Advances in Atomic Spectroscopy, Volume 5

ADVANCES IN ATOMIC SPECTROSCOPY

Volume5

9 1999

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ADVANCES IN ATOMIC SPECTROSCOPY Editor: JOSEPH SNEDDON Department of Chemistry McNeese State University Lake Charles, Louisiana

VOLUME5

9 1999

JAI PRESS INC.

Stamford, Connecticut

Copyright 91999 by JAI PRESSINC 1O0 Prospect Street Stamford, Connecticut 06904-0811 All rights reserved. No part of this publication may be reproduced, stored on a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, filming, recording, or otherwise without prior permission in writing from the publisher. ISBN: 0-7623-0502-9 ISSN: 1068-5561 Manufactured in the United States of America

CONTENTS

LIST OF CONTRIBUTORS

vii

PREFACE

Joseph Sneddon

ix

SPECIATION STUDIES BY ATOMIC SPECTROSCOPY M. de la Guardia, M.L. Cervera, and

A. Morales-Rubio

NEW TYPES OF TUNABLE LASERS

Xiandeng Hou, Jack X. Zhou, Karl X. Yang, Peter Stchur, and Robert G. Michel

99

DEVELOPMENTS IN DETECTORS IN ATOMIC SPECTROSCOPY

Frank M. Pennebaker, Robert H. Williams, John A. Norris, and M. Bonner Denton

145

GLOW DISCHARGE ATOMIC SPECTROMETRY

$ergio Caroli, Oreste Senofonte, and Gianluca Modesti

173

LASER-INDUCED BREAKDOWN SPECTROMETRY

Yong-II! Lee and Joseph Sneddon

INDEX

235 289

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LIST OF CONTRIBUTORS

Sergio Caroli

Applied Toxicology Department Istituto Superiore di Sanit~ Rome, Italy

M.L. Cervera

Department of Analytical Chemistry University of Valencia Valencia, Spain

M. Bonner Denton

Department of Chemistry University of Arizona Tuscon, Arizona

M. de la Guardia

Department of Analytical Chemistry University of Valencia Valencia, Spain

Xiandeng Hou

Department of Chemistry University of Connecticut Storrs, Connecticut

Yong-lllLee

Department of Chemistry Changwon National University Changwon, Korea

Robert G. Michel

Department of Chemistry University of Connecticut Storrs, Connecticut

Gianluca Modesti

Applied Toxicology Department Istituto Superiore di Sanit~ Rome, Italy

A. Morales-Rubio

Department of Analytical Chemistry University of Valencia Valencia, Spain vii

viii

LIST OF CONTRIBUTORS

John A. Norris

Department of Chemistry University of Arizona Tuscon, Arizona

Frank M. Pennebaker

Department of Chemistry University of Arizona Tuscon, Arizona

Oreste 5enofonte

Applied Toxicology Department Istituto Superiore di Sanit~ Rome, Italy

Joseph Sneddon

Department of Chemistry McNeese State University Lake Charles, Louisiana

Peter Stchur

Department of Chemistry University of Connecticut Storrs, Connecticut

Robert H. Williams

Department of Chemistry University of Arizona Tuscon, Arizona

Kad X. Yang

Wandsworth Research Center New York State Department of Health Albany, New York

Jack X. Zhou

CVI Lasers Corporation Putnam, Connecticut

PREFACE Element speciation determines the different forms or species a chemical metal can have within a given compound. It is well known that different forms of a metal have different toxicity effects. Chapter 1 by Miguel de la Guardia and coworkers describes the use of various atomic spectroscopic methods and applications of speciation studies in atomic spectroscopy. The emphasis is on combining atomic spectroscopy with gas and liquid chromatography. While the laser has been around for close to 40 years, new lasers are becoming available which have the potential to directly impact atomic spectroscopy. In Chapter 2, Bob Michel and coworkers describe new developments in tunable lasers for use in atomic spectroscopy. Traditional methods of detection such as photography and the photomultiplier tube are being replaced by new detectors which have potential for multielement detection and more. Chapter 3 describes the developments in detectors in atomic spectroscopy from M. Bonner Denton and coworkers. Chapter 4 is on the very active area of glow discharge atomic spectrometry presented by Sergio Caroli and coworkers. After a brief introduction and historical review, the use of glow discharge lamps for atomic spectrometry and mass spectrometry is discussed. This includes a discussion on the geometry and the use of radiofrequency power. A discussion on recent applications including metals and alloys, nonconductive solid materials, and liquid and gaseous samples is presented. Finally the future of this source in atomic spectrometry is discussed.

x

PREFACE

Chapter 5 covers the use of a laser-induced or laser-ablated plasma as a spectrochemical source for atomic emission spectrometry. The technique is referred to as laser-induced breakdown spectrometry (LIBS). A brief introduction is followed by a description of the instrumentation, in particular the laser and detection device. A brief outline of the plasma physics is followed by a description of the applications of LIBS, particularly where it is advantageous over conventional atomic emission techniques. Joseph Sneddon Editor

SPECIATION STUDIES BY ATOMIC SPECTROSCOPY

M. de la Guardia, M.L. Cervera, and A. Morales-Rubio

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Definition and Several Approaches . . . . . . . . . . . . . . . . . . . . . . . Importance of Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . Speciation in Waters: The Methodology . . . . . . . . . . . . . . . . . . . . . Metal Speciation in Biological Fluids: Some Specific Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Speciation in Blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Speciation in Urine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Speciation in Milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Speciation of Miscellaneous Biological Fluids . . . . . . . . . . . . . . . VI. Speciation in Solid Samples: The Challenge . . . . . . . . . . . . . . . . . . . . A. Speciation of Soil and Sediment Samples . . . . . . . . . . . . . . . . . . B. Speciation of Solid Biological and Food Materials . . . . . . C. Speciation of Miscellaneous Solid Samples . . . . . . . . . . . . . . . . . VII. Speciation Studies of Different Metals . . . . . . . . . . . . . . . . . . . . . . A. Aluminium . . . . . . . . . . . ....................... B. Antimony . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 4 6 8

I. II. III. IV. V.

Advances in Atomic Spectroscopy Volume 5, pages 1-98. Copyright 9 1999 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0502-9

. ......

15 18 25 28 30 41 41 49 56 57 58 68

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M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO C. Arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Cadmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Calcium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E Chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. Iodine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K. Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L. Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O. Selenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P. Tin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q. Zinc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70 71 72 72 74 75 76 76 77 79 81 81 82 83 85 85

ABSTRACT The term speciation is used to describe the oxidation state or chemical form of a metal in a sample. The importance of speciation, particularly in clinical or biological and environmental samples, is described. Sample preparation and preservation is considered to be the key to accurate determination of the species and is discussed in detail. The use of various atomic spectroscopy techniques, coupled with both gas and liquid chromatography, to allow speciation studies of natural or real samples is presented. The application of these hyphenated techniques to selected metals is further presented.

I. A D E F I N I T I O N A N D SEVERAL APPROACHES The term speciation is commonly employed in geochemistry to differentiate dissolved metals from particulated solid forms. In the past it has been used to describe the different oxidation states of a metal in the same sample. Furthermore, speciation has been used to interpret the results obtained in the electrochemical analysis of complex samples as a function of their chemical forms. This is achieved through the availability of metals to be reduced on the electrodes. In the analysis of soils, the use of sequential extraction schemes based on different reagents provide an excellent way to discriminate the chemical form of metals as a function of their suitability to be extracted by plants. In clinical analysis and toxicological studies, it was clear that the availability of metals to be absorbed by humans is a function of the specific chemical form of the metal. This is used to explain the biogeochemical cycle of trace metals and dramatic accidents, such us the 1954 methyl mercury intoxication in Minamata (Japan) (Smith and Smith,

Speciation Studies

3

1975). A series of different approaches, based on toxicity, bioavailability, bioaccumulation, mobility, or biodegradability of metals are of interest to analytical chemists for speciation studies. Recently, the International Union of Pure and Applied Chemistry (IUPAC) has defined speciation as the process yielding evidence of the atomic or molecular form or configuration in which a metal can occur in a compound or a matrix. From this point of view it is clear that to perform actual speciation studies, it is absolutely necessary to quantitatively determine the amount of a chemical form of a metal in a sample (Krull, 1991). The analytical process offers tremendous possibilities to perform speciation studies, based on the use of selective detection systems, chemometrics, and the exploitation of analytical data. Additionally, it is possible to improve speciation during the sampling and sample pretreatment by introducing specific collector devices or appropriate chemistries, as indicated in Figure 1. Using the IUPAC definition, it is clear that selectivity in front of the determination of a species is the key problem. Conventional atomic spectroscopy, because of the high temperatures used in the measurement cells, offers only limited applicability for speciation. The atomic signals are based on the presence of free atoms in the fundamental or the excited states, or on the previous formation of free gaseous ions. It makes it difficult to obtain different signals as a function of the chemical form of the metal to be determined. However, it is possible to improve the performance of atomic spectroscopy, from the ability to accurately determine the total metal content to the specific determination of species at trace levels by means of hyphenation between the atomic detector and some separation processes. The main objective of this chapter is to provide an analytical perspective about the state-of-the-art of speciation by atomic spectroscopy. Furthermore, a look at the

SAMPLING

l

SAMPLE PRETREATMENT

l l

SEPARATION DETECTION

l

CHEMOMETRICS

* Specific collection

and/or preconcentration of species

* Selective leaching * Derivatization

* E x t r a c t i o n in specific conditions * V o l a t i l i z a t i o n of compounds

* Chromatographic separations

* Specific molecular m e t h o d s * Selective atomic spectrometry methods hyphenated with separation procedures

* data deconvolution

Figure 1. Possibilitiesoffered by the analytical process to do speciation studies.

4

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

future of speciation by considering the tremendous possibilities offered by flow injection analysis (FIA)-microwave-assisted sample treatments, and the combination of different approaches will be presented. It has been proposed that the hyphenation between chromatography and the most sensitive of atomic detectors is the main approach to achieve speciation studies. However, other approaches, such as the old practice of sequential extraction schemes, and some specific leaching or chemical treatments, could also be suitable to improve atomic measurements. They must be considered in order to make speciation available for complex samples and eventually control laboratories. In fact, the challenge is to provide suitable methodologies, far from the reduced perspective of pure academic studies. We will try to harmonize the rigorous application of the IUPAC definition with a touch of imagination in order to incorporate as many possible analytical tools from both classical chemical knowledge and modern instrumentation.

II.

IMPORTANCE OF SPECIATION

The main characteristics of the behavior of chemical metals, such as their solubility, mobility, availability or toxicity, are strongly dependent on their specific form. It is clear that for life science studies, it is necessary to know the concentration of each species, as well as determination of the total metal content. The interest in speciation studies has been growing from the 1950s and particularly during the last two decades. A series of conferences have specifically focused on speciation and related topics. In April in 1983 in Gtittingen (Germany), a symposium on "Forms of Binding of Chemical Elements in Environmental and Biological Materials" was organized by Schwedt (1983). In September 1984 in Berlin (Germany), a new congress organized by the Association of Sponsors of German Science was devoted to "The Importance of Chemical Speciation in Environmental Processes," thus highlighting the tremendous interest on this aspect of environmental pollution. In September 1988 in Karlsruhe (Germany), EH. Frimmel organized a symposium about "Elements and their Chemical State in the Environment." In 1989, a North Atlantic Treaty Organization (NATO) workshop organized in Izmur (Turkey) focused "International Conference on Metal Speciation in the Environment." Since 1990, BCR have organized several meetings and workshops on speciation. These include those at Arcachon (France) on "Improvements in Speciation Analysis in Environmental Materials," in 1992 at Sitges (Spain) on "Sequential Extraction of Trace Metals in Soils and Sediments" and in February 1994 in Rome (Italy) on "Trends in Speciation Analysis." In March 1993, Schwedt organized a new meeting in Clausthal on "Advances in Elemental Species Analysis: Concepts, Findings and Evaluation" which focused on the methodological problems of speciation. In June 1991 in Loen (Norway) it was organized as a post-symposium of the XXVII Colloquium Spectroscopic International (C.S.I), entitled "Speciation of Elements in Environmental and Biological Sciences." In June 1994,

Speciation Studies

5

the 2nd International Symposium on "Speciation of Elements in Toxicology and Biological Sciences" assured the continuity of international conferences on this topic. From the pioneering German interest in this field, the international scientific community has been quick to accept speciation as a major topic of current analytical chemistry. This is not only due to its extreme importance in evaluating the environmental impact and mobility of metals and their toxicological behavior, but also from the methodological point of view in order to assure the accuracy, repeatability and ruggedness of methods of speciation. A good parameter for the evaluation of interest in a research topic is the number of published books. The first publication was in 1983 of the book edited by Leppard on "Trace Element Speciation in Surface Water and its Ecological Implications" (Leppard, 1983). Since that time several books have been published: 1984 (Kramer and Duinker), 1986 (Bernhard et al.), 1987 (Lander), 1988 (Buffle; Kramer and Allen), 1989 (Batley; Harrison and Rapsomanikis), 1990 (Patterson and Passino), 1991 (Krull), and 1996 (Caroli). This regular appearance is a strong indication of the continuous attention paid to this topic by major scientific publishers. Figure 2 shows the accumulated number of published papers from 1980 until the present (late 1998). There is an exponential growth in this area, with the total number of papers published during the period of time considered (from Analytical Abstract Data Base) of more than 700. Considering the metals studied, it can be seen from Figure 3 that the major interest has focused on metals with different stable oxidation states such as As, Se, Cr, and Sb, or with highly stable specific organic forms, commonly used in industrial applications, such as Hg, Sn, and Pb.

Figure 2. Chronological evolution of the scientific literature about speciation (from Analytical Abstracts January 1980-March 1998).

6

M. DE LA GUARDIA, M.L. CERVERA,and A. MORALES-RUBIO As

Sb 2%

Se 8%

Pb

..........

Miscellaneous 19%

Cr 11%

~~N Sn 14%

g 12%

Figure 3. Distribution of the literature on speciation as a function of the elements determined.

Throughout this chapter the reader will find a guide to follow the scientific literature about speciation studies. It includes a final section providing a summary of speciation of individual metals.

III.

FACTORS AFFECTING SPECIATION

All analytical steps preceding speciation are extremely critical to assure the stability of a chemical species present in natural samples. Sampling, sample preservation, storing, and sample treatment must be carefully controlled in order not to disturb the equilibria established among the species. Therefore, practical studies on speciation must involve several protocols to ensure the accuracy and representivity of laboratory data. Additionally, typical problems which can occur during the determination of total concentration of trace metals, such as the contamination of samples from the material employed for sampling by reagents employed for sample preservation or losses during storing and sample pretreatment, must be taken into account. The instability of redox species changes in a sample erwironment, may also be significant. Due to theses concerns, it is necessary that changes involved by sample handling do not modify or change the ratio between species. A good example of this potential problem can be found during the speciation of Fe in deep lakes. The absence of 0 2 and the presence of high quantities of dissolved CO 2 involve a sample environment totally different than that found outside; Fe (II) being frequently oxidized to Fe (III) during the sampling step. Redox conditions, macroconstituents, ionic strength, pH, temperature, and pressure are some of the factors affecting speciation and species preservation. Additionally, synergistic and antagonistic effects between trace compounds present in the sample must also be taken into consideration. When biological fluids are to be analyzed, the conditions of a

Speciation Studies

7

minimum and soft pretreatment procedure must be applied to attain quick and accurate analysis, paying particular attention to the type of compound used for calibration (Dawson, 1986). During the sampling step, a number of potential problems should be considered during the collection of biological fluids. The major problem in such work is due to the contamination and losses of the trace metal. The effect of contaminants may alter the amount of analyte bound to a given fraction. For example, zinc and copper ions added in-vitro to a serum sample result in an increase in the albumin-bound fraction (Cornelis et al., 1993). It must also be considered that some factors, such as person-to-person differences, region-to-region variations, occupational exposure, and physiological state of the subject, would influence the chemical speciation of a given metal in body fluids (Negretti de Br~itter et al., 1995). Sampling of urine in studies devoted to metal speciation is commonly performed on urine collected during a 24-h period. One reason for this is that many constituents exhibit diurnal variation, with variable peak excretions as a result of variation in drinking patterns. At the beginning of the collection period (usually when the person awakens), the bladder should be emptied, the specimen discarded, and the time noted. All urine specimens passed during the next 24-h period are collected in a pre-cleaned polyethylene or polypropylene container. At the end of the collection period the bladder is emptied and the specimen is added to those already collected. Transfer of the urine from the body into the container may introduce contamination from clothes and skin (Robberecht and Deelstra, 1984). The timing of body fluids taken from a subject can have a significant influence on the concentration of total metal and species. Thus, the diurinal variation of zinc concentration in human blood plasma, and the history of the tissue before sampling, e.g. the use of cosmetic agents (shampoo or conditioner) in the treatment of hair, affects the result (Dawson, 1986). It has been shown that the concentration of protein-bound serum zinc in human blood plasma varies depending on whether the sample was taken from a patient who was standing or lying in a recumbent position (Behne, 1981). Based on the previous comments, it is absolutely necessary in the determination of speciation of metals in biological fluids to include a detailed description of both, the samples analyzed and the procedures for sampling and storage, in order to make possible a critical evaluation of the published literature. The key to successful speciation work is the preservation of the species. Often this is impossible and the integrity of an organometallic compound is violated (Woittiez et al., 1991). Nevertheless, collected samples are usually frozen at -20 ~ until required. Next, the frozen substance is allowed to thaw to room temperature. At this stage, pretreatment needs the addition of preservatives, stabilizers, and other additives. If these substances are complexing agents or their presence could change the pH, ionic strength, redox potential, and dielectric constant of the medium, it could result in some changes in the distribution of chemical species. For example, methylmercury may be lost from blood on freeze-drying (Horvat and Byrne, 1992).

8

M. DE LA GUARDIA, M.L. CERVERA,and A. MORALES-RUBIO

Lyophilization is a common process applied to natural or real samples. It converts the material into a form that can be easily stored for longer periods. It is necessary to ensure that the analytes (metals) are not lost and the material characteristics as well as the stability of the analyte species are maintained. In fact, it has been reported that lyophilization could cause the destruction of lipoproteins (Kroll et al., 1989) and also result in denaturation and aggregation of other proteins. Evidently, this will change the speciation of metals like selenium which is associated with lipoproteins (Gardiner, 1993). A problem, studied in depth, is the effect of storage conditions of sediments for species of Cd, Cu, Fe, Mn, Ni, Pb, and Zn. Typical methods of sample preservation available are (1) wet at room temperature, (2) wet at low temperature, (3) frozen, (4) freeze-drying, (5) oven-drying, (6) microwaveassisted drying, and (7) air-drying at room temperature. It was found that wet preservation methods produced a reducing soil or sediment environment, whereas drying procedures provided an oxidizing environment with important consequences for speciation. Drying at 90 ~ promotes the crystallization of amorphous oxides and the formation of new solid mineral phases. It appears that freeze-drying and microwave-assisted drying could be more appropriate for geochemical material preservation. During a study on preparation of reference materials of clays and sediments, it was concluded that microwave-assisted drying provided excellent precision but did not produce identical results than those found after conventional drying. The composition changes and sample instability could be produced during the microwave treatment (Beary, 1988). However, in spite of changes introduced by drying, due to the inevitable oxidation of samples, which can be dramatic in the case of oxidizing sediments, it is clear that drying inhibits further changes in speciation mediated by microbiological action. It was concluded, in a rigorous study, that air-drying preservation of soils and sediments facilitates sample handling, homogenization, and preservative subsampling without affecting chemical species (Ure, 1994). In order to improve the methodologies for species separation and determination, efforts must be made in order to assure correct protocols, including all the steps of the analytical process. This will lead to accurate results in speciation studies. Appropriate methodologies for sampling, which avoid species transformation, and a complete guide for sample preservation, homogenization, and subsampling must be developed, and accompanied by a careful check on their effect on species stability. In the following sections problems regarding speciation in complex liquid samples, such biological fluids, and particularly in solid samples, for which a drastic sample pretreatment is often required, will be presented. These areas will be discussed in practical applications of speciation studies.

IV.

SPECIATION IN WATERS: THE M E T H O D O L O G Y

All atomic spectrometric methods work for the direct analysis of liquid or dissolved samples. All the main atomic spectrometry detectors have been employed in

Speciation Studies

9

speciation studies. From the techniques indicated in Figure 4, cold vapor atomic absorption spectrometry (CVAAS) is preferred for Hg determination and microwave-induced plasma-atomic emission spectrometry (MIP-AES) can be applied to easy volatile compounds or easily derivatizable metals, from which a gaseous phase, free from the solvent, can be obtained. On the other hand, the lack of an appropriate commercially available atomic fluorescence spectrometry (AFS) instrumentation (this has been changed recently) requires a strong reduction of the metals to determine Hg, As, Se, Sb and Te (Corns et al., 1993). In fact, the evolution of the literature about speciation in atomic spectrometry clearly shows that, in the case of atomic emission measurements, the recent development of hot atomization systems such as plasma, has completely replaced flame emission procedures. The inductively coupled plasma (ICP) is the most commonly and widely employed plasma source. Regarding the use of atomic absorption techniques, flame atomic absorption spectrometry (FAAS) continues to be employed to a larger extent than electrothermal atomization atomic absorption spectrometry (ETAAS). This is due to the easily coupling between FAAS and dynamic separation systems. This is despite the reduced sensitivity of FAAS as compared with ETAAS. An important aspect to be considered in order to improve the sensitivity of speciation studies by FAAS is the possibility to generate on-line volatile derivatives, like covalent hydrides. This avoids problems related to the reduced nebulization efficiency of classical continuous introduction systems. This dramatically increases the sensitivity and will add

/EMISSION

[FLAME PHOTOMETRY(FP) I PLASMA EMISSION | MICROWAVEINDUCED PLASMA (MIP) | INDUCTIVELY COUPLED PLASMA (ICP) I DIRECT COUPLEDPLASMA (DCP)

/FLUORESCENCE I ATOMIC FLUORESCENCE (AFS)

I/ION COUNTING

i ICP.MASSSPECTROMETRY ~

lioBsORPTiON

LD VAPOURATOMICABSORPTION(OVAAS) AME ATOMICABSORPTIOM(FAAS) LECTROTHERMALATOMICABSORPTION(ETAAS)

Figure 4.

Atomic spectrometrymethods currently employed in speciation studies.

10

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

new variables. These can be suitable to improve the selective determination of different species, as a function of their ability to form hydrides and the kinetics of this process. However, as indicated previously, the low residence time of sample particles in the measurement zone and the high temperature of the atomization cells can cause problems in discriminating the chemical forms of a metal. As a consequence, the single use of atomic spectrometry is not convenient for "in situ" analysis of chemical species with only a few exceptions (Arpadjan and Krivan, 1986; de la Guardia, 1996). This has led to the general strategy for speciation by atomic spectrometry being based on its hyphenation with separation techniques. Figure 5 summarizes the techniques most often employed to preconcentrate selectively or to isolate a specific chemical form of a metal to be determined by atomic spectrometry. From these techniques, a general consensus has been reached that gas chromatography (GC) (Schwedt and Russel,1973; Fernandez, 1977; Van Loon, 1979; Segar, 1984; Ebdon et al., 1986; Chan and Wong, 1989) and liquid chromatography (LC) (Van Loon, 1981; Fuwa et al., 1982; Chau, 1986; Irgolic, 1987; Ebdon et al., 1987a and 1988) are the best alternatives to provide accurate determination of the different species of an metal. Recent studies have focused on the development of appropriate interfaces between chromatography and flame spectrometers (Aue and Hill, 1973; Kawaguchi et al., 1973; Jones and Managhan, 1976; Parris et al., 1977; Ebdon et al., 1988), electrothermal atomizers (Brickman et al., 1977; Ebdon et al., 1982 and 1987b) or plasmas (Beenakker, 1977; Windsor and Denton, 1979; Gast et al., 1979; De Jonghe et al., 1980; Krull and Jordan, 1980; Hansler and Taylor, 1981; Duebelbeis et al., 1986; Bushee, 1988; Bushee et al., 1989; Crews et al., 1989; Heitkemper et al., 1989). Gas chromatographic (GC) separation requires that the species to be determined are volatile and thermally stable under the conditions employed for separation.

Figure 5. Separation methods commonly employed in speciation studies classified as a function of the phases involved (reproduced from de la Guardia, 1996).

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11

Speciation through GC-atomic spectrometry is limited to the analysis of volatile organic compounds such as lead or mercury alkylderivatives, which commonly occur in natural samples. Additionally, volatile derivatives can be prepared before the chromatographic separation, as was the case in the arsenic speciation through methyl thioglycolate formation (Haraguchi and Takatsu, 1987; Ebdon et al., 1988). Compared with GC, procedures based on liquid chromatography (LC) separation followed by atomic spectrometry detection are more simple and suitable to the direct speciation of many natural occurring species. In general, LC-AS speciation studies involve separations on C18 bonded silica columns, or the use of ionexchange resins. Porous gels based on the use of size exclusion mechanisms (Van Loon and Barefoot, 1992) are suitable to be applied to differentiate between inorganic and organic species, cationic and anionic species, and also species of the same type but with different molecular sizes. However, the simplest applicability of LC for speciation studies in atomic spectrometry is compounded by difficulties found in the development of appropriate interfaces between the LC and the atomic detector. The flow rate of the carrier gas or liquid phase through the chromatographic column must be adjusted to the gas or liquid uptake of the detector. A simple heated stainless steel tube, with minimal dimensions, can be employed in the case of GC. For LC separations, an auxiliary supply of the mobile phase is commonly required to supplement the column effluent. New nebulizers (Gustavsson and Nygren, 1987), jet separator interfaces (Gustavsson, 1987) and thermospray interfaces (Robinson and Choi, 1987) have been proposed for this reason. The time of species separation must be reduced to a minimum in order to save both time and gas consumption, particularly when inductively coupled plasma-atomic emission spectrometry (ICPAES) or inductively coupled plasma-mass spectrometry (ICP-MS) determination is performed. In the literature there are examples on fast chromatographic separation and atomic spectrometry detection of several species of a metal. Figure 6 depicts a gas chromatogram of seven organotin species separated after ethylation with NaBET 4 (Martin-Lecuyer and Donard, 1996). It can be seen in Figure 7 by an appropriate selection of both, the chromatographic column and the mobile phase, it is possible to clearly separate different mercury species, by liquid chromatography. This allows the quantitative analysis of traces of these species by using an extremely sensitive detector such as ICP-MS (Pastor, 1998). The hyphenation between chromatography and atomic spectrometry provides suitable tools for speciation in liquid and dissolved samples. However, additional efforts must be achieved in order to improve the multimetal capabilities of some atomic detectors, like ICP-AES, or ICP-MS in order to develop methodologies suitable for multimetal speciation studies in natural samples. There are some precedents on the use of typical single metal techniques, such as ETAAS, for multi-speciation analysis. These are based on the use of a gold trap for retention of Se and Te species and sequential leaching with H20, 1 M HC1 and 3 M HNO 3,

12

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO (MV) 20,0 19,5

-

19,0 18,5! 18,0 17,5~ 17,0 16,5

-

3

16,0 15,5 15,0 14,5 o

i

I

10

5

I

15

T i m e (min)

Figure 6. Gas chromatogram of seven organotin species separated by GC after ethylation with NABEt4 and detected by flame photometry (reproduced from MartinLecuyer and Donard, 1996). (1) BuSn 3+ , (2) Bu2Sn2 + , (3) PhSn3+ , (4) Bu3Sn+ , (5) Bu4Sn, (6) Ph2Sn2+, (7)Ph3Sn+, and (e) Internal Standard (Pr3Sn+). combined with ion chromatography (Muangnoicharoen et al., 1988) or based on extraction in CHC13 and CC14of As(III), Sb(III), Se(IV) and Te(IV) with 4% APDC followed by a separate treatment of another sample aliquot after an appropriate reduction of oxidized forms of these four metals (Chung et al., 1984a, b). However, multi-speciation studies has been achieved using ICP-AES (McCarthy et al., 1983; Gjerde et al., 1993; Sanz-Medel et al., 1994), MIP-AES (Sadiki and Williams, 1996), and ICP-MS (Haraldsson et al., 1993; Jantzen and Prange, 1995; Kumar et al., 1995; De Smaele et al., 1996; Krupp et al., 1996; Guerin et al., 1997). Gas chromatography has been employed in the multimetal speciation of organotin and organolead compounds. It is based on their extraction with pentane. This is followed by derivatization with pentylmagnesium bromide and butylmagnesiun

13

Speciation Studies 60000

HgCI2 2.56

50000 Ions/s 40000 EtHg 4.36

30000 -

20000 -

1 MeHg I~ ~3.15 JI

10000

PhHg

0 0

200

400

600

800

Time (s)

Figure 7. Liquid chromatogram of four mercury species separated by HPLC and direct ICPMS detection of 25 ng of each specie (from Pastor, 1998).

chloride, and MIP-AES (Sadiki and Williams, 1996). During the direct separation of organotin, organolead and organomercury species in environmental samples (De Smaele et al., 1996) and sediment samples (Jantzen and Prange, 1995), ICP-MS was used in the determination. Methylated and alkylated species of As, Sn, Sb and methylated derivatives of Bi, Hg, Ge and Te can be also separated and determined by GC-ICP-MS (Krupp et al., 1996). Supercritical fluid chromatography (SFC) with ICP-MS detection has been proposed for the separation of trimethylarsine, triphenylarsine, triphenyl arsenic oxide, triphenylantimony, and diphenylmercury (Kumar et al., 1995). Liquid chromatography coupled to ICP-AES (McCarthy et al., 1983; Gjerde et al., 1993; Sanz-Medel et al., 1994) and coupled to ICP-MS (Haraldsson et al., 1993; Guerin et al., 1997) provides an excellent way for multimetal speciation. Direct injection nebulization in ion chromatography-ICPAES provides a good way for As, Se and Cr multispeciation (Gjerde et al., 1993). Using a nucleosil dimethylamine column and a mobile phase of ammonium

14

M. DE LA GUARDIA, M.L. CERVERA,and A. MORALES-RUBIO

phosphate buffer with gradient elution from pH 4.6 to pH 6.9, it has been possible to separate AsO]-, SeO~-, AsO 3-, and SeO 2-. Absolute detection limits of 52, 140, 57, and 91 ng, respectively, were obtained in these studies (McCarthy et al., 1983). The use of a C18 bonded silica column, modified with didodecylodimethylammonium bromide in 50% methanol, has been proposed to achieve the separation of As, Se, Hg, and Sn species using surfactant vesicles mobile phase and ICP-AES determination (Sanz-Medel et al., 1994). The technique of ICP-MS provides a highly selective and sensitive detector for multimetal speciation. It has been proposed as a method for A1, Cd, Co, Cu, Pb, Mn, Mo, Ni, Zn, and Fe speciation in waters using Chelex 100, Fractogel TSK DEAE-650 and C18 to fractionate the free metal ions or easily dissociate complexes, humic complexes on nonpolar organic compounds (Haraldsson et al., 1993). Monomethylarsonic acid, dimethylarsinic acid, selenite, selenate, tellurate and antimonate can be separated using a PRP • 100 anion exchange columns and determined accurately by ICP-MS (Guerin et al., 1997). Figure 8 shows an example of the chromatograms which can be obtained for simultaneous speciation of arsenic and selenium compounds using a microcolumn of anion exchange for the quantitative separation of monomethylarsonic, As (III) and As (V) and that of Se (IV) and Se (VI) using ICP-AES as a detector. The excellent sensitivity of AFS and ICP-MS detection and, in the latter case, its exciting possibilities for multimetal and isotopic analysis offer excellent possibilities for a rigorous speciation of liquid samples. An additional effort is required in order to improve both analyte separations and detection, in order to be I

000-

S e +4

800 t--

600

MMA :~

Se+6

-

400<

200 0

0

I

50

I

100

I

150

200

Time (s) Figure 8. Simultaneous speciation of As and Se species by using anion exchange chromatography directly coupled to ICP-AESthrough a direct injection nebulization system (reproduced from CETAC, 1993).

Speciation Studies

15

able to perform direct multimetal speciation in natural samples. For that reason it will be necessary to develop additional strategies for on-line precolumn and postcolumn derivatization and a careful validation of the academic methodology by the analysis of complex samples with the use of reference and spiked samples.

V. METAL SPECIATION IN BIOLOGICAL FLUIDS: SOME SPECIFIC PROBLEMS Speciation of trace metals in biological fluids and tissues has been approached in many different ways during the last decades, but it is extremely difficult because of the complexity of biological systems. Metal speciation in biological fluids implies investigation of the bond between trace metals and available ligands, mostly proteins or compounds with relatively low molecular mass, as a basis of kinetic and metabolic studies (Cornelis and De Kimpe, 1994) and must take into consideration the complexity of clinical matrices. The most common way for speciation of metal ions in biological fluids is the identification and quantification of the biologically active compounds to which the metal is bound and the quantification of the metal in relation to those particular molecules. Initially, many difficulties were encountered while determining the total content of trace metals in a biological fluid or tissue sample, such as the elimination of matrix interferences, the development of effective and fast sample decomposition methods, or the control of sample contamination. These problems are now understood and under control. However, a new problem is to define the various biocompartments to which the trace metals are linked. As a matter of fact, speciation of trace metals in biological fluids consists of defining the various biocompartments to which these metals are linked and to explain their mobility, storage, retention, and toxicity. The toxicity of chemical species of different metals is a function of the target metal and the chemical structure of the compounds considered and depends on the absorption path of the metals. Most of the species of interest in the toxicology of trace metals are small molecules. Many metals are capable of forming organometallic compounds and their toxic effects, in some cases, exceed by far those of the inorganic forms of the metals or the compounds formed with large molecules. In general, in the investigation of the toxic effects, the speciation of small molecules is of concern, whereas in the investigation of biological functions, the determination of large molecules has priority (Das et al., 1995). In the case of speciation studies it is very important to maintain the integrity of the metal-ligand binding and to check the mass balance of the protein and the trace metal throughout the isolation steps (Cornelis et al., 1993). Again, in view of the recent problems in environmental and clinical fields all over the world, fast and reliable analytical techniques for chemical speciation in biological fluids are needed urgently (Dunemann, 1992).

16

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Chemical speciation of trace metals in biological matrices has been discussed in some review articles. Metal ion speciation in biological fluids (Das et al., 1996) and in solid matrices including foodstuff and biological materials (Das et al., 1995) have been discussed recently by our group. Gardiner (1988) discussed the role of atomic spectrometry techniques in chemical speciation in biology and medicine. They also described considerations in the preparation of biological and environmental reference materials for use in the study of the chemical speciation of trace metals (Gardiner, 1993). Behne (1992) depicted the trends and problems of speciation of trace metals in biological materials. Van Loon and Barefoot (1992) reviewed a selection of major developments in the field of analytical methodology for metal speciation in various matrices including biological samples. Ebdon et al. (1986) has extensively reviewed the coupling between GC and atomic spectrometry and of LC and atomic spectrometry. An excellent review is available on flow injection and speciation (Luque de Castro, 1986). Although there are some review articles on speciation of a particular metal, like selenium in human urine (Robberecht and Deelstra, 1984), and arsenic in biological fluids (Violante et al., 1989), the literature on speciation of metal ions in biological fluids is rather limited. In Figure 9 the number of published papers as a function of the year of their publication is shown. From this data it is evident that the scientific literature

Figure 9. Development of the literature published about metal speciation in biological fluids.

Speciation Studies

17

concerning this topic has been increasing in recent years. The papers published could be classified into four groups depending on the type of biological fluid: blood, urine, milk, and miscellaneous biological fluids. In Figure 10 the percentage of analytical papers devoted to different body fluids is shown. The use of atomic spectrometric methods in clinical samples is well established. Alcock (1993) has published a review on this aspect. It is clearly evident from Figure 11 that the feasibility of various atomic spectrometric techniques for the determination of chemical species in biological fluid samples are in the order: ETAAS > cold vapor atomic spectrometry (CVAAS) > hydride generation atomic absorption spectrometry (HGAAS) > FAAS > ICP-AES = ICP MS > direct current plasma-atomic emission spectrometry (DCP-AES) = MIP-AES. All these techniques are useful for the analysis of liquid samples (except MIP-AES which is more convenient for gaseous samples). In recent years, hyphenated methods like those involving chromatographic methods linked to atomic ones have emerged. All of the hyphenated methods have shown high promise for specific metal species in specific sample matrices, with different degrees of selectivity, specificity and sensitivity/detection limit. All these techniques have provided the analyst a choice of methods for trace metal species determination. Actually, there is no simple way to decide which specific hyphenated or direct method of analysis is the best for a particular metal species in a particular sample matrix. Perhaps that remains as a lacuna in the whole scheme of trace metal speciation. Urine

Milk 11%

Blood 36 %

~%~176 "/~o / -

~'*~.,@'~q'r162 e9

~ ~ LBr~176176

1%

..g

d Figure 10. Distribution of published papers about speciation in biological fluids as a function of the type of sample considered.

18

M. DE LA GUARDIA, M.L. CERVERA,and A. MORALES-RUBIO ETAAS 22.4%

ICPMS 1

8

~

.

1

~

~

i

.

~

x

~

DCPAES 1.7%

~:I~HGAFS

2.6%

i![,~,PAES

3.4*/.

FAAS 9.5% ICPAES 16.4% HGAAS 13.8%

2.1%

Figure 11. Atomic spectrometric methods applied to speciation of biological fluids.

In several cases, atomic spectrometric measurements can be employed for speciation purposes without requiring the use of chromatographic techniques (de la Guardia, 1996). In these cases, the procedures could be based on (1) the different atomization yields obtained for different chemicals, (2) the use of selective extraction, (3) derivatization procedures performed previously to the measurement step, (4) a selective volatilization of the different chemical forms of the elements to be determined, or, (5) other less commonly used separation methodsme.g, ultrafiltration, coprecipitation, electrodeposition, or electrophoresis. The development of automated procedures of analysis based on flow injection analysis (FIA) techniques offers new possibilities for the on-line treatment of samples. It can be expected to provide the availability of simple and low-cost procedures for the metal speciation based on on-line separation and atomic spectrometric determination (Luque de Castro, 1986; de la Guardia, 1996). Thus, by introducing FIA, a 5.5- to 60-fold increase in the sensitivity is obtained for FAAS and a 15- to 50-fold increase for ETAAS. Examples for other speciation methods proposed for the analysis of various trace metal species are based on high-performance liquid chromatography (HPLC) separation followed by determination by differential pulse anodic stripping voltammetry (DPASV) (Michalke and Schramel, 1990) or radiotracer labeling (Cornelis, 1992). To date, there is no generalized method for speciation of protein-bound trace metals (Cornelis et al., 1993). To provide an idea of such a method, an outline of procedures applied to trace metal speciation in blood is presented in Figure 12.

A. Speciation in Blood Blood is the most important indicator for humans and domestic animals as an illness diagnostic. For clinical purposes, serum is commonly used but in some cases

19

Speciation Studies

,I, I Se arat,on o. t.e prote,ns!

~On-l inemonitoring-1 |UV and~or

~ o., 0erm.a,,on-, .n,on-. ca,,on-, a,,in,,,-ch,oma,o0r,0hy..,o~ i

iii i

~1

9

!

$

ii

i M,no;,raot,on I

Major fraction for trace element analysis

~Atomicspectrometry I

,,

L Visible spectrometry

1

i

I Protein identification ~ Nephelometry 1

i

1

I Protein quantification i1

~. Electrophoresi~s

Figure 12. Scheme for trace element speciation in blood (reproduced from Das et al., 1996).

whole blood samples may be analyzed (Pais, 1994). Blood contains thousands of different compounds, although with little variability in total ionic strength. A series of procedures have been proposed in the bibliography for trace metal speciation involving aluminum, arsenic, chromium, copper, iron, mercury, lead, platinum, selenium, silicon and zinc. Most studies have been made with aluminum and mercury followed by selenium and chromium and then the other metals. There are only a few studies focused on simultaneous speciation of several metals (Br~itter et al., 1988a).

Aluminum Aluminum toxicity is now well recognized and gaining more and more interest. The only aluminum oxidation state in biological samples is (+III). Metal speciation is a crucial feature in directing the biological effects of aluminum. In blood plasma, citrate is the main small carrier and transferrin the main protein carrier for AI(III or +3). In fluids where the concentrations of these two ligands are low, nucleoside mono- and diphosphates become aluminum binders and when they are absent, then catecholamines are the major ligands. Double-stranded deoxynucleic acid (DNA) binds AI(III) weakly, and in general, is unable to compete with other ligands for its complexation. In the cell nucleus, AI(III) is probably bound to nucleotides or to phosphorylated proteins (Martin, 1992). Nevertheless, organically complexed forms of aluminum appear to be much less toxic than inorganic forms. An aluminum

20

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

species of particular concern are A13§ AI(OH) 2§ AI(O H)~, and AI(O H)4 (Gardiner et al., 1984). Aluminum species present in human blood serum have been separated by gel permeation chromatography (GPC) and by ultrafiltration (Blanco Gonzalez et al., 1989). The former procedure was applied for dialysis patients to fractionate aluminum-bound species. Aluminium determination was performed by ETAAS. Several inferences have been drawn by these authors including: (1) albumin and probably transferrin are the major proteins that bind aluminium; (2) added aluminium is taken up by serum constituents only after incubation and a slow exchange of aluminium between the species occurs in serum; (3) the low molecular mass fraction is made up of mainly inorganic aluminium complexes; and, (4) the pH of the serum determines the level of ultrafiltrable aluminium. In experiments with patients undergoing desferrioxamine (DFO), chelation therapy showed that the ultrafiltrable aluminium in their serum increased by up to 74% because of the formation of a relatively low molecular mass chelate with the DFO. HPLC separation of serum proteins was performed with an ion-exchange column ofTSK DEAE-3SW using a sodium acetate gradient (0-0.5 M) at pH 7.4 (Tris-HC1 buffer). Proteins were detected spectrophotometrically at 280 nm and the aluminium determined by ETAAS in 0.5 mL fractions collected from the HPLC column. Results obtained with this system suggests that transferrin is the only aluminium binding protein in normal serum, but in the presence of DFO in serum, most of the aluminium was found to be bound to this drug. Further work in this area using ultramicrofiltration and HPLC techniques by Sanz-Medel and Fairman (1992) has revealed that more than 90% of the aluminium in serum is bound to the protein transferrin. To have a clear idea about the presence of A1 species in blood the following studies at clinically relevant concentrations must be performed: (1) identification of the aluminium binding serum proteins; (2) quantification of the amount of these protein-bound aluminium as well as non-protein bound fraction; (3) study of the effect of the Fe-transferrin saturation on the transferrin binding of aluminium which is important in view of the large number of iron-depleted patients; and, (4) investigation of the effect of DFO on the binding of A1 and Fe to transferrin (Van Landeghem et al., 1994).

Mercury There has been a continuous work on mercury species present in biological fluids. For the determination of mercury one of the most selective and sensitive methods is the use of CVAAS based on the low boiling point of Hg ~ and the easy reduction of mercurials to the zero-oxidation state (Magos, 1971). From a number of published papers on mercury speciation in blood, it can be concluded that the reduction of mercurials has been selectively carried out by using different reduction steps with SnC12 and with a mixture of SnC12 + CdC12. Recently, most of the speciation methods for mercury are based on chromatographic separations with detection by means of atomic spectrometry. A procedure

Speciation Studies

21

for determination of organic and inorganic mercury in various biological materials including blood by ETAAS has been reported (Filippelli, 1987). Organic mercury was extracted as the chloride in benzene and reextracted by a thiosulphate solution. The organic mercury thiosulfate extract was next treated with cupric chloride, reextracted in the benzene layer, and analyzed by GLC for speciation. Inorganic mercury was converted into a methyl chloride derivative by methanolic tetramethyltin prior to extraction. Speciation of mercury in human whole blood by capillary gas chromatography with a MIP-AES system following complexometric extraction and derivatization has been described by Bulska et al. (1992). In this method, methyl- and inorganic mercury were extracted in toluene from whole blood samples as their diethyldithiocarbamate (DDTC) complexes. The product was butylated and the mercury species were then separated and detected.

Selenium The accurate speciation of selenium has been a major challenge for analytical chemists and knowledge of its pathways in the environment and living organisms is still limited. The metal can either be considered as essential; 10 to 40 lxg mL -1 in serum and 0.1 I.tg mL -1 in urine or can be toxic when it is in excess. Br~itter et al. (1988a) have developed a procedure for establishing profiles of selenium protein in various body fluids via the on-line coupling of gel permeation chromatography and ICP-AES after performing the acid digestion treatment and the formation of Se(IV) with the subsequent hydride generation in the connecting flow. Recovery studies have been performed to examine and optimize the wet ashing conditions with the aid of various selenium compounds. To demonstrate the usefulness of this technique for the speciation of selenium, its distribution in samples of human origin has been measured. In serum, selenium was found distributed among three different fractions. The location of these peaks seemed to be similar to those of zinc. The highest selenium peak at 90 + 15 kDa is in the same region but shows a broader shape when compared to zinc. This may be due to the presence of selenium containing enzyme glutathione peroxidase (molecular mass 88 kDa) which elutes in the same region. The selenium content bound to different fractions were as follows: 90 kDa fraction 77 + 4%, 200 kDa fraction 10 + 3% and the high molecular mass (>600 kDa) fraction 13 + 5%.

Chromium Speciation of chromium in blood has become an important area of research after it was known that patients with terminal renal failure treated with hemodialysis or continuous ambulatory peritoneal dialysis become iatrogenically loaded with chromium. The fact was revealed through observation of very high chromium concentrations (4.25 ng mL -1) in their serum in comparison to normal healthy persons (0.16 ng mL -1) (Wallaeys et al., 1986). Urasa and Nam (1989) developed a method for chromium speciation using both anion- and cation-separator columns. The

22

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

procedure used DCP-AES detection and required a preconcentration step to achieve a detection limit of 1 ng mL -1. The method was applied to human serum and other samples. The authors found 0.05 gg mL -1 of Cr(VI) and 0.06 l-tg mL -1 of Cr(III) in a standard reference material of human serum in freeze-dried form obtained from the National Institute of Standards and Technology (SRM 909); although the reducing capacity of saliva and stomach involves that any intake of Cr(VI) can be readily reduced to Cr(III) (De Flora and Wetterhahn, 1989). Chromium speciation in plasma was reported by Cornelis and her group. Chromium is known to be mainly bound to two plasma proteinsmtransferrin (molecular mass = 80 kDa) and albumin (molecular mass = 64.5 kDa). A suitable separation procedure was developed for these proteins with in vitro 51Cr(III) labeled plasma from healthy persons as well as from dialysis patients (Cornelis et al., 1992). Speciation studies were also undertaken with the aid of in vitro and in vivo 51Cr-labeled rat and rabbit plasma. It consisted of a combination of fast protein liquid chromatography with cation- and anion-exchange system, ensuring a complete resolution of both proteins and a total recovery of chromium. The metabolized Cr was measured by using NaI (T1) detector of the 51Cr label. On the other hand, identification and quantification of proteins were done by isoelectrofocusing and nephelometry, respectively (Cornelis and De Kimpe, 1994). The 51Cr has been found to distributed as follows: 85% is associated with the transferrin, 8% seems to be bound to albumin and 6% appears to be spread over the other components (Cornelis et al., 1992). Since transferrin is usually saturated at 30% only with iron, this protein should indeed be considered as a potential binding site for chromium (Wallaeys et al., 1987). Recent studies demonstrated that 51Cr can shift from albumin to an unidentified low molar mass complex in ambulatory peritoneal dialysis patients (Borguet et al., 1995).

Arsenic Arsenic occurs in both inorganic and organic forms which exhibit large differences in their metabolism and toxicity. Elimination kinetics has shown that arsenic is removed very quickly from blood to urine with a half-life in the body of about 30 h (Chana and Smith, 1987). The determination of inorganic arsenic and organoarsenicals in biological fluids was reviewed by Violante et al. (1989). This chapter emphasizes the necessity for distinguishing between As of nutritional origin and that from water or the environment and for guarding against possible interconversion of the inorganic oxidation states during sample treatment. Speciation studies for arsenic in blood are comparatively less numerous than in urine. Speciation of As (III) and As (V) in biological materials including blood was studied by HNO3-H2SO 4 digestion followed by hydride generation AAS technique. It was found that the extent of change of the original valency of As was not reproducible (Weigert and Sappl, 1983). Fast protein liquid chromatography cation and anion exchange separation scheme (Cornelis et al., 1993) were applied to serum incubated

Speciation Studies

23

in-vitro with carrier-free 74ASO43 for 24-hr. All 74As was completely eluted from the cation exchanger together with the negatively charged unbound proteins. The ultraviolet (UV) responses of the separated species in combination with the metalspecific responses can be used for correlating the arsenic species with the bulk amount of potential arsenic-binding partners in serum. The protein fractions were identified as asialotransferrin, sialotransferrin and albumin carrying, respectively, 17.7, 25.3, and 56.3 of total 74As radioactivity, i.e. arsenic is bound to these proteins in these proportions. However, when the albumin fraction was subjected to gel permeation chromatography on a Superose column for differentiating the molecules according to molecular mass instead of charge, the elution patterns of the albumin and the 74As did not coincide anymore.

Copper and Zinc Recently, several reports have appeared in the literature on the speciation of zinc. Faure et al. (1990) separated the human serum fractions by ultrafiltration with the use polyacrylonitrile membranes. Loosely bound zinc (bound to albumin and some other proteins) was separated with ultrafiltrable zinc after treatment of the serum with ethylene diamine tetraacetic acid (EDTA). The difference between the loosely bound and total zinc gave the content of strongly bound zinc, i.e. to t~2-macroglobulin. The zinc content in each fraction was measured by conventional ETAAS. Zinc in serum is bound to macroglobulin (720 kDa) and albumin (66.5 kDa) whereas Cu is bound to ceruloplasmin (160 kDa) and albumin (66.5 kDa) (Gardiner et al., 1981). Sch/3ppenthau and Dunemann (1994) have reported the separation of serum for characterization of metals (including copper and zinc) and nonmetal species by size-exclusion chromatography (SEC). The coupling of HPLC to ICP-AES was performed by connecting the column outlet of the chromatographic system with the nebulizer of the metal-specific detection systems of ICP-AES or ICP-MS. The metal distribution patterns in serum samples indicate a Cu maximum at 68 kDa which again correlates with the first major sulfur maximum at 75 kDa. Thus Cu may be bound to the albumin fraction. The Zn maximum has been recorded at the 49 kDa region. The absence of the high molecular mass proteins in the investigated samples was explained by the method of sample preparation (i.e. centrifugation and filtration through a 0.2 ~m membrane).

Iron In a pioneering study, for the determination of iron species in serum samples, an HPLC-ETAAS hyphenated method with an on-line metal scavenger for studying protein binding has been reported (Van Landeghem et al., 1994). Due to the currently introduced procedures for the treatment of anemia in dialysis patients involving a relative iron deficiency in these subjects, the study on the competition between A1 and Fe for binding to transferrin presents a renewed interest. As shown in this study, over 80% of the total serum A1 and over 97% of the total serum Fe is

24

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

bound to transferrin, indicating both elements do not dissociate from transferrin during gradient elution. The postelution Fe recovery was found to be 97.3 + 2.0%.

Lead The speciation of lead has attracted great deal of interest particularly with respect to organolead compounds. A high-resolution GC-ETAAS method for the determination of trimethyllead salts in human blood samples has been described by Nygren and Nilsson (1987). The method involves several steps including complexation with sodium diethyldithiocarbamate (pH 9), extraction with pentane, evaporation, and butylation by using a Grignard reagent in tetrahydrofuran.

Platinum In recent years, platinum compounds, especially cisplatin, are used for the treatment of cancer. In the analysis of biological fluids of patients treated with platinum salts, it is desirable to identify free platinum species and those bound to macromolecules. Ion exchange (Bannister et al., 1978) and ultrafiltration (Pinta et al., 1978) techniques coupled to traditional atomic spectrometry have been applied to study the different platinum species in blood plasma and serum. Cisplatin, its hydrolysis products, and two methionine-platinum complexes were studied by reversed phase ion-pair chromatography with on-line ICP-AES and applied to the analysis of (bio)transformation products originating from cisplatin in human and rat plasma in vitro and in vivo (De Waal et al., 1987). Free platinum (not bound to proteins) in plasma has been separated by ultrafiltration and the metal concentration was determined by ICP-AES (Dominici et al., 1986).

Silicon Silicon is becoming a biological trace metal of increasing scientific interest, particularly in connection with neurological disorders associated with aluminium in dialysis encephalopathy and in Alzheimer's disease. Relatively few analytical data exist on the concentration of Si in physiological fluids in health and disease (P6rez Paraj6n and Sanz-Medel, 1994). To provide evidence on the possible correlation between A1 and Si levels in the serum of renal failure patients, and the possibility of the reduction of aluminium bioavailability by the presence of silicon in biological fluids, the effects of different factors including storage conditions, administration of desferrioxamine, and kidney transplantation on the total A1 and Si contents and on their distribution in the same serum samples were examined and compared by Wr6bel et al. (1994). Ultramicrofiltration was used for the separation of low molecular mass and high molecular mass serum fractions and ETAAS for the determination of Si. Distribution of Si in serum proved to be affected only by the storage conditions. When the sample is stored properly (pH < 7.8), the ultrafiltrable Si content results were consistent and reproducible. It was found that 43 +

Speciation Studies

25

3% of total serum Si in the low molecular mass fraction was ultrafiltrable. Nevertheless, Si binding to serum proteins must be different from that observed for aluminium.

Various Metal Speciation Speciation of various metals in human serum by anion exchange and size exclusion chromatography (SEC) with detection by ICP-MS was reported by Shum and Houk (1993). A direct injection nebulizer was used with packed microcolumns for anion exchange chromatography and SEC. Proteins in human serum were separated by SEC without sample pretreatment. The metals present in each molecular mass fraction were determined by ICP-MS with detection limits of 0.5 to 3 pg of metal. Six metal-binding molecular mass fractions determined in human serum were assigned as follows: (1) >650 kDa fraction for Pb, Cd, Zn, Cu; (2) 300 kDa fraction for Pb, Zn, Cu; (3) 130 kDa fraction for Pb, Cd, Zn, Ba, Cu, Na; (4) 85 kDa fraction for Fe; (5) 50 kDa fraction for Zn, and (6) 15 kDa fraction for Pb and Zn. There was only one Fe-binding molecular mass fraction found at 85 kDa and this could be serum transferrin. The proteins responsible for the other molecular mass fractions required identification. In summary, speciation studies in blood must take into consideration the low metal contents in clinical samples, the presence of free ions and protein-bounded metals, and the effect of added compounds, like EDTA or DFO. Additionally, natural occurring organometalic species must be considered.

B. Speciation in Urine In the case of intoxication or to provide information on the balance between intake and output, the level of trace metals like arsenic, mercury, or selenium in urine is frequently taken as an indicator, since the kidney is an important feature in body homeostasis. Like blood, urine is a complex matrix often causing analytical problems because of its high salt content and a range of organic constituents. When applied to a chromatographic system, the urine matrix may create column overload problems. This can result in peak splitting or broadening of the analyte signals. Thus samples must be subjected to a desalting process, before subsequent separations, to allow control of the elution parameters. The major work that has been performed on metal speciation in urine samples concerns speciation of arsenic followed by selenium and then mercury. Reports on the speciation of other metals like tin, lead, cadmium, chromium, zinc, iron, and magnesium in this biological fluid have been also reported.

Arsenic The presence of As in the human body mainly occurs through food and/or occupational exposure. After absorption in the gastrointestinal tract or in the lungs,

26

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

this metal is eliminated through urine. Inorganic arsenic undergoes considerable biotransformation in the body; both monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) derivatives are formed. The proportion of inorganic and methylated species in urine may vary, although DMA is generally the major metabolite (Buchet et al., 1981). Although total urinary arsenic determinations are often used to assess occupational exposure to inorganic arsenic, specific measurements of DMA, MMA, and inorganic arsenic provide a more reliable indicator or exposure than total urinary arsenic levels (Chana and Smith, 1987). Again, organoarsenic compounds such as arsenobetaine have been found to be present in urine following the ingestion of seafood. The decreasing order of toxicity of arsenic compounds is now well known: arsenite > arsenate > MMA > DMA > As ~ > arsenobetaine. Different types of methods have been investigated for the speciation of arsenic compounds in urine. Initially for speciation studies of arsenic compounds by atomic spectrometry, a series of procedures based on the use of a selective liquid-liquid extraction or the use of chromatography were reported. Several reports deal with the separation and detection of arsenic species present in urine. This has been performed by HPLC coupled with hydride generation AAS for the determination of arsenobetaine (AB), arsenocholine (AC), and tetramethylarsonium cations in human urine (Momplaisir et al., 1991), and also for the determination of arsenite, arsenate, DMA, and MMA at Ixg L -l As level (Chana and Smith, 1987). There are several reports (Jimenez de Bias et al., 1994a; Le et al., 1994) on the determination and speciation of arsenic in human urine by ion-exchange chromatography-flowinjection analysis with hydride generation AAS. Eight arsenic compounds (four anionic, such as arsenite, arsenate, monomethylarsonate, and dimethylarsinate, and four cationic, such as arsenobetaine, trimethylarsine oxide, arsenocholine, and tetramethylarsonium ion) in urine were separated by anion- and cation-exchange HPLC and detected by ICP-MS at m/z = 75. Hexahydroxyantimonate(III) was used as an internal standard for their qualitative analysis. Arsenite was unstable in both urine samples and standard mixtures when diluted with a basic (pH 10.3) mobile phase used for anion chromatography. Interference due to 4~247 was eliminated by chromatographic separation of the chloride present in the sample from the arsenic analytes (Larsen et al., 1993a).

Selenium Nearly all information on selenium species in urine refers to rats and mice. Studies on selenium metabolites in human urine are less numerous. Trimethyl selenonium ion (TMSe§ a detoxification metabolite of selenium that is excreted in the urine, was first isolated and identified in rat urine (Palmer et al., 1969). The determination in human urine by cation exchange chromatography and ETAAS was reported by Tsunoda and Fuwa (1987). Fodor and Barnes (1983) were able to speciate selenate and selenite from urine samples by using different pH values for

Speciation Studies

27

complexation with a poly(dithiocarbamate) resin. It was found that for the urine of 11 healthy persons, the selenite content (8.6 I.tg L-1) was on average about three times more than the selenate concentration (3.1 lag L-l). Laborda et al. (1993) analyzed chromatographic effluents containing selenium species of urine samples by ETAAS using a sampling procedure based on fraction collection and hot injection into an electrothermal atomizer. The HPLC separation of TMSe § SeO32, SeO42 was performed by anion-exchange chromatography using 0.01M ammonium citrate at pH 3 and 7 as eluent.

Mercury For mercury speciation in urine samples similar methods have been suggested. In work on mercury speciation in urine and blood, the reduction of mercurials were selectively performed by using different reduction steps with SnC12 and with a mixture of SnC12+ CdC12. Several workers (Robinson and Wu, 1985; Seckin et al., 1986) have separated the organomercury species of urine primarily by gas chromatography followed by AAS detection. Samples of urine were injected directly on to a Chromosorb W AW-DMCS column interfaced with ETAAS. Inorganic Hg was the major form of mercury excretion. The results indicated the presence of unidentified nonvolatile Hg species in the samples (Robinson and Wu, 1985). Mercury and its species have been determined in urine samples from humans after breathing air in a dental workplace. The urine was treated with concentrated nitric acid and digested in a polytetrafluorethylen (PTFE) reactor at 140 ~ for 90 min. The resulting solution was mixed with SnC12 and the Hg vapor produced was analyzed by CVAAS. For GLC analysis of urine, samples were extracted with toluene and then with benzene. The extract was analyzed with temperature programming from 110 to 220 ~ at 15 ~ min -l. Nitrogen as carrier gas and 63Ni-electron capture detection was used (Seckin et al., 1986). Urine samples have also been analyzed by Shum et al. (1992) after separation with ion-pair liquid chromatography followed by ICP-MS detection with direct injection nebulization. A 24 h urine specimen was analyzed by this method and Hg 2§ methylHg§ and ethylHg§ species were found.

Other Metals Trialkyltins are the organotins having the greatest biocidal activity in mammalians. A purge and trap flame photometric gas chromatographic technique for speciation of trace organotin and organosulphur compounds in human urine standard reference material was reported by Olson et al. (1983). Urine samples were purged with nitrogen, with or without prior treatment with NaBH 4, and the volatile compounds were trapped on Tenax GC. These compounds were desorbed by heating the Tenax and were analyzed by GLC. The flame photometric detector was used in either the sulfur mode (394 nm) or the tin mode (600 to 2000 nm) with a hydrogen-rich flame. Neutral volatile organotin and organosulfur compounds did not require a pretreatment with NaBH4. This is necessary for alkylchlorotins.

28

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Speciation studies of mono-, di- and tributyltin compounds in urine have been performed by extraction, GC separation, and ETAAS determination (Dyne et al., 1991). Preconcentration and determination of ionic alkyl lead compounds in human urine has been described by Neidhart and Tausch (1992). Preconcentration and cleanup steps were followed by HPLC. Ionic species were preconcentrated by solid-phase extraction and detection was performed on-line. The alkyllead species were eluted from the HPLC column, partially dealkylated by iodine solution to form dialkyllead species, and detected as [4-(2-pyridylazo) resorcinol] complexes after the reduction of excess iodine with thiosulfate. Various cationic species of lead as Pb 2§ Me3Pb § and Et3Pb § were separated as ion pairs by reversed-phase liquid chromatography-ICP-MS with a direct injection nebulization system (Shum et al., 1992). The method was tested for measurement of Pb species in National Institutes of Science & Technology-Standard Reference Material (NIST-SRM) 2670 freezedried human urine. Since this reference material contained only Pb 2§ and did not contain measurable levels of Me3Pb § and Et3Pb § these compounds were spiked in the NIST urine to test the suitability of the method. Reasonable chromatographic peaks were seen for Pb 2§ Me3Pb § and Et3Pb § species in the spiked urine sample. A thermospray-interfaced HPLC-flame AAS system was developed for studies of cadmium speciation in human body fluids by Chang and Robinson (1993). The cadmium compounds in urine were separated by HPLC on a Zorbax C 8 column with water as the mobile phase. The column eluate was fed directly to the thermospray interface and the analyte delivered directly into the base of the flame for effective atomization and detection by AAS. Successful separations were achieved for a large number of nonvolatile cadmium compounds.

C. Speciation in Milk Human milk, cow milk, or infant formulas are the main nutrient fluids for newborn infants. Assuming that the composition of breast milk may satisfy the growing demand of healthy babies during their early months of age, it could be a reference to evaluate the nutritional value of alternative milk formulas. The protein and composition as well as the kind of trace metal of breast milk have been known for the last several years. But the absorption and utilization of the trace metals depend not only on the total amount in the milk but also on the availability of the chemical form in which they occur. Hence the speciation of metals in milk is very meaningful. Few metal speciation studies have been performed in this body fluid. Only several reports on speciation of zinc, cadmium, selenium, mercury, lead, and a few other metals in milk may be found in the literature. Babies at their early ages are susceptible to selenium deficiency. This may arise due to intake of milk or infant formulas with low selenium content. Again, the total amount of this metal does not ensure the overall utilization or bioavailability of selenium. Thus the chemical forms of selenium and its distribution in food are

Speciation Studies

29

important factors of selenium bioavailability. The distribution patterns of selenium species in cow and human milk were compared by Van Dael et al. (1988). Milk samples were fractionated into fat, whey, and casein parts by ultracentrifugation. In order to separate the lipid components, milk fat globule membranes were solubilized with sodium dodecyl sulphate. Further centrifugation separated the outer and inner fat globule membranes from triglycerides. After separation all fractions were lyophilized and stored at -20 ~ Samples were digested with a mixture ofHNO 3 and HC10 4 and then analyzed for selenium by hydride generation AAS. The study revealed that the whey fraction represented 40 and 72% of the total selenium content of cow and human milk, respectively. The lipid fraction contained approximately 10% of either cow or human milk's total selenium. After solubilization of milk fat globule membranes, 61 and 80% of selenium was found in the outer fat globule membrane for cow and human milk, respectively. Determination of the selenium content in individual proteins of cow's milk revealed that the highest selenium concentrations were present in 13-1actoglobulin and in K-casein. Br~itter et al. (1988a) developed a procedure to determine selenium protein profiles in skimmed human breast milk via on-line coupling of gel permeation chromatography and ICP-AES after performing the acid digestion procedure and the formation of Se(IV) with the subsequent generation of selenium hydride in the connecting flow. In total, four selenium peaks (molecular mass at >600, 90, 25, and 10 kDa) have been detected in breast milk. A method for the preparative separation of human breast milk proteins was developed by Michalke (1993), keeping metal-protein complexes intact, especially with respect to zinc and cadmium species. Separations were performed on TSK columns, using HW 55 gel, with double distilled water as the mobile phase. The metals were determined in native human milk, the protein pellet, and supernatant (without fat fraction) as well as in peak related HPLC-fractions of protein pellet and supernatant with differential pulse anodic stripping voltammetry (DPASV) for cadmium and DCP-AES for zinc, respectively. Cadmium content of whole breast milk, the protein pellet, and the peak-fraction corresponding to metallothionein was determined to be 1 Ixg L -1, whereas no cadmium was found in the supernatant. On the other hand, the amount of zinc was found to be about 3.5 mg L -1 in human milk and only a small quantity (160 ~g L -1) could be detected in the protein pellet. Zinc content could be related to several breast milk proteins (e.g. metallothionein) in different amounts. In the case of the supernatant, zinc was related only to citrate. On-line combination of gel filtration chromatography and ICP-AES has been applied for the fractionation and identification of metal-containing species in skimmed human milk including background control and its subtraction. Subtraction yielded a selenium peak of lower intensity and its shift to position around 10 kDa. At this position, iron, manganese, and zinc elute exactly in the same fraction. Another chromatogram of defatted human milk showed the distribution of copper, iron, manganese, and zinc. Fromthis study, the binding of citrate was established

30

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

with zinc. There was also some evidence for the binding of iron to citrate (Br~itter et al., 1988a). The technique of ICP-AES coupled to a gel filtration chromatography system was used by Br~itter et al. (1988b) to characterize the species containing copper, zinc, iron, manganese, magnesium, and calcium in human milk and milk formulas based on cow's milk. The distribution profiles clearly indicate that the metal (Cu, Zn, and Fe)-protein binding in human milk and milk formulas are different. Investigations of protein-bound trace metals in human milk before and after the birth of a baby showed marked differences in zinc and iron bound to citrate. An increase of citrate concentration by a factor of 25 + 3 takes place after birth at the first day of milk secretion. About 80% Cs, 22% Sr, and 1.5% Eu were recovered in pectin when fresh pasteurized skimmed cow's milk spiked with the respective radionuclides (85Sr, 137Cs,and 152Eu) was shaken with a 4% aqueous solution of apple pectin at an initial milk/pectin volume ratio of 7:3. The recovery fraction was proportional to the abundance of radionuclides in milk. Extraction of the spiked milk was performed with Aerosol OT in isooctane (Mac~igek and Gerhart, 1994). Binding of added strontium by milk proteins under native conditions was also investigated using pectin of various degrees of esterification. Upon partition of Sr, Cs, and Eu in aqueous two-phase milk-pectin system performed by membraneless dialysis, it was compared with the distribution between cation exchanger and milk, milk formula, or pectin solutions. The low molecular mass fraction of added Sr in milk assessed from Dowex 50 x 8 sorption data was found to be 31% and that of Cs and Eu were 58 and 40%, respectively (Mac~igek et al., 1994).

D. Speciation of Miscellaneous Biological Fluids Some papers concerning different biological fluids in which speciation studies have been carried out other than blood, urine, and milk have been also published. Some examples are on blood cell lysate, sweat, saliva, cerebrospinal, seminal, tear, and bronchoalveolar fluids which have been developed in recent years are presented below. Several reports varying in manipulative complexity have been proposed for determining As, Fe, Zn, and Se species of blood cell lysate. The binding of arsenic in the red blood cells of rat were investigated by Cornelis and De Kimpe (1994) because it was thought that most of this metal binds to hemoglobin (Hb). The radiotracer 74As was used throughout the experiment. The red blood cells lysate was submitted to size exclusion chromatography (SEC) using Superose HR 12 column and 0.15 M NaCI + 25 mM Tris (pH 8) eluent and cation-exchange chromatography using Mono S HR 5/5 column and 10 mM malonic acid (pH 5.8) and 10 mM malonic acid + 0.3 M LiC1 (pH 5.8) eluents. The study of binding of As in the red blood cells lysate with SEC showed that 87.7% of the metal is associated with a protein with a relative molecular mass of around 60 kDa and a

Speciation Studies

31

strong absorption band at 420 nm. This compound is Hb. Cation exchange of the SEC fraction showed that the signal peak observed in the SEC chromatogram consisted of several Hb species, each carrying part of the 74As. On-line combination of gel filtration chromatography and ICP-AES for studying metal speciation in blood serum and human milk has already been discussed in previous sections. The same technique was also used for red blood cell lysate (Br~itter et al., 1988a). From the gel filtration chromatography of human erythrocyte-lysate on a TSK column, it may be found that selenium is eluted in two peaks which correspond to a molecular mass of 90 kDa and 33 kDa. The high molecular mass compound could be classified with the selenoenzyme glutathione peroxidase. Iron and zinc were also monitored with selenium. Fe indicates the position of Hb in erythrocytes and Zn being in the position of the enzyme carbonic anhydrase. Owing to the narrowness of these two markers, a clear identification of the selenium binding complex at 33 kDa is not possible. Accumulation of iron in the myocardium in circumstances of transferrin saturation is associated with heart failure in iron-loaded patients. To characterize the underlying causes of this phenomenon, Parks et al. (1993) measured the flux as well as the speciation of iron in normal and iron-loaded cultures of rat myocardiocytes. Iron loading of cultured myocytes induced shifts in iron speciation. Thus the ratio of iron bound in hemosiderin-like compounds to ferritin-bound iron increased twofold from a range of 0.84 to 1.44 in control cells to 1.96 to 3.3 in iron-loaded cells. Only few analytical methods are known for determining chemical species of mercury, cadmium and sodium in sweat (Robinson and Wu, 1985; Chang and Robinson, 1993). Calcium and magnesium species have been studied in saliva (Lagerlof and Matsuo, 1991). Speciation of iron, potassium, sodium, and calcium has been reported in cerebrospinal fluid (Gutteridge, 1992). Ferrous ion has been detected in cerebrospinal fluid by using bleomycin and DNA damage. Normal cerebrospinal fluid samples were centrifuged and the supernatant liquid was stored a t - 2 0 ~ For the determination of Fe(II) species the following reagents were added in order, into metal-free plastic tubes: (1) 0.4 mL of DNA solution (1 mg mL -1) stored over 0.05 volumes of Chelex-100 resin, (2) 0.1 mL of bleomycin sulphate (1.5 mg mL -1) stored over a solution of conalbumin (5%, w/v) where 120 mM sodium azide was added to inhibit the ferroxidase activity of ceruloplasmin, (3) 0.1 mL of cerebrospinal fluid, and (4) 0.4 mL of sodium phosphate buffer (pH 7) treated with conalbumin. The mixture was incubated at 37 ~ for 30 min to allow Fe2§ ent degradation of DNA and then treated with 0.5 mL of thiobarbituric acid (1%, w/v in 50 mM NaOH) and 0.5 mL of HC1 (25%, v/v). The mixture was heated at 100 ~ for 5 min, cooled and extracted with 1.5 mL of butan-1-ol. The mixture was centrifuged and the thiobarbituric acid-reactive material in the clear upper organic layer was determined spectrofluorometrically.

