Detectors in Gas Chromatography (Journal of chromatography library)

Journal of Chromatography Library - Volume 4

DETECTORS IN GAS CHROMATOGRAPHY

JOURNAL OF CHROMATOGRAPHY LIBRARY Volume 1 Chromatography of Antibiotics by G. H. Wagman and M. J. Weinstein Volume 2 Extraction Chromatography edited by T. Braun and G. Ghersini Volume 3 Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Janak Volume 4 Detectors in Gas Chromatography by J. SevCik

Journal of Chromatography Library - Volume 4

DETECTORS IN GAS CHROMATOGRAPHY JIkf SEVcfK Department of Analytical Ciieunistrj., Ciiarles University, Prague

ELSEVIER SCIENTIFIC PUBLISHING COMPANY AMSTERDAM - OXFORD - NEW YORK 1976

Distribution of this book is being handled by the following team of publishers for the U.S.A. and Canada AMERICAN ELSEVIER PUBLISHING COMPANY, INC. 52 Vanderbilt Avenue New York. New York 10017 for the East European Countries, China, Northern Korea, Cuba, Vietnam and Mongolia SNTL, PUBLISHERS OF TECHNICAL LITERATURE Prague for all remaining areas ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P. 0. Box 211, Amsterdam, The Netherlands

Library of Congress Cataioging in Publication Data

“

4’

.(’

Sevcik, Jiri. Detectors i n gas chromatography. (Journal of c h r m t c g r a p h y l i b r a r y ; v. 4 ) Includes b i b l i o g r a p h i c a l referenees and index. 1. Gas chranatography. I. T i t l e . 11. Series. 0 7 9 . C45548 544’ .926 75-30850 ISBN 0-444-9985 7 -8

Q JIRI SEVCfK 1975 Translation 0 KAREL STULfK 1976

ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED, STORED IN A RETRIEVAL SYSTEM, OR TRANSMITTED I N ANY FORM OR BY ANY MEANS, ELECTRONIC, MECHANICAL, PHOTOCOPYING, RECORDING OR OTHERWISE, WITHOUT PRIOR WRITTEN PERMISSION OF THE PUBLISHERS

Elsevier Scientific Publishing Company, Jan van Galenstraat 335, Amsterdam

PRINTED IN CZECHOSLOVAKIA

CONTENTS

List of symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

9

.

1.1 Concentration distribution of the eluted substance at the column outlet . 1.2 Detector signal . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Detector response . . . . . . . . . . . . . . . . . . . . . . 1.3 Effect of the measuring device on signal changes . . . . . . . . . . . 1.4 Sample injection . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Parameters characterizing detectors . . . . . . . . . . . . . . . . . 1.5.1 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Detector linearity . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Linear dynamic range . . . . . . . . . . . . . . . . . . . . . 1.5.4 Lowest detectable amount . . . . . . . . . . . . . . . . . . 1.5.5 Detector selectivity . . . . . . . . . . . . . . . . . . . . . . 1.6 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. The Thermal Conductivity Detector (TCD)

2

. . . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

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

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

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

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

2.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 TCD signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 TCD background current . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 TCD response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Effect of experimental parameters on the magnitude.and shape of the TCD signal . . . 2.3.1 Carrier gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Construction of the TCD . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2.1 Sensor heating voltage . . . . . . . . . . . . . . . . . . . . . . . 2.3.2.2 Sensor parameters . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2.3 Cell geometric constant . . . . . . . . . . . . . . . . . . . . . . . 2.3.2.4 Temperatures of the sensor and the cell walls . . . . . . . . . . . . 2.3.2.5 Time constant of the TCD . . . . . . . . . . . . . . . . . . . . . . 2.3.2.6 Measuring circuits . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Applications of the TCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Ionization Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

3.1 Physical principles of the detection . . . . . . . . . . . . . 3.1.1 The collision . . . . . . . . . . . . . . . . . . . . 3.1.2 Effect of the electric field intensity . . . . . . . . . . .

