procedure based on the boiling point of the species trapped in the column (Figure 12.4). Helium flows used as a carrier or desorbing gas are very important to get optimum sensitivity. In general, helium flow rates used range from 50 to 200 ml min -~. After cryofocusing, controlled warming of the column optimises the separation of the analytes avoiding thermal decomposition and non-quantitative stripping of the species. The reproducibility obtained in the retention times is less than 2%. The interfaces from the reaction vessel to the column and from the column to the detector must be as short as possible in order to minimise dead-volumes and, consequently, to avoid lack of reproducibility, peak tailing, memory effects and loss of sensitivity.

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Cryofocusing can also be used for other purposes, mainly to improve quality in sampling procedures. Off-line preconcentration of non-volatile analytes in liquid samples can be attempted directly in the field by cryofocusing techniques. A large volume of liquid sample can be treated by any derivatization technique and the volatile species formed can then be cryotrapped directly in the field on a large array of supporting materials (carbon trap, Tenax, silicon based stationary phases, etc.). The column can be stored in commercially available portable cryogenic containers. Analysis can be performed off-line in the laboratory after flash desorption of the column. Recently, a field compact system for on-line preconcentration of liquid environmental samples directly in the field has been developed in our laboratory. 47 It provides quantitative transfer of the volatile organometallic compounds from the water sample (up to 1 1) to the cryotrap. In addition, field determination is of growing interest, so that these general procedures have been assembled in a compact portable set-up for preconcentration of volatile organoselenium species in sea-water samples followed by AFS on-line detection. 47 In this system (Figure 12.5), a flow of helium is directly passed through a column of water

E

Figure 12.4 Speciation of mercury compounds by Eth-CT--GC-QFAAS: temperature profile in the column showing the relationship among molecular weight, desorption temperature and retention time of MeHgEt (first peak) and HgEt2 (second peak).

A. de Diego et al.

388

::_

I

:.i

He oOo

.~

1,2 ~l

:.~ P 9

q

2~ go 9

....

~

Figure 12.5 Schematic description of a compact portable set-up for in-field preconcentration and detection of organoselenium volatile species in sea water. 1, Transfer of the sample into the stripping unit; 2, extraction and cryofocusing of the analytes; 3, chromatographic separation and AFS detection; 4, calibration by injection of standard solutions. sample and, after removing traces of water, the volatile compounds are collected in a cryogenic trap and finally detected by cold vapour atomic fluorescence spectrometry. A similar device (Figure 12.6) has also been adapted for air sampling. 51A volume of air is aspirated by means of a pump and passed through a column held at the temperature of nebulized liquid nitrogen ( - 175~ after removal of suspended particles by filtration and elimination of water traces in a water trap (column immersed in an acetone+ice). The column containing the trapped volatile species is then stored in a portable cryogenic bath until analysis in the laboratory. These different approaches illustrate the potential existing in the cryogenic handling of the analytes. An important part of the future of trace and speciation analysis will certainly make improved use of cryotrapping techniques for environmental purposes. 12.2.3

Atomisation and detection

Detection of the analytes after their elution from the cryotrap and the separation unit is also a fundamental process to be controlled for optimal sensitivity. Most of the earlier

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Figure 12.6 Schematic description of an air sampler for volatile metal and metalloid species collection in the atmosphere. applications made use of atomic absorption (AAS) as detection technique. However, this hyphenated system can also be used with many detectors using atomic spectrometry signal. The most critical condition to obtain a good sensitivity response with any type of detectors is to obtain efficient atomisation of the analytes prior to detection. Since the analytes arrive to the detector under molecular formulation, atomisation processes need to be well understood and controlled. Detectors other than atomic absorption spectrometers such as atomic fluorescence (AFS) or inductively coupled plasma-mass spectrometry (ICP/MS) have been successfully used.

12.2.3.1

Atomic absorption

The most popular techniques were developed for AAS. In this case, the end of the cryotrap is directly connected to a quartz furnace. Graphite furnace systems have also been attempted but their environmental applications have been limited. 3 An efficient and inexpensive alternative to the graphite tube atomiser consists of a heated quartz cell aligned in the beam of the spectrophotometer. To optimise the sensitivity, a large array of cell designs has been developed. All the designs tend to increase the residence time of the atoms generated. A typical cell design is presented in Figure 12.7. This atomiser is generally electrically heated up to 900~ This temperature is high enough to obtain dissociation of most of the organometallic species due to the weakness of the metal-C or metal-H bounds, and to generate a cloud of metal atoms for the absorbance measurement. Temperature ranging from 700~ to 900~ is not sufficient for full atomisation of selenium and arsenic species or large organometallic molecules. 3 In order to obtain complete atomisation, addition of gases, such as 02 and H2, is generally required. Addition

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A. de Diego et al.

S

t

t

Figure 12.7 Design of a typical cell used in quartz furnace atomic absorption spectrometry. of H2 and 02 leads to the generation of a dynamic population of OH" and H" radicals and to strip the molecule to the bare metal. Atomisation of molecules under these conditions has been suggested to occur via free radical mechanisms by Dedina and Rubeska 52 for SeHz (see Chapter 8 for more detailed information) and by Welz and Melcher 53 for AsH 3. The design of the quartz atomiser should avoid any dead volume and the introduction of the atomising gas should be as close as possible to the light-path of the atomic absorption spectrometer. Oxygen is generally provided in small quantities (few ml min-') compared to the hydrogen introduction flow rates (100 to 200 ml min-~). However, the optimal ratio of these atomisation gases is closely dependent on the cell design. 54 For an organotinhydride species, the following mechanisms may take place in the quartz cell: R2SnHz + 4H" ~ 2RH + 2H2 + Sn ~

(12.4)

The simultaneous introduction of other molecules from the matrix with the analytes results in a competition in the consumption of free radicals available for atomisation and, hence, will lead to an overall depletion of the absorbance signal. In general, the use of a background correction system for non-specific absorbances is not required and alters the signal-to-noise ratio of the overall instrumentation. The overall signal response registered by the detector is a combination of all the processes involved in this hyphenated system. A typical schematic signal response for an

Cryofocusing for on-line metal and metalloid speciation in the environment

391

organotin chromatogram of inorganic, methyl- and butyltin compounds is presented in Figure 12.3. Theoretically, for the same amount of Sn metal injected, the same surface area should be observed as response. This is not generally the case and the observed surface response certainly takes into account the summary of mechanisms occurring throughout the analytical procedure of the hyphenated system. In general (see Figure 12.8), hydridisation or ethylation of the analytes shows a high yield, close to 100% (Process 1). However, the stripping (liquid to gas partitioning of the analytes (Process 2)) may be less efficient with respect to the formed compounds presenting the highest boiling points. Both desorption kinetics from the column (Process 3) and atomisation efficiency (Process 4) will be less efficient with these highest boiling point species, leading to an overall poorer signal response expressed both in peak height and surface area measurements. However, this flow through the hyphenated system performs all the different procedures on-line and yields very reproducible responses (RSD less than 5% for a solution of organometallic species of 50 ng 1-~ and a total sample volume of 25 ml). It allows the rapid and quantitative determination of a large array of metals and metalloids species in the environment. 12.2.3.2

Atomic fluorescence

Atomic fluorescence detectors are usually more compact than atomic absorption spectrometers. They can be efficiently hyphenated to the cryofocusing device for sensitive environmental species determination. The first simple system using the hyphenation of a cryotrap to an AFS detector has been reported for mercury determination. 4~ Earlier systems with detection by atomic fluorescence made use of a water trap and a carbotrap trapping system operating at room temperature. 4~The system presented by Ritsema et al. considerably simplifies the hyphenation between the different parts compared to that reported by Bloom. The reaction vessel had variable content, ranging from 250 ml up to 1 1. All parts were made of borosilicate and Teflon. Water was removed with a Naphion drier tube fitted before the cryotrap. The derivatisation reaction used for the determination of mercury species in sea water was hydride generation. Atomisation of the mercury species was achieved thermally by means of a small oven (a quartz tube filled with quartz wool) heated to 900~ just at the entrance of the detector. Atomic fluorescence detectors are usually compact, robust and provide excellent detection limits. Under routine conditions this system gives absolute detection limits less than 2 pg for both methyl- and inorganic mercury. The concentration detection limits obtained when analysing environmental samples are around 0.1 ng 1-1. These simple systems are very effective for low detection limits in environmental waters. The combination of large sample volumes to be stripped with the use of sensitive and cheap detectors such as atomic fluorescence provides excellent low level capabilities for environmental species determination.

392

A. de Diego et al.

Recent work using the combination of a large stripping vessel (1 1) and cryofocusing after water removal in a water trap kept at - 2 0 ~ gave excellent results for volatile selenium species determination in the environment. 47 Routine detection limits in this case are also very low, less than 7 pg 1-! (90 fmol) for real samples. Non-dispersive AFS is a quite selective technique but, like any other spectroscopic method, it can be affected by

SnI-h

MeSn~ BuSnI-13

3SnH

y

::.-...... .....

~

ii~i::::i~::

Time

o

........... i:iii~ii:::~

i'iii:.i':ii !i~:,i! ..... ......

o [-.,

i'ii:,i:i~,:i i:iii:;i:i .......... i".-.: :~: :!ii~ ?:e

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i!ii/:i!!:.! i~!i!:i!i

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Process I

<

Process 3 <

i!!i~i~i~i

excellentderivatisati~ (hydride~thyl) iii'iii:ii exceUem stripping effi!~ncy ...

Time y

~,!ii,'i ~i:i:.~.~.'

excellentdesorption co~itions ~,~.~:~

Process 4 excellent atomisation condit!ons Figure 12.8 Quality of the peaks corresponding to species with increasing boiling points as a consequence of the efficiency in the different processes occurring during determination of organotin species by HG--CT-GC-QFAAS.

Cryofocusing for on-line metal and metalloid speciation in the environment

393

interferences taking place at the detection level. 4~ Detection is based on emission properties of selenium atoms formed in the flame and excited with a selenium discharge lamp. A multireflectance filter centred at 200 nm (bandpass at half-light 10 nm) collects the emitted light from the most important atomic fluorescence lines of the selenium atoms (196.02 nm, resonance, 203.98 nm and 206.28 nm). However, some spectral interferences can be observed under certain conditions. These interferences were recently evidenced by P6cheyran et al. for selenium species determination. 47 The presence of volatile carbon containing species in environmental samples leads to the production of radicals in the flame. Possible electronic transitions originating from thermal excitation and de-excitation processes of radicals produced in the flame may emit light in the same wavelength as the band path selected for observation. This phenomenon has been particularly well evidenced for the detection of selenium species in environmental samples. Interference signals discrimination may be achieved by analysing first the sample in normal AFS conditions (e.g. lamp switched on) and then with the lamp switched off. As illustrated in Figure 12.9, when the AFS lamp is turned on, the signal recorded is the sum of the signal generated by selenium atoms and that generated by carbon interfering species likely to occur in the sample. When the lamp is turned off, only the interfering signal is recorded since no fluorescence processes can be involved in these conditions. Despite the lack of selectivity, hyphenated systems based on cryofocusing and AFS should know further development in the future and allow the promotion of reliable "onsite" species determination. These procedures by-pass many of the problems associated with sample storage and potential interconversion among species. They will certainly bring a new insight to biogeochemical processes.

I Lamp ON

Figure 12.9 Selenium speciation analysis in marine water samples from the Gironde estuary (France) by cryofocusing and AFS detection: evidence of CO2 interference in the selenium signal.

A. de Diego et al.

394

m

Ar

Figure 12.10 Schematic description of a CT-GC-ICP/MS hyphenated system for volatile metal species determination.

12.2.3.3

Inductively coupled plasma-mass spectrometry

New information with respect to metal and metalloid species occurrence will also be brought forward with the use of ICP/OES or ICP/MS as detector after cryofocusing systems. No work has currently been reported using ICP/OES but the multielemental detection capabilities of plasma sources using mass spectrometry has recently received increasing attention. 5~58 These authors described the use of ICP/MS as detector after cool trapping conditions. A trapping temperature of - 80~ was used to avoid the condensation of CH4 in the trap. These developments were applied to the detection of volatile species (hydrides of methylated metal or metalloid species) naturally formed in the environment. Cryotrapping hyphenated to ICP/MS detection has also recently been described for the determination of biogeochemically formed metals or metalloids in the environment or of anthropogenic species (alkylleads) (Figure 12.10). 5~'59 The combination of the cryotrapping (bringing important preconcentration factors) to

Cryofocusing for on-line metal and metalloid speciation in the environment

395

the sensitivity and selectivity of ICP/MS provides a very powerful tool for environmental research. Further, after the preconcentration step brought by cryofusing of the analytes, desorption results in the direct introduction of the analytes in gaseous form to the detector, greatly minimising the formation of interfering species in the plasma compared to results obtained with wet plasmas. This system provides extremely low detection limits in the pg level for most of the species studied. 51'59A large number of isotopes can be monitored (up to 30) by setting the appropriate data acquisition parameters of the detector. The number of channels to be used is only dependent on the type of information which is required. A good chromatographic peak definition shows evidence of the occurrence of new species in environmental samples. However, data acquisition should allow enough isotope accuracy to unravel species discrimination from interferences. A large number of volatile and unstable species will certainly be trapped in the cryotrap. They will sometimes generate the formation of interferences that will be detected in the mass spectrometry mode of the instrument. The simultaneous recording of several isotopes of the same element (when possible) and the comparison with the theoretical ones easily rules out the occurrence of spurious species. Figure 12.11 shows the chromatograms obtained in the simultaneous analysis of different volatile metal species by CT-GC-ICP/MS. After elution from the cryotrap, the volatile analytes carried by a helium stream (100 ml

80000

0

E

0

_= 0

Figure 12.11 Simultaneous detection of volatile metal species in standard aqueous solution by CT-GC-ICP/MS. The temperature (T) profile of the column during the desorption step is also provided. 1, 2~ pg; 2, 2~ pg; 3, I2~ pg; 4, I2~ 100 pg; 5: 2~ 150 pg; 6, 2~ pg; 7, 2~ 75 pg; 8, 78SeMe2:150 pg.

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min-~) are mixed just before the torch with additional gases such as Ar, a mixture of Ar/Xe and 02. All the gases are mixed with a special laboratory made 5-ways Teflon connector prior to being introduced in the plasma via the injector tube of the torch (see Reference 51). The additional inlet of Ar is necessary in order to introduce a significant amount of gas in the torch. The additional inlet of Ar/Xe is required to allow good quantitative detection capabilities. The continuous injection of Xe represents continuous sensitivity monitoring during the whole analytical run. Introduction of 02 also compensates for sensitivity drift occurring with the combustion and ionisation of carbon containing gas. All natural gaseous samples contain significant amounts of CO2 o r C H 4. Compared to the work of Feldmann, 5~58 the samples are all trapped at very low temperature ( - 196~ assuring the entrapment of any volatile species present (hydrides are expected to occur naturally in the environment and their boiling points are always low). CO2 and C H 4 will be released first and their abundance in real samples can certainly extinguish the plasma or severely alter the sensitivity of the instrumentation. They are, therefore, eluted first and vented away from the plasma. As soon as they have been removed from the column, the gaseous eluents from the trap are instantly reconnected to the ICP/MS. Despite these precautions, sensitivity responses may fluctuate simultaneously to the elution of important species. The addition of 02 brings to the plasma a more complete combustion of carbon-containing species, improving both sensitivity and reproducibility. 5 This hyphenated system has been used to determine volatile naturally or anthropogenically formed hydrides or organometallic species in different gaseous environments. 5~'55-59 After on-site cryotrapping, both Hg and alkyleads have been monitored simultaneously in an urban European area (Figure 12.12). The levels recorded have shown that a significant decrease in the total volatile lead content in air has been achieved over the last decade. 59 Up to now, all the results presented have been focused on the occurrence of new volatile species in the environment. Air samples have been, therefore, simply cryogenically trapped on the field and flash desorbed in the cryotrap of the hyphenated CT-ICP/MS system. Despite the small amount of authors working on this technique, a large array of new information has been brought forward. Many mono-isotopic light elements have been reported to form volatile species. 5~ However, potential inteferences may still exist due to the monoisotopic character of the element studied. These questions will certainly be solved in the future by the use of ICP/OES or high resolution ICP/MS detectors. Nevertheless, a significant body of new information has been seen, suggesting the occurrence of more complex and novel pathways for metals and metalloids in the environment. Natural liquid and solid samples have not yet been submitted to this type of detection

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systems. The extraction of metals, metalloids and organometallic compounds from liquid and solid matrices and their derivatization and detection by CT-ICP/MS will certainly also bring a new understanding on the simultaneous, synergic or antagonist pathways of elements in the global ecosystem.

12.3

Interferences

These hyphenated flow analytical systems rely on the combination of four important analytical steps that need to be fully controlled in order to achieve precise and accurate species determination in real samples. Compared to many other analytical procedures, all the potential intefering reactions during the different steps have been carefully investigated. They most likely take place during:

(a) (b) (c) (d)

the the the the

hydrydisation/ethylation reactions in the solution transport of the volatile compounds to the column preconcentration/separation step in the column atomisation in the detector.

Figure 12.12 Chromatograms corresponding to the analysis of an air sample collected in Bordeaux (France): a volume of 15 1 of air were on-line filtered, dried and trapped at - 175~ and later analysed in the laboratory by CT-GC-ICP/MS. 1, Hg0:1.5 ng m-3; 2, Me4Pb, 2.5 ng m-3; 3, Me3EtPb, 0.3 ng m-3; 4, Me2Et2Pb, 0.5 ng m-3; 5, MeEt3Pb, 0.1 ng m-3; 6, Et4Pb, 6.9 ng m-3.

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A large amount of literature has been published on hydride generation and ethylation procedures. Interfering reactions may take place in the reaction vessel prior to the stripping of the analytes. The main inteferences likely to take place during the derivatization procedure will, therefore, be reviewed first. More specific types of interferences related to the speciation and taking place in other parts of the hyphenated system will be described in the second part of this section.