32

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

In seminal fluid, iron and zinc species have been studied by various authors (Caldini et al., 1986; Gavella, 1988). Atomic absorption techniques for direct determination of multimetal species in whole tear film were described by Giordano et al. (1983). Dissolved sodium, lithium, potassium, calcium, and magnesium were determined by flame AAS. No matrix effect was observed. ETAAS was used for determination of chromium, manganese, iron, lead, zinc, copper, nickel, cobalt, arsenic, and aluminium. These metals were present in the range between less than 0.01 to 1 l.tg mL -1 and probably, following this methodology the total content is determined. The reader is invited review Table 1 for the analytical details of the selected metal ion species in biological fluids. It suggests that chromatography is the method of choice for the preferential separation of various components in biological fluids. After separation, metals are usually analyzed by atomic spectrometry techniques suitable for trace analysis such as AAS (flame and electrothermal) inductively coupled plasma spectrometry (ICP-AES and ICP-MS). However, in some work, radiochemical methods like neutron activation analysis (NAA) and electrochemical techniques like DPASV have been used. It is clearly evident from Table 1 that serum and urine are the more commonly studied matrices. There has been more interest in speciation studies of arsenic, mercury, and selenium in different biological fluids. Aluminium in blood has also been studied widely. Only few reports are available on metals like cadmium, chromium, iron, lead, platinum, zinc, copper, and others in these matrices. An examination of Table 1 also reveals that the detailed information about the analytical performance of the developed methodologies is not very often reported. Speciation studies of biological fluids are very significant in clinical chemistry to understand the physiological behavior of trace elements in the living system. We have tried to highlight some of the achievements realized until now for the separation and quantification of the metal species which might be present below sub-lxg L -1 level in biological fluids. From the published literature it can be concluded that it is not possible to detect a general methodology for trace metal speciation in biological fluids. However, some important aspects of the most appropriate methodology could be taken into consideration, such as the importance of mechanical separation. This is a useful preliminary step to identify the different size of the chemical species. This provides information about the molecular mass of the proteins or the other molecules to which the metal ions are bound. This step, commonly performed by ultrafiltration or centrifugation, can be considered as a specific methodology for speciation studies in biological materials as a difference of similar studies carried out in other matrices, such as sediments, soils, or environmental samples. After the physical separation of different molecular mass fractions, the combination between chromatographic method and sensitive atomic spectrometric detection, as indicated for water analysis, seems to be the best alternative for the identification and quantitative analysis of different species. However, there is a lack

Table 1. Analytical Details of the Atomic Spectrometry Procedures Developed for Speciation in Biological Fluids

OJ

Element

Species

Matrix

Methodology

DetectionLimit

RSD

AI

Transferrin

Serum

HPLC-ETAAS

AI

Albumin, transferrin

Serum

GPC-ETAAS

AI

Proteins with different MM

Serum

SEC-ETAAS

AI

Albumin, transferrin

Serum

Ultrafiltration, HPLC-ETAAS

As

Urine

AS

AsC~3-,AsC)34-, MMA, DMA, AB, AC, TMA +, As-sugars AsO~3-,AsO34-, MMA, DMA, AB, AC AsC~3-,AsO34-, MMA, DMA

0.12 l~g L-1

8%

HPLC-HGAFS

--

--

Urine

HPLC-HGAAS

8-15 l~g L-1

2.5-5.3%

--

Blood

IEC-HGAAS

0.44-0.92 Ilg L-1

3.6-6.8%

90-110%

As

AsO~3-,AsO34-, MMA, DMA

Urine

--

--

94.9-107.7%

As

MMA, DMA, AB, AC

Serum

Extraction-ETAAS HPLC-ICPMS IEC-HGAAS

1-1.5 Mg L-1

4.2-4.8%

>90%

As

AsO~3-,AsO34-, MMA, DMA

Urine

HPLC-MIPAES

AsO~3-, DMA Toxic-As (AsC~3- + AsOa4- + MMA + DMA) Non-toxic-As (AB + AC)

Urine Urine

IC-ICPMS Microwave digestion-HGAAS

2.8% (AsC~-) 2.5% (AsO34-) 3.1% (MMA) 2.1% (DMA) -4-7%

--

AS As

1 ng mL-1 (AsO:]-) 5 ng mL-1 (AsCP4-) 6 ng mL -1 (MMA) 1.2 ng mL-1 (DMA) -4-6 I~g L-1

AS

Recovery

Ref.

101.1 + 15.3% Van Landeghem et al. (1994) -Gardiner et al. (1984) 60-125% Keirsse et al. (1987) Blanco Gonzalez et al. (1989) Wrobel et al. (1994) -Le et al. (1996)

---

Lopez Gonzalvez et al. (1996) Zhang et al. (1996a) Bavazzano et al. (1996) Zhang et al. (1996b) Costa Fernandez et al. (1995) Feldmann (1996) Lopez Gonzalvez et al. (1995)

(continued)

Table 1. Continued Element

Species

Matrix

Methodology

Detection Limit

RSD

As

AsO]3-, AsO~-, MMA, DMA

Urine

90-300 pg

3.4-5%

~

Ding et al. (1995)

As

Inorganic-As, MMA, DMA

Urine

Micellar LCICPMS IEC-HGAAS

0.9 ~g L-1 (Inorg-As)

3.5% (Inorg-As)

~

Jimenez de Bias et al. (1994b)

Urine

IC-ICPMS

1.3 l~g L-1 (MMA) 0.5 I~g L-1 (DMA) 0.22-0.44 l~g L-~

3.2% (MMA) 4.6% {DMA) 3.2-4.9%

Urine

HPLC-HGAAS

2 14g L-1

--

Chana and Smith (1987)

Blood

Digestion-HGAAS

m

5.1% (AsC~3-}; 3.8% (MMA); 5.3% (DMA); 7.2% (AsO34-) --

90-105%

m

--

Weigert and Sappl (1983) Dix et al. (1987)

As As

AsO]-, AsCP4-, MMA, DMA, AB, TMAO AsO]-, AsCP4-, MMA, DMA

,~o~-, , ~ -

4~

0.1-5 ng L-1 Blood, Derivatization-GC urine Urine HPLC-ICPMS 3-6 ng m1-1 (cations) 7-10 ng m1-1 (anions) Urine Extraction-ETAAS 10 l~g L-1

As

Inorganic and organic-As

As

AsO:]3-, AsO43-, MMA, DMA, AB, AC, TMAO, TMA+

As

Inorganic and organic-As

As

Urine

IEC-HGAAS

As

Inorganic, MMA, hydroxydimethylarsine oxide AB, AC, TMA +

Urine

HPLC-HGAAS

As

Inorganic, MMA and DMA

Urine

IEC-HGAAS

0.5 l~g L-1 I 2 l~g L-1

Recovery

Ref.

Inoue et al. (1994)

23% (AB}; 96-108% Larsenet al. (1993) 5.4-8.9% (others) 7.9% (inorg-As); 93% (inorg-As}; Fitchett et al. 3.6% (DMA} 88.4% (DMA) (1975) 3-6% 95-102% Buratti et al. (1984) --

85-97%

3.2-4.6%

85-93%

Momplaisir et al. (1991} Jimenezde Bias et al. (1994a)

As

AsO~3-, AsO34-, MMA, DMA

Urine

HPLC-ICPAES

10 I~g L-1 (AsO~3-, DMA, AB); L-1 (MMA); 15 l~g 20 llg L-1 (AsC~4-) 0.5 l~g L-1

As

AsO~3-, AsO34-, MMA, DMA

Urine

HPLC-ICPMS

36-96 pg

As

AsO~3-, AsO34-, MMA, DMA

Urine

HPLC-ICPMS

Cd

Proteins with different MM

HPLC-FAAS

Cr

Cr(lll), Cr(VI)

Cr

Cr(lli), Cr(VI)

Urine, sweat Blood, urine, serum Urine

63 pg (AsC~-); 37 pg (AsC~4-); 80 pg (MMA, DMA) 1 I~gL -1

Cr

Cr(lll), Cr(VI)

Urine

Cr

Cr(Vl)

Cr

Cr(lll), Cr(VI)

Blood, urine Serum

Fe

Transferrin

Hg Hg

As

M'I

AsO~3-, AsO43-, MMA, DMA, AB Urine

HPLC-ICPMS

Le et al. (1994)

Low et al. (1986 and 1987) Heitkemper et al. (1989) Sheppard et al. (1992)

10%

Chang and Robinson (199 3) Gaspar et al. (1996)

FAAS

24 l~g L-1 (Cr Itt) 75 llg L-1 (Crvl)

HPLC-ICPMS

3 pg

IC-ICPAES IC-ICPMS Extraction-AAS

12-15 I~g L-1 35-47 l~g L-1

IEC-DCPAES

3-15 l~g L-1

Serum

HPLC-ETAAS

0.17 l~gL-1

1.4%

Hg, MeHg

Urine

CV-ICPAES

4 ng mL-1

5%

Hg, MeHg

Urine

HPLC-MIPAES

0.15 ng mL-1 35 ng mL-1

6.7% 6.8%

Zoorob et al. (1995) Jensen and Bloedorn (1995)

0.9% (Crlll), 1.3% (Crvl) 0.3-1.6%

Devoto (1968)

97.3 + 2%

Urasa and Nam (1989) VanLandeghemet al. (1994) Menendez Garcia et al. (1996) Costa Fernandez et al. (1995)

(continued)

Table T. Continued

M,;

Element

Species

Matrix

Methodology

DetectionLimit

RSD

Recovery

Hg

MeHg, EtHg, PhHg

Urine

HPLC-CVAFS

5-7 ng

m

>95%

Hg

Hg, MeHg

Urine

HPLC-CVAAS

Hg

Hg, MeHg, MeEtHg, diEtHg

Blood

GC-CVAFS

0.3-0.6 ng g-1

6.9% 3.5% 5%

90-114% 89-103% m

Hg

Inorganic and organic-Hg

Hg Hg

Inorganic and organic-Hg Inorganic and MeHg, PhHg

1-8% 2.8-5.2%

86-106% 97-102%

Hg

Inorganic and MeHg

5-10%

Hg

Inorganic and organic-Hg

Urine Reduction-CVAAS

95.2 + 2.7% (MeHg); 99.5 + 4.3% (inorg-Hg)

Hg

Inorganic and organicoHg

Hg

. Inorganic and MeHg, EtHg, PhHg

Urine, sweat Urine

Hg Pb

Inorganic and organic-Hg TrimethyI-Pb

Pb

Ionic alkyI-Pb

O~

Blood, Reduction-CVAAS urine Blood Reduction-CVAAS Blood, GC-ETAAS milk Whole GC-MIPAES blood

3-5 ng L-1

GC-ETAAS

0.3 ng

GC-CVAAS

1.7 ~g L-1 (total); 12 ~g L-' (MeHg); 2.4 gg L-' (EtHg); 21 Mg L-1 (PhHg) 7 pg 3 l~g L-1

Urine Ion-pair LC-ICPMS Blood GC-ETAAS Urine

<4 gg L-1 3 ~g L-1 (MeHg); 125 ~g L-' (PhHg) 1 pg

HPLC

11%

93%

Ref. Yoshino et al. (1995) Aizpun et al. (1994) Palmisano et al. (1993) Coyle and Hartley (1981) Lind et al. (1993) Filippelli, 1987 Bulska et al. (1992)

Oda and Ingle (1981) Robinson and Wu (1985) Seckin et al. (1986)

Shum et al. (1992) Nygren and Nilsson (1987) Neidhart and Tausch (1992)

Se

Cisplatin, hydrolysis products

Plasma

HPLC-ICPAES

Free, complexed

Plasma

Se-Cys, Se-Met, TMSe §

Serum, urine

UltrafiltrationETAAS HPLC-ICPMS

Inorganic-Se, total selenoamino acids

Urine

L-1 (Se-Cys), 0.2 ~g L-1 0.6 l~g (Se-Met), 0.2 l~g L-1 (TMSe § 6.8 l~g L-1 (inorg-Se) 30 l~g L-1 0.16 l~g L-1 1 ~gL -1 5 l~g L-~ 2 llg L-1

SeO~-, SeO~-,

Urine

Se

SeO]3-

Blood

HPLC-HGAAS -ICPAES -ICPMS HPLC-HGAAS HPLC-ETAAS GC

Se

Inorganic-Se

Blood

GC-MIPAES

Se

TMSe §

Urine

IEC-ETAAS

1.2 ILtg L-1

Se

TMSe +, total Se TMSe § selenonocholine

Urine Spiked urine

IEC-ICPMS HPLC-AAS

Se

TMSe § SeO]3-, SeO24-

Urine

HPLC-ETAAS

40 ng Se 44 ng (TMSe+); 31 ng (selenonocholine) 1.83 ng (TMSe+); 1.15 ng (SeO24-)

Se

Proteins with different MM

Milk

Proteins with different MM

Serum

UltrafiltrationHGAAS HPLC-ETAAS

Organotins Mono-, di-, and tri-alkyltins

Urine Urine

GLC GC-ETAAS

Se

Se

selenoamino acids

De Waal et al. (1987) Dominici et al. (1986) Muffoz Olivas et al. (1996)

35 Mg L-1

5%

m

96-104%

",4

Se

Sn

Sn

1-3 pg L-1

91 +12% m m

B

Gonzalez LaFuente et al. (1996a) Marchante Gayon et al. (1996) Kurahaschi et al. (1980) Harrison and Rapsomanikis (1989) Tsunoda and Fuwa (1987) Sun et al. (1987) Blais et al. (1991 ) Laborda et al. (1993) lyengar (1987) Wrobel et al. (1994) Olson et al. (1983) Dyne et al. (1991 )

Table 1. Continued Element Zn Cu, Zn

Species Albumin, 0s Dissolved species

Cu, Zn

Qo

Matrix

Methodology

Serum

Ultrafiltration . ETAAS HPLC-ETAAS

Blood

DetectionLimit .

.

1 ~g L-1 (for both)

RSD

Recovery

Ref. Faure et al. (1990)

. --

--

Gardiner et al. (1981) SchOppenthau and Dunemann (1994) Wu and Robinson (1986) Buchberger and Rieger (I 989)

m

Shum and Houk (1993)

Ceruloplasmin and albumin (for Cu); macroglobulin and albumin (for Zn) Mg, Zn Mg 2§ predominant, zinc species could not be identified Ca, Mg, Dissolved species Na, K

Serum

HPLC-ICPMS, HPLC-ICPAES

--

Urine

HPLC-FAAS

J

Tear

Centrifugation-IC

Cd, Cu, Fe, Pb, Zn Cu, Fe, Mn, Se, Zn

Proteins with different MM

Serum

HPLC-ICPMS

Proteins with different MM

HPLC-ICPAES

Br~tter et al. (I 988a)

Ca, Cu, Fe, Mg, Mn, Zn

Proteins with different MM

Serum, milk and red blood cell lysate Milk

GPC-ICPAES

Arts and Hafkenscheid (1984)

Ca (0.60 l~g mL-1); Mg (0.25 I~g mL-1); Na (0.07 ~g mL-1); K (0.12 ~g mL-1) Cu (0.7 pg); Fe (3 pg); Cd and Pb (0.5 pg); Zn (1 pg)

m 7% (Ca); 3-4% (Mg, Na, K) m

AI, As, Ca, Co, Cr, Cu, Fe, Li, Mg, M n , Na, Ni, K, Pb, Zn

t,D

Dissolved species

Tear

FAAS, ETAAS

E

7-I 2%

93-I 05%

G i o r d a n o et al. (1983)

Notes: Abbreviations: RSD - relative standard deviation; HPLC - high performance liquid chromatography; ETAAS - electrothermal atomic absorption spectrometry; GPC gel permeation chromatography; $EC - size exclusion chromatography; HGAFS - hydride generation atom ic fluorescence spectrometry; HGAAS - hydride generation atomic absorption spectrometry; IEC - ion exchange chromatography; ICP MS - inductively coupled plasma mass spectrometry; MIP AES - microwave induced plasma atomic emission spectroscopy; IC - ion chromatography; LC - liquid chromatography; GC - gas chromatography; ICP AES - inductively coupled plasma atomic emission spectroscopy; FAAS - flame atomic absorption spectrometry; AAS - atomic absorption spectrometry; DCP AES - direct current plasma atomic emission spectroscopy; CVICP AES - cold vapor inductively coupled plasma atomic emission spectroscopy; CVAFS - cold vapor atomic fluorescence spectrometry; CVAA5 - cold vapor atomic absorption spectrometry; GLC - gas liquid chromatography; ~ - Molecular Mass; M M A - monomethyl arsonic acid; D M A - dimethyl arsinic acid; AB - arsenobetaine; AC - arsencx:holine; TMA + - tetramethylarsonium ion, T , ~ O - trimethylarsine oxide; MeHg - methylmercury; EIHg - Ethylmercury; PhHg - phenylmercury; MeEIHg methylethylmercury; diEtHg - diethylmercury; Se-Cys - selenocystine; Se-Met - selenomethionine; TMSe + - trimethyl selenonium ion.

40

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

of information regarding the development of adequate chemistries for leaching small molecules which could be retained or adsorbed in some protein fractions, and the development of structural studies which provide specific and complete information on the nature of the binding between chemical species and biomolecules. It can be stated that work on metal ion speciation in biological fluids is incomplete. It continues to be a very active and topical area of research in analytical chemistry due to the absence of studies on a large number of important metals (V, Mo, Ni, B) and the few studies carried out in some important body fluids. Evidently metals can take a variety of forms, particularly in biological systems which is complicated by itself, but as yet there is no good analytical procedure for separating, detecting, and quantifying many of these forms. Moreover, the reliability of speciation results may be impaired by the factors such as contamination, losses, stability, metal transfer effects, or identification problems. A critical examination on this subject reveals that available major work has been done in blood and urine. Although several papers may be found on milk, only a few references are available on blood cell lysate, sweat, saliva, cerebrospinal, seminal, tear, and bronchoalveolar fluids. The study of trace metal species in all these different biological fluids is expected to improve the insight into the trace element kinetics and metabolism to a great extent. Regarding the analytical features of methods proposed in the literature, it can be found that the detection limits (LOD) reported for most of the metal species are of the order of ktg L -1. For arsenic, comparatively more LOD values were reported. In urine or blood the value lies in the range 0.1 to 20 Ixg L -1, depending on the species of arsenic. Various species of mercury in urine have typical LOD value of 2.4 lag L -1 (EtHg+), 12 ktg L -1 (MeHg+), and 21 lag L -1 (PhHg§ In blood and milk the LOD reported values are different as 3 ~g L -1 (MeHg § and 125 lag L -1 (PhHg§ In the case of selenium in urine determination, the LOD values vary from 1.2 lag L -1 for trimethyl selenonium ion to 40 lag L -1 for total selenoamino acids. For aluminium, cadmium, chromium, lead, platinum, and tin in various forms, the LOD values are respectively, 0.12, 1, 3 to 15, 3, 35, and 1 to 3 lag L -1. For iron in serum by ordinary spectrophotometric method LOD is 50 gg L -1, whereas by the ETAAS method it is 0.17 ~tg L -1. For copper and zinc in blood, a detection limit of 1 ktg L -1 is reported. In tear, the LOD values of dissolved species of Na § K § Mg 2§ and Ca 2§ are 0.07, 0.12, 0.25, and 0.60 lag mL -1, respectively. In a work on HPLC-ICP MS in serum detection limits have been reported for various in proteins metals are as follows: Fe (3 pg), Cu (0.7 pg), Zn (1 pg), Cd and Pb (0.5 pg each) (Shum and Houk, 1993). The precision, where reported, is usually adequate for these determinations. The overall relative standard deviation (RSD) values are in the range of 1 to 11%. Accuracy data has been reported in only a few papers. Some generalizations could be made for various metals in terms of recovery percentages: aluminium in serum, 60 to 125% (Blanco Gonz~ilez et al., 1989); arsenic in blood, 90 to 105% (Weigert and Sappl, 1983); arsenic in urine, 85 to 108% (Momplaisir et al., 1991; Larsen et

Speciation Studies

41

al., 1993a; Jimenez de Blas et al., 1994a); calcium in saliva, 97 to 101% (Lagerlof and Matsuo, 1991); iron in serum, 97.3 + 2%; iron in semen, 82 to 107% (Caldini et al., 1986); mercury in blood and milk, 86 to 106% (Bulska et al., 1992); lead in blood, 93% (Nygren and Nilsson, 1987); selenium in urine, 91 + 12% (Tsunoda and Fuwa, 1987); sodium in sweat, 85.2 to 116.3% (Barbour, 1991); and zinc in semen, 105.6% (Gavella, 1988). In the case of tear fluid for quantitation of 15 different metal ions, the accuracy has been reported to be between 93 and 105% (Giordano et al., 1983).

VI. SPECIATION IN SOLID SAMPLES: THE CHALLENGE Although it is now possible to determine various metals at the nanogram per gram level in different samples with the aid of currently available analytical instrumentation, the same measure of success has not yet been achieved with the determination of the same metals in solid matrices. This is currently due to the changes of the species during the pretreatment of samples. Also, inaccurate results are sometimes obtained because of the differences in the analytical response of the various physical and chemical forms of the same metal. A further problem that is encountered in chemical speciation of solids is that the analytical sensitivity for certain metal is inadequate, as we are dealing with only fractions of the total content. Thus it will be highly interesting to study different aspects of speciation studies in various types of solid samples in order to find the current state of the art. The papers that have been found in the literature could be classified into three groups depending on a matrix criterion: (1) speciation of soil and sediment samples; (2) speciation of solid biological and food materials; and (3) speciation of miscellaneous solid samples which includes other types of matrices. From Figure 13, the distribution of published papers about speciation in solid matrices as a function of the samples considered is presented. It is tremendously heterogeneous corresponding to 43.7% of published studies to soils and sediments, 48.5% in biological and food samples, and reduced percentages in other solid samples such as atmospheric particulates (5.2%), wood (0.4%), coal (1.7%), and PVC (0.4%). In Figure 14, the number of published papers as a function of the year of their publication is shown, and from this it can be shown the increasing interest in this area and the delay and reduced number of published works as compared with other studies carried out on fluids.

A. Speciation of Soil and Sediment Samples Soils and sediments constitute concentrated reservoirs of trace metals in which amounts are more magnified than those present in other adjacent phases of the environment, as for example water or air. In the literature, speciation studies of soils and sediments are usually reported independently. This is because most natural soils are characterized by oxidized conditions, whereas unpolluted sediments are usually

42

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Figure 13. Distribution of published papers about speciation in solid samples as a function of the type of sample considered.

Figure 14. Development of the literature published about metal speciation in solid

matrices.

Speciation Studies

43

deposited under oxygen-deficient conditions (Hirner, 1992). In the environment, different situations as a consequence of the anthropogenic activity may also occur. Thus waste deposits with high coal ash content may show high diversity in surface layers, resulting in reducing conditions underneath; or dredged anoxic sediments may be deposited under aerobic conditions. Hence the composition of soils and sediments reflect the nature of the original base rock, the degree of degradation and leaching introduced by weathering cycles, and the influence of external inputs (Pickering, 1981). Metal cations in soils may be present in several different physiochemical forms. These include simple or complex ions as easily exchangeable ions; as organically bound; as occluded by or coprecipitated with metal oxides, carbonates, phosphates, and secondary minerals; or as ions in crystal lattices of primary minerals (Lake et al., 1984). From the chemical point of view, the chemical compounds of a sediment may be carbonates, hydroxides, silicates, sulfides, phosphates, and organics in various stages of crystallization, stoichiometry, water contents, and so on (Sager et al., 1990). Again, the principal metal forms in sludge are soluble being present as precipitated or coprecipitated in metal oxides, adsorbates, and associated with biological residues (Lester, 1983). Storage and preparation of soil or sediment samples prior to leaching with selective reagents may affect the extractability of metals. For example, exchangeable potassium can be converted into a nonexchangeable form when samples are dried (Hesse, 1971). Manganese and iron solubilities decreased dramatically on air drying, but after oven drying the solubility of manganese exceeded by several times that found in moist samples (Breward and Peachey, 1983). Nevertheless, soils and sediments all vary in their moisture content depending on the location and the time of sampling. Provided that the metal species are stable the matrix should be dried either by freeze drying or by air drying at 40 ~ or, as proposed by Ure (1994), at room temperature. Most trace organics associated with them are bound to the organic surfaces of the matrix. The metal species may be leached from such samples by soaking and stirring or sonicating the sediment with HC1 and extracted into the organic solvent with ethyl acetate (Tusuda et al., 1987). In other methods reported (Harrison and Rapsomanikis, 1989) a similar use of acid to leach the compounds from the sediment along with methanol as an additive. Alternately, the metal species may be exhaustively extracted in a soxhlet apparatus after acidification with HC1. A weakly bonded/adsorbed fraction may be removed by sonication with an electrolyte such as NHaC1, followed by extraction with solvents of increasing polarity. The strength of the extraction medium can be further increased with diluted HC1 and then with more concentrated HC1, followed by organic solvent and acetic acid mixtures. Additional solvent extraction, by boiling with 2-mercapto ethanol in tris-buffer will disrupt the S-S linkage and release the more strongly bounded organometallic compounds. Nevertheless, the concentration of many metal species in soil and sediment iscompletely dependent on the method used for extraction (Wells, 1992).

44

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

The effects of pretreatments on the leaching of metals vary from metal-to-metal and seem to depend on the form of occurrence of the metal in the sediment sample. Drying of organic stream sediment tends to decrease the concentration of readily extractable elements (R~iis~inen et al., 1992). Various approaches to sampling and sample pretreatments for metal speciation in soils and sediments has been reviewed by Rubio and Ure (1993). They have discussed the avoidance of contamination and loss, field sampling, and subsampling of air-dried soils and sediments. Location of sampling points, kinds of sample, and sampling apparatus for suspended sediments, bottom sediments and interstitial water, measurement in situ, and pretreatment and storage of the sample have also been presented. In order to assess the reactivity of the species or binding forms of heavy metals in solid soil, sediment and sludge samples, extraction procedures have been applied both as single leaching steps and combined in sequential extraction schemes with representative sample size and without the need of any sample preparation (Salomons and Forstner, 1980). Extraction media could be neutral electrolytes, like CaC12 or MgC12; buffers of weak acids, like acetic acid or oxalic acid; chelating agents, like EDTA or DTPA; redox agents, like NH2OH; strong acids, like HC1, HNO 3, HC10 4, or HF; or bases, like NaOH or Na2CO 3. The ability of various extracting agents to release metal ions depends on its association with particular soil fractions. Extractants like electrolytes, weak acids, and chelating agents release metals from coordination sites, while strong acids and other redox agents are capable of releasing additional quantities of metal as a result of the decomposition of the soil matrix. Sequential extraction schemes have been described by several workers (F&stner et al., 1981) for sediments and dredged material. Consecutive leaching techniques allow information to be obtained about the mobilities of major and trace constituents under different environmental conditions such as acidic or alkaline, oxidizing or reducing behavior, or the action of chelating agents. From this study, the actual chemical form of occurrence of investigated species in soil or sediment can be concluded, hence leading to predictions about both origin and bioavailability (Sager, 1986). Subjecting soil and sediment samples to multiple extraction has not proved to be an attractive alternative to sequential extraction, possibly because the latter approach tends to more readily identify the sediment component(s) responsible for retaining the majority of the metals of interest. The most appropriate sequence can be determined by the type of soil or sediment being examined, the horizon involved, and the metals of interest (Pickering, 1986). As a matter of fact, sequential extraction provides more information than single extraction and has several advantages (Hirner, 1992): extractive procedures applied are comparable to those occurring in nature. In natural environments, soil and sediments are subject to similar leaching procedures by (1) natural and anthropogenic electrolytic solutions, (2) the total sum of all fractions should be more or less 100% and so the results are self-checking, (3) it is an essential tool in establishing

Speciation Studies

45

metal partitioning within natural samples, and (4) chemical extraction sequences can be used for the estimation of the potential remobilization of metals under changing environmental conditions. Microwave-assisted extraction techniques have been very useful in the study of the speciation of metals not only in geological and environmental samples but also for biological samples (Nakashima et al., 1988; Bettinelli et al., 1989). This is because the use of microwave ovens in chemistry provides a very effective means for supplying high temperature in a short time. Extraction rate experiments using conventional and microwave-assisted heating showed that microwave-assisted heating produces results comparable to the conventional procedure. Sequential microwave extraction procedures were established from the results of the extraction rate experiment (Mahan et al., 1987). Other operationally defined speciation schemes include the use of the combination of a chromatographic fractionation of the soil solution followed by the analysis of the fraction by a sensitive analytical procedure. Thus Gardiner et al. (1987) used size exclusion HPLC with ETAAS determination to identify aluminium species in soil solutions. Speciation schemes also make use of combinations of anion and cation exchange columns to fractionate various species of metals like cadmium and lead in soil solution (Tills and Alloway, 1983). Supercritical fluid extraction is fast and becoming an alternative technique to the more conventional methods for soils and sediments (Berger and Perrut, 1990). Table 2 shows, as an example, the most frequently employed sequential extraction procedures for speciation of soils. No actual speciation is performed in terms of IUPAC definition, because it is not possible to attribute the concentration of a metal extracted with a reagent to a chemical form of this element. An intelligent combination between this traditional approach and the chromatographic separation and spectrometric detection, as commented before for water analysis, could contribute to an rigorous picture about the presence of specific chemical forms of the metals extracted in each one of the different fractions of the traditional sequential schemes. One serious concern about the extraction schemes stated in Table 2 and other similar schemes is that the rate of extraction is slow and takes several days to complete the analysis. For example, the procedure of Teisser et al. (1979) needs five overnights for the extraction steps to be completed during the analysis of a sediment sample, whereas the procedure of Miller and McFee (1983) needs more than 77 h. Use of PTFE reactors and microwave-assisted digestion procedures at high pressures and temperatures have usually been effective to overcome this problem (Pickering, 1986; Nakashima et al., 1988; Bettinelli et al., 1989). Another way by which speciation studies have been made rapid is by using the on-line treatment. In an experiment for studying speciation of heavy metals like Cd, Zn, Cu, and Pb by Scokart et al. (1987), a small column was packed with soil and eluted successively with water, BaC12, acetate buffer/EDTA, and HNO 3 furnished by a peristaltic pump. The leachates were directly injected into a ICP-OES in order to analyze immediately the extracted solution. Actually, it gives a kind of chromato-

46

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Table 2. Commonly Used Sequential Extraction Procedures for Extracting Various

Components of Soils and Sediments

Authors

Gatehouse et al. (1977)

Tessier et al. (1979)

Sposito et al. (1982)

Miller and McFee (1983)

Psenner et al. (1984)

Extractant

Fraction

H20 N H4Ac/HAc NH2OH.HCI/HAc H202/HNO3 N2H4.HCI HCIO4 MgCI2 NaAc/HAc NH2OH.HCI/HAc H202/HNO3/NH4Ac HF/HCIO4 KNO3 NaOH EDTA HNO 3 H20 KNO3 Na4P207 EDTA

Water solubles Exchangeables Oxides Sulfides and organics Nonsilicate Fe phases Residuals Exchangeables Carbonates Oxides Organics Residuals Exchangeables Sorbed components Organics Carbonates and sulfides Water solubles Exchangeables Organics Carbonates, Fe occluded (amorphous) Mn-oxide occluded Crystalline Fe-oxide occluded Sulfides Residuals Organics and humics (partially) Humics Carbonates, Fe-hydroxides, sulfides (partially) Kaolinite (partially), sulfides Exchangeables Organics Mn-oxides Fe-oxides (amorphous) Fe-oxides (crystalline) Residuals Exchangeables Carbonates Mn-oxides Fe-oxides (amorphous) Sulfides and organics Residuals

NH2OH.HCI/HNO3 Na-citrate/NaHCO3/Na2S204 HNO 3 HNO3/H202 HCO~/S202NaOH HCI

Hot NaOH Shuman and Hargrove Mg(NO3)2 (1985) NaOCl NH2OH.HCI/NH4Ac (NH4)2Ox Ascorbic acid/oxalate buffer HCI/HF/HNO3 Kersten and Ftrstner NH4Ac (1986) NaAc/HAc NH2OH.HCI/HNO3 Oxalate buffer H202/HNO3/NH4Ac HNO 3

(continued)

47

Speciation Studies Table 2. Continued A u th o r s

Extrac ta n t

Fraction

Zeien and Br0mmer

NH4NO3

Exchangeables (non specifically adsorbed) Exchangeables (specifically adsorbed) Mn-oxides Organics Fe-oxides (amorphous) Fe-oxides (crystalline) Residual Water solubles Exchangeables Soluble organics (solvent extractables) Soluble organics (humic and fulvic acids) Mineral matrix (easily soluble) Mineral matrix (hardly soluble) Insoluble organics Exchangeables Acid solubles Easy reducibles Organics Fe-oxides and hydroxide

(1989)

NH4Ac

Hirner et al. (1990)

NH2OH.HCI/NH4Ac (NH4)2EDTA (NH4)2Ox Ascorbic acid/oxalate buffer HF/HCIO4/HNO 3 H20 NH4Ac C6H6/CH3OH C6H6/CH3OH/KOH

Silva et al. (1993)

Legret (1993)

Nowak (1995)

Campos et al. (1998)

HCI HF HCI/HCIO4/HNO3 Ca(NO3)2 HAc NH2OH.HCI

K4P207 (H N4)2C204/H2C204/ascorbic acid HAc/Ac -1 Na2C204/citrate buffer HNO3/H202 HCIO4/HF MgCI2 NaAc NH2OH.HCI/HNO 3 NH2OH.HCI/HAc HNO3/H202 + NH4Ac/HNO3 HCIO4/HF NaAc (Microwave assisted) NH2OH.HCI/HAc (Microwave assisted) HNO3/H202 (Microwave assisted) + NH4AcJHNO3 HCIO~HF

Note:

Carbonates Fe-oxides, Mn-oxides Organics Residual Exchangeables Carbonates Mn-oxides Fe-Mn-oxides Organics Residual Carbonates, Exchangeables, Water solubles Fe-oxides, Mn~ Organics Residual

The differentextractantsare employedsequentially and eachone providesthe leachingof the noted metal fractions, as indicatedby the authors.

48

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

gram of the chemical species of the metals studied and may be considered as a picture of their availability. When the requirement is not the complete analysis of the soil or sediment but to study the speciation of specific component(s), the modification of the usual extraction sequence has been proposed. An improvement of analytical measurements for single and sequential procedures applied to soil and sediment analysis has been reported by Quevauviller et al. (1993). The study indicated that broadly acceptable extraction schemes are established, e.g. a single-extraction (EDTA/acetic acid/ammonium acetate) method for soil and a sequential extraction (with acetic acid, hydroxylammonium chloride, ammonium acetate and H202) method for sediment. A few specific examples are cited here. For Cd speciation, sequential extractions were performed with H20, Ca-acetate, Na-acetate, NaOH, and Na-citrate (Viereck et al., 1989). Columns packed with Amberlite IR-120 cation exchange resin were used together with Amberlite XAD-2 adsorbent resin to investigate Cd speciation in solutions of sewage sludge-amended soils (Butterworth and Alloway, 1981). Similarly, Cu species may be sequentially extracted using CaC12, HAc, K4P207, oxalate buffer, and HF (McLaren and Crawford, 1973). For marine sediments, extraction of the organic-bounded fraction by hot (100 ~ dodecylsulfatesodium bicarbonate at pH 9.2 has been recommended (Robbins et al., 1984). Speciation studies of aluminium were performed by extracting soil samples with CaC12, and then with KC1 with the separation of aluminium species by the ion-chromatographic method (Gibson and Willett, 1991). For the evaluation of the speciation of tin in finely grained sediments of the Danube fiver, a leaching sequence was presented (F&stner et al., 1981) which consisted of a treatment with ethanol, NH2OH, HC1/acetic acid, oxalate buffer, ~nO4ffrINO3, NH4I, and tartaric acid. Owing to the very high toxicity for marine organisms of tributyltin there is a great interest in its speciation in environmental samples. The sediment samples collected from Porto Vecchio Bay (Corsica, France) were analyzed by three different speciation methods of which extraction of tin species in acetic acid followed by hydride generation, cold trapping, separation by GC column, and determination by ETAAS furnished the best results (Astruc et al., 1989). The extraction of tributyltin from spiked sediment by supercritical fluid (CO 2 containing 5.1M methanol) has been reported by Dachs et al. (1994). Extracts were derivatized with C2HsMgC1 in tetrahydrofuran and analyzed by GC with flame photometric detection. Extraction and speciation of inorganic arsenic, As(III), and As(V) in soil and sediment samples involves extraction with 60% HNO 3 followed by hydride generation-AAS to determine the amount of arsenic compounds such as arsenate, arsenite, monomethyl arsonate, and dimethyl arsinate (Bombach et al., 1994; Gonzalez Soto et al., 1994). In Japanese soils, Takamatsu et al. (1982) developed an analytical technique that consisted of sequential extraction with HC1, HC1/KI, benzene, and H20/H202 followed by anion-exchange chromatography and final determination of arsenic by ETAAS.

Speciation Studies

49

Speciation of Ca 2§ and particulate calcium as Ca3(PO4) 2 needs neither sample preparation nor extraction step, and the species were studied by flame AAS using slurries (Martinez Avila et al., 1991). Speciation of Sb(III) and Sb(V) is difficult in the solid samples because quantitative recovery is only obtained in a mixture of HF+HNO3+H2SO4+HC10 4 which alters the oxidation state. To determine Sb in solid samples the batch hydride generation method is recommended in preference to the automated methods because a clear solution is not obtained in any of the sample treatments used. In FIA and continuous methods, a previous filtration of the suspension is required before analysis to avoid clogging of teflon tubing of the systems (de la Calle Guntifias et al., 1992b). For studying speciation of Fe(II) and Fe(III) in contaminated aquifer sediments, single-step extraction including 1M CaC12, Na-acetate, oxalate, dithionite, Ti(III)EDTA, 0.5M HC1, 5M HCI, hot 6M HC1, and a sequential extraction by HI and Cr2§ were tested on saturated iron minerals and nine aquifer sediments (Heron et al., 1994). Analytical information reported in speciation studies is scarce. The limits of detection obtained are in the parts-per-million to parts-per-billion range. The precision is usually adequate for these determinations (relative standard deviation between 0.4 and 8%). Accuracy data have been reported in a few papers and, in general, these data relate to the determination of the total content of metals and not to the accuracy of the determination of each one of the species or fractions analyzed. Recoveries of total metals in certified sediment samples ranged from 76 to 120% for the conventional procedure and 62 to 120% for the microwave procedure. Recoveries of total metals using microwave and conventional techniques were reasonably comparable except for iron (62% by microwave as against 76% by conventional). Substitution of an aqua regia/HF extraction for total/residual metals results in essentially complete recovery of metals. Precision obtained from 31 replicate samples of the California Gulch (Colorado) sediment yielded about an average of 11% RSD excluding the exchangeable fraction which was more variable (Mahan et al., 1987). In the case of the speciation studies of Sb(III) and Sb(V), very low recoveries (18.8 + 8.4% to 48.2 + 2.2%) have been reported. Again, when quantitative recovery (107.1 + 6.1%) is obtained, the oxidation state of antimony is altered yielding errors in speciation (de la Calle Guntifias et al., 1992b).

B. Speciation of Solid Biological and Food Materials In contrast to inorganic materials where the number of metal species is relatively small, trace metals can be present in living matter in a large variety of chemical forms. The purpose of most investigations into chemical speciation of trace metals in biological systems has been to identify the physiologically active forms, the

50

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

storage and transport species, the metabolites of trace metals containing drugs, and to find sensitive indices of the status of trace metals and their chemical species (Gardiner, 1988). In general, in the investigation of the toxic effects, the speciation of small molecules is of concern, whereas in the investigation of the biological functions the determination of large molecules has priority. Again most of the essential species of trace metals, such as metalloenzymes, are produced in the organism after transformation of the different chemical forms of the metals present in the diet. The content of an essential compound decreases with insufficient dietary metal supply and is thus related to the total metal content. Although speciation of metals is widely recognized as a desirable goal in the analysis of biological materials, it has been particularly the case for clinical and environmental samples, but interest is now being widened to include foods (Dawson, 1986). This is due to the fact that the bioavailability of metal species in food has been correlated with the extractability obtained by enzymolysis with gastric and intestinal fluids (Crews et al., 1983). Quality control measures of toxic metals neglected in many cases in food analysis should be rigorously followed. No effort should be spared in determining accuracy and precision for each separate food matrix, since each foodstuff is a different chemical matrix and requires special control of interferences (Cervera and Montoro, 1994). For speciation studies of solid biological materials, more stringent conditions have to be fulfilled during sample collection, pretreatment, and storage (Gardiner, 1988). It is necessary not only to collect a representative sample and to avoid chemical contamination, but also the integrity of the chemical species has to be maintained. As it has been indicated during and after sampling, changes in parameters such as temperature, ionic strength, pH, redox potential, oxygen level, and irradiation with UV light to which the sample is exposed, can influence the distribution of chemical species. The sampling itself may create a condition that could induce a change. This may affect the types and prevalence of species found for a given metal. In order to minimize the effect of the above changes, it is essential to choose analytical conditions, in particular pH and ionic strength of the working media and do not differ markedly from those found in the original system. The choice of sample storage containers is also a critical factor. In general, the most appropriate action is to deep freeze samples immediately on collection to minimize any bacterial or enzyme degradation, loss through volatility or contamination of the sample. According to Uthe and Chou (1988), biological tissue should be dissected at the sampling site and frozen in liquid nitrogen and then stored in a deep freeze to reduce enzyme release of heavy metals. Most studies on biological matrices are undertaken on wet tissue, rather than lyophilised material, which is either extracted as fresh material or has previously been dissected and rapidly frozen. Pretreatment techniques can only be chosen when the nature of the problem and the information required from the analysis of the sample has been clearly under-

Speciation Studies

51

stood. Again the effect of storage on the distribution and the genuinity of the chemical species are both associated with such problems. For example, as stated above, biological tissue samples after collection are usually frozen until required. But such freezing of samples slows down ongoing biochemical reactions. These would otherwise disrupt the complexes or completely changes the distribution profile of a given element. The extent of damage to any constituent of interest would depend upon the conditions and time period of storage. Mechanical and physical effects of the ice crystals formed could denature proteins and may deactivate enzymes. Also oxidation or reduction of side chains and light catalyzed degradation of various proteins could continue to occur in the material during storage, particularly when transition metals are present (Gardiner, 1993). Usually sample preparation procedures for biological materials fall into two categories. First, those methods where the sample is analyzed after a minimal pretreatment by using slurry or solid sample introduction systems, or alternately after dilution with an appropriate reagent for deproteinization, metal release, or cell disruption. Second, there are methods where considerable sample pretreatment such as separation, extraction, or destruction of the organic component by heat, solubilizing agents, or oxidizing agents is necessary before determinations can be carried out. The information required for speciation studies in case of biological materials may be the oxidation state of the metal, the concentration of molecular species, or the identity of the organometallic complex. The most common technique for obtaining such information involves separation procedures which isolate selectively the species of interest (Yu et al., 1983), or a separating column which generates a series of fractions containing organometallic complexes (Chilvers et al., 1984). Most of the separation modes available for liquid chromatography such as size exclusion, normal phase, reversed phase, paired ion reversed phase, and ion-exchange should be applicable to biological systems. Of these, fractionation according to size has been most frequently used. When chromatographic techniques are used to separate organometallic complexes, information on the concentration of organic molecules may also be obtained concomitantly from the elution pattern. For efficient separation of sensitive nonstable species, gel permeation chromatography with aqueous eluents is used. Other techniques like ion exchange may have the problem of loading the species with separation of specific charges and/or substrates from either the column matrix or the buffer systems. This may result in changes of the original species. Thus, leaching procedures are not usually used in speciation analysis of biological samples. For stable species only, electrophoretic separation can be applied in the form of continuous flow-through or a capillary system (Dunemann, 1992). Biological samples can also be extracted by blending as a paste or slurry with a selected solvent or solvent mixtures (Gui-Bin et al., 1989; Wells, 1988). Sample tissue or plant material can be pulverized in liquid nitrogen to break up the matrix.

52

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Ionic lead species in grass and tree leaves may be solubilized with tetramethylammonium hydroxide from the crushed leaves in sodium diethyldithiocarbamate prior to extraction with pentane (Van Cleuvenbergen et al., 1990). Organometals are more strongly bound to the protein and for studying these systems, rupturing of the protein-metal linkage is required. In such cases, enzymes can be used to destroy the substrate and release the organometal intact. For example, pepsin can be used to rupture the S-S bonds and bacterial protease K can be used to release organometals bound to proteins (Wells, 1992). Recently, successful use has been made of pressure digestion in a microwave oven as an alternative procedure for solubilizing samples because of the advantages it offers in reducing digestion time and reagents, and decreasing contamination problems. Unfortunately, under these conditions, some species can be changed. Supercritical fluid extraction is more suitable to isolate labile compounds and is an ideal technique for on-line automation. The method has been applied for speciation studies in biological tissues (Lopez-Avila et al., 1990). For most speciation analyses, it is important to remove the preferred species from other undesirable components of the sample matrix by selective extraction of either fat or water-soluble forms. After selective separation, sensitive detection procedures followed by effects on biological systems, uptake, and toxicological experiments to mimic real processes are performed for complete speciation studies. Various schemes like that shown in Figure 15 are used to fulfil the requirements of speciation analysis. Evidently, there is no unique procedure for speciation studies of all different types of biological materials. Depending on the nature of the material, some direct or combined method may be suggested. In some cases chemical species can be measured directly in the sample without interference from other components. For example, with several metal-containing enzymes, the enzymic activity can be taken as a measure of the concentration of the compound. Again enzyme systems specifically affected by the toxic form of the metal may be monitored so that the inhibition of the enzymic activity can be taken as a measure of the toxic species. Immunoassay may be another direct way for the speciation of trace metal compounds with antigenic properties. On the other hand, various combined methods can be used in speciation studies. Some metal species can lend themselves to relatively simple procedures, e.g. volatilization of compounds or a single absorption or extraction step. For the speciation of small molecules like organometallic compounds, gas chromatography or high-performance liquid chromatography have been combined with analytical methods such as AAS and applied for routine analysis. A critical examination on this subject reveals that major work has been done on the speciation of arsenic followed by that of mercury. Work on the speciation of other metals in solid biological materials has not been achieved in many cases. For the speciation of arsenic chemicals by atomic spectrometry, a series of procedures based on the use of a selective liquid-liquid extraction have been

Speciation Studies

53

[ Homogenate [ [Digestion for total met'alcontents 1 ~

~%~

Direct spectroscopic investigations]

~Extraction of water soluble species]

Characterization, Stability, , Capacity

j

01uble ecies I

[ Detection of all chemical species I

"x

[Effects on biological and geological~stems !

!

[ Mobility', Availability, Toxicity I Figure 15. General scheme that may be applied for metal ion speciation studies in solid biological materials (reproduced from Das et al., 1995). developed. The selective determination of inorganic arsenic and organic compounds in foods has been performed by Mtinz and Lorenzen (1984) by liquid-liquid extraction and hydride generation. Previously mineralized samples with HNO 3 were treated with ascorbic acid to reduce As(V) to As(III) extracted from 7-9 M HC1 into CHEC12. Inorganic arsenic was determined after a back extraction of the CH2C12 phase with water; the determination of organic arsenicals was achieved by a evaporation of the remaining HCI phase, decomposition with HNO 3, and subsequent measurement by ETAAS. Inorganic arsenic species found in foods may also be selectively extracted with diluted H2SO4 from the lyophilized solid samples. Under these conditions organically bound arsenic remains stable (Vaessen and Van Ooik, 1989). For the determination of total inorganic arsenic in fish, solid samples were treated with NaOH and Na2SO 4 on a boiling water bath. Subsequently HC1 was added and arsenic was extracted with APDC in methyl isobutyl ketone (MIBK). The arsenic was back-extracted with HNO 3 and then treated with HESO4 before measurements performed by a hydride generation technique (Brooke and Evans, 1981). Inorganic As(III) and As(V) were determined in biological materials after solubilization with HC1, extraction into toluene, and back-extraction with water. Organic arsenic was retained in the acidic solution. The total arsenic was determined after digestion with HNO 3, HESO4, and HC10 4 (Yasui et al., 1978). For selective determination of As(III) in foods, in the presence of As(V), the liquid-liquid extraction of AsC13 in

54

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

CHCI 3 was proposed (Holak and Specchio, 1991). A method for the separation of arsenic species from solid samples of orchard, tomato, and oak leaves has been described by Sarx and B~ichmann (1983). The volatilized arsenic species were separated into various fractions and determined by ETAAS and MIP-AES. The major form of arsenic in fish and crustaceans is arsenobetaine. Although this compound has been shown to be nontoxic, for reasons of toxicological reassurance its concentration and that of other arsenicals in foodstuff of marine origin must be determined by a routine method (Morita and Edmonds, 1992). Extraction of arsenobetaine from fish tissue homogenates can be performed using CHCI3/CH3OH mixtures. Recovery of 101 + 4% was reported for the 1:2 solvent mixture extraction of freeze-dried fish muscle spiked with arsenobetaine (Beauchemin et al., 1988). For routine analysis of arsenobetaine in marine samples, HPLC/ICP-AES would be a good choice in terms of sensitivity, selectivity, and ease of operation (Morita et al., 1981). Better sensitivity could be achieved with HPLC/ICP-MS (Morita and Edmonds, 1992). In the determination of organoarsenic species by HPLC/ICP-MS, dried fish was extracted with CHC13 or aqueous trypsin, and the extracts were analyzed on a HPLC column containing anion-exchange resin with a mobile phase of alkaline potassium sulfate and ICP-MS detection (Branch et al., 1994). Recoveries ranged between 80 and 115% for arsenobetaine and arsenate. No monomethylarsonic acid was observed in any of the fish, though dimethylarsinic acid was detected. The method proposed by Lunde (1973) for the separation and analysis of organic-bound and inorganic arsenicals in marine organisms by NAA and X-ray fluorescence spectrometry (XRF) was based on the extraction of arsenic as AsC13 and subsequent removal by distillation at 100 ~ It can be adapted for the analysis of inorganic arsenic in seafood products by ETAAS after in-line microwave-assisted distillation of AsC13 (L6pez et al., 1994). Although the most popular way for the speciation of mercury is based on the selective reduction to Hg ~ following the procedure proposed by Magos, organic mercury can be determined by solvent extraction into toluene and ETAAS. For this purpose, the solid residue of an extract in acetone from homogenized biological materials is treated with acid bromide and CuSO 4 and then the mercury extracted into toluene (Shum et al., 1979). The extraction of CH3Hg+ as bromide from fish samples by CHC13 and its direct determination in organic medium by cold vapor atomic absorption spectrometry was described by Rezende et al. (1993). Fresh fish muscle was cut into small pieces, prepared as a homogenate, and mixed in a centrifuge tube with water, H2SO 4, and KBr. Extraction with CHC13 were performed and methylmercury was determined by mixing the organic extract with 50% HNO 3 in DMF and 2% NaBH 4 in DMF (1:2:1). A procedure for the determination of organic and inorganic mercury in biological materials like fish, oyster, meat, milk (dried), banana (dried), and wheat flour by ETAAS has been reported (Filippelli, 1987). To the homogenate tissue, NaC1 in HC1 and benzene were added. Organic mercury extracted as chloride was reextracted by thiosulphate solution. The organic

Speciation Studies

55

mercury thiosulphate extract was next treated with CuC12, reextracted in the benzene layer, and analyzed by GLC for speciation. Inorganic mercury was converted into a methyl chloride derivative by methanolic tetramethyltin prior to extraction. Arakawa et al. (1981) reported the determination of tetra- and trialkyl-tin homologues in mammalian tissues. This technique involves isolation of alkyltins from tissue homogenates as chlorides by simultaneous extraction with HC1 and ethyl acetate, followed by extraction with n-hexane and stepwise elution with n-hexane+ethyl acetate on a silica gel column. The column eluate is injected directly into a gas chromatographic column. The method involves at least nine steps requiring six transfers of sample. The time-consuming process was improved by Means and Hulebak (1983). In their method, methyltin compounds (monomethyl-, dimethyl-, trimethyl-, and tetramethyl-) are purged from freshly homogenized mouse kidney and brain tissues using NaBH 4. The volatile organotin hydrides produced are cryogenically trapped on the head of a gas chromatographic column (at-40 ~ and eluted. The compounds are detected using selected ion monitoring in a mass spectrometer. The use of enzymes in the extraction procedures of chemical species from solids indicates clear differences in the solubility of species as iron and manganese compounds by pectinase from oat flakes and iron by papain/lipase from wheat germ (Schwedt and Neumann, 1992). The application of gradient gel electrophoresis provides a useful method for the separation of zinc, nickel, and copper species in soybean flour extracts (Dunemann and Reinecke, 1989). The separated substances were electrophoretically eluted, followed by flame AAS determination of the metals. It is a sensitive separation method with a high-resolving power that can provide information of the metal species, e.g. molecular size, charge, and stability. Speciation of zinc and cadmium in different vegetable foodstuffs has been reported (Gtinther and Waldner, 1992) where the cell breakdown was carried out by liquid shearing of the crushed plants in a Tris-HC1 buffer (pH 8). The homogenates were separated into cytosols and pellets by centrifugation. The cytosols were again separated by gel permeation chromatography. The metal in each fraction was determined by ETAAS after wet digestion in HNO 3. Chau et al. (1984) used tetramethylammonium hydroxide as tissue solubilizer to dissolve biological samples like fish without altering the chemical species of the alkyllead species. The various alkyllead species were isolated quantitatively by chelation extraction with sodium diethyldithiocarbamate, followed by n-butylation to their corresponding tetraalkyl forms, R,,PbBu(4_,,) and BuaPb, respectively (R = Me, Et), all of which were determined by GC/AAS method. The method could determine simultaneously the following species in one sample: tetraalkyllead (Me4Pb, Me3EtPb, MezEtzPb, MeEt3Pb, Et4Pb), ionic alkyllead (MezPb 2+, EtzPb 2+, Me3Pb § Et3Pb+), and Pb 2§ ion.

56

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Methyl- and butyl-tin and inorganic tin in oysters were extracted from frozen tissue with methanol and hydrochloric acid. Then the extracts were treated by a hydride generation-AAS method (Han and Weber, 1988).

C. Speciation of Miscellaneous Solid Samples Different studies have been described on metal speciation in atmospheric particulates, wood, and coal in recent years and are described below. Several experimental procedures, varying in manipulative complexity, have been proposed for determining several chemical species of particulate trace metals. Thus atmospheric particulates collected on a glass fiber filter can be acidified by soaking in HC1 and extracted by shaking with toluene, ethyl acetate, or methanol, or by refluxing in a soxhlet. The continuous soxhlet extraction is sometimes preferred since the cycling time can be controlled and the exhaustive technique exposes fresh solvent, which can percolate into the pores of the particles like fly ash which can have a hollow structure (Wells, 1992). A sequential extraction procedure, originally applied for the study of soil sampies, has been used for the analysis of urban dust particles (F&ster, 1986). Exchangeable metals were determined by extraction with MgC12, metals bounded to carbonate by leaching with and acetic acid buffer, metals bounded to Fe-Mn oxides by extraction with hydroxylamine at 96 ~ those bounded to organic matter by extraction with H202 at 85 ~ and residual metals by digestion with HF-HC104 mixture. Chemical species of iron as iron metal, iron soluble (H20, H+), Fe 2§ Fe2§ Fe3§ Fe304(magnetite ), Fe203(hematite ), and FeO(OH) (goethite) have been determined in atmospheric particulate samples. Filtration, ultrafiltration, ion chromatography, extraction (H20, 0.02M citric acid, 0.1M HC1), and Mtissbauer spectroscopy have been used for their characterization (Dedik et al., 1992; Hoffmann et al., 1994). Speciation of Cu, Pb, Zn, and Cr in dust of different origins and particle diameters has been reported by Gao et al. (1993). Major parts of these metals were bounded to organic matter, Fe-Mn oxides, and the rest as residuals. The exchangeable metals bound to carbonates and Fe-Mn oxides of these four heavy metals in small particles were superior to those in large particles. This illustrated that heavy metals in small particles are more active and toxic; the activity order being Zn > Pb > Cu > Cr. Determination and speciation of ionic alkyl-lead compounds have represented for the analytical chemists a serious challenge. In the environment these compounds have an anthropogenic origin but they may also be produced from biological transformations of Pb 2§ Triethyl-lead, tetraethyl-lead, and other alkyl-lead species in environmental samples have been studied by GC-AAS after extraction (Wong et al., 1989). Possanzini et al. (1992) developed a method for the speciation and separate determination of inorganic NH~ salts in atmospheric aerosols. The speciation and

Speciation Studies

57

quantification of organosilicon compounds at subparts-per-million levels in environmental and industrial samples require the separation of the compounds of interest coupled with a sensitive and selective detector (Dorn and Skelly Frame, 1994). The combination of HPLC for separation and ICP-AES for detection is ideal for the determination of various organosilicon compounds. Separation of nonpolar, high molecular mass poly(dimethylsiloxane) polymers was done by size-exclusion chromatography with tetrahydrofuran or xylene as the mobile phase. A gain separation of polar, low molecular mass silanols was accomplished by reversedphase HPLC with water-acetonitrile as mobile phase. Methods for the determination of Cu, Cr, and As and for speciation of Cr(VI), As(III), and As(V) in wood and dust from impregnated timber (with those metal ions preservatives) were given by Nygren and Nilsson (1993). Weak-acid-soluble Cr(VI) was determined by soaking a dust sample in Na acetate buffer. For speciation studies of arsenic, a dust sample was leached in 6M HC1 for 1 h and the extract treated with 12M HCI and toluene. As(III) was reextracted from the organic phase and determined by ETAAS. After separation of As(III), in the remaining solution As(V) was determined after reduction with KI and followed the same procedure. Brown coal samples from different deposits have been analyzed (Vogt, 1994) for the binding forms of their inorganic components. Besides the analysis of dried coals, ashing techniques and extraction procedures with different solvents (acids, bases, complexing agents, organic solvents with different polarity) have been investigated. ESCA, photon-induced X-ray emission (PIXE) spectrometry, NAA, ICP-AES, nuclear magnetic resonance (NMR) spectrometry, and ion chromatography have been applied to the analysis of coals, ashes, wet ashes, and extraction products. The bonding behavior of more than 40 metals were characterized. Species identified in such cases were kaolinite, muscovite, calcite, apatite, iron pyrites, siderite, mica, rutile, quartz, bassanite, gypsum, anhydrite, and hematite. On the other hand, major species in the coal ash were SiO 2, CaO, Fe203, A1203, and MgO. Although the methodologies of the speciation studies included in this section are described in details, except in a few cases, the analytical characteristics are not stated, e.g. LOD (Dorn and Skelly Frame, 1994): 1-4 ng Si for methylsilanediols, 4-5 ng Si for polydimethylsiloxane. Recovery percentages and coefficient of variation values are indicated only in some cases (Nygren and Nilsson, 1993). For Cr(VI), As(III), and As(V) they are, respectively, 94%, 1-8%; 82%, 4-5%; and 89%, 4-12%.

VII.

SPECIATION STUDIES OF DIFFERENT METALS

Through this section the reader will find, alphabetically by metals, a series of comments on the methodologies developed in the scientific literature for the isolation, identification, and quantification of different species of a single metal in different types of samples.

58

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

We have tried to provide a guide to the literature in practical applications of speciation studies by atomic spectrometry. This includes some important details on complex procedures and a complete as possible list of references for studies in which natural or real samples were analyzed, and not only synthetic mixtures of standard solutions. As can be seen, the studies performed until now were based on different methodological approaches and, in some cases, not true speciation was carried out. However, as indicated in the first section, some of these studies based on sequential extraction or on the grouping of species as a function of their reactivity can be improved by an appropriated selection of chromatographic or electrophoretic conditions applied to the crude extracts, thus providing a complete picture of the chemical species included in each group. For those metals, on which several studies have been published in the scientific literature, it has been considered a specific subsection. However in Table 3 it can be seen some details on speciation studies for a miscellaneous metals, concerning uncommon speciated compounds.

A. Aluminium Aluminium is present in water at different concentrations; usually its concentration is lower in groundwaters than in surface waters, but extremely dependent on the pH conditions. Furthermore, aluminium salts are employed in the therapeutic treatment of hyperphosphatenia that arises in renal failure. Due to this causetreatment, A1 speciation has been mainly performed in waters (Noller et al., 1985; Hirayama et al., 1994) and in clinical samples (Wang and Barnard, 1992; Danielsson et al., 1995; Wu et al., 1996). Other attempts have been performed in biological samples (Oughton et al., 1992; Noelte et al., 1995) and in soil samples (Mitrovic et al., 1996). The most commonly employed instrumental techniques have been ETAAS (Noller et al., 1985; Wu et al., 1996), ICP-AES (Hirayama et al., 1994; Danielsson et al., 1995; Fairman et al., 1995), and ICP-MS (Lu et al., 1994). Also used include the coupling between chromatography and atomic spectrometry, such as LC-ICP-AES (Mitrovic et al., 1996), HPLC-ICP-AES (Noelte et al., 1995), HPLC-ETAAS (Van Lendeghem et al., 1994; Wrobel et al., 1995), and SECETAAS (Keirsse et al., 1987). Aluminium species have not been well established and authors refer to them as labile aluminium (Wu et al., 1996), monomeric A1 (Mitrovic et al., 1996), or fast reactive A1 (Fairman et al., 1995). However it has been shown that the presence of A1 can be complexed to some molecules such as Al-transferrin (Van Lendeghem et al., 1994; Wrobel et al., 1995), Al-dopamine (Wang and Barnard, 1992), or Al-fulvic acid (Morrison, 1990). Furthermore, there are discussions of different species of aluminium but without identifying them (Noller et al., 1985; Hirayama et al., 1994; Danielsson et al., 1995).

Table 3. Analytical Procedures Proposed in the Literature for Speciation of Miscellaneous Elements Analytical Figures of Merit Species

Matrix

Ag

Galena

Borate monoester and diester, boric acid

Radish roots

kO

Technique FAAS

Procedure

Galena was mixed with EDTA-NH 3 solution, and acetone cyanohydrin solution. The mixture was stirred for 15 min and filtered. Isomorphic Ag in the filtrate was determined. For the determination of metallic Ag, the residue was further treated with HNO3-Fe(NO3) 3 solution and stirred at room temperature for 1 h. The mixture was filtered and the filtrate was analyzed. For the determination of Ag in AgS, the residue was treated with H2SO4-thiourea, stirred at room temperature for 30 min, filtered and the filtrate was analyzed. The resulting residue was treated with HNO 3 and H2SO4 for determining other Ag. HPLC-ICP-MS Radish roots were grated and the homogenate was centrifuged. Portions of the supernatant juice was dialyzed against H20 at 4 ~ The dialyzed juice was freeze-dried and dissolved in 1 mL H20. The solution was filtered through a membrane and a portion of filtrate was injected onto a YMC-Pack Diol-120 SEC column with 0.2 M-ammonium formate buffer of pH 6.5 as mobile phase and ICP MS detection. The mass spectrometer operated in selected-ion moring mode at m/z 11 ; only 1.4% remained in the dialysis tube. Thus the amount of B-macromolecule complexes present was very small and most B compounds were of low molecular weight.

RSD

RANGE/L OD

Reference Zhang and Shao (1990)

Matsunaga et al. (1996)

(continued)

Table 3. Continued

Analytical Figures of Merit Species

Matrix

Bromide Bromate

CIO~ Br-/ BrO~

Cl-

ClO~

Drinking water Human urine

Technique

Procedure

IC-NTI-IDMS A spiked sample was applied to an anion-exchange IC-ICP-MS column of Dowex Agl-X8. Elution was with NH4NO3/NH 3. The fractions were acidified with HNO 3 and sodium sulfite was added to the bromate fraction. Next, AgNO 3 was added to both fractions followed by PTFE microfiltration. The precipitate was dissolved in 25% NH 3 and deposited on the evaporation filament of a thermal ionization source for analysis by negative thermal-ionization isotope dilution MS. After evaporation to dryness, approximately 300 ng lanthanum was deposited on the filament. The filament was heated stepwise to 1100-1200 ~ Measurements were made at m/z 79 and 81. ICP MS was also used with a channeltron electronmicro tiplier detector. HPLC-ICP-MS Sample solution was subjected to HPLC on a gel permeation column adjusted to pH 6.8 with NH 3. The column outlet was connected by a PTFE tube to the inlet of the pneumatic nebulizer for ICP MS. This method permitted the simultaneous determination of IO~, BrC~, CI-, CIC~3, Br- and I-; the MS response did not depend on the chemical form of the element. ICP-MS The ICP mass spectra of 0.2 M HCl and 0.2 M HClO 4 were compared and differences were noted. Similar studies were made of solution of salts of each acid with nine common rock-forming metals.

RSD 0.3-1.2% (n = 4-5) 0.4-6% (n = 4-5) ICP-MS 5.5%

RANGE/L OD

Reference

10-500 ng/mL Diemer and 0.09 ng/mL Heumann 1-50 ng/mL (1997) 0.03 ng/mL

(n = 3)

13% (n = 3)

25pg/36ng/-

o.sng/-

Salov et al. (1992)

Longevich (1993)

C N- total CN- free

Co Cobalamins

Waters

AAS

LC-FAAS

A method is described for the determination of total and free CN-, together with weak acid cyanides and cyanides not amenable to chlorination. To sample (pH 12) were added NaOH and NaH2PO4, then the solution was pumped through a Ag filter and on to the spectrometer. The signal from the Ag(CN)~ complex was compared with that of CN- standards to yield the free CN-. With the sample still being pumped, a photocell was placed inline before the Ag filter until a steady signal was obtained, pumping was stopped, then the solution was irradiated for 31 rain and analyzed as above to give the total CN-. A further sample was adjusted to pH > 12 with KOH and chlorinated with Ca(OCl) 2, with the solution protected from light. Residual CI 2 was eliminated with ascorbic acid, the solution was aged > I h, the ppt. was removed, the filtrate was mixed with NaH2PO 4, and the solution was analyzed as for total CN-. Aqueous Co and four naturally occurring cobalamins, namely, cyanocobalamin (vitamin BI 2), hydroxocobalamin (vitamin B12a), 5'deoxyadenosylcobalamin (coenzyme BI 2) and methylcobalamin (methylcoenzyme B I 2) were determined. LC was carried out on a Spherisorb ODS-2 column coupled to a guard column packed with the same stationary phase. Methanol and phosphoric acid adjusted to pH 5.2 with triethanolamine were used as the mobile phases with gradient elution from 25-60% methanol in 5 min. The column was connected directly to the nebulizer of the FAAS.

Rosentreter and Skogerboe (1991)

lOm~L (n = 1O)

5% 25mg/L (n = I O) 4-6.7%

0.5-25 mg/L Vinas et al. 0.35mg/L (1996)

12-300mg/L 4.2-5mg/L

(continued)

Table3. Continued Analytical Figures of Merit Species

Matrix Wine

Technique FIA-AAS

Procedure

Wine was injected into the H20 carrier stream and merged with the ionic strength adjustor. The stream leaving the electrode system was merged with a H20 diluting stream and mixed in a 60 cm coiled tube. The sample plug flowed through to the flame AAS system for measurement of total K. Mn +2 non Noncolloidal MnO 2 was removed by membrane Culture media HPLC-AAS colloidal filtration and the material on the filter was examined by several chromatographic techniques. Manganese in the MnO 2 various fractions was determined by polarography or AAS. Water HPLC-ICP-MS A quantitative chromatographic separation of the Mo-disanalytes with at least two stable or long-lived radioactive solved organic matter isotopes of the analytes free from spectroscopic interference. Advantages of the method include: (i) high sen; (ii) ideal internal standardization using enriched spike isotopes of the same element; (iii) elimination of matrix effects and calibration drifts; (iv) easy calibration; (v) analysis of species with unknown structure and composition is possible; (vi) better characterization. ParaFAAS Samples containing either paramolybdate or thiomolybdate standard solutions at different molybdate Thioconcentrations were divided into six groups. With the exception of the first group, the sample (10 mL) was molybdate digested with 4.5 mL HNOg/HCIO4/H2SO 4 (4:4:1) followed by heating. After cooling to room temperatures the digest (0.5 mL) was neutralized with 4 M NaOH. The third, fourth, fifth and sixth group were then treated with 5 mL 0.56 M NH4OH, 1.2 M HNO 3, 0.32 M M3PO 4 and 300 mg/L aluminium nitrate before being diluted to 20 mL with H20. Analysis was by AAS using a nitrous oxide-acetylene flame. The proposed oxidative treatment ensured the quantitative conversion of TTM into PM or probably the conversion of both of them to another but the same species. K(free) K Total

RSD

RANGE,/L OD 300-2000 mg/L

Reference Rangel and Toth (1996)

Hoffmann and Schwedt (1983) Rottmann and Heumann (1994)

0-1 O0 mg/L

Vougaropolulos et al. (1995)

p034P20~-Tri phosphates Polyphosphates (P4PI2)

DCP-AES

Combustion Phosphate pyrophosphate trimetaphosphate

ICP-AES UV NMR

SiO 2 soluble Pure water SiO 2 coloidal

ICP-AES

The sample of polyphosphoric acid, adjusted to pH 7 to 8 with tetra-alkylammonium hydroxide, was injected on to a column of Hamilton PRP-I (styrene divinylbenzene copolymer) and an 18 min gradientelution program was carried out with I to 40% [0.01Mtetraethylammonium nitrate (pH 9) containing 0.1 M KNO3]. Known amounts of phenylphosphonic acid were contained within a portion of ashless filter paper and ignited within an 02 combustion flask containing 10 mL aqueous 0.6% H20 2. After shaking and allowing the solution to stand for a few minutes, the flask contents were quantitatively made up to 25 mL with H20. Portions of this solution were analyzed on a 10 micro m Waters IC-PAK column with gluconate/borate/acetonitrile as mobile phase and conductivity detection. Analysis of total P in the absorbing solution by ICP AES, spectrophotometry and 31P NMR showed the speciation of P as phosphate, pyrophosphate, and trimetaphosphate. Water was concentrated by freezing. Soluble and colloidal silica were determined spectrophotometrically at 800 nm by the heteropoly blue method. Total silica in the H20 concentrate was determined by ICP AES.