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

17 22 23 24 28 30 31 32 33 34 36 37 39

.

39 42 43 43 46 46 47 49 49 51

.

52 52 55

56 57 59 59 60 62

3.2 Ionization energy sources . . . . . . . . . . . . . . . 3.3 Reactions in the ionization detector . . . . . . . . . . . 3.3.1 The slow-down mechanism . . . . . . . . . . . . 3.3.2 Recombination . . . . . . . . . . . . . . . . . 3.3.3 Background current of the ionization detector . . . 3.4 Literature . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . . . 65 68 . . . . . . . . . . . . . . 68 . . . . . . . 69 . . . . . . . . 71 . . . . . . . 71

.

4 The Electron Capture Detector ( E C D ) . . . . . . . . . . . . . . . . . . . . . .

72

4.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 ECD signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 ECD background current . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 ECD response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2.1 Linearity and linear dynamic range . . . . . . . . . . . . . . . . . 4.2.2.2 Sensitivity and selectivity of the ECD . . . . . . . . . . . . . . . . . 4.3 Experimental conditions affecting the ECD signal . . . . . . . . . . . . . . . . . 4.3.1 Carrier gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Construction of the ECD . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Applications of the ECD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

72 74 74 76 77 77 79 79 80 82 85

. The Flame Ionization Detector (FID) . . . . . . . . . . . . . . . . . . . . . . .

5

87

5.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 FID signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 FID background current . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 FID response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2.1 Linear dynamic range and linearity of the FID . . . . . . . . . . . . . 5.2.2.2 Sensitivity and selectivity of the FID . . . . . . . . . . . . . . . . . 5.3 Experimental conditions affecting the magnitude and character of the FID signal . . . 5.3.1 Gas flow-rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Geometry of the FID . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 FID applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

87 91 92 92 94 95 95 95 97

.

6

101 102

The Thermionic Detector Using an Alkali Metal Salt (TIDAj . . . . . . . . . . . . . 105

6.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 6.2 TlDA signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.2.1 TlDA background current . . . . . . . . . . . . . . . . . . . . . . . . . 110 6.2.2 TIDA response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 6.2.2.1 Linearity and linear dynamic range of the TlDA . . . . . . . . . . . . 115 6.2.2.2 Sensitivity and selectivity of the TIDA . . . . . . . . . . . . . . . . . 116 6.3 Effect of the experimental conditions on the magnitude and character of the TIDA signal . 117 117 6.3.1 Gas flow-rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Detector geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.4 TIDA applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 121 6.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

7 The Photoionization Detector (PID) . . . . . . . . . . . . . . . . . . . . . . .

123

7.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123

7 7.2 PID signal . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 PID background current . . . . . . . . . . . . . . 7.2.2 PID response . . . . . . . . . . . . . . . . . . . 7.3 Effect of the experimental conditions on the PID signal . . 7.3.1 Carrier gas . . . . . . . . . . . . . . . . . . . . 7.3.2 Geometric arrangement of the PID . . . . . . . . . . 7.3.2.1 Discharge compartment . . . . . . . . . . . 7.3.2.2 Detection compartment . . . . . . . . . . . 7.4 PID applications . . . . . . . . . . . . . . . . . . . . 7.5 Literature . . . . . . . . . . . . . . . . . . . . . . .

.

8

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

. . . .

. . . .

. . . .

. . . .

i25 i27 . . . . . . . 127 . . . . . . . . 128 . . . . . . . 128 . . . . . . . 129 . . . . . . . 129 . . . . . . . 130 . . . . . . . 131 . . . . . . . 133

The Helium Detector (HeD) . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.1 Detection mechanism . . . . . . . . . . . . . . . . . 8.2 HeD signal . . . . . . . . . . . . . . . . . . . . . . 8.2.1 HeD background current . . . . . . . . . . . . . 8.2.2 HeD response . . . . . . . . . . . . . . . . . . 8.2.2.1 Linearity and linear dynamic range of the HeD 8.2.2.2 Sensitivity and selectivity of the HeD . . . . . 8.3 Effect of experimental conditions on the HeD signal . . . . 8.3.1 Carrier gas . . . . . . . . . . . . . . . . . . . 8.3.2 Construction of the helium and argon detectors . . . 8.4 HeD applications . . . . . . . . . . . . . . . . . . . 8.5 Literature . . . . . . . . . . . . . . . . . . . . . .