12.3.1

Interferences during the derivatization reactions

Both spectral and chemical interferences may happen during analysis by derivatization (HG or Eth) followed by atomic detection. 46 Spectral interferences are, however, essentially minimised since the analytes are stripped from the vessel in a gaseous form while the matrix remains in the solution. In addition, the specificity of atomic detectors reduces the possibility of overlapping between the atomic lines of the analytes and the spectral lines of molecular bands due to volatile matrix compounds. Chemical interferences are more likely to occur in solution or in the gaseous state, as pointed out by Dedina (see References 60 and 61 and Chapter 8 of this book). 12.3.1. I

Inorganic and organic interferents

Most of the interference studies are devoted to the negative effect of metals (transition or hydride/ethyl-derivative forming elements) present in solution during the derivatization step. Liquid phase interferences taking place in solution most usually involve competitive reactions of metals with the reducing agent minimising, therefore, the efficiency of the derivatization of the target analytes. 6~Transition metals may also be reduced to their nonsoluble metallic form. These finely dispersed metal precipitates can entrap or adsorb the derivatized analytes and prevent their gaseous removal from the solution. 62-66Very reactive metal borides may also be produced after reaction with NaBH4 or NaBEt4, which could promote the decomposition of the analytes due to their high reactivity. 67-69The formation of such borides has been known for a long time. 7~ Several types of interfering agents such as organic (e.g., diesel fuel, gasoline, pigments, fulvic and humic acids, solvents) and inorganic compounds (e.g., thiol groups, chloride, bromide, nitrate, nitrite, perchlorate) can also modify the yield of the derivatization reactions.a0. 71-74The interferent effect of diesel fuel in organotin speciation by AAS after derivatization by hydride generation was first reported in 1986. 75 Interferences due to high amounts of gasoline have also been observed in the determination of tributyltin (TBT) in sediments. TM In both cases the interference was overcome by addition of a higher amount of derivatization agent. Poor butyltin recoveries were obtained by Quevauviller et al. 73 in sediments displaying high sulphur, hydrocarbon or chlorophyll pigment contents. It was

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399

concluded that poor recoveries were due to an inhibition of the hydride generation rather than to interference at the atomisation stage. Organic and sulphide-rich matrices may certainly react with NaBH4 and compete with the analyte compounds during the derivatization step in selenium speciation analysis by HG coupled to cryofocusing and QFAAS. 77 The presence of nitric acid and/or nitrites also affects the analysis by forming volatile nitrogen oxide species which interfere in the gas phase. 67'78. This interference can be avoided by addition of sulphanilamide or sulphamic acid. In the case of selenium determination, the use of cyano-borohydride leads to a general improvement in sensitivity, while the signal is completely suppressed by the presence of nitrite. 79

12.3.1.2 Masking agents Different approaches have been attempted to reduce the effect of interferences on metal analysis by volatilization-atomic detection combined techniques. The addition of some masking/chelating agents such as EDTA, 71'80-85potassium iodide, 86'87 ascorbic acid, 87"88 Lcysteine and L-cystine 89-93has been reported to effectively eliminate the interfering effects of most of the transition metals. Special care must be paid, however, since these agents can also react with the target analytes preventing their derivatization in the solution. 71'72 Welz and Shubert-Jacobs 94 proved that increasing the acid concentration generally extends the range of tolerance of transition metals in hydride generation. The interfering effect of metals can also be overcome by adding a higher amount of derivatizating reagent to the reaction vessel. 95 For ethylation, this recourse must be handled with care since a high concentration of NaBEt4 can give rise to non-identified peaks in the chromatogram, due to impurities in the reagent. This fact has recently been confirmed in our laboratory for mercury speciation analysis (unpublished data). Howard and Arbab-Zavar 87 individually considered the effect of different metals, metaloids, anions and oxianions during the determination of arsenic species by hydride generation, cryofocusing and atomic absorption spectroscopy. They tried to overcome the effect of the interfering agents by (a) off-line ionic exchange with Kelex 100 resin and (b) addition of EDTA masking agent. It was concluded, however, that the concentration of potential interfering agents in natural waters is not high enough to affect the analysis. The reduction of the interfering effect by transition and hydride forming metals has also been attempted by addition of L-cysteine 89'92 and of potassium iodide in a solution of ahydroxiacids. 95 Although interferences may be limited in water sample determinations, this is certainly not the case when polluted sediment extracts have to be analysed. In order to understand the type of mechanisms involved in signal suppression with contaminated sediment extracts, investigations have been performed with model mixtures presenting inorganic levels similar to what can be expected in polluted sediment extracts. This study was

400

A. de Diego et al.

performed for tin species determination. Martin et al. 71"72 investigated the effect of a large list of potential interferents in simultaneous analysis of synthetic aqueous solutions of monomethyltin (MMT), monobutyltin (MBT), dibutyltin (DBT) and tributyltin (TBT) by HCv-CT-GC-QFAAS at 50 ng 1-1 level. A multielemental metal solution caused severe signal suppression for all the organotin compounds at a level of 100 mg of each element in a 50 ml reaction vessel. Individual studies of each interfering metal showed that different mechanisms could take place in the reaction vessel. The presence of EDTA suppressed most of the interferences by forming complexes with the inorganic species and improving the formation of DBT and TBT hydrides. However, EDTA also formed complexes with MMT and MBT. L-cysteine improved the reproducibility and sensitivity of MBT determination and prevented the interfering effect of metals on the speciation of organotins. The addition of organic compounds such as organic solvents, polychlorinated biphenyls, pesticides and n-alkanes had little effect on the signal of all the organotin species. However, reproducibility of the analysis was significantly affected by the presence of humic acids in solution. A recent comparative study on the types of interferences generated by NaCI, a mixture of metals and EDTA on the simultaneous determination of inorganic mercury (Hg 2+) and methylmercury (MeHg§ by derivatization-CT-GC-QFAAS has been performed together with a comparison between hydride generation and ethylation techniques. 96 For hydride generation derivatization reactions, the sensitivity of MeHg § determination is affected by the presence of NaCI in the sample. NaCI could catalyze the decomposition of MeHg § to Hg 2§ prior to reaction with NaBH4. This process is prevented if EDTA is added to the solution. NaCI however, has no influence on the determination of Hg 2§ . A signal decrease for both Hg 2§ and MeHg § is observed in the presence of a multielemental solution. Here again, EDTA totally suppresses the interfering effect of metals when hydride generation is used in the derivatization stage. When dealing with ethylation reactions, a significant decrease in sensitivity is observed during the determination of Hg 2§ and MeHg § in the presence of high ionic strength solutions. The direct reduction to Hg ~ of both MeHg § and Hg 2+ is enhanced by the addition of NaCI. This effect is much more significant than what has been observed for hydride generation. Addition of EDTA also allows the recovery of full efficiency for Hg 2§ determination. Surprisingly, the presence of metals in solution do not induce any significant effect when ethylation reactions are used. Addition of EDTA enhances the sensitivity for Hg 2+ and decreases it for MeHg § The interfering effect of chloride in the analysis of MeHg § in marine water samples by EthG-CT--GC-AFS had already been previously reported] ~ 40 Horvat et al. 2~ noticed that the addition of copper to the solution resulted in suppression of the chloride interference. Finally, off-line separation procedures, such as selective precipitation, 26 liquid-liquid

Cryofocusing for on-line metal and metalloid speciation in the environment

401

extraction, 87 ionic exchange 87 and clean-up procedures by solvent extraction 97 have also been attempted to eliminate potential interfering metals from the matrix.

12.3.2

Other types of interferences

The quality and the dead volume of the different interfaces in the hyphenated system is a potential source of error in the determination of metal species in environmental samples. Careful attention should be paid to the design of the interfaces between

(a) (b)

the reactor and the column and the column and the detector.

A correct design and efficient heating of the interfaces should however, minimise losses of analytes by condensation. Re-arrangements, transalkylation processes or decomposition of the analytes may also happen in the column, giving rise to the appearance of new peaks, changes in the retention times or memory effects. These aspects can be suppressed by careful conditioning of the trap column. Finally, interferences occurring during the gaseous state are mainly associated with atomisation procedures. As mentioned earlier in this chapter, atomisation efficiency depends on the effectiveness of the collisions between free radicals and the volatile analytes. The short lifetime of radicals 98 and competition with other volatile compounds in the quartz furnace 99 mainly account for low atomisation efficiency. Decomposition of the quartz furnace and poisoning of this surface by high metal deposition can also lead to significant losses of sensitivity. 48 Most of the problems associated with changes in the atomisation yield can be overcome with the use of an internal standard.

12.4

Applications

The different flow techniques relying on cryofocusing and separation upon heating have been applied successfully for two decades to the determination of a wide array of species for elements such as As, Sn, Se, Ge, Sb, Hg, etc. in a wide variety of matrices. Table 12.2 presents a summary of the different techniques and applications of this simple, low-cost and effective method.

12.5

Conclusions

Investigations on the fate of metals and metalloids in different environmental compartments asks for the development of analytical techniques able to distinguish among the different chemical forms the metal presents in a variety of environmental samples.

4~ to Table 12.2 Summary of applications reported in the literature using atomic detection coupled to cryofocusing after volatilization of the target analytes for speciation analysis of several elements in different environmental samples Species

Reagent

Packing material

Detector/Detection Limit

Sample

Reference

Inorganic Sn MexSnt4-x)§

NaBH4

Chromosorb GAW-DMCS 45-60, 3% SP2100

QFAAS 20-50 pg as Sn

[30]

n-BuxSnt4-x~§

NaBH4

Chromosorb GAW-DMCS 45-60, 3% SP2100 Chromosorb GAW-DMCS 45-60, 3% SP2100

Water samples Sediments Biota Water samples Sediments Oysters

[ 100]

Synthetic solutions

[39]

Water samples Pore waters

[ 101 ]

Water samples Biota Seawater

[40]

Fish tissues

[ 19]

Biological tissues

[32]

Sediments

[ 13]

Biological tissues

[33]

Water samples

[47]

MexSn~4-x)§ n-BuxSnC4-x~§

NaBH4

Me~Pb~4-,o+

NaBEt4

As "t, Asv Monomethylarsine Dimethylarsine Hg 2§ MeHg+, Me2Hg

NaBH4

NaBH4

Hg2§ MeHg +

NaBH4

Hg2§ MeHg §

NaBEt4

Hg 2§ MeHg § Me2Hg, EtEHg MeHg §

NaBH4 NaBEt4

Hg2§ MeHg §

NaBH4

Volatile Me2Se, Me2Se2

(-)

Chromosorb WAW-DMCS 80-100, 10% SP2100 Glass beads, 40 mesh

QFAAS I 1-45 pg as Sn

QFAAS I. 1-2.5 ng for 0. l g of wet tissue QFAAS 9-10 pg as Pb QFAAS 19-61 pg as As

[36]

>

o Chromosorb WAW-DMCS 60-80, 15% OV- 3 Chromosorb GNAW 604580, 10% SP2100 Chromosorb WAW-DMCS 45-60, 10% OV-101 Chromosorb GAW-DMCS 45-60, 10% SP2100 Chromosorb W-HP 60-80, 10% SP2100 Chromosorb W-HP 60-80, 10% SP2100 Chromosorb W-HP 60-80, 10% SP2100

CVAFS 0.6 pg as Hg AFS 0.l-l.0 ng l -I as Hg QFAAS 4-75 ng g-~ as Hg, wet tissue QFAAS 50-1 l0 pg as Hg QFAAS 0.5 ng g-~ as Hg QFAAS 3 ng g-~ as Hg, wet tissue AFS

[41 ]

Cryofocusing for on-line metal and metalloid speciation in the environment

403

Cryofocusing coupled to atomic detection after derivatization of the target analytes provides a versatile, sensitive and selective tool for metal speciation. The automation of such a hyphenated on-line system highly increases the overall reproducibility of the technique, being on average less than 5% for a 50 ng 1-~ solution. Derivatization of the analytes by hydride generation or ethylation minimises further interferences in the detection step, since most of matrix interferents remain in the liquid phase. Cryofocusing allows on-line preconcentration of large volumes of sample, so that the technique matches the low concentrations usually found in real samples. The incorporation of the preconcentration step into the hyphenated system simplifies sample manipulation, avoiding contamination and keeping to a minimum potential sources of error. Atomic detection provides both selectivity to the analysis and very low detection limits (about 10-100 pg of metal). Analysis of sediments, soils and biological tissues requires previous treatment of the sample, in order to quantitatively extract the target analytes to a liquid phase. The recent use of open focused microwave assisted extraction techniques allows the prediction for the near future the on-line implementation of the solid sample pre-treatment step. Furthermore, once derivatized, the gaseous handling of the analytes provides important sensitivity and unsuspected stability. The introduction of ethylation reactions has extended the field of applications. Other derivatizing reagents will also certainly further extend the potential of this analytical approach. The expansion of this interesting technique is limited due to the poor efficiency of the separation step. Despite its already long use, future optimisation of the efficiency of the chromatographic separation step and automation of the whole analytical chain could certainly considerably expand the field of application of this very promising technique for environmental research.

Acknowledgement A. de Diego is grateful to the Spanish Government for his post-doctoral fellowship. C. M. Tseng acknowledges the Taiwan Government for his PhD. grant.

12.6

References

1 Bernhard, M., Brinckman, E E. and Irgolic, K. J. In: The Importance of Chemical Speciation in Environmental Processes, SpringVerlag, Berlin, 1986, p 7. 2 Elinder, C. -G. In: ChangingCycles and Human Health, SpringVerlag, Berlin, 1984, pp 265. 3 Donard, O. F. X. and Ritsema, R. In: Environmental Analysis: Technique, Applications and Quality Assurance, Elsevier Science Publishers,Amsterdam, 1993, p 549. 4 Craig, P. J. In: Organometallic Compoundsin the Environment, Longman, Harlow, 1986, p 65. 5 Hewitt, C. N. and Harrison, R. M. In: Organometallic Compounds in the Environment, Longman, Harlow, 1986, p 160. 6 de Mora, S. J., King, N. G. and Miller, M. C., Environ. Technol. Lett., 1989, 10, 901. 7 Martin, F., Developmentof Analytical Methods for Speciation of Organometallic Compounds of Tin and

404

A. de Diego et al.

Mercury in the Environment. PhD Thesis. University of Bordeaux, 1993. 8 Donard, O. F. X. and Martin, F., Trends Anal. Chem., 1992, 1, 47. 9 Harrison, R. M. and Rapsomanikis, S. (Eds.), Environmental Analysis Using Chromatography Interfaced with Atomic Spectrometry, Ellis Horwood, Chichester, 1989. 10 Chau, Y. K. and Wong, P. T. S., Fresenius J. Anal. Chem., 1991, 339, 640. 11 Chau, Y. K., Wong, P. T. S. and Goulden, P. D., Anal. Chem., 1975, 47, 2279. 12 Donard, O. F. X. and Pinel, R. In: Environmental Analysis Using Chromatography Interfaced with Atomic Spectrometry, Ellis Horwood, Chichester, 1989, 189. 13 Tseng, C. M., de Diego, A., Martin, F. and Donard, O. F. X., J. Anal. Atom. Spectrom., 1997, 12, 629. 14 BORWIN, Logiciel intuitif pour la Chromatographie, version 1.2, JMBS DEVELOPPEMENTS, 9 Copyright 1994. 15 Reamer, D. C., Veiilon, C. and Toukousbalides, P. T., Anal. Chem., 1981, 53, 245. 16 Donard, O. F. X. and Pedemay, P., Anal. Chim. Acta, 1983, 153, 301. 17 Campbell, A. D., Pure appl. Chem., 1992, 2, 227. 18 Ashby, J. R. and Craig, P. J., Sci. Total Environ., 1989, 78, 219. 19 Fisher, R., Rapsomanikis, S. and Andreae, M. O., Anal. Chem., 1993, 65, 763. 20 Horvat, M., Bloom, N. S. and Liang, L.,Anal. Chim. Acta, 1993, 281, 135. 21 Craig, P. T., Mennie, D., Ostah, N., Donard, O. F. X. and Martin, F., Analyst, 1992, 117, 823. 22 Craig, P. J., Mennie, D., Ostah, N., Donard, O. F. X. and Martin, F., J. Organomet. Chem., 1993, 447, 5. 23 Holak, H.,Anal. Chem., 1969, 41, 1712. 24 Nakahara, T., Prog. Anal. Atom. Spectrosc., 1983, 6, 163. 25 Braman, R. S. and Tompkins, M. A., Anal. Chem., 1979, 51, 12. 26 Campbell, A. T. and Howard, A. G., Anal. Proc., 1989, 26, 32. 27 Cutter, G. A., Anal. Chem., 1985, 57, 295 !. 28 Braman, R. S., Johnson, D. L., Craig, C., Foreback, C. C., Ammons, J. M. and Bricker, J. L., Anal. Chem., 1977, 49, 621. 29 Hambrick, G. A., Freolich, P. N. andreae, M. O. and Lewis, B. L., Anal. Chem., 1984, 56, 42 !. 30 Donard, O. F. X., Rapsomanikis, S. and Weber, J. H., Anal. Chem., 1986, 58, 772. 31 Filippeili, M., Baldi, F., Brinckman, F. E. and Olson, G. J., Environ. Sci. Technol., 1992, 26, 1457. 32 Puk, R. and Weber, J. H.,Anal. Chim. Acta, 1994, 292, 175. 33 Tseng, C. M., de Diego, A., Amouroux, D., Martin, F. and Donard, O. F. X.,J. Anal. Atom. Spectrom., 1997, 12, 743. 34 Hodge, V. F., Seidel, S. L. and Goldberg, E. D., Anal. Chem., 1979, 51, 1256. 35 Andreae, M. O. and Byrd, J. T., Anal. Chim. Acta, 1984, 156, 147. 36 Randall, L., Donard, O. F. X. and Weber, J. H.,Anal. Chim. Acta, 1986, 184, 197. 37 Cai, Y., Rapsomanikis, S. and Andreae, M. 0., .1. Anal. Atom. Spectrom., 1993, 8, 119. 38 Cai, Y., Rapsomanikis, S. and Andreae, M. O., Anal. Chim. Acta, 1993,274, 243. 39 Rapsomanikis, S., Donard, O. F. X. and Weber, J. H., Anal. Chem., 1986, 58, 772. 40 Bloom, N., Can. J. Fish. Aquat. Sci., 1989, 46, I 131. 41 Ritsema, R. and Donard, O. F. X., Appl. Organomet. Chem., 1994, 8, 571. 42 Ceulemans, M. and Adams, F., J. Anal. At. Spectrom., 1996, 11,201. 43 Andreae, M. O. In: Trace Metals in Sea Waters, Plenum Press, New York, 198 !, pp !. 44 Rapsomanikis, S., Donard, O. F. X. and Weber, J. H., Anal. Chem., 1986, 58, 35. 45 Honeycutt, J. B. and Riddle, J. M., J. Am. Chem. Soc., 1961, 83, 369. 46 Ritsema, R., Environmental Applications of Hyphenated Techniques for the Speciation of Tin, Arsenic and Mercury: Monitoring Butyltin Levels in Marine Environments of the Netherlands. PhD Thesis. University of Pau, 1997. 47 Pecheyran, C., Amouroux, D. and Donard, O. F. X., J. Anal. At. Spectrom., 1998, 13, 615. 48 Donard, O. F. X. In: Sonderdruck aus Colloquium Atomspektrometrische Spuren Analytik (5), Bodendeewerk Perkin-Elmer, Uberlingen, 1989, pp 395. 49 Apte, S. C., Howard, A. G. and Campbell, A. T., in Environmental Analysis Using Chromatography Interfaced with Atomic Spectrometry, Ellis Horwood, Chichester, 1989, pp 258. 50 Howard, A. G. In: Environmental Analysis Using Chromatography Interfaced with Atomic Spectrometry, Ellis Horwood, Chichester, 1989, pp 318.

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405

P6cheyran, C., Quetel, C. R., Martin, F. M. and Donard, O. F. X., Anal. Chem., 1998, 70, 2339. Dedina, J. and Rubeska, !., Spectrochim. Acta, 1980, 35B, 119. Welz, B. and Meicher, M.,Analyst, 1983, 108, 213. Sarradin, P. M., Leguille, F., Astruc, A., Pinel, R. and Astruc, M., Analyst, 1995, 120, 79. Feldmann, J., Griimping, R. and Hirner, A. V., Fresenius J. Anal. Chem., 1994, 350, 228. Feldmann, J. and Hirner, A. V., Int. J. Anal Chem., 1995, 60, 339. Krupp, E. M., Gr~mping, R., Furchtbar, U. R. R. and Hirner, A. V., Fresenius J. Anal, Chem., 1996, 354, 546. Feldmann, J., Riechmann, T. and Hirner, A. V., Fresenius J. Anal. Chem., 1996, 354, 620. P6cheyran, C., Lal6re, B. and Donard, O. F. X., submitted to Environ. Sci. Technol. Dedina, J., Anal Chem., 1982, 54, 2097. Dedina, J., Prog. Anal, Spectrosc., 1988, 11, 251. Smith, A. E.,Analyst, 1975, 100, 300. Kirkbright, G. F. and Taddia, M., Anal. Chim. Acta, 1978, 100, 145. Welz, B. and Melcher, M.,Analyst, 1984, 109, 569. Welz, B. and Melcher, M., Analyst, 1984, 109, 573. Welz, B. and Melcher, M.,Analyst, 1984, 109, 577. Brown, R. M., Fry, R. C., Moyers, J. L., Northway, S. J., Denton, M. B. and Wilson, G. S., Anal, Chem., 1981, 53, 1560. Yamamoto, M., Yamamoto, Y. and Yamashige, T., Analyst, 1984, 109, 1461. Bye, R., Talanta, 1986, 33, 705. Schlesinger, H. I., Brown, H. C., Finholt, A. E., Gilbreath, J. R., Hoekstra, H. R. and Hyde E. K., J. Am. Chem. Soc., 1953, 75, 215. Martin, F. M. and Donard, O. E X., J. Anal At. Spectrom., 1994, 9, 1143. Martin, F. M., Tseng, C. M., Belin, C., Quevauviller, P. and Donard, O. F. X., Anal Chim. Acta, 1994, 286, 343. Quevauviller, Ph., Martin, F., Belin, C. and Donard, O. F. X., Appl. Organomet. Chem., 1993, 7, 149. Story, W. C., Caruso, A., Heitkemper, D. T. and Perkins, L., J. Chromat. Sci., 1992, 30, 427. Valkirs, A. O., Seligman, P. F., Stang, P. M., Homer, V., Lieberman, S. H., Vafa, G. and Dooley, C. A., Mar. Pollut. Bull., 1986, 17, 319. Desauziers, V., Leguille, F., Lavigne, R., Astruc, M. and Pinel, R., Appl. Organometal. Chem., 1989, 3, 469. Evans, W. M., Jackson, F. J. and Delan, D.,Analyst, 1979, 104, 16. Cutter, G. A., Anal Chim. Acta, 1983, 149, 391. Mufioz, R., Donard, O. F. X., C/lmara, C. and Quevauviller, P., Anal. Chim. Acta, 1984, 286, 357. Belcher, R., Bogdanski, S. L., Henden, E. and Townshend, A., Analyst, 1975, 100, 522. Drinkwater, J. E.,Analyst, 1976, 101,672. Uthus, E. O., Collings, M. E., Comatzer, W. E. and Nielsen, F. H., Anal Chem., 1981, 53, 2221. Jin, K., Terada, H. and Taga, M., Bull. Chem. Soc. Jpn., 1981, 54, 2934. Henden, E.,Analyst, 1982, 107, 872. Al-Zamil, I. Z. and Townshend, A., Anal. Chim. Acta, 1988, 209, 275. Water, G. A., Anal. Chim. Acta, 1978, 98, 59. Howard, A. G. and Arbab-Zavar, A. M.,Analyst, 1981, 106, 213. de la Calle, M. B., Madrid, Y. and C/lmara, C., Mikrochim. Acta, 1992, 109, 149. Boampong, C., Brindle, I. D., Le, X. C., Pidwerbesky, L. and Ceccarelli, C. M., Anal. Chem., 1988, 60, 1185. Brindle, I. D. and Le, X. C., Anal, Chim. Acta, 1990, 229, 239. Le, X. C., Cullen, W. R., Reimer, K. J. and Brindle, I. D., Anal Chim. Acta, 1992, 258, 307. Chen, H., Brindle, I. D. and Le, X. C., Anal Chem., 1992, 64, 667. Le, X. C., Cullen, W. R. and Reimer, K. J., Anal, Chim. Acta, 1994, 285, 277. Welz, B. and Shubert-Jacobs, M., J. Anal. At. Spectrom., 1986, 1, 23. Welz, B. and Melcher, M., Spectrochim. Acta, 1981, 36, 439. de Diego, A., Tseng, C. M., Stoichev, T., Amouroux, D. and Donard, 0. F. X., J. Anal. Atom. Spectrom., 1998, 13, 623.