0.2 l~g

6.6%

(n = 8)

6%

(n=8)

Biggs et al. (1984)

Umali et al. (1995)

5-200 ng/mL Miwa et al. (1988)

(continued)

Table 3. Continued Analytical Figures of Merit Species

4~

Matrix

Technique

Procedure

Si Si-proteins Human serum HPLC-ETAAS Serum was diluted with 0.02 M Tris hydrochloride of pH Electro7.4 containing 0.01 M NaHCO 3. A portion of the resulting solution was analyzed by HPLC on a I 0 micro phoresis m Protein-Pak DEAE-5PW column gradient elution. Serum proteins were detected by UV absorption at 220 or 280 nm. For AI and Si detection, fractions of column eluate were collected and analyzed by ETAAS. Binding of serum proteins to Si was studied by further separating the HPLC column fractions by SDS-PAGE. Si was not bound specifically to any one serum protein. The effect of desferrioxamine (DFO) on Si speciation in serum was also studied. DFO had no effect on Si speciation. Poly(diLC-DCP-AES Poly(dimethylsiloxane) oligomers and TMS derivatives of methylsiloxpolysilicates in Na silicate solution were separated by ane) HPLC on a Zorbax ODS column, with gradient elution oligomersSi with ethyl acetate in acetonitrile - acetone (7:3) and hexamethyldisiloxane as external standard. The eluted silicon species were detected by dc plasma AES. Sulfate S2O2- Waste water Electrospray Solutions were prepared by dissolving sulfur salts in H20 Thiosulfate MS to form stock solutions and subsequent dilution of these solutions with methanol. Electrospray mass spectrometry S40~S2Og(ESMS) was employed for selected ions monitoring. Te(IV) Natural water HG-GFAAS Method involves the selective reduction of Te(IV) by Total Te Aerosol NaBH 4 to H2Te, which is trapped in a modified graphite particles furnace for determination by AAS. Total Te is determined after reduction of Te(Vl) to Te(IV) by boiling with HCI, and Te(Vl) is determined by difference.

RSD

RANGE/L OD

Reference Wrobel et al. (1995)

1-5%

0.25 ng s-1

Biggs et al. (1987)

Vinas et al. (1996) 5.1% 20 pg 2.5% 200pg

2-4 pg

Yoon et al. (1990)

Te(IV) Total Te

Sea water Rain water

GFAAS

TI(1) TI(III)

Sea river waters

FAAS

Te(IV) and Te(Vl) were co-precipitated with Mg(OH) 2. The ppt. was redissolved in 6 M HCl, the solution was diluted to 3 M HCI and NaBH 4 was used to convert Te(IV) into Tell4, which was trapped inside a graphite tube. Sexavalent Te was reduced to Te(IV) by boiling with 3 M HCl, followed by determination of total Te. An acidified sample containing TI(III), was mixed online with I M acetic acidlNH4Cl buffer of pH 10.2 and the solution passed through a column containing of controlled pore glass beads with immobilized quinolin-8ol (Q-8). The sample line was washed with buffer. Analytes were eluted with I M HClII M HNO 3 before nebulization and flame AAS. The major ions in seawater, namely Na(1), Ca(ll) and chloride, did not interfere with TI(III) preconcentration and the tolerable limits of I0 mglL of Cu(ll) and I0 mg/L of Fe(lll) or AI(III) in the presence of 0. I M fluoride permitted analysis of river or seawaters. TI(1), present in most samples was not retained on immobilized Q-8. Oxidation with Br to TI(III) and removal of excess Br with phenol proved satisfactory. Synthetic mixtures of TI(1) and TI(III) in 3% HCl were analyzed satisfactory and the procedure was applied to the determination of thallium in K-enriched table salt.

Andreae (1984)

10-20%

up to 400 ng/mL 3 ng/mL

Mohammad et al. (1994)

(continued)

Table 3. Continued

Analytical Figures of Merit Species u(Iv) u(vl)

Note:

Matrix

Technique

Natural waters ICP-MS

Procedure Samples were prepared to contain 0.125 M oxalic acid and 0.25 M HNO 3. They were applied to a column of Dowex AG 50W-X8. Fractions were collected and analyzed with Bi as internal standard. The U(IV) was eluted as a sharp peak in the first four fractions and the U(VI) as a broader peak in fractions 9-19. The separation was due to the U(IV) being mainly present as tetraoxalatouranate and the U(Vl) being mainly present as uncomplexed UO 2§ The presence of Ca(ll) in the sample caused a broader U(VI) peak and low recovery, probably because of competition for the exchange sites.

RSD --

RANGE/LOD 1 l.tg/L

Reference Duff and Amrhein (1996)

In this table only elements for which less than two papers were found in the scientific literature are included. For other elements latter studied than these the reader can found details on the text. RSD: Relative standard deviation in percentage. LOD: limit of detection.

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67

In natural waters it has been found that A1 is bound to a broad size range of humic substances and inorganic A1 was detected unrepeatable by ETAAS, possibly due to the presence of polymeric forms (Zernichow and Lund, 1995). An interesting work employs kinetic studies to differentiate A1 components and demonstrates that snow contains two kinetically distinguishable A1 components and riverwater contains three A1 species (Lu et al., 1994). Total dissolved A1 has been determined by direct analysis of filtered and acidified water by ICPMS (Morrison, 1990). For the determination of inorganic A1 in the presence of model and neutral organic ligands, a pyrocatechol violet solution was added to filtered water followed by addition of hexamine buffer solution (pH 6.1) and the mixture was set aside for 5 to 69 min. The absorbance of the solution was measured at 585 nm. The measurement of A1 is critically dependent on reaction time. Short reaction times do not indicate the presence of inorganic monomeric A1, but the complex reaction can be used to indicate A1 bound to fulvic acid. In urine, A1 exists in three forms as (1) nonlabile AI(III) complexes, (2) labile monomeric AI(III), and, (3) nonlabile monomeric + labile AI(III) complexes (Wu et al., 1996). For measurement of (2) + (3), as total labile AI(III) [TAL]: potassium hydrogen phthalate was added to urine and the pH adjusted to 4.7; lumogallion reagent was added and, after 20 min, the fluorescence intensity was measured at 574 nm (excitation at 497 nm). For the determination of (3), urine (15 mL) was applied to an ionexchange column (3 mm i.d.) packed with Amberlite IR120 Plus resin in Na § form and 5 mL of the middle fraction was collected and analyzed by fluorimetry as above. The concentration of the nonlabile complexes (1) was found by subtraction of TAL from the total A1 concentration determined by ETAAS. Serum has been analyzed for A1 speciation (Wrobel et al., 1995). The sample was diluted fourfold with 0.02 M Tris hydrochloride at pH 7.4. A portion of the resulting solution was analyzed by HPLC on a Protein-Pak DEAE-5PW column. Serum proteins were detected by UV absorption at 220 or 280 nm. For A1 and Si detection, fractions of column eluate were collected and analyzed by ETAAS at 309.3 and 251.6 nm for A1 and Si, respectively. Binding of serum proteins to A1 and Si was studied by further separating the HPLC column fractions by SDSPAGE. Transferrin was the only serum protein that bound A1, whereas Si was not bound specifically to any one serum protein. The effect of desferrioxamine (DFO) on A1 and Si speciation in serum was also studied. DFO affected A1 speciation but had no effect on Si speciation. No interrelations between A1 and Si species were found to justify significant aluminosilicate formation in human serum. The speciation of A1 bound to serum transferrin was studied by Van Landeghem et al. (1994). A1 was bound in vitro to transferrin. After filtration, the solution was analyzed by HPLC on a poly(ether ether ketone) column of Biogel TSKDEAE5PW with gradient elution. The mobile phase was passed through a scavenger column of ODS in order to remove extraneous A1 before elution. Fractions were then acidified with HNO 3 and analyzed by ETAAS. Protein recoveries were in the range 95-105%.

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M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

Some attempts have been carried out to control analytical procedures by interlaboratory studies (Fairman et al., 1994). The value of a quality control program whereby solutions containing 11 and 55 ktg L -1 of A1 in 0.01 M HNO 3 are analyzed periodically over the test period was demonstrated. Samples containing 100-200 ktg L -1 of A1 gave the best speciation results. The DriscollPCV method was fully portable and gave an RSD of 15% for toxic labile monomeric A1 in the more stable samples. Five methods have been described for the determination of aluminium in waters by the United Kingdom Department of Environment in 1988. These involve spectrophotometry with catechol violet or with bromopyrogallol red, fluorimetry with lumogallion, DPASV with a hangingmercurydrop electrode, and ICP-AES. The limit of detection, upper limit of rectilinear range, and time taken per sample, respectively, are: 13 ~g L -1, 0.3 mg L -1, and 90 min for 10 samples; 16 l.tg L -1, 0.8 mg L -1 and 30 samples per hour; 0.17 l.tg L -1, 55 ktg L -1, and 16 samples in 270 min; 27 ng L -1, 13.5 ktg L -1, and 20 min per sample; for ICP AES the limit of detection was 2 ktg L -1 with detection at 396.15 nm, and 10/.tg L -1 at 167.08 nm. The A1 species determined, the types of water sample, and each method were discussed. Size exclusion chromatography is considered to be a valid technique for studying the speciation of A1 in biological fluids, but one must be careful about contamination of the eluent buffer by A1 because it is the principal source of error (Keirsse et al., 1987).

B. Antimony Antimony acts by bonding irreversibly to thiol-containing enzymes, and it is known that the antimony metabolism depends on its oxidation state. The inorganic species are more toxic than the methylated ones, and Sb(III) is 10 times more toxic than Sb(V). Sb species studied are mainly inorganic antimony, Sb(III) and Sb(V), and organic antimony-like methylated species (mono-, di- and trimethyl-antimony) (Andreae et al., 1981; Yamamoto et al., 1981; Dodd et al., 1996) and triphenyl-antimony (Kumar et al., 1995). Two review papers have been published on Sb speciation covering the principles of hydride generation, particularly of SbH 3, atomization systems, sources of interferences, chemical speciation, and analytical applications (Castillo et al., 1987) and the antimony speciation in water (Smichowski et al., 1998). Sb(III) and Sb(V) have been determined in freshwater and seawater samples (Apte and Howard, 1986a; Clark and Craig, 1988; Menedez Garcia et al., 1995; Smichowski et al., 1995), plants (Dodd et al., 1996), and soils and sediments (Smichowski et al., 1994), and methylated species have been determined in plants (Dodd et al., 1996) and water (Andreae et al., 1981).

Speciation Studies

69

Some of the inorganic antimony speciation procedures involve the determination of Sb(III) and total inorganic antimony, Sb(V) being determined by the difference between using both selective hydride generation and AAS detection (Apte and Howard, 1986a; de la Calle Guntifias et al., 1992a) or by solid-phase extraction and ETAAS detection (Smichowski et al., 1994). For the direct determination of Sb(III) and Sb(V) separation methods, such as HPLC (Smichowski et al., 1995), solvent extraction (Chung et al., 1984; Castillo et al., 1986; de la Calle Guntifias et al., 1992b; Menedez Garcia et al., 1995) or column hydride generation gas chromatography (Clark et al., 1987; Clark and Craig, 1988) have been employed. Also used for detection were ICP-MS (Smichowski et al., 1995), ETAAS (Chung et al., 1984; de la Calle Guntifias et al., 1992b), HG AAS (Castillo et al., 1986; Clark et al., 1987; Clark and Caig, 1988; Smichowski et al., 1995), ICP-MS (Smichowski et al., 1995), and ICP-AES (Menedez Garcia et al., 1995). For methylated Sb species determination, hydride generation coupled to GC and detection by MS (Dodd et al., 1996) and ICP-MS (Krupp et al., 1996) were used. Triphenylantimony has been separated from other organoarsenic and organomercury compounds by SFC and determined by ICP-MS (Kumar et al., 1995). A method for determination of Sb(III), monomethyl-antimony, dimethyl-antimony, and trimethyl-antimony in plants by HG-GC-MS has been proposed (Dodd et al., 1996). Plant samples were extracted with 0.2 M acetic acid. After filtration, the filtrate was pumped, together with 1 M HC1 and 2% NaBH 4, to a reaction coil. The Sb hydrides were retained in a liquid N 2 U-trap. Then the U-tube was heated at 70 ~ to volatilize the stibines, which were analyzed by GC on a pre-silanized PTFE column of Porapak-PS, using a temperature programming from 70 to 150 ~ and MS detection. Sb(III) and total inorganic antimony in water were determined by HGAAS (Apte and Howard, 1986a). For total Sb, samples were injected into a hydride generator, the pH was then adjusted with HC1, a KI solution was added, and the generator was connected to a cryogenic preconcentration trap. To determine Sb(III), the procedure was repeated but with citric acid to adjust the pH and with the omission of KI. Sb(III) and Sb(V) were determined in seawaters by ICP-AES using a continuous tandem online separation device (Menendez Garcia et al., 1995). For Sb(III) determination a stream of sample mixed with 0.2 M ammonium citrate/NaOH buffer solution of pH 6 was merged with a stream of 0.1% pyrrolidine- 1-dithiocarboxylic acid ammonium salt and the resulting stream then merged with IBMK (0.5 mL/min) at a solvent segmenter. After extraction and phase separation the aqueous phase was collected for Sb(V) determination and the organic phase was mixed with glacial acetic acid and sodium tetrahydroborate in DMF for stibine generation. For Sb(V) determination the aqueous phase collected above was acidified with HC1 to give a final concentration of 3 M HC1 and mixed with 2% KI. The resulting solution was used for hydride generation ICP-AES with 1% NaBH 4.

70

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

C. Arsenic

Arsenic compounds are used for a variety of industrial and agricultural purposes, including herbicides, pesticides, wood preservatives, and as additives in glass. The determination of arsenic species in environmental and biological samples has been considered by the IUPAC. This review covers the distribution of As in the biosfere, the various As species, and the methods of identification and speciation (IUPAC, 1992). An overview about present possibilities for the determination of inorganic arsenic and organoarsenical compounds in biological fluids is presented. The work emphasizes the necessity for distinguishing between As of nutritional origin and that from water or the environment (Violante et al., 1989). Arsenite (AslII), arsenate (AsV), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AB), arsenocholine (AC), tetramethylarsonium ion (TMA+), and arseno-sugars are the main As species determined in natural samples. Toxicity and metabolic fates of these arsenic species differ dramatically. MMA and DMA are the most commonly detected species in marine waters, mainly as a consequence of phytoplankton activity, DMA being the dominant organoarsenic compound; and MMA probably an intermediate in the arsenic methylation sequence. Occupational exposure to arsenic often involves arsenite, arsenate, MMA, and DMA released through activities such mining, smelting, glassmaking, and pesticide manufacturing. Urinary excretion is the major pathway for the elimination of arsenic from the body. Therefore, speciation of these species in workers' urine is useful for assessing occupational exposure to arsenic. However, arsenosugars, ingested by means of seafood, are metabolized to species as DMA, which is also excreted into urine. Speciation analysis of arsenic has been carried out in waters (40%), biological and food solid samples (26%), urine (18%), soils and sediments (11%), atmospheric particulate (3%), and blood (2%). The separation process most employed was HPLC. After the chromatographic separation, detection has been performed by ICP-MS (Beauchemin et al., 1989; Sheppard et al., 1990; Larsen et al., 1993b; Demesmay et al., 1994; Inoue et al., 1994; Ding et al., 1995; Larsen, 1995; Thomas and Sniatecki, 1995a, b; Teraesahde et al., 1996), ICP-AES (Morita and Shibata, 1987; Violante et al., 1989; Rubio et al., 1993; Lagarde and Leroy, 1995; Moll et al., 1996), and HG-AAS (Gonzalez Soto et al., 1995, 1996a, b; Lagarde and Leroy, 1995; Zhang et al., 1996a). Some methodologies include a microwave or UV light-digestion of the species previously to the hydride generation (Martin et al., 1995; Le et al., 1996; Lopez Gonzalvez et al., 1996; Stummeyer et al., 1996; Zhang et al., 1996b). Other separation techniques employed were solvent extraction (Puttemans and Massart, 1982; Subramanian, 1988a; Russeva et al., 1993; Chappell et al., 1995), solid-phase extraction (Jimenez de Blas et al., 1994; Chwastowska et al., 1996), and SFE (Cleland et al., 1994); in some cases separation

Speciation Studies

71

and differentiation were carried out by hydride generation (Howard and ArbabZavar, 1981; Anderson et al., 1986; Van Cleuvenbergen et al., 1988; Burguera et al., 1992; Howard and Comber, 1992; Michel et al., 1992; Ruede and Puchelt, 1994; Gonzalez Soto et al., 1996c; Howard and Salou, 1996). Teraesahde et al. (1996) proposed a method for the simultaneous determination of arsenic species by ion chromatography-ICP-MS. The method used a Waters IC-Pak an HC anionexchange column and a Waters Guard-Pak CM/D cation exchange column. Samples were injected and the gradient elution was started with a 0.3 mM NaHCO3/0.3 mM Na2CO 3 buffer of pH 9.3 for 1 min. During this time arsenite, arsenate, dimethylarsinic acid, monomethylarsonic acid, and arsenocholine were retained on the anionexchange column, while arsenobetaine was eluted in the void volume. The retained species on the first column were eluted between 1.5 and 6 min with a gradient up to 2.5 mM carbonate buffer and 4-6 mM HNO 3. Between 7 and 11 min, the cationic species retained on the second column were eluted with 40 mM HNO 3. The flow rate was 2 mL/min and the column outlet was connected to the inlet hole of the nebulizer for ICP-MS measurement of As at m/z of 75. A clean up method for determination of six arsenic species in urine was based on their treatment with 20 mL ethanol in a solid CO2/acetone bath, centrifugation, and precipitation of high molecular mass organic compounds and salts. The precipitate was washed with ethanol and recentrifuged. The combined ethanolic extract containing the As species was evaporated and the residue was dissolved in mobile phase and injected into an HPLC-microwave oxidation-HG-AAS system. Separation was performed on a Hamilton PRP-X100 column with 17 mM NaH2POa/Na2HPO 4 pH 6 mobile phase. Microwave oxidation was performed in a 700 W microwave oven and the thermo-oxidized effluent was passed through an ice bath before hydride generation. The hydrides were transported by argon to an FAAS instrument. The method was used to determine As(III), As(V), MMA, DMA, AB, and AC (Lopez Gonzalvez et al., 1996).

D. Cadmium Some of the effects of cadmium poisoning in human are high blood pressure, kidney damage, destruction of testicular tissue, and destruction of red blood cells. These effects produced by the replacement of Zn in some enzymes alter a threedimensional structure of the enzyme and subsequently its catalytic activity (Vallee and Ullmer, 1972). Determination of free divalent Cd and labile, slowly labile, and stable Cd complexes have been performed by ion exchange in soil, compost, and land waste (Holm et al., 1995). In some matrices, speciation of Cd was performed by equilibrium dialysis for 48 h and Cd in the sample and outer solutions was determined by FAAS, enabling high and low molecular weight Cd complexes to be determined (Holm et al., 1995). Cd bound to metallothioneins has been determined from rabbit

72

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

liver extraction using tetrabutylammonium dibutyldithiocarbamate as complexing agent by supercritical fluid extraction (SFE)-AAS (Wang and Marshall, 1995) and from cell cultures by HPLC-ICP-MS (Takatera and Watanabe, 1992). Studies of Cd determination in body fluids such as urine and perspiration revealed that the half-life of Cd in the body was 1.5 years and confirmed that the highest Cd concentration occurs in perspiration (Chang and Robinson, 1993). Successful separations for numerous nonvolatile Cd compound can be achieved by HPLCFAAS. Generation of volatile Cd species with use of sodium tetraethylborate as reagent is very useful for determination of this metal in potable water (Ebdon et al., 1993) and seawater (D'Ulivo and Chen) by AFS. The technique of ETAAS has been employed for the determination of Cd in different cytosol fractions of food (Guenther and Waldner, 1992) and for different protein fractions from crabmeat (Olayinka et al., 1989). E. Calcium

Calcium is an essential element for metabolism of every living organism. Basically calcium was determined as Ca 2§ together to the anionic part of the salt; that is as Ca3(PO4)2 (Martinez-Avila et al., 1991), Ca(OH)2 (Mauri et al., 1994), and lactate, carbonate, citrate, ascorbate, or HPO 2- (Schuette and Schwedt, 1993). Furthermore, all of these studies were performed onto synthetic aqueous samples, and employing FAAS as the detection technique, except for calcium determination in serum (Zhang et al., 1994) which employed a capillary electrophoretic potential detector. F. Chromium

Chromium speciation studies have been, until now, clearly focused on the different oxidation states (Cr(III) and Cr(VI)) of this metal (Fukushima et al., 1995; Kabil, 1995; Lam et al., 1995; Garcia-Pinto et al., 1996; Saleh et al., 1996). Chromium (III) is a micronutrient, with deficiency in mammalians causing a decrease of ( 1) glucose tolerance, (2) insuline receptor number, and (3) high-density lipoprotein cholesterol levels and an increase of circulating insulin, cholesterol, and triglyceride (Anderson, 1985). On the other hand Cr(VI) is a potent carcinogen and irritant, and can cause bronchitis, pneumonia, hypersensitivity, and gastrointestinal, hepatic, and renal impairments (Nriagu and Nieboer, 1988). Studies are concerned with the determination of free Cr § or CrO 2-. In some cases, the specific determination of one of these species and the determination of total Cr were established arbitrarily. Cr(III) and Cr(VI) have been determined in waters (Manzoori and Shemiran, 1995; Mena et al., 1995a; Pasullean et al., 1995; Cespon et al., 1996; Demirata et al., 1996; Stewart and Horlick, 1996), foods and nutritional supplements (Ding et al., 1996; Gaspar et al., 1996), clinical samples and biological materials (Dungs et al., 1985; Gammlgaard et al., 1992; Milacic and

Speciation Studies

73

Stupar, 1994; Zoorab et al., 1995), soils and sediments (Posta et al., 1993; Elmahadi and Greenway, 1994; Tomlinson et al., 1994; Flores-Velez et al., 1996), dust and welding fumes (Neidhart et al., 1990; Girard and Hubert, 1996), and industrial samples (Stein and Schwedt, 1994; Timerbaev et al., 1996). A Cr(III) and Cr(VI) interaction study with rat plasma erythrocyte lysate and liver supernatant solutions evidenced, after sample in vitro incubation with chromium at time intervals up to 2 h at 37 ~ that Cr(VI) is reduced to Cr(III) with relatively higher reduction rates in liver supernatant solutions and erythrocyte lysate than in plasma samples. In this study the use of HPLC-FAAS evidenced the presence of other Cr species bounded to the biological materials (Suzuki, 1987). Cr determination in welding fumes generated by a plasma metal sprayer showed the presence of Cr(VI), soluble Cr(III), and Cr203 present in corundum structural form. The total Cr(VI) was determined by leaching by treatment of fumes for 1 h with aqueous 7% Na2CO 3 at 90 ~ Cr(III) obtained after digestion of the residue with hot diluted H2SO4 for several hours, and remaining corundum undissolved after digestion with boiling HC10 4 and H2SO4 (Sawatari and Serita, 1986). Analytical separation of Cr species using resins showed the presence of complexed Cr(III) in water samples as well as free Cr(VI) and Cr(III) (Grasso and Ummarino, 1980). This demonstrates that it is incorrect to deduce the concentration of a Cr species from the determination of a single one of the two oxidation states of Cr and the total content. From the methodological point of view, Cr(III) and Cr(VI) are commonly determined through specific solid-phase preconcentration by ionic exchange using alternatively cationic or anionic exchangers (Cox et al., 1985; Horvath et al., 1994; Naghmush et al., 1994; Sule and Ingle, 1996) or after complexation of Cr(III) with TOPO (Yamada et al., 1996), methanolic quinolin-8-ol (Beceiro-Gonzalez et al., 1992; Beceiro-Gonzalez et al., 1993), EDTA (Fong and Wu, 1991), or Cr(VI) complexation with diethyldithiocarbamate (Wai et al., 1987; Sperling et al., 1992), erythrocytes (Neidhart and Tausch, 1992), tetrabutylammonium (Groll et al., 1995; Posta et al., 1996), pyrrolidino-l-carbodithioate (Subramanian, 1988b; Andrle and Broekaert, 1993), dibenzyldithiocarbamate (Leyden et al., 1985), and 1,5-diphenylcarbazide (Lynch et al., 1984). The best results were obtained through the use of powerful separation techniques, such as HPLC (Jakubowski et al., 1994), and capillary electrophoresis (Semenova et al., 1996) hyphenated with atomic spectrometry detectors like FAAS (Jimenez et al., 1996; Syty et al., 1988), DCP-AES (Krull et al., 1983; DeMenna, 1986), ICP-AES (Krull et al., 1982; Roychowdhury and Koropchak, 1990; Pantsar and Manninen, 1996a) or ICP-MS (Powell et al., 1995; Pantsar and Manninen, 1996b; Tomlinson and Caruso, 1996). Traditional studies based on sequential chemical extraction have shown that the use of a three-stage extraction procedure based on (1) 1 M ammonium acetate at pH 5 for 20 h at room temperature, (2) 1 M hydroxylamine in 25% acetic acid for 20 h at room temperature, and (3) 8 M HNO 3 at 120 ~ for 20 h, are suitable for speciation of Cr in marine suspended particulate matter (Baffi et al., 1995). On the

74

M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

other hand, the five steps Tessier's method applied to the speciation of Cr in compost samples showed that most of the 50% of Cr remained in the fifth fraction of insoluble compounds and that the main form of Cr in compost is Cr(III). The Cr(VI) content was about 12% (Sterlinska and Golebiewska, 1994). It is interesting to indicate the efforts performed for the preparation of reference materials suitable for Cr speciation. One of these studies, based on the impregnation of diatomaceous earth with BaCrO 4 and Cr203 and mixture with soils, observed Cr(VI) instability after six months (Solano et al., 1994). Another project was based on the preparation of reference materials of water and welding dust from aqueous solutions of Cr(III) and Cr(VI) in HCO3-H2CO 3 pH 6.4 and pH 9.6 buffer media, respectively. The solutions were kept under a CO 2 blanket in sealed quartz ampoules at 5 ~ or lyophilized by freeze-drying in glass ampoules in a pure N 2 atmosphere; it was observed that both materials are homogeneous and stable (Dyg et al., 1994). G.

Copper

Copper is an essential metal for humans because it participates in some important enzymes, such as ferroxidases, cytochrome oxidase, or amine oxidases. Generally there is no toxic action when excess Cu intake occurs in man, but it is toxic to many bacteria, viruses, and fungi. Major studies performed on Cu speciation involved water (Miller and Bruland, 1994; Rottmann and Heumann, 1994), soil and sediment (Brown et al., 1983; Sturgeon et al., 1996), biological tissues (Dunemann and Reinecke, 1989; Wang and Marshall, 1995), and fuel (Taylor and Synovec, 1993 and 1994). Studies have focused on different complexes of Cu(II) with fulvic or humic substances (Morrison et al., 1990; Taga et al., 1990), protein (Dunemann and Reinecke, 1989; Wang and Marshall, 1995), or organic matter in general (Taylor and Synovec, 1993, 1994). Even an unidentified volatile Cu species was obtained when marine sediments were treated with NaBH 4 (Sturgeon et al., 1996). The most employedanalytical technique for Cu determination is FAAS (Miwa et al., 1989; Itabashi et al., 1994; Geiger et al., 1996), ETAAS (Brown et al., 1983; Moffett and Zika, 1987; Mackey and Zirino, 1994) and coupled with HPLC (Morrison et al., 1990; Taylor and Synovec, 1994). Other instrumental techniques employed are SFE-AAS (Wang and Marshall, 1995), ICP-AES (Sturgeon et al., 1996), polarography (Dundar and Haswell, 1995), HPLC-ICP-MS (Rottmann and Heumann, 1994), voltametry (Donat et al., 1994), and electrophoresis (Dunemann and Reinecke, 1989). For the speciation of Cu in water (Miller and Bruland, 1994), samples were filtered through polycarbonate filters and the Cu was extracted with acetylacetone and toluene and the organic phase was backextracted with HNO 3. The acid fraction was evaporated to dryness and redissolved in 1 M HNO 3. Copper was determined in the final fraction by standard additions using ETAAS. Model ligand (DTPA) titrations were performed on UV-oxidized seawater. Natural hydrophobic Cu

Speciation Studies

75

complexes were determined in each sample by extracting with acetylacetone. The amount of Cu determined in the organic fraction was subtracted from the extracted Cu signal in the titration calculations. Total Cu was determined by pyrrolidine dithiocarbamate extraction. The results agreed with those obtained from analyses of the open ocean but there were discrepancies with estuarine samples. In fuel samples (Taylor and Synovec, 1994), the identification and determination of five classes of Cu compounds formed by interaction between jet fuel and Cu in the presence and absence of a DuPont metal deactivator (active component N, N'-disalicylidene propylenel, 2-diamine (I)) was studied. For HPLC, an Asahipak GS-510M column of poly(vinyl alcohol) was used with a gradient mobile phase of propan-2-ol acetonitrile and methanolic 10 mM potassium trifluoromethane sulfonate. Detection was by FAAS. Detection limits were 10 ppb for the Cu complex of Cu(I), 10 ppb for copper cyclohexanebutyrate, and 16 ppb for copper dimethyldithiocarbamate. Measurements were not obtained for copper tetramethylheptanedionate. Copper N, N'-disalicylidene-1,2-propylenediamine (I) and several copper carboxylates were preconcentrated from a hexane solution by using a column of poly(vinylpyrrolidone) from Dionex Onguard-P solid phase extraction cartridges (Taylor and Synovec, 1993). Elution was performed with propan-2-olhexane (7:3) from 0 to 3 min followed by 5% acetic acid and 0.5% H20 in propan-2-ol to the end of the run; the sample (1.25 mL) was injected at 0.5 min. Copper complexes were detected by UV absorbance and FAAS detection. The detection limits obtained for Cu(I) and the copper carboxylates were 7 and 40 ppb, respectively. Speciation of Cu in low-fat soya flour was achieved by extraction with 45 mL water at 60 ~ for 2 h. The mixture was centrifuged and filtered, then sucrose and NaN 3 were added and this solution analyzed by chromatography. For gradient gel electrophoresis, a mixture of the extract and Tris-HC1 buffer (pH 8.8) was heated for 2 to 3 min in a boiling-water bath, then mixed with 2-mercaptoethanol and bromophenol blue solutions, and portions of this solution were applied to a gradient polyacrylamide. Determination of Cu, Ni, and Zn extracted electrophoretically from the isolated protein fractions was carried out by FAAS. Gradient gel electrophoresis affords the better separation of the protein fractions, but does not detect proteins of molecular weight 6500. These can be detected by gel chromatography (Dunemann and Reinecke, 1989). In soils, Cu determination by HPLC-ETAAS demonstrated that between 60 to 80% of Cu in soil is bound to humic and fulvic acids (Morrison et al., 1990).

H. Germanium Inorganic germanium has been determined in waters (Hambrick et al., 1984; Jin et al., 1991), sediments (Krupp et al., 1996), brine (Padro et al., 1995), and nutritional oral liquid, together with 13-carboxyethyl-Gesexquioxide (Zhang and Ni, 1996). Small organic speciesof Ge have been studied in water, as MeGe (Hambrick

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M. DE LA GUARDIA, M.L. CERVERA, and A. MORALES-RUBIO

et al., 1984) and in brine, as diMeGe, diEtGe, triMeGe, and triEtGe (Padro et al., 1995). Instrumental techniques employed in all cases involved hydride generation by NaBH 4, separation of volatile species generated by GC (Krupp et al., 1996) or LC (Padro et al., 1995), trapping in a liquid-N-trap (Hambrick et al., 1984; Jin et al., 1991) or into a palladium-coated graphite tube (Zhang and Ni, 1996), and detection by ETAAS (Hambrick et al., 1984), ICP-AES (Padro et al., 1995), or ICP-MS (Krupp et al., 1996). I. Iodine The most used instrumental technique for the determination of iodine species has been HPLC coupled with ICP-MS. The species I- and 10 3 were determined in waters (Salov et al., 1992; Heumann and Rottmann, 1994) and wine (Salov et al., 1992), and I-aminoacids in bovine thyroblobulin (Takatera and Watanabe, 1993) enzymic digest. Total iodine was determined after filtration of water (Anderson et al., 1996) by oxidation to 12 in situ with H2SOa/NaNO 2 in a continuous flow manifold and ICP-AES detection. Iodine was determined without the use of the oxidation reagent and iodide was calculated by difference. Iodide and total iodine have been determined in serum (Michalke et al., 1996a, b). Both studies present the same results in which a size exclusion pretreatment was necessary before analysis by ionic chromatography and iodide determined by pulsed electrochemical detector employing a Ag electrode. The total iodine was determined by analyzing the same fractions by ICP-MS. J. Iron Iron is an essential micronutrient in living organisms because it is involved in metabolic processes and is a very important component of hemoglobin. Speciatiori studies have been performed mainly on inorganic (Fe(II), Fe(III)) species involving atomic detection such as FAAS (Lynch et al., 1984; Kabacinski et al., 1996; Tawali and Schwedt, 1997) or coupled with chromatographic techniques such as HPLC-ICP-AES (Noelte et al., 1995), HPLC-FAAS (Weber, 1991), IC-FAAS (Ajlec and Stupar, 1989), IC-DCP-AES (Lewis et al., 1989; Urasa and Mavura, 1992), LC-FAAS (Weber, 1993), electrochemical detection as stripping voltametry (Milosavljevic et al., 1988; Naumann et al., 1994), spectrochemical detection (Lynch et al., 1984; Kabacinski et al., 1996), and even kinetics measurements (Abe et al., 1986). On the other hand, organic species of iron were determined as Fe bounded to protein (Stuhne et al., 1992; Van Landeghem et al., 1994) or to porphyrins (Zeng and Uden, 1994). Iron(II) and Fe(III) have been determined in wine (Weber, 1991). Wine was filtered and a buffer of pH 2-7/phenanthroline solution/ferron solution added, and applied to a nonionic Amberlite XAD-4 column. The direct determination was performed by AAS of Fe(III) from the anion

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exchange eluate and the Fe(II) from the nonpolar RP-18 phase (Tawali and Schwedt, 1997). A plastic column of Spherisorb $ 5 0 D S - 2 has been employed for separation of inorganic Fe species, with in-line detection. Iron (II) in the eluate was detected electrochemically and total Fe was detected by FAAS. On the other hand, separation of Fe(II) and Fe(III) was not necessary because the electrochemical detector was specific for Fe(II) (Weber, 1991). In another study, wine was diluted with HNO 3 and applied to a glass column of Dowex 50-X8 cation exchange resin. Iron (III) was determined by off-line AAS after retention on the column and elution with 2 M HC1, with the Fe(II) not being retained (Ajlec and Stupar, 1989). The speciation of Fe bound to serum transferrin has been studied (Van Lendeghem et al., 1994). Samples were analyzed by HPLC on a poly (ether ether ketone) column of Biogel TSK-DEAE-5PW with gradient elution from Tris-hydrochloride buffer to Tris/NaC1; fractions were then acidified with HNO 3 and analyzed by ETAAS. Speciation transferrin and ferritin has been performed in liver cells (Stuhne et al., 1992). Cells were homogenized and applied to a column of carboxymethyl cellulose with elution with different equilibration buffer solution. Fractions were diluted with HNO 3 and analyzed by ICP-MS. Studies for Fe(II)-Fe(III) speciation as Fe(CN) 4- and Fe(CN)63- employing an electrochemical-AAS coupled detection has been achieved (Milosavljevic et al., 1988). In natural waters there were some works on this topic (Kabacinski et al., 1996). An interesting application of kinetic measurements (Abe et al., 1986) for Fe speciation in waters has been achieved in which the method required only a single sample, no precise temperature control, and no premeasurement of rate constants. It was based on air oxidation of Fe(II)in the presence of tiron and acetate and complexation of the Fe(III) formed with tiron. The Fe(II) oxidation was first-order, and a kinetic equation could be used to treat the absorbance-time data to give the initial and equilibrium absorbances, which were measures of Fe(III) and the total Fe, respectively. The concentration of Fe(II) and Fe(III) were calculated by nonlinear leastsquares regression. The calibration graphs were rectilinear up to 12 lag L -1 of each Fe species and, at levels of 50 laM of each species, the coefficient of variations (n = 9) were approximately 2.1%.

K. Lead Lead poisoning is produced mainly by ingestion and can produce appetite loss, vomiting, or even renal malfunction, anemia, disorders in the nervous system, and brain damage. However, the absorption of lead by humans is low owing to formation of insoluble phosphate that is accumulated in bones, which serve as a detoxification place (Bruce and King, 1994). In contrast to inorganic forms of lead, organolead species are easily absorbed and are extremely toxic. This has resulted in speciation studies of lead (Debeka, 1992; Wang and Marshall, 1994; Enger et al., 1995; Minganti et al., 1995; Brenner et al., 1996; Halicz et al., 1996) being performed

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mainly on organolead species, which is, approximately 80% of the works published in the bibliography. Organolead species have been determined extensively in waters (Van Cleuvenbergen et al., 1986; Neidhart and Tausch, 1992), petroleum products (gasoline, fuel) (Borja et al., 1990; Olson et al., 1996), air (Hewitt et al., 1986), sediments and soils (Chan et al., 1984), dust and particulates (Ceulemans and Adams, 1996), environmental samples (fish, plant) (Van Cleuvenbergen et al., 1990), and wine (Lobinski et al., 1993). Inorganic lead has been determined in waters (Naghmush et al., 1995; Pyrzynska, 1996; Rapsomanikis et al., 1996) and in sediments and soils (Cordos et al., 1995). Organolead species determined include monoalkyl- (MePb), dialkyl- (MeEtPb, diMePb, diEtPb), trialkyl- (triMePb, triEtPb, triPhPb, diMeEtPb), and tetraalkyllead (tetraMePb, tetraEtPb, tetraBuPb, triMeEtPb, triMeBuPb, triMePhPb, diMediEtPb, diMediBuPb, MetriEtPb, diEtdiBuPb, triEtBuPb). The most studied compounds are diMePb, diEtPb, triMePb, triEtPb, tetraMePb, and tetraEtPb. Instrumentation employed for the determination of different organolead species involves the separation by gas chromatography coupled to atomic detectors; e.g. AES (Lobinski and Adams, 1992a; Szpunar et al., 1994), AAS (Forster and Howard, 1989), ETAAS (Dirkx et al., 1992), MIP-AES (Lobinski and Adams, 1992b; Sadiki and Williams, 1996) and ICP-MS (DeSmaele et al., 1996), or by HPLC hyphenated with atomic detectors, FAAS (Robinson and Boothe, 1984; Ebdon et al., 1987), ICP-AES (A1-Rashdan et al., 1991), and ICP-MS (Shum et al., 1992). The most common method for the GC separation of organolead species employs alkylation of these compounds with buthyl-magnesium (Chan et al., 1984), tetraethyl-borate (Jantzen and Prange, 1995; Tutschkn et al., 1996) or propyl-magnesium (Radojevic et al., 1986). Previous extraction with a solvent, such as pentane (Bergman and Neidhart, 1996), hexane (Allen et al., 1988; Waldock, 1992), nonane (Chakraborti et al., 1989), ethylether (Metcalfe, 1989), isobutylmethyl ketone (IBMK) (Arai, 1986), carbon tetrachloride (Diehl et al., 1984), or benzene (Chau et al., 1984) has been used. Two pH conditions were used for lead derivatization: at pH 5 (Prange and Jantzen, 1995) using citrate (Bergmann and Neidhart, 1996), Na-acetate (Tutschku et al., 1996), NHaAc (Campanella et al., 1995), or Na-acetate/NH4-acetate (Brown et al., 1994); and at pH 8-9 (Lobinski et al., 1993b) employing citric acid or ammonia (Lobinski et al., 1993a). Complexing agents have been employed as well to separate organolead species, such as dithiocarbamate (Lobinski et al., 1993a; Johansson et al., 1995) or diethyl dithiocarbamate (Chakraborti et al., 1987; Allen et al., 1988). Lead determination in urban particulate material, spiked with 20 Ixg of Pb(II), 20 ~g of triEtPb, and 5 ktg of triMePb, shows that sonication at 50/60 Hz for 30 min gives recoveries between 100-101% of these three species. Sonicated solutions were filtered and 100 lxL of diluted filtrate were injected onto a 3 lam C 8 HPLC column with acetic acid/ammonium acetate buffer of pH 4.7 containing methanol as mobile phase. The eluate was mixed with streams of H202 and HC1, and the

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solution was merged with a stream of NaBH 4 in NaOH, determining the lead hydride produced by ICP-MS (Yang and Jiang, 1995). Some methods employ six sequential extraction stages for the selective extraction on sediments of the metals bound to organic matter from those present as the sulfide based on (1) mixing with ammonium acetate (pH 5) and stirring for 24 h at room temperature; (2) extracting with hydroxylammonium chloride/acetic acid; (3) extracting with HC1; (4) extracting with NaOH, and (5) extracting with HNO 3 at 85 ~ for 3 h. The fraction corresponds to the insoluble part of metals. Moreover, total sediment metal dissolution was performed with microwave-assisted digestion with HNO 3 and HF in a PTFE reactor (Campanella et al., 1995). On the other hand, fly ash or sediment samples were separated in seven fractions by sequential leaching: (1) exchangeable metals and metals bound, respectively (2) carbonate, (3) manganese oxides, (4) organic matter, (5) poorly crystallized iron oxides (6) crystallized iron oxides, and (7) silicates (Hlavay et al., 1995). Tetraethyl- and tetramethyl-lead are the most frequent organolead species determined in gasolines, querosene, and fuel in which capillary GC is coupled to ICP-MS (Kim et al., 1992), GC-AAS (Harrison and Hewitt, 1985; Bai et al., 1987; Brunetto et al., 1992), or HPLC-FAAS (Xia et al., 1989). Borja et al. (1990) determined these species directly by employing a FIA-FAAS system in which 100 txL of samples were injected into a carrier stream of 0.1% emulsogen M. These were demetallated by merging with another channel in which it has been injected 200 gL of an iodine solution. After emulsification in a PTFE stirred dilution chamber, lead was determined in an air-acetylene flame. L.

Mercury

Mercury occurs in four basic states; Hg(0), Hg(I), Hg(II), and organo mercuric forms. The species Hg(I) is the most unstable. It disproportionates spontaneously into Hg(0) and Hg(II). Organomercury compounds include monoalkyl- (MeHg, EtHg), monoaryl- (PhHg, p-TolHg), dialkyl- (diMeHg, diEtHg, MeEtHg, MeOxiEtHg, EtOxiEtHg), and diarylmercury (diPhHg). All of the mercury forms are toxic for any biological system. Mercury vapor inhaled accumulates in the brain, kidney, and testicles and may lead to nervous system damage, fatigue, and trembling. Ingested Hg(II) also accumulates in the kidney and causes pain, vomiting, and diarrhea. Organomercury compounds are highly toxic, e.g. R-Hg § is quickly absorbed and accumulated in the liver, kidney, and brain causing an irreversible damage to the central nervous system. The well-known disaster at Minamata Bay (Harada, 1978) has been attributed to mercury exposure. Other mercury species, such us HgS, HgSe, Hg(0), and diMeHg, are much less toxic than Hg(II) and R-Hg § The number of publications in which different species of mercury are considered shows the importance of each one; the most studied species are MeHg (34%) and Hg(II) (32%), followed by EtHg (11%), diMeHg (6%), and PhHg (5%).

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Speciation of mercury has been performed in waters (Krull et al., 1986; Wu, 1991; Madrid et al., 1995), air (Lupsina et al., 1992), natural gas (Snell et al., 1996), sediments (Ebinghaus et al., 1994; Jantzen and Prange, 1995; Schmitt et al., 1996), soils (Hempel et al., 1995), fish (Rapsomanikis and Andreae, 1992; Rezende et al., 1993), seafood (Holak, 1995), clinical samples (Robinson and Boothe, 1984; Shum et al., 1992), and tissue samples (Huang and Jiang, 1993; Lind et al., 1993; Saouter, 1994). Speciation of mercury has been performed by AAS using both electrothermal (Aller et al., 1995, 1996) or flame atomization (Bzezinska et al., 1983; Wang et al., 1983; Yuan and Zhong, 1993) and by AFS (Schroeder and Jackson, 1985 and 1987) and AES employing MIP hyphenated with HPLC (Costa-Fernandez et al., 1995). Determinations have been performed by ICP-MS coupled with LC (Bushee, 1988; Bloxham et al., 1996) and SFC (Kumar et al., 1995). However, the most employed procedure for mercury determination is based on the high volatility of elemental mercury. In this sense, cold vapor generation has been frequently used as a sample introduction procedure in AAS (Horvat and Byrne, 1992; Lind et al., 1994), AFS (Schroeder and Jackson, 1984; Jian and McLeod, 1992; Waldock, 1992) and ICP-AES (Menendez-Garcia et al., 1996); or by the hyphenation of different chromatographic techniques with atomic spectrometry, such as high-performance liquid chromatography coupled with cold vapor AAS (Munaf et al., 1990; Aizpun et al., 1994; Sarzanini et al., 1994), super critical fluid chromatography-CV-AAS (Holak, 1995), or gas chromatography coupled with cold vapor AAS (Seckin et al., 1986) or AFS (Bloom and Fitzgerald, 1988). In these approaches, a reducing reagent is employed to generate Hg(0) from the mercury species; typically NaBH 4 (Palmisano et al., 1993) and SnC12 (Ergucyener et al., 1988; Saouter, 1994) are the most commonly employed reagents. In the determination of organomercury species, gas chromatography has been employed hyphenated with different atomic detectors, such as GC-ICP-AES (Kato et al., 1992), GC-MIP-AES (Carro et al., 1994), GC-ICP-MS (De-Smaele et al., 1996), GC-AAS (Emteborg et al., 1996), GC-ETAAS (Robinson and Wu, 1985), and GC-AFS (Wilken, 1992). Alkylation of organomercury species has been performed using butyl MgC1 (Emteborg et al., 1996), tetramethyl-tin (Filippelli, 1987), sodium tetraethyl-borate (Rapsomanikis and Craig, 1991; Hintelmann et al., 1995) and sodium tetraphenyl-borate (Mena and McLeod, 1996) before organic extraction with hexane (Mena et al., 1995b; Prange and Jantzen, 1995), CH3CN (Falter and Schoeler, 1996), xylene (Menendez-Garcia et al., 1996), toluene (Wilken, 1992; Ebinghaus et al., 1994), or benzene (Jiang et al., 1989). The use of different trapping systems to retain mercury species, due to them being sequentially released by temperature control, has been used. Amalgamation trap (Frech et al., 1995), fused-silica-liner trap (Ceulemans and Adams, 1996), cryogenic trap (Puk and Weber, 1994), or room-temperature traps (Liang et al., 1994a and 1994b) have been employed.

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In some cases, in which the concentration of some of the mercury species is excessively low, speciation studies also include a preconcentration step. This can be performed on resins modified by different compounds, such as thiocarbamate (Emteborg et al., 1996), dithiocarbamate (Johansson et al., 1995), pyrrolidine dithiocarbamate (Falter and Schoeler, 1994, 1995), diethyldithiocarbamate (Bulska et al., 1992), dimethylpolysiloxane (DeSmaele et al., 1996), didodecyldimethylammonium (Sanz-Medel et al., 1994), or sulphydryl cotton (Mena et al., 1995b; Mena and McLeod, 1996). An interesting method for speciation of Hg(II) and PhHg in certified solids has been developed by Bryce et al. (1996) in which microwave-assisted prevaporation of Hg was used for AFS detection. Depending on the reagent added into the prevaporation cell, both species were determined sequentially. For the determination of Hg(II) it was injected with 0.25 mL of 5% SnC12 and the vapor obtained was conveyed to the detector. For the determination of PhHg reagents employed, 0.125 mL of 0.5% KBr/0.14% KBrO 3 plus 0.125 mL of 5% SnC12 was used. In both cases a microwave treatment of 1 min was necessary for mercury reduction.

M. Nickel Nickel has been speciated successfully, together with A1 and Fe, in soybean extract by HPLC-ICP-AES (Noelte et al., 1995). In soya flour it was extracted electrophoretically from isolated protein fractions and detected by FAAS (Dunemann and Reinecke, 1989). In waters, dissolved Ni speciation was carried out by capacitive ligand equilibration cathodic stripping voltametry and by chelating resin column partitioning GF-AAS (Donat et al., 1994). Nickel has been determined as soluble Ni 2§ Ni metal, and nickel oxides in airborne particulate by FAAS (Wong and Wu, 1991). In this case, air was sampled on a glass fiber membrane or on a cellulose ester membrane. Water-soluble nickel was determined after filtration and sonication of particulates into water. The residue was resuspended in water and extracted electromagnetically and the Ni metal adhering to the rod was digested in boiling HC1 and determined after evaporation; and the remaining suspension was filtered and nickel oxides determined. Furthermore, Ni has been determined in petroleum as Ni-porphyrin. The method was used to characterize metal distribution in different oils and to study the decomposition of petroleum metal species (Zeng and Uden, 1994).

N. Platinum Platinum has been determined in drugs such as cisplatin (Cairns et al., 1994), in milk as hexachloroplatinate, tetrachloroplatinate, and platinum methionine complex (Michalke, 1996), and in plasma as methionine complex and cisplatin (DeWaal et al., 1987). Separation of different species of platinum has been performed by HPLC (Cairns et al., 1996) or by capillary electrophoresis (Michalke, 1996)

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followed by UV detection (Michalke, 1996) or atomic detection using ICP-AES (DeWaal et al., 1987) or ICP-MS (Cairns et al., 1994, 1996). There is an increasing interest on this subject due to the extensive use of Pt catalysts in automobiles. This will probably cause an increase in the presence of this metal in the urban atmosphere. O. Selenium

Selenium may be released in the environment as a result of anthropogenic activity such as fossil fuel combustion and industrial and agricultural processes, and also by natural processes such as weathering of minerals. Selenium compounds are widely used in industry: glass manufacture, electronic applications, photocopy machines, inorganic pigments, rubbers, ceramics, plastics, and lubricants. Selenium is an essential trace metal for human health at a concentration ranges from 0.8 to 1.7 ktmol L -1, but exhibits toxicity outside of this range of concentration. Selenium is known to play an important role in mammalian systems such enzyme glutathione peroxidase. In the last few years, Se has been suspected to be an agent of cancer inhibition. Se also has protective actions towards cell membranes, and preventing oxidative damage. On the other hand, there is some evidence of mutagenic effects of selenite and selenate. Both the bioavailability and toxicity of Se depends on its chemical form. Selenium occurs under oxidation states: (VI), (IV), (0) and (-II). The species Se(IV) and Se(VI) are essentially present as selenite and selenate ions in waters and soils. Inorganic selenides Se(-II) and metallic selenium Se(0) are mainly insoluble compounds. The species Se(-II) is present as organic selenides such as methylated volatile compounds. It occurs also as seleno amino acids, mainly selenocystine, selenomethionine, and selenoethionine, and covalently bound to proteins. A recent review of the methods used for the speciation of Se in waters (Russeva and Havezov, 1996) is available. Selenium speciation has focused on the identification of the oxidation state of the inorganic species (Apte and Howard, 1986b; Larraya et al., 1994; Bryce et al., 1995; Burguera et al., 1996; Jakubowski et al., 1996), and the determination of selenoaminoacids (Sanz-Medel et al., 1994; Gilson et al., 1996; Mufioz Olivas et al., 1996; Quijano et al., 1996). The methylated species has been less studied (Masscheleyn et al., 1991), except the trimethylselenonium ion, and the more toxic organic species (Cobo Fernandez et al., 1995a; Mufioz Olivas et al., 1996). In some cases, the total selenium and inorganic selenium (Gonzalez LaFuente et al., 1996b), or only inorganic (Yang et al., 1996) or organic selenium (Block et al., 1996) was studied. Selenium speciation has been performed mainly in waters (51%), followed by biological and food solid samples, including plants, fish, liver, kidney, etc (18%), soils and sediments (8%), urine (8%), and blood (6%). Lower percentages (2%) correspond to milk, dialysis fluids, atmospheric particulate, fly ash, or citrus juice.

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For the determination of Se(IV) and Se(VI), selective hydride generation with AAS (Cutter, 1985; Apte and Howard, 1986b; Burguera et al., 1996) or AFS detection (He et al., 1998), SPE (Larraya et al., 1994; Pyrzynska, 1995; Jakubowski et al., 1996), solvent extraction (Chung et al., 1984) or HPLC (Thompson and Houk, 1986; Muangnoicharoen et al., 1988; Gjerde et al., 1993; Shum and Houk, 1993; Pitts et al., 1994 and 1995; Cai et al., 1995) coupled to different spectrometric techniques, such as ETAAS, ICP-AES, ICP-MS or HG-AAS, and HG-AFS have been employed. Inorganic species and seleno amino acids are commonly separated by HPLC and detected by ICP-MS (Mufioz Olivas et al., 1996; Quijano et al., 1996), ETAAS (Sanz-Medel et al., 1994; Gilson et al., 1996; Marchante Gayon et al., 1996), and microwave-assisted reduction hydride generation AAS (Cobo Frenandez et al, 1995; Marchante Gayon et al., 1996). The determination of total inorganic selenium, Se(IV), selenomethionine, and selenoethionine in Water and urine can be performed by on-line HPLC-focused microwave digestion-hydride generation and measurement by ICP-MS, ICP AES, and quartz-tube AAS detection (Gonzalez LaFuente et al., 1996a). The employed manifold includes a Spherisorb C18 column, a system for microwave digestion of the separated compounds with KBrO3/HBr, and a laboratory-designed gas-liquid separator. A second injection with the microwave heating discontinued was necessary in order to determine selectively Se(IV). A study of the effect of physicochemical parameters on trace inorganic selenium stability (Cobo et al., 1994), stability of organoselenium compounds (Mufioz Olivas et al., 1995), and an interlaboratory study for the quality control of Se(IV) and Se(VI) determination in simulated freshwater (Cobo Fernandez et al., 1995b) have been also published. P. Tin

Organotin compounds, such as R4Sn, R3Snx, R2SnX2, and RSnX 3 (where R is an an alkyl or aryl group, and X an anionic group such as halogen,-OH, or acetate), have a great variety of applications as catalysts (R4Sn), industrial biocides, in antifouling paints, in wood preservatives, agricultural fungicides (R3SnX), and PVC stabilizers (R2SnX 2, RSnX3). Due to their continuous use, they can be released in the environment in which they undergo a variety of degradation reactions and biomethylation. The toxicity of organotin compounds depends on the alkyl(aryl) group and the anionic group. Tributyltin (TBT) and triphenyltin (TPhT) are the most toxic. The main part of studies has focused on water samples (42%), soils and sediments (29%), and biological and food solid samples (19%), especially fish and environmental materials. The speciation in biological fluids (urine) concerns only 2%. Due to the use of tin for packaging materials or as polyvinyl chloride (PVC) stabilizers,

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there are some studies on canned foods (Krull and Panaro, 1995) and PVC packaging (Forsyth et al., 1993). A series of review articles have been published on tin speciation. The use of GC hyphenated with AES or AAS detection, and that of various procedures for extraction and derivatization of organotin species for sample preconcentration and cleanup, have been compared (Dirkx et al., 1994). Examples of the microwave-assisted sample preparation for capillary GC with quartz furnace AAS detection, flame photometric detection, and MIP AES have been reported by Szpunar et al., (1996a). The application of SFE to the recovery of organotin pollutants in a variety of matrices has ben reviewed by Bayona and Cai (1994). Speciation of Sn can be performed directly by hydride generation-cold trapping and AAS detection (Donard et al., 1986; Chamsaz et al., 1988; Martin and Donard, 1994), ICPMS detection (Sato et al., 1996), or ETAAS detection (Andreae and Byrd, 1984). A chromatographic separation by GC or HPLC coupled to an atomic spectrometric detector has been employed extensively. Although derivatization is needed for gas chromatographic separation, in order to transform the mostly Sn ionic species into volatile and thermally stable compounds, the application of GC is superior to liquid chromatography. The derivatization processes employed were ethylation with sodium tetraethylborate (Ceulemans and Adams, 1995; Rodriguez Pereiro et al., 1996; Smith et al., 1996; Szpunar et al., 1996b), hydride derivatization with NaBH 4 (Valkirs et al., 1987; Martin et al., 1994; Krupp et al., 1996), or Grignard reactions with penthylmagnesium bromide (Forsyth et al., 1993; Szpunar-Lobinska et al., 1994; Ceulemans and Adams, 1995; Minganti et al., 1995). Interest in speciation analysis by HPLC is growing due to the coupling with ICP-MS (Suyani et al., 1989; Dauchy et al., 1993; Kumar et al., 1993; Rivas et al., 1995, 1996; Yang et al., 1995) and ETAAS detection (Jewett and Brickman, 1981; Astruc et al., 1989, 1992). For the extraction of organotin compounds from solid matrices, SFE (Bayona and Cai, 1994), and microwave-assisted extraction procedures (Schmitt et al., 1996; Szpunar et al., 1996a, b) have been proposed. Rodriguez Pereiro et al. (1996) proposed the speciation of mono-, di- and tributyl-tin and triphenyl-tin by GC-AES after an integrated process including dissolution, extraction, and derivatization in a focused microwave from fish samples. For that 2,2,4-trimethylpentane or nonane, with tetrabutyltin as internal standard, and sodium tetraethylborate were added to a portion of acetic acid containing an organotin compound. The mixture was diluted and microwave-heated at 40 W for 2 min. A portion of the cooled supernatant was injected into a DB-210 column using He as carrier gas, and AES detection at 303.419 nm. SFE was evaluated as a sample preparation technique for the determination of individual organotin compounds in fish by HPLC-ICP-MS (Vela and Caruso, 1996). The HPLC was carried out on a PRP-1 column fitted with a PRP-1 guard

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column and operated with 46 mM acetic acid/12 mM ammonium acetate/4 mM sodium pentanesulfonate in aqueous 94% methanol as the mobile phase. P. Zinc

Zn is an essential micronutrient due to its association with many enzymes and with certain other proteins. Because of that, studies of Zn speciation have been performed mainly on biological samples such as liver (Wang and Marshall, 1995), foodstuffs (Guenther and Waldner, 1992), soya flour (Dunemann and Reinecke, 1989), serum (Faure et al., 1990); and waters (Lu et al., 1994; Xue and Sigg, 1994). Zn has been determined in different protein fractions bounded to metallothioneins (Wang and Marshall, 1995), albumin (Faure et al., 1990), or other proteins (Dunemann and Reinecke, 1989; Guenther and Waldner, 1992). The methodologies employed for Zn speciation involve atomic techniques such as ETAAS (Faure et al., 1990; Kallithrakas, 1996), ICP-MS (Lu et al., 1994), FAAS coupled to electrophoresis (Dunemann and Reinecke, 1989), and SFE (Wang and Marshall, 1995), or electrochemical determination (Xue and Sigg, 1994). An interesting application employs kinetic studies for Zn speciation in natural waters (Lu et al., 1994). Data were analyzed using an interactive convolution method with a nonlinear least-squares algorithm and showed that three kinetically distinguishable components can be found in rainwater.

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NEW TYPES OF TUNABLE LASERS

Xiandeng Hou, Jack X. Zhou, Karl X. Yang, Peter Stchur, and Robert G. Michel

I. II.

III.

IV.

V.

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Parametric Oscillator (OPO)-Based Lasers . . . . . . . . . . . . . . .

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A. Fundamentals of Optical Parametric Oscillation . . . . . . . . . . . . . . B. Examples of Commercial Optical Parametric Oscillator Lasers . . . . . . C. Characteristics of Optical Parametric Oscillator Lasers . . . . . . . . . . Vibronic, Tunable, Solid-State Lasers . . . . . . . . . . . . . . . . . . . . . . A. Fundamentals of Vibronic Solid-State Lasers . . . . . . . . . . . . . . . B. Examples of Vibronic Lasers . . . . . . . . . . . . . . . . . . . . . . . . Diode Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Basics of Diode Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Characteristics of Diode Lasers . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

103 110 113 121 122 122 133 133 135 141 142

Advances in Atomic Spectroscopy Volume 5, pages 99-143. Copyright 9 1999 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0.7623-0502-9 99

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ABSTRACT Many laser-based atomic spectrometric techniques have excellent analytical characteristics, particularly from the points of view of sensitivity and spectral resolution. Unfortunately, this has not resulted in particularly successful routine applications of lasers to analytical atomic spectrometry. Primarily, this has been due to the lack of flexibility in the spectral tunability of lasers, and their lack of reliability, or ruggedness, in operation. In the past, dye lasers have been most frequently involved in laser atomic spectrometry for they were the only commonly available tunable lasers, but dye lasers have some disadvantages, such as a limited tuning range within each dye, and the toxicity of organic dyes and their solvents. In this chapter, relatively new types of tunable lasers, including solid-state optical parametric oscillator (OPO) lasers, titanium:sapphire lasers and semiconductor diode lasers, are discussed. The principles of each type of laser, are described, and their optical characteristics with respect to atomic spectroscopic applications are reviewed. It has been shown that solid-state OPO lasers have the advantages of flexible and broad tunability, rapid wavelength scan speed, high output energy, and relative ease of operation. Titanium:sapphire lasers have similar characteristics to OPO lasers, but it is more complicated to achieve wide tunability throughout the UV-vis and near-IR, and the output energy in the ultraviolet region below 220 nm is relatively low. Finally, semiconductor diode lasers look promising because of their compactness and potential for relatively low price. Much of the success of diode lasers lies in the future commercial availability of long-lived, blue diode lasers.

I.

INTRODUCTION

For analytical atomic spectrometry, excimer lasers, Nd:YAG lasers, dye lasers, semiconductor diode lasers, noble gas ion lasers, nitrogen lasers, carbon dioxide lasers, and ruby lasers have been used for atomic absorption, atomic fluorescence, laser enhanced ionization, and laser ablation (Sneddon et al., 1997). The discussion here is focused primarily on pulsed lasers, rather than CW lasers, because pulsed lasers have been the most successful due to their wide tunable wavelength range and high-pulse energies. It turns out that most laser-based analytical atomic spectrometric methods are still at the research stage, mainly due to the lack of lasers with adequate performance and operational characteristics. Lasers for atomic spectrometry should have broad wavelength tunability, and they should emit in an appropriate spectral region, usually in the ultraviolet (UV) and visible. In addition, they should have high output energies (l.tJ to mJ) in the case of pulsed lasers, and high average power in the case of continuous wave lasers. Further, excitation of atomic lines requires a narrow spectral linewidth in the picometer range in order to achieve freedom from spectral interferences and maximum excitation efficiency. Good long- and short-term stability is necessary to allow for reliable generation of calibration curves and quantitative analysis of large numbers of samples. Lasers for atomic spectrometry should also approach the ideals of compactness, low price,

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and ruggedness with respect to ease of operation and maintenance. In the case of pulsed lasers, additional parameters such as pulse-to-pulse stability, pulse duration, and pulse repetition rate should also be taken into account. Two main problems have become evident over the years of development of laser-based atomic spectrometric techniques. These are the lack of facile wavelength tunability and lack of ruggedness of the technologies involved. Among the lasers mentioned above, the number of available wavelengths is limited, except for dye lasers for which the wavelength is tunable over about 30 nm with each dye in the visible, and more than 10 dyes are required to cover the entire visible range. The practical difficulties that are encountered during dye changes prevent effective implementation of those applications that require a wide tunable wavelength range for accessibility to different wavelengths or for continuous wavelength scans. In addition, the lifetimes of dyes are limited, and the disposal of potentially hazardous, flammable, and toxic dye solutions is a problem (Ledingham and Shinghai, 1991). For high-energy and high-repetition rate dye lasers, the dye circulation system can be noisy, complex, and bulky. Various advances in solid-state laser systems have indicated that they possess some advantages over traditional types of laser. Therefore, these lasers are the focus of this chapter. The word "solid state" refers to the solid-state laser gain media, which are often transition metal-doped crystals. Several types of solid-state lasers, such as optical parametric oscillator (OPO)-based lasers (Radunsky, 1995, 1997; Rowland, 1997), titanium:sapphire (Ti:sapphire) lasers (Ledingham and Singhai, 1991), and alexandrite lasers, are commercially available. Compared with dye lasers, these lasers have the advantages of wide spectral tunability (220-2000 nm) good long- and short-term stability, long lifetime of the solid-state gain media, and ease of operation, although maintenance issues are still a problem. These new types of solid-state lasers have been demonstrated to have promising analytical applications in the areas of analytical atomic spectrometry, Raman spectroscopy, and near-IR absorption spectroscopy. There is no population inversion involved in the OPO process, although the output of an OPO-based laser is, indeed, laser radiation (see later). Ti:sapphire lasers have a wide tuning range from about 700 to 900 nm, which can be frequency doubled into UV between 350 and 450 nm. This is a fairly restricted range when compared to an OPO laser, especially for atomic spectrometry where the most useful wavelengths are below 350 nm. The remaining gaps in the visible and UV can be filled by use of frequency-doubling and mixing techniques, or Raman shifting, and/or optical parametric frequency conversion, but a fully tunable laser system based on a Ti:sapphire laser is complex, expensive, and of somewhat low output energy, especially in the deep UV region. Nevertheless, the solid-state nature of a Ti:sapphire laser results in considerable practical advantages over dye lasers. Semiconductor diode lasers are attractive because of their small size, potential for being low cost and easy to use, and for their ability to have their wavelength controlled electronically.

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II. OPTICAL PARAMETRIC OSCILLATOR (OPO)-BASED LASERS The history of optical parametric oscillation is almost as long as that of the laser itself (Armstrong et al., 1962). In fact, the basic principles of the parametric process had been known long before the invention of the laser, and date back to the invention of the maser (microwave amplification by stimulated emission of radiation). The first successful research and theories of optical parametric oscillation were reported in 1965 (Giordmaine and Miller, 1965). However, it is only recently that visible and UV OPO lasers have become commercially available and successful. There are two reasons for the resurgence of interest in OPO lasers. First, suitable pump lasers have become available that are acceptably stable and of spectrally narrow linewidth. Second, suitable nonlinear optical crystals have become available, such as [3-barium borate that can support the OPO process. These advances have led to the development of a variety of practical optical parametric oscillators and amplifiers from continuous wave systems, to nanosecond, picosecond, and femtosecond pulsed lasers. Simon and Tittel (1994) have summarized some properties of nonlinear optical crystal materials. The ideal crystal for the optical parametric process possesses the following properties: broad optical transparency to ensure low absorption over the entire tuning range; high nonlinear coefficient and sufficient birefringence for phase matching over a wide tuning range; high damage threshold to withstand the high pump fluence that is required for the nonlinear interaction; good temperature and chemical stability with no significant degradation over time; and ease of sample fabrication which includes the availability of large homogeneous crystals and compatibility with crystal polishing and coating procedures. Several types of nonlinear crystal, such as ~-BaB204 (BBO), LiB305 (LBO), and KTiOPO 4 (KTO), have been used as OPO media. For example, the BBO material developed by Chen et al. (1985) has a damage threshold of about 13.5 GW/cm 2 for a 1 ns pulse; a large transparency window with over 50% transmission from 200 nm to 2.6 l.tm; and a high nonlinear coefficient and birefringence for several phase-matching configurations over a wide wavelength range. 13-barium borate was one of the first nonlinear crystals to satisfy the requirements for optical parametric oscillator-based laser systems. Owing to recent developments in crystal growth techniques such as the immersion-seeded method, the availability of large BBO crystals of good optical quality is no longer problematic (Tang and Cheng, 1995). The initial patents for BBO, applied to visible wavelength OPO systems, were issued to Tang et al. in the early 1990s (United States Patents #5,033,057, #5,047,668). Meanwhile, the performance of pump lasers, such as injection-seeded nanosecond-pulsed Nd:YAG lasers, has been improved to the point that they have the desired temporal and spatial characteristics: high output energy; narrow spectral linewidth; spatial beam pointing stability; and small spatial divergence (Tang and Cheng, 1995). In 1993, the combination of all these factors

New Types of Tunable Lasers

103

led to the development of the first commercial narrow spectral linewidth BBObased OPO system for the UV-vis (Rowland, 1997).

A. Fundamentals of Optical Parametric Oscillation An OPO laser system usually consists of a pump laser, and one or more types of optical parametric devices (OPD). OPDs can be categorized into the optical parametric generator (OPG), the optical parametric amplifier (OPA), and the optical parametric oscillator (OPO). Optical parametric devices are based on a nonlinear optical process in a nonlinear optical medium. A laser beam of frequency COp, propagating in the medium, is converted simultaneously into two lower energy laser beams called the signal, at frequency cos, and the idler, at frequency c0i. The conversion can start from either spontaneous or stimulated emission. Figure l a shows a representation of the process in comparison with the optically pumped, three-level laser of Figure lb. Figure l a indicates that the output wavelengths can be manipulated by changing the frequency of the pump beam and/or by changing the position of the virtual level between the idler and signal beams such that COp= cos + coi. Manipulation of the virtual level is the most common approach. Although the output of an OPD-based laser system is indistinguishable from a laser, the two are generated in a fundamentally different fashion. In principle, an OPD is a nonlinear frequency conversion device, rather than a laser in which the gain originates from population inversion between discrete energy levels such as those represented in Figure lb. The resultant signal and idler outputs are at the expense of a decrease in energy or power at the pump frequency.

upper energy level

~

pump

....... r

idler

i signal

~

"",,,

optical pump

cos

~1~

(a)

non-radiative

transition

ground state (b)

Figure 1. (a) A three-level representation of a three-photon parametric process. A high-energy photon, (or), from the pump laser breaks down into a lower energy signal frequency at cos and idler frequency at (oi. The energy levels in dashed lines are virtual and tunable, but no specific atomic or molecular transitions are involved. (b) Energy level representation of a conventional three-level laser, which relies on specific energy levels in its gain medium.

104

HOU, ZHOU, YANG, STCHUR, and MICHEL

Figure 2 gives a schematic indication of the three major types of OPD. An OPO is contained in a laser cavity, but no end mirrors are required for either an OPG or an OPA. In an OPG (Figure 2a) only pump radiation is required to generate tunable signal and idler outputs (Zhang et al., 1993; Petrov et al., 1994). In an OPA (Figure 2b) both an input signal beam at fos, and pump beam at fop are directed into the nonlinear medium, where they undergo the optical parametric process and generate the idler beam at foi' where foi = fOp -- fOs" T h u s , the signal beam at fOs is amplified. In an OPO, a nonlinear crystal is placed in an optical resonator to form feedback for the signal, or idler, or both (Figure 2c). A simultaneous oscillation develops at the signal fOs, or idler fOi, or both fOs and fOi, depending upon the design of the cavity.

pump o ~

BBO

.... ..~..~..... ~ cos ~

idler signal

r

pump

(a)

pump cop

f

........................... 99

cos or

BBO

r

idler signal

pump

(b)

M1

P~'~I~

.... .~! ..... .~ cos ...~ ...... ~.~'; .... ~.=

M2

BBO

~____~.~..;idler ........... signal

COp > p u m p

(c) Figure 2. (a) An optical parametric generator (OPG) where a pump wave at frequency O)p is used to interact with a nonlinear optical medium, the BBO crystal, to generate tunable signal radiation at frequencies C0sand idler radiation at frequencies 0ai. (b) An optical parametric amplifier (OPA) where weak signal, or idler, seed radiation is amplified while an idler or signal, beam is generated simultaneously. (c) An optical parametric oscillator (OPO) in which a resonant cavity is employed for one or more of the waves that are generated, and tunable output results from the same nonlinear process as in (a) and (b).