. The Flame Photometric Detector (FPD) .

9

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

133

. . . . . . . . . . . . 133 . . . . . . . . . . . . 135 . . . . . . . . . . . 135 . . . . . . . . . . . 136 . . . . . . . . . . . 139 . . . . . . . . . . . 140 . . . . . . . . . . . 140 . . . . . . . . . . . 140 . . . . . . . . . . . . 141 . . . . . . . . . . . . 143 . . . . . . . . . . . . 144

. . . . . .

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

9.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 FPD signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 FPD background current . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 FPD response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2.1 Linearity and linear dynamic range of the FPD . . . . . . . . . . . . . 9.2.2.2 Sensitivity and selectivity of the FPD . . . . . . . . . . . . . . . . . 9.3 Effect of experimental conditions on the magnitude of the FPD signal . . . . . . . . 9.3.1 Composition of the gases and their flow-rates . . . . . . . . . . . . . . . . 9.3.2 Detector temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.3 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Use of the flame photometric detector . . . . . . . . . . . . . . . . . . . . . . 9.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

10 The Coulometric Detector ( C D ) . . . . . . . . . . . . . . . . . . . . . . . . .

10.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . 10.2 CD signal . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 CD background current . . . . . . . . . . . . . . . . . . 10.2.2 CD response . . . . . . . . . . . . . . . . . . . . . . . 10.2.2.1 Linearity and linear dynamic range of the CD . . . . 10.2.2.2 Sensitivity and selectivity of the CD . . . . . . . . 10.3 Effect of experimental conditions on the magnitude of the CD signal . 10.3.1 Gas flow-rate . . . . . . . . . . . . . . . . . . . . . .

. . . . . . .

. . .

. . . . . .

145 149 151

152 153 155 155

155 156 157 159 162 165

165 169 . . . . 170 . . . . 170 . . . . . 171 . . . . . 171 . . . . . 172 . . . . 172

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

145

8 10.3.2Construction of the detector . . . . . . . . . . . . . . . . . . . . . . . 10.3.2.1 Bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.3 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Applications of the CD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

11 The Electrolytic Conductance Detector ( E l C D )

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

172 175 175 175 179 181

11.1 Detection mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 ElCD signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 ElCD background current . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 ElCD response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Construction of the ElCD . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Applications of the ElCD . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

181 183

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

189

Index

185

185 186 187

188

LIST OF SYMBOLS

A

A A A a

- atom - radioactive source activity -

experimental constant

- radiative transition probability - geometric constant

a aa

- probability ratio for two processes

b B B

- optical path length - atom

bc

bci C

- analytical property of a - constant - background current - ionization background current - carrier gas

C

- instantaneous concentration

co

- initial concentration

p i n

CmaX

c'

C, Cy

D e

E EP E, F G H I I ic

ic, i ic,,,

minimum instantaneous concentration - maximum instantaneous concentration - instantaneous concentration in the effective volume of the detector - heat capacity at constant pressure - heat capacity at constant volume - diffusion coefficient - secondary electron, slow electron - voltage - excitation potential - excitation energy of the eluted substance - Faraday constant - mass - height equivalent to a theoretical plate - current - atom or molecule of impurity - ionization current - ionization current of alkali metal ~ - ionization ~ ~ current . ~ of ~de-excited atomic states - ionization current due to electron capture -