406 97 98 99 100 101

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speciation in biological systems

13.1

Introduction

It has become accepted worldwide nowadays that atomic spectrometry methods dominate the elemental inorganic analysis in most fields. This is particularly true for biological systems where the complexity of samples and the low levels of many elements to be analysed demand the use of detectors exhibiting both very high selectivity and sensitivity, as required for the determination of minor, trace and ultratrace elements in biological material. Moreover, such methods can play a critical role today for the speciation of trace elements (Chapter 12). Aiming at a general appraisal of the relevance of atomic spectrometric methods for clinical and biological analysis, it should always be borne in mind that atomic spectrometry relies on atoms (or ions), that is to say, on measurements of the examined matter brought into its elemental form via a high temperature device. As only elemental information can be expected, in principle, it appears useful to examine which chemical elements are present in living organisms.

13.1.1

The importance o f trace elements in biology and medicine

From the above perspective, we can say that out of the 90 elements naturally occurring in the earth and its atmosphere, four non-metals (C, H, O, N) account for 96% of the total 407

A. Sanz-Medel

408

Table 13.1 Natural chemical elements in man and their analysis by atomic spectrometry Atomic spectrometric direct analysis Major elements (C, H, N, O)

Not very good

(organics) (P, S, CI)

Yes, elemental (anions & organics)

Alternative techniques

- Molecular spectroscopy - Organic elemental analysis - Molecular spectroscopy - HPLC

Minor elements "Electrolyte" metals (Na +, K § , Ca 2+, Mg 2+)

Yes

- Colorimetry Potentiometry (ISE)

-

Trace and ultratrace elements

Yes

- Neutron activation

- Voltamperometry

weight of the organism (see Table 13.1 to realise which analytical techniques are used to determine each group of elements). Three other non-metals (P, S, CI), combined with the so-called "electrolyte" metals (Na § K § Ca 2§ Mg2+), account for another 3.6% of the total weight. If we exclude the six noble gases, which are unlikely to have a physiological function, 73 chemical elements remain: the so-called "trace" and "ultratrace" elements. These elements at trace (l~g g-~) or ultratrace (ng g-~) concentrations quantitatively amount to less than the remaining 0.4% of the total organism weight. In spite of these almost negligible concentrations the roles of trace and ultratrace metals in the organism are so varied, and in many instances so essential to life, that their clinical importance has grown at a dramatic pace in recent years. ~ It should be stressed how present biological research on the role of trace elements in living organisms 2 and in laboratory medicine, 3 in which chemical and biochemical methods are applied to the study of disease, are demanding reliable analytical techniques to provide total trace element levels information. The clinical interest of such determinations stems mainly from the necessity to ascertain and control: (a) deficiencies of elements considered essential (e.g. Fe, Cu or Zn in serum); (b) overloading of particular exogenous elements which could result in toxicological problems (e.g. Hg or Cd); (c) conversely, monitoring adequate levels of metals with favourable pharmacological effects, but toxic at higher concentrations (e.g. Au compounds in rheumatoid arthritis). It follows that today many nutritional, toxicological and/or pharmaco-toxicological disorders could be identified/confirmed more clearly by resorting to the analytical results provided by the clinical laboratory. 3 Table 13.1 summarizes the natural chemical elements found in man as related to the common techniques used for their chemical analysis. A more detailed account of atomic techniques preferred today for

Chromatographic separations coupled to atomic detectors

409

routine analysis of major, minor, trace and ultratrace elements in biological samples was given in Chapter 11. 13.1.2 The determination o f total trace elements in clinical samples with atomic spectrometric detection Any spectroscopist more or less familiar with routine clinical practice knows intuitively that the importance of atomic spectrometry for the diagnosis and control of diseases related with trace and ultratrace elements has been continuously growing over the last twenty years. It is common practice in science, however, to try to assess a given phenomenon through numerical values. For this purpose the data published by Oster and Prellwitz 4 of the clinical chemistry division of Maguncia University in Germany, may be most enlightening: in that hospital (with 1600 beds), about 1000 determinations per day were carried out by atomic spectrometric methods. Considering that we are speaking about a laboratory where around 10 000-14 000 analyses per day are usually carried out, it follows that about 10% of all determinations per day require the use of atomic spectrometric methods, a figure which clearly documents the important role of such methodologies in a modern clinical chemistry laboratory. According to those authors "electrolyte" metals determinations account for 50% of the total number of determinations carried out with atomic methods: In the case of trace elements, the frequency of this type of analysis is still remarkable (100-200 per day) and probably more related to the diagnosis of nutritional deficiencies than to toxicity problems. Comparatively less importance seems to be given in routine clinical chemistry to the ultratrace elements because the relative number of analyses performed is very low: a few per month. 4 It has to be stressed, however, that accurate diagnosis of acute poisoning or therapeutic treatment control rely heavily on the analysis of these ultratrace elements. The present necessity for such analysis to be reliable cannot be over-emphasized: very few clinical laboratories are aware of the fact that the drastic gain in analytical sensitivity necessary to cope with ultratrace analysis is always accompanied by a proportional increase in the difficulties to obtain reliable results. Aluminium determination in serum is a typical example: we know now that accumulation of A1 in the body plays a fundamental role in the pathogenesis of some clinical disorders identified in patients undergoing regular dialysis (encephalopathy, anaemia, and osteodistrophy are altogether clinical abnormalities associated to A1 loading in the human body). As for other trace metals, it is necessary to establish normal or reference serum levels 5 (i.e. the values range of that particular element's normal concentrations in the organism) to diagnose deficiencies or toxic overloading. Contamination problems should explain the amazing range of normal

A. Sanz-Medel

410

Table 13.2 Some "normal" or basal levels of aluminium in human serum published in bioanalytical literature up to 1985

Authors D'Haese, 1985 De Baets et al., 1978 Kaehny et al., 1977 Salvaedo et al., 1972 Valentin et al., 1976 Gorsky et al., 1978 Fuchs et al., 1974 Clarkson et al., 1972 Niedermeier et al., 1971 Seibold, 1960 Panteliadis, 1975 Julshamn et al., 1978 Berlyne et al., 1970 Butt et al., 1964 Niedermier et al., 1962 Berlyne et al., 1970

Technique

Mean (ng ml-t)

S.D. (ng ml-t)

A.A.S. A.A.S. A.A.S. A.A.S. A.A.S. A.A.S. A.A.S. N.A.A. E.S. Spect. E.S. A.A.S. A.A.S. E.S. E.S. N.A.A.

2.0 3.7 6 12.0 14.2 28 38 72 109 172 175 210 240 400 800 1460

0.4 1.2 3 4.0 7.1 9 70 159 80

Interval (ng ml-~)

2.1-6.2

4-34.5 12--46 10-92 <60-790

200-300 277 103-1930 1100-1800

A.A.S.: Atomic Absorption Spectrometry N.A.A.: Neutron Activation Analysis E.S.: Emission Spectrometry Spect.: UV-Vis Spectrophotometry

values published for A1 in serum (Table 13.2): from above 1.000 Ixg 1-~ in 1970 to a few txg 1-~ in present years (it is believed today that a reasonable value for normal serum A1 should be around 2 Ixg 1-~). Similar events have occurred in the case of urinary Cr or Mn, as well as for Ni and Cd normal serum levels, and they indicate that contamination and other methodological problems during pre-analytical steps: sample collection, sample storage, preparation of the sample for analysis, etc. have been ruining the results published on the analysis of the basal (normal) levels of ultratrace elements in biological samples. Rigorous protocols should be designed and followed in order to obtain the required reliability of our results 6 focusing our attention particularly on these ill-named preanalytical steps. Of course, such steps are as analytical as the final instrumental measurement and should be carefully designed and carried out for reliable final results. It seems obvious that, in order to minimise contamination risks, sample preparation should be kept to a minimum. In the best of worlds we should have an analytical technique at our disposal so selective and sensitive that no sample manipulations were required; in other words, just by placing an aliquot of uncontaminated sample in the "ideal" instrument we could obtain a direct measurement/result of the sought analyte. Unfortunately that situation is rather utopic so far, because even being close to such an ideal situation (e.g. using automatic direct Electrothermal Atomic Absorption Spectrometry, ETAAS)

Chromatographic separations coupled to atomic detectors

411

contamination risks from atmospheric dust are still possible and serious precautions should be taken to avoid them. 7 Flow manifolds and techniques can be invaluable, as sample presentation systems to atomic detectors, in order to minimise contamination risks in clinical and biological total analysis (see Chapter 11).

13.2 The need for chemical speciation information in biological systems As clearly stated in the preceding chapter by de Diego et al., the determination of total concentrations of metals is not sufficient nowdays for most environmental applications because physico-chemical behaviour, toxicological risk and bioavailabity of metals and metalloids are highly dependent on the chemical species present. As we will see in the following, a similar situation has developed in the biological field. In particular, information on the exact nature of the particular chemical species and their concentrations of a given trace element in biological systems is more and more needed and increasingly demanded. Let us consider at least two different areas of this important biological field where chemical speciation of trace and ultratrace elements are becoming essential: clinical and bioinorganic chemistry.

13.2.1

Speciation for clinical chemistry

Analytical information of total element content, as directly provided today by atomic spectroscopy techniques, is strongly demanded in environmental and clinical monitoring programmes. Of course, the data on total trace element levels has become of prime importance to aid both the clinician in the diagnosis and treatment of a variety of diseases ~'2 and the life science researcher to investigate the role of trace elements in health and disease. 3 Unfortunately for the analyst, the high degree of evolution experienced by living systems after about 4x 10 9 years 8 has determined the great (bio)chemical complexity possible for constituent trace elements. Leaving aside the four non-metal major elements of living systems (C, H, O, N) and perhaps with doubts about the three minor non-metals (P, S, C1) there is no doubt that atomic spectrometry provides the techniques and methods of choice to determine electrolyte metals (Na, K, Ca, Mg) as well as trace and ultratrace elements in biological materials (see Table 13.1). Of course, such atomic methods are most useful and reliable for total element determinations. Unfortunately these techniques are unable to provide

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additional information needed about the different possible forms (chemical, physical or morphological) in which the sought-for element occurs in the biological material. Without that information total element data are at best incomplete and in many instances misleading (e.g. toxicity of arsenic in a given fish can be grossly overestimated if analysed by ETAAS; in fact arsenobetaine, being a main arsenic species in fish, is not toxic). Speciation information is thus urgently needed in biological/biomedical research and also in clinical medicine. Considering biological research, we have seen that the role of some trace and ultratrace elements in the body are so rich and varied 2 that in many instances they are essential to life (essential elements) while in others they are lethal even at very low concentrations (toxic elements). The role, metabolic behaviour, bioavailability and toxicity of an element, in a biological system is determined by its actual chemical form present in the particular environment considered. Regarding clinical laboratories, there is no question that just total element determinations are demanded for patients so far, as we have seen in the previous section. However, as pointed out before, that information is not sufficient, or may even be misleading, because the element bioavailability, metabolism and toxicity will depend on the particular chemical species of the element.

13.2.2 Speciationfor bioinorganic chemistry Bioinorganic chemistry is a recently established discipline, at the interface of the more well-known areas of inorganic chemistry and biology, consisting of two major components: the investigation of naturally occurring inorganic elements in biology and the introduction of non-naturally occurring metals into biological systems as probes and drugs. 9 Progress in this new science requires reliable information on metal functions in metalloproteins, enzymes and nucleic acids, on the messenger/communication roles for metals, on metal-ion transport and storage in target organs, on uptake/availability/ metabolism of exogenous metals as used in modem medicine (e.g. anticancer drugs as "cisplatin", cis [Pt (NH3) C12] or "imaging reagents" of heart, such as complexes of Technetium). All these problems call for basic chemical speciation information of formed metal compounds (complexes with small ligands and high molecular weight biomolecules alike, adducts, oxidation states, etc.). We need more basic speciation knowledge about metal associations with low molecular mass (LMM) or high molecular mass (HMM) compounds present in the body, possible metabolism and detoxification pathways, fate of the different species with time, etc. This knowledge will lead eventually to proven clinical conclusions and, in this way, new

Chromatographic separations coupled to atomic detectors

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speciation techniques could be implemented in the long run in our clinical routine laboratories.

13.3 Chemical speciation in biology and medicine: a challenge for modern analytical chemists It follows from the above narrative that new and innovative chemical speciation strategies and technologies are increasingly demanded in the field of life sciences. It should be stressed, however, that we are dealing with "chemical speciation" of biological systems (not to be confused with "biological speciation" or identification of different biological species of existing living organisms and creatures) and this is a formidable task, considering their great complexity, even for modem analytical chemistry. The challenge stems from, at least, three main reasons: First, we have the omnipresent problem of "lability" of the sought-for species; modifying ambient conditions from those prevailing in the biological systems to be monitored often disturbs the existing physicochemical equilibria and so the species finally determined may not fully represent those which really occur in the biological sample analyzed. Secondly, particularly with toxic elements we have to deal with extremely low concentrations which, after fractionation, will require extremely sensitive and selective detectors to be actually measured in real-life metal speciation analysis (e.g. control of 0.001-11xg 1-1 concentration levels may be demanded for toxic species). Thirdly, to make matters worse, those minute amounts of the sought-for species are sort of "buried" in the very complex matrices (e.g. serum, urine, soft tissues, etc.) typical of biological systems. Thus, it is not surprising that on many occasions just a sensitive detector is not enough. It should be pointed out that the majority of the methods developed so far for analytical speciation usually involve half a dozen distinct steps, including extraction, preconcentration, cleaning, derivatization, chromatographic separation and element specific detection. Such processes will probably put the integrity of the sought-for species at risk, unless its stability can stand for the changing ambient conditions experienced along these processes and pretreatments. From this point of view, a few years ago we proposed considering three different categories of analytical speciation 1~depending on the physico-chemical properties of the sought species. Such features, which would condition the species stability and so the difficulty of analysis, could be as follows:

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Category I. Analysis of thermodynamically stable/kinetically inert species (e.g. organometallics !1 such as Tributyltin, TBT). Category H. Analysis of stable/not inert species (e.g. relatively high stability constant metal complexes in solution, such as Al-citrate, or known oxidation states as As(III) or As Category III. Analysis of weakly bound/not inert compounds (e.g. the so-called "labile monomeric aluminium", ~3 the more toxic AI fraction in natural waters, comprising A13§ AI(OH) 2§ A1C12§ AI(OH) 2§ etc., which relates to a known group rather than to a single species). ~4 Analytical requirements necessary to preserve the integrity of the sought species in each of the above-mentioned categories will be increasingly demanding from I to III as lability increases in the same order. Of course, we could add a fourth type of analytical speciation: the analytical processes allowing the evaluation of fractions of the total element which are "leachable" from a solid matrix e.g. from a sediment, 15 with leaching reagents of increasing chemical strength. Notwithstanding the fact that these processes are probably one of the first described with the term "metal speciation", this is now a controversial issue. Its inclusion as a sort of fourth type of analytical speciation, Category IR could be a matter of convention and agreement, but, strictly speaking, it seems that the wording "speciation" should be discouraged to refer to the determination of "extractable forms" of metals. ~6 The more accurate and descriptive terms of "sequential extraction" or "sequential leaching" are now recommended to describe those analytical processes. ~7 Analytical literature shows a certain activity trying to tackle the problem of element speciation in biological systems. Most of the work carried out could be classified in one of the three main speciation categories established above. An attempt of this classification is shown in Table 13.3. Finally, in Biology and Medicine there has to be concern as to whether our current analytical measurements, usually carried out in the laboratory, truly reflect metal speciation as it occurs in living organism being monitored. Hence the recent interest, and the challenge, of carrying out measurements in vivo to avoid important speciation changes occurring after removing the biological sample from the living system.

13.4 Analytical approaches to chemical speciation in biological samples There is no doubt that speciation strategies are more developed in the field of environmental analysis (see Chapter 12) than in clinical analysis.

Chromatographic separations coupled to atomic detectors

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In fact, analytical strategies and tools developed so far trying to tackle speciation problems in biological systems follow more or less closely, patterns established for environmental speciation, TM a topic addressed thoroughly in the preceding chapter. To tackle the problem, there are at least three main directions in recent literature: computational approaches, direct species-specific detection and hybrid techniques. 13.4.1

Computational approaches

The more theoretical approach consists of using the existing information on ligands and thermodynamic constants to predict the speciation of an element in a given sample. An excellent example of this approach is the paper by Harris 19 on A1 speciation in human serum. Notwithstanding its usefulness for a first approximation to a biological speciation problem, the limitations of such an approach are obvious: in complex biological matrices we do not know all the ligands able to interact neither the corresponding constants of interaction, nor the data for kinetics involved, etc. Therefore the scope o f this computational approach is restricted to very well-known systems and so the results should always be treated with care in real life situations. Table 13.3 An attempt to classification of research on analytical speciation of inorganic trace elements in biological systems

Category I (Thermodynamically and kinetically favourable) LMM (Low Molecular Mass) compounds with C-Me bonds. (e.g. tin organometallics, trimethylselenonium, etc.) HMM (Higher Molecular Mass) compounds having metal centres as building blocks. (e.g. prostetic groups, cobalamine)

Category H (Thermodynamically stable but kinetically not inert) Inorganic element oxidation states (e.g. As(Ill), As(V), Se(IV) or Se(VI), etc.) LMM metal complexes, with drugs and ligands (e.g. AI(III) citrate, Al-Desferroxiamine) HMM biopolymers complexing metals sometimes with "folding" of proteins by metals (Al-Transferrin, Fe-Transferrin)

Category llI (Thermodynamically weak/kineticaUy fast) LMM "group" species (Similar toxic effects, such as "toxic Aluminum" AP +, AI(OH)2§ AI(OH)~, AI(OH)4, AICI2+, etc) HMM biopolymers weakly and trasiently associated to metals (association of Ca 2§ Na § or K§ to biopolymers for cell communications) Note: Sequential extractions (Category IV, if admitted) are still unexploited in the biological field, but they could prove very useful, in the near future.