New Typesof Tunable Lasers

105

Several commercial OPO lasers are available that are based on various permutations and combinations of OPDs (see Section II.B).

Three-Photon Optical Parametric Process The three-photon optical parametric process in OPDs is one of many nonlinear phenomena that result from second-order optical nonlinear susceptibility. In that process, the electrical field of a pump laser interacts directly with the charged species inside the nonlinear crystal, and to create oscillating dipoles. The negatively charged electrons respond with significant displacement, while the motion of the positively charged ion cores can be neglected due to their greater mass. If the pump laser fluence is strong, the response to the driving field is no longer linear. The oscillations take place in an anharmonic potential well, and nonlinear polarization is induced. The magnitude of the induced polarization P, per square volume, depends on the magnitude of the applied electric field E (in cgs electronic units). Macroscopically, the polarization P can be expressed as,

P = Z O) E + Z (2)E2+...

(I)

where Z Cm) are dimensionless coefficients termed "susceptibility" coefficients, which are dependent on frequency and temperature. The second-order nonlinear susceptibility7,42)is the foundation of many important nonlinear effectsincluding second harmonic generation (SHG) and the various three-photon parametric processes. It is only noncentrosymrnetric crystalsthat display second-order nonlinear susceptibilities,as second-order nonlinear susceptibilityreduces to zero for crystals with inversion symmetry. For a plane light wave that consists of two differentfrequencies, cos and COp,and amplitudes, E s and Ep, respectively,the electricfieldmay be expressed as: E = Ep cos ((apt) + E s cos (COst)

(2)

Combining Eqs. 1 and 2, the second-order term at an arbitrary point in space, becomes: 2 X(2)Ep E s cos

((apt) cos (COst)

(3)

Expression 3 can be rewritten as 2 ZC2)Ep E s {cos (COp+ (as) t + cos (cop - (as) t}

(4)

From the above expression it can be seen that the second-order nonlinear susceptibility will give rise to a nonlinear polarization, and reemit radiation at cop + (as (sum) and cop - COs (difference) frequencies when two primary beams, at frequencies (av and cos, interact with a nonlinear crystal. In a three-photon parametric process, a signal beam at (as can thus be amplified by the presence of a strong

106

HOU, ZHOU, YANG, STCHUR, and MICHEL

pump beam at C0p, while an idler beam at fop - fOs is generated simultaneously, as described in expression 4. The three-photon parametric process can be imagined as an energy conversion process in which a pump photon with wave vector Kp at a circular frequency fOp breaks down to two lower frequencies signal fOs and idler fOi with wave vectors K s and K i, respectively, as shown in Figure 3. The parametric interaction is considered between the pump photon and the second-order nonlinear susceptibility Z(2) and it couples the energy from the pump radiation into two new waves. The term "parametric" comes from the trigonometric relationship between the x and y coordinates of a moving point and a third parameter. Two parametric equations relate the coordinates to the third parameter.

Energy and Momentum Conservation Energy conservation is maintained during the three-photon parametric energy conversion process: fOp = COs + foi

(5)

This process can be represented by analogy with an optically pumped three-level laser (Figure lb). The upper level (Figure 1a) could be tuned by adjustment of the pump frequency, C0p, but the tuning is normally achieved by adjustment of the middle level. This implies the generation of infinite combinations of output frequencies, as numerous frequency pairs can satisfy Eq. 5. Consequently, OPO lasers can produce wavelengths that are difficult to generate by conventional laser sources that depend on specific transitions in a material. This infinite combination of output K,

Ki (gp = (Os + (Oi

and

Kp = K, + Ki

Figure 3. The three-photon parametric process, which is governed by energy and momentum conservation, is a process in which a pump photon with wave vector Kp, at a circular frequency cop, breaks up into two lower frequency signal and idler photons with wave vectors Ks and Ki at COsand CObrespectively.

New Typesof TunableLasers

107

frequencies is the origin of the broad tunability of OPDs. However, it is the momentum conservation, or phase matching, that governs the process to yield a specific frequency pair, Kp = K s + K i

(6)

where Kp, K s, and K i are the momentum vectors or wave vectors for the pump, signal, and idler waves, respectively, as defined previously. The magnitude of the vector K depends on refractive index, n0~ K= r

(7)

where n is the refractive index that the radiation at frequency co is experiencing, and c is the speed of the lightmfrom which, the momentum conservation, Eq. 6, can be rewritten as, np O)p-" n s ~s + ni 0~i

(8)

where np, n s, and n i are the refractive indices for pump, signal, and idler radiation, respectively. The phase-matching condition is said to be satisfied, when Eqs. 5 and 8 can be combined to give t.0p(np - ni)

(9)

COs --

ns - ni Equation 9 shows how the output of an OPD can be tuned, although it is a somewhat oversimplified model. There are two ways to tune the output frequencies. One is to change the pump frequency; the other is to manipulate one or more refractive indices. The latter is almost always used in practical OPO lasers, as the change of refractive indices through the manipulation of either the temperature or the orientation of the crystal can be achieved in a nonlinear, anisotropic medium, such as a BBO crystal. Moreover, the phase-matching characteristics and broad wavelength coverage of available nonlinear optical materials give OPO lasers a wide wavelength tuning range.

Angle Phase Matching The angle phase-matching method was first proposed by Maker et al. (1962) and, independently, by Giordmaine (1962). The approach employs the birefringence of a uniaxial crystal. If an object is viewed through such a birefringent crystal, in unpolarized light, two images are seen which result from different refractive indices experienced by light of different orthogonal polarizations. Manipulation of the angle of incidence of light on the crystal changes the refractive index experienced by one polarization of the light, called the extraordinary ray, but not the orthogonal polarization called the ordinary ray. Hence, one image moves when the crystal is

108

HOU, ZHOU, YANG, STCHUR, and MICHEL

rotated while the other remains stationary. Consider the wave vector constructions for refraction shown in Figure 4. For the ordinary ray, the refractive index is independent of the direction of propagation, as shown in the solid locus of Figure 4, regardless of whether the material is isotropic or anisotropic. For the orthogonally polarized extraordinary ray in an isotropic material, the locus of the wave vector is also a sphere, and independent of the direction of propagation, but in an anisotropic material the locus is an ellipse depicted as a dotted line in Figure 4. For the extraordinary wave, the different refractive indices of an anisotropic material at different angles of incidence lead to the double-refraction phenomenon called "birefringence". It is the birefringence that is exploited to enable phase matching in an anisotropic material. If the value ofn e - n o is larger than zero, the birefringence is said to be positive; and for n e - n o smaller than zero, the birefringence is negative. The corresponding crystals are termed positive or negative uniaxial, respectively. Figure 5a illustrates a typical curve of refractive index as a function of wavelength. The dispersion causes the refractive index to drop with increased wavelength. As a result, the pump wave vector Kp is always too large and causes the phase mismatch, AK, that prevents conservation of momentum. However, for a nonlinear crystal, the refractive index for extraordinary rays varies elliptically, depending on the angle between the direction of propagation of the incident light, and the axis of symmetry, or optical axis, of the crystal. In a negative uniaxial nonlinear crystal, such as a BBO, the refractive index that an extraordinary ray experiences is always lower than, or at most equal to, that for an ordinary ray. The maximum index is reached when the extraordinary ray is propagated along the crystal's axis of symmetry. Such birefringence can be used to selectively vary the vector magnitude of the extraordinary wave to compensate for the dispersion and

i

optical axis q 4

i o

~o~.a1.

no . I

ss

p

Figure 4. In an anisotropic medium, such as BBO crystal, the wave vector of an ordinary ray follows the spherical locus, while the wave vector of an extraordinary ray follows the ellipsoidal locus.

109

N e w Types of Tunable Lasers

~

~K

e-

x" (D "10 _= ._> 15

Kp

t Ki

Ks

Q) n,"

UV

Vis

IR

(a)

t"

1

>~ r

~dinary

'1o

KpT I Ki

.>_ (D n,'

(op, np extraord~'na'01.:--_- . . . . . . . . . F-o~-r~ UV

Vis

IR

(b)

Figure 5. (a) Dispersion exists in any optical material and causes a phase mismatch, AK, which prevents conservation of momentum in an isotropic medium. (b) The phase mismatched condition can be obviated because the dispersion of the extraordinary pump beam can be slowed in an anisotropic medium.

allow phase matching (Figure 5b). If the pump is introduced as an extraordinary wave, the resultant signal and idler can emerge as ordinary waves, while the K vector for the pump, Kp, can be manipulated through the angle of incidence, (0 in Figure 6) which results in the required phase match (AK =~ zero). Another approach is to vary the crystal temperature and induce index changes in order to achieve the desired wave vectors. As depicted in Figure 6, phase matching occurs by variation of the vector magnitude of the pump wave until the point where the resultant extraordinary wave vector is equal to that of the signal plus idler wave vectors for the two ordinary rays. The corresponding phase matching condition, Kp(e) --~ Ks(~ + Ki(~ is termed Type I phase matching. If either the signal or the idler wave shares the same polarization as the pump wave, then it is called Type H phase matching.

110

HOU, ZHOU, YANG, STCHUR, and MICHEL

S. ~f

i.==== l= ====Q ~

~,

optical axis

s ~'

pump(e-ray)

,-0-0-I-

signal + idler (o-rays) pump(e-ray)

Figure 6. Phasematching occurs at the intersection of the spheres that correspond to the combined spherical vector surfaces of the two ordinary rays, which are the signal and idler beams, with the ellipsoidal vector of the extraordinary ray, which is the pump beam. This figure is only an approximate representation of the phase matched vectors. B. Examples of Commercial Optical Parametric Oscillator Lasers With the advent of the BBO crystal and the maturity of spectrally narrow pump lasers, several OPO lasers have become commercially available in recent years. The dominant commercial narrow-band OPO lasers are based on diode injection-seeded Nd:YAG laser pumped BBO OPDs. These lasers are usually pumped with the third harmonic of Nd:YAG laser at 355 nm with a repetition rate of 10 Hz. The wavelength tuning range is in the range of 220 nm to 2 l.tm. Figure 7 shows the optical layout of one commercial pulsed OPO laser system (Johnson et al., 1995; Zhou et al., 1997). In this system, a frequency-tripled Nd:YAG laser, with a 6 ns pulse width and injection-seeded with a laser diode, is used to pump a tunable OPO equipped with a second harmonic generator (SHG). The OPO section consists of a master oscillator and a power oscillator. While the master oscillator is a true OPO, the power oscillator is no more than an OPA in a resonant cavity seeded by coherent monochromatic radiation from the master oscillator. It can also be thought of as a combination of an OPA and an OPO. Type I phase matching is used for the master and power oscillators in combination with a "collinear pump" configuration that aligns the pump and output beams colinearly. Typically, the 355 nm pump energy is in the range from 450 to 540 mJ, with 450 mJ being a threshold level below which the OPO does not oscillate efficiently. By use of a dichroic beam splitter about 30% of the pump energy is directed to the master oscillator, while the rest is directed to the power oscillator after delay in an optical delay line. In this design, a singly resonant scheme was adopted for both

New Types of Tunable Lasers

111

delayed 355 nm

i

+

,., ...... .i.I ............. }. ..... ii i I ' : : ' ; :;I "% rl BBOI ~" I::; I : " BS | BS v I ........................... ..... Pow Oscillator .. ..

,Y~ " I ,-

9i ; ; ............

;; .... i ...... ; ;

~~IBBOI

~.............. Master Oscillator

~,

i

' ......................... 3 (o[ BP BP. l ,~i \ / ~-?--;;--)~ i '-I -'~ to beam dump ,#, l ; ~ =~ ! I "5 ,";~ l;. ~ it',.... ...""",..""'"""",,.,, ";...,,~.:. ,:.i:.!": ,,,,,,I ,' D.._ ,.~, o

.a...,i

iBBOt_~BBOI.~[

- .............................. :355 nm

P.B.P" SHG

Figure 7. A schematic diagram of a Spectra Physics OPO laser system equipped with a second harmonic generator (SHG). This 10 Hz pulsed OPO laser consists of a master oscillator and a power oscillator (MOPO). BS = beam splitter, BP = beam steering prism, PR = beam steering and polarization rotation prism, GT = grating, TM = tuning mirror, PBP = Pellin-Broca prism. cavities, in which only one output, the signal, is resonant, which allows for relatively simple cavity designs. An intracavity grating set at grazing incidence is used in the master oscillator to achieve both a narrow linewidth and a wide tuning range. The master oscillator crystal is mounted on the same shaft as the power oscillator crystal, which allows the output wavelengths of the master oscillator and power oscillator to be tuned synchronously. Output from the power oscillator is directed into the SHG by use of beam steering and polarization rotation prisms. Polarization rotation is used to satisfy the phase-matching requirements of the SHG crystals. Two BBO crystals cut at 56 ~ and 36 ~ are used in the SHG to cover the wavelength range from 220 to 440 nm. The output wavelength of this laser system can be tuned, under computer control, from the UV to the NIR via synchronous rotation of the four BBO crystals and the tuning mirror. The tuning mirror is driven linearly by use of a sine bar. There are two basic modes provided by the manufacturer for wavelength tuning. One is the "track" mode which uses a photodetector to monitor the output power and allow feedback to control the crystal position, while the other is a "table" mode which employs a lookup table to locate a previously optimized crystal angle for each wavelength. In general, the "table" mode provides more short-term stability, as long as the laser has been aligned correctly. Two high-damage threshold 355 nm dichroics route the pump beam directly through the BBO crystal, then out into a beam dump. This design avoids any necessity to direct the pump beam through the cavity resonator optics which have a low-damage threshold due to

112

HOU, ZHOU, YANG, STCHUR, and MICHEL tuning

sealed OPO cavity

SHG

mirror [..................................................................i[ ............. ~

! :

t

~k

I ,

grating~i j

,

.

I

(

~ip

BBO

il "~,

:

/" I" " ....... .............................., ....................

pump retro-reflector /

half'halfw a v e plates

I

i

9

[,;

355 nm pump ;; ;

. ............................................. "

i .

delayed 355 nm

BBO

9

~

BBO

i

.!

A IX>,

i

i ;

iUV ~

r ,--v,,,

i

--

/

...... 3

-B

............................................. .: ~r ; sealed OPA cavity 355 nm to beam dump

!

'o?V] m

..... :

i 1

"r" LI~ } [

J'ii

o/2= O m m

[~TJ"

m

~ . . . . . . . . . . . . ....~... ......

~

_L.. I. /

prism

"9 '[

[ !!

t'

!

! i

: !

Figure 8. A schematic diagram of a Continuum Sunlite OPO laser system with a second harmonic generator (SHG). The laser system consists of an extraordinary resonance OPO and a single-pass OPA, with all crystals sealed in temperature stabilized enclosures. constraints imposed by the necessary broadband optical coating materials in the resonant cavity. A somewhat different commercial design is shown in Figure 8, and involves Type I phase matching, and an OPA pumped with a 10 Hz pulsed Nd:YAG laser. For this design, a typical pump energy at 355 nm is 320 mJ, of which 20% is directed to the OPO and the rest to the OPA. An attenuator and various telescopes are used to provide the oscillator with the proper pump beam divergence and energy density. Both the OPO cavity and OPA cavity are sealed in temperature-controlled enclosures to protect the hygroscopic BBO crystals. This minimizes the influence of ambient temperature, and stabilizes the crystals' phase-matching conditions. An interesting aspect of this laser system is its OPO cavity design (Figure 9) in which the polarization of the signal is rotated by 90 ~ with a half-wave plate, for dispersion by the grating in the extraordinary plane. This results in higher diffraction efficiency, and the small acceptance angle of both the grating and the crystal in their extraordinary planes combine to reduce the spectral linewidth of the signal. The tuning mirror retro-reflects the first-order diffraction of the signal onto the grating so that it returns into the cavity in the usual way. The cavity geometry is not designed to allow the idler to resonate in the cavity. After exiting the OPO cavity the signal is rotated back to the vertical polarization by another half-wave plate to seed the OPA. The active elements of the OPA are a pair of phase-matched Type I BBO crystals, the optical axes of which are a mirror image of each other. Therefore, when

New Types of Tunable Lasers

113

Figure 9. The extraordinary resonance design of the OPO cavity of Figure 8, showing the BBO crystal dispersing light along its extraordinary plane, and its grating further dispersing the light in the same plane. The half waveplate rotates the polarization of light that strikes the grating, which increases its diffraction efficiency. (Reproduced with permission from Radunsky, M.B. (1995). Copyright 1995 Pennwell Publishing).

they counter rotate during a wavelength scan, the pointing direction of the laser beam at the output is kept constant. Compared to the resonant design described in Figure 7, the objective of the single-pass OPA design of Figure 8 is to produce less degradation of beam quality by the back-conversion that occurs during resonance.

C. Characteristics of Optical Parametric Oscillator Lasers Output Energies of the Laser Pulses To pump an OPO laser, the output energy of a YAG laser at 355 nm may vary from 320 mJ to more than 450 mJ, depending upon the design of the OPO laser and the spatial and temporal characteristics of the YAG pump beam. For an OPO laser system that was characterized in our laboratory, the YAG pump energy for a beam size of 9-10 mm was between 460 and 540 mJ per pulse over about 4 months of 40 hour weeks. It was found that when the energy of the YAG laser dropped below 400 to 450 rnJ, the OPO began to stop oscillating at a gradually increasing number of wavelengths. Although the oscillation could often be restarted, by tweaking the alignment of the BBO crystals and other optics at each wavelength of interest, this condition generally indicated that the flash lamps should be replaced and the OPO laser realigned. There was also a fear that the beam quality of the YAG laser might have deteriorated to the point that it might damage optical components, or that refocusing the 355 nm radiation more tightly into the BBO crystals to compensate for energy loss might also damage the optics. Figure 10 shows the average output

114

HOU, ZHOU, YANG, STCHUR, and MICHEL 120 100

80 60

(a) signal

40 "-3

E

>~

G) t--

w

0

20 0 400

500

600

700

60

45

idler

30

(b)

15

500

750

1000 1250 1500 1750 2000 Wavelength, nm

Figure 10. Average output energy of the OPO laser system of Figure 7 as a function of wavelength: signal (a) and idler (b). The error bars represent the long-term energy stability of the laser, and are the difference between the minimum and maximum energy of four scans obtained during the course of 6 days. The mean of the four scans was plotted. energy per pulse of four scans of the OPO laser system of Figure 7. The scans were obtained over a period of six days. The output energies per pulse were 35-90 mJ for the signal beam, and 1-40 mJ for the idler beam. The error bars in Figure 10 represent the extremes of the range of energies that were measured. The energy levels were stable at most wavelengths with variations in range between +2% and +10%, although the variations were up to +15% at the blue end, around 440 nm. The tuning range was extended by use of a second harmonic generation (SHG) section which had typical output energies in the range 1-11 mJ (Figure 11). The SHG wavelength range was 220-440 nm, with a gap around 355 nm due to the degeneracy of the OPO's fundamental signal and idler beams around 710 nm. For the laser of Figure 8, also characterized in our laboratory, the output of the idler beam had energy levels in the range of 1-40 mJ, while the signal beam had

New Types of Tunable Lasers

115

12-

-:

E

crystal change

9-

>~

eIii

6-

=3 t'3

I degeneracygap O

3-

0 200

I

I

I

I

I

250

300

350

400

450

Wavelength, nm

Figure 11. Output energy of the frequency doubled radiation for the laser of Figure 7 in the wavelength range from 220 to 440 nm. Two crystals cut for 56 ~ and 36 ~ were used to cover the measured output range. The first part of the curve (e) was the frequency doubled output, obtained by use of the signal beam, while the second part (0) was obtained by use of the idler beam. Reproduced with permission from Zhou, J.X. et al. (1997). Copyright 1997 American Chemical Society.

energies in the 10-55 mJ range, and the YAG pump energy was about 320 mJ. The output energy of the SHG section was between 2 and 14 mJ in the wavelength range from 225 to 450 nm. This represented a high SHG conversion efficiency of about 20%. The stability of the output of the SHG section was 10-20% over a period of three days. The high energies that are obtained from these laser systems exceed those required for atomic spectroscopy, and it is useful to use an optical attenuator to control output energies. For laser ablation, these energies are the same or more than is required.

Pulse-to.Pulse Energy Stabifity The pulse-to-pulse stability can be obtained by measurement of the relative standard deviation (RSD) of the energy of the laser pulses. For commercial OPO lasers, pumped with 355 nm radiation from a Nd:YAG laser, there are several factors which affect the pulse-to-pulse stability of an OPO laser. These factors include the inherent pulse-to-pulse stabilities of the Nd:YAG laser, and the signal and idler laser beams. Other variables include the ambient temperature which must be maintained within about +2.5 ~ and the accuracy of the optical alignment of the OPO laser

116

HOU, ZHOU, YANG, STCHUR, and MICHEL

system. Typical pulse-to-pulse RSDs of the 355 nm pump radiation are normally better than 2%. For the OPO part of the system, and through most of the wavelength range in the visible (signal) and infrared (idler), the RSD is 2-6%, but the RSDs degrade at the extreme wavelengths of operation to a level as high as 12-14%. Typical RSDs for the second harmonics of the signal and idler are around 3-12% for most of the wavelength range. Figure 12 shows the RSD of the second harmonic of the signal beam of the OPO of Figure 8. For the doubled radiation there appears to be no relationship between the RSD and wavelength, in contrast to the situation with the signal and idler beams which show increases in RSDs at the extremes of the wavelength range. A summary of approximate laser beam size, pulsed output energies, and pulseto-pulse RSDs for the lasers of Figures 7 and 8 appears in Table 1.

Signal Spectral Linewidth Figure 13 shows a typical set of interference fringes recorded with a linear charge coupled detector (CCD) array during a laser linewidth measurement at 595 nm with the laser of Figure 7. About three laser pulses were averaged to obtain the fringes, but significant multiple peaks in the interference fringes were observed when a larger averaging time was used at the CCD, which could indicate mode hopping or beam pointing variations between pulses. The spectral linewidth of the OPO laser of Figure 7 is typically between 0.1 and 0.25 cm -1 for the signal beam. Figure 14 16

l~" 1 2 oo 9

00

9

9 90

6 oo

9

a.

0

"

200

I

I

I

I

I

250

300

350

400

450

500

Wavelength, n m Figure 12. Pulse-to-pulserelative standard deviation (RSD)of the frequency doubled output of the signal beam from the OPO laser of Figure 8. The RSD was measured and calculated for 50 pulses at each wavelength.

117

New Types of Tunable Lasers

Table 1. Comparison of Some Features of Two Designs of Nd:YAG Pumped Nanosecond OPO Laser

OPO Laser

Typical Beam Diameter (mm)

Laser Energy (mJ/pulse)

Typical Pulse-toPulseStability (RSD, %)

10

>450

2

8

35-90

5

idler SHG

7 8

1-40 1-11

5 5

YAG pump at 355 nm signal

8

320

2

7

10-55

5

idler SHG

5 6

1-40 2-14

8

Laser Beam

Spectra Physics MOPO (Figure 7)

Continuum sunlite (Figure 8)

YAG pump at 355 nm signal

5

1000 -

:j

800 -

c::

600 -

r

> :~

400

o) n~

200

I

I

I

I

I

I

200

400

600

800

1000

1200

CCD Pixel Figure 13. Typical interference fringes for the laser of Figure 7, recorded at 595 nm with a linear CCD array. The fringes were formed by an ~talon and imaged with a biconvex lens of 30 cm focal length onto a linear CCD array. Three consecutive interferograrns were averaged and recorded. The calculated laser spectral line width was about 0.2 cm -I.

118

HOU, ZHOU, YANG, STCHUR, and MICHEL 0.4

'

,7, 0.3E o

.=-o 0.2..... t.d .m

0.1-

.0

450

I

I

I

I

500

550

600

650

700

W a v e l e n g t h , nm

Figure 14. Linewidth versus wavelength for the signal beam from the laser of Figure

7. The interferograms were recorded at wavelength intervals of 5 nm from 460 to 690 nm for the signal beam. The integration time for the interference fringes was such that the average of two or three consecutive interferograms was recorded. The linewidth was calculated by use ofthe distances between the interference fringes and their width, together with the free spectral range of the ~talon.

shows a series of measurements of the same laser's linewidth as a function of wavelength. For the OPO laser system of Figure 8, the different cavity design leads to a narrower spectral linewidth by about a factor of 2.

Wavelength Calibration Accuracy The wavelength calibration accuracy of the output of an OPO laser can be measured by centering the laser's wavelength at known atomic lines and observing atomic fluorescence. Comparison of the known wavelength with the wavelength on the readout of the OPO laser system controller allows an estimate of the calibration accuracy of the laser. For the laser of Figure 8, the results of such an experiment are shown in Figure 15, where the deviation of the readout from the known atomic wavelength is within the laser linewidth. This degree of calibration accuracy allows an atomic line to be found, in the first instance, by a simple "go to" command on the laser controller.

Rapid ScanSpeed One of the most important advantages of an OPO laser is that it can be rapidly tuned from wavelength to wavelength over a broad range from 220 to 2000 nm. If the laser is slewed at high speed between wavelengths, typically at up to 2.5 nm

New Types of Tunable Lasers

119

40 E 30,.,t" <1

Mn

~" 2 0 -

0

. ....=

(D

a

Pb

10-

270

Cu

Co

. ,,..,

280

I

I

I

I

290

300

310

320

330

Wavelength, nm

Figure 15. Accuracy of the wavelength calibration for the laser of Figure 7. The deviation, AX, was calculated as the difference between the wavelength reading of the laser and the known atomic wavelength of each element. The known atomic wavelength was located by use of flame laser excited atomic fluorescence spectrometry (flame-LEAFS).

S-1, the destination wavelength is reached accurately, as indicated by Figure 15, although the laser may not lase reliably when slewed. If it is required to maintain lasing during a scan of a desired wavelength range, these lasers can also be scanned continuously at slower scan speeds. However, in present commercial versions of pulsed OPO lasers the accuracy of the wavelength calibration is compromised if the laser is continuously scanned while a spectrum is recorded. This is illustrated, for sodium fluorescence, in Figure 16 where the laser of Figure 7 was continuously scanned through the sodium "D" lines, at two different speeds. An apparent shift in wavelength is observed, which is probably a result of serial port communication delays between the OPO laser's host computer and the user's data collection computer. High scan speeds may be desirable for atomic spectrometry to allow screening for multiple elements, although a rapid slew scan to predetermined wavelengths is a useful compromise.

Sensitivity to Ambient Temperature OPO lasers have been found to be sensitive to environmental temperature changes. The observed effects include a steady drop in power and degradation of the RSD in the energies of pulses as the ambient temperature changes. Overall these effects can be ascribed to changes in optical alignment within the laser, of which the two main problems appear to be a shift in beam position of the Nd:YAG laser

120

HOU, ZHOU, YANG, STCHUR, and MICHEL _

5 0.01 nm s"1

e"

o t-

_

t~ 0 U.. _

0.05 nm s "1

~

n,' _

I

I

I

589.0

589.5

590.0

Wavelength, n m Figure 16. Sodium excitation spectra that result from a continuous scan and flame LEAFS at two different scan rates, 0.01 and 0.05 nm/s. An apparent shift in wavelength was observed at the higher scan rate.

and changes in refractive index or alignment of the OPO crystals or other optics. For example, if the position of the Nd:YAG laser's beam changes, the angle of incidence of the beam at the OPO crystal will no longer be at an optimum for the desired wavelength. In some OPO lasers the OPO crystals are placed in temperature-stabilized enclosures which provide improved short-term stability in output power, but cannot compensate for beam-pointing changes at the Nd:YAG laser. Despite these problems, adequate performance is maintained with an ambient temperature that is within +2.5 ~ of the temperature at which the laser was last optically aligned. Another aspect of beam pointing is that, despite careful attention to the optical configuration, it is difficult to prevent the output beams from moving while the laser wavelengths are being scanned. This can occur for either of the signal or idler beams, as well as for the output of the second harmonic generator that might follow the OPO section. Often, the SHG section is followed by a pellin-broca prism, which is primarily designed to separate the signal and idler from the SHG output, but which can also be programmed to compensate for changes in the direction in which the beam points. Over the full wavelength scan range of the OPO the shift in beam position can be as large as a few millimeters, or less than a mm, depending upon the design of the laser and which beam is being used.

New Types of Tunable Lasers

121

III. VIBRONIC, TUNABLE, SOLID-STATE LASERS In recent years, tunable solid-state lasers that are based on materials that contain transition metals, such as titanium and chromium, have been of considerable interest due to their solid-state nature, and broad wavelength tunability. These lasers are often referred to as "vibronic" lasers because they have transitions in which the lasing species exhibit changes in both vibrational and electronic states. The first commercial vibronic solid-state laser appeared in the late 1970s, and was based on alexandrite, which is chromium doped BeA1204. The tuning range of an alexandrite laser is approximately 700-800 nm. In 1982 Moulton demonstrated the first Ti:sapphire (Ti3§ A1203) laser, which could be tuned from 670 nm to over 1100 nm. The Ti:sapphire laser is attractive for many applications because it combines a broad tuning range and a large gain cross section (-3 x 10-19 cm2). The sapphire host has very high thermal conductivity, which allows for high-power CW pumping. The crystal is also chemically inert and mechanically rigid. Some characteristics of alexandrite, Ti:sapphire, and BBO crystals are summarized in Table 2 (Tang and Cheng, 1995; Eimerl et al., 1987; Imai, 1998) together with some characteristics of the 13-barium borate crystal that is used in OPO lasers. The latter are not directly comparable because the principles of the lasing action are different, but the information is included for the sake of completeness.

Table 2. Comparison of Characteristics of Alexandrite, Ti:Sapphire, and 13-Barium

Borate Crystals

Crystal

Alexandrite

Emission band, nm Emission peak, nm Peak of absorption band, nm Chemical formula Crystal structure Cross section, x 10-20 cm 2 Radiative lifetime, l~S Thermal conductivity, W/m.K Thermal expansion, x I 0-6/C Refractive index, n Notes: aOptical transmission range.

700-800 755 440, 640 Cr:BeAl20 4 orthorhombic 0.5-5 270 23 6 1.74

buy intrinsic absorption edge at room temperature. CN/A: not applicable.

77:Sapphire ~-BariumBorate 660-I 200 800 490 Ti:AI20 3 rhombohedral 30-40 3.2 38 8.4 1.76

190-2600 a N/A 190 b I]-BaB20 4 hexagonal N/A c N/A c 0.8 4 1.66

122

HOU, ZHOU, YANG, STCHUR, and MICHEL

A. Fundamentals of Vibronic Solid-State Lasers Tunable, vibronic, solid-state lasers are similar to nontunable solid-state lasers in that excitation of the light-emitting specie, which is doped inside a transparent host material, is brought about by absorption of external radiation. However, in nontunable solid-state lasers, such as a ruby laser (Cr3+:A1203), only discreet transitions are involved (Figure 17a). In vibronic lasers, such as alexandrite lasers, electronic energy levels are broadened into bands by vibrational sublevels (Figure 17b) that arise from vibrations of the crystalline host lattice. Hence, there exists a range of possible energy differences that allows for tunability of the laser. When a light-emitting specie is pumped by radiation to an upper vibronic energy band, it first relaxes nonradiatively to the lowest energy level of the band. The laser transition occurs when the specie then drops to a vibrational sublevel of the lower state. Further release of vibrational energy brings the atom back to the lowest state. Tuning of vibronic lasers can be achieved with wavelength-selective optics such as birefringent filters that restrict the oscillation within a narrow wavelength range.

B. Examples of Vibronic Lasers Alexandrite Lasers Alexandrite lasers have very similar energy levels to those of ruby lasers (Figure 17). The excited state lifetime is about 270 Its, and laser transitions take place

'T'I + 2T2

'T i '--,i v exciation

2T1 2E

2~ excitation

laser

emission

laser emission ground

.,

f (a)

~A2 ground

2E

] ....

"'"'~'" [

4A2

(b)

Figure 17. (a) Energy-levels of chromium in a ruby laser (Cr3+:AI203). (b) Energy-levels for chromium in an alexandrite laser (Cr3+:BeAI204).

New Types of Tunable Lasers

123

between the aT2 and 4A2 levels (Walling, 1979). A tuning range of approximately 700 to 800 nm has been demonstrated with 6talons and birefringent filters in a similar fashion to dye lasers. The lasing specie in the alexandrite laser rod is chromium, and the dopant has a concentration of 0.01-0.4%. Pump bands at 380-630 nm and at 680 nm can be used for optical and semiconductor laser pumping, respectively. Often, commercial alexandrite lasers are pumped optically by flashlamps or arc lamps. Alexandrite lasers can be operated in continuous wave mode (CW), pulsed, or they can act as a laser amplifier. The emission cross section for alexandrite is relatively low--about 6 x 10-21 cm 2 at room temperaturemwhich causes a high pump threshold, but allows for more energy storage. In combination with the long excited-state lifetime, 270 I~s, this allows for the production of high-power Q-switched pulses. An alexandrite laser can also be operated as an amplifier, which allows for the generation of high peak powers. In Q-switched, pulsed operation, commercially available alexandrite lasers (Hecht, 1992)can have an average pulse output energy over 500 mJ, in a 012 nm bandwidth, for most wavelengths in the tuning range shown in Figure 18. Overall, the tuning range and output energy level of alexandrite lasers can be readily covered by Ti:sapphire lasers or OPO lasers, and this is probably the main reason that limits the further development of alexandrite lasers.

Ti:sapphire Lasers Similar to OPO lasers, the commercial availability of Ti:sapphire lasers has largely been dependent on the maturity of the technology of crystal growth. A variety of techniques have been developed to grow large Ti:sapphire single crystals that possess the necessary spectroscopic, chemical, mechanical, and thermal prop1000 750 "

E >~ t-LU

500" 250 0

7O0

|

750

!

800

Wavelength, nm

Figure 18. The output wavelength range and pulse energy of a 20 Hz pulsed Alexandrite laser at 50 ~ Reproduced with permission from Hecht, J. (1992). Copyright 1992 McGraw-Hill.

124

HOU, ZHOU, YANG, STCHUR, and MICHEL

erties for lasing. Postgrowth annealing treatments have also been developed to further ensure high-quality, low-loss materials. Ti:sapphire lasers are available that range from CW, to pulsed, and to ultrafast, with output wavelengths between the UV and IR. A Ti:sapphire crystal has about 0.1% Ti3§ doped into the sapphire host, which replaces octahedrally coordinated A13§ ions in the crystalline lattice at sites with trigonal symmetry. The Ti 3§ ion has a single 3d electron outside the closed electronic shell of an argon core. Undoped A1203 crystals are very transparent in the nearinfrared and visible regions from approximately 2000 nm to 400 nm, and in the UV region from 400 nm to 200 nm, although there is somewhat increased absorption towards shorter wavelengths. Ti3§ A1203 crystals are pink, which results from the broad, double-humped absorption band that extends from about 400 nm to 600 nm in the blue-green region of the visible spectrum. This absorption band is due to photon-assisted excitation of the 3d electron of the Ti 3§ The various peak absorption bands at 266 nm, 216 nm, and 185 nm are of unknown origin (Sanchez et al., 1988). The free-space, five-fold degenerate, d-electron levels of Ti 3§ are split by the crystal field of the host A1203, which can be viewed as the sum of cubic and trigonal symmetry components. The cubic field dominates and splits the Ti 3§ energy levels into a triply degenerate 2T2 ground state and a doubly degenerate 2E excited state. The trigonal field splits the 2T2 ground state into two levels, of which the lower level is split further into two levels by the spin-orbit interaction. Figure 19 shows a schematic energy level diagram for Ti3§ in a Ti:sapphire crystal. Interactions between the Ti 3§ electronic energy levels and the vibrational energy levels of the crystal are responsible for the vibronically broadened 2E-2T2 transition (Figure 20). Although the laser emission is tunable from 660 to over 1100 nm, the maximum efficiency is in the wavelength range from 700 to 900 nm. The large tuning range is the result of the simple, 3d 1, energy-level structure, which avoids possible

2Eg

2E

~176176176~

i !

i

2D /

I

free ion

2A1

2T2g ,:....... 9 cubic field

2E

52

$~1 I trigonal ........... field

o~.........

spin-orbit

Figure 19. Schematic energy-level diagram for Ti 3+ ion in Ti:AI203.

125

New Types of Tunable Lasers 2E

relaxation

(9 C UJ

.o

.o

o

E

x

2~

relaxation

Figure 20. Interactions between the Ti 3+ electronic energy levels and the vibrational energy levels in the sapphire crystal produce a vibronically broadened 2E-2T2 transition, which is the origin of the tunability of a Ti:sapphire laser.

redirection of energy by reabsorption of the laser radiation. There are two key propertiesnthe concentration and the optical cross section of the Ti 3+ ionsmthat are responsible for the laser action. The absorption cross section at a wavelength ~. is given by cr= ~[Ti3+], where c~ is the absorption coefficient at g., while [Ti 3§ is the ionic concentration. Figure 21 shows the polarized absorption cross section for

~E 6.0 o 0

~" 4.0

o 2.0 L)

400

500 600 Wavelength, nm

700

Figure 21. Polarized absorption cross section for the 2T2---~2Etransition in Ti:AI203. The baseline was arbitrarily set to zero for both polarizations at 700 nm. Reproduced with permission from Moulton, P.F. (I 986). Copyright 1986 Optical Society of America.

126

HOU, ZHOU, YANG, STCHUR, and MICHEL

the 2T2-2E transition in the Ti:sapphire crystal (Moulton, 1986), while a generalized absorption-fluorescence spectrum is shown in Figure 22. For rr polarization, which is the polarization that yields the strongest absorption, the absorption cross section is 6.5 x 10-20 cm 2 at 490 nm. The absorption band from 400 nm to 650 nm allows for convenient optical pumping by a number of blue-green sources including argon lasers, frequencydoubled Nd:YAG lasers, copper vapor lasers, and flashlamp pumped dye converter systems. The most commonly used pump sources are argon ion lasers and frequency-doubled Nd:YAG lasers because the temporal pulse width of the Nd:YAG laser is appropriate for the Ti:sapphire material's short fluorescence lifetime at room temperature, about 3.2 Its. Also, the output wavelength of both these pump lasers matches the narrow range for pumping at the peak wavelength near 500 nm (Figure 22). Efficient CW laser operation requires sufficiently high intensity within the gain medium to maximize the stimulated emission process. For the Ti:sapphire laser's transition, which peaks at 800 nm, the calculated saturation intensity is 2.6 x 105 W/cm 2. This high saturation intensity necessitates tight focusing within the crystal, especially at low pump power levels. Tight focusing requires the use of a physically short laser crystal with a high concentration of dopant. The concentration must not be too high, in order to minimize potential losses associated with scattering, concentration quenching, and parasitic absorption that overlaps with the emission region of the Ti:sapphire laser. The parasitic absorption is caused by Ti3§ 4+ defect pairs in the host material. After crystal growth, and under a reducing atmosphere of hydrogen gas, carefully controlled treatment during annealing at high tempera-

1.0 s

absorption

fluorescence

t"

r 0.6-

0.2-

400

600 800 Wavelength, nm

1000

Figure 22. Generalized absorption and fluorescence spectra of a Ti:sapphire crystal.

New Types of Tunable Lasers

127

ture has been developed to minimize the number of Ti3+:Ti 4+ defect pairs. An absorption figure of merit (FOM), defined as the ratio of the absorption coefficients at the pump and emission wavelengths, has been used to assess the quality of Ti:sapphire crystals by Pinto et al. (1994). Figure 23 shows the dependence of laser performance on the crystal's FOM. FOM values for Ti:sapphire crystals decrease with increased Ti 3§ concentration. Since the Ti:sapphire medium has a relatively low gain, a compromise must be made between low-threshold operation and high-output efficiency. Laser rods with Ti 3+ ion concentration in the range of 0.1-0.15 wt% have been demonstrated to give superior CW laser performance. The product of the excited-state lifetime and the cross section for stimulated emission is approximately proportional to the inverse of the spectral bandwidth over which useful laser gain is obtained. A tunable solid-state laser, with its broad gain bandwidth, cannot match both the high cross section and long excited-state lifetime of a laser such as the Nd:YAG laser with its relatively narrow spectral output. In addition, the balance between lifetime and cross section affects the choice of pump laser. A Ti:sapphire laser has a cross section comparable to that of Nd:YAG, but its radiative lifetime is only 3.2 lets, and this is the reason that it must be pumped with short-pulsed, Q-switched, or CW lasers, or with short-pulsed flashlamps. Ti:sapphire lasers are usually designed for laser pumping and resemble tunable dye lasers in this regard. Both linear standing wave cavities and ring cavities similar to those for CW lasers are employed for CW Ti:sapphire lasers. Figure 24 shows a schematic diagram of a typical ring configuration for a CW Ti:sapphire laser

C

3 b 0

n 0

2

1

==

a

=,,,

0

2

4

6

8

10

Input Power, W

Figure 23. Laser performance data as a function of FOM, as defined in the text, for several Ti:AI203 rods. FOM = 90, 560, 1000, for a, b, and c, respectively. The wavelength of the argon ion laser pumped CW Ti:sapphire laser was at 800 nm. Reproduced with permission from Pinto, J.F. et al. (1994). Copyright 1994 IEEE.

128

HOU, ZHOU, YANG, STCHUR, and MICHEL output

birefringent crystal

lens

pump

Figure 24. Schematic diagram of a ring configuration for a CW Ti:sapphire laser cavity. Reproduced with permission from Cunningham, R. (1991). Copyright 1991 Gordon Publications.

(Cunningham, 1991). In this design, the Ti:sapphire rod is cut and polished at Brewster's angle. The broad-band folding mirrors are dielectrically coated for both high reflection at around 800 nm and high transmission in the pump wavelength range. A birefringent filter, which restricts the oscillation within a narrow wavelength range, is used for wavelength tuning, while an 6talon is employed for subsequent narrowing of the linewidth. The absorption band of Ti:sapphire is a maximum near 500 nm, which allows for argon laser pumping, or pumping with frequency-doubled Nd:YAG lasers, or copper vapor lasers. Gallium aluminum arsenide (GaA1As) diode lasers cannot pump Ti:sapphire lasers directly, but they can be used to pump a Nd:YAG laser that can be frequency-doubled to pump a Ti:sapphire lasers. Figure 25a shows the tuning curve of a Ti:sapphire laser pumped by an argon laser. The tuning range is from 665 nm to 1070 nm by use of four sets of cavity optics. For some commercial versions, five sets of mirrors are employed to maximize performance (Gray, 1989; Figure 25b). Higher pump powers from a CW argon laser produces higher output powers (Figure 26). Commercial Ti:sapphire lasers can generate CW powers of up to several watts of fundamental output. Ground-state absorption by Ti 3§ limits tunability at short wavelengths, and tunability at long wavelengths is limited by a lower gain cross section for stimulated emission. While dye lasers can be tuned across a broad range only by changing the dye, Ti:sapphire lasers can be tuned across the entire range by exchanges of cavity mirrors. Compared to a change of dye, switching mirrors in a Ti:sapphire laser is easier to realize because it can be computer-controlled.The gain curve also depends on the temperature because the vibrational sublevels of the ground electronic state of the lasing levels are thermally populated. Pulses produced by a Ti:sapphire laser pumped with a Q-switched Nd:YAG laser are eminently suitable for conversion to other wavelengths in order to extend the tuning range, due to the high beam quality and relatively short pulses. Extension of the tunable wavelength range is possible with harmonic generation or other

New Types of Tunable Lasers

129

2.0

1.5

(a)

1.0

~

0.5

700

0

0

800

4.0

900

1000

~ **~

2.r

!

650

i I i, 750

850

Wavelength,

950

1100

(b)

1050

nm

Figure 25. Typical tuning curves for CW Ti:sapphire lasers with four mirror sets (a) and five mirror sets (b), at 5 W and 20 W pump power for (a) and (b), respectively. Reproduced with permission from Pinto, J.F. et al. (1994) and Gray, T. (1989). Copyright 1994 IEEE and 1991 Gordon Publications, respectively.

nonlinear wavelength conversion techniques. The use of Raman shifting and second and third harmonic generation is possible for extension of the tuning range to encompass 202 to 3180 nm (Funayame et al., 1993). From 260 nm, over 3 mJ of pulse energy is obtained for most of the spectral range, but the pulse energy drops rapidly in the deep UV, ranging from less than 1 mJ at 250 nm to 1 l.tJ at 202 nm (Figure 27a). Including Raman shifting, many stages of frequency conversion (Figure 27a) are required to cover the wavelength range from 202 nm to about 700 nm. In contrast, only two SHG crystals are needed to extend the tuning range of an OPO laser over the wavelength range of 220 nm to around 2 l.tm, and higher output energy levels are available for most of this wavelength range (Figure 27b). Also, it is worth noting that the tuning range of OPO lasers can be up to 2000 nm in the red, while it is difficult for Ti:sapphire lasers to produce wavelengths longer than 1 l.tm. The broad-gain bandwidth of Ti:sapphire laser has been used to generate ultrashort pulses directly. Figure 28 shows a block diagram of a commercial 200 fs

130

HOU, ZHOU, YANG, STCHUR, and MICHEL 2.0

_ 1.5

0

n

1.0

0

0.5

a

700

800

900

1000

Wavelength, nm Figure 26. Higher CW argon-ion pump powers produce higher Ti:sapphire output powers, but they vary as a function of wavelength. Three sets of mirrors were used. Pump powers were, 3, 5, and 7 W for a, b, and c, respectively. Reproduced with permission from Hecht, J. (1992). Copyright 1992 McGraw-Hill.

pulse 76 MHz repetition rate mode-locked Ti:sapphire laser (Fisher et al., 1997). Mode locking is used to produce ultrashort pulses in Ti:sapphire lasers. With mode locking, a phase relationship is created such that completely constructive interference between all the modes is realized at just one point, with destructive interference everywhere else. By use of pulse compression techniques, together with mode locking, Ti:sapphire lasers can produce ultrashort pulses down to the femtosecond level. In order to amplify short pulses, three main requirements have to be satisfied by the amplification medium. First, the bandwidth of the amplifier must be large enough to accommodate the full spectrum of the short pulse. Second, the fluence of the pulse has to be near the saturation fluence of the medium. Finally, the intensity within the amplifier has to stay below a critical level at which nonlinear effects can distort the spatial and temporal profiles of the pulse (Maine et al., 1988). A new technique, chirped pulse amplification (CPA), has been used to amplify short pulses to saturation energies while maintaining low power levels in the amplifier (Maine and Strickland, 1988). In CPA, a short optical pulse is first temporally stretched, thus allowing it to be amplified to saturation while maintaining relatively low peak power. Then the original short pulse is restored by use of an optical compressor, which produces a short pulse at its Fourier-transform limit, and at high energy. CPA instrumentation includes four basic components: a short pulse oscillator, a pulse stretcher, an amplifier, and a pulse compressor. The CPA technique is schematically shown in Figure 29. The stretcher can be a single-mode optical fiber, a prism pair, or an antiparallel pair of gratings. The compressor can

131

New Types of Tunable Lasers

~~ lo'1

"

ShG

THG

"

/ " ~ S,

S, (

~"~ ~\

100

(a)

10"2~ [ 1

AS1

1 0 3 . IAS2 >~ 200 (l) eLU

10 3

Q.

102

O

101

.

doubled signal

, 400

~

. . . . . 600 800 1000

signal

idler

(b)

100 10-1

doubled idler

10.= 10.3

200

=

500 Wavelength,

=

1000

2000

nm

Figure 27. (a) Several stages of frequency conversion are needed for extension of the wavelength tuning range of a Ti:sapphire laser to 202-1 txm. Reproduced with permission from Funayama, M. et al. (1993). Copyright 1993 Elsevier Science Publisher B. V. (b) Only one stage of SHG frequency conversion, with two BBO crystals, are involved for extension of the wavelength tuning range of an OPO laser to cover 220-2 t.tm.

be a parallel pair of gratings, or a glass-block compressor. The CPA method greatly reduces the peak power of the optical pulse during amplification by avoiding any nonlinear interaction with the gain medium which can result in catastrophic damage. The tuning range of ultrafast Ti:sapphire lasers has been extended to the deep UV by various optical methods. For a 1 kHz femtosecond Ti:sapphire laser, a spectral range from 173 nm to 1.5 txm has been demonstrated (Petrov et al., 1994) which involved the use of cascaded second-order nonlinear frequency conversion processes. Extension of the wavelength tuning range of ultrashort pulses generated by Ti:sapphire lasers Can also be obtained by use of an optical parametric oscillator where the nonlinear crystal of such an arrangement is usually in a fixed position. The tunable OPO output wavelengths are achieved by tuning the Ti:sapphire pump wavelength rather than angle tuning the OPO. By tuning a Ti:sapphire laser in the range 720 to 853 nm, a synchronously pumped near-infrared OPO can produce a

132

HOU, ZHOU, YANG, STCHUR, and MICHEL

M8 /1 . . . . . . . . .alignment cavity ........... P1

M7 pump beam

~

riM9

L

M5

P2

M6

BRF

M1

M3L/slit

Figure 28. Optical schematic diagram ofthe Coherent Model 900-F Ti:sapphire laser. M1, output coupler; M2, M3, M6, and MT, high reflecting mirrors; M4, focusing mirror coated to pass the pump beam and reflect the intracavity beam; M5, focusing, high-reflector mirror; XTL, Ti:sapphire crystal; P1, and P2, prism pair for introducing negative GVD; L, lens. Reproduced with permission from Fisher, W.G. et al. (1997). Copyright 1997 Society for Applied Spectroscopy.

signal wave from 1.05 to 1.2 l.tm and an idler wave from 2.28 to 2.87 l.tm with a maximum average power of about 700 mW (Nebel et al., 1993). In an arrangement comprised of a 250 kHz Ti:sapphire regenerative amplifier and an OPA, it is possible to achieve (Reed et al., 1994) tunable outputs in the signal beam of 460 to 700 nm and in the idler beam from 2.4 to 0.9 I.tm with a peak pulse energy of about 150 nJ. In order to improve the tuning speed of a pulsed Ti:sapphire laser, an acousto-optical tunable filter (AOTF) can be incorporated into an OPO cavity (Chang, 1981) which enables rapid and randomly accessible wavelength tuning, with signal and idler tuning ranges typically from 1.06 to 1.31 and 2.97 to 2.27 ~m, respectively, and very rapid tuning speeds as high as 4 kHz (Akagawa et al., 1997).

Nd:YAG"'

argonlaser

Ti:sapphirelaser

I

"1 amplifier I

oompressor l .

Figure 29. Block diagram of the chirped pulsed amplification technique.

New Types of Tunable Lasers

133

Other Vibronic Lasers Other promising materials suitable for vibronic lasers include cobalt-doped magnesium fluoride (Co:MgF 2, tunable from 1750 to 2400 nm) (Welford and Moulton, 1988), chromium-doped crystals such as chromium-doped LiSrA1F6 (tunable from 750 to 1000 nm) (Payne, 1989), and chromium-doped LiCaA1F6 (tunable from 700 to 900 nm). The Co:MgF 2 laser was first commercialized in 1989 with a pulse energy of 75 mJ at its peak wavelength. Many other hosts have also been investigated. Practical problems that are encountered with some of the crystals include poor mechanical, thermal, or optical properties. For example, in some host materials absorption by excited states is found, which reduces laser efficiency.

IV.

D I O D E LASERS

Some significant advances in diode laser technology have been made in recent years. In many ways, diode lasers are the most promising lasers for commercial laser atomic spectrometric instruments because of their small size and potentially low cost. Presently, the main limitation is the lack of reliable lasers that emit in the blue part of the spectrum.

A. Basicsof Diode Lasers A simplified schematic diagram of a semiconductor laser is shown in Figure 30a in which the typical small size, 300 x 250 x 150 ktm, is indicated. The active laser region is a spatially confined layer. When an injection current is sent through the active pn junction region of the diode, which is between the n- and p-type cladding layers, electrons and holes move to the pn junction where they recombine and emit photons. If the densities of electrons and holes are large enough, this radiation can stimulate the recombination of electrons and holes, and laser action can be realized if the amplification of radiation exceeds the loss. The ends of the substrate act as resonator mirrors. The spatial mode of the laser can be defined either by varying the spatial injection current density through the active region, which is called gain guided, or by changing the semiconductor material to change the spatial distribution of the index of refraction, which is called index-guided. The beam divergence directly from the laser diode is always large because the light is emitted from a small rectangular region, in the order of 0.1 lxm by 0.3 lxm. The typical divergence angle is 30 ~ in the direction perpendicular to the junction and 10 ~ in the parallel direction. Therefore, a lens with a smallf number is used at the exit of the laser diode to collimate the laser beam. In addition, the output beams of most diode lasers are also astigmatic. However, a laser wavefront with a good Gaussian profile can be obtained by use of relatively inexpensive lenses and spatial filtering.

134

HOU, ZHOU, YANG, STCHUR, and MICHEL

current blocking

layer

250 pm

active layer

(pn junction)

r

0

p-type material

2

laser output

\

n-type material polished end

polished end

(a) electron flow

J

"10 t-. c~

T

Eg=band gap

I

conduction bands

VV~~

p-type

~I~

laser emission

n-type

(b) Figure 30. (a) Simplified diagram of a semiconductor laser. The active layer (pn junction) is formed when heavily doped p and n materials are joined. The thickness of the layer is about 1 l~m. When an injection current is applied to the laser diode, holes in p type material and electrons in n type material will migrate to the pn junction to recombine and produce laser radiation. The laser radiation is emitted from a well-defined region in the active layer because of the stripe design. (b) Schematic diagram for the energy levels of the p and n-type semiconductor. The laser transition occurs between the conduction band of the n-type semiconductor and the valence band of the p-type semiconductor.

New Typesof Tunable Lasers

135

The semiconductor material determines the wavelength range of the laser. For example, A1GaAs devices provide radiation in the near-IR spectral range, 770-810 nm, with a typical output power of 3 mW; InGaAsP lasers operate at around 670 nm with a power of 3 mW, while InGaN lasers can provide radiation at 420 nm, but these lasers are still under development to improve the lifetime. Among diode lasers that are available in the market, typical output powers are in the range from 2 to 200 mW. At a current that is higher than a threshold value, the output power increases abruptly with increased injection current at a fixed temperature. On the other hand, if the injection current is fixed, the output power increases rapidly when the temperature drops.

B. Characteristics of Diode Lasers Diode lasers have excellent practical and spectroscopic characteristics, including compactness, narrow bandwidth, wavelength tunability, and facile wavelength modulation. In addition, they are inexpensive compared to other lasers, but they have low peak power and relatively limited tuning range. Also, they are temperature-sensitive, as they require a temperature stability of a few mK, and they have a somewhat limited lifetime especially for blue wavelength diode lasers (Niemax et al., 1996).

Compactness The small size and low cost of diode lasers make it possible to install and operate several units simultaneously in a single spectrometer to realize multielement determination. Up to six laser diodes have been used simultaneously, and independently tuned by temperature and locked to the wavelength of several analytes of interest (Niemax et al., 1993). A limiting factor in the simultaneous use of a number of laser diodes is the number of independently operated power supplies that is required.

Narrow Linewidth The linewidth of a diode laser is much narrower than other lasers such as a tunable dye laser or an OPO laser. Typically, at constant temperature and injection current, the spectral linewidth of a diode laser is 30 fm. Whether the characteristic of the narrower linewidth is an advantage or disadvantage in elemental analysis depends on the pressure conditions in the sample atomizer. For example, for laser-excited atomic fluorescence in a graphite furnace, LEAFS, the linewidth of a pulsed dye laser is usually wider than the linewidth of the homogeneously broadened atomic transition. The unabsorbable radiation in the wider bandwidth increases the stray light and degrades the detection power of the technique. The narrower linewidth of a laser diode may be an advantage in this respect. For the acquisition of qualitative spectroscopic information, the biggest advantage of a narrow linewidth is that it

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HOU, ZHOU, YANG, STCHUR, and MICHEL

can be used to allow higher resolution of atomic and molecular spectral features in the gas phase. For example, one important application is isotope detection by Doppler-free laser spectroscopy (Lawrenz et al., 1987).

Wavelength Tunability The wavelength of a laser diode is determined primarily by the band gap of the semiconductor material, while the tunable range is determined by the width of both the conduction and valence bands, and by the injection current and pn junction temperature (Figure 30b). Generally, this means that the wavelength of a laser diode can be tuned over an interval of about 20 nm. Tuning by variation of the temperature is much slower than by variation of the injection current because it takes time for the heat to transfer to the semiconductor chip. At constant current, the behavior of temperature tuning is illustrated in Figure 3 l a. The temperature tuning curve is a staircase with sloping steps in which the smooth tuning section spans about 0.25 to 0.4 nm, and is due to the continuous change of the optical path length of the cavity (0.06 nrn/K) with temperature. The jump between steps corresponds to a hop from one longitudinal mode to another, which occurs because the slope of the laser gain curve is steeper than the change in cavity length caused by the change in temperature. The spectral gaps that result are a drawback in the application of diode lasers to atomic spectrometry, although a technique that involves external optical feedback can be used to reduce the problem. Changes in the injection current affect both the diode temperature and the index of refraction, both of which affect the wavelength. At a stable temperature, the injection current can be modulated in such a way that the variation of the laser wavelength is within the smooth tuning section, and the wavelength is modulated around a center wavelength (Figure 31 b). The depth of the wavelength modulation depends on the modulation frequency. For example, the

E t-

J

.c"

J

t(!,1

Case Temperature,~

(a)

Injection Current, mA

(b)

Figure 31. (a) Temperature tuning of a diode laser (see text). The typical total tunable wavelength range is 20 nm, while the continuously tunable range at each segment is about 0.25 to 0.4 nm. (b) Injection current tuning of a diode laser. Reproduced with permission from Niemax, K. et al. (1996). Copyright 1996 American Chemical Society.

New Types of Tunable Lasers

13 7

wavelength of a typical A1GaAs diode laser can be changed at a rate of 1 prn/mA for modulation frequencies below 1 MHz, then it drops to 100 fm/mA for frequencies from 1 to 3000 MHz. The modulation frequency can rise as high as 3 GHz, after which point the wavelength barely changes as the modulation frequency is raised (Telle, 1993).

Extension of Wavelength TuningRange Laser diodes are now commercially available to cover the spectral range from 625 to 1600 nm, although each laser diode has a limited tuning range of about 20 nm. Both external and internal cavity SHG techniques have been demonstrated for the wavelength extension of diode lasers. Internal methods are advantageous because the SHG beam is automatically created with the same narrow linewidth as the diode fundamental wavelength, and can be directly used for spectroscopic applications, or for pumping another frequency doubler to produce UV radiation. By use of SHG techniques, near-infrared diode lasers with an output wavelength around 780 nm can be frequency-doubled to 390 nm with a potassium dihydrogen phosphate (KDP) crystal (Okazaki et al., 1988). The power achieved is around 50 nW for a 20 mW diode laser, which is a conversion efficiency of 2.5 • 10-6. The conversion efficiency depends on the angle and temperature of the crystal because phase matching is necessary. The conversion efficiency is not very sensitive to the focal length of the lens and optical configuration of the SHG system. Most frequently, potassium niobate (KNbO3) has been used as the SHG crystal for frequency-doubling of diode lasers. It possesses the highest nonlinearity among the commercially available crystals, but high conversion efficiency is not usually realized with an external SHG cavity (Lodahl et al., 1997). One of the main reasons for this is the process of blue light induced infrared absorption (BLIIRA). After accounting for BLIIRA, the conversion efficiency, as measured, is in good agreement with calculated values (Lodahl et al., 1997; Figure 32). Generally, it is easier to obtain high conversion efficiency at higher input power due to the nonlinear character of the process. The data in Figure 32, which were obtained with a 20 mW laser diode, are encouraging because 10 mW of coherent tunable blue light can be generated by using only 20 mW of diode laser radiation. At higher powers, the conversion efficiency reaches 60% and then levels off, which can be explained by the effect of BLIIRA. To take advantage of the compactness of diode lasers, the frequency conversion system should be as small as possible. For this purpose, a technique based on internal cavity SHG has been used (Imasaka et al., 1989) with gallium arsenide diode lasers. The second harmonic can be generated even under phase-mismatched conditions because of the high radiation field inherent in an internal cavity design. This approach allows SHG from a diode laser without the use of extracavity optics and a nonlinear crystal, so the advantages of compactness and convenience of diode laser are retained. The main problem is low conversion efficiency, which can be as low as 10-11 for a CW diode laser. By use of a 1000 Hz

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HOU, ZHOU, YANG, STCHUR, and MICHEL

80

60

40

g

O

20

'2b

4b

6'0

8'o 160

Input Power, mW Figure 32. The measured conversion efficiency as a function ofthe input fundamental power (N) for a laser diode. Also, the conversion efficiencies were calculated with BLIIRA taken into account (A) (see text). The solid curve was calculated in the absence of BLIIRA. Reproduced with permission from Figure 4 of Lodahl, P. et al. (1997). Copyright 1997 Springer-Verlag.

pulsed laser, operated at a peak power of 10 W, the output is about 0.4 mW at 452 nm, which is a conversion efficiency of about 10-5. The lowest output wavelength, 625 nm, from a commercial diode laser can be frequency-doubled down to 313 nm. With the fundamental input at a level of 50 mW, typical frequency-doubled output power is 50 nW, with a maximum of 3 l.tW (Niemax, 1997). While the wavelength tuning range can be extended by SHG methods, generally, the conversion efficiency is low, especially when converted to blue radiation. Therefore, efforts have been made to extend the short wavelength coverage by development of blue diode lasers. New Type II-VI diodes (ZnSe) with a lasing wavelength in the blue-green range (470-515 nm at room temperature) have been developed in research laboratories. Blue diode lasers have been made that emit at wavelengths as low as 416 nm with an output power of 50 mW, but the lifetime of 300 h is too short. Wavelengths produced by diode lasers have been obtained as low as 400 nm, but with a lifetime of only 0.5 h at room temperature. This type of diode

New Types of Tunable Lasers

139

laser is made of InGaN with a multiquantum well structure which consists of lasers of various types of n- and p-doped semiconductor materials, grown by a two-flow organometallic chemical vapor deposition method. Further improvement of the lifetime may be achieved in the future by reducing the threshold current and voltage (Nakamura et al., 1997).

Wavelength Modulation and Wavelength Stabilization The wavelength of a diode laser can be easily and rapidly modulated by variation of the injection current. For common diode lasers, the modulation can be as high as a few gigahertz, while the depth of modulation can be greater than a few picometers. Wavelength modulation allows for discrimination against 1/f noise, which enables measurements to be made near the shot noise limit. Also, for atomic spectrometry, this rapid tunability allows the laser wavelength to alternate between resonance with an atomic line and off-resonance, which can be used for rapid background corrected measurements. Modulation of the injection current also changes the amplitude of the output, but the change is much weaker than that caused by the wavelength change. For many applications of atomic spectrometry, the change in output amplitude can be ignored during the wavelength modulation process. Optical and electronic devices can also be used to improve the spectral quality of diode lasers. By use of a small mirror or a glass plate to provide optical feedback, a multimode laser can be forced to oscillate in a single mode, which reduces the spectral linewidth (Andrews, 1985). For electronic feedback, a small portion of the injection current is injected back to control the laser wavelength. If a very fast electronic feedback is applied, the spectral linewidth can be improved to 100 kHz (Saito et al., 1985). However, these simple feedback devices cannot be applied generally to all types of diode lasers. More sophisticated devices such as external cavity lasers (Fleming and Mooradian, 1981), pseudo-external cavity lasers (Wieman and Hollberg, 1991), and feedback from a high-Q optical cavity (Dahmani et al., 1987) have been developed. It is essential that the output wavelength remains constant for spectroscopic applications, but the wavelength of a distributed feedback diode laser drifts with temperature at 0.1 nm/K. Accordingly, the temperature of these lasers must be stabilized carefully. This is usually done by mounting the laser on a thermoelectric cooler, and by provision of a temperature-sensing feedback loop. Although this approach works, the coolers are bulky, consume extra power, and increase the cost of the diode laser. Stress from differential thermal expansion can be used (Cohen et al., 1996), to counteract the effects of temperature. The temperature sensitivity of the wavelength of a 1.55 mm GalnAsP/InP laser can be reduced by 50%, through use of differential thermal expansion between various fluids incorporated into the laser package. This method automatically generates a temperature-dependent pressure.

Table 3. Summary of the Output Characteristics of Alexandrite, Ti:Sapphire, OPO, and Semiconductor Lasers Laser Type

Alexandrite

Typical pump

flashlamps

Repetition rate, Hz

10-250 (Sam et al., 1988)

Pulse duration

ps-CW (Hecht, 1992)

Fundamental tunable range, 701-826 (Hecht, 1 9 9 2 ) nm

Titanium:Sapphire Nd:YAG or CVL (copper vapor laser) 10-250K and around 100 MHz (Petrov et al., 1994) fs-CW (Padgett and Dunn, 1994) 680-930 (Funayama et al., 1993) to 202 (Funayama et al., 1993)

Wavelength extension, nm, incl. Raman shift, SHG, THG Typical av CW power, W

to 248 (Hobbs, 1993)

Typical av pulsed power, W

up to 5.5 (Knowles and 10-150 (Walling et al., Webb, 1993) 1985; Hecht, 1992) 10-2500 (Sam et al., 1988; 4-100 (Funayama et al., 1993) Walling et al., 1985) 10-s-4 (Padgett and Dunn, 0.003-30 (Bruneau et al., 1994: Knowles and 1994; Volker et al., 1991) Webb, 1993) wide tunable range with high output power, but several stages of relatively narrow tuning frequency conversion; range several sets of mirrors needed for tuning

Typical pulse energy, mJ Linewidth, cm -1 Comments

2-60 (Samelson et al., 1988; 1-43 (Erbert et al., 1991) Walling et al., 1985)

OPO Nd:YAG

Semiconductor Diode electrical

10-250 K (Reed et al., 1994) can be modulated fs-CW (Padgett and Dunn, 1994) 400-2500 with degeneracy gap (Zhang et al., 1 9 9 3 ) to 220 (Zhou et al., 1997) 0.01-0.8 (Padgett and Dunn, 1994; Gerstenberger and Wallace, 1993) 0.01-0.4 (Padgett and Dunn, 1994) 1-100 (Zhou et al., 1997)

continuous wave (CW), usually 625-1600 many lasers/wavelength gaps (Niemax et al., 1996) to 313 nm (Niemax, 1997) 1-50 mW (Wieman and Hollberg, 1992) (up to several W for diode arrays) N/A N/A

0.02-300 (Radunsky, 1995) <0.05 (Wieman and Hollberg, 1991 ) wide tunable range; ease of operation; room temperature needs to be stable within +2.5 ~

compact and inexpensive; narrow tunable range for each diode laser; blue and UV output not yet available routinely

New Types of Tunable Lasers

141

V. CONCLUSION Table 3 summarizes the general characteristics of the new types of tunable laser that have been discussed in this chapter (Walling et al., 1985; Sam et al., 1988; Samelson et al., 1988; Erbert et al., 1991, Gerstenberger and Wallace, 1993; Hobbs, 1993; Knowles and Webb, 1993; Zhang et al., 1993; Padgett and Dunn, 1994). The laser has engendered broad interest in analytical atomic spectrometry, since many laser-based atomic spectrometric techniques have excellent analytical characteristics. Yet, no significant atomic spectroscopic instruments have been commercialized, although fixed-wavelength lasers have been used for commercial laser ablation instrumentation. The lack of use of tunable lasers is due to one or more of the disadvantages of poor reliability, complicated maintenance, limited tuning range, large size, and high cost of the lasers that are suitable for atomic spectrometry. Many of these disadvantages stem from the basic requirements of a light source for atomic spectrometry which are wide tuning range, narrow spectral linewidth, and fairly high peak energy. These requirements necessitate large and complex power supplies to provide the necessary energy, and complicated optics to provide the narrow spectral linewidth and wavelength tunability. When equipped with second harmonic generation, the tuning range of a dye laser covers the ultraviolet and visible range, and provides the energy and spectral linewidth that is suitable for atomic spectroscopy. As a result, the dye laser has been involved in most analytical atomic spectrometry. However, it is necessary to change dyes frequently because of the limited tuning range for each dye. Therefore, the change from wavelength to wavelength is not always an easy thing to do. In contrast to dye lasers, an OPO laser can be considered to be a "turn key" laser system, and it is a trivial to dial from wavelength to wavelength over a wide wavelength range from 220 to 2000 nm. Instead of hours, days, or even weeks, the change of wavelength becomes a matter of minutes. In addition, the narrow spectral linewidth, and high-output pulse energy levels are more than enough for analytical atomic spectrometric applications. However, an OPO laser system is still bulky, and expensive, and the never-easy work of optical alignment is always needed when new flashlamps are installed every few months. The sensitivity of the output energy to ambient temperature, together with their general lack of ruggedness in the engineering, remain the main problems for OPO laser systems. Nevertheless, the OPO laser should be considered a significant step forward in laser technology for use in analytical instrumentation. The solid-state Ti:sapphire laser has a wide tuning range of 700 to 900 nm that can be frequency-doubled into the UV from 350 to 450 nm. The shorter wavelengths below 350 nm and the gap between 450 and 700 nm can be obtained through further frequency-doubling and/or mixing techniques; although at the cost of making the laser system more complex. The pulsed Ti:sapphire laser might have played a more important role in laser atomic spectrometry if it had not been followed almost immediately by the commercialization of the OPO laser. In any event, short-pulse Ti:sapphire lasers have proven to be a major advance in laser technology for fast

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spectroscopic measurements outside atomic spectrometry. The tunable wavelength range of a single laser diode is close to that of a single dye in dye lasers, and commercial diode lasers at the blue end of the spectrum are still not routinely available. Blue diode lasers with wavelengths as short as 400 nm have been demonstrated, albeit with short lifetime. Although the wavelength range can be extended by use of second harmonic generation, the output energy is generally too low to saturate the excitation transitions for atomic fluorescence, which degrades sensitivity. However, diode laser-based atomic absorption spectrometry, which does not require as high laser energies as fluorescence, is presently the most promising application for the commercialization of laser diode-based atomic spectrometry. The most recent development in Nd:YAG lasers has been the replacement of the flashlamps with diode laser arrays. The lifetime of a diode laser array should be longer than a set of flashlamps, but the initial cost of such an array is higher. It remains to be seen whether or not this development will result in Nd:YAG pump lasers with superior operational characteristics for pumping the solid-state lasers described here.

