Selective Gas Chromatographic Detectors

JOURNAL OF CHROMATOGRAPHY LIBRARY - Volume 36

selective gas chromatographic detectors

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JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 36

selective gas chromatographic detectors M. Dressler Institute of Analytical Chemistry, Czechoslovak Academy of Sciences, Leninova 82, 61142 Bmo, Czechoslovakia

ELSEVIER Amsterdam - Oxford - New York - Tokyo

1986

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, NY 10017, U.S.A.

Library of CO'lgress Cataloging-in-PubIiClltion Data

Dressler, M., 1940Selective gas chromatographic detectors. (Journal of chromatography library ; v. 36) Includes bibliographies and index. 1. Gas chromatography. I. Title. II. Series.

QD79.c45D74 1986 ISBN 0-444-42488-1

543'.0896

86-13366

ISBN 0-444-42488-1 (Vol. 36) ISBN 0-444-41616-1 (Series)

© Elsevier Science Publishers B.V., 1986

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./Science & Technology Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. Printed in The Netherlands

CONTENTS Journal of Chromatography Library (other volumes in the series) Preface 1. Introduction References .

IX XIII

3

2. Basic terms relating to detectors 2.1. Detector sensitivity 2.2. Minimum detectability 2.3. Detection limit. . . 2.4. Detector noise 2.5. Dependence of detector response on amount of compound 2.6. Selectivity of response References . . . . . . . . • • .

5 5 6 7 8 10 12 13

3. Alkali flame-ionization detector 3.1. Introduction . . . • . . . 3.2. Detector design. • . • . . 3.3. Detector life, reproducibility of response 3.4. Background current (hydrogen flow-rate) 3.5. Negative response. . • . . . . • . . . . . 3.6. Response to individual heteroatoms 3.7. Influence of compound structure on detector response 3.8. Influence of main operational parameters on detector response 3.9. Detection mechanism References . . . '.

15 15 16 23 24 29 33 41 43 54 59

4. Flameless alkali sensitized detectors 4.1. Introduction . . . • • • • . 4.2. The Perkin-Elmer detector .• 4.3. The Hewlett-Packard detector 4.4. The Tracor detector . . . . . 4.5. The Varian detector . . . • . 4.6. The Detector En~ineering Technology detector 4.7. The chemi-ionization detector • . • • . . • . 4.8. Detector life and reproducibility of response 4.9. Detectors for halogen compounds References • . . . • . . . • • • • • • • . . . . .

63 63 64 72

73 74 78 83 84 87 90

VI

5. Flame-ionization detector . • • . . • • • . • . • . 5.1. Introduction . . . . . . • . . . • . . . . . . 5.2. Hydrogen atmosphere flame-ionization detector 5.3. Hydrogen atmosphere flame-ionization detector for silicon compounds 5.4. Flame-ionization detector with hydrocarbon background 5.5. Selective detection of halogen compounds References

91 91 92

102 105 106 106

6. Photoionization detector 6.1. Introduction. 6.2. Response model 6.3. Sensitivity of response and minimum detectability 6.4. Selectivity of response 6.5. Carrier gas References

109 109 111 112 118 126

7. Flame photometric detector 7.1. Introduction . . . . . 7.2. Response model . . . . 7.3. Detector sensitivity and minimum detectability 7.4. Selectivity of response . . 7.5. Tin and germanium compounds 7.6. Halogen compounds • . . . . 7.7. Other detection possibilities 7.8. Linearity of response 7.9. Sulphur background. 7.10. Response quenching 7.11. Flame stability . . 7.12. Other identification possibilities References ••. . . . . .

133

131

133

136 137 144 145 147 149 150 152 152 157 157 158

161 8. Chemiluminescence detectors 8.1. Introduction . . . . . 161 161 8.2. Detector for N-nitroso compounds 169 8.3. Detector for nitroaromatic compounds 170 8.4. Detector for nitrogen-containing compounds 8.5. Ozone chemiluminescence detector for compounds not containing 174 nitrogen . • . . . . . . • • • . . . . . . . . 174/312 8.5A. Redox chemiluminescence detector . . . . . . 174 8.6. Chemiluminescence detector with sodium metal 177 8.7. Fluorine-induced detector 179 References

VII

9. Electrolytic conductivity detector 9.1. Detector construction. 9.2. Selectivity of response 9.3. Response 9.4. Solvent . . 9.5. Gases . . . 9.6. Temperature References . . .

181 181 186 189 196 201 203 206

10. Coulometric detector 10.1. Introduction. 10.2. Response . . . 10.3. Quantitative results References . . • • • . • •

209 209 210 212 215

11. Electron-capture detector 11.1. Introduction . . . 11.2. Design . . . . . • 11.3. Sources of primary electrons 11.4. ~lethods of measuring detector current 11.5. Response theory . • . 11.6. Response . . . . • . • . • . • . . . 11.7. Linearity of response . . . . . . . . 11.8. Selective electron-capture sensitization 11.9. Coulometric and hypercoulometric response 11.10. Use of the electron-capture detector with capillary columns References . . . . . .

217 217 218 219 224 230 235 249 251 263 266 269

12. Ion mobility detector 12.1. Introduction .. 12.2. Principle of the technique 12.3. Detection principles 12.4. Effect of background References • • . • • • .

275 275 275 279 286 288

13. Miscellaneous detectors 13.1. Introduction • . • • . . . • 13.2. Plasma-emission spectrometry 13.3. Atomic-absorption spectrometry 13.4. Ion-selecti~e electrodes •.• 13.5. Piezoelectric sorption detector 13.6. Mass and infrared spectrometry References . . . . • . . • . . . • . .

291 291 291 294 294 295 296 305

VIII

14. Conclusion References

311 311

List of abbreviations

313

Subject index . . . .

315

IX

JOURNAL OF CHROMATOGRAPHY LIBRARY

A Series of Books Devoted to Chromatographic and Electrophoretic Techniques and their Applications Although complementary to the Journal of Chromatography, each volume in the Library Series is an important and independent contribution in the field of chromatography and electrophoresis. The Library contains no material reprinted from the journal itself.

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Chromatography of Antibiotics (see also Volume 26) by G.H. Wagman and M.J. Weinstein

Volume 2

Extraction Chromatography edited by T. Braun and G. Ghersini

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Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Janak

Volume 4

DetectoR in Gas Chromatography by J. ~ev
Volume 5

Instrumental Liquid Chromatography. A Practical Manual on High-Performance Liquid Chromatographic Methods (see also Volume 27) by N.A. Parris

Volume 6

Isotachophoresis. Theory, Instrumentation and Applications by F.M. Everaerts, J.L. Beckers and Th.P.E.M. Verheggen

Volume 7

Chemical Derivatization in Liquid Chromatography by J.F. Lawrence and R.W. Frei

Volume 8

Chromatography of Steroids by E. Heftmann

Volume 9

HPTLC - High Performance Thin·Layer Chromatography edited by A. Zlatkis and R.E. Kaiser

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Gas Chromatography of Polymers by V.G. Berezkin, V.R. Alishoyev and LB. Nemirovskaya

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Affinity Chromatography by J. Turkova

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Instrumentation for High·Performance Liquid Chromatography edited by J.F.K. Huber

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Radiochromatography. The Chromatography and Electrophoresis of Radiolabelled Compounds by T.R. Roberts

Volume 15

Arltibiotics. Isolation, Separation and Purification edited by M.J. Weinstein and G.H. Wagman

Volume 16

Porous Silica. Its Properties and Use as Support in Column Liquid Chromatography by K.K. Unger

Volume 17

75 Yean of Chromatography - A Historical Dialogue edited by L.S. Ettre and A. Zlatkis

x Volume 18A Electrophoresis. A Survey of Techniques and Applications. Part A: Techniques edited by Z. Deyl Volume 18B

Electrophoresis. A Survey of Techniques and ApPlications. Part B: Applications edited by Z. Deyl

Volume 19

Chemical Derivatization in Gas Chromatography by J. Drozd

Volume 20

Electron Capture. Theory and Practice in Chromatography edited by A. Ziatkis and C.F. Poole

Volume 21

Environmental Problem Solving using Gas and Liquid Chromatography by R.L. Grob and M.A. Kaiser

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Volume 23A Chromatography of Alkaloids. Part A: Thin-Layer Chromatography by A. Baerheim Svendsen and R. Verpoorte Volume 23B

Chromatography of Alkaloids. Part B: Gas-Liquid Chromatography and High-Performance Liquid Chromatography by R. Verpoorte and A. Baerheim Svendsen

Volume 24

Chemical Methods in Gas Chromatography by V.G. Berezkin

Volume 25

Modern Liquid Chromatography of Macromolecules by B.G. Belenkii and L.Z. Vilenchik

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Chromatography of Antibiotics Second, Completely Revised Edition by G.H. Wagman and M.J. Weinstein

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Instrumental Liquid Chromatography. A Practical Manual on High·Per· formance Liquid Chromatographic Methods Second, Completely Revised Edition by N.A. Parris

Volume 28

Microcolumn High-Performance Liquid Chromatography by P. Kucera

Volume 29

Quantitative Column Liquid Chromatography. A Survey of Chemometric Methods by S.T. Balke

Volume 30

Microcolumn Separations. Columns, Instrumentation and Ancillary Techniques edited by M.V. Novotny and D. Ishii

Volume 31

Gradient Elution in Column Liquid Chromatography. Theory and Practice by P. Jandera and J. Chunicek

Volume 32

The Science of Chromatography. Lectures Presented at the A.J.P. Martin Honorary Symposium, Urbino, May 27-31, 1985 edited by F. Bruner

XI

Volume 33

Liquid Chromatography Detectors. Second, Completely Revised Edition by R.P.W. Scott

Volume 34

Polymer Characterization by Liquid Chromatography by G. Glockner

Volume 35

Optimization of Chromatographic Selectivity. A Guide to Method Development by P.J. Schoenmakers

Volume 36

Selective Gas Chromatographic Detectors by M. Dressler

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XIII

PREFACE In the last decade, specialized chromatographic literature and chromatographic practice have placed emphasis on the identification of individual compounds from complicated gas chromatograms. Of course, a gas chromatograph-mass spectrometerdata system has been the most efficient combination. However, much simpler techniques are provided by use of the so-called selective detectors. Selective detectors give a response only to certain heteroatoms, resulting in a simplified chromatogram. Many selective systems exist and some of them are manufactured commercially and employed in routine chromatographic practice. Recently, new selective detectors have been developed and known detector designs have been innovated. The aim of this book is to collect and to collate up-to-date information on this topic to give the reader a detailed understanding of selective detectors in general, their principles, designs and analytical possibilities. Throughout the preparation of the manuscript, I have appreciated the assistance of many people from the Institute of Analytical Chemistry of the Czechoslovak Academy of Sciences. Special acknowledgements are due to my colleague Dr. Josef Novak who read the manuscript and made valuable suggestions, and to Mrs. Melita Radevova for translation into English. Brno, Mareh 1986

M. DRESSLER

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1

Chapter 1

INTRODUCTION Qualitative analysis by gas chromatography (GC) is based on the concept of the retention characteristics of sample compounds. The absolute values of the retention characteristics such as retention times, retention volumes or specific retention volumes, and also the relative values of these quantities such as relative retention times or volumes or Kov6ts retention indices are used. The retention volume, VR' is determined by the quantities that characterize the chromatographic system, i.e., the dead volume of the column, VO' the volume of the stationary phase, VS' or the surface area of the adsorbent and the partition coefficient, K. The value of K depends on the substance being analysed and the stationary phase employed: ( 1.1)

As the partition coefficient is a function of the thermodynamic properties of the system, the retention volume of a given solute in a given chromatographic system is constant at constant temperature and pressure, but it is neither selective nor specific. In addition, the separation power of any chromatographic column, even the best, is limited. This means that the column has a limited capacity for peaks and, therefore, the separation number (Trennzahl, TZ)l, given by the number of the separated peaks that can be placed between two successive peaks of neighbouring n-alkanes: (1. 2)

(where d2 - d1 is the distance between the peak maxima of the n-alkanes and Y i are the peak widths at half-heights) is finite. For complex mixtures (of natural or biological origin, for instance) not all of the peaks can be separated by the column even if the partition coefficients of the compounds differ. Consequently, from a theoretical point of view, only a negative result can be considered as conclusive in chromatography, i.e., that a compound a is not identical with b when VRa F VRb . I~hen performing qualitative analyses on the basis of retention data only, one should be aware of the risk that the chromatographic peak may not pertain to the

2 substance selected for calibration, although the latter has the same retention value, and even that it may not be due to a single substance but to two or more substances that have the same partition coefficients in the given system. Hence it has been generally accepted that the identification power of GC (and of chromatography in general) is far less than its excellent separation power. Therefore, a number of auxiliary techniques have been used for identification purposes, such as methods utilizing regularities in the partition coefficients within a homologous series which are known or can be predicted from experimental data. The retention values can be correlated with values characterizing the homologous series. Such values are either those that cannot be determined in any way from the chromatogram (e.g., molecular weight, number of carbon atoms or boiling point) or those found by chromatographic experiments (e.g., the ratio of the retention values on two stationary phases differing in polarity, or retention values measured at different temperatures)2,3. By interpolating these relationships, the retention value for a particular member of a homologous series can be obtained and compared with the retention value obtained experimentally. The agreement between the two sets of data increases the probability that the predicted identity wfll agree with that of the substance being analysed. However, these identification approaches are labourious and time consuming. Reaction gas chromatography3,4 is another approach used to facilitate the identification of individual components on a chromatogram. With this approach, the sample is subjected to selective reactions intended to remove selected types of substances from the chromatographic spectrum or to convert them into different substances. Subsequently, the chromatogram of the original sample is compared with those obtained after reaction. By introducing chemical reactions into the system, additional information on the identity of the sample compounds is obtained from the chromatogram, and the possibility of confusing the identities of the substances is again reduced. The utilization of the detector itself for the identification of substances is an efficient approach to the application of auxiliary techniques for qualitative purposes. From the viewpoint of quantitative analysis, chromatography requires a detector that responds as far as possible to all types of sample compounds. If it is sensitive enough, the detector provides a record of all the solutes. thus making possible their subsequent determination. If, in addition, the detector response per unit solute mass (weight or number of moles) is similar for different types of compound, which is very advantageous for quantitative analysis. the detector itself provides no data for qualitative purposes. The availability of a detector that gives a response that differs in some way for a certain type of compound from that for other compounds is obviously advantageous in qualitative analysis. Therefore, let us consider the ways in which the response of a certain detector can differ for different types of compounds.

3

It can differ, first of all, in the level of the response per unit solute mass. The ideal case would be represented by a detector responding to a certain type of compound only (e.g. to a certain kind of heteroatom in a molecule of these compounds). As will be seen later, no gas chromatographic detector meets this requirement. However, selective detectors 5- 8 are available the response of which per unit mass to compounds containing a certain heteroatom differs 'considerably from the response to other compounds. In addition to the level of the response, the detector response to various compounds can also differ in polarity. Spectral detectors such as the mass or infrared spectrometer supply, in addition to the common chromatographic record, data for each peak that allow one to characterize the compound. It is therefore evident that the detector itself can contribute to the identification of chromatographed compounds, if a suitable selective detector is properly selected for a particular case and if optimum operating conditions for the chosen detector are maintained. In order to function properly, selective detectors must be operated under optimum conditions. In other words, there are a number of operating variables that can either adversely affect or even nullify the function of the selective detector. Descriptions of the individual selective detectors and of the principles providing the basis for their operation, an,analysis of the effects of the various operating conditions on the basic parameters of the selective detectors and consideration of the potential use of these detectors in qualitative analysis are the subjects of this book. REFERENCES 1 R.E. Kaiser, Z. Anal. Chem., 189 (1962) 1. 2 J.H. Purnell, Gas ChFomatogFaphy, Wiley, New York, 1962. 3 R.C. Crippen, Identification of OFganic Compounds with the Aid of Gas ChFomatogFaphy, McGraw-Hill, New York, 1973. 4 V.G. Berezkin, AnaZytical Reaction Gas ChFomatogFaphy, Plenum Press, New York, 1968. 5 M. Krejc1 and M. Dressler, ChFomatogF. Rev., 13 (1970) 1. 6 M.L. Seluck9, ChFomatogFaphia, 4 (1971) 425. 7 D.F.S. Natusch and T.M. Thorpe, Anal. Chem., 45 (1973) 1185A. 8 L.S. Ettre, J. ChFomatogF. Sci., 16 (1978) 396.

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5

Chapter 2

BASIC TERMS RELATING TO DETECTORS CONTENTS 2.1. Detector sensitivity 2.2. Minimum detectability 2.3. Detection limit . . . 2.4. Detector noise . . . . . . . . . . . . . . . . . . . 2.5. Dependence of detector response on amount of compound 2.5.1. Detector linearity 2.5.2. Dynamic detector range 2.5.3. Linear dynamic detector range 2.6. Selectivity of response References . . . . • . . . • . . .

5 6 7 8 10 10 11 11

12 13

2.1. DETECTOR SENSITIVITY Detector sensitivity is the basic term used in describing any detector. In spite of the frequent use of this term, there is great non-uniformity in expressing detector sensitivity in the literature l - 9 , which has resulted in misleading interpretations in many instances. Frequently, terms such as molar response, detection limit and minimum detectable amount are used to express detector sensitivity. In addition, different terms have been used for the same thing. For instance, the terms minimum detectable amount, minimum detectable quantity, minimum detectable limit, detection limit, minimum detectability and limit of detectability have been used to express the minimum detectable rate of introduction of solute mass into the detector. The detector responds to an eluted sample compound, to the carrier gas and to other compounds that may be present and the response (H) can be expressed by R=R.+H '1.-

m +Rx

(2.1)

Under stabilized chromatographic conditions, the response to the carrier gas (H ) and to the impurities (H ) is constant and can be compensated to zero by m x the applied counter voltage. Thus, the net response given by the detector during the passage of an eluted sample compound through the detector is equal to the response to that compound, Hi'

6

The detector sensitivity (s) to a compound in the effluent from the chromatographic column can generally be defined as the change in the net detector response, i.e., in response to a substance i, with the change in the concentration (and/or mass flow) of this compound in the fluid supplied: S=dR./da. 1.-

1.-

(2.2)

or S = dR./(dm./dt) 1.1.-

(2.3)

depending on whether a concentration-sensitive or a mass-rate-sensitive detector one is used 10 . Detector sensitivity is generally expressed as the detector response per unit mass, i.e., as the molar response, R~Ol (response per mol), or the weight response, R~ (response per gram), of a given compound. With ionization detectors, for example, it is coulombs per mol or coulombs per gram of the compound. The detector sensitivity itself does not enable one to determine the smallest amount of the compound that could be detected by means of a given detector. For these purposes the detector noise must also be considered. 2.2. MINIMUM DETECTABILITY A peak observed over the detector noise should be large enough relative to the noise level to allow the reliable determination of the compound concerned. It follows from Young's statistical studies 11 that a peak that is at least twice as high as the width of the noise zone almost certainly belongs to a solute. The larger the multiple of this minimum value, the greater is the precision of determination. Karger et al. 12, for instance, considered a quintuple value, but generally a multiple of two is used. Thus, if the peak height in the chromatogram is at least twice the detector noise band width, Rn , this peak can safely be considered to be the peak of an eluted compound. This value then corresponds to the minimum detectable solute concentration in the column effluent, a~in (g/ml), or the minimum detectable solute mass rate, m~in (g/sec), 1.1.for a concentration-sensitive detector and/or a mass rate-sensitive detector, respectively:

(2.4)

7

With a selective detector, the detector response is conditioned by the presence in the solute molecule of the heteroatom to which the detector is selective, i.e. the detector responds predominantly to d certain type of compound. For this reason, the minimum detectable mass rate should be expressed in grams of the heteroatom per second rather than in grams of the whole compound per second (provided that a mass-rate-sensitive detector is used). For instance, with an alkali flame-ionization detector in the P mode (see Chapter 3), the minimum detectable mass rate of 6.1 • 10- 13 g/sec for ethion (16.3% P) corresponds to 1.0 • 10- 13 g/sec of P. With most selective detectors, the minimum detectable mass rate, in grams of the heteroatom per second, is approximately the same for different types of compound containing the same heteroatom. However, this finding does not hold for detectors that give a substanceselective response. For instance, the response of an electron-capture detector to halogen-containing compounds varies over several orders of magnitude (see Chapter 11), depending on the structure of the solute molecule. Hence the minimum detectability of a heteroatom also varies over a range of several orders of magnitude in this instance. It should be borne in mind that the minimum detectable solute concentration and/or mass rate refers to the concentration or mass rate of the compound in the detector and not in the sample injected. This fact is frequently neglected, which results in discrepancies when comparing different detectors. 2.3. DETECTION LIMIT The term detection limit, expressed in grams of solute compound, refers to sample injection into the chromatograph. It denotes the smallest solute mass, w~in, that still yields a discernible peak on injection into the chromatograph. This value is always larger than the minimum detectable mass rate because of dilution of the sample in the chromatographic column during the chromatographic process. Owing to zone dispersion, the peak height of the solute compound is smaller at the end of the chromatographic column than that at the inlet. Provided the solute peak is Gaussian, the concentration of the solute compound at the peak maximum, a~ax, is related to the mass of the solute injected, ws ' or to the original concentration of the solute in the sample, as, by

(2.5)

8

where N is the number of theoretical plates of the column, VR is the retention volume of the solute and vs is the volume of sample injected. Similarly, the mass rate of the solute at the peak maximum, m~ax, is given by 'Z-

(2.6) where F is the carrier gas flow-rate as measured in the detector. Hence, in contrast to the minimum detectability, the detection limit is not constant but depends on the column conditions, i.e., the length and kind of the column packing, the column temperature and the retention time of the sample compound. The relationships between the minimum detectable concentration, minimum detectable mass rate and detection limit can be written as

= C.'min Z-

(2.7)

VR 1271 = m.'min • IN F Z-

(2.8)

and

As far as the solute concentration in the original sample is concerned (in ppm, for instance), the detection limit for the solute concentration, c~in, is the lowest solute concentration in the material being analysed that gives a discernible peak on injecting the maximum permissible volume, v~ax, of this sample material into the chromatograph:

(2.9)

2.4. DETECTOR NOISE The detector response fluctuates owing to lack of constancy of experimental parameters such as temperature, gas flow-rates, power line voltage, thermal stability of the electrometer circuitry and stability of the detector components. All perturbations of the detector output that are not due to an eluted solute are referred to as noise. These perturbations can be divided into three types (Fig. 2.1). The first type is the short-term noise, consisting of baseline perturbations the frequency of which is substantially higher than that of the

9

Long term noise

Short term noise

Drift: I I

I I

-----------j

Fig. 2.1. Detector perturbations. eluted peaks. Frequently, this type of noise can be eliminated by means of a filter. The long-term noise consists of perturbations that have a frequency similar to that of the eluted peaks. This type of noise can neither be distinguished from the eluted peaks nor be filtered, because solute peaks having widths commensurate with the noise period would also be filtered out. Baseline perturbations with frequencies substantially lower than those of the eluted peaks are called drift. The expression of detector noise is also ambiguous. Some authors base it on the fact that the noise oscillates around the average signal, and they express the noise value by the baseline fluctuation about this average value, i.e., by half the value of the average noise band width. In order to evaluate the level of the detector noise correctly, the total noise band width should be considered in any case, however. Sometimes the short-term noise only is supposed to be the detector noise, but this concept is not correct either. The value of the detector noise should be represented by the maximum amplitude of the combined short- and leng-term noise measured over a period of about 10 min. The noise of a certain detector may not be constant, as it usually depends on the experimental conditions such as the type of stationary phase, column temperature and detector temperature. For instance, the higher the background current in the flame-ionization detector, the higher is the noise. With selective detectors, such as the alkali flame-ionization detector (AFID)13 and the flame photometric detector (FPD)14, the noise level also increases with increasing background response. This means, of course, that the minimum detectability with a given detector is not constant, but varies as a function of the experimental conditions; with a given detector type, these variations may also depend on the kind of hetero-

10 atom present in the sample molecule. For instance, the noise of the FPD increases with increasing detector temperature. The response of this detector to phosphorus compounds also increases with temperature, but the response to sulphur compounds decreases. Therefore, for phosphorus compounds, the minimum detectable mass rate remains about constant, whereas for sulphur compounds it increases from 1.6 • 10- 12 g/sec of S at a detector temperature of BOoC to 3.B • 10- 12 g/sec of S at 1520 C14 . 2.5. DEPENDENCE OF DETECTOR RESPONSE ON AMOUNT OF COMPOUND 2.5.1. Deteetor linearity

The relationships between the amount of solute (concentration, mass, mass rate) in the sensor of the detector and the detector response for different types of detectors, and the linearity of response, have been discussed in detail by Novcik 15 . The dependence of the detector response on the amount of solute compound can generally be expressed by (2.10)

R. = ken '/..

where k is a constant. A detector for which n = is then a perfectly linear detector. This dependence can be expressed graphically in two ways: on a logarithmic scale or as a semi-logarithmic dependence in which the response to unit amount of solute (e.g., the molar response) is represented as a function of the amount of solute (Fig. 2.2). If the detector response is linear, the slope of the dependence is unity in the former instance, whereas in the latter instance the response per unit amount of solute is constant. In both instances, tolerances up to ±5% are allowed. However, sometimes a response is considered to be linear when n is not unity but is constant, i.e., when represented graphically on a logarithmic scale log R.'/..

=

log k

+ n

log e

(2.11)

a straight line is obtained for the dependence of the response on the amount of solute. In this book, the term linear response will be applied only for the case when n = 1 ± 0.05.

11

:!: 5 %

Envelope

r~-----.-]l----------i--------f------~---CI C

GI

.... C

~

~

[iL---------

-----~

I I I I I

:,-

Linear range

I I I I I

: Minimum detectability

V-

Upper limit I of linearity :

~

Fig. 2.2. Dependence of the response per gram on amount of compound. 2.5.2. Dynamic detector range

The dynamic detector range is the range of the solute concentrations and/or mass rates over which the detector yields a concentration-dependent or a massrate-dependent output. The lower limit of this range is given by the amount of solute for which the response is twice the noise (i.e., the minimum detectability) and the upper limit is given by the pOint from which the detector no longer responds to increasing amounts of solute. 2.5.3. Linear dynamic detector range

This is the range of solute concentrations and/or mass rates over which the detector response is linear. With most detectors, the response ceases to be linear with large amounts of solute, but continues to increase with increasing amount of solute within a certain range. Hence the linear dynamic range differs from the dynamic range. The lower limit of the linear dynamic range equals that of the dynamic range, i.e., that given by the minimum detectable concentration or mass rate. The upper limit of the linear range is represented by the point at which the plot crosses the -5% envelope (see Fig. 2.2). It is the dependence of the response on the amount of substance injected that is usually used in plots illustrating the linearity of the detector response. This approach is not correct in principle, as the figure expressed in this way does not relate to the amount of the compound in the detector (see

12

section 2.3) and, consequently, it indicates nothing about the operation of the detector. In any event, the true concentration and/or mass rate of the solute in the detector should be taken into account. These data can be calculated from the known amount of the substance injected, ws' by means of the following equations: eomax

(2.12)

'!..

and (2.13) where s is the recorder chart speed (cm/min) and Y is the peak width (cm) at 0.607 of the peak height. The linear range can be expressed numerically as the ratio of the amount of solute corresponding to the upper limit of the linear range to the minimum detectable mass rate or concentration. 2.6. SELECTIVITY OF RESPONSE A detector is considered to be selective if its response to a certain type of compound differs markedly from that to another type of compound. By the term "markedly" a factor higher than 10 is usually meant 4 ,16. Thus, the selectivity 8 is expressed by the ratio of the responses per unit amount of the compound under consideration and a standard compound. These responses are expressed as the relative molar response, RMRir : R~ol 8 =

RMRo

H

=_'!..-

goo 1

(2.14 )

r

or as the relative weight response: R~ s = RWR

0

H

= -1:... RW

(2.15)

r

Hydrocarbons, i.e., compounds without heteroatoms, are commonly used as reference compounds. When comparing chromatograms obtained with a selective and a nonselective detector, in the former chromatogram we can observe substantially reduced and/or entirely lacking peaks of the compounds (compared with the latter

13 chromatogram) to which the selective detector is not sensitive and, conversely, in most instances an increased response to compounds to which the selective detector is sensitive. Occasionally, the term specific detector is used, but we should distinguish between the terms selective detector and specific detector 17 . Specific detection is the identification of a single substance and/or an element. In fact, the responses of GC detectors giving a common chromatographic record cannot be classified as specific. It is not true that a selective detector responds only to a certain heteroatom, e.g., the FPO to phosphorus or sulphur or the AFIO to phosphorus or nitrogen, and, consequently, that the detector is specific for this element. Although the selectivity of some detectors is high, e.g., 10 7 :1 for the AFIO, a response to hydrocarbons is also always obtained. Thus, it is only the concentrations of the components in the original sample that will decide the nature of the chromatographic record. Provided that the concentrations of the heteroatom-containing compounds to which the detector is sensitive and those of the hydrocarbons are commensurate, the ratios of the peak areas of the former to those of the latter will equal the ratios of the corresponding response selectivities. However, if the concentrations were inversely proportional to the selectivity of response, the peak areas would be commensurate. The only exception may be the specificity of response in the sense of a negative response (e.g., with the AFIO). In this instance, the response of some detectors to compounds that contain a certain heteroatom show opposite polarity to the response to other compounds. Additional, non-chromatographic data can be considered as specific, e.g., spectral data obtained with a mass or infrared spectrometer. In this instance, the spectrum is characteristic of the compound and can be used to identify the latter. Certain selective detectors, e.g., the electron-capture detector, give a response that vari es over a range of several orders of magnitude, dependi ng on the structure of the compound, with the same heteroatom in the molecule. In this instance we can speak of substance-s~lective detection. REFERENCES 1 S. Oal Nogare and R.S. Juvet, Jr., Gas-Liquid Chromatogl'aphy, Wiley, New York, 1962, p. 183. 2 O. Jentzsch and E. Otte, Detektol'en in del' Gas-Chl'omatogl'aphie, Akademische Verlagsgesellschaft, Frankfurt am Main, 1970, p. 18. 3 O.J. David, Gas Chromatogl'aphic Detectol's, Wiley, New York, 1974. 4 J. 5ev~fk, Detectol's in Gas Chromatography (Journal of Chromatogl'aphy Libl'al'Y, Vol. 4), Elsevier, Amsterdam, 1976, p. 31.

14

5 R.P.W. Scott, Liquid Chromatography Detectors (Journal of Chromatography Library, Vol. 11), Elsevier, Amsterdam, 1977, p. 5. 6 L.S. Ettre, J. Chromatogr. Sci., 16 (1978) 396. 7 American National Standard, ANS1/ASTM E 685-79, American Society for Testing and Materials, Philadelphia, PA, 1979. 8 L.S. Ettre, J. Chromatogr., 165 (1979) 235. 9 J. Novak, Kvantitativnt Analyza Kolonovou Chromatografit, Pokroky Chemie (Quantitative Analysis by Column Chromatography, Advances in Chemistry),

10 11 12 13 14 15 16 17

Academia, Prague, 1981. I. Halasz, Anal. Chem., 36 (1964) 1428. I.G. Young, in H.J. Noebels, R.F. Walland and N. Brenner (Editors), Gas Chromatography, Academic Press, New York, 1961, p. 75. B.L. Karger, M. Martin and G. Guiochon, Anal. Chern., 46 (1974) 1640. M. Dressler and J. Janak, Collect. Czech. Chem. Commun., 33 (1968) 3970. M. Dressler, J. Chromatogr., 262 (1983) 77. J. Novak, Quantitative Analysis by Gas Chromatography, Marcel Dekker, New York, 1975, pp. 25-45. M. Krej~f and M. Dressler, Chromatogr. Rev., 13 (1970) 1. H. Egan, ~oc. Soc. Anal. Chem., 9 (1972) 283.

15

Chaptel' :3

ALKALI FLAME-IONIZATION DETECTOR CONTENTS 3.1. 3.2. 3.3. 3.4. 3.5. 3.6.

Introduction. . . . . . . . . . . . . . • Detector design . . . . . . . . . . . . . Detector life, reproducibility of response Background current (hydrogen flow-rate) Negative response . . . . . . . . Response to individual heteroatoms 3.6.1. Phosphorus compounds 3.6.2. Nitrogen compounds 3.6.3. Halogen compounds 3.6.4. Hydrocarbons 3.6.5. Sulphur compounds. 3.6.6. Arsenic compounds. 3.6.7. Boron compounds. . . 3.6.8. Tin and lead compounds 3.6.9. Silicon compounds. . • . . . 3.7. Influence of compound structure on detector response 3.8. Influence of main operational parameters on detector response 3.8.1. Voltage and polarity of the electrodes 3.8.2. Height and shape of the collector electrode 3.8.3. Diameter of the tip bore . . . . . . • . . 3.8.4. Cations and anions of the alkali metal salt 3.8.5. Detector temperature 3.8.6. Carrier gas, air . . . 3.9. Detection mechanism . . . . . 3.9.1. Solid-phase reactions. 3.9.2. Gaseous-phase reactions 3.9.3. Photoeffects 3.9.4. Negative response References . . . . . .

15 16 23 24 29 33 33 34 36 37 38 38 39 39 40 41 43 43 45 48 49 52 53 54 54 56 59 59 59

3.1. INTRODUCTION The alkali flame-ionization detector (AFID) is a modification of the flameionization detector (FID). An alkali metal (Na, K, Rb, Cs) salt is introduced into the detection system and heated to volatilize it. Thus, an alkali metal enters the flame, where it is ionized. If a compound containing a certain heteroatcm (P, N, halogens, S, As, B, Si, Pb, Sn) is introduced into the system, variations in the background current of the detector occur. The response obtained in this way differs in magnitude and, with certain heteroatoms, in polarity from that of the FlO. The detector response to compounds that do not

16

contain the particular heteroatom, and/or that contain a heteroatom under operating conditions that are not optimum for that heteroatom, is lO\ler than that of the FID. Hence the selectivity of the AFID is given both by the level and the polarity of response. The response of the AFID depends strongly on all the operating conditions. In addition, these dependences differ significantly for various AFID designs. For this reason, data obtained with the individual types of AFID differ considerably in both magnitude and quality. It is difficult or even impossible to assess the common features of these interrelations for the different designs. Therefore, a generalization will be made in the following sections wherever possible. The conditions under which the results were obtained are specified for the design variants where the interrelations can be affected by additional parameters. The flame in the AFID serves a dual purpose: (1) it volatilizes the alkali metal salt and (2) it produces the ions that are collected. The amount of hydrogen determines the flame temperature and, therefore, the extent of the above processes. Therefore, slight variations in the hydrogen flow-rate strongly affect the detector response. Difficulties with maintaining very accurately a stable hydrogen flow-rate and some problems with the service life of the source of the alkali metal salt have resulted in recent years in a reduced interest in the AFID in favour of the so-called flameless alkali sensitised detectors (see Chapter 4) that were developed later. This trend is not general, however, evidence for which is provided by the fact that some manufacturers continue to equip their gas chromatographs with AFIDs (e.g., pye Unicam and Carlo Erba). In addition, the low cost and the ease of converting an FID into an AFlD make this detector still attractive. There is no unanimity in the designation of the alkali flame-ionization detector, as it is often also called the thermionic detector (TID) or nitrogenphosphorus detector (NPD). However, the term alkali flame-ionization best describes the character of this type of detector. 3.2. DETECTOR DESIGN The individual AFID designs mainly differ in the means of inserting the alkali metal salt into the detection system and in the method of signal sensing. A schematic representation of the original AFID deSign 1,2 is shown in Fig. 3.1a. Sodium sulphate is moulded on one of the electrodes and heated by the flame. This simple design was modified in minor detail by moulding the salt to a carrier (platinum wire, coil, etc.)3-5. Compared with the FID, the sensitivity of the detector to halogen compounds is increased by a factor of about 10 and

17

a

b

Ion collector!+)

-Probe!-)

-i- -Ignitor

Sodium

sulpha te Ceramic ------. bead

Quartz

t

flame tip

_

c

d

e

f

Carrier gas + hydrogen

E E

t'l

E

Alr--==*~::JJ~~~='Column L H2 Fig. 3.1. Schematic diagrams of some AFIOs. a, From ref. 21; b, from ref. 6; c, from ref. 7; d, from ref. 17; e, from ref. 13; f, from ref. 21. Reproduced with permission. to phosphorus compounds by a factor of about 300. The main disadvantages of this AFID design are a reduced useful life and stability (see section 3.3). This poor life and stability are the fundamental drawbacks of the AFIO that are responsible for the difficulties in the quantitative interpretation of the chromatograms. The elimination of these disadvantages has been the principal objective of all other designs of the AFID.

ex>

TABLE 3.1 PRINCIPAL PARAMETERS OF DIFFERENT TYPES OF AFID Compound

Reference

Atom Salt detec- used ted

Methyl pa ra th i on 8 Hydrocarbon 8 Diisopropyl 12 methanephosphonate Pyridine 12 Triethoxymethyl 12 silane Bromobenzene 12 12 Chlorobenzene Iodobenzene 12

P C P

Br Cl I

Thiophene Tetraethyllead

12 12

S· Pb

Tetraethyl tin

12

Sn

Phosphorus-containing compounds (1 P atom per molecule) Azobenzene

11

P

11

N

N Si

Type of AFID (Fig.3.1)

CsBr

c

Na S04 K2 04 K2S03 Na2S 4 K2S03 Na2S 4 Na2S04 Na2S04 K2 S04 K2 S04 Na2S04 K2 S04 Na2S04 K2 S04 Rb2S04

c

S

Three electrode

Sensitivity BackNoise Minimum (C/mol) ground (pA) detectable current mass rate (nA) (g/sec)

220 350 3.7 1.9 6.7 1.9 1.1 0.64 1.6 -6.0 -3.7 -4.2 -8.9 -44

3

2

6 6

5 5.8

3.10- 13 1.10-8 8.3.10- 12 2.5.10- 10

Recommended flow-rate (ml/min) H2

Air

Carrier gas

16

170

70 40

660 660

60 60

38

210

19

1.10- 10

0.03

0.02

1.5-10- 14 1.0.10- 12

Eicosane Methyl parathi on Methyl parathi on

11 13 14

C P P

Propasin Hexane Di isopropyl methanephosphonate Tetraethylpyrophospha te Triethylarsine Arsine Hydrogen phosphide p-Dichlorbenzene Tetrapropyl ti n Benzene

14 14 15

N C P

15

P

15 15 15 15 15 15

As As P Cl Sn C

*

C/g of X.

CsBr CsBr

Na2S04

e In gaseous phase f

8.8* 0.33* 5.4.10- 5* 130 160 0.23 0.35 60 2.9 -7.0 4.3.10- 2

0.01

4.5

75

1.0.10- 7 1.0.10- 13 2.0.10- 13

21 25

200 250

2.1.10- 10

34

1000

2.6.10- 10

lower flame (FlO)

7.0.10-8 2.2.10- 8 9.0.10- 11

81 102 upper flame (AFID)

7.7.10- 7

28 20-40

33

20 Fig. 3.1b shows the design of Coahrane6 , who placed a ceramic cup filled with sodium bromide crystals on the detector jet. The hydrogen-carrier gas mixture flows through the salt bed to be burned at the surface of the latter. In the next step, the salt was compacted to fit the shape of the tip of the detector jet (Fig. 3.1c). Here, the gases flow through a channel in the centre of the tip and again they burn on the surface of the salt 7- 9 . The useful life of the types of AFIO that utilize a compacted salt as the source of alkali metals 1 substantially exceeds that of the original design by Giuffrida and Karmen ,2; for this reason, some commercial detectors employ this type of alkali metal source. Many studies concerned with the AFrO were made with this particular type of detector (for the principal parameters, see Table 3.1). This design also differs 10 in detail, e.g., the tip may be moulded on to the detector jet • The sensitivity of the AFIO to compounds that contain a heteroatom is higher than that of the FlO (1-3 orders of magnitude, depending on the heteroatom), 12 14 but the noise of the AFIO is also higher than that of the FlO (10- _10- A); thus, the minimum detectable mass rate is comparable to that of the FlO or 1-2 orders of magnitude lower. TABLE 3.2 COMPARISON OF FlO ANO AFIO SENSITIVITIES Responses are given in C/mol (signal measured as peak area) and in A/ng (signal measured as peak height and without any correction for the different retention times of the test compounds); the latter are given in parentheses. (From ref. 18.) Parameter

Air flow-rate (ml/min) Nitrogen flow-rate (ml/min) Hydrogen flow-rate (ml/min) Probe gap Background current (A) Background noise level (A) Triethyl phosphite response Oibutyl sulphide response o-Toluidine response Tetradecane response

AFIO*

FIO

Phosphorus and nitrogen

Sulphur

550 68.0 34.0 O' 1.2.10- 9 1.0,10-12 3150 (3.1,10- 9) 3.0 (2.5,10- 12 ) 13.5 (9.2,10- 12 ) 0.5 (7.5,10- 14 )

450 68.0 34.5

o

9.0.10- 10 2.0,10- 13 400 (4.0'10- 10 ) 15.0 (1.2.10- 11 ) 2.2 (1.5,10- 12 ) -0.2 (-3,10- 14 )

400 25 25 8.0.10- 12 2.0,10- 14 0.8 ( 1.0,10- 12 ) 1. 50 ( 1.3,10- 12 ) 1.0 (7.5,10- 13 ) 2.4 (7.0-10- 13 )

* Oetector set to maximum sensitivity, but this does not necessarily correspond to a setting recommended for optimum detector performance.

21 Attempts to reduce the high AFID noise led to the design of three-electrode detectors in which the signal of the compound is monitored by means of a circuit separated from that of the noise. In the Pye three-electrode detector (shown schematically in Fig. 3.1d), the compacted salt (CsBr, RbCl) is placed in a metallic cylinder to which a negative voltage is applied. The ions produced by the burning hydrogen flame and by the ionization of organic compounds that-do not contain a heteroatom occur only a few millimetres above the flame; these ions are collected in circuit 2. The ions of the compounds that contain a heteroatom can be monitored by the collector electrode up to a distance of 30 mm above the flame (circuit 1). In this manner, the detector noise is separated from the signal (only a minor part of the background ionization current is measured by circuit 1)17. Table 3.2 lists the principal parameters of this detector under operating conditions that result in the maximum response for a given heteroatom. Data obtained with an FlO are also given for comparison. For routine work, Hoodless et al. 18 often recommend operating conditions that make allowances for useful life, noise, selectivity and critical positions of the electrodes, and in many instances these conditions differ from those given in Table 3.2. A similar detector was described by Brazhnikov and Shmidel 13 (Fig. 3.1e). In contrast to the above-mentioned design, the compacted-salt cylinder is placed in the upper collector electrode while the central electrode with the applied voltage is made in the shape of a ring that is covered with a platinum

Height of C peak

HeightofN peak

Fig. 3.2. Effect of position of salt crystal on the response to carbon and nitrogen compounds. 1,2 and 3 are positions of the alkali source with respect to the flame. (From r~f. 19.)

22 screen. The detector jet is earthed. The principal parameters are listed in Table 3.1. The Hewlett-Packard N-FID type B15161 detector 19 was also designed as a three-electrode detector. The upper collector electrode, containing a compactedsalt cylinder with a central bore, is movable. At the top of the jet (+350 V) there is a gate electrode to which the corresponding negative voltage (0 to -350 V) is applied so as to compensate the background ionization current induced by the rubidium ions. In this manner, the noise is reduced by a factor of 10. The response to nitrogen compounds between positions 1 and 3 of the collector electrode is approximately constant, whereas there is a decrease in the response to compounds that do not contain a heteroatom to which the detector is sensitive (Fig. 3.2). Thus, the detector displays the highest selectivity in position 3 with the following ratios: N:C, Cl, Br = 5000:1, N:I = 200:1 and N:P = 1:10. The detector noise is the lowest in position 1, and the minimum detectabi1ity of the detector is also the lowest in this position 19 . Hartmann 11 described an AFID (Varian) in which the alkali metal salt is pressed into an earthed stainless-steel cup that forms part of the flame support. A polarizing voltage (-300 V) is applied to the ignitor coil placed below the upper surface of the salt source.

I

:

o

NP

I I

I I I I L_

I I

®

Air

0-i

H2

Column

t: * ' '1u :

,0

I

I

p:-orl:

~

t- erw- ~. N MODE

P MODE

NP MODE

r-ITl

I

I

I

Air

N

:-EW-0

I I

I

C

H2

Column

N

I

Air

L_

I

__J

1-\

Column

Fig. 3.3. Operational modes of the NPD-40 (Carlo Erba) • (From ref. 20. ) The NPD-40 detettor (Carlo Erba) employs20 an AFID (Fig. 3.3) in which three different configurations inside the detector allow its operation in the NP. P and N modes. The NP mode is used for detection of either nitrogen- or phosphorus-containing compounds. The P and N modes show an enhanced response to phosphorus and nitrogen, respectively, in comparison with other heteroatoms (see a1so Fig. 3.21).

23

All the above types of design relate to the so-called single-flame AFID in which the detector operates as an AFID only. Double-flame detectors 21 consist of two detection systems placed one above the other (Fig. 3.1f). The lower system functions as a FlO and the upper system as an AFID. An alkali metal salt, mainly heated by the lower flame (or electrically22), is positioned on a carrier between the two systems. Two simultaneous chromatograms, an FlO and an AFID chromatogram, are obtained from the double-flame detector. The response to phosphorus, halogen and tin compounds is similar to that obtained with the single-flame AFID (cf., Table 3.1); the response to hydrocarbons, alcohols and sulphur compounds is approximately two orders of magnitude lower than that of the FlO. The response of the upper system to nitro compounds is low 23 • In all of the AFID designs described, the alkali metal salt is located within the detector space proper. However, the atoms of the alkali metal can also be brought into the detector compartment in the gaseous phase. The alkali metal salt is heated (to above 500 oC) in the temperature-controlled compartment and transported into the detector proper with the flow of inert gas. A stable flow of alkali metal results in this way24. The advantage of this type of detector is a low dependence of the response on the variations in gas flow-rate and the drawback is its bulky construction, because the whole space from the salt source to the flame has to be thermostated. 3.3. DETECTOR LIFE, REPRODUCIBILITY OF RESPONSE As the alkali metal salt is heated in the AFID it is volatilized, which leads to a loss of the salt in the source. By the service life of the AFID is meant the time span during which the detector behaves as an AFID, i.e., the time span during which enough alkali metal can be introduced into the detection system to make the detector respond to compounds that contain heteroatoms. Hence the detector life is determined by the exhaustion of the alkali metal source. The reproducibility of the response is qualified by the constancy of the detector response during the service life of the detector. With the first AFID designs that had the alkali metal salt moulded on a carrier (Fig. 3.1a), the source life was usually only a few days and the reproducibility of the response became poor soon after the operation had started 2 ,25,26. According to Coahrane 6 (Fig. 3.1b), the detector life was a few weeks and the decrease in response with time was slower than that with the preceding type. Mounting the compacted salt on the jet in the form of a tip (Fig. 3.1c) extends the service life to 1000 h9 ,20 and the response is reproducible over approximately 8 h9.

24 The background ionization current gradually decreases during detector operation, which results in a decrease in sensitivity27,28. If the background ionization current is kept constant by increasing the hydrogen flow-rate, a . 27 . G' . d even after 18 h 0 f operatlon reproducible response can be 0 btalne rln d'lng of the salt surface also results in the background ionization current being restored to its original value and, thereby, to the original response. With a compacted salt source placed into the flame above the jet (Fig. 3.1d), the compensation of the decrease in response to the original value could be achieved by increasing the hydrogen flow-rate even after operation for 2 months 17 . The decrease in response with time is often caused by deposits on the detector electrodes 29 ,30 (cleaning restores the response to its original value), by deposits of silica from the polysilicone stationary phases or silylating agents 31 on the alkali metal source or by the deposition of products generated by the combustion of lead and tin compounds on the surface of the salt source 12 . The life of an AFID in which the alkali metal is introduced into the flame by the flow of an inert gas is several thousand hours 14 • No changes in the structure of the salt source occur due to the contact of the flame with the source, and the reproducibility of the response should be very high. The stability of the flow-rates of the gases, mainly that of hydrogen, is of great importance with regard to the reproducibility of the response. Temperature variations in the flame and, as a result, changes in the concentration of the alkali metal in the flame occur with slight variations in the flow-rate of hydrogen (cf., section 3.4). Precise regulation, mainly that of the hydrogen flow-rate, is essential for this reason (at least to 0.1 ml/min). 3.4. BACKGROUND CURRENT (HYDROGEN FLOW-RATE) As the function of the AFID is conditioned by the presence of an alkali metal in the flame, the temperature of the flame and that of the source of the alkali metal salt housed in the detector are the most important factors affecting the detector response. The temperature of the salt determines the amount of alkali metal emitted from the source into the flame, and the temperature of the flame determines the degree of ionization of the alkali metal. The higher the hydrogen flow-rate, the greater is the heat released by the combustion of hydrogen. With increasing hydrogen flow-rate, the flame temperature increases in all zones of the flame (cf., temperature distribution in the flame and in the source, Fig. 3.4) and, as a result, the temperature of the alkali metal salt source also increases. For instance, the temperature at the surface of a jet tip made of CsBr increases from 400 0 C at a hydrogen flow-rate of 14.1 ml/min to 750 0 C at 63.8 ml/min, causing the CsBr saturation vapour pressure to increase

25 (mm)

13 41

11

E

c

'0

~

9

t =Temperature

7

of salt surface

CI

41 I

5 )

41

U

c

't:

"

-a.e UI

-----MlO"'C'...--.....!.l..s. 1-

0

A

SOO 900 1000 ( °C I Temperatures of flame and salt tip

UI

E

2

.J::.

)

41 0

(mm)

t.c.) t.c.1.

Fig. 3.4. Temperature distribution in flame and in the salt tip depending on the hydrogen flow-rate. Carrier gas flow-rate, 30 ml/min; air flow-rate, 250 ml/min. Hydrogen flow-rate: 1 = 63.8; 2 = 46.8; 3 = 33.9; 4 = 14.1 ml/min. A. CsBr melting point. t.c.1-t.c.4 are positions of four thermocouples pressed into the salt tip. (From ref. 32.) from 5'10- 5 to 1 mmHg 32 Hence the amount of the salt evaporated from the source and. at the same time, the number of ionized atoms of the alkali metal increase with increasing hydrogen flow-rate and, as a result, the background ionization current also increases. Therefore, the hydrogen flow-rate is considered to be the principal parameter determining the AFID response, and all response interrelations for the individual compounds are often related to the hydrogen flowrat/ ,33. As mentioned in the section on the detector life and reproducibility of response, the decrease in response with the operating time of the detector can be compensated for by increasing the hydrogen flow-rate so as to maintain a constant background ionization current 27 . In this instance, the molar response is independent of the hydrogen flow-rate within a certain range. With the threeelectrode detector the response remains constant 13 even if the hydrogen flowrate is changed from 20 to 28 ml/min. The temperature differs at various axial distances from the base of the flame (see Fig. 3.4); the distance of the collector electrode or its dimensions should affect the response (see section 3.8). It has been found for an AFID fitted with a jet tip of compacted alkali metal salt that the background ionization current also changes with variation in the distance of the electrode from the source

TABLE 3.3 DEPENDENCE OF AFID RESPONSE ON THE DISTANCE AND SHAPE OF THE COLLECTOR ELECTRODE, Na 2S0 4 From ref. 34. Electrode shape Ring, diameter 3 mm

Ring, diameter 8 mm

Cylinder, diameter 15 mm

Distance from the jet ti p (mm)

H~

flow-rate ( l/min)

Background (nA)

Relative response Cl

0.3 1 3 3 6 6 10 1 3 3 6 10 10 13

81.5 72.0 65.0 72.0 66.5 72.0 67.5 68.5 68.0 72.0 67.0 67.5 72.0 69.0 65.0

5.8 5.8 5.8 8.4 5.8 9.3 5.8 5.8 5.8 7.1 5.8 5.8 6.9 5.8 5.8

0.80 1.01 0.97 1.24 1.02 1.30 1.00 1.03 0.97 1.15 0.98 0.99 1. 11 1.00

P

0.97 1.01 1.01 1.00

27 C/moi

C/mol

A

2

B

C/mol

c

-1rP

A

10-10

A

Fig. 3.5. Dependence of the response on the background current for halogen compounds. A, Sodium salt; B, potassium salt; C, caesium salt. 1 = Chlorobenzene; 2 = bromobenzene; 3 = iodobenzene; xl. x2 and x3 = FlO responses at optimum hydrogen and nitrogen flow-rates. (From ref. 37.)

28

and with the diameter of the ring-shaped electrode. If this current is kept constant by varying the hydrogen flow-rate, the response is constant again for phosphorus, halogen and sulphur compounds (Table 3.3)34. It seems to be obvious from the above results that, in some instances, it is the background ionization current (as a measure of the concentration of the alkali metal and the temperature in a given location) rather than the hydrogen flow-rate that constitutes the principal factor determining the AFID response. For this reason, the individual relationships between the response and the operating conditions are often reported with reference to the background ionization current. If, for a given compound. the AFIO response displays the same polarity over the whole range of the investigated background ionization currents, the response follows the variations in the background ionization

CARRIER NITROGEN 101

"",

b

'\

1rP'>~ Bromine '\,

\, .11,

I

'.~

II!

\ ,

~\\.\

l'o I d' \\orne

5a. III

Q)

'0,

~

v ;'i\ p ~

~

:g

.

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'102

Hycjrogen flow 25ml/min

'iii o a.

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%,

''0

\ '~

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c

,

30ml/min

,, cf'

33ml/min

35ml/min

38 ml/min

CI.,

101

q.~ Bromine loDo.~

\,Iodine c o a.

T''. '"

1~

,~

f'~~ine

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r:I'

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,

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\ C\,.~

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CARRIER HELIUM Fig. 3.6. Halogen response profiles for varying carrier gas flow-rates (nitrogen and helium) and five selected hydrogen flow-rates. Electrode height, 2 mm. Bead bore, '.0 mm. Duplicate injections of 1 ~l of 0.01% chlorobenzene. bromobenzene and iodobenzene. (From ref. 39.)

29 current 1,27,35. For a compound whose response changes its polarity, the response level and polarity are functions of the background ionization current and these functions are characteristic of the given heteroatom and the alkali metal applied 12 ,34,36,37 3.5. NEGATIVE RESPONSE The AFID yields a response that is selective also with regard to the polarity of the response. Under certain operating conditions of the detector (flow-rates of the gases, alkali metal employed, detector design), halogen 8 ,9,36-41, sulphur 12 ,36,42-45, tin 12 , lead 12 and nitrogen 27 ,39 compounds, and hydrocarbons 8 ,9, 17,19,20,36,44 give negative responses, i.e., the background current is de-

"~

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o c.

tJ2 CARRIER NITROGEN ,.........

;,;

.. 1cfrrr~T"TTTTT'~'-r\n~" ~ ,-........

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~....Hydrogen flow 25ml/min l)ml/min~ 33~l/min --......,

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1cY

r

Fig. 3.7. Nitrogen and carbon response profiles for varying carrier gas flowrates (nitrogen and helium) and three selected hydrogen flows-rates. Electrode height, 2 mm. Bead bore, 1.0 mm. Single injections of 1 ~l of 1% aniline (~---~) and 1% each of p-xylene (6), n-decane (a), p-cymene (0), l-octanol (~) and anisole (0). (From ref. 39.)

30

creased. At low background currents (low hydrogen flow-rates), all the mentioned compounds yield a positive response that gradually changes to negative on increasing the background ionization current (increasing hydrogen flow-rate or decreasing carrier gas flow_rate B,9,12,34,36,37,39,43; Figs. 3.5-3.7). As the response changes from a positive to a negative value, changes in the peak shape occur, as can be seen from the example illustrated by thiophene (Fig. 3.B). The background ionization current (hydrogen flow-rate) at which the response of an AFID with a jet tip of compacted salt is negative is different for each heteroatomB,9,12,34,36,37,39,43,49 and usually increases in the order Sn
A

1.8.10 -loA

All

2.2.10 -loA

3.10 -lOA

-9

1.2.10 A

)(

1.3.10

-8

A

Fig. 3.B. Dependence of the response to thiophene on the background current. x indicates that the scale range is reduced by a factor of 5. (Reproduced from ref. 42 with permission.)

31 change stepwise the polarity of the response to compounds containing other heteroatoms. The type of alkali metal used affects the negative response in such a way that the lower the atomic number of the metal, the higher is the background ionization current that induces a negative response 12 ,36,37. An example of this relationship for sulphur compounds is shown in Fig. 3.10. Halogen compounds (cf., Fig. 3.5) yield a positive response when using a sodium salt; when a potassium salt is employed, the AFID response is positive only with iodine compounds36 ,37 The diameter of the jet tip bore also affects the conditions under which a negative response occurs. With halogen compounds, an increased diameter shifts the occurrence of a negative response to the region of smaller hydrogen flowrates 37 (Fig. 3.11). At a constant hydrogen flow-rate, a decrease in the carrier gas flow-rate leads to a negative response (cf., Figs. 3.6 and 3.7). When a constant background ionization current is maintained while increasing the flow-rate of the carrier gas (by increasing the hydrogen flow-rate), a negative response occurs 27 at higher flow-rates of the carrier gas with lower ionization currents .

C

Cl

5 5

10

min

Fig. 3.9. Chromatogram of a model mixture. S thiophene; C chlorobenzene. (Reproduced from ref. 42 with permission.)

toluene; Cl

32 Negative responses can also be induced by decreasing the potential of the gate electrode in three-electrode detectors (see section 3.B.1, Fig. 3.2)46 or by modifying the electrical configuration inside the detector 20 (P mode in Figs. 3.3 and 3.21; see also section 3.B.1). Negative AFIO responses have been described only for detector designs in which the flame either burns on the surface of the jet tipB,9,12,27,34,36-40, 42-45 or where the salt cylinder is situated above the jet, but very deeply in the flame 19 ,20,41. Only tin and lead compounds also display the negative response with a double-flame detector 22 .

C~

~8

A

Fig. 3.10. Plot of AFIO response for thiophene against ba~kground current for various alkali metals. 1 = NaCl; 2 = KC1; 3 = RbCl; 4 = CsCl. x = FlO response at optimum hydrogen and nitrogen flow-rates. (From ref. 36.)

33 IV

.~ iii

0 0-

Bead: 05 ,·0 103 fbdine

'1

0

a.

II! IV

a:

'1'

10' ci 30 Hyd

1.0 9 ;

p

i

d /,

m c:

075

I

i ; ; i

.rr.j

,

dp i

d

1ti

p

J

1

50 10 fLowmlhnin

1.5

d

I

I

2.0

3.0mm 1.0.

,

./'"

p

t)

i

.

i

!

I

. I .

I . I

I

!

!

;

j

50 10

\

i I'Iodinei.

;

i ;

i

\

.

50 10

0

let

Fig. 3.11. Halogen response profiles at various hydrogen flow-rates and bead bores. Electrode height, 2 mm. Triplicate injections of 1 ~l of a 1% solution of chlorobenzene, bromobenzene and iodobenzene. Nitrogen flow-rate, 50 ml/min. (From ref. 39.) 3.6. RESPONSE TO INDIVIDUAL HETEROATOMS 3.6.1. Phosphorus oompounds

A high sensitivity to phosphorus compounds was found with the earliest versions of the AFID. The minimum detectability and selectivity of all types of AFIO are highest for phosphorus compounds 1,2,14,16,21,27,36,47,48 (e.g., 1.5.10- 14 g/ sec 11 , 1.10- 12 g/sec 13 , 5.10- 13 g/sec 9; see also Tables 3.1 and 3.2). In comparison with the FlO, the sensitivity is 3-4 orders of magnitude higher. Similarly to with compounds containing other heteroatoms, the sensitivity of the AFID to phosphorus compounds depends mainly on the hydrogen flow-rate (background ionization current); the response increases with increasing background ionization current (hydrogen flow_rate)l,7,8,27,36. The response is positive, attaining a maximum at a certain ba~kground ionization current (hydrogen flow-rate). The course of the decrease in response at high hydrogen flow-rates in characteristic, for instance, of the Pye three-electrode system 17 (Fig. 3.12) (an analogous course was also reported for other heteroatoms 49 - 51 ). The molar response varies between 10 2 and 10 3 C/mole of the compound for all of the AFID types 14 ,18,27,36 and the selectivity varies from 10 3 to 10 5•

34

l~

+10

+5

/

- 5

-10

Fig. 3.12. Var3ation of the response with hydrogen flow-rate. 1 = Background current (x 10- A); 2 = peak height of trimethyl phosphate (x 10-9 A); 3 = noise level (x 10- 11 A); 4 = peak height of hexadecane (x 10-9 A). (From ref. 17.) 3.6.2. Nitrogen aompounds

In 1966, Ruyle et al. 52 and Wells 53 found that the AFIO yielded a selective response to nitrogen compounds. The response is positive and 2-3 orders of magnitude higher than that of the FI0 11 ,48,51,54, depending on the detector design. The AFIO requires the presence of potassium, rubidium or caesium salts to give a selective response, no increase in ionization current being found when a sodium salt is used 36 ,48,52-54 (Fig. 3.13). Rubidium has been found to be the most suitable alkali metal for nitrogen compounds (mainly due to the stability, source life and simplicity of the preparation of the source, particularly that of the jet tips)11 ,23,28,30,49,51 ,52,54-56. The AFID response to nitrogen compounds is negative 27 ,39 (see Fig. 3.7) at high hydrogen flow-rates (high background ionization currents) or at low air flow-rates. The response to nitrogen compounds is greatly affected by the distance of the electrode from the alkali metal source 52 ,54,55 (Fig. 3.13), this relationship being affected by the hydrogen flow-rate (Fig. 3.14). With increasing hydrogen flow-rate (background ionization current), the response of an AFID

35

...c>. ... ...

.Q

UJ (/I

z

o

CL (/I

IJJ 0::

2

3

4

5

6

ELECTRODE POSITION tnm abave bead)

Fig. 3.13. Dependence of nitrobenzene response on the collector electrode position. (Reproduced from ref. 54 with permission.)

c: ::J

...>. ...CI

-... .c C

w (/I

z

o

a. VI

IJJ 0::

2

3

4

5

6

ELECTRODE POSITION (mmabove bead)

Fig. 3.14. Effect of hydrogen flow-rate on nitrobenzene response profile. (Reproduced from ref. 54 with permi.ssion. J provided with a jet tip increases to a maximum, than decreases again 10 ,11,27,29, 36,51,56. A similar relationship also holds with variations in the air flowrate 39 ,56 (cf., section 3.8.6). Under the conditions stipulated by the manufacturer, the minimum detectability of the Pye detector is 1.1.10- 11 g/sec of azobenzene 51 , the selectivity for

36 2 3 hydrocarbons is 10 4-10 5 and that for phosphorus compounds is 1:10 to 1:10 (see also Table 3.2). 12 The minimum detectability of the detector provided with a jet tip is 1.10- gl 3 sec 11 ,55,57 and its N:C selectivity is 10 (ref. 55). 3.6.3.