4 16

A. Sanz-Medel

More reliable approaches for speciation in such real life systems are probably those using analytical experimental measurements (theory guides, experience decides). 13.4.2

Direct speciation approaches

The so-called direct methods would be ideally suited for speciation purposes because, in principle, they would provide means to determine in situ a given species in the complex matrix directly without treatment. In this way disturbances of the equilibria (change in concentrations) or of the nature of the species sought for, should be avoided. Of course, such species-specific technique should exhibit sufficient selectivity to recognize only the sought species and enough sensitivity to respond to trace and ultratrace levels of that species in the sample (a difficult goal, no doubt!). Probably the techniques more extensively used in this context of direct speciation have been of electroanalytical nature. 2~ Species-specific techniques such as potentiometry with ion-selective electrodes (ISE) have been widely used in natural waters speciation and also in biological fluids for Ca 2§ and Mg 2§ speciation in serum. 2~ Unfortunately selectivity and sensitivity of ISEs are not enough for trace and ultratrace levels of Hg, Cd, Pb, etc. in biological material. More sensitive electroanalytical techniques such as modem voltamperometry and particularly stripping voltammetric methods 2~have also enjoyed certain popularity in this field. They have proved useful to distinguish stable and labile forms of metals or their different oxidation states and occasionally to provide information about toxicity of a metal in natural water samples by relating labile forms contents with actual toxicity of the sought metal] 2 In general terms, however, these electrochemical species-specific techniques lack the necessary selectivity to provide reliable information for trace elements in biological systems (they work well in model solutions but problems arise when the species is buried in a biological material). Similar to classical VIS-UV molecular spectrophotometry, fluorimetry, etc., also proposed for direct speciation purposes, they show serious selectivity and sensitivity limitations hindering their practical use as "direct" speciesspecific detectors in real life situations. A modem approach which could hold some potential for speciation work is the use of optical (bio)sensors] 3 The required selectivity is secured via compound-recognition processes (e.g. a selective chemical reaction with the sought compound) integrated within a physicochemical sensitive transducer (e.g. a fluorimetric detector); in this way the basic processes of recognition-transduction-electronic read-out can be carried out in a small compact device capable of in situ and reversible monitoring of the species with minimal sample disturbance. So far the gathered experience using chemical optical sensors for direct speciation in environmental samples is rather scarce and hardly successful. 24 There is room to investigate and use bio-sensors, both optical and electrochemical, as direct speciation approaches in biological systems. The achievement of the selectivity

Chromatographic separations coupled to atomic detectors

417

required is a difficult goal, but the high sensitivity provided by luminescent detection encourages investigations in that direction, exploiting the expertise and experience on luminescent sensors for clinical fluids. 25 Of course, direct speciation, if at all possible with any detector, should be aimed at. In the present state of the art, however, only in rare occasions can that ideal be reached and probably some forms of sample preparation may nevertheless be required to solve real-life speciation problems, both in environmental and biological systems.

13.4.3 Hybrid or hyphenated techniques The complexity of the sample, the low levels of analyte or perhaps both, determine in most cases the need to combine a separation step with a specific detection of the physically separated species via the so-called "hybrid" techniques (see Figure 13.1), which are nowadays taking the lead in inorganic trace element speciation in real life systems,

Figure 13.1 Schematic of a typical real-life system for ultratrace levels quantification of a given element species, including an expansion of the "hybrid technique" concept.

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A. Sanz-Medel

particularly in connection with atomic spectrometric detection of the element. Moreover, in many real-life situations a previous extraction and/or a preconcentration step are also required before the final separation/detection. Figure 13.1 shows schematically a typical speciation strategy at very low concentrations (e.g. speciation of some volatile toxic metal species present in an aqueous solution). Hybrid techniques are undoubtedly the more widely used and so the preferred approaches of analysts at present. Therefore, we will come back to them in greater detail later on.

13.4.4 Physico-chemical characterization Interestingly, it appears that the now popular hybrid techniques mentioned earlier are ignored or perhaps unknown to practitioners of Bioinorganic Chemistry. In fact, physical methods included in a very recent book on this topic 9 appear to deal only with X-ray, magnetic resonance, ),-ray M6ssbauer techniques and well-known molecular VIS--UV spectroscopic methods based on electronic, vibrational or circular dicroism spectra of the investigated biological species. There is no mention in this book of efforts devoted to hybrid coupling of separation/atomic detection strategies. Clearly, we are witnessing a sort of divorce today between bioinorganic chemists, biochemists and analytical chemists devoted to chemical speciation in biological systems. The two former type of researchers have concentrated on the study at length of proteins, enzymes, nucleic acids, etc. Their objective is the characterization of the main components and structures in the previously isolated biocompounds. In an apparently parallel track, analytical scientists are now focusing on developing techniques to analyze quantitatively very small amounts of metals and their associations with the biomolecules in real-life samples (e.g. serum, urine, tissues) by convenient analytical methods. Of course, structural knowledge of associations metal-biomolecule are invaluable to design new strategies able to identify and determine those particular species in a sample. They, in turn, could eventually drive to improved bioinorganic basic developments or perhaps easier analytical speciation schemes for real life matrices. The multidisciplinary character of speciation becomes obvious in the life sciences and the need for real cross-fertilization among different scientists evident from the above. Thus, practical actions should be devised and carried out as soon as possible to overcome the present lack of inter-relations existing among the different professionals involved (analytical chemists, chemists, biochemists, clinical chemist, pathologists, etc.) in order to allow a more effective development of basic elemental speciation and its applications in the life sciences.

Chromatographic separations coupled to atomic detectors

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13.5 Hybrid techniques for speciation of biological systems As mentioned before, in most cases of real-life analytical speciation we have to resort to hybrid techniques, that is to say, to combine an adequate species separation technique with a sensitive-enough atomic detector. 26'27 There are excellent reviews on this topic. 18'z6'27 In order to develop a speciation strategy, the selection of an effective separation technique is crucial and will depend on the nature of the species to analyze and that of the biological matrix. The more common speciation techniques published for biological systems studies have been summarized in Table 13.4, along with their separate principle Table 13.4 materials

Separation techniques commonly used for trace element speciation in biological

NON--CHROMATOGRAPHIC Techniques

Separation Principle

Illustrative examples

Filtration

Particle size

Ultrafiltration

Molecular size

Dialysis

Molecular size

Centrifugation

Particle density

Extraction

Solubility

Electrophoresis

Electrical mobility solution

Urine filtration: soluble fraction (0.45 I~m pore filters) Serum analysis: species distribution between ultrafiltrable and non-filtrable fractions Clinical analysis: toxic species fraction eliminated during dialysis Clinical analysis: toxic species fraction in blood red cells Tissue analysis: sequential extraction of different Se species of the analyte Serum analysis: different Se species bound to proteins

CHROMATOGRAPHIC Techniques

Separation Principle

Illustrative examples

Gas Chromatography

Hydrophobic/hydrophilic interactions Different possible mechanism (size exclusion, reverse phase, ion-exchange, ion-pair formation, affinity, adsorption...)

Volatile Organometallics analysis (e.g. methylmercury) Non--volatile species in urine or serum (e.g. Al-transferrin)

Liquid Chromatography

ELECTROPHORETIC Capillary Electrokinetic Chromatography Capillary Electrophoresis

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A. Sanz-Medel

and some illustrative examples. It should be stressed how electrophoretic separations, in different jackets, are becoming more and more popular in this field in the last years. Detection techniques could be spectrochemical and electrochemical in nature, but the more reliable and increasingly used detectors are based on atomic spectrometry. They are closer to the required ideal specific detector of a given trace element (e.g. via simultaneous detection of several confirmatory masses, or emission lines, of the same element). Particularly plasma detectors allow sensitive, matrix-free, on-line, real-time continuous monitoring of the separated species. 27 Further enhancements of sensitivity can be achieved by resorting to on-line volatile species generation, 28 the use of higher sensitivity plasmas such as ICP-MS 29 or High Resolution-Sector Field-ICP-MS, 3~ liquid flow preconcentration techniques developed previously for AAS, 31 volatile species formation and cryogenic trapping 32 etc. As pointed out before, there are a considerable number of approaches which have been proposed to solve environmental speciation problems (see Chapter 12) and, in principle, those approaches could be extended to tackle speciation issues in biological material. Excellent books and reviews on this particular topic of hybridations of separation techniques with atomic detectors have appeared 18'26'27'32'33 and the reader is referred to them for detailed descriptions. However, it will be useful to revise here the main general analytical strategies at our diposal to try to solve biological problems needing speciation information, illustrated by examples worked out in the last ten years or so in the author's laboratory.

13.5.1 Off-line techniques Notwithstanding the fact that continuous or on-line detection is desirable after an adequate separation (e.g. chromatographic separations), there are important examples of biological speciation which have made impressive use of discontinuous separation procedures. In other words, we can collect one or various fractions obtained after a discontinuous process (e.g.: see Table 13.4 to get a glimpse of possibilities for discontinuous nonchromatographic separations available) and then carry those sample fractions obtained to the atomic detector for analysis. As shown by Table 13.5, ETAAS still holds at the present time about 40% of the total publications on analysis of toxic trace elements in biological materials. The discontinuous nature of ETAAS operations made this technique appropriate as a detector for off-line analysis of the fractions separated by discontinuous, nonchromatographic techniques. A good example of the utility of such off-line systems is the chemical speciation of aluminium 34'35 and silicon 36 in human serum carried out in our laboratory. Figure 13.2 is a plot selected to resume our results for A1 speciation (fractionation in LMM and HMM compounds) in human sera, while Figure 13.3 shows the results for

Chromatographic separations coupled to atomic detectors

421

Table 13.5 Trendsin development of atomic spectroscopic techniques for the analysis of biological materials (%) Technique

1971

1985-87

1988--90

1991-93

1994--96

65 15 5

9.5 58 7

7.5 62.5 10.5

12 45 8

16 39 8

10 5 20.5 1

4 11.5

2.5 17

1 14

1

1

2

Optical Absorption Flame ETAAS Hydride/Cold/Vapour

Optical Emission Flame Arc/Spark/DCP ICP-OES MIP-OES

Mass Spectrometry ICP-MS

Total Number of Papers

4.5 139

307

627

16

19

795

859

ETAAS: Electrothermal Atomic Absorption Spectrometry. DCP: Direct Current Plasma. ICP: Inductively Coupled Plasma. MIP: Microwave Inductively Plasma. OES: Optical Emission Spectrometry. MS" Mass Spectrometry

silicon fractionation in the same sera samples. Off-line fractionation was achieved by ultramicrofiltration of serum with 30 000 Daltons pore size membranes and the metals were detected in the ultramicrofiltrate and in total serum by off-line ETAAS detection in a "clean room". Results demonstrated that just about 10% of total serum aluminium is ultrafiltrable and this fraction did not change with age, sex, renal failure status or even after kidney function. This result is most useful because it indicates that 90% of serum aluminium should be bound to HMM proteins; in other words, such speciation information shows that direct aluminium detoxifying in normal haemodyalisis sessions is most unlikely. 35 Not only discontinuous separations, chromatographic continuous separation processes can also be monitored by ETAAS, notwithstanding the need for collecting the fractions of serum exiting the column and of measuring off-line the content of metal(s) in each separate fraction, as illustrated in o u r l a b o r a t o r y 36 to investigate the nature of proteins binding A1 and Si in human serum. Of course, other authors have also stressed the usefulness of these off-line approaches in connection with the high sensitivity and availability of ETAAS for specific detection 35'37 in solving metal speciation problems in clinical fluids. Along with the ETAAS detection approach, the use of radiochemical neutron activation analysis and "radiotracer" techniques for alternative off-line detections of previously separated fractions have also proved to be an invaluable trace element speciation tool in samples of biological fluid and t i s s u e s . 38'33 Chemical speciation for the study of As (V)

A. Sanz-Medel

422

30 25 -1

20

=k

o

15

o

9 t_

O0

O

10

< 0

40

60

80

100

120

140

160

180

A! (total)/~tg.L -~

12.3 • 2.7 141 11.3 • 2.3 11.6 4- 1.9

,

I ::~th::

:!:! !:'-:I 9

,

9

.

9

.

; I~ !ia ~b~d

12.3 • 1.8

.... j ;.

i: slier :!

i: Mii~r: i

-, .,

.

Figure 13.2 Ultramicrofiltration-ETAAS results for serum aluminium: (A) Aluminium distribution in serum between low molecular weight and high molecular weight fractions, for Al-spiked normal sera and non-spiked uremic sera. (B) Ultrafiltrable Aluminium (Al-low molecular weight species) in healthy persons, kidney patients and transplantated kidney patients (reproduced from Reference 34, with permission).

Chromatographic separations coupled to atomic detectors

423

metabolism using radiochemical approaches detection should be cited here as an excellent example of biological speciation using radiochemical techniques. 39

Figure 13.3 Aluminum and Silicon speciation in human serum by HPLC-ETAAS ("spiked" serum): (a) 1.0 Ixg mL -~ of A1; (b) 5.0 Ixg mL -l of Si. Shaded bars show metal contents as measured in each fraction of the HPLC column. Elution profiles of proteins are followed by absorbances at 280 nm.Elution times: Transferrin, 13 min; albumin, 25 min (reproduced from Reference 76, with permission).

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13.5.2 The potential of non-chromatographic techniques coupled to atomic detectors for the speciation of trace elements The use of FI strategies as a means for matrix modification and analyte preconcentration for atomic spectrometry is rapidly growing and gaining popularity owing to their capability of providing an efficient, easily automated, chemical pretreatment at a microscale. These features of FI could also be exploited for speciation purposes. 4~ In many instances only one or two species of an element in a given sample have to be determined (e.g. routine analysis of methylmercury and inorganic mercury). For these cases a simple non-chromatographic separation coupled to an atomic detector, via FI procedures, might solve such uncomplicated routine speciation problems (see Chapter 9, Section 9.6.3). Flow-injection strategies typically developed for preconcentration/separation of an analyte (species) could be easily extended and exploited to tackle simpler speciation cases. The great number of analytical strategies developed today for non-chromatographic flow separation and preconcentration 4' can be widely exploited for this purpose. Solid-liquid non-chromatographic separations using packed minicolumns are well known and probably the most popular ones because they afford maximum preconcentration capacity and flexibility. 3'' 4o Preconcentrations of 50-100 times are possible for some species able to be retained in the packed columns (e.g.: Cr(VI) preconcentration/speciation in an activated alumina 4~ column). Retained species can afterwards be eluted by leaching the column with the adequate reagent, e.g. sodium hydroxide, to liberate Cr(VI), previously retained, as illustrated in Chapter 14 (Fig. 14.4). Other possible FIA approaches, beautifully illustrated by Prof. Fang in Chapter 6, could also be tried in this context. Of course, liquid-liquid extractions can also be employed advantageously for such purposes 4~ of straightfoward speciation of just one species, even if the preconcentration factors achievable are generally poorer than using solid-liquid separations. This separation can be further developed by resorting to the concept of"tandem on-line continuous liquidliquid separation" as discussed in greater detail in Chapter 9 of this book. In this last case a continuous separation unit (e.g.: a liquid-liquid extraction, designed to select the soughtfor species) is coupled on-line to a second separation device (e.g. a continuous hydride generator, designed to increase final selectivity and sensitivity) before detection. In this way the two-steps separation allows both selectivity and sensitivity to be enhanced as illustrated for the speciation of methylmercury and inorganic mercury in human urine. 42 Table 13.6 summarizes the type of results obtained in our laboratory using this tandem concept and ICP--OES specific detection 42 as compared with results for such determinations using vesicular HPLC-AAS detection. 34This tandem on-line approach could also be specially useful for speciation of trace clement oxidation states, e.g. arsenic or antimony,43 as highlighted earlier in Chapter 9.

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425

Table 13.6 Non-chromatographic (A) and chromatographic (B) speciation of inorganic mercury and methylmercury in "spiked" human urine samples (A) Tandem on-line liquid-liquid continuous separation ICP-OES. 42

Sample

MeHg added

Hg 2§ added

MeHg found a (Ixg 1-t)

1

25 50 100

25 25 25

23 + 3 48 • 4 98 • 2

2 3

Hg 2§ found a 23 + 1

24 • 1 25 + 2

(B) Vesicle-mediated H P L C - C V - A A S speciation in "spiked" human urine. 34

Sample

MeHg added

I

0.0 100 300 3.0 4.0

2 3 4 5

Hg 2+ added 0.0 103 95 103 b 101 b

MeHg found a (Ixg 1-~)

Hg 2+ found a

0.0 100 300 3.0 4.0

0.0 114 90 91 b 93 b

a. Each value is mean of 3 determinations b. After microcolumn preconcentration (100" 1)

13.5.3 Chromatographic separations on-line detection coupled techniques As we have seen before (Table 13.4), so far off-line techniques are quite common in biological research. There is no doubt, however, that the greatest potential for metal speciation purposes, both in environmental and biological research, is held by on-line couplings where a powerful chromatographic separation technique is interfaced with a sensitive and ideally specific atomic detector. This allows continuous operation and realtime detection as is commonplace in modem chromatography. In brief, as explained in the preceding chapter on environmental speciation, chromatographic separation with specific on-line detection constitutes the most versatile and powerful approach to tackle speciation problems so far. Gas chromatography (GC) and HPLC separations are preferred and the exit of the chromatographic columns can be coupled with any type of atomic detectors in the most varied ways via adequate interfaces. 18'26'44 Of course, supercritical fluid chromatography (SFC) has also been recommended for speciation of some environmental samples; however, its application to the speciation of biological systems is still to be exploited. Concerning detectors to be interfaced to the exit of the chromatographic columns, flames, heated quartz tubes and plasmas are the three most common specific detectors

426

A. Sanz-Medel (1) (5)

(4) :

(2)

1

8 (4')

~

Time (min)

5)

(2)

(4)

(1)

(l')

....

~ ....

(4')

1 ....

(3)

(3')

~ .... ~ .... Time (min)

,'0 ....

,'~ ....

'

Figure 13.4 Chromatograms obtained for basal and spiked urine by: (a) HPLC-MW-HG-AAS and (b) HPLC-MW-HG-ICP-MS. 1. Selenocystine, 2. Selenomethionine, 3. Selenoethionine, 4. Selenite, 5. Selenate, 4' Peak observed when selenomethionine or selenoethionine are spiked to human urine (reproduced from Reference 50, with permission).

Chromatographic separations coupled to atomic detectors

427

used. Spectrochemical plasmas are particularly well suited for on-line coupling to chromatography. 27 When the speciation of volatile species is pursued, GC is probably the technique of choice for separation and the direct coupling GC-MIP-OES 45 is becoming commonplace. Considering that the vast majority of biological species are charged, polar, non-volatile and so more amenable to liquid chromatography than to GC separations, it is not surprising that the coupling HPLC-atomic detectors appears more promising at present for analytical speciation of biological samples. 3~ This type of approach provides today a fruitful field of research and probably the most popular tool for analytical speciation. 18"26'44 Just as an example of this popular coupling, let me mention here the speciation of toxic arsenic species 46 and selenium species in human urine developed in our laboratory using a new approach for HPLC separation based on the use of vesicles. 47 When a Cl8 column is modified conveniently by passing through it a mobile phase containing vesicles of didodecyldimethyl ammonium bromide (DDAB), a new vesicular separation mechanism is possible which allows adequate separation of charged, polar and non-polar analytes in just a single chromatographic run. Figure 13.4 gives a chromatogram showing the good separation of at least six different selenium species in spiked human urine using vesicles of DDAB in the mobile phase and a DDAB-modified C1 8 column. This new type of separations allows very convenient interfacing, via hydride generation, with plasmas such as MIP--OES for Hg and As speciation, 48 ICP-OES for As toxic species 46 or ICP-MS for selenoaminoacids in urine. 49'5~ As a rule, the sensitivity of ICP--OES (and, of course, flame-AAS) with conventional nebulization is not sufficient to tackle the problem of speciation of trace elements in clinical samples. 44 Therefore, improved sensitivity techniques and strategies have to be frequently sought for. For instance, resorting to volatile species generation, with an on-line interface between the column exit and the atomiser 47-5~ has proved to be a very effective means of improving (e.g. up to two orders of magnitude) the detectability of some species. Figure 13.5 illustrates a most convenient set-up of this type of coupling for speciation purposes which has been developed in our laboratory. Of course, if available, to resort to the use of extremely sensitive atomic detectors, such as the I C P - M S 44 is becoming more and more commonplace in many laboratories, as this instrumentation is getting wider acceptance. In fact, HPLC-ICP-MS techniques for the speciation of As, Hg, Sn, Pb, Se, Sb, Cd, etc. in biological materials have been published 44'5i-53 and continue to be widely used and increasingly reported in scientific meetings and conferences. The great potential of High Resolution-Sector Field-ICP-MS, regarding selectivity (spectral resolution), great sensitivity at low resolution and improved isotopic ratios and capabilities, continues to be evaluated and assesed. 54

428

A. Sanz-Medel

Figure 13.5 Schematic diagram of the coupling of HPLC-focused MW assisted digestion-HG(QC-FAAS/ICP-OES/ICP-MS) (reproduced from Reference 49, with permission).