REFERENCES Akagawa, K., Wada, S., Tashiro, H. Appl. Phys. Lett. 1997, 70(10), 1213-1215. Andrews, J.R. Appl. Phys. Lett. 1985, 47(2), 71-73. Armstrong, J.A., Bloembergen, N., Ducuing, J., Pershan, P.S. Phys. Rev. 1962, 127(6), 1918-1939. Bruneau, D., Amaud des Lions, T., Quaglia, P., Pelon, J. Appl. Opt. 1994, 33(18), 3941-3950. Chang, I.C. Opt. Eng. 1981, 20, 824. Chen, C., Wu, B., Jiang, G., You, G. Sci Sin., Ser. B. 1985, 28, 235-243. Cohen, D.A., Heimbuch, M.E., Coldren, L.A. Appl. Phys. Lett. 1996, 69(4), 455-457. Cunningham, R. Lasers & Optronics 1991, February, 31-38. Dahmani, B., Hollberg, L., Drulling, R. Opt. Lett. 1987, 12(11), 876-878. Eimerl, D., Davis, L., Velsko, S., Graham, E.K., Zalkin, A. J. Appl. Phys. 1987, 62(5), 1968-1983. Erbert, G., Bass, I., Hackel, R., Jenkins, S., Kanz, K., Paisner, J. Conference on Lasers and Electro-Optics, Optical Society of America; Washington, DC, 1991, pp. 390-393. Fisher, W.G., Wachter, E.A., Armas, M., Seaton, C.Appl. Spectrosc. 1997, 51(2), 218-226. Fleming, M.W., Mooradian, A. IEEE J. Quantum Electronics 1981, 17(1), 44-59. Funayama, M., Mukaihara, K., Morita, H., Okada, T., Tomonaga,N., Izumi, T., Maeda, M. Opt. Comm. 1993, 102(5/6), 457-460. Gerstenberger, D.C., Wallace, R.W.J. Opt. Soc. Am. B. 1993, 10(9), 1681-1683. Giordmaine, J.A. Phys. Rev. Lett. 1962, 8(10), 19-20. Giordmaine, J.A., Miller, R.C. Phys. Rev. Lett. 1965, 14(24), 973-976. Gray, T. Lasers & Optronics 1989, December, 49-55. Hecht, J. Laser Focus World 1992, 28(10), 93-103. Hobbs, J.R. Laser Focus World 1993, 29(7), 20-22. Imai, S., lto, H. IEEE J. Quantum Electronics 1998, 34(3), 573-576. Imasaka, T., Hiraiwa, T., Ishibashi, N. Mikrochim. Acta 1989, II, 225-231. Johnson, B.C., Newell, V.J., Clark, J.B., McPhee, E.S.J. Opt. Soc. Am. B. 1995, 12(11), 2122-2127. Knowles, M.R.H., Webb, C.E. Opt. Lett. 1993, 18(8), 607-609. Lawrenz, J., Obrebski, A., Niemax, K. Anal. Chem. 1987, 59(9), 1232-1236. Ledingham, K.W.D., Singhal, R.P.J. Anal. Atom. Spectrom. 1991, 6(2), 73-77.

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Lodahl, P., Sorenson, J.L., Polizik, E.S. Appl. Phys. 1997, B64(3), 383-386. Maine, P., Strickland, D., et al. IEEE J. Quantum Electronics 1988, 24(2), 398-403. Maker, P.D., Terhune, R.W., Nisenoff, M., Savage, C.M. Phys. Rev. Lett. 1962, 8(1), 21-22. Moulton, P.E Solid State Res. Rep. MIT Lincoln Lab. Lexington, MA, 1982, 15-21. Moulton, P.E J. Opt. Soc. Am. B 1986, 3(1), 125-133. Nakamura, S., Senoh, M., Nagahama, S., lwasa, N., Yamada, T., Matsushita, T., Sugimoto, Y., Kiyoku, H. Appl. Phys. Lett. 1997, 70(7), 868-870. Niemax, K., Groll, H., Schnurer-Patschan, C. Spectrochimica Acta Rev. 1993, 15(5), 349-377. Niemax, K., Zybin, A., Schnurer-Patschan, C. Anal. Chem. 1996, 68(11), 351A-356A. Niemax, K. 24th Annual Conference of the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS); October 26-30, Providence, RI, 1997. Nebel, A., Fallnich, C., Beigang, R., Wallensiein, R. J. Opt. Soc. Am. B. 1993, 10(11), 2195-2200. Okazaki, T., lmasaka, T., lshibashi, N. Anal. Chim. Acta 1988, 209 (1/2), 327-331. Padgett, M.J., Dunn, M.H. Laser Focus Worm 1994, 30(9), 69-76. Payne, S.A.J. AppL Phys. 1989, 66(3), 1051-1056. Petrov, V., Seifert, E, Noack, E Appl. Phys. Lett. 1994a, 65(3), 268-270. Petrov, V., Seifert, E, Kittelman, O., Ringling, J., Noack, E J. Appl. Phys. 1994b, 76(12), 7704-7712. Pinto, J.E, Esterowitz, L., Rosenblatt, G.H., Kokta, M., Peressini, D. IEEE J. Quantum Electronics 1994, 30(11), 2612-2616. Radunsky, M.B. Laser Focus Worm 1995, 31(10), 107-116. Radunsky, M.B. Laser Focus World 1997, 33(1), 79. Reed, M.K., Steiner-Shepard, M.K., Negus, D.K. Opt. Lett. 1994, 19(22), 1855-1857. Rowland, A. Lasers & Optronics 1997, March, 13-15. Saito, S., Nilsson, O., Yamamoto, Y. Appl. Phys. Lett. 1985, 46(1), 3-5. Sam, R.C., Reh, J., Leslie, K.R., Rapoport, W.R. IEEEJ. Quantum Electronics 1988, 24(6), 1151-1166. Samelson, H., Walling, J.C., Wemikowski, T., Harter, D.J. IEEEE J. Quantum Electronics 1988, 24(6), 1141-1150.

Sanchez, A., Strauss, A.J., Aggarwl, R.L., Fahey, R.E. IEEE J. Quantum Electronics 1988, 24(6), 995-1002. Simon, U., Tittel, EK. Laser Focus World 1994, 30(5), 99-110. Sneddon, J., Lee, Y.I., Hou, X., Zhou, J.X., Michel, R.G. In Lasers in Atomic Spectrometry; Sneddon, J., Thiem, T.L., Lee, Y.I., Eds.; VCH Publishers: New York, 1997, Chap. 2, pp. 41-81. Tang, C.L., Cheng, K.L. Fundamentals of Optical Parametric Processes and Oscillators; Letokhov, V.S., Shank, C.V., Shen, Y.R., Walther, H., Eds. Laser Science and Technology: An International Handbook; Hardwood Academic: The Netherlands, 1995, Vol. 20. Telle, H.R. Spectrochimica Acta Rev. 1993, 15, 301-327. United States Patents 5,033,057, 5,047,668. Volker, E, Lu, Q., Weber, H. J. Appl. Phys. 1991, 69(6), 3432-3439. Walling, J.C. Opt. Lett. 1979, 4(6), 182-183. Walling, J.C., Heller, D.E, Samelson, H., Harter, D.J., Pete, J.A., Morris, R.C. IEEE J. Quantum Electronics 1985, 21 (I 0), 1568-1581. Welford, D., Moulton, EE Opt. Lett. 1988, 13(11), 975-977. Wieman, C.E., Hollberg, L. Rev. Sci. Instrum. 1991, 62(1), 1-20. Zhang, J.Y., Huang, J.Y., Shen, Y.R.J. Opt. Soc. Am. B. 1993, 10(9), 1758-1764. Zhou, J.X., Hou, X., Tsai, S-J.J., Yang, K.X., Michel, R.G. Anal. Chem. 1997, 69(3), 490-499.

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DEVELOPMENTS IN DETECTORS IN ATOMIC SPECTROSCOPY

Frank M. Pennebaker, Robert H. Williams, John A. Norris, and M. Bonner Denton

Abstract

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

Introduction

II.

III. IV. V. VI.

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

A.

Detectors of the Past

B.

Early Attempts at Simultaneous Multichannel Detection

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

146 146 146 147

Charge-Transfer Device Basics . . . . . . . . . . . . . . . . . . . . . . . . .

150

A.

151 158

C h a r g e - C o u p l e d Devices . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Charge-Injection Devices . . . . . . . . . . . . . . . . . . . . . . . . . . C o m p a r i s o n of Charge-Transfer Devices and Photomultiplier Tubes for A t o m i c Emission Spectrometry . . . . . . . . . . . . . . . . . . . . . . . Readout and M e a s u r e m e n t with Charge-Transfer Device Systems . . . . . . . Charge-Transfer Device Detection for Absorption and Spectral Imaging Multielement Detection Systems o f the Future . . . . . . . . . . . . . . . . .

. . . 169

Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advances in Atomic Spectroscopy Volume 5, pages 145-172. Copyright 9 1999 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0502-9

145

162 164 169 170 171

146

PENNEBAKER,WILLIAMS, NORRIS, and DENTON ABSTRACT

Recent developments in detectors for atomic spectroscopy are presented. In particular, charge-transfer devices, comprising the charge-coupled device (CCD) and the chargeinjection device (CID), solid-state systems based on an internal photoelectron effect, are described. Their application in multi-element or multichannel spectrometers is discussed and their performance is compared with that of the more traditionally used photomultiplier tube (PMT).

I.

INTRODUCTION

Analyses in atomic spectroscopy are made by measuring characteristic absorptions or emissions of light from an atomic source. The wavelengths of these absorptions or emissions are representative of the identity of the analyte species present, whereas the magnitude of a particular absorption or emission is proportional to the concentration of analyte species. Both quantities must be measured accurately to determine the identity and amount of analyte present. Detectors for these types of measurements can be as simple as the eye or as complicated as a modern highresolution electronic camera system. Distinctions among detectors are typically found in sensitivity, response time, wavelength range, quantum efficiency, dynamic range, noise, and cost. Until recently, detection of atomic lines has been performed in most instances using the photomultiplier tube (PMT) and photographic emulsions. Recently, solid-state devices based on the internal photoelectron effect have begun to replace traditional detectors in many spectroscopic fields including atomic spectroscopy. Multielement spectrometers have been developed using charge-transfer devices (CTDs) as detectors. Simultaneous multielement measurement using CTDs has revolutionized atomic spectroscopy by decreasing analysis time and increasing sensitivity. This chapter will discuss the variety of detectors that have been and will be employed in atomic spectroscopy. Furthermore, the development of modern multichannel atomic systems will be discussed, with a strong emphasis on CTDs. The performance of current CTD atomic emission systems will be evaluated based on the device characteristics for each system. Also, a direct comparison will be made between CTD systems and scanning PMT systems. The chapter will close with contemplation on the enhancement of detectors for future atomic spectroscopic measurements.

A. Detectors of the Past Until recently, much of analytical atomic spectroscopy has focused on the use of two principal detectors: the photographic emulsion and the photomultiplier tube.

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147

For different reasons, each detector found a niche in qualitative and quantitative analysis of atomic lines. The photographic emulsion is able to obtain a large amount of chemical information during a single exposure. Figure 1 is a small portion of a photographic emulsion that was exposed to the emission lines from a DC arc. However, photographic emulsions have a limited linear response with concentration. Precision and accuracy are further limited by development time and processing methodology. The PMT, on the other hand, is able to make more sensitive and more accurate chemical determinations. However, the PMT is limited by its single-channel nature. Discrete wavelengths must be analyzed individually, resulting in significant measurement times for a scan of an entire spectrum. Furthermore, if the source of the atomic signal is transient, such as a DC arc or spark, single-channel scanning systems will not optimally detect all wavelengths of interest. Because of the limitations of both detectors, research has focused on the development of multichannel detectors, which retain the large amount of spectral information achieved by the photographic emulsion while also maintaining the sensitivity and linear response of the PMT.

B. Early Attempts at Simultaneous Multichannel Detection In many spectroscopic situations, changing from a single-channel to a multichannel or multiplexed detection scheme significantly decreases measurement time. Numerous attempts have been made to develop multichannel or multiplexed detection systems suitable for atomic emission measurements. A successful system must meet several criteria. First, the detection scheme must have wide dynamic range. Atomic spectra consist of a wide range of intensities based not only on the concentration of elements but also on the sensitivity and spectral throughput of emission wavelengths. Second, the detection scheme must have high sensitivity. Third, there must be a sufficient number of detector elements to provide both the required resolution and spectral coverage, ranging from K (769 rim) to A1 (167 rim). Unfortunately, the sensitivity of the PMT has been difficult to match in multichannel detectors. One obvious solution is the use of multiple PMTs in the form of a direct reader. The Rowland Circle shown in Figure 2 can be outfitted with a series of slits and PMTs at the positions of useful analytical lines. By using multiple PMTs, the direct reader maintains the high sensitivity and wide dynamic range that made the PMT valuable in scanning systems. Furthermore, the nearly simultaneous measurement of a variety of wavelengths reduces the time required to analyze a given sample. When using a direct reader, however, flexibility in wavelength selection is lost. Nevertheless, for routine analyses involving a similar matrix, the direct reader has proven to be very valuable. In comparison to photographic emulsions, the direct

o~

Figure 1. Photographic emulsion of dc arc approximately covering 278--269 nm. From top to bottom the spectrum consists of Mn, Mg, In, Ge, Ga, and Fe.

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Com~ve

Atomic Som'e~ Figure 2. Lightdispersed in the Rowland Circle will fall along a curved focal plane incident with the circle as shown above.

reader is superior in terms of sensitivity and dynamic range; however, more spectral information is achieved with emulsions. Another possible alternative is the use of a multiplexing scheme based on either interferometry or Hadamard spectroscopy. In fact, early work by Michelson, Fabry, and Perot dealt with the measurement of atomic lines through interferometry (Sawyer, 1963). Modern use of multiplexed systems has primarily focused on the infrared (IR) region of the spectrum, since IR detectors normally have high noise and poor sensitivity. Thus, noise in the signal-to-noise ratio (SNR) is dominated by detector noise, and an increase in SNR is achieved through signal averaging. While multiplexed spectroscopic systems work well in the IR, they are not as successful in the visible and ultraviolet regions of the spectrum. Due to the high level of sensitivity, PMT emission systems are limited by either shot or flicker noise. An SNR advantage is not achieved in either shot or flicker noise limited situations. Furthermore, multiplexing is disadvantageous under flicker noise limited conditions, with the extent of the disadvantage based on the complexity of the line spectrum (Billhorn et al., 1987a). Subsequently, ICP and other atomic emission systems using multiplexing experience a decrease in sensitivity compared to single-channel detection systems. While the routine use of multiplexing is hampered by poor SNR, there are situations in which it is useful for atomic spectroscopy.

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The extremely high resolution achievable with multiplexing schemes has enabled high precision measurements of wavelengths for many elemental lines and examination of fundamental atomic line shapes. While the use of multiplexing is not common, FT and Hadamard ICP have been used in a number of instances (Plankey et al., 1974; Horlick and Yuen, 1975; Stubley and Horlick, 1985; Marra and Horlick, 1986). Research has also focused on linear photodiode arrays (PDAs) placed at the focal plane of scanning monochrometers using ICP systems (Van der Plas et al., 1985; Keane and Fry, 1986; Winge et al., 1988; Wirsz and Blades, 1988). These arrays, which generally consist of a few hundred to a few thousand elements, provide a compromise between the single wavelength detection of PMTs and the full wavelength coverage of the photographic plate. To obtain reasonable spectral resolution in these systems, only a small continuous spectral window can be obtained at one time. Since important spectral lines are scattered between ultraviolet and visible wavelength radiation, only a small percentage of the multielement advantage is achieved with the PDA. Also, system signal-to-noise for the PDA is extremely poor in element-to-element comparison versus the PMT (Billhorn et al., 1987a,b; Pilon et al., 1990). Accordingly, photodiode arrays have not provided a suitable alternative to the direct reader or scanning PMT systems. Another major area of research has focused on the use of two-dimensional imaging detectors developed for television applications. These detectors, composed of vidicons, orthicons, and intensified vidicons (Wood et al., 1975; Felkel and Pardue, 1977, 1978a,b; Gustavsson and Ingram, 1979; Furuta et al., 1980), have proven to be less sensitive than PMTs (Sweedler et al., 1989). In addition, these devices suffer from a variety of other problems, including hysteresis, lag, cross-talk, blooming, 1 limited dynamic range and linearity, and low spectral coverage (Sweedler et al., 1989; Sims and Denton, 1990). Image dissectors (Felkel and Pardue, 1978a,b) realize better single element detection limits, but also have high spectral background and problems with stability. To avoid the multitude of problems with PDAs and certain imaging-type detectors, research has turned to the use of CTDs to improve atomic spectroscopic measurements.

II. CHARGE-TRANSFER DEVICE BASICS Charge-transfer devices, consisting of the charge-coupled device (CCD) and the charge-injection device (CID), are rapidly replacing PMTs in a number of optical spectroscopies, including atomic emission spectroscopy (AES). Although originally developed as logic and memory devices, CTDs have proven to be effective imaging devices and are thus widely used in the television industry. When operated in the scientific mode, 2 these integrating devices offer low noise, large dynamic range, high spatial resolution, high quantum efficiency (QE), and a linear response from the soft X-ray to the near-IR. These qualities have made CTDs attractive spectroscopic multichannel detectors in a number of chemical applications.

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Both the CCD and CID are derived from metal oxide semiconductor (MOS) technology. Figure 3 illustrates the basic structure of a CTD. Both CCDs and CIDs contain four basic structures--gate electrodes, an insulating layer, an epitaxial layer, and a substrate--and are fabricated on either p-type (CCDs) or n-type (CIDs) epitaxial silicon, which has an indirect band gap energy of 1.1 eV. When a photon of energy greater than the bandgap hits and is absorbed by the epitaxial layer, an electron-hole pair is created. 3 The electrons (CCD) or holes (CID) 4 measured are then proportional to the number of incoming photons multiplied by the quantum efficiency of the detector. Polysilicon electrodes (conducting gates), which sit above the epitaxial layer, provide the electrostatic potential necessary to collect and move the charge within a pixel. An insulating layer approximately 100 nm thick composed of SiO 2 or SiO2/SiaN4 sits between the gates and the epitaxial layer. The substrate is at the bottom of the device and acts as the mechanical support and electrical common contact for the structure.

A. Charge-Coupled Devices Conventional Devices The structure and operation of a CCD are shown in Figures 4 and 5, respectively. The typical device has a two-dimensional format of individual detector elements called pixels, as shown in Figure 4. The operation of a three-phase device is the simplest to understand and is illustrated in Figure 5. Above each pixel are three separate overlapping gates, with every third gate connected to the same voltage driver. Initially, positive potential is applied to the first electrode, t~l, which creates a charge inversion or potential well. When a photon of light strikes and is absorbed by the epitaxial layer, an electron-hole pair is created. The photogenerated electrons will collect under the nearest positively biased gate, ~1, while the holes are driven to the substrate. In this manner, light intensity is integrated through the collection and storage of charge beneath the gates.

Figure 3. A CTD consists of four layers: gate electrodes, an insulating layer, an epitaxial layer, and a substrate.

M'I

Figure 4. Rows and columns of pixels make up the structure of a CCD. A blowup of a 3-pixel region with gate electrodes and potential wells is also shown.

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Figure 5. (a) Initially, positive potential is applied to the first electrode, ~1, creating a "potential well." (b) The potential of ~2 is then made positive creating a potential well under both ~1 and ~2. (c) The potential under ~1 is then removed, thus collapsing the well under ~1 and driving the electrons into the well under ~2. Charge has been transferred from ~1 to ~2. In a similar fashion, charge can be transferred from electrode to electrode and pixel to pixel until reaching the serial register.

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After a specified integration time, spectral light is blocked from the detector, usually by a shutter. The collected charge is then propagated to the serial register for measurement. The charge is transferred through the series of steps shown in Figure 5. In this manner, charge has been transferred from one electrode to the next. By cycling through this process, the entire image is sequentially shifted to the serial register, as shown in Figure 6. At the serial register, charge is shifted to the output amplifier. A voltage change is then measured and amplified according to the number of electrons gathered in the original pixel. CCDs have many properties that make them almost ideal for spectroscopic analysis. When run in the scientific mode, these devices can achieve very low read noise and almost nonexistant dark current (Sweedler et al., 1989; True et al., 1999). Other desirable features of CCDs are the large number of individual detector elements and wide dynamic range set by a pixel's capacity for charge, the full well potential for the device. A large number of detector elements allows the device to adequately cover the wavelength range of analytically important emission lines while also maintaining the necessary resolution to avoid spectral overlap. Unfortunately, several improvements in conventional CCD technology are required to successfully implement CCDs in atomic spectroscopic measurements. Movement of charge between pixels presents a potential problem. If charge cannot be quantitatively transferred between electrodes, and subsequently pixels, an inaccurate measurement will occur at the output amplifier. The measure of transfer efficiency is termed charge-transfer efficiency (CTE). CTE is particularly important for CCDs because charge can be transferred thousands of times before measurement. Thus, a CTE of 0.99999 or better is required. For the most part, charge is lost at trap sites in or between pixels and particularly at the surface interface between the epitaxy and insulating layer. For this reason, buried channel CCDs have become popular. In this configuration, an n-type region is added just below the silicon surface. This n-type region or buffed channel creates a potential energy minimum for charge to reside below the surface. The result is a device with much better CTE. Therefore, modem CCDs are manufactured with buffed channels to enhance CTE. While CCDs have the sensitivity for atomic emission, these devices must also be able to detect a wide range of emission intensities. The full well capacity for a typical CCD is approximately 3 x 105 e-, giving the CCD a wide dynamic range. When the full well potential of a pixel is reached, charge can spill over into nearby pixels in a process called blooming. In atomic emission, where strong and weak lines occur in close spatial proximity, blooming from strong analyte or background (Ar) lines will typically cause interference and decrease the sensitivity and dynamic range of the device. If enough light hits a pixel, an entire row or even the serial register can be filled with charge blooming from an intense line, eliminating the usefulness of the affected pixels. While AES measurements can be limited by blooming during normal operation of a CCD, two possibilities exist to prevent it in conventional CCDs. First, CCD architecture can be modified by the addition of n-type diffusion drains within the

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Figure 6. A CCD image is sequentially shifted toward the shift or readout register. Charge in the shift register is moved in a perpendicular direction.

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pixel. These drains can be placed horizontally, adjacent to the buried channel, or vertically, in the fabrication of the substrate. In either case, QE is reduced, particularly in the case of horizontal drains, which create fairly large dead regions within the pixel. Thus, diffusion drains are not generally used in scientific devices. Second, modifications in readout can be used to prevent blooming. This approach, called clocked recombination antiblooming (CRAB), uses the process of pinning to remove excess charge. In pinning, one electrode is biased with a negative potential to invert the surface charge, allowing holes to collect above the n-type buried channel region. During charge integration, the other electrodes are alternately clocked positive and negative to force excess charge to the surface. Holes at the surface can then recombine with excess charge to prevent blooming. While CRAB is not as effective as overflow drains, it does give conventional CCDs the ability to control blooming without modifications in architecture, which would result in lower quantum efficiency. Conventional antiblooming CCDs have been investigated for atomic spectroscopy, with mixed results (Sweedler et al., 1989; Billhorn et al., 1992). Since CCDs are integrating detectors, optimum integration times will differ between strong and weak analytical lines. Longer integration times are especially critical for short wavelength lines of Se, S, P, A1, and As, particularly at lower concentrations. While the CCD can be made antiblooming, quantitation of the collected charge is not possible for strong lines in which the full well has been surpassed. Thus, during a multicomponent analysis, multiple measurements can be required at a variety of different integration times. The need to perform multiple measurements translates into extra analysis time. Furthermore, caution must be used in selecting internal standards, because internal standard lines may have different integration times than analytical lines. One other concern in applying conventional CCDs to atomic emission is achieving the highest possible spectral throughput, especially in the UV portion of the spectrum. The presence of overlying gate electrodes and reflection at multiple layer surfaces significantly reduces the quantum efficiency of CTDs. An alternative to the traditional method of light collection is backside illumination with back-thinned CCDs. In these devices, the substrate on a typical CCD is removed mechanically and/or chemically, and an antireflection coating is applied. When the CCD is illuminated from the backside, light can interact directly with the epitaxial layer without traversing overlying electrodes and the insulation layer. CCDs realize significant QE improvements with backside devices, particularly in the UV and near-IR. Another means of increasing UV quantum efficiency in both conventional CCDs and CIDs is to use organic phosphor coatings. These coatings act as fluorescent down converters, in which short wavelengths of light are converted to longer wavelengths optimally detected by the CTD, and are commonly thin to prevent significant loss in spatial resolution. Similar increases of QE in the UV can be achieved with frontside-illuminated devices by altering the gate structure and readout of the device. The addition of

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virtual gate structures just below the buried channel eliminates the need for some of the overlying structure. As in backside devices, light can reach the epitaxial layer without first encountering overlying gate structure, thus increasing UV QE. One successful example of a virtual device used commercially in atomic spectroscopy is the segmented array.

Segmented Array Charge-Coupled Devices Segmented array charge-coupled devices (SCDs) are CCD arrays specially designed to overcome problems found in applying conventional CCDs to atomic emission. The SCD consists of a number of linear CCD arrays (Figure 7) placed at the positions of important analytical lines as dispersed in an echelle spectrometer. Two sets of SCD arrays are used to cover visible and ultraviolet wavelengths. Using 224 separate arrays, the segmented array positions up to four primary analytical lines per element. In the SCD, light is collected only over the region of a single p-doped virtual electrode. Electrons created in this region then migrate to a potential well in a second register, designated the storage register, which has overlying

Figure 7. The segmented array (SCD) consists of virtual phase linear CCDs placed at the positions of important emission wavelengths. Each linear array has three registers, the largest of which is the photoactive region under a virtual electrode.

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PENNEBAKER,WILLIAMS, NORRIS, and DENTON

electrodes for electron movement. Charge is then shifted into a third register from which readout occurs (Barnard et al., 1993a). The use of a virtual device allows the SCD to collect light without an overlying gate structure. Thus, the quantum efficiency of the device is improved, particularly in the UV, which circumvents the need for a fluorescent down converter. Since the SCD is composed of individual linear CCDs, each spectral region is analyzed by a linear array isolated from the other arrays. This configuration prevents much of the blooming possible in atomic emission; however, it does not prevent blooming from interferences within the array analyzed. The major limitation for antiblooming CCDs is the difficulty in quantitatively measuring strong and weak lines in the same spectrum. This limitation is overcome in the SCD by having control over each individual linear array. Integration time can be selected based on the sensitivity of the atomic line. Strong spectral lines can be read at early integration times while weak analytical lines can be integrated for long periods of time. The drawback to this system is that much of the spectrum is ignored; in fact, only 5.7% spectral coverage from 167 to 782 nm is achieved (Barnard et al., 1993b). Also, alignment of the spectrometer is critical to obtaining the specified lines, and internal standard lines may not be integrated over the same period as analytical lines. Despite these concerns, the SCD has been used in atomic spectroscopy, achieving very good detection limits with subnanometer resolution and a large increase in spectral coverage compared to the direct reader.

B. Charge-Injection Devices While the structure of the CID as shown in Figure 8 is similar to the CCD, operation of the device is considerably different. The CID is capable of reading charge both destructively and nondestructively. In either case, charge is not moved between pixels. ACID pixel consists of a pair of crossed electrodes, designated row and column electrodes. 5 To facilitate explanation, these electrodes are drawn side by side. The operation of a CID requires four separate steps, which are illustrated in Figure 9 for the operation of a CID 38 in "column storage" mode. The voltages on the electrodes are set to create a potential well beneath the column electrode "deeper" than the potential well under the row electrode. In a fashion similar to the CCD, photons that strike and are absorbed by the epitaxial layer create electron-hole pairs. Charge, consisting of holes rather than electrons, collects under the more negatively biased column electrode while electrons are attracted to the substrate. The pixel will collect charge until either the potential well is filled or the source is blocked from the detector. The next process, nondestructive charge readout, consists of steps (b) and (c) shown in Figure 9. First, the row electrode is disconnected from the row reference voltage and measured. Second, the potential well under the column electrode is collapsed, causing charge to migrate to the row electrode. This movement of charge results in a change in potential at the row electrode. A second measurement of the

Developments in Detectors in Atomic Spectroscopy

Figure 8. Photograph of a CID 38 with a blowup of four pixels.

159

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PENNEBAKER, WILLIAMS, NORRIS, and DENTON

Figure 9. ClD operation and measurement. (a) Initially, charge is collected under the more negatively biased column electrode. (b) Charge is measured by first sensing the voltage under the row electrode. (c) Charge is then moved under the row electrode by increasing the potential under the column electrode. The potential under the row electrode is then measured a second time. The combination of these 3 steps constitutes a nondestructive read. (d) Charge is cleared by moving both electrode potentials to +8 V.

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PENNEBAKER,WILLIAMS, NORRIS, and DENTON Table 1. Characteristicsof CID and SCD Detectors

CTD Pixel Dimensions Vertical Dimension Horizontal Dimension Dark Current (e-/pixel/sec) Full Well Capacity Detector Read Noise (e-) b

O D Syste~

SCD Syste~

28~t 281x <0.014@-80 ~ 1.04 a 10 6 18 e-

80-170~t 12.5lEt 120@-40 ~ 9.80 a 105 13 e-

Notes: aThesedevices are used by Thermo larrell Ash and Perkin Elmer, respectively.

bCID read noise is reported using 100 rereads. CDetection limits are reported using commercially available radial systems.

new version of the CID discussed later, true random access will be available. As in the segmented array, low wavelength lines are read with long integration times while visible and near-IR lines can be integrated for short periods. A short pre-exposure, approximately 0.2 s, is made prior to a measurement to determine the precise time at which the full well capacity 6 in a pixel will be reached. Based on the pre-integration signal, strong spectral lines can be continuously read and cleared while weak lines are measured at the end of the integration period, as illustrated in Figure 10. Instead of using short discrete integration times, the RAI feature effectively extends the full well capacity of the device for strong spectral lines. Device characteristics of the CID and the SCD are shown in Table 1. Although in practice read noise of a CID is slightly higher than that of an SCD, background noise from the source is typically the limiting factor in ICP emission spectroscopy (Boumans, 1987). As will be demonstrated in the next section, read noise has little effect on either system except when short wavelengths are measured at short integration times. However, based on device design, the CID is able to achieve full spectral coverage from 167 to 800 nm in a single measurement. More detailed information regarding the operation of CTDs can be found in a variety of other references (Epperson et al., 1988; Sweedler et al., 1988, Sweedler et al., 1994; Hanley et al., 1996).

III. COMPARISON OF CHARGE-TRANSFER DEVICES AND PHOTOMULTIPLIER TUBES FOR ATOMIC EMISSION SPECTROMETRY As discussed, a CTD gives atomic spectroscopists significant advantages over PMT-based systems. First, CTDs have the ability to collect either a significant fraction (SCD) or the entire spectrum (CID) in a single measurement. The wealth of lines collected by a CTD enables its user to analyze multiple elements at one time and to identify elements that would not normally be measured. Also, these

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devices have a large linear dynamic range with the ability to integrate light over variable periods of time. In other words, part per trillion level concentrations can be detected in the same spectrum as percent level concentrations. Beyond the wealth of lines, high sensitivity, and large dynamic range, CTDs can offer simultaneous analysis of all wavelengths. In many spectroscopic situations, changing from a single channel to multiple channels or to a multiplexed system will bring about a significant decrease in measurement time. Based on the time saved, multiple measurements of the entire spectrum can be made with a multichannel or multiplexed system. In certain cases, these measurements can be co-added, as in an FT-IR, to increase SNR of a measurement. This increase in SNR is based on two assumptions. First, both the single-channel and multichannel detector must be equivalent in terms of sensitivity. Second, the limiting noise in the measurement must be either shot noise or detector noise. As described earlier, atomic emission systems are normally background flicker noise limited. In this case, multiplexed systems experience a disadvantage and multielement systems experience no SNR advantage 7 versus a single-channel detector. In other words, CTDs should experience no SNR advantage compared to PMTs for atomic emission under equivalent circumstances. Emission measurements using a CTD, however, are not made in the same manner as emission measurements using a PMT. For CTDs, the entire spectrum (CID) or individual fractions (SCD) are collected over the same integration period. This type of analysis means that background and signal are measured at the same time. Therefore, when a background subtraction is made, the flicker component normally present between the measurement of background and background + signal is removed during the subtraction. Thus, background shot noise rather than background flicker noise is the major limiting factor for measurements made near the detection limit with simultaneous background subtraction. Using the concept of relative standard deviation of the background (RSDB) developed by Boumans (1987, 1994), the effect of the change in noise on the system is observable. The RSDB for the CID and PMT are plotted in Figure 11, along with the shot noise limit, for relative comparison. Initially, the CID is detector noise limited for low photon fluxes and short integration times. As the background signal is increased, the RSDB approaches the theoretical shot noise limit, surpassing the capabilities of a PMT scanning system. Improved RSDB results in higher SNR and better detection limits. For this reason, integration times for low wavelength lines (<200 nm) are typically set for 30 s or longer to insure superior detection limits. Also, an increase in either spectrometer throughput or integration time results in lower detection limits. Similar results have been obtained for measurement of RSDB with a segmented array (Barnard et al., 1993b; Ivaldi and Barnard, 1993).

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PENNEBAKER, WILLIAMS, NORRIS, and D E N T O N

10 .......... 9 CID System - - - Shot Noise Limit PMT System

1"-. I ""--.

m 1Of)

~""" _

"'~'~..

~-.,,.,,. "q.~

0.1

....... O Q

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I O O

0 0

. . . . . . . .

I

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. . . . . . . .

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Equivalent Background Signal (electrons)

Figure 11. Log-log plot of RSDB for CID system versus background intensity compared with theoretical PMT values and shot noise limit. PMT does not actually determine signal in electrons, but is shown as such for a comparison against the CID.

IV. READOUT AND MEASUREMENT WITH CHARGETRANSFER DEVICE SYSTEMS To apply a CTD to atomic analysis, two conditions must be met by the spectrometer system: the grating must have high dispersion, which gives the spectrometer sufficient resolution to avoid many spectral interferences, and the output of the spectrometer must match the format of a particular CTD. Echelle spectrometers, originally described by Harrison (1949), fulfill both of these conditions. While originally thought to be of little analytical significance (Laitinen and Ewing, 1977), echelle spectrometers create a high-resolution two-dimensional dispersion pattern, enabling CTDs to fulfill their potential as AES detectors. Traditional methods for creating high dispersion include large focal lengths and finely ruled gratings. Thus, spectrometers capable of producing high dispersion can be bulky (0.79 to 3 m focal lengths). On the other hand, echelle spectrometers use large incidence angles and high orders to produce high dispersion. An echelle grating (Figure 12) is a coarse grating with spacing of typically less than 1001/mm and is illuminated at high incidence angles on the "short" side of the grating to disperse light at high orders. Since orders of dispersed light overlap, order separation is performed with a second dispersion device, either a second grating or a prism. Light will then be dispersed in two dimensions, an ideal format for large pixel count two-dimensional CTDs. An echelle spectrometer is depicted in Figure 13. Light passes through an entrance pinhole and is collimated by mirror A. This collimated light is then

Developments in Detectors in Atomic Spectroscopy

165 /

hv diffracted

/ / /

// /

f

t

/

.. hv incident t

~

Figure 12. An echelle grating is illuminated at high angles on the "short" side.

cross-dispersed, first with a prism (y-direction) and then with an echelle grating (x-direction). The dispersed light is then focused with mirror B onto the imaging detector. The resulting echellogram from continuum light is shown in Figure 14. In this configuration, the grating produces high dispersion while the prism acts as the "order sorter." A typical setup for a CTD-AES system, also shown in Figure 13, consists of an atomic source (arc, spark, plasma, etc), focusing optics, an echelle spectrometer, and a CTD cooled to reduce dark current. The echelle spectrometer creates high-resolution cross-dispersed radiation, which is detected by the CTD. An echelle spectrum consists of wavelength dispersion in the horizontal direction and order dispersion in the vertical direction. An example of part of an ICP spectrum is shown in Figure 15. This spectrum of Mg illustrates the same five lines seen earlier in the left side of the second line of Figure 1. Various wavelengths are represented as 3 x

Figure 13. Diagram of echelle-CTD atomic emission system.

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PENNEBAKER, WILLIAMS, NORRIS, and DENTON

Figure 14. Continuum source arc lamp dispersion on a high-resolution echelle spectrometer with collection by CID over a spectral range of 410-195 nm.

3 sets of pixels rather than lines due to the pinhole structure of the entrance slit. A full spectrum of Fe (Figure 16) demonstrates the wealth of spectral information obtained in a single read of a CID. Qualitative analysis for atomic emission is often quite simple. The presence or absence of characteristic wavelengths of light indicates the occurrence of a particular analyte. In the simplest case, only one wavelength is indicative of an analyte; however, the presence of a spectral interference at the same position can inaccurately predict the presence of the element analyzed. For example, if Co is examined at 341.4736 nm, the presence of Fe at 341.4765 nm could interfere and yield a false positive without a spectral resolution of better than 3 picometers. Due to the possibility of interference, multiple lines are used to identify species. The primary

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Figure 15. Magnesium lines collected with low resolution CID echelle system illustrated in Figure 13. Wavelengths from left to right: 278.297, 278.142, 277.983, 277.829, and 277.669 nm.

wavelengths for each species in a particular atomic source are known, and their positions can be identified after wavelength calibration of the echelle spectrometer. Each of these positions on the CTD is examined for the presence of spectral lines. If each of the positions examined has sufficient spectral intensity, then the particular analyte is present. An example can be seen in Figure 16. The positions of five sensitive Fe lines are indicated by boxes in the echelle spectrum. Note that each box is filled, indicating the presence of Fe. The level of sufficient spectral intensity indicating the presence of a line varies according to the analyst. This level can be defined as the detection limit, 2-3 times the level of noise (c0, or as high as 5a as described by Bilhorn et al., 1992c. The absence of lines indicates that an element is not present at the defined

Figure 16. Low-resolution echelle spectrum from 800 to 190 nm of 1000 ppm Fe collected with a ClD. The indicated lines are as follows. (A) 259.940 nm (B) 259.837 nm (C) 240.588 nm (D) 238.204 nm (E) 239.562 nm. A total of 9 lines are present due to multiple orders of the same line detected by the CID.

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detection limit for the strongest line examined. Complications arise if only some of the boxes are filled. The presence of some lines indicates either the possibility that the element is present at a low concentration or that there is interference from any of the matrix elements in the sample analyzed. Difficulties arise from matrix elements with a multitude of lines at high concentration, such as Fe and rare earth elements, which present a complex spectrum on the CTD chip. It might be possible in the future to develop a smart system to correct for interferences and give approximate concentrations, judgments attainable due to the wealth of information provided by the CID. To date, no emission system has evolved to this level. In quantitative analysis, the actual intensity of a particular line is measured. The greater part of this discussion will focus on the use of a CID; however, segmented array readout is similar except that the horizontal dimension is variable and the vertical dimension is only one pixel high. Based on the spectral positioning and focus of a particular line, 90-95% of the light will fall on a 3 x 3 pixel area. Each individual line measurement comprises a 3 x 15 subset of pixels, called a subarray, based at the center of the pinhole image. An example of this readout is shown in Figure 17. For an atomic emission measurement, background intensity occurs fairly evenly across the spectrum, while the line signal will occur at the 3 x 3 center subarray. Background is obtained from the average background intensity on either side of the peak and subtracted from the intensity average obtained at the center. A bias subtraction is also required to measure any residual charge and correct for fixed pattern noise (True et al., 1999a,b).

Figure 17. 3 x 15 subarray from a CID. Signal + background is collected from the center 3 x 3 set of pixels while background is the average from both sides.

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V. CHARGE-TRANSFER DEVICE DETECTION FOR ABSORPTION AND SPECTRAL IMAGING This chapter has focused on traditional atomic emission detection techniques, neglecting many other areas of analysis. CTDs have been used in a number of other areas of atomic analysis. CIDs (True and Denton, 1999b) and SCDs (Harnly et al., 1997) have been used for multielement detection in graphite furnace atomic absorption (GFAA). Absorption in graphite furnace is a transient event and difficulties have been encountered with readout speed necessary to analyze it. Also, intensified CCDs have been used in the detection of laser-induced breakdown spectroscopy (LIBS) (Yamamoto et al., 1996; Castle et al., 1997; Ng et al., 1997). In these measurements, intensifiers such as multichannel plates are used to gate the detection of the CCD to avoid large background signals occurring early in the analysis. The high-resolution full-wavelength coverage of an echelle-CTD system has not been used to date with LIBS. CTDs have also been used in atomic spectral imaging. In these cases, both dimensions of a CTD detector are used to spatially image an atomic source rather than to collect spectral information. A filter or spectrometer is then used to isolate the wavelength region of interest, although it could be replaced with an appropriate AOTF to provide greater flexibility in atomic imaging of plasmas and other species. Spectral imaging has been used to study Pb (Castle et al., 1997), Na, Li, and Rb (Ng et al., 1997) in LIBS, and in fundamental studies on plasmas (Bye and Scheeline, 1994; Olesik et al., 1997) and flames (Georgiev and Alden, 1997).

VI. MULTIELEMENT DETECTION SYSTEMS OF THE FUTURE Array detectors provide atomic spectroscopists with a formidable amount of analytical data. Even with the data processing capabilities of today's computers, most of this data is considered superfluous; consequently, it is normally discarded. Several new chemometric software techniques are only now beginning to be applied to the area of atomic spectroscopy. These technologies should provide pattern recognition subroutines that will vastly improve qualitative analysis. At this time, a new generation of larger, faster CIDs is being developed, with better readout capabilities than current models. These devices, called random access CIDs (RACIDs), will be 1024 x 1024 or over 1,000,000 pixels, four times the size of the largest current CID. Also, the RACID will provide true random access of individual detector sites along with a new mode of detector readout called "collective readout" which is similar to binning in CCDs. In binning, charge from two or more pixels is physically combined and measured. Binning can cause three effects on spectral readout: increased SNR, increased read speed, and decreased spatial resolution. Binning differs from digital summing only if read noise is dominant in a measurement. In this case, fewer reads result in an SNR improvement of N rather

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than N ~/~.In collective readout, multiple row and column electrodes are used to move charge at the same time in several chosen pixels. This charge is read out as a combined result with similar effects as seen in binning. However, collective readout differs from binning in that charge is not physically combined. If the signal from a subarray is too high, which would then over-range the measurement voltages, or if a large spurious signal (cosmic ray event) occurs, then pixels can be read out individually, a possibility not available once a group of pixels has been binned in a CCD. When used for atomic emission, these modifications promise greatly improved performance. The increase in size will allow both increased resolution and the capability of simultaneously measuring all wavelengths from 167-900 nm in a single measurement. The application of collective readout should provide CIDAES measurements with lower read noise and increased read speed. As an example, the 3 x 15 subarray shown in Figure 17 would make 4500 reads for the measurement of 15 pixels not including bias subtraction. With collective readout, only 200 reads would be made on 2 pixel groupsmbackground and signal + background--which would result in both decreased detector noise and decreased measurement time. During this time saved for measurement, multiple rereads can be performed, resulting in even lower detector noise. Improved performance should also result for graphite furnace instrumentation. At the present time, measurements of a maximum of 8 subarrays can be made with the CID (True and Billhorn, 1999b). With faster readout speed and collective readout, the maximum number of lines should increase by 2 orders of magnitude or better. Another important future innovation is the development of a new generation of array detectors: pre-amp per pixel devices. Whether developed as CCDs or CIDs, these detectors should have reduced read noise while maintaining the RAI and antiblooming abilities of the CID. Currently, these devices are limited by the loss of device territory, with pre-amplifiers taking up as much as 30% of the pixel area. Hopefully these devices can be developed with larger pixel sizes for increased spectral throughput, which will result in even better detection limits. CTDs are only now being applied to a number of areas in atomic spectroscopy. CTDs will provide fresh capabilities and improved versatility to many "outdated" and other types of atomic sources, such as direct current plasma (DCP) and microwave-induced plasma (MIP). With improvements in new technology such as accousto-optical tunable filters (AOTF), CTDs should provide diagnostic abilities for monitoring plasma etching and other industrial processes. The future of array detector technology and its atomic spectrochemical application seems bright, especially when combined with the rapidly expanding powers of modern computers.

NOTES 1. Bloomingwill be defined and discussedlater in the text. 2. Scientificmodeof operationconsistsof slow scan speedsto increase chargetransferand reduce readout electronic noise and coolingto reduce dark current.

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3. Multiple electron-hole pairs may be created by higher energy photons according to AE/3.65 eV, the direct bandgap energy of silicon. 4. When either holes or electrons are physically measured, the charge is referred to as electrons. 5. In the past, these electrodes have been designated the "sense" and "collection" electrodes to describe their actual function. 6. Charge is actually integrated until 3/4 full well is reached. Nonlinear effects are realized above this value. 7. In some cases, a reduction in flicker noise is realized by the decrease in measurement time, as described by Bilhom et al. (3).

REFERENCES Bamard, T.W., Crockett, M.I., Ivaldi, J.C., Lundberg, EL. AnaL Chem. 1993a, 65, 1225. Barnard, T.W., Crockett, M.I., Ivaldi, J.C., Lundberg, P.L., Yates, D.A., Levine, P.A., Sauer, D.J. Anal. Chem. 1993b, 65, 1231. Boumans, P.W.J.M. In Inductively Coupled Plasma Emission Spectroscopy Part I; Boumans, P.W.J.M., Ed.; John Wiley and Sons: New York, 1987, Chap. 4. Boumans, P.W.J.M. AnaL Chem. 1994, 66, 459A. Bilhorn, R.B., Epperson, P.M., Sweedler, J.V., Denton, M.B. Appl. Spec. 1987a, 41, 1125. Bilhorn, R.B., Epperson, P.M., Sweedler, J.V., Denton, M.B. Appl. Spec. 1987b, 41, 1114. Bilhom, R.B., Denton, M.B. Appl. Spec. 1989, 43, 1. Bilhom, R.B., Pomeroy, R.S., Denton, M.B. Computer-Enhanced Analytical Spectroscopy; Plenum Publishing: New York, 1992, Vol. 3, Chap. 11, p. 281. Bye, C.A., Scheeline A. Charge-Transfer Devices in Spectroscopy; Sweedler, J.V., Ratzlaff, K.L., Denton, M.B., Eds.; VCH Publishers: New York, 1994. Castle, B.C., Visser, K., Smith, B.W., Winefordner, J.D. Appl. Spec. 1997, 51, 1017. Epperson, P.M., Sweedler, J.V., Bilhorn, R.B., Sims, G.R., Denton, M.B. AnaL Chem. 1988, 60, 327A. Felkel, H.L., Jr., Pardue, H.L. AnaL Chem. 1977, 49, 1112. Felkel, H.L., Jr., Pardue, H.L. AnaL Chem. 1978a, 50, 602. Felkel, H.L., Jr., Pardue, H.L. Clin. Chem. 1978b, 24, 602. Furuta, N., McLeod, C.W., Haraguchi, H., Fuwa, K. Appl. Spec. 1980, 34, 211. Georgiev, N., Alden, M. Appl. Spec. 1997, 51, 1229. Gustavsson, A., Ingram, E Spectrochim. Acta 1979, 34B, 31. Hanley, Q.S., Earle, C.W., Pennebaker, EM., Madden, S.M., Denton, M.B.Anal. Chem. 1996, 68, 661A. Hamly, J.M., Smith, C.M.M., Wichems, D.N., Ivaldi, J.C., Lundberg, P.L., Radziuk, B. JAAS 1997,12, 617. Harrison, G.R.J.O.S.A. 1949, 37, 522. Horlick, G., Yuen, W.K. AnaL Chem. 1975, 47, 775A. Ivaldi, J.C., Bamard, T.W. Spectrochint Acta. 1993, 48B, 1265. Keane, J.M., Fry, R.C. Anal, Chem. 1986, 58, 790. Laitinen, H.A., Ewing, G.W. (Eds.) A History of Analytical Chemistry; Maple Press: York, PA., 1977. Marra, S., Horlick, G. Appl. Spec. 1986, 40, 804. Ng, C.W., Ho, W.E, Cheung, N.H. Appl. Spec. 1977, 51,976. Olesik, J.W., Kinzer, J.A., McGowan, G.J. Appl. Spec. 1997, 51,607. Pilon, M.J., Denton, M.B., Schleicher, R.G., Moran, P.M., Smith, S.B. Jr. Appl. Spec. 1990, 44, 1613. Plankey, EW., Glenn, T.H., Hart, L.P., Winefordner, J.D.Anal. Chem. 1974, 46, 1000. Sawyer, R.A. Experimental Spectroscopy; Dover Publications: New York, 1963. Sims, G.R., Denton, M.B. Multichannel Imaging Detectors; Talmi, Y. Ed.; American Chemical Society: Washington DC, 1983, Vol. 2, Chap. 5. Sims, G.R., Denton, M.B. Talanta 1990, 37, 1. Stubley, E.A., Horlick, G. Appl. Spec. 1985, 39, 805.

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Sweedler, J.V., Bilhom, R.B., Epperson, P.M., Sims, G.R., Denton, M.B. Anal, Chem. 1988, 60, 282A. Sweedler, J.V., Jalkian, R.D., Pomeroy, R.S., Denton, M.B. Spectrochim. Acta 1989, 44B, 683. Sweedler, J.V., Ratzlaff, K.L., Denton, M.B. (Eds.). Charge-TransferDevices in Spectroscopy; VCH Publishers: New York, 1994. True, J.B., Hanley, Q.S., Denton, M.B. 1999. In preparation. True, J.B., Williams, R.H., Denton, M.B. Appl. Spec. 1999, 53, 273. Van der Plas, P.S.C., Uilbeyerse, E., De Loos-Vollebregt, M.T.C., De Galan, L. Spectrochim. Acta 1985, 42B, 1027. Winge, R.K., Fassel, V.A., Edelson, M.C. Spectrochim. Acta 1988, 43B, 85. Wirsz, D.E, Blades, M.W.J. AnaL At. Spectrom. 1988, 3, 363. Wood, D.L., Dargis, A.B., Nash, D.L. Appl. Spec. 1975, 29, 310. Yamamoto, K.Y., Cremers, D.A., Ferris, M.J., Foster, L.E. Appl. Spec. 1996, 50, 222.

GLOW DISCHARGE ATOMIC SPECTROMETRY

Sergio Carol i, Oreste Senofonte, and Gianluca Modesti

I.

II.

III.

IV.

V.

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Present Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Progress in Mechanism Elucidation . . . . . . . . . . . . . . . . . . . . . . . A. Fundamental Aspects of Planar Glow Discharges . . . . . . . . . . . . . B. Fundamental Aspects of Hollow Cathode Discharges . . . . . . . . . . . Innovative Source Models and Assemblies . . . . . . . . . . . . . . . . . . . A. Planar Cathode Glow Discharge Sources . . . . . . . . . . . . . . . . . B. Hollow Cathode Discharge Sources . . . . . . . . . . . . . . . . . . . . Recent Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Analysis of Metals and Alloys . . . . . . . . . . . . . . . . . . . . . . . B. Analysis of Nonconductive Solid Materials . . . . . . . . . . . . . . . . C. Analysis of Liquid Samples . . . . . . . . . . . . . . . . . . . . . . . . D. Analysis of Gaseous Samples . . . . . . . . . . . . . . . . . . . . . . . Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advances in Atomic Spectroscopy Volume 5, pages 173-234. Copyright 9 1999 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0502-9

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Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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ABSTRACT The use of the hollow or glow discharge sources in spectrochemical analysis and mass spectrometric analysis is described. Following a historical discussion the progress in mechanism of excitation is discussed. Types and variations of the systems to improve performance both for atomic and mass spectrometry is presented 9Finally, recent applications of the systems in such areas as metals and alloys, nonconductive solid materials, liquids, and gases showcases the versatility of these sources 9

I. I N T R O D U C T I O N A. Historical Background The vitality of glow discharge (GD) sources in analytical chemistry is a splendid (if not unique) example of how a spectrochemical approach can keep its validity throughout the years, adapt itself to new needs, and survive in its best shape the rise of other entirely new techniques. To date, several generations of spectrochemists have had (and still have) an opportunity to discover and appreciate the multipurpose nature and outstanding capabilities of this class of devices. The role in fields as diverse as analytical spectrochemistry, theoretical physics, laser technology, and surface treatment has not yet been fully explored and exploited. The long journey of GD sources started in the second half of the past century when the first systematic studies on electroluminescent phenomena generated by noble gases in low-pressure discharges (LPDs) were reported. In this context, the names of scientists such as Hittorf, Crookes, Geissler, and Townsend are too well known to require any further comments. It was only some decades later, however, that the applicability of LPDs to the realm of spectroscopy began to be recognized. The first milestone in this direction, by universal acknowledgment of the scientific community, is attributed to Friedrich Paschen. In 1916, he published a paper centered on the exploitation, for the interpretation of emission spectra, of a special GD version named hollow cathode discharge (HCD) because of the particular configuration given to the cathode (Paschen, 1916). More precisely, in this work Paschen clearly referred to previous findings by Bartels and coworkers, who in 1914 reported on the dramatic increase of the Fowler spectral lines, i.e. those attributable to the He spectrum emitted from a cylindrical cathode inside a Geissler tube. Wrote Paschen on that occasion: Es wurde weiter bemerkt, dass dieses Glimmlichtbei niederem Gasdrucke auf das Innere einer hohlenZylinderkathodebeschr~lktbleibtund dortdurchVermehrungder Stromstr~kezu intensiven Leuchten gebracht werden kann. Die ~iusserenW~indeder Zylinderkathodebleiben, 9

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wohl infolge von Ladungen auf den umschliessenden Glasw~inden, dunkel. Die hiernach konstruirte Heliumlampeenthielteine kastenf'6rmigeKathode aus dtinnemAluminiumblech... (... It was also noted that this glow is confined, at low gas pressure, in the cavity of a hollow cylindrical cathode, where the enhancementof current intensity can give place to intense light emission. The outer surface of the cylindrical cathode remains dark in spite of the electrical charges on the glasshousing. The heliumlampset up afterwardswas equippedwith a rectangular cathode made of a thin aluminumfoil .... ) The official birth of the HCD as a special type of GD was thus marked and brought to the attention of the practicing spectroscopist. Once this new avenue was opened, it was a matter of time to spark other basic investigations aimed at clarifying and ascertaining the actual properties of GD sources in general and of HCD sources in particular. Among these, especially worth mentioning are a study on the spectrum emitted by A1 (Paschen, 1923a,b) and a number of works promoted by Schiller and his coworkers (Schiller, 1923; Schiller and Keyston, 1931; Schiller and Gollnow, 1935). This latter author capitalized on the progress made by Paschen in understanding the advantages brought about by the HCD and its peculiarities; in doing so he shed further light on the candidate applications of this emission source. Among others, it was possible to elucidate the effects of excitation conditions on the characteristics of the emitted spectra and the formation of the dark space and negative glow (Schiller, 1923). The hyperfine structure of the Hg spectrum was also ascertained by using a new GD lamp, with particular emphasis on the separation of spectral lines emitted by isotopes of the metal (Schiller and Keyston, 1931). The experimental evidence accrued through these pioneer researches led Schiller to envision a bright future for HCD in terms of microanalytical uses. The lamp model developed a few years later clearly testified to the validity of this assumption and to the suitability of this technique to analyze not only gaseous samples, but also tiny amounts of solids, such as the very metal forming the hollow cathode (Schiller and Gollnow, 1935). Schiller continued to foster investigations in this field up to the 1950s by, for example, conceiving new designs for HCD lamps cooled by liquid nitrogen to further enhance the relative simplicity of the emitted spectra (Schiller and Michel, 1952). The period around World War II was undoubtedly a time of advancement and great accomplishments and the foundation to its present uses. The pilot work by Pahl and Weimer (1958) on the combination of a mass spectrometer with a GD source is a brilliant intuition of a hyphenated system. Today this system shows much promise for the direct ultratrace analysis of solids. For the sake of clarity, however, it should be stressed that these authors adopted this approach with the purpose of measuring ion fluxes in He and Ne and quantifying differences in energy among various types of noble gas ions formed during the discharge. Their findings revealed that ions with highly differing mobilities do not diffuse with equivalent energy, whereas the isotopes of Ne possess the same energy at the precision level permitted by the instrumental assembly. Finally, volume recombination processes in He and Ne were not detectable.

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The bloom of work that followed from the 1960s onward cannot be definitely summarized in a few pages. More information on the early years of LPDs can be found elsewhere (Caroli, 1983, 1985, 1987). Here it will suffice to recall the turning point marked by the planar cathode GD design conceived by Grimm in 1967, which gained immediate popularity for mineralogical and metallurgical applications and promoted quite a number of studies in other fields (Grimm, 1967, 1968). Of primary importance was also the work of McNally et al. (1947) who for the first time ever provided unequivocal experimental evidence of the suitability of HCD sources for reliable quantitative determinations, even in the case of difficult-to-excite elements such as halogens and sulfur. All this pioneering activity set the stage for the progress made so far by this branch of spectroscopy, a necessarily condensed account of which will be presented in the following sections. B. Present Trends

Although the versatility of LPD sources implies that their advancement is characterized by a different pace depending on the specific sector, a number of general features can be boiled down on which current efforts are concentrated. One of these hot issues regards the possibility of performing direct analyses of solid samples with none or minimal pretreatment. This dramatically reduces the determination time and the risk of contamination inherent in the dissolution of solid specimens. It can offer a valid alternative to solid sampling analysis by, for example, inductively coupled plasma-atomic emission spectrometry (ICP-AES) or inductively coupled plasma-mass spectrometry (ICP-MS) techniques combined with electrothermal volatilization or laser ablation (Boumans, 1972; Broekaert et al., 1983; Caroli et al., 1984; Koch et al., 1985; Winchester and Marcus, 1990). This spectroanalytical mode can easily be applied to electrically conducting samples and, with some adaptation, also to refractory materials (Caroli et al., 1983, 1986, 1993a; Duckworth and Marcus, 1989; Winchester et al., 1993a). In turn, the ability of LPDs to produce an atomic cloud from the test material can be expediently used to introduce the sample into the torch of ICP-AES or ICP-MS systems (Broekaert, 1987; Hess and Marcus, 1987). On the other hand, one of the major obstacles to the wider exploitation of LPDs in analytical chemistry is their relatively poor detection power when operated in the conventional dc mode. This gives inadequate ability to satisfy current needs in the determination of trace and ultratrace elements for environmental, clinical, toxicological, and technological purposes. Plenty of time and resources were thus invested over the years to try to overcome this drawback. This includes potentiating the emission output from an LPD tube by superposing to the discharge an additional form of energy (magnetic field, auxiliary electric discharge, radiofrequency, microwaves) or by modifying the lamp geometry (Farnsworth and Waiters, 1982; Ferreira et al., 1982; Czakow, 1985; Rudnevsky and Maksimov, 1985; Szilv~sy-V~imos, 1985; Zhechev, 1985; Caroli et al., 1988). Another way to indirectly circumvent

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this limitation resorts to the exploitation of the ability of GD and HCD devices to generate ionic populations which can thus be supplied to a mass spectrometer for subsequent quantification (Marcus et al., 1986). The success of this approach rests on the fact that the pseudo-vapor generated by the discharge is representative of the composition of the solid under test. On the other hand, while intrinsically suited to the analysis of solid and gaseous samples, LPDs appear to be less efficiently prone to the straightforward investigation of solutions. This is due to the quenching effect exerted on the plasma by the introduction of substantial amounts of solvent into the discharge zone. Various strategies, more or less satisfactory, have been elaborated to date to alleviate this shortcoming, as in the case of the cryogenic HCD system (Foss et al., 1983). Finally, surface analysis is a field where the importance of this spectroscopic discipline is bound to substantially increase thanks to the layer-by-layer ablation mechanism inherent to these discharges (Broekaert, 1987). The penetration rate depends on gas pressure, voltage, and current intensity as well as on the type of material examined. Problems are generally encountered in the initial phase of the surface attack because the plasma is still unstable. This means that it is difficult to derive quantitative information from the layer close to the surface. All these aspects are at the same time a clear testimony to the challenges posed today to analytical chemistry and unequivocal evidence supportive of the capabilities of LPDs to adapt themselves to novel and more demanding tasks. Although with no claim of completeness, the major contributions to this end done in the past decade will be illustrated and discussed hereafter.

II. PROGRESS IN MECHANISM ELUCIDATION A. FundamentalAspects of Planar Glow Discharges The advancement made recently by LPDs in terms of practical exploitation for analytical and technological purposes finds its driving force in the significantly better comprehension of the basic processes governing the plasma in these sources. The wealth of information achieved so far is exhaustively illustrated in some reviews on such processes as well as on novel designs and recent applications of the radiofrequency (RF)-powered sources (Marcus, 1992; Fang and Marcus, 1993; Marcus, 1993; Mei and Marcus, 1993). As revealed by a Fourier transform (FT)-based study, one of the major components of noise in GD-AES was found to be that caused by photons, while at extremely low frequencies this could be accompanied by a drift-noise component (Winchester et al., 1993b). In addition to this, four major components were identified in the background signal (Payling et al., 1994). The gas flow rate in GD-AES is a critical parameter that should be fully controlled to obtain reliable relative sensitivity factors (RSFs) (Tanaka et al., 1994). To this end, a diaphragm valve was inserted between the vacuum pump and the GD source to master

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independently from each other the in- and outflow rates of the discharge gas. Unfortunately, a self-consistent interpretation of the dependence of RSFs on gas flow rate could not be elaborated. Various techniques, namely atomic emission spectrometry (AES), atomic absorption spectrometry (AAS), and mass spectrometry (MS) were used to study the temporal signal profiles of some analytes in a modulated GD plasma (King and Pan, 1993). Among others, it was found that, using Ar as the filler gas, peaks were obtained 2 ms after the discharge power was switched off. Recent Langmuir probe measurements of electron temperature and species densities in the GD plasma provided additional evidence for the absence of local thermodynamic equilibrium (Boegaerts et al., 1995). A two-dimensional model of dc GD plasma was proposed to interpret the processes of formation of Ar metastable atoms and of pseudo-vapor generation (Boegaerts and Gijbels, 1996). The same authors also worked out a model to account for the behavior of Ar ions, fast Ar atoms, and electrons (Boegaerts et al., 1996). A He-CH 4 dc GD plasma was investigated and a simplified kinetic model proposed to accommodate all experimental observations (De la Cal et al., 1993). The negative ion formation in a GD source sampling from the atmosphere was investigated in dependence of gas dynamics (Kasthurikrishnan and Koropchak, 1993). Autoionization of neutral and metastable Ar atoms in the accelerating space of a mass spectromer was examined with the purpose of contributing to the interpretation of some key aspects of the GD plasma (Mason et al., 1993). A sharp increase in the production of various species of analyte ions was noted in the GD plasma immediately after the termination of the applied power (Pan and King, 1993a). This phenomenon was ascribed to the increase in the formation of Ar metastables resulting from the recombination of Ar ions with electrons and the ensuing enhancement in Penning ionization of sputtered atoms. The possibility of exploiting this fact to improve by 1 to 3 orders of magnitude the signal-to-noise ratio through the use of gated detection was envisaged. Charge-exchange mechanisms in a GD were studied to measure the emission intensity of three Cu transitions (Levy et al., 1991). The dependence of the etching rate of Mo in a GD source on applied power density (0-10 W cm -2) and Ar pressure was investigated (Van Straaten et al., 1991). Experimental results were interpreted by means of a one-dimensional Boltzmann equation to take into account the role played by ions, neutral species, and electrons. On the basis of calculated diffusion profiles of sputtered material, it could be concluded that the abundance of neutral atoms is the highest in front of the sample. Also the effects of He added to Ar and Ne were elucidated by measuring the intensities of Cu ionic lines (Wagatsuma and Hirokawa, 1991). The formation of metal argides in the GD plasma was studied by AES and MS (Ohorodnik et al., 1993). The influence of reactive species (e.g. air and water vapor) could be thus clarified. The excitation efficiency in the case of various mixtures ofNe, Xe, and halogens was further clarified (Golovitskii, 1992). The highest ultraviolet (UV) radiation output was obtained with Xe at 4.8 torr and C12 at 0.8 torr. Wagatsuma and coworkers carried out extensive studies to characterize the emission of A1, Ca, Fe, and In with different filler gases (Ar, He, Ne)

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(Wagatsuma and Hirokawa, 1995a, 1996; Wagatsuma, 1996). The role played by the carrier gas in GD-AES was explored in another study (Wagatsuma and Hirokawa, 1994). In particular, alloys of Cu-Ni and Ni-Co were sputtered by He-Ar gas mixtures. When He predominated (92% or more), the dependence of the voltagecurrent intensity characteristics of the discharge was found to cease. In another paper the same team reported on the effects of small amounts of O 2 added to Ar (Wagatsuma and Hirokawa, 1995b). An 800-fold reduction in the Cu line at 324.7 nm was observed when O 2 was present at a concentration of 4%. The energy distribution of ions bombarding the cathode surface in a GD source was examined by Van Straaten et al. (1995). Using Ar and Ne as the discharge gases and A1, Cu, Mo, and Ta as the cathode materials the conclusion was reached that bombarding particles are both low-energy singly charged gas ions and high-energy singly charged ions from the cathodic material itself. While for the former group the motion in the dark space was dominated by charge-exchange collisions, the latter ions could impact onto the cathode surface with almost all the energy of the full discharge potential. Asymmetrical charge exchange, in fact, is known to have a low probability. The interferences caused by contaminants in the discharge gas as well as plasma diagnostic measurements were reported by Ohorodnik and Harrison (1993) and Ohorodnik and Harrison (1994). They used a GD unit with a cryogenically cooled coil. To this end, data on Fe § excitation, N § rotational emission spectra and physical temperature were obtained. What is still unclear are the consequences on the analytical performance of a plasma possessing a temperature lower by 50-300 K than normal. By moving the sample insertion probe inside the GD cell it was possible to map the discharge atom populations for Ar, Cu, and Fe (Hoppstock and Harrison, 1995). The techniques used for this purpose were AES, AAS, and MS. The dependence of the atomic profiles on discharge voltage and distance between sample and exit orifice was clarified. It was shown that N 2 and 02 could seriously affect spectral line emission intensity of analytes in GD-AES if they were present above 0.1% m/m in the Ar cartier gas (Fischer et al., 1993, 1994). Ionization and atomization in the GD plasma may be seriously affected by the presence of water vapor, thus leading to a decrease in analytical signals, the extent of which is hardly foreseeable (Ratcliff and Harrison, 1994). The major consequences would be oxidation of the sample surface, quenching of Ar metastables, lower sputtering, and loss of analyte atoms due to enhanced gas-phase reactions. It was therefore recommended to minimize the water vapor content and to keep it constant as much as possible by using a high-purity discharge gas, optimized vacuum systems, and getters as appropriate. Also cryogenic cooling of the GD unit was deemed to be beneficial. In a subsequent study, pulse-injected water into the discharge to clarify the behavior of various cathodic materials (Cu, Fe, and Ti) under such conditions was used (Ratliff and Harrison, 1995). Depending on the reactivity of these metals, oxides were formed in the course of the pulsed injection and shortly afterwards, thus affecting the analysis. From this standpoint, Ti was found to be

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less perturbated than Cu and Fe. The effect of water vapor in the Ar plasma gas was investigated in a study of the GD source as a sputtering atomizer for AAS (Larkins, 1991). It was concluded that in the case of A1, Zn, brass, and steel alloys both lower sample erosion rates and combination of the analyte atoms in the gas phase with the H20 molecules led to a decrease in the number of free atoms. Such adverse effects on atom production have been observed at water vapor concentrations in Ar as low as 10 lag g-1. The benefits deriving from rapid pulsing of GD sources were explored by Harrison and Hang (1996) and Pollmann et al. (1996). When compared to dc- and RF-operated units, the emission in the pulse mode from Cu samples was characterized by a different atom-to-ion ratio. Several research teams endeavored to boost the performance of GD sources by a number of approaches. Among these, the superposition of MW irradiation on the GD plasma was found to be a promising powerful analytical tool. Atomic lines of the carrier gas (Ar, Ne) as well as many atomic lines of the sputtered material were found to increase in a MW-boosted GD source when compared to the pattern characteristic of the conventional GD discharge (Steers and Leis, 1991). The charge-exchange mechanism were assumed to be affected significantly by the superposition of the MW field, thus leading to a decrease in the intensity of ionic lines. Further improvements in the investigation of the fundamental properties of MW-boosted GD-AES were reported by Leis and Steers, 1994. The excitation temperatures of a MW-boosted GD source were found to be higher than in the conventional discharge mode (Li et al., 1996a,b). The proposed analytical system was used for the direct analysis of bulk solids as well as for depth profiling. A MW-boosted GD source was used to investigate the excitation of Cr and Fe spectra by UV/vis FT spectrometry (Steers and Thorne, 1993). The view that excitation temperatures do not follow a self-consistent pattern was supported by the experimental findings. The true line profiles of A1, Cr, Cu, and Fe were also reported with both Ar and He, this pointing to unambiguous line identification even when relative intensities are profoundly altered. Finally, the applicability of the FT emission spectrometry to achieve high-resolution measurements with an MW-boosted Grimm GD source was reviewed by Weiss, 1995a. Another way to boost the GD plasma is coupling with a magnetic field. The performance of the Grimm GD source was reported to substantially benefit from this kind of superposition (Heintz et al., 1995a). The experimental design did not alter the lamp configuration, but was limited to the mere addition of a N d - F e - B magnet on the back of samples. Provided that samples were sufficiently thin (1-2 mm), for bulk Cu and a Cu-Ni-Fe alloy the sputtering rate of Cu was found to double from the initial 100 l.tg min -1 in the absence of the magnetic field, although this also caused an increase in noise. On the other hand, for several lines (e.g. Cu at 324.7 nm and Cr at 425.7 nm) the enhanced intensity led to a lower signal due to an increase in self-absorption phenomena.