Ha~ogen

aompounds

Giuffrida and Karmen found that the AFIO yielded selective responses to chlorine, bromine and iodine compounds for both single- 1,2 and double-flame 21 detectors. This response is higher than that to hydrocarbons and about an order of magnitude higher than that of the FlO. The positive response (increase in the ionization current of the detector in the presence of halogen compounds) when using a sodium salt is higher with bromine compounds than with chlorine and iodine compounds2,21 ,26,37,47; when employing potassium, rubidium and caesium salts, the order of the positive response levels depends on the detector design and the distance of the electrode 38 ,47. The level of the positive response depends on the cation used; it increases in the order47 Na+
37 surface of the salt source and, consequently, the background ionization current is higher at equal flow-rates of the gases. The flow-rate dependences of the response related to the bore of the jet tip again qualitatively follow the relationship between the response and the background ionization current. In accordance with the operating conditions, the selectivity of the AFlO response to halogen compounds is determined by an increase in their response in comparison with hydrocarbons or by a negative response that may be up to three orders of magnitude higher than that for hydrocarbons 38 . The detection limit with the negative response is about 1 ng 38 and the minimum detectable mass rate with a positive response is 1 • 10- 9 g/sec 27 • Fluorine compounds behave in the same manner as hydrocarbons in the AFlO, i.e., fluorine does not increase the ionization current. An exception to this rule is the lower response of the double-flame detector when using a caesium salt 21 ,47. Karmen and co-workers 58 ,59 described a double-flame AFlD in which CaC1 2 is situated on a screen between the two systems, below the source of the alkali metal salt. HF is produced from the organic fluorine compounds in the lower flame and it reacts with CaC1 2 to give HC1. The HCl vapour then causes an increase in the ionization current in the upper system. 3.6.4. HydPoaarbons

The sensitivity of the AFlO to compounds that contain no heteroatom to which the AFlO responds (hydrocarbons, alcohols) is, at best, as high as that of the Fl0 14 - 17 ,21,36,39,47. The response and character are again primarily functions of the background ionization current (hydrogen flow-rate). With increasing background ionization current (hydrogen flow-rate), the response decreases 1,8, 9,17,36 below the response of the FlO and becomes negative8 ,9,17,36 (Fig. 3.11), starting at a certain value of the background current (hydrogen flow-rate). The relationship between the response and increasing flow-rate of the carrier gas 39 (see Fig. 3.6) displays the same course. The degree of the decrease in the positive response of the AFlO in comparison with that of the FlO also differs in accordance with the detector design, amounting to 1 - 3 orders of magnitude 14 - 16 ,36,54, whereas the negative response is approximately commensurate with the positive response8 ,9,17,36. The increase in the selective response to compounds with a heteroatom compared with hydrocarbons in thus given both by the increase in the response to compounds with heteroatoms compared with the FlO response and by a decrease in the hydrocarbon response compared with the FlO.

38 3.6.5. SuLphur aompounds

Dressler and Jan~k42 found that the response of an AFID with a jet tip of a compacted alkali metal salt could be negative. The sensitivity depends on the background ionization current and on the kind of alkali metal, and it increases with increasing background current and atomic number of the cation 42 ,45. The course of the changes in the polarity of the response and peak shape for sulphur compounds as a function of the background ionization current is described in section 3.5 (Fig. 3.8). When using K2S04 and at a background ionization current of 6 • 10- 9 A, the molar response of thiophene is 6 C/mole42 ,43 and the minimum detectable mass rate is 1 • 10- 10 g/sec (detector noise 4 • 10- 12 A). The AFID response to sulphur compounds is negative under these conditions, whereas it is positive for other heteroatoms (except lead and tin) (Fig. 3.9). The AFID response is about 20 times higher than the FID response. The negative peak of sulphur compounds displays tailing in some instances, which seems to be connected with the character of the surface of the jet tip. It was observed when the upper surface of the jet tip was flat, but not with conical shapes 45 . A positive response (RbCl) has been described for the pye three-electrode AFID (see Fig. 3.1d)41,49. The dependence of the response on the hydrogen flowrate reaches a maximum at higher flow-rates compared with the situation with nitrogen compounds 43 , or at lower air flow-rates 18 ,41. The best selectivity over phosphorus and nitrogen compounds (1:1 and 10:1, respectively) occurs at a low air flow-rate (100 ml/min)18,41; the minimum detectable mass rate is about 3 • 10 12 g/sec (noise 5 • 10- 14 A) under these conditions 41 . A positive response was also found with the double-flame detector when potassium, rubidium and caesium salts were used (the response ratios, compared with the FID, were 2.6, 11.6 and 13.8, respectively)47. 3.6.6. Arsenia aompounds

Ives and Giuffrida 48 reported the response of a Single-flame AFID to arsenic compounds to be five times (KCl) to thirty times (esCl) greater than that of the FID (Fig. 3.1a). The double-flame detector 16 also responds to arsenic compounds. The response is lower than that of the FID (about 3 • 10- 1 C/mole), but is approximately ten times greater than that to hydrocarbons. A response is also obtained with arsine, for which the minimum detectable mass rate is 2.2 • 10-8 g /sec.

39

3.6.7. Boron oompounds

At the optimum hydrogen flow-rate (which is substantially higher than the optimum flow-rate for phosphorus compounds; when using CsBr it is 42 ml/min for boron compounds and 32 ml/min for phosphorus compounds), the pye threeelectrode AFlD yields a response to boron compounds that is about 50 times greater than that of the FlD. The minimum detectable mass rate for decaborane is 1.6 • 10- 12 g/sec and the response ratio of phosphorus and boron compounds is 330:1 50 • 3.6.B. Tin and lead oompounds

The dependence of the response of an AFlD equipped with a jet tip of compacted alkali metal salt on the background ionization current for tin and lead compounds displays a course similar to that for sulphur compounds (Fig. 3.15). The negative response increases with increasing background current (hydrogen

AFlD/FID

B

A AFID/F1D

-10'

10"111

1(f'

10'"

A

A

Fig. 3.15. Dependence of the response on the background current. A, Sodium salt; B, potassium salt. 1 = Triethoxysilane; 2 = tetraethyllead; 3, tetraethyl tin. (From ref. 12.)

40

FlO

I0:

~

VI

AFIO (Na)

I0:

<{

UJ VI

IVI

Z

o a.

VI UJ 0:

TIME

Fig. 3.16. Chromatograms of tetraethyltin (1). Column, stainless steel (68 cm x 6 mm I.D.), packed with 20% PEG 1500 on Chromosorb W, 55 0 C. (From ref. 12.) flow-rate) and, at a constant background current, it increases with increasing atomic number of the cation used (with a sodium salt the response to lead compounds is still positive at a background ionization current of 10- 10 A). Tin compounds have higher molar responses than lead compounds (Table 3.1). The peaks of compounds containing these two heteroatoms show tailing, and the elution time is prolonged (Fig. 3.16; cf., negative response 3.5). After repeated injections of larger amounts of these compounds, a black deposit appears on the surface of the jet tip while the background ionization current is decreasing 12 A negative response to tin was also reported for the double-flame detector; the response is commensurate with the values obtained with the single-flame detector, but the reproducibility of the response is poor 15 3.6.9. Sirieon evmpounds

The response of the AFID with a jet tin is positive within the range of background ionization currents from 1 • 10- 10 to 2 • 10-8 A12. Except for the sodium salt and background currents up to 1 - 10- 9 A, the AFID response always surpasses that of the FID (Fig. 3.15). The decrease in response, in comparison with the FID, is due to the fact that, in the range of background ionization currents mentioned, the detector still functions as an AFID (see section 3.7), but with

41

a hydrogen flow-rate that is not optimal. A comparison of the molar responses with those of compounds containing other heteroatoms is given in Table 3.1. 3.7. INFLUENCE OF COMPOUND STRUCTURE ON DETECTOR RESPONSE In the quantitative interpretation of chromatograms obtained by means of the AFID, it has often been assumed that the response is due to the heteroatom only and that, consequently, the response is proportional only to the percentage of the heteroatom contained in the molecule of the sample compound. However, the results reveal that this is not always so and that the detector design and operating conditions seem to play an important role. Aue et al. 40 showed that the response of the AFID with a jet tip (Rb 2S04) to phosphorus, chlorine, bromine, iodine and sulphur compounds is invariably proportional to the percentage of the heteroatom contained in the molecule with both a positive and a negative response. Nitrogen compounds do not behave in this manner, however40 ,60. In addition, it was found for another version of this type of AFID that the structure of sulphur compounds affected the molar response 43 . Ives and Giuffrida 48 also reported for a type a detector (Fig. 3.1) that the response is proportional to the number of nitrogen atoms in the molecule and independent of the structure. Karmen 21 found a proportionality of the responses to mono-, di-, tri- and tetrachloromethane with a double-flame detector, but this proportionality was not found with another double-flame detector 15 . The decrease in the contributions of the second and the other chlorine atoms to the total molar response of multi-substituted chlorine compounds is attributed to the decrease in the number of chlorine atoms that reach the upper system as a result of diffusion processes 34 . The molar responses of various bromine compounds are not identical; the responses to phosphorus compounds also depend on the structure, and the presence of two phosphorus atoms in the tetraethyl pyrophosphate molecule does not substantially increase the molar response in comparison with compounds containing one phosphorus atom 15 • The response of the pye three-electrode detector (Fig. 3.1d) also depends . on the solute structure for nltrogen, phosphorus 51 ' 61 and boron 50 compounds. However, in the series of dimethyl-, diethyl- and diisopropylnitrosamines, the response is proportional to the percentage content of nitrogen 30 • The response of the AFID in which the alkali metal is brought into the flame in the gaseous phase also depends on the structure of the compound 14 • The frequently conflicting conclusions reached in studies of the effects of the structure of sample compounds on the molar response may be due to the fact that with an AFID with a jet tip, the contributions of the AFID and FID responses are combined for compounds with heteroatoms where the AFID response is not too

42

different from the FlO response (up to one order of magnitude difference in the response levels). Hence, in addition to the heteroatom, the carbon skeleton also contributes to the total molar response of the compound. The smaller the amount of the alkali metal in the flame (lower background ionization current), the greater is the contribution of the carbon skeleton, and the molar responses of compounds with the same heteroatom differ considerably. The contribution of the carbon skeleton decreases only at higher background ionization currents (the FlO contribution to the total response) and the molar responses of various bromine compounds are about the same (Fig. 3.17). When substracting the contribution of the carbon skeleton from the total response of a compound, the contribution of the halogen atoms is invariably the same even with multi-substituted compounds (Table 3.4)37.

R

1.5

to

Fig. 3.17. Dependence of the relative response on the background current using a sodium salt. 1 = Bromocyclohexane; 2 a bromotoluene; 3 = bromocymene. (From ref. 37.) Fig. 3.18 shows the dependence of the responses of compounds with an SH group on the heat of combustion. The decomposition of the compounds in the flame leads to increased flame temperatures and to an increase in the ionization current that, to a certain extent, compensates for the variations in the ionization current caused by specific interactions of the particular heteroatom62 •

43 TABLE 3.4 MOLAR RESPONSES OF HALOGEN COMPOUNDS From ref. 37. Compound

Ionization efficiency

Chlorobenzene 1,4-Dichlorobenzene 1,3,5-Trichlorobenzene 1,2 ,4 ,5-Tetrachlorobenzene Bromocyclohexane 4-Bromotoluene B-Bromostyrene Bromobenzene 2-Bromocymene

Clmol compound

Clmol Cl

1.38 2.79 4.10 5.53 2.4 2.5 2.2 2.4 2.5

1.38 1.40 1.37 1.38

elmol 6

4

2

o

2000

4000 k J Imol

Fig. 3.18. Dependence of the ionization efficiency of some mercaptans on increase in the heat of combustion. (From ref. 62.) 3.8. INFLUENCE OF MAIN OPERATIONAL PARAMETERS ON DETECTOR RESPONSE 3.8.1. Voltage and polarity of the electrodes

The AFID response to phosphorus compounds increases with increasing voltage applied to the detector electrodes 33 ,35,39,47,63,64, commonly attaining a constant value at a certain voltage (saturation current range), after which the current increases again. The course of the potential-current characteristic of

44

the detector (interrelation between ionization current and applied voltage) depends, however, on the background ionization current35 ,63 (which is determined by the hydrogen flow-rate or by electrical heating of the alkali metal source); the higher the background ionization current, the higher is the potential causing the saturation of the current (Fig. 3.19). For compounds with other heteroatoms, the course of the potential-current characteristic of the AFIO depends on the detector design. In double-flame detectors, the potential-current characteristic for chlorine, bromine and iodine compounds is similar to that for phosphorus compounds 64 . With a single-flame AFIO with the jet tip made of compacted alkali metal salt, the response to halogen compounds also increases with increasing voltage, but it is negative (depending on the alkali metal used) (see section 3.6.3); the response to nitrogen compounds remains approximately the same 39 •

+ I.1Q-l0( .10- 8 )

15

(4.5) , ___

10(3)/

/

I

5

/

I

-E IV 1200 100/~'

/

2 3

/" .

,,,". I

/

I Al

,/

/

100

200 +EIVI

I 1 ;~

.1/

.I

,I /

,I I

-I-~

I

/

I

10 ( 3

)

15 (4.5) - 1.10-10 (.10- 8 ) I Al

Fig. 3.19. Potential-current characteristics of AFIO with jet tip salt. 1 = H2 11.7 ml/min, range 5.10-10-15.10- 10 A; 2 = H2 = 12.9 ml/min, range 5.10- 0_ 15,10-10 A; 3 = H2 = 13.7 ml/min, range 1.5,10-8-4.5,10- 8 A. (From ref. 63.) The polarity of the double-flame detector fundamentally affects both the level and shape of the responses of both the upper system (AFIO) and the lower system (FlO) of this type of detector. The electrical systems affect each other; the least interaction occurs with a + - + - connection (jet and collector electrode of the lower system, jet and collector electrode of the upper system). With a + - - + connection minor distortion of the peaks in the AFIO system

45 occurs and in connection with a negative polarity of the FlD jet distortion and inversion of the peaks in the FlD system occur 16 ,47. Peak distortion and inversion also occur with single-flame detectors if the system contains a third "gate" electrode (cf., Fig. 3.2), depending on the voltage applied (Fig. 3.20). The voltage that induces peak distortion and inversion depends on the structure of the compound 46 .

inversion zone specific inversion potential

A.,:

Ju

I

-240

potential IV)

Fig. 3.20. Peak s~ape deformation and inversion as a function of gate electrode potential. (From ref. 46.) Different electrical configurations inside the Carlo Erba detector (Fig. 3.3) affect the response level and polarity of phosphorus and nitrogen compounds and hydrocarbons 20 (see Fig. 3.21). 3.8.2. Height and shape of the aolleator eleatrode

With other operating conditions of the detector constant, the sensitivity of the jet-tip AFlD for phosphorus, nitrogen, sulphur, chlorine, bromine and iodine compounds depends on the distance of the collector electrode from the surface of the salt source 18 ,23,34,38-40,54,55,65. The dependence of the response on the electrode distance is different for each heteroatom. At hydrogen, nitrogen and air flow-rates of 33, 50 and 215 ml/min, respectively, and with a jet tip of Rb 2S0 4 with a 1-mm bore diameter, the negative response of sulphur compounds decreases with increasing electrode distance; the negative response to chlorine compounds first increases and then decreases; for other heteroatoms

46 P

0\ C

0\ C N

d

CIJ C CIJ

c

o

N

:c

C

CIJ

o

.0

:I:

«

o

"5

N mode N

N

Pmode

NPmode

N

0\

C 0 0

~ <Xl

P

In Lll N

U ~

In X

Fig. 3.21. Analysis of a test sample in the NPD-40 working modes. (From ref. 20.) the response increases with increasing distance of the electrode. However, the character of these interrelations depends on the operating conditions of the detector: the hydrogen flow_rate 38 ,39,54 (Fig. 3.14), the cation used 54 (Fig. 3.13), the shape of the collector electrode and the shape of the jet tip66. As the dependence of the response on the electrode distance from the source differs for each heteroatom, the selectivity of a compound containing a certain element compared with a compound with another element attains its maximum value at a certain distance. As noted in section 3.4, under certain conditions the AFID response can be a function of the background ionization current. Comparisons of the responses of phosphorus, chlorine and sulphur compounds at different electrode distances

47 from the salt source (Table 3.3) have shown that if the background ionization current is kept constant for the AFIO with a jet tip when changing the distance of the electrode (by changing the hydrogen flow-rate), the response of these compounds also remains constant 34 . Hartmann 11 also reported that for this type of AFIO the position of the electrode in the detector is not critical for the responses of phosphorus and nitrogen compounds, but that the flow-rates of the gases have to be adjusted so as to attain the optimum response. Mostly, data on the relationships between the response and the position of the electrode have not been accompanied by data on the background ionization current. However, it follows from Figs. 3.6 and 3.10 that, also for this type of design, the AFIO response to compounds with various heteroatoms is a certain (always different) function of the background ionization current (given by the hydrogen flow-rate), and that these relationships do not differ in quality from those established by Dressler and Janak 37 (see Fig. 3.5). It can be concluded from Table 3.3 that the background ionization current increases to its maximum with increasing distance of the electrode at constant flow-rates and that the course of this relationship varies for different shapes of the electrode and different hydrogen flow-rates. So, considering these variations in background ionization current with varying distance of the electrode from the salt source, the variations in the responses to compounds with various heteroatoms at different distances of the electrodes can be related to the corresponding variations in the background ionization current at the respective points in the flame. There is a qualitative resemblance of the dependence of the response on the background ionization current to that on the electrode distance. The influence of the hydrogen flow-rate on this relationship can also be related to the background ionization current. At higher hydrogen flow-rates the above relationships shift into the region of higher background ionization currents. Again, qualitative agreement can be observed. With AFIO designs in which the mobile polarizing electrode is situated above the detector jet, the background ionization current decreases with increasing distance of this electrode from the salt source, while the response ratio of phosphorus and nitrogen compounds shifts with respect to nitrogen compounds. In contrast, the P:N response ratio increases at a close distance of the electrode to the source (higher background current)65. The lateral distance of the electrode from the flame also affects the response to nitrogen compounds. Increased distances result in decreased responses to nitrogen compounds, whereas the response to phosphorus compounds remains unaltered 23 . The position of the electrode relative to the flame seems to be related to the dependence of the AFIO response on the diameter of the collector

48 10

Phosphorus

0.1

Background

o

.§ .D

~

~

51c:

o ~ Q)

a:

1.1O-11~..,

~ ::J a.

g.., ..,

to ::J

0.01

0~--~---7~~3~--~4

1 . 10-12 -;

Probe gap, mm

Fig. 3.22. Variation of detector response with probe height. Flow-rates: air, 100 ml/min; nitrogen, 30 ml/min; hydrogen, 30 ml/min. (From ref. 18.) electrode. The response is higher for larger diameters of the electrode 34 ,66; however, this relationship also depends on other operating parameters, such as the hydrogen flow-rate and the distance of this electrode from the source. As a result, the response can also decrease with increasing diameter of the electrode under different operating conditions. In this instance, the relationships also follow the variations in the background ionization current and, at a constant background current, the response may be constant 34 . Fig. 3.22 shows the dependence of the response to phosphorus, nitrogen and sulphur compounds and that of the background ionization current on the distance of the probe electrode for the Pye three-electrode detector. These relationships vary considerably depending on the flow-rates of hydrogen, air and nitrogen, even though the basic character depicted in Fig. 3.22 remains unaltered. The maximum responses (attained at various ratios of the flow-rates of the gases) for the compounds with all the mentioned heteroatoms decrease with increasing distance of the electrode form the flame 18 . 3.B.3. Diameter of the tip bore

The diameter of the jet tip bore affects the shape of the flame and, thereby, the size of the salt surface that is contacted by the flame and hence the flame temperature at the point of contact with the salt. The larger the diameter of the jet bore, the broader is the flame at its base where it covers a larger

49

,

"

,

"

!

,

!

I

mm

Fig. 3.23. Alkali salt designs and approximate flame configurations. (From ref. 66. ) surface of the salt. A greater number of alkali metal atoms are introduced into the flame, and the background ionization current is increased at constant flowrates of the gases 27 . With a constant background ionization current (due to the decreased hydrogen flow-rate with increasing diameter), the response to nitrogen compounds increases with increasing diameter of the jet bore (by a factor of about 4 from 0.6 to 3.00 mm). With halogen and phosphorus compounds the dependence of the response on the diameter of the jet bore produces a maximum for both positive and negative responses (halogens)39. The effect of the diameter of the jet tip bore on the negative response is described in section 3.5. The detector response to organophosphorus compounds with salt sources having a large flame-to-salt (Rb 2S04 ) contact surface (Fig. 3.23, II, IV and V) is about 50 times greater than that for salt sources with a small contact surface [the hydrogen and air flow-rates are adjusted so as to result in maximum response; the hydrogen flow-rate is always lower for types II, IV and V (27-30 ml/min) than for the other types (45-50 ml/min)6~. 3.8.4. Cations and anions of the atkati metat satt

The vapour pressure of alkali metal compounds increases with increasing atomic number of the alkali metal and the ionization potential of the alkali metal decreases in the same order. This means that, with a constant hydrogen flow-rate, different amounts of salts of the individual alkali metals are

01

o

TABLE :1.5 DEPENDENCE OF AFrO: FlO RESPONSE RATIOS ON THE TYPE OF ALKALI METAL AT CONSTANT GAS FLOW-RATES (A) Sulphates; (B) chlorides. Compound

Li

Na

(A) Double-flame detector (from ref. 47): 0 0 Fluorobenzene Chlorobenzene 0.26 2.30 Bromobenzene 0.16 3.85 lodobenzene 6.93 2.82 144.70 Diisopropyl methanephosphonate 7.59 Thiophene 0.12 0.76 0.11 Pyridine 0.13 (B) Single-flame detector, type a (Fig. 3. 1) (from ref. 48) : Triphenylamine Triphenylphosphine Triphenylarsine

K

Rb

Cs

0.35 15.00 5.00 8.35 247.20 2.62 0.98

0.35 30.81 14.83 69.10 418.80 11.63 1.18

7.80 117.50 67.60 242.20 751.40 13.80 3.10

30 10 000 5

60 11 000 10

150 12 000 30

51 brought into the flame and, in addition, the proportion of the ionized atoms of the individual alkali metals is also different. Therefore, if all other operating parameters of the detector remain constant, the background ionization current will increase in the order NaK>Rb>Cs) allows us to compare the responses with equal amounts in the system. Also in this instance the sensitivity of the AFID with a jet tip for phosphorus, nitrogen, sulphur, silicon, lead and tin compounds increases in the order Na
ClBrS024 C023 N0 3

AFID response

H2 flow-rate

Chlorobenzene

Diisopropyl methanephosphonate

(ml/min)

1. 00 1.47 1.60 2.28 2.02

1. 00 2.56 2.23 4.62 5.05

54.0 36.0 50.0 36.5 33.0

52 The same dependence on the atomic number of the alkali metal as for compounds with other heteroatoms is valid for the detector sensitivity in the case of halogen compounds. However, the type of alkali metal also affects the polarity of the response (Fig. 3.5). When using a sodium salt, the AFrO response to chlorine, bromine and iodine is positive over the whole range of the observed background current (10- 10 _10- 8 A). When a potassium salt is used, the response is positive within this range of background currents only to iodine compounds, the response to bromine and chlorine compounds becoming negative at certain values of the background current. With rubidium and caesium salts the responses are negative for all three halogens 37 . The higher the atomic number of the alkali metal employed, the lower is the background ionization current for which the response becomes negative 12 ,36,37. For the double-flame detector, an approximately constant ratio of the responses to phosphorus and chlorine compounds has been reported for all alkali metals at the same background current67 For the AFrO with a jet tip, the influence of the salt anions on the response at a constant beckground ionization current can be seen from Table 3.6. The response for phosphorus and chlorine compounds increases 36 in the order Cl- < Sr - <S042-
With increasing temperature of the detector, the background ionization current and the detector response to phosphorus and nitrogen compounds increase, whereas the response to hydrocarbons decreases. With increasing temperature the selectivity increases by a factor of about 5 in the range from 200 to 400 0 C19 • With the Pye three-electrode detector the response decreases, starting from a temperature of about 250 0 C29 With a detector having the jet tip moulded into the detector jet, the response to phosphorus compounds also decreases, at optimum N2 : H2 flow-rate ratios, with increasing temperature of the detector (by almost three orders of magnitude within the range 200-300 0 C), whereas it does not vary very much for nitrogen compounds. The N2 : H2 flow-rate ratio at which the maximum sensitivity is obtained decreases with increasing detector temperature 1D

53

,). 8. 6. Carrie1' gas, a'ir

When helium is used as the carrier gas, the sensitivity of the AFID for phosphorus and nitrogen compounds is greater than the sensitivity in nitrogen 4 ,48,57,65 (using a jet of about 1 mm I.D.; this difference levels off when a larger diameter is used, e.g., 1.5 mm48 ) , it decreases with chlorine compounds 4 (using KC1) and hydrocarbons 65 and, therefore, the selectivity of the responses to phosphorus and nitrogen compounds is higher when helium is used. The minimunl detectable mass rate is also lower when employing helium65 • The optimum hydrogen flow-rate with respect to the sensitivity of the AFID is different, being lower with helium65 . With nitrogen compounds, detectors with jet tips of compacted salt yield negative responses in helium at lower hydrogen flow-rates than they do in nitrogen. For this reason, the positive response is lower at equal flow-rates of the gases with helium (particularly at higher hydrogen flow-rates) and the negative response is roughly the same. With helium, the negative response (at the optimum air flow-rate) is about 1.5 orders of magnitude higher than the positive response and the two responses are roughly the same with nitrogen (Fig. 3.7). With halogen compounds, both the positive and negative responses with helium are higher than those with nitrogen (Fig. 3.6)39. The positive response to phosphorus, nitrogen, chlorine, bromine and iodine compounds and hydrocarbons initially increases with decreasing flow-rate of the carrier gas (Figs. 3.6 and 3.7). At a certain flow-rate (depending on the kind of heteroatom and the hydrogen flow-rate), the response attains a maximum and then changes gradually with further decrease in the flow-rate to reach a negative response that subsequently continues to increase. The positive response to chlorine and iodine compounds is about 2-3 orders of magnitude and that to hydrocarbons about one order of magnitude lower than the maximum negative response 39 The increased flow-rate of the carrier gas cools the flame and causes a decrease in the background ionization current. This means that, at the same time, the mentioned relationships for the response variations in connection with the decreased flow-rate of the carrier gas follow the increase in the background ionization current. Negative responses for halogen compounds occur at flow-rates of the carrier gas that are lower (higher background currents) the higher is the atomic numbers of the halogen. The same is true for the dependence of the response on the background ionization current. The higher the hydrogen flow-rate, the higher is the flow-rate of the carrier gas at which a negative response occurs. The same holds for nitrogen compounds and hydrocarbons (Fig. 3.7).

54 A comparison of the responses obtained at equal background currents (by regulating the hydrogen flow-rate) has shown that, also in this instance, decreased flow-rates of the carrier gas lead to increased responses to nitrogen, phosphorus and chlorine compounds' ,27, However, the variations are substantially lower than those at a constant hydrogen flow-rate, The dependence of the negative response for sulphur compounds on the flowrate of the carrier gas displays a maximum that, for various hydrogen flow-rates, approximately matches the same flow-rate of the carrier gas (about 60 ml/min)40, A maximum has also been observed for the dependence of the response on the flowrate of the carrier gas with the Pye three-electrode detector (65 ml/min with nitrogen compounds)56, At a constant background current, the response increases' with increasing air flow-rate at low flow-rates (roughly up to 250 ml/min) where the supply of oxygen is inadequate, The response subsequently remains constant' ,27 at flowrates up to about 500 ml/min, after which it increases again with increasing flow-rate 27 , The dependence of the response on the air flow-rate exhibits a maximum (475 ml/min) with the three-electrode detector (type d, Fig, 3,,)56, This dependence also shows a maximum for detectors in which the alkali is supplied to the flame in the gaseous phase'4, With double-flame detectors, the dependence of the response on the air flowrate is similar to that of the FID, i ,e" the response increases for flow-rates up to about 500 ml/min and remains constant at higher flow-rates 47 , The use of oxygen instead of air in the double-flame detector leads to a suppression of the response to phosphorus compounds and an increase in the response to halogen compounds 22 3,9, DETECTION MECHANISM Similar difficulties to those experienced in attempts to generalize the experimental results obtained form the studies on the AFID are encountered when trying to arrive at an unambiguous explanation of the response mechanism with this detector, The existing theories can be classified into the following groups, 3.B.1. Solid-phase peaations

2 Karmen ' found in one of his first studies on the AFID that, on introduction of chloroform into the lower burner of the double-flame detector, both the ionization current and the light emission characteristics of the alkali metal used increased in the upper system. When a halogen compound was introduced into the upper burner, no emission occurred 23 , Karmen concluded that halogen compounds

55 or their decomposition products increase the volatility of the alkali metal salts. In the flame the ionization current is increased owing to the ionization of newly formed atoms of the alkali metal. The AFIO responds to halogen compounds only if the detector allows-contact of the halogen compounds with the alkali metal salt. Hence the generation of the AFIO response is attributed to the reactions of the heteroatom-containing combustion products with the alkali metal in the solid phase. However, the presence of halogen and phosphorus compounds in the flame of the single-flame AFIO also leads to the reduction of the light emission of the alkali meta1 24 ,34. Depending on the hydrogen flow-rate, the emission of the flame is higher (at low flow-rates) or lower (at high flow-rates) in the presence of halogen compounds (except fluorine)68,69. In contrast, the emission is reduced for phosphorus compounds at low hydrogen flow-rates 69 . The variation in the ionization current of the AFIO during the passage of halogen compounds through the flame also depends on the hydrogen flow-rate, in that the positive AFIO response (i.e., the increase in ionization current) changes to a negative response at higher hydrogen flow-rates (see section 3.6.3). Owing to the dissimilar courses of the variations in emission and ionization current (see also section 7.6), these variations cannot be compared directly, however. Brazhnikov et al .32 studied the weight variations of the alkali metal salt source of the AFIO when the flame was burning and, later, also when? constant amount of tributyl phosphate vapour was supplied to the flame. The evaporation rate of caesium bromide (characterized by the weight loss of the source with time) remained constant regardless of the presence or absence of the phosphorus compound (Fig. 3.24). During the elution of tin and lead compounds from the chromatographic column, the time-related course of the signal of an AFIO with a jet tip of compacted salt differs from that of the FI012. With the AFIO, both the retention value and the standard deviation of the tetraethyl tin peak exceed those of the FlO (Fig. 3.16). Therefore, it is believed that with certain compounds reactions take place on the surface of the alkali metal salt (see section 3.5 for attempts to explain the negative responses of the other heteroatoms). According to Olah et al. 70 , a mechanism describing the ionization process as a catalytic reaction can be applied for the AFIO in some instances (see section 4.4.2).

56

a

III

ai

III

U

-....

1.64600

a

..c

Cl

41

3

......

1.64500

5

10

15

20

25

Time

(h)

30

35

.........

40

Fig. 3.24. Dependence of salt tip weight on its performance time. Flow-rates: hydrogen, 33 ml/min; nitrogen, 30 ml/min; air, 250 ml/min. (From ref. 32.) 3.9.2. Gaseous-phase peactions

An alkali metal from the source of the alkali metal salt is brought into the AFID and ionized in the flame, generating a constant ionization current either thermally:

(3.1) or by a three-body reaction: (3.2) where A is the atom of the alkali metal, and M represents a general flame-gas molecule. In explaining the mechanism of the AFID response by reactions in the gaseous phase, Page and Woolley?1 proceeded from the assumption that in the oxygenhydrogen flame there is a higher concentration of hydrogen atoms than that which would correspond to the equilibrium state (the influence of the slow three-body recombination of hydrogen atoms):

(3.3) H + H + M- H2 + M

(3.4)

57

The ionization of the alkali metal in the flame is accomplished according to reaction 3.1, because reaction 3.2 is slow. However, the rate of reaction 3.2 can be increased (in proportion to the amount of the heteroatom, X) in the presence of a compound containing the heteroatom X. The degree of ionization of the alkali metal increases in the range from 1 to y2, where y is the ratio between the true concentration of hydrogen atoms and the equilibrium concentrati on, y = [H] / [}le;J : (3.5) X + H ~HX + e

(3.6) (3.7)

According to this theory, the ionization current increases in the presence of a compound with a heteroatom owing to the increased ionization of the atoms of the alkali metal that is already present in the flame. The decrease in light emission is caused by the decrease in the concentration of neutral atoms due to this increased ionization. Baldwin 72 ascribed the increased emission (observed at higher alkali metal concentrations in the flame) to self-absorption. As during the passage through the flame of a compound with a heteroatom the neutral atoms of the alkali metal are decreasing, self-absorption decreases and, consequently, light emission increases. Brazhnikov and Shmidel 13 explained the increase in ionization current by the production of heavy ions during the combustion of phosphorus and nitrogen compounds. The mobility of heavy ions is about 1000 times lower than that of light ions; they combine with the ions of the alkali metal giving rise to even heavier ions. The alkali metal salts are inhibitors of combustion. The concentration of the alkali metal salts decreases owing to the production of heavy ions, thereby reducing the inhibiting effect of this salt (the flame temperature increases). The temperature increase leads to greater ionization of the alkali metal atoms and, thereby, to an increase in ionization current. Sevcik 62 explained the AFrO response by specific interactions. With halogen compounds, the electrons produced as.a result of ionization in the flame are captured and the ionization current decreases owing to the increased recombination of ions. The reaction of the decomposition products with the alkali metal atoms that gives thermostable compounds is dominant for sulphur compounds. The concentration of the alkali metal in the flame decreases and, for this reason, the ionization current also decreases. The two preceding mechanisms are not substantial for phopshorus compounds. Various phosphorus compounds with a low ionization ~otential ar~ produced in the flame and the ionization current increases.

58

Add P d

c No P

b

a

CPIB~------

6

1. min

2

6

Fig. 3.25. Effect of addition of phosphorus vapoufi to the upper flame. CPIB = chlorophenoxy isobutenate; a = lower flame (3'10- A f.s.); b = upger flame (10- 8 A f.s.); c = lower flame (10- 8 A f.s.); d = upper flame (10- 7 A f.s.). (Reproduced from ref. 23 with permission.) Experiments have shown that the ionization current of the AFID increases in the presence of phosphorus and nitrogen compounds, even if no reactions of the combustion products of the compounds with the source of the alkali metal salt can occur, i.e., when the alkali metal is supplied to the detector in the gaseous phase 14 ,23,24,71,72. The response of a detector provided with this means of supplying the alkali metal to the flame is comparable to the response of other types of AFID, viz., about 5 C/g of phosphorus and 0.3 C/g of nitrogen. Karmen 23 also attributed the response of phosphorus compounds to reactions in the gaseous phase. If a constant concentration of phosphorus vapour is supplied to the upper burner of the double-flame detector, the response of the upper system to halogen compounds brought to the lower burner is higher than if no phosphorus is supplied to the upper burner (see Fig. 3.25). This increase in response is ascribed to the increased volatility of the alkali metal of the source caused by the halogen compound in connection with the increased ionization of the alkali metal in the upper flame owing to the presence of phosphorus. This effect was not observed when phosphorus compounds were supplied to the lower burner.

59 3.9.3. Photoeffects

Brazhnikov et al .73 assumed that the light generated from the burning phosphorus compounds is responsible for the photoevaporation of the salt, i.e., the photons emitted by the flame are absorbed on the salt surface and lead to evaporation of the alkali metal salt. These atoms of the alkali metal are ionized in the flame. 3.9.4. Negative response

The causes of negative responses are even more obscure than those of positive responses. Maier-Bode and Riedmann 19 explained the negative response by changes in the shape of the flame during elution of the compounds. During the growth of the flame, when the compounds elute, the salt source is actually situated in the zone of the flame in which the temperature is lower than it was before the elution of the compound. Consequently, less salt is evaporated into the flame and this results in a decrease in the ionization current. ~evtfk62 explained the negative response to halogen compounds by electron capture and that to sulphur compounds by the generation of stable compounds (see section 3.9.2). REFERENCES 1 2 3 4 5 6 7

L. Giuffrida, J. Ass. Offic. Agr. Chern., 47 (1964) 293. A. Karmen and L. Giuffrida, Nature (London), 201 (1964) 1204. L. Giuffrida and N.F. lves, J. Ass. Offic. Agr. Chern., 47 (1964) 1112. J.H. Ford and M. Beroza, J. Ass. Offic. Anal. Chern., 50 (1967) 601. J.R. Wessel, J. Ass. Offic. Anal. Chern., 50 (1967) 430. D.R. Coahran, Bull. Environ. Contam. Toxicol., 1 (1966) 141. D.M. Oaks, K.P. Dimick and C.H. Hartmann, Aerograph Phosphorus Detector, W-122, Varian Aerograph, Walnut Creek, CA, 1966. . 8 C.H. Hartmann, Aerograph Research Notes, Varian Aerograph, Walnut Creek, CA, Summer 1966. 9 C.H. Hartmann, Bull. Environ. Contam. Toxicol., 1 (1966) 159. 10 M. Mraz, R. Nemetek, V. ~edivec and J. Flek, Chern. Listy, in press. 11 C.H. Hartmann, Varian Aerograph Tech. Bull. 136-69, Varian Aerograph, Walnut Creek, CA. 12 M. Dressler, V. Martinu and J. Janak, J. Chromatogr., 59 (1971) 429. 13 V.V. Brazhnikov and E.B. Shmidel, J. Chromatogr., 122 (1976) 527. 14 V.V. Brazhnikov, V.M. Poshemansky, K.K. Sakodynskii and V.V. Chernjankin, J. Chromatogr., 175 (1979) 21. 15 J. Janak, V. Svojanovsky and M. Dressler, Collect. Czech. Chern. Comrrmn., 33 (1968) 740. 16 V. Svojanovsky, J. Janak and M. Dressler, Collect. Czech. Chern. Commun., 31 (1966) 3925. 17 F.P. Speakman and C. Waring, Co Zumn , 2, No.3 (1968) 2. 18 R.A. Hoodless, M. Sargent and R.D. Treble, J. Chromatogr., 136 (1977) 199. 19 H. Maier-Bode and M. Riedmann, Residue Rev., 54 (1975) 113. 20 G.R. Verga, J. Chromatogr., 279 (1983) 657.

60 21 A. Karmen, Anal. Chem., 36 (1964) 1416. 22 K. Abel, K. Lanneau and R.K. Stevens, J. Ass. Offic. Anal. Chem., 49 (1966) 1022. 23 A. Karmen, J. Chromatogr. Sci., 7 (1969) 541. 24 W.A. Aue, D.L. Stalling, C.W. Gehrke, R.C. Tindle and S.R. KOirtyohann, Organophosphate-Alkali Interaction, 5th National Meeting of the Society for Applied spectroscopy, Chicago, IL, June 1966. 25 L. Giuffrida, N.F. Ives and D.C. Bostwick, J. Ass. Offic. Anal. Chem., 49

26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

(1966) 8. D. Jentzsch, H.G. Zimmermann and I. Wehling, Z. Anal. Chem., 221 (1966) 377. M. Dressler and J. Jan~k, Collect. Czech. Chem. Commun., 33 (1968) 3960. D.F.K. Swan, Column, No. 14 (1972) 9. R. Greenhalgh and J. Dokl~dalova, Column, No. 14 (1972) 4. T.A. Gough and K. Sugden, J. Chromatogr., 86 (1973) 65. R.F. Coward and P. Smith, J. Chromatogr., 61 (1971) 329. V.V. Brazhnikov, M.V. Gurev and K.I. Sakodynskii, Chromatographia, 3 (1970) 53. H. Beckman and W.O. Gauer, Bull. Environ. Contam. Toxicol., 1 (1966) 149. M. Dressler, Alkali Flame Ionization Detector, Thesis, Institute of Analytical Chemistry, Brno, 1969. R.A. Mees and J. Spaans, Z. Anal. Chem., 247 (1969) 252. M. Dressler and J. Janak, Collect. Czech. Chem. Commun., 33 (1968) 3970. M. Dressler and J. Janak, J. Chromatogr., 44 (1969) 40. S. Lakota and W.A. Aue, J. Chromatogr., 44 (1969) 472. K.O. Gerhardt and W.A. Aue, J. Chromatogr., 52 (1970) 47. W.A. Aue, K.O. Gerhardt and S. Lakota, J. Chromatogr., 63 (1971) 237. R.A. Hoodless, M. Sargent and R.D. Treble, Analyst (London), 101 (1976) 757. M. Dressler and J. Janak, J. Chromatogr. Sci., 7 (1969) 451. M. Dressler and J. Janak, Collect. Czech. Chem. Commun., 34 (1969) 1797. W.H. Stewart, Anal. Chem., 44 (1972) 1547. J. Novotny and A. MUller, J. Chromatogr., 148 (1978) 211. F.K. Martens, M.A. Martens, T. Soylemozoglu and A.M. Heyndrickx, J. Chromatogr., 140 (1977) 86. .J. Janak and V. Svojanovsky, in A.B. Littlewood (Editor), Gas Chromatography 1966, Institute of Petroleum, London, 1967, p. 166. N.F. Ives and L. Giuffrida, J. Ass. Offic. Anal. Chem., 50 (1967) 1. N. Mellor, J. Chromatogr., 123 (1976) 396. R. Greenhalgh and P.J. Wood, J. Chromatogr., 82 (1973) 410. R. Greenhalgh and M. Wilson, Column, No. 15 (1972) 10. C.D. Ruyle, W.A. Aue, C.W. Gehrke, D.L. Stalling and R.C. Tindle, Application of the Alkali Flame Detector to Nitrogen Containing Compounds, 5th National Meeting of the Society for Applied Spectroscopy, Chicago, 1L, June 1966. C.E. Wells, Application of the Potassium Chloride Thermionic Detector to the Analysis of Nitrogen Containing Drugs, U.S. Food and Drugs Administration Pesticide Workshop, Kansas City, MO, 1966. W.A. Aue, C.W. Gehrke, R.C. Tindle, D.L. Stalling and C.D. Ruyle, J. Gas Chromatogr., 5 (1967) 381. C.H. Hartmann, J. Chromatogr. Sci., 7 (1969) 163. M. Butler and A. Darbre, J. Chromatogr., 101 (1974) 51. W. Ebing, Chrornatographia, 1 (1968) 382. A. Karmen and E.L. Kelly, Anal. Chern., 43 (1971) 1992. A. Karmen and H. Haut, J. Chrornatogr., 99 (1974) 349. J.F. Palframan, J. Macnab and N.T. Crosby, J. Chrornatogr., 76 (1973) 307. R. Greenhalgh and W.P. Cochrane, J. Chrornatogr., 70 (1972) 37. J. Sev~ik, Chrornatographia, 6 (1973) 139. V. Svojanovsky, M. Dressler and J. Janak, in H.G. Struppe (Editor), Gas CJwornatographie 1968, Deutsche Akademie der Wissenschaften DDR, Berlin,

1968, p. 545. 64 A. Karmen and H. Haut, Anal. Chern., 45 (1973) 822.

61 65 66 67 68 69 70 71 72

G.R. Verga and F. Poy, J. ChPomatogr., 116 (1976) 17. H.K. Deloach and D.O. Hemphill, J. Ass. Offic. Anal. Chem., 52 (1969) 533. A. Karmen, J. GaD ChY'OmatogY'., 3 (1965) 336. A.V. Nowak and H.V. Malmstadt, Anal. Chem., 40 (1968) 1108. W.A. Aue and R.F. Moseman, J. ChPomatogY'., 61 (1971) 35. K. Olah, A. Sz6ke and Z. Vajta, p. ChY'omatogY'. Sci., 17 (1979) 497. F.M. Page and D.E. Woolley, Anal. Chem., 40 (1968) 210. J. Baldwin, Alkali Metal-Halogen InteY'actions in the Sensitized Flame Ionization/Flame PhotometY'ic DetectoY', Thesis, University of Illinois, Urbana, Il, 1968. 73 V.V. Brazhnikov, M.V. Gurev and K.I. Sakodynsky, ChPomatogY'. Rev., 12 (1970) 1.

This Page Intentionally Left Blank

63

Chapter 1

FLAMELESS ALKALI SENSITIZED DETECTORS CONTENTS 4.1. Introduction . . • • • • • • 4.2. The Perkin-Elmer detector •• 4.3. The Hewlett-Packard detector 4.4. The Tracor detector •••• 4.5. The Varian detector . • . • . • . . . • . • 4.6. The Detector Engineering Technology detector. 4.7. The chemi-ionization detector ••• 4.8. Detector life and reproducibility of response 4.9. Detectors for halogen compounds References • . • • . . • • • • . . • • • • •

63 64 72 73

74 78 83

84 87 90

4.1. INTRODUCTION Detectors that utilize the different ionization of phosphorus and nitrogen compounds in the presence of alkali metals are commonly called alkali flameionization detectors (AFIDs), thermionic detectors (TIDs) or nitrogen-phosphorus detectors (NPDs), frequently regardless of whether a flame is present in or absent from the detector system. If detectors bringing about ionization of compounds in a flame are involved, the term (AFID) (Chapter 3) should be used. For other detectors, in which ionization occurs in the absence of a flame, the term flameless alkali sensitized detectors (FASDs) has been used recently. With some types of these detectors, the thermal emission is thought to be the basic process taking place in the system. It is not possible, however, to consider this process as a general explanation of the response mechanisms in all FASDs, because other supposed processes exist with some types. Therefore, the term FASD actually seems to be the most suitable for flameless detectors with an alkali metal. Compared with the AFID, the FASD is distinguished by the following main features: (1) although hydrogen and air are mostly required, the amount (flowrate) of the hydrogen used (1-6 ml/min) is not sufficient to sustain an ordinary flame; (2) FASDs of the type described so far exhibit selective responses only to nitrogen- and phosphorus-containing compounds (for this reason they are also called NPDs) and are not sensitive to halogen-containing compounds; (3) the alkali metal source is heated electrically.

64

4.2 THE PERKIN-ELMER DETECTOR The detector developed by Kolb and Bishoff 1 and manufactured by Perkin-Elmer represents a transition from an alkali flame-ionization to a flameless alkali sensitized detector. The hydrogen flow-rate is 35 ml/min for detection in the P mode and therefore should belong to AFIDs. On the other hand, the hydrogen flow-rate is low (2 ml/min) in the NP mode and, consequently, the presence of a flame as classically conceived is not likely.

r

,VENT

COLLECTOR ELECTRODE RUBIDIUM BEAD

I

I ~ f A{HEATI"d_ AIR -~PtH,L-q~ JET

I

COLUMN EFFLUENT

POLARITY SWITCH

Fig. 4.1. Simplified schematic diagram of the Perkin-Elmer detector. (Reproduced from ref. 2 with permission.} A schematic diagram of a three-electrode detector is shown in Fig. 4.1. The source of the alkali metal is glass containing an alkali metal (rubidium silicate). A negative voltage is always applied to the source, which is heated electrically. The jet polarity can be changed; if it is positive the detector operates in the P mode, and if it is negative it operates in the NP mode. The function of this detector can be seen from Fig. 4.2, which shows the electrode connections and compares the AFID and FID chromatograms. In Fig 4.2a the alkali metal source is not heated and the detector operates as an FID, not being affected by the mere presence of the source. Electrical heating of the source leads to alkali metal emission and to increased sensitivity to phosporus compounds (Fig. 4.2b). Earthing of the jet (Fig. 4.2c) causes the detector to become selective to phosphorus compounds; the decomposition products of the phosphorus compounds react with the alkali metal, giving rise to electrons above the source with a negative potential, which are sensed by the collector electrode. The electrons resulting from the decomposition of substances in the flame below the alkali metal source cannot pass

65

a

P

FI

b

,A,

rB:@

0

r~@

til-e

x

~

tV

FI+P

~e

(X)

FlO

r-1-@

~

fU-e

0

x tV c.J1

en

P .J....

C

P

P

rtt~ tll-G>

.J....

d

N+P

r:.1=~ u.-Le

N

Fig. 4.2. Operational modes of the Perkin-Elmer detector, exemplified by the analysis of the ester oil with nitrogen- and phosphorus-containing additives. Glass column (1 m) packed with 3% OV-1 on Chromosorb WHP. Temperature, programmed from 270 to 300 0 C at 80 C/min. Compounds: P = P-containing additives (trieresyl phosphates); N = N-eontaining additives (first N peak: diphenylamine). In (a)-(d) with FID; H2 flow-rate = 35 ml/min; air flow-rate = 400 ml/min. (a) NPD, FI mode: H2 flow-rate = 35 ml/min; air flow-rate = 400 ml/min; heating position, "off". (b) NPD, P + FI mode: H2 flow-rate = 35 ml/min; air flow-rate = 400 ml/min; heating position, 250. (e) NPD, P mode: H2 flow-rate = 35 ml/min; air flow-rate = 400 ml/min; heating position, 280. (d) NPD, NP mode: H2 flowrate = 2 ml/min; air flow-rate = 100 ml/min; heating position, 710. (From ref. 3. )

66

the barrier of the negative potential of the source and therefore the two groups of electrons are separated. Selectivity for nitrogen compounds is attained by reducing the hydrogen flow-rate to about one tenth of the value used with the FID and P-AFID and the air flow-rate to about 100 ml/min (Fig. 4.2d). This low hydrogen flow-rate (1-3 ml/min) can neither heat the alkali metal source nor sustain a stable flame burning at the jet; the hydrogen burns around the glowing source (the latter is electrically heated). The energy of this flame is not high enough to induce the ionization of hydrocarbons. Kolb and Bischoff 1 described the response mechanism of this three-electrode detector in the following way. Nitrogen compounds enter a relatively cool and diluted zone of the flame around the alkali metal source and are pyrolysed therei n: I

- C - N=

-+

·C

= NI

(4.1)

The radical formed takes an electron from the excited atom of the alkali metal, giving a cyanide ion: Rb* + ·C;; NI

[IC ;;

NI

r

(4.2)

The cyanide ion moves to the collector electrode, releasing an electron that is sensed by the collector electrode. The reaction scheme can be seen from Fig. 4.15 (see section 4.7). This explanation of the selective response to nitrogen compounds is supported by the zero (or low) response to compounds having the nitrogen bonded in a way that does not allow the formation of a radical [barbiturates with a -CO-NH-CO structure, amides (-CO-NH 2 ) and nitrate esters (0-N0 2 )]. l'li th these compounds, the oxygen atom cannot be extracted from the carbon atom; however, if the nitrogen atom in barbiturates is alkylated, the R-CH 2-N= structure allows the formation of the 'C ;; NI radical and the response is normal. It has been reported 2 , as another supporting fact for the above mechanism of the response of nitrogen compounds, that no selective response to nitrogen compounds occurs in a hot oxidation flame that favours the formation of nitrogen oxides (the response to nitrogen compounds in the P mode is about two orders of magnitude lower, in fact 4 ). An analogous reaction mechanism giving PO or P0 2 radicals is considered 1 to exist with phosphorus compounds: I· P = 0) (0 ='p

+ eO~

+ e-

->

[(P = 0)

->

[~O =

r

P - QI]-

(4.3)

°/r

(4.4)

67 The response of the detector is approximately proportional to the nitrogen content in the compound. However, the response is also dependent on the compound However, the response is also dependent on the compound structure, as is obvious from Table 4.1, which gives relative responses to nitrogen compounds converted to the mass content of nitrogen. An alternative theory that explains the long-term stability of the PerkinElmer NPD was eleborated by Ol~h et al. 6 . The mechanism for a nitrogen compound is as follows: k'

N-comp·(vapour) N-comp·(ads.) ~

CN(ads)

+

Rb+ + e

(4.5)

(degradation)

(4.6)

(desorption)

(4.7)

Rb+ + CN(adS) (ionization)

(4.8)

k

~

CN(ads)

products

Rb + CN(ads) CN(adS)

(adsorption)

N-comp·(ads)

~

+

~

CN(gas) +

Rb

(emission)

(4.9)

(recombination)

(4.10)

The essential parts of the reaction sequence are the following: 4.5 and 4.6: non-ionic, voltage-independent reaction, rate-determinating step at high voltages and limiting current; 4.7: process concurrent with 4.8, branching to non-ionic desorption and degradation, at least to CO 2 and N2 ; 4.8 and 4.9: ionization and ion emission, activated and exponentially becoming a voltage-dependent process, rate-determining step at low voltages; 4.10: Rb atom recombination on the bead surface; the electrons originate from the platinum wire, migrating through the glass phase. The charge carriers are electrons and emitted negative ions. The former migrate inside the glass from the platinum to the surface and the latter move from the glass surface through the gas phase to the collector. According to this theory, the ionization process is catalytic, the Rb atoms being only surface catalysts of the electron transfer. The alkali glass is not a source, because the Rb atoms do not leave the glass surface. The idea of electrically heating the alkali metal source originated in order to make the source temperature independent of the gas flow-rates and to provide for setting optimum flow-rates of these gases. As already mentioned in Chapter 3,

en 00

TABLE 4.1 RELATIVE RESPONSES OF NPD AND FID TO DIFFERENT TYPES OF NITROGEN COMPOUNDS (From ref. 5 with permission.) Compound

Pyridine 2,6-Dimethylpyridine 4-Methylpyridine 2,4,6-Trimethylpyridine 4-tert.-Butylpyridine Quinoline 2-Methylquinoline 6-Methylquinoline 2,6-Dimethylquinoline 2,4-Dimethylquinoline Indole 3-Methylindole 2,3-Dimethylindole N-Ethylcarbazole N-r~thylcarbazole

5,6-Benzoquinoline l,2,3,4-Tetrahydrocarbazole 6-Methyl-1.2,3.4-tetrahydrocarbazole Carbazole

Blended weight ratio

Relative area response ratio

Relative response factor, weight/area

Nitrogen

Carbon

NPD

FID

NPD

FID

1.278 0.9494 1.076 0.8720 0.7551 1.008 0.9857 0.7154 0.6445 0.9146 1.000 1.017 0.7061 0.5466 0.6129 0.7819 0.6224 0.5481 0.7472

0.7988 0.8308 0.8082 0.8759 0.8515 1.133 1.235 0.9154 0.8853 1.257 1.000 1.145 0.8815 0.9567 0.9981 1.272 0.9360 0.8928 1.120

1.617 1.120 1.564 1.109 1.145 1.101 1.054 0.8450 0.6985 0.9732 1.000 0.9628 0.6260 0.5942 0.6596 0.8492 0.5204 0.4831 0.6621

0.7122 0.8450 0.7534 0.9438 0.8995 1.037 1.2S2 0.8880 0.9255 1. 318 1.000 1.184 0.8730 0.9525 0.9257 1. 284 0.9078 0.8889 1.072

0.7904 0.8477 0.6880 0.7863 0.6595 0.9155 0.9352 0.8656 0.9227 0.9398 1.000 1.056 1.128 0.9199 0.9292 0.9207 1. 196 1.135 1.128

1.122 0.9832 1.073 0.9281 0.9466 1.093 0.9864 1.031 0.9566 0.9537 1.000 0.9671 1.010 1.004 1.078 0.9907 1.031 1.004 1.045

69

the background ionization current increases with increasing flame temperature owing to the higher temperature of the source and the greater ionization of the alkali metal. When increasing the temperature of the source by electrical heating, without changes in the flow-rates of the gases, the background ionization current increases exponentially depending on the vapour pressure. The response then also increases with increasing background ionization current 4 ,7,8. If the background ionization current of a three-electrode detector is increased by increasing the hydrogen flow-rate with a constant heating current, the responses to nitrogen compounds also increase, but they do so faster than when the background ionization current is changed by heating it electrically at a constant hydrogen flow-rate. Even if the background current is kept constant (by changing the heating current), the response increases with increasing hydrogen flow-rate. This means that the variations in response caused by the hydrogen flow-rate cannot be compensated for by changing the heating current so as to maintain a constant background current. The same is also true for the flow-rates of the carrier gas and air7. However, on changing the heating current so as to keep the background ionization current constant, the decrease in response with time can be compensated for (see section 4.7). In contrast to Lubkowitz et al. 7, who reported an increased sensitivity when the hydrogen flow-rate was increased in the NP mode with a constant background current (adjusted by changing the heating current), a decrease in sensitivity was also found 1,2,8. In the P mode, the detector can also respond to nitrogen compounds: about 2 • 10- 3 C/g of azobenzene, this sensitivity being essentially independent of the hydrogen flow-rate. By reducing the nitrogen flow-rate the response can be increased (relative to the optimum for phosphorus compounds) to a value as high as 1.3 . 10- 2 C/g 4. At a constant heating current, this optimum is about 74 ml/min for phosphorus compounds 4• In the NP mode, the dependence of the sensitivity on the flow-rate of the carrier gas reveals a maximum at low flow-rates, about 7 ml/min (similarly to the background current 7). The replacement of nitrogen with helium leads to changes in the background current owing to a difference in the thermal conductivity of the two gases 9 . The dependence of the sensitivity on the air flow-rate shows a maximum at flow-rates of about 200 ml/min (the same as for the background current) in both the P mode 4 and the NP mode 7. The basic parameters of the detector are listed in Table 4.2. Table 4.3 gives the basic parameters of a pure rubidium-quartz source of the same detector construction prepared from ground quartz mixed with rubidium hydroxide 8.

TABLE 4.2 CHARACTERISTIC OPERATION SPECIFICATIONS OF THE PERKIN-ELMER DETECTOR IN THE THREE MODES OF OPERATION (Reproduced from ref. 2 with permi ss ion. ) Parameter

Sensitivity Minimum detectable mass rate Linearity Selectivity Noise

FI mode

5.10- 3 C/g C 3.10 12 g C/sec 2.10 7

P mode

1 C/g P 5.10- 14 9 P/sec 10 5 10 6 g C/g P ca. 5.10- 14 A

NP mode P

N

5 C/g P

0.5 C/g N

1.10 14 g P/sec 5

10 1.25.10 5 g C/g P

1.10- 13 g N/sec 105 2.5.10 4 g C/g N ca. 7.10- 14 A

TABLE 4.3 PERFORMANCE CHARACTERISTICS OF RUBIDIUM/QUARTZ BEADS (Reproduced from ref. 8 with permission.) Compound

Atom X Sensitivity (C/g X)

Diethylphenyl phosphorothioate P Ch 1orpyri fos P Atrazine N Lindane Cl Hexadecane C

Selectivity (g X/g C)

2.4% Rb

7.6% Rb

19.6% Rb

2.4% Rb

7.6% Rb

19.6% Rb

3.1

4.1

9.2

3.6 0.4 2.9.10- 3 5.2.10- 5

3.3 0.2 1.2.10- 3 2.3.10- 5

6.4 0.3 2.2.10- 3 1.2.10- 5

6.10 4 7.10 4 7.10 3

1.8.10 5 1. 4.10 5 9.10 3

57 1

50 1

6.1.10 5 5.4.10 5 2.5.10 4 1.9.10 2 1

mmin (g X/sec) (19.6% Rb)

Noise (A)

1.1.10- 15 1. 7.10- 15 3.4.10- 14 4.4.10- 12 7.2.10- 10

1.1.10- 14

72

4.3 THE HEWLETT-PACKARD DETECTOR The NPD manufactured by Hewlett-Packard uses a ceramic cylinder coated with an alkali metal salt activator as a source of alkali metal. The source is electrically heated. A voltage of -240 V is applied to the cylindrical collector electrode enclosing the source (shown schematically in Fig. 4.3). A low-temperature plasma is generated around the source 10 . The basic parameters of this detector are given in Table 4.3. The use of an 8% mixture of hydrogen in helium, instead of pure hydrogen, improves the sensitivity of the detector. The detector also responds to those nitrogen compounds which do not contain HCN bonding (cf., Fig. 4.2). The response mechanism described by Kolb and Bischoff 1 for the Perkin-Elmer detector does not apply to the Hewlett-Packard detector, because the detector potential is negative. In addition, it responds fairly well to barbiturates and pesticides which contain vicinal carbonyl groups and no HeN bonding lO •

POLARIZING VOL TAG E TO RECORDER

lOj r::J AIRHYDROGEN

I

(1

:lECTRODES

"'"ALKALI METAL SOURCE JET

COLUMN EFFWENT

Fig. 4.3. Schematic diagram of the Hewlett-Packard detector. (From ref. 11.) TABLE 4.4 PERFORMANCE CHARACTERISTICS OF THE HEWLETT-PACKARD DETECTOR (From ref. 10.) Parameter

Compound Nitrogen

t1inimum detectable mass rate: 8% H2-He H

Sel~ctivity (g X/g C) Linear dynamic range Background current (A) Flow-rate of hydrogen (ml/min): 8% H2-He Air

<1.10- 13 g N/sec <4.10- 13 g N/sec 3.4.10~ >10

Phosphorus P/sec P/sec 1.6.10- 11 1.0-5.0

15 -60 30 -100

73

4.4 THE TRACOR OETECTOR The source of alkali metal in the Tracor detector is a mixture of alkali metal salts in a silica gel matrix 12 • The detector is provided with a quartz jet, above which the alkali metal source and the collector electrode are positioned. The heating element for the alkali metal source is a short loop of platinum wire, which constitutes one arm of the Wheatstone bridge 13 • As the resistance of platinum is directly proportional to its temperature, the voltage drop across the source can be related to the power required to heat the source in order to maintain a fixed resistance (temperature) 14. Theoretically, the rapid response of the alkali metal source heater makes the source temperature independent of the carrier and plasma gas flow-rates. In fact, small variations in the source temperature occur, however13. TABLE 4.5 PERFORMANCE CHARACTERISTICS OF THE TRACOR DETECTOR (From ref. 12.) Parameter

Compound Nitrogen

Phosphorus

Minimum detectable mass rate Selectivity (g X/g C) Linear dynamic range

The basic parameters of this detector are listed in Table 4.5. The sensitivity to phosphorus and nitrogen compounds, hydrocarbons, the background current and noise increase as the source setting (temperature) is increased. The sensitivity to nitrogen compounds and hydrocarbons decreases with increasing hydrogen flowrate (1-7 ml/min) at a constant background current, whereas the sensitivity to phosphorus compounds increases. Therefore, nitrogen compounds can be distinguished from phosphorus compounds at different hydrogen flow-rates or due to a change in the negative polarizing voltage. A decrease in voltage reduces the phosphorus response by 30% while maintaining the normal response for nitrogen.