13.6 Trends in trace element speciation research in the biological and biomedical fields Chemical speciation research in biological and biomedical sciences is very active nowdays. Here we will try to delineate some particular areas where rapid progress can be envisaged in the near future. 13.6.1

Elements and

samples

Future directions would be modulated mainly by efforts to solve pending problems in the bioinorganic, biological and biomedical fields, needing speciation information. Basic research in these fields is keenly involved to understand: (a) bioavailability of the inorganic elements on exposure of organisms to them; (b) transport in the body; (c) element cellular uptake; (d) metabolism and excretion, (e) toxicity mechanisms; (f) selective tissue deposition in target organs; (g) detoxifications (e.g. using drugs such as metal chelators); (h) interactions and synergisms among different inorganic elements in the organism. Chemical speciation will be required to unveil such open questions. Therefore analytical techniques and strategies will develop to speciate preferentially the so-called essential (V, Cr, Fe, Mn, Mo, Co, Ni, Cu, Zn, B, Si, As, Se, I, F) and toxic (Hg, Pb, Cd, Tl, Sn, etc.) trace elements. However, speciation work on therapeutical drugs' metabolism

Chromatographic separations coupled to atomic detectors

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(e.g. gold compounds, cis-platin) is also likely in the near future. Of course, the preferred samples will be more likely typical clinical specimens such as urine, whole blood, serum, etc. From these more readily available biological fluids interest will likely derive to not-soavailable biological tissues (e.g. soft and hard tissues). Additional care should be exercised to develop methods to speciate solid biological samples (e.g. bone) which, using mild reagents and conditions and keeping sample manipulation at a minimum, were able to extract the sought species out of the sample without any noticeable change of their nature and relative concentration in the solid. As developments in bioinorganic chemistry O c c u r 9 this knowledge could permeate and be exploited to guide analytical strategies aiming at routine control of species of importance (nutritionally, toxicologically or therapeutically) at very low levels and, desirably, by on-line techniques.

13.6.2 Analytical speciation tools Atomic spectrometry will continue to play a predominant role, coupled to separation techniques, even if perhaps bio-sensors could eventually solve particular problems of particular species in the future 23 while computational approaches continue to grow steadily. Following developments in biotechnology and other areas for enhanced resolution and specificity, new fractionation and separation techniques for biological material (see Table 13.4) could be tried for speciation purposes. For instance, the great separative potential of capillary electrophoresis and other low volume and low flow techniques, e.g. microbore HPLC, 55 could now be favorably utilized for speciation in biological systems because appropriate interface designs with Plasma-MS detectors of different types 56-59are being developed at an incredible pace in recent years. Specially promising is the hybridization of such separative techniques with a High Resolution-Sector Field--ICP-MS. This sophisticated on-line detector 6~could be used after the electrophoretic separation in the low resolution mode allowing for 102 or 103 times better detection limits. Of course, as in HPLC columns, the separation conditions (e.g. the high electric field used for electrophoretic separations) should not break or even displace the original metal-biopolymers bonds. A mild separation technique which might hold analytical potential in this area is field flow fractionation (FFF), recently coupled to ICPMS detection for environmental purposes 61 but still waiting for application to biological systems in spite of its suitability to separate macromolecules, colloids and related materials such as cells. 62 At present it is the coupling HPLC-Atomic Detector which is the more common approach for liquid samples and, as pointed out before, ICP-MS should become the more powerful detection, 44 especially now that wider spread of these instruments is observed worldwide. New designs and couplings of HPLC with ICP-MS by means of new high

430

A. Sanz-Medel

efficiency nebulizers, the use of double focusing magnetic sector ICP-MS 63 or the coupling on-line via volatile species generation 64 are offering better convenience, sensitivity or freedom from matrix effects. However, even with these modem systems many basic analytical problems remain to be solved in the near future for reliable speciation using HPLC. I would stress in the first place, the problem of adsorptions of trace elements in column packings; secondly, columns or buffer solutions competing and displacing bioinorganic ligands from the sought trace metal during separation; thirdly, likely exogenous contamination and omnipresent blank problems; and, finally, we have to be aware of incomplete separations in complex matrices. The atomic detector is unable to distinguish between two different species of the same element leaving the column unseparated (a simple fact often forgotten by researchers). Regarding detectors, perhaps AAS detection modes will be superseded by plasma emission detectors as GC-MIP-OES systems65 or recent GC-ICP-MS developments. 66 In any case, the enormous sensitivity attainable today stresses once more the extreme importance of minimizing blank values 67 for this type of work.

13.6.3 Urgentneed for quality a s s u r a n c e Only validated speciation data will allow the drawing of valid conclusions about toxicity, bioavailability, transport, accumulation mechanisms in target organs, etc., of the trace element considered. Chemical speciation of biological material is undoubtedly more difficult and critical than total metal analysis, as carried out customarily by atomic spectrometry methods. Thus, additional sources of error, apart from well-known contamination and losses, have to be taken into account. In brief, analytical validation of our speciation results is now a most urgent, and challenging, issue which is lying ahead at this stage for analytical chemists. There is a lot of effort these days directed to quality assurance (QA) and control of chemical analysis in routine measurements. 68 Such quality assurance management concepts and guidelines should be now extended to speciation. The pitfalls to that extension are numerous. Thus, whichever efforts in this direction of establishing guidelines for QA for non-routine analysis, 69 as chemical speciation is at present, are most welcome. In this vein, two main approaches may be used to validate new methodologies: the use of certified reference materials (CRM) 68 and, when a suitable CRM is not available, the use of alternative analytical methods (based on different principles) to carry out the same determination. Unfortunately, very few CRMs are available for chemical speciation. 6s' 70 Of course, spiked samples and surrogates could be used as reference samples, but this approach is less satisfactory in itself. What is more, sometimes it is not possible today to obtain pure standards of all possible chemical species of an element in a biological system

Chromatographic separations coupled to atomic detectors

431

simply because many of those forms are not available or may be unknown. Thus, in the present situation, the use of different-principle-based complementary techniques and analytical strategies appear to be a more versatile and short-term approach. The agreement of results with those obtained using additional independent methodologies can provide validation or at least useful complementary information, to strengthen conclusions of speciation results as we will illustrate in the next section.

13. 6.4 Complexity of speciation in biological systems calls for complementary analytical strategies and techniques The idea that the great complexity of biological systems complicates the interpretation of speciation results, and therefore we have to resort to the use of different principle (complementary though) analytical techniques, has been developed in our laboratory to tackle the modem problem of understanding and controlling aluminium toxicity to patients with chronic renal failure suffering long-term haemodialysis. 34'35,71-78 In an attempt to investigate and understand the real possibilities of toxic aluminium ultrafiltration (detoxification) with dialysis, to identify the protein(s) binding and the influence of metal-releasing drugs like DFO (desferroxiamine) administered to patients for A1 detoxification we started non-chromatographic-based speciation studies almost ten years ago. Different complementary techniques have been employed including ultrafiltration and ultramicrofiltration, 7mexclusion chromatography,72 ion exchange HPLC with silica-based 73-75 and polymeric resins, 34 fast-protein liquid chromatography (FPLC) 77 and polyacrilamide-gel electrophoresis (PAGE). TM Fluorimetric detection initially used in early HPLC s t u d i e s TM w a s gradually replaced by atomic (specific) detectors including off-line ETAAS, on-line ICP-OES and most recently ICP-MS. TM As progressively good matching of our fractionation (non-chromatographic ultramicrofiltration results previously referred to in Figures 13.2 and 13.3) and chromatographic speciation results has been observed, 34 repeated HPLC experiments demonstrated that HMM-A1 fraction (around 90% of serum A1) is in fact A1-Transferrin, while the ultrafiltrable LMM-A1 fraction (10+3%)could be predominantly Al-citrate, as indicated graphically by Figure 13.6, which also shows the effect of adding DFO to form LMM-A1 compounds. Most importantly, the use of FPLC and the use of a Kelex-100 impregnated silica C!8 precolumn, as an on-line A1 scavenger from reagents, allowed aluminium speciation studies at clinically relevant concentrations, i.e. in unspiked serum of dialysis patients, 77 confirming the above-mentioned findings and the efficiency of DFO as a drug to bind Al in serum (displacing the metal from other compounds, as transferrin). Moreover, PAGE results TM confirmed quite conclusively that Transferrin is the protein

A. Sanz-Medel

432

=.

LDFO

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Column fractions, rain Figure 13.6 Effect of DFO (0.01 mol 1-~): aluminium elution profiles (fraction volume 0.6 ml) and chromatograms of diluted serum (1+4): (a) model solution (4 g l -~ Tf, 1.0 i~gml -~ A1, 0.1 mmol 1-~ citric acid, 5.0 mg ml- i Si); and (b) spiked serum (1.0 Ixg ml-~ A1, 0.1 mmol 1-~ citric acid, 5.0 Ixg ml-z Si) (reproduced from Reference 76, with permission). which binds A1 in human serum (other serum proteins' role to transport the metal to target organs such as brain or bones, if any, should be negligible). The use of High Resolution-

Chromatographic separations coupled to atomic detectors

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Table 13.7 Some important biomedical conclusions from our work on aluminium and silicon speciation in human serum Aluminium Only 11 :i:3% of AI is ultrafiltrable (citrate being the more likely LMM-AI species) 90% of AI bound to Transferrin (AI-Tf) a very HMM protein (not ultrafiltrable). Metal overload cannot be efficiently cleared up in haemodialysis DFO administration, in time, originates a LMM-AI complex from AI-Tf (75% ultrafiltrable). Thus, aluminium detoxification is so possible

Silicon Ultrafiltrable Si fraction observed depends on conditions (age, pH, etc.) (45-16%) In HMM-Si the element is probably only adsorbed to serum proteins (weak interactions) Si distribution in human serum is completely unaffected by DFO therapy

No change in relative A! "ultrafiltrable", AI-LMM compounds, in serum occurs for increasing Silicon concentrations, age or small serum pH variations I C P - M S as a detector on-line with FPLC separations is now being addressed in our laboratory in order to approach similar A1 speciation studies even at basal (normal) levels of healthy subjects (total A1 serum levels around 2 Ixg 1-~).78 It is interesting to note that silicon has been implicated in A1 toxicity of Alzheimer's disease patients 79 and also as a possible protecting agent o f A1 toxicity by formation o f hydroxyaluminosilicates. 8~ Therefore parallel speciation studies o f Si speciation were carried out in the same serum samples speciated for A1. 34'76 Table 13.7 summarizes important biomedical conclusions derived from these studies, in a comparative manner for A1 and Si speciation in human serum. Such results illustrate the completely different behaviour and speciation observed for silicon as compared with serum aluminium 75' 76 and also demonstrate the usefulness of validated speciation information in biological and clinical issues.

13.6.5 Isotopic ratio and isotope measurements for establishing definitive or reference techniques It is known in analytical chemistry the existence o f definitive or primary methods which, as gravimetry or titrimetry, provide analytical results of proven and demonstrated accuracy for a given element in a given matrix. In the search for validating new developed speciation methods the possibility of using such definitive methods as reference alternative methods, with which to compare our results, is highly attractive at this stage o f development of speciation technology. Perhaps highly qualified reference methods for speciation could be provided by Isotope Dilution-Mass Spectrometry (ID-MS), today considered as a definitive or primary 8~ analytical method. As for other recognized definitive techniques (e.g. gravimetry), I D - M S can provide methods of demonstrated accuracy. 8~ Moreover, I D - M S is able to determine, with such proven accuracy, extremely

434

A. Sanz-Medel

low levels of most chemical elements. Therefore, it appears that ID-MS could provide a measurement technique for developing definitive methods for ultratrace analysis and so for species accurate determinations. Thermal Ionization-Mass Spectrometry (TIMS) has been the technique of choice for precise isotopic ratio measurements so far. Thus, this could provide the instrumental basis for accurate ID techniques to determine chemical species at low levels in biological systems. However, TIMS is a time-consuming expensive technique requiring careful and labour intensive sample preparation and comparatively long machine times for the measurements. It has recently been shown, however, that for certain elements and experimental conditions ICP-MS could represent a favourable alternative to TIMS for those applications requiring precise isotopic ratio measurements. 82 Comparatively poor plasma stability and precisions can be overcome using a multiple collection detector MS system able to collect many isotopes simultaneously and display isotopic ratios. This type of instrument has already been described using a Double Focusing Magnetic Sector-Mass Analyser for ion separation and multicollector detection. 82 Thus, ID-ICP-MS could eventually be used on a routine basis to provide a convenient instrumental tool to develop definitive methods of analysis for trace element concentration levels in environmental and biological samples. Should this aim be achieved, the technique ID-ICP-MS could probably play a very important role in the near future to attain the needed quality assurance for element speciation 83'84 both in environmental and biological samples. The potential of using isotopes for chemical speciation purposes should again be mentioned here in connection with "radiotracers". The measurement of the radiochemical activity of a spiked tracer 85 has proved to be invaluable to evaluate and assess the efficiency of the different steps of speciation processes (e.g. extraction, preconcentration, separation and final determination). This practice of assessment of the efficiency of every different step needed for final speciation should be encouraged in publications. In this sense, the possibility exists of replacing unwanted radiotracers, in such particular applications, by enriched stable isotopes measurable by non-radiactivity-based techniques such as Ion Chromatography-ICP-MS.

13.7

Concluding remarks

People entering the field of trace element speciation in biological systems, in its present stage of development, should be aware of a few important points I would like to stress as conclusion of this chapter. First of all, many relevant problems in biological systems can be better understood with speciation information; therefore, problems pending should be tackled first by developing

Chromatographic separations coupled to atomic detectors

435

adequate sensitive and selective analytical strategies and techniques. The risk exists of going too far in the techniques themselves and forget the basic aim of solving problems via chemical speciation data. The necessary balance can be approached by crossfertilization of analytical chemists with other professionals (biochemists, clinical chemists, pathologists, etc.) helping us to identify actual problems and to approach them adequately to obtain really useful chemical speciation information. At present the amount of data on speciation of biological systems is scarce compared with that existing today for environmental issues, as illustrated in the recent excellent review on speciation by Lobinsky. 86 It should be stressed, however, that principles and analytical strategies or techniques developed for environmental samples (Chapter 12) could easily be extended to solve speciation problems in biological material. Secondly the complexity of biological systems can be huge. Of course, to tackle adequately such complexity we have to interact with other professionals again. Particularly cross-fertilization with bioinorganic chemists can be most helpful these days, as the book edited recently by S. Caroli, Element Speciation in Bioinorganic Chemistry, 87 stresses in the very title. Moreover, such potential complexity of biological systems calls for complementary data, coming from different-principle-based techniques, as the more general way of validating chemical speciation results in biological systems. Of course, the availability of adequately speciated CRMs in the market would be most welcome to alleviate the present need for validating quantitative results. Thirdly, the more effective speciating analytical techniques today seem to be hybrid techniques. Thus, more efficient separations are needed for accurate chemical speciation, e.g. capillary electrophoresis holds a great potential provided that sensitivity limitations are circumvented, and they should be coupled to sufficiently sensitive and selective detectors on-line with the separation unit (e.g. ICP-MS and ICP-HR-MS coupled to HPLC, GC, CE or SFC). 88At this point it should be borne in mind that many applied and routine speciation works ahead of us will not need such sophisticated separations nor those expensive detectors. In fact, if only one or two species are sought in a sample (e.g. methylmercury in urine), perhaps a non-chromatographic separation 4~ using the simple and convenient strategies of FIA for speciation purposes could solve some serious routine problems. The review paper by Campanella et al. 89 of chemical speciation by flow analysis can be consulted for information on FIA speciation of different oxidation states, coordination compounds and organometallics. However, at this stage many element species at trace levels in biological systems are still u n k n o w n . 49 Basic research is needed here to unveil the nature of such species and their concentration levels in tissues and biological fluids. This type of frontier work might demand highly sophisticated hybrid techniques with improved interfaces 9~ between a

436

A. Sanz-Medel

powerful separation and the sensitive detection unit. Double focusing Mass Spectrometers 91 could have an extraordinary future here by overcoming spectroscopic interferences and providing extremely sensitive detection at low resolutions. At the same time better understood and controlled preliminary steps of sample pretreatments for species extraction, preservation and preconcentration must be developed without delay. 86 In brief, in order to take full advantage of available present knowledge and powerful tools for the advancement of speciation in biological systems, at least complementary people work (crossfertilization), complementary hybrid techniques and complementary/ reliable sample pretreatment steps should be aimed at. In this way the complexity of biological materials could be progressively unveiled via new speciation data and this, in turn, would promote the development of a host of analytical techniques and strategies for speciation which, no doubt, are lying ahead and will be commonplace to analysts of the next millenium.

13.8

References

1 Frieden, E. ed., Biochemistry of the Essential Ultratrace Elements, Plenum Press, New York, 1984. 2 Mertz, W. ed., Trace Elements in Human. and Animal Nutrition; 5th edn, Vol. 1 and 2, Academic Press, Inc., San Diego, 1987. 3 Gaw, A. et al., Clinical Biochemistry, Churchill, Livingstone, Edinburgh, 1995. 4 0 s t e r , O. and Prellwitz, W., Forstscht: Atomspektrom. Spurenanal., 1987, 2, 185. 5 Versieck, J. and Cornelis, R.,Anal. Chim. Acta, 1980, 116, 217. 6 Cornelis, R., Mikrochim. Acta, 1991, i!!, 37. 7 Sanz-Medel, A., Rodriguez, R. and Cannata, J. B., d. Anal. At. Spectrom., 1987, 2, 177. 8 Williams, R. J. P., Chem. Brit., March issue, 1996, 42. 9 Lippard, S. J. and Berg, J. M., Principles of Bioinorganic Chemistry, University Science Books, Mill Valley, California, ! 994. 10 Fairman, B. E. and Sanz-Medel A., Mikrochim. Acta, 1992, 109, 15711. 11 Thayer, J. S., Organometallic Compounds. and Living Organisms, Academic Press Inc, Orlando, Florida, 1984. 12 Larsen, E. H., Arsenic Speciation, Ph.D. Thesis, National Food Agency of Denmark, Soborg, Denmark, 1993. 13 Fairman, B. E. and Sanz-Medel, A., Determination of Aluminium Species in Natural Waters, Chapter 9. In: Quality Assurance for Environmental Analysis, Quevauviiler, P., Maier, E. A. and Griepink, B. eds., Elsevier, Amsterdam, 1995. 14 Spar6n, A., Analytical Methodology for Aluminium fractionation in Aqueous Media, Ph.D.Thesis, The Royal Institute of Technology, Stockholm, Sweden, 1994. 15 Tessier, A., Campbell, P. G. C. and Bisson, M., Anal. Chem, 1979, 51,884. 16 Thomas, R. P., Ure, A. M., Davidson, C. M., Littlejohn, D., Rauret, G., Rubio, R. and L6pez, J. F., Anal. Chim. Acta, 1994, 286, 423. 17 Quevauviller, Ph. and Maier, E. A., Environment. and Quality of Life, European Commission, Report-EUR 16000 EN, pp. 53 and 69, Luxemburg, 1994. 18 Harrison, R. M. and Rapsomanikis, S. eds., Environmental Analysis using Chromatography interfaced with Atomic Spectroscopy, Ellis Horwood Ltd. (Wiley)Chichester (U.K.) 1989. 19 Harris, W. R., Clin. Chem., 1992, 38(9), 1809. 20 Mota, A. M. and SimOes, M.L, Direct Methods of Speciation of Heavy Metals in Natural Waters, Chapter 2. In: Element Speciation in Bioinorganic Chemistry, S. Caroli ed., Wiley lnterscience, New York, 1996.