10 ic,,,

- ionization current of impurities in the carrier gas

icHe - ionization current of the carrier gas due to direct ionization ic,, - ionization current of carbonaceous substances due to the FID mechanism - intensity of emitted light I, - intensity of fluorescence emitted light 1, - initial intensity of emitted light Z: ZP - ionization potential J - Joule's constant k - constant k 1 , 2 , ,3, , . - rate constant of reaction 1, 2, 3, . . . kd.i - rate constant of dissociation, ionization - statistical constant dependent on the number of measurements, n k, K - absolute temperature, Kelvin K - thermal conductivity Ka,b - dissociation constant of acid, base 1 - electron free path 1 - length of heated filament I - detector linearity L - column length M - atom M - molecular weight M" - metastable atomic state M* - excited atomic state n - number of electrons - number of theoretical plates n n - number of measurements N - number of moles N' - number of moles in the effective volume of the detector ORG - polyatomic molecule of organic compound CI(0RG) - chlorine-containing ployatomic organic molecule p - pressure pc,s - partial pressures of the carrier gas, the eluted substance ppb - part per billion (American billion = lo9) Q - electric charge Q - conductivity cell constant Q - amount of heat Q E - cross-section for electron capture ionization Qc,s - cross-section of direct ionization of carrier gas, eluted substance r - column radius r - radius of the sensor R universal gas constant R - amplifier resistance

-

11 - heated filament resistance - detector response to property a

- estimated standard deviation - eluted substance -

signal of analytical probcrty a, of eluted substance S

- time -

the Student distribution

- time of the beginning and the end of elution -

time corresponding to the inflection points of the elution curve

- elution time

- elution curve width - time of passage of gas through the effective volume of the detector - temperature

temperature of the detector walls temperature of the heated filament linear flow-rate of gas volume flow-rate of gas flow-rate expressed in moles - mean molecular velocity - velocity of electrons v1 , 2 . 3 . ._.- rate of reaction 1 , 2, 3, . . . I/ - volume - effective volume of detector I/,,, X - molar fraction Z - constant Z i - ionic charge Z - the third particle in a three-body collision z,, za - number of non-elastic collisions -

a c! c1

6

r A

q,, 1"

e T

@

recombination coefficient temperature coefficient degree of dissociation thermistor material constance selectivity - change in parameter - photoionization efficiency - mean free path, mobility - density - time constant - fluorescence yield -

PREFACE

Much attention has been paid to the theoretical aspects of gas chromatographic separation processes. Considerably less theoretical work has been devoted to elucidating the function of the detection part of the instrument, the “black box” that yields the actual results for interpretation. This leads to a situation in which, for example, experimenters are prepared to calculate Kovfits retention index values to hundredths of a unit without considering the time constant of the measuring device and recorder employed. An ever increasing demand for high-precision measurements has evolved as a result of rapidly developing research techniques. Thermostats maintaining the temperature constant to within a few hundredths of a degree have been constructed and gas flow-rates are controlled with a precision of kO.1 cm3/min. Simultaneously, detectors are regarded as ideal measuring devices; this assumption is obviously false and has been the cause of many erroneous conclusions. The principle aim of this book is to draw attention to a number of experimental conditions that exert a considerable effect on the magnitude and character of the detector response. At present, there are only two bboks on gas chromatographic detectors. The treatment in German by Jentzsch and Otte appeared in 1970; that by D.J. David in English was published just as the manuscript of this book was submitted to the publishers in the beginning of 1974. These two books survey experimental work carried out up to 1969 and 1970 respectively. This book brings the subject up to date (papers published up to May 1974 are covered) but, in contrast to the above two books, is not intended to give an exhaustive survey of the literature on gas chromatographic detectors. Attention is centred rather on the clarification of detection mechanisms and explanation of the dependences of the detector response on experimental conditions. General conclusions which are intended to serve as criteria in designing new detectors are drawn whenever possible. In the Introduction, response formation is considered in general terms and relationships that characterize all detectors are derived; although some of these relationships differ from those commonly used, it is felt that they represent a closer approximation to the actual situation that obtains because of the consistent general approach followed in their derivation. The formation of such a general theory is based on the large amount of published data and the author wishes to acknowledge the important role played by these data. The literature cited is by no means exhaustive;

13 only those references which have a direct bearing have been included. There is no doubt that much of what has been written here will have to be considerably modified as a result of further research; it is the author’s hope that this book will prove a stimulus for such work. The author is grateful to a1 those who assisted him during the preparation of the book, to all the authors and publishers who gave permission to reproduce original figures and tables, to Dr. J. Novik for critical discussions and to Dr. M. Stulikovi and Dr. K. Stulik for translating the manuscript into English and for comments on the text.