74 4.5 THE VARIAN DETECTOR Fig 4.4 is a schematic diagram of the detector manufactured by Varian. A ceramic alkali metal source containing an alkali metal salt in its entire volume is placed above the detector jet, through whi ch the effl uent together wi th hydrogen leaves the chromatographic column. The source is heated electrically and a cylindrical collector electrode is situated around it •

•

_ _~SCREEN

-I~I---COLLECTOR

~~~~~~~~~~:i~~~~t-

WITH

ERAMICBOTTOM BEAD WITH HEATER COIL FLAME TIP

l-----DETECTOR BASE 1 1 1 4 - - - - GC COLUMN

CARRIER GAS

Fig. 4.4. Schematic cross-section of the Varian detector. (Reproduced from ref. 16 with permission.) TABLE 4.6 PERFORMANCE CHARACTERISTICS OF THE VARIAN DETECTOR (From ref. 16.) Parameter

Compound Nitrogen

Minimum detectable mass rate Selectivity (g X/g C) Linear dynamic range Background current (A) Noise (A) Flow rates: Hydrogen (ml/min) Air (ml/min)

Phosphorus

5'10~14 9 P/sec 4.10- 12 1_10- 14

4.5 200

2·10 10 4

75

The sensitivity of the detector to nitrogen and phosphorus compounds is about five orders of magnitude higher than that to other compounds (see Table 4.6). \~ith a low negative voltage applied to the alkali metal source, the response (negative ions) is high at a low background ionization current. On passing from a negative to a positive voltage the polarity of the response is changed, and the response is relatively low at a relatively high ionization current 15 ,16 (see Fig. 4.5). A voltage of about -4 V is applied to operate the detector 16 • The temperature of the source is the main parameter affecting the response of the detector (Fig. 4.6). The detector response drastically increases with increasing heating current of the source, with both phosphorus and nitrogen compounds. However, the background ionization current and, consequently, the detec-

z UJ

Z

~

z~

~~ OUJ

~~ U

UJ

z

-22V

12

-6J, • 1(f

A

a.. za..

o

~ a..

UJ

I

-1.85V 12 + 64·10· A -1.80 V

_12

+64'10 A

Fig. 4.5. Chromatograms of detector test sample at different settings of bead bias voltage. IB = background current. Column, glass, 200 cm x 2 mm 1.0., packed with 5% OV-IOI on Gas-Chrom W. (Reproduced from ref. 17 with permission.)

76 1D 0.9 0.8 0.7 0.6 0.5 0.4 t-

Z

0.3

UJ

c::: c:::

0.2

:J

U

Z

Q Cl

0.1 0 2.50

2.70

2.80

BEAD CURRENT (A)

UJ

N

2.90

~

toO ~ 0.80 0.60

c::: ~

0.40 0.20 0.10 0.08 0.05 0.04 0.02 0.115

0.120

0.125

0.130

0.135

0.140'

0.145

(BEAD CURRENTr 2 (A- 2)

Fig. 4.6. Variations of the background current (I B), peak height of azobenzene (IN) and peak height of mal~thion (Ip) as a functlon of bead heating current and (bead heating current)- • In tne top graph the ion current scale in linear and in the bottom graph it is logarithmic. (From ref. 15.) tor noise increase more rapidly, so that the minimum detectable mass rate increases. It is recommended, therefore, that one should work at a source temperature that is low enough to be still suitable for the decomposition of the sample; in fact, a background ionization current of 4'10- 12 A is appliedl~. The hydrogen flow-rate is low and, therefore, it exerts a negligible influence on the temperature of the source surface 15 ,16. However, changes in the hydrogen flow-rate can result in substantial changes in the radical concentrations in the boundary layer of the gases (see later); hence the hydrogen flow-rate strongly affects the response level and the selectivity of the detector (Fig. 4.7). At higher hydrogen flow-rates, the detector response to hydrocarbons increases whereas that to phosphorus and nitrogen compounds decreases. At increased air flow-rates (100-200 ml/min), the background ionization current and, therefore,

77

the detector response decrease. If nitrogen is used as the carrier gas, the decrease occurs more rapidly than with helium. A similar dependence also holds for the flow-rates of the carrier gas (10-60 ml/min); the decrease in response with increasing flow-rate is faster in helium. However, if the background ionization current is kept constant by changing the heating current of the alkali metal source, the response is constant at varying flow-rates of air and carrier gas.

1.0 0.8 0.6

r

Z

0.4

UJ

0:: 0:: 0.2 ~

u

z

0

0 1.0

0

He

UJ

N

::::i

0.8

,,


L 0.6 0:: 0

Z

I

0.4

,, ,,

/

,,

;f

"

p"

0.2

..... ~' "

"

0 0

2

4

6

10

H2 FLOW (mllminl

Fig. 4.7. Variations in the background current (IB)' peak height of azobenzene (IN), peak height of malathion (Ip) and hydrocarbon response current (IC) as a function of H2 flow-rate for He and NZ carrier gases at 25 ml/min. Bead heating current held constant. (From ref. 15.) The sample leaving the chromatographic column impinges on the hot surface of the alkali metal source (700-900 oC), where it decomposes. A boundary layer of high-temperature gases is produced around the hot source. The layer consists of radicals, similarly as in a flame (H, OH, 0), which play an important role in the decomposition of the compounds and in subsequent ionization. In this detector, the ionization mechanism is ascribed to surface ionization processes 15 ,16. Negative ions are emitted from the surface of the alkali metal source. The work function of the beads is about 3.4 eV, whereas it drops to some 2.3 eV in the presence of nitrogen and phosphorus compounds (as calculated from the dependence

78 of the response and the background ionization current on the heating current of the source and on the detector temperature). Hence the detector response to nitrogen and phosphorus compounds is due to the reduction of the work function of the bead. This reduction is attributed to the formation of decomposition products that are highly electronegative (probably CN, N0 2 and P0 2 ) and to the subsequent generation of gas-phase negative ions owing to extraction of electrons from the surface of the heated source. Both the background ionization current and the detector response depend on the temperature of the source surface (see Fig. 4.6) in a way that can be expected for thermionic emission 15 • 4.6 THE DETECTOR ENGINEERING TECHNOLOGY DETECTOR In 1982, Patterson et al. 17 described another detector, developed for the Spectra-Physics gas chromatograph. A significant difference between this detector and the previous version 15 ,16 is that the source is in the form of a cylindrical collector electrode. This detector has been further modified 18 ,19 so that the alkali metal source possesses a separate non-alkali metal/ceramic sublayer and an alkali metal/ceramic surface layer (Fig. 4.8). The sublayer protects the metallic heating wire in the source from chemical attack by corrosive alkali metal atoms or by corrosive decomposition products of the sample compounds. The detector is provided with a built-in temperature-sensing element, which is used in an electronic control circuit that varies the heating current so as to maintain a constant source temperature. Two different surface coatings were described for the detector 17 ,19. One of them contains a low concentration of a caesium compound that is used for operation in dilute hydrogen/air environment. The other coating contains a much higher concentration of the caesium compound and nitrogen is the only gas supplied to the detector. The mechanism of detection is thought to be the same as already described in section 4.5 for the Varian detector, i.e., a surface ionization process that can be described schematically as follows: Sample + Hot surface + Chemically reactive gases

+

Electronegative species + Hot surface

Electronegative decomposition products

+

(4.11)

Negative ions

NP selectivity is obtained from the very hot surface (600-800 0 C) of moderate work function (so-called TID-2-H /air). In the TID-I-N 2 mode, the low-work-fonc2 tion source operates at a moderate temperature (400-600 0 C). In this instance, the hydrogen (gas 1) and air (gas 2) flows to the detector are replaced with

79

/n"(/ rfi\ .,.

....

-'"

'\

I I

I I

I

I

I

I

I

SA~PLE

GAS 2

SAMPLE CONDUIT

GAS 1

Fig. 4.8. Schematic diagram of the TID detector. (From ref. 19.) nitrogen flows (both gas 1 and gas 2). The gaseous boundary layer of the source is no longer chemically reactive. Therefore, if samples are to decompose into electronegative products, the electronegative functional groups must originate within the sample itself l7 , i.e., the detector is selective to certain compounds containing electronegative functional groups: Sample

+

Hot surface

+

Electronegative decomposition products

Electronegative species + Hot surface

~

Negative ions

(4.12)

The source is biased at a negative potential (-15 V for the Spectra-Physics, -5 V for the TID-2-H 2/air, and -15 or -45 V for the TID-1-N 2 ). For the TID-2-H 2/air, the basic parameters of the detector at flow-rates of hydrogen = 3 ml/min and air = 60 ml/min are as follows 19 : sensitivity, 0.35 C/g of N (azobenzene) and 1.1 C/g of P (malathion); minimum detectability, 1,10- 13 g of N/sec (azobenzene); linear dynamic range, 10 5 ; N:C selectivity, 2.10 4 ; P:C selectivity, 4'10 4. Also with this type of detector the above basic parameters depend on the temperature of the source (heating current) and on the hydrogen flow-rate. I~ith i ncreas i ng source temperature the background current ; ncreases, the selectivity slightly increases and the minimum detectability decreases. The dependence of the selectivity and minimum detectability for nitrogen compounds on the hydrogen flow-rate displays an optimum value in the vicinity of flow-rates

80

TID

FID

H2 =6 ml/min 1024 • 1(j"12 A

N.P

P

c

c N N

N.P

TID

P

TID

H2=3 ml/min 128' 1012A

~

a...

Z·

H2 = 9 ml/min 11 512 '10 A

C a...

z 0

N,P

P

N I

0

2

4

6

MIN

0

2

4

6

MIN

Fig. 4.9. Comparison of FlO and Spectra-Physics TIO chromatograms of the detector test sample. (Reproduced from ref. 17 with permission.) of 5-6 ml/min. At a hydrogen flow-rate above 5.5 ml/min, the background current increases very rapidly and the detector loses its selectivity and starts to behave like an Fl0 20 • Fig. 4.9 shows chromatograms of the FlO and those of the above detector at different hydrogen flow-rates. As can be seen, the phosphorus compounds behave in a different way to nitrogen compounds 17 when the hydrogen flow-rate is changed. At a hydrogen flow-rate of 6 ml /mi n the sens iti vity for phosphorus compounds is enhanced as compared with that at a flow-rate of 3 ml/min, but that for nitrogen compounds is suppressed. The detector becomes much less selective at hydrogen flow-rates of 9 ml/min and above. The basic parameters of the TlO-1-N 2 at a total nitrogen flow-rate of 50200 ml/min are as follows 19 : noise, 1'10- 14 _2'10- 14 Ai sensitivity, 0.6 C/g of

81

2,4-dinitrotoluene; minimum detectability, 1.10-13 g of 2,4-dinitrotoluene/sec; selectivity, >10B (methylparathion/C 15 ); linear dynamic range, 10 3_10 4• The minimum detectability is again dependent on the heating current, detector temperature and other instrumental parameters 21 Peak tailing was observed when using a TID-1-N 2 (heating current 3.2-3.4 A). TID (L-Cs) H2/AIR 128 .1O-12 A Cl.

~

ii:_

TID(H-Cs) N2 16·1Q-11A

Fig. 4.10. Comparison of chromatograms of the test sample for the two different modes of TID operation. L-Cs, low-concentration Cs source; H-Cs, hiqh-concentration Cs source. (Reproduced from ref. 17 with permission.) The sensitivity of the above detector depends not only on the presence of an electronegative atom or group in the molecule of the compound (Fig. 4.10), but also on the structure of this molecule. This is obvious from Fig. 4.11, which shows the chromatograms obtained from the analysis of base neutrals that are of concern as water pollutants. The catalytic flame ionization detector (CFID) is a detector that resembles the TID-1 and TID-2 in design, but its source consists of a nickel-ceramic composition for both the sub-surface and surface layers. The hydrogen and air flow-rates to the detector are 25 and 100 ml/min, respectively. The CFID provides universal responses to most organic compounds and is similar to but not identical with an FID lB ,19. The TID-l-N 2 mode yields a selective response only to 2,6-nitrotoluene and 3,3'-dichlorobenzidine. If the detector gas environment of the TID-l source is changed from nitrogen to oxygen, the TID-l-0 2 mode yields an enhanced relative response to chloro compounds and a diminished relative response to nitro compounds. The TID-2-H 2/air mode responds to all nitro-

82

BASE NEUTRALS CFID a:

a:

:J:

:J:

UJ

.... UJ

::; ,..

200 ng

16'10-10 A

UJ

.... UJ

UJ

Z UJ

:J:

:J:

.... UJ o a:

g :J:

~

,.. ...J

x

UJ

UJ

Z

:I:

,..

UJ

...J

N Z

:I:

.... ~

UJ

CD

o

a:

....

.... Z

..: a:

o

~

...J U.

E

II)

a;

Z

I J TID-1-N2

8' 10- 9 A

J_'-----------'~~

I ~~

TI

.D1~1_~OO~

~~l---------.J!~' TID -2-H2/AIR

16 .10- 9 A

Fig. 4.11. Chromatograms of base neutrals. (From ref. 19.) gen compounds. Fig. 4.12 depicts other modes of the TID-I. Comparison of the response to 4-nitrophenol and 2-nitrophenol illustrates a significantly greater TID-I-N 2 sensitivity for the isomer with the nitro group in the 4-position. If the source is operated in gas environment composed of approximately equal flows of nitrogen and air. the sensitivity to certain compounds is suppressed whereas that to others is enhanced. If both detector gases 1 and 2 are air. responses are obtained for all of the chloro- and nitrophenols. with the dominant response for dinitro compounds (FiJ. 4.13). The sensitivities to 2- and 4-nitrophenol are commensurate. For nitro 'aromatics the TIO-I-N 2 provides a negligible response to nitrobenzene and a selective high response to 2,4- and 2,6-isomers of dinitrotoluenes with about a tw;'~e as high response to the 2,4_isomer 1B ,19 The sensitivity for nitrated polycyclic aromatic hydrocarbons is also very different. It differs from solute to solute by as much as a factor of 100 (ref. 21).

83 CFI D 3· 10-10 A

...J

o

z

UJ

:t

"o

II:

....

.¥

TI D-l-N

3 '10- 8 A

L TID-l-N/AIR

3 ·10-s A

Fig. 4.12. Chromatograms of phenols. (From ref. 19.) 4.7 THE CHEMI-IONIZATION DETECTOR A detector that yields a selective response in a nitrogen environment when an alkali metal is present was described by Scolnick 22 . In this detector (called the chemi-ionization detector), caesium bromide in the gas phase is brought into the reaction space of the detector. The detector gives a selective response to phosphorus compounds and this response is 20-60 times smaller than that of the AFID. A schematic diagram of the device is presented in Fig. 4.14. The effluent from the column enters the reaction zone of the detector (temperature 800-850 0 C), where it mixes with nitrogen saturated with CsBr vapour in the saturation zone at about 500 0 C. The tubular cathode is earthed and the collector electrode is biased at +50 V. The results of the measurements showed that the ionization reactions between the alkali metal salt vapour and the phosphorus compounds can occur in the gas phase and in the absence of combustion products or hydrogen and oxygen radicals.

84 TIO-l-AIR 8 • 10-9 A

-' 0

-' 0

Z

8•

.!t

~

0

Z UJ

0 Z

-' 0 Z

UJ

:I:

J: IL 0

iE

:I: IL 0

It:

0 -' :I:

<..)

It:

Y N

-'

UJ

0

-'

:I:

~

.,

<..)

'"N

-t'

-'

0 .... It: t: z Z z UJ :I: ;:; IL

-'

-'

It:

'r I' 0

It:

:I: IL 0 It: 0

UJ

:I: IL 0

UJ

10-1OA

'"

-' 0

N

-i

'" E UJ

It:

<..)

I

0

It:

0

-'

:I:

y..,

N'

0

It:

0

-'

:I:

<..)

....'z"

UJ

IL

..J

0

Z

UJ

:I: IL 0 It:

....

~

Fig. 4.13. Chromatograms of phenols. Both TID gases 1 and 2 are air. (From ref. 19. )

4.8 DETECTOR LIFE AND REPRODUCIBILITY OF RESPONSE As has already been mentioned in Chapter 3, the popularity of FASDs over AFIDs is due mainly to the long service life of the alkali metal source in the FASD. The life of the FASD has been reported to amount to 1000 h of operation. A high long-term stability has been reported for the Perkin-Elmer threeelectrode detector and is ascribed to the following mechanism 1• Transport of the alkali metal cation to the gas phase from a source having a negative potential is impossible, and this transport is also unlikely with the silicate of an alkali metal. Hence only the neutral atom of the alkali metal, which can be formed by the alkali metal ion accepting an electron (at the temperature applied the glass behaves like an electrolyte), can be transported to the gas phase. Owing to ionization in the flame, alkali metal ions are produced, which are immediately collected by the negatively charged source, neutralized and re-evaporated (see Fig. 4.15). The relative standard deviation of the response for methamphetamine (NP mode) was 1.5% over 30 days9. Lubkowitz et al. 7 showed, however, that the service life of the above detector in the NP mode, particularly with background ionization currents exceeding 5.10- 12 A, is markedly shorter. When the source is heated electrically, the back-

85 INSULATING MATERIAL

BIAS VOLTAGE ....=1---11-+-1111-- E LE CTROMETER

VYCOR TUBING

CsBr+ GLASS BEADS - - -...

COLUMN EFFLUENT

Fig. 4.14. Schematic of the chemi-ionization detector. (Reproduced from ref. 22 with permission.)

Flame space Surface of the bead Fig. 4.15. Alkali metal recycling mechanism in the Perkin-Elmer detector. e- = electron; A' = alkali metal atom; A* = excited alkali metal atom; A+ = alkali metal ion; R' = eN or P02' (Reproduced from ref. 2 with permission.) ground ionization current decreases from 51'10- 12 to 0.8'10- 12 A within 16 days, and higher heating currents are required to attain the same background ionization current. Table 4.7(A) reveals the rapid decrease in response during the first 6 h, simultaneously with the reduction in the background ionization current when the heating current is kept constant. By maintaining the background ionization current constant by increasing the heating current, a relative standard deviation of 0.94% can be obtained [Table 4.7(8)J.

86 TABLE 4.7 REPRODUCIBILITY STUDIES OF PEAK AREAS (From ref. 7.) Time elapsed (h)

Heati ng current (dial setting)

A. Constant heating current: 6.600 o 6.600 6 24 6.600 6.600 48 B. Constant bead current: 0"* 7.236 1 7.242 2 7.264 3 7.278 4 7.298 5 7.320 6 7.345

Bead current (pA)

Area response * (arbitary units)

14.3

7320 5823 3462 1685

11.3

7.20 4.17

4622 4606 4718 4659 4690 4704 4705

14.3 14.3 14.3 14.3 14.3 14.3 14.3 Av~rage

R.S.D.***

4672 0.94%

*Constant azobenzene injection of 11.8 ng. "*Initial setting not changed over a period of 48 h. ***R.S.D. = relative standard deviation. A smaller decrease in sensitivity with time (to about a quarter after 30 days) was found by Schulte and Shive 23 . The inconsistency of the determined servi ce 1i fe wioth the mechani sm of the a1ka 1i metal ci rcul ati on has been ascri bed to the fact 4 that ageing o~ the source is caused by changes in the properties of the platinum wire carrying the silicate. The reducing flame that burns around the alkali metal source induces embrit°tlement and cracking of the platinum wire. In contrast to the NP mode, the flame burns at the jet in the P mode and there is no reducing atmosphere. The response does not change during 7 days4. The lowest reproducibility of the response was found for sources with high boron and sodium contents 24 • The stability of the Hewlett-Packard detector has been reported to be excellent 10 ; the standard deviation of 199 analyses of samples containing nanogram amounts of amphetamine and methamphetamine was less than 5%. The service life of the ceramic alkali metal source used by Varian is several thousand hours. However, the detector sensitivity decreases during the operation of this source and changes in the selectivity of the response occur. This de-

87

crease can be offset by adjusting the heating current of the source 15 (Fig. 4.16). Detectors with sources consisting of an inert ceramic core with a surface coating of an alkali metal-ceramic activating material also exhibit decreases in background current, sensitivity and selectivity with operating time 20 • The decrease in sensitivity and selectivity can be restored to some extent by periodic adjustments of the source heating current. A decay of background current and sensitivity was also observed with the Tracor detector 13 • Therefore, it is obvious that almost all commercial FASDs (NP mode) also suffer some decrease in sensitivity with operating time.

1.0 w VI

Z

~

VI W 0::

w Z

5

-----_+_

ACETONE

1 - - - - - - - - ACETONE

SOLVENT

AND CHLOROFORM

0

50:50

W

U. U.

~ 1.0 o W N

:;

5

<{

~

0::

o Z

-----+--METHANOL EXTRACT OF COLA DRINK-

o

50:50

o

200

400

600

BOO

1000

OPERATING TIME

1200 (H)

1400

1600

lBOO

2000

Fig. 4.16. Change of detector response as a function of bead operating time. Discontinuous steps in data correspond to increases in bead heating current to the values indicated. (Reproduced from ref. 16 with permission.) 4.9 DETECTORS FOR HALOGEN COMPOUNDS All the types of FASDs described in the previous sections, except for the TID-2-N2 (section 4.6), are selective for phosphorus and nitrogen compounds. However, as this chdpter deals with the FASD it should also include the surface ionization detector, which is selective for halogen compounds and is also called the thermo-ionization detector in the literature. In 1961, Cremer et al. 25 employed a surface ionization detector as a halogen-sensitive device in the search for leakages 26 and as a detection device in the gas chromatography of halogen

88 To heater To anode

Glass envelope

Silicone-rub

sleeve IIII!I--- Brass adapter

From column

Fig. 4.17. Halogen-sensitive detector. (From ref. 27.) compounds. The detector (Fig. 4.17) consists of an anode containing an alkali metal (ceramic body, alkali metal-coated platinum) and of a cathode usually in the shape of a cylinder enclosing the anode. The anode is electrically heated (800 oC). If the electron affinity of the anode material is higher than the ionization potential of the alkali metal, emission of positive ions from the heated anode occurs 28 ,29. The ion emission increases in the presence of halogens. This phenomenon is attributed 30 to the formation of a K-O complex on the surface of the anode metal that splits off K+ at higher temperatures. The probability of cleavage of the complex increases in the presence of halogens. A Cl-K-O complex is produced that considerably alters the work function. The sensitivity of the detector depends on the heating current of the anode and on the detector temperature 25 ,31. The response to halogen compounds increases with increasing heating current 27 ,31-33 (Table 4.8) and, at the same time, the response to hydrocarbons 27 , the background ionization current and the noise also increase 31 ,33. The response also increases with increasing voltage of the anode, and the detector noise also increases again 27 • The response increases with increasing carrier gas flow-rate (40-120 ml/min)34. The response is higher for chlorine compounds than for other halogen compounds 25 , and is about four orders of magnitude higher than for halogen-free compounds 25 ,32. The ratios of the molar

89 TABLE 4.8 CHANGE IN RESPONSE OF A PLATINUM THERMIONIC DIODE TO SOME PESTICIDE COMPOUNDS WITH INCREASING INCREMENTS OF FILAMENT HEATER CURRENT (From ref. 33.) Pesticide

Lindane Heptachlor Aldrin Tel odri n Kelthane Heptachlor epoxide p,p'-Dichlorodichlorophenylethylene Dieldrin Methoxychlor Parathion Melathion Trithi on

Amount injected (ng) 0.18 0.38 0.36 0.36 0.11 0.36 0.36 0.36 9.0 4.0 8.0 8.0 16.0 4.0 8.0

Response- as peak height (mm) lo7A *

1.8 A*

1.9 A*

10

30 32 17 26 14 22 9 9 30 0 0 4

91 116

15 7 11

7 11

4 4 13 0 0 7

17

72

97 75 95 40 39 90 0 0 8 13 41 81

*Filament current.

responses of fluorine, chlorine, bromine and iodine compounds are 0.9 : 1 : 0.8 : 0.1 31 . The ratios of the molar responses of mono-, di-, tri- and tetrachloromethane are 1 : 2 : 3 : 431 and those of mono- and trichlorobenzene are 1 : 2 to 1 : 2.2 32 • The response decreases with operating time and the ratio of the molar responses of the individual halogens also changes (but that for chlorine compounds is always the highest)31. The detection limit is 1.10- 10 g of chlorine compounds 27 ,33 and the linear dynamic range of the response covers 4-5 orders of magnitude 32 ,34,35. The distance between the electrodes is critical (usually it is about 1.5 mm). loJith an increase in distance above 1.5 mm the response decreases rapi dly and becomes zero at an electrode di stance of 4 mm 33 . Cremer and co-workers 32 ,35 applied pre-combustion of the sample prior to its introduction into the space of the detector proper. Pre-combustion overcomes the problems related to the variations in the baseline in the presence of excess of solvent, enhances the reproducibility of the response and correlates the ratio of the molar responses of the halogen compounds concerned with the number of halogen atoms in the molecule of the compound.

90 REFERENCES 1 B. Kolb and J. Bischoff, J. Ch~omatogr. Sai •• 12 (1974) 625. 2 B. Kolb, 11. Auer and P. Pospisil, J. Chromatogr. Sai., 15 (1977) 53. 3 B. Kolb, M. Auer and P. Pospisil, J. Chromatogr., 134 (1977) 65. 4 B.P. Semonian, J.A. Lubkowitz and L.B. Rogers, J. Ch~togl'., 151 (1978) 1. 5 O.K. Albert, Anal. Chern., 50 (1978) 1822. 6 K. OUh, A. Szoke and Z. Vaita. J. Chromatogr. Sai., 17 (1979) 497. 7 J.A. Lubkowitz, J.L. Glajch, B.P. Semonian and L.B. Rogers, J. Chromatogr., 133 (1977) 37. 8 R. Greenhalgh, J. MUller and W.A. Aue. J. Chromatogr. Sai., 16 (1978) 8. 9 M.J. Hartigan, J.E. Purcell, M. Novotny, M.L. McConnell and M.L. Lee, J. Chromatogr •• 99 (1974) 339. 10 C.A. Burgett. D.H. Smith and H.B. Bente, J. Chromatogr., 134 (1977) 57. 11 H.B. Bente, Appliaation Note ANGC 2-77, Hewlett-Packard, Avondale. PA, 1977. 12 Traaol' MOdel 702 N-P Deteator, Austin. TX. 1977. 13 B.J. Ehrlich, Ind. Ree. Dev •• April (1980) 107. 14 R.C. Hall, CRC Crit. Rev. Anal. Chem., 8 (1978) 323. 15 P.L. Patterson, J. Chromatogr •• 167 (1978) 381. . 16 P.L. Patterson and R.L. Howe, J. Chromatog~. Sai., 16 (1978) 275. 17 P.L. Patterson. R.A. Gatten and C. Ontiveros, J. Chromatogr. sai., 20 (1982) 97. 18 P.L. Patterson, Chromatographia. 16 (1982) 107. 19 DET Deteatore, Detector Engineering Technology, Walnut Creek, CA. 20 A. Nohl, Speat.ra-Phyeiae Chromatogr. Rev •• 9, No.1 (1983) 11. 21 C.M. White, A. Robbatt, Jr •• and R.M. Hoes, Anal. Chem •• 56 (1984) 232. 22 M. Scolnick. J. Chromatogr. Sai •• 8 (1970) 462. 23 B.K. Schulte and L.W. Shive. Anal. Chern., 54 (1982) 2392. 24 J.A. Lubkowitz. B.P. Semonian, J. Galobardes and L.B. Rogers, Anal. Chern., 50 (1978) 672. 25 E. Cremer, T. Kraus and E. Bechtold, Chern.-Ing.-Teah., 33 (1961) 632. 26 C.W. Rice. U.S. Pat., 2 550 498 (1951). 27 R. Goulden. E.S. Goodwin and L. Davies, Analyet (London), 88 (1963) 951. 28 K.H. Kingdon and I. Langmuir, Phye. Rev., 21 (1923) 380. 29 I. Langmuir and K.H. Kingdon, Proa. R. Soc. London, Sera A. 107 (1925) 61. 30 H. Moesta and P. Schuff, Ber. Bunaengee Phye. Chern., 69 (1965) 895. 31 E. Bechtold, Theeie, Innsbruck. 1962. 32 E. Cremer, H. Moesta and K. Hablik, Chem.-Ing.-Teah., 38 (1966) 580. 33 H.A. McLeod and W.P. McKinley, J. Ass, Offia. Anal. Chern., 50 (1967) 641. 34 G.G. Oevjatych. N.C. Agliulov and V.V. Lucinkin, Zavod. Lab., 33 (1967) 901. 35 E. Cremer, J. Gae ChromatogF., 5 (1967) 329.

91

Chapter 5

FLAME-IONIZATION DETECTOR CONTENTS 5.1. Introduction . . • • • . . • • . • • . . • . . • . . . . • • . . " 5.2. Hydrogen atmosphere flame-ionization detector . • . . . . . . . " 5.3. Hydrogen atmosphere flame-ionization detector for silicon compounds 5.4. Flame-ionization detector with hydrocarbon background 5.5. Selective detection of halogen compounds References . . . . . . . • . . . . . . . . . . . . .

91 92 103 105 106 106

5.1. INTRODUCTION The basic reactions taking place in the oxygen-hydrogen flame can be described by the following equations t : H + O2 o + H2

~

OH + 0

(5.1)

~

OH + H

(5.2)

OH + H2

H20 + H (5.3) The radicals formed in the reaction zone of the flame react exothermically as follows (M represents a general flame-gas molecule): ~

H + H + M~ H2 + M

(5.4)

OH + H + M ~ H20 + M and, if organic compounds are present, chemi-ionization occurs:

(5.5)

CH + 0

~

CHO+ + e-

(5.6)

Various substituents on the individual carbon atoms in the molecule of a compound affect ionization in different ways (commonly reducing the ionization compared with hydrogen). This effect has led to the determination of the socalled number of effective carbon atoms in a molecule of a compound 2 , the ionization of an alkane carbon atom being taken as a reference, i.e., a value of unity is ascribed to the response due to an alkane carbon atom. For this reason, the flame-ionization detector (FID) is expected to respond only to organic compounds containing an effective (ionizable) carbon. Inorganic gases, carbon oxides and carbon disulphide should yield no response (the effective carbon number for C=O and C=S being zero). It has been found, however, that,

92 at increased hydrogen flow-rates 3 and with an electrode geometry that causes the electrodes to be heated by the flame 4 , carbon disulphide gives a response that is a thousandth to a hundredth of that of hydrocarbons. At increased hydrogen flow-rates, the detector gives a response to nitric oxide 5 that is commensurate with the methane response. Under these conditions, the detector gives a lower response 5 ,6 also to carbon dioxide, carbon monoxide, nitrous oxide, nitrogen dioxide, helium, oxygen and hydrogen sulphide. None of the above-mentioned responses has the character of a selective response, because the molar responses to organic carbon compounds differ from each other only in the number of the effective carbon atoms in the solute molecules; the slightly anomalous responses to inorganic gases are small compared with the hydrocarbon responses and they display the same polarity (the number of ions is increasing). The aim of this chapter is to describe modifications of the flame-ionization detector that give a selective response to certain types of compounds with regard to a higher response and opposite polarity. In fact, this chapter should also include the alkali flame-ionization detector (AFIO), because it is a modification of the FlO. However, the modifications of the AFIO cover a broader spectrum (e.g., additional electrode, the method of heating the alkali metal source) and, essentially, the AFIO is so widely and manufactured distributed that it has been considered as a separate type of detector. In 1969, an FlO was described 7 the electrodes of which were made of 0.051 mm diameter high-purity platinum wires. These microelectrodes were adjustable and it was with their varying distances from the detector jet that a selective response, i.e., different responses for different compounds, was obtained. The responses to ketones and aliphatic and aromatic hydrocarbons displayed maximum values at a vertical distance of the electrodes from the jet of ca. 6 mm, whereas the response to polychloroalkanes was maximal at a distance of ca. 2 mm. When the electrodes were placed in the region of the flame corresponding to the optimum response for aliphatic or aromatic hydrocarbons, the chloroalkane response was four orders of magnitude lower. 5.2. HYDROGEN ATMOSPHERE FLAME-IONIZATION OETECTOR Aue and HillS found in 1972 that, when the hydrogen and air flows were interchanged in the flame-ionization detector, i.e., when air was introduced into the jet while hydrogen was flowing around it, this detector gave a selective response to organometallic compounds. In this instance the flame burns in a hydrogen atmosphere and the hydrogen flow is an order of magnitude higher than the air flow (hence the name hydrogen atmosphere flame-ionization detector,

93

Igniter Collector electrode

assembly

~~~--90V

from GC

Fig. 5.1. Cross-section of a silane-doped HAFID. (From ref. 10.) HAFlD). Further investigations showed that the introduction of silicon compounds into the detection system (e.g., by the carrier gas from the stationary phase, the silicon septum, etc.) is essential for the HAFlD response to organometallic g compounds to occur . On the basis of these findings, a detector was designed 10 to which gaseous silane was supplied (Fig. 5.1). The carrier gas from the column and an oxygennitrogen mixture enter the 1.2 mm quartz jet of the detector, which is 70 mm from the base of the detector body. The hydrogen and silane inlet is situated at the detector base near the jet. The collector electrode is of the shape of a 5-mm diameter ring, 50 mm from the jet. Under these conditions, the HAFlD responds to organometallic compounds·and this response, depending on the presence of a metal in a molecule of the compound, is several orders of magnitude higher than the response to hydrocarbons (see Table 5.1). The detector is most sensitive to manganese and aluminium compounds, the minimum detectable mass rate for these compounds and for iron, tin and chromium compounds being lower than or commensurate with the FlD minimum detectability of hydrocarbons (1.7 I 10- 14 g/ sec of Mn for manganese"). The background current of the HAFlD is higher than that of the FlD'2,13. Therefore, the noise of the HAFlD is increased by approximately an order of magnitude'3. The detection limit and selectivity are also evident from Figs. 5.2 and 5.3. A comparison of the gasoline chromatograms

TABLE 5.1 RESPONSES OF MODEL COMPOUNDS Selectivity against tetradecane. taken as the ratio of injected amounts of tetradecane and the organometallic that produce the same peak area of 1.10- 11 C. A minus sign preceding a selectivity value indicates a negative response. (From ref. 10. ) Compound

Column temperature (oC)

Aluminium(III) hexafluoroacetylacetonate Ferrocene Tetravinyltin Tetrapropyltin Tetraethyl ti n Chromium(III) hexafluoroacetylacetonate Chromium hexacarbonyl Chromium(III) trifluoroacetylacetonate Tetraethyllead Tetrabutyll ead Triphenylantimony Tungsten hexacarbonyl Molybdenum hexacarbonyl Tetrabutylgermane Tri-n-butyl phosphate Chlorobenzene Bromobenzene Triphenylarsine Triphenylbismuth

50 145 110 140 90 60 30 170 100 190 250 50 30 140 190 70 80 250 250

Sel ecti vity 3.2 1.9 2.6 1.9 1.6 1.6 1.5 1.1 1.1 6.0 1. 2 8.6 5.0 3.5 -3.2 -3.2 -3.2 -3.2 1.3

• • • • • • • • • • • • • • • • • • •

10 5 10 45 10 10 4 10 4 10 4 10 4 10 44 10 3 10 10 3 102 10 2 10 2 2 10 2 10 10 2 10 2 2 10

Detection 1 imit (g) 6.0 2.1 1.0 7.8 2.1 1.5 2.3 1.7 5.1 7.5 1.3 5.3 3.5 5.5 1.1 1.4 1.4 2.3 6.3

• • • • • • • • • • • • • • • • • • •

10- 13 10- 12 10- 11 10- 12 10- 11 10- 11 10- 11 10- 11 10- 11 10- 11 10- 10 10- 10 10-9 10- 10 10-9 10-9 10-9 10- 9 10- 9

Piaselenole Iron(III) trifluoroacetylacetonate Di-n-butyl disulphide Fluorobenzene Nitrobenzene Di ethyl mercury Tetraethylsilane Tetradecane

165 175 140 30 110 70

50 140

-9.2 8.0 -2.0 7.1 3.1 2.9 1.9 1.0

• • • • • • • •

10 1 10 1 10 1 10 0 10 1 10 0 10 0 10 0

5.7 • 10- 9 4.1 • 10- 9 2.3 • 10-8 6.7 • 10-8 1.8 • 10- 7 1.3 .10- 7 4.1.10- 7 3.7 • 10- 7

96

(Fig. 5.3) obtained by means of an HAFlD and a FlD clearly shows the advantage of the HAFlD in detecting organometallic compounds in mixtures with hydrocarbons. The molar response of the compounds seems to be given by the number of heteroatoms in the molecule of the compound 13 (Table 5.2). Similarly to the FlD, the linear dependence of the molar response on the number of effective carbon atoms in the molecule is retained l2 . The wide gap between the jet tip and the base of the detector body is of great importance with regard to the selectivity of the HAFlD response 13 • Table 5.3 compares the response selectivities and the detection limits for a common massive jet and a jet tip modified with quartz or stainless-steel tubing (10 mm), with the same distance of the collector electrode from the jet tip. Compared with a normal jet, the response of a detector with a jet tip is substantially lower for hydrocarbons and, therefore, the selectivity of the response to hydrocarbons is higher. The reduction in the response to hydrocarbons in the HAFlD compared with the FlD is believed to be due to an oxidation step that occurs in the oxygen-rich precombustion zone of the HAFlD I2 ,13, in addition to

a

b

c

Fig. 5.2. Chromatograms of model compounds: (a) 5 pg of aluminium(lll) hexafluoroacetylacetonate, 50 0 C; (b) 10 pg of ferrocene, 145 0 C; (c) 50 pg of chromium(lll) trifluoroacetylacetonate, 170 0 C. Glass column (1 m) packed with 6% OV-17 on Chromosorb W, temperature as indicated. Flow-rates: hydrogen 1600 ml Imi n pl us 7 lJ 1Imtn silane, oxygen 125 ml 1m; n, ni trogen 100 ml Imi n, ni trogen fron1 a GC column 40 ml/min. (From ref. 10).

97

«

FlO

FlO

HAFID

HAFID

~ '0

iii ~

« en

4

'0

6

~

3

~

5 2

6 .J

30

IJ'

A

J>.

I

I

I

I

II

50 00 120

·C

50 90120 ·C

Fig. 5.3. HAFIO and FlO analyses of alkyl lead compounds in gasoline. Left, regular gasoline; right, premium gasoline. Glass column (6 ft. x 2 mm 1.0.) packed with aO-l00 mesh Ultrabond 20 M. Temperature, programmed from 300C (5 min) to 120 0C (5 min) at 20 0C/min. HAFIO flow-rates: hydrogen 1600 m1/min, silane 34 ppm, oxygen 150 m1/min, air 120 m1/min. (From ref. 11 with permission.) the effect of the position and potential of the collector electrode. The massive jet dissipates the heat from the precombustion zone, decreasing the oxidation therein. The jet tip then reduces the cooling effect and enhances oxidation 13 . The linearity of response of the detector in which oxygen enters the detection space at the detector base covers about three orders of magnitude for aluminium, chromium, tin, lead and iron compounds 10 • If hydrogen is supplied to the detector above the jet tip only, the response is not a linear function of the amount of compound 11 .

\D (Xl

TABLE 5.2 EVALUATION OF RESPONSE (From ref. 13 with permission.) Compound

Dodecane Diallyldibutyltin Tetraethylti n Tetrabutyltin Tetra-n-propyltin Hexabutylditin

* Due to the column.

Detection limit (pg)

Sensitivity (C/mol)

Selectivity

FlD

HAFID

FlD

HAFID

FID

HAFID

300 800 2000 250 1400 760

48,000 800 38 70 22 13

1.4 0.38 0.52 1.8 1.4 2.1

0.034 12'" 29 31 38 55

1.00 0.27 0.37 1.3 1.0 1.5

1.00 3.5 • 8.5 • 9.1 • 1.1 • 1.6 •

10 2 10 2 10 2 10 3 10 3

TABLE 5.3 COMPARISON OF JET TIPS (From ref. 13 with permission.) Compound

Tetraethyl tin Dodecane

Stainless-steel jet tip

Massive jet

Quartz jet tip

Detection 1 imit

Sel ecti vity

Detection 1imit

Sel ecti vity

Detection 1imit

Selectivity

38 pg 48 ng

110

15 pg 0.2 llg

1. 2 • 10 4

10 pg 0.5 llg

5.9 • 10 4 1

1

1

\0 \0

100 -0

.... _.-----. ~ 1

,

t

2

4

5 3

2

-l~j

1

104 -

~i~i-ri,i-'i-i~i-'i-ri'i-'i-i~i-'i-'i'i-'i-'-i

-600 -500 -400 -300 -200 -100

o

+100 +200 V

Fig. 5.4. Background current and responses of model compounds at different collector electrode potentials. 1, Background current; 2, tetradecane (2 ~g); 3, tetrabutyll ead (lOng); 4, tetrabut}'lti n (10 ng); 5, ferrocene (1 na). h = Peak height (arbitrary units, logarithmic scale). (From ref. 10.) Fig. 5.4 shows the relationships between response and collector electrode potential for tin, lead and iron compounds and for a hydrocarbon. The negatively polarized electrode is clearly more advantageous; compared with the positive collector electrode, the response to organometallic compounds in higher and the hydrocarbon response and background current are lower. The responses of all of the investigated compounds and the background ionization current increase to maximum values as the potential increases. The optimum electrode potential with regard to the selectivity and the minimum background current is -90 V. For a constant concentration of a silicon compound entering the detector, the response of the HAFID depends on the distance of the collector electrode from the jet9 ,10. The response to organometallic compounds increases to maximum values with increasing distance of the electrode. This response enhancement is 1.5 orders of magnitude for a distance ranging from 10 to 50 mm. On the other hand, the hydrocarbon response decreases by approximately an order of magnitude within this range. The background current also decreases with increasing distance of the collector electrode from the jet. The dista~ce of the collector electrode from the jet also affects the amount of silane that has to be supplied to the detector. As mentioned above, the HAFID gives a selective response to organometallic compounds only in the presence of a silicon compound. The HAFID response and the background current increase to

101

0.4

ELECTRODE HEIGHT

0.3

50 mm

« 0.2

90 mm

'"52 l-

I

l!)

u:i

I

0.1

70 mm 30 mm 110 mm

~

4:

LJ.J

a..

0.0 10 mm -0.1

Fig. 5.5. HAFID response as a function of silane concentration for various electrode heights. Flow-rates: hydrogen 1500 ml/min, oxygen 120 ml/min, air 165 ml/min. (From ref. 14.) maximum values with increasing concentration of silane in the detector. The response to hydrocarbons and the background current attain this maximum value at higher silane concentrations than does the response to organometallics 10 The optimum silane concentration, however, is different for each distance between the collector electrode and the jet 14 (Fig. 5.5). The greater the electrode distance, the lower is the optimum silane concentration. With optimum silane concentrations the HAFID response to organometallics increases with increasing distance of the electrode. Greater electrode distances (70 mm) are advantageous for this reason, because both minimum detectability for organometallics (improved sensitivity of response, lower background current and noise) and selectivity (higher sensitivity of response to organometallics and reduced sensitivity to hydrocarbons) are increased. As can be seen from Fig. 5.5, the response of an HAFID of the type used is negative at a distance of the collector electrode from the jet of 10 mm 14 This negative response is related to the shape and position of the collector electrode. The electrode was pin-shaped and situated out of the jet centre in this instance. When using a ring-shaped electrode situated above the jet, the response is also positive for a distance of 10 mml0. The optimum distance for negative response depends on the entire detector design 15 . Negative responses also occur at low positive potentials of the collector electrode (see Fig. 5.4) and at high silane concentrations 10 . The detection limits for tetraethyltin and tetraethyllead in the negative mode are lower by about one order of magnitude than those in the positive mode 15 . However, parameter settings for the negative response must be carefully controlled and a relatively high silane concentration may prove to cause long-term degradation of response due to silicon dioxide

102 deposits on the electrode. Thus for routine analyses the positive mode seems to be more suitable 15 • The HAFID requires substantially higher hydrogen flow-rates than the FlO. The HAFID response to organometallics increases to a maximum at about 3 1/min 14 with increasing hydrogen flow-rate (4 ppm of silane, 70 mm electrode distance), but the hydrocarbon response drastically increases with flow-rates below 650 ml/min 9 • The means of supplying hydrogen to the detector affects the amount of silane needed to achieve maximum response, both in positive 11 and negative mode 15. In the positive mode, if hydrogen enters the detector 21 mm above the jet tip this concentration is 34 ppm, and if hydrogen is supplied to the detector base the optimum silane concentration is below 10 ppm (see Fig. 5.5). The dependence of the response of an organometallic compound on the percentage of nitrogen in the air-oxygen mixture supplied also shows a maximum (at a 1:1 nitrogen to oxygen ratio). Therefore, for a total flow-rate through the jet of 305 ml/min and a nitrogen carrier gas flow-rate of 20 ml/min, the optimum air and oxygen flow-rates are 165 and 120 ml/min, respectively14. With higher nitrogen to oxygen ratios the response to organometallics decreases, the hydrocarbon response increases and, consequently, the detection selectivity is diminished 10 . 5.3. HYDROGEN ATMOSPHERE FLAME-IONIZATION DETECTOR FOR SILICON COMPOUNDS If a silicon compound is introduced into the HAFID, it gives a selective response to metallic compounds. If an inverted system is used, i.e., if an organometallic compound is supplied to the detector, the HAFID responds to organic silicon compounds. Hence this is again an HAFID, but an iron compound, commonly ferrocene, is supplied in the hydrogen flow in this instance 16 • The detector geometry plays an important role in peak tailing 17 • Pronounced tailing occurs with larger inner diameters of the detector (above 11 mm). Hydrogen is introduced into the detector space below the jet level and oxygen is supplied to the detector jet together with the carrier gas from the column. The collector electrode (-90 V) is 110 mm from the jet. The amount of iron compound supplied to the detector fundamentally changes the response16~18-21 (Fig. 5.6). With increasing amounts of an iron compound the response to silicon compounds increases to a maximum (about 5 ppm in this instance) and then decreases, passing to a negative response (at about 12 ppm). The specific course of this dependence is affected by the distance of the point of the inlet of the iron compound from the detector flame 20 (the inversion from a positive to a negative response is shifted to higher amounts of the iron compound at larger distances) and by the type of iron compound supplied 18 ,20 The maximum negative response (about 35 ppm of ferrocene) is three times greater than the maximum positive response. The detector noise increases at the same

103 ~ I

0 ~

4: Q.o 0/1

c: 0

a. 0/1

20 15 10 5

Q.o

L-

III

Amount of ferrocene (ppm)

0

I

Cl

~ 4:

30

-5

40

x 10

-15 -20

Fig. 5.6. Effect of amount of ferrocene on the HAFID-Si response. (From ref. 18 with permission.) TABLE 5.4 DETECTION LIMITS FOR THE HAFID-Si (From ref. 21.) i·lode of HAFI D operation Non-doped Positive mode Negative mode

Coumpounds containing Silicon

Phosphorus

Iron

Chlorine

4 ng (ref. 16) 50 pg (ref. 17) 1 ng (ref. 18)

7 ng 0.5 ng 1 ng

0.6 ng 1 ng 14 ng

50 ng 2.5 ng 9 ng

time, so that the detection limit is smaller by a factor of about 20 in the negative mode (see Table 5.4). Linearity of detection covers a range of about three orders of magnitude in both mOdes 21 . Hence, the selectivity of response of this detector,· called the HAFID-Si, is conditioned by the response level and response polarity (Fig. 5.7). With smaller amounts of doping iron compounds in the positive mode, the HAFID response to silicon compounds is higher than that of a detector in the absence of a doping agent and approximately commensurate with the response of a common FID, whereas the response to hydrocarbons is reduced. The selectivity of response of Si to C is 18,21 about 10 4 :1. With higher concentrations of iron compounds in the negative mode, the response to silicon compounds is negative whereas that to hydrocarbons remains positive. The change from a positive to a negative mode is easy to accomplish, by increasing the flow-rate of the gas containing the

104 124

FlO

b

a 5 8

HAFIO lSi) positive mode

9

3

5 2

5

8

70

B4

98

112

1

126 "C

70

84

98

112 126 "C

C

2

B

HAFIO lSi) ne9ative mode

5

Fig. 5.7. Chromatograms of equal amounts of compounds using different detection methods. Chromatograms obtained with a standard FID (a); a HAFID doped with 5 ppm of ferrocene (b); a HAFID doped with 30 ppm of ferrocene (c). Peaks: 1 = Hexane (solvent); 2 = chlorobenzene; 3 = decane; 4 = octanol; 5 = triethyl phosphate; 6 = dodecane; 7 = decanol; a = ferrocene; 9 = tetradecane. Methylsilicone-coated fused-silica capillary column (10 m x 0.2 mm I.D.). Temperature, programmed from 70 to 150 0 C at aOC/min. Helium flow-rate: 1.2 ml/min, HAFID gas flow-rates: hydrogen 1600 ml/min, oxygen 130 ml/min. FID gas flow-rates: hydrogen 30 ml/min, air 240 ml/min. (From ref. 21.) doping iron compound. A selective enhancement of the response to silicon compounds compared with hydrocarbons was also observed 18 when other metals were used as doping agents, such as aluminium, nickel, chromium, molybdenum, brass, platinum, copper and magnesium. The lifetimes of the sources of these metals are short, however.

105 The HAFIO-Si gives a response approximately commensurate with that of silicon compounds also to phosphorus, iron and chlorine compounds '8 ,2' (see Table 5.4). The response to the latter compounds also depends on the amount of doping agent applied. The course of this dependence for phosphorus and iron compounds is similar to that for silicon compounds, i.e., the response to these compounds is negative for larger amounts of dopants supplied to the detector (Fig. 5.7). Alcohols, ketones, ethers and nitrogen and fluorine compounds give a low positive response 20 . The HAFIO responds to organic silicon compounds '6 ,17 and to phosphorus, iron and chlorine compounds 21 even if no iron compound is supplied to the detector. In this instance, the sensitivity of detection is lower than in the presence of ferrocene in the positive mode (see Table 5.4), being approximately two to three orders of magnitude lower than that of the FlO. The detector does not differ in design from that with ferrocene '7 , the Si to C selectivity is about 3.5 orders of magnitude and the detection limit is 4 ng tetraethylsilane. 5.4. FLAME-IONIZATION OETECTOR WITH HYDROCARBON BACKGROUND Fritz et al. 22 found with an FlO that the response to silicon compounds showed an inversion starting from a certain mass flow-rate of the silicon compound. They ascribed this effect to two contradictory phenomena: the production and the loss of charged particles in the system. Based on these findings, they created conditions causing the ion loss to prevail in the detector. They used a hydrogen-hydrocarbon flame (by supplying acetylene to the detector burner) and found that the detector response to organic silicon compounds was negative when the hydrocarbon concentration in the detector exceeded a certain value. On modifying this detection system by introducing methane into the hydrogen flow 23 (tenths of a millilitre per minute of methane) to the FID (oxygen-hydrogen flame), the negative response of this detector attained a value of 0.2 C/g of silicon. This value considerably exceeds the response of the common FlO as expressed in coulombs per gram of carbon. The FlO to which a hydrocarbon is supplied gives, at certain hydrogen flowrates, a negative response to inorganic gases to which the common FlO mostly is not sensitive (see section 5.1). On supplying methane to the detector, either in the hydrogen or in the carrier gas in amounts causing the background current due to hydrocarbon ionization to attain values of ca. 3 • 10- 10 A, the response of this detector 24 to hydrogen sulphide, sulphur dioxide and carbon disulphide is about 2 • 10- 4 C/mole (which represents a detection limit of 1 • 10-8 mole), the response to oxygen and nitrous oxide is about 4 • 10- 5 C/mole 25 and the response to carbon oxide and carbon dioxide is about 3 • 10-6 C/mole 6 . When

106 perfluoromethane was used as the carrier gas, the response to nitrogen, helium and carbon dioxide was about 5 • 10-6 C/mole 26 . 5.5. SELECTIVE DETECTION OF HALOGEN COMPOUNDS Under certain conditions (higher hydrogen flow-rate, the electrode positioned near the flame), the sensitivity of the flame-ionization detector for halogen compounds may be higher than it is under common conditions 4 When hydrogen is employed as the carrier gas and oxygen instead of air, the sensitivity of the FID towards halogen compounds is approximately two orders of magnitude higher than that of the common detector 27 , The response is in direct proportion to the number of chlorine atoms in the molecule. For instance, with flow-rates for hydrogen of 100 ml/min and oxygen of 50 ml/min, and an applied potential of +170 V, the molar responses of mono-, di- and trichlorobenzene were 23, 45 and 69 C, respectively, i.e., in the ratio 1:2:3. However, the background current and the detector noise are also several orders of magnitude higher than they are with the standard FID28 (1 • 10- 9 and 5 • 10- 10 A, respectively). REFERENCES 1 T.M. Sugden, Ann Rev. Phys. Chem., 13 (1963) 369. 2 J.C. Sternberg, W.S. Gallaway and D.T.L. Jones, in N. Brenner, J.E. Callen and M.D. Weiss (Editors), Gas Chromatography, Academic Press, New York, 1962, p. 231. 3 B.L. Walker, J. Gas Chromatogr., 4 (1966) 384. 4 M. Dressler, J. Chromatogr., 42 (1969) 408. 5 P. Russev, T.A. Gough and C.J. Woolam, J. Chromatogr., 119 (1976) 461. 6 B.A. Schaefer and D.M. Douglas, J. Chromatogr. Sci., 9 (1971) 612. 7 R.W. McCoy and S.P. Cram, J. Chromatogr. Sci., 7 (1969) 17. 8 W.A. Aue and H.H. Hill, Jr., J. Chromatogr., 74 (1972) 319. 9 H.H. Hill, Jr. and W.A. Aue, J. Chromatogr. Sci., 12 (1974) 541, 10 H.H. Hill, Jr. and W.A. Aue, J. Chromatogr., 122 (1976) 515. 11 M.D. DuPuis and H.H. Hill, Jr., Anal. Chem., 51 (1979) 292. 12 J.H. Wagner, C.H. Lillie, M.D. DuPuis and H.H. Hill, Jr., Anal. Chem., 52 (1980) 1614. 13 D.R. Hansen, T.J. Gilfoil and H.H. Hill, Jr., Anal. Chem., 53 (1981) 857. 14 J.E. Roberts and H.H. Hill, Jr., J. Chromatogr., 176 (1979) 1. 15 D.R. Hansen and H.H. Hill, Jr., J. Chromatogr., 303 (1984) 331. 16 H.H. Hill, Jr. and W.A. Aue, J. Chromatogr., 140 (1977) 1. 17 M.A. Osman, H.H. Hill, Jr., M.W. Holdren and H.H. Westberg, Anal. Chem., 51 (1979) 1286. 18 M.A. Osman and H.H. Hill, Jr., J. Chromatogr., 213 (1981) 397. 19 M.A. Osman and H.H. Hill, Jr., J. Chromatogr., 232 (1982) 430. 20 M.A. Osman and H.H. Hill, Jr., Anal. Chem., 54 (1982) 1425. 21 M.A. Osman and H.H. Hill, Jr., J. Chromatogr., 264 (1983) 149. 22 D. Fritz, G. Garz6, T. Szekely and F. Till, Acta Chim. Hung., 45 (1965) 301. 23 G. Garz6 and D. Fritz, in A.B. Littlewood (Editor), Gas Chromatography 1966, Institute of Petroleum, London, 1967, p. 150.

107 24 25 26 27

B.A. Schaefer, Anal. Chem., 42 (1970) 448. B.A. Schaefer, J. Chromatogr. Sci., 10 (1972) 110. W.C. Askew, Anal. Chem., 44 (1972) 633. A.E. Karagozler, C.F. Simpson, T.A. Gough and M.A. Pringuer, J. Chromatogr., 158 (1978) 139. 28 C.F. Simpson and T.A. Gough, J. Chromatogr. Sci., 19 (1981) 275.

This Page Intentionally Left Blank

109

Chapter> 6

PHOTOIONIZATION DETECTOR CONTENTS 6.1. Introduction • • • • • • • • • • • • • . • • • • • 6.2. Response model • • • • . • • • • • • • • • • • • • 6.3. Sensitivity of response and minimum detectability 6.4. Selectivity of response 6.5. Carrier gas References ••• . • • • • •

109

111 112 118 126 131

6.1. INTRODUCTION When a compound AB is irradiated with UV light, ionization occurs if the iozization potential of the compound is equal to or smaller than the photon energy, hv. The photoionization process is initiated by the absorption of UV light: (6.1 ) At present, commercial photoionization detectors (PIDs) (Fig. 6.1) are equipped with a radiation source, viz., a UV lamp separated by a wirdow from the ionization chamber. Interchangeable lamps with various energies of the emitted photons (commonly 9.5, 10.2 and 11.7 eV) have been employed as radiation sources. The window material consists of MgF 2 and LiF (for photon energies exceeding 10.9 eV). The ionization chamber contains two electrodes to which a voltage is applied. The ions formed flow to the collector electrode in the electric field, and the generated current is measured. In earlier types of PIDs, a glow 2- 7 or microwave 8 discharge with no separation of the radiation source and the ionization chamber (see Fig. 6.2) was used as the source of UV radiation. The discharge occurred at sub-atmospheric pressure; therefore, the ionization chamber was run at a'low pressure and the chromatographic column terminated in a space at sub-atmospheric pressure. These detectors have several other drawbacks 9 : they need a continuous flow of highly purified gas into the discharge zone, they are of a cumbersome design (necessitating a vacuum pump), contamination of the electrodes resulting in aging occurs and the temperature limit is 170 oC8. The source of UV radiation produces 9 , in addition to photons, ionized particles and excited molecules. The ionization of the

110

5

2

3

Fig. 6.1. Schematic diagram of a PIO manufactured by HNU Systems. 1 = Ceramic internals; 2 = ionization chamber; 3 = glass-lined inlet; 4 = glas-lined exhaust; 5 = UV lamp. (From ref. 1.)

I Fig. 6.2. Schematic diagram of a glow discharge PIO. 1 = Gas inlet to source of UV radiation and discharge cathode; 2 = discharge anode; 3 = sensing chamber cathode; 4 = carrier gas inlet and anode of sensing chamber; 5 = outlet to pump. (From ref. 2, with permission.) substance can be initiated not only by photons and therefore photoionization is not the only process occurring in the detector, which leads to difficulties in interpreting the mechanism of detection. A PIO provided with a separated source of UV radiation lO eliminates most of the disadvantages that occur with the windowless detector. This detector can be run at atmospheric pressure. A combination of this PIO with an electron-capture detector (ECO) (see also Chapter 11) has been described 11 • In this instance, the detector works as a PIO or as an ECO based on photoionization.

III

6.2. RESPONSE MODEL The processes taking place in the PIO are represented by the following kinetic scheme lO : AB + AB" AB* + e AB+ + AB+ + AB" +

h

->->->-

anode cathode e- + C

->-

C

->-

+ +

AB" AB+ + eA+ B current AB AB + C AB + C

Rl R2 R3 R4 R5 R6 R7

=I0 - I = K2 [AB"] = K3 [AB*] = K4 [e-] = K5 [AB+] = ex [AB+] [e-] = K7 [AB"] [C]

(6.2) (6.3) (6.4) (6.5) (6.6) (6.7) (6.8)

where R is the reaction rate, [AB] is the concentration of substance AB, C is the carrier gas of concentration [e], I O is the initial photon flux, I is the intensity of transmitted radiation, I O - I is the number of photons absorbed per second and ex is the recombination coefficient. The following expression applies for the equilibrium state:

(6.9) The probability of the photon being absorbed (eqn. 6.2) depends on the absorption cross-section cr of the substance (defined by the Beer-Lambert law). The probability of ionizing of the excited state depends on the photoionization efficiency, n, where

(6.10) If the recombination reaction (eqn. 6.7) is suppressed by a high enough voltage, we can write for the ionization current (il of the detettor 12 ([C] ~ 1): (6.11 )

where N is Avogadro's number, F is Faraday's constant, and L is the path length. For a given detector and lamp, the m0lar response of the PIO is proportional to the molar concentration and the photoionization cross-section, cr n: R = if [AB] = kan

(6.12)

112 There is a direct dependence of the photoionization cross-section on the photon energy and ionization potential 13 . Even though this is a complex dependence, the FlO signal is related to the ionization potential by means of the photoionization cross-section 12 • 6.3. SENSITIVITY OF RESPONSE ANO MINIMUM OETECTABILITY The molar response of the PIO depends on the photon energy14 (Fig. 6.3). The dependence is given by the relationship between the photoionization efficiency and photon energy (Fig. 6.4). As can be seen, the low alkane response at a photon energy of 10.2 eV is associated with the low photoionization efficiency; similarly, the high response to aromatic hydrocarbons is related to the high ionization efficiency at this energy. Near the ionization potential, alkenes, cyclic alkenes and alkylbenzenes also show a similar rapid increase to the maximum ionization efficiency. This course is typical of photoionization involving TI electrons 15 .

RMR

o 10

15

20

eV

Fig. 6.3. Relative molar responses per mole of carbon (RMR) of an alkane (hexane) and an aromatic hydrocarbon (benzene) as a function of the photon energy. 1 = Hexane; 2 = benzene. (From ref. 14.) As mentioned above, with a constant-energy source (e.g., a 10.2-eV lamp), the sensitivity of response of the photoionization detector depends on the ionization potential of the compounds and increases as their potential decreases 2 ,6,7, 10,12,14,16. Fig. 6.5 shows this dependence for aromatic hydrocarbons together with the dependence of the response on the number of carbon atoms in the molecule of the compound. Fig. 6.6 shows the dependence of the PIO sensitivity on the number of carbon atoms in the molecule for other homologous series. The detector response increases with increasing number of carbon atoms in the molecule of the compound. The idea 17 that the molar response of the photoionization detector may be related to the carbon number of the compound (correlation coefficient 0.67),

113

9.0

9.5

eV

100

Fig. 6.4. Relationship between the photoionization efficiency (n) and the photon energy. E indicates the energy of the UV lamp. I, Alkane (hexane); 2, aromatic (benzene). (From ref. 14.)