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21 Wang, J., Electroanalytical Techniques in Clinical Chemistry. and Laboratory Medicine, VCH eds., New York, 1988. 22 Florence, T. M., Lumsden, B. G. and Fardy, J. J.,Anal. Chim. Acta, 1983, 151,281. 23 Boisd6, G. E. and Harmer, H., Chemical and Biochemical Sensing with Optical Fibers. and Waveguides, Artech House, Boston, 1996. 24 C~imara, C., P6rez, C., Moreno, M. C. and Rivas, C., Fiber Optical Sensors applied to Field Measurements, Chapter 7. In: Quality Assurance for Environmental Analysis, Quevauviller, Ph, Maier, E.A. and Griepink, B., eds., Elsevier, Amsterdam, 1995. 25 Diaz, M. E., Alava, F. and Sanz-Medel, A., Mikrochim. Acta, 1994, 113, 211. 26 Ebdon, L., Hill, S. and Ward, R. W.,Analyst, 1986, 111, 1113; lb. 1987, 112, 1. 27 Uden, P. C., ed., Element Specific Chromatographic Detection by AES, ACS Symposium Series 479, Washington, DC, 1992. 28 Dedina, J. and Tsalev, D. L., Hydride Generation Atomic Absorption Spectrometry, Wiley. and Sons, Chichester, U.K., 1996. 29 Barnes, R. M., Anal Chim. Acta., 1993, 283, 115. 30 Vanhaecke, F., Moens, L. and Dams, R., Anal. Chem., 1996, 68, 567. 31 Fang, Z., Flow Injection Separation. and Preconcentration, VCH, Veinheim, 1993. 32 Donard, O. F. X. and Ritsema, R., Hyphenated Techniques applied to the Speciation of Organometallic Compounds in the Environment, Chapter 16. In: Environmetal Analysis:Techniques, Applications and Quality Assurance, Barcel6, D. ed., Elsevier Science Pub., Amsterdam, 1993. 33 Cornelis, R. and Kimpe, J. D., J.Anal.At.Spectrom., 1994, 9, 945. 34 Sanz-Medel, A., The Analyst, 1995, 120, 799. 35 Van Laudeghem, G. F., Haese, P. C. and De Broe, M. E., Anal Chem., 1994, 66, 216. 36 Wr6bei, K., Blanco, E. and Sanz-Medel, A., J. Anal At. Spectrom., 1994, 9, 281. 37 D'Haese, P. C., Van Landeghem, G. F., Lamberts, L. V. and De Broe, M., Mikrochim. Acta, 1995, 120, 83. 38 Cornelis, R., Borguet, F. and De Kimpe, J., Anal. Chim. Acta, 1993, 283, 183. 39 Cornelis, R., De Kimpe, J., Zhang, X., Mees, L. and Vanholder, R., Fourth International Symposium Metal Ions in Biology. and Medicine, OA 23, p. 55, Barcelona, Spain, 1996. 40 MacLeod, C. W.and Jian, W., Spectroscopy World, 1990, 2(1), 32. 41 Valcarcel, M. and Luque de Castro, M. D., Non-Chromatographic Continuous Separation Techniques, RSC, Cambridge (UK), 1991. 42 Men6ndez, A., Fern~indez, M. L., S/mchez Uria, J. E. and Sanz-Medel, A., Mikrochim. Acta., 1996, 122, 157. 43 Men6ndez, A., P6rez, M. C., S~nchez Uria, J. E. and Sanz-Medel, A., Fresenius J. Anal. Chem., 1995, 353, 128. 44 Hill, S. J., Bloxman, M. J. and Worsfold, P. J.,J. Anal. At. Spectrom., 1993, 8, 557. 45 Sullivan, J. J., Trends Anal. Chem., 1991, 10(1), 24. 46 Liu, Y. M., Fern~indez, M. L., Blanco, E. and Sanz-Medel, A., J. Anal. At. Spectrom., 1993, 8, 815. 47 Sanz-Medel, A., Aizpfin, B., Marchante Gay6n, J. M., Segovia, E., Fern/mdez S~inchez, M. L. and Blanco Gonz~ilez, E., J. Chromatogr., 1994, 683, 333. 48 Costa Fern/mdez, J. M., Lunzer, F., Pereiro, R., Bordel, N. and Sanz-Medel, A., J. Anal. At. Spectrom., 1995, 10, 1019. 49 Gonz/dez La Fuente. J. M., Fern~indez S~inchez, M. L. and Sanz-Medel, A., J. Anal. At. Spectrom., 1996, 11, 1163. 50 Gonz~ilez La Fuente, J. M., Fern~indez S~inchez, M.L. and Sanz-Medel, A., J. Anal At. Spectrom., 1998, 13, 423. 51 Larsen, E. H. and Pritzl Hansen, G., J. Anal. At. Spectrom., 1993, 8, 557. 52 Vela, N. P. and Caruso, J. A., J. Anal At. Spectrom., 1993, 8, 787. 53 Quijano, M. A., Guti6rrez, A. M., P6rez-Conde M. C. and C~imara, C., J. Anal. At. Spectrom., 1996, 11, 407. 54 Riondato, J., Vanhaecke, F., Moens, L. and Dams, R., J. Anal At. Spectrom., 1997, 12, 933. 55 Shum, S. C. K., Pang, H. and Houk, R. S., Anal Chem., 1992, 64, 2444. 56 Olesik, J. W., Kinzer, J. A. and Olesik, S. V., Anal. Chem., 1995, 67, 1. 57 Liu, Y., L6pez-Avila, V., Zhu, J. J., Widerin, D. R. and Beckert, W. F., Anal Chem., 1995, 2020.

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58 Lu, Q., Bird, S. M. and Barnes, R. M., Anal. Chem., 1995, 67, 2949. 59 Caruso, J. A., New departures for elemental speciation at ultra-trace levels, Lecture T.I, Book of Abstracts, p. 60, 1996 Winter Conference on Plasma Spectrochemistry, Florida, USA. 60 Vanhaecke, F., Verstraete, D., Moens, L. and Dams, R., Book of Abstracts, FP54, p. 310, 1996 Winter Conference on Plasma Spectrochemistry, Florida, USA. 61 Taylor, H. E., Garbarino, J. R., Murphy, D. M. and Beckett, R., Anal. Chem., 1992, 64, 2036. 62 Giddings, J. C., Unified Separation Science. John Wiley & Sons, New York, 1991, p. 189. 63 Becker, J. S. and Dietze, H. J., d. Anal. Atom. Spectrom., 1997, 12, 881. 64 Gonz~ilez La Fuente, J. M., Fern~indez S~inchez, M. L., Marchante Gay6n, J. M., S/mchez Uria, J. E. and Sanz-Medel, A., Spectrochim. Acta B, 1996, 51(14), 1849. 65 Sullivan, J. J. and Quimby, B. D., Interferencees affecting selectivity in GC-AES, Chapter 4. In: ElementSpecific Chromatographic Detection by AES, P. C. Uden, ed., ACS Series 479, Washington, DC, 1992. 66 Evans, E. H., Pretorius, W., Ebdon, L. and Rowland, S., Anal. Chem., 1994, 66, 3400. 67 Cornelis, R., Mikrochim. Acta, 1991, !il, 37. 68 Quevauviller, P., Mikrochim. Acta, 1996, 123, 3. 69 Holcombe, D. and Steck, W., Quality Assurance Best Practice for Research. and Development and NonRoutine Analysis Eurachem/CITAC, Guide 2, Working Draft, Prague, 1996. 70 Okamoto, K., Yoshinaga, J. and Morita, M., Mikrochim. Acta, 1996, 123, 15. 71 P6rez Paraj6n, J. M. and Blanco Gonzhlez, E., Trace Elem. Med., 1989, 6(1), 41. 72 Wr6bei, K., Blanco, E. and Sanz-Medel, A., d. Anal. At. Spectrom., 1994, 9, 281. 73 Wr6bel, K., Blanco, E. and Sanz-Medel, A., Mikrochim. Acta., 1993, 10, 97. 74 Garcia-Alonso, J. I., L6pez, A., Blanco, E., P6rez, J. and Sanz-Medel, A., Clin. Chim. Acta, 1990, 189, 69. 75 Sanz-Medel, A., Fairman, B. and Wr6bel, K., Aluminium. and Silicon Speciation in Biological and Clinical Materials, Chapter 7. In: Element Speciation in Bioinorganic Chemistry, S. Caroli ed., Wiley Interscience, New York, 1996. 76 Wr6bel, K., Blanco Gonzfilez, E., Wr6bel, Kz. and Sanz-Medel, A., Analyst, 1995, 120, 809. 77 Soldado Cabezuelo, A. B., Blanco-Gonzalez, E. and Sanz-Medel, A., Analyst, 1997, 122, 573. 78 Soldado Cabezuelo, A. B., Montes Bay6n, M., Blanco Gonzalez, E., Garcia Alonso, J. I. and Sanz-Medel, A., Analyst, 1998, 123, 865. 79 Zatta, P. F., Trace Elem. Med., 1993, 10, 120. 80 Birchal, J. D., Chem. Br., 1990, 26, 146. 81 De Bigvre, P., Anal. Proceed., 1993, 30, 328. 82 Walder, A. J., Platzner, 1. and Freedman, P. A., J. Anal. Atom. Spectrom., 1993, 8, I. 83 Heumann, K. G., Quality Assurance in Trace Elemental Speciation by Isotope Dilution Mass Spectrometry, Plenary Lecture, 8th JAI (Expoquimia), Barcelona, 1996. 84 Kingston, H. M., Huo, D., Lu, Y. and Walter, P. J., Pittsburg Confference on Analytical Chemistry, Atlanta, USA, 1997. 85 Gallorini, M., Birattari, C., Bonardi, M., Magon, L. and Sabbioni, E., J. Radioanal. Nucl. Chem., Articles, 160(2), (1992), 549. 86 Lobinsky, R., Appl. Spectrosc., 1997, 51(7), 260A. 87 Caroli, S., Element Speciation in Bioinorganic Chemistry, John Wiley and Sons, New York, 1996, pp. 1-20. 88 Zoorob, G. K., Mckiernan, J. W. and Caruso, J. A., Mikrochim. Acta, 1998, 128, 145. 89 Campanella, L., Pyrzynska, K. and Trojanowick, M., Talanta, 1996, 43, 825. 90 Magnuson, M. L., Creed, J. T. and Brockhoff, C. A., J. Anal. Atom. Spectrom., 1997, 12, 689. 91 Moens, L., Jakubowski, N., Anal. Chem., 1998, April 1, 251A.

enhanced trace analysis and element speciation

14.1

Introduction

Over the last two decades remarkable advances have been made in analytical technologies that provide rapid, sensitive and selective determination of inorganic moieties. In particular, element specific techniques, such as ICP--OES and ICP-MS have become established and are widely used for ultratrace analysis of diverse samples. ~ Instrumental measurement, however, constitutes only one stage in the overall analytical strategy. In the context of water analysis, a major effort in terms of time and resources is given over to sampling and sample pretreatment operations, 2-5 which are an integral part of the analysis. Thus in conventional water monitoring programmes, discrete volumes of sample are typically collected in pre-cleaned storage containers, reagents are then added for sample preservation and, after shipment back to the laboratory, further chemical processing may be necessary before instrumental measurement. The procedures tend to be labour intensive, time consuming and barely compatible with the highly developed and automated instrumentation that is used in the final measurement stage. In addition to possible sample contamination during storage, changes in metal concentrations can occur as a result of sorption processes, precipitation, colloid formation and bacterial growth. 439

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Furthermore, in the context of speciation analysis any procedure adopted for sample preservation should not disturb the equilibria existing among the species of interest. 5 At an early stage, Paulsen et al. 6 recognised that problems in trace element analysis of sea water were generally not in the quantitative analytical techniques used to obtain data, but in the methods used to obtain a representative sample free from errors introduced during sampling and storage. In an attempt to improve data quality, they advocated the development of in situ sample processing techniques and proposed the following: 9 the system must be sampled in a manner so that it is rendered physically and chemically inert 9 the sample must be in a form to be analysed by a direct non-destructive methodpreferably with one step or no chemical preparative steps 9 the analysis method must be one that is potentially applicable for shipboard or field use in order to get rapid analytical results 9 the in situ instrumentation must be inexpensive, rugged and durable 9 the analytical method must provide high quality resolution for simultaneous identification of several elements 9 the analytical method must also be quantitative with respect to initial concentration of each element in the original environment 9 the sample must occupy a small space for convenient storage. Based on these considerations, an electrodeposition sampling device was devised whereby metals were deposited on a graphite working electrode which could then be subsequently removed from the sampler and retained until laboratory analysis. The aforementioned researchers also proposed the use of in situ column preconcentration techniques whereby a known volume of sea water, at the sampling site, was pumped through a small column (15 cm• mm) packed with chelating-agent immobilised on controlled-pore glass to collect the analytes of interest. The column was then returned to the laboratory for subsequent analysis. Since particulate matter could be either trapped on the column or removed by filtration, data reflecting dissolved and total trace element concentrations could, in principle, be obtained. Other researchers have also documented similar approaches for collection and processing of waters. 7-9 In the mid-1970s the concept of FIA was introduced and has become an accepted and versatile technique for solution handling and information gathering. 1~ The FIA technique has created a revolution in analytical methodology directed at automation, miniaturisation and mechanisation of routine operations. Main features of FIA include good precision, low risk of sample contamination, low consumption of sample and reagents, ease of automation, and high sample throughput. As shown in detail by J. Tyson

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in Chapter 3, in combination with atomic spectrometry, FIA is extremely versatile in that a range of analytical operations can be readily implemented, e.g., direct sample introduction, calibration, analyte preconcentration/matrix removal and speciation measurement. 12Microcolumn preconcentration systems based on FI methodology, in particular, have received considerable attention. One of the first such FI systems utilised a microcolumn of Chelex-100 for on-line preconcentration of lead in sea water. ~3Since then a wide range of column packing materials have been deployed for enrichment purposes, including activated alumina, Muromac A-l, immobilised 8-hydroxyquinoline, dithiocarbamate resin, sulphydryl cotton fibre, Amberlite XAD, activated carbon and reversed-phase Cz8 sorbent. ~4 Enrichment factors of one to two orders of magnitude are readily achieved by processing sample volumes of 5--50 ml. The versatility of FIA is such that developments should not be confined to the laboratory and experiments of the type proposed by Paulsen e t al. 6 lend themselves to adaptation by FlA. In microcolumn field sampling, water samples are processed in flow systems at the sampling site and trace elements of interest are immobilised on microcolumns. The microcolumns may then be returned to the laboratory and directly inserted into a FI system for on-line elution and quantitative analysis. The approach can exploit a wide range of chemistries and potentially offers significant analytical and economic benefits, e.g., more efficient, rapid, convenient, cost-effective and environment-friendly sampling and analysis; also there is the potential for improved data quality in speciation/ultratrace analysis. This chapter discusses the basic equipment and analytical requirements for implementing microcolumn field sampling with FI atomic spectrometry. Reference to recent literature is also used to highlight scope and capability in aquatic studies.

14.2

Equipment and analytical strategies

Microcolumn field sampling is readily implemented using basic FI equipment, as illustrated in Figure 14.1. Water samples are passed through the microcolumn at flow rates of 2-5 ml min- ~to immobilise trace elements of interest. A small volume of eluent is then injected to release the immobilised species for final analysis. Different analytical strategies associated with microcolumn field sampling can be realised (Figure 14.1): (1) After sampling the microcolumns may be retumed directly to the laboratory and the elution step performed in an on-line manner. (2) The elution step can be accomplished, in the field, immediately after sample processing. Then the extracts, contained in small vials (sample volume 1-2 ml), are shipped back to the laboratory for analysis.

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Figure 14.1 Analyticalstrategies for microcolumn field sampling. (3) The microcolumns, after shipment back to the laboratory, are subjected to off-line elution and extracts analysed as in 2. Concerning the relative merits of the three different strategies, transporting microcolumns back to the laboratory may be advantageous in terms of maintaining and preserving the analyte species. On-line elution eliminates the use of additional storage containers, thus reducing the risks of sample contamination and loss; also due to use of relatively small eluent volumes (100-500 I~1) high analyte preconcentration factors are realised. Off-line elution in the field or in the laboratory is compatible with conventional laboratory practices, in that a fixed sample volume is available for use with autosampler for high throughput unattended operation. The steady state response associated with offline elution is clearly compatible with standard data acquisition software. This contrasts with on-line elution (strategy (1)) which because of the transient nature of the signals does require specialised data acquisition software. If not available commercially, microcolumns may be constructed from thick-walled Teflon tube and dry packed or filled with slurry. Small pieces of sponge/foam, quartz/glass wool, nylon gauze or membrane filter are placed at both ends of the columns and the packing is retained by inserting additional tubes with leak proof push-fit. The column dimensions and physical characteristics of the packings are critical in terms of analytical performance. ~4--t6Analyte deposition efficiency will depend on the column aspect ratio and generally the use of long narrow columns is favoured. Typical column lengths range from

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3-10 cm with diameter of 1-7 mm. Larger column capacity may provide an enhanced breakthrough capacity, particularly in the case of processing large sample volumes of complex composition. Use of column packings with small particle size should result in an improved breakthrough capacity. The scope for rigorous optimisation is, however, limited since high back pressure may be generated at relatively high sample loading rates. The column packing materials function through ion exchange, chelation or adsorption and the chemistry of the deposition process can be tailored to suit the particular application. A typical example is activated alumina, whose ion exchange properties are strongly pH dependent. 17 In the acidic form, alumina has a high affinity for oxyanion species such as chromate, molybdate, selenate, phosphate, sulphate, etc., 18whilst in basic form alumina has a high affinity for cations. ~9This difference in retentive behaviour may be exploited to provide a speciation capability as in the case of chromium. ~2 Other common packings include chelating resins utilising the iminodiacetic acid (IDA) moiety, which offer selective retention of transition and heavy metals. Chelex-100 is readily available but the resin undergoes substantial volume changes with change in pH, 13'n5 making it difficult to maintain column integrity and prevent leaking. Muromac A-1 resin, an alternative to Chelex-100, does not swell and shrink with changes in pH. 2~ Sulphydryl cotton fibre (SCF) exhibits a high affinity for soft Lewis acids, like mercury and precious metals. 22 Depending on application candidate columns should exhibit high affinity for the species/analytes of interest and in particular display rapid and efficient exchange characteristics. In addition microcolumns should be free from contaminants, e.g., metallic impurities and/or if necessary be amenable to decontamination. This aspect is particularly relevant to ultratrace applications such as the analysis of sea water. The field sampling system can be purpose-built and configured from basic hardware components. The sample delivery system can be a syringe, in the simplest case, or a peristaltic pump. Battery-operated pumps are available. Although not specifically designed for the purpose commercially available flow injection systems can be deployed in the field (subject to availability of power). One such system is the TRACECON unit supplied by Knapp Logistic Automation. 23 In testing a new application feasibility studies can be performed in the laboratory and typically involve some of the following: 9 effect of FI/solution parameters on analyte deposition/elution, e.g., sample pH, flow rates, sample volume, nature of eluent, presence of co-existing species etc. 9 analyte preconcentration and matrix rejection capability 9 basic analytical performance (enrichment factor, limits of detection, precision) 9 column to column variability 9 stability of immobilised species.

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T

@ lye

Figure 14.2 FI manifolds for method development. The FI manifolds, illustrated in Figure 14.2, can generally be used to undertake initial method development. The single-line system (a) with a microcolumn in series with the detector, is suitable for initial work with standard solutions and for analyte breakthrough studies. With a two-line system (b) and by incorporating the microcolumn within an injection valve, matrix constituents, at the sampling stage, can be prevented from entering the detector. It is normal practice to use an injection or switching valve to effect on-line elution. Based on results from such studies, a protocol including calibration strategies can be drawn up for field sampling/analysis trials.

14.3

Field investigations

There is an increasing demand for monitoring trace and ultratrace inorganic contaminants in waters as a result of new legislation aimed at the protection of the environment. In the context of trace element analysis, few advances in water sampling and sample handling operations have been seen in routine practice, which currently remain time-consuming, labour-intensive and hardly compatible with the advanced and automated instrumentation that is a feature of modem environmental laboratories. Although there are only few published reports on microcolumn field sampling to date, further research activity is warranted, given its potential value in key areas including monitoring of the marine environment, remote analysis/geochemical prospecting and speciation analysis.

Microcolumn field sampling and flow injection analysis

14.3.1

445

Speciation studies

It is generally accepted that, for elements such as aluminium, arsenic, chromium, lead, mercury and tin, the determination of total element content in natural waters is not adequate for an accurate assessment of environmental impactd As pointed out in Chapter 12, additional data on the chemical forms of elements are necessary in order to improve knowledge and hence provide better understanding of bio-geo-chemical pathways. From a quality assurance standpoint, if there is a significant time lapse between sampling and analysis and if storage conditions do not maintain the original species distribution there is cause for concern regarding the significance of reported data. Isolation and immobilisation of the analyte species at the time of sampling by deploying microcolumns in the field may be an effective way to preserve the analyte species distribution until the analysis step is performed in the laboratory. Several groups have demonstrated the potential value of this approach in speciation measurement.