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As regards the RF-coupled GD source, electron average temperature and electron and ion densities in the plasma were derived after Langmuir probe measurements (Ye and Marcus, 1995). The characteristics of charged particles in a RF-GD plasma were further investigated by the Langmuir probe approach (Ye and Marcus, 1996). In particular, the figures obtained for a conductive material (Cu) and an insulator (Macor| were compared. In view of potential applications in AAS, the sputtering and spatial distribution of atoms in the RF-GD plasma were investigated as a function of discharge conditions (Absalan et al., 1994). Oxygen-free Cu was used as the cathodic material. The emission characteristics of a RF-powered side-on viewed GD unit were investigated by Heintz et al. (1994). Emission features of Ar at 420.5 nm, Cu at 324.7 nm, and OH bands at a pressure of 0.1-1 torr of the filler gas, and an RF power of 20-50 W were reported. The effects of a transverse magnetic field above the cathode surface were also elucidated as regards the voltage source, sputtering rate, and spatial emission characteristics. When the magnetic field was applied, the detection power of the boosted source became much the same as the conventional GD source by Grimm; e.g. in the range of 10 to 50 l.tg g-1 for trace components (A1, Cr, Mg, and Mn) in a Monel 400 Ni-Cu alloy. In another study following the same line of investigation, it was concluded that sample ablation and analyte emission intensities increased significantly when compared to those of the plain RF-GD mode (Heintz and Hieftje, 1995b). This held true independent of whether the samples were conductive or not. Attempts to clarify the dependence of the RF-GD plasma on excitation frequency have been reported for the interval 2-30 MHz (Lazik and Marcus, 1994; Marcus et al., 1995). Results showed that the lower the frequency is, the more the discharge energy is dissipated at the sample surface (rather than in the negative glow zone, with the attendant enhancement in sputtering) and the more high-lying transitions are favored. On the other hand, higher frequencies were characterized by lesser background and better stability of the plasma. Parker and Marcus (1996) investigated the characteristics of a power-modulated or pulsed RF-GD source. By performing AAS measurements, the atom production process could be studied and concluded that the large populations of excited states with high instantaneous power make the use of such sources for AAS not very convenient because of the unfavorable resonant transitions. The influence of working conditions and limiting orifice diameter in RF-GD-AAS was further explored (Parker and Marcus, 1994). Moreover, self-absorption in RF-GD-AES is an aspect that still awaits clarification (Winchester and Marcus, 1996). With particular reference to the GD-MS technique, specific reviews on its properties and applications were recently published (Dean et al., 1990; King and Harrison, 1990; Steuwer, 1990a, b). Regarding significant progress in this field, mention should be made of the work done by Vieth and Huneke (1990) who developed a rate model for electron ionization-three-body recombination fairly consistent with experimental measurements of densities of singly- and multiplycharged Ar ions. Penning ionization predominated when the ion species had

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SERGIO CAROLI, ORESTE SENOFONTE, and GIANLUCA MODESTI

ionization energies lower than the energy of Ar metastables. The limited dynamic range was found to be the major drawback of GD ion sources when coupled to quadrupole ion traps (Duckworth et al., 1994). An electrostatic probe was used to clarify how GD-MS plasma parameters were influenced by the voltage of an auxiliary electrode immersed in the plasma itself (Taylor et al., 1990). While positive bias on the sample orifice correlated well with plasma potential and ion energy, electron temperature displayed a more complex pattern. An increase in ion transmission to the quadrupole analyzer ensued from enhanced ion energy. An exhaustive treatment of the exploitation of the formation of doubly charged and polyatomic ions in GD-MS when the singly charged analyte ions are seriously interfered was presented by Goodner et al. (1995). Especially useful from this point of view were analyte argides of the type MAr + as well as singly charged analyte dimers and doubly charged analyte ions. Matrix matching was rather important to obtain accurate results. A comparative study of Ar, He, Kr, Ne, and N 2 was done to identify which of them gave place to lesser interferences in GD-MS (Giglio and Caruso, 1995). The worst gas in terms of analyte intensities, memory effects, and cost was Kr, while Ar was the best as regards sputtering rate. The formation of polyatomic ions turned out to be facilitated, as demonstrated by a study focused on the behavior of three discharge gases, i.e. Ar, N 2, and Xe (Douglas et al., 1994). The relatively low ionization potential along with the high dissociation efficiency of polyatomic species point to Xe as the most effective target gas. In order to elucidate the dependence of ion intensity and energy distribution on power, carrier gas pressure, and sampling distance, a magnetron RF-GD-MS apparatus with a borosilicate glass cathode was assembled. The MS moiety could be aligned either in the axial or radial position with respect to the discharge (Molle et al., 1995). The only parameters significantly affecting ion intensity were found to be pressure and distance in the axial mode, whereas ion energies varied as a function of changes in electron temperature.

B. FundamentalAspects of Hollow Cathode Discharges Commercial HCD lamps were the subject of a study aimed at examining their emission line profiles by means of high-resolution FT spectrometry (Kang et al., 1995). The Mg line at 285.2 nm and the Pb line at 283.3 nm showed the expected self-absorption broadening effects with increasing lamp current intensity. On the other hand, the AI line at 308.2 nm was practically independent of current intensity in the range of 1-30 mA. A microcavity HCD source was used to study the breakdown characteristics of an Ar-He mixture (Chen and Williams, 1996). The behavior of conventional cylindrical cavity cathodes was compared with that of fiat cathodes and cathodes with a spherical hole. Also the size of the cavity and the cathode material (Cu, stainless steel) were examined. Results showed that the hole size determined the minimum discharge current necessary for the plasma to take place inside the cavity. By properly shaping the cathode hole it was possible to

Glow Discharge Atomic Spectrometry

183

minimize the breakdown voltage and have access to a wider range of pressure for the filler gas (Ar, He, and their mixtures). The aging of hollow cathodes due to the sputtering process influences the analytical precision attainable by this technique. A systematic study was conducted to investigate in detail the changes occurring in the cavity after prolonged sputtering (Tseng et al., 1992). A bulb-like hollow was thought to be the final stable configuration. Conditioning of cathodes is extremely beneficial as short-term stability of ca. 1.8% RSD and long-term stability of better than 5% RSD could be easily attained after several hours of predischarge. This rather time-consuming procedure lends itself to automation and can therefore be conducted overnight. Cathodes thus conditioned can be reused for more than 140 analytical runs. The procedure was tested in the case of Li and Na in view of its applicability to microsamples of biological fluids. A study on the spatial distribution of spectral lines emitted by HCD lamps for AAS use was undertaken (Gilmutdinov et al., 1995). In the case of cathodes of small size, the distribution pattern was parabolic in shape, independent of whether the emission was from atoms or ions of the cathodic material or the carrier gas. The minimum emission took place along the cathode axis. The spectrum of a Pt-Ne HCD reference lamp was reported in an atlas (Sansonetti et al., 1992). A vidicon videocamera was used to clarify the axial evolution of the negative glow in a HCD plasma (Bartlow et al., 1992). Results confirmed previous studies in that the optimum pulse width was 7-11 Its. The electrons' motion in a high-voltage segmented HCD system was studied by a Montecarlo approach (Donko et al., 1996). The focusing ability of the cathode geometry was calculated to enhance the ion production in the discharge center. The superposition of a magnetic field led in turn to the formation of two regions of high ionization rate. The dependence of these phenomena on the number of cathode segments (up to six) was also examined. The energy distribution of the electrons absorbed by the anode and the fraction of oscillating electrons were finally studied under different discharge conditions. Laser photoionization and galvanic detection were applied to HCD dark space diagnostics (Babin and Gagn6, 1992). Photoionization was produced to directly measure dark space widths of linear field distribution. A theoretical model was developed and its predictions were verified with experimental findings in the case of a uranium HCD assembly operating with Ne or Xe. Variations in neutral ground-state density of sputtered material in the dark space were also measured. The excitation mechanism of metal vapor spectra was studied (Chera et al., 1992). The HCD plasma characteristics and the effects of easily ionizable elements were investigated (Szilv~issy-V~.mos et al., 1991). A decrease in the emission intensities of Sr II at 430.5 and 407.7 nm, A1 1 at 494.4 and 396.1 nm, and He I at 412.1 nm was observed. It was reported that the addition of N 2 to the filler gas of HCD lamps for use in AAS resulted in higher excitation and emission intensity (Niemczyk et al., 1994). The extent of the enhancement as a function of total gas pressure and

184

SFRGIOCAROLI, ORESTESENOFONTE,and GIANLUCA MODESTI

lamp operating current was clarified. This phenomenon could be ascribed to the formation of native N with the ensuing enhanced efficiency ofpromoting transitions for elements such as Ca, Cd, and Fe. The presence of N 2 spectral bands was an obvious drawback. Moreover, proper exploitation of the fundamental parameters of an electrothermal HCD allowed for a 4-6-fold enhancement of the radiation emitted in a furnace atomic nonthermal emission spectrometry (FANES) system (Papp, 1990). Last, but not least, the applicability of the HCD source in MS looks promising. Two groups of ions (respectively with high and low kinetic energy) were detected in the plasma of a reversed HCD source combined with a MS apparatus (Deng and Williams, 1994). The former was produced in the negative glow region, while the latter formed in the extraction orifice of the cathode base. Discrimination between these two groups by setting the MS acceleration voltage so as to separate the high-energy group (analyte atoms) from the low-energy group (Ar carrier gas and cluster ions) could lead to analytical benefits.

III.

I N N O V A T I V E SOURCE MODELS A N D ASSEMBLIES

A. Planar Cathode Glow Discharge Sources

TypesIntended for Atomic Spectrometry The revolutionary GD lamp devised by Grimm in the late 1960s has never ceased stimulating the imagination of spectroscopists who since then have continuously endeavored to further improve its performance and to tailor it to specific needs. Worth mentioning from this standpoint is the version recently proposed by Ruste and Schwoehrer, by which these authors succeeded in obtaining concentration profiles from nonflat oxide surfaces of nonductile alloys (Ruste and Schwoehrer, 1996). Another interesting modification of the classical dc Grimm lamp for the direct examination of solids is the microsecond-pulsed model proposed by Hang et al. (1996). The sample was placed on the tip of a direct insertion probe. The configuration adopted (Figure 1) permits current intensities as high as 500 mA with 200 ns pulses to be applied. During the active cycle of a pulsed operation a larger net signal (even up to 2 orders of magnitude) can be produced than would be generated through a comparable power level in the dc mode. Furthermore, the fast pulsed discharge operation may permit a diagnostic evaluation of plasma processes and ultimately may lead to tangible analytical advantages. The same setup was also used to ascertain its suitability in conjunction with MS since the short-term nature of the discharge appears to be particularly attractive in the time-of-flight (TOF) mode. In addition to this, the response of the system to AAS and atomic fluorescence spectrometry (AFS) measurements was examined. Recently, an atmospheric-sampling GD lamp was presented which has some merit as a soft ionization source for MS (McLuckey et al., 1988). The system also

185

Glow Discharge Atomic Spectrometry A

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Figure 1. Representation of the GD system used for AES, AAS, AFS, and MS measurements. (A) direct insertion probe; (B) GD chamber showing GD as shaded region; (C) monochromator; (D) preamplifier; (E) boxcar; (F) data aquisition; (G) focusing lenses; (H) xenon flashlamp; (I) hollow cathode lamp; (J) microsecond pulsed power supply. Reproduced by permission of the American Chemical Society, with acknowledgment to Hang et al., 1996. proved to be adaptable to the determination of nonmetals in gases and vapors in the AES mode (Starn et al., 1991; Broekaert et al., 1993). This last version, shown in Figure 2, performed well in the determination of As in solutions after its conversion to arsine by means of a continuous-flow hydride-generation attachment (Winchesster et al., 1993). Stability of discharge posed no problems whatever the support gas used (Ar, He, Ne). A detection power in the region of a few tens of ng mL -1

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186

SERGIO CAROLI, ORESTE SENOFONTE, and GIANLUCA MODESTI

could be thus achieved; these figures being however still too high when compared with those offered by other more popular techniques, e.g. electrothermal atomization AAS (ETA-AAS). For sputtering to take place with LPD sources in the conventional dc mode, solid samples must be, by definition, electrically conductive. This constraint has been for a long time a problem for users of such spectroanalytical techniques. Various approaches were developed to expand their applicability to refractory materials. The preparation of pellets by mixing the ground sample with metal or graphite powder was quite successful, although time-consuming and contamination-prone. This went against one of the primary benefits brought about by LPDs in the case of conducting specimens (i.e. very little on no pretreatment). A jet-assisted GD source was employed to investigate the effects of power and Ar flow rate and pressure (Banks and Blades, 1991). Serious self-absorption problems were encountered with the most sensitive spectral lines. The introduction of RF-powered LPD sources has triggered unprecedented progress both in AES and MS (Marcus et al., 1994d). A prototype of the RF-GD tube is detailed in Figure 3 (Lazik and Marcus, 1992). With this particular construction, moreover, it was possible to elucidate the effects on the emission characteristics of limiting anode orifice geometry and anode thickness. In the case of brass samples, it could be ascertained that orifices with a diameter of 2 mm minimize self-absorption interferences and enhance sputtering, whereas emission intensity improves only slightly with thinner anodes. Moreover, the bias potential showed a direct proportionality to RF power and an inverse relationship to discharge gas pressure. Along the same line of investigation, experimental evidence has accrued as to the advantage to pulsing a RF field in that better stability of the plasma is achieved even at high applied power. An example of pulsed RF-GD arrangements as described by Pan and King is illustrated in Figure 4 (Pan and King, 1993b). Reportedly, this operation mode allowed emission signals for a number of analytes in coal fly ash and graphite (Ca, Co, Cr, Cu, Fe, and V) to be enhanced by a factor of 4 over those obtained through the conventional steady-state RF-GD source. No external cooling was found necessary because the cathode can dissipate heat sufficiently between two consecutive pulses. A flow assembly to be mounted onto a RF-GD source was developed with the purpose of delivering a uniform radial flow of discharge gas onto the cathode surface (Lazik and Marcus, 1993a). Pros and cons on the use of this instrumental scheme were outlined. Among others, in fact, the sputtering efficiency of the plasma turned out to decrease at higher gas flow rates, whereas self-absorption and self-reversal phenomena were found to increase. Under the reported experimental conditions, on the other hand, emission intensities for ionic analyte species are enhanced by higher gas flow rates. The possible advantages ensuing from gas entrainment of sputtered atoms or plasma positioning deserve however further exploration. The same authors carried out some additional work to characterize the

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Figure 3. I. Schematic diagram ofthe RF-powered GD source. (A) stainless steel body; (B) thermocouple gauge; (C) vacuum port; (D) O-rings; (E) Macor| spacer; (F) brass torque bolt; (G) glass insulator; (H) RG-213/U coaxial cable (to matching network); (I) male coaxial connector; (I) copper conductor; (K) female coaxial connector; (L) sample; (M) orifice disk; (N) negative glow; (O) fused silica windows; (P) argon inlet ports. II. Enlargement of the anode orifice disk. (A) orifice diameter; (B) O-ring groove; (C) orifice thickness. Reproduced by permission of Elsevier Science B.V., with acknowledgment to Lazik and Marcus, 1992.

behavior of the RF-GD lamp in the analysis of nonconducting materials (Lazik and Marcus, 1993b). A major factor in the performance of the discharge was identified in sample geometry. The dependence of self-absorption phenomena in RF-GD-AES on discharge conditions were further clarified by means of FT spectrometry (Winches-

188

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Figure 4. Illustration of the experimental setup for pulsed RF-powered GD measurements. (A) HV; (B) microprocessor; (C) computer; (D) pump; (E) argon cylinder; (F) pressure monitor; (G) RF power supply; (H) automatic matching network; (I) adjustable bellows assembly; (J)lens; (K) photomultiplier; (L) monochromator. Reproduced by permission of the Society for Applied Spectroscopy, with acknowledgment to Pan and King, 1993. ter and Marcus, 1996). It was concluded that self-absorption exerts severe effects with RF-powered tubes and no suitable set of parameters can appreciably reduce it. It is therefore recommended to resort to nonresonant lines in the analysis of major elements. The quantification of minor and trace analytes was not expected to be seriously affected by this drawback. The possibility of viewing either the glow region together with the sample surface or only the glow region is one of the features of another RF-GD emission source (Winchester et al., 199 l a). Although basically equivalent, the former optical geometry results in higher emission intensity as a consequence of the more complete viewing of the energetic parts of the plasma. Some information on the sputtering process of Macor | samples was also given. Substantial increase in emission intensity with respect to the conventional GD source was attained by means of the microwave-induced plasma (MIP)-boosted microsecond-pulse GD tube (Su et al., 1997). The assembly, shown in Figure 5, performed efficiently at support gas pressure less than 200 pa, with optimal coupling between MIP and electrical discharge, appreciable reduction of self-absorption, and enhancement of atomic emission by more than 1 order of magnitude

Glow Discharge Atomic Spectrometry

189

Figure 5. Diagram of the MIP boosted lxs-pulse GD system. (A, B, and F) O-rings; (C) quartz tube; (D) Y-tuning screw; (E) to vacuum detector; (G) sample; (H) quartz ring; (I) argon inlet; (I) coaxial line; (K) coupling loop; (L) X-tuning screw; (M) to vacuum pump; (N) to monochromator. Reproduced by permission of the Royal Society of Chemistry, with acknowledgment to Su et ai., 1997. for selected spectral lines. Above 240 pa, on the other hand, resonance lines gave place to two independent peaks caused by the separate excitation of sputtered atoms by MIP and pulsed discharge. Another important factor in the optimization of the performance of GD sources is the identification and removal of optically induced analytical errors. This aspect was dealt in a study involving a parabolic distribution of species in the GD plasma with an axial peak aligned with the center of the crater, if the sputtered surface was chemically homogeneous (Hoffmann and Ehrlich, 1995). Imaging the plasma directly onto the spectrometer entrance slit was a potential source of error in the case of chemically heterogeneous samples (Winchester, 1996). As ascertained experimentally by means of the lamp shown in Figure 6, this is basically the consequence of the localization of the atomic emission of sputtered surface components close to their origin combined with the fact that only a small, slit-shaped volume of plasma is imaged onto the entrance slit. To solve this problem, the so-called spatially averaging optical transfer method should be used. Many other important improvements in configuration, construction, and electronics have been conceived. Among these, the detection power of the Grimm GD

190

SERGIO CAROLI, ORESTESENOFONTE, and GIANLUCA MODESTI

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Figure 6. Schematic view of the GD source used to study the optically induced error in GD-AES. (A) polypropylene flange; (B) stainless steel body; (C) hose connection for Baratron gauge; (D) 175 txm gap; (E) disk with 9.5 mm diameter sampling orifice; (F) O-ring; (G) bolt for mounting sample; (H) sample; (I) O-rings; (J) vacuum hose connection; (K) argon hose connection; (L) quartz window; (M) O-ring. Reproduced by permission of the Society for Applied Spectroscopy, with acknowledgment to Winchester, 1996. lamp was reported to increase substantially by using a photon-counting dynamic digital lock-in amplifier system which allowed spectral background to be discriminated from the emission signals (Gokmen et al., 1996). Macroscale elemental mapping was possible by means of a new GD device, which permitted a spatial resolution of 1 mm to be obtained (Winchester and Salit, 1995). Multiple discharges could in fact be performed simultaneously over a Macor | restrictor plate bearing a grid-like series of holes. The emission signals were processed by Hadamard transform spatial imaging. To obtain the individual emission intensifies again, the matrix multiplication approach was applied. The use of the internal standard was mandatory to minimize mapping errors. Chakrabarti et al. (1991) designed a cathodic-sputtering atomizer for AAS in which both the flow rate and pressure of the Ar discharge gas could be controlled, this resulting in a substantial enhancement of the sensitivity by a factor of 5 when compared with commercial devices. The construction of a new cathodic-sputtering atomizer for use in AES and AAS was described which permitted the independent control of Ar flow rate and pressure

Glow DischargeAtomic Spectrometry

191

(Hutton et al., 1991). Some pilot studies were performed by means of this device on the atomic emission of massive Cu and Cr and Cu in steel, as well as on the atomic absorption of Ni in brass. Sputtering and transport efficiencies were found to greatly influence the ground-state population within the analysis volume, while the excited-state population depended mostly on transport efficiency and discharge voltage, a fact consistent with an electron impact excitation mechanism. The decrease in sensitivity noted at higher Ar flow rates was attributed to a decreased residence time of sputtered atoms in the excitation zone. Current intensity-voltage curves were recorded for a new type of atmospheric pressure GD lamp working in the 50-250 l.tA range at approximately 1 kV and using He as the carrier gas (Sofer et al., 1990). The source was found to be particularly suited to the investigation of organic compounds since ionization occurred through proton transfer from water clusters. A jet-assisted GD lamp was optimized for use in direct solid analysis by AAS (Park et al., 1992). A new and more versatile design of the MW-boosted GD source was proposed which is based on a slab-line MW cavity (Outred et al., 1994). The planar magnetron GD source was further developed and cathode current densities of more than 100 mA cm -2 for Ar pressures of 0.1-350 pa and applied voltage lower than 500 V were reported (Trivedi et al., 1991). The effects of working parameters were monitored through the emission intensity of spectral lines from pure Cu and a Zn-based alloy.

Types Intended for Mass Spectrometry Part of the considerations made in the preceding section on the analytical benefits of RF-GD systems in AES are also valid for applications in MS since a more energetic plasma implies a larger population of ions. The analytical figures of merit of the RF-GD source in MS, also with reference to depth profiling, have been recently summarized (Marcus et al., 1994). Two current versions of the lamp which incorporate much of the experience gained thus far in this field are the direct insertion probe designs for pin and flat sample types illustrated in Figure 7. The first solution allows for easy sample interchange with no vacuum breaking and keeps machining of the material under test to the minimum. The second variant can alternatively accommodate larger fiat samples. Crucial to the further acceptance of the RF made in GD-AES are the assessment of atomization rates as a function of sample thickness and oxygen content and a better understanding of the time vs. ablation depth relationship. A RF-GD-TOF mass spectrometer was developed by simple modification of the interface of a commercial ICP-TOF-MS apparatus (Myers et al., 1994). The interface allowed for rapid exchange of the two sources. The ion optics which focus ions toward the entrance of the TOF instrument are the same as those used in the original ICP-TOF-MS design. By means of pin-shaped brass samples of varied lengths, the sample-skimmer distance in the RF-GD-TOF-MS instruments was optimized at 4 mm, while discharge pressure and power provided the best results

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at 50-60 W and 40 pa (ca. 0.3 torr), respectively. The application of small negative potentials to the skimmer cone (extraction orifice) was found to improve signals only slightly. However, higher negative potentials reduced both signal levels and resolving power. The skimmer potential was found also to affect the final kinetic energy of the ions before their extraction into the TOF system. At 40 pa all ions extracted for mass analysis have approximately the same kinetic energy. Detection power was at the level of 1 I.tg g-I or less. Following this research line, the same team investigated the capabilities of the RF-planar-magnetron GD ion source for TOF-MS displayed in Figure 8 (Heintz et al., 1995). To allow for rapid changing of samples without venting the mass spectrometer, a sliding Teflon | seal was placed at the interface. The seal, in turn, hosts a Macor | ring and shields it from the plasma and supports the sample. The afforded detection power for conducting and insulating materials was in the order

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of magnitude of 0.1 and 10 lag g-1 for B and Mg in Macor | and Bi, Cr, Mn, Ni, and Pb in aluminum, respectively. It was pointed out that the source-spectrometer combination still needs improvements in the interface to extract the analytes ions closer to the sample surface, in the scattered-ion noise, and in the extraction repetition rate. The RF-GD was also coupled to a high-resolution double-focusing mass spectrometer (Saprykin et al., 1995a). This required problems of RF shielding, grounding, and coupling to the accelerating potential to be faced. To this end, a source was built capable of handling both pin and fiat samples (Figure 9). It operated stably at 13.56 MHz, 10-50 W, and 1O- 100 pa of Ar. In this configuration total ion currents of up to 10-9 A and a mass resolution of up to 8000 could be attained. Depth profiling was facilitated by a radial flow of support gas onto the sample surface. The optimization of extraction and focusing of ions from the RF-GD source into the mass spectrometer requires full knowledge of the ion kinetic energies. A study demonstrated the effects of RF modulation on ion kinetic energy values and distributions (Cable and Marcus, 1995). In particular, it was shown that the location of ion formation together with RF-only quadrupole transmission properties, and averaging degree of ions approaching the sampler can strongly influence the kinetic energy curves. Some evidence was also provided that ions possessing a higher m/z ratio are characterized as a rule by slightly higher ion kinetic energies; this fact further supports the view that ions are transported electrostatically and not in a bulk gas flow. The behavior of another variant of the planar magnetron GD to be used as a low-pressure sampling device was studied (Shi et al., 1995). Its exploitation as an ion source for MS of conducting solids was examined. A special sample holder allowed up to five sample plugs to be analyzed without source chamber vacuum being broken. Sample ion current and sample axial position relative to the MS sampling cone were closely correlated.

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Figure 9. RF glow discharge source for coupling to high resolution double focusing MS. (A) gas input; (B) tube; (C) back cover; (D) pressing bar; (E) RF probe; (F) insulator spacer; (G) flat sample (cathode); (H) diaphragm; (I) cell body (anode); (I) extraction orifice; (K) ion beam; (L) accelerating plate (20 kV). Reproduced by permission of Springer-Verlag KG, with acknowledgment to Saprykin et al., 1995. A new dc GD unit to be used in combination with a magnetic sector double-focusing ICP-MS instrument was designed especially for the analysis of high-purity Si wafers (Milton et al., 1992). The basic scheme of this cell is given in Figure 10. The Si § ion yield was observed to increase with gas pressure, probably in consequence of the enhancement in Ar metastable population. The opposite is true for the Si~ dimer ion, perhaps reflecting an increase in the number of dissociative collisions. Detection limits for elements such as A1, As, B, C1, Fe, P, and U fell in the interval 6 x 101~ - 6 x 1013 atoms cm -3. The expanding horizon of elemental speciation studies prompted the development of a RF-GD-MS system to serve as the detector of gas-chromatographic (GC) separation (Olson et al., 1996). The setup, depicted schematically in Figure 11, featured a temperature-controlled transfer line consisting of a stainless steel tube which was connected to a GC oven with the interior of the GD source. The system was tested in the case of tetraethyl-Pb, tetraethyl-Sn, and tetrabutyl-Sn and provided useful structural information for the identification of these compounds through the observation of fragment peaks. A modified version of the conventional Grimm GD lamp was proposed by Shao and Horlick (1991). The source incorporates a floating restrictor and is designed so as to replace an ICP-AES torch in a commercial ICP-MS apparatus. To this end, the anode is slightly positive with regard to the earthed skimmer-interface plate of the MS system. The simultaneous analysis of an unknown sample and a reference material was carried out by means of a system based on two pulsed GD sources housed within the same tube (Klingler and Harrison, 1991). Optimization of the

Glow Discharge Atomic Spectrometry A

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C Figure 10. Discharge unit for the analysis of high purity Si wafers. (A) spring; (B) spring loading probe; (C) anode/cathode insulator; (D) anode plate; (E) flat cell; (F) ion exist slit; (G)liquid nitrogen coolant; (H) insulator; (I) flat sample. Reproduced by permission Of Springer-Verlag KG, with acknowledgment to Milton et al., 1992.

relative position of the two cathodes was achieved by evaluating the signals produced in GD-MS when using the same specimen for them both [National Bureau of Standards-Standard Reference Material (NBS-SRMs) 1262]. The RSDs under these conditions were in the range of 2.2-5.2%; this interval broadened to 0.759.1% when two different materials were used (NBS-SRMs 1262 and 1263). A pin-type direct insertion probe was developed along with a RF-GD source intended for use with commercial quadrupole GD-MS instruments (Shick et al., 1993). Performance of this system was shown to be equivalent to that of dc-powered sources. The discharge cell design in terms of ion sampling distance and sample dimension as well as plasma parameters as regards RF power, power density, and flow rate and pressure of the discharge gas were later optimized. The RF noise levels, cryocooling efficiency, and the use of Ta in the source were also thought to be crucial for further improvements (Shick et al., 1994). A RF-GD cell with a confined geometry was designed to be immediately compatible with commercial types of spark source (SS)-MS instrument (Saprykin et al., 1995b). Radial gas flow to prevent redeposition, increased sputtering rate, and enhanced intensity of the analyte signals were the major merits of this unit and could be best exploited by coupling with high-resolution spectrometers. The configuration of a GD-MS instrument was modified so as to allow for its use in a glove box when nuclear material is analyzed (Betti et al., 1994). Large use of plug-in components and filters on all supplies could practically solve all containment problems. A new GD-TOF-MS system was devised in which the source could work in the microsecond-pulsed mode (Wei et al., 1994). The circular profile of the ion beam was shaped as an elliptical profile by a dc quadrupole, resulting in a more compacted spatial distri-

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SERGIO CAROLI, ORESTE SENOFONTE, and GIANLUCA MODESTI

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Figure 11. Schematic diagram of the GC-RF-GD interface for speciation studies. (A) front plate sampling cone; (B) skimmer; (C) expansion stage vacuum pump; (D) heated helium flow; (E) 1/16" T-connection; (F) heated transfer line; (G) GC; (H) RF matching network and RF generator; (I) vacuum fitting; (J) GD probe; (K) six-way cube. Reproduced by permission of the Royal Society of Chemistry, with acknowledgment to Olson et al., 1996.

bution of the ions. The kinetic energy distribution effect was lowered by resorting to a two-stage acceleration field. Ion transmission improved sensibly over that of the dc mode. In spite of these advantages, the overall approach was seriously affected by a huge Ar ion packet arriving onto the detectors. B. Hollow Cathode Discharge Sources

Types Intended for Atomic Spectrometry Much of the advances made by GD-AES are mirrored, not surprisingly, by the resurgence of the twin technique of HCD-AES. The still partly unexplored possibilities offered by the HCD mode of excitation stimulated in recent years numerous investigations which significantly contributes to the solution of real-life problems. A study to reduce self-absorption effects in the HCD spectral emission and thus ameliorate the analytical response of HCD lamps was reported (Phillips et al., 1988). To address this aspect, an HCD tube was built in which a boosted positive column forms in front of the cathode cavity to trap therein the sputtered atoms (Figure 12). Excitation was also augmented at the center of the cavity as the electron flow was pipelined by the positive column. Further advantages ensue from the enlarged volume of the cell, the central region of which is characterized by a

197

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cathodic material atom-electron density ratio that is easily adjustable. The utilization of this versatile source for AAS and AFS applications was highlighted. Long ago it was amply demonstrated that when the HCD cavity diameter is less than 2 ram, the emission signal increases dramatically. To elucidate such aspects, a systematic comparison among two-piece HCD lamps (different in size and shapes) intended for the discharge of microsamples was investigated (Williams et al., 1995). They studied the behavior of these HCD lamps using both dc and the pulsed discharge conditions. Details of this source are shown in Figures 13 and 14. The temporal emission profiles were investigated as a function of sputtering effects on the cathode geometry by means of optical and scanning electron microscopy (SEM). It was also emphasized that the two-piece construction permits the change in shape with aging to be minimized, this being particularly true for spherical cathode cavities. Another microcavity HCD unit was proposed with the basic design shown in Figure 15 (Morgan et al., 1994). This assembly was tested in the case of Cu and Pb

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after drying within the cavity aqueous solutions containing these analytes. The two metals could be thus assayed down to 10 pg and 10 fg, respectively. Calibration curves were fully reproducible from day to day and from cathode to cathode of equal characteristics. The major shortcomings of this approach were the poor power supply stability, the limited access to emission lines for multielemental analysis due to narrow window on the photodiode array, and the resulting rather large uncertainty of measurements (from 7 to 18%). Enhancing the emission output of microcavities HCDs by boosting the discharge with a magnetic field was achieved (Raghani et al., 1996a). The design of the miniature cylindrical magnetron sources is shown in Figure 16. In this configuration the magnetic field is parallel to the cathode axis. In order to prevent the attack of the discharge on the magnets, these and the microcavity cathode were covered with a heat-shrunk Teflon | sleeve. In the case of cathodes machined from an A1 alloy containing 4.54% Mg, the emission intensity of this latter metal was found to increase up to a factor of 3 depending on the support gas pressure. This behavior was ascribed to the fact that the ambipolar diffusion coefficient in the magnetic field decreases with decreasing pressure of the filler gas, thus efficiently trapping the electrons inside the microcavity and enhancing their collisions with sputtered atoms.

Glow Discharge Atomic Spectrometry 1.S

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Figure 14. Various models of two-piece hollow cathodes. (a) cylindrical cathode cavity with a fiat bottom; (b) cylindrical cathode cavity with a conical bottom simulating the shape of the hollow cathode bottom of the one-piece cylindrical hollow cathode made by simply drilling; (c) spherical hollow cathode with dimensions similar to those approached by cathodes after being aged for hundreds of hours; (d) spherical cathode cavity of arbitrary dimensions. The cathodes in a, b, and d were made of 304 stainless steel; the cathode in c was made of graphite. Reproduced by permission of the Society for Applied Spectroscopy, with acknowledgment to Williams et al., 1995.

As already touched upon, the direct analysis of liquids with LPDs still suffer from many disadvantages. To tackle once more this problem, a solution sample introduction interface of wide applicability was proposed (Schroeder and Horlick, 1994). The HCD cell used to this purpose is schematically given in Figure 17, while the complete system for sample presentation is illustrated in Figure 18. In this configuration the flow of carder gas (He) was matched to the uptake rate of the pumping system. Noteworthy is the fact that the carrier gas with the nebulized solution is kept at atmospheric pressure to avoid sample deposition until it reaches the plasma zone where the transition to the discharge pressure can take place. The generally adverse effects exerted by the presence of water vapor in the discharge were expediently minimized by adopting a high filler gas flow rate (1.0-3.7 L min -1)

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SERGIO CAROLI, ORESTESENOFONTE, and GIANLUCA MODESTI

Figure 15; Schematicdiagram ofthe microcavity HCD device. (A) 19/22 quartz inner tapered joint; (B) ceramic sleeve; (C) metallic cathode; (D) electrical connection. Reproduced by permission of the Society for Applied Spectroscopy, with acknowledgment to Morgan et al., 1994.

and discharge current values (150-300 mA). This remains, however, a crucial aspect and more efficient removal of the aqueous vapor from the sample aerosol should still be achieved. The real analytical capabilities of this HCD source were ascertained by assaying solutions containing A1, Ca, Cd, Ce, K, Li, Mg, Mn, Na, Rb, and Zn, with detection limits in the range of 0.03 (Li) to 200 (Zn) ng m1-1. Another way to approach the problem of the analysis of liquid samples by means of the HCD source was reported by You et al., 1994. A particle beam liquid chromatography (LC)-MS device was interfaced to a heated HCD unit, as represented in Figure 19. The high efficiency of the thermal-concentric nebulization system ensures that analyte particles from the aqueous solution can be transported into the heated HCD cell for subsequent vaporization and excitation. The discharge characteristics were investigated in the case of Ar and He as the support gas, the latter resulting preferable for the achievement of simplified emission spectra and the excitation of high-energy lines. To explore in detail the behavior of this assembly, the determination of Cs and Na (as nitrates) was conducted as a function of discharge pressure, current intensity, solvent flow rate, and tip and block

Figure 16. Schematic diagram of the microcavity cylindrical magnetron cathode. I. Side view. (A) embedded magnets; (B) microcavity hollow cathode. II. Cross section. Reproduced by permission of the Society for Applied Spectroscopy, with acknowledgment to Raghani et al., 1996.

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T Figure 17. Sectional view of the HCD source for continuous solution sample introduction. (A) carrier gas and sample aerosol; (B) 2 mm o.d. glass tube; (C) tube holder (brass); (D) Teflon | insulator; (E) anode (brass); (F) Macor | insulators on both sides of hollow cathode; (G) hollow cathode; (H) cathode housing (stainless steel); (I) water cooling jacket; (J) 3/4 o.d. glass tube; (K) quartz window; (L) to vacuum pump and pressure. For the sake of clarity, parts A and B are retracted from the holder C. Reproduced by permission of Elsevier Science B.V., with acknowledgment to Schroeder and Horlick, 1994. temperature. Optimal results were obtained at low flow rates (even down to 0.2 mL min -1) and temperatures of 200-220 ~ The background equivalent concentrations were 0.74 and 0.53 mg L -1, respectively. Possible uses of this setup in combination with flow injection or LC instrumentation appear rather promising, in particular for the quantification of chemical species after separation. Also the determination of Cs in nuclear wastewater solutions was highlighted as a real-life application, with a detection limit in the vicinity of 8 lxg L -1. Further experimental findings from the same team gave a better insight in the mechanism of analyte particle transport in

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Figure 18. Schematic view of the liquid sample introduction system into the HCD source by Schroeder and Horlick. (A) to vacuum pump and pressure gauge; (B) hollow cathode; (C) heated desolvation chamber; (D) water cooled condenser; (E) peristaltic pump; (F) solution sample; (G) helium; (H) to drain and gas bubbler; (I) to photodiode array spectrometer; (l) imaging lens. Reproduced by permission of Elsevier Science B.V., with acknowledgment to Schroeder and Horlick, 1994. the particle beam-HCD combination (You et al., 1997). In all cases, the transport of analytes was enhanced by the addition of HC1 to the solutions in consequence of the enhancement in analyte particle size favored by this reagent, as revealed by SEM. Typically, the particle size was in the range of 2-8 ~m with transport efficiencies of 4-18%, as ascertained for Cu, Fe, and Na test solutions. As regards advancement of layer-by-layer analysis through HCD devices, an excellent survey was prepared by Djulgerova and Mihailov (1993b). A general scheme of the configuration adopted for this quickly expanding area of investigation for HCD-AES is given in Figure 20. Three different approaches were discussed, namely spectral line intensity, laser optogalvanic, and photoelectric optogalvanic layer-by-layer analysis of surfaces of metals, semiconductors, and dielectric materials. All of them can substantially contribute to the obtainment of much needed additional information on the mutual diffusion of atoms of two neighboring layers, the two-way diffusion between substrate and layer, technological peculiarities, physical processes in the plasma, and cathode sputtering rates.

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Figure 19. Complete Thermabeam nebulizer and particle beam interface coupled to the heated HCD atomic emission source. (A) momentum separator; (B) aerosol; (C) analyte/solvent flow; (D) thermal-concentric nebulizer; (E) expansion chamber; (F) stage 1 pump; (G) stage 2 pump; (H) particle beam; (I) discharge gas; (J) thermoblock assembly; (K) 5-way cross-source housing; (L) hollow cathode discharge; (M) to monochromator. Reproduced by permission of the American Chemical Society, with acknowledgment to You et al., 1994.

Not to be overlooked are a variety of other developments. The potential of an HCD-combined spectroscopic atomizer and radiation source for qualitative and quantitative elemental microanalysis was illustrated by Papp and Teljes, 1989. A current-controlled switch was found to enhance the analytical precision achievable with an HCD (Ho/ynska and Lankosz, 1991). A high-current Its-pulsed HCD lamp was devised which was characterized by an increase in the intensity of atomic and--even more--of ionic emission lines with respect to that of the dc-operated unit, this being a definitive advantage for applications in ionic fluorescence measurements (Gong et al., 1995). A low-pressure HCD source for element-selective GC detection was devised by Klemp et al., 1992. The advantages of a boosted HCD source for AAS were discussed (Wilson, 1992).

Types Intended for Mass Spectrometry For the time being, the combination of the HCD source with MS appears to be dormant. After some encouraging preliminary studies with the so-called HCD plume system for solids MS, in fact, no further significant advances were published (Marcus et al., 1986).

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SERGIO CAROLI, ORESTESENOFONTE, and GIANLUCA MODESTI A

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Figure 20. Schematic diagram ofthe experimental setup by Djulgerova and Mihailov. (A) power supply; (B)laser; (C)lens; (D) hollow cathode tube; (E) resistance; (F) preamplifier; (G) selective nanovoltmeter or oscilloscope; (H) condenser; (I) spectrograph. Reproduced by permission of ISSP, with acknowledgment to Djulgerova and Mihailov, 1993.

What has been detailed above did not take into account the wealth of minor modifications and adaptations to more consolidated HCD versions which also characterized the past decade. As these last were more directly oriented to the solution of specific analytical problems, wherever necessary reference to them will be made in the following sections. IV.

RECENT A P P L I C A T I O N S

A. Analysis of Metals and Alloys The most traditional and consolidated field of analytical exploitation for LPDs is still receiving attention from the spectrochemist, although not at the pace it was used to in the pioneering years of such sources. In this, as well as in the other sections centered on the major sample categories, an overview of the most interesting and promising applications of today is provided, covering as a rule what has been published in the 1990s. Whenever limits of detection (LoDs) are mentioned, it should be noted that these cannot be easily compared from study to study but for their order of magnitude because different criteria were used to calculate them.

Analysis of Metals and Alloys by Planar Glow Discharge Sources A better understanding of the basic phenomena in the GD is still a driving force behind improved and novel analytical applications. This fertile field of research is reflected by the abundant number of surveys issued on this subject matter in a few years. In this context, the detection power of GD-AES and HCD-AES was critically scrutinized (Boumans, 1994). Analytical applications of GD spectrometry were reviewed (Van Straaten and Gijbels, 1993). The perspectives of GD spectrochemical analysis were outlined (Winefordner et al., 1996). Further reviews on the exploitation of GD sources for atomic spectrometry were published (Marcus, 1994a; Pereiro et al., 1994; Broekaert, 1995; Heyner et al., 1995; Leis, 1995;

Glow Discharge Atomic Spectrometry

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Marshall and Valensi, 1995; Bengtson, 1996). A survey of the applicability of magnetic field-boosted GD sources was also presented (Raghani, 1996b). A review was published on the properties of very-high-frequency GD sources (Keppner et al., 1995). An updated review of the characteristics of the RF-GD source and its applicability to nonconductors and insulators was set up (Marcus, 1996). A survey of present capabilities and future prospects of RF-GD-AES and RF-GD-MS for elemental analysis was worked out (Marcus, 1994b). The versatility of applications of GD-AES and GD-MS were vividly illustrated along with it drawbacks (Harrison, 1992). An updated review on GD-MS with particular emphasis on the MS aspects was also published (King and Harrison, 1993). In general terms, the analytical potential of GD-AES and GD-MS is well documented by a significant amount of published material. The relative atomic mass and efficiency of ion extraction from a dc GD source were found to be fairly correlated (Ronan et al., 1989). This behavior was ascribed to gas flow transport dominating over the extraction of charged material by the electric field. The ratio of matrix ion current to contamination ion current could be exploited to quantify impurities down to the ng g-1 range. The properties of the GD source as an atom source for AFS were exploited (Kim et al., 1994; Walden et al., 1994). Active pressure regulation systems were introduced in the operation of GD sources to adjust current intensity and voltage during the discharge and thus greatly improve stability, typically with detection power and precision of 1-10 ~g g-1 and of 0.1-1%, respectively (Bengston, 1996). A substantial improvement (6 to 9 times) was obtained in the detection power achievable by GD-AES when a pulsed supply of square-wave form at 400 and 700 V was used along with a lock-in amplifier (Ulgen et al., 1993). The intensities of Ar at the low voltage were factorized and subtracted from the background signal at the high voltage. The direct analysis of solids was found to largely benefit from the so-called gas-jet GD-AES approach (Kim et al., 1992, 1996; Lundholm and Baltzer, 1992). New avenues are opened to GD spectrometry by boosting the discharge with a MW field (Leis and Steers, 1996). The possibility of using an ion trap as a mass analyzer for ions produced in the Ar plasma gas of a GD source was found particularly promising (McLuckey et al., 1992). In no case, however, LoDs better than a few tens of l.tg g-1 could be expected. Samples even smaller than 5 mm in length were analyzed by resorting to GD-MS with a direct insertion probe which allowed for specimen changing without breaking the vacuum (Duckworth and Marcus, 1992). Careful alignment of the discharge was crucial to minimize spatial variations in the populations of the various ionic species. Cell cryocooling by liquid nitrogen allowed high-temperature superconductors such as YBa2Cu30 x to be investigated by pin GD-MS with no matrix effects (Tigwell et al., 1992). A FT ion cyclotron resonance MS instrument was coupled to a GD unit to achieve ultrahigh mass resolution GD-MS (Watson et al., 1993). This allowed heavy isotopes to be directly analyzed without any pretreatment of the samples.

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Jakubowski and Stuewer (1992) described the application of GD-MS in the low-resolution mode to the in-depth analysis of technical surface layers. A multilayer system of Ni and Cr on a low resistivity n-type Si wafer was studied and the influence of discharge voltage on crater profile was elucidated. Depth resolution was about 10 nm. A mixed layer of Ni and Pd on a Cu substrate was also characterized. The technique showed promise for the analysis of coatings with a thickness in the ~m region. Marcus et al., 1994 summarized recent experimental work with GD-AES and GD-MS and concluded that the former is characterized by a detection power of 10-100 ng g-1 for most metals, while the latter can span the range 1-500 ng g-1. An overview of applications of GD-MS to high-purity metals and semiconductors was set up (Mykytiuk et al., 1990). Evidence was presented that this technique can now effectively replace SS-MS affording at the same time better detection limits (often at the ng g-1 level) and greater independence from the matrix composition. The applicability of GD-TOF-MS to the elemental analysis of solids was reviewed (Harrison and Hang, 1996). These authors described a newly developed combination of a lxs-pulsed GD source with a TOF-MS apparatus. This allowed for maximum efficiency of sample utilization especially for small-volume specimens, thin films, and nonconducting materials. A substantial improvement by several orders of magnitude of the analyte-to-interfering Ar gas intensity ratio was obtained by Oksenoid et al. (1996) in the case of GD-MS. This enhanced response was obtained by optimizing lamp geometry and working voltages and gas flow. This allowed elements such as Br, K, and Rb to be determined down to 1 lag g-l, 1 Ixg g-1 and 10 ng g-l, respectively. A 1.5-15-fold enhancement of ion intensity signals was achieved in GD-MS when 0.1-0.5% H Ewas added to Ar, probably in consequence of the formation of a higher number of Ar metastable atoms (Luo, 1990). If the H 2 content exceeded 1%, the signals of Nb, S, Ti, and Zr dropped. This behavior was ascribed to the reaction of these elements with H 2. LoDs in the sub-ng g-1 in solid samples for many elements were claimed to be easily achievable. Routine bulk analysis of trace components by GD-MS in the framework of a comparative study among various laboratories using the same equipment revealed differences of 1 order of magnitude or more (Wilhartitz et al., 1990). This disappointing pattern was ascribed to heterogeneous distribution of trace analytes at the low lxg g-1 level in the sample. Six different alloys and metals were analyzed for 56 elements by high-resolution GD-MS (Vieth and Huneke, 1991). A good agreement was found between the RSFs and those calculated on the basis of a semiempirical model accounting for diffusion, ionization-recombination, and ion-extraction phenomena. This investigation further corroborated the independence of GD determinations from matrix composition. More specifically, a number of successful analytical applications to the determination of trace components in metals and alloys were described. The sputtering pattern of Ag, A1, As, Cu, and Zn with a 1:1 discharge gas mixture of Ar and CF 4 was reported (McCaig et al., 1992). Independent of the gas pressure, quenching of

Glow Discharge Atomic Spectrometry

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numerous spectral characteristics was observed. The determination of isotope ratios for Ag, B, Cu, Pb, Re, Sb, and Sr in solid samples could be achieved with precision ofbetween 0.1 and 1% for element concentrations in the range of 10 to 5000 lxg g-1 (Riciputi et al., 1995). Pellets were accomplished using Ag or Cu as the binder. The major problems arose from the strong dependence on gas pressure, samples geometry, and the need for frequent recalibration. Nondestructive methods are of major importance in the goldsmith sector. Harville and Marcus (1995) have developed a method to analyze several trace elements (Ag, Bi, Cr, Cu, Fe, Ir, Ni, Pb, Pd, Pt, Rh, Sn, and Zn) in Ag and Au alloys. The analyte signal intensity was ratioed to the spectral background in the close vicinity of the spectral line selected. These studies show rapid plasma stabilization time and satisfactory precision of the signal together with LoDs in the range of ng g-1. Trace impurities of Ag, A1, Au, Cd, Cu, Fe, Ir, Ni, Pb, Pt, Rh, Si, and Zn in Pd metal powders were quantified by GD-MS (Wayne et al., 1996). Unlike analyses in solid metal specimens, problems are encountered because of inhomogeneities at the mg level that can seriously impair measurements. LoDs range from 0.02 t.tg g-1 for Pb to 20 ~g g-1 for A1. Comparable results were obtained by ICP-AES (for Fe and Si) and quadrupole ICP-MS (all other elements but C). The distinct variations shown by certain elements (Ag, Au, Cu, Ir, Pb, Pt, and Rh) appeared to be related to the provenance of the high-purity host matrix. A number of elements (A1, Mn, Ni, and Si) were quantified in a Cu-based alloy by means of ~s-pulsed GD-AES (Gong et al., 1996). Results were affected by RSD of 1 to 6% and were in good agreement with the expected values. Further exploration of the inherent merits of MW-boosted GD-AES was reported (Leis et al., 1991). Sputtering rates and crater shapes were elucidated in the case of A1, Cu, and Pb. Spectral lines were compared with those characterizing the conventional GD source. Background equivalent concentrations were found to be in the range of 1-25, 0.4-110, and 0.4-11 lxg g-1 for A1, Cu, and Pb, respectively. The increase in emission intensity for many spectral lines and in the linear dynamic range, the improvement in detection power, and the decrease in self-absorption were the major advantages brought about by this analytical approach. Thin films of A1-Zn on steel were investigated by RF-GD-AES (Bordel-Garcia et al., 1995). An A1 alloy as well as Au, Cu, and brass were analyzed by a planar magnetron de GD system (Dehghan et al., 1994; Shi, 1994). Sputtering increased with discharge current and element atomic number, while an inverse relation was found with carrier gas pressure. A number of hillocks were detected by microscopy, the location of which, however, had nothing to do with inclusions. The optimum frequency for the quantification of trace elements in an A1 matrix by means of RF-GD-AES was found to be 20 MHz, with LoDs in the sub-txg g-1 range (Heintz and Hieftje, 1995). In the analysis of A1 and Cu alloys by RF-GD-AES, a thorough evaluation of the signal-to-background ratio was undertaken prior to line selection and assessment of accuracy, precision, and detection power (Harville and Marcus, 1993). High purity (>99.999%) and alloyed A1 [Pechiney and National Institutes of Standards

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(NIST) standards] was tested regularly in a semiconductor factory by means of GD-MS (Vassamillet, 1989). A presputtering time of 30 min at a voltage higher than 1000 V and a current density of 5.5 mA were adopted. For the actual analysis, the discharge conditions were set at 1000 V and 2 mA. In each standard, 30 elements were determined and as many as 71 elements were cumulatively assayed in different runs. Operative aspects regarding the full implementation of a quality control scheme were particularly accounted for. Other authors had different views on this issue. The investigation of background spectral interferences in GD-MS of A1 alloys showed little or no influence from gaseous, aqueous, and solution species, although the formation of metal argides and multiply charged Ar species could not be disregarded in the evaluation of mass spectra (Feng and Horlick, 1994). Possible compensation for effects in the analysis of A1 alloys would ensue from the similar patterns possessed by analyte ions and MAr § On the other hand, with the RF-GDMS approach, the LoDs achieved for some trace elements in an A1 matrix were worse than those afforded by commercially available double sector MS instruments, i.e. (in l.tg g-l) 0.61 for Mn, 0.53 for Ni, 0.19 for Pb, 0.17 for Cr, and 0.15 for Bi (Heintz et al., 1995). LoDs in the ng g-1 and sub-ng g-1 with precisions between 7 and 30% were obtained in the analysis by GD-MS of A1- and Co-based alloys (Venzago and Weigart, 1994). The importance of rigorously clean working conditions was stressed. The use of RF-GD-AES for the analysis of high-purity Au and Pt was reported by Marcus et al. (1994c). Saito et al. (1995) used GD-MS to determine B in high-purity Mo down to 0.1 ng g-1. The use of RSFs derived from the analysis of steel was thought to be at the root of the relatively unsatisfactory results. The curcumin method, SS-MS and GD-MS were used. High-purity Mo, shaped as a rod, was assayed for trace impurities by GD-AES (Mizota et al., 1992). Metals could be quantified after 30 min sputtering. Spectral overlap from the Mo matrix made difficult the assay of Ti. Carbon, C1, and O down to the ng g-1 levels could be determined after 2 h sputtering with a RSD of 3% or less. Transient and stable gas contaminants accumulated in a F-laser gas mixture were detected and monitored on-line by resorting to high-pressure He and He-F GD-AES (Treshchalov et al., 1996). The mechanism of formation of radicals (C 2, CF 2, CH, CN, OH, and SiF) of excited atoms (C, H, N, O, Si) and of molecules (CO, CO 2, N2) was discussed. The GD-MS quantification of C, N, and O in high-purity steels was achieved at amounts of 20 btg g-1 and below (Tanaka et al., 1991). The approach consisted of cooling the GD cell to reduce the background signal, in evacuating the ion chamber for 20 min to allow for the determination of N, in presputtering the samples to remove surface contaminants that would hamper the determination of C, and in purifying Ar with a ZrO catalyst to improve the quantification of O. A study by Tidblad and Lindbergh (1991) provided good evidence of the versatility of LPDs for a variety of applications. According to a common practice, small amounts of chromate are added to the electrolytic bath in the chlorate process

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in order to form a thin Cr film on the electrode and thus inhibit the undesirable reduction of the intermediate hypochlorite ions. GD-AES and electron spectroscopy chemical analysis (ESCA) was used to clarify the structure of the Cr film. Both techniques agreed in indicating that its composition during the growth is given by the formula Cr(OH)3 • H20, with a variable maximum thickness depending on the cathodic substrate (5 nm on Pt and 8 nm on Au). Singly ionized Cu emission lines produced by a GD source in an Ar-He plasma were studied in the visible region (Wagatsuma and Hirokawa, 1993). A sample loss rate of 0.23 mg min -1 was measured for Cu in a jet-type GD ion source (Woo et al., 1992). Relative ion yields were 0.57 for Fe and 3.5 for Cr. Frictional brass-coated steels were investigated by GD-AES and quantification of continuously varying concentrations of Cu, Fe, and Zn was performed by using in an iterative way equations based on the linear combination of the RSFs or sputter rates for both Fe and brass (Behn et al., 1994). The already described magnetron GD plasma was used to test pure Cu and Mg as well as A1, Zn-base, and Mn alloys (Shi et al., 1995). Although in the preliminary stage, this approach showed much promise for highprecision analysis of alloys. Copper-base alloys were analyzed by means of a novel quadrupole-type GD-MS system with optimized discharge current, orifice-to-cathode distance, energy analyzer setting, and bias voltage (Lee et al., 1989). Concentrations of trace elements down to the level of ~g g-1 were detected with a repeatability of about 2-5%. The determination of impurities in Cu and In by GD-MS was the object of a round robin which pointed to an inter-laboratory RSD of 17% or less (Nakamura et al., 1991). The direct analysis of high-purity Cu bars and pins by means of a quadrupole GD-MS system could achieve LoDs in the sub-~g g-l, with accuracy and precision comparable to those typical of the more conventional arc and spark AES methods (Hutton and Raith, 1992). The emission intensities of Ar and F were measured in a RF-GD plasma using Ar mixed with CF 4 as the discharge gas (Lee et al., 1993). The determination of Fe in brass in the subt.tg g-I range with a total uncertainty of 15% or less was performed by means of laser-induced fluorescence of the atom cloud generated by a cathodic sputtering cell (Travis et al., 1991). The major problem in the analysis could be ascribed to the noise associated to the laser-induced background. The surface of an A6063 A1 alloy was cleaned by sputtering in a GD device using H 2 as the discharge gas (Hsiung et al., 1991). Contaminants containing C, C1, F, and Na could be removed. An artificial neural network and multivariate calibration of spectra emitted by the GD source allowed for the bulk analysis of quite a number of Fe and Ni alloys and to distinguish the various alloy classes (Glick and Hieftje, 1991). Weiss and Cizek (1990) resorted to GD-AES to analyze a Fe-based alloy. Exogenous substances were removed from the surface of steel samples just before depth profiling by stopping the gas flow and sparking a preliminary discharge (Mega et al., 1990). The analysis of powdered samples of steel was performed by preparing pellets with Cu powder at a percentage in weight of 80 to 99% (Luft, 1990). The disks (10-mm across and 0.3-mm thick) were further embedded in Cu powder and pressed again.

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This approach was considered promising for studying precipitation phenomena in steel. It allowed trace analytes in amounts of steel as low as 1 mg to be easily quantified. Steel samples were analyzed by GD-AES in combination with a photodiode array spectrometer with detection power for a number of analytes worse as a rule by an order of magnitude than that obtainable with multichannel photomultiplier systems (Brushwyler and Hieftje, 1991). Heat-treated (annealed, hardened, and tempered) steels were analyzed by GD-AES and spark AES. The former technique resulting in a wider applicability provided that sputter rate correction was applied (Dem6ny, 1992). Heating between 400 and 500 ~ reduced the influence of the metallographic structure on the intensity of the C line at 155.7 nm. The precision afforded by the two approaches was similar. Various aspects of the analysis of steel surfaces were also examined by several authors (Liu et al., 1992; Angeli et al., 1993; Koshu, 1993; Miyawaki et al., 1994). Trace elements in high-speed tool steel were quantified by GD-AES (Weiss, 1994). The suitability of depth profile analysis of thermochemical treated surfaces by the GD approach was demonstrated by Boehm (1994a). The same author also characterized coated steel sheets (Boehm, 1994b). Steel was analyzed by GD-AES to quantify its content in P and S (Matsumoto, 1995). The best emission-to-background ratios were obtained with anode inner diameter of 4 mm; this parameter is also convenient in terms of accuracy. The performance of GD-AES in the analysis of graphitized cast irons and low- and high-alloyed steels was compared to that of spark AES (Sommer and Flock, 1996; Weiss, 1996). Relative ion yields in the analysis of steels were obtained by normalizing on the basis of Fe (Cross and Augenstine, 1991). Ranges of 0.1 to 5 and 0.2 to 2.5 were obtained in Ar and He, respectively. A novel type of RF-GD source was employed to analyze low-alloyed steels (Woo et al., 1994). A comparative study of spark-ablation ICP-MS and GD-MS in the analysis of steel was reported (Jakubowski et al., 1992). The RSF sensitivity for some trace elements and measurement precision were similar in both cases. Steel analysis by GD-MS greatly benefited from the addition of 1% H 2 to the Ar discharge gas, even up to a factor of 1.3 (Smithwick et al., 1993). No valid interpretation of the mechanism responsible for this improvement could be provided. In the depth profiling analysis of steel by GD-MS, initial outgassing caused serious interference problems independent of whether fast or slow erosion rates were adopted (Pichilingi et al., 1993). Thermal degassing under vacuum conditions in the ion source before igniting the discharge was recommended. Certified reference materials (CRMs) of Fe and steel of five different types for a total of 30 samples were investigated by GD-MS (Itoh et al., 1994). The study could ascertain the RSFs and thus improve the analysis of alloying elements in steel. In the case of the heat-resistant superalloy JSS CRM-680-3, the experimental concentrations were found to coincide with the certified values. Rare earth elements were assayed in metallic Gd, La, Nd, Pr, and Tb by GD-MS (Hirose et al., 1991). Surface contamination of the samples had to be removed by a 10 min predischarge. Careful assessment of polyatomic interferences was also necessary. Precision worsened to 40% at concentrations lower than 1 lag g-1. By

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optimizing the experimental conditions, Mn and Ni in Zn-based alloys were reliably quantified with a magnetron GD device (Brewer et al., 1991). The two metals could be detected at levels of 60 and 5.5 l,tg g-l, respectively. The determination of Mo, Nb, and Zr in steel by GD-MS was found to be affected by the formation of multiply-charged cluster ions (metal argides) (Takahashi and Shimamura, 1994). A correction procedure based on the assumption that the rate of formation of the singly charged argide is the same for all analyte and coincident with that of FeAr + was set up. The content of N in steel was ascertained by GD-AES (Lundholm and Baltzer, 1992). Nickel coatings on a steel substrate were subjected to sputtering in order to elucidate the dependence of the sputtering rate on the GD power (Bohmer and Nel, 1990). Metal alloys were assayed for their O content down to a few lxg g-l, although major drawbacks were caused by the lack of suitable CRMs and instrumental background (Huneke et al., 1989). Isotope ratios in solid samples could be ascertained for Os and U by GD-MS (Ecker and Pritzkow, 1994). To avoid interferences, Kr was selected as the discharge gas; results were however still heavily affected by multiply-charged ions of Ca, Kr, and Si. The presence of Kr § was also detected. Isotopic measurements of Pd charged electrolyticaUy with protium and deuterium were performed with precision better than 0.07% (Donohue and Petek, 1991). The occurrence of polyatomic species of Pt bound to protium and deuterium was found to significantly affect analysis so that the use of a high-resolution ICP-MS instrument became mandatory. Also cooling of the discharge cell was essential for elimination of hydride interferences from adsorbed H20. Trace elements were determined in Pt powder by GD-MS after ascertaining their RSFs (Van Straaten et al., 1994). The interference of PtH could not be disregarded in the case of Au and Ir, so that cryocooling became mandatory to remove it. Accuracy and precision were in the range of 10-15% and 5-10%, respectively, while the detection power turned out to be better by 1 to 2 orders of magnitude than that afforded by ICP-AES. High-purity Ti was analyzed by GD-MS to ascertain the content of Sc (Held et al., 1995). The interference of 5~176 hampered the detection power of the method. It was no better than 25 ng g-1. When compared to ICP-MS, this approach presented a higher throughput, but the need for solid calibrants made it less flexible. Titanium alloys were analyzed by GD-MS (Itoh et al., 1993). The determination of V in steels was achieved by GD-AES using a dual cathode tube to avoid background emission from the carrier gas (Wagatsuma and Hirokawa, 1991b). The interference of the Ar line at 438.0 nm on that of V at 437.9 nm was thus avoided. The useful range for V was 0.1-2.5% (m/m). A place of its own is occupied by the application of GD sources to depth profiling. In an extensive report, Bengtson and Saric (1991) evaluated the state-of-the-art of the applicability of GD-AES for surface and depth profile analysis of constructional steels (hot rolled, pickled, and cold rolled), stainless steels (hot rolled and pickled), and galvanized steel sheets (coatings of Zn and Zn-base alloys). In general, accuracy of measurements for major and alloying elements was better than +10%,

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whereas repeatability tended to improve with increasing analytical depth, although the spread of results was generally much wider than those for bulk materials. Quantitative evaluation of depth profiles was achieved by means of an ad hoc developed method. The analysis of Zircoloy 4 assembly cladding tubes was reported as a good example of how nonflat samples can be presented to GD sources (Ruste and Schwoehrer, 1996). Depth profiling is in this context also of primary importance. The potential of GD sources for depth profiling is well documented by several surveys (Baudoin, et al., 1993; Bengston, 1994; Boehm, 1994c). The potential of GD spectrometry for depth profiling was also reviewed by Bengtson (1994). In general, it was pointed out that GD-MS has better detection power and broader element coverage over GD-AES. On the other hand, it was concluded that quantitative bulk analysis with depth calibration through sputtering rate measurements is still in the developmental stage. Layer analysis by GD-AES was reviewed and the radial distributions of electron density and temperatures in a GD plasma were reported (Drobyshev, 1992). How the crater shape was influenced by the anode geometry and hence by the electric field distribution was investigated (Demrny et al., 1991; Demrny, 1992). An almost fiat crater was obtained by restricting the burn spot through a ceramic spacer in the anode tube and by optimizing gas pressure and discharge voltage. Depth profiling and the inherent difficulties of controlling the crater shape were reported (Quentmeier, 1994). When layers thinner than a few l.tm are to be examined, then low Ar pressure and constant applied voltage become crucial. The so-called approximation of matrix independent yields was at the basis of the approach to obtain quantitative data in GD-AES depth profiling (Weiss, 1995b). A multielement calibration algorithm was developed to this end. The information needed to run the system is substantially less than that necessary for other methods. From an analytical point of view, sample composition could be ascertained with an accuracy level comparable to that of a single matrix bulk-mode approach. Improved sampling efficiency was provided by directed support gas flows, although this led to reduced depth profiling resolution (Banks and Blades, 1992). The dependence of depth resolution on the crater formation process with both dc and RF-GD sources was also clarified in the case of a multilayer metal surface (Pr~sler et al., 1995). The two excitation modes achieved the best resolution in the same interval of power and gas pressure settings. Efforts were also devoted to developing a computer-aided system for the interpretation of ion sputtering depth profiling in GD-AES (Ostwald et al., 1994). Concentration values as a function of depth were thus directly obtained from the analytical signals. The model could be adapted to specific working conditions, primarily the sample nature. To better quantify GD depth profile measurement, a number of spectral lines from several analytes hosted in A1, Cu, Fe, and Ni matrices were selected to ascertain their emission yields (Weiss, 1993). Depth profiling of Co in an Fe silicide layer on Si was plagued by matrix effects (Heyner et al., 1995). Similarly, matrix effects were encountered in depth profiling minor elements in a Ti-A1 intermetallic phase (Branagh et al., 1995).

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Other important uses of the GD source are emerging. Among these, the dependence of the etching on power density attracts much interest. Moreover, a new dc GD plasma polymerization system was reported (Suwa et al., 1996). In the GD sputtering the relative ions yields of some elements were found to vary depending on their chemical form (e.g. deposited solution or component of a conductive solid) (Hess et al., 1994). Although no satisfactory explanation could be found, it was suggested that reactive etching could play a role along with collisional dissociation of the sputtered polyatomic species. The addition of H 2 to the Ar discharge gas was reported to increase the sputtering rate of C and Si in a GD source (Tsuji and Hirokawa, 1991). Also of interest is the formation of nitrides on the surface of steel (Rusnak and Vlcek, 1993). The authors investigated the excited states of H and N in the plasma and their effects on the above process.

Analysis of Metals and Alloys by Hollow Cathode Discharge Sources The advantages inherent in HCD-AES were critically reviewed, particularly as regards electrical and thermal properties and ionization processes (Maksimov et al., 1993). Analytical applications of HCD-AES were briefly reviewed (Szilv~syV~imos et al., 1991b). With regard to the applicability of HCD-AES to conducting solids, new possibilities are continuously unveiled, thus mirroring the progress made by GD-AES in the same field. Solids analysis by HCD-AES and the role of vaporization and excitation processes were further elucidated (BorkowskaBurnecka and Zyrnicki, 1993). The ablation process in a water-cooled HCD system was followed by radioactive tracers for elements of different volatility (Cd, Ce, Co, Cr, Cs) (Szilv~sy-V~imos et al., 1991c). The ablation rate and the emission intensity of the spectral lines were found to be independent of whether ablation-excitation took place separately for each element or simultaneously. Boosting the HCD source with a MW field resulted in enhanced emission from Ag, A1, C, Cu, Fe, and Ni with the additional advantage of a reduced loss of the cathode mass (Senofonte et al., 1991). Emission intensity from atoms of A1 and Cu as well as from ions of Mg was found to increase with the strength of a magnetic field (up to 2000-2500 G) applied perpendicularly to a miniature quartz discharge tube equipped with a microcavity HCD unit (Raghani et al., 1995). The samples examined were bulk A1 and Cu and an A1-Mg alloy. This enhancement was explained on the basis of an increase in electron temperature and in the radial diffusion of the electrons from the cathode axis. The addition of N 2 to the He plasma gas resulted in an improvement of the detection limits for Bi, Pb, and Sn when analyzed by HCD-AES (Dubinkina et al., 1991). Electrochemical separation followed by HCD-AES allowed Cu amounts as low as 10 ng g-1 to be quantified in high-purity U (Kolev, 1990). An HCD atomizer for laser-excited FAS was described (Dashin et al., 1991). Amounts of Pb down to 3 ng g-1 could be thus quantified in state reference materials of pure Cu.

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B. Analysisof Nonconductive Solid Materials

Analysis of Nonconductive Solid Materials by Planar Glow Discharge Sources Although grinding and mixing with a powered metal is often the method of choice to prepare conductive pellets, this approach can be the cause of serious contamination. An alternative to this approach is the so-called secondary cathode technique (also known as mask technique), as described by Milton et al., 1992. This method resorts to a thin conducting diaphragm with a hole of 3-12 mm in diameter, placed on the top of the sample. The principle of the secondary cathode is to sputter-deposit in situ a very thin conducting layer on the nonconductive samples. The sampling depth of the ion bombardment is large enough (0.5 nm) to atomize the conducting redeposited layer and the underlying nonconducting samples as well. It was found that the discharge pressure is a very critical parameter in obtaining the equilibrium between sputtering and redeposition of the conducting layer. In fact, working at a pressure that is too low results in an unstable discharge. This is because of insufficient coating of the exposed nonconducting sample, while a pressure that is too high causes an excess of redeposition and therefore a thick conducting coating, which makes it impossible to obtain a signal from the sample. This method was used to carry out a survey analysis of Si wafer for a number of transition metals and rare earth elements. The concentrations measured were as follows (in l.tg g-l): Fe < 1.5; Ni < 1.4; Co < 1.0; Gd < 0.7; Sm < 0.6; Yb < 0.5; Cr, Cu, Mn, and Ti < 0.4; Dy, Er, and V < 0.3; Eu < 0.2; Lu, Nd, and Pr < 0.1; Tb < 0.09; and Ho and Tu < 0.08. By placing a Ta secondary electrode directly on the surface of glass samples, a thin conducting layer was formed on these last which made their analysis possible by GD-AES (Yang et al., 1994). Quantitative figures within 30% of certified values were obtained in the analysis of a glass CRM. To be analyzed by means of LPDs, refractory samples generally have to be previously mixed with conducting matrices such as Cu or graphite powders. The analysis of sediment from the Mediterranean Sea was successfully carded out by GD-AES after grinding the samples and mixing with Cu powder to make them conductive (Caroli et al., 1993). The mixture was then pelletized. The choice of Cu as a binding material was particularly convenient due to the high efficiency of sputtering of this metal and because of the excellent electric and mechanical properties of the obtained disks. These were then encapsulated in a brass support plate to assure a good electric contact and adequate vacuum tightness. An innovative approach to make powders conductive was developed by Battagliarin et al. (1995). It was based on the heating of the powdered sample together with a high-purity In rod for 2 h at 190 ~ and atmospheric pressure. Liquid In was then forced to infiltrate the sample by raising pressure to 70 bar with N 2. By this technique, after infiltration, the sample could also be confined to a specific portion

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of the pin cathode, while problem-causing water could be easily removed. Although further research is necessary, the method appears to be very promising. A tandem system consisting of pulsed dye laser ablation and ionization in a GD source for MS was described by Barshick and Harrison (1989). The role played by support gas type (Ar, He, Ne) on redeposition of sputtered material was also clarified. Removal of interfering species in GD-MS was possible through the use of getters such as Ag, C, Ta, Ti, and W (Mei and Harrison, 1991). This approach, useful only under specific analytical conditions, was applied successfully in the case of rare earth determination to minimize the adverse consequences of the presence of O that would lead to an interference by oxide ions. The coupling of RF excitation is becoming one of the most useful and appreciated arrangements for the direct analysis of nonconducting samples. The RF-GD source allows glasses, electric insulators, ceramics, and geological sediments to be conveniently analyzed. In terms of analysis of refractory solids, the RF coupling and the matrix modification through grinding and mixing with a conductive material method are two different approaches, of which the former is capital-intensive, while the latter is labor-intensive. The placement of a high voltage on the surface of a nonconductor induces a capacitor-like response so that the surface acquires the applied potential only to be neutralized by charge compensation by ions or electrons in dependence of polarity. The analysis of metal oxides was accomplished by dc GD-MS adopting the conducting pellet approach (De Gendt et al., 1995a). The influence of the host matrix type (Ag, Cu), cooling, and cathode shape (pins, disks) was investigated in the case of some Fe ore CRMs. While binder and samples matrix had little effect on experimental RSFs, cooling and samples geometry played a significant role. Careful optimizing and controlling the working parameters allows accuracy and precision of the measurements to be easily kept at the level of 10% or less. The use of a secondary cathode for GD-MS of nonconducting samples such as solid glasses or sinterized iron ores was reported (Schelles et al., 1995). A methodological evaluation of the secondary cathode approach was made. It was reported that different sample types require ad hoc optimization of the measurement parameters, e.g. by changing the ratio between the sample signal intensity and the secondary cathode signal intensity. This ratio and the purity of the secondary cathode can directly influence the blank values and thus the detection power. Finally, an additional determining factor from the analytical point of view can be ascribed to the electrical resistivity of the samples. Iron meteorites were analyzed for major (Co, Fe, Ni) as well as minor and trace elements (up to 53) by using GD-MS (Shimamura et al., 1993). Isotopic composition was also ascertained in an attempt to obtain some information on the origin of this outer space material. The meteorite, Yamato 791694, presented an anomalous elemental composition with very high C and N concentrations and isotopic composition of Pb close to the primordial one. Detection limits in the order of magnitude of ng g-1 and below were attained.