55

\

0

\1F

50

0

a:: a:: :l:

45

40--~--~~

____

8.5

6

o~o

______ ______- L__ 89 9.1 8.7

~

~

~~~

9.3

IP eV

7

c

8

9

Fig. 6.5. Relative molar response (RMR) of aromatic hydrocarbons as a function of the ionization potential (curve IP) and the number of carbon atoms (curve C) in a molecule. RMR of benzene = 1. c = number of carbon atoms. (From ref. 12.) as with the flame-ionization detector, does not agree with the results, however. The increase in the detector response with increasing number of carbon atoms in the molecule invariably applies to particular homologous series, and is different for different homologous series 12 ,18. Obviously, it is mainly due to the decrease

114

60

Fig. 6.6. Relative molar response (RMR) as a function of the number of carbon atoms (C) in a molecule. R/4R of benzene = 1. 1 = Styrene; 2 = xylene; 3 a aromatics; 4 = cyclohexanes; 5 = cyclohexenes; 6 = 1-alkenes; 7 = n-alkanes. (From ref. 12.) in the ionization potential with increasing carbon number of the compound {see Fig. 6.5)12. In addition, the relationship between the pro sensitivity and the carbon number of the members of a homologous series is not always linear. When comparing, for instance, the responses to hydrocarbons with six carbon atoms in the molecule, the response is found to increase with increasing degree of unsaturation: benzene> cyclohexene > cyclohexane (a linear dependence of the response on the number of double bonds in the molecule is valid for CI8 fatty aCidsl ). For different types of compound, the sensitivity increases in the following orders l8 : alkanes < alkenes < aromatics; alkanes < alcohols < esters < aldehydes < ketones; non-eye 1i c compounds < eye 1i c compounds; fl uorine-substituted < chl ori ne-substi tuted < bromi ne-substituted < i odi nesubstituted compounds; benzenes substituted with electron-withdrawing groups < benzene < benzenes substituted with electron-releasing groups. The theoretical relationship between the photoionization cross-section and the ionization potential is complex, including other factors such as molecular geometry and symmetryl9,20. For instance, the relative molar responses (RMR) (relative to benzene) for 0- and m-xylene, which have the same ionization potential (IP) of 8.56 eV, are 1.14 and 1.15, respectively18,19. The RMR for the more symmetric p-xylene, with an IP of 8.445 eV, is 1.2. The RMR of the isomeric ethyl-

115

3

R 2

1

a

7

8

9

10

11 IP

12

eV

Fig. 6.7. Molar response (R) as a function of the ionization potential (Langhorst's data). (From ref. 19.) benzene, with a much higher IP of 8.76 eV, is 1.16, similar to those of 0- and m-xylene. In spite of the complexity of the overall situation, the ionization potential is believed to be the most important factor determining the PIO response 19 • The dependence of the molar response on the IP for 50 compounds from the Langhorst set 18 of response data gives a linear regression line with a correlation coefficient of 0.89 (Fig. 6.7). The assumption 21 that the number of n electrons may be a factor important to the response of the individual compounds seems to be less acceptable 19 , because the mentioned dependences yield low correlation coefficients in this instance (0.25 for the dependence from Fig. 6.7). The large discrepancy can also be caused by the fact that the 10.2-eV source is not monochromatic 20 (see Table 6.5). The response level also depends on the lamp intensity setting. The response improves with increasing intensity but, at the same time, the lamp life decreases. The detector noise also increases with increasing intensity. Table 6.1 lists the basic parameters of various types of PIOs. As can be seen, the commercial PIO with a separated radiation source manufactured by HNU Systems 9 ,22,23 displays the most favourable detection limit (2.10- 12 g of benzene), detector noise (2.10- 14 A) and dynamic linear range (10 7) (similar parameters apply to pros manufactured by Tracor 24 ). The sensitivity of the HNU Systems detector is17 0.3 C/g for aromatic hydrocarbons, i.e., about 24 C/mole for benzene (flow-rate of helium carrier gas 30 ml/min). Hence the response is about 20 times greater than that of the flame-ionization detector (FlO). A ca. 10-fold greater molar resprnse was found for the Photovac detector, which operates with a high-

......

...... 0'1

TABLE 6.1 COMPARISON OF PID OPERATING CHARACTERISTICS AND DESIGN From ref. 9. Ionization efficiency

10- 4 Lovelock 2 10- 3 Price et al. 7 8 10- 3 Freeman and Wentworth 1.6.10- 4 Krysl and Sevcik 10 6 Locke and Meloan Ostojic and Sternberg 16 5.10- 5 Driscoll and Spaziani 9 8.10- 4

Background current (pA)

Linear dynamic range

Detection limit (pg)

Noise (pA)

500 200 6000 90 500 300 8

10 4 10 5 10 4 105 10 5 10 5 10 7

1 40 2 200 300 2

0.1 40 0.3 1 0.5 0.02

Type of discharge

Sealed lamp

Maximum temperature (0C)

Ar Ar He H2 Ar Ar, H2 H2

No No No Yes No Yes Yes

250 100 350 250

117

frequency electrodeless source 25 . However, its noise is also about ten times higher, so that the minimum detectability is approximately comparable to that obtained with other detectors. This detector, operating at ambient temperature, is mounted in a portable gas chromatograph designed mainly for analyses of atmospheric samples. The detection sensitivity for other compounds is indicated only as the detection limit for the individual compounds or it is compared with the sensitivity of the FID. Table 6.2 shows the values for a 10.2-eV lamp. The sensitivity indicated in this way is greater than that for the FID in all instances, the magnitude of the PID:FID ratio depending on the structure of the compound. The high sensitivity to inorganic compounds is of prime importance. The'detection limit is in the range of tens of picograms for H2S, PH 3 , 12 , AsH 3, CS 2 and NO, and is 200 pg for NH 3 • Thus, all mentioned compounds can be analysed using one detector whose sensitivity surpasses by several orders of magnitude that of the only other detector that could be used for all of these compounds, viz., the thermal conductivity detector. Compared with other selective detectors (flame photometric detector, Hall detector), the detection limit is approximately one order of magnitude lower. TABLE 6.2 DETECTION LII1ITS FOR PID Compound

Detection limit (pg)

Aromatic hydrocarbons Barbiturates Polyaromatic hydrocarbons Tetraethyllead ( CH 3)2 S ( CH 3 )2 S2 CH 3SH

1-10 100-1000 50-100 150 20 22 20 30 15 20 25 200 52 25

~SS P~3

AsH 3 NH3 NO 12

Ref. 1 1 1

26 26 26 26 26 26 26 26 26 26 26

The dynamic linear range is claimed to be about 6.5, 5.75, 5 and 4.5 orders of magnitude for photon energies (lamps used) of 10.2, 9.5, 11.7 and 10.9 eV, respectively. This is due to the fact that the detection limit increases for the individual lamps in the mentioned sequence, while the upper limit remains approx-

118

imately the same22 • However, the upper limit of the linear response for n-octane was found to be about 1 ~g for the 10.2-eV source 20 • 6.4. SELECTIVITY OF RESPONSE Under certain experimental conditions, the photoionization detector is also a selective detector. Its selectivity is due to the ability of the substances to be photoionized. This ability varies with the energy of UV radiation, i.e., it differs when different lamps are used. Fig. 6.8 gives a schematic illustration of the detector response when using 11.7-, 10.2- and 9.5-eV lamps. The detector invariably responds to compounds that have ionization potentials that are lower than the photon energies of the individual lamps. Hence the use of different lamps makes it possible to achieve the selective detection of various compounds. The lamp with the lowest energy displays the highest selectivity. The relative responses of a PID for about 30 compounds when using 9.5- and 10.2-eV lamps, normalized to benzene (R = 100) for both lamps and related to the ioniza-

11.7 eV

R

~.romatics

Amines

NR

Mercaptans

Formic acid ---1--- Alkenes

Formaldehyde

(except C H4) 2

Ac rylonit ri le

12

11

10

Fig. 6.8. PID response for various UV lamps. (From ref. 1.)

9

eV

119 Ii' 100

50

10

100

9.2 IP

eV

Fig. 6.9. Relative sensitivity of a PID with a 9.5-eV lamp and a 10.2-eV lamp as a function of the ionization potential of the compounds. R = relative responses (normalized to benzene = 100 for both lamps). (Reprinted from ref. 28, with permission.) TABLE 6.3 COMPARISON OF SOME HYDROCARBON RESPONSE RATIOS FOR PIDs WITH 10.2- AND 9.5-eV LAt1PS AS A FUNCTION OF THE IONIZATION POTENTIAL Normalized to benzene (for the low-boiling aromatics) or benzophenone (for the higher-boiling polyaromatics) for 9.5- or 10.2-eV lamp. (Reprinted from ref. 27, with permission.) Compound

Benzophenone Benzene Toluene Xylene Naphthalene Anthracene

Response rati 0 IP (eV)

PID (10.2 eV)

PID (9.5 eV)

Improvement in sel ecti vi ty, 9.5 eV/1O.2 eV

9.45 9.25 8.82 8.45 8.12 7.23

6.4 1.0 8.4 5.3 12.1 3.8

6.4 1.0 11.6 22.3 89.3 34.2

1.0 1.0 1.4 4.2 7.4 9.0

tion potential, are listed in Fig. 6.9. The responses (relative to benzene) of several compounds are given in Table 6.3 and indicate the possibility of using the response ratio of the two lamps in order to obtain some structural information. As can be seen from Table 6.4, the 10.2-eV lamp can also yield a selective response (see also Fig. 6.10). The selectivity of the response for the 10.2-eV source is independent of the intensity of the source, whereas this is not so for the 9.5-eV source 20 •

120 TABLE 6.4 COt1PARISON OF RESPONSES TO UNSATURATED COMPOUNDS FOR VARIOUS DETECTORS Normalized to benzene. (Reprinted from ref. 27. with permission.)

Response

Detector

PID (lO.2-eV lamp) PID (11.7-eV lamp)

Benzene

Cyclohexene

Cyclohexane

1.0

0.30

0.15

1.0

1.05

1.0

FIO

1.00

1.00

1.00

R

2

6

9.SeV 7

1a.2eV

7

1tZeV

Fig. 6.10. Comparison of normalized PID responses (R) for seven selected naphtha peaks. (Reprinted from ref. 27. with permission.)

121 Fig. 6.10 illustrates the case of using three different lamps for identification purposes. It shows the results of analysing a naphtha sample, depicting the responses of seven selected peaks normalized to benzene (10.2-eV lamp). Peak responses 1 and 5 are the same for the three lamps, which indicates an aromatic hydrocarbon (the 10.2- and 11.7-eV ratios being approximately identical) with an IP of about 9 eV or higher (the 9.5- and 10.2-eV ratios being approximately identical). Peaks 2 and 3 have responses with the 11.7-eV lamps that are approximately double those with the 10.2-eV lamp, the analysed substance thus being an olefin (cf., Table 6.4). The higher response obtained with the 9.5-eV lamp compared with the 10.2-eV lamp indicates an IP smaller than 9 eV. Peak 6 shows an 11.7-eV to 10.2-eV response ratio of about 1, indicating an aromatic hydrocarbon, but its high 9.5-eV to 10.2-eV ratio indicates a polyaromatic hydrocarbon. Peak 7 has a high 11.7-eV to 10.2-eV response ratio and a low 9.5-eV to 10.2-eV response ratio, so that a straight-chain hydrocarbon can be assumed to be the substance being analysed 27 • In practice, the PID also responds to compounds the ionization potentials of which are slightly higher than the photon energyl2. The following factors cause this phenomenon: (a) the energy difference between the molecules in excited vibrational states and those in the ground state can be up to 0.4 eV less than the IP and (b) contamination in the lamp that can give rise to small side-bands with photon energies between 10.2 and 10.9 eV (for the 10.2-eV lamp). For the Photovac detector, equipped with a high-frequency source and using photons with an energy slightly below 11 eV, the response is delivered also by compounds with an IP greater than 12 eV. The explanation lies in the formation of negative ions and in some type of functional exchange between these large negative species and the negative oxygen ions that commonly serve as negative carriers in the ion cell, and/or in the reaction of the ozone produced in the ion chamber with non-ionizable species, forming new compounds that can be ionized later 25 . The narrower the energy spread of the incident photons, the greater is the selectivity. The characteristic lines of four HNU Systems sources (Model PI 52-02 detector) are listed in Table 6.5. It follows from these data that (1) of the four different sources, only the 8.3-eV lamp is monochromatic within the range examined, but the energy of this line is 8.44 eV, not 8.3 eV; (2) the 9.5-eV source is the worst, as nearly all the light emitted is. at 8.4 eV; of the remaining ing lines, the most intense is at 9.57 eV, but others are present up to 10.88 eV; (3) the 10.2-eV source has two sharp lines at 10.03 eV (83%) and 10.64 eV (17%); (4) the 11.7-eV source again has two sharp lines (11.62 and 11.82 eV), but as they are closer together than those of the 10.2-eV source, they do not create as much of a problem when interpreting the detector response; (5) the output of the lamps does not include an emission continuum, implying that the gases are at low

122 pressure; (6) the gases used are very pure; and (7) the importance of the lamp window material can be seen in the spectra of the 8.3- and 9.5-eV sources: both are filled with xenon, but the 8.3-eV source uses a window of a material that cuts off completely above about 8.5 eV. TABLE 6.5 CHARACTERISTIC LINES FOUND IN VACUUM UV OUTPUT OF HNU SYSTEMS LAMPS (From ref. 20.) Lamp designation (eV) 8.3 9.5

10.2 11. 7

Wavelength

Energy

Output

(nm)

(eV)

(%)

147.0 114.0 117.2 119.3 125.0 129.6 147.0 116.6 123.6 104.9 106.6 121.6

8.44 10.88 10.58 10.40 9.92 9.57 8.44 10.64 10.03 11.82 11.62 10.20

100 0.03 0.01 0.18 0.05 2.1 97.6 17.1 82.9 26.2 71.8 2.0

Lamp type

Xenon Xenon

Krypton Argon

Table 6.6 gives a list of solvents that, owing to their ionization potential, do not give a positive response, i.e., increase in ionization current, in the photoionization detector. Fig. 6.11 compares the chromatograms of drug solutions in acetonitrile obtained by means of a PIO and an FlO. The PIO yields a smaller (negative) response to acetonitrile than the FlO. However, the drop of the baseline is pronounced during the passage of the acetonitrile peak 28 ,29. Just as with oxygen, the drop is due to electron capture by the acetonitrile molecule and to the subsequent neutralization of the positive and negative ions (see eqns. 6.15 and 6.16). As this acetonitrile peak, and also the peaks of tetrachloromethane, trichloromethane, methanol and water, diminish the response of the simultaneously eluting compounds 30 , the latter should be separated from acetonitrile. The PIO has frequently been used connected in parallel to an FI0 14 ,20,32-34. The FlO responses to an effective carbon atom in alkanes, alkenes and aromatics are approximately the same. The PIO response to a carbon atom of aromatic hydrocarbons is several times greater than that of the FlO. The values found by

123 TABLE 6.6 SOLVENTS USED WITH PID THAT PRODUCE NO POSITIVE DETECTOR RESPONSE (Reprinted from ref. 28, with permission.)

Lamp 9.S eV

10.2 eV

11.7 eV

H2O CH 3CN CH 30H C2HSOH CH 3Cl CH 2C1 2 CHC1 3 CC1 4 C2H4C1 2 Freons

H2O CH 3CN CH 30H CH 3Cl CH 2C1 2 CHC1 3 CC1 4 C2H4C1 2 Freons

H2O CH 3CN Freons (some)

CSH12 C6H14 C7H16

Driscoll et al. 14 for the PID to FlO response ratio are <2, 2-4 and S-10, for alkanes, alkenes and aromatics, respectively. This ratio also differs considerably for sulphur-containing and chlorinated pesticides. It has been found to be 12-16 for aromatic compounds and 2 or less for aliphatic compounds. Fig. 6.12, depicting the chromatograms of automobile exhaust gases, illustrates the application of this method of determining compound types. Table 6.7 shows the response ratios of other groups of substances under different experimental conditions. The response ratio of an electron-capture detector (ECD) and a PID has been used for the selective differentiation of nitro compounds 11 • This ratio differs by several orders of magnitude for polyaromatic hydrocarbons and their nitro derivatives (see Table 6.8). Fig. 6.13 again indicates the possibility of selectively distinguishing certain groups of compounds by simultaneously employing an FlO and a PID. For nitro toluenes, the PID yields a relative response to o-nitrotoluene that is about two orders of magnitude smaller than

TABLE 6.7 PID/FID TOLUENE NORMALIZED RESPONSES (TNR) FOR VARIOUS COMPOUND CLASSES (Reprinted with permission from ref. 33.) Class

Retenti on times Number of species tested

Halogenated alkanes Simple alkanes Cycloalkanes + trimethylalkanes Alkynes Alkenes Aldehydes Ketones Aromatics Chlorinated aromatics Chlorinated alkenes Sulphur-containing hydrocarbons * **Does not include ethylene. Does not include acetaldehyde.

9 13 2 3 23 4 2 0 0 3 2

<

17 min TNR (mean

±

S.D.)

o± 0

3 3 3 70 69 157

± ± ± ± ± ±

3 1 3 11* 10** 5

218 500

± ±

180 210

Retention times

>

Number of species tested

TNR (mean

±

1 12 27

± ± ±

1 4 8

55 56 123 87 141 211 129

±

10

± ± ± ± ±

25 18 12 97 37

6 10

5 0 20 1 2 21 7 4 2

17 min S.D.)

125

9 10

16

2 13

PIO

7

11

FlO

16

o

5

10

o

. 15

min

5

10

min 15

Fig. 6.11. Drug chromatograms. Glass column (6 ft. x 2 mm I.D.) packed with 3% OV-17 on Gas-Chrom Q deactivated with phosphoric acid. 1 = Barbital; 2 = Methyprylon; 3 = Aprobarbital; 4 = Allylisobutyl; 5 = Amobarbital; 6 = Pentobarbital; 7 = Secobarbital; 8 = Meprobamate; 9 = Doriden; 10 = Mephobarbital; 11 = Phenobarbital; 12 = Darvon; 13 = Methaqualone; 14 = Primidone; 15 = Dilantin; 16 a Valium. Temperature programmed from 200 to 285°C at 12 0 C/min. (From ref. 31.) TABLE 6.8 RELATIVE RESPONSE FACTOR RATIOS ON ECD/PID FOR POLYAROMATIC HYDROCARBONS AND THEIR NITRO DERIVATIVES (From ref. 35.) Compound pair

PID/PID

ECD/ECD

ECD/PID:ECD/PID*

Indan/5-nitroindan Fluorene/2-nitrofluorene Naphthalene/2-nitronaphthalene Anthracene/9-nitroanthracene Pyrene/3-nitropyrene

0.46 1.01 0.23 0.38 1.00

3.1.10- 5

6.80.10- 5 ** 7.41,10-6 4.88,10- 3 6.99.10- 3

**

1. 70.1O-~ 2.70·107.00,10- 3

*Calculations made using peak heights. Relative response factor normalizations using o-nitrotoluene = 1.00 on both detectors. **Not possible to obtain any measurable ECD response for fluorene at microgram levels or above.

126

PIO

26 24

23

29

2~ 27~\ )30 2122

18

1920~

61

1m

_ill

LL

11

15

FlO 12 14 13 7

8910

56

1

liUill o

5

10

15

20

25

30 min

Fig. 6.12. Analysis of automobile exhaust. SE-30 silica capillary column (60 m). 1 = ethane; 2 = propane; 3 = isobutane; 4 = n-butane, 5 = 2-methylbutane, 6 = n-pentane; 7 = 2-methylpentane; 8 = 3-methylpentane; 9 = n-hexane; 10 = methylcyclopentane; 11 = 2-methylhexane; 12 = 2,2,4-trimethylpentane; 13 = n-heptane; 14 = methylcyclohexane; 15 = n-nonane; 16 = propylene; 17 = I-butene; 18 = trans- and ais-butenes; 19 = 1-pentene; 20 = 2-methyl-1butene; 21 = trans- and ais-pentenes; 22 = cyclopentene; 23 = benzene; 24 = toluene; 25 = ethyl benzene; 26 = m- and p-xylene; 27 = a-xylene; 28 = npropylbenzene; 29 a l,3,5-trimethylbenzene; 30 = 1,2,4-trimethylbenzene. Temperature programmed from -50 0 C (2 min) to 100 0 C at 60 C/min. (Reprinted from ref. 33 with permission.) that to dinitrotoluene. The PIO response to dinitrobenzenes is so small that levels below 1 vg are not detectable. The ECD and PIO response ratios characterize all three groups (Table 6.9). 6.5. CARRIER GAS Helium has been used as carrier gas because it does not absorb UV radiation (10-eV photons). The PIO with nitrogen as the carrier gas (absorbing the UV radiation)9 gives a signal similar to that with helium. In this instance, other reactions can take place in addition to the set of reactions 6.2-6.8:

127

FlO

min

3

2

4

3

PIO

12

min

Fig. 6.13. Gas chromatograms obtained with simultaneous use of an FID and a PID of four nitro compounds. Glass column (6 ft. x 2.00 mm I.D.) of Ultrabond 20M. 1 = Nitropentane; 2 = nitrocyclohexane; 3 = nitrotoluene; 4 = 2,6-dinitrotoluene. Temperature programmed from 50 0 C (2 min) to 180 0 C at lOoC/min; split, 70:30 between FID and PID. (From ref. 35.) N2 + hv

+

N2 *

(6.13)

N2* + AB

+

N2 + AB+ + e-

(6.14)

where reaction 6.14 can be highly efficient. When using argon as the carrier gas, the PID response is about 40% higher than that when nitrogen or helium is used (Table 6.10). This is due to the higher drift velocity of the electrons in argon; the efficiency of electron collection in the PID is higher and the recombination of electrons and ions decreases (reaction 6.7)36. The response in hydrogen is also by about 25% greater than that in nitrogen 10 • The response in air is lower than that in helium 10 ,12,25, and which is due to the more efficient neutralization of the positive ions from photoionization by anions compared with that by electrons 12 (owing to the comparable velocities of the anions and cations). The oxygen itself decreases the background ionization current (negative response)17:

128 TABLE 6.9 RELATIVE RESPONSE FACTORS FOR NITRO-AROMATICS OBTAINED BY GAS CHROMATOGRAPHY WITH ECD AND PID (From ref. 35.) Compound

o-Nitrotoluene m-Nitrotol uene p-Nitrotoluene 2,3-Dinitrotoluene 2,4-Dinitrotoluene 2,6-Dinitrotoluene 3,4-Dinitrotoluene o-Dinitrobenzene m-Dinitrobenzene p-Dinitrobenzene

Relative response factor * PID

ECD

ECD/PID

1.00 1.06 1.56 2.92 10- 2

1.00 1.11 1.06 5.44 4.86 5.47 4.94 4.86 3.31 5.81

1.00 1.05 0.68 186.3 540 222.4 277.5 -** -** -**

0

9.00.1O-~

2.46·101. 78.10- 2 -** -** -**

*Relative response factors were obtained by measuring peak heights and dividing by the absolute amount reaching the detector. o-Nitrotoluene was assigned an arbitrary value of 1.00 cm/ng, and other relative response factors were calculated relative to o-nitrotoluene. **There was no measurable PID response for sub-microgram amounts of the dinitrobenzenes. TABLE 6.10 RELATIVE PID RESPONSES (From ref. 36.) Carrier gas (flow-rate 15 ml/min)

Relative response Benzene

Toluene

p-Xylene

m-Xylene

o-Xylene

Argon Carbon monoxide Tetrafluoromethane Nitrogen Helium Air Carbon dioxide

1.401 1.03 1.010 1.000 0.987 0.542 0.285

1.44 1.02 0.998 1.000 0.987 0.525 0.269

1.49 1.03 0.848 1.000 1.126 0.487 0.278

1.40 1.00 1.098 1.000 0.970 0.456 0.249

1.42 1.00 1. 27 1.000 0.982 0.433 0.179

129 e

+ O2

O - + AB+ 2

+

O2

(6.15)

+

AB + O 2

(6.16)

The response in carbon dioxide is about five times smaller than that in helium, which, according to Freedman's explanation l2 , is because carbon dioxide is much more efficient in the deactivation of collision-excited states (see eqn. 6.8). Carbon monoxide and tetrafluoromethane do not lead to a decrease in response and, therefore, Senum36 ascribed the lower responses in carbon dioxide to electron capture, similarly to the case with oxygen (eqns. 6.15 and 6.16). Table 6.9 lists the relative responses in various carrier gases. A response decrease similar to that in air and carbon dioxide has also been found for methane and nitrous oxide. The absorption coefficient at 10.2 eV is very high for these two gases (Table 6.11). The responses to the analysed compounds in nitrogen, helium, carbon monoxide and tetrafluoromethane are commensurate 36 . It is evident from the above that the use of argon as a carrier gas is advantageous, whereas the use of methane is disadvantageous (for instance, for the connection of a PIO and an ECO). TABLE 6.11 IONIZATION POTENTIALS AND ABSORPTION COEFFICIENTS FOR VARIOUS CARRIER GASES (From ref. 36.) Carrier gas

Ionization potential (eV)

Absorption coefficient at 10.2 eV (cm- 1 )

Oxygen Nitrous oxi de Carbon dioxide Methane Tetrafluoromethane Carbon monoxide Argon Helium

12.063 12.894 13.769 12.6 15.67 14.013 15.67 24.58

0.27 100 2

400 Negligible 3

Negligible Negligible

In the PIO in which the UV radiation source is not separated from the ionization chamb~r and where photoionization is not the only reaction to occur (see section 6.1), the response decreases in the order He > N2 > Ar > air. Here, the type of carrier gas also affects the background ionization current and the detector noise 7.

130 TABLE 6.12 CHROMATOGRAPHIC EFFICIENCY (N) AS A FUNCTION OF FLOW-RATE USING THE FID AND THE PID (From ref. 37.) Carrier gas flow-rate (ml/min)

Gas velocity (em/sec)

0.32 0.52 0.91 1.60 2.13

5.16 7.13 10.77 15.23 18.74

PID (30 m)

FlO (30 m)

86 76 47 28 20

102 97 68 45 33

917 990 577 972 508

A

o

5

10

Di fference in N

N

(%)

389 720 182 455 520

17 .8 26.9 43.31 56.89 63.45

B

min 0

5

10

min

Fig. 6.14. Comparison of the efficiencies of chromatographic columns with (A) a PID and (B) an FID. (From ref. 37.)

131 The PID is a concentration detector and therefore the detector response (peak area) decreases with increasing flow-rate of the carrier gas 9• If the carrier gas flow-rate is varied from 10 to 30 ml/min, the ppak area diminishes to 25% or 20% of the initial value 9 ,17,22. Hence the response is lower when using make-up gas with capillary columns in connection with a PID. For instance, the PID response increases by about 50% if the flow-rate of the make-up gas is changed from 30 to 20 ml/min 33 • The optimization of the HNU Systems PID for capillary columns was described by Jaramillo and Driscol1 37 (for applications of PIDs with capillary columns see, e.g., refs. 20, 32, 33, 38 and 39). The deterioration of the chromatographic efficiency compared with the application of an FID is a function of the flow-rate of the carrier gas (see Table 6.12). The decrease in peak separation is very small for low flow-rates (see Fig. 6.14, the group of peaks with braces). I~hen using make-up gas with a flow-rate of 30 ml/min, the peak width at half-height is reported to be very similar for both detectors 33 . The smallest detector volumes vary20,24,32 from about 35 to 50 ~l, the effective volume of the 225-~1 unmodified HNU Systems detector being only 54 ~l at a flowrate of 1.6 ml/min 37 • REFERENCES 1 HNU Systems, New High Tempepatupe Photoionization Detectop fop Gas Chpomatogpaphy, Industrieregler GmbH, Vienna, 1978. 2 J.E. Lovelock, Natupe (London), 188 (1960) 401. 3 t1. Yamane, J. ChPOmatogp., 11 (1963) 158. 4 M. Yamane, J. Chpomatogp., 14 (1964) 355. 5 J.F. Roesler, Anal. Chem., 36 (1964) 1900. 6 D.C. Locke and C.E. Meloan, Anal. Chem., 37 (1965) 389. 7 J.G.W. Price, D.C. Fenimore, P.G. Simmonds and A. Zlatkis, Anal. Chem., 40 (1968) 541. 8 R.R. Freeman and W.E. Wentworth, Anal. Chem., 43 (I971) 1987. 9 J.N. Driscoll and F.F. Spaziani, Res./Develop., 27, May (1976) 50. 10 J. Sevc1k and S. Kr~sl, Chpomatogpaphia, 6 (1973) 375. 11 S. Kapila, D.J. Bornhop, S.E. t1anahan and G.L. Nickell, J. ChPOmatogp., 259 (1983) 205. 12 A.N. Freedman, J. Chpomatogp., 190 (1980) 263. 13 A. Schweig and W. Thiel, J. Chem. Phys., 60 (1974) 951. 14 J.N. Driscoll, J. Ford, L.F. Jaramillo and E.T. Gruber, J. Chpomatogp., 158 (1978) 171. 15 K. Watanabe, J. Quant. Spectposc. Radiat. Tpansfep, 2 (1962) 369. 16 N. Ostojic and Z. Sternberg, Chpomatogpaphia, 7 (1974) 3. 17 J.N. Driscoll, J. Chpomatogp., 134 (1977) 49. 18 M.L. Langhorst, J. Chpomatogp. Sci., 19 (1981) 98. 19 A.N. Freedman, J. Chpomatogp., 236 (1982) 11. 20 J.N. Davenport and E.R. Adlard, J. Chpomatogp., 290 (1984) 13. 21 M.E. Casida and K.C. Casida, J. Chpomatogp., 200 (1980) 35. 22 Instpuction Manual, Model PI-52-02 Photoionization Detectop, HNU Systems, Newton, MA, 1979. 23 J.N. Driscoll, Amep. Lab., 8 (1976) 71. 24 Tpacop Model 703 Photoioniaation Detectop, Gas Chpomatogpaphy, Tracor Instruments, Austin, TX.

132 25 R.C. Leveson and N.J. Barker, in Proceedings of the 27th Annual ISA Analysis Instrumentation Symposium. St. Louis. MO. March 1981. Instrument Society of America, Research Triangle Park, NC, p. 7. 26 J.N. Driscoll, Ind. Hygiene News. 3, No. I, March (1980). 27 J.N. Driscoll, J. Chromatogr. Sci., 20 (1982) 91. 28 J.N. Driscoll, J. Ford, L. Jaramillo, J.H. Becker, G. Hewitt, J.K. Marshall and F. Onishuk, Amer. Lab •• 10 (1978) 137. 29 D.B. Smith and L.A. Krause, Amer. Ind. Hyg. Ass. J., 39 (197B) 939. 30 M. Dressler, J. Chromatogr •• in preparation. 31 L.F. Jaramillo and J.N. Driscoll, J. Chromatogr., 186 (1979) 637. 32 S. Kapila and C.R. Vogt. J. High Resolut. Chromatogr. Chromatogr. Commun., 4 (l981) 233. 33 R.D. Cox and R.F. Earp, Anal-. Chem •• 54 (1982) 2265. 34 J. Winskowski. Chromatographia, 17 (1983) 160. 35 J.S. Krull, M. Swartz, R. Hilliard, K.H. Xie and J.N. Driscoll, J. Chromatogr., 260 (1983) 347. 36 G.I. Senum, J. Chromatogr., 205 (1981) 413. 37 L.F. Jaramillo and J.N. Driscoll, J. High Resolut. Chromatogr. Chromatogr. Commun., 2 (1979) 536. 38 W.G. Jennings, S.G. Wyllie and S. Alves, Chromatographia, 10 (1977) 426. 39 J. Meili, P. Bronnimann, B. BrechbUhler and H.J. Heiz, J. High Resolut. Chromatogr. Chromatogr. Commun., 2 (1979) 475.

133

ChapteY' 7

FLAME PHOTm1ETRI C DETECTOR CONTENTS Introduction • • • • • • • • • • • • • • • • • Response mOdel • . • • • • • . • • • • • . • • • Detector sensitivity and minimum detectability 7.3.1. Flow-rates of gases •••• 7.3.2. Structure of compounds •• 7.3.3. Concentration of compounds 7.3.4. Detector temperature 7.3.5. Interference filter. 7.3.6. Photomultiplier ••• 7.4. Selectivity of response •• 7.5. Tin and germanium compounds 7.6. Halogen compounds • • • • . 7.7. Other detection possibilities 7.8. Linearity of response 7.9. Su 1phur background • • • • • . 7.10. Response quenching • . • • • • 7.11. Flame stability • • • • • • • • • 7.12. Other identification possibilities References • . • • 7.1. 7.2. 7.3.

133 136 137 137 138

139 142 142 143 144 145 147 149 150 152 152 157 157 158

7.1. INTRODUCTION In an oxygen-rich flame, the decomposition of substances that contain a heteroatom generates excited species that, during their transition to the ground state, emit radiation characteristic of the given heteroatom. The principle of the flame photometric detector (FPD) is based on the measurement of the characteristic emission in the flame. In gas chromatography, a flame photometric detector for sulphur and phosphorus compounds has found the widest use and has also been produced commercially. As early as in 1869, Salet 1 found that, on introduction of an aerosol of a solution of a sulphur compound into a low-temperature, fuel-rich hydrogen flame, an intense blue emission near the surface of the cold object near the flame core could be observed. Salet's phenomenon became the basis for the technique of detecting sulphur and/or phosphorus compounds by means of a flame photometric detector in a hydrogen-rich flame 2• Dagnall et al. 3 and Syty and Dean 4 developed this technique for cool flames in flame emission spectrometers, while Brody and Chaney5 developed it for gas chromatography (Juvet and Durbin 6 for organometallic compounds in 1963).

134

..... COLUMN EFFLUENT(N2)

Fig. 7.1. Schematic diagram of flame photometric detector. 1 = Flame-ionization burner tip; 2 = burner; 3 = mirror; 4 = glass window; 5 = optical filter; 6 = photomultiplier tube. (Reprinted from ref. 5, with permission.)

<0 N

a

If)

600

490

WAVELENGTH (nm )

b

WAVELENGTH ( nm I

Fig. 7.2. Emission spectra of (a) HPO species, slit width 0.3 mm (from ref. 4); and (b) S2 species, slit width 0.4 mm (from ref. 7). The design of an FPD for use with a gas chromatograph to detect phosphorus and/or sulphur compounds is illustrated in Fig. 7.1. The carrier gas at the chromatographic column outlet is mixed with oxygen while hydrogen is fed directly

135

PHOSPHORUS

1

2

4

3 6

START

~A

x 64 000 1mV

L START

r

10 min

~

V

t

5

10 min

x 6 400 1mV

4

2

6

SULPHUR 3

Fig. 7.3. Simultaneous detection of phosphorus- and sulphur-containing compounds. 1 = disyston; 2 = methyl parathion; 3 = malathion; 4 = parathion; 5 = methyl trithion; 6 = ethion. Glass column (6 ft. x 0.16 mm 1.0.) packed with 3% OV-101 on Gas-Chrom Q; temperature, 190 0 C. (From ref. 9.)

C

B

A

H'~ I

/

II

t

02+ N2

II

/

°2

II

/

N2+AIR

H/AIR

H/N2

E

0

II

/

O2/ AIR

I

t

N2

H2

I N2+ H2

Fig. 7.4. Schematic diagram of the gas supply lines leading to the FPD. A, From ref. 5; B, from ref. 10; C, from ref. 11; 0, from ref. 12; E, from ref. 13. to the detector body (see also Fig. 7.4A). The compounds decompose in the detector flame giving rise to excited HPO * species (phosphorus compounds) or S2 * species (sulphur compounds); the spectra of the emitted radiation of the above compounds are displayed in Fig. 7.2. From the given spectra, wavelengths of about 526 nm for phosphorus and about 394 nm for sulphur are selected for detec-

136

tion by the FPD. The appropriate interference filter is placed between the emission chamber of the FPD and the photomultiplier tube. The light emission is viewed by the photomultiplier tube. The original construction by Brody and Chaney5, equipped with a single optical system (526 or 394 nm), was complemented by Bowman and Beroza 8 by an additional system that made it possible to apply phosphorus and sulphur filters simultaneously. At present, this construction is currently also used with commercial detectors. It allows two simultaneous selective records to be obtained from one chromatographic analysis (Fig. 7.3). In addition to the gas inlet .system shown in Fig. 7.1, many other configurations of the FPD have been used (Fig. 7.4). Their potential advantages are discussed in the respective sections. 7.2. RESPONSE MODEL The flame in the FPD serves three basic functions in the sulphur response mechanism 14 ,15: (a) the initial sulphur-containing molecules are decomposed in the hot regions of the fl arne; (b) the sulphur species formed then produce sulphur atoms, either directly: sulphur compound

heat

S atoms

(7.1)

H2S

(7.2)

or indirectly: sulphur compound H2S + H

heat

HS + H2

(7.3)

---'" HS + H .......S + H2

(7.4)

(c) subsequently, excited S2* species are formed in the cool outer cone. The excitation energy for S2 is believed to come from three-body recombinations 16 ,17: S + S + r1 -

S2 * + 11

(7.5)

(where r1 is the thi rd body) with the correspondi ng emi ssi on: (7.6) Syty and Dean~ believe that the excitation energy for the S2 ~ S2* transition is due to recomb.ination reactions:

137 (7.7)

or H + OH +

S2

(7.8)

The contribution of reactions 7.7 and 7.8 to the total emission of 52 * species is thought to be small, however 16 ,17. The following set of reactions has been proposed for the phosphorus response mechanism 4: P + 0 2

P + PO

(7.9)

P + OH

PO + H

(7.10)

H + PO + /·1 H + PO

~

~

HPO * +

r~

HPO *

(7.11)

(7.12)

7.3. DETECTOR SENSITIVITY AND MINIMUM DETECTABILITY The detector response to phosphorus and sulphur compounds is a function of (1) gas flow-rate; (2) structure of the solute compounds; (3) concentration of the solute compound in the carrier gas; (4) detector temperature; (5) filter application; and (6) photomultiplier. 7.3.1. Flow-rates of gases

The dependence of the detector sensitivity and minimum detectability on the gas flow-rates varies according to the detector design and the configuration of the conduit leading the gas into the detector 17 ,18-23, and it is difficult to generalize. Evidently, optimum flow-rates have to be found for each detector (Table 7.1) and there are substantial differences in some instances. For instance, if the carrier gas is fed outside the jet, the response decreases with increasing air flow-rate 18 , as with the type E configuration of gas inlets (see Fig. 7.4)17. For the type C configuration, the response increases 18 with increasing air flow-rate or displays a maximum 24 in the same way as with a detector with a separate emission chamber 25 • With hydrogen, the dependence of the response on the increasing flow-rate of this gas can also decrease 17 ,24, increase 25 or reach a maximum 21 ,23. All these interrelations are affected by the 02:H2 flowrate ratio (e.g., refs. 20,23 and 25) and by the type of carrier gas used 23,26 (nitrogen or helium). The minimum detectabilities with various types of FPD are approximately commensurate at optimum gas flow-rates; however, the gas inlet con-

138 TABLE 7.1 SUMMARY OF OPT! MUr1 FLAME PARAMETERS Reprinted from ref. 21, with permission. Source

Flow-rate (ml/min) H2

Brody and Chaney5 Mi zany 22 Greer and BYdal~~33 Sugiyama et al. Pes car and HartT~nn24 Eckhardt et al. Burgett and Gre g13 Kapila and Vogt2

200 50 90 168 80 100 50 60-80

O2 40 20 18 0 0 24 10

6-8

Air

N2

o /H

0 10 15 195 100 0 50 0

160 70 30 22 0 126 60

0.20 0.44 0.25 0.23 0.25 0.24 0.44 0.10

* 2 2

* Including oxygen from the air. figuration in which the carrier gas is pre-mixed with hydrogen seems to offer certain advantages. The minimum detectable mass rate ranges from about 1'10- 13 g/sec 20 ,26-28 to 2'10- 12 g/sec 5 ,9,29,30 of P for phosphorus compounds and from about 2'10- 12 g/sec 20 ,27,29,31,32 to 5'10- 11 g/sec of S28,30-32 for sulphur compounds. 7.3.2. Structure of compounds

Studies on the effects of the structure of sample compounds on the FPD response have produced different results. According to Maruyama and Kakemoto 18 , the detector response to sulphur compounds depends solely on the sulphur content in the molecule of the compound. The response is constant for equal amounts, of sulphur, i.e., it is independent of the structure of the compound. However, most workers have demonstrated that the response and, hence, the minimum detectability depend on the structures of both sulphur compounds 19 - 22 ,28,34 and phosphorus compounds 20 ,35,36. This means that the response depends substantially on the efficiency of the production of the emitting HPO* and S2* species. Studies on the intensity of this influence have led to contradictory conclusions: Mizany22 reported that the response decreases in the order disulphide > sulphate> sulphone> sulphide> sulphite, whereas Sugiyama et al. 34 reported the order buthanethiol > thiolane > thiophene> butyl sulphide > thiophenol > benzothiophene > phenyl sulphide> pentyl disulphide. In the former instance disulphide

139 gives a greater response than sulphide, whereas in the latter the contrary is true. The response decreases in the order 36 phosphane > phosphite> phosphate phosphate for phosphorus compounds (see, for instance, triphenyl compounds in Table 7.2). For compounds that contain both a phosphorus and a sulphur atom in the molecule, the response ratio Rp/RS is given 8 ,9,30 by the phosphorus to sulphur ratio in the molecule of the compound. The Rp/IRS ratio is recommended 8 ,9,30 for calculating the phosphorus and sulphur contents. As n (see section 7.3.3) does not alway equal 2, and in view of the non-constancy of n, this relationship cannot be used generally. 7.3.3. Concentpation of compounds

The dependence of the FPD response on the amount of a phosphorus compound is linear over a range of about four orders of magnitude. Thus, the response per unit amount of solute (molar response, weight response) is constant within this range. The dependence of the response on the amount of a sulphur compound is given by the equation R = k[S]n. As S2 species are responsible for the emission, the response should theoretically be linearly related to the square of the amount of sulphur compound. Measurements exist that support this assumption. Maruyama and Kakemoto 18 reported n a 2, irrespective of the structure of the compound. Several experimental results have shown, however, that n varies between 1 and 2 (e.g., refs. 20, 31, 34, 37 and 38). In addition, it changes with the hydrogen flow-rate and the 02:H2 and 02:N2 flow-rate ratios 17 ,19,22, with the structure of the sulphur compounds 19 ,21,22,34 and with the detector temperature 39 . An exponential relationship between the response and the amount of the compound was also found for selenium 40 •41 and tellurium 41 compounds. Hence the response per unit amount of solute increases with increasing amount of the solute compound in any case. However. if n varies with the structure of the compound, the relative responses of different compounds can vary with changing concentration. Fig. 7.5 demonstrates this effect for dimethyl disulphide (n = 1.78) and diphenyl sulphide (n = 1.96). With solute supply rates exceeding about 0.4 ng/sec of S, the response for sulphide is greater than that for disulphide, whereas the disulphide response is greater than the'sulphide response below this rate. The dependence of n on the structure of the compounds may be responsible 21 for the discrepancies that occur in the literature for the response level of individual types of sulphur compounds (see section 7.3.2). Sugiyama et al. 34 , who found that the detector response to sulphides was greater than that to disulphides, worked in the range 16-32 ng/sec of S, whereas Mizany22 reported the disulphide response to exceed that to sulphides. When recalculating the investigated amounts of compounds with regard to the sample charge and the

......

TABLE 7.2

~

o

RESPONSE OF ORGANOPHOSPHORUS COMPOUNDS IN SINGLE- AND DOUBLE-FLAME PHOTOMETRIC DETECTOR Reprinted from ref. 36, with permission. Compound

Amount of compound injected (ng)

Amount of P (ng)

Tri ethyl phosphate Tri-n-butyl phosphate Triphenyl phosphate

20 20 20

3.4 2.3 1.9

7.5 4.2 2.0

28.1 18.1 7.2

3.7 4.3 3.6

Triethyl phosphite Tri-n-butyl phosphite* Triphenyl phosphite

20 20 20

3.7 2.5 2.0

21.1 5.8 11.1

31.1 7.5 14.1

1.5 1.3 1.3

Triethyl thiophosphate Methyl parathion Parathion

20 20 20

3.1 2.3 2.1

28.1 16.1 15.1

27.9 16.2 15.3

1.0 1.0 1.0

Triphenyl phosphine

20

2.4

18.2

19.4

1.1

*Showed impurity peak.

Single-flame response (R ): peak area x110-5

Double-flame response (R ): 5 peak area x210-

141

UI Vl

Z

a

a. Vl w

cr:

0.01

0.1

1.0

ngS ISec

10

100

Fig. 7.5. Effect of structure of sulphur compounds on the dependence of the FPO response on the amount of the compound. (Reprinted from ref. 21, with permission.)

carrier gas flow-rate, it becomes evident that Mizany actually applied about 0.04 ng/sec of S, so that both results appear to be correct. Other discrepancies are also likely to be explained in this manner; for instance, Mizany22 noted that the relative response varies with the concentration of the compounds for the sulphite-sulphide pair. The sulphide response is greater for 5 ng of S whereas the sulphite response is greater for 10 ng. As n varies with the flow-rate, the relative response additionally also varies with the gas flow-rates at different concentrations. For an 02:H2 ratio of 0.44 (2 ng of S) the disulphide response is greater than the sulphate response and at an 02:H2 ratio of 0.15 the response i.s the same.

142 7.3.4. Deteator temperature

Whereas the FPD response to sulphur compounds decreases with increasing detector temperature 18 ,24,39, with phosphorus compounds it increases 39 • The character of the variations in the response to sulphur compounds differs slightly for individual types of detector. Whereas Maruyama and Kakemoto 18 found that the dependence of wl'h (W is peak width at half height, h is peak height) on the detector temperature was linear in the range 100-300 0 C with a decrease in the response by about a third, Dressler 39 noted a decrease in the peak height by approximately two thirds in the range BO-160 0 C (see Table 7.3). Under certain experimental conditions. this decrease changes to an increase in response at detector temperatures exceeding 160 0 C. The above relationships are also slightly affected by the flow-rates of the gases 39 and by the structure of the compound 24 , 39. However. the detector noise also increases exponentially with increasing temperature of the FPD 39 • Hence the minimum detectable mass rate increases with increasing detector temperature with sulphur compounds, whereas it remains approximately constant with phosphorus compounds. TABLE 7.3 PEAK HEIGHT OF THIOPHENE OBTAINED USING VARIOUS FPD TEMPERATURES Amount injected: 1.50.10-10 g. From ref. 39. TD (OC)

Peak height (arbitrary units)

BO 100 120 140 160 1BO 200

136 90 74 47.5 43.5 51.5 62

*A, TD = 160 0 C taken as the basis; B, TD taken as the basis.

Difference in peak heights (%)* A

B

+212.6 +106.9 +70.1 +9.2

+B3.B +31.6

+18.4 +42.5 =

-35.B -41.2 -30.4 -16.2

C

-33.B -45.6 -65.1 -68.0 -62.1 -54.4

1200 C taken as the basis; C, TO

=

BOoC

7.3.S. Interferenae filter

Monitoring of the emission without using a filter yields a greater response than that obtained when a filter is used 42 - 44 (Fig. 7.6). Under these conditions, the selectivity for hydrocarbons is retained even though it is smaller, but the selective resolution between phosphorus and sulphur disappears. As the noise

143

10 6

10 5

0

-0

10 4

CII 111

.0

c

-

10 3

0

Ci 10

C'4 H30 ( 52 5 )

2

C'2H26 (393 )

c

Ol

III

10 '

-1

10

Fig. 7.6. Relationships between the signal-to-noise ratios and the amounts injected for filter and filterless modes. Closed symbols, 393 or 525 nm interference filters used; open symbols, no filters used. (From ref. 42.) also increases, the minimum detectable mass rate is only slightly decreased 42 • If the dimensions of the slit of the black disc placed in the detector instead of the interference filter are optimized. the minimum detectable mass rate is about half that obtained when a filter is used 45 • 7.3.6. Photomultiplier

The detector sensitivity and detector noise depend on the type of photomultiplier and on the voltage applied to the latter. The detector response increases with increasing voltage applied to the photomultiplier. However, the noise increases at the same time, so that the minimum detectable mass rate remains approximately constant 24 •25 . The detector noise and. consequently, the minimum detectable mass rate increase with increasing temperature of the photomultiplier tUbe 46 .

144 7.4. SELECTIVITY OF RESPONSE The selectivity of the detector response for phosphorus (interference filter of about 394 nm) is about four to five orders of magnitude relative to hydrocarbons. With a detector in which the emission space is separated from the compartment in which combustion takes place, a selectivity as high as seven orders of magnitude is reported 25 ,47. Filters with slit widths as narrow as possible should be used in order to obtain minimum interference between sulphur and phosphorus emissions. As the FPD response d~pends on the amount of the compound present in the detector, the selectivity for sulphur increases with increasing concentration of the compound. Hence the selectivity for sulphur compounds has to be expressed for the minimum detectable mass rate. Owing to the different dependences of the responses to phosphorus and sulphur compounds on concentration, the S:P response ratio is also a function of the concentration of the substances when a single filter is used. When using an S filter the S:P response ratio is 100:1 to 1000:1 and when using a P filter the S:P response ratio is 4:1 for amounts of 100-200 ng 29 ,30. For a detector with separated combustion and emission compartments, Joonson and Loog47 described the possibility of compensating electrically for the interfering sulphur response in the P channel; part of the signal from the S channel, corresponding to the interfering sulphur signal in the P channel, is fed to the output of the P channel amplifier (Fig. 7.7). Hence the response to the phosphorus compounds only is recorded. As the detector response to hydrocarbons is approximately constant at various temperatures of the detector, the selectivity varies with the detector temperature in the same way that the peak height of the respective compound varies depending on the temperature of the detector 39 • The P:S response ratio then increases with increasing detector temperature. The selectivity of the FPD response for sulphur and phosphorus compouns is also a function of the gas flow-rate. WP

Fig. 7.7. Dual-channel FPD system with S-interference compensator. 1 = FPD; 2 = dual-channel photometric block; 3 = light conductor; 4 = light divider; 5 = interference filters; 6 = photomultipliers; 7 = electrometric amplifiers; 8 = signal divider; 9 = separating network; 10 = recorders. (From ref. 47.)

145 This is also due to the varying dependence of the hydrocarbon response and the sulphur and/or phosphorus response on the flow-rates of the gases. For instance, with air flow-rates in the range 35-92 ml/min, the SIC selectivity decreases by a factor of about 100 with detectors having separated combustion and emission compartments 39 • 7.5. TI NAND GERt1AN IUM COMPOUNDS In a cool hydrogen-air diffusion flame, tin compounds produce a red molecularband emission 48 with a sharp SnH peak at 609.5 nm. With the aid of a Shimadzu FPD with a modified quartz tube containing the flame gases (chimney) and using a 610-nm interference filter, about 0.1 ng of tetrapropyltin could be determined by gas chromatography44. The application of a filterless technique (see section 7.3.5) makes it possible to determine about 30 pg of a tin compound 49 ,50. A detector of special design 51 has a detection limit of approximately 10 pg when using an interference filter. Carbon dioxide and arsine give rise to response interference due to the CO and CO+ molecular band emissions and AS 2+ molecular band emission. The more sensitive tin luminescence produces a broad emission ranging from 360 to 490 nm and appearing on the quartz surface (surface luminescence 44 ,49,50,52). t10nitoring of this emission without using a filter provides for the sensitive and selective detection of tin. The detection limit is about 2.10- 13 _4'10- 14 g for tetrapropyl tin; the Sn:C selectivity is 10 4:1 to 10 5:1 and the Sn:S and Sn:P selectivity is 10 2:1 to 10 3:1. The emission is considerably

GAS PHASE (py rex tube)

i

-12

i

-11

-9

i

-8

-7

-6

LOG GRAMS INJECTED

Fig. 7.8. Response of tetrapropyltin by two different mechanisms. (Reprinted with permission from ref. 50.)

146

Surface

phase

att :

I

40

I

80

I

120 • C

Fig. 7.9. Chromatogram of a standard mixture in surface- and gas-phase modes. S, 3 ng of (tert,-C4H9)2S2; Sn, 100 pg of (n-C3H7)4Sn; C, 5 pg of n-C15H32; Ge, 100 pg of (n-C4H9)4Ge. (Reprinted from ref. 49, with permission.) affected by the shape of the quartz chimney in which the flame is burning 44 ,49,53. The peaks or the compounds are tailed and peak tailing is increased by the presence of silicon 44 (the response also increases, however), sulphur 45 and hydrocarbons 44 in the flame and increases with increasing diameter of the quartz tube 44 • A decrease in the response was reported 54 ,55 with injections of large amounts of organotin ~ompounds (more than 100 ng), tropolone as the extracting solvent and organic co-extractives. In the most advantageous design of the conventional detector a piece of quartz wool is placed above the detector flame 50 • Fig. 7.8 compares the responses of both types of detection, i.e., monitoring either surface luminescence or gas-phase luminescence with tin compounds. Fig. 7.9 compares both methods of detection for sulphur compounds, germanium compounds, and hydrocarbons (filterless mode). With the surface mode the minimum detectable mass rate for tin compounds is approximately two orders of magnitude smaller, but the linear dynamic range is smaller 50 ,52. The sulphide dominates the gas phase while the response is small in the surface mode. Tin and germanium compounds show the opposite behaviour.

147 TABLE 7.4 FPD SELECTIVITY FOR GERMANIUr1 COMPOUNDS From ref. 53. Heteroatoms compared

Ge:Sn Ge:S Ge:P Ge:C

Selectivity Surface

Gas phase

1:5 50:1 - 500:1 ca. 5'10 4 :1

100:1 10:1 - 1000:1 17'1 106:1

A situation similar to that with tin is also observed with germanium. Surface luminescence is the most sensitive emission mode for organic germanium compounds. The minimum detectable mass rate of tetrabuty1germanium is 8.10- 15 g/sec of Ge 53 ; the selectivity of germanium relative to other elements is obvious from Table 7.4. The chromatographic peaks are broadened. however (even more than with tin compounds). Monitoring of the gas-phase luminescence (650 nm) yields less sensitive detection (2'10- 13 g/sec of Ge). The peaks are not broadened, however, the selectivity of germanium relative to tin and hydrocarbons increases (see Table 7.4) and the linearity of response covers about three orders of magnitude 53 • 7.6. HALOGEN COMPOUNDS Halogen compounds combusted in the presence of indium produce an intense emission of indium ha10genides in the range 340-420 nm 56 • By positioning indium near the flame of the detector and using a suitable interference filter (359.9 nm for InC1, 372.7 nm for InBr and 409.9 nm for InI), a detector selective to halogen compounds (except fluorine) is obtained 56 - 61 • A dual-flame detector was found to be the most suitable design 60 ,62 (see Fig. 7.10). The indium source is positioned between the two flames. The decomposition products containing C1, Br, I react with indium giving halogenides, the emission of which is monitored in the upper flame. The design according to Fig. 7.10 also provides for simultaneous photometric detection 62 ,63 (from the lower flame) of phosphorus compounds (by means of a 526-nm filter) and sulphur compounds (by means of a 394-nm filter). Similarly to the above-mentioned modes of flame photometric detection, the response is highly dependent on the flow-rates of the gases. The minimum detectable amount

148 with the dual-flame detector is 1.10- 11 g/sec for chlorine compounds, and the linearity covers four orders of magnitude. Compared with the single-flame detector, the sensitivity of this detector type is increased 58- 61 , owing to the separated electrical heating of the indium source. The detector selectivity is 1000:1 to 10 000:1 relative to hydrocarbons 60 ,61 and about 100:1 relative to sulphur and phosphorus compounds 60 ,62. The system of the FPD sensing the sodium emission is based on the principle of the alkali flame-ionization detector (AFID). A constant amount of an alkali metal salt is introduced into the flame. The Na emission (589 nm) is either increased or decreased in the presence of halogen compounds (except for fluorine). The character and the level of the detector response, as with the AFID, are basically affected by the hydrogen flow-rate (the flame temperature)64-67. Starting from a certain hydrogen flow-rate, the basic Na emission seems to decrease, even though several workers reported solely a decrease in basic emission 64 ,68 in the range of hydrogen flow-rates examined, or, on the contrary, solely an increase in basic emission 67 • The differing results can be ascribed to the fact that the detector design plays an important role, as with the AFID, and, particularly, the way in which the source of the alkali metal salt is positioned in the detector. The molar response decreases 64- 66 in the order I > Br > Cl

quartz tube

filter 360 nm.CI-channel

__ photomultiplier h 0 u sin g

-t:t=::., r

=:::~~

indium heater upper flame air inlet

1:;""",__-

flame shielding glass window filter, 394 nm. 5- channel filter, 526 nm,P-channel mix ed hydrogen-air inlet column

Fig. 7.10. Dual-flame photometric detector for chloro compounds. (Reproduced from ref. 62, with permission.)

149 compounds, even though Bowman et al. 66 reported the order Br > Cl > I compounds. The minimum detectable mass rates of iodine compounds vary in the range from 66 1'10-9 t0 65 2'10- 11 g/sec and the minimum detectable mass rate is about 1'10- 10 g/sec for Br and Cl compounds. Depending on the experimental conditions, the detection selectivity relative to hydrocarbons ranges 65 ,67 from 5000:1 to 10 000:1 and is 100:1 relative to phosphorus compounds 65 • The responses of the sodium FPD and the AFID display many similar features. As mentioned in section 3.9, one of the theories dealing with the mechanism of the function of the AFID ascribes the response of the latter to the increased volatility of the sodium and/or an other alkali metal salt in contact with the flame 68 • The response of both detectors depends strongly on the hydrogen flowrate (flame temperature) and on the detector design. With both detectors. the response to halogen compounds can be positive (increased ionization current or emission) or negative (decreased current or emission), being a function of the experimental conditions, particularly the hydrogen flow_rate 65 •67 ,69-72. The hydrogen flow-rate at which the positive response begins to decrease depends on the cation applied 66 ,69,71 and on the heteroatom-in the molecule of the compou~d67,69,71,72. However, a direct relationship between the decrease or increase in the emission in the FPD and the decrease or increase in the ionization current in the AFID cannot be derived from these results. For instance, the Na emission is decreased (the response is negative) when the AFID response is positive, but it remains negative also when the positive AFID response is decreasing (with the hydrogen flow-rate)64 or when it is negative already70. The Beilstein test for halogens is utilized with the detector in which the flame emission (526 nm) is sensed in the presence of copper 73 •74 • The detection limit is about 1'10- 8 g for halogen compounds 74 • For the detection of fluorine compounds an FPD in which calcium in argon is introduced into an acetylene-oxygen flame has been described 75 . The CaF emission is sensed. 7.7. OTHER DETECTION POSSIBILITIES Selenium compounds display a dominant Se 2 emission spectrum between 450 and 500 nm. The minimum detectable mass rate for selenium compounds is 2.10-12 g/sec of Se with a 484-nm interference filter, The selectivity relative to C varies 40 between 1000 and 10 owing to the exponential dependence of the selenium compound response on concentration. Of other heteroatoms, arsenic compounds (3.10- 11 g)26,42, boron compounds (3'10- 10 g)76 and Cr 77 and Fe 42 ,78 compounds (1'10-9 g) can be detected by the FPD in amounts below 1 ng. The remaining heteroatoms can be detected in amounts larger than 1 ng (Sb, Pb, Bi, Ni, Hg, Cu, Ti, Zr, Rh, W, Al)6,79-81.

150

6

Fig. 7.11. Flame-ionization detector modified for simultaneous FPO detection. 1 = Photomultiplier tube; 2 = interference filter; 3 = quartz window; 4 = detector jet tip assembly; 5 = GC column; 6 = collector electrode. (From ref. 61.) The positioning of two electrodes in the FPO system makes it possible to monitor the ionization current of the detector 30 ,82 and to obtain another nonselective record of the compound. On the other hand, a commercial flame-ionization detector (FlO) can simultaneously be operated as an FP061 ,78,83, as can be seen from Fig. 7.11. As the optimum operating conditions differ for the two detectors, an FPO operated under FlO conditions yields a lower response. 7.8. LINEARITY OF RESPONSE As noted above, the dependence of the FPO response on the amount of the phosphorus compound is linear, whereas this dependence for sulphur. selenium and tellurium compounds is exponential. If a constant amount of a sulphur compound is fed into the flame to create a background, the detector response becomes linear also for sulphur, selenium and tellurium compounds 23 ,27,41 within a certain concentration range. The effect of the amount of the sulphur compound supplied on the linearity range is evident from Fig. 7.12. The linearity range covers about two orders of magnitude (the sulphur background should be sufficiently high in comparison with the peak height).

151

.!1 'c ::J

>. <-

a<-

A ODED SULPHUR BACKGROUND

.0 <-

a

.4

tI
3 -.

UJ I

-

2

,

::.:: oCt UJ D-

O

--.


n- C1s H38 3 2

...J

o•

o - 11

-10

-9

-S

-7

-6

-5

- 4

LOG (GRAMS INJECTED)

Fig. 7.12. Calibration graphs on different sulphur backgrounds. No interference filters used. Octadecane peaks inverted. (From ref. 41.) The non-linearity of the response to sulphur compounds as a function of the amount of the compound can be compensated for with the aid of an electronic linearization devide 19 that converts the response to a linear function of the sulphur concentration rather than the normal relation R = k[S]n. Commercial linearization devices are commonly based on the quadratic dependence of the detector response on the amount of the compound (n = 2). As mentioned in section 7.3, n does not always equal 2 and, in addition, it varies as a function of the flow-rates of the gases, the structure of the compound and the detector temperature. The errors due to this device that arise during quantitative analysis increase with increasing concentration of the sulphur compounds and with decreasing n, varying between 40 to 200% at the tens of nanograms leve1 37 . For this reason, the linearization device should provide a variable exponential functi on 19.

152

7.9. SULPHUR BACKGROUND Owing to the exponential dependence of the FPD response to sulphur compounds on the amount of the compound, the response per unit amount of solute is the lowest for the smallest amounts of the compound. This is disadvantageous from the point of view of trace analysis. The supply of a certain constant amount of a sulphur compound to the flame (commonly sulphur dioxide) increases the background of the sulphur emission. Therefore, the detector response is higher at this increased background 12 ,27,84. The relationship between the response level and the amount of the sulphur compound supplied (the emission background) is apparent from Fig. 7.12. The noise also increases with increasing detector response and, therefore, the minimum detectability increases by a factor of only 5-10. Of course, the ratio of the responses without and with the sulphur emission background is higher for small amounts of sulphur compounds. For the surface mode (see section 7.5), carbon disulphide doping also results in an increased response to sulphur, selenium and tellurium compounds. The tin and germanium responses are unaffected, however, except that peak tailing worsens. A similar increase in response can also be obtained by doping the flame with selenium or tellurium compounds 49 • It should be realized that in trace analysis the unwanted presence of a sulphur compound in the flame (detector contamination, column bleeding) can lead to erroneous results owing to the effect mentioned above. As mentioned later (section 7.10), sulphur-free organic compounds decrease the detector response to sulphur compounds. Hence, if a sulphur compound is supplied constantly into the flame, the signal decreases during hydrocarbon elution 12 ,13,27,46,47,84. When using this mode, the minimum detectable mass rate of hydrocarbons is about 10- 7 g/sec 45 • 7.10. RESPONSE QUENCHING

The presence of a volatile sulphur-free compound simultaneously with sulphur compounds in the cool flame results in a decrease in the FPD response 3 ,23,38 and in the FPD response 7 ,10,18,25,32,40,44,46,84-86. The decrease in the response occurs regardless of the type of interfering compound (hydrocarbons, alcohols, aldehydes, ketones, esters, acids), but the extent of the decrease depends on this compound (Fig. 7.13). With an equal amount of the interfering compound. however, the extent of the quenching effect is independent of the amount of the sulphur compound 46 ,87. The decrease in response is attributed to the inactivation of the excited S2* species owing to their combination or to their collision with organic compounds or the degradation products of the latter:

153

100

50

~--------1

~------2

o L-________ 5 o

~

________

Organic

~

________

~

________

10 15 substance (10-6mol/min)

~

_____

20

Fig. 7.13. S2 emission quenching of benzo[b]thiophene. Interfering substances: 1 = ethanol; 2 = methanol; 3 = acetone; 4 = cyclohexane. ¢ = (response of FPD with interfering substance present)/(response of FPD without interfering substance). (From ref. 7.) k

--+

(7.13)

where X is the sulphur-free compound. The S2* concentration actually emitting radiation is given by the equation (7.14) where X » S2 *, tis the mean 1ife of the excited S2 speci es and [S2 *] 0 is the concentration of the excited S2 species in the absence of X7 The response decreases approximately exponentially with increasing concentration of sulphur-free compounds 7 ,18,85; this effect arises only starting from a certain concentration of the compound in the carrier gas 18 ,25,46,86-88. The critical mass rate is 5.10- 8 to 5·10-6 g/sec and depends on the type of detector and the experimental conditions applied 7 ,25,40,44,46,86,87. The extent of the quenching effect depends on the detector design; it is smaller with type A than type B89. With type A the sample first diffuses from the oxygen-rich zone into the reduction zone of the S2 emission through the part of the flame with the highest temperature. The organic compounds are oxidized to carbon dioxide prior to S2 formation; carbon dioxide has a low quenching effect 10 ,32,90. With type B, the sample first diffuses through the region of lower temperature and lower partial oxygen pressure, giving rise to products that cause considerable quenching 89 • The quenching range depends on the 02:H2 ratio; the effect is less pro-

a 9

8 10.0 pi 16' 10-8 A

7 6

5 4 z

o

3

:cI-

2

z 1-0 ..J_ >-:I: :I: I-

i'i:

I-IJJ IJJ ~

1

o !

o

2

4

o

I

2

4

6

MINUTES Fig. 7.14. Chromatograms of samples containing 5 ppm each of five pesticides. (a) Single flame; (b) dual flame. Glass column (2 m x 2 mm 1.0.) packed with 5% OV-IOI on Chrom W; temperature, 60 0 C. (Reprinted with permission from ref. 91.)

155 nounced at higher values of this rati0 89 • The detector temperature also influences the extent of the quenching effect 86 • In the detector temperature range 100-190 0C, the thiophene peak height varies within 5.7-10% of the initial peak height (without hydrocarbons) during coelution with always the same amount of cyclohexane. Under certain conditions the background due to stationary phase bleeding can also induce quenching of the response 86 • Quenching occurs if the column temperature is so high that the mass flow-rate of the stationary phase molecules through the detector exceeds the threshold value causing the quenching effect. Water does not cause response quenching during coelution with sulphur or phosphorus compounds 86 • In some instances, at lower concentrations of water in the carrier gas, the response even increases irregularly by about 20%. The quenching effect has also been observed with phosphorus, selenium and tin compounds 40 ,44. The largest decrease was noted with sulphur compounds and the smallest with selenium compounds 40 • This phenomenon markedly affects the results of quantitative analysis if the compounds to be detected elute immediately after a large zone of solvents or if complex mixtures are analysed. In the former instance, the quenching effect is caused by tailing of the solvent zone and in the latter it is due to incomplete separation of the individual compounds on the chromatographic column and, consequently, coelution of sulphur compounds and hydrocarbons. Fig. 7.14a shows this phenomenon for the analysis of 10 ~l of a mixture of five thiophosphate pesticides. Compared with the injection of 1 ~l of the same mixture, the first three compounds show decreased responses (injection volume 10 times larger, recorder range 100 times wider; the peaks should be of equal heights assuming the validity of the quadratic dependence of the response on concentration). Fig. 7.14b shows the chromatograms of the same mixture as in Fig. 7.14a, but obtained with a dual-flame FPD. The decrease in response does not occur in this instance. The response is not influenced with small concentrations of coeluting substances. The design of a dual-flame FPD is shown in Fig. 7.15. The compound to be detected is first combusted in the lower flame. The decomposition products and the uncombusted hydrogen of the lower flame proceed to the other burner, in which the upper flame is generated with air from the second source. Hence the lower flame serves to decompose the compounds leaving the chromatographic column into relatively simple molecules. The radiation of the $2* and HPO* species is emitted in the upper flame. The response of the dual-flame detector is greater than that of the single-flame detector 36 ,91. With phosphorus compounds, the response of the dual-flame detector is higher for phosphates and phosphites 36 (Table 7.2). The conversion of various types of phosphorus compounds to PO requires different amounts of energy. Phosphate, with a P=O bond energy of 140 kcal/mole, will be

156

FLAME 2

FLAME TIP2

--r/7

WINDOW

-~.,L,+--.f'..

FLAME 1 ---+'h4-~1-01

FLAME TIP1

-~~~H..I

Fig. 7.15. Schematic diagram of the dual-flame detector. (Reprinted with permission from ref. 92.) the most difficult to decompose. In the single-flame detector where PO formation and detection as HPO species occur almost simultaneously, phosphate consequently gives a low response. The response is greater if the compound is first dissociated in one flame and then detected as HPO in another flame. Phosphine, with a P-C bond energy of only 63 kcallmole, yields the same response in both detectors. The minimum detectable mass rate is 5.10- 13 glsec of P and 5'10- 11 glsec of S. The following advantages of the dual-flame FPD have been reported 92 : (I) zero quenching effect due to small concentrations; (2) reduced influence of the solvent; (3) increased selectivity, PIC> 5'10 5 g Clg P, SIC from 10 3 to 106 g Clg S; (4) a response independent of the structure of the compound; (5) a quadratic dependence of the response on the concentration of the sulphur compounds; and (6) no quenching of the flame. The small volume of this detector {O.17 mll and the short residence time of the effluent in the detector at the usual flow-rates (O.7 msec) make it possible to use the detector with capillary cOlumns 93 •

157 7.11. FLAME STABILITY The injection of large sample volumes extinguishes the detector flame. To avoid this, a valve has to be placed between the detector and the chromatographic column in order to displace the solvent from the effluent. If the column effluent is mixed with hydrogen prior to entering the flame, the flame is not extinguished owing to changing large liquid samples, either when hydrogen mixed with the carrier gas enters the burner (oxygen around the burner 13 ) or when both gases are supplied around the burner (oxygen into the burner 27 ). No flame extinction has been noted with the dual-flame detector either92 • Another type of design that does not suffer from flame extinction is an FPD having its decomposition compartment separated from the emission compartment 47 ,88. The design of this detector, which exhibits further advantageous features (see sections 7.3 and 7.4). is shown in Fig. 7.16.

3

2

Fig. 7.16. FPD with separate combustion chamber. 1 = Combustion chamber; 2 oxidant and gas inlet orifices; 3 = emission chamber. (From ref. 47.) 7.12. OTHER IDENTIFICATION POSSIBILITIES The fundamental identification information provided by the FPD is the selective response to sulphur and phosphorus compounds and/or to other heteroelements. as described earlier in this chapter. However. two additional auxiliary techniques allowing sulphur compounds to be differentiated from phosphorus compounds. if necessary. can be inferred from the above analyses of the features of this detector. The response to sulphur compounds increases exponentially with increasing amount of these compounds whereas the response to phosphorus compounds increases

158 linearly. By means of repetitive analyses of the same sample. but injecting different volumes, the two types of compounds can be distinguished by virtue of the different increases in the peak heights. What is true for sulphur compounds is also true for selenium compounds, whereas compounds of the other elements behave similarly to phosphorus compounds. The dependence of the FPD response on the detector temperature tends to decrease with sulphur compounds. whereas it tends to increase with phosphorus compounds. Duplicate analyses. this time· at two different detector temperatures, enables one' to distinguish the two types of compounds. because a smaller peak height will be obtained for the sulphur compound at an elevated detector temperature compared with a larger peak height for the phosphorus compound. REFERENCES 1 G. Salet, Ann. PhYB •• 137 (1869) 171. 2 H. Draeger and B. Draeger. Ger. Pat •• No. 113918. 1962. 3 R.t1. Dagnall, K.C. Thompson and T.S. West, Analyst (London), 92 (1967) 506. 4 A. Syty and J.A. Dean, Appl. Opt., 7 (1968) 1331. 55.5. Brody and J.E. Chaney, J. Gas Chromatogr., 4 (1966) 42. 6 R.S. Juvet and R.P. Durbin, J. GaB Chromatogr., 1 (1963) 14. 7 T. Sugiyama, Y. Suzuki and T. Takeuchi. J. Chromatogr •• 80 (1973) 61. 8 M,C. Bowman and t1. Beroza, Anal:. Chem •• 40 (1968) 1448. 9 R. Pigliucci, W. Averill. J.E. Purcell and L.S. Ettre, Chromatographia. 8 (1975) 165. 10 W.E. Ruprecht and T.R. Phillips. Anal. Chim. Aata. 47 (1969) 439. 11 H.A. 11oye. Anal. Chem •• 41 (1969) 1717. 12 \~.L. Crider and R.~'i. Slater, Jr •• Anal. Chem •• 41 (l969) 531. 13 C.A. Burgett and L.E. Green. J. Chromatogr. Sci., 12 (1974) 356. 14 S.O, Farwell and R.A. Rasmussen. J. Chromatogr. Sci •• 14 (1976) 224. 15 S,O. Farwell, n.R. Gage and R.A. Kagel. J. Ch.romatogr. Sci., 19 (1981) 358. 16 T. Sugiyama. Y. Suzuki and T. Takeuchi. J. Chromatogr., 85 (1973) 45. 17 T. Sugiyama, Y. Suzuki and T. Takeuchi. J. Chromatogr •• 77 (1973) 309. 18 M. Maruyama and M. Kakemoto, J. Chromatogr. Sci •• 16 (1978) 1. 19 J.G, Eckhardt. M.B. Denton and J.L. 11oyers, J. Chromatogr. Sci •• 13 (l975) 133. 20 R. Greenhalgh and M.A. Wilson, J. Chromatogr •• 128 (1976) 157. 21 C.H. Burnett, D.F. Adams and S.D. Farwell, J. Chromatogr. sci •• 16 (1978) 22 23 24 25 26 27 28 29 30 31 32

68.

A.D. 11izany, J. Chromatogr. Sci., 8 (1970) 151. T.J. Cardwell and P.J. t1arriott. J. Chromatog'J'. Sai •• 20 (1982) 83. R.E. Pecsar and C.H. Hartmann, J. Ch'J'omatogr. Sci •• 11 (1973) 492. M. Dressler, Chem. Liety. 78 (1984) 645. S. Kapila and C.R. Vogt, J. Chrornatog'J'. Sci •• 17 (1979) 327. A. R. L. r1oss. Scan~ 4 (1974) 5. li.P. Cochrane and R. Greenhalgh. Ch'J'omatographia. 9 (1976) 255. C.H. Hartmann, Anal:. Chem., 43 {1971} 113A. H.W. Grice. M.L. Yates and D.J. David, J. Chromatogr. Sci., 8 (1970) 90. J.F. licGaughey and S.K. Gangwai. AnaZ. Chern •• 52 (1980) 2079. B.J. Ehrlich. R.C. Hall. R.J. Anderson and H.G. Cox. J. Chromatogr. Sai •• 19 (l981) 245. 33 D.G, Greer and T.J. Bydalek. Environ. Sai. Teahnoi •• 7 (1973) 153. 34 T. Sugiyama. Y. Suzuki and T. Takeuchi. J. Ch~omatogr. Soi •• 11 (1973) 639.

159 35 S. Sass and G.A. Parker. J. Chromatogr •• 189 (1980) 331. 36 C.R. Vogt and S. Kapila. J. Chromatogr. Sci., 17 (1979) 546. 37 C.H. Burnett, D.F. Adams and s.n. Farwell, J. Chromatogr. Sai, 15 (1977) 230. 38 R.K. Stevens, J.D. Mulik, A.E. O'Keefle and K.J. Krost, Anal. Chem., 43 (1971) 827. 39 M. Dressler, J. Chromatogr., 262 (1983) 77. 40 C.G. Flinn and \~.A. Aue, J. Chromatogr •• 153 (1978) 49. 41 W.A. Aue and C.G. Flinn, J. Chromatogr •• 158 (1978) 161. 42 W.A. Aue and C.R. Hastings. J. Chromatogr •• 87 (1973) 232. 43 J. Sev~'k and N.P. Thao, Chromatographia. 8 (1975) 559. 44 I~.A. Aue and C.G. Flinn. J. Chromatogr •• 142 (1977) 145. 45 C.R. Hastings, D.R. Younker and W.A. Aue, Environ. Health, 8 (1974) 265. 46 L. Blomberg, J. Chromatogr., 125 (1976) 389. 47 V.A. Joonson and E.P. Loog, J. Chromatogr., 120 (1976) 285. 48 R.M. Dagnall, K. C. Thompson and T.5. I'lest, Analyst (London), 93 (1968) 518. 49 C.G. Flinn and W.A. Aue, J. Chromatogr. Soi., 18 (1980) 136. 50 W.A. Aue and C.G. Flinn, Anal. Chem., 52 (1980) 1537. 51 R.S. Braman and M.A. Tompkins, Anal. Chem •• 51 (1979) 12. 52 S. Kapila and C.R. Vogt, J. Chromatogr. Soi., 18 (1980) 144. 53 C.G. Flinn and I~.A. Aue, J. Chromatogr •• 186 (1979) 299. 54 R.J. Maguire and H. Huneault, J. Chromatogr., 209 (1981) 458. 55 R.J. Maguire and R.J. Tkacz, J. Chromatogr •• 268 (1983) 99. 56 C.V. Overfield and J.D. Winefordner, J. Chromatogr. Sci., 8 (1970) 233. 57 B. Gutsche and R. Hermann. Fpesenius Z. Anal. Chem., 245 (1969) 274. 58 B. Gutsche and R. Hermann. Fresenius Z. Anal. Chern., 249 (1970) 168. 59 B. Gutsche and R. Hermann, FreseniuB Z. AnaZ. Chern., 253 (1971) 257. 60 M.C. Bowman, M. Beroza and G. Nickless, J. Chromatogr. Sci., 9 (1971) 44. 61 R.F. 110seman and W.A. Aue, J. Chromatogr., 63 (1971) 229. 62 B. Versino and H. Vissers, Chromatographia, 8 (1975) 5. 63 B. Vers;no and G. Rossi, Chromatographia, 4 (1971) 331. 64 W.A. Aue. C.W. Gehrke, R.C. Tindle, C.D. Ruyle, D.L. Stalling and S.R. Koirtyohann, Characteristios of the Alkali Flame Detector, 5th National 65 66 67 68 69 70

Meeting for Applied Speotroscopy, Chioago, IL, June 1966. A.V. Nowak and H.V. Malmstadt, Anal. Chern •• 40 (1968) 1108. M.C. Bowman, ri. Beroza and K.R. Hill, J. Chmmatogr. Soi., 9 (1971) 162. W.A. Aue and R.F. Moseman, J. Chromatogr., 61 (1971) 35. A. Karmen. Anal. Chem •• 36 (1964) 1416. M. Dressler and J. Jan&k, Colleot. Czeoh. Chern. Commun., 33 (1968) 3970 .. M. Dressler, AZkali Flame Ionizaaon Detector, Thesis, Institute of Analytical

Chemistry, Brno, 1969. 71 M. Dressler and J. Janak, J. Chmmatogr., 44 (1969) 40. 72 K.O. Gerhardt and W.A. Aue, J. Chromatogr., 52 (1970) 47. 73 J.L. Monkman and L. Dubois, in H.J. Noebels, R.F. Wall and N. Brenner (Editors) Gas Chromatography, Academic Press, London, 1961, p. 333. 74 tt C. Bowman and M. Beroza, J. ChT'omatogr. sci., 7 (1969) 484. 75 R. Gutsche and R. Hermann, Fresenius Z. Anal. Chern •• 259 (1972) 126. 76 E.J. Sowinski and I.H. Suffet, J. Chromatogr. Soi., 9 (1971) 632. 77 R. Ross and T. Shafik, J. Chromatogr. Sci., 11 (1973) 46. 78 W.A. Aue and H.H. Hill, Jr., AnaZ. Chern., 45 (1973) 729. 79 F.M. Zado and R.S. Juvet. Jr .• Anal. Chern •• 38 (1966) 569, 80 R.S. Juvet, Jr. and R.P. Durbin, Anal. Chern., 38 (1966) 565. 81 H.H. Hill, J~ and W.A. Aue, J. Chromatogr •• 74 (1972) 311. 82 J. Sevctk, Chromatographia,'4 (1971) 195. 83 W.A. Aue and H.H. Hill, Jr., J. Chromatogr., 70 (1972) 158. 84 J.M. Zehner and R.A. S1monanaitis, J. Chromatogr. Sd., 14 (1976) 348. 85 S.G. Perry and F.W.G. Carter, in R. Stock (Editor), 8th International Gas Chromatography Symposium, Dublin, 1970, Prooeedings, Institute of Petroleum, London, 1971, p. 381.

~o

86 87 88 89 90 91 92 93

M. Dressler. J. Chromatogr •• 270 (1983) 145. D.A. Ferguson and L.A. Luke, Chromatographia. 12 (1979) 197. S. Hasinski. J. Chromatogr •• 119 (1976) 207. S.A. Fredriksson and A. Cedergren. Anal. Chern •• 53 (1981) 614. S.A. Fredriksson and A. Cedergren. Anal. Chim. Acta. 100 (1978) 429. P.L. Patterson. AnaL. Chern •• 50 (1978) 345. P.L. Patterson. R.L. Howe and A. Abu-Shumays. Anal. Chern., 50 (1978) 339. F.J. Yang and S.P. Cram, J. High Reaolut. Chromatogr. Chromatogr. Commun., 2 (1979) 487.

161

Chapter 8

CHEMILUMINESCENCE DETECTORS CONTENTS 8.1. Introduction • . . . . • • . • . 8.2. Detector for N-nitroso compounds 8.2. 1. Response . . . . • • • . 8.2.2. Selectivity of response . . . 8.3. Detector for nitroaromatic compounds . • . 8.4. Detector for nitrogen-containing compounds . . 8.5. Ozone chemiluminescence detector for compounds not containing ni trogen . . . . . . . • . . . • • • . • . . 8.SA.Redox chemiluminescence detector . • • • . . 8.6. Chemiluminescence detector with sodium metal 8.7. Fluorine-induced detector References . . . • . . . • • . . . . . . .

....

161 161 163 167 169 170 · . 174 174/312 174 • . 177

• . 179

8.1. INTRODUCTION Chemiluminescence detectors (CLDs) are based on emission spectroscopy. This chapter deals with detectors utilizing (1) the reaction of ozone with nitrogen oxide, the reaction of sodium metal vapour with nitrous oxide and halogenated compounds, and the reaction of fluorine with sulphur compounds; and (2) the emission of radiation due to the transition of excited species formed by these reactions to their ground states. 8.2. DETECTOR FOR N-NITROSO

Cor~POUNDS

The selective CLD designed to detect N-nitroso compounds utilizes the technique developed for the analysis of nitrogen oxides 1 ,2 based on the reaction 3 of nitrogen oxide with ozone combined with the preceding pyrolysis of nitroso compounds. Fig. 8.1 shows a schematic diagram of a CLD. The effluent from the chromatographic column enters the pyrolyser where the selective catalytic decomposition of N-nitroso compounds takes place, giving rise to a nitrosyl radical and an organic radical. The N-NO bond is the weakest in these compounds 4:

(8.1)

162

3 6

2

8 9 Fig. B.1. Schematic diagram of a CLD. 1 = Sample injection point; 2 = chromatograph; 3 = pyrolysis chamber; 4 = cold trap; 5 = filter; 6 = vacuum; 7 = photomultiplier; B = electronics; 9 = recorder. (Reproduced with permission from ref. 5.)

where R1 and R2 are organic radicals. The pyrolyser effluent expands into the evacuated reaction chamber in which the nitrogen oxide (the nitrosyl radical) reacts with ozone, giving excited nitrogen dioxide: (B.2) The excited nitrogen dioxide rapidly decays back to its ground state, emitting light in the near-infrared region of the spectrum: k

N0 2* -L NO 2 + h\!

(B.3)

The emitted radiation is detected by a photomultiplier through a red optical fi lter. The decomposition of substances with a catalyst at lower temperatures (about 300 0 C) is more advantageous than pyrolysis at elevated temperatures, because the decomposition of the N-NO bond is more selective, even though the detector response to nitrosamines is diminished at lower temperatures 6 • The response level depends on the temperature of the pyrolysis chamber also in decompositions without a catalyst (Fig. B.2). The maximum response was observed 7 at temperatures in the range 300-400 0 C. A mixture of W0 3 and W20 05B was found to be the most

163 100

80

60 • 1

40

Cl

2

o 3 .4

'" 5 A 6

20

0 200

300

400

500

600'C

Fig. B.2. The N-nitroso compound response as a function of pyrolytic chamber temperature. 1 = N-Nitrosodibutylamine; 2,= N-nitrosodiethylamine; 3 = N-nitrosodimethyl amine; 4 = N-nitrosodipropylamine; 5 = N-nitrosomorpholine; 6 = N-nitrosopyrrolidine. R = Response. (Reprinted with permission from ref. 7.) suitable catalyst 4• Gough B recommended W0 3 adsorbed on the walls of a porous ceramic tube as a catalyst, because this arrangement results in an increase in the catalyst lifetime. The thermal energy analYSer9 ,10 (TEA), manufactured by Thermo Electron Corp., U.S.A., connected to a gas chromatograph, is a commercial detector. However, it is much more expensive than conventional detectors. A luminescence detector laboratory-made from available parts was described by Gough et al. B• B.2.1. Response

The fundamental reaction of the nitrosyl radical in the detector is reaction B.2. However, nitrogen oxide also reacts with ozone according t0 4

(B.4) The NO fraction converted to the excited state, p*, is given by .

164 1

(8.5)

and depends on the temperature (about 7% at 20 oC, about 13% at 100 0 C). The excited nitrogen dioxide can lose its energy not only by reaction 8.3, but also due to collisions with other molecules in the system (M):

(8.6)

N0 2" + t·1 + M ......

N0 2 + M + M

(8.7)

The contribution of reaction 8.7 to the deactivation of N0 2" is negligible under vacuum. Then, the NO fraction decaying back to the ground state with the emission of radiation, F, is given by

F(NO) = F" •

• [M]

• [t1]

(8.8)

x -4 10

0.51.0

5 10

50 100

500 1000 mmHg

Fig. 8.3. Effect of pressure on the fraction X of nitrosyl radicals emitting light. (From ref. 4.)

165 The change of F with pressure (at T = 20 0 C) is shown in Fig. 8.3. At a pressure of 2 mmHg (the range within which the detector is operated). the NO fraction that contributes to light emission is 3.10- 4. The number of efficient light quanta is also reduced by approximately an order of magnitude owing to the detector geometry and photomultiplier characteristics 4•

R DEN DMN

DPN

-

min

Fig. 8.4. Chromatogram of a 100-~1 solution containing 5 ng/ml of N-nitroso compounds. SARCOSN = N-nitrososarcosinate; PYRN = N-nitrosopyrrolidine; NIP = N-nitrosopiperidine; DBN = N-nitrosodibutylamine; DPN = N-nitrosodipropyl amine; DEN = N-nitrosodiethylamine; DMN = N-nitrosodimethylamine. R = response. Column. 6.5 m x 2 mm 1.0 •• gacked with 15% free fatty acid phase on Chromosorb WAI~ DMCS; temperature, 185 C. (From ref. 12.) The detection limit for nitrosodimethylamine is about 5'10- 11 g 6,8,11 (Fig. 8.4). The response is linear over a concentration range of five 12 to six 5 orders of magnitude. The detector response expressed per nitrosyl group in the molecule of the compound is not constant 13 , apparently depending slightly on the structure of the compound (see Table 8.1).

...... 0'1

TABLE 8.1

0'1

TEA RESPONSE FACTORS FOR DIFFERENT N-NITROSO COMPOUNDS Reprinted with permission from ref. 13. N-Nitroso compound

Mol. wt.

Concentration (llg/ml)

Measured response (i ntegrated units)

Response per nitrosyl group

Relative response (nitrosyl mole basis)

N-Nitrosodimethylamine N-Nitrosodi ethyl amine N-Nitrosodipropylamine N-Ni trosodi phenyl amine N-Nitroso-N-ethylaniline 9-Nitrosocarbazole N-Nitroso-N-methyl urethane N-Ni troso-N-phenyl benzyl amine Ethyl N-nitrososarcosinate N-Methyl-N-nitroso-N-nitroguanidine N-Nitrosopiperidine Dinitrosopiperazine

74 102 130 198 150 196 132 212 146 147 114 144

0.964 1.07 0.84 1.86 1.17 1.99 0.52

235 204 128 189 145 167 74 166 258 590 172 434

18.1 19.4 19.8 20.1 18.6 17.6 18.8 16.7 17.5 18.4 21.2 15.7

1.00 1.07 1.09 1.11 1.03 1.03 1.04 0.92 0.97 1.02

2.10

2.15 4.71

0.93 1.99

1.17

0.87

167 8.2.2. Selectivity of response

The detection selectivity for N-nitroso compounds with a CLD is based on the selective decomposition of the compound at relatively low temperatures and on the generation of excited nitrogen dioxide molecules by the reaction of nitrogen oxide with ozone. Some other substance, e.g., carbon monoxide and ethylene, also react with ozone to produce luminescence, but the wavelength of this radiation is in the blue and visible light regions l2 • A filter that does not transmit radiation up to 0.6 ~m eliminates these inferferences. In order to provide high detection selectivity, the part of the detector situated between the pyrolyser and the reaction chamber is maintained at temperatures below _150 0 C13 ,14. The NO vapour pressure exceeds 1 atm at this temperature, whereas the vapour pressure of most organic compounds is much lower than 1 atm. Therefore, compounds that could react with ozone giving luminescence in the IR region are collected just ahead of the reaction chamber 13 . In addition, a short column packed with the porous polymer Tenax is sometimes placed ahead of the reaction chamber6 ,7. Under certain conditions, the nitro compounds can also yield a nitrosyl radica1 4 : (8.9) -N0 2

+

C

~

CO

+

NO

(8.10)

At equilibrium under ideal conditions, the conversion range according to reaction 8.9 varies from 10% (300 0 C) to 94% (700 0 C). In practice, however, the reaction kinetics are so slow that no conversion can be observed during detection even at 400 0 C4. When using metallic catalysts (gold, molybdenum), the reduction of nitrogen dioxide to nitrogen oxide (reaction 8.10) proceeds rapidly at 200 0 C. For this reason, active forms of carbon and possible contact with metallic catalysts should be eliminated. When studying how the temperature of the pyrolysis· chamber affects the response of compounds other than N-nitroso compounds (Fig. 8.5), it has been found 7 that the detector also responds to nitrohexane, nitrotoluene, hexyl nitrite and dipropylnitramine. The course of this dependence differs markedly for the individual compounds. Hexyl nitrite pyrolyses to give nitrogen oxide at relatively low temperatures of 200-300 0 C; the response decreases at temperatures above 300 0 C. Nitrotoluene and nitrohexane respond only at higher temperatures. The molar response of dipropylnitramine at 400 0 C is very high, amounting to 50-96%, according to the experimental conditions, of the molar response to N-nitrosodialkylamine 7 ,15. Table 8.2 lists the relative molar

168 100 R a/a

80 60 40 20

Fig. 8.5. Detector response as a function of pyrolytic chamber temperature. 1 = Nitrohexane; 2 = nitrotoluene; 3 = hexyl nitrite; 4 = dipropylnitramine. R = response. (Reprinted with permission from ref. 7.) TABLE 8.2 tl0LAR RESPONSES OF NITRAtUNES RELATIVE TO THE CORRESPONDING NITRASAMINES From ref. 16. Nitrami ne

Molar response relative to nitrosamine

N-Nitrodimethylamine N-Nitrodiethylamine N-Nitrodipropylamine N-Nitromethylpentylamine N-Nitrodibutylamine N-Nitropiperidine N-Nitropyrrolidine

0.87 0.82 0.78 0.81 0.75 0.80 0.73

responses of other nitramines with regard to the corresponding nitrosamines. These values approach 8~% for a pyrolyser temperature of 500 0 C. For this reason, misleading interpretations could be made when analysing a mixture of nitro and nitroso compounds 16 (see also section 8.3). N-Dimethylbenzylamine 17 , several C-nitroso compounds 18 and nitrites do also respond. However, nitrites decompose in the hot zone of the injection chamber of the chromatograph (200 0 C) giving nitrogen oxide, which elutes in the dead vOlume 12 • The initial statement that the CLD responds only to N-nitroso compounds, so that no preliminary purification of the samples (extracts) is necessary14, should be treated with caution.

169 N-Nitrosodimethylamine (NOMA) dissolved in a mixture of hexane and ethanol gives a higher response than its normal response. This anomalous response increases with decreasing concentration of ethanol in the mixture; it is approximately an order of magnitude higher at a 10% ethanol concentration. The alcohol or the contaminants contained therein act as catalysts of the interaction of NOMA (or of its pyrolysis products) with hexane or its contaminants, giving a highly volatile product that either responds itself or improves the efficiency of the reaction of nitrogen oxide with ozone 19 • Amines have been found to reduce the detector response to N-nitroso compounds 20 • The nitrogen oxide formed by cleavage of the N-NO bond can react with amine molecules at the chamber outlet, giving rise to another nitrosamine which therefore cannot react with ozone. The initial response to the N-nitroso compound is reduced or even zero, depending on the degree of recombination. However, Parees and Prescott6 found that the response for N-nitrosodimethylamine in the presence of excess of diethylamine (in tailing the amine peak) equalled the response of this compound in the presence of excess of dichloromethane. The same is also true for N-nitrosomorpholine in morpholine. 8.3. DETECTOR FOR NITROAROMATIC COMPOUNDS The response of the TEA detector to nitroaromatic compounds increases rapidly with increasing temperature of the pyrolyser5 (see Fig. 8.6). The fact that nitrogen oxide is also generated by the pyrolysis of nitro compounds (reaction 8.9) was employed by Lafleur and Mills 5 to determine nitroaromatic compounds with the aid of a CLD. The pyrolysis temperature was 800-900 0 C (the response decreased drastically by several orders of magnitude at lower temperatures), the other experimental conditions being the same as for the nitroso compound detector. The detection limit of this detector is 0.6 ng of nitroaromatics and the dynamic linear range covers four orders of magnitude. When using a capillary column inserted deeply into the ceramic pyrolysis tube, the detection limit was found to be tens of picograms 21 or even picograms 22 • This arrangement of the column is of great importance, because it prevents losses of polar substances due to adsorption. If the column is inserted only to the transfer line or into the relatively cold zone of the pyrolysis tube, the detection limit is only nanograms of nitroaromatics 21 • The Thermo Electron TEA Model 543 analyser is designed for both nitroso and nitro group detection 23 •

170 log R ~

___

~1

2

5

·C Fig. 8.6. Response of nitroaromatic compounds as a function of pyrolyser temperature. 1 = 2,4,6-Trinitrotoluene; 2 = 2,3-dinitrotoluene; 3 = 2,6-dinitrotoluene; 4 = 3-nitrotoluene; 5 = 2-nitrotoluene. (Reprinted with permission from ref. 5.) 8.4. DETECTOR FOR NITROGEN-CONTAINING COMPOUNDS The chemiluminescence analyser for nitrogen-containing compounds is based on the catalytic decomposition of these compounds in the presence of oxygen at temperatures of 900-IOOO oC and on the subsequent ozone oxidation of the nitrogen oxide formed 24 ,25, as with nitroso compounds (see eqns. 8.2 and 8.3): (8.11)

The detector consists of a pyrolyser with an oxygen supply and an NO analyser 26 - 29 ; the analyser manufactured by Antek 28 and/or the Thermo Electron TEA Model 610 29 are commercial detectors of this type. The only difference from the nitroso detector is in the oxidative decomposition of the substance. The detection limit is in the nanogram region. Also with this detector, the response level depends on the pyrolyser temperature, which increases to 900 oC. The detection sensitivity is given by the efficiency of the conversion of nitrogen-containing compounds to nitrogen oxide. The molar responses of ammonia, acetonitrile and nitromethane are about 1.5 times greater than those of amines 26 ,27,30. Ethanol, benzene and acetone in I-~l amounts yield signals equivalent to several nanograms of trimethylamine 26 ,27, and this also enables one to determine nitrogen-containing compounds eluting in the peaks of these solvents (Fig. 8.7).

171

r(c)--: I I I

I I I

I

I I I

TMA

DMA

I I I

i-PA

!,

,, ,, ,, ,

I

\

i-SA

24

28 min

Fig. 8.7. Chromatogram of mixture of amines. TMA = trimethylamine; DMA = dimethylamine; i-PA = isopropylamine; DEA = diethylamine; i-SA = isobutyl amine; BA = butylamine. (a) Ethanolic solution of the sample; (b) ethanol only; (c) ethanol only, flame-ionization detection. Glass column (2 m x 3 mm I.D.) packed with 5% squalane plus 2% potassium hydroxide on Chromosorb 104. Temperature, 130 oC. (From ref. 27.) A modified 31 commercial TEA can also be used as a gas chromatographic detector (/1ode 1 610). The effl uent from the col umn is oxi di zed at 690 0 C by a metal oxide to give the nitrosyl radical. The metal oxide is continuously regenerated by the oxygen passing through the pyrolyser. The linearity of response covers four orders of magnitude and the detection limit is about 3'10- 13 mole i.e., 2'10- 11 g, of pyridine. Hence the detection sensitivity is commensurate with that of the original CLD for .N-nitroso compounds (the TEA). The molar responses of individual compounds are listed in Table 8.3. The response depends on the number of nitrogen atoms in the molecule of the compound; the response to N-nitrosamines is twice that of pyridine. (8.12) Neither carbon dioxide nor water interfere; 1 ~l of an organic solvent gives only a small negative response caused by the passage of a large volume of organic vapours through the detector (Fig. 8.8).

172 TABLE 8.3 r~OLAR

RESPONSE OF THE CHEMI LUMI NESCENCE DETECTOR

From ref. 31. Compound

Relative molar response

pyri di ne Ammonia Trimethylamine Triethylamine Diethylamine Morpholine 3-Methylpiperidine n-Propylamine Isopropylamine sec.-Butylamine Anil i ne Acetonitrile N-Nitrosodimethylamine N-Nitrosodiethylamine N-Nitrosopropyl ami ne

N mode

Nitro mode

1.00 1.04

1.00

None None None None None None None None None None None None

2.01 2.08

1.03 1.00

1.96

0.98

1.03 1.00 1.04

0.97 1.05

0.95 0.92 1.05

1.00

(8 )

(A)

cD

100

(C)

100

100

~

(0)

100

Z lI-

«

UJ VI

z a

a. VI

UJ

.:.e a:

hr-0

2

4

r-V 0

2

4

~ 0

2

4

k 0

2

4

TIME (MINUTES)

Fig. 8.8. Chromatograms of 1 ~l of orgdnic solvents. TEA, N mode. (A) Acetone; (B) toluene; (C) benzene; (D) methanol. Glass column (l.7 m x 2 mm 1.0.) packed with 20% potassium hydroxide on Chromosorb W. Temperature, programmed from 90 nC (2 min) to 140 0 C at 12 oC/min. (From ref. 31.)

173

690'C

DETECTOR

)I------
N MODE

600'C

DETECTOR

)i----
NITRO MODE

Fig. B.9. Schematic diagram of the GC-TEA interface for the N mode and the nitro mode (From ref. 31.)

100

NITRO

2

R 0/0

50

o

2

4

o

6

2

4

6

min

Fig. B.10. Comparison of the N mode and nitro mode chromatograms. 1 = Pyridine; 2 = nitrosodimethylamine; 3 = N-nitrosodiethylamine; 4 = N-nitrosodi-n-propylamine. Glass column (1.6 m x 2 mm 1.0.) packed with 20% Carbowax 20M. Temperature, program~~d from 160 0 C (1 min) to 190 0C at 100C/min. (From ref. 31.)

174 The positioning of a four-way valve in the oxygen flow upstream of the pyrolyser enables the detector to be operated in the N mode or in the nitro mode 31 (Fig. 8.9). In the N mode, oxygen is supplied to the pyrolyser and the detector responds to all nitrogen-containing compounds. No oxygen is supplied to the detector in the nitro mode and, at a pyrolyser temperature of 600 0 C, the detector responds only to the compounds that produce an NO radical, such as N-nitroso compounds, and nitroaromatic compounds (see also Table 8.3). The change from the N mode to the nitro mode is accomplished by switching over the valve. An example of an analysis performed with both modes is shown in Fig. 8.10. 8.5. OZONE CHEMILUMINESCENCE DETECTOR FOR COMPOUNDS NOT CONTAINING NITROGEN The reaction of a solute with ozone, which gives rise to chemiluminescence, is also used for the selective detection of hydrocarbons and some sulphur compounds 32 ,33: solute + 03 ~ A* + further products A* ~ A + hv

(8.13) (8.14)

In this instance, the reaction occurs at normal pressure. The response selectivity is provided by variations in the reactivity of different classes of compounds towards ozone and can be varied by changing the detector temperature (Table 8.4). The response increases with detector temperature up to about 300 0 C, after which it rapidly decreases owing to ozone decomposition. The response selectivity is the greatest at temperatures up to 150 0 C. The minimum detectable mass rate also depends on the detector temperature and is fairly high (the noise also increases with temperature); of the compounds listed in Table 8.4, it is the lowest for propadiene at 250 0 C, being 8.2'10- 12 mole/sec 32 . 8.5A. REDOX CHEMILUMINESCENCE DETECTOR (see p.312) 8.6. CHEMILUMINESCENCE DETECTOR WITH SODIUM METAL Reactions producing chemiluminescence proceed between sodium metal vapour and nitrous oXide 34 : (8.15) (8.16) (8.17) Na*

~

Na + hv

(8.18)

175 TABLE 8.4 DETECTOR SENSITIVITY FOR DIFFERENT TYPES OF HYDROCARBONS Calculated as peak area/mole (counts/mole). (From ref. 32. ) Temperature

Propane

Benzene

Ethylene

Acetylene

Propadiene

- 6 2.2.10 9 3.8.10 10 3.8.10 3.0.10 11 1.2·10 12

- 9 1.0.10 10 1.2.1010 4.1.10 1.2.10 11 3.3.10 11 7.4·10 11

2.9.10 10 1.4·lOH 2.4.10 11 3.4.10 11 6.2.10 12 1.3.10 2.3·10 12

4.3.10 10 1.7·lOH 5.7.10 11 9.0'1012 1. 8.10 12 2.9.10 12 3.9·10

1.3.1011 6.9·lOB 1.2.10 12 1.6.10 12 2.2.10 3.4.10 12 5.0,10 12

(0C)

50 100 150 175 200 225 250

TABLE 8.5 RELATIVE RESPONSES Reprinted from ref. 35 with permission. Compound

Concentration (ppm)

Response rati 0 against N20 of the same concentration

NO N02 CO CO 2 0 Sb2 H2S

50 85 50 1000 50 1000 1000

1:400 1:200 1:200 0 1:200 0 0

Araki et al. 35 used these reactions for the gas chromatographic detection of nitrous oxide. The sodium metal vapour generated in the vaporization cell of the detector at 310 0 C is carried into the reaction cell by nitrogen. A pressure of several Torr is maintained in the detector. Chemiluminescence appears at the tip of the nozzle through which the effluent from the chromatographic column is supplied to the detector. The minimum detectability is 1.9.10- 12 mole/sec and the response selectivity relative to other gases covers about two orders of magnitude (Table 8.5). The response is a function of the vaporization cell temperature (maximum at 320 oC). reaction cell temperature (maximum at 260 oC). and reaction

176

cell pressure (maximum 4-8 Torr), and is also affected by the flow-rate of the sodium metal vapour carrier gas and the flow-rate of the carrier gas from the chromatographic column. Sodium vapour also reacts with volatile aliphatic hydrocarbons containing more than two halogen atoms in the molecule 36 ,37. RX 2 + Na

~

·RX + NaX

(8.19)

·RX + Na ~ (R) + NaX*

(8.20)

NaX * + Na

(8.21)

~

NaX + Na *

Chemiluminescence then again arises from the transition of Na* to the ground state (eqn. 8.18). A simplified and improved metallic version 38 of the above detector is shown in Fig. 8.11. Argon was used as the carrier gas. The sensitivity of this type is better than that of the original Pyrex model. The chloroethane molar response (Table 8.6) varies with the position of the chlorine atoms in the molecule, which can be explained by the following reaction mechanism 37 : Pump

5 cm

~

Union Tee ( 112 in.' Effluent from GC

t

Stainless - steel tube

\ ~

t

Quartz window

Reducer (1116 in.)

Fig. 8.11. Stainless-steel CLD with sodium metal. (From ref. 38.)

177

TABLE 8.6 DETECTION LIMITS AND RELATIVE MOLAR RESPONSES (RMR) OF CLD WITH Na From ref. 38 Polyhalogenated hydrocarbon

Detection limit (ng) (SIN = 3)*

RMR**

CH~C12

0.3 0.2 0.003

1 3 230 0.04 1 5 4 3 430 540 25 26 0.4 0.7 4 2 2 230 590

CH 13 CC14 CF3C1

10

CF~C12

0.5 0.1 0.2 0.3 0.0009 0.0009 0.03 0.02 0.9 0.8 0.2 0.2 0.2 0.002 0.001

CF 13 CHBr3 CHBr2Cl C1CH2CH~Cl C1CH~CH 12

C12C CHC12 C1CH2CC13 CH3CHC12 CH3CC1~

C1CF2C Cl~ eis-C1CH= HCl tl'ans-C1CH=CHCl C1CH=CC1 2 C12C=CC12

*S/N = signal-to-noise ratio. **Normalized with respect to the value for CH 2C1 2• 'CH 2-CH 2Cl + Na CH 3-CHCl + Na

+

+

NaCl + CH 2=CH 2 NaCl + CH 2=CH 2

(8.22) (8.23)

The (R) in reaction 8.20 is the unsaturated molecule formed by the closure of a double bond (reaction 8.22) or by the migration of a hydrogen atom (reaction 8.23). Reaction 8.23 has a much smaller light yield (e.g., for CH 3CHC1 2, CH 3CC1 3) than reaction 8.22 (e.g., for C1CH 2CH 2Cl, C1CH 2CHC1 2 ). The other chloroethanes investigated show medium sensitivity, with both reactions being responsible for the chemiluminescence reaction. Fluorine atoms are inactive in the chemiluminescence reaction. The linearity of response is 10 4-10 6• The atomic sodium vapour reacts to produce chemiluminescence also with monohalogenated compounds and with several types of nitrogen and oxygen-containing compounds. The light yields are low, however (Table 8.7). 8.7. FLUORINE-INDUCED DETECTOR This detector monitors the chemiluminescence resulting from the reaction of the gas chromatographic effluent with molecular fluorine at reduced pressure 39 •

178 TABLE 8.7 RELATIVE MOLAR RESPONSES OF OTHER COMPOUNDS RELATIVE TO DICHLOROMETHANE (= 1) From ref. 38. Compound

RMR

Compound

RMR

n-C 3H I

0.02 0.009 0.007 0.007 0.007 0.001 0.0009 0.0002

C ~CHO ( ~ B)2 NH C 5 C2 H5 ( RAe)2 S C6 5 1 C6 H6

CH~COCH3

0 0 0 0 0 0 0

6 ~R~~~C~CH3

)t n-Ct7 r ( CH 30

CH~ N

n- 3H~Cl

C2 H50

t t

TABLE 8.8 DETECTION LIMITS AND RELATIVE RESPONSE COMPOUNDS

FACTORS FOR VARIOUS SULPHUR-CONTAINING

Reprinted with permission from ref. 45. Compound

Detection limit (pg)

Relative response factor

Allyl sulphide Ethanethi 01 Ethyl sulphide 1-Butanethi 01 n-Butyl disulphide 1-Hexanethiol n-Butyl sulphide Isopentyl disulphide 1-0ctanethiol

24 24 46 35 73 84 180 200 257

8.3 4.6 6.0 4.6 4.3 2.5 1.4 1.8 1.0

The emission spectrum of sulphur compounds has been identified as vibrational overtone bands of the ground electronic state of HF 40 • Whereas many organic compounds react with fluorine to give excited-state HF products 39 ,41-44, only sulphur compounds were found to react to yield HF in vibrational levels as high as ~ = 5 and 6. Emission is detected with a red-sensitive photomultiplier tube, and an optical filter passing wavelengths between 660 and 740 nm is used. The detector is op-

179 TABLE 8.9 SELECTIVITY RATIOS OF n-BUTYL DISULPHIDE OVER VARIOUS ORGANIC COMPOUNDS Reprinted with permission from ref. 45. Compound

Anil i ne Carbon disulphide tpans-Cinnamaldehyde Diethylamine Hexane I-Hexene l-Iodohexane

Selectivity ratio

Compound

Selectivity ratio

Methylene chloride I-Dctanol Propanol Propionaldehyde Tetrahydrofuran Toluene m-Xylene

erated at pressures below 2 Torr and the reagent gas is a mixture of 5% fluorine in helium. If the flow-rate of the carrier gas (helium) is kept constant at 30 ml/min, the detector response is the highest for a reagent gas flow-rate of 15 ml/min 45 • Detection limits and relative responses are listed in Table 8.8 and selectinity ratios in Table 8.9. The fluorine-induced chemiluminescence detector does not respond to sulphur gases such as S02' COS, H2S, CS 2. The linearity of response for sulphur compounds covers three orders of magnitude 45 • A iodine-selective detector based on fluorine-induced chemiluminescence and with a detection limit of 1 ~g has also been described 46 • In this instance, chemiluminescence is produced by reaction of the gas chromatographic effluent with the decomposition products from a microwave discharge of SF6 in helium. REFERENCES 1 R.K. Stevens and J.A. Hodgeson, Anal. Chem., 45 (1973) 443A. 2 A. Fontijn, A.J. Sabadell and R.J. Ronco, Anal. Chem., 42 (1970) 575. 3 P.N. Clough and B.A. Thrush, Tpans. Papaday soc., 63 (1967) 915. 4 D.H. Fine, D. Lieb and F. Rufeh, J. Chpomatogp., 107 (1975) 351. 5 A.L. Lafleur and K.ll. Mills, Anal. Chem., 53 (1981) 1202. 6 D.ll. Parees and S.R. Prescott, J. Chpomatogp., 205 (1981) 429. 7 T.J. Hansen, 11.C. Archer and S.R. Tannenbaum, Anal. Chem., 51 (1979) 1526. 8 I.A. Gough, K.S. Webb and R.F. Eaton, J. Chpomatogp., 137 (1977) 293. 9 D.H. Fine, F. Rufeh and B. Gunther, AnaZ. Lett., 6 (1973) 731. 10 TEA Uodel 502A Analyzep, Thermo Electron Corp., Waltham, MA, May 1980 11 K.S. Webb, I.A. Gough, A. Carrick and D. Hazelby, Anal. Chem., 51 (1979) 989. 12 D.H. Fine and D.P. Rounbehler, J. Chpomatogp., 109 (1975) 271. 13 D.H. Fine, F. Rifeh, D. Lieb and D.P. Rounbehler, AnaZ. Chem., 47 (1985) 1188.

180

14 D.H. Fine. D.P. Rounbehler and P.E. Oettinger. Anal. Chim. Acta. 78 (1975) 383. 15 J.H. Hotchkiss. J.F. Barbour, L.M. Libbey and R.A. Scanlan. J. Agr. Food Chern., 26, (1978) 884. 16 E.A. Walker and M. Castegnaro. J. Chromatogr., 187 (1980) 229. 17 T.A. Gough and K.S. Webb, J. Chromatogr., 154 (1978) 234. 18 R.W. Stephany and P.L. Schuller, in B.J. Tinbergen and B. Krol (Editors). 19 20 21 22

Proceedings of 2nd International symposium on Nitrite in Meat Products, Zeist, September 1976. Pudoc. Wageningen. 1977. p. 249. G.V. Alliston, K.S. Webb and T.A. Gough, J. Chromatogr •• 175 (1979) 194. K.S. Webb and T.A. Gough, J. Chromatogr •• 177 (1977) 349. J.M. Douse. J. Chromatogr •• 256 (1983) 359. D.H. Fine, W.C. Yu. U. Goff. E. Bender and D. Reutter, J. Forensic Sci.,

28 (1983) 29.

23 TEA Model 543 AnaZyzer. Thermo Electron Corp •• Waltham, MA. July 1980. 24 R.E. Parks, presented at 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. Cleveland. OH. March 1976. 25 H.V. Drushel, Anal. Chern., 49 (1977) 932. 26 N. Kashihira, K. Kirita and Y. Watanabe, Bunseki Kagaku (Jap. AnaZ.). 29

(1980) 35. 27 N. Kashihira. K. Makino, K. Kirita and Y. Watanabe, J. Chromatogr., 239 (1982) 617. 28 The Antek Nitrogen AnaZyzers, Antek Instruments, Houston, TX, 1983. 29 TEA Model 610 Nitrogen Analyzer, Thermo Electron Corp •• Waltham, MA. 30 N. Kashihira, K. Makino, K. Kirita and Y. Watanabe, Bunseki Kagaku (Jap. Anal.). 31 (1982) E13. 31 D.P. Rounbehler. S.J. Bradley. B.C. Challis, D.H. Fine and E.A. Walker. Chromatographia. 16 (1982) 354. 32 W.B. Bruening and F.J.M. Concha. J. Chromatogr •• 112 (1975) 253. 33 W.B. Bruening and F.J.M. Concha, J. Chromatogr., 142 (1977) 191. 34 C.E.H. Bawn and A.G. Evans, Trans. Faraday Soc., 33 (1937) 1571. 35 S. Araki, S. Suzuki, M. Yamada, H. Suzuki and T. Hobo, J. Chromatogr. Sei., 16 (1978) 249. 36 C.E.H. Bawn and R.F. Hunter. Trans. Faraday Soc., 34 (1938) 608. 37 C.E.H. Bawn and W.J. Dunning. Trans. Faraday Soa., 35 (1939) 185. 38 M. Yamada, A. Ishiwada, T. Hobo, S. Suzuki and S. Araki. J. Chromatogr., 238 (1982) 347. 39 W.H. Duewe and D.W. Setser, J. Chern. Phys., 58 (1973) 2310. 40 D.E. Mann. B.A. Thrush, D.R. Lide. J.J. Ball and N. Acquista, J. Chern. PhY8., 34 (1961) 420. 41 K.C. Kim and D.W. Setser, J. Phys. Chern •• 77 (1973) 2493. 42 D.J. Bogan and D.W. Setser, J. Chern. Phys., 64 (1976) 586. 43 D.J. Bogan, D.W. Setser and J.P. Sung, J. Phys. Chem., 81 (1977) 888. 44 D.J. Smith. D.W. Setser. K.D. Kim and D.J. Bogan, J. PhY8. Chem., 81 (1977) 898. 45 J.K. Nelson, R.H. Getty and J.W. Birks. AnaZ. Chern., 55 (1983) 1767. 46 R.H. Getty and J.W. Birks, Anal. Lett •• 12 (1979) 469.

181

Chapter 9

ELECTROLYTIC CONDUCTIVITY DETECTOR CONTENTS 9.1. Detector construction. 9.2. Selectivity of response 9.3. Response • 9.4. Solvent •• 9.5. Gases ••• 9.6. Temperature References • • •

181 1~

189 196 201 203 206

9.1. DETECTOR CONSTRUCTION In 1962, Piringer and Pascalau 1 described an electrolytic conductivity detector (ELCD) designed to determine organic compounds. In a quartz capillary containing copper(II) oxide, the compounds eluted from the chromatographic. column are decomposed, giving rise to carbon dioxide. The carrier gas with the carbon dioxide produced contacts flowing deionized water in a long glass capillary (about 15 cm). and the conductivity of the aqueous solution of carbon dioxide is measured. The minimum detectability with this detector was found 2 to be 6'10- 11 g/ml of methane. For the determination of hydrogen and its iso? topes, a modification of this detector was described" in which a PdC1 2-containing reactor (80 0 C) was placed ahead of the contact space of the carrier gas with water. Hydrogen chloride was produced; the detection limit was 4'10- 12 g of the hydrogen isotope. Sternberg et al. 4 in 1962 described an ELCD complementing the flame-ionization detector (FlO). A particulate material with water flowing down on its surface was positioned above the burner of the FlO. The decomposition products from the FlO were trapped in the thin liquid film and the conductivity changes of the liquid were measured. The detector is selective (Cl:C ~ 105; S:C ~ 10 4 ); the minimum detectable mass rate is 2.5.10- 10 g/sec of dichloromethane and 1.5.10- 9 g/sec of carbon disulphide. The selectivity relative to hydrocarbons is given by the relatively low absorption capacity of carbon dioxide in the rapidly flowing water and also by the low ionization of the carbon dioxide absorbed 5•

182 PYROLYSIS ZONE CATALYST

INLET

"

HELIUM -

~=~-LIQUID VENT

<;EPARATOR PLATINUM ELECTRODES

LIQUID COLUMN

Fig. 9.1. Coulson electrolytic conductivity detector. (From ref. 6.) The above selective detector was modified by Coulson 5 for routine work (see Fig. 9.1). Under oxidizing or reducing conditions, the solute is decomposed in the reaction space and the decomposition products are supplied to the space where the gas phase comes into contact with the liquid (deionized water). The decomposition products are absorbed in water, giving an electrolyte. The gas phase is separated from the liquid phase in a separator. The change in the water conductivity due to the electrolytes is sensed by platinum electrodes. The distance between the electrodes of the glass detection cell is about 1 cm and the cell geometry is optimized so as to display minimum polarization. Coulson's detector was commercially produced until about 1974. Later, this detector was modified by several workers. Under reducing conditions, the use of a PTFE tube for the reactor outlet reduces the sorption losses of the ammonia produced and increases the sensitivity of detection when using hydrogen as the carrier gas and a nickel wire as the catalyst 7• The use of a water-jacket and modified electrodes (larger electrodes, shorter distances between the electrodes)8 also results in a higher detection sensitivity than with Coulson's original design. A similar effect is obtained by reducing the amount of water that enters the water-gas contact space 9 by inserting a wire into the capillary feeding the water to the gas-water contact space (Fig. 9.2). The main disadvantage of Coulson's detector is its size: the detector has to be mounted on the chromatograph as a separate unit, which requires heating of the conduit leading to the gas chromatograph. A microdetector (the Hall ELCD) (Tracor Instruments, Model 310) was described by Hall 10 in 1974. This is substantially smaller and displays a higher sensitivity than the Coulson detector (Fig. 9.3). The conductivity cell also

183 FURNACE EFFLUENT

HYDROSTATIC PRESSURE COLUMN

MIXING CHAMBER

DETECTOR CELL

TYGON TUBING