14.3.1.1

Aluminium

The very labile inorganic monomeric aluminium species (AI 3§ AI(OH) 2§ AI(OH)~, AI(OH)4, etc.) in the aquatic environment are highly toxic to aquatic biota. 24 There is clearly a need for methodology that provides quantitative data on aluminium species in addition to total element concentrations. It has been reported that conventional sample manipulations, such as filtration, temperature changes, change in pH, choice of container material, all have the potential for changing the species equilibria in water samples. 25 Fairman and Sanz-Mede126 demonstrated that the "fast reactive" or toxic aluminium fraction in waters, based on short time (3 s) reaction with 8-hydroxyquinoline (0.0005 M) at pH 5.0, could be isolated by an Amberlite XAD-2 non-ionic resin packed microcolumn (5 c m x 3 mm, resin particle size range 80-160 Ixm) in a FI system. The retained aluminium was then eluted by back-flushing the column with 1 M hydrochloric acid for on-line detection by ICP-OES or ICP-MS. 26'27 The fast reactive aluminium fraction was shown not to include the slower reactive/non-toxic A1F2§ and AIF 2§ even at low F to A1 molar ratios (0.3: 1). No difference was found between columns loaded with aluminium standard solutions and then stored for up to two days before analysis and columns freshly loaded and analysed immediately. Between-column precision was 8% RSD for 2 ml of 100 ~g 1-1 standard solution and the limit of detection was 10 Ixg 1-1 with ICP--OES and 1.7 ixg 1-~ with ICP-MS detection, respectively. The signal-to-blank ratio was improved from 4.6 with ICP-OES to 10.7 with ICP-MS detection. For field sampling trials, 27 the microcolumns (5 cm• 1.5 mm, 90 mg resin) were washed with methanol (5 • 1 ml) and then with hydrochloric acid (1 M, 5 x 1 ml). The water sample, buffer solution (0.01 M NaAc/HAc at pH 5) and 8-hydroxyquinoline (0.005 M in 0.01 M NaAc/HAc at pH 5.0) were continuously pumped through the

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--5

Figure 14.3

il-

Field sampling manifold for aluminium speciation.

manifold as shown in Figure 14.3. After loading of between 2-5 ml of sample and the residual internal fluid being drawn off, the microcolumn was removed and reserved for analysis. Microcolumn calibration standards were prepared in the same way by pumping through the appropriate standard solutions. The final analysis was achieved by injecting 330 ILl of 1 M hydrochloric acid as eluent. As shown in Table 14.1 data were consistent with the toxic aluminium fraction as measured by an established HPLC-fluorimetric procedure. 27

14.3.1.2

Chromium

A great deal of attention has been paid to the speciation of chromium in water samples at the low or sub-txg 1-z level because of the high toxicity of chromium(VI). Currently Table 14.1 Determination of the fast reactive aluminium fraction using microcolumn field sampling with FI-ICP-MS/OES (from Reference 27) Sample Burrator feed water Moorland marsh water Princetown tap water Moorland spring water Plymouth tap water Spring water De Lank river water Colliford reservoir water

Fast reactive aluminium fraction

HPLC

< DL 3.8 130• 22 77.9• 40.6• 27 38 47

7.7 22.0 162 76.9 40.6• 17.3 16.7• 16.1 • 1.7

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European Community (EC) regulations do not stipulate the determination of chromium (VI) and the maximum admissible concentration for total chromium in drinking water is 50 txg 1-!. Furthermore, conventional sampling, storage and manipulation may create problems related to the preservation and stability of chromium species. 28-3~For instance sample acidification and increase in temperature cause reduction of chromium(VI) to chromium(III). 28 Sample filtration and the removal of humic substances may lead to adsorption of chromium(Ill) on the container walls. 29'3~ Syty et al., 3] who used a C18 bonded silica column for preconcentration of chromium(VI), found that the recovery of chromium(VI) from pond waters was 100% on the day of spiking, but in the range 81--85% one day after spiking. Cranston and Murray, 32'33 using in s i t u co-precipitation with iron(II) hydroxide for both chromium species and iron(Ill) hydroxide for chromium(Ill) only, found very large analyte losses over a period of 100 hours from the time of spiking, 78% for chromium(Ill) and 14% for chromium(VI). The losses were ascribed to adsorption on both the filtration equipment and the container walls. Cox et al. 34 developed a FI system with a microcolumn of acidic alumina ( 3 c m x 1.5 mm, particle size range 120-150 Ixm) in combination with ICP--OES to provide a rapid speciation capability for chromium(VI) and chromium(Ill) in waters. On sample injection, chromium(VI) was retained by the acidic alumina whilst the chromium(Ill) was unretained and passed directly to the detector. A subsequent injection of ammonium hydroxide (200 ~1, 1 M) resulted in elution of chromium(VI). In this way a time-resolved sequential monitoring capability for chromium(Ill) and chromium(VI) was realised, as shown in Figure 14.4. Limits of detection (20-) were 1.4 and 0.2 Ixg 1-

iI

0

II

160

I .~

= 120 E

-~,

~ ~

I.="'~,Cr(ll , "l=

0

I

[ o=

I

Figure 14.4 Emission-time responses for chromium(III) and (VI) using acidic alumina microcolumn.

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for chromium(III) and chromium(VI), respectively, based on processing a 2 ml sample. Later in subsequent experiments basic alumina was shown to retain and offer a preconcentration capability for chromium(III), whereas chromium(VI) was unretained. In that system nitric acid (200 Ixl, 2.0 M) was the eluent. 35 Field survey analysis for two river water systems in South Yorkshire (UK) was undertaken over a one-month period using alumina microcolumns. A set of acidic alumina microcolumns were used for determination of chromium(VI) whereas basic alumina microcolumns were deployed to retain the chromium(Ill) fraction prior to analysis. 36 The microcolumns, previously conditioned with nitric acid or ammonium hydroxide (0.02 M, 2 ml) in the laboratory, were dipped into the river water samples (unfiltered, unacidified) and 2 ml of the sample was drawn through the microcolumn by syringe and then passed back in the reverse direction. The microcolumns with retained analyte were returned to the laboratory and inserted into a FI-ICP-OES system for elution and quantitation. As shown in Table 14.2 there was close agreement between the summations of the chromium(Ill) and (VI) data and the total chromium data (unfiltered, acidified to pH 2 with nitric acid) as determined by ICP--OES with conventional pneumatic nebulisation of samples. For tests on general acceptability, the column to column variability (n= 10, 250 Ixl, 100 ~g 1-!) for both chromium species was comparable with that of single-column replicates, 2-4% (RSD), indicating that the efficiencies for deposition/elution were column independent. 36

14.3.1.3

Mercury

It is well known that organomercury compounds, particularly methylmercury, are highly toxic relative to inorganic forms. Yet in official monitoring programmes which are directed at total mercury determination, (the upper limit for total mercury concentration in drinking water, as recommended by the EC, is 1 txg 1-1), the speciation information is actually Table 14.2 Speciation of chromium in river waters via microcolumn field sampling/FI-ICP-OES (from Reference 36) River Don

River Rother

Sample batch

Cr(lll)

Cr(VI)

Total Cr

Cr(lll)

Cr(VI)

Total Cr

1 2 3 4 5

19.4 12.8 13.2 8.5 10.2

3.2 3.3 3.2 4.5 3.1

22.0 16.8 16.3 13.4 13.4

12.5 8.6 16.2 16.1 19.5

3.1 1.7 2.7 2.8 3.0

15.5 10.0 19.2 18.4 22.5

I~g !-t. Total Cr by conventional ICP analysis.

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destroyed at the time of sampling since powerful oxidants are added to samples for preservation purposes. Preservation of organomercury species in solution has been shown to be problematic. The long-term stability of methylmercury in standard solutions under various storage regimes was reported to be strongly dependent upon concentration, temperature, container material and treatment. 37'38 Losses of methylmercury in sea water at the ixg 1-139 and ng 1-! 40 levels were reported to be caused by adsorption onto the container walls and conversion to the inorganic form. It has been reported that organomercury can be preconcentrated on sulphydryl cotton fibre (SCF) from fresh waters and separated from inorganic mercury. 22 This approach was subsequently adapted for FI cold vapour atomic fluorescence spectrometry by Wei and McLeod, 41'42 the SCF microcolumn (15 mg, 5 cmx 1.5 mm) allowing rapid on-line separation (at pH 3) and sequential determination of inorganic and organo-mercury species. The FI manifold is shown in Figure 14.5. The eluent was 3 M hydrochloric acid (0.5 ml); an oxidant, 0.5% (m/v) potassium bromide + 0.14% (m/v) potassium bromate, was used to convert organomercury to inorganic mercury; and the reductant for vapour generation was 3% (m/v) tin(II) chloride in 15% hydrochloric acid. The detection limit (30-) for methylmercury based on processing of 0.5 ml sample was 6 ng 1-1, although improvement in the method sensitivity could be realised by increase of the sample volume. Field trials were performed on waters of the Manchester Ship Canal and the River Rother (UK), both water courses known to be contaminated by mercury as a result of extensive and prolonged discharges from a chloro-alkali plant. 43'44 The field sampling kit included an on-line filter (0.45 ~m), a SCF microcolumn and a syringe. The system was first conditioned by passing through 2 ml of 3 M hydrochloric acid (twice) followed by

l

Figure 14.5 Manifold for mercury speciation.

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2 ml of high purity water (twice). Water samples, on collection, were acidified to pH 3-3.5 with concentrated hydrochloric acid and then processed through the SCF microcolumn (once in each direction) to ensure retention of organomercury species. The microcolumns were then disconnected, placed in a light-tight box and returned to the laboratory for online elution/quantification. Elevated organomercury concentrations of 0.006---0.058 and 0.21-1.7 t~g Hg l-~ were obtained for the two waterways, respectively, confirming the extensive contamination by organomercury species. Organomercury species immobilised on SCF microcolumns can be further separated and quantified by gas chromatography-microwave induced plasma-OES (after elution, derivatisation and extraction steps) to provide full speciation information. 45'46 14.3.1.4

Tin

There has been a continuing interest in the determination of organotin compounds at the low ng l -~ level in natural waters. Of all the organotin species, it is mainly tributyltin (TBT) and triphenyltin (TPT) which present the greatest hazard to living organisms. Organotin species are subject to degradation processes down to inorganic tin, leading to a progressive loss during storage. Change of TBT concentration in frozen unfiltered natural sea water has been monitored, falling by about 50% after two years. 47 It has also been reported that butyltin species are not readily stabilised in turbid water samples with high suspended matter. 4s For sea water filtered through 0.45 Ixm filter and acidified to pH 2 with hydrochloric acid, monobutyltin (MBT) and dibutyltin (DBT) concentrations were not stable at ambient temperature (20-25~ as Hyphenated techniques based on chromatographic separation with sensitive elementspecific detection have been widely used for the speciation of organotin compounds. Traditionally liquid-liquid extraction is used to achieve preconcentration and matrix removal. Immobilisation of ionic organotin species by solid phase extraction using hydrophobic CI8 bonded sorbent has been successfully applied to waters. 49-5~ In a flow system, TBT, DBT, TPT and diphenyltin (DPT) in river waters were quantitatively adsorbed on a Ct8 bonded silica microcolumn over a wide pH range of 2 to 12. In the case of MBT and monophenyltin (MPT) retention efficiency dropped somewhat owing to an increase in polarity and ionic character of the analytes. 51 In a batch procedure developed by Kohri e t al., 52"53 a metacrylic ester derivatised styrene polymer (EXCELPAK, Yokogawa, Japan) was used as sorbent for the extraction. The recoveries of TBT and TPT were almost 100%, those of DBT and DPT were lower, whilst MBT and MPT were not retained. In a collaborative study, Woods et al. 5a carried out microcolumn field sampling of sea waters in Tokyo Bay (Japan) for organotin determination using microcolumns of EXCELPAK (30mg, 5 c m x 1.5 mm). Samples, acidified to p H 2 by addition of

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Deposition Volume(ml) Figure 14.6 Signal response as a function of sample volume for organotin standard and spiked sea water. 100 ng 1-' TBT, TPT, DBT and DPT. concentrated hydrochloric acid, were processed at 2 m l m i n - ' using a 4-channel peristaltic pump before storage in the dark at 4~ The microcolumns, on return to the laboratory were then inserted, one at a time, into a single-line FI-ICP-MS system and online elution was performed by injection of methanol (120 I~l). As shown in Figure 14.6 signal response increased linearly as a function of sample volume for both standard solution and spiked sea water. No significant difference in deposition efficiency between the two matrices was evident for sample volumes up to 100 ml. The recovery of total organotin (100 ng 1-~) was about 86% for 100 ml sample deposition. In addition to determination of total organotin, full speciation information was achieved through interfacing of micellar liquid chromatography (LC) with ICP-MS. 52'53 For this analysis, microcolumns were subjected to off-line elution with methanol (120 I~l) prior to injection into the LC-ICP-MS. A typical ion-time response for a spiked sea water extract is given in Figure 14.7.

!

!

Retention Time (s) Figure 14.7 Ion-time response for spiked sea water (100 ng 1-' organotin species, 100 ml).

452

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Marine environmental monitoring

The difficulty of determining elements at the trace/ultratrace levels in sea water is well known, hence considerable efforts have been expended over the last two decades on quality control initiatives concerning sampling and laboratory measurement, i.e. intercomparison exercises and production of certified/standard reference materials. As pointed out by Paulsen et al.6 there is a need to improve the methodology for sampling and sample preservation, and much of what was said by these authors 25 years ago remains valid to this day. As highlighted in the Introduction there are very significant gains (analytical and economic) to be realised if microcolumn field sampling can be successfully deployed for environmental monitoring. The key to success is essentially to be able to transfer, from the laboratory to the ship, appropriate microcolumn chemistry. Sample size and the degree of analyte enrichment will depend on the sensitivity of the laboratory measurement technique. Clearly in the case of ICP-MS only a modest enrichment factor of 5-10 fold would be required. More critical from an ICP-MS measurement standpoint is the extent to which microcolumn processing achieves rejection of sea-water salts. Currently in our laboratory two candidate column packings are receiving attention-activated alumina and Muromac A-1 and preliminary work has been undertaken with A. Otsuki of Tokyo University of Fisheries. Target elements include titanium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, molybdenum, cadmium, lead, uranium and radionuclides. A 4-channel peristaltic pump was employed on ship. Prior to sample processing, the microcolumns (3 cmx 1.5 mm) were washed with nitric acid (2 M, 3 minutes at 2 ml min-~) and conditioned with ammonium hydroxide (0.02 M) in the case of alumina or high purity water (pH 8) for Muromac A-1 (1 minute at 2 ml min-~). Natural sea water collected at Sagami Bay (Japan) was pumped through the microcolumns at 2 ml min-~ to effect analyte deposition. Thereafter the columns were washed with pure water (1 ml, pH 8) to discard residual seawater salts before removal from the flow system and storage. On return to the laboratory (Sheffield, UK), microcolumns were inserted into a FI-ICPMS for elution (2 M nitric acid, 250 Ixl) and quantitative analysis. Figure 14.8 illustrates typical ion-time response for lead corresponding to on-line elution. A total of four injections of nitric acid was performed per column. A relatively high blank contribution for lead from the alumina column in comparison to Muromac A-1 was however noted. A key experiment in method development is to be able to demonstrate a proportional increase in signal response with increase in sample volume (see Figure 14.9). In the case of Muromac A-1 such behaviour was noted for processed volumes up to at least 50 ml for many elements including manganese, cobalt, nickel, copper, zinc, cadmium and lead, but not for titanium and chromium. For high sample loading of 50 ml, curvature in response

Microcolumn field sampling and flow injection analysis

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was noted for vanadium, molybdenum and uranium. Elements which exhibited poor sensitivity included arsenic and selenium and this suggested a relatively inefficient deposition step. The basic alumina microcolumn offered a good preconcentration capability for vanadium, arsenic and uranium, but unfortunately determination of many elements at the sub-I~g 1-~ level were compromised by relatively high blanks thought to originate from impurities in the column packing. 14.3.3

Gold determination in lake waters and streams

Due to potential use in geochemical prospecting, there is a demand for new procedures that permit rapid and sensitive determination of gold in natural waters at the ng 1-l level. A practical limitation to undertaking extensive survey work in remote areas relates to the difficulty of collecting and transporting water samples (say 0.5-1 1) back to a central laboratory. This is over and above the key analytical concern of being able to preserve and maintain sample integrity until analysis is performed. Recently a FI system utilising a microcolumn of sulphydryl cotton fibre (5 cm x 1.5 mm) was combined with ICP-MS for on-line preconcentration and determina-

Figure 14.8 Ion-time response for 2~

using (a) Muromac A-1 and (b) alumina microcolumns

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o

=

~~J--W'-*-~-~*

Figure 14.9 Signalresponse as a function of sample volume for lead in sea water, (a) Muromac A1 and (b) alumina. tion of gold in natural waters. 55 Hydrochloric acid (0.01 M) served as the carrier stream whilst potassium cyanide (0.01 M, 250 IJ,l) was the eluent. Concentrations of gold determined in natural waters ranged from not detected (<0.2 ng 1-!) to 57 ng 1-~. Analyte enriched microcolumns, after seven days storage, yielded a mean recovery of 97.8% (9.0% RSD, n= 7, 20 ng 1-~ Au, 10 ml), indicating the feasibility of utilising SCF microcolumns in the field for the collection/immobilisation of gold. 56 In order to assess the potential in geological prospecting, survey work was performed with colleagues of the Ontario Geological Survey (OGS) in the Kirkland Lake region of Northern Ontario (Canada) as part of the OGS regional geochemical mapping project. 56 The SCF microcolumns were pre-cleaned and conditioned by pumping through the following solutions at 1 ml min -~ in sequence: hydrochloric acid (0.01 M) for 2 min; potassium cyanide (0.02 M) for 2 min; and hydrochloric acid (0.01 M) for 10 min. Microcolumns were then disconnected from the flow system and stored in a light-tight box at 4~ until deployed in the field. Lake and stream water samples (10 ml, unfiltered and unacidified), on collection, were drawn through the microcolumns using a plastic syringe to retain and preconcentrate gold species. At each site, microcolumn sampling was performed in duplicate. Microcolumns were then kept in a light-tight box at 4~ until shipment back to the laboratory and FI-ICP-MS analysis. Instrument calibration was based on processing of gold standard solutions at pH 7 containing 100 mg 1-1 chloride and gold concentrations for the waters are presented in Table 14.3. In general results for lake waters were at the low ng 1-~ and the ion-time response of Figure 14.10 is a typical result.

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Table 14.3 Determination of gold in lake and stream waters via microcolumn field sampling/FIICP-MS (from Reference 56) Sample No. 1

2 3 4 5 6 7 8 9 10

Lake water 1.59 1.40 2.21 1.46 1.75 4.57 4.73 2.68 3.30 3.53

1.61 2.32 1.75 1.56 1.71 4.04 4.16 4.62 3.08 3.00

Sample No. 1 2 3 4 5 6 7 8

Stream water 119.4 63.1 29.2 37.0 56.8 24.8 76.7 26.7

119.6 56.6 33.6 37.2 56.5 27.6 89.6 36.9

ng I-'

Significantly elevated concentrations (1-2 orders of magnitude) were obtained for streams located upstream o f regions o f known gold mineralisation. A g r e e m e n t in duplicate values for samples was acceptable, suggesting that the field sampling t e c h n i q u e / I C P - M S s c h e m e was reliable and robust. It is concluded that further activities directed at g e o c h e m i c a l prospecting are warranted.

Figure 14.10 Ion-time response (197Au) for lake water (S1) after microcolumn field sampling. Arrow refers to injection of eluent (250 i~1, KCN). Au concentration 1.6 ng 1-~.

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14.4

R. Ma et al.

Conclusions and future developments

Water sampling, analyte preconcentration/isolation and species preservation are key elements in trace environmental analysis programmes. It is clear that microcolumn field sampling strategies can offer realistic alternatives to current sampling practices. Implementation of microcolumn field sampling with FI procedures is highly advantageous because of the in-built preconcentration, automation, ease for use, convenience and versatility. The state-of-the-art is such that processing of modest sample volume, e.g., 10 ml, in combination with FI atomic spectrometry provides an integrated means not only for rapid sampling, but also for enabling direct quantification of trace elements in environmental waters down to the ng 1-1 level. The integration of microcolumn field sampling with ICP-MS will impact heavily on future water monitoring programmes, particularly if standards relating to speciation are introduced. The merits of microcolumn field sampling have yet to be fully appreciated and exploited. Despite the considerable research activity in FI microcolumn enrichment/ separation, it is not at present possible to judge from the literature the most appropriate microcolumn systems for water processing and preservation, since some of the approaches are not fully developed and validated. Intercomparison tests are needed for critical evaluation of each laboratory's preferred methodology. Exploring the feasibility of various packing materials for field sampling and using available certified aqueous reference materials would lead to standardisation of methodologies and allow critical assessment of the quality of measurement and laboratory performance. Although not referred to in this chapter there is a potential to develop external calibrants and reference materials in microcolumn f o r m a t 46"57 given the good long-term stability of some immobilised species and good column-to-column reproducibility. A further possibility which deserves study is use of a tandem FI system employing two or more different chemistries in series. Finally, prospects for new developments and routine applications would be stimulated if custom sampling kit and microcolumn chemistries were commercially available.

Acknowledgements The authors acknowledge the important contributions of A. Cox, W. Jian, M. Mena, M. M. Gomez and J. Fortescue, in compiling this article.