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The effects of cryogenic cooling, power, pressure, and distance between sample and exit orifice were investigated to improve the performance of RF-GD-MS in the analysis of oxides (De Gendt et al., 1995b). The first parameter was crucial to remove gaseous interfering species. After careful optimization of the discharge conditions, a precision as low as 5% could be obtained. The RSFs were in the interval of 0.5-3 depending on matrix. Only semiquantitative analyses could be carried out. Teng et al. (1995) examined some factors affecting the quantitative determination of trace elements in soils, in particular red clay and forest soil. Satisfactory analytical results were gained by resorting to RSFs from soils CRMs. The influence of sample oxygen content and host binder identity for the preparation of pellets and soil type were examined. It was concluded that soil composition has with GD-MS less matrix effects than observed with ICP-MS or laser ablation solid-sampling ICP-MS. Betti et al. (1996) used GD-MS to analyze trace isotopes in soil, sediment, and vegetation by blending with a conductive host matrix, namely Ag, and using the secondary cathode technique to achieve stable discharge. In this way LoDs in the order of magnitude of pg g-1 were attained for the radioisotopes 137Cs,239pu,241pu, 9~ and 232Th using an integration time of 1 h and a mass resolution of 100. Both total elements and organometallic species were analyzed in soil by means of GD-MS and GC-MS (Barshick et al., 1996). Suspended particulate matter was analyzed by GD-MS after depositing it on high-purity In (Takahashi et al., 1994). Data obtained for 53 elements in a 10 mg sample, showed that for 34 of them the agreement with measurements done by other techniques was within a factor of 2. The detection power was in the sub-lxg g-1. Atmospheric particulate matter was subjected to analysis resorting to GD-MS (Schelles et al., 1995). Air was pumped through a single-orifice impactor stage in which the aerosol was collected on a metal support. This plate was then used as a cathode in the GD unit. The analysis of ceramics and metallic layers by GD plasma sources was reviewed, respectively, by Nickel et al. (1992a) and by Koch (1993). Decorative coatings were analyzed by means of GD-AES (Boehm, 1994d). Dielectric materials were investigated using RF-GD-based approaches (Parker and Marcus, 1995). Signals, detected either in the AES or in the AAS mode were found to stabilize after 20 s or less from plasma ignition, with RSDs of less than 5% over several minutes. Samples thickness played a major role in the atomization process in that thinner materials presented higher ablation rates. This increase in sputtering was also achieved at lower frequency. With a moderate power RF-GD source, it was possible to compare the shape of craters formed on nonconducting samples with that obtained by the conventional dc-operated source (Woo et al., 1994). No remarkable differences were observed but for the case of Pyrex. Sputtering rates were in the range of 30-100 Ixg min -1, i.e. 1 order of magnitude lower than for metals. The properties of the RF-GD plasma as both an emission and an ion source were investigated (Kawaguchi et al., 1991). Sintered ceramics were assayed for their contents in SiN,

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alumina, and zirconia by RF-GD-MS. The dependence of analytical signals on RF power, gas pressure, and sample thickness was described. The applicability of RF-GD-based techniques to glass, silicate paint layers, and massive Teflon | as well as to metal films was surveyed (Marcus et al., 1994d). Dielectric materials were assayed by means of RF-GD-AES and the electrical and optical features of the plasma were described (Lazik and Marcus, 1993b). The maturity of GD-MS as a technique capable of providing routinely complete chemical analyses at the ultratrace levels for insulating solid materials was clearly demonstrated by quantification of a full range of elements (from Li to U) in coal and coal fly ash (Luo and Huneke, 1991). Samples were mixed with a conducting binder (high-purity Ag) and pressed into pin shape by means of a polypropylene mold. Critical steps in the determination process were the inhomogeneous distribution of elements within and among the coal fly ash particles and purity of the conducting host binder. The presence of highly volatile compounds, on the other hand, hindered the applicability of the method to sub-bituminous coal. LoDs were in the ng g-l-lxg g-1 range depending on the element. Nonconducting materials were analyzed by GD-MS (Tong and Harrison, 1993). The ability of RF-GD sources to directly sputter or atomize nonconductive samples was further demonstrated in a series of papers based on the use of GD-MS for the analysis of such materials (Duckworth and Marcus, 1989, 1990; Winchester et al., 1993a). The RF-GD-MS analysis of glass and ceramic samples was also reported (Saprykin et al., 1996). By adjusting the ion transfer optics of the double-focusing mass spectrometer the ratio of analyte-to-background contaminant ion intensities could be optimized. Ceramic materials were analyzed by RF-GD-MS (Leis, 1992). Macor | ceramic samples were investigated by RF-GD-AES at a frequency of 6 or 13 MHz (Heintz and Hieftje, 1995). Under these conditions, the detection power for a number of elements ranged from 30 to 110 Ixg g-l, the major limitation to better performance being posed by the detector noise. In another study, dc GD-MS in combination with the secondary cathode technique was used for the analysis of Macor, a hardly dissolvable nonconducting glass ceramic used as an insulating material capable of standing high potential differences (Schelles and Van Grieken, 1996). This material contains as the major components (in decreasing order of concentration) O, Si, Mg, A1, and B. Although simple and low-cost, the method still suffers from the disadvantages of possible blank contribution and restricted discharge conditions. Pigmented polymer coatings on steel were analyzed by RF-GD-AES by Jones et al. (1994). The applicability of RF-GD-MS to analyze polymers such as Teflon | and Teflon | -based copolymers in order to assess the fingerprinting ability of this technique was investigated (Shick et al., 1996). One of the most remarkable advantages of this kind of analysis compared to secondary ion (SI)-MS spectrometry and X-ray photoelectron spectroscopy is that the assay is fast and does not require dissolution of the sample, whereas thermal volatilization processes do not appear to take place. Sample preparation involves only rinsing with methanol, air

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drying, and loading the polymer onto a flat sample holder. Short stabilization times (<3 min) and excellent signal stability (RSD <5%) characterize this approach. Textile surface-polyvinyl chloride (PVC) coatings were investigated by GD-AES (Knitell and Schollmeyer, 1996). The analysis of some minor elements (A1 and Fe) as well as of several trace elements (Co, Cr, Cu, Mn, Ni, Pb, and Zn) in Antarctic marine sediment was accomplished (Caroli et al., 1996). After grinding in an agate ball mill, sediment aliquots were mixed with high purity Cu powder and disk pellets were obtained by pressing the resulting mixtures at 10 tons. Measurements were performed by GD-AES and HCD-AES. The two sets of results were in full agreement and were also completely confirmed by independent ICP-AES and ICP-MS determinations. Impurities in SiC powdersnin particular, A1, B, Ca, Cr, Fe, and Vmwere determined by GD-AES after pelletizing with Cu powder at a ratio of 3 to 7 (Florian et al., 1994a). Amounts as low as 50, 44, 27, 8.5,557, and 646 lxg g-1 could be detected for the above elements in the given order. These figures were 1 to 2 orders of magnitude worse than with the conventional dc arc AES, although precision of measurements was superior with the GD approach. Further findings from the same authors were reported later (F16ri~in et al., 1994b). Powdered samples were mixed with Cu as the binding material (Ehdich et al., 1991). The host metal was used as the internal standard to quantify A1, Cr, Fe, Mn, Mo, Nb, Ni, Si, Ti, and V by GD-AES. To prevent the influence of air included in the pellet, Ar was kept flowing over the sample. In a preliminary study it was reported that Au, Cu, and Zn could still be reliably determined in glass down to concentrations of 1.2, 20, and 18 l.tg g-l, respectively, by means of RF-GD-AES (Lazik and Marcus, 1993b). Heintz et al. (1995) used the already described RF-planarmagnetron GD ion source for TOF-MS to quantify B and Mg in Macor | with LoDs of 8 and 17 l.tg g-l, respectively. Coal fly ash and graphite were assayed by pulsed RF-GD-AES (Pan and King, 1993b). The LoDs for Ca, Co, Cr, and Cu in the former material were, in the order, 62, 3, 21, and 12 l.tg g-l, while those for Cr, Fe, and V in the latter material were 13, 15, and 35, respectively. It was stressed that the cathode-anode distance in the sampling geometry adopted is critical to optimize the analytical signal. An investigation of oxides on Cu-Ni and Fe-Ni alloys was performed by GD-AES (Tsuji and Hirokawa, 1990). An exploratory study showed that F could be quantified by GD-AES by using Ne as the filler gas in a unit consisting of a hollow anode placed at a distance of 0.2-0.3 mm from the cathode surface (Wagatsuma et al., 1996). The latter consisted of NaF dispersed in a Cu-powder pellet and was predischarged for 30 min before emission stabilized. By using the atom line at 685.6 nm (or in alternative the ion line at 402.5 nm) as low as 100 l.tg g-1 could be detected. Films of Fe oxides were investigated by GD-AES and data were confirmed by Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS), or SIMS (Suzuki et al., 1991). A comparative study of the performance of dc and RF-GD-MS in the analysis of rare earth element oxides revealed that the latter required higher power levels to

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attain signal intensities comparable to those of the former. It was more affected by variations in carrier gas pressure, more plagued by gaseous impurities, and characterized by more variable RSFs (De Gendt et al., 1995). Only when sample distance and gas pressure were strictly kept constant, reproducibilities could improve to 5% (between samples) and 15% (within sample). Pulsed dc GD-MS was applied to the elucidation of the equilibrium between La and LaO in the plasma (Mei and Harrison, 1996). The rare earth oxide (10%) was diluted in a host matrix (90%) of either Ag or Ti powder and pressed into small disks. Water contamination was found to arise primarily from the bulk of the cathode and to shift the equilibrium toward the formation of LaO molecules. This phenomenon may have deleterious consequences for the analytical performance. A GD-sputtering atomizer was developed which showed promise for the analysis of Na in refractory metal targets in microelectronic materials (Grazhulene et al., 1991). Trace concentrations of Pt and Rh in y-A1203 were quantified by GD-AAS after pelletizing with Cu powder (Winchester et al., 1991b). Precision of 7.5% or better could be achieved. Quantitative depth profile measurements with GD-AES were performed (Nickel et al., 1991, 1992b). Pellets of Cr203, MnO 2, and TiO 2 hosted in a powdered Cu matrix were prepared. Using pure Cr, Mn, and Ti as the base, corrections for thesputtering rate and discharge current could be applied. Oxide scales on Ni-Cr alloys were thus ascertained. Surface coatings were depth-profiled by GD-AES (Hamada et al., 1995). The method was based on the generation of emission spectra differentiated as a function of the sample depth. The depth profiles of C, H, N, and O of C and Cr-Si layers deposited on Si and of TiC on WC were investigated (Hoffmann, 1993). Quantification of many elements was possible provided that their concentrations were not lower than 10-100 I.tg g-l. Figures of merit of this GD-AES method were sharpness of spectral lines, low background intensity, and linearity of the calibration graphs over several orders of magnitude for concentration.

Analysis of Nonconductive Solid Materials by Hollow Cathode Discharge Sources Only a few examples of this type of application for HCD-AES have been recorded so far. Among these worth mentioning is that rocks and metals were analyzed for trace elements (Szilv~sy-Vhrnos et al., 1990). The performance of HCD in the analysis of marine sediments in the case of both the conventional source and a version boosted through superposition of a MW field at 2450 MHz was tested (Caroli et al., 1996). Total concentration and layer-by-layer distribution of Ag in chalcogenide glass films based on Ge selenide were ascertained by HCD-AES (Drobyshev et al., 1995). Applications for clarifying photostimulated diffusion in photoresistive materials were outlined. Trace elements (As, Cd, Cs, Hg, Mo, Pb, Se, and Zn) were detected in coal ash by a novel HCD design (conical inner contour) based on the emission from the

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cathode plume. A unique cooling system was set up in which use was made of transformer oil to insulate the cathode and of tap water to cool the dielectric fluid in an outer heat exchanger. Coal ash was compacted with Cu powder at a 9:1 weight ratio and the resulting disks were placed at the bottom of the cathode. Detection capabilities went from a few hundred ~g g-1 for As to 2 ktg g-1 for Cd (Collet et al., 1993). Nitrogen layers were studied by HCD optogalvanic analysis (Djulgerova and Mihailov, 1993a). The rationale behind this approach is that the replacement of spectral line intensity by the photoelectric optogalvanic signal leads to an improvement in the direct layer by layer analysis of surfaces. A number of Ti nitride layers of various thickness supported by Ti alloys were investigated. This detection mode appeared particularly convenient when the elements of interest were present also in the substrate and when the layers were not very homogeneous.

C. Analysisof Liquid Samples

Analysis of Liquid Samplesby Planar Glow DischargeSources Although traditionally the direct analysis of liquid samples has been of little relevance for the LPD plasma, there is a growing interest for these techniques also in this area as very low volumes of sample are generally requested. The rather limited applications described to date can be primarily ascribed to the detrimental effects that solvents in general (and water in particular) have on the ionization process. From this standpoint, it has been suggested to drop solutions on a Cu or graphite supporting cathode, with subsequent drying under IR irradiation (Caroli et al., 1993; Mei and Marcus, 1993). This has an additional benefit in that analytes undergo a noticeable concentration. Several other alternatives have recently been proposed. Among these, a GD lamp operating with He flowing at atmospheric pressure was reported (Alzate Londono, 1990). The system allowed for the introduction of the aqueous solution into the GD plasma and for the ensuing solvent vaporization, charring, atomization, excitation, and emission. Determinations were performed in the case of Ag, Cd, Cr, Cu, Hg, K, Na, Pb, and Zn. A technique defined as electrolyte-cathode discharge spectrometry was developed to allow for direct analysis of solutions by GD-AES (Cserfalvi and Mezei, 1994). A 2-6 mm air gas between a W rod anode and the electrolyte solution cathode was the site of the GD plasma. At pH 2.5 or lower the discharge could be ignited. The electron temperature was estimated to be 5000 K. Besides atomic lines of several analytes, Ca and Mg ion lines and intense N 2, NH, and OH bands were also observed. A particle beam LC-MS interface as a viable method for the introduction of liquid samples into a GD device was used (Strange and Marcus, 1991). The efficiency-of the desolvation process was clarified. No significant contribution from residual water vapor was observed on the plasma stability nor did the plasma discharge conditions change during the sample introduction. On the

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other hand, a strong influence of the cathode surface temperature was inferred. An additional advantage of the proposed configuration is that the system can be switched to the conventional solid sample analysis mode without removing the attachment for solutions. Analysis of microliter samples of biological fluids is often needed for forensic purposes. This kind of problem was faced by means of a miniature magnetron RF-GD-AES system with a demountable device for the rapid analysis of liquids (Raghani et al., 1996b). The system consisted of a stack of three vacuum flanges separated by Viton O-rings, a water-cooled cathode assembly, and a set of disk and ring permanent magnets. This device allowed analyses of nanoliter samples to be performed with better detection power than with the conventional GD approach. The same set of discharge conditions could be used for several analytes. Optimization of the discharge conditions for each element (Ag, B, Cu, Eu, and Mg), however, increased the LoDs as much as 40 times when compared to the conventional discharge. A GD-MS method was developed to analyze microliter volumes of aqueous solutions that permitted the long-term acquisition of data (Barshick et al., 1993). Samples were either adsorbed on pin-shaped electrodes prepared by pressing high-purity Ag powder or by preparing a slurry of Ag powder and solution to be subsequently dried and pressed as a pin. In both instances homogeneity and particle size of the individual materials are critical. Tests were carried out by using the NIST SRMs 3171 and 3172 multielement solutions in which A1, Ba, Be, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Se, Sr, and Zn were quantified. The average relative error of measurements of about 14% (from 2 to 30%) indicated the need for further refinement of this approach. The GD-MS method for the analysis of solution residues and crude oils was further investigated (Barshick et al., 1994). Several elements (Cr, Cu, Fe, Mg, Na, Ni, Pb, Si, Sn, and Ti) were quantified in a number of NIST SRMs, SPEX organometallic oil standards, and refined oil composites. It was stressed that the method described is particularly suitable when limited samples are available as well as in field applications. Polyatomic interferences arising from residual organic constituents were a major drawback. Accuracy was always better than 5%. In spite of recent significant progress, the determination of trace elements in biological materials after digestion as appropriate still has little advantage from the development of GD-MS, as clearly demonstrated by a review (Aggarwal et al., 1994). Au and Fe were quantified in solution residues by using both dc and RF-GD sputtering to atomize the samples and AAS to determine the two metals (Absalan et al., 1996). To simulate the behavior of seawater sample, the NaC1 interference on such determinations was elucidated. Sensitivity and precision of measurements was strongly influenced by the distribution of the solution residue on the cathode surface. Moreover, the analyte type and its ratio to the NaC1 matrix influenced more the analytical pattern than the absolute amount NaC1.

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Low-volatility elements (Eu, Tm, and Y) were measured in aqueous solutions by depositing and drying nanoliter amounts of sample on the Ni cathode of a miniature GD source (Davis et al., 1995). The atomic cloud thus formed was excited by a Cu-vapor laser-pumped dye laser to directly detect fluorescence. In other terms, in this analysis mode the GD mechanism was exploited to produce an atom reservoir for another technique. Absolute LoDs of 2 fg for Eu, 0.08 fg for Tm, and 1.2 pg for Y were achieved. The total time of analysis (from sample probing to final acquisition of data) did not exceed 5 min. Elements such as A1, As, B, Co, Cr, Cu, Mn, Mo, Sn, V, and W were determined in steel after acid dissolution of the samples and adsorption onto a graphite cathode (Saito, 1996). Jakubowsky et al. (1991) reported on an analytical procedure by GD-MS for determining pg quantities of Ir and Pt in aqueous solutions. Cementation from the liquid phase onto a less electropositive substrate (Cu) was carried out. Although satisfactory, measurement precision may be further improved. As in microchemical applications pin-shaped cathodes are considered to be more convenient, a comparison of the merits of the two geometries was made through which it was ascertained that the useful yield was 2 orders of magnitude worse for disks. This fact could be ascribed to a substantial increase of the sampling distance in the latter configuration. Samples of waste oils from the crankcases of vehicles were subjected to acid digestion. A few l.tL of the resulting aqueous leachates were pipetted into Ag powder (purity better than 99%) and the slurries were dried and pressed in polyethylene slugs to produce pins which could finally be submitted to GD-MS analysis for the assay of their Pb content (Barshick et al., 1994b). Determinations were performed by the isotope dilution approach and concentrations of Pb as low as 3 l.tg g-1 could still be easily detected with an internal precision better than 5 %. Experimental data agreed with those obtained with ICP-AES. Urine samples from patients under antiblastic therapy with Cisplatin were dried, the residues were dissolved in water and aliquots of this solution were deposited on the carbon tip of cathodes for GD-MS analysis (Evetts et al., 1991). Both Pb and Pt were quantified at the ng g-1 level. It was thus possible to conclude that Pb is displaced from body compartments and mobilized upon administration of Pt. Olson et al. (1996) exploited the tandem GC-CD-MS system to speciate and quantify organometallic compounds. Both tetraethyl- and tetrabutyl-Sn resulting from the fragmentation of Sn-containing species of higher molecular weight were detected down to 1 pg (absolute detection limits). With the same approach, the tetraethyl-Pb compound was measured in NIST-SRM 2715 Reference Fuel.

Analysis of Liquid Samplesby Hollow Cathode DischargeSources It was observed that when the HCD source is operated in combination with a voltage-controlled power supply, the precision obtained vary erratically day by day (Cai and Williams, 1995). By carefully optimizing the experimental conditions this

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still may range in the percent range. These authors described the application of pulsed HCD-AES to the analysis of microsamples. The discharge current was electronically controlled during the intermittent pulses and the periods of dc discharge. Five repetitive determinations of 186 pg each of Li, K, and Na as chlorides gave RDSs of 5-8% for Li, 4-10% for K, and 3-5% for Na. Furthermore, calibration curves obtained in different days with different cathodes conditioned in the same manner were very similar. The capability of working with microsamples and low LoDs allowed samples of renal fluids to be assayed for Ca, C1, K, Mg, Na, and P at amounts as low as nanoliters. It was clearly shown how proper preconditioning of the cathodes was critical to the optimal performance of the analysis. Cathodes with smooth inner surface gave place to greater emission intensities than the rough ones. An absolute detection power of 3.2 pg for K, 0.35 pg for Li, and 0.32 for Na was reported, respectively. When pulse width, pulse height, frequency, pressure, gas flow rate, and cathode material are optimized high precision can be attained. A hot HCD source was optimized so as to reach temperatures as high as 2100 ~ in consequence of the ion bombardment (250 mA, 4 torr of Ar), thus fully achieving the conditions for the vaporization of solution residues (Tanguay and Sacks, 1991). An axial magnetic field was also applied to the cathode in an attempt to improve the emission performance. With a pulsed microcavity HCD unit it was possible to improve by 1 to 2 orders of magnitude the detection power for several analytes in solution (Mixon et al., 1994). In the analysis of fiver water by FANES, heavy metals could be determined with a RSD increasing to between 12 and 45%, depending on matrix composition (Geiss et al., 1990). A set of elements (Ag, A1, As, B, Bi, C1, Co, Cr, Cu, F, Fe, Mn, Na, Ni, P, Pb, S, Sb, Se, Sn, Te, Ti, and Zn) in pure acid were determined by a solution dry residue method using a new type of demountable HCD device (Matschat and Czerwensky, 1992). This version was intended to minimize the risk of contamination. Detection limits ranging from 0.1 to 4 lxg L -1 were achieved (50 I,tg sample size). With their continuous-operation HCD system Schroeder and Horlick (1994) succeeded in quantifying a number of elements in aqueous solutions. The discharge gas was He and current densities of 640 mA cm -2 could be maintained. LoDs (in ng mL -1) of 100 for A1, 3 for Ca, 50 for Cd, 0.6 for Cs, 4 for K, 0.03 for Li, 20 for Mg, 40 for Mn, 0.2 for Na, 10 for Rb, and 200 for Zn could be achieved. The matrix effects, on the determination of rare earth elements in solution residues of their halides by HCD-AES was investigated (Mierzwa and Zymicki, 1992). Barium, Ca, and Sr matrices were studied. The second element was the most disturbing and should therefore be removed prior to analysis. The FANES approach was again exploited to quantify Cd and Mn (DemCny and Radziuk, 1992). The emission characteristics of these analytes and the influence of working conditions were examined. The analytical advantages brought about by the particle beam HCD-AES approach were highlighted (You et al., 1996). Analyte particles were introduced into a heated HCD system by resorting to a high-efficiency thermoconcentric nebulizer connected to a particle beam LC-MS interface. Either the flow injection or the

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continuous flow mode was adopted. The interferences normally noted with water vapor were thus eliminated. Aqueous solutions (200 l.tl) of Cu, Fe, Mg, and Pb were analyzed with LoDs of 12, 20, 15, and 25 ~tg L -1, respectively. HCD sources can be used as atomizers for AFS. With this technique trace concentrations of Ir and Pb in liquid samples could be detected at levels as low as 2 pg and 15 fg, respectively. Volumes of more than 40 mL did not further improve sensitivity. A pulsed HCD lamp tuned to a Cu-vapor laser-pumped dye laser (6 kHz) was used (Womack et al., 1991). The temporal behavior of Pb emission in the HCD mode was studied to further elucidate the merits of the analysis of liquid microsamples (Becerra et al., 1992). By loading Cu cathodes with 0.5 lxL of solution containing 50 ng of Pb, it was observed that all metal was sputtered in less than 1 s and that the maximum signal intensity took place in 0.1 s. Linear response and precision were quite good and sub-ng detection limits could be achieved. A decrease in the Pb signal under the same experimental conditions was observed when 5 lxg NaC1 was added to the Pb solution in consequence of a reduction in the sputtering rate of the analyte. Finally, an interesting example of application of the HCD plasma source was reported, i.e. human serum could be analyzed for Se (Szvilv~sy-V~imos et al., 1990).

D. Analysisof Gaseous Samples Planar Glow Discharge Sources It is to a certain extent surprising that LPds are in practice much less exploited for the analysis of gaseous materials than would be expected from their inherent properties. However, several recent papers described the introduction of gaseous samples into modified GD sources. The glow is exploited, in such arrangements, to decompose the gaseous species and to excite or to ionize the resulting atoms or fragments. Helium is sometimes used as the discharge gas since the Ar plasma is not particularly successful in the determination of nonmetals, mainly because of the high excitation energy generally required by these dements. The GD technique was employed to analyze a variety of gas mixtures, e.g. to determine the C amount of C-containing molecular gases and for the continuous determination of C, C1, F, and S in molecular gases and organic vapors (Starn et al., 1993). A gas sampling GD system was described (Starn, 1994; Pereiro et al., 1995). This allowed for the continuous determination of C, C1, F, and S in molecular gases and organic vapors using He as the discharge gas. An exponential dilutor was used to convey discrete aliquots of the samples through a silica capillary tube into the GD source. LoDs (in ng s-1) were reported to be 0.6, 7, 114, and 2 in the given order of analytes, with uncertainties comprised between 3.0 and 5.5%. The linear dynamic range extended over 2-3 orders of magnitude for concentration. This approach was thought to be of value also for determining elemental ratios in organic compounds.

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The As content of aqueous solutions could be quantified after conversion of the element into arsine and direct introduction of the gaseous sample into a GD-AES system (Broekaert et al., 1993). Depending on the nature of the discharge gas, i.e. Ar, He, or Ne, LoDs were found to be 20, 54, or 30 ng L -1, respectively. Volatile organic compounds (benzene, styrene, toluene, xylene, isoprene, carbon tetrachloride, acrylonitrile, n-butanol, 2-propanol, acetone, etc.) were assayed in air through direct atmospheric sampling and introduction into a GD-MS system with ion trap (Gordon et al., 1996). LoDs were typically in the low ng g-1 range. In this last investigation area, an overview of the current capabilities of GD sources coupled with quadrupole ion-trap MS for the determination of high-explosive substances in the vapor phase was presented (McLuckey et al., 1996). The atmospheric sampling GD system was found to be quite effective in forming negative ions from, e.g., mono-, di-, and trinitrophenols, mono-, di-, and trinitrotoluenes, S, and still others.

Hollow Cathode Discharge Sources Similarly to what has been said for GD sources, only a few examples of applications of the HCD source to gases have been published. Worth mentioning in this context is a HCD lamp for gas analysis patented in Germany (Rolski and Zoechbaner, 1996). An electrothermal HCD source was used to study air emission spectra (Lee et al., 1995). Traces of sulfides and SO 2 were quantified by combining a HCD source with vapor molecular absorption spectrometry (Jin et al., 1992). Silicon was determined by HCD-AES coupled to hydride generation (Fujiwara et al., 1996). Aqueous solutions of silicates were dried and the salt residues were mixed with powdered LiA1H4. The silane thus generated was introduced into the plasma through a pinhole at the center of the hollow cathode. LoDs in the 6-30 Ixg range were reported. Moreover, organic vapors were directly injected into a HCD source using He as the carrier gas to quantify Br, C1, F, I, and S (Ng et al., 1991).

V. FUTURE PERSPECTIVES As never before, analytical chemistry has experienced impressive achievements in the 1990s regarding the ability to detect ever more elusive analytes in matrices of daunting complexity. This is obviously the consequence of the development of new techniques and the refinement of those already in wide use. Glow discharges have a place of their own in this context in that they are absolutely no newcomers in the spectroscopy world, while at the same token they have demonstrated from the very beginning the ability of keeping the pace with increasingly sophisticated challenges. A unique case of longevity, this branch of spectrometry will continue to successfully cope with the needs of a society marked by accelerating evolution. It is regretful that, in spite of all the merits of this analytical approach, users still form a relatively small (while not exclusive) club. Not a minor role from this standpoint

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is played by the limited interest shown by most manufacturers in further expanding this sector of the market. A feedback mechanism thus takes place which, on the one hand, offers modest selection to the practitioners and, on the other hand, does not increase the confidence of manufacturers in promoting this analytical technique. This vicious circle nowadays can and should eventually be broken on the basis of the rich and diversified evidence available of the versatility and high potential of glow discharge spectroscopy.

ACKNOWLEDGMENT The skill and forbearance of Miss Clarissa Ferreri and Mr. Massimo Delle Femmine in dealing with the various drafts of this manuscript are gratefully acknowledged.

REFERENCES Absalan, G., Chakrabarti, C.L., Hutton, J.C., Back, M.H., Lazik, C., Marcus, R.K.J. Anal AtomSpectrom- 1994, 9, 45. Absalan, G., Chakrabarti, C.L., Headrick, K.L. Can. J. Appl. Spectrosc. 1996, 41, 51. Aggarwal, S.K., Kinter, M., Fitzgerald, R.L., Herold, D.A. Crit. Rev. Clin. Lab. Sci. 1994, 31, 35. Alzate Londono, H. Rev. Colomb. Qufm. 1990,19, 81. Angeli, J., Haselgruebler, K., Achammer, E.M., Burger, H. Fresenius' J. Anal. Chem. 1993, 346, 138. Babin, EJ., Gagn(~,J.M. Appl. Phys. B 1992, 54, 35. Banks, ER., Blades, M.W. Spectrochim- Acta 1991, 46B, 501. Banks, P.R., Blades, M.W. Spectrochim- Acta 1992, 47B, 1203. Barshick, C.M., Harrison, W.W.Mikrochim- Acta 1989, 3, 169. Barshick, C.M., Duckworth, D.C., Smith, D.J.J. Am. Soc. Mass Spectrom. 1993, 4, 47. Barshick, C.M., Smith, D.H., Hackney, J.H., Cole, B.A., Wade, J.W. Anal. Chem. 1994a, 66, 730. Barshick, C.M., Smith, D.H., Wade, LW., Bayne, C.K.J. Anal. At. Spectrom. 1994b, 9, 83. Barshick, C.M., Barshick, S.C., Matthew, M.L., Britt, Ph.E, Smith, D.H. Rapid Commun. Mass Spectrom. 1996, 10, 341. Bartlow, R.B., Griffin, S.T., Williams, J.C.Anal. Chem. 1992, 64, 2751. Battagliarin, M., E. Sentimenti, E., Scattolin, R. Spectrochim- Acta 1995, 50B, 13. Baudoin, J.L., Chevrier, M., Herman, B., Passetemps, R., Hunault, P. Spectra 2000-Analyse 1993, 22, 47. Becerra, E.R., Deavor,J., Winefordner,J.B. Spectrosc. Lett. 1992, 25, 1257. Behn, U., Gerbig, EA., Albrecht, H. Fresenius' J. Anal. Chem. 1994, 349, 209. Bengtson, A. Spectrochim- Acta 1994, 49B, 399. Bengtson, A. J. Anal. At. Spectrom. 1996, 11, 829. Bengtson, A., Saric, A. Analys av Ytbeliiggningar och Oxider pd Valsade Material, Institutet f6r Metallforskning; Forsningrapport IM-2722, 1991, 76 pp. Betti, M., Rasmussen, G., Hiemaut, T., Koch, L., Milton, D.M.P., Hutton, R.C.J. Anal. At. Spectrom. 1994, 9, 385. Betti, M., Giannarelli, S., Hiemaut, T., Rasmussen, G., Koch, L. Fresenius'J. Anal. Chem. 1996, 355, 642. Boehm, H. Stah11994a, 58. Boohm, H. Blech, Rohre, Profile 1994b, 41,375. Boohm, H. Oberfl~hen Werkst. 1994c, 35, 8. Boehm, H. J. Oberfl~hentech. 1994d, 34, 124.

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LASER-INDUCED BREAKDOWN SPECTROMETRY

Yong-III Lee and Joseph Sneddon

I. II.

III.

IV.

V.

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals in Laser-Induced Plasma . . . . . . . . . . . . . . . . . . . . . A. The Interaction of a Laser B e a m with Target Materials . . . . . . . . . . B. Laser-Induced Plasma Production . . . . . . . . . . . . . . . . . . . . . C. Factors Influencing Plasma Formation . . . . . . . . . . . . . . . . . . . D. Excitation Temperatures and Electron Densities of the Plasma . . . . . . Laser-Induced Breakdown Spectrometry . . . . . . . . . . . . . . . . . . . . A. Basic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Analytical LIBS Techniques . . . . . . . . . . . . . . . . . . . . . . . . Applications o f LIBS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Metallurgical and Solid Samples . . . . . . . . . . . . . . . . . . . . . . B. Liquid Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Environmental and Gaseous Samples . . . . . . . . . . . . . . . . . . . D. A d v a n c e d Materials and Others . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A dvances in Atomic Spectroscopy Volume 5, pages 235-288. Copyright 9 1999 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0502.9

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236 236 237 237 240 241 247 250 250 251 257 260 261 265 271 280 284 284

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ABSTRACT When a pulsed high-powered laser beam is focused on a solid surface material, breakdown of the sample occurs and eventually results in the formation of a transient, and highly energetic plasma. This chapter presents theoretical and experimental results of this laser-induced plasma including excitation temperatures and electron densities, and factors which effect the laser plasma production such as laser parameters (wavelength, energy and type) and effect of solid sample. The use of plasma in atomic emission spectrometry-laser-induced breakdown spectrometry (LIBS) is discussed including basic principles and instrumentation. The application of LIBS to a wide variety of solid and liquid samples is presented. Novel applications such as remote sensing and underwater are discussed.

I. I N T R O D U C T I O N When a high-powered laser beam (normally > 1-10 mW/cm 2) is focused on a certain spot of a target (usually solid) material, breakdown of the sample occurs and eventually results in the formation of a transient, and highly energetic plasma, often referred to as a laser-induced plasma (LIP). Spectrochemical analysis based on the light given off by LIP emission is often called laser-induced breakdown spectrometry (LIBS). In recent years, LIBS has been growing steadily and it has shown a strong potential for performing direct spectrochemical elemental analysis of materials including solids, liquids, and gases with no or little sample pretreatment procedures. Significant research and progress has been achieved regarding its potential application in areas such as metal and metal alloys, geological analysis, and environmental monitoring, and has been recently reviewed (Rusak et al. 1997; Song et al. 1997b). Such progress in this field is being driven mainly by a rapid improvement in available equipment such as lasers and detection systems. The principle advantage of LIBS is in the minimal sample preparation required for a solid sample, resulting in increased throughput and reduction of tedious and time-consuming preparation procedures that can also lead to contamination. Further advantages include: (1) ability to analyze extremely hard materials that are difficult to digest or dissolve, such as ceramics and superconductors, (2) potential for multielement determinations, and (3) potential for direct detection in aerosols (a solid or liquid particle in a gaseous medium) or ambient air using the transient plasma. Disadvantages include: (1) increased cost and complexity of the system, difficulty in obtaining suitable standards, (2) interference effects (including matrix and, in the case of LIBS in aerosols, the potential interference of particle size), and (3) poorer sensitivity than conventional atomic spectrometric techniques using solutions such as inductively coupled plasma atomic emission spectrometry (ICPAES) and graphite furnace atomic absorption spectrometry (GFAAS). The specific advantages and disadvantages of LIBS over conventional atomic spectrometric techniques are summarized in Table 1.

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Table 1. Advantages and Disadvantages of LIBS for Direct Spectrochemical Analysis

Advantages 1. No or little sample preparation is necessary (resulting in increased throughput and reduction of tedious and time-consuming preparation procedures which can lead to contamination) 2. Versatile sampling of all media (analyze nonconducting as well as conducting materials) 3. Very small amount of sample (about 0.1 lag-0.1 mg)is vaporized. (sometimes called "nondestructive" method) 4. Ability to analyze extremely hard materials which are difficult to digest or dissolve such as ceramics and superconductors 5. Local analysis in microregions offers a spatial resolving power of about 1-100 lam

Disadvantages 1. Increased cost and complexity of the system

2. Difficulty in obtaining suitable standards (for this reason the technique must be regarded as semiquantitative) 3. Large interference effects (including matrix and, in the case of LIBS in aerosols the potential interference of particle size) 4. Detection limits are generally not as good as established solution techniques

5. Poor precision, typically 5 to 10% depending on the sample homogeneity, sample matrix, and excitation properties of the laser 6. Possibility of simultaneous multielemental 6. Possibility of ocular damage by the highanalysis energy laser pulses 7. Potential for direct detection in aerosols (a solid or liquid particle in a gaseous medium) or ambient air using a transient plasma which is only piece for fraction of 8. Simple and rapid analysis (ablation and excitation processes are carried out in a single step)

This chapter describes some fundamental studies on the interaction of a laser beam with a target material, the characteristics of the LIP, development in instrumentation, analytical detection techniques used in LIBS, and selected versatile applications to a wide variety of samples.

II.

FUNDAMENTALS IN LASER-INDUCED PLASMA A. The Interaction of a Laser Beam with Target Materials

The interaction of high-power laser light with a target or solid sample has been an active topic not only in plasma physics but also in the field of analytical chemistry. From a practical standpoint, the use of lasers to vaporize, dissociate, excite, or ionize species on solid surfaces has the potential of becoming a powerful analytical tool. When a high-power laser pulse is focused onto a solid target, the irradiation in the focal spot can lead to rapid local heating, intense evaporation, and

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degradation of the material. A description of laser-solid interaction and a comprehensive review of analytical techniques has been published by several authors (Radziemski and Cremers, 1989; Majidi and Joseph, 1992; Majidi, 1993; Thiem et al. 1993; Radziemski, 1994; Sneddon et al. 1997). The interaction between a laser beam and a solid is a complicated process dependent on many characteristics of both the laser and the solid. Numerous factors affect ablation, including the laser pulse properties, such as pulse width, spatial and temporal fluctuations of the pulse, and power fluctuations. The mechanical, physical, and chemical properties of the sample also influence the ablation process. The phenomena of laser-target interaction have been investigated by several authors. Ready (1971) gave a comprehensive description of melting and evaporation at metal surfaces. Anisimov and coworkers (1968, 1973) related the thermal conductivity mechanism to the boundary condition of free vaporization of the solid into a vacuum. Caruso et al. (1966) found three different regions existing in a metal and the hot plasma formed on the outer surface expanding towards the light source at a supersonic speed. The hot vapor plasma interacts with the surrounding atmosphere in two ways illustrated in Figure 1 and involves (1) the expansion of high-pressure vapor which drives a shock wave into the atmosphere, and (2) energy is transferred to the atmosphere by a combination of thermal conduction, radiative transfer, and heating by the shock wave. The subsequent plasma evolution depends on irradiance, size of vapor plasma bubbles, target vapor composition, ambient gas composition and pressure, and laser wavelength. The history of important quantities such as radiative transfer, surface pressure, plasma velocity, and plasma temperature are strongly influenced by the nature of

Figure 1. Featuresof the interaction between vapor plasma and the ambient gas.

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239

the plasma, as is the final steady-state nature of the plasma. The three major types of a laser absorption wave are: (1) laser-supported combustion waves (LSC), (2) laser-supported detonation waves (LSD), and (3) laser-supported radiation waves (LSR) (Radziemski and Cremers, 1989). The difference in the waves arises from the different mechanisms used to propagate the absorbing front into the cool transparent atmosphere. The characteristics in distinguishing the waves are velocity, pressure, and the effect of radial expansion on the subsequent plasma evolution. At low irradiation, LSC waves are produced. Razier (1970) examined the long-time propagation of the LSC waves at 1 atm of pressure. Thermal conduction was assumed to be the primary propagation mechanism. Subsequent authors (Boni and Su, 1974; Jackson and Nielsen, 1974) proposed that radiative transfer could contribute. The major mechanism causing LSC wave propagation is radiative transfer from the hot plasma to the cool high-pressure gas created in the shock wave. The plasma radiation is primary in the extreme ultraviolet (UV) region of the wavelength and it is generated by photorecombination of electrons and ions into the ground-state atom. At intermediate irradiance, the precursor shock is sufficiently strong so that the shocked gas is hot enough to begin absorbing the laser radiation without requiring additional heating by energy transport from the plasma. The laser absorption zone follows directly behind the shock wave and moves at the same velocity. This is the analog of the chemical detonation wave and has been modeled by Ramsden and Savic (1964) and Razier (1965). The propagation of the LSD is entirely controlled by the absorption of the laser energy. Several workers (Maher et al., 1974; Steverding, 1974; Bergel'son et al., 1975; Weyl et al., 1981) studied theoretically and experimentally the ignition and propagation of the LSD wave off metal surfaces. Plasma energy transfer to the metal surface and the breakdown times were calculated and modeled. At sufficiently high irradiance, the plasma radiation is so hot that, prior to the arrival of the shock wave, the ambient gas is heated to temperatures where laser absorption begins. In the idealized configuration, laser absorption is initiated without any density change, and the pressure profile results solely from the strong local heating of the gas rather than a propagating shock wave. This configuration is an example of an overdriven absorption wave (Razier, 1965) These supersonic waves were modeled numerically by Bergel'son et al. (1975). Their numerical results confirm the basic structure that once the transient plasma initiation and formation process is completed the quasi-steady approximation is suitable. The LSR wave velocity increases much more rapidly with irradiance than that of the LSC and LSD waves. The temperature and pressure increase, conversely, is quite slow. This behavior illustrates that the LSR wave is effective in channeling the absorbed energy into heating a large amount of the gas rather than increasing the local enthalpy.

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B. Laser-Induced Plasma Production By increasing the energy deposited into the sample surface, the temperature reaches a point where material transfer across the surface becomes significant. In this type of experiment, target erosion appears in the form of craters. Theoretical considerations on plasma production and heating by means of a laser beam has been proposed by several workers (Maher et al., 1974; Steverding, 1974; Weyl, 1981). These models produce essentially similar solutions for the plasma temperature, density, and expansion velocity, and are broadly in agreement with experimental results. Cottet and Romains (1981) studied the formation and decay of laser-generated shock waves by a hydrodynamic model. Measurements of shock-wave velocities were performed on copper foils for incident intensities between 3 • 1011 and 3 • 1012 W/cm 2 with the use of piezoelectric detectors. Balazs et al. (1991) calculated the time development of density, velocity, temperature, and pressure profiles below and above the plasma ignition threshold. Below the plasma ignition threshold, the temperature of the expanding plume never exceeds the surface temperature, and in vapor, thermal ionization is almost completely absent. The plume expands into the vacuum, and its flow becomes supersonic. In the high-fluence case, the energy delivered to the plume through electron-neutral inverse Bremsstrahlung processes was enough to elevate the temperature close to the surface value. This gives rise to high electron density as well as intense light absorption. Between the creation of a localized vapor plasma and the steady-state lasersustained plasma, a plasma evolves through several transient phases. The initiation of a plasma over a target surface begins in the hot target vapor. Absorption generally commences via electron-neutral inverse Bremsstrahlung, but when sufficient electrons are generated, the dominant laser absorption mechanism makes a transition to electron-ion inverse Bremsstrahlung. Photoionization of excited states can also contribute for short wavelength interactions. The same absorption processes are also responsible for the absorption by the ambient gas. The basic progression of interaction (from absorption through compression) is, however, preserved. In recent years, research has revealed a strong dependence of absorption and scattering processes on the laser wavelength (Garban-Labaune et al., 1985). The critical densities for some common laser wavelengths are shown in Table 2. The aforementioned trend towards short wavelength research thus implies investigation of plasma processes at a much higher density where collisional effects will be emphasized. Recent work by Lee and coworkers (1992a, b, 1997) dealt with the characterization of the laser-ablated plasma formed from various pure metals under a controlled atmosphere by various laser systems. They observed that the LIP consists of two distinct regions when the pressure is reduced below 50 torr in an air or argon atmosphere. In He atmosphere, two distinct plasmas are observed even at 760 torr. One is near the target surface, called the "inner sphere plasma," which emits a strong background continuum signal. The other is called the "outer sphere plasma"

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Table 2. Critical Densities for Some Important Lasers Wavelength (lxm) Critical Density (cn'1-3)

Laser CO 2 Nd glass (coo) Nd glass (3(00)a KrF Note:

10.6 1.06 0.35 0.25

1019 1021 9 x 1021 1.6 x 1022

a3to0: third harmonic generation of the laser beam.

surrounding the inner sphere, shown in Figure 1, which gives the brilliant bluegreen emission of copper with a relatively low background continuum.

C. Factors Influencing Plasma Formation Laser Parameters Influence o f the irradiation wavelength. In practice, different types of lasers, mainly Nd:glass resonators, (~, = 1064 nm), Nd:YAG laser (~. - 1064 nm), CO2-TEA---carbon dioxide-transversely excited atmospheric pressure laser---(~, 10.6 micron), nitrogen-laser (~, = 337 nm), and dye lasers (~, = 220-740 nm) are used in the production of laser plasmas. The interaction of near and mid-infrared pulsed laser with metals has been studied extensively. The influence of the laser wavelength on the material removal was studied by several authors. Bingham and Salter (1976) investigated ion production in mass spectrometry using three different lasers: CO 2, ruby, and Nd:YAG. They obtained the highest sensitivity for the elements (P, S, Ti, V, Cr, Mn, Ni, Co, Cu, As, Zr, Mo, Nb, S, Ta, and W) with a steel standard using ruby (~. - 694 nm) laser ablation. However, the CO 2 laser (~, - 10.6 l.tm) showed poor sensitivity for high-boiling point elements (Ti, V, Zr, Mo, Nb, Ta, and W). Fabbro et al. (1980) used a Nd:YAG laser that was frequency doubled and quadrupled to give wavelengths of 1064, 532, and 266 nm to study the effect of wavelength and postulated the following equation for the mass ablation rate [m (kg/s cm2)], its dependence on wavelength, (~,), and the absorbed flux [F a (W/cm2)] 9 m = 110(F~/3)/(1014)~, -4/3

(1)

They found that the mass ablation rate would increase strongly at shorter wavelengths. Measurements of the ablation pressure generated by the ablating plasma has been obtained at a number of laboratories (Ripin et al., 1980; Yaakobi et al., 1981; Maaswinkel et al., 1984). These results confirmed the expected higher ablation pressure with shorter wavelength laser irradiation. Kwok et al. (1988) investigated the optical emission produced by laser ablation of YBaECU307 targets using a wide

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YONG-ILL LEE and JOSEPH SNEDDON

range of laser wavelengths and showed that 193 nm radiation produced mostly neutral atomic species while 1064 nm and 532 nm radiation produced mostly ionic species. The comparative work on the plasma emission characteristicsmspecifically self-absorption, line broadening, emission intensity, and metal ion formationmwas obtained by the use of three different laser wavelengths (XeC1 excimer; ~, = 308 nm, Nd:YAG; ~, = 1064 and 532 nm) (Lee et al., 1997). Lee et al. (1997) found that the degree of self-absorption and line-broadening strongly depends on the surrounding atmosphere and irradiation wavelength. This phenomenon was explained by shock-wave excitation of atoms in the outer-sphere plasma. The wavelength dependence of laser-induced breakdown in air, CO, and CO 2 was studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm, and 1064 nm) by Simeonsson and Miziolek (1994). A significant reduction in the breakdown thresholds for both CO and CO 2 was apparent when comparing 193 nm with the four Nd:YAG harmonics which was attributed to the resonance enhanced two-photon ionization of metastable carbon atoms.

Influence of irradiation energy. There are two main mechanisms for electron generation and growth. The first mechanism involves absorption of laser radiation by electrons when they collide with neutrals. If the electrons gain sufficient energy, they can impact and ionize solids. The electron concentration will increase exponentially with time due to the cascade breakdown. The second mechanism, called multiphoton ionization (MPI), involves the simultaneous absorption by an atom or molecule of a sufficient number of photons to cause its ionization. Multiphoton ionization is important only at short wavelengths (< 1 ILtm). Both cascade and multiphoton ionization require high laser irradiances, usually in excess of 108 W/cm 2. For most materials the power density required for evaporation is in the range of 104 to 109 W/cm 2. In the range of 104 to 107 W/cm 2, the resulting vapor consists of polyatomic particles (Lincolin and Kenneth, 1974). Selter and Kunze (1982) studied the degree of atomization in laser-produced vapor from titanium targets. At power densities below 7 • 107 W/cm 2, no atoms were observed in the vapor, whereas the evaporated material became partially ionized above 5 x 108 W/cm 2. It was assumed that the power density in the range of 106 to 108 W/cm 2, depending on the solid target, was sufficient for analytical measurements in a laser-ablated plume (Opauszky, 1982). Carroll and Kennedy (1981) found that the threshold power density for the formation of a plasma plume by laser irradiation was typically in the neighborhood of 108 W/cm 2. The threshold power density varied with the wavelength of a laser primarily because the absorbance of the target surface depended on the wavelength of the incident light. Dyer (1989) also determined the threshold energy of 108 W/cm 2 for the generation of a plasma on a copper target with KrF-excimer laser (~, = 248 nm).

Laser-InducedBreakdownSpectrometry

243

In order to describe the fate of the laser energy during laser-solid interaction, several processes should be considered. Due to the character of the target, a fraction of the energy is absorbed from the laser pulse while the rest is reflected by the surface. The deposited part of the laser energy is converted into local heat instantaneously, which can in turn diffuse by heat conduction. An increase in temperature may induce appreciable changes in optical and thermal properties of the target material, thus influencing the rate of energy deposition and heat transfer. If the surface temperature is sufficiently high, a phase change (melting) may occur and part of the absorbed laser power is expended into the latent heat of transition. Further heating results in the translation of the solid-liquid interface into the bulk, while the surface temperature continues to rise until evaporation commences. Hydrodynamic effects in which droplets and particulates are expelled from the molten surface layer and ejection of the melt caused by vapor recoil are among the mechanisms which contribute to the ablation of the target. Vertes et al. (1989) established a criterion for the plasma ignition threshold as a relation of the plasma absorption coefficient by the adiabatic absorption model for CO 2, ruby, and quadrupled Nd:YAG lasers and for different materials. They found an increase of the threshold for materials with increasing ionization potential. It was also noticed that under similar circumstances the threshold temperature for the frequency-quadrupled Nd:YAG (~, = 266 nm) laser were always largest, while for the CO 2 laser they exhibit the lowest value. This is in accordance with the widely known observation that UV lasers produce sharply etched craters in the target. Increasing the laser wavelength creates a molten crater rim. Laqua (1979) distinguished two different cases of vaporization depending on the irradiation, at an irradiation of less than 108 W/cm 2 and higher than 109 W/cm 2. In the first case, the stream of vapor leaves the surface at a velocity of 104 cm/s. After the initial vaporization, the process changed to a melting-flushing mechanism as a result of the heat conduction of the material. At higher irradiation, the temperature of the vapor leaving the surface is higher than the boiling point of the target material. The gas molecules above the target were ionized with a velocity of 106cm/s near the surface.

Physical Properties of the Target Material The physical properties of the target have an important influence on the shape and size of craters in target materials. The reflection of part of the laser energy is an important consideration in determining the fraction of laser energy absorbed by sample materials. The change in reflectivity may be due in part to the result of phase changes that occur during intense heating. In any case, reflectivity measurements indicate that laser energy that can be coupled effectively into a target is initially highly reflective, if the irradiance is high enough (Piepmeier, 1986). Recognition of the fundamental differences of the interaction of a burst (single) and Q-switched modes of operation with materials led to the conclusion that craters produced by

244

YONG-ILL LEE and JOSEPH SNEDDON

the latter mode might be less material-dependent than those for the former case (Neuman, 1964; Ready, 1965). However, Klocke (1969) and Baldwin (1970) found that the sample by a laser beam is strongly target material-dependent, whether the laser is Q-switched or not. Allemand (1972) showed that the reflectivity of the sample surface, density, specific heat, and boiling point of the pure metal target have an important influence on the shape and size of the craters and derived a relationship to the physical constants of pure materials, D=A(1 - R ) / O C p T b

(2)

where D = diameter of total splash (crater); A = proportionality constant (in energy per unit area); R = reflectivity of the surface at 1 mm; p = density; Cp = specific heat; and Tb = boiling temperature. Ishizuka (1973) studied the size and depth of the crater in samples of rare earth oxides, aluminum oxide, and sodium salts by using a Q-switched ruby laser. The crater produced by a laser shot was about 1 mm in diameter regardless of the composition of the matrix, but the depth of the crater depended on the type of matrix. A comparison of the crater size of the homogeneous material revealed that thermal conductivity is an important parameter. The depth of the crater increased with this value. The volume heated depends on the thermal conductivity of the material for the same laser conditions (Prochorov et al., 1973). On the other hand, heating of material around the crater increases with incident light intensity because evaporation only depends on the boiling point of the material at fixed pressure. Dimitrov et al. (1981, 1984) investigated the substance evaporation processes and the kinetics of plasma plume development depending on target orientation with respect to the laser radiation source direction. When the metal target was irradiated by laser radiation, the erosion products emerged nearly perpendicularly to the target surface. When the target surface was inclined with respect to the direction of laser radiation, the path length of the radiation in the plasma was shortened, which resulted in decreased absorption of the laser produced plasma. Lee and Sneddon (1994b) investigated the ambient gas breakdown phenomena during and after laser irradiation and developed a relationship between the degree of ambient gas breakdown and the physical properties of selected metals. The authors deduced the following relationship between breakdown intensity and the properties of metal species with five different metal species, I B = K [ 1 / ( C p x CT • p]

(3)

where, I B = intensity of gas breakdown emission (counts); K = proportional constant; Cp = specific heat (JK-1 mol-1); CT = thermal conductivity (Wm -1 K-l); and 19= density (kg m-3).

Laser-Induced Breakdown Spectrometry

245

Ambient conditions. The atmospheric influences on the LIP were concerned with the mass loss, crater formation, and plasma emission characteristics. The work was mostly performed in air, Ar, He, or N 2. Iida (1989) investigated the emission of the LIP with the use of a Q-switched ruby laser of energy 1.5 J in 20 ns duration in an argon atmosphere at reduced pressure. The emission intensities of atomic wavelengths increased several-fold in an argon atmosphere, in comparison with those obtained in air at the same pressure. Moderate confinement of the plasma and a resultant increase of emission intensities were achieved at 50 torr. They also used a Q-switched Nd:YAG laser (150 mJ/pulse, 10 ns pulse) to study the effect of atmosphere and power density on plasma generation (Iida et al., 1991). They found that tight focusing of laser radiation did not directly bring about a plasma of high emission intensity because of the absorption of laser energy by the plasma itself. The importance of prevention of a gas breakdown before sample vaporization was also indicated. Grant and Paul (1990a) studied the laser-induced plasma by in'adiation of a steel target with an XeCl-excimer laser (~, = 308 nm) with an energy of 40 mJ/pulse in an atmosphere of air, argon, nitrogen, and helium at pressures from 0.5 to 760 torr. The maximum spectral intensity and line-to-background (L/B) ratio occurred in an atmosphere of argon at a pressure of 50 torr. Lee et al. (1992a, b) investigated the effect of pressure over the range 10 to 60 torr and various atmospheres (air, Ar, and He on an Ar-F excimer laser-(~, = 193 nm, 100 mJ/pulse) induced plasma created above the surface of a copper target. The use of neutral copper wavelengths and reduced pressure from 760 to 10 torr resulted in a seven-fold increase in air and 11-fold increase in an argon atmosphere. The use of a helium was only a 1.5-fold increase over that obtained at 760 torr. They also observed nitrogen in air and argon breakdown in the plasma and concluded gas breakdown influences the laser energy coupling to the metal target. However, helium gas breakdown was not observed because of higher ionization potential and high thermal conductivity compared to argon and nitrogen. This characteristic peaks of neutral copper atom lines are shown in Figure 2. Kagawa et al. (1994) used an XeC1 excimer laser (15-70 mJ/pulse, 20 ns pulse duration) induced shock wave plasma on a Zn plate in a surrounding gas at low pressures (0.75-11.3 torr) and defined the role of the surrounding gas as only damping material to prevent the free expansion of the propelled atoms. The total emission intensity of the atom emission wavelengths was determined mainly by the amount of propelling atoms and the entire amount of kinetic energy they produce. Mao et al. (1993) also demonstrated the shielding effect on the coupling of laser energy to a target surface during the interaction of picosecond-pulsed laser beam and target material using a He and Ar gas atmosphere. They concluded that Ar is easier to ionize than He and plasma; shielding is more severe in Ar than in He. Kuzuya et al. (1993) studied the effect of laser energy and atmosphere on the emission characteristics of LIP with the use of Q-switched Nd:YAG laser over a laser energy range of 20 to 95 mJ/pulse. The experimental results showed that the maximum spectral intensity was obtained in argon at around 200 torr at high laser

246

YONG-ILL LEE and JOSEPH SNEDDON

(a)

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(b) lz

(b)

o

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energy of 95 mJ/pulse, whereas the line-to-background (L/B) ratio was maximized in He at around 40 torr at a low energy of 20 mJ/pulse.

Influence of electric and magnetic fields. There has been an interest on the use of electric or magnetic fields for enhancing the analytical characteristics of plasma sources for spectrochemical analysis. Pulsed magnetic fields have been used to alter the properties of microwave plasmas (Goode and Pipes, 1981), spark discharges (Klueppel and Waiters, 1980; Majidi and Coleman, 1987) and exploding conductor plasmas (Albers et al., 1986, 1987; Johnson and Sacks, 1987, 1988). A few recent studies have looked at the effect of a static electric field on UV emission enhancement and breakdown threshold. Hontzopoulos et al. (1988) used a 3-20 mJ

Laser-Induced Breakdown Spectrometry

247

KrF excimer laser to study the effect of UV emission lines of gold from a laser-induced plasma on the surface of gold target in fields up to 13 kV/cm. They found a significant enhancement, up to a factor of 100, for some lines above 6.6 kV/cm, saturating for some lines at 20 kV/cm. They tentatively interpreted this enhancement on the basis of recombination processes taking place near the surface of the gold electrode. Kumar and Thareja (1988) studied the breakdown in Ne, Ar, and Xe gas at different pressures using a XeCI excimer laser (60 mJ, 8 ns of pulse length) ranging up to a maximum field strength of 1000 V/cm. They concluded that their results were similar to those observed using a high-power laser alone. Mason and Goldberg (1991a) designed and constructed a new capacitive discharge system. The pulsed magnetic field, produced by capacitive electrical discharge through a specially designed solenoid, was oriented normal to the laser axis. Temporally integrated emission enhancements due to the magnetic field were found to be most significant when the plasma was formed about 1 mm below the magnetic field axis. The degree of confinement of the plasma increased with magnetic field strength and they also found an increase in line-broadening, neutral atom self-reversal, and minor constituent emission intensities (Mason and Goldberg, 1987). Subsequently, their work (Mason and Goldberg, 199 lb) was conducted on the dynamic effects of a high-intensity pulsed field having a maximum strength of 85 kG, by using time-resolved emission and absorption measurements. Spatial and temporal discrimination of emission enhancement indicated that radial compression was due to static magnetic field interactions with the LIP and that mild Joule-heating from the small induced current was most likely responsible for emission enhancements later in time. They concluded that more efficient coupling of energy from the magnetic field to the plasma would require low-pressure operation in a controlled atmosphere and/or a pulsed magnetic field having a greater dB/dt (B is the magnetic field intensity). The influence of a magnetic field on the plasma emission of Mg induced by a KrF excimer laser was reported by Dirnberger et al. (1994) They observed a dramatic enhancement of emission signal from neutral and ionized species over the field-free case, in certain spatial regimes.

D. Excitation Temperatures and Electron Densities of the Plasma Temperature is one of the more important properties of any excitation source. Knowledge of the temperature of an excitation source is vital to the understanding of the dissociation, atomization, ionization, and excitation processes occurring in the source and is helpful in attempts to utilize the source to its maximum analytical potential. The methods most frequently used for determination of excitation temperatures are the two-line method (Kirkbright et al., 1970, Mehs and Niemczyk, 1981; Hood and Niemczyk, 1987) and the Boltzmann plot method (Kalnicky et al., 1977; Uchida et al., 1981; Faires et al., 1984; Lee et al., 1992a). Grant and Paul (1990a) determined the electron temperature and density of an XeCl-excimer LIP. The relative atomic intensities of the 11 Fe(I) wavelengths used

248

YONG-ILL LEE and JOSEPH SNEDDON

in the Boltzmann plot of electron temperature were used. The temperature ranged from 9000 to 22,000 K depending on the ambient conditions. Temperature decreased with distance from the surface and with decreasing ambient pressure. Electron densities were calculated according to the Saha equation and the fitted values of temperature with the assumption local thermodynamic equilibrium (LTE). The density profile exhibited features similar to those for temperature, ranging from 3 x 1019 to ~ 1016 cm -3. Kagawa et al. (1984) calculated the excitation temperature in a high-power nitrogen laser-induced plasma with the two-line method by the line pair of Zn(I). The temperature ranged from 8000 to 9000 K, and the region of maximum was at a point some distance from the center of the plasma rather than at the center. Ursu et al. (1989) studied the optical breakdown plasma in a gas in front of various solid samples by a TEA-CO 2 laser source. They measured the energy absorbed into the blade calorimeter, placed at various distances from the center of the plasma. An upper limit of the vapor temperature of ~ 14,000 K was inferred from the characteristic darkening curve of the photographic film. They found that the initial maximum corresponding to the breakdown plasma in gas having a temperature of ~20,000 K is followed by a luminescence tail due to vapors acting with the fireball and having a temperature of ~ 10,000 K. Radziemski and coworkers (1983a, b, 1985) measured the temporal variation of temperature and electron density in an air plasma induced by a CO 2 laser operating at 0.5 and 0.8 J/pulse. The excitation temperature was determined by the Boltzmann plot, and ranged from 19,000 K at 1 ~s to above 11,000 K at 25 Its. The electron density was measured at 500 rnJ/pulse and determined to be from 3.6 x 1017 cm -3 at 1 Ixs and 4 x 1016 cm -3 at 25 bts. Lee et al. (1992a, b) calculated the excitation temperature of an ArF-excimer (100 mJ/pulse, 11 ns pulse length) LIP by the Boltzmann plot method with Cu(I) and Pb(I) wavelengths. The temperature of the excimer LIP were quite high, ranging from 13,200 to 17,200 K in the plasma formed with copper and from 11,700 to 15,300 K for the plasma formed with lead depending on the location in the plasma. They also measured the excitation temperatures of the plasma under different pressure and atmosphere. The temperatures ranged about 14,000 K at 10 torr to 18,000 K at 760 torr for an air atmosphere, from about 13,400 K at 10 torr to 14,200 K at 760 torr for an Ar atmosphere, and from 12,600 K at 760 torr to 14,800 K at 200 torr for a He atmosphere. The excitation temperature profiles of the plasma induced by 308, 532, and 1064 nm irradiation under helium atmosphere, and varying the pressure from 760 to 10 torr as well as the location in the plasma are shown in Figure 3. Measurement of precision and accuracy in the plasma temperature is highly dependent on sample composition, homogeneity, surface condition, and particle size. Precision is typically 5 to 20%, but values of less than 1% have been achieved under certain conditions (Fabbro et al., 1980). However, the accuracy of the estimation by the Boltzmann equation is uncertain. It depends on the existence of LTE assumed for this method and the accuracy of spectroscopic constants of neutral atom lines.

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YONG-ILL LEE and JOSEPH SNEDDON 13000

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i11. LASER-INDUCED BREAKDOWN SPECTROMETRY A. Basic Principles

Elemental analysis based on the emission from a plasma-generated by focusing a powerful laser beam on a sample (solid, liquid, or gas) is known as laser-induced breakdown spectrometry (LIBS). The principles of LIBS are similar to those of conventional plasma atomic emission spectrometry (AES), such as inductively coupled plasma (ICP)-AES, microwave induced plasma (MIP)-AES, direct current plasma (DCP)-AES, flame-AES, arc-AES, or spark-AES. In atomic emission spectrometry (AES), the light from an excited sample is spectrally resolved and, frequently temporally resolved as in the case of pulsed-light sources, to yield qualitative and quantitative information about the elemental constituents. What distinguishes the LIBS technique from conventional plasma-AES or other AES, is that the sample need not be transported to the plasma source; rather, the plasma is formed in or on the sample in situ. It is a simple method because the ablation and excitation processes are carried out by the laser pulse in a single step. However, the mechanism of laser ablation is complex and not clearly defined with research into laser/solid interactions continuing. In addition, the LIBS technique can be used to analyze gases, liquids, and solids directly without sample preparation, because the plasma is produced by optical radiation. This method also provides, in principle, simultaneous multielement analysis without increased instrumental complexity and cost. Atomic ions as well

Laser-Induced Breakdown Spectrometry

251

as neutral atoms will be produced in plasma sources having high temperatures. The energy levels are unique for each element and, therefore, it is possible to perform simultaneous qualitative analysis by examining each wavelength in the emission spectrum of an unknown. A spectrometer is used to disperse the plasma emission, and from the wavelengths of individual elements in the emission spectrum, it is possible to determine the elemental composition of the sample material. For quantitative analysis, a measurement involves integration of the analytical signals from a wavelength or set of wavelengths of the elements of interest over a certain period of time. The amount of an element present is determined by constructing a calibration curve of signal versus concentration by the use of standard reference materials. Over 1000 different standard reference materials (SRMs) are obtained from the National Institutes of Standards and Technology (NIST), formerly known as the National Bureau of Standards (NBS) (Gaithersberg, Maryland). These materials are certified for their chemical composition, chemical properties, or physical properties by various reliable analytical methods. However, the standard references are often not available with some matrix. Other standards are available from various international government agencies such as the National Research Council (NRC) (Ottawa, Ontario, Canada), and National Institutes for Environmental Studies (NIES) (Ibaraki, Japan). B. Instrumentation The ability of LIBS to determine elemental composition of solid, liquid, and gaseous samples without any sample preparation has been demonstrated as a new and versatile analytical technique in the past decade. As shown in Figure 4, the instrumentation for LIBS generally consists of a pulsed laser beam for sample ablation or breakdown, the optics for focusing the laser beam and plasma emission, an ablation chamber, and a system for detection, and analysis of the specific elements in the plasma.

Laser Systems The laser systems used for LIBS must generate pulses of sufficient power to produce the plasma. Commercially available lasers that meet this criterion include solid-state lasers (e.g. Nd:YAG and ruby), gas lasers (e.g. CO 2 and excimer) and Nd:YAG pumped-and flashlamp pumped-dye lasers. Each laser has advantages and disadvantages that include properties of the laser itself such as wavelength, mode quality, and characteristics of operations. The most frequently used lasers are solid-state lasers such as the Nd:YAG [~, = 1064 nm, 532 nm (frequency-doubled) and 355 nm (frequency-tripled), pulse length of 5-10 ns] and ruby (~, = 693 nm and pulse length of 20 ns) gas lasers such as the CO 2 (~, = 1060 nm and pulse length of 100 ns), N 2 laser (~, = 337 nm, pulse length of 30 ps-10 ns) and excimer (~, = 193nm (ArF), ~, = 248 nm (KrF), ~ = 308 nm (XeC1) and pulse length 10-20 ns]. For any of these laser types, it is typical that many tens of millijoules can be

252

YONG-ILL LEE and JOSEPH SNEDDON

Figure 4. Typical instrumental setup for LIBS system. From Song et al. (1997b), with permission.

produced in time frames of a few nanoseconds. As a result, peak power output is measured in the range from 1 to 100 mW. Focusing the laser pulse to a few tens of micrometers in diameter with a simple lens can produce fluxes on the order of 101~ to 1012 W/cm 2. The laser pulse energies required to form a plasma depend on many factors including properties of the pulse (such as the energy, mode quality, wavelength, and pulse length) and the material (i.e. reflectivity at the laser wavelength for solids and density for gases). Typically, a 100 mJ/pulse from a Q-switched laser is sufficient to generate the plasma for most types of analysis. Recently, there has been an increase in the use of the UV excimer laser because of the low UV reflectivity for most metals, resulting in improved energy coupling efficiency, and the high optical resolution offered by the relatively short wavelength (Iida, 1990; Lee et al., 1992a, b, 1997; Multari et al., 1996). A comparison of the characteristics of different types of lasers are shown in Table 3.

Ablation Chamber Although a LIP can be produced in an atmospheric condition, much work has been performed on the influence of different buffer gases (air, Ar, He, and N2) and gas pressure. An ablation chamber can be constructed with glass (quartz), polymer block, or stainless steel for LIBS studies. Some considerations concerning the construction of an ablation chamber include the following: (1) extending the spectral range into the far-UV field, particularly to perform C, P, and S determina-

Laser-Induced Breakdown Spectrometry

253

Table 3. Comparison of the Characteristics of Some Lasers Used for LIBS Laser

Lasing Medium

Wavelength PulseWidth P u l s e Divergence (nm) (ns) Energy (J) (mrad)

Nd:YAG Nd 3+ ion in a 1064 host crystal 532 (Y3AI5012) 355 Ruby

AI20 3 doped with 0.05% by Cr20 3 Carbon CO 2 in gas dioxide mixture (N2 and He)

694

Nitrogen N2

337

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193 248 308

10.6 ~tm

5-10 5-8 4-8

0.3-1.0

Comments

1-10

Four-level laser, Compact, Low maintenance, 0.25 Three-level 25 (Q-switch) > I0 laser, High power, 0.5-500 TEA: 0.5-10 Simple 1-200 ILtS Sealed: 1-2 design, Good gain and very high efficiency (up to 30%), ideal for industrial applications. 6-14 Short pulse 300psat760 < I 0 duration, torr, I 0 ns at compact, 20 torr repetition rate 1-100 Hz, 0.25 3-10 UV 7-20 wavelength, toxic gases, quartz optics required, repetition rate to 250 Hz.

tion in metallic and mineral specimens, (2) easy-to-control gas inlet/outlet system with gas purging during the ablation, evaporation, and excitation/breakdown process, and (3) convenient and reproducible positioning of an analyte material to a sample holder. Most LIBS work has been investigated with the laboratory-constructed stainless steel ablation chamber for precise control of buffer gas (for solid analysis) or sample gases. Kagawa et al. (1984, 1982) used a rotating sample holder at 2 rpm in a chamber which could control the buffer gas and pressure to obtain the uniformity in the shape and emission intensity of the plasma during successive irradiation with the laser light. Nordstrom (1995) used a hexagonal sample container with 25 mm diameter ZnSe lens used to focus the laser beam at the center of the chamber to make a gas breakdown in air. Kagawa et al. (1993) put a rectangular tube close to the target surface to limit the free expansion of the LIP and to confine the plasma to a limited space.