~~~~~~=~f--~WATER

FROM IONEXCHANGE RESIN

If

STAINLESS-STEEL WIRE RESERVOIR WATER LEVEL

Fig. 9.2. Partial diagram of the Coulson detector with wire in place. (Reprinted with permission from ref. 9.)

2

3

Fig. 9.3. t1icroelectrolytic conductivity detector cell assembly. 1 = Gas-liquid contactor; 2 = PTFE solvent delivery tube; 3 = PTFE reaction products delivery tube; 4 = stainless-steel detector block; 5 = solvent vent; 6 = PTFE insulator sleeve; 7 = gas-liquid exit tube and center electrode. (Reprinted with permission from ref. 11. Y

184

comprises the space for the separation of the gas and liquid phases. The contact of the liquid and the gas from the reactor takes place in a small section of the PTFE tube. The heterogeneous gas-liquid mixture produced enters the interior of the conductivity cell where the phases separate on contacting the inner stainlesssteel detector wall. The liquid phase flows down on the cell surface, forming a sheath with the gas phase as the core. In this manner, the liquid passes between the inside wall of the outer electrode (cell wall) and the outside wall of the inner electrode (gas outlet). Then the liquid again contacts the gas through the port in the inner gas-outlet tube (E), where it mixes with the gas and flows out through the bottom of the tube 12 • The electrolytic conductivity is measured by means of a conductivity bridge with synchronous detection. The temperature of the reaction chamber is adjustable, and the liquid is a non-aqueous solvent. The adhesion of the liquid phase to the surface of the detector is critical with regard to the function of the detector and, therefore, the material used for the construction of the detector is an important factor. A stainless-steel surface is more suitable than a glass surface and glass, in turn, is more suitable than plastics' for the separation of gases from organic solvents. On the other hand, a clean glass surface is slightly superior.to metal for the separation of a gas from aqueous mixtures 10 • A further improvement in the sensitivity of the ELCD is achieved by a bipolar pulse differential detector 13 in which the conductivity of the solvent is measured in the first detector cell and that of the solvent with the decomposition products in the second cell. In this way, the factors particularly affecting the detector noise (temperature variations, solvent purity) are common for both cells and become reduced to the minimum by differential sensing. The classical determination of electrolytic conductivity is performed by means of direct current or alternating current techniques. These methods, however, are limited because of electrode polarization, capacitance effects and cell heating. The Hall 700A electrolytic conductivity detector (Tracor Instruments, Austin, TX, U.S.A.) is based on bipolar pulse programmed to reach full amplitude for only one out of every ten pulses. The output of the differential amplifier is caused only by the change in conductivity produced by the gaseous reaction products formed in the microreactor. A cross-section of the detector is shown in Fig. 9.4. Deactivated nickel reaction tubes have been used for most applications; the microreactor also accepts, however, quartz reaction tubes. The conductivity solvent enters through the solvent inlet and flows through the reference conductivity cell. The latter consists of the top and outer electrode assemblies. Subsequently, the solvent flows into the gas-liquid contactor where it is mixed with the gaseous reaction products entering through the gas inlet. The mixture is then separated in the gas-liquid separator. The gas leaves through the hollow bottom electrode. The liquid flows be-

185 Top Electrode Solvent Inlet Gas Inlet

Insulator Outer Electrode Gas-Liq u id Contactor Gas-liquid Separator Solvent Exit Hole Insulator Bottom Electrode Exit Line

Heating Element Reactor Assembly

Reaction Gasc:f7W7W7¥IT1

Thermocouple

Column

Fig. 9.4. Cross-section of microreactor and differential cell. (Reprinted from ref. 13, with permission.) tween the outer wall of the bottom electrode and the inner wall of the gas-liquid separator. The liquid passes into the hollow bottom electrode through a small hole in the wall of this el~ctrode. At this point, the gas and liquid phases are recombined and returned to the solvent reservoir. This Hall 700A detector offers nitrogen, halogen, sulphur and nitrosamine modes. Recently, a new design of an ELCD was described 14 that is especially suitable for high-resolution capillary gas chromatography. The phase separator is eliminated and the gas-liquid mixture passes directly through the conductivity cell.

1~

9.2. SELECTIVITY OF RESPONSE The ELCD can be used as a selective detector for halogen-, sulphur- and nitrogen-containing compounds. The selectivity is determined by the products formed in the detector reactor, the scrubber used and the chemical properties of the electrolyte. The reaction furnace is operated under one of the following conditions: reductive (hydrogen reaction gas), oxidative (oxygen or air reaction gas) or pyrolytic (inert reaction gas). The post-furnace scrubber removes acidic or basic products. The following cond~tions are essential for a selective response: (I) the compounds of interest should give a decomposition product that is soluble and ionized in the conductivity solvent; (2) the interfering compound decompose to products that are either insoluble or non-ionized in the solvent or can be chemically subtracted without interfering with the product of interest; and (3) the dissolution and ionization processes should display discrimination against interferences that have not been removed previously. Under reducing conditions (800-900 0 C) and when using a nickel catalyst, ammonia is produced by the decomposition of organic nitrogen compounds:

R-CN

Ni ,H 2

----+

NH3

+

lower alkanes

(9.1)

Under these conditions, the organic halogen-, sulphur-, oxygen- and phosphoruscontaining compounds are converted into HX, H2S, H20, PH 3 and lower alkanes. The water gives a low or no response, because it is already present in the aqueous solvent. The lower alkanes (mainly methane) are poorly soluble in the solvent under the given conditions and, in addition, they exhibit a low ionization. Hydrogen sulphide gives a low response owing to the low ionization constant in water 15 (see Table 9.1). Ammonia is the only base formed during the reductive catalytic decomposition of the organic compounds. For this reason, the acidic products HX and H2S can be removed with a basic scrubber containing, for instance, strontium hydroxide, placed between the reaction chamber and the conductivity cell (see Fig. 9.1). Under these conditions, the N:C response selectivity7,13, 15,17,18 is 10 4-10 6 (nitrogen mode). In the reductive version in the absence of a catalyst, the organic nitrogen compounds are converted into ammonia to only a small extent. Therefore, the detector becomes selective to halogen compounds l9 • If a slightly acidic electrolyte, such as I-propanol or n-butanol, is used the response of the weak acid, hydrogen sulphide, and that of the weak base, ammonia, is levelled l3 (halogen mode).

187 TABLE 9.1 DISSOCIATION CONSTANTS OF SOME ACIDS AND AMMONIA From ref. 16. Species

Dissociated from

Ka

H2S0 3 HSOj H2SO 4 HS0 HN0 2 H2C0 3 HCOj H2S HSHCl

HSOj S023 HS0 S024 N0 HCOj C023 HS S2-

1.54.10- 2 1.02.10- 7

NH 40H

NH+ 4

4

4

2

pK a

1.81 6.91

Temperature (0C)

18 25

1.20.10- 2 4.6.10- 4 4.3.10- 7 5.61.10- 11 9.1.10- 8 1.1.10- 12

1. 92 3.37 6.37 10.25 7.04 11.96

12.5 25 18

Cl 1.97.10- 5*

4.75**

25

Pyrolysis with an inert gas without a catalyst (quartz reaction tube) gives a selective response to certain types of nitrogen compounds. In the temperature range 400-600 oC, ammonia is formed from the amines and nitrosamines, whereas other types of organic nitrogen compounds produce little ammonia, if any20. The selectivity is 21 1:10 7 and the selectivity relative to pyrazine exceeds 10 5• Fig. 9.5 gives a comparison of the detector responses under (A) pyrolytic conditions and (B) reducing conditions. The nitrosamine mode of the Hall 700A detector operates under reducing conditions, however, in the absence of a catalyst (gold reaction tube)22. At about 700 oC. the nitrosamine is reduced as follows:

(9.2)

188 B

LU Vl

A

3

Z

o a.. Vl LU

a:

2

I

LU Vl Z

o a.. Vl LU

45

II

a:

"""' Z

r

a

I

5

z"""'

I

10 min

a

5

'-

10 min

Fig. 9.5. Comparison of detector response in (A) pyrolytic mode (550°C) and (B) reductive mode (B20 0 C). 1 = N-Nitrosodimethylamine; 2 = N-nitrosodiethylamine; 3 = N-nitrosodi-n-propylamine; 4 = N-nitrosodi-n-butylamine; 5 = N-nitrosopiperidine. Stainless-steel column (3 m x 1/8 in. 0.0.) packed with 10% earbowax 20M-terephthalic acid on Gas-Chrom Q; temperature, programmed from 100 to 2I0 0 e at 10 0 C/min. (From ref. 21.) When using a 'fJater-n-propanol (1:1) electrolyte, the selectivity over nitrogenous compo~nds ranges from 200 to 500. At temperatures above lOOoe and under noncatalytic reducing conditions, the detector (Fig. 9.6) with a hydrogen flowrate of 5 ml/min gives a selective response to barbiturates 15 ,18, allowing barbiturates to be distinguished from other drugs 23 in biological extracts. Under oxidizing conditions S02 and S03' HX, N0 2 , CO, CO 2 and H20 are formed by organic sulphur-, halogen- and nitrogen-containing compounds. Water and carbon monoxide give a low or no response; the responses to carbon dioxide and nitrogen dioxide are also low (short contact time with the solvent or use of a non-aqueous solvent). The detector can be made selective to sulphur (sulphur mode) or halogen compounds by using a scrubber displacing either HX (silver salt at lOOoC) or

189 50 h

40

~

z

30

::>

> cr: cr:


~ 20

cr:


10

700 600 900 1000 ·C Fig. 9.6. ELCD furnace temperature profiles for barbiturates (0- and S-barbs.) and azobenzene in the reductive mode. 1 = Butabarbital; 2 = amobarbital; 3 = pentobarbital; 4 = secobarbital; 5 = thiopental; 6 = thiamylal; 7 = azobenzene. (From ref. 15.) sulphur oxides (calcium oxide at 800 0 C; the oxygen is saturated with water vapour before entering the reaction tube in order to hydrolyse the possibly produced calcium chloride)24. When using ethanol as a solvent, the selectivity of Cl:C 10 is about 10 5 , that of S:C 10 ,13,25 is about 10 5 and that of S:C1 26 is about 3 5'10 .

Depending on the temperature of the decomposing unit (see section 9.6), selective differentiation can sometimes be achieved between different types of compounds containing the same heteroatom. The detector can also be converted into the non-selective carbon mOde 13 • Carbon is detected with a nickel reaction tube containing a platinum catalyst, air as reaction gas, a potassium hydrogen carbonate scrubber and an aqueous conductivity solvent. The detection limit of heptadecane is 6.10- 9 g. 9.3. RESPONSE The sensitivity of the ELCD depends on the amount of the element investigated (see eqn. 9.4) and, for this reason, it should be independent of the structure of the compound. A direct proportionality between the response and the content of the element in the molecule was indeed found for compounds decomposed under oxidizing conditions 5 and for nitrogen compounds decomposed under catalytic

190 TABLE 9.2 RELATIVE RESPONSES OF N-NITROSAMINES

x = mean peak area (mm2); s = standard deviation; V = coefficient of variation; P = 100' (peak areo. of given compound)/(peak area of NOMA); M = 100· (mol. wt. of given compound)/(mol. wt. of NDMA). (From ref. 21.) N-Nitroso derivative

-X

S

V

P

M

Dimethylamine (NDMA) Diethylamine Di-n-propyl ami ne Di-n-butylamine Piperi di ne Pyrrolidine

120 106 100 64 32 17

7.3 6.7 4.9 2.0 1.0 1.3

6.1 6.3 4.9 3.1 3.2 7.7

100 87 82 53 26 14

100 73 57 48 65 74

TABLE 9.3 RESPONSE QUENCHING COMPARISON OF FLAME PHOTOMETRIC DETECTOR (FPD) AND ELCD Reprinted from ref. 25 with permission. Percentage of quenching compound in N2 matrix

o (reference) 10% 10% 50% 1% 10% 50%

Carbon dioxide Methane Methane Ethylene Ethyl ene Ethylene

FPD response (%)

ELCD response (%)

H2S

COS

H2S

COS

100.0 95.7 74.3 66.5 83.8 70.2 38.2

100.0 100.0 72.1 66.8 93.3 75.4 70.4

100.0 89.6 92.4 90.0 99.0 94.4 77 .4

100.0 97.2 100.0 100.0 95.0 96.0 98.0

reduci ng conditi ons 5 ,27. However, a 1arger vari ance in the mol ar response was found by Greenhalgh and Cochrane 28 • A chlorine atom in the aniline molecule affects the response level in the N mode. Derivatives substituted in the paraposition give a lower response. No decrease in detector response was found as more nitro groups were placed on the aromatic ring in chloronitroanilines 29 • A nitrogen atom in the molecule of 'a sulphur compound decreases the response in the S mode. whereas that of chlorine increases the response 30 • The structure of the compound affects the response with barbiturates 15 • The response of thiobarbiturates ;s lower than that of their oxygen analogues (Fig. 9.6). In the

191 h em

p~

20

0

°

1

• •

:

3

A

A

t.

2

10

'"

: : /./ 700

Fig. 9.7. catalytic chloride; thiazine.

• 5

1000

900

800

°c

ELCD furnace temperature profiles of phenothiazines in the N-selective reducing mode. 1 = Trimeprazine tartrate; 2 = methdilazine hydro3 = thioridazine hydrochloride; 4 = phenothiazine; 5 = 2-chloropheno(From ref. 15.)

30

25 1/1 U C

~ 20

::> o

.c c 1/1

C 15 ::> o u

II 1/1 C

~ 10 1/1 II

a::

5

400

500

600

700

800

900

1000

1060·C

Fig. 9.B. Effect of furnace temperature on the response of three sulphur and three chlorine compounds. BHC = 1.2.3.4.5.6-hexachlorocyclohexane. (From ref. 33.)

192

.-._k

h

N'

30

<>

20 A

10

•

500

600

700

800

900 'C

Fig. 9.9. ELCD temperature profiles of lmlpramine in the N-selective cata1 tic reducing mode, illustrating differences in catalytic activity. (A) 500-9006C sequence; (8) 550-900 oC sequence; (C) 900-550 oC sequence. (From ref. 15.) pyrolysis version, the response of nitrosamines also depends on their structure 21 ,31, but there is no clear relationship between the relative nitrogen content and the relative sensitivity for each nitrosamine 21 (Table 9.2). The structure of the molecule also affects the sensitivity of detection in the Cl mode. If the chlorine atom is in the para-position in mono-, di- and trichloroanilines, the response of this compound is always lower than that of the other derivatives. The introduction of a nitro group into the chloroaniline molecule appears to decrease the detector response 29 . The differences in the sensitivities seem to be given by the degree of conversion of the compounds in the reactor chamber. The extent of conversi on depends on the temperature in the decomposition unit, and this dependence is affected by the structure of the compound 15 ,2o,32-34 (see Figs 9.7-9.11). If another compound is simultaneously present in the detector (hydrocarbons, carbon dioxide), the sensitivity of the ELCD is decreased. It follows from Table 9.3 that this decrease depends on the type of interfering compound (the influence of carbon dioxide is less than that of hydrocarbons) and on the amount of the in:erfering compound (the influence increasing amount). This decrease is always lower than with the flame photometric detector.