14.5

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Alumina, 443,452 Aluminium, determination of: in bovine liver, 292 in food samples, 141 in human blood serum, 292 in sediments, 141 in soil extracts, 329 in waters 313 speciation of: 445 aquatic environment, 445 biological material, 422-424 Analysis of air samples, 381-383,389, 396 Analysis of solid samples, 381-383, 397-400, 402, 403 Anion determination, 220, 326 Anodic stripping voltammetry 47, 347, 349, 351,355, 356 Antimony, determination of: in electrolytic solutions, 297 in river waters, 314 in sea waters, 313 in sediments, 329 in soil, 329 in waters, 183, 199 speciation of, 427 Approaches to chemical speciation: computational, 414 direct speciation, 415 hybrid or hyphenated tecniques, 417, 418-428 physico-chemical characterization, 418 Arsenic, determination of: arsenobetaine, 96 arsenocholine, 96 in beer, 297 in biologicals, 191, 291 in blood samples, 149

in cobalt-based alloys, 176 in copper-based alloys, 176 in fly ashes, 334 in industrial effluents, 123 "in-situ" trapping, 60 in liver, 297 in mussels, 297 in nickel-based alloys, 176 in organised media, 294 in organoarsenic compounds, 53 in seafood, 95 in tomatoes, 297 in urine, 149, 157 in waters, 314 in working atmospheres, 334 sample volume, 94 speciation of: 155, 156, 427 biological materials, 427 environmental samples, 155 urine, 427 Atomic detectors: AAS, 274, 275 AFS, 15, 57, 347, 353,370, 371 Arc-OES, 15 DC-OES, 15 ETAAS, 15, 19, 44, 54, 57, 59, 60 FAAS, 15, 16, 18 FAES, 342, 358--360, 364 general concepts, 14-30 GD-OES, 15, 54, 58, 277-285 ICP-MS, 15, 28-30, 48, 54, 57, 58 ICP-OES, 15, 25-27, 52, 54, 57, 58, 285-287 MIP-OES, 56, 57, 58 Quartz tube atomization, 22 SCP-OES, 54, 57 Spark-OES, 15 459

460

Subject index

Atomisation, 388-391,397, 401 Automatic samplers, 39 Automation, 3, 31, 32 Batch hydride generation, 238, 249-251,255, 256, 265, 267, 269, 270, 278, 279 Beryllium, determination of: in copper alloys, 226 in water, 315 in urine, 356 Biological systems, 407 atomic spectrometric detection, 409 Bismuth, determination of: in urine, 283,350, 353 in waters, 183, 315, 353 Boron, 250, 315, 350, 356 Bromide, 57, 286, 287 Cadmium, determination of: in biologicals, 191,226, 228, 291 in coal, 140 in coal fly ash, 334 in cocoa powder, 140 in cod, 364 in environmental samples, 283 in food samples, 283 in geological, 215, 225 in horse kidney, 140 in lobster, 364 in mussels, 364 in oysters, 364 in rain water, 198 in river water, 315 in sea waters, 315,452 in sediments, 191, 331 in slurried sewage sludge, 146 in soil, 331 in waters, 191 in whole blood, 195, 198 speciation, 427 Calcium, determination of: in biological tissues, 353, 354 in blood serum, 105 m ceramic materials, 105, 111 in cerebrospinal fluid, 363 m vegetable material, 77 in vitreous humour, 363 m waters, 111, 116 m wines, 91, 231

in soil extracts, 126, 231 in whole blood, 353 Calibration, 98, 99 Carbon, 287, 316, 408 Cascade dilution, 103, 118 Cerium, 191 Chloride, 57, 287, 327 Chromate speciation, 443 Chromatographic separations, 407 Chromatography with detection by: flame, 425 heated quartz tubes, 425 ICP-OES, 427 ICP-MS, 427 MIP-OES, 425 Chromium, determination of: in biologicals, 191 in bovine liver, 292 in human blood serum, 292 in river water, 447, 448 in slurried sewage sludge, 146 in steels, 124 in waters, 316 speciation of: waters, 316, 447, 448 with alumina, 447 Cobalt, 346, 349, 353,361,364 in biological materials, 226, 228, 291 in geological, 215 in lobsters, 364 in oysters, 364 in sea water, 191,452 in sediments, 191 in steels, 215 in waters, 191, 317 in whole blood, 54, 155 Column packings, 172 Column preconcentration system, 171 guidelines for system design, 171 Column sorption, 170 Compound interferences, 266, 268 Continuous aspiration FAAS, 274, 275 ICP-OES, 275, 277 MIP-OES, 278 Continuous flow, 281 nebulization ICP--OES, 281 hydride generation, 249--253,255, 256, 259, 265--267, 270, 281

Subject index Copper, determination of: in artichoke, 144 in biologicals, 191,226, 227-228 in bovine liver, 292 in brines, 191 in chocolate, 144 in coal fly ash, 334 in dietary, 53, 145 in gelatins, 215 in geological, 215 in industrial effluents, 231 in mussels, 364 in olive oil, 124, 125 in ores, 215 in plant, 145 in plasma, 142 in rain water, 198, 215 in rice samples, 199 in sea water, 56, 191, 199, 317, 452 in sediments, 191, 331 in serum, 142, 292 in sludge samples, 53, 145, 146 in soil, 147, 331 in tap water, 215 in tomato leaves, 144 in vitreous humour, 363 in waters, 191, 317 in whole blood, 54, 142, 147 Coprecipitation, ETAAS, 196 FAAS, 193 HGAAS, 195 ICP-OES, 198 preconcentration systems, 192, 193, 195, 198 Cryogenic collection, 247, 250 Cyanide, 327 Cysteine, 240, 242, 243,269 Derivatization, by sodium tetraethylborate, 383 by sodium borohydride, 382 foaming problems, 379 liquid to gas conversion, 377, 379, 391, 402 new reagents for-, 381,382 optimum conditions for-, 380, 382, 383, 384, 402 reaction vessel for--, 379, 380, 391,392

461

reactions, 382, 383 Desorption, based on boiling points, 377, 386 flash --, 387, 396 Detection: addition of auxiliary gases, 389, 390, 396 AAS, 382, 389-391,398, 399 AFS, 377-388, 389, 391-394 ICP-MS, 377, 389, 394-397 multielemental-, 377, 382, 394, 396 optimum conditions for, 389-391,395, 396 Detection limit, 13, 14, 31, 69, 71, 72, 74, 79, 80, 87, 88, 92 Diffusion, cofficient, 10, 13 secondary flow, 11 Dispersion, coefficient definition, 7, 8 coefficient reagent, 14 flow rate, 12 sample volume, 11 temperature, 12 tube dimensions, 12 Dialysis, active (Donnan) dialyser, 232, 233 definitions, 229 passive dialyser, 231,232 Electronic dilution, 80, 102, 104 Element speciation tools, 429 Extraction, 205-229 Extraction coil, 207, 208 Field sampling, 439 Flow injection, reactions, 13, 14 hydride generation, 249--253, 255, 259, 261,265, 267, 270, 279, 281 merging zones, 281 microvolumes, 281 column preconcentration, 179, 180, 183 separations, 4 Fluoride, 228, 327 Fly ashes, 334 Gas--liquid separators: hydrostatic, 251,259--261 membrane, 44, 248, 261,263-265

462

Subject index

spray chambers, 259 types, 45, 258 with forced outlet, 261,262 Germanium, 60, 318, 331,350, 356, 366 Gold, determination of: in geological samples, 225 in jewellery, 116 in lake waters, 453, 455 in minerals, 191 in ores, 215 in streams 453,455 in water, 191, 318 with sulphydryl cotton fibre, 453 Gradient ratio, 119 Hafnium, 191 Halides, 286 Hybrid techniques, 418 Hydraulic high pressure nebulizer, 55, 78 Hydride generation: definition, 238 efficiency, 240, 248 electrochemical-, 246, 264, 265 from alkaline solutions, 244 interferences, 265 mechanism, 241-243 optimum conditions, 239, 244 on-line generation systems, 152 Hydride release efficiency, 244, 246, 248, 255, 265, 266 Hydrodynamic injection, 40 Hyphenation, 319, 322, 328, 376-397, 400, 402, 403 Indirect determinations of: alkaloids, 367 amphetamines, 220 amylocaine, 220 anionic surfactants, 220 anions, 326 bromazepan, 220 bromhexine, 220 cationic surfactants, 220 chloraniphenicol, 367 chlordiazepoxide, 367 cocaine, 220, 301,369 glycine, 367 isoniazid, 367 levamisole, 367

lidocaine, 367 methadone, 367 nitrate, 220 nitrite, 220 ondansetron, 367 papaverine, 367 perchlorate, 220 procaine, 367 quinine, 369 salycilic acid, 368 strychnine, 367 sulphonamides, 362, 368 surfactants, 328 tetracaine, 367 ziram, 220 Indium, determination of: in brines, 191 in geological samples, 215 "In-situ" trapping, 60 Interferences, cryofocusing: chemical -, 386, 390, 398 during derivatization, 380, 397, 398-401 in the column, 386, 397, 401 in the detector, 390, 397, 401 masking agents, 399--401 memory effects, 380, 386, 401 spectral, 378, 392, 393, 395, 398 transalkylation processes, 401 Internal standard, 99, 127 Iodide, determination of: in sea water, 327 in table salt, 229, 286, 300 in waters, 327 Iron, determination of: in adipose tissue, 154, 353,365 in bovine liver, 292 in gelatins, 215 in geothermal fluids, 155 in ceramic materials, 105, 111 in coal, 140 in cocoa powder, 140 in horse kidney, 140 in plasma, 142 in rain water, 198 in serum, 142, 292 in soil, 147 in vegetable material, 77 in vitreous humour, 363 in water, 318

Subject index in whole blood, 142 Isotopic dilution, 48, 128, 355, 356, 364 Knotted reactors, 47, 199 Lake waters and streams, 453 Laminar flow, 9 Lanthanum,318 Lead, determination of: in air filters, 141 in air samples, 335 in biological materials, 153, 191,199, 349, 350, 353, 356 in drinking water, 91,232 in dust, 141 in environmental samples, 199 in hair, 350 in leaves, 141 in organized media, 283,294 in paint, 141 in rain water, 198 in reagents, 214, 215 in sea waters, 191, 199, 452 in sediments, 331 in sewage sludge, 144, 146 "in-situ" trapping, 60 in sludge, 14 lspeciation of, 427 in urine, 141, 356 in water, 232, 318 in whole blood, 353 in wine, 149 Liquid-liquid extraction: continuous configurations, 45, 210-214 direct determinations, 214-218 ETAAS coupling, 221-225 Extraction, 206-210 Extraction coil, 207-208 FAAS coupling, 211-221 general considerations, 88 ICP coupling, 225-229 indirect determinations, 218-221 limitations, 205, 206 membranes, supported liquid, 216-218 phase separator, 208-210 solvent segmenter, 207 Lithium, 359, 360 Magnesium, determination of:

463

in alloys 215 in blood serum, 105, 359 in body fluids, 358 in brines, 118 in ceramic materials, 105, 111 in cerebroespinal fluid, 363 in coal, 140 in cocoa powder, 140 in horse kidney, 140 in milk, 353 in soil extracts, 331 in urine, 353 in vegetable material, 77 in vitreous humour, 363 in waters, 111 in wines, 231 Manganese, determination of: in alloys, 214 in artichoke, 144 in biologicals, 191 in bovine liver, 292, 363, 364 in chocolate, 144 in dietary, 53, 145 in hair, 215 in human blood serum, 292 in milk, 353 in mussels, 364 in pharmaceuticals, 215 in plant, 145, 353 in plant digest, 127 in sea water, 191,452 in sediments, 191 in sludge samples, 53, 145 in tomato leaves, 144 in urine, 353, 356 in vegetable material, 77 in waters, 191,215, 318 Matrix interferences, 268 Mercury, determination of: in dogfish muscle, 303 in environmental samples, 152, 402 in human hair, 303 in natural gas, 335 organomercury, 355, 356, 363,402, 448 in pid kidney, 303 in river water, 449 in sea water, 183 in sediments, 331 in slurried solid samples, 149

464

Subject index

in soil, 331 in tap water, 183 in urine, 149, 150 in waste, 183 in waters, 150, 183, 319 in whole blood, 149, 151 speciation of, 160, 402, 426, 427, 443,448 in sulphydryl cotton fibre, 449 Merging flow, 48, 49, 103, 116 Methylation, 376 Methylmercury, 348, 426, 448 speciation of, 426 Microcolumn, 439 Microcolumn preconcentration, 441 Microwave-assisted treatments, 311,320, 332, 334 Microwave heating, 52, 53, 136, 137, 138, 352, 360 Mixing chamber, 102, 103, 109, 110, 112, 113, 116 variable volume, 125 Mixing devices, 256 Model for dispersion: general, 67 peak area, 68 peak width, 68 detection limit, 69 Molybdate speciation, 443 Molybdenum, determination of: in biological tissues, 356 in bovine liver, 364 in brines, 191 in high purity reagents, 198 in human hair, 198 in sea waters, 453 in soils, 332 in urine, 350 in water, 198, 320 coprecipitation, 365 Multielemental determination, 323,324, 326, 333 Nation dryer, 24 Need for chemical speciation, biological systems, 411 clinical chemistry, 411 bioinorganic chemistry, 412 biology and medicine, 412 Network manifold, 117

Nickel, determination of: in biological, 225, 226, 291,349, 351 in brines, 191 in geological, 215 in rain water, 198 in rocks, 332 in sea waters, 452 in shellfish, 364 in slurried sewage sludge, 146 in steels, 215 in urine, 349, 350, 356 in water, 320 in whole blood, 195 Niobium 191 Non-metals determination: volatile species, 285 MIP--OES, 285 ICP-OES, 285 GD-OES, 287 GD-MS, 287 On-line microwave sample mineralization, acidic digestion mixture, 140 automation, 143 irradiation time, 145 microwave energy, 138 mineralization coil, 154 on-line sample digestion, 141 on-line sampling processing, 141 reaction coil, 143 residence time, 139 slurries, 140 solid samples, 143 with continuous detection, 142 with discontinuous detection, 138 On-line preconcentration, 313, 315-317, 320, 322, 334 On-line sample dissolution/decomposition, 135 Ordered media, 244 Organic solvents, 77 Organic pollutants, 328 Organomercury, 449 Palladium, 321, 350 Partial overlapping zone, 117 Peak width, 102 Perchlorate, 362 Pesticides, 328

Subject index Phosphate, 327, 443 Phosphorus, 321 Photo-oxidation, 52, 53 Physical constants of hydrides, 239 Platinum, 191,321,350, 356 Potassium, determination of: in biological material, 358 in blood serum, 105, 359 in ceramic materials, 111 in soil extracts, 231 in vegetables, 231 in vitreous humour, 363 in waters, 111 in wines, 91, 231 Precipitation, 89, 192 Preconcentration, air sampling, 388 cryofocusing, 377, 378, 380-383, 385--388, 391,394, 396, 399 off-line -, 377, 379, 387, 396 water removal, 385, 388, 391,392 Pre-reduction, 239, 240 Programmed pump rates, 107 Reactions coils, 253,257, 258, 263, 265, 267, 270 Response time, 66 Rhodium, FI-ICP-MS determination of: number of elements measured effect, 73 peak heigh and peak area, 75 standard deviation, 74 volume injected effect, 71 Sample loading, kinetic features, 170 time-based, 169 volume-based, 169 Sampling, 135 Sample preparation, decomposition, 135 digestion, 136 disolution, 135 microwave heating, 136, 137 Sampling systems, 39, 40 Savitsky--Golay procedure, 73 Sea water, 452 Selenate speciation, 443 Selenium, determination of: co-precipitation, 91

465

in biologicals, 57, 191,347, 349, 350, 353, 356, 357, 363, 364 in cobalt-based alloys, 176 in copper-based alloys, 176 in nickel-based alloys, 176 in sediments, 332 in serum, 357 in soil, 183,332 in urine, 159, 350, 351,353,356 in waters, 183, 195, 321,353, 402 Se, detection limits, 58 speciation of: biological materials, 158, 159, 427 fruit juices, 158, 159 hemodialysis fluids, 370, 371 water, 158, 183, 402 selenoaminoacids, urine, 426, 427 with alumina, 452 Space charge, effects, 30 Separation: off-line procedures, 400, 401 optimum conditions for--, 386, 402 packed column chromatography, 377, 379, 386 silanization of the column, 386, 401 Separation techniques coupled to: Arc/spark/DCP, 420 ETAAS, 420 High resolution Sector Field--ICP-MS, 419 ICP-MS, 419 ICP-OES, 420 MIP--OES, 420 Silicon, 421 Silver, 321 Single standard, 102, 106, 116 Slug injection, 81 Slurries, 76, 140 Smoothing, 73 Sodium, determination of: in biological materials, 358, 359 in blood serum, 105, 342, 349 in ceramic materials, 111 in soil extracts, 231 in vegetables, 231 in vitreous humour, 363 in waters, 111 in wines, 231 Soil analysis, 329 Soil and sediment multielemental, 330

466

Subject index

Solenoid valve, 104, 105, 122 Solid phase extraction, 83, 313, 318, 327, 331,333 Sorption, columns, 170, 176, 181 preconcentration, KR, 199 Sorption column preconcentration systems: AFS, 371 biological, applications, 350 concept, 47, 349 dual-column system, 173 ETAAS, 180 FAAS, 173, 369 ICP-MS, 188, 355, 364 ICP--OES, 173, 361,369 single-chanel, 173,344, 364 VG-AAS, 176 VG-ICP-OES, 176 Speciation, aluminium, 421,422, 445 arsenic, 351 antimony, 301 chromatographic, 407, 424 chromium, 356, 424 complexity of biological systems, 435 crossfertilization, 434 cryogenic trapping, 157 cryofocusing: arsenic, 402 lead, 402 mercury, 402 selenium, 402 tin, 402 effective, 435 hybrid techniques, 95, 343,435 improved interfaces, 435 mercury, 301,302, 348, 356 non-chromaatographic, 301-303, 310 on-line HPLC-microwave oxidation, 155 organotin compounds, 333 sample pretreatments, 435 selenium, 351,370 silicon, 421 speciation information, 434 sulphur, 443 Split flow, 118 Standard additions: by flow rate variation, 127 interpolative, 124

merging zones, 123 on-line, 125 partial overlapping zones, 124 time based electronic, 126 zone sampling, 127 Standardization, 98, 100 Strontium, 321 Sulphate, 327, 443 Sulphide, 284 Sulphur, 285-287, 322, 335, 350, 356 Sulphur compounds, 335 Supply rate of: analyte, 254 hydride, 249, 251,253--255, 259, 267 tetrahydroborate, 246, 267-269 Syringe, 111, 114, 115 Tandem on-line continuous aspiration: automation, 299 concept, 297 indirect determinations, 300 non-chromatographic speciation, 301 schematic diagram, 298 Tantalum, 191 Tellurium, 60, 322, 349 Tetrahydroborate decomposition, 244, 246 Thallium, 356, 366 Thermospray, 55, 78 Time based injector, 104 Time injection, 81 Tin, determination of: with C 18 bonded silica, 450 with EXCELPAK, 450 in harbour sediments, 333 by ICP-MS, 451 with micellar liquid chromatography, 451 in sea water, 450 "in-situ" trapping, 366 in urine, 363 in waters, 322, 333,353 speciation of, 183,402, 427, 450 Titanium, 153,322 Toxicity, 375 Trace analysis, 439 Trace element speciation research, complementary analytical strategies, 431 elements, 428 isotopic ratio and isotope measurements, 433

Subject index need for quality assurance, 430 samples, 428 tools, 429 Transport interferences, 270 Tungsten, 191 Uranium, determination of: in brines, 191 in sea water, 453 in soil, 333 in urine, 356 in water, 191,322 Vanadium, determination of: in biologicals, 191,350 in sea water, 453 in urine, 356 in water, 322 Volatile compounds, organic solvents, 288 Volatile species-forming reactions, acetylacetonates, 283 alternative volatile species, 291 carbonyl, 283 methyl borate, 283 halogens, 283, 285 Silicon tetrafluoride, 283 Volatile species from organised media: efficiency of reactions, 295 kinetics, 293 selectivity, 295

Water analysis, 312, 324, 439, 443 Zinc, determination of: in adipose tissue, 154, 353,365 in biologicals, 154, 191, 215, 228, 346, 350, 356 in bovine liver, 292, 364 in brines, 191 in coal, 140 in cocoa powder, 140 in environmental, 215, 323 in geological, 215 in hair, 350, 356 in horse kidney, 140 in mussels, 364 in plants, 127, 350, 356 in plasma, 140 in rain water, 198 in sea water, 191,450, 452 in sediments, 191,333 in serum, 140, 292, 359 in slurried animal tissue, 354 in slurried botanicals, 354 in slurried sewage sludge, 146 in soil, 147 in vegetable material, 77 in vitreous humour, 363 in whole blood, 54, 140, 147 Zirconium, 191 Zone penetration, 117 Zone sampling, 117, 127

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