254

YONG-ILL LEE and JOSEPH SNEDDON

Optical Fiber System In the application of LIBS to the remote analysis of materials, a beam delivery system utilizing reflective optics, or on having a miniature laser in situ to provide the laser radiation necessary for the plasma generation, has been the most widely used system. The optical emission from the plasma was often collected and delivered to the spectrometer for determination via a fiber-optic system. Transport of a high-pulse power laser beam (several GW/cm 2) through optical fibers for various applications have been successfully developed due to the recent development of fiber-optic materials (Trott and Meeks, 1990). These advances in optical fiber technology have been adapted in the LIBS system to transfer the laser beam to a target material or transfer the plasma emission to the entrance slit of the spectrometer by placing it near the LIP or beneath additional optical components (Geyer and Weimer, 1990; Aragon et al., 1993; Wisbrun et al., 1994; Cremers et al., 1995; Ernst et al., 1996; Pakhomov et al., 1996; Yamamoto et al., 1996). In comparison to the fiber-optic collection, the components for the provision of the laser beam at the remote site were rather large and cumbersome. Thus, it would be advantageous for the realization of easy-to-handle remote LIBS if the laser beam could be delivered using a relatively simple fiber-optic system. The ability to perform LIBS measurements with the use of fiber optics opens up many new areas for the application of the technique. Examples include analysis in hostile environments and monitoring of soil contamination inside a borehole. A typical experimental arrangement for remote LIBS measurements made with a fiber-optic cable (FOC) is shown in Figure 5. The pulses transmitted by the fiber were collimated and then focused on the sample by a pair of lenses. The same fiber was used to transport the laser pulses to the sample and to collect the plasma emission. Recently, Marquardt et al. (1996) applied a fiber-optic-based LIBS system to the determination of Pb content in dry paint. They used two different fibers: one for excitation (transport the laser pulse to the sample), and the other for collection (transfer the emission signal to the spectrometer). Nyga and Neu (1993) also used a pair of optical fibers for analysis of solid samples submerged in a liquid.

Dispersing Systems and Detectors Dispersive spectrometers used in LIBS normally make use of a diffraction grating as the dispersive element. For sequential multielement determinations, monochromators are used whereas simultaneous multielement analysis are performed with polychromators. In LIBS, the use of high-resolution gratings as well as a spectrometer with sufficient focal length (f) and narrow slit widths is required to resolve the emission linewidths and obtain maximum signal-to-background (S/B) ratios. Generally, both Ebert and the Czerny-Turner mountings are used for sequential spectrometers. In Ebert mountings, optical aberrations especially when using narrow slits and a focal length > 0.5 m are minimized by using curved slits. Using a Czerny-Turner mounting, this problem does not occur as two mirrors with slightly different focal length and slightly different incident angles can be used.

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Figure 5. Typical experimental setup for fiber-optic LIBS system.

256

YONG-ILL LEE and JOSEPH SNEDDON

Polychromators used for simultaneous measurements have a Paschen-Runge arrangement where the concave grating, the entrance slit, and many exit slits are on the Rowland circle. These instruments have a focal length between 0.5 and 1 m and are used in most commercial equipment. For the simplest detection, the LIP can be photographed. The exposure time will be determined by the lifetime of the plasma and the picture is obtained by simply holding the camera shutter open in a darkened laboratory while the laser is being fired at the sample target. However, direct photography provides only information of physical interest and, in particular, gives no information of the evolution of the plasma in time. Such information can be obtained by the use of either streak or framing cameras. The spectrally resolved signal can be detected using either a photomultiplier (PMT) or a photodiode array (PDA) or recently developed chargetransfer device [charge-coupled device (CCD) or charge-injection device (CID)]. The classical spectrometer consists of arranging slits in the focal plane of a spectral apparatus that screens off the wavelengths. Photomultiplier tubes (PMTs) are placed behind the slits as detectors, which produce a photocurrent proportional to the intensity of the incident radiation. Current techniques using PMTs are able to analyze for a large number of elements over a concentration range spanning 6 or more orders of magnitude (Boumans, 1987). Early studies of LIBS conducted by Radziemski et al. (1983a) and Cremers and Radziemski (1983b) used a variety of photomultiplier tubes with extended UV or infrared (IR) responses. However, the single nature of the PMT, or limited multichannel capabilities of small banks of PMTs, have often reduced the ability of the analyst to monitor multiple spectral wavelengths for each element, as well as simultaneously monitoring the spectral region surrounding the analytical wavelengths of interest. One of the most promising approaches to AES involves the use of an echelle spectrograph and an array detector to measure a large number of wavelength intervals simultaneously. To minimize the overlap of emission spectral wavelengths in the analytical spectral range from UV to near-IR regions, a large number of resolution elements is required. An echelle spectrograph can provide both the rectangular format and the required resolution (normally 0.01 nm). Recently, the performances of CCD and CID were compared for use in AES by Sweedler et al. (1989). The strong background and matrix emission greatly limit the CCD while having little effect on CID performance. The ability of the CID to measure hundreds of spectral wavelengths simultaneously allowed extremely complex samples to be measured with little or no prior knowledge of sample composition. Sweedler et al. (1989) also found that the detection limits do not change significantly even in complex matrices with a CID detector.

Advances in Instrumentation The technique of spectrochemical analysis by a LIBS system is now an established method for direct analysis of solids, liquids, and gases of widely different origin and composition. The development in recent years of more compact and

Laser-Induced Breakdown Spectrometry

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reliable laser systems combined with the increased need for field-deployable instrumentation has increased interest in laser-based analytical methods. Work conducted by Cremers and Radziemski (1985) at Los Alamos National Laboratory (Los Alamos, New Mexico) reported the success of a LIBS system to determine airborne beryllium (Be) collected on filters. Based on this work, a Be monitoring instrument was developed to automatically analyze filters. The basic components were the laser, spectrograph, detection system, PMT, filter rotating device, and data acquisition/processing system. Filters were analyzed by placing them on a platform that rotates and translates the filter under the laser pulses. The LIP was produced on the filter surface and was about 3 mm long. The LIP emission was spectrally resolved using a 0.125 m spectrograph and detected by a PMT. The PMT signal was processed by a gated integrator to produce a voltage that was digitized and stored in a computer memory. A recent commercial LIBS instrument was developed by MelAk, Inc. (El Paso, Texas). This MA 5095 laser analyzer system is portable (dimensions: 22 x 48 x 40 cm), light weight (30 kg), and air-cooled with a YAP (yttrium aluminate) laser (~ = 1079 nm, 5-10 ns pulse width). Plasma emission was collected by a collimating lens and projected onto a CCD by a holographic grating polychromator. It is ideally suited for rapid and accurate quality control checks requiring precise qualitative and quantitative analysis in any production or field location at which alternating current (ac) power (110 or 220 V) is available. Other portable instrumentation, based on LIBS using a compact Nd:YAG laser and nongated CCD detection system has been developed for the determination of metal contaminants on surfaces by Yamamoto et al. (1996). The instrument has a weight of 14.6 kg, fits completely into a small suitcase (46 x 33 x 24 cm), and operates from 115 V ac. The instrument consists of a sampling probe connected to the main analysis unit by electrical and optical cabling. The hand-held probe contains a small laser to generate a LIP on a sample surface and a fiber-optic cable to collect the plasma emission. This instrument was evaluated for the analysis of several metals in the environment: Ba, Be, Pb, and Sr in soils; Pb in paint; and Be and Pb particles collected on filters.

C. Analytical LIBS Techniques LIBS is a simple, rapid, and in situ analytical technique based on laser-induced plasma emission. For analytical purposes, the plasma emission is spectrally resolved and the atomic wavelengths are subsequently analyzed in order to determine elemental concentration in the sample. This technique has, however, several limitations such as high continuum background, line broadening, and self-absorption. This consideration limits the use of this plasma emission for practical spectrochemical analysis. Work in minimizing these limitations has been investigated by the use of time-resolved LIBS (TRELIBS) (Ciucci et al., 1995; Barbin et al., 1997; Castle et al., 1997; Song eta!., 1997a; St-Onge et al., 1997) or time-integrated spatially

258

YONG-ILL LEE and JOSEPH SNEDDON

resolved LIBS (TRELIBS) (Lee et al., 1992, 1994, 1997; Aragon and Aguilera, 1997) in a controlled atmosphere. In most LIBS work, TRELIBS has been generally utilized in order to avoid the intense initial continuum emission and improve the line-to-background (I.JB) ratio by gating off the early stage of the plasma emission. A suitable choice of time delays in detecting the emission spectra allows selective assignment of the resolved line emission to different elements. However, it has been noted that the spatially resolved signal along the vertical direction from the target surface can provide a similar spectrum to that which is time delayed without using the expensive gated detection system required for TRELIBS. Multari et al. (1996) verified the resolveable properties of the plasma emission between TRELIBS and spatially resolved spectra with the many images with various different geometric factors. This result showed the potential application in the construction of a less expensive LIBS apparatus and portable LIBS instrumentation. More recently, Mao et al. (1995) characterized the emission spectra and excitation temperature spatial profiles within a LIP from a copper target as a function of laser power density with the use of time-integrated emission spectroscopy. This work showed how the measured axial spatial emission intensity of the expanding plasma can be influenced by time integration. They concluded that the ability to accurately measure spatial emission intensity and temperature behavior was shown to be related to the integration time versus plasma expansion velocity. Recent work by Lee et al. (1997) also showed that the selection of the location in the plasma is crucial for obtaining the best operating conditions; in particular to achieve the highest signal-to-background ratio of analytical atomic wavelengths and to avoid self-absorption and line-broadening. They also found that the degree of self-absorption and line-broadening strongly depends on the surrounding atmosphere and irradiation wavelength. Figure 6 shows the comparison of copper emission spectra obtained at 4.0 mm (called "inner sphere") and 5.6 mm (called "outer sphere") from the target surface generated by a XeC1 excimer laser. The other limit to the use in LIBS for quantitative elemental analysis is related to the stability of the laser-induced plasma emission which originated from the laser intensity fluctuation (1-5%) and the scattered light depends on the local matrix effects and on physical and chemical characteristics. The most common method to compensate for signal fluctuations in LIBS is based on calculating the ratio of the spectral peak intensity to that of a reference intensity. However, this currently used internal calibration provides relative, rather than absolute, concentrations. Recently, Xu et al. (1997) developed a new data acquisition approach followed by a suitable data analysis for LIBS. It provides absolute (rather than relative) concentration of elements in particular materials, e.g. industrial dusts and soils using a sequence of signals from single-pulse breakdown events. This method does not require an internal standard element of known concentration; however, standard samples are still needed to obtain calibration curves. Recently, a spectroscopic imaging technique has been applied to LIP based on modern charge-coupled devices (CCD) that are coupled to spectroscopic utilities

Laser-Induced Breakdown Spectrometry

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Figure 6. Time-integrated spatially resolved copper plasma emission generated by the XeCI excimer laser under 400 torr of helium and taken at different positions; (A) 0.4 mm (inner sphere) and (B) 5.6 mm (outer sphere) from the target surface. Wavelengths (a through s and I through q) are given in original paper. From Lee et al. (1997), with permission. and to fast data transfer capabilities (Bulatov et al., 1996; Romero and Laserna, 1997). Spectroscopic maps of the LIP were obtained and used for allocation of chemical components in plasma. Imaging spectroscopy provides a new and powerful tool for chemical imaging of the LIP. It combines the analytical power of traditional spectroscopy with two-dimensional (2-D) visualization. Bulatov et al.

260

YONG-ILL LEE and JOSEPH SNEDDON

Figure 7. A result of the classication algorithm. Each image is collected at different focusing conditions; however, in all cases Cu is located at an inner shell while Zn is an outer shell. A lens of f = 400 mm was used, and the distances of the target from the lens were 400, 385,360, and 250 mm (from top to bottom). From Bulatov et al. (1996), with permission. (1997) investigated the spectroscopic images of the LIP by TRELIBS. The system was based on a mechanically driven interferometer coordinated with a CCD camera. The gray-scaled classification maps induced onto a brass (Cu and Zn) sample are shown in Figure 7. These maps show that Zn is present at the outer shell and in the tail of the plasma, while Cu is present mainly in the inner shell. The authors found the optimum location in the plasma that provides the best spectral signal-to-noise ratio. Romero and Laserna (1997) used multielemental chemical imaging to generate selective chemical imaging for Ag, Ti, and C from silicon photovoltaic cells. Lateral resolution of 80 l.tm and depth resolution of better than 13 nm were obtained.

IV. APPLICATIONS OF LIBS There are many applications of LIBS for qualitative and quantitative elemental measurements in a wide range of samples such as metallurgical and solid samples, environmental samples, solution or colloidal phase samples, gases, and advanced materials. The literature which has appeared on this subject area during recent years is increasing. Among the applications of LIBS, elemental composition analysis of metallurgical samples has been most popular. Recent applications, however, are

Laser-Induced Breakdown Spectrometry

261

more inclined to environmental samples and liquid samples, which includes biological samples.

A. Metallurgical and Solid Samples A driving force for LIBS has been due to the need for the direct and rapid determination of trace metals in various types of samples. This is because no other analytical technology can do trace analysis (determination) as efficiently as LIBS without complicated sample preparation. Elemental analysis of solid samples has been the most popular among the applications of LIBS. Radziemski and Cremers are one of the pioneers in this field. Loree and Radziemski (1981) examined copper, aluminum-alloy, and steel samples by using LIBS as well as time-resolved LIBS (TRELIBS) for the trace elemental analysis of a solid-sample surface. The contents of Be in a Be-Cu alloy was determined by Radziemski et al. (1986), and excitation temperature within the spark was determined by Cu(I) and Cu(II) emission wavelength in that report. Leis et al. (1989) also applied TRELIBS with a Nd:YAG laser to find optimum conditions for laser ablation and atomization for analytical purposes. Strong temperature changes were observed depending on the sample matrix composition. Calibration curves for two analytes (Si and Cr) in homogeneous and low-alloyed standard steel samples were developed. The detection limits calculated on a 3-a basis were 24 lxg/g for Cr 425.2 nm, 30 lxg/g for Si 288.2 nm, and about 200 lxg/g for Si 251.4 nm. The standard deviation measured on the analytical lines were typically 6%. When using an appropriate iron wavelength as the internal standard, the relative standard deviation (RSD) of the intensity ratio was only 2.4%. Nemet and Kozma (1995) studied an application of TRELIBS to the direct, undamaged qualitative and quantitative analysis of tertiary high-alloys (gold jewels). The investigation was performed over a wide spectral range in the UV and visible region for the analytical wavelength pairs of copper-gold, silvergold, and copper-silver. The RSD of the background was <4%, and the RSD of the line pairs was <7-8%. With most of the line pairs, a linear correlation could be obtained, when the concentration of Ag and Cu changed between 0 and 45%. Ko et al. (1989) obtained excellent analytical results almost independent of the plasma temperature and the state of evaporation of the ablated sample material for Fe and Cr by the use of the internal standardization in LIBS. The Fe/Cr wavelength pairs selected for the measurements were 300.304/298.865, 300.957/301.371, and 449.457/450.030. The standard deviation of the data from the straight line was only 3 %. Belliveau et al. (1985) tested NIST steel standards at ordinary conditions (atmospheric pressure and air) and calibration for Cr, Mn, and Ni have been performed by using a Nd:YAG laser. In this report the effect of laser power, critical alignment/focusing procedures, and number of laser pulses per analysis were also studied. Determination for Mn, Si, and Cr in solid steel was performed by Cremers and Romero (1986). They examined several factors, such as lens-to-sample distance, laser pulse energy, and position of the imaging lens, which affects the LIBS analysis. These affects are minimized by ratioing the absolute elemental signal to adjacent

262

YONG-ILL LEE and JOSEPH SNEDDON

Fe wavelengths. Grant et al: (1990b, 1991) studied the determination of Fe, Si, Mg, Ti, A1, and Ca in i r o n o r e b y using TRELIBS with an XeC1 excimer laser; the optimum period in the plasma lifetime for spectrochemical determination was obtained in this work. As a result, a 1 to 2 Its delay was appropriate to obtain the best signal-to-noise ratio. Observed calibration curves were linear except for A1. At a higher concentration range of A1 (AI/Fe > 0.01) the curves started to level out resulting in a decrease in sensitivity. This is due to the self-absorption of the AI(I) 396.15 nm resonance wavelength. The element/iron ratio detection limit was estimated from 0.001 to 0.00003 depending on the elements. They also estimated oxide concentration detection limits and the results were 0.003% for CaO, 0.15% for SiO 2, 0.023% for MgO, 0.013% for A1203, and 0.023 % for TiO 2. This study indicated that the detection limit can be lowered by using resonance wavelengths, but self-absorption may degrade the precision and accuracy of the measurement. They also pointed out if a LIBS instrument is used for on-site analysis of iron ore, emission wavelengths located in the UV region with a high cross section cannot be adopted. Aguilera et al. (1992) applied a fiber-optic-based LIBS for the determination of carbon content in steel by using a Q-switched Nd:YAG laser (~, = 1064 nm). The emission wavelength at 193.09 nm was chosen for the determination because the 247.86 nm wavelength suffers an interference from the 247.86 nm wavelength of Fe(II). The detection limit was estimated as 65 ppm and the precision was 1.6%. In this study, a CO2-free environment was necessary in order to improve the accuracy and detection limit since atmospheric CO 2 can be dissociated in air. For this purpose, nitrogen was used as a buffer gas. Matrix effects for the studied steels were identified to be small by comparing the calibration curves for stainless steel samples to that of nonstainless steels. This group applied these results to determine carbon content in steel samples melted in a laboratory induction furnace and observed a precision of 10% in the concentration range of 150-1100 ppm (Aragon et al., 1993). The wavelengths selected for the LIBS determination were the 193.09 nm wavelength of C(I) and the 201.07 nm wavelength of Fe(II). With the use of the UV region, the strong thermal emission from the molten sample was avoided. The selection of a time window of 1 to 6 Its provided high line-to-background and signal-to-noise ratios. The detection limit for carbon was obtained as 250 ppm in an argon atmosphere and 65 ppm in an nitrogen atmosphere. This study demonstrated that LIBS can be used for direct composition analysis in molten alloys. The removal of carbon content produced by the decomposition of CO 2, which is produced by combustion reactions with the molten surface, is a major obstacle in applying LIBS to the on-line analysis of molten metals. Owens and Majidi (1991) used an Nd:YAG laser-(~, = 1064 nm) based TRELIBS for the investigation of a NBS powder sample containing 0.02% A1 by placing the sample between two layers of tape attached to a Plexiglas plate. In this study, one of the goals was to develop a good signal-to-noise ratio with a single laser pulse, because multiple laser firings generate increased amount of sample material. This means a large consumption of sample, and the composition of the sample changes after each plasma sampling for solid samples of alloys. They adopted TRELIBS and monitored emission only

Laser-lnduced Breakdown Spectrometry

263

during a small portion of the plasma lifetime to increase signal-to-noise ratio. They also adopted an alternative calibration technique using embedded A1 in a nonconductive resin. Advantages of resin standards include uniformity of analyte distribution and shot-to-shot reproducibility. The lowest detectable concentration of A1 in the resin was 16.9 I.tg/g. Kagawa and Yokoi (1982) also performed the determination of Cr in iron-steel samples by using a N 2 laser with good precision. The minimum detectable concentration of Cu in iron and Mg in aluminum were not lower than 0.05% due to the lack in performance of the monochromator. This group also investigated Zn plasma intensity by observing several emission wavelengths of Zn with XeC1 excimer laser as well as TEA-CO 2 laser. A calibration curve was obtained for Cr in carbon steel by observing 425.4 nm emission wavelength and the detection limit was estimated as 20 ppm with a background equivalent concentration (BEC) of 12 ppm (Kagawa et al. 1994). Thiem et al. (1994) studied the simultaneous quantitative elemental analysis of NIST transition metal alloy samples by using a Nd:YAG laser. Linear calibration curves were obtained for elements with the composition of A1 (0.2-1.2%), Cu (0.021-0.49%), Fe (4.5-51.0%), Ni (30.8-80.3%), and Zn (6-12.8%) using nonresonance wavelengths. The detection limit varied with sample composition complexity from 0.0001% for Ni in a copper alloy (SRM 111) to 0.16% for A1 in a complex granular sample (SRM 349a). The spectral wavelengths used in the analysis were 309.3 nm for AI, 357.9 nm for Cr, 351.8 nm for Co, 324.7 nm for Cu, 372.0 nm for Fe, 403.3 nm for Mn, 352.5 nm for Ni, 390.5 nm for Si, 399.9 nm for Ti, and 334.5 nm for Zn. Lee and Sneddon (1992c) used an ArF excimer laser (~, = 193 nm) for ablation to perform quantitative analysis of elements in solids. A calibration curve was developed which related the Cr concentration in a solid-steel matrix to the intensity ratio of Cr(I) 520.84 nm to Fe(I) 516.75 nm. The Cr concentration determined ranged from 0.062% to 1.31%. A detection limit of 20 I.tg/g (approximately 0.002%) was estimated. Sabsabi and Cielo (1995a) obtained a calibration curve for Mg, Mn, Cu, and Si in aluminum alloy samples. The calibration curves for Cu and Mg exhibited a curvature for higher concentration due to the self-absorption of the Cu(I) 327.4 nm and Mg(I) 285.2 nm resonance wavelengths. The strong resonance wavelengths can be used, however, for lower concentrations with improvement of the detection limit. The detection limits for each element were estimated as 0.5 ppm for Mg, 10 ppm for Cu, 14 ppm for Si, and 2 ppm for Mn. The precision of 50 consecutive measurements of Mg, Mn, Cu, and Si was 4% for strong wavelengths at high concentrations and 6% RSD at low concentrations. Sabsabi and Cielo (1995b) also performed quantitative analysis of copper alloys by LIBS using a Nd:YAG laser (~, = 1064 nm). Calibration curves for Fe, Ni, and Ag were produced. The precision ranged from 2 to 10% of the analyte concentration. The detection limits were 20, 10, and 1 l.tg/g for Fe, Ni, and Ag, respectively. These values were comparable to conventional atomic emission spectrometric

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YONG-ILL LEE and JOSEPH SNEDDON

methods. This group also performed work on A1 and Mg in an aluminum alloy (Sabsabi and Cielo, 1993). Noll et al. (1994) performed quantitative elemental analysis in an iron matrix. This study was to try to improve the detection limits of LIBS for multielement analysis. The influence of the laser pulse structure on the emission of the laser-induced plasma was also studied. Sattmann et al. (1995) studied the effect of single and double pulses from a Q-switched Nd:YAG laser on the intensity of emission signal for trace elements. Line intensities were increased by a factor of about 2 using double pulses. Quantitative microchemical analysis of low-alloy steel was also performed with single and double pulses. Cremers (1987) published the results of the determination of trace elements (Si, V, Cu, Mn, Cr) in metal samples at a distance from 0.5 to 2.4 m by using a fiber-optic cable. This study motivated further work on element determination by LIBS at a remote distance up to few hundred meters. Observed concentrations of elements were comparable to the predicted concentrations for the concentration range of 0.05 to 2.95%. Precision of the measurements were estimated from 9% for Si to 28% for Cu. The simultaneous determination of Mg, Mn, Fe, and Pb in A1 was performed by Bescos et al. (1995). In this study, linear calibration curves for those elements over 0.01-1% concentration range were obtained. They suggested the potential application of this method to on-line industrial analysis. Gonzalez et al. (1995) studied the determination of sulfur content in steel by LIBS using a Q-switched Nd:YAG laser. They measured the emission intensity ratio of S(I)/Fe(II) in the wavelength region of 550 nm (543.23, 545.38, and 547.36 nm lines) and 180 nm (180.73, 182.03, and 182.62 nm). A detection limit of 70 ppm and a precision of 7% was obtained. Calibration curves were linear over the range of 0.008 to 0.28% of sulfur concentration. No noticeable matrix effect had been observed in this study. Lorenzen et al. (1992) also applied LIBS to the in-process quality assurance and process control in different industrial branches such as steel production and plant making. Their instrument, which is called LIESA (laser-induced emission spectral analysis) is equipped with a high-power laser source (irradiance: 1 x 108 - 5 x 109 W/cm 2) and optical fiber bundle for the detection of emission signals. Relative detection limits of between 10 to 100 ppm were demonstrated for most of the detectable elements in various matrices (steel, rubber, rock, and glass). They also developed procedures to convert relative measurements with a precision of between 1 and 2% into absolute concentration values with relative accuracy of about 3%. Talmi et al. (1981) used a LIBS system using a ruby laser (~, = 694 nm, 0.6-1.2 J, repetition rate: 4 pulses/min) to monitor both surface and depth profiles of elemental content of a variety of sample types including a ruby rod and ceramic material with blemishes, electrical capacitors, integrated circuits, and surfacecoated electrical conductors. Detection limits obtained were in the range of 2-500 ppm depending on the element, the wavelength used, and the matrix. Ernst et al. (1996) applied LIBS as a means to assess radiation embrittlement by the determination and quantification of copper in A553b steel. The LIBS results were comparable to those from atomic absorption spectrometry (AAS) in precision

Laser-Induced Breakdown Spectrometry

265

and accuracy. Copper content < 0.02% was unresolvable for AAS, whereas LIBS was able to detect and quantify Cu down to 0.01%. The average error for levels < 0.50% was 0.010 wt% for LIBS and 0.026 wt% for AAS. Anderson et al. (1995) performed depth profile measurements of coatings on steel using LIBS. Linear calibrations against coating thickness for Zn/Ni (2.7 to 7.2 l.tm) and Sn (0.38 to 1.48 Ixm) on steel were achieved with good precision (3.5% RSD). An ultrathin coating of Cr (20 nm) on steel was also detected by LIBS. Marquardt et al. (1997) determined lead concentration in paint in houses using the fiber-optic probe LIBS technique. The measurement required less than 1 min to perform and the detection limit of Pb was 0.014% in latex paint, on a dry weight basis, with a precision of 5-10%. Song et al. (1997a) applied the TRELIBS technique to determine various trace elements including Mg, Cu, and Cr in zinc-based alloys, and Si and Ca in rock samples. The analytical signals for trace elements were integrated within 20 Ixs after an optimum gate delay time (200 ns). The detection limit (S/N ratio = 3) was element-dependent and varied with complexity in sample composition but was in the order of 5-100 ppm. Precision was typically 5 to 10%. St-Onge et al. (1997) determined several elements (A1, Cu, Fe, Pb, and Sn) in solid zinc alloys using LIBS. In this experiment, optimal conditions for determination were evaluated with variation of time-gating parameters and distance from focusing lens to target. Detection limits lower than about 60 ppm were achieved for all elements except Cu, for which a 544 ppm 3 o-limit was found for the wavelength used. We can conclude that almost all of the metal and transition metal elements in different matrices have been target elements in trace determination by LIBS, and ppm to parts per billion (ppb) range of detection limits have been obtained. Further work in this direction will be the application to on-site analysis of elements, improvement of precision in measurement, development of accurate calibration procedures, and, of course, improvement of detection limits. A typical LIBS spectrum of metallurgical sample is shown in Figure 8. This spectrum is for Cu in zinc-based alloy. The calibration curve for Cu in zinc-based alloy is presented in Figure 9. Application of LIBS to trace determination in metallurgical samples are summarized in Table 4. The tabulated information includes details of the elements determined, sample matrices, aim of the analysis, range of concentration, detection limits, and reproducibility.

B. Liquid Samples Application of LIBS to liquid samples, including colloidal and biological sampies, has been a popular area and solution analysis continues to be a relevant part of metal analysis. Boiron et al. (1991) applied LIBS to the determination of mono-atomic ions in individual fluid inclusions. They determined major cations (Ca2+, Mg 2+, Na +, and K+) contained in macroscopic electrolyte solutions. For Ca 2+ and Mg 2+ calibration curves were obtained in the concentration range of 0.1 to 1.2 M using synthetic solutions, and the detection limits were estimated as 0.005 M

266

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Table 4. Application of LIBS in Metallurgical and Solid Samples Elements

Mg, Mn, Cu, Si

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AI alloy

Object of Analysis

Laser U s e d

AI(80), Cr(10), Cu(10), Fe(540), Mn(60), Ni(1), Si(40), Ti(110), Zn(44)

Thiem et al. (1994)

0.15-1.1

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Excimer (308 nm) CAO(0.01-10.8), SIO2(0.06-2.0), MgO(O.O05-1.47), AI203(0.11-10.61 ), TIO2(0.05-0.48) XeCl 0.05-5

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Nd:YAG

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Nd:YAG

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Reference

Sabsabi et al. (1995a)

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Composition analysis

RDS (%)

Mg(0.5), Cu(lO), Si(14), Mn(2)

Nd:YAG

Steel, rubber, rock, glass Ca, Si, Mg, AI, Ti Iron ore

LOD flimit of detection,ppm)

Mg(0.05-4.6), Cu(0.05-0.3), Si(0.5-1), Mn(0.05-1.5) AI(0.2-I .2), Cr(0.05-2.19), Cu(0.021-0.49), Fe(4.5-51 ), Mn(0.02-0.55), Ni(30.8-80.3), Si(0.02-0.4), Ti(0.26-3), Zn(6-12.8%)

Composition analysis

Steel sample

Range of Concentration(%)

170

Aragon et al. (1993) Lorenzen et al. (1992) Grant et al. (1990b)

Kagawa et al.

(1982) Kagawa et al. (1994) (continued)

Table 4. Continued Elements

Matrices

Object of Analysis

Laser Used

Range of Concentration (%)

LOD (limit of detection, ppm)

RDS (%)

Reference

Composition analysis

Nd:YAG

Ag(0-41.5)

15 for 324.75nm 160 for 521.83 nm

Nemet et al. (1995)

Fe, Ni, Ag

Au-Ag-Cu alloy, gold jewel etalons Copper alloy

Compositio n analysis

Nd:YAG

Fe(20), Ni(10), Ag(1) 2-10

Sabsabi et al. (1995b)

Cr, Fe

Solid s t e e l

Excimer (ArF)

Cr(20)

Lee et al. (1992c)

C

Steel

Nd:YAG

C(0.04-1.85)

C(65)

1.2

S

Carbon steel

Nd:YAG

S(0.008-0.28)

S(0.008)

S

Cr, Si

Nd:YAG

Cr (24) Si (200)

6

Trace analysis

Nd:YAG

Si(0.001-0.74), Cr(0.0059-16.76) 0-45% for Cu, Ag

Evalutate detectors,trace anlysis

Ruby

Cu

AS33b steel

Assessment of radiation enbrittlement Quantitative analysis Quantitative analysis

Nd:YAG

Cu(15:324.75 nm 160:521.83 nm) Ag (<15), Cr(2), Ga (<10), Cu(<20), Mn(<10), Sn (<500), Pb(<500), Pd(<50) 0.01%

<7-8

Ag, Cr, Ga, Mn, Sn, Pb, Pd

Low-alloyed standard steel Gold jewel etalons Metal alloy and gold wire

Composition analysis Composition analysis Quantitative analysis Trace analysis

Fe(0.008-0.2), Ni(0.004-0.37), Ag(0.002-0.04) Cr(0.062-1.31 )

Ag

Cu, Ag

AI, Cu, Fe, Pb, Sn Zinc alloys Mg, Cu, Cr, Si, Ca Zinc alloys (Mg, Cu, Cr), Rocks (Si, Ca)

Nd:YAG Nd:YAG

0-5

Aguilera et al. (1992) Gonzalez et al. (1995) Leis et al. (1989)

10-14

Nemet and Kozma (1995) Talmi et al. (1981 )

<10

Ernst et al. (1996)

AI(9), Cu(544), average St-Onge et al. Fe(22), Pb(54), Sn(33) 13% (1997) Song et al. (1997) Mg(20), Cu(<100), 5-10 Cr(5), Si(<1%), Ca(50)

Laser-lnduced Breakdown Spectrometry

269

(200 ppm Ca) for Ca 2+ with 393.336 nm and 396.847 nm emission wavelengths and 0.001 M (25 ppm Mg) for emission wavelengths of 279.553 nm and 280.270 nm. Essien et al. (1988) determined the concentration of Cd, Pb, and Zn in aerosols by LIBS. Samples were generated in an aerosol form by a nebulizer heat chamber arrangement. Calibration curves for these elements in dry aerosols were developed using the ratio of the intensity of the analyte wavelength to the adjacent background. Detection limits for Cd, Pb, and Zn in aerosols were 0.019, 0.21, 0.24 lxg/g, respectively, at a signal-to-noise ratio of 3. Wachter and Cremers (1987) used LIBS to detect uranium in solution for possible application to process control in nuclear fuel reprocessing facilities. A pulsed Nd:YAG laser was focused on the surface of the liquid in order to generate sparks. The detection limit for U in 4 M nitric acid was 0.1 g/L. Measurement precision was 1-2% for a 4.2 g/L solution with the use of 1600 laser sparks, corresponding to a measurement time of about 3 min. A calibration curve was prepared that covered U concentrations from 0.1 to 300 g/L. The effect of some experimental parameters on the analysis was also discussed in this work. Archontaki and Crouch (1988) used an isolated droplet generator (IDG) as a novel sample introduction system for LIBS. The IDG converts liquid samples to equally spaced, uniform-sized droplets. Calibration curves linear over 3 orders of magnitude have been obtained for several elements. The detection limits for solutions with the FIA (flow injection analysis)/IDG/LIBS system were in the low ppm range. Cremers et al. (1984) used the LIBS technique with a Nd:YAG laser (15 ns duration, 45 mJ/pulse) to analyze liquids spectroscopically for element constituents. Emission from once-ionized and neutral atoms and simple molecules were observed. Detection limits for Li, Na, K, Rb, Cs, Be, Mg, Ca, B, and A1 in aqueous solutions were established. Most of these elements were only detectable at levels above 1 ppm, although the detection limit for Li was 0.006 ppm. The RSD for replicate sample analysis was 4-8%. Ito et al. (1995) used a Q-switched Nd:YAG laser for the LIBS measurement to detect FeO(OH) in water and concentration of the turbid solution down to a few ppm. In this study, cell-less measurement was achieved using a coaxial nozzle flow system which allowed the study of the effect of ambient gas on emission intensity and decay lifetime of the breakdown plasma. Ito et al. (1995) also developed the coaxial nozzle configuration system to determine the colloidal iron in water by LIBS. Figure 10 shows the experimental setup where the sample solution flowed down the central nozzle at a constant flow and ambient gas flowed through the outer shroud. The stream of water was irradiated by a laser pulse and emission spectra were taken from the direction perpendicular to both the water flow and the laser beam. The estimated detection limit of FeO(OH) in water was 0.6 ppm. This group also published the results of quantitative detection for colloidal and particulate Fe in water (Nakamura et al, 1996). Using sequential laser pulses from a Q-switched Nd:YAG laser as the excitation source, the FeO(OH) concentration in the tens of

270

YONG-ILL LEE and JOSEPH SNEDDON

Sample Solution

IIlJ Lens

-

1

Laser Beam

l0 mm Optical Fiber Head

Figure 10o Experimental setup for detection of colloids in a liquid jet. From Ito et al. (1995), with permission.

ppb range was demonstrated. With sequential pulse excitation, the detection limit was decreased to 16 ppb compared to the 0.6 ppm observed with a single-pulse excitation. Knopp et al. (1996) performed the quantitative determination of ions of Li, Na, Ca, Ba, Pb, Cd, Hg, and Er in aqueous solution by using an excimer-pumped dye laser-based LIBS. The detection limit was dependent on the elements and was estimated at 500 mg/L for Cd § 12.5 mg/L for Pb 2§ 7.5 mg/L for Na § 13 mg/L for Li § 6.8 mg/L for Ba 2§ and 0.13 mg/L for Ca 2§ No Hg and Er emission could be detected even at Hg 2§ and Er a§ concentrations up to the g/L range. On the other hand, for Er in suspensions of ErBa2Cu30 x particles, a better than 103 times higher sensitivity was found than for dissolved Er a§. This result offers the possibility to analyze colloid-borne metal ions with an increaseA sensitivity. A similar technique for elemental analysis of heavy metal colloids with particle diameters between 0.1 and 1 Ixm was obtained by Haisch et al. (1997). A liquid jet system was used to reduce the interaction of the plasma with water molecules. The sample was pumped through a micro orifice (0.5 mm i.d.) at a flow rate of 160 mL/min and a sheath flow of nitrogen (1/L min) was maintained around the liquid jet. The detection limit of Fe in the colloidal solution was of the order of mg/L.

Laser-Induced Breakdown Spectrometry

271

Pichahchy et al. (1997) evaluated LIBS for the detection of metals located under water. Repetitive laser spark (RSS) and repetitive spark pair (RSP) were performed on the metals by focused pulses from a Q-switched Nd:YAG laser ()~ = 1064 nm). They found that the plasma on the second pulse was useful to monitor the element composition of the metals. Calibration curves for Cr, Cu, Mn, and Si in steel were prepared by analyzing a set of certified steel reference standards positioned under water. The detection limits of 367,520, 1200, and 1190 ppm were obtained for Cr, Cu, Mn, and Si, respectively. Arca et al. (1997) checked the feasibility of quantitative determination of trace element concentrations in water by LIBS. The Nd:YAG laser beam was focused on the free surface of the water sample and then collected the plasma emission by an optical system coupled with an optical fiber to a 1 m spectrometer. The concentrations for the main components of water (Mg, Ca, C1, Na, Si, K) determined by LIBS were in very good agreement with those from standard chemical analysis, thus confirming the reliability of the LIBS method for precise quantitative analysis of traces in water up to a few parts per million. Table 5 summarizes the applications in this section.

C. Environmental and Gaseous Samples Recent applications of LIBS are more focused towards the elemental analysis of environmental and industrial samples. LIBS has been applied to elemental analysis in environmental samples such as air, soil, sewage sludge, and solutions. Among the applications of a growing interest are in situ analysis, on-line monitoring, and remote sensing of the pollutants or specific elements. Fiber-optic-based LIBS has the potential of becoming standard equipment in these applications. In addition, much effort is devoted to develop a portable and sensitive LIBS instrument for monitoring or analysis as necessary. Radziemski's group (1981) was one of the pioneers in this research area, as was the case in previous section of this review on metallurgical applications of LIBS. They detected phosphorus atoms from diisopropylmethyl phosponate in air and C12 in air. A concentration of 690 ppm of P was detected. The projected detection limit was 60 ppm for C12 and 15 ppm for P. This group also used LIBS to analyze F 2 and C12 in air by monitoring emission wavelengths at 837.6 nm for C12 and 685.6 nm for F 2. Minimum detectable concentrations of C12 and F 2 in air were 8 and 38 ppm (w/w), respectively. Minimum detectable concentrations of C12 and F 2 were, respectively, 80 and 200 ng in air and 3 ng for both atoms in He. The precision for replicate sample analysis was 8% (Cremers and Radziemski, 1983b). Detection of Be was one of the specific topics for application of LIBS because it is known to be a significant inhalation health hazard to workers exposed to dusts of Be and its alloy. Radziemski et al. (1983a) demonstrated the development of a system which could routinely monitor concentrations of Be in air. They published the results for the determination of Be, Na, P, As, and Hg in air. Beryllium in

Table 5. Application of LIBS on Liquid, Colloid, and Biological Samples Elements

Particulate iron FeO(OH)

Matrices

Water

Li+, Na+, Ca2+, Aqueous Ba2+, pb2+, solution Cd 2+, Hg2+, Er3*

Object of Analysis

Laser Used

Quantitative detection

Nd:YAG

Qualitative detection

Excimer pumped dye laser

Process control

Nd:YAG

Range of Concentration (%)

Solution

Ca+, Mg § Na+, K+ Cd, Pb, Zn

Solution Aerosols

Li, Na, K, Rb, Cs, Solution Be, Mg, Ca, B, AI

Nd:YAG Quantitative detection

Nd:YAG

Elemental analysis

Nd:YAG

RSD (%)

Cd (I 5-80) Pb (3-50), Zn (8-I 00)

Ca (200 ppm) Mg (25 ppm) Cd (0.019), Pb (0.21), Zn (0.24) Li (0.006), Na (0.014), K (1.2), Rb (0.2), Cs (1.0), Be (10.0), Mg (100.0), Ca (0.8), B (80.0), AI (20.0)

Reference

Nakamura et al. (1996)

single pulse(0.6) sequential pulse(0.016) Cd2+(500 mg/L), pb2+(12.5 rag/L), Na+(7.5 lig/L), Li+(13 l~g/L), Ba2+(6.8 rag/L), Ca2+(0.13 mg/L) 0.1 g/L

"4

Uranium

LOD (ppm)

Knopp et al. (1996)

Watcher et al. (1987) Boiron et al. (1991) Essien et al. (1988) 4-8

Cremers et al. (1984)

Laser-Induced Breakdown Spectrometry

273

atmospheric pressure air had been detected at 0.7 mg/m 3, which is 0.6 ng/g of air (RSD = 30%). Limit of detection for Na, P, As, and Hg were estimated as 0.006 lxg/g, 1.2 Ixg/g, 0.5 lxg/g, and 0.5 lxg/g, respectively. The observed limit of detection for these elements are comparable with those oflCP-AES. Detection limits of LIBS for Be was comparable with that of ICP-AES, but ICP-AES showed a much better detection limit for other elements, especially for Hg (0.5 lxg/g for LIBS, 0.0006 Ixg/g for ICP-AES). Direct determination of Be on filters was also tested by this group (Cremers and Radziemski, 1985) because a direct monitoring of air with LIBS can determine Be concentration down to only one-third of 2 lxg/m3 which is the U.S. standard in Be monitoring. In this study the calibration of the detecting system was performed by depositing a known Be mass on the surface of the filter by a nebulizer/heat-chamber apparatus or by passing an aqueous suspension of the particles, which is prepared by positioning Be metal under water and ablating material off with a focused laser beam; through the filters, or by passing a certain volume of the well-stirred suspensions through the filters after mixing of a known mass of Be particles. The surface detection limit for particles 0.5-5 mm in diameter was 0.45 ng/cm 2, which corresponds to 3.6 ng total Be mass on the exposed area (32 mm diameter) of a 37 mm diameter filter. The precision for replicate sample analysis was 4%. They also detected Na and K in a coal gasifier product stream, airborne Be and P, S, and CI2 in various organic molecules. The detection limit for Be in air was measured as 0.5 ng/g, which is one-third of the OSHA limit for the 8 h average exposure to Be (Radziemski et al., 1983b). Approximate linear working curves were obtained over the concentration range of 5-20 lxg/g. Chemical warfare agents are another area of interest and potential for LIBS with its detection limit of ppm or ppb obtained by Quigley et al. (1981) in Radziemski's group. Loree et al. (1982) demonstrated the capability of LIBS for remote probing into hostile environments. Potential applications included prospecting, oil shale analysis, and pollution monitoring. Radziemski's group also published results on the real-time detection in monitoring of coal gasifiers (Hartford, Jr. et al., 1983). Further efforts on real-time LIBS application on particulates in combustion environments were made by Ottesen et al. (1989a, 1991). They developed an instrument which could determine the size of particles by measuring scattering and determine the composition of particles by LIBS. In this study, quantities of 520 femtogram (fg) of Fe were detected. The overall detection limit for trace elements including Si, AI, Ti, Ca, and Na in single coal particles were estimated at below 100 ppm. Casini et al. (1991) proposed a TRELIBS system for the quantitative determination of small amounts of pollutants in gas. Flower et al. (1994) applied LIBS technology to continuously monitor metal aerosol emissions in industrial process vents (e.g. exhaust stacks from electroplating baths), waste treatment processes (incinerators), and boilers and industrial furnaces (coal-fired power plants). The approach used to measure total Cr concentrations in laboratory simulations of

274

YONG-ILL LEE and JOSEPH SNEDDON

electroplating aerosols was also described in that report. Chronium concentrations less than 1 mg/cm 2 had been measured. This work formed the basis for a future application to incineration and fossil power plants. Singh et al. (1995) and Zhang et al. (1997) used LIBS to monitor the concentration of toxic metals in the off-emission of waste-processing facilities. Toxic metal concentration measurements from three plasma torch test facilities had been used to evalutate the possiblity of employing LIBS as a process monitor. Special attention was given to the spectral region with Pb, Cr, Cd, and Ce atomic transitions. Typical LIBS spectra at a central wavelength of 360 and 412 nm are shown in Figure 11. Nordstrom (1995) applied LIBS to evaluate the spectral characteristics of the interference from the nitrogen and oxygen components in air by using a CO 2 laser. Of primary interest in this study was the atomic and molecular origin of the emission 350 300 A

= 250

~9m 200 Cr

_.=~ 150

Pb

IO0 5O 0 352

'P-

~

i

w

357 362 Wavelength (nm)

367

400 350 A 300 ~9m 250 cln

-~

~ 200 150

M=

K

100 50 0 400

i~= w

405

9

_

__JL

1=r ~... l

410 415 Wavelength (nm)

420

Figure 11. Typical LIBS spectrum of off-gas from a Westinghouse Savannah River Company (WSRC) surrogate feed at the central wavelength of (a) 360 nm, and (b) 412 nm. From Singh et al. (I 997), with permission.

Laser-Induced Breakdown Spectrometry

275

features. Comparison of the LIBS spectra with NIST atomic emission data was presented in this paper. Davies et al. (1995) applied optical-fiber-based LIBS using a Nd:YAG laser system to determine concentrations of Cr, Cu, Mn, Mo, Ni, and V in NIST standards up to distance of 100 meters. In this investigation, fused optical fiber with a core diameter of 550 lxm was chosen to deliver the laser beam and detect the emission signal. The measured concentration of elements was approximately 200 ppm. Relative detection limits (3-a error) were estimated as 150 + 50 ppm for Cr, 100 + 30 ppm for Cu, 210 + 70 ppm for Mn, 200 + 50 ppm for Mo, 150 + 50 ppm for Ni, 380 + 90 ppm for Si, and 200 + 50 ppm for V. In this report they pointed out several potential sources of error in the LIBS technique such as inhomogeneous distribution of trace elements in the target and variation in the alignment of the light-collecting optics. Marquardt et al. (1996) performed on-site determination of leaded paint in houses by LIBS with 532 nm from a Nd:YAG laser. The measurement took less than 1 min to perform, required no sample preparation, and could be made through overlayers of non-lead-containing paint. The limit of detection was 0.014% Pb in latex paint, on a dry weight basis, with relative sample standard deviation of 5-10%. Figure 12 shows a typical LIBS spectrum of latex paint sample containing 3.05%, added as Pb(NO3) 2. Figure 13 illustrates the Pb response, added as Pb(NO3) 2 latex paint 1400

-

1200

-

Pb

"e ~ 1000 -,4

~ 0

i

800

9

600

400 200 1 1

398

,,

I

400

.

I

.

402

wavelength

.

.

I

.

404

(run)

.

.

I

406

Figure 12. LIBSspectra of 3.05% Pb, added as Pb(NO3)2, in latex paint with the 600 I~m/600 I~m fiber-optic probe using 4.2 mJ of 532 nm excitation at the sample. Both spectra were measured with 100 laser pulses (e.g. 50 s acquisition). (a) Leaded paint. (b) Leaded paint with two overlayers of non-lead-containing paint approximately 0.26 mm thick. From Marquardt et al. (1996), with permission.

276

YONG-ILL LEE and JOSEPH SNEDDON 200 -

'~9 1 5 0 II

i 9, 4

100-

U

.~

50

-

_

l

0.00

I

0.02

I

0.04

,,,

I

0.06

concentration of lead

I

0.08

(~

I

0.10

w/w, dry basis)

Figure 13. LIBS calibration curve showing the intensity of the 405.78 nm Pb line versus dry weight concentration. (w/w%) of Pb, added as Pb(NO3)2, in latex paint. From Marquardt et al. (1996), with permission. paint samples versus concentration below about 0.11% measured with the 600 mm probe and using 4.2 mJ of 532 nm excitation. The precision of these measurements, as measured by +1 sample standard deviation, was 5-10%. The limit of detection, 0.014% Pb (w/w dry weight) was calculated from the calibration plot. Arnold and Cremers (1995) presented the results of the investigation on T1 particles on filters. They detected T1 particles (< 20 mm diameter) in less than 1 ,fin by LIBS. Thallium was detected by forming a series of laser sparks across the filter surface. The detection limit for T1 was 40 ng/cm 2 on a filter surface. The useful dynamic range extended from 0.2 to 40 btg/filter using the strong 535.05 nm neutral T1 emission line. Based on this investigation, LIBS has the potential to provide a method for rapidly analyzing multiple samples on-site at a low cost. Determination of metals in the environment using a portable LIBS instrument was performed by Yamanoto et al. (1996) to monitor Pb in paint, metals in soils, and Pb and Be particles collected on filters. The authors primarily focused their research to evaluate the portable LIBS which was equipped with a compact laser of low-pulse energies (10-20 mJ/pulse) and repetition rate (<1 Hz), for a wide range of samples. Detection limits in ppm for metals in soils were 265 (Ba), 9.3 (Be), 298 (Pb), and 42 (Sr). The detection limit for Pb in paint was 0.8% (8000 ppm), corresponding to 0.052 mg/cm 2. Direct field screening in the environment for metal contamination with a portable LIBS system can significantly reduce the time and costs associated with the sample collection, transportation, and preparation steps required by conventional laboratory methods. The authors also discuss, in detail,

Laser-Induced Breakdown Spectrometry

277

on the advantages of LIBS for environmental applications compared to X-ray fluorescence spectrometry (XRF) which is currently the method of choice for many types of field screening measurements. Poulain et al. (1995) generated a laser-produced plasma (LPP) in an aerosol spray and the emission wavelengths of Na and Ha were monitored with an optical multichannel analyzer (OMA) to characterize the plasma spatially and temporally. The electron temperature of the plasma induced by an excimer laser was estimated to be 12,600 + 4600 K. A calibration curve relating the Na(I) (589 nm) to Ha (656.3 nm) intensity ratio as a function of Na concentration ranging from 100 to 10,000 ppm was constructed. The limit of detection for Na by the current method under the experimental conditions was estimated to be approximately 165 ppm for monodisperse sprays and 925 ppm for one case involving a polydisperse spray. They also observed that the size of droplet strongly influenced the observed emission intensity ratio. Recently, Eppler et al. (1996) studied the effect of chemical speciation and matrix composition on Pb and Ba measurements in sand and soil matrices with the use of LIBS. The measured detection limits for Pb and Ba spiked in a sand matrix were 17 and 76 ppm (w/w), respectively. In spiked soil, the detection limits were 57 and 42 ppm (w/w) for Pb and Ba, respectively. Calibration curves were linear over 2 orders of magnitude for both elements, and measurement precision was highest at 2.3% RSD when a cylindrical lens was used. Both Ba(II) and Pb(II) emissions were dependent on analyte speciation (e.g. nitrate or oxide) in the sample. Wisburn et al. (1994) determined selected heavy metals in soils, sand, and sewage sludge samples by TRELIBS. Various factors such as aerosol production, crater formation, size effect, timing effect, laser intensity, and humidity affecting the detection limits and the quality of the analysis were investigated. Detection limits for several metals (Zn, Cr, Pb, Cu, Ni, and Cd) are in the 10 ~g/g range, which are usually below the ecological requirements. Vadillo and Laserna (1996) used TRELIBS to obtain the spectra of minerals from different families including sulfides, vanadates, and silicates, and demonstrated the capability to conduct geological taxonomy to analyze a field sample. Detection of heavy metals had been performed in different standard soil samples by using a mobile lidar LIBS system. Barbini et al. (1997) had developed the semiquantitative determination method of several metallic elements in different soils even at lowresolution LIBS spectra by the use of an internal standard reference method. For most of the elements in soils, the detection limit proved to be 10-100 ppm, depending on the matrix. As was mentioned earlier in this review, application of LIBS to environmental samples are receiving more and more attention from researchers. There have been several efforts to develop a portable LIBS instrument and the results were somewhat successful. Further development of a portable instrument will result in a reliable and compact on-site analytical tool in the very near future. Representative examples of the application of LIBS to environmental samples are summarized in Table 6.

Table 6. Application of LIBS on Environmental Problems

Elements

",,I O0

Matrices

Object of Analysis

Ba, Be, Pb, Sr

Soil

Pb, Be, Pb

Paint collected in filter Paint In situ analysis

Pb Ca, Fe, AI, Ti, Sr

Preliminary evaluation of portable instrument

LaserUsed

Range of Concentration (%)

Nd:YAG

LOD (limit of detection,ppm) Ba(265), Be(9.3), Pb(298), Sr(42) Pb(800) Be(21-63 ng/cm 2) Pb(5.6 IJ.g/g) 14

Nd:YAG

MHD coal-fired flow facility Aerosol (Zn), soil (Cd) Ferrous specimens

Pollution monitoring Evaluation of absolute analysis Trace analysis

Nd:YAG

Zn (10-12 g), Cd (18)

Nd:YAG

200

N2, 02

Air

Identification

TEA CO 2

Na, H

Aerosol spray

Quantitative analysis Pollution monitoring

Excimer (KrF)

Zn, Cd Cr, Cu, Mn, Mo, Ni, Si, V

Diisopropylmethyl Phosphonate in air

Nd:YAG

Nd:YAG

0.1-10

Monodisperse (165) Polydisperse (925) P(15)

RDS (%)

Reference Yamamoto et al. (1996)

Marquardt et al. (1995) Zhang et al. (1995) Xu et al. (1997) Davies et al. (1995) Nordstrom et al. (1995) Poulain et al. (1995) Radziemski et al. (1981)

Cl Be

Air Filters

Monitoring

Nd:YAG

C1(60) 3.6 ng/32 mm

Si, AI, Ti, Ca, Na

Alumina

Trace analysis

Nd:YAG

100 ppm, Fe(520 fg)

F, CI

Air

Pollution monitoring Pollution monitoring

Nd:YAG

C1(8 ng), F(3 ng)

Nd:YAG

Be(0.6 ng/g), Na(6 ng/g) P (1.2 ng/g) Hg (0.5 ~g/g)

CH4-air flame, shale oil, CH 4 [74-82-8]-air jet Soil

Pollution monitoring

Nd:YAG

Pollution monitoring

Nd:YAG

Pb, Ba

Soil and sand

Pollution monitoring

Nd:YAG

Zn, Cr, Pb, Cu, Ni, Cd

Soil, sand, sewage sludge

Pollution monitoring

Nd:YAG

Be, Na, P, As, Hg Air

CH 4

As, Ba, Be, Cd, Cr, Pb, Sr, Zr, Hg, Sb

Pb (3601080 ppm) Ba (0.1-1.0)

30

As(1500), Ba(63), Be(2), Cd(73), Cr(23), Pb(14), Sr(24), Zr(86) Pb (17 in sand, 57 <10% in soil) Ba (76 in sand, 42 in soil) Zn (30), Cr (10), Pb (10), Cu (20), Ni (20), Cd (3O)

Cremers et al. (1985) Ottesen et al. (1989) Cremers et al. (1983b) Radziemski et al. (1983b) Schmieder et al. (1981) Koskelo and Cremers (1994) Eppler et al. (1996) Wisburn et al. (1994)

280

YONG-ILL LEE and JOSEPH SNEDDON 2I

Time/min 4I 6 , I

.,

15 000

,..,'"""..,... .

r

."

,. 10000-

8I

10

....

........................ iiiiii . . . . "'........ . . . . . . ..."

. / - - Cu (327.4 nm) .

357.8 nm) ../

Zn (334.5 nm)~.//

L_

c 9 5000

(9 t,-

\,,..-:

..""

,. . . . . .

..:

Ni (341.4 nm) /

..... " .............

,.....

,

I i

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

150

300 450 Laser shots

600

750

Figure 14. Variation of emission profiles of Cr, Ni, Cu, and Zn lines with depth. Sample consisted of a commercial brass (typically a zinc-copper alloy with minor percentages of different elements). Copper, nickel, and chrome platings were deposited following an electrochemical process. From Vadillo and Laserna (1996), with permission.

D. Advanced Materials and Others One of the potential uses of LIBS in the field involves depth-profiling of surface coatings. The composition of each individual layer, particularly at the interface, is much more informative than the composition of the average over a range of depths. Hidalgo et al. (1996) investigated the emission spectra of a laser-generated plasma from titanium dioxide antireflection coatings in solar cells. A method for measuring time, TiO 2 films between 40 and 400 nm within the typical values used in solar cells based on the LIBS technique was developed. A pulsed nitrogen laser at 337.1 nm was used with a pulse width of 10 ns and a laser fluence of 8.6 J/cm 2 on the sample. The capability of LIBS to resolve complex depth profiles was also demonstrated by Vadillo and Laserna (1997) using electrolytically deposited brass samples. Ablation depths of 6.5 ng per pulse were obtained, which imply absolute detection limits of the order of fg per pulse for an element present in the sample at a concentration of a few ppm. The results of the depth profiles of the brass sample produced by LIBS are shown in Figure 14. As shown, the intensity profile corresponds exactly to the spatial pattern of the sample (Cr, Ni, Cu, and Zn from outer to inner layer).

Laser-Induced Breakdown Spectrometry

281

Anglos et al. (1997) employed the LIBS technique for the in situ analysis of pigments used in painting. Appropriate emission wavelengths for the identification of the metallic elements in the pigments examined were proposed. Furthermore, a test of an 18th century oil painting was examined by LIBS and the different pigments used in the original and in the restored part of the work were clearly identified. The LIBS technique has also been applied as a real-time diagnostic technique for the laser cleaning of a natural marble surface by Maravelaki et al. (1997). They demonstrated that LIBS can be used for the on-line control to the extent of laser cleaning at each spot of the surface by analyzing the spectra of the plasma emission. Lee and Sneddon (1994a) analyzed potassium (I) in solid glass by observing the emission wavelength at 764.6 nm in the laser-produced plasma. The detection limit for K in solid glass was estimated as 0.13 ~tg/g and the precision was estimated as +10%. Ciocan et al. (1993) applied laser-ablated microwave-induced plasma atomic emission spectrometry (LA-MIP-AES) to the direct determination of trace elements (Mg, A1, Si, and Fe) in a high-temperature superconducting ceramic (YBa2Cu3OT) and the determination of low Na concentration in high-purity natural and synthetic quartz used for the production of optical fibers. They used internal standardization for the normalization of emission signal and calibrated with relevant spectra of standard reference samples (A1, Cu, borax glass). The measured concentrations for elements (Si, Fe, A1, Mg) by LA-MIP-AES and ICP-AES were compared to that of LIBS. Both methods revealed very similar results (e.g. for Si, 2670 + 330 ~g/g by LA-MIP-AES and 3300 + 300 I.tg/g by LIBS). The concentrations for other elements in YBa2Cu30 7 were 114 + 40 Ixg/g for Fe, 114 + 8800 l.tg/g for A1, and 26 + 5 btg/g for Mg. For A1, a strong inhomogeneity was found in different spots of the target. This resulted in the wide variation in this result. In this study internal standards such as a known concentration of Cu were used for the determination. The concentration of Na on natural and synthetic quartz was 0.28 to 2.0 ~g/g. The detection limit for Na was estimated as 40 ng/g by adopting the square root of 3-o. Although the precision of the measurement was not as good as ICP-AES, it was comparable to that of LA-MIP-AES. Uebbing et al. (1991) applied a Nd:YAG laser (~, = 1064 nm, 13 mJ) as an atomizer coupled with another Nd:YAG laser (~, = 1064 nm, 115 mJ) for reheating the LIP to increase the analyte wavelength intensifies and to improve the detection limit. Calibration curves for A1 and Mn in glass and steel samples and for Mg and Mn in glass, copper, and aluminum were obtained by internal standardization. Generally, detection limits of these elements were in the higher g/g range. However, detection limits were 4 to 10 times better than those from experiments without reexcitation. Ottesen (1992) applied the LIBS technique to analyze the elemental composition of contaminants found on electronic microcircuits fabricated on alumina substrates and obtained spatially resolved data with some degree of depth profiling informa-

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YONG-ILL LEE and JOSEPH SNEDDON

tion. Kakawa et al. (1982) detected B and Si in borosilicate glass and Cr in iron-steel samples by using N 2 laser-based LIBS. Kurniawan et al. (1995) in Kagawa's group, used an XeC1 excimer laser and TEA-CO 2 laser for the detection of Li, Be, Na, Mg, A1, K, Ca, Ti, Zn, Zr, and Ba in glass. The detection limit for Li in glass was estimated at 10 ppm by using the emission wavelength observed at 670.7 nm. Detection limits for other elements were dependent on the buffer gas pressure. Detection limits at 1 torr of buffer gas pressure were 30 ppm for B(I), 14 ppm for Na(I), 130 ppm for Mg(I), 54 ppm for AI(I), 190 ppm for K(I), 85 ppm for Ca(I), 410 ppm for Ti(I), 160 ppm for Zn(I), 290 ppm for Zr(I), and 180 ppm for Ba(I). The detection limit required for ordinary glass analysis were 100 ppm and the obtained results were good enough for the real sample applications. Franzke et al. (1992) investigated minor components contained in the naturally occurring minerals by using a UV laser pulse as an ablation source (370 nm). The minerals investigated in this study included FeS, ZnS, PbS, ZrO 2, PbCrO 4, K2S, Sb2S 3, and Mo2S 3. For the more sensitive elements, such as Fe and Pb, the detection limit was estimated at approximately 10 ppm or 35 mg/cm 2. Cheng et al. (1991) applied LIBS for the determination of trace concentrations of column II and V hydrides, such as phosphine (PH3), arsine (ASH3), and diborane (B2H6). These three gases are widely used in the semiconductor industry and the toxicity of each has impeded the development of diagnostic procedures. In MOCVD (metal organic chemical vapor deposition) Ge impurity is introduced to the reactor in the form of germane (GeH4) as a contaminant in AsH 3. Detection limits of the order 1 ppm were estimated. Stoffels et al. (1991) detected time-resolved emission from laser-ablated uranium by using a Q-switched Nd:YAG laser (~. = 1064 nm). They found the dependence of the emission intensity on time was strongly affected by the nature and pressure of the buffer gas. Jensen et al. (1995) performed mechanistic studies of LIBS to model environmental samples to detect the cation content of a physical mixture of Eu203 and K2CrzO7 with granular, crystalline SiO 2. Excitation of the powered specimens was focused by a KrF excimer laser light (~, = 248 nm, 30 ns pulse width, fluence/pulse 0.3-30 J/cm 2, irradiance 10-100 Mw/cm 2) in one atmosphere of air. The detection limit for the quartz matrix for their detection system were 100 ppb for Eu and 2 ppb for Cr. They claimed these limits could be easily lowered by a factor of 10 or more through more efficient light collection alone. Experimental studies on laser-ablated ZrC was performed by Wantuck et al. (1992) for the investigation of fuel corrosion diagnosis in nuclear fuel. Monitoring of the fuel corrosion products is important not only for understanding corrosion characteristics, but to assess the performance of an actual, operating nuclear propulsion system. Table 7 summarizes results on the analysis of trace elements in non-metal samples by LIBS.

Table 7. Application of LIBS on Advanced Materials and Related Fields Elements

co

Matrices

Pb, Zn, Fe

PbS, FeS2, ZnS

B, Si, Cr K

Borosilicate Glass, Iron-steel Solid glass

EU203, K2Cr20 7

Crystalline SiO 2

B, P, AS

B2H6, PH3, AsH 3

Range of Object of Analysis LaserUsed Concentration(%) Surface analysis

Quantitative analysis Trace analysis

Excimer (XeCl) N2

Nd:YAG

B2H6(0.001 --0.1 ), ASH3(0.001-0.1), PH3(0.05--0.4) Mg(0.97-20)

Zr

ZrC

Mg, AI, Fe, Si

Yb2Ca30 7 superconductor

Fuel corrosion diagnosis Material analysis

Sn, As

Metal hydrides of Sn, As Glass, s t e e l , Evaluationof Nd:YAG copper, aluminum reheating Microcircuits Elementalanalysis Nd:YAG of surface contaminants

AI, Mn, Mg AI, Cr, Ca, Mg, Si, Bi

Cr(0.1-1) K(0.03-0.461 )

Trace analysis

TEA CO 2

RSD (%)

Pb (10 ppm)

Excimer (ArF) KrF

B, Na, Li, Mg, AI, Si glass K, Ca, 15, Zn, Zr, Ba

LOD

Cu in Fe (50 ppm), Mg in AI (50 ppm) K(0.13 ppm) Eu(0.1 ppm) Cr(0.002 ppm) B2H6(1 ppm), PH3(3 ppm), ASH3(1 ppm) B(30 ppm), Na(14), Li(10), Mg(130), A1(54), K(19), Ca(85), Ti(410), Zn(160), Zr(290), Ba(180) Si (3300 ng/g), Fe (114 ng/g), AI (114 ng/g), Mg (26 ng/g) Na (40 ng/g) higher ~g/g range

10

Reference Franzke et al. (1992) Kagawa et al. (1982) Lee et al. (1994) Jensen et al. (19~ Cheng et al. (1991 ) Kurniawan et al. (1995)

Wantuk et al. (1992) Ciocan et al. (1993)

Singh et al. (1996) Uebbing et al. (1991) Ottesen (1992)

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YONG-ILL LEEand JOSEPHSNEDDON V. C O N C L U S I O N A N D FUTURE PROSPECTS

The present chapter has discussed and reported on select applications of LIBS. LIBS can be applied to the analysis of trace elements in any matrix such as gas, liquid, solid, nonconducting metals, nuclear samples, chemical warfare samples, environmental samples, and biological samples without complicated sample preparations. All these results of the investigation will be a base for the development of the LIBS technique and the application of LIBS as a routine sensitive analytical technique. We have attempted to describe the fundamental aspects of a laser-induced plasma and the interaction between a laser beam and a target, advanced LIBS analytical techniques, instrumentation, and some selected applications which have recently been developed. In general, there is no indication that the LIBS technique can match the detection limits and precision achieved by conventional analytical techniques at this time. However, the reported overall precision is satisfactory (5-20%). The major advantages of LIBS would appear to be its ability to directly analyze solid samples with no sample pretreatment. LIBS is particularly suitable for application to real-world problems since optical fibers can be used to transport the laser pulse to the sample and the emission signal to the spectrometer, permitting in situ analysis of relatively inaccessible samples (e.g. painted surfaces). When considering the recent developments and instrumentation described above, the future of LIBS lies with the development of a fully automated portable system, with data acquisition facilities, for remote, on-line and in situ analysis of versatile samples from refractive solid to biological samples.

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INDEX

Detectors, 145-172 charge transfer devices, 150-162 absorption and spectral imaging, 169 charge-coupled devices, 151-158 conventional devices, 151-156 segmented array, 157-158 charge-injection devices, 158-162 comparison of charge transfer devices and conventional photomultipiers, 163-164 multielement detectors for future, 169-170 past, 146-147 readout and measurement, 164-169 simultaneous multichannel, 147-150 Glow discharge, 173-234 applications, 204-225 liquids by hollow cathode discharge, 222-224 liquids by planar glow discharge, 220-222 gases by hollow cathode discharge, 225 gases by planar glow discharge, 224-225 metals and alloys by hollow cathode discharge, 213

metals and alloys by planar cathode discharge, 204-213 non-conductive solids by planar glow discharge, 214-219 non-conductive solids by hollow cathode discharge, 219-220 future, 225-226 historical, 174-176 hollow cathode glow discharge, 196-204 atomic spectrometry, 196-203 mass spectrometry, 203-204 mechanism elucidation, 177-184 fundamental aspects of hollow cathode discharges, 182-184 fundamental aspects of planar glow discharge, 177-182 planar cathode glow discharges, 184-196 atomic spectrometry, 184-191 mass spectrometry, 191-196 present trends, 176-177 Lasers, 99-143 commercial optical parametric oscillator lasers, 110-113 diode laser, 133-139 basic types, 133-135 characteristics, 135-139 compactness, 135

289

290

extension of wavelength tuning range, 137-139 narrow linewidth, 135-136 wavelength modulation, 139 wavelength stabilization, 139 wavelength tunability, 136-137 optical parametric oscillator, 102-110 angle phase matching, 107-109 energy and momentum conservation, 106-107 fundamentals, 103-110 output energies of laser pulses, 113-115 pulse-to-pulse energy stability, 115-116 rapid scan speed, 118-119 sensitivity to ambient temperature, 119-120 signal spectral linewidth, 116-118 three-photon process, 105-106 wavelength calibration accuracy, 118 solid-state lasers, 121-133 tunable, 121 vibronic lasers, 122-132 alexandrite 122-123 fundamentals, 122 other types, 133 Ti:sapphire, 123-132 Laser-induced breakdown spectrometry, 235-288 analytical LIBS techniques, 257-260 applications, 260-283 advanced materials, 280-283 environmental samples, 271-280 gases, 271-280 liquid samples, 265-271 metallurgical and solid samples, 260-265

INDEX

basic principles, 250-251 fundamentals, 237-249 ambient conditions, 245-246 electric and magnetic field, 246-247 electron densities, 247-250 excitation temperature, 247-250 factors influencing plasma production, 241interaction of laser with target materials, 237-239 irradiation energy, 242-243 irradiation wavelength, 241-242 laser-induced plasma production, 240-241 laser parameters, 241-247 physical properties of target material, 243-244 instrumentation, 251-257 advances in, 257 ablation chambers, 252-253 detectors, 254-256 dispersion systems, 254-256 laser systems, 251-252 optical fiber systems, 255 Speciation, 1-98 biological fluids, 15-40 blood, 18-20 aluminum, 19-20 arsenic, 22-23 chromium, 21-22 copper, 23 iron, 23-24 lead, 24 mercury, 20-21 platinum, 24 selenium, 21 silicon, 24 zinc, 23 milk, 28-30 urine, 25-28 arsenic, 25-26

Index

mercury, 27 other metals, 27-28 selenium, 26-27 factors affecting, 6-8 importance of, 4-6 metals, studies of, 57, 85 aluminum, 58, 67-68 antimony, 68-69 arsenic, 70-71 cadmium, 71-72 calcium, 72 chromium, 72-74 copper, 74-75 germanium, 75-76

29i

iodine, 76 iron, 76-77 lead, 77-79 mercury, 79-81 nickel, 81 platinum, 81 selenium, 82-83 tin, 83-85 zinc, 85 solid samples, the challenge biological and food, 49-56 miscellaneous, 56-57 soil and sediments, 41-49 waters, 8-15

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