193

100

80 ~

8 CXl C 60

:c01 'Qj

J: ~

cQJ 40 a.. ~

0

20

0L-L-6~00~~~~7~00~~~~8~0~0~

Furnace Temperature (·C) Fig. 9.10. Relative peak heights versus furnace temperature for PCBs, chlorinated pesticides and reference compounds. 1 = Chlorocyclohexane; 2 = average for pesticides (lindane, heptachlor epoxide, dieldrin, DDT, o-chlordane and o,p'-DDT); 3 = Aroclor 1254; 4 = pentachloroanisole; 5 = a,3,4-trichlorotoluene; 6 = average of eleven isomers of polychlorobenzene; 0, common to 4,5,6. (Reprinted with permission from ref. 32.) Table 9.4 gives the detection limits for individual types of ELCD. The response may decrease gradually during the operation of the detector, owing to contamination of the reaction tube, solvent impurities and the ion-exchange resin 10 ,21,32. Silicon dioxide from the stationary phase is a source of contamination in the oxidizing mode and can be washed out with 10% hydrogen fluoride. In the reducing mode, contamination is mainly due to the condensation of non-volatile substances in the cold section of the reaction tube and in the PTFE connecting tube (bleeding from the septum, column, etc.). Carbon contamination is related to the inner diameter of the tube. In the oxidizing mode, the injection of 3 ~l of hexane severely contaminates the 4 mm 1.0. reaction tube, whereas no contamination was observed with 0.5-1 mm 1.0. tubes even after 300 injections 10 • The linear dynamic range is about four orders of magnitude 5 ,10 and >10 5 for Pi ri nger and \~o lff IS detector 14 • The electrolytic conductivity (c) of the solution is directly proportional to the specific conductance (Csp ) of the solution and inversely proportional to the cell constant 10 • The specific conductance depends on the kind of ion in the solution and on the concentration and temperature of the latter. The cell

..,.

\0

TABLE 9.4 DETECTION LIMITS FOR ELCDs Detector type

Heteroatom

Detection limit or minimum detectability

Mode

Solvent

Ref.

Coulson

Oxidative Oxidative Reductive Reductive

Water Water Water Water

Modified Coulson Hall 350

N Cl S N N S Cl Cl

Reductive Oxidative Reductive Oxidative Reductive Pyrolytic Oxidative Reductive Reductive

Water Ethanol Ethanol Methanol Propanol-water (1: 1) Water Methanol

8

Hall 700A

0.5 ng of lindane 1 ng of Systox 1 ng of azobenzene 0.4 ng of nitrotoluene 2 pg//sec of N 0.1 ng of atrazine 20 pg of lindane 20 pg of lindane 1 pg/sec of S 10 pg of nitrosamine 50 pg of dialkylnitrosamine 0.4 pg/sec of S 0.5 pg/sec of Cl 1. 5 pg/sec of Cl

5,6 5 35

Modified Coulson

Cl S N N

Water

14

Hall 310 Hall 700A Hall 700A Pi ri nger and Wolff

7

10 10 25 36 21 26 13

195

70

60

50 f-

:J: (.!)

40

w :::c

::.::

i;5 30 n..

20

10

0 300

400

500

600

°c

700

800

900

Fig. 9.11. Effect of furnace temperature on the response of 3 ppm of ethyl mercaptan (1).3 ppm of hydrogen sulphide (2), 3 ppm of dimethyl sulphide (3) and 100% methane (4). (Reproduced from ref. 34 with permission.}

constant depends on the electrode area (A) and the distance between the electrodes (d):

c = Csp /k

(9.3)

k = d/A

(9.4)

The detector response depends on the amount of the compound entering the detector per unit of time (M), the weight percentage of the element monitored (w), the conversion efficiency (E) of the sample, the solvent flow-rate (f), the cell temperature (T c )' the voltage (V) and the cell constant: (9.5)

196 where K is a constant and a is the coefficient of temperature dependence. The cell with the largest electrode area per unit volume, i.e., with concentric cylindrical electrodes, will give the greatest response. The theoretical dependence of the response on the radius of the inner electrode and the interelectrode distance can be seen from Figs. 9.12 and 9.13 (the Hall detector).

o o

~

N

~

•

~

0 0

~

N

r

2

2r

3r

4r

Sr

6r

Fig. 9.12. Calculated detector response versus radius of inner electrode (r). Inter-electrode distance fixed at 0.0139 cm. (Reprinted from ref. 10 with permission.) The response of the Coulson detector increases by a factor of about 12 in the voltage range 5-50 V. However, peak tailing and detector noise also increase at the same time. The signal-to-noise ratio remains virtually constant 32 •

9.4. SOLVENT Water undergoes autoprotolysis 19 : (9.6)

197

~

2

d

2d

3d

4d

5d

6d

Fig. 9.13. Calculated detector response versus inter-electrode distance (d). Radius of inner electrode fixed at 0.0826 cm. (Reprinted from ref. 10 with permission.) If an acid or a base is added to water, the autoprotolysis is suppressed. Hence the increase in conductivity is smaller than would correspond to the addition of an electrolyte alone: (9.7) (9.8) The hydronium or hydroxyl ions formed shifts the equilibrium according to eqn. 9.6 to the left. As a result of this phenomenon, a non-linear relationship exists between the equivalent conductance and the electrolyte concentration at low concentrations (Fig. 9.14). This problem can be essentially eliminated by maintaining the water slightly basic by means of an ion exchanger 7• which considerably reduces the effect of autoprotolysis. This problem can also be eliminated by using a non-aqueous solvent (ethanol, methanol, butanol)10,15.25.34.

198

A

10- 6

•

10-5

10- 4

MOLARITY OF ELECTROLYTE

Fig. 9.14. Conductivity (A, ~-1 cm- 1) of several electrolytes in water. (From ref. 19.) When using water, the CO~ response is about three orders of magnitude lower than that of sulphur di/trioxide and hydrogen chloride. The use of absolute ethanol almost eliminates the CO 2 response 10 • increasing the selectivity to 10 5 In the reducing mode, the use of ethanol increases the selectivity to halogen response, because the H2S response is reduced. However, the absolute response in water is about four times greater than that in absolute ethanol. Mixtures of isopropanol and water 37 and n-propanol and water 36 have also been used as solvents for the Hall detector. Hall recommends a hydrogen chloride-ethanol mixture for the highly sensitive detection of nitrogen compounds 10 • Methano1 26 has been used for the oxidative decomposition of sulphur compounds (S mode). l~hen using dilute hydrochloric aCid 38 , the conductivity of the solvent is decreased: (9.9) The ionic conductance of the NH4+ ion is about five times smaller than that of the H30+ ion ll . The response to nitrogen compounds in dilute hydrochloric acid (5 ppm) is about four times greater than that in water. Hence the neutralization reaction 9.9 produces greater changes in conductance than simple dissolu-

199 TABLE 9.5 COMPARISON OF DETECTION LIMITS From ref. 38. Compound

Detection limit (ng)

Nitrobenzene Chlorobenzene Chloronaphthalene Hexachlorobutadiene

20 1

0.5 0.05

tion and partial ionization of ammonia (reaction 9.8). When using dilute hydrochloric acid, the response to chlorine compounds is one to two orders of magnitude greater than that to nitrogen compounds (Table 9.5). Fig 9.15 shows a chromatogram of nitrogen and chlorine compounds after reductive decomposition using hydrochloric acid (1 ppm). All nitrogen compounds give negative peaks; the conductance is reduced, whereas the responses to all the chlorine compounds are positive, i.e., the conductance is increased. The peaks of compounds containing both nitrogen and chlorine in the same molecule are doubled (this fact was explained by the different rates of desorption of ammonia and hydrogen chloride from the catalyst)38.

80

~

~

~ ~

~ ~ ~

c

.~

~ ~

~

~

~

70 60

C

~

U

~

50

~

0~ 0 ~

u

c

~ ~

0 ~

u

I

I

Q

0

~

c c

~

~

c

~

~

£

~

c c

0 ~ 0 ~

~ ~

~

0~ 0 ~

u I

U

N

~

c

0

~ ~

40 ~

~

~

c

30 20

~

~

N C

~

~

N

C ~

c

0

Z

~

3 D ~I ~

Fig. 9.15. Chromatogram of nitrogen and chlorine compounds using 1 ppm of hydrogen chloride. Stainless-steel column (2 m x 0.2 cm 1.0.) packed with 3% Carbowax 20M on Celite; temperature, programmed from 75 to 170 0 C at 7.5 0 C/min. (From ref. 38.)

200 In a dilute iodine solution, rapid oxidation of sulphur occurs, glvlng two highly conducting hydrogen ions per molecule of oxidized hydrogen sulphide: (9.10) The response to sulphur compounds in a solution composed of 0.04% iodine, 2% ethanol and 1 ppm hydrogen chloride is about an order of magnitude higher than that in a 1 ppm hydrogen chloride solution 38 • Hydrocarbons are not ionized. However, the stability of the baseline is low and strong drifting occurs (the solvent is rapidly contaminated and ion exchangers cannot be used). For this reason, this technique is unsuitable in practic~34. The response of the Hall detector decreases with increasing flow-rate of the solvent. In the flow-rate range 1-2 ml/min, the atrazine response remains approximately constant. The response increases drastically if the flow-rate is reduced below 1 ml/min. There is an approximately 10-fold increase in response 37 for a flow-rate of 0.2 ml/min. However, the detector noise increases in the same proportion at low flow-rates. With regard to the noise, the optimum flow-rate is 0.7 ml/min 21 ,37. The flow-rate of the solvent determines the ion concentration and, theoretically, the detector response should be inversely proportional to the flow-rate of the solvent lO (eqn. 9.5). Hall 11 explained the non-linearity of this relationship by the evaporation of the solvent and the limited capacity of the gas-liquid contactor for exposing the entire solvent entering the cell to the stream of the reaction products. When using a solvent consisting of isopropanol-water (50:50)11, the solvent composition changes owing to the partial evaporation of the alcohol. The amount of solvent actually flowing through the cell is also reduced. Hence the solvent polarity increases and the actual amount of the solvent decreases if the flow-rate is decreased. The invariability of the response at flow-rates exceeding 1 ml/min can be explained 11 by the assumption that large solvent drops, rather than a fine mist, are formed at these flowrates. The solvent drops are expelled through the hole in the gas exit tube without touching the electrode working surfaces. There is also a non-linear decrease in response with increasing flow-rate of the solvent (methanol) in the oxidative determinations of sulphur compounds. The minimum detectability shows a maximum at a flow-rate of 1.9 ml/min 26 • The daily variations in the sensitivity were found to be due to the decrease in the concentration and temperature variation of the conductivity solvent. When using a mixed solvent, the isopropanol concentration and minimum detectability decrease with time and the short-term noise increases. The use of an undiluted alcohol as the conductivity solvent in conjunction with a temperature-controlled water-jacket gives the most reproducible detector response from day to day39.

201 The solvent conductivity and the noise of the Coulson detector increase with increasing temperature of the detection cell. The response of this detector decreases by tens of percent 40 in the temperature range 8-45 0 C, whereas the response of the Coulson detector modified by Lawrence and Moore 8 remains the same. 9.5. GASES Under reducing catalytic conditions, the response of the Hall detector to atrazine increases drastically to a maximum at about 25 ml/min with increasing hydrogen flow-rate (carrier gas helium) and then remains essentially constant up to a flow-rate of 100 ml/min 37 • The influence of the hydrogen flow-rate is low when using hydrogen as the carrier gas (Table 9.6)11. In the pyrolysis version (carrier gas nitrogen), the response to barbiturates reveals a temperature-dependent maximum between hydrogen flow-rates of 0 and 5 ml/minl1. TABLE 9.6 INFLUENCE OF HYDROGEN REACTION GAS FLOW-RATE ON RESPONSE TO NITROGEN-CONTAINING COMPOUNDS Furnace temperature, BOOoC. Reprinted with permission from ref. 11. Flow-rate (cm 3/min)*

10 20 40 60 80 100

Peak height (mm) Chlorpropham

Atrazine

Simazine

27 32 37 32 38 31

89 109 121 108 131 108

78 95 109 95 116 96

*Total flow also contained 40 cm 3/min of H2 carrier. The Coulson detector shows an increase in the atrazine response (maximum about 40 ml/min) similar to that of the Hall detector, but with a decrease in response at hydrogen flow-rates exceeding 40 ml/min41. In the oxidative version, oxygen flow-rates within the range 20-40 ml/min were generally required in order to obtain the maximum response of the Coulson detector to chlorine and sulphur compounds. The response profiles are dependent

202 on the structure of the compound 33 (Fig. 9.16). The detector noise increases with increasing flow-rate of the air26.

30

25

Promctryne

,, ,, Aldrin'-------___ /' ,,/, ",'"

..........

--......

/.;1'

I

~/

'......

/'"

,", ", ....~~BHC

-, /

/ I

II 1/

,

~

Diethyl S-phenyl phosphorodithioate

,\ 10

': "\

",.,..---

I

---------

/'

I

I

I

I

Heptachlor

," /

/

5 :

510 20 30 40

60

80

100

160ml/min

Fig. 9.16. Variation of the detector response with oxygen flow-rate at a furnace temperature of 850 0 C. BHC = 1,2,3,4,5,6-hexachlorocyclohexane. (From ref. 33.) At high separation speeds, such as occur in capillary gas chromatography, especially at low solute concentrations, peak tailing was opserved as a result of the low desorption rate of HCl from the surface of the nickel catalyst. The doping of the auxiliary gas with vinyl chloride completely eliminated this effect 14 (Fig. 9.17).

203 5

4

5

2

4

3

3

2

a

b

Fig. 9.17. Effect of doping on the peak shape. a, Un doped gas; b, auxiliary gas doped with 20 ppm of vinyl chloride. 1 = 4-Chlorobiphenyl; 2 3,4-dichlorobiphenyl; 3 = 3,5-dichlorobiphenyl; 4 = 4,4'-dichlorobiphenyl; 5 = 2,4,5-trichlorobiphenyl. (From ref. 14.) 9.6. TEMPERATURE With increasing temperature of the reaction cell, the response of the electrolytic conductivity detector to nitrogen compounds (the Hall detector, Model 310, catalytic reducing mode) increases to the maximum l4 ,37. The temperature at which the maximum response it attained depends on the compound to some extent (see Fig. 9.7, for example) and varies within the range 700-900 oC. The course of the temperature dependence of the response in the region that follows after attaining the maximum (BOO-I000 oC) also depends on the structure of the compound. For substituted phenotiazines, diphenylmethanes and tricyclic antidepressants, the response remains constant 15 reaching the maximum value. The response to atrazine decreases slightly in the range 900-100 oC36 • The response to barbiturates, in the reducing mode without catalyst (poisoning of the catalyst occurs if Ni is used) also decreases with increasing temperature after attaining the maximum 15 (Fig. 9.6). The maximum response to N-nitrosamines is attained at 600 oC21 .

204 TABLE 9.7 INFLUENCE OF REACTION CONDITIONS ON RESPONSE OF COULSON DETECTOR TO MODEL CQt·1POUNDS Reprinted with permission form ref. 32. Compound

Relative response * 700 0C

800 0 C

Chlorobenzene 1.2-Dichlorobenzene 1.3-Dichlorobenzene 1.4-Dichlorobenzene 1.2.3-Trichlorobenzene 1.3.5-Trichlorobenzene 1.2.3.4-Tetrachlorobenzene 1.2.4.5-Tetrachlorobenzene Pentachlorobenzene Hexachlorobenzene Chlorocyclohexane 1.2.3.4.5.6-Hexachlorocyclohexane 1-Chlorotetradecane Pentachloroanisole a.3.4-Trichlorotoluene

H ** 2

No H2***

H ** 2

No H2***

1.000 2.520 1.642 2.180 3.080 1.842 2.861 2.082 1.495 0.296 22.168 16.074 17.168 14.392 5.907

0.701 0.982 0.607 0.794 0.904 0.537 0.738 0.572 0.861 0.398 27.815 17.188 27.516 14.963 8.631

0.078 0.166 0.082 0.132 0.115 0.073 0.062 0.052 0.026 0.009 21.806 10.420 7.824 1.221 0.344

0.038 0.028 0.017 0.021 0.006 0.002 0.013 0.009 0.012 0.009 26.449 17.750 21.798 4.514 0.372

*Response expressed relative to chlorobenzene. **80 ml/min of hydrogen reaction gas. ***80 ml/min of helium substituted to maintain constant residence time. Similar relationships are also valid for the response of the Coulson detector 18 •41 ; compared with the Hall detector, the difference consists in a lower increase in the response in the range 500-900 0C (by a factor of 2-3) and in the higher temperature required to obtain the maximum response (about 900 0C). Similar relationships can also be observed in the response to sulphur and chlorine compounds in the pyrolysis version 31 (Fig. 9.8). However, the response again increases at temperatures above 10000C. Fig. 9.9 shows the temperature profile of the response to imipramine as a function of the sequence in which the furnace temperature was varied. As can be seen, the temperature profile of the response changes substantially, depending on the starting temperature and on whether the temperature was gradually increased or decreased. Pape et al. 15 ascribed this dependence to the deactivation of the catalyst at low temperatures.

205 At a given temperature, the efficiency of the reactions giving hydrogen chloride from polychlorinated biphenyls (PCBs) and chlorinated pesticides depends on the dissociation energy of the C-Cl bond 32 • The dissociation energy of the aromatic C-Cl bonds (PCBs) is 4-13 kcal/mole higher than that of th~ aliphatic C-Cl bonds (chlorinated pesticides). The dependence of the responses on the temperature of the reductive decomposition differs for the two groups of substances (Fig. 9.10). The response to pesticides in the temperature range 600-700 0 C is greater than that to PCBs, attaining the maximum at 800 0 C. With increasing temperature, the response of PCBs continues to increase up to 900 0 C. Table 9.7 gives the relative responses of several chlorine compounds at various temperatures under reducing and/or pyrolysis conditions. It is obvious that the elimination of hydrogen from the system considerably reduces the production of hydrogen chloride from chlorobenzene, whereas it is increased with chlorinated aliphatics. Hence a selective response to chlorinated pesticides relative to PCBs can be attained. The selectivity and sensitivity are shown in Table 9.8. TABLE 9.8 SELECTIVITY OF CHLORINATED HYDROCARBON PESTICIDES OVER PCBs AS A FUNCTION OF REACTOR TEMPERATURE Reprinted with permission from ref. 32. Temperature (OC)

610 710 810

Detecti on 1i mi t

Selectivity

Chlorinated pesticides

PCBs

5-10 ng 1-5 ng 0.2-0.5 n9

>50 119 >5 119

10-20 n9

Fig. 9.11 shows the temperature dependence of the sulphur response in the oxidative mode. A selective response to mercaptans 34 can be obtained at various temperatures. The detector response depends on the temperature of the column if chromatographic columns with a stationary phase containing nitrogen (e.g., the cyanopropylsilico,ne polymer OV-225) are used. The resistance of the solvent either decreases (water) or increases (dilute hydrochloric acid) with temperature owing to an increase in the rate of dissolution of ammonia in the liquid (influence of stationary phase bleeding)42. Low loadings should be applied with these

206 phases. The use of phases containing nitrogen or a halogen should be avoided, as they can give rise to acidic or basic products that would "poison" the electrolyte and drastically reduce the sensitivity43.44. Columns deactivated with phosphoric acid should also be avoided, especially at temperatures of I80 0C or higher. The phosphoric acid bleed has a great adverse effect on the nickel reaction tube and on the alkaline scrubber. REFERENCES 1 O. Piringer and M. Pascalau, J. Chromatogr., 8 {1962} 410. 2 O. Piringer. E. Tataru and M. Pascalau. J. Gas Chromatogr •• 2 (1964) 104. 3 M. l1ohnke, O. Piringer and E. Tataru, J. Gas Chromatogr., 6 (1968) 117. 4 J.C. Sternberg, D.T.J. Jones and R.A. Morris, A Method for SeLective Halogen and Detection in Gas Chromatography, presented at 1.3th Pi t tsburgh

5 6

Confepence on Analytical Chemist~J and Applied Spectroscopy, March 1962. D.M. Coulson, J. Gas Chromatogr •• 3 (1965) 134. D.M, Coulson. The DeTektor. 1, No.1 (1968). f1ikro Tek Instrument. Austin,

TX.

7 G.G. Patchett, J. Chromatogr. Sai •• 8 (1970) 155. 8 J.F. Lawrence and A.H. tloore, Anal. Chem •• 46 (1974) 755. 9 J.F. Lawrence and N.P. Sen, Anal. Chern., 47 (1975) 367. 10 R,C. Hall, J. Chromatogr. Sci •• 12 (1974) 152. 11 R.C. Hall. Crit. Rev. Anat. Chern., 8 (1978) 323. 12 R.C. Hall, U.S. Pat., No.3 934 193 (1976). 13 R.J. Anderson and R.C. Hall, Amer. Lab., 12 (1980) 108. 14 O. Piringer and E. Wolff. J. • 284 (1984) 373. 15 B.E! Pape. D.H. Rodgers and T.C. Flynn. J. Chromatogl"., 134 (1977) 1. 16 J. Sevcik, Deteatol"S in Gas Chromatography, Elsevier, Amsterdam, 1976, p. 182. 17 S.L Pape and t4.A. Ribick, J. Chr·omatogl" •• 136 (1977) 127. 18 R.C. Han and C.A. Risk, J. Sai., 13 (1975) 519. 19 a.M. Coulson, lh:-tmgen, and Cal"bon Detection Electroehemical Methods, presented at Eastern Symposium, Net.) YOl?k, 1968. 20 J.II. Rhoades and D.L Johnson, J. Sci., 8 (1970) 616. 21 E. von Rappard, G. Eisenbrand and R. Preussmann. J. Chl"omatogr •• 124 (1976) 247. 22 R.J. Anderson, Semi-specific Detepmination of llitl"osamines Eleotl"olytic Cond:>J.ctivity Deteato'l'~ presented at on AnaZytioal Chemist~ and Applied'Speatl"oscoPY, 1980, paper 412. 23 S.E. Pape, C~in. Chern., 22 (1976) 739. 24 D.H. Coul son, Advan. Chromatogl"., 3 (1966) 197. 25 B.J. Ehrlich. R.C. Hall, R.J. Anderson and H.G. Cox, J. Chl"Omatogl". Sci •• 19 (1981) 245. 26 S. Gluck, J. Ch~omatog~. Sci., 20 (1982) 103. 27 J.F. Palframan, J. Macnab and N.T. Crosby. J. Chromatogr •• 76 (1973) 307. 28 R. Greenhalgh and W.P. Cochrane, J. Chromatogr., 70 (1972) 37. 29 V. Lopez-Avila and R. Northcutt. J. Reso2ut. Chromatogr, Chrornatogr>. Commun •• 5 (1982) 67. 30 W.P. Co~hrane and R. Greenhalgh. Int. J. Environ. Anal. Chern •• 3 (1974) 199. 31 p. Issenberg and S.R. Tannenbaum. presented at JARG Meeting on Analysis and Formation of (Ii tY'osamines ~ Heide lber>g. 1971.

32 J.W. Dolan and R.C. Hall, Anat. Chern •• 45 (1973) 2198. 33 W.P. Cochrane, B.P. \-I11son and R. Greenhalgh, J. Chl'omatogr., 75 (1973) 207. 34 R.G. Schiller and R.B. Bronsky, J. Chromatogr. Sci •• 15 (1977) 541.

207 35 D.t4. Coulson, J. Gas ChY'Omatogl'., 4 (1966) 285. 36 R.J. Anderson, Tr'acor' Chl'omatogl'aphy. Appl. 79-3. N Selective Detection in Gas Chl'omatogl'aphy, Tracor Instruments, Austin, TX, 1979. 37 B.P. I~ilson and W.P. Cochrane, J. ChY'Omatogl'., 106 (1975) 174. 38 P. Jones and G. Nickless, J. Chl'omatogl'., 73 (1972) 19. 39 R.K.S. Goo, H. Kanai, V. Inouye and H. Wakatsuki, Anal. Chem., 52 (1980) 1003. 40 G. Winnett and \~.L. Illingsworth, J. ChY'Omatogr·. Sci., 14 (1976) 255. 41 J.F. Lawrence, J. Chl'omatogl'., 87 (1973) 333. 42 D.tt Hailey, A.G. Howard and G. Nickless, J. ChY'Omatogr>., 100 (1974) 49. 43 ttA. Luke, J.E. Froberg, G.t1. Doose and H.T. Masumoto, J. Ass. Offic. Anal. Chem., 64 (1981) 1187. 44 Tl'acol' Chl'omatogl'aphy, Appl. 78-5, Selective Detection in Gas Chl'omatogl'aphy, Tracor Instruments, Austin, TX, 1978.

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209

Chapter 10

COULOMETRIC DETECTOR CONTENTS 10.1. Introduction • . . . • 10.2. Response • • . • • • • 10.2.1. Oxidative mode 10.2.2. Reductive mode 10.2.3. Nitrogen mode 10.3. Quantitative results References • • • • • • • . .

209 210 210 210 211 212 215

10.1. INTRODUCTION The column effluent is mixed with oxygen in a combustion tube where organic compounds are converted into CO 2 , H20, HX, S02 and oxides of nitrogen. H2S, HX, and PH 3 conversion products are obtained in the reductive mode, when using hydrogen as the reaction gas. The resulting products then enter a titration cell where they are absorbed in an appropriate solution and titrated automatically with coulometrically generated ions. Thus, the coulometric detector. (CD) consists of three parts: the combustion tube, the titration cell and the coulometer (Fig. 10.1). The titration cell consists of four electrodes that function as a sensor-reference pair and an anode-cathode generator pair. The input signal from the sensing electrode is the difference between the sensor

Gas Chromatograph

Recorder

Combustion

Coulometer

Fig. 10.1. Schematic diagram of the coulometric detector.

210 and reference electrodes and it is biased so as to give a zero signal across the input of the amplifier for a given concentration of the titrant ion in the titration cell. As this titrant concentration is decreased by the conversion products, additional titrant is generated to maintain a balance. For sulphur compounds, which yield sulphur dioxide in the combustion tube, the titrant is generated in a iodide solution at a platinum anode. For halides, the titrant is generated at a silver anode. 10.2. RESPONSE 10.2.1. Oxidative mode

In the oxidative mode, oxygen reactant gas is fed into the combustion tube of the detector. Oxidative degradation is most commonly followed by iodimetric titration of sulphur dioxide with coulometrically generated iodine l - 8 : (10.1) Bromine can also be used as a titrant 9- 11 . Large samples of compounds that produce strong oxidizing agents during combustion, e.g., compounds containing nitrogen, chlorine and bromine (with iodine titrants), give negative responses because they are stronger oxidizers than the titrant 2 ,6,9. The detection limit is 10-8 g for sulphur and the linear dynamic range covers three orders of magnitude 2 • For halides, internally generated silver ions serve as the titrant 4 ,8,12-18. Organic bromides are converted into bromine and the latter yields half the response of the hydrogen halide owing to the hydrolysis of Br 2 to HOBr and HBr19; only the latter precipitates silver ions 15 . The detection limit is approximately 1.10- 9 g of chlorine 15 ,17. Sulphur compounds can also be determined without combustion. However, it is necessary to standardize the titration cell against each type of compound 20 Sevcik 21 described a combination of a CO and a flame-ionization detector (FlO). The FlO served as a burning space for the substances eluted. It is possible to determine compounds containing sulphur and chlorine in this way, the minimum detectability being 8.10- 9 and 1.10-8 g/sec, respectively. 10.2.2. Reductive mode

In the reductive mode, sulphur compounds are reduced 20 to hydrogen sulphide. The sample is pyrolysed6 in hydrogen at about 11500 C over a catalyst (e.g.,

211 10% platinum on Alundum). The hydrogen sulphide formed by the pyrolysis is automatically titrated with silver ions in a microcoulometric cell: (10.2) Ag

+

Ag +

+

e-

(10.3)

The sulphur reduction method suffers from nitrogen interference, the extent of which is proportional to the content of hydrogen cyanide formed by pyrolysis. The reductive method can also be used for phosphorus compounds 22 ,23. In the oxidative method the phosphate moiety is probably converted to P4010 in the combustion tube, but it fails to leave the tube. Hence, the column effluent is reduced with hydrogen at 950 0 C with the conversion of phosphates to phosphine. Organically bound sulphur and chlorine are converted to hydrogen sulphide and hydrogen chloride, respectively. These three gases precipitate silver ions. They are measured with relative sensitivities of 2:2:1. However, a scrubber containing aluminium oxide quantitatively subtracts hydrogen sulphide and -chloride, whereas phosphine passes through the packing unchanged. The temperature range for the optimum yield of phosphine is 925-1000 0 C. Sulphur bonded directly to phosphorus can be measured directly without interference from phosphine in the reductive mode at a lower temperature. Compounds that contain only phosphorus do not yield any response at 70U oC. When sulphur is bonded to carbon, the yield of hydrogen sulphide is low at this temperature. Mercaptans can be determined directly in the silver cell without combustion 24 - 26 . 10.2.3. Nitrogen mode

Nitrogen compounds are converted into ammonia in a stream of hydrogen over a catalyst. The ammonia is automatically titrated in the titration cell to a constant pH in a sodium sulphate solution with hydrogen ions: (10.4) (10.5) The decrease in hydrogen ion concentration is sensed by the sensor-reference electrode pair. The second electrode pair serves for hydrogen ion generation. The titrant ion thus restores the original titrant ion concentration:

212 + H2 ~ 2 H + 2 e

( 10.6) (10.7)

Nickel deposited on magnesium oxide of the type described by Ter Meulen 27 was used as catalyst 28 ,29. The temperature limit for this catalyst is 440-450 oC; at higher temperatures nickel turnings or granules are used 29 - 31 . As the detector response is given by the concentration change of hydrogen ions, any substances that can change this concentration will produce a response. Acidic compounds, such as hydrogen chloride or hydrogen sulphide (from sulphur- and halogen-containing compounds) are removed by absorption methods. A hot (350-450 oC, to allow the ammonia formed to pass quantitatively) alkaline scrubber is used 30 ,31 with nickel catalysts. With the Ter Meulen catalyst a separate scrubber is not needed because of the alkalinity of the magnesium oxide catalyst support. The detection limit is approximately 3 ng for nitrogen and the linear dynamic range covers three orders of magnitude 32 • The selectivity relative to hydrocarbons is 106 , and relative to other elements such as halogens and sulphur it is at least 10 4 . 10.3. QUANTITATIVE RESULTS

The CD is theoretically a quantitative detector based on the amount of electricity required for the internal generation of the titrant. Coulometric titration proceeds according to Faraday's law, and calculation of the results TABLE 10.1 CONVERSION OF SULPHUR COMPOUNDS TO SULPHUR DIOXIDE AT DIFFERENT TEf4PERATURES Reprinted with permission from ref. 2. Combustion tube temperature (OC)

Conversion to sulphur dioxide (%)

550 660 650 700 750 850 950

70 80 91 93

89 74 63

TABLE 10.2 MICROCOULOMETRIC DETECTION OF HALOGEN COMPOUNDS From ref. 18. Reactant gas

Pyrolysis tube temperature (oC)*

O2

820 920 1020 820 920 1020

CO 2

Recovery ± S.D. (%) (n = 3) CHC1 3 (66 ng Cl-)**

C1CH 2CH{1 (53 ng Cl-)**

CHBr 3 (137 n9 Br-)**

CH Br 2 2 (138 ng Br-)**

42 47 47 40 48 66

41 73 64 45 76 92

82 55 39 68 82 81

77 ± 1.7

1.5 1.2 1. 0 ± 2.0 ± 2.3 ± 3.0

± ± ±

2.1 1.5 2.5 ± 5.0 ± 1.5 ± 3.5 ± ± ±

2.6 3.6 4.7 ± 3.8 ± 2.3 ± 2.9 ± ± ±

53 43 79 91 87

4.2 2.1 ± 4.5 ± 2.3 ± 0.6 ± ±

* **Deviation from specified temperatures ±10%. Amount equivalent to complete conversion of organic halogen compound to titratable halides.

N W

TABLE 10.3 MICROCOULOMETRIC DETECTION OF HALOGENATED BENZENES WITH OXYGEN AS THE REACTANT GAS From ref. 18. Pyrolysis tube temperature (oC)

820 870 920 1020

Recovery ± S.D. (%) (n Chlorobenzene (87 ng Cl-)** 5

14 ± 0.6 28 ± 3.6 26 ± 2.1

=

3) Bromobenzene (186 ng Br-)** 5

16 ± 1.2 47 ± 5.7 50 ± 1.0

1,2,4-Trichlorobenzene (36 ng Cl-)** 5

3 ± 1.2 18 ± 3.2 26 ± 1. 7

* **Deviation from specified temperatures ±10%. Amount equivalent to complete conversion of organic halogen compound to titratable halides.

1,2,4-Tribromobenzene (175 ng Br-)** 5

10 ± 2.5 40 ± 2.9 47 ± 4.4

215 is carried out in terms of coulombs required for the titration divided by the Faraday constant, i.e., 96 500 coulombs/equivalent 13 . Hence the amount of titratable material in an eluted peak is given by (10.8) where I is the titrant generator current in amperes and t the time in seconds 17 . The results are dependent, of course, on the level of conversion in the combustion tube. The conversion of sulphur compounds into sulphur dioxide depends on the temperature 2 ,4,9 (Table 10.1). 'High temperature favours the formation of sulphur dioxide rather than sulphur trioxide. It is not necessary for the conversion to sulphur dioxide to be quantitative, it is only necessary that the 50 2/50 3 ratio be kept constant 9 . The pyrolysis efficiency of halogen compounds changes with temperature, reactant gas and the type of compound 4 ,18 (Table 10.2) and gas flow-rate 4. For CHC1 3 and C1CH 2CH 2Cl with oxygen as the pyrolysis gas, the recovery increases with increasing pyrolysis tube temperature. The opposite effect occurs for CHBr 3 and CH 2Br 2 • When carbon dioxide is used as the pyrolysis gas the temperature dependence is stronger for both types of compounds 18 . For both chlorinated and brominated benzenes the recoveries increase with increasing pyrolysis tube temperature; the recovery of bromobenzenes is better (Table 10.3). The microcoulometric detector suffers from a number of disadvantages. The CD system is relatively complex and difficult to operate when attached to a gas chromatograph. The peaks are broad and tailing and the time constant is higher than with other detectors2,3,21 ,32. The catalytic properties of the catalyst change with time, as the catalyst becomes poisoned by condensed aromatics and sulphur compounds 28 • REFERENCES

2 3 4 5 6 7 8 9 10 11 12

D.M. Coulson, L.A. Cavanagh, J.E. DeVries and B. Walther, Agr. Food Chern., 8 (1960) 399. R.L. Martin and J.A. Grant, Anal. Chern., 37 (1965) 644. H.V. Drushel, Anal. Chern., 41 (1969) 569. L. Giuffrida and N.F. Ives, J. Ass. Offie. Anal. Chern., 52 (1969) 541. 5.1. Kricmar and V.E. Stepanenko, Zh. Anal. Khim., 24 (1969) 1874. L.D. Wallace, D.W. Kohlenberger, R.J. Joyce, R.T. Moore, M.E. Riddle and J.A. McNulty, Anal. Chem., 42 (1970) 387. V.E. Stepanenko and 5.1. Kricmar, Zh. Anal. Khim., 26 (1971) 147. D.M. Coulson, J. Forensie Sei., 17 (1972) 678. P.J. Klass, Anal. Chern., 33 (1961) 1851. D.F. Adams and R.K. Koppe, J. Air Pollut. Control. Ass., 17 (1967) 161. R.J. Robertus and M.J. Schaer, Environ. Sei. Teehnol., 7 (1973) 849. D.M. Coulson and L.A. Cavanagh, Anal. Chern., 32 (1960) 1245.

216 13 D.M. Coulson and L.A. Cavanagh, Theory and Equipment for Microcoulometric Gas Chromatography, presented at 140 Meeting of the American Chemical Society, Division of Analytical Chemistry, Chicago, IL, September 1961.

14 J. Burke and W. Holswade, J. Ass. Offic. Anal. Chem., 47 (1964) 845. 15 H.P. Burchfield and R.J. Wheeler, J. Ass. Offic. Anal. Chem., 49 (1966) 651. 16 H.P. Burchfield, J.W. Rhoades and R.J. Wheeler, in L.R. Mattick and H.A. Szymanski, Lectures on Gas Chromatography 1964, Plenum Press, New York, 1965, p. 59. 17 D.M. Coulson, Nitrogen, Halide, Sulfur and Carbon Detection by Electrochemical Methods, presented at Eastern Analytical Symposium, New York, November 1968. 18 J.A. Sweetman and E.A. Boettner, J. Chromatogr., 212 (1981) 115. 19 E.E. Storrs and H.P. Burchfield, Contrib. Boyce Thompson Inst., 21 (1962) 423. 20 D.F. Adams, G.A. Jensen, J.P. Steadman, R.K. Koppe and T.J. Robertson, Anal. Chem., 38 (1966) 1094. 21 J. Sevcfk, Chromatographia, 4 (1971) 102. 22 H.P. Burchfield, J.W. Rhoades and R.J. Wheeler, Agr. Food Chem., 13 (1965) 511. 23 H.P. Burchfield, D.E. Johnson, J.W. Rhoades and R.J. Wheeler, J. Gas Chromatogr., 3 (1965) 28. 24 A. Liberti, Anal. Chim. Acta, 17 (1957) 247. 25 E.M. Fredericks and G.A. Harlow, Anal. Chem., 36 (1964) 263. 26 V.T. Brand and D.A. Keyworth, Anal. Chem., 37 (1965) 1424. 27 H. ter Meulen, Recl. Trav. Chim. Pays-Bas, 43 (1924) 1248. 28 R.L. Martin, Anal. Chem., 38 (1966) 1209. 29 D.K. Albert, Anal. Chem., 39 (1967) 1113. 30 R.F. Cook, R.P. Stanovick and C.C. Cassil, Agr. Food Chem., 17 (1969) 277. 31 C.C. Cassil, R.P. Stanovick and R.F. Cook, Residue Rev., 26 (1969) 63. 32 R.C. Hall, CRC Rev. Anal. Chem., December (1978) 323.

217

ChapteY' 11

ELECTRON-CAPTURE DETECTOR CONTENTS 11.1. 11.2. 11.3. 11.4.

Introduction............. Design... . . . . . • . . . . . . • Sources of primary electrons. . . . . Methods of measuri ng detector current 11.4.1. Direct-current mode •. . . . . • 11.4.2. Pulse mode with constant frequency 11.4.3. Pulse mode with constant current 11.4.4. Other modes .• . . . . . . 11.5. Response theory ••• • • • • . • . 11.5.1. Recombination theory. . . . 11.5.2. Positive space-charge model 11.5.3. Negative space-charge theory 11 .6. Response.............. 11.6.1. Effect of compound structure. 11.6.2. Effect of detector temperature •• 11.6.3. Derivatization for electron-capture detection 11.6.4. Effect of impurities. . . . . • . . . . 11.7. Linearity of response • . . . . . . . • . . . . 11.8. Selective electron-capture sensitization. . . . 11.8.1. Nitrous oxide doping of the carrier gas 11.8.2. Oxygen doping of the carrier gas. . . . 11.8.3. Sensitization of aromatic hydrocarbons. 11.9. Coulometric and hypercoulometric response . . . . . . • • 11.10. Use of the electron-capture detector with capillary columns References . . . . • • . • • . • . • • . . . . . . • • • .

217 218 219 224 224 225 227 229 230 230 233 234 235 235 239 243 248 249 251 252 257 263 263 266 269

11.1. INTRODUCTION The electron-capture detector (ECD) is the oldest of the selective detectors. Owing to its high sensitivity, which is the highest of all gas chromatographic detectors, it is highly attractive for chromatographers. The number of original papers dealing with this detector is very large, and it is included in all reviews on detectors l - 8 . An excellent book 9 is devoted to particular aspects of the ECD. For this reason, it is difficult to treat the topic of ECDs in an exhaustive manner without repeating material that is already well known.

218 11.2. DESIGN The basic arrangement of the ECD consists of an ionization chamber containing a source of particles (generally a radioactive source) and two polarized electrodes. By applying a potential difference to the ECD electrodes it is possible to collect the thermal electrons. The following detector types can be distinguished according to the position and shape of the electrodes. The parallel-plate detector 10 - 13 (Fig. 11.1A) has a very simple geometry and its design enables the relative configuration of the radioactive foil and the collector electrode to be changed. The volume of this detector is large, particularly if a 63Ni source is used, the specific activity of which is lower as than that of tritium. Therefore, a large foil area is required with 63Ni . This foil is more easily accommodated in the coaxial design 15 ,16, (Fig. 11.1B) where the anode is positioned inside the cylinder

B

A 63 Ni FOIL

c

PTFE

L . . - _...... Q

Fig. 11.1. Basic designs of the electron-capture detector. A, Parallel-plate detector. 1 = Carrier gas inlet and anode; 2 = diffuser; 3 = source of ionizing radiation; 4 = carrier gas outlet and cathode. B, coaxial detector. C, asymmetric (pin-cup) detector. a = Anode; c = cathode; s = source. (From ref. 14.)

219 INSULA

DETECTOR TOWER CAP

FOILCYLINDER DETECTOR ASSEMBLY COLLECTOR CYLINDER

DETECTOR - - - TOWER

'--NA,RRC)W CLIP

~

SWAGE FERRULE

Fig. 11.2. Asymmetric (displaced coaxial cylinder) electron-capture detector. (From ref. 14.) formed by the source. The overall detector volume is 2-4 ml with the higher energy and high-temperature 63Ni source. In the asymmetric configuration 17 - 22 (Figs. 11.1C and 11.2), the cylindrical cathode, which may serve as the detector body, is separated by a glass, ceramic or PTFE insulator from a small anode. If the system of electrodes is arranged so as to maximize their spacing, the applied electric field is longitudinally asymmetric and minimizes the effect of the positive space charge by concentrating the field in the vicinity of the anode, while making the field near the cathode less intense 14 . In the displaced coaxial cylinder design, the cell geometry makes direct collisions of particles with the anode unlikely. Smaller diameters are possible, provided the collisions of particles with the radioactive source itself are minimized 23 . A cell with a total volume of 0.3 ml has been described 22 • 11. 3. SOURCES OF

PRII~ARY

ELECTRONS

As a rule, the ECD utilizes a radioactive emitter, generally in the form of a metal foil, as a source of primary ionizing particles. An a-emitter, which would produce 10 5 ion pairs per 1 cm of travel, would generate high detector noise. The ion-pair yield per 1 cm through a gas from a very high-activity y-emitter is extremely low and meets the requirements for an ideal source. However, the health hazard inherent in such an emitter excludes it from use.

N N

o

TABLE 11. 1 SOME PROPERTIES OF ELECTRON SOURCES FOR THE ECD From ref. 23. Properties

Source 63 Ni foil

B-Particle energy (keV) B-Particle range (mm) Maximum activity (mCi/cm2 ) Upper temperature limit (OC) Maximum current (pA) Rate of electron production, Rv (sec- 1 ) Noise level* (pA)

66 -10 10 350 9

(15-mCi source) 6.10 10 1.5

*Measured at ambient pressure in nitrogen at 21 0 C.

3H titanium foil 18 -2.5 170 220 30 (500-mCi source) 2.10 11 3

3H in scandium foil 18 -2.5

55 Fe on nickel alloy foil 5.387-5.640 -0.5 3

325

400 0.5 (5-mCi source) 3.10 9 0.1

221 The best compromise was found in isotopes that emit low-energy S-particles (minimum number of ion pairs per disintegration) at relatively high specific activities (maximum total ion pair formation)16. The choice of an irradiation source is governed 3 by (1) the emanation rate of the radioactive material at elevated temperatures, (2) the energy of the radioactive particle, (3) the availability of adequate specific activity, (4) the radiochemical form and (5) the costs. 3H and 63Ni are the most frequently used materials. Tritium is usually preferred owing to its lower energy S-radiation (18 keV for 3H, 67 keV for 63 Ni )24,25 and the fact that foils of higher specific activities which provide a denser radiation (9800 and 65 Ci/g for 3H and 63Ni , respectively)26 can be manufactured. The disadvantage of tritium sources is their low operational stability owing to a loss of activity at elevated temperatures. The temperature limit for tritium is 200-225 0 C in the case of a titanium- 3H foi1 24 ,27,28 and 300-325 0 C for tritium embedded in a rare earth, e.g., scandium 16 ,20. The maximum temperature that can be used with 63Ni is as high as 400 0 C15 , however. The temperature limits quoted for the tritium source are valid for nitrogen, helium, argon and argon plus 5% methane carrier gases. When hydrogen is used, the tritium emanation rate is as much as ten times higher owing to the exchange between hydrogen gas and bound tritium 20 ,28. The use of hydrogen as the carrier gas is not recommended, therefore, particularly at higher detector temperatures. The application of high temperatures in the detector has two aspects: (1) it decreases the possible contamination of the source (for this reason 63 Ni sources are preferred in practice) and (2) the detector sensitivity increases or decreases depending on the nature of the capturing process (see section 11.6.2). 147 pm29 and 99 Tc 24 have also been used as ionization sources. The properties of the ECD with promethium are similar to those of the nickel detector, but the 147 pm foil is much less affected by the nature of the sample 30 . In comparison with other emitters, technetium shows 24 a disadvantageous signal-to-noise ratio. Hence the use of radioactive sources has several disadvantages 31 : (1) the radioactive source can be contaminated by column bleed or by compounds of low volatility eluted from the GC column, (2) the radioactive sources have an upper temperature limit related to the thermal stability of the foil, (3) the radioactive metal foil appears to react with the electron-capturing species, as evidenced by the discoloration of the surface of the foil after continued use and (4) the disadvantages arising directly from the application of the radioactive emitter proper. The Auger electron emitter with 55Fe has been employed 32 as a source that gives low noise with an operating current lower that that with S-emitters. In

222 this case, the electrons are not produced by decay, but formed in extra-nuclear readjustments that follow radioactive decay by orbital electron capture. Some properties of the above-mentioned electron sources are listed in Table 11.1. As a non-radioactive electron source for the ECD, a thermionic emitter 33 with a barium zirconate cathode protected by a guard gas has been decribed (Fig. 11.3). A directly heated thermionic cathode supplies electrons, which are attracted towards a mesh-like anode. The electrons diffuse through the anode and are attracted with a small potential towards a collector. The column effluent flows in the outer cylinder and the guard gas in the inner cylinder. The guard gas prevents excessive penetration of the solvent into the filament chamber. The reaction chamber proper consi sts of the .annul us between both cyl i nders. Thi s new mode of operation is based on the phenomenon of space charge amplification. The detection limit for lindane is 3.2.10- 16 g. An ECD based on photoionization was described by Wentworth et al. 31. The lamp was a laboratory-made device exploiting the Lyman a-resonance line of hydrogen (10.2 eV) and was provided with a lithium fluoride window. In the photoelectron-capture detector, UV lamps cannot bring about the ionization of

EXHAUST

HEATER WE

L..---+--FILAMENT K---f---CIJLLECTOR GRID '-~--STRUCTURE

FLOW "t---·STRAIGHTENER

Fig. 11.3. Schematic diagram of a non-radiative (thermionic) electron-capture detector. (From ref. 33.)

223 the carrier gas like that with an ECD having a radioactive source, because the ionization potential (IP) of these gases exceeds the energy from the lamp. For this reason, a compound with an IP lower than the photon energy and at higher concentration than any electron-capturing species should be added to the carrier gas ahead of the detector. After the production of primary electrons ( 11.1)

the remaining electron attachment and neutralization reaction are similar to those in the radioactive ECD (section 11.4). In the d.c. mode with triethylamine (IP 7.50 eV), the detection limit for carbon tetrachloride is 50 pg (10-150 times less than that with a tritium ECD in the pulsed mode). Fig. 11.4 shows a schematic diagram of a detector 34 using a commercial UV lamp (HNU Systems) as the ionization source. The detector cavity contains five openings, two of which serve as the electrode ports, two as the inlets and the last as the outlet. When operating in the ECD mode (the detector can also work as a photoionization detector (PID); cf., section 6.1), the column effluent is brought to the bottom of the detector through inlet 1; easily ionizable substances (naphthalene or tri-n-propylamine) are introduced with a nitrogen stream through the top inlet in close proximity to the magnesium fluoride window of the UV lamp. The operation in the PID mode requires a reversal of the inlets. The bottom electrode was polarized with a d.c. power supply (+20 V) and acted

exit

-~~ tL--_ _ inlet 1 BN Detector Cell

Top View

·,: :· · D

Linlet2

o·fI'~"""'''''~

~,,::::::::::..: Linlet1

~F==== Top electrode

0--Bottom electrode

Stainless-Steel Cell Housing

Fig. 11.4. Schematic diagram of combined photoionization-electron capture detector. (From ref. 34.) BN = boron nitride.

224

as the anode. The detection limit is 1 pg for lindane and the linear dynamic range is similar to that of the ECD in the d.c. or constant-frequency pulse modes. 11.4. METHODS OF MEASURING DETECTOR CURRENT The electron concentration in the ECD can be measured continuously by applying a d.c. voltage or intermittently by applying pulses of short width, long period and sufficient amplitude to collect all the electrons available. 11.4.1. Direat-aurrent mode

In the d.c. mode 35 , a constant potential is applied to the detector electrodes and the detector is exposed to this potential throughout the operation. The detector current increases with increasing potential applied to the ECD. At a certain voltage, the saturation plateau is attained at which all the electrons produced are collected (Fig. 11.5). The presence of an electron-capturing compound in the detector reduces the concentration of free electrons, thus inducing a drop of the standing current at all potentials until the field becomes strong enough to collect both the electrons and negative ions simutaneously, and no effect due to electron capture is noted subsequently. The voltage range for the ECD operation extends up to the onset of the saturation plateau region. The optimum applied voltage occurs at the knee of the currentvoltage curve, approximately at 85% of the detector saturation current 6 ,36,37. The exact optimum-voltage value is influenced by a number of factors, e.g., the flow-rate of the make-up gas 38 or the pulse width 39 in the pulse method. The

IZ

LU

0:: 0::

:::>

u

z

Q

APPLIED POTENTIAL

Fig. 11.5. Relationship between current and applied potential in a d.c. ECD. A, Pure carrier gas; B, carrier gas containing a trace of a strongly electroncapturing compound. (From ref. 14.)

225 maximur,l sensitivity of the ECD in the d.c. mode can be observed at low applied voltages. However, owing to space-charge effects, contact-potential effects and non-electron-capture ionization processes 12 , the detector may behave anomalously in this region. Space charge is produced owing to differences in the mobilities of positive ions and electrons in the applied field 12 ,40. A slowly moving positive-ion drift to the cathode generates a cloud of positive ions in the vicinity of the cathode, the potential of this cloud being inverse to the potential applied. Changes in electron concentration occur. The secondary electrons produced by the collisions of the B-particles with the carrier gas molecules do not have enough time to attain thermal equilibrium and are rapidly collected at the anode. This reduces their life times in the detector and makes ther,l inaccessible to reaction with the solute. The negative molecular ions formed by electron capture may also be collected at the anode, thus producing an erroneous value for the detector current. Detectors with asymmetric geometry are less influenced by space-charge phenomena. The eluted solute can be adsorbed on the electrode surface, which can result in the generation of a contact potential that is either complementary or inverse to the potential applied 12 ,40. At a value of several volts, the contact potential may cause errors in the electron attachement process. If the potential is inverse, the chromatographic peak area is anomalously large and this peak often tails. If the contact potential is complementary, the response decreases and a negative deflection at the tailing edge of the peak occurs. A solute that generates a potential on the electrode surface, even if it does not absorb electrons itself, may also create false responses. The use of high detector temperatures and higher voltages (the contact potential being low in comparison with the applied voltage) reduces the problems resulting from the generation of contact potentials. 11.4.2. Putse mode with eonstant frequeney I~any of the prob 1er,lS encountered wi th the d. c. ECD can be overcome by us i ng the pulse mode 41 ,42. When a voltage pulse is applied, the electron concentration drops to zero owing to the collection of all electrons at the anode (Fig. 11.6). After each pulse, the electron concentration is restored, attaining a constant value as a result of the irradiation of the gas. Hence the detector is voltage free for a larger proportion of the working period, leaving enough time for the concentration of the thermal electrons to be replenished by the ionizing B-radiation and to attain thermal equilibrium. The amplitude and width of the pulses should be of adequate size to provide for the complete collection

226

I-

Z

UJ

a:

Q::

;:)

U

a: a

I-

u

UJ

IUJ

a

Vl

~

a >

!

o

100

!

200

TIME (MICROSECONDS)

Fig. 11.6. Effect of a pulsed voltage supply on electron concentration in an ECD. w = pulse width in ~sec. (From ref. 3.) of electrons (Fig. 11.7A and B), i.e., for the withdrawal of the standing current from the detector. However, they should not be too large to cancel the advantages of field-free operation. The relationship between the electron concentration and the pulse period, t p' is shown in Fig. 11.7C. The optimum tp value exceeds 1000 ~sec. In view of the low detector currents, lower values are frequently preferred in practice. As a rule, the sensitivity of the detector increases with increasing intervals between the pulses, because enough time is allowed for the recombination of positive and negative ions. A limit is set by the increase in the recombination of positive ions and electrons 43 . During voltage-free time periods, the electrons do not drift out of the plasma. Negative ions are generated in the region where positive ions are also present simultaneously, and the recombination of these ions is more effective for this reason. The duration of the brief pulse is insufficient for collecting the

227 6

A

4· 2 0

OS

1.5 2.0 10 PULSE WIDTH ( tw) IJ SEC

2.5

6 Z

5 4

>a:

3

Vl

I-

B

:J

« a:

I-

2

m a:

«

IC])

z

0

10 20 30 40 50 60 70 80 90 100 PULSE AMPLITUDE ( VA) VOLTS

100

C

80 60 40

o

1000 2000 3000 PULSE PERIOD (t p ) IJ SEC

Fig. 11.7. Dependence of electron concentration (N e ) on (A) pulse width, (B) pulse amplitude and (C) pulse period for a pulse sampled electron-capture detector. (Reprinted with permission from ref. 15.) negative molecular ions. In comparison with the d.c. mode, the sensitivity is higher with the constant-frequency pulse mode, and the noise is approximately the same. 11.4.3. FUZee mode with aonetant aurrent

At present, the most frequently used ECD mode is the constant-current ECD 11 ,44, i.e., by changing the pulse frequency the detector current is kept constant during the run. The base pulse frequency, fo' is low in the presence of carrier gas alone. When an electronegative species enters the ECD, some of the electrons are removed by the electron-capture processes to form negative

228

ECD CELL RADiO-liONIZED ACTIVE GAS FOIL

ELECTRON COLLECTOR

II I

I

NEGATIVE PULSE

L..J VOLTAGE

VARIABLE FREQUENCY PULSER

I

f--

SIGNAL 0 UT

-I D (I s - IDl

ELECTROMETER

Is

REFERENCE CURRENT

Fig. 11.8. Schematic diagram of the electronic components of a constant-current pulse electron-capture detector. (From ref. 22.) ions. The current drop is matched by an increase in frequency in order to keep the cell current constant. The output signal is represented by the frequency difference fA - fa, where fA is the frequency corresponding to the sample concentration A. The change in pulse frequency is a measure of the concentration of the electron-capturing solute passing through the detector. A schematic diagram of the pulse-modulated constant-current ECO is given in Fig. 11.8. The circuit forms a closed-loop electronic feedback network in which an external reference current, Is' is compared with the cell current, 1 0 , so as to maintain the relation 10 - Is = O. The base frequency, fa, giving the optimum limit of detection for a particular solute in this mode is identical with the frequency of operation that gives the optimum limit in the constantfrequency mode. The optimum fa varies from solute to solute depending on its electron-capturing ability in both pulse mOdes 23 . The main advantage of this mode is claimed to be a larger range of linearity of the dependence of the function fA - fa on concentration, A. The difference in the principles of the two pulse modes is obvious from Fig. 11.9. The vertical line AS represents the change in detector current with changing solute concentration at a constant pulse frequency. The horizontal line CO represents the change in applied pulse frequency with changing sample concentration at a constant detector current.

229

d.t.

__~~===="--~1~~ 10. 0(

~ 5!

-... !z i

it

1.0

PURE H2 FREQ= (PI..lSE WlDTHrl

0.1

1.0

10.

100.

1000.

FREQIENCY (11Hz)

Fig. 11.9. Current versus frequency operating curves illustrating operation in a constant-current pulse mode and in a d.c. mode (or constant-frequency pulse mode). (From ref. 22.) 11.4.4. Other modes

By changing the applied voltage, constant-current operation is also possible in the d.c. mode 45 ,46. The current drop due to electron capture is set to the initial value by increasing the voltage. Fig. 10.10 shows a schematic diagram of a d.c. constant-current system. The output current of the ECD is proportionally converted to voltage and the latter is subtracted from a reference voltage. The voltage difference is then amplified and fed back to the ECD. The monitored detector output represents the variable polarizing voltage that is applied to the ECD in order to maintain the current at a set level. Also in this mode the linear dynamic range surpasses that of the d.c. mode with constant voltage or that of the constant-frequency pulse mode and covers about four orders of magnitude 46 . An ECD with an a.c. input has also been described 47 , its sensitivity being similar to that of the d.c. and pulse modes.

2~

ECD

RECORDER

Fig. 11.10. Schematic diagram of d.c. constant-current system. I/V = I/V converter, KEPCO = KEPCO OPS operational power, AMP = amplifier. (From ref. 46.) 11.5. RESPONSE THEORY 11.5.1. Recombination theory

The standing current in the electron-capture detector arises from the production of secondary electrons through non-elastic and elastic collisions between primary electrons (a-particles) and molecules of the carrier gas (nitrogen, argon or helium). A plasma of positive ions (p+), radicals (R) and thermal electrons homogeneous through most of the detector cell is generated: B + P + p+ + e- + a* + energy

(11.2)

A direct process of ionization of the carrier gas is most probable with nitrogen. Metastable atoms 3 may be formed with argon: a + Ar

+

Ar+ + e- + a* + energy

(11.3) (11.4)

Ar*

+

X + Ar + X

(11.5)

231 8* represents 8-particles with reduced energy as a result of thermal electron production and X is a polyatomic quencher (usually methane). The 8-particles lose their energies during their collisions with argon and the quench gas until their energy becomes lower than that necessary for the generation of ion pairs. Each 8-particle may generate 10 2-10 3 thermal electrons before its kinetic energy is reduced to the thermal level, as about 30 eV are expended for the generation of an electron-positive ion pair. The rate of production of thermal electrons is assumed to be constant, being neither increased nor decreased by the presence of capturing species 43 . Pure argon or helium is not suitable for the attachn~nt of electrons to solute molecules, because the molecules of these gases are readily converted into metastable forms that would produce considerable ionization of the solute molecules 48 . p+ represents any of the positive ions in the plasma, e.g., Ar+, ArH+, ArCH+, ArCH;, ArCH;, ArCH:, CH:, CH;, CH; and R is any radical, e.g., H·, CH 3, :CH 243 ,49 The thermalization (cooling) of electrons coming from high-energy 8-particles is essential in order to allow or enhance the capturing process while minimizing solute ionization. The thermalization of fast electrons is brought about by a polyatomic gas in 5-10% concentrations. Hence, the addition of a quenching gas serves two purposes: (1) it reduces and maintains the electron energy at a constant thermal level and (2) it removes, by deactivating cOllisions 43 , metastable argon species as fast as they are formed. Each electron with an energy of 10 keV is cooled to 10% above the thermal energies (2.10- 2-5.10- 2 eV) in 0.076 ~sec43. Under these conditions, the detector can work neither as an argon ionization detector nor as an electron mobility detector. If an electron-capturing solute enters the ECD, the thermal electrons are captured giving negative molecular ions (non-dissociative reaction 11.6) or fragment ions (dissociative mode, eqn. 11.7). This can be described by the following set of reactions 43 ,49-52 e-

+ AB

( 11.6)

AB

-+

(11.7) ( 11.8) AB-

-+

A- + B·

e-

T

PT

PT

T

AB -

-+

-+

neutra 1s neutrals

(11.9) (11.10) ( 11.11) ( 11.12)

232

Fig. 11.11. Potential energy diagrams for four electron-capture mechanisms. EA = Electron affinity, E* = activation energy, ~E = overall change in the internal energy for the process. (Reprinted from ref. 50 with permission.)

The attachment of an electron to a solute molecule is related to the electron affinity and to the requirement for sufficient energy to cause attachment at a given temperature. The larger the activation energy necessary for attachment, the slower is the attachment reaction. The activated complex AB* represents an intermediate form during which the electron is being accommodated by the solute molecule. In order to produce this activated complex, AB has to absorb an energy Ea' Fig. 11.11 shows potential energy diagrams for four capture mechanisms 50 Mechanism I depicts a non-dissociative electron attachment. The overall change in the internal energy for the process, ~, equals the electron affinity. Aromatic hydrocarbons and carbonyl moieties serve as examples of compounds reacting in this way. Mechanism.II depicts a single bimolecular electronattachment step, leading immediately to dissociation via a dissociative potential

233 energy curve (alkyl halogens except C_F)ll. In mechanism III, Ea is greater than 6£ (aromatic halogens Cl, Br, I). This is a two-step dissociative process via a dissociative potential energy curve. The two-step dissociative process that first involves the formation of a molecular negative ion followed by dissociation by means of the same potential energy curve represents mechanism IV. Ea equals 6£. Mechanisms I and III involve a negative ion curve with a large dissociation energy, but with mechanism III this curve is crossed by a dissociative curve. In mechanism II the negative ion curve has a small or zero dissociation energy, while the dissociation energy of the negative ion is thermally accessible for mechanism IV. The kinetics of the electron-capturing process is more complex than described here in a simplified way; detailed studies can be found elsewhere 23 ,43,49-58. The negative ions formed have a higher mass than the original electrons. Hence, they display a lower drift velocity and a substantially higher rate of recombination with positive ions 42 ,43. The conditions for operating the detector are optimized by arranging that thermal electrons rather than negative ions be collected. The detector response is then given by the background current drop due to the loss of thermal electrons by attachment to solute molecules. 11.5.2. Positive space-charge modeZ

Wentworth's classical mode1 43 ,49,50 assumes the [CD to be a well mixed reactor, where a single concentration expression describes the presence of each species throughout the cell. The model described by Grimsrud et al. 59 for the pulse mode differs from the previous model in that it considers the electrostatic forces between the charged particles as the dominant force 60 in determining the concentrations and locations of the charged particles within the [CD. Thermal electrons are not evenly distributed throughout the [CD volume at all tir,~s, but are concentrated in a local zone where charge neutrality exists (plasma). The size of this plasma increases with time after the end of a pulse. The plasma is separated form the cell boundaries by positive ions, forming a sheath. This sheath decreases in size with time after the end of a pulse. All thermal electrons are removed from the cell by the anode during the application of each pulse, which results in a momentary excess of positive ions. The posi~ tive charge created in the cell by electron removal tends to dissipate itself by space-charge-driven migration to all grounded surfaces of the cell during the periods between pulses. A fraction of these positive ions strikes the anode and causes a reduction in the time-average negative current, Ie' indicated by the electrometer 59 ,61. Hence the [CD current measured need not necessarily be

234 dttributed to the collection of electrons alone. The observed current I is given by I = (1-o)Ie , where 0 is the fraction of the excess positive ions that migrate to the anode 61 • The magnitude of 0 depends on the cell geometry: it is 0.25 for the pin-cup detector61 and 0.01 for the displaced coaxial geometry62. 11.5.3. Negative speae-ahapge theopy

In the classical theory, the neutralization of electrons (response generation) occurs via the intermediary negative ions being neutralized. In an alternative (and/or complementary) theory63 of the response in the d.c. mode, the recombination of electrons and positive ions in the ionization region is increased owing to the migration of negative ions outside this region. The centre of ionization is very close to the foil (as close as 1 mm63 ,64 for 63Ni and 0.2 mm for 3H, depending on the detector shape). Electrons (and negative ions) migrate over a relatively long distance from this region, setting up an opposing or counter field. Owing to a space charge, the field gradient in the ionization region decreases and, consequently, the electron-positive ion reco,mbi nation increases. As electrons mi grate through all the vol ume, negati ve ions can be formed in regions where no .positive ions are available for neutralization. Neutralization of these negative ions can occur only by contact with the counter electrode or with any other conducting surface63 • Maximization of the counter field by having the centre of ionization .situated as close as possible to the cathode and as far away as possible from the anode (either by moving the anode farther away from the radioactive foil or by increasing the pressure in the detector cell) aids in maximizing the response. The response increases with increasing electrode distance 65 ,66 (the pulse response follows approximately the same trend 66 as the d.c. response; see Fig. 11.12) and also with increasing pressure 65 ,67. The extent of electron capture, however, remains constant66 • The same applies for the increasing pulse interval in the pulse' mode 66 . According to this mechanism, such an ECD should be able to function, even if it were not possible for the negative ions to be neutralized by positive ions. Such a situation may occur if the anions are generated far away from the cations and if the two species are kept apart 68 (Fig. 11.13). In the "separated" mode, the column effluent enters and leaves the anode chamber. The cathode chamber is flushed with pure nitrogen only. Hence, the cations should be located only in the cathode chamber and the anions only in the anode chamber. Slow anions set up a sizable counter field. The cations and electrons present in the ionization zone of the cathode chamber are slowed down. The

235 'A_ _ ~_l'00 ",,"'''''

'I. OF MAXIMUM CURRENT

" ~

"'" .....MU" RE5"'",:/// ty/

/

20 d. c. RESPONSE

PULSE RESPONSE

~

I

/'-0'

.......

/

!

i

Ii

o o

4 ELECTRODE OISTANCE

8

12

(mm)

Fig. 11.12. Variation of response with inter-electrode distance for d.c. and pulse conditions. Scandium tritide. (From ref. 65). second-order recombination rate increases and less electrons (and cations) reach the electrodes: the d.c. system produces current drops typical of the ECD 68 ,69; the same detector operated in the pulse mode 70 also produces a response. 11.6. RESPONSE 11.6.1. Effect of compound structure

The ECD response depends on the substance-specific term K, the electroncapture (also electron-absorption) coefficient. This represents the degree to which the compound is able to capture thermal electrons. The probability of electron capture by different types of molecules spreads over a range of 10 6 depending on the presence of so-called electrophores (some atoms, groups and structures) in the molecule 3 ,42,53,71,72 (see Table 11.2).

ptwires

ANODE CHAMBER

....

steel

nickel cathode

purgegas plus .., column effluent in'conventional mode'

63 N i foil

borosilicate space. reducer

t

col umn effluent in 'separated mOde'

Fig. 11.13. Schematic diagram of the ECD with separated ionization and capture regions. (From ref. 68.)

237 TABLE 11. 2 RELATI VE ATTACHI4ENT COEFFICI ENTS K' FOR VARIOUS COMPOUNDS 41 ,42,73-75 From ref. 3. Chemical class Alkanes, alkenes, alkynes, aliphatic ethers, esters and dienes

K'* scale

0.01

Selected example Hexane Benzene Cholesterol Benzyl alcohol

1

Naphthalene

0.10

Aliphatic alcohols, ketones, aldehydes, amones, nitriles, monofluoro and monochloro compounds ~

]:

Vinyl chloride Ethyl acetoacetate Chlorobenzene

1.0 Enols, oxalate esters, monobromo, dichloro and hexafluoro compounds

1

cis-Stilbene trans-Stilbene Azobenzene Acetophenone

}

Allyl chloride Benzaldehyde Tetraethyllead Benzyl chloride

10.0

Trichloro compounds, chlorohydrates, acyl chlorides, anhydrides, barbiturates, tha 1i domi de and a1kyll eads

Azulene

300 Monoiodo, dibromo and trichloro compounds, mononitro compounds, lachrymators, cinnamaldehyde, fungistatic compounds and resticides

J

Cinnamaldehyde Nitrobenzene Carbon disulphide 1,4-Androstadiene-3,11,17-triene Chloroform

J

Dinitrobenzene Diiodobenzene Dimethyl fumarate Tetrachloromethane

1000

1,2-Diketones, fumarate esters, pyruvate esters, quinones, diiodo, tribromo, polychloro, dinitro compounds and organomercurials

10 000

*Values for K' are relative to chlorobenzene, which is arbitrarily given a value of 1.0.

238 The highest response is usually found with electronegative compounds containing halogens or nitro groups, with organometallic compounds 76 - 78 and compounds characterized by the presence of two or more weakly electron-capturing groups connected by some specific bridge promoting a synergic interaction between the two groups79 (conjugated carbonyl compounds, some polycyclic aromatic hydrocarbons and certain steroids). The sensitivity of the ECD depends strongly on the structure of the compounds 42 ,52. With halogen compounds the response decreases in the sequence 41 I>Br>Cl>F. The sensitivity of detection is affected by the position of the electrophores and their number in the molecule of the compound 3 ,39,41,42,53, 72-75,80-84. The sensitivity increases synergistically with multiple substitution on the same carbon atom (Tables 11.3 and 11.4). TABLE 11.3 RELATIONSHIP BETWEEN MOLECULAR STRUCTURE AND RELATIVE CAPTURE COEFFICIENT 41 ,42,73-75 From ref. 3. Parameter

K'*

Halogen series I

Br Cl F

Substitution on carbon atom Tertiary Secondary Primary Frequency on carbon atom TetraTriDiMonoPositional isomer (di-, tri-, etc.) Alphahb-

DeltaGeometrical isomer

10 2 1

10 5

1

Trans-

4

Cis-

1

*Capture coefficients are relative to the lowest value of the series, which is arbitrarily given a value of 1.0.

239 TABLE 11.4 EFFECT OF THE POSITION AND THE MULTIPLE SUBSTITUTION OF ELECTROPHORES ON ELECTRON ABSORPTION Reprinted with permission from ref. 42. Electrophore

Compound or class

Cl

Vinylic Aromatic Aliphatic Allylic Benzylic Benzene, 0Benzene, mBenzene, p-CHC1 2 Benzene, 1,2,3Benzene, 1,2,4Benzene, 1,3,5- CC1 3 Hexachlorobenzene

Absorption coefficient 0.2 1

0.3 55 110 42 30 11 1

113 75 60 500 1100

The minimum detectability and detection limit of the ECD for compounds with the highest electron-capture coefficients are the lowest of all gas chromatographic detectors. Examples for some selected compounds are as follows: 3.10- 16 mole/sec for tert.-butyliodide83 , 1.10- 13 g for lindane and aldrin 22 , 1.10- 14 g/sec for chloropyrifos 85 , 1.10- 15 g for tetrachloromethane (capillary column)86, 3.7.10- 14 mole/sec for tetraethyllead 76 • 3.1.10- 15 mole/sec for benzophenone87 , 2.2.10- 16 mole/sec for the heptafluorobutyramide derivative of S-phenylethylamine 88 and 1.3.10- 16 mole/sec for chromium(III) trifluoroacetylacetonate89 . Many other data can be found in Zlatkis and Poole's book on ECD 9. 11.6.2. Effect of detector temperature

The ECD response is highly temperature dependent. In principle, the character of this dependence is given by the reaction mechanism of electron attachment 43 ,49. Mechanism I gives a stable negutive molecular ion, the potential energy curve of which (Fig. 11.11) l~es below that of the neutral molecule. On electron attachment, this energy difference must be liberated either by radiation or through collisions with other molecules by energy exchange. A temperature increase in the detector would increase the population of higher

240 vibrational levels and, therefore, the probability of attachment would decrease. In contrast, with mechanism II, representing dissociative attachment with the simultaneous production of negative ions and radicals in a single step, a temperature increase in the detector increases the probability of attachment due to an increase in the population of excited levels. In this instance, the potential energy curve would cross that of the neutral molecule at a level corresponding to the vibrational excited state. The activation energy of this process would be the energy that is necessary to populate those states where the dissociative surve crosses. Mechanisms III and IV also describe dissociative attachment 90 • The character of the detection mechanism can be inferred43 ,49,50 from the plot of ln KT3/ 2 vs. l/T, where K is the capture coefficient and T is the absolute detector temperature. With compounds that capture electrons in a nondissociative manner, the plot shows a positive slope at higher temperatures, i.e., the response increases with decreasing detector temperature. On the other hand, a negative slope of the plot characterizes the dissociative type of electron capture; the response increases with increasing detector temperature in this instance. Idealized plots related to the four basic types of electron capture are shown in Fig. 11.14. The presence of a positive slope region for mechanism IV is evidence of the formation of a negative molecular ion intermediate. From a practical point of view, the mechanism of electron capture is deduced from the plot of ln AT3/ 2 vs. l/T, where K is replaced by the peak area A. The two plots have the same shape and their interpretations are similar.

N M

;:;

I

l-

:.: c

..J

n

I-

:.: c

r

..J

"'---lIT

1fT N M

m

;;;

l-

I-

..J

..J

:.: c

Ii

:.: c

'"

lIT

VII T

Fig. 11.14. Idealized plots of ln KT3/2 vs. l/T for the four basic mechanisms of electron capture. (From ref. 91.)

241 Clearly, the nature of the electron-capture mechanism can also be assessed from the dependence of the ECD response on the detector temperature. Generally, aromatic compounds display a non-dissociative type of electron attachment, whereas chlorine, bromine and iodine compounds show a dissociative type 15 ,43,44, 49,50,56. The constant-current mode can also be applied to establish the mechanism of electron capture 92 • It is evident from this analysis that the detector temperature substantially affects the detection sensitivity of the ECD. Hence, it follows that (1) accurate control of the detector temperature is necessary - it should vary within 44 ,SO ±0.3-0.1 0 C in order to obtain a 1% precision in the measurement of response; (2) comparisons of the responses to various compounds or various derivatives of the same compounds at the same temperature and/or without temperature indication (as well as quoting the detection limits or minimum detectability) are misleading 93 ,94, as this gives no idea of the maximum possible detection sensitivity, viz., the temperature dependences of the responses to different compounds can follow entirely different courses; Table 11.S gives an idea of the extent to which the sensitivity varies with detector temperature. This variation may be as large as up to three orders of magnitude with a temperature difference of 250 0 C93 , the temperature changes in the ~esponses to individual substances are also outlined in section 11.6.3; (3) by optimizing the detector temperature it is possible to increase the selectivity of the ECD response. Provided the responses to individual compounds display different temperature dependences, the responses to the compounds to be detected can be increased by increasing the detector temperature, thereby suppressing the response to the other compounds present. TABLE 11. 5 LIMIT OF DETECTION FOR 1 ml OF SAMPLE From ref. 93. Compound

Detection 1imit (ppb) BOoC

227 0 C

3S0 0 C

0.01 1.0 1000.0 1000.0

0.01 0.10 40.0 20.0

0.01 O.OS B.O 1.0

N

~

N

TABLE 11.6 RELATIVE RESPONSE OF THE ELECTRON-CAPTURE DETECTOR TO SOME HALOACYL DERIVATIVES Reprinted with permission from ref. 96. Derivative

Compound Amphetamine 100 Testosterone 101 Thymol 102

Diethylstilbesterol Ref. 103

Acetyl Monofluoroacetyl Monochloroacetyl Chlorodifluoroacetyl Dichloroacetyl Trichloroacetyl Tri fl uoroacety 1 Pentafluoropropionyl Heptafluorobutyryl Perfluorooctyl Pentafluorobenzoyl

Ref. 104 1.0

1.0 1.0 540 0.1 40 90 230 770

40 340 4 50 190 600

Benzylamine 105

0.007 0.3

2.6 2.1 1.7 1.3 1.0 6.9

750

2.7

1.5 15 23 21

200 5725 17875

243

11.6.3. Derivatization for electron-capture detection

The electron-capture detector is the most sensitive detector in gas chromatography as far as the compounds to which it responds are concerned. With compounds having a low ECD response, efforts have been made to increase the response so as to attain the highest ECD responses and lowest possible detection limits. Advantage is taken of the fact that the detector responds to certain electrophores. Compounds with low responses are therefore converted into electron-capturing derivatives whose responses are many times greater than those of the parent compound 90 ,95,96. Generally, the derivatizing agent contains a reactive group that provides for bonding with the substrate, and an organic chain that provides for detector sensitivity, i.e., an electrophore. Of course, this chain must meet certain chromatographic criteria, i.e., it must render sufficient volatility, thermal stability and chemical stability. The problems of derivatization proper, i.e., the methods of preparation and the choice of the reactive groups with regard to the above chromatographic requirement, are broad and exceed the scope of this section. Their detailed analysis can be found elsewhere 97 - 99 • Naturally, derivatives with the highest ECD responses are most advantageous for detection with an ECD. A comparison of the individual derivatives made from this viewpoint is mostly based on a comparison of the responses of the resulting products at the same detector temperatures. An example is given in Table 11.6. As mentioned in section 11.6.2, such a comparison can be misleading owing to different courses of the temperature dependence of the response. Halogen, nitro, trialkylsilyl, haloacyl, pentafluorophenyl and boronic groups are mostly introduced in derivatization for the ECD 96 The sensitivity of the ECD to chlorine, bromine and iodine compounds is greater than that for fluorine compounds, but the larger molar masses of the former result in less volatile derivatives, which can be undesirable in the analysis of high-molecular-weight compounds. The presence of fluorine atoms in alkyl and acyl compounds causes a slight increase in the boiling point relative to hydrocarbons with the same carbon number. This makes it possible for multiple fluorine substitution to be carried out, thus increasing the ECD response (Table 11.7). An increase in response by a factor as large as 10 4 can be attained 106 if chlorine is replaced with iodine atoms through the reaction of the chlorine compounds with sodium iodide directly ahead of the ECD. Trimethylsilyl reagents are frequently applied derivatization agents in gas chromatography, but the trimethylsilyl group shows no particular electroncapture properties. By introducing a halogen atom (Cl, Br, I) into one of the

244 TABLE 11.7 RELATIVE VOLATILITY OF A SERIES OF RR 1(CH 3)Si-CHOLESTEROL ETHERS Reprinted with permission from ref. 96. R

R1

Relative retention time

CH 3 CF 3(CH 2)2 CF 3(CF 2)2(CH 2)2 CH 2Cl C6F5 C6F5 CH 2Br C6F5 C6F5 CH 2I

CH 3 CH 3 CH 3 CH 3 CH 3 CH(CH 3)2 CH 3 CH 2Cl C(CH 3)3 CH 3

1.00 1.26 1.37 2.10 3.14 4.57 5.13 6.26 6.30 12.82

12.0 , . . - - - - - - - - - - - - - ,

In AT¥2 11.5

c

11.0

10.5

10.0

L--'----.l._~.........__'_""'____..............J

1.5 1.6

1.7

1.B

1.9

2.0 2.1

1.10 T

2.2 3

Fig. 11.15. Temperature dependence of the ECO response to some octanol derivatives. (a) Pentafluorophenyl dimethylsilyl ether; (b) pentafluorophenyl isopropylmethylsilyl ether, (c) tert.-butylpentafluorophenyl methylsilyl ether; (d) chloromethylpentafluorophenyl methylsilyl ether. (From ref. 109.)

245

30 29 28

~

27

I-

:.: 26 c

...J

25

24 1.5

20 3 1fT .10

Fig. 11.16. Plots of ln KT3/2 vs. liT for the (1) pentafluoropropionate, (2) heptafluorobutyrate, (3) chloroacetate, (4) chlorodifluoroacetate, (5) pentafluorobenzoate, (6) pentafluorophenacetate and (7) pentafluorophenoxyacetate derivatives of n-hexanol. (From ref. 91.) methyl groups or by rep 1ac i ng the methyl group wi th a pentafl uorophenyl group, the detection sensitivity is increased107-113. In Fig. 11.15 the response levels of the individual silyl derivatives of octanol and their temperature dependences are shown 109 . The haloacyl anhydrides rank among the most frequently studied reagents 91 , 100-105, 114-116. The temperature dependence of a series of haloacyl derivatives of n-hexanol is shown in Fig. 11.16; a comparison of the responses at nonoptimized temperatures of the detector is given in Table 11.6. Generally, the Illonochloroacetyl and chlorodifluoroacetyl derivatives are more sensitive than the trifluoroacetyl derivatives. An increase in the fluorocarbon chain length of fluorocarbonacyl derivatives brings about an increa?e in their ECD response without inconveniently increasing their retention times. In accordance with the above properties, heptafluorobutyryl derivatives have found most universal use. A comparison of the properties of electron-capturing boronic acids as derivatizing reagents at optimum detector temperatures is given in Table 11.8. 3,5-Bis(trifluoromethyl)benzeneboronic acid, 2,4-dichlorobenzeneboronic acid, 4-bromobenzeneboronic acid, and 4-iodobutaneboronic acid seem to be the best reagents 117 ,118. The temperature dependences are given in Fig. 11.17.

TABLE 11.8 COMPARISON OF THE VOLATILITY AND ECD SENSITIVITY OF THE ELECTRON-CAPTURING BORONIC ACIDS Reprinted with permission from ref. 96. Boronic ester

Relative vol atil ity*

3,5-Bis(trifluoromethyl)benzeneboronate Benzeneboronates 4-Iodobutaneboronates

0.3 ± 0.05 1.0 1.8 ± 0.5 3.9 ± 0.8 4.3 ± 2.0 4.7 ± 1.7 5.0 ± 1.1 6.9 ± 1.8 11.7±3.4 18.5 ± 4.6

4-Bromobe~zeneboronates

2,6-Dichlorobenzeneboronates 2,4-Dichlorobenzeneboronates 3,5-Dichlorobenzeneboronates 2,4,6-Trichlorobenzeneboronates 3-Nitrobenzeneboronates Naphthaleneboronates

Detection limit (x 10 -12 9 pinacol)

Optimal detector temperature (oC)

3.0 200.0

180 150

3.0 18.0 4.0 11.0 4.0 4.0 2550.0

350 380 380 380 380 300 350

*Based on a comparison of the retention times for a series of bifunctional compounds compared with the benzeneboronate derivative.

247 11.0

9.0'-------'-----'--t50 2n0

Fig. 11.17. Plot of 1n AT3/2 versus liT for the (A) 4-bromobenzeneboronate. (B) naphthaleneboronate, (C) 2,4-dichlorobenzeneboronate, (D) 3.5-dichlorobenzeneboronate, (E) benzeneboronate. (F) 3-nitrobenzeneboronate derivatives of pinacol. ( From ref. 117.)

32

/i

31

1 5

30

);

.2 • _______4

29

.~:

I-

I<:

5

28

"'-----6

27 26 25 1.5

I/T·10

2.0 3

Fig. 11.18. Plots of 1n KrJ/2 vs. liT for some acetyl derivatives of phenol. For the identification of the derivatives see Fig. 11.16. (From ref. 91.)

248 The response mechanism of the ECO is given not only by the properties of the derivatizing agent - the same derivatives of different compounds can manifest very different mechanisms (c.f., Figs. 11.16 and 11.18 for the pentafluoropropionate, heptafluorobutyrate and pentafluorophenacetate of n-hexanol and phenol) and, consequently, would have different optimum temperatures. The plots are similar 91 for similar types of compounds (e.g., n-hexanol, cyclohexanol or n-hexylamine, cyclohexylamine). Therefore, the detection limits depend on both the properties of the reagent and the structure of the parent compound and hence are compound dependent. 11.6.4. Effect of impurities

It has been known from the early days of the ECO that it is extremely sensitive to impurities. Impurities either contained in the carrier gas or originting from column bleeding may contaminate the surface of the radioactive source foil (see contact potential, section 11.4.1). A reduction in the standing current 22 ,26,36,39,45,119 in the d.c. mode or an increase in the base frequency in the pulse mode 22 ,120 occurs, followed by'a decrease in sensitivity. The presence of the stationary phase molecules or their decomposition products diminishes the probability of the capture reaction between the solute molecules and electrons 36 . It is advisable, therefore, that the maximum column temperature applied be lower than that with other detectors for conventional (non-crosslinked) stationary phases. This phenomenon can be suppressed by increasing the detector temperature 15 with some stationary phases, particularly non-polar ones. However, this leads to changes in the response level due to the change in the detector temperature, depending on whether the dissociative or non-dissociative type of electron attachment is involved (see section 11.6.2). The simultaneous elution of an electron-capturing and a non-capturing (alkane) solute can induce changes in the detection sensitivity in both the d.c. and the pulse modes, the extent of the changes depending on the detector shape and the carrier gas employed 121 (see Table 11.9). The presence of oxygen and water molecules has an appreciable effect on the magnitude of the standing current and, consequently, on the sensitivity of the EC0 12 ,122,123. Oxygen reduces the standing current due to electron capture, giving rise to clustered species 124 (H20)n02' This effect is greater at lower temperatures and is related to the water concentration. For this reason, thorough removal of oxygen and water from the carrier gas is recommended with the ECO. On the other hand, oxygen can increase the ECO response (see section 11.8.2) .

24')

TABLE 11.9 INTERFERENCE EFFECT (%) OF ALDRIN Reprinted with permission from ref. 121. Carrier gas

Nitrogen

Argon-methane(95:5)

Concentration of i nterferi ng n-C 20 alkane 0.01 0.1 1.0 10.0 0.01 0.1 1.0 10.0

Constant voltage

Pul sed voltage

Concentric Parallel tube plate

Concentric Parallel tube plate

3 - 13 + 36 +

+

8

+ +

15 39

+ + +

11 43 65

+

-

2

3

7 - 17 + 24

- 15 - 19 + 14

- 5 - 18 36

+

11.7. LINEARITY OF RESPONSE In the conventional operation of the ECD, Ib - Ie is measured, where Ib is the detector current in the absence of electron-capturing species and Ie is the detector current in the presence of electron-capturing species. Except for low solute concentrations over a range of about two orders of magnitude, the above difference is not linearly proportional to the solute concentration 125 ,126 Under certain conditions, the following equation for the detector response is valid in the pulse sampled mode 50 : = :'a

( 11.13)

where:. is the electron-capture coefficient and a is the concentration of the species. In order to satisfy eqn. 11.13, the detection system can be made to give an output linear with a over a range of 1.10 5 by the use of a reaponse converter 125 ,126. The pulse periods for which this equation is valid are 10ng 126 ,127, i.e., 1000-2000 ~sec, which implies a need for extremely stringent conditions of cleanliness to ensure an adequate electron concentration in the detector for satisfactory operation. Hence the disadvantage of the ECD in the d.c. and constant-frequency modes is a narrow range of the linear dependence of the response on the solute concentration.

250 In the constant-current mode of the ECO, the measured term is the change in pulse frequency44, fA - fO' where fO is the applied pulse frequency required to keep the current constant when the electron-capturing species are not present in the detector. In this instance, the linear dynamic range of the response is much larger, amounting 22 ,44 to about 5.10 4 • The dynamic range 22 is about 7.10 5 The linear dynamic range depends on the flow-rate of the carrier gas 22 , an additional stream of 70 ml/min of make-up gas shifts the linear range towards higher sample weights, which represents the behaviour of a concentration-type detector. The problem concerning the relationship between the linear dynamic range of the constant-current ECO and the type of carrier gas used seems to depend on the geometry of the detector. The linear dynamic range is generally reported to be smaller 22 ,86,128 in nitrogen than argon plus 5% of methane. However, with the detector described by Patterson 22 , the linear dynamic ranges are commensurate with both gases. The linear dynamic range is slightly decreased by decreasing the gas flow-rate or increasing the standing current 55 with the constant-current mode, and by changing the detector temperature or decreasing the pulse period 126 with the constant-frequency mode. However, the dependence of the response on the solute concentration of a constant-current ECO can also be non_linear 23 ,55,56,129-131 for some of the compounds for which the ECO is most sensitive, i.e., for highly capturing compounds such as tetrachloromethane, trichlorofluoromethane, aldrin, lindane, dieldrin and mirex. The course of this dependence is linear in the region of low solute concentrations the sensitivity increasing at higher concentrations. This course is frequently S-shaped 129 , with a linear section again in the region of the highest concentrations (Fig. 11.19). Alkyl chlorides show 132 a non-linear response dependence in the low concentration region, whereas the response is linear at higher concentrations. For highly halogenated compounds, this course has been explained as follows 130 ,131: at low solute concentrations, a significant fraction of the solute is destroyed by the electron-capture reaction, because the electron-capture rate constant and the average electron density are both large. As long as the electron population remains high and relatively constant, causing the same fraction of solute molecules to react over this limited concentration range, the response is linear. However, as the solute concentration increases, a smaller population of electrons consumes a successively smaller fraction of the solute entering the cell. Also, as the response is directly proportional to the instantaneous concentration of the solute within the detector, detector, the response becomes non-linear. It is possible to minimize the problem by employing 22 ,133 a small effective detector volume (e.g.,22 in which the response to lindane, aldrin and mirex is linear over a range of 5.5 orders

251 RESPONSE FACTOR

mV/pg

0.3

ALDRIN

0.2

0.1

10

100

1000

10 000

PICOGRAMS

Fig. 11.19. Calibration graph for aldrin. Constant-current ECD. (From ref. 129.) of magnitude. Bipolar pulsed microvolume detector extends the linear dynamic range to higher concentrations 134 . Knighton and Grimsrud 131 suggested the function (f-fO)(H+f)/F instead of the common function f-fO in order to improve the dependence of the linearity of the response on the solute concentration, especially using high baseline pulse frequencies (ca. 2000 Hz)135. However, it could be inferred from Connor's theoretical conclusions 23 that operation in the constant frequency mode rather than in the constant current mode should give optimum linearity of response. A linear dynamic range covering four orders of magnitude can also be attained in the constant-current d.c. mode 45 ,46 and the advantage of the latter over the constant-current pulse mode is the fact that no deviation from linearity has been found for strong electron capturers 46 . 11.8. SELECTIVE ELECTRON-CAPTURE SENSITIZATION The sensitivity of the ECD is strongly influenced by many factors. Impurities in the carrier gas usually reduce the detection sensitivity when using the ECD. For a long time, it had beer. required for the ECD that the carrier gas be inert to the electron-capture process. Later it was found, however, that the presence of certain impurities could increase the detector response to certain compounds. This effect has been proved for nitrous oxide and oxygen in the last few years.

252 11.8.1. Nitrous oxide doping of the carrier gas

The molecule of nitrous oxide is electron attracting and nitrous oxide itself responds in the ECD136-13S. If nitrous oxide is used as a dopant in the carrier gas, the following reactions take place in the ECD 139 : e-

+

N20 + 0-

N2

0-

+

N0 2

+

NO-

NO- + N2

+

NO + N2 + e-

+ +

NO

(11.14) (11.15) (11.16)

The concentrations of electrons, 0- and NO depend on the rate constants in eqns. 11.14-11.16, the concentration of nitrous oxide in the carrier gas and the operating temperature of the detector. Any solute that can react with 0 or NO-, giving a stable negative ion, interrupts the reaction cycle, causing a reduction in the electron density and a concomitant detector response: 0-

+

A + stable negative ion

( 11.17)

The result of summing the steady-state ion chemistry outlined in the above equations represents the electron-catalysed conversion of nitrous oxide to nitrogen and nitrogen oxide: ( 11.1S) The formation of nitrogen oxide in the ECD could be demonstrated by comparing the NO levels in a carrier gas containing 20 ppm of nitrous oxide in the inlet and outlet of the detector 142 . Whereas the NO level in front of the ECD was ~ 0.5 ppb, the level behind the detector was 5.5 ppb. Hence, compared with the common ECD, the presence of nitrous oxide causes response sensitization (selective electron-capture sensitization, SECS) for compounds that do not rapidly attack the electron, but that do react with 0 to form stable negative ions and, hence, also for compounds that do not directly capture electrons (see Table 11.10). The detection limit for compounds that rapidly convert electrons to stable negative ions is slightly increased by addition of nitrous oxide, owing to the increased baseline noise 140 • Compared with the value at zero nitrous oxide concentration, with 20 ppm of nitrous oxide the noise level is increased 141 by a factor of 4-5. The detection limits for N20sensitized ECDs are in the picogram range: 1.4 pg for vinyl chloride 141 , 16 pg for carbon monoxide 142 and 30, 16, 15, 1S, 35 and 62 pg for methane, ethane, propane, butane, pentane and hexane, respectively143. Carbon monoxide does not

253 TABLE 11.10 ELECTRON-CAPTURE DETECTOR RESPONSE ENHANCEMENT FACTORS WHEN NITROUS OXIDE IS ADDED TO THE CARRIER GAS Detector temperature, 350 0C. From ref. 140.

Compound

Formula

Enhancement factor*

Hydrogen Carbon dioxide Methane Ethane n-Propane n-Butane n-Pentane n-Hexane Dich10rodif1uoromethane Dich10rotetraf1uoroethane Trich10rof1uoromethane 1,1,1-Trich10roethane Trich10rotrif1uoroethane Carbon tetrachloride Dich10romethane Chloroform Tetrach10roethene Carbonyl sulphide Methyl chloride Ch1orodif1uoromethane Methyl f1 uori de Acetaldehyde Vinyl chloride Carbon monoxide

H2 CO 2 CH 4 C2H6 C3H8 C4H10

40 10 000 13 27 31 34 29

C5H12 C6H14 CC1l2 CC1F 2CC1F 2 CC13F CH 3CC1 3 CC1lCC1F2 CC1 4 CH 2C1 2 CHC1 3 C2C1 4 COS CH 3C1 CHC1F 2 CHl C2H4O C2H3C1 CO

11

0.23 0.29 0.40 0.51 0.33 0.43 0.32 0.40 0.45

63

782 11 760

100

* detection limit with 20 ppm N20 in N2 carrier Enhancement factor = detection limit without N 0 in N2 carrier 2

N

<.n

.:>0

TABLE 11.11 SECS ENHANCEMENT FOR POLYNUCLEAR AROMATIC HYDROCARBONS From ref. 141. Compound

Anthracene 9-Methylanthracene Phenanthrene Tetracene l,2-Benzanthracene Chrysene Triphenylene Pyrene Benzo [gJ pyrene Benzo [iiJ pyrene Perylene Acenaphthene Fluorene Dibenzofuran Dibenzothiophene Carbazole Acridine

Detector temperature 250 oC*

Detector temperature 350°C

Signal enhancement

S/N** enhancement

Signal enhancement

S/N** enhancement

3.2 1.5 9.7 1.4 1.3 5.5 9.5 6.8 0.6 3.5 3.2 4.0 1.2 17 18 3.2 3.7

0.8 0.4 2.4 0.4 0.3 1.4 2.4 1.7 0.2 0.9 0.8 1.0 0.3 4.3 4.5 0.8 0.9

6.3 5.9 8.1 1.6 3.2 6.5 17 11 1.7 34 6 4.0 1.5 29 25 8.4 14.4

1.6 1.5 2.0 0.4 0.8 1.6 4.3 2.8 0.4 8.5 1.5 1.0 0.4 7.3 6.3 2.1 3.6

*Selective electron-capture sensitization response at a detector temperature of 350 0 C divided by the ECD response at **250 0 C. SIN = signal-to-noise ratio.

255

:;;DIMETHYLPHENOL

E CD

Fig. 11.20. Identification of phenols in shale oil waste water (steam distillate); 2-~1 split injection, temperature programmed from 50 to 200 0 C at 20 C/min. Top, selective electron-capture sensitization; bottom, without nitrous oXide. Capillary fused-silica column, 25 m, Carbowax 20M. (From ref. 141.) directly react with 0- to form a stable negative ion. For this compound, the sensitivity is explained 142 by the catalytic conversion of carbon monoxide to carbon dioxide in the presence of nitrous oxide on the hot detector wall. The practical effect of adding nitrous oxide (or other dopant gases) to the carrier gas consists in converting the relatively selective ECD into a much less selective detector with no change in instrumentation. The increase in the response of certain compounds in a complex chromatogram due to the addition of nitrous oxide to the carrier gas in the ECD can also be utilized to identify unknown peaks 140 ,141,143 (Fig. 11.20). Even isomers of compounds 141 can be distinguished in this way (c.f., Table 11.11, isomers of toluidine, benzo[gJpyrene vs. benzo[eJpyrene). In addition to the compounds listed in Table 11.11, an enhancement of response was also observed 143 for benzene, ethanol, methyl isobutyl ketone and other groups of compounds 141 such as phenols, amines, polynuclear aromatic hydrocarbons (PAHs), aromatic and heteroaromatic compounds and water. A negative response was observed for benzene and furan 143 and for some other PAHs and aromatic hydrocarbons 141 , frequently as a W-shaped peak.

256 The dependence of the sensitivity and minimum detectability for vinyl chloride on the concentration of nitrous oxide in the carrier gas in shown in Fig. 11.21. The increase in sensitivity with increasing nitrous oxide concentration up to about 20 ppm results in an increase in the minimum detectability. Both parameters stop changing 144 at higher nitrous oxide concentrations. Fig. 11.22 shows the dependence of the response to chloroethane on the detector temperature. The sensitivity decreases with decreasing temperature so that the minimum detectability at 190 0 C is increased by a factor of about ten with respect to the value at the maximum operating temperature of 350 0 C'40. Similar relationships are also valid for other compounds 139 ,140,142-144. The optimum concentration range is from 15 to 70 ppm of nitrous oxide in nitrogen; nitrous oxide is introduced into the carrier gas between the column outlet and the ECD 140 ,144.

:~: 0 : 0:

f]

;

:

J

I

, ,, ,,, ,

.!!! 'c ,

I

~

t'

I

~

ii 0... o

I

-;;1(J'" \/I

c: o

0. \/I

Q)

I

a:

I

,~L-__L -__L-~L-~L-~L-~L-~

o

10

20 30 40 50 60 Concentration of ~o (ppm,vlv)

70

Fig. 11.21. Response of the sensitized ECO to vinyl chloride and the signal-tonoise ratio as a function of nitrous oxide concentration. (Reprinted with permission from ref. 144.)

257

>

~20 QI

11\ C

o

Co 11\ QI

0::

.... o '0 QI

Gi10

Cl

350

300

250

200 'C

O~--,-~~----~--~----~--~ ~ ~ ~ ~

W

v

W

10001T ( K-1)

Fig. 11.22. Response to vinyl chloride as a function of the detector temperature with carrier gas nitrogen containing 17 ppm of nitrous oxide. (From ref. 140.) The degree of sensitization is significantly affected by the presence of impurities in the ECD, the source of which can be the carrier gas transfer lines, controllers, valves and other equipment, column bleed and leaks 140 ,144. 11.8.2. Oxygen doping of the

ea~~ie~

gas

Oxygen present in the ECD carrier gas considerably reduces 12 ,122,123 the detector sensitivity and linearity by producing O2 ions, and in a wet carrier gas also by molecular complexes 124 (see eqn. 11.20). This removes a significant proportion of the electrons and prevents them from performing their function of reacting with solute molecules. However, Van de Wiel and Tommassen 123 found, using a constant-frequency ECD, that, compared with pure nitrogen, the electroncapture coefficient of a weakly responding compound such as n-butyl bromide increased and that its ECD response increased at high oxygen concentrations in the carrier gas and at high temperatures. These findings were later extended to other compounds: halogen compounds 132 ,145-149, polycyclic aromatic hydrocarbons 150 ,152, polycyclic aromatic amines 152,153, polycyclic aromatic

258 154 hydroxides 152 , carbon dioxide 138 and bis(chloromethyl)ether • For all these compounds, both the sensitivity and the linearity of response were improved by oxygen doping. The reaction scheme is as follows 145 ,147,149: ( 11.19) or, if the carrier gas is not dry, ( 11.20) The overall reaction is O2 + sample molecule

~

negative

2

+

neutral species

(11.21)

Reaction 11.21 consumes O and shifts the position of the equilibrium of reaction 11.19 to the right. Similarly to nitrous oxide doping, the reaction of compounds that have very low rates of electron capture with 0; can be faster than that with the free electron. Then, for instance, highly chlorinated molecules show little response enhancement, because their rate of direct electron capture is much faster. In the constant-current mode of the ECD, the baseline frequency increases 145 at oxygen concentrations above ca. 100 ppm with increasing oxygen concentration and decreasing detector temperature. The response also increases with increasing oxygen concentration 145 ,146,149. The increase begins mainly at concentrations of about 1% (Fig. 11.23). With decreasing detector temperature and an oxygen concentration at a ~oo level, the response for halogen compounds increases 145 ,146 (Fig. 11.24), but it decreases for anthracene and 1-chlorobutane 145 . In the constant-frequency mode, the course of the dependence of the response increases on the oxygen content displays different features (Fig. 11.25). The maximum for methyl chloride is attained 155 at relatively low dopant concentrations. The value of the response increase is about 4 with 0.030% of dopant, but the response decreases at higher concentrations. At the same detector temperature, the sensitivity increase is about 200 for the constantcurrent mode. Hence the constant-current mode is more advantageous for oxygen doping. When oxygen is added to the detector, both the sensitivity and noise increase. Like the response, the noise becomes the greater the lower is the temperature of the detector 146 • Hence the signal-to-noise ratio for a given compound does not increase on oxygen doping as much as the response does 122 . For instance, for methyl chloride at a detector temperature of 250 0 C and a 0.2% oxygen con-

259

/

300'C

+

60

CH 3 CI

GJ

~40 o

1/

0Il)

....GJ GJ

.~

C

~20

+

6

~

/

/o/O

.1 ,4

6/0

CHCI 3

1:>/0

o

_x

x~-

o~ __ ~==b~~~~-DO.--~~--O­

o

CCI4

123 O2 concentration (%0)

4

5

Fig. 11.23. Effect of oxygen doping on the ECD response to halogenated methanes. (From ref. 149.)

/

C~CI

300

o

2000cj +

GJ

~200

o

0Il)

GJ

....

GJ

>

GJ

a:: 100

a

1

234

O2 concentration

(0/00)

Fig. 11.24. Effect of detector temperature on the oxygen-induced response enhancements of chloromethane. (Reprinted with permission from ref. 146.)

N

m o

TABLE 11. 12 ECD RESPONSES AND RESPONSE ENHANCEMENTS CAUSED BY 2.0% OF OXYGEN IN THE CARRIER GAS ECD responses under the normal conditions of clean carrier gas are the first values listed under each detector temperature. These are relative molar responses normalized with respect to the case of methyl chloride at 300 oC. Oxygen-induced response enhancement are listed in parentheses under each detector temperature. Reprinted with permission from ref. 147. Compound

Detector temoerature 0

CH 3Cl CH 2C1 2 CHC1 3 CC1 4 CH 3CH 2Cl C1CH 2CH 2Cl CH 3CH 2CH 2Cl CH 3CHCl CH 3 C1CH 2CH 2CH 2Cl CH 3CH 2CH 2CH 2Cl CH 3CHC1CH 2CH 3 (CH 3)3 CCl

0

0

250 C

300 C

350 C

1.6( 189) 3.8( 108) 459(4.8) 9100( 1.90) 2.1(228) 4.6(161) 2.0(201) 2.0(195) 2.2(180) 2.3(145) 3.5(132) 1. 7(95)

1.0(113) 4.6(32) 662(1.71) 10 500 (1. 20) 1.5(135) 5.1(69) 1.3(127) 1.4(113) 3.6(81) 1.6 (84) 2.2(65) 1.3(47)

1.0(56) 8.3(10.9) 815(1.42) 11400(1.15) 1.4(57) 5.2(22) 1.2(57) 1.2(50) 5.1(34) 1.4(34) 2.3(25) 1.8( 13)

Relative oxygencaused response, *RO 1.0 ± 0.3 1.3 ± 0.4 4.1 ± 1.6 18 ± 9 1.8 ± 0.6 3.1±1.0 1.4 ± 0.5 1.4 ± 0.5 2.6 ± 0.9 1.2 ± 0.4 1.2 ± 0.4 0.53 ± 0.20

2

C1CH 2CH 2CH 2CH 2Cl CH 2=CHCl CH 2=CC1 2 tmns-Cl CH=CHC1 cis-Cl CH=CHC1 C1CH=CC1 2 C1CH=CHCH 3 CH 2=CC1CH 3 CH 2=CHCH 2Cl trans-C1CH 2CH=CHCH 2Cl C6H5CH 2Cl C6H5Cl o-Cl-C 6H4C1 m-Cl-C 6H4Cl C1 2C=CC1 2 p-Cl-C 6H4C1 CH 3Br CF C1 3 CHF 2Cl CF 2C1 2 CFC1 3

3.4(197) 0.0068(107) 197(1.78) 1.7(17) 1.2(20 ) 505(2.9) 0.0036( 161) 0.180(3.0) 5.7(153) 380(8.5) 42(44) 0.029( 14.7) 20(3.9) 29(10.2) 4000(1.79) 10.6(8.8) 18(55) 6.9(2.7) 2.0(190) 174(3.4) 4450(2.2)

3.2(94) 0.0068(69) 373(1.13) 3.7(3.9) 2.3(5.5) 732(1.42) 0.0031 (91) o•190 ( 1.88) 4.9(61 ) 500(2.9) 67(10.3) 0.068(3.7) 43(1.52) 60(2.4) 4880( 1.16) 26(2.0) 24(13.3) 9.3(1.11) 1.35(90) 253(1.52) 5210 (1. 31)

2.9(53) 0.013(29) 675( 1.02) 8.3(1.59) 4.9(2.0) 1070( 1.11) 0.0056(33) 0.21(1.11) 6.3(20) 580( 1.69) 97(4.5) 0.16( 1.84) 77 (1.26) 103( 1.39) 6160(1.02) 47(1.25) 39(5.0) 12.8( 1.00) 0.67(62) 361(1.10) 5850(1.03)

2.6 ± 1.0 0.0041 ± 0.0014 0.3 ± 0.3 0.09 ± 0.03 0.09 ± 0.03 2.6 ± 1.5 0.0024 ± 0.0007 0.0015 ± 0.0006 2.6 ± 0.9 8.4 ± 3.0 5.5 ± 2.3 0.0017 ± 0.0006 0.18 ± 0.07 0.75 ± 0.26 7±5 0.23 ± 0.09 2.6 ± 0.9 0.008 ± 0.008 1.1 ±0.4 1.1 ± 0.5 14 ± 7

*The relative oxygen-caused response is the contribution to an overall response provided by 0.2% of oxygen. These values have been normalized with respect to methyl chloride using the ECD data at 300 0 C.

N

0'1

262

Oxygen

Concent rat ion (0/00)

Fig. 11.25. Response enhancement as a function of the oxygen concentration in the carrier gas for (a) constant-frequency ECO (®) and (b) constant-current ECO (x). (Reprinted with permission from ref. 155;) centration, the signal-to-noise ratio increases by about 16 times, whereas the sensitivity increases by about 200 times. If' the response increase with oxygen doping is very low, such as that with carbon tetrachloride, the signal-to-noise ratio even decreases. The number of chlorine atoms per molecule of solute significantly affects both the response enhancement and the ECO response (see Table 11.12). For instance, with chlorinated methanes from methyl chloride to carbon tetrachloride, the ECO response increases by almost four orders of magnitude, while the corresponding response enhancement decreases from 113 to 1.20. The highest response increase has generally been observed for halogen compounds that yield a low reaponse in the ECO, e.g., the monohalogenated alkanes. An exception is the relatively small response increase with some monohalogenated alkanes without any hydrogen atom on the carbon to which the chlorine atom is attached 147 (tert.butyl chloride, 2-chloropropene, chlorotrifluoromethane). Isomers display different response enhancements 147 ,151-153,156 (cf., for instance, Table 11.3, trans-, ais- and 1,1-dichloroethylenes, 0-, m- and p-dichlorobenzenes and also 151 ,152 1-, 2- and 9-chloroanthracenes, 2- and 7-methylanthracenes, benzopyrenes, aminobiphenyls, dihydroxynaphtalenes, etc.), which suggests an instrumentally simple means for compound verification in analysis by gas chromatography - electron-capture detection, particularly with the simultaneous use 157 of two ECOs, with oxygen added to one of them. For inxtance, the response enhancement is 3.9, 5.5, and 1.13 for trans-, ais- and 1,1-dichloroethylenes, respectively. The response enhancement is independent of the solute concentration over a range of at least two orders of magnitude 156 ,157

263 No enhancement is generally observed with compounds that do not yield a normal ECD response (e.g., aliphatic hydrocarbons). The method of adding oxygen provides a nreans of enhancing certain responses that may already be faintly evident in preference to the response of the strongly responding compounds which normally dominate the chromatogram 149 11.8.3. Sensitization of aromatic hydrocarbons

The response of a pulsed ECD to some aromatic hydrocarbons is anomalous141 ,150. The direction of this response is opposite to that of the normal ECD response. The peaks are often "M-shaped". In pure carrier gas, the identity of the positive ions changes as the aromatic hydrocarbon (e.g., anthracene, acridine) passes through the detector, with a simultaneous increase in the population of both positive ions and electronsI58-160. Increase in the electron population is due to the smaller rate constant for the recombination of electrons with the protonated aromatic positive ions compared with those for the positive ions normally presented l58 . This anomalous effect can be eliminated by adding a compound (e.g., methylamine, trimethylamine, diethylamine) with a gas-phase to the carrier gasI58-160. Doping of the carrier gas with 10 ppm of ethyl chloride (or isopropyl chloride) enhances the detector response to anthracene by ca. one order of magnitude. Alkyl chlorides react with short-lived negative anthracene ions to form Cl and neutral species 159 (reactions 11.22 and 11.23).

(11.22) ( 11.23) The use of both alkyl chloride and alkylamine as dopants altogether produces useful and well behaved responses 159 11.9. COULOMETRIC AND HYPERCOULOMETRIC RESPONSE The calibration of the detector response in a range of very high sensitivies with corresponding small amounts of compounds is a difficult problem. In order tosolvethis problem, Lovelock 161 suggested that the ECD itself could be used as an absolute calibrant. This means using a detector in the coulometric mode in which, at 100% ionization efficiency, the solute concentration can be calculated via Faraday's law directly from the number of the electrons absorbed. With

264 intensely electron-absorbing substances, the ECD tends to become a destructive detector in which a large proportion pf the substance entering the detector is ionized irreversibly. Coulometry requires the equilibrium of reaction 11.5 to be shifted almost entirely to the right or AB- to be rapidly scavenged from the system by reactions 11.7, 11.8 and 11.10. The concentration of positive ions within the detector is about 1000 times higher than the electron concentration, and this would favour reaction 11.10. Therefore, under certain conditions, the ionization of most of the molecules entering the ECD is favoured and, hence, a cou10metric response 23 ,54-56,62,161-168 is obtained. The cou10metric method of detecting electronegative compounds enables one to decrease the detection limit, because at 100% ionization efficiency the ECD shows its maximum sensitivity. If an ECD is to function successfully as a gas-phase cou10meter, the following requirements should be envisaged 167 • (1) The concentration of electrons must be in excess of the concentration of sample molecules (under typical conditions of operation, the average electron concentration in the pulse mode is in the range 10- 13 mole/lor 6.10 7 e1ectrons/m1). The minimum detectable concentration of intensely electron-absorbing substances, e.g., sulphur hexafluoride, tetrach10romethane and, halogenated pesticides, is about 10 6 mo1ecu1es/ m1. (2) The electron-capture reaction must be nearly complete. (3) The stoichiometry of the reaction of thermal electrons with the sample must be 1:1 or some other known ratio. (4) No side reactions of the sample molecules may occur. (5) The current monitored from the ECD must reflect the absolute amount of electrons present in the entire ECD cell, and the ECD response must absolutely reflect the loss of these electrons due to electron-capture reactions. The ventilation of the detector by the flow of the carrier gas also removes the electron-absorbing substances from the detector. Hence, the amount of the substance ionized decreases 161 ,162 with increasing flow-rate of the carrier gas, and the highest cou10metric response is observed at the lowest flow-rate of the carrier gas. At an ionization efficiency of less than 100%, the application of several identical detectors in series makes it possible to determine the portion ionized in either of them and, hance, the signal corresponding to complete ionization 161 ,162. The cou10metric mode is difficult to attain with a common ECD. The ionization of most of the emerging molecules is provided for by a detector of special design with a long ionization chamber 55 replacing the arrangement of several detectors in series. This detector operates in the cou10metric mode for dibromodif1uoromethane at flow-rates of up to about 60 ml/min, whereas the conventional detector approaches this mode giving a cou10metric response only

265 at very low flow-rates. In practice, the detector never yields a signal that is truly coulometric; a small portion of the electron signal is always lost by recombination 56 • This loss is about 10% in argon-methane or argon-hydrogen carrier gas and at a pulse period of 250 ~sec. The loss is even higher in nitrogen, about 30%. Even with the long coulometric detector the signal efficiency attained is 90-97%. According to the "classical" theory (section 11.5.1), the [CD in the coulometric mode of operation should yield the maximum response, and the ratio of the peak area in Faradays to the amount injected in moles should not exceed unity. This ratio can be 2-3 if the products of solute ionization possess greater electron affinities than the reactants 2. Aue and Kapila 169 , however, found far greater ratios for many compounds with commercial d.c. mode [CDs at elevated pressure. The ratio attained a value of 20 with a detection limit of 10 fg for hexachloroethane. This hypercoulometric response increases with increasing pressure 63 ,67; the ratio reaches a value of 50 for tecnazene at 5 atm. This effect also exists in the pulse [CD63 ,170 (Fig. 11.26) and increases with increasing pressure. The hypercoulometric response is explained by a mechanism based on migrating negative ions 63 (section 11.5.3). Coulometric detection forms the basis 55 ,171-174 for solute switching using the [CD. Two [CDs coupled in series are used. The first detector, having a much higher density of thermal electrons available to attach compounds, operates on the coulometric principle and serves as a solute switch. This detector switches between the destruction and the free passage of solute by switching the applied potential on or off. The second [CD is locked into the switching frequency of

d.c.

~

8 E ..... Q)

6

Q)

III

.J;.- - -

C

a. 4 0

Q)

2

Pulse

.....t>- - - - -

'Zl.-

.... ~'"

III

0::

-

~

l:(

,. "

Cell

Pressure

Fig. 11.26. Hypercoulometric response vs. cell pressure for d.c. and pulse modes. elm = Faradays (peak areal/moles of substance injected. (From ref. 63.)

266 the first and is used to determine the sample concentration. The signal from the detector is modulated with an a.c. component at the switching frequency. This may be amplified and synchronously demodulated. The signal-to-noise ratio is increased owing to noise filtering. The selectivity is also increased 173 . With respect to the required high flow-rates (excluding the use of capillary columns) that reduce the sensitivity of the detector, the improvement in the signal-to-noise ratio is not yery high if the detector itself is the source of noise. The improvement is significant if the noise is caused by interferences 174 . 11.10. USE OF THE ELECTRON-CAPTURE DETECTOR WITH CAPILLARY COLUMNS The volume of most commercial ECDs is so large that they cannot be used directly in connection with capillary columns. In order to prevent the dispersion of chromatographic zones in the detector and, as a consequence, deterioration of the separation of substances on the capillary chromatographic column, the volume of the ECD should be as small as possible. A peak with a baseline width of 1 sec would elute from the capillary column in a gas volume equal to 16.7 ~18 at a flow-rate of 1.0 ml/min. If the peak distortion due to the detector volume were not to exceed 1%, the residence time of the peak in the detector would have to be less than 0.05 times the peak width at half-height 175 . This would require a detector with a cell volume of about 0.5 ~l for the above peak. The make-up gas used to reduce the effective volume of the detector dilutes the sample and decreases the sensitivity, because the ECD is a concentration-sensitive detector. The limit to the ECD volume is conditioned by the source type and the geometry of the cell. The penetration depth of the B-particles in pure carrier gas is 3 ,8 ca. 6-10 mm for 63Ni and ca. 2 mm for 3H• In order to ensure full deactivation of all B-particles by collisions and their reduction to thermal energies without colliding with the anode, the source-to-anode distance should be larger than the penetration depth. For this reason .the tritium source is better suited for ECD miniaturization (cf., section 11.3). However, the distance should not be so large as to make efficient electron collection impossible when a narrow, low-voltage pulse is applied to the anode 23 . It is evident, therefore, that the above requirement of 0.5 ~l for the beta ionization source is impractical. A small-volume ECD using a plane-parallel electrode configuration was described by Devaux and Guiochon 38 . The ECD with this configuration has a larger volume than the cylindrical design with the same source area, which causes both the standing current and the sensitivity to be lower. The coaxial design of the

267 high-temperature scandium tritide EC0 16 results in a volume of about 0.4 ml. Even though the volume of the 3H cell is several times smaller than that of the 63Ni cell, the standing current is higher because higher specific activity foils can be produced with tritium. The detection limit for testosterone diheptafluorobutyrate was 13 about 2 pg. The volume of the cylindrical scandium tritide microdetector is 120-160 ~1133,176. The asymmetric geometry of the cell (displaced coaxial cylinder) with 63Ni allows the volume to be reduced to 350 ~122, 177 to 100 ~1178 (the cell volume is smaller than that with the symmetrical cell). The volume of the non-radioactive ECO with a thermionic emitter is 50 ~133 (c.f., section 11.3). However, detectors with the above volumes are still too large in view of the peak width quoted in the first part of this section. For this reason, makeup gas has still been used in this instance in order to reduce the effective detector volume. The influence of the gas flow-rate through the detector on the initial column efficiency and on the peak shape with the microvolume 63Ni EC0 177 (380 ~l) is shown in Fig. 11.27. Although the minimum flow-rate through the cell should be about 68 ml/min in order to maintain the 90% efficiency with a l-sec peak when assuming perfect mixing within the ECO chamber 179 , the compromise flow-rate is about 30 ml/min 177 ,180,181. Complete mixing usually does not occur in the detector, and with a cell designed to attain plug-like flow 169 ,182 (Fig. 11.28),

6

25 DIBJ)RIN

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M

I

0

.- 4

1.5 ~

~

...J

Z

UJ ~

3

Will

2

05

10

10

20

30

40

50

60

'70 80 TOTAL FLOW-RATE, ml/min

0

Fig. 11.27. Effect of total detector gas flow-rate on the column efficiency (NIL, plates/m) and peak shape (~) for capillary ECO. 25 m x 0.25 mrn 1.0. column with QV-l01, 170 0 C. k' = Capacity factor. Skewness in (m3/m2)3/2 where m3 and m2 are third and second moment, respectively. (From ref. 177.)

268

INSERT

SAMPLE SIDE PORTS ANODE

CERAMIC COLUMN INSULATOR

Fig. 11.28. Micro-volume electron-capture detector cell. (From ref. 182.)

lpAa-II .A.

30

" ! \

\/ \

{

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t-.i "'..

10

~

I j> r-I' If

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:

10 20 30 40 50 60 mil min

DETECTOR FLOW

Fig. 11.29. Peak height as a function of total detector flow-rate at different pulse intervals: • = 160 ~sec; 0 = 290 ~sec; ~ = 530 ~sec; • = 750 ~sec. (From ref. 180.)

269 the real flow-rates need not be extremely hi9h 182 In this 63Ni microdetector the active region of the cell (the part of the cell below the insert from which electrons can be extracted by the anode) is 100 ~l. The nitrogen make-up gas flows from the bottom concentrically around the column. The sample flows from the middle hole of the anode into the centre of the cell. In addition, the sample flows along the sides of the cell through the cross-drilled holes in the anode. This results in a nearly plug-like flow through the active region of the cell. As the column effluent is not pre-mixed with the purge gas, the gas flowing through the side ports and up along the sides of the cell walls contains no sample and forms a boundary layer 182 The dependence of the response on the gas flow-rate through the ECD differs in its nature from that of the common concentration-sensitive detectors, reaching the maximum value 180 ,181,183,184. This course also depends on the pulse intervals 180 (Fig. 11.29). As the maximum detectability for the ECD is proportional to the concentration of the sample molecules at the peak maximum, the detection limit is lower for capillary columns than for packed columns and can be as low as a few femtograms 87 ,177,182,185. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

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272

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122 123 124 125 126 127 128

129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156

G.G. Guilbault and C. Herrin, Anal. Chim. Acta, 36 (1966) 252. H.J. van de Wiel and P. Tommassen, J. Chromatogr., 71 (1972) 1. F.W. Karasek and D.M. Kane, Anal. Chem., 45 (1973) 576. D.C. Fenimore, A. Zlatkis and W.E. Wentworth, Anal. Chem., 40 (1968) 1594. D.C. Fenimore and C.M. Davies, J. Chromatogr. Sci., 8 (1970) 519. E. Chen and J. E. Lovelock, J. Phys. Chem., 70 (1966) 445. Electron Capture Detector for SP 7100 Gas Chromatograph, 195D 10/82, SpectraPhysics, San Jose, CA, 1982. J.J. Sullivan and C.A. Burgett, Chromatographia, 8 (1975) 176. E.P. Grimsrud and W.B. Knighton, Anal. Chem., 54 (1982) 565. W.B. Knighton and E.P. Grimsrud, AnaL Chem., 55 (1983) 713. E.P. Grimsrud and D.A. Miller, J. Chromatogr., 192 (1980) 117. B. BrechbUhler, L. Gay and H. Jaeger, Chromatographia, 10 (1977) 478. R. Simon and G. Wells, J. Chromatogr., 302 (1984) 221. W.B. Knighton and LP. Grimsrud, J. Chromatogr., 288 (1984) 237. W.E. Wentworth, E. Chen and R.R. Freeman, J. Chem. Phys., 55 (1971) 2175. W.E. Wentworth and R.R. Freeman, J. Chromatogr., 79 (1973) 322. P.G. Simmonds, J. Chromatogr., 166 (1978) 593. M.P. Phillips, R.E. Sievers, P.D. Goldan, W.C. Kuster and F.C. Fensenfeld, AnaL Chem., 51 (1979) 1819. F.C. Fehsenfeld, P.D. Goldan, M.P. Phillips and R.E. Sievers, in A. Zlatkis and C.F. Poole (Editors), Electron Capture - Theory and Practice in Chromatography, Elsevier, Amsterdam, 1981, Ch. 4, p. 69. M.A. Wizner, S. Singhawangcha, R.M. Barkley and R.E. Sievers, J. ChroI7atogl'., 239 (1982) 145. P.D. Goldan, F.C. Fehsenfeld and M.P. Phillips, J. Chromatogr., 239 (1982) 115. R.E. Sievers, M.P. Phillips, R.M. Barkley, M.A.Wizenr, M.J. Bollinger, R.S. Hutte and F.C. Fehsenfeld, J. Chromatogr., 186 (1979) 3. P.D. Goldan, F.C. Fehsenfeld, F.C. Kuster, M.P. Phillips and R.E. Sievers, AnaL Chem., 52 (1980) 1751. E.P. Grimsrud and R.G. Stebbins, J. Chromatogr., 155 (1978) 19. E.P. Grimsrud and D.A. Miller, Anal. Chem., 50 (1978) 1141. D.A. Miller and E.P. Grimsrud, Anal. Chem., 51 (1979) 851. G. di Pasquale and T. Capaccioli, J. Chromatogr., 206 (1981) 589. LP. Grimsrud, in A. Zlatkis and C.F. Poole (Editors), Electron Capture Theory and Practice in Chromatography, Elsevier, Amsterdam, 1981, Ch. 5, p. 91. E.P. Grimsrud, D.A. Miller, R.G. Stebbins and S.H. Kim, J. Chromatogr., 197 (1980) 51. D.A. Miller, K. Skogerboe and E.P. Grimsrud, Anal. Chem., 53 (1981) 464. J.A. Campbell, E.P. Grimsrud and L.R. Hageman, Anal. Chem., 55 (1983) 1335. J.A. Campbell and E.P. Grimsrud, J. Chromatogr., 284 (1984) 27. C.J. Kallos, Anal. Chem., 53 (1981) 963. E.P. Grimsrud, S.W. Warden and R.G. Stebbins, Anal. Chem., 53 (1981) 716. J.A. Campbell and E.P. Grimsrud, J. Chromatogr., 243 (1982) 1.

273 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184

J.A. Campbell and E.P. Grimsrud, J. Chromatogr., 291 (1984) 13. E.P. Grimsrud, Anal. Chem., 56 (1984) 1797. E.P. Grimsrud and O.A. Valkenburg, J. Chromatogr., 302 (1984) 243. E.P. Grimsrud, J. Chromatogr., 312 (1984) 49. J.E. Lovelock, R.J. Maggs and E.R. Adlard, Anal. Chem., 43 (1971) 1962. D. Lillian and H.B. Singh, Anal. Chem., 46 (1974) 1060. H.B. Singh, D. Lillian and A. Appleby, Anal. Chem., 47 (1975) 860. E. Bros and F.M. Page, J. Chromatogr., 126 (1976) 271. J. Rosiek, I. Sliwka and J. Lasa, J. Chromatogr., 137 (1977) 245. J. Lasa, E. Bros, I. Sliwka, A. Korus and J. Rosiek, Report No. 1029/C4, Institute of Nuclear Physics, Krakow, 1978. E.P. Grimsrud and S.H. Kim, Anal. Chem., 51 (1979) 537. P. Popp and J. Leonhardt, Isotopenpraxis, 19 (1983) 160. W.A. Aue and S. Kapila, J. Chromatogr.{ 112 (1975) 247. D.C. Legget, J. Chromatogr., 133 (1977) 83. J.E. Lovelock, J. Chromatogr., 112 (1975) 29. P.G. Simmonds, A.J. Lovelock and J.E. Lovelock, J. Chromatogr., 126 (1976) 3. P.R. Boshoff and B.J. Hopkins, J. Chromatogr. Sci., 17 (1979) 588. G. Well s, J. Chrornatogr., 285 (1984) 395. J. Sevcik and J.E. Lips, Chromatographia, 12 (1979) 693. H.R. Buser, Anal. Chem., 48 (1976) 1553. F.J. Yang and S.P. Cram, J. High Resolut. Chromatogr. Chromatogr. Commun., 2 (1979) 487. G. Wells and R. Simon, J. High Resolut. Chromatogr. Chromatogr. Commun., 6 (1983) 427. A.P.J.M. de Jong, J. High Resolut. Chromatogr. Chromatogr. Commun., 5 (1982) 213. J.J. Franken and H.L. Vader, Chromatographia, 6 (1973) 22. L. Rejthar and K. Tesarik, J. Chromatogr., 131 (1977) 404. G. Wells, J. High Resolut. Chromatogr. Chromatogr. Commun., 6 (1983) 651. P. Devaux and G. Guiochon, J. Chromatogr. Sci., 7 (1969) 561. K.P. Dimick and H. Hartmann, Aerograph 1/63 W-106, Wielkens Instrument, Walnut Creek, CA, 1963; presented at the ACS Winter Meeting, Cincinnati, Ohio, January 1963.

185 G. Eklund, B. Josefsson and C. Ross, J. Chromatogr., 142 (1977) 575.

This Page Intentionally Left Blank

275

Chapter 12

ION MOBILITY DETECTOR CONTENTS 12.1. Introduction . . . . . • • 12.2. Principle of the technique 12.3. Detection principles 12.4. Effect of background References . . . .

275 275 279 286 288

12.1. INTRODUCTION The ion mobility spectrometer was developed in 1970 by Karasek and Cohen 1- 3 for organic trace analysis. The original name of this technique, plasma chromatography4, relates to the application of ions (plasma) and to the analogy between ion mobility separation and chromatography. The term plasma chromatography has been used until now. 12.2. PRINCIPLE OF THE TECHNIQUE The principle of the method is apparent from the schematic diagram of an ion mobility detector (IMD) Fig. 12.1. A mixture of the effluent from the chromatographic column and the reactant gas (nitrogen or air) enters the detector space, passing adjacent to a radioactive 8-particle source (63Ni , about 10 mCi). In the ionization space of the detector a weak plasma consisting of positive and negative ions is generated. Ions such as (H20)nH+ where n = 2-4, and (H 20) NO+ and (H 20) NH4+ ions where n = 0-2 are generated in nitrogen in the n n 4-7 presence of a small amount of water . The value of n depends on the water content of the gas and on temperature. The negative particles are low-energy electrons (about 0.5 eV). (H20)n02 ions are formed additionally when air is used as the reactant gas. When molecules of the sample enter the reaction space, the mentioned ions and electrons react with the sample molecules, giving product ions. The product ions formed, together with the unreacted initial reactant ions, are released in pulses (for 0.2 msec in intervals of about 10 msec) through the shutter grid into the drift space of the detector. Under the effect of ca. 200-300 V/cm electric field, the ions move to the collector

276 DRIFT GAS

I~ 7

•

0

• • 0 0

6

• • • 0

5

0

0

4

•

2

3 PLASMACHROMATOGRAM

Fig. 12.1. Schematic diagram of the ion mobility spectrometer (plasma chromatograph). 1 = Polarization electrode; 2 = shutter grid; 3 = gate grid; 4 = collector electrode; 5 = signal space; 6 = drift space; 7 = reaction space; 8 gas exhaust. (Reprinted from ref. 2, with permission.) electrode. Within this drift (separation) space, the ions are separated at normal pressure on the basis of their different mobilities, the heavier ions moving more slowly. Nitrogen has commonly been used as the drift gas, because it prevents additional reactions of the ions with the molecules. Positive or negative ions can be observed by selecting the polarity of the electric field. Various ions arrive at the gate grid at various times. If the gate grid is open, the ions pass through it and are collected at the electrode. The drift speed Vd (the speed of ion movement through the drift space) is proportional to the intensity of the electric drift field E (ref. 4):

(12.1) where"K is the linear ion mobility. The IMD measures the time required for the ion to migrate to a fixed distance. Hence, Vd can be replaced with drift length, d, divided by drift time, T, between the ion injected grid and the collector: K =

.E....

(12.2)

TE

The following equation applies for the ion mObi1ity8: ..l.

K

e rl

= 16 • N Lm

+

ll1/2 [21TJ 1/2 1 + Il MJ LkTJ • 2n{ 1, 1)* l'm"

(12.3)

277

2

1

4.8 em

3

1

4

10----1 5

9-=;:=13 8---1

6 7

Fig. 12.2. Schematic diagram of the modified ion mobility spectrometer. 1 = Sample inlet; 2 = shutter grid; 3 = protective stainless~steel ring; 4 = gate grid; 5 = passive grid electrically connected to the protective ring; 6 = col~ lector electrode; 7 = electrometer; 8 = PTFE insulation; 9 = drift gas inlet; 10 = glass insulation; 11 = high voltage (e.g. +3000 V plate; 12 = gas exhaust. (Reprinted with permission from ref. 7.) where N is the molecular number density, k is the Boltzmann constant, e is the electronic charge, T is the absolute temperature, ~ is a small correction term for higher approximations, Pm is the position of minimum potential for the interaction between the ion and molecule, Q(1,1)* is the first~order collision integral, M is the molecular mass and m is the ionic mass. For the standard conditions T = 273 K and p = 760 mmHg, the following rela~ tion is valid:

278

R

12 10 11 13

9

15

14

8

4

40

7

·c

Fig. 12.3. Gasoline chromatogram, obtained in non-selective reactant ion mode. 1 = Heptane; 2 = methylcyclohexane; 3 = toluene; 4 a m,p-xylene, 5 = a-xylene; 6 = trimethylbenzenes; 7 = ethyltoluene; 8 = tert.-butylbenzene, 9 = naphthalene; 10 = dodecane; 11-13 = unknowns; 14 = 2-methylnaphthalene; 16 = l-methylnaphthalene; 17,18 = unknowns. 15 m quartz capillary column, SE-54; temperature, programmed from 40 to 100 0 e at 8oe/min, held for 10 min at 100oe. Ion monitoring with drift time between 8 and 9 msec, gradient 215 V/cm; drift gas, nitrogen, 600 ml/min; detector temperature, 140 oe. R = response. (Reprinted with permission from ref. 7.) K

o

=!l... TE

76PO • 273 T

(12.4)

where Ko is the reduced mobility, depending on the ionic size, charge and mass, on the molecular size and mass, and on the drift gas composition and polarizability4. Values of Ko have been published for a number of compounds: n-alkyl halides 9 , substituted aromatics 10 , isomeric halogenated nitrobenzenes 11 , trinitrotoluene 12 , n-alkanes 13, phthalic acids 14,15, lysergic acid diethylamide and ~9_tetrahydrocannibinol16, n-alkyl acetates 17 , heroin and cocaine 18 , n-alkanols 19 , aliphatic n-nitrosamines 20 , barbiturates 21 and polychlorinated biphenyls22.

279

R 567

8

Fig. 12.4. Gasoline chromatogram, obtained with an FlO. For description and chromatographic conditions, see Fig. 12.3. (Reprinted with permission from ref. 7.) 12.3. DETECTION PRINCIPLES The equipment described above has been used as a detector combined with a gas chromatograph2,14,21 ,23-25. Commercially produced IMD (plasma chromatographs), when used in gas chromatography, have shown some disadvantages, mainly owing to the large inner volume, absorption phenomena occurring in the detector and the influence of the stationary phase molecules on the spectra. Baim and Hill? described a detector (Fig. 12.2) modified so as to be usable with capillary columns. The modifications are as follows. (1) The gas flow through the ionization space is reversed and, as a result, the gas flows through the entire detector in only one direction. The drift gas enters the detector near the collecting electrode and flows through the drift and ionization spaces towards

2S0

FID

~

A

II

(I !~~KGR

E-5

~~msec

(~

E-8

~~08~~' x4

I

I

20

';' (io~x1c:2C~c: :t: :~lOXo4

r~'

~ 40msec p 20

OL-~=-~~~----~~~--

o

2

,4

6

8

10

12

14 16

40msec

18

'-2

40msec

__~~~~__----20 22 24

26

28 30 32 min

Fig. 12.5. Chromatogram and ion mobility spectra for freons (E-1 to E-10). negative ion monitoring; drift and reaction gas, air; detector temperature, 1990 C; 6% SE-30 on Anakron ASS, 1.S3 m x 3.2 mm 1.0. column; temperature, 100 0 C; BKGR, background spectrum. (Reprinted from ref. 24, with permission.)

the detector outlet. In earlier equipment the gases were introduced at the opposite sides of the detector, leaving the detector near the shutter grid. (2) The volume of the ionization cell is reduced to 1 ml, in contrast to the original 7 ml. (3) The detector inlet is situated 'between the ionization cell and the drift space. Hence the sample is removed from the ionization space by means of the drift-gas flow through the ionization space. Therefore, neutral sample molecules, neutral products or radicals cannot interact with the product ions moving through the drift space. (4) The rings forming the wall of the drift tube are separated from each other by rings made of borosilicate glass. This closed drift tube increases the efficiency of removal of neutral species from the detector. The chromatograms obtained with this detector and those obtained with a flame-ionization detector (FID) are compared in Figs. 12.3 and 12.4. The IMO can be used as a detector in gas chromatography in several modes. (1) If the gate grid is closed, no detection takes place. If this grid is opened at intervals gradually increasing compared 'with opening the shutter grid, the ion mobility spectrum is monitored. This means that for each peak leaving

281

c

a-.............-...I'--I

iii

i

I

50 62 74

86·C

Fig. 12.6. Chromatograms of a mixture of 30 ng of iodobenzene and 180 ng of chlorobenzene. (a) Selective record of the negative ions, 1- drift time 7.14 msec; (b) selective record of the negative ions, Cl- drift time 6.07 msec; (c) electron monitoring. Drift time, 0.2 msec; 214 V/cm; drift gas, nitrogen; detector temperature 140 0 C; 1.22 m x 4 mm 1.0. glass column, packed with UltraBond; temperature, programmed from 50 to 80 0 C at 60 C/min. 1 = Chlorobenzene; 2 = iodobenzene. Arrows indicate injection. (From ref. 26.) the chromatograph the corresponding spectrum characterizing the given compound is obtained from the spectrum, being either positive or negative according to the polarity chosen. An example of these spectra for commercial freons is given in Fig. 12.5. (2) If the gate grid is opened only for certain fixed intervals subsequent to the shutter grid, only ions of a certain mobility are monitored 7 ,26-29. Hence the IMD becomes a selective detector responding only to compounds producing ions that move for the chosen drift time. An example showing the application of this mode of detection is given in Figs. 12.6 and 12.7 for the negative product ion mode. The positive ions can be detected in a'similar manner 7 •29 • (3) If the gate grid is opened so as to monitor the drift time corresponding to the time of the reactant ions, the detector background current corresponds to the detection of these ions. As the solute molecules react with these ions, giving product ions of different mobility, the presence of organic molecules is associated with a decrease in the background current and the response is negative. The resulting chromatogram is either the record of the decrease in negative ions 26 ,28 (Figs. 12.6 and 12.7) or a non-selective record of the decrease in positive ions 7,26. When oxygen is used as the reactant gas, (H20)n02

282 NON-SELECTIVE

4: N

I 0

MODE

iii ...:

iii ,.!

SELECTIVE

MODE

IU

4:

UJ

...,

~

~

0

~

~

L/'I

L/'I

...!

100

150

200

HOLD

100

150

200

HOLD

TEMPERATURE,OC Fig. 12.7. Chromat09ra~ of soil sample extracted with acetone-hexane. (a) Monitoring of the (H20)n02 reactant ions; (b) selective record of 2,4-dichlorophenoxyacetic acid product ion. Drift time, 7.90 msec; 230 V/cm; 15 m SE-54 fused-silica capillary column. Temperature, programmed from 100 to 200 0 C at 10 0 C/min. 1 = 2,4-Dichlorophenoxyacetic acid. (From ref. 28.) ions are formed, the monitoring of which again gives the complete recording 27 ,28_ In the negative reactant ion mode, the detector responds only to compounds that capture an electron. The response is therefore selective again, analogous to the electron-capture detector in this instance. An example of the non-selective positive reactant ion mode using a capillary column is shown in Fig. 12.3. The chromatogram resembles that obtained with the same mixture by means of an FID (Fig. 12.4). (4) The detector can operate in the non-selective mode also with a positive response 7 ,29. All product ions with drift times within a certain interval are monitored. The IMD when used as a gas chromatographic detector in mode 3 (the nonselective positive reactant ion mode 7) and in mode 1 (the IMD spectrum23 ,25) is one order of magnitude more sensitive, and in mode 3 (the selective negative reactant ion mode with the use of oxygen 27 ) up to two orders of magnitude more sensitive than the FID. The dependence of the response on the amount of the compound is not linear 25 ,27,28. The utilization of a photoionization source (UV lamp) instead of a 63Ni ionization source results in a lack of reactant ions and uncomplicated fragmenta-

283 1.0 N

o

0.5

0 0

I

5 REACTANT ION REGION

I

10

15

20

PRODUCT ION REGION

1.0 ~

~ <{

0.5

J , t t

f

0 0

10

5 DRIFT

15

f 20

T I ME, msec

Fig. 12.8. Comparison of benzene ion mobility spectra using 63Ni (top) and photoionization (bottom) sources. (Reprinted with permission from ref. 29.) tion patterns 29 (see Fig. 12.8). As the energy of the 10.00-eV Kr lamp is substantially lower than the ionization potential of nitrogen (15.58 eV), no ionization occurs and, hence, no background peaks can be observed. The advantage of this fact is obvious from Fig. 12.8. The reactant ions present in the 63Ni ionization spectrum obscure a portion of the ion mobility scan from 6 to 8 msec. Product ions of any kind having mobilities similar to those of the reactant ions cannot be observed in the form of discrete peaks separate from the reactant ions. The first two peaks from the three product ion peaks present in the 63Ni spectrum of benzene are not completely resolved from the (H20)nH+ reactant ion peak. When using a UV lamp, a single large production peak with a drift time of 7.31 msec is observed. This drift time is virtually identical with that of the (H20)nH+ reactant ion with 63 Ni ionization. Laser multi-photon ionization (MPI)30,31 was also used as an ionization source. The MPI process allows the direct ionization of organic compounds with the production of only one peak, which is either the molecular ion or MH+. Multiwavelength-selective ionization of organic compounds in the IMD can be obtained

284 TABLE 12.1 IONIZATION AS A FUNCTION OF WAVELENGTH IP = Ionization potential; + = high response; Reprinted with permission from ref. 31.

±

Compound

Anil i ne N-Methyl anil i ne m-Toluidine 2,4-Lutidine p-(n-Butyl)aniline N,N-Diethylaniline N,N,3,5-Tetramethylaniline N,N-Dimethyl anil ine 2,4-Dimethylaniline Hexylamine . Diisopropylamine Triethylamine tert.-Butylamine sec.-Butylamine n-Butylamine Methylamine Formamide Dimethylformamide Benzene Toluene Xylene Phenol Cresol Cyclohexane Hexane Indene Pyridine Pyrrole Naphthalene Azulene Anthracene Phenanthrene Tetracene p-Nitrotoluene 2,6-Dinitrotoluene Methanol Ethanol l-Nonanol 1-0ctanol Acetone Benzophenone Benzaldehyde p-Dioxane Ethyl formate

= weak response; -

Mol wt.

IP (eV)

Expected cut-off (nm)

Wavelength (nm)

93 107 107 107 149 149

7.7 7.32 7.5 8.85 7.53 6.99

322 338 330 280 329 354

±

149 121 121 101 101 101 73 73 73 31 45 80 78 92 106 94 108 84 86 116 79 67 128 128 178 178 228 137 182 32 46 144 130 58 182 106 88 74

7.25 7.14 7.44

320 310 293 280 266 249 194 + +

+ + +

+ +

+ +

342 347 333

+ + +

+ + +

7.73 320 7.50 330 8.64 287 8.70 285 8.71 284 8.97 276 10.25 242 9.12 ' 271 9.23 268 8.82 281 8.5 291 8.51 291 8.52 291 9.8 253 9.45 262 8.81 281 9.3 266 8.2 302 8.13 305 7.42 334 7.43 333 7.80 317 7.01 353 9.82 252

± ;!:

± + +

10.84 10.49

no response.

+ +

+ + + + + +

+ + + + + +

+ + + + + +

+ + + + + +

+ + + +

+ + + +

+ + +

+ + +

+ + +

+

+

+

+ +

+

+ + + ± +

+

+

+

+

+ +

+ +

+ + +

±

±

+ +

+ +

± ± +

+ + + +

+ + + + +

+

+ + + + +

+ +

+ +

+ ±

± +

+

+ ± +

± + ± +

+

+

+

±

+ + +

+

+

+

+

± +

+

+ + + +

+

+ +

+

+ + + +

+ +

+ +

±

228 236 ± ±

9.98 9.4 9.52 9.13 10.61

248 263 260 271 233

+ +

±

+

+

±

+ ±

+ + +

+ + + + + + + + +

+ + + +

+ + + + + + + + + + + + + + + +

+

285

Drift Time (msec) 5

10

15

20

25

r--+--+--4~Ir-4------

0.5

19.1 msec 2QOmsec

A

to DriftTime (msec) 5

VJ

:;

10

15

20

25

0

>

"5

c .21 0.5

(/)

B

c

0

:.;:; 0

N

·c

to 20.0 msec

.2

Drift Time (msec) 5

10

15

20

25

13.6 msec

0.5

C

to Fig. 12.9. Ion mobility spectrum of p-xylene and N-methylamine. (A) 266 nm; T = 220 oC; input laser energy, 0.1 mJ; laser beam size, 2 x angle; electric field, 170.3 V/cm; drift gas flow-rate 600 cm 3/min. source, 310 nm; input laser energy, 0.25 mJ; other conditions as in source. (Reprinted with permission from ref. 31.)

Laser source, 6 mm rect(B) Laser (A). (C) 63Ni

with MPI 31 . In resonant two-photon ionization, a molecule will ionize if the two-photon energy is greater than the ionization potential of the molecule and if there is a real intermediate state resonant with the first photon. Table 12.1 presents the ionization results for 44 organic compounds in an IMD based on eight different wavelengths in the UV region. It is evident that amines, for instance, can be distinguished from aromatic hydrocarbons by this wavelength selectivity. This effect is illustrated in Fig. 12.9 for the N-methylaniline and p-xylene

286 TABLE 12.2 ISOMER REDUCED ION MOBILITIES (Ko) AND AVERAGE COLLISIONAL CROSS-SECTIONS

(~D)

Ko is in cm 2/V-sec and nD in ~2. Reprinted with permission from ref. 32.

Compounds

Fluorotoluenes Dimethoxybenzenes Methylphenetoles Toluic acid methyl esters

Ion

+ 1*+ 2*+ + +

Acetotoluidides

+

Phthalic acid methyl esters

+

Ortho

Para

Meta

Ko

~D

Ko

~D

Ko

~D

1.939 1.733 1.738 1.706 1.680 1.691 1.623 1.610 1.540 1. 527

115.9 127.0 126.7 129.2 130.2 129.3 134.8 135.9 138.5 140.6

1.951 1.761 1.757 1.703 1.649 1.663 1.602 1.587 1.450 1.481

115.2 125.0 125.3 129.5 132.6 131.5 136.6 137.9 148.0 145.0

1. 951 1.766 1.763 1. 716 1.671 1.674 1.614 1.599 1.466 1.497

115.2 124.7 124.8 128.5 130.9 130.6 135.5 136.8 146.5 143.5

* 1, The original samples were analysed by gas chromatographic injection of the peaks into the plasma chromatograph; 2, the same samples injected in methanol solvent directly into the plasma chromatograph inlet three days later. pair. Both compounds ionize at 266 nm and the two peaks differing in their mobilities partially overlap. The N-methylaniline peak only appears at 310 nm. The combination of gas chromatography with an IMD allows us to distinguish 25 ortho-, meta- and para-isomers and also ais- and trans-isomers ,32. Isomers give different drift times. Meta-substituted isomers are larger than para- and ortho-isomers (compare their reduced ion mobilities and average collision crosssections in Table 12.2). Therefore, plasma chromatography is isomer selective 25 ,32,33. The sensitivity of detection is also structure dependent 25 ; for instance, the detection limit for p-chlorodiphenyl oxide is about 0.01 ng and for o-chlorodiphenyl oxide it is about 0.4 ng. 12.4. EFFECT OF BACKGROUND Column bleed from different liquid stationary phases exhibits different degrees of reactivity towards the reactant ions, positive or negative 25 ,34. This contributes to the baselines and results in discrete peaks of ions that can show different reactivity toward the sample molecules compared with the original reactant ions.

287 a

Dexsil410 yDexsil300 c It. OY-210 0.9 OY-101 ......; UCW-98 -30 -410 SP-525 08 ---- ... ...-

tO

0.7 06

QY-17 .

05

HI-EFF 2GP(EGP)

04

0.3 0.2

Carbowax E-20M

OY-25

0.1 40

50

60

Time(min)

b

Carbowax E-20M Carbowax E-4ooo

....-___~~,..a:::=--.-6- OY-7

RL--:,=--~~~~~~------;.,20

30

40

50

60

--

_J.

70

~-;!;;-'------;!', 80

90

Time(min)

Time!mini

Fig. 12.10. Relative conditioning rates for GC stationary phase in plasma chromatograph. c, Normalized reactant ion concentration. (a) Positive mode in air; (b) positive mode in nitrogen; (c) negative mode in air. Temperature, 200 0 C. DEGS = Diethylene glycol succinate. EGP = Ethylene glycol phthalate. (Reprinted from ref. 34, with permission.)

288 Fig. 12.10 shows plots of the fractional return of the reactant ion signal to its initial intensity as a function of time following the introduction of 0.1 mg of stationary phase into the sampling port of the plasma chromatograph for positive and negative modes 34 • For the positive mode with air as carrier and drift gas, the plasma chromatograph gives little response to Dexsil 410, OV-210, Dexsil 300 and OV-l0l. These stationary phases, together with DC 410. UCW-98, SE-30 and OV-7, if thermally conditioned to remove volatiles, should be acceptable for plasma chromatography (the faster the return of the reactant ion concentration, the less is the interaction of the plasma chromatograph with the bleed from the stationary phase). With nitrogen as carrier and drift gas, Carbowax E-20M is also acceptable. A comparison of the relationships in the positive and negative modes for OV-210 and SP-525 clearly indicates differences in reactivity for volatiles towards the corresponding reactant ions. The highly electronegative trifluoropropyl group in OV-210 shifts this phase to the right in the negative mode with air. However, the curve for SP-525 shifts to the left, which demonstrates its applicability to plasma chromatography in the negative mode. The introduction of a dopant compound (hexane or tetrachloromethane) into the detector usually causes a decrease in the detection sensitivity in both the selective positive and selective negative mode 35 . At a hexane mass rate of 3.10- 7 g/sec this decrease amounts to ca. 50% in the positive mode with naphthalene. In the negative mode the decrease is ca. 66% at a mass rate of 8.10- 10 g/sec of carbon tetrachloride with hexachloroethane. REFERENCES 1 2 3 4 5 6

F.W. Karasek, Res./DeveZop., 21, March (1970) 34. M.J. Cohen and F.W. Karasek, J. Chromatogr. Sei., 8 (1970) 330. F.W. Karasek, Res./DeveZop., 21, (1970) 25. F.W. Karasek, AnaZ. Chem., 46 (1974) 710A. F.W. Karasek and D.W. Denney, AnaZ. Chem., 46 (1974) 633. 0.1. Carroll, R.N. Dzidic, R.N. Stilwell and E.C. Horning, AnaZ. Chem., 47 (1975) 1956. . 7 M.A. Baim and H.H. Hill, Jr., AnaL. Chern., 54 (1982) 38. 8 LA. Mason and H.W. Schamp, Jr., Ann. Phys. (N.Y.), 4 (1958) 233. 9 F.W. Karasek, 0.5. Tatone and D.W. Denney, J. Chromatogr., 87 (1973) 137. 10 F.W. Karasek, D.M. Kane and 0.5. Tatone, AnaZ. Chem., 45 (1973) 1210. 11 F.W. Karasek and D.M. Kane, AnaZ. Chem., 46 (1974) 780. 12 F.W. Karasek and D.W. Denney, J. Chromatog~., 93 ,tl9]4) 141. 13 F.W. Karasek, D.W. Denney and LH. DeDecker, Anaz'\ Ch~m., 46 (1974) 970. 14 F.W. Karasek and S.H. Kim, J. Chromatogr., 99 (1974) ~: 15 F.W. Karasek and S.H. Kim, AnaZ. Chem., 47 (1975) 1166. 16 F.W. Karasek, D.E. Karasek and S.H. Kim, J. Chromatogr., 105 (1975) 345. 17 F.W. Karasek, A. Maican and 0.5. Tatone, J. Chromatogr., 110 (1975) 295. 18 F.W. Karasek, H.H. Hill, Jr. and S.H. Kim, J. Chromatogr., 117 (1976) 327. 19 F.W. Karasek and D.M. Kane, J. Chromatogr. Sei., 10 (1972) 673. 20 F.W. Karasek and D.W. Denney, AnaZ. Chem., 46 (1974) 1312.

289 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

D.S. Ithakissios, J. Chromatogr. Sci., 18 (1980) 88. F.W. Karasek, Anal. Chem., 43 (1971) 1982. F.W. Karasek and R.A. Keller, J. Chromatogr. Sci., 10 (1972) 626. S.P. Cram and S.N. Chesler, J. Chromatogr. Sci., 11 (1973) 391. J.C. Tou and G.U. Boggs, Anal. Chem., 48 (1976) 1351. F.W. Karasek, H.H. Hill, Jr., S.H. Kim and S. Rokushika, J. Chromatogr., 135 (1977) 329. M.A. Bairn and H.H. Hill, Jr., J. High Resolut. Chromatogr. Chromatogr. Commun., 6 (1983) 4. M.A. Bairn and H.H. Hill, Jr., J. Chromatogr., 279 (1983) 631. M.A. Bairn, R.L. Eatherton and H.H. Hill, Jr., Anal. Chern., 55 (1983) 1761. D.M. Lubman and M.N. Kronick, Anal. Chern., 54 (1982) 1546. D.M. Lubman and M.N. Kronick, Anal. Chern., 55 (1983) 867. D.F. Hagen, Anal. Chem., 51 (1979) 870. T.W. Carr, J. Chromatogr. Sci., 15 (1977) 85. T. Ramstad, T.J. Mestrick and J.C. Tou, J. Chromatogr. Sci., 16 (1978) 240. M.A. Bairn and H.H. Hill, Jr., J. Chromatogr., 299 (1984) 309.

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291

Chapter 13

t4ISCELLANEOUS DETECTORS CONTENTS 13.1. 13.2. 13.3. 13.4. 13.5. 13.6.

Introduction . . • . • • . • . Plasma-emission spectrometry. Atomic-absorption spectrometry Ion-selective electrodes ••• Piezoelectric sorption detector Mass and infrared spectrometry . . • • . . . . . • • • • • . 13.6.1. Interfacing gas chromatography and mass spectrometry 13.6.2. Mass spectrometer as a GC detector 13.6.3. Methods of ion production 13.6.4. Infrared spectrometer References . . . . . . . . . • . . . . .

291 291 294 294 295 296 297 297 298 300 305

13.1. INTRODUCTION The previous chapters described selective detectors that are commonly used in gas chromatography (GC) and manufactured commercially. In addition to these detectors, many other physico-chemical principles can be applied for selective detection, e.g., polarographyl, nuclear magnetic resonance 2 ,3, atomic-fluorescence spectrometry4, fluorescence spectrometry5-15, ultraviolet spectroPhotometryI6-21, elemental analysis 22 ,23, and laser absorption 24 . However, these detectors have so far been used for research purposes. Detectors that have found wider use, e.g., atomic-absorption spectrometers, ion selective electrodes, piezoelectric detector and, above all, plasma emission spectrometers, are briefly described in this chapter, which also contains concise data on infrared and mass spectrometry. 13.2. PLASMA-EMISSION SPECTROMETRY Detection by means of emission spectrometry is based on the same principle as in flame photometric detection, but it is a plasma that serves as the emission source in this instance. The plasma consists of a mass of predominantly ionized gas at a temperature of 4000-10 000 K. This state can be maintained by an electrical discharge through the gas [d.c. plasma (OCP)] or indirectly via inductive heating of the

292

Argon-Hydrogen Gas Supply

Microwove

I

Generator

Oscilloscope

Stobilized Power Supply

Strip Chart Recorder

Fig. 13.1. Schematic diagram of the GC-MIP system. 1 = Microwave cavity and plasma; 2 = quartz lens; 3 = sample injection port; 4 = reflected power meter; 5 = diffraction grating; 6 = photomultiplier power supply; 7 = vibrating plate. (Reprinted with permission from ref. 52.) gas by means of an electromagnetic field established using, power generated at radiofrequencies [inductively coupled plasma (rCP)] or microwave frequencies [microwave-induced plasma (MIP)J. The analyte,excitation results from electron impact and from collision with metastable atoms of the plasma support gas (usually argon or helium). Emissions from these species, selected by means of a monochromator, are used to detect the presence of the compound in the effluent rare gas stream25 - 79 • In order to achieve selective detection, it is necessary to form predominantly atomic rather than molecular species, as the latter often overlap the major atomic spectral lines. Most of the literature is concerned with MIP emission detectors. A schematic diagram of a GC-MIP interface is given in Fig. 13.1. The GC-MIP combination has been used for the determination of phosphorus 26 ,28, 31,38,54,68, halogens25,27-31 ,33,37,38,54,68,71 ,76,77, nitrogen 35 ,39, chromium41 ,47,63,68, 1ead 44 ,52 ,54 ,67 ,68 ,72,74, mercury36 ,43 ,46,54,61,68,75, s il icon 48 , 54,68, selenium42 ,68, arsenic44 ,45,68 , manganese54 ,68 , boron 68 ,73, sulphur 25 ,29, 32,35,38,39,54,62,68,70, iron 63 ,68, tin68,72, molYbdenum68 , vanadium68 , germanium68 ,72, niobium68 , tungsten 68 , ruthenium68 , osmium68 , cobalt68 , nicke1 68 , aluminium68 , gallium40 compounds, including metal chelates, and hydrogen isotopes39,51 ,68. Examples of the sensitivity, linear dynamic range and selectivity for individual elements are given in Table 13.1. Of course, the detector can be made non-selective by observing C lines25,35,38,39,61 ,63,67,68,76 . If a polychromator/microcomputer system is used, the simultaneous monitoring of multiple atomic-emission wavelengths can be carried out through an entire chromatographic run 75 •

293

TABLE 13.1 DETECTION LIMITS, SELECTIVITIES AND LINEAR DYNAMIC RANGES Reprinted with permission from ref. 68. Element

Emission wavelength (nm)

Detection 1imi t (pg)

Minimum detectabil ity (pg/sec)

Selectivity

Linear dynamic range

H H 2H( I) V(I I) Nb(I1) Cr(I1) Mo(I1) W(II) Mn(I I) Fe(I!) Ru (I I) Os (I I) Coli) Ni (I 1) Hg(I) B(I) Al (I) C(I) Si (I) Ge(I) Sn( I) Pb(I) Pb(I) P(I) As(I) S(I I) Se(I ) F( I) C1{II) Br (I 1) Br(I I) I{I)

486.1 656.3 656.1 268.8 288.3 267.7 281.6 255.5 257.6 259.9 240.3 225.6 240.7 231.6 253.7 249.8 396.2 247.9 251.6 265.1 284.0 283.3 405.8 253.6 228.8 545.4 204.0 685.6 479.5 470.5 478.6 206.2

45 22 20 26 335 19 25 646 7.7 0.89 35 34 18 5.9 60 27 19 12 18 3.9 6.1 0.71 7.2 56 155

16 7.5 7.4 10 69 7.5 5.5 51 1.6 0.28 7.8 6.3 6.2 2.6 0.60 3.6 5.0 2.7 9.3 1.3 1.6 0.17 2.3 3.3 6.5

5-10 2 5-10 2 5-10 2 10 2 10 2 10 3 5-~02 10 10 3 10 3 10 3 10 3 5-10 2 10 3 10 3 5-10 2 5 -10 2 10 3 5-10 2 10 3 10 3 10 3 10 3 5-10 2 5-10 2

62 64 155 106 106 56

5.3 20 43 33 34 21

74 160 194 5.69 -1 04 3.21-10 4 1.08-10 5 2.42-10 4 5.45 -10 3 1.11-10 5 2.80-10 5 1.34-10 5 5.00-10 4 1.82'10 5 6.47-10 3 7.69-10 4 9.25-10 3 3.90-10 3 1.00 1.58-10 3 7.57 -10 4 3.58-10 5 2.46-10 5 2.00-10 5 1.06 -10 4 4.70-10 4 76 1.09-10 4 573 610 274 599 5.01-10 4

10 3 5-10 2 5-10 2 5-10 2 5-10 2 5-10 2

. 56 57 .. 55-57 55-57 The GC-ICP system has been used for t1n ' , slllcon , lead , 65 compounds. The GC-DCP system has been used .1ron 56,57. G6 d , n1trogen an oxygen f or c hrom1um . 50 , copper 50. d· 53 , 1ea d60 , t1n . 60 , Sl·1 lcon . 60 , , me ke153 , pa 11 alum 62 49 60 germanium , sulphur and manganese compounds •

294 13.3. ATOMIC-ABSORPTION SPECTROMETRY The interfacing of an atomic-absorption spectrometer (AAS) to a gas chromatograph is accomplished 58 ,80 through the atomizer (direct connection of the column to the burner gas flow, connection of a heated transfer line from the column to the injection port of the electrothermal device, or cold vapour tube atomizer interface for mercury compounds). Thus, both flame 81 - 84 and flameless 85 - 91 atomization are used. However, the minimum detectability with the flameless mode is two to three orders of magnitude 10wer80 The AAS is particularly attractive for. detection of organometallic compounds. This detector has been used for lead81 ,83-85,87 (217 nm), tin 88 ,90,91 (224.6 nm), mercury92-94 (253.7 nm), chromium95 ,96 , arsenic 88 (193.7), silicon 82 (251.6 nm) and selenium86 ,88 (196 nm), the detection limits being 0.1,0.1,0.02, 1,5, 100, and 1 ng for Pb 87 , Sn 90 , Hg 93 , Cr 95 , As 88 , Si 82 and Se 86 compounds, respectively. The linear dynamic range covers about four orders of magnitude 91 13.4. ION-SELECTIVE ELECTRODES The gas chromatographic effluent is drawn through a reaction tube at 8001000 0C. The separated components undergo hydrogenolysis in the presence of a catalyst, forming hydrogen sulphide, hydrogen chloride, hydrogen fluoride and ammonia. The gases are dissolved in a suitable absorption solution and the concentrations of the ions produced are monitored continuously in a flow-through a cell with an appropriate ion-selective electrode (Fig. 13.2). Non-absorbed

:-----l I I I

I

L ____ --.J

1

Fig. 13.2. Schematic diagram of the combination of gas chromatograph, reaction tube and ion-selective electrode. 1 = Gas chromatograph; 2 = reaction tube; 3 = tube consisting of a gas-liquid contact area and a gas-liquid separation area; 4 = absorption solution; 5 = micropump; 6 = stream buffer; 7 = ionselective electrode; 8 = reference electrode. (Reprinted with permission from ref. 99.)

295 gases, such as methane, are separated together with the carrier gas from the absorption solution in a gas-liquid separator and vented from the system. Selective responses are obtained for sulphur 97 ,98, chlorine 97 , fluorine 99 , 101 bromine 100 and nitrogen compounds • A detector equipped with two ion-selective electrodes allows the simultaneous determination of two types of compound through dual-channel operation (chlorine/fluorine-containing compounds 102 or bromine/fluorine-containing compounds 103 ). The products of hydrogenolysis can also be split into two parts. One part passes through a granular silver absorber into a flame-ionization detector and the other passes through a dual ionselective electrode cell (chloride and bromine). Thus, the atomic ratios of chlorine, bromine and carbon in halogen-containing organic compounds can be determined 104 in this way. Mercaptans can be detected selectively without hydrogenolysis by using a silver/sulphide ion-selective electrode 105 • The detection limits are 10- 12 mole of sulphur compounds 103 , 10- 11 mole of fluorine compounds, 10- 10 mole of chlorine compounds and 5.10- 11 mole of bromine compounds 100 • The linear dynamic range is about 10 4 and the selectivity for sulphur compounds 97 and the N:Cl response ratio 101 are 2.10 3• The response time of the detector is high 99 • 13.5. PIEZOELECTRIC SORPTION DETECTOR A quartz crystal vibrating at a constant frequency in the megacycle range exhibits a decrease in frequency when substances are adsorbed directly on the surface of the crystal coated with a thin film of a liquid or solid sorbent. . 0 bta1ne . d When used as a detector for gas chromatography 106-116 , the response 1S by coating the crystal with a film of the same liquid phase as that commonly used as the stationary phase for the column. The eluted compounds passing over the quartz crystal surface dissolve in the coating, thus changing the resonant frequency of the oscillating piezoelectric crystal. The detector response is given by the equation ( 13.1) where w is the total weight of the eluent, y is the activity coefficient of the eluent in the crystal coating, po is the vapour pressure of the eluent at the operating temperature, F is the carrier gas flow-rate and c is a constant that is characteristic of the detector temperature, the crystal and the liquid phase used to coat the crystal. Thus, the detector response increases with increasing molecular weight and boiling point of the eluting compounds, making the wide, low peaks at the end of the chromatogram more detectable.

296

THERMAL DETECTOR

Fig. 13.3. Chromatograms from differential piezoelectric sorption detector and thermal conductivity detector. (From ref. 107.) The detector can be made selective by the choice of the crystal coating, " ge 1 , mo 1ecu 1ar Sleve " 106 or a hygroscoplC "1 e.g., Sl"1 lca po ymer 107 f or th e de t ermination of water, lead acetate for the determination of hydrogen sulphide 106 and inorganic salts 111 for organophosphates. It is obvious that the detector selectivity is limited by being given by the relative sorption properties of the system only. The chromatogram in Fig. 13.3 shows the results obtained with a differential piezoelectric sorption detector. The response of the polar detector (dinonyl phthalate coating) is made equal to that of the non-polar detector (DC 200 silicone oil) for alkanes. The response is obtained for polar compounds only. The detection limit 106 ,114 is about 10- 9 g and the linear dynamic range 107 is 10 4 . 13.6. MASS AND INFRARED SPECTROMETRY As already stated in Chapter 1, a great advantage of GC is its high separation efficiency. This means that GC is able to separate mUlti-component mixtures into single components. However, the identification ability of GC does not match the level of its separation ability. On the other hand, mass spectrometry (MS) is an analytical technique with a high identification ability. By combining these two techniques, we obtain an almost ideal analytical unit where MS operates as a chromatographic detector. Much time has passed since combined GC-MS was used for the first time 117 , and nowadays it is a highly developed independent technique. There is a large literature on this subject and to deal with it in detail would require a separate book. Therefore, only the principles are briefly described here and readers are referred to the numerous books available, e.g., refs. 118-128.

297 13.6.1. Interfacing gas chromatography and mass spectrometry

It has been stated that "The development of a gas chromatographic-mass spectrometric analysis system is not a mere combining of the two techniques. There are inherent incompatibilities of operational procedures when the techniques are performed separately that must be resolved when they are combined"121,129. The GC-MS interface has frequently been a point of difficulty in combining these techniques. The difficulty arises from the fact that a mass spectrometer must operate with sufficient vacuum for the ions to traverse the analyser without collisions (10- 4-10- 5 Torr), whereas a typical gas chromagraph operates with its exit at atmospheric pressure. With packed columns it is mostly necessary to use so-called molecular separators as interfaces between the GC and MS instruments 125 ,127,128,130-132. Thelr task is to remove as many molecules of the carrier gas as possible from the column effluent and to transport the maximum amount of the organic solute into the mass spectrometer ion source. These two functions are measured by an enrichment factor N, indicating the amount of the carrier gas removed from the GC peak, and by the yield Y, indicating the percentage of the sample that actually reaches the MS ion source. The jet separator133-136 is a based on the fractionation of gases in an expanding jet stream. Effusion-type separators137-142 are based on selective effusion through fine pores or through a narrow slit. In membrane separators, the preferential diffusion of the carrier gas or of the sample takes place through a semi-permeable membrane (PTFE separator 137 ,143, palladium-silver separator144-146 or silicone separator147-150). GC-MS interfacing without a molecular separator can be accomplished either as an open split coupling or as a direct coupling. The former seems to be the preferred interfacing technique at present151-153. Direct coupling can be used for a well pumped chemical ionization system and also for packed columns, whereas capillary columns can be connected directly to the most modern GC-MS systems. 13.6.2. Mass spectrometer as a GC detector

There are two ways in which chromatograms can be obtained from the output of a mass spectrometer: by recording either the total ion current or the ion current of a selected ion mass. A general chromatographic record, similar to those provided by other nonselective GC detectors, can be obtained by measuring the total ion current (TIC). The TIC is formed from the solute molecules eluting from the gas chromatograph and is recorded as a function of time. In sel'ected ion monitoring (SIM), the

298 intensities of pre-selected ions, characteristic of a class or of a particular compound, are recorded as a function of time, SIM is made possible by rapid switching from one mass to another in a very short time. Chromatographic peaks appear only if the compound producing the chosen ion is present. In this mode the mass spectrometer operates as a classical selective detector responding only to a certain type of compound. The sensitivity that can be obtained in the single ion mode is up to two orders of magnitude higher than that with the TIC (the ion current is integrated for a longer time), e.g., the detection limit for steroids 154 (mass 436.319) is 2.10- 14 g, for benzophenone 155 with chemical ionization (mass 183) 1.10- 14 g and for dopamine derivatives 156 with negative ion chemical ionization 1.10- 14 g. It is possible to obtain a mass spectrum for every chromatographic peak in real time. Hence, in this mode, a mass spectrometer is a highly specific GC detector. The spectra produced are compared with those of known compounds by a computer system. The information obtained indicates what compound has eluted in an observed peak. The chromatograms produced by a GC-MS system may either be generated in real time or reconstructed-by a computer. With computer control, the spectra can be scanned and acquired repetitively every few seconds during the chromatographic run. In this way, three-dimensional ion signals are generated, each characterized by mass, intensity and time. All the spectra are stored in the computer. The total ion chromatogram can be reconstructed by summing the ion intensities obtained in each repetitive scan. By retrieVing the intensities of the chosen ion from each scan and by plotting them as a function of time, mass fragmentograms (selected ion chromatograms) can be reconstructed. Fourier transform Ms 157 ,158 and MS_MS 161 can also be used in connection with gas chromatography150,160. 13.6.3. Methods of ion produation

The following methods of ion production are mostly used in GC-MS: electronimpact ionization (EI), chemical ionization (CI), field ionization (FI), and atmospheric pressure ionization (API)123,128,162. EI is the commonest method. Electrons of 70-eV energy are produced in a collimated stream by thermionic emission from a filament. This electron beam falls on neutral molecules entering the ion source, and the energy transmitted brings about ionization and fragmentation of the sample molecules. The molecular ionization is initiated when the electron energy is greater than the ionization potential of the compound. A disadvantage of this mode of ionization

299 is that in most instances very complicated spectra arise in which the molecular ion is mostly missing. For structural analyses this is, however, an advantage. Methane and isobutane are the most often used reactant gases in CI_MS163-167 The CI mass spectra often show abundant characteristic protonated molecular ions, even when the corresponding EI mass spectra show no detectable molecular ions. The primary ions produced in CI by the EI of the reactant gas are, in comparison with the analyte, in a large excess at pressures of about 1 Torr. The ionized reactant gas undergoes ion-molecule self-reactions to form a steadystate plasma. The ions produced react by proton and hydride transfer to give M+1 and M-1 ions that may further dissociate. The CI mass spectrum of the sample is dependent on the ions of the reactant gas. The ion molecular reaction is a much milder process than EI. CI mass spectra are less complex and, therefore, easier to interpret than El mass spectra. Negative CI gives a lower detection limit for certain types of compounds, because the negative quasimolecular ion spectra of some compounds are one to three orders of magnitude more intense than the positive ion spectra1~2. It is possible to obtain both the positive and negative spectra at the same time by pulsing the polarity of the ion source potential and focusing the lens potentia1 168 . In the FI source 169 ,170, soft ionization is promoted by an extremely high potential gradient (about 10 7-10 8 V cm- 1). The molecules passing through this gradient will modify the field and allow an electron to escape from the molecule, thus producing an ion with a small excess of energy available for fragmentation. i~ass spectra are generally characterized by the presence of prominent ion or "parent ion" peaks with only a few minor fragment ion peaks. FI is five to ten times less sensitive than EI {ca. 50 nm)171. In the API source ion-molecule reactions occur in an inert gas weak plasma at a pressure of 1 atm '56 ,172,173. The source can work in both positive and negative ion modes. Two mass detectors, small compact units, are currently commercially available. The ions of a certain mass can be stored in stable paths for many seconds in an ion trap174,175. A GC detector 176 , the so-called ion trap detector 21 ,177 (ITO) (Finnigan-MAT), has been constructed on this principle. The ion trap is scanned over a mass range whereby the ions are ejected from the ion storage region sequentially from low to high mass. The ejected ions are detected by a conventional electron multiplier. The ITO also gives a universal gas chromatogram (the instrument is scanned repetitively over a user-selected mass range). complete mass spectra of all the eluting compounds and selected ion detection (up to sixteen pre-selected ions during each scan). The Hewlett-Packard massselective detector 178 ,179 with a hyperbolic quadrupole analyser can also operate in the TIC mode so that a spectrum of any peak can be obtained, or in the SIM mode with twenty channels.

300 13.6.4. Infrared spectrometer

In the last few years interest has focused on the combination of GC with infrared (IR) spectrometry180,222, particularly with the use of capillary columns. The wider application of modern GC-IR for analyses of multi-component mixtures has resulted especially from: (1) the development of Fourier transform (FT) IR spectrometry, (2) the introduction of a narrow-range mercury-cadmium telluride photodetector and (3) the construction of small, gold-coated glass "light pipes,,193. The path length is very long, which results in a relatively low detection limit. At present, the detection limit 182 ,184,191 ,202,215 ranges from 10 to 100 ng. A schematic diagram of the GC-FTIR system is shown in Fig. 13.4. There are two ways to obtain chromatograms from interferometric data. (1) In the Gram-Schmidt reconstruction each interferogram is treated as a vector and the orthogonal components of the chromatogram vectors are computed with respect to a set of background basis vectors representative of the GC baseline. A plot of the length of the orthogonal component against the interferogram number will form a reconstructed chromatogram for a particular set of interferograms 181. Specific functional groups cannot be monitored. (2) 512 (or 1024) point sections of each interferogram are transformed to obtain a low-resolution IR absorbance spectrum, which can then be integrated to determine a chromatogram

IR Source Interferometer

/

/

MGT Detector

/ /

Paraboloid

Inlet

Gas Chromatograph

Fig. 13.4. Schematic diagram of GC-FTIR instrumentation. MCT = mercury-cadmium telluride photodetector. (Reprinted with permission from ref. 212.)

301 detector response. The total absorbance is calculated for various bands or windows in the spectrum and plotted atainst the interferogram number. The socalled chemigram l85 ,21o is the plot of the integrated absorbances within five user-selected spectral windows as a function of elution time. Therefore, selective detection (analogous to SIM in GC-MS) is possible 21o (with the earlier IR spectrometers a desired wavelength could also be selected by a filter and thereby a selective chromatogram obtained I8o ,195,211). The reconstructed chromatogra~ can be compared to total ion monitoring in GC-MS. The peak heights in TIC chromatograms are often similar to those of the peaks obtained from an FLO, but this is not the case when comparing·the IR reconstructed chromatogram and the FLO chromatogram, as the IR molar absorptivities of bands of polar molecules tend to be much greater than those of non-polar molecules (see Fig. 13.5).

a

b

Scan Sets

91

182

i

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2.47

279 I

170

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4.94

453

543

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7.41

8.64

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Time (min)

Fig. 13.5. Comparison of (a) the chromatogram measured o~ an FLO located ~fter the light pipe with (b) IR reconstructed chromatogram uSlng the Gram-Schmldt algorithm. (Reprinted with permission from ref. 212.)

302

A very successful search algorithm for GC-FTIR spectra involves first normalizing the absorbance of each measured spectrum, so that the most intense band has an absorbance of unity. Each spectrum in the reference library is normalized in the same fashion. The sample spectrum is then subtracted from each reference spectrum, and the sum of the squares of each point in the differential spectrum is computed. This sum is sometimes known as the "hit index"; the reference spectrum giving the smallest hit index has the greatest 212 probability of yielding the identity of the component of interest • A subnanogram detection limit, i.e •• a limit 100 times lower than that obtained with a commercial light pipe GC-IR, is reported for the use of matrix isolation 223 . The combined use of gas chromatography and matrix isolation IR spectrometry was first demonstrated in 1979 224 , 225. A commercial unit, Cryolect. has been developed 226 • Each molecule of the GC effluent is captured into an argon cage at 12 ~ (up to 5 h of chromatography can be frozen indefinitely). Thus each molecule is isolated from the others and intermolecular interactions that give rise to band broadening are eliminated. The Cryolect unit is schematically presented in Fig. 13.6. The GC effluent is directed onto a gold-plated copper disk. After spraying helium. argon and the separated GC components onto the disk surface, helium is removed by the vacuum system. Argon and the GC components condense in a solid spiral band.

VACUUM CHAMBER r="";;::::::::::;;"-=

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303 The optical beam from the spectrometer passes through the sample matrix, is reflected by the gold disk, passes back through the sample to the second mirror and passes into an IR detector. All the IR measurements are performed after the chromatograms have been deposited 223 ,226 The complementary nature of the GC-MS and GC-FTIR techniques is well recognized 189 ,197,200,203,219. Both mass and IR spectrometry suffer from certain weaknesses in identifying compounds: the former in distinguishing chemical isomers and the latter in distinguishing long-chain homologues, as illustrated in Figs. 13.7 and 13.8, demonstrating the mass and IR spectra of tetrachlorobenzene isomers and n-alkanes, respectively. The mass spectra of the tetrachlorobenzene isomers are identical whereas the IR spectra yield data that can he uspd for identification. In contrast, for n-alkanes the IR spectra are B 1.2000

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304

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305

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Fig. 13.9. Schematic diagram of GC-FTIR-MS system. HP = Hewlett-Packard; SCOT = support-coated open tubular. (Reprinted with permission from ref. 203. Credit is given to the University of California, Lawrence Livermore National Laboratory, and to the Department of Energy under whose auspices the work was performed.) identical whereas the MS spectra differ. Hence it is obvious that the use of the two spectral methods combined with gas chromatography offers the optimum solution to the analysis of complex mixtures and the identification of individual compounds. There are two approaches to this problem: either sample analysis separately with both instrument combinations using the same chromatographic column, or tandem connection of the two spectral devices at the column outlet, i.e., GC-FTIR-MS. The latter solution is more advantageous, of course. The latest publications 196 ,203,207,220 dealing with this problem are based on the tandem arrangement (see Fig. 13.9). REFERENCES 1 2 3 4 5 6 7 8 9

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311

Chapter 14

CONCLUSION Chapter 1 discussed the advantages of using selective detectors. Another important fact follows from the chapters dealing with the individual selective detectors. With most of these detectors the minimum detectability is either approximately equal to or, in many instances, lower than that obtainable with the commonly used non-selective flame-ionization detector (FlD). Thus, in addition to a selective response, selective detectors usually also yield a lower detection limit. The application of selective detectors is most advantageous if they are used in an on-line combination with a non-selective detector. Two chromatograms can thus be obtained from a single chromatographic analysis; a non-selective record containing all peaks, and a selective record. This combination of two detectors is commonly realized with parallel connection, where the effluent from the chromatographic column is split and supplied to two detectors. With series connection of the detectors the effluent passes first through a non-destructive detector ~lectron-capture detector 1 (ECD)]. A direct dual non-selective-selective record is possible with detectors that simultaneously combine an FlD and a selective detector in a single detection system: an alkali flame-ionization detector (AFlD) or flame-photometric detector (FPD) (see the respective chapters). The second way to obtain simultaneously selective and non-selective records is to use a detection system that provides both records. Plasma emission spectroscopy can give carbon- and element-selective chromatograms simultaneously2. REFERENCES 1 P. Gagliardi and G.R. Verga, J. Chromatogr., 279 (1983) 323. 2 K.J. Slatkavitz, P.C. Uden, L.D. Hoey and R.M. Barnes, J. Chromatogr., 302 (1984) 227.

312

The following section was added in proof (see p. 174)

8.5A. REDOX CHH1ILUt1INESCENCE DETECTOR The redox chemiluminescence detector 1 (RCD) is based on catalyzed redox reactions of nitrogen dioxide with reducing agents to form nitroge~ oxide, which is detected by an ozone chemiluminescence technique. Nitrogen dioxide in helium (ca. 100 ppm) is introduced post-column just prior to a heated reaction zone, where a catalyst (gold) is present. In the presence of any solute that reduces nitrogen dioxide, nitrogen oxide is produced 2 : reducing solute

+

N0 2

Au 0" oxidized species 150-400 C

+

NO

Nitrogen oxide produced is re-oxidized with ozone and the emission arlslng from the decay of excited N0 2 back to its ground state is detected. The RCD thus provides response to the compounds that serve as reducing agents. Compound that can be detected are 1 alcohols, aldehydes, ketones, acids, phenols, olefins, aromatic hydrocarbons, amines, thiols, sulphides, hydrogen, ammonia, hydrogen peroxide and sulphur dioxide. No response is obtained with low-molecular weight paraffins, water, carbon dioxide and argon. Common compounds such as methylene chloride, l,2-dichloroethane, tetrachloroethylene and tetrahydrofuran do not generate any appreciable response. Not only the type of compound, but also the reaction temperature 3 affects the selectivity of response. For instance, in the analysis of a gasohol only methanol is detected at 360 0 C. At 390°C other components of gasoline, such as the aromatic fractions, are detected, but not the saturated alkanes. At 420°C the chromatogram becomes very complex and resembles that obtained with an FID. When palladium used as a catalyst instead of gold, alkanes produce a response at a temperature as low as 250 0 C. The response to unsaturated hydrocarbons is much greater than that to saturated ones. The detection limit 1 is 200 pg and the linear dynamic range is three orders of magn itude. REFERENCES 1 S.A. Nyarady, R.M. Barkley and R.E. Sievers, Anal. Chem., 57 (1985) 2074. 2 S.A. Nyarady and R.E. Sievers, J. Am. Chem. Soc., 107 (1985) 3726 3 R.E. Sievers, S.A. Nyarady, R.L. Shearer, J.J. DeAngelis, R.11. Barkley and R.S. Hutte, J. Chromatogr., 349 (1985) 395.

313

LIST OF ABBREVIATIONS

AAS

AFIO API CO CFID CI CLO d.c. OCP ECO EI ELCD FASO FI FlO FPO FTIR Fn1S

GC ICP IMO IP IR

ITO HAFIO HAFID-Si M

MIP MPI MS NPO N mode NP mode PIO P mode

atomic absorption spectrometer alkali flame-ionization detector atmospheric pressure ionization coulometric detector catalytic flame-ionization detector chemical ionization chemiluminescence detector direct current d.c. plasma electron-capture detector electron impact ionization electrolytic conductivity detector flameless alkali sensitized detector field ionization flame-ionization detector flame ohotometric detector Fourier transform infrared spectroscopy Fourier transform mass spectrometry gas chromatoqraohy inductively coupled plasma ion mobility detector ionization potential infrared ion trap detector hydrogen atmosphere flame-ionization detector hydrogen atmosphere flame-ionization detector for silicon compounds general flame gas molecule microwave-induced plasma multi-photon ionization mass spectrometry nitrogen-phosphorus detector nitrogen mode nitrogen-ohosphorus mode photoionization detector phosphorus mode

314

R

RCD RMR RWR S

SECS SIM TEA TIC TID TZ

UV

response redox chemiluminescence detector relative molar response relative weight response sensitivity selective electron-capture sensitization selected ion monitoring thermal energy analyser total ion current thermionic detector Trennzahl (separation number) ultraviolet

315 SUBJECT INDEX

A

Alkali flame-ionization detector 15-59, 63 - , air in 18-20, 34, 53, 54 - , carrier gas 18-20, 28, 29, 31, 37, 38, 53, 54 - , collector electrode 25, 26, 28, 34-36, 45-48 - , design 16-23 - , double flame 23, 32, 37, 38, 40, 44, 54 - , effect of anions 49-52 - , effect of cations 49-52 - , effect of compound structure 41-43 , effect of temperature 52 - , flame 15, 16, 21 - , gate electrode 22, 32, 45 - , hydrogen in 16, 18-20, 24-26, 28-31, 33-35, 38, 39 - , jet tip bore 31, 36,48,49 - , life 16, 23, 24 - , negative response 29-33, 34, 36-38, 40, 45, 52-54, 59 , peak tailing 40 - , principle parameters 18-21 - , single flame 23 - , three-electrode construction 21, 22, 25, 32, 34 - , temperature of the flame 24, 25 - , voltage 21, 22, 32, 43-45 Amines ,CLD 169, 171, 172, 312 - , ECD 223, 255, 257 - , IMD 284 - , RCD 312 Arsenic compounds , AFID 19, 38, 50 - , FPD 149 - , HAFID 94 - , PID 117 Atomic-absorption spectrometer 294

- , FPD 149 - , HAFID 93-95, 100 - . PID 116 Barbi turates ,ELCP 188 - , FASD 66 - , IMD 278 -,PID117,125 Boron compounds AFID 39 ,ECD 245-247 --, FPD 149 C

Chemigram 301 Chemi-ionization detector 83, 85 Chemiluminescence detector 161-179, 312 - , design 162, 173, 176 - , effect of temperature 163, 168, 169,312 - , fluorine-induced detector 177-179 - for nitroaromatic compounds 169, 170 -- for nitrogen-containing compounds 170-174,312 for N-nitroso compounds 161-168 - with sodium metal 174-177 Concentration-sensitive detector 6 Coulometric detector 209-215 - , design 209 , effect of temperature 212-214 - , nitrogen mode 211 - , oxidative mode 210 --, reductive mode 210 Coulometric response in ECD 263-265 Coulson detector 182, 183, 196, 201, 204 D

Detection limit B

Background current - , AFID 18-21, 24, 29, 30, 32, 36-40,44,47,48,51-54 - , FASD 69, 72-74, 76, 77, 79, 85

7,8

,AAS 294 ,CD 210 CLD 165, 177, 178,312 ECD 222, 224, 239, 241, 246, 252 ElCD 181, 194, 199, 205 FPD 145, 149

316 HAFID 94-96, 98, 99, 101, 103 ,IMD 286 IR 300 ion-selective electrode 295 ,MS 298 --, PID 116, 117 --, piezoelectric sorrtion detector 296 --, plasma-emission spectrometer 293 --, RCD 312 Detector characterization parameters 5-13 Detector --, halogen compounds 87-89 --, nitroaromatic compounds 170, 171 --, nitrogen-containing compounds 171-174 --, N-nitroso compounds 161-168 --, reducing compounds 312 Detector linearity 10 Detector noise 6, 8-10 AFID 20-22 ,ECD 220 ,FASD 70,71,73, 74 HAFID 93 PID 116 , temperature dependence 10 Detector response 5, 6 Detector sensitivity 5, 6 Detector, specific 13 Detector, substance-selective 13 Drift speed 276 Drift time 276 Dynamic detector range 11, 12 E

Electrolytic conductivity detector 181-206 --, design 181-185 --, effect of compound structure 109-192 --, effect of tempera ture 187, 189, 191-193, 195,201,203-206 --, electrode 195, 196 , gases in 186-188, 201-203 --,life 193, 205, 206 --, principal parameters of 194 --, solvent in 196-201 Electron capture coefficient 235, 237, 249 Electron capture detector 217-269 --, bipolar pulsed mode 251 --, contact potential 225 --, design 218, 219 --, direct-current mode 224, 225, 249

--, direct current mode with constant current 229 --, effect of compound structure 235-239, 243-248 --, effect of detector volume 266-269 --, effect of temperature 239-241, 244, 247 --, effect of voltage 224, 226 --, gases in 221, 250, 267-269 --, impurities in 248 -- pulse mode with constant current 227-229, 249, 262 --, pulse mode with constant frequency 225-227, 249, 262 --, pulse period 226 --, source of electrons 219-223 --, space charge effect 225 F

Flame-ionization detection 91-106 Flame-ionization detector 15, 20, 27 37,64,91-106 --, flow-rate of hydrocarbon in 105, 106 Flameless alkali sensitized detector 63-89 --, bead current 86 --, design 64, 72, 74, 79, 85, 88 --, effect of compound structure 66, 68, 81-83 --, effect of voltage 64, 73, 75, 76, 88 --, gases in 65,69,72,74,76,77, 79, 80, 88 - , heatin9 current 69, 86 -,life 84-87 - , principal parameters 70-74,80,81 - , temperature of alkali source 75, 79 Flame photometric detector 133-158 --, design 134, 135 --, dual channel 135, 144 - , dual flame 147, 148, 154-156 - , effect of compound concen tra t ion 139-141 - , effect of compound structure 138, 139 - , effect of temperature 142 - , flame stability 157 --, gases in 135, 137, 138 - , interference fil ter 142, 143 -,photomultiplier 143 --, sul phur background 151, 152 Fluorine-induced detector 177-179 J

317

G

Germanium compounds - , FPD 146, 147 - , HAFID 94 Gram-Schmidt reconstruction

300

H

Hall detector 182-184, 196, 200, 201, 204 Halogen compounds - , AFID 18,19,26-29,31,33,36, 37,41-43,49,50,52,53,55,57, 59 - , CD 210, 213, 214 --, CLD 176, 177, 312 - , ECD 238, 239, 242-244, 246, 247,253,257,259-262 ELCD 186, 188, 199, 204 FASD 71, 87-89 FID 106 FPD 147-149 HAFID 94,95, 103 IMD 278, 280, 281, 286 PID 114, 123, 124 ,RCD 312 Heteroatom 7,9, 13, 15,20,21, 30 Hydrocarbons AFrD 19, 22, 29, 37, 53 CLD 174, 175, 312 ECD 253, 254, 257 FASD 71,88 FPD 144 HAFID 93, 95, 96, 98-100, 102 IMD 278, 284 PID 112-114, 117, 119, 120 ,RCD 312 Hydrogen atmosphere flame-ionization detector 92-104 air in 102 carrier gas 102 collector electrode 100, 101 design 93, 102 for silicon compounds 102-104 hydrogen in 102 iron compounds 102, 103 , negative response 100, 101, 103, 104 oxygen in 102 , peak tailing 102 - , potential 100 - , principal parameters 98 - , silicon compounds 93, 101, 102 Hypercoulometric response in ECD 265

Infrared spectroscopy 300-305 - , matrix isolation 302,303 Ion mobility detector 275-288 - , different modes of operation 280-282 - , source of ionization 275, 282, 283, 285 Ion-selective electrodes 294, 295 Ion trap detector 299 Iron compounds ,FPD 149 - , AFID 94,95, 103 L

Lead compounds ,AFID 18, 29, 39, 40, 55 - , FPD 149 - , HAFID 94, 97, 100 - , PID 117 Linear dynamic range 11 AAS 294 CD 212 CLD 165, 169, 171, 177, 312 ECD 249-251 ELCD 193 FASD 70, 72-74, 81 FPD 150, 151 HAFID 97, 103 ion-selective electrode 295 PID 115-117 , piezoelectric sorption detector 296 - , plasma-emission spectrometer 293 - , RCD 312 Long-term noise 9 M

Mass detector 299 Mass spectrometry 296-299 - , atmospheric pressure ionization 299 - , chemical ionization 299 - , electron impact ionization 298 , field ionization 299 . - , mass fragmentography 298 - , selected ion monitoring 297 - , total ion current 297 Mass-rate sensitive detector 6 Matrix isolation, IR spectroscopy 302, 303 Minimum detectability 6,8, 9 - , AFID 18-20, 33, 35-39

318

-, -, -, -,

CLD 175 ECD 239 ELCD 194 FASD 70-74, 81 FlO 20 FPD 137-143 HAFID 93-95 PID 117

- , plasma-emission spectrometer 293 ~1inimum detectable solute concentration 6-8, 11 Minimum detectable solute mass-rate 6-8, 10 .. 11

Molecular separators 297

Photoionization detector 109-131 carrier gas 126-131 design 109, 110 principal parameters 115 radiation source 109, 110 , solvents with no response 123 Photoionization cross-section 111, 112, 114

Photoionization efficiency 11, 112, 113

Piezoelectric sorption detector 295, 296

Plasma chromatography 275 Plasma-emission spectrometry 291, 293

N

Nitro compounds AFID CLD ECD ELCD FASD IMD ,PlO

23, 35 167-170 237 199 66 278, 284 125

Nitr0gen compounds - , AFID 18-22, 29, 34-36, 49, 50, 53, 57 ,CD 211,212 - , CLD 170-174 ,ELCD 186-188, 199 - , FASD 66,68,69, 71,73 - , HAFID 95 - , PID 117, 128 Nitrogen-phosphorus detector 16, 63

Nitroso compounds

,CLD 163-169 - , ELCD 187-190

o Organometallic compounds - , ECD

237, 239 149 - , HAFID 93-96, 102 - , FPD

P

Phosphorus compounds - , AFID 18-22, 26, 33, 34, 41, 43, 49, 50, 53, 55, 57, 58 CD 211 ELCD 186 FASD 64,65,68,71,73 FPD 140, 144, 157 ,HAFID 94, 103 - , PID 117

Q

Qualitative analysis

1-3

R

Reaction gas chromatography 2 Redox chemiluminescence detector 312 --, effect of catalyst 312 --, effect of reaction temperature 312

Response --, molar 6 --, negative 13,29-34,36-38,40, 45, 52-54, 59, 100, 101, 103, 104

of detector 5, 6 ,polarity 13 --, relative molar 12 - , relative weight 12 --, specific 13 Response mechanism --, AFID 54-59 --, CD 210-212 --, CLD 161-164, 170, 174, 176, 177, 312 ECD 230-235, 252, 258 --, ELCD 186, 187 --, FASD 64-67,72,77-79,84,85,88 FlD 91, 92 FPD 136, 137 HAFID 92, 102 lMD 275-278 ion selective electrode 294 --, lR 300, 301 MS 297, 298

, piezoelectric sorption detector 295

--, plasma-emission spectrometer 291, 292 --, RCD 312

319 Response quenching ECO 248, 249 ,ELCO 190, 192 --, FPO 152-156 --, IMO 287, 288 --, PID 122 Response reproducibility --, AFIO 23,24 --, ELCO 200 --, FASO 84-87 Response selctivity 12, 13 --, AFIO 13, 33, 36, 37 --, CLO 167-169, 312 --, ECO 237-239, 242, 244, 253, 254, 260, 261 ELCD 186-189 FASO 70-74, 76, 81 FPO 144, 145 HAFID 94, 94, 98, 99, 102-105 Hm 281, 282 ,IR 301 --, MS 298 --, PIO 118-126 --, piezoelectric sorption detector 296 --, plasma-emission spectrometer 293 --, RCD 312 Response sensitization in ECO 251-263 aromatic hydrocarbons 263 , effect of nitrous oxide 252-256 --, effect of oxygen 257-263 S

Selenium compounds in FPO 149, 155 Separation number 1 Short-term noise 8,9 Silicon compounds --, AFID 39, 40 --, FlO 105, 106 --, HAFIO 102-105 Solute switching 265, 266 Substance selective response 7 Sulphur compounds AFIO 18, 20, 29-32, 38, 41, 53 CD 210,211 CLO 174, 178, 179, 312 ELCD 188, 200 FPO 141, 144, 157 HAFID 95 PID 117 ,RCD 312 Surface ionization detector 87

T

Thermal energy analyzer 163, 169, 170 Thermionic detector 16, 163 Tin compounds --, AFID 18, 19, 39, 40, 55 --, FPD 147-147 --, HAFIO 94, 98-100 Trennzahl (see Separation number) W

Weight response 6

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