This Page Intentionally Left Blank

1.

Introduction

Progress in gas chromatography very frequently depends on the development of measuring techniques that are capable of monitoring substances after their separation. While the separation process has often been treated in detail, less attention has been devoted to the accuracy of results and the stability of various measuring devices, The latter are termed ‘‘detectors” and form an independent part of the gas chromatograph. Their use is based on the various physical and chemical properties of the eluted substances or products formed by their reaction inside the detector. The method first employed for the monitoring of substances separated gas chromatographically was titration [35]. Later, gas volumes were measured [36] or the eluted substances were weighed after deposition on active charcoal placed on the pan of a balance [5]. None of these methods required calibration or electronic circuitry and thus contributed to the development of gas chromatography in the 1950s. However, simple detection methods are still employed, such as visual observations [74]. The progress of gas chromatography has led to the application of numerous physical detection principles. Changes in the electrical properties of materials are utilized in the thermal conductivity detector, in which changes in the resistance of a heated wire, thermistor or transistor are measured. The piezoelectric detector [37, 39, 431 with a quartz crystal, the pyroelectric detector [68] containing lithium tantalate and semiconductor detectors employing the p-n junction of a silicon diode [18,28, 51, 711 or tunnel junction supra conductivity [131 have also been used in gas chromatography [17]. A detector utilizing variations in the electric conductivity of a TiO, layer [27] is based on an analogous principle. In the literature can also be found descriptions of catalytic detectors [2] and detectors that measure differences in the dielectric constants of substances [ 5 5 , SO]; the sorptiothermal detector [16] and thermal flux meters [64] respond to heat changes in the system being studied, in a similar manner to the thermal conductivity detector. Detectors based on the measurement of variations in the gas density utilize the inverse proportionality between the flow-rates of gases and their molecular weights [25, 531. These detectors are among the oldest gas chromatographic detectors; they have been modified in various ways, e.g., the measurement of changes in the gas density has been performed on a diaphragm [59] or combined with other detectors [22]. Changes in electronic energy states and in rotafional and vibrational energies of niolecules have also been utilized for the construction of detectors [12]. Atomic emission spectra [76], atomic absorption [3, 321, chemiluminescence [75,77] and

16 fluorescence [8] of molecules or quenching of excited states in very sensitive measurements [lo] have been employed. The sensitivity of measurements increased considerably when the measurement of ionization currents was introduced. In the ionization current detection mechanism, various processes are involved, are mutually combined and cannot be separated, as follows from the additivity of ionization cross-sections [41]. In addition to the ionization detectors discussed below (ECD, FID, TIDA, PID, HeD), an electron mobility detector [52]: a cross-section detector [14, 661 and a surface ionization detector [ll, 231 have been described. The function of these detectors is based on collisions of particles of the eluted substances with high-energy particles. The latter are obtained from radioisotopes or from an electric discharge. However, such a discharge causes not only ionization but also excitation of molecules and atoms, resulting in emission of light. Thus detectors employing an electric discharge as a source of energy are often called emission detectors [63]. In addition to measuring the intensity of the emitted light, it is possible to measure the voltage at which the discharge begins [61], the discharge current [30, 501 or the ionization current [47-491. The ionization is affected by the space charge [56] and by the intensity of the electric field in which the ions move [69]. All of the above quantities have been employed in the design of GC detectors. For the elucidation of the ionization mechanism, plasma chromatography [40] has great importance for monitoring the fragments formed during ionization processes under conditions identical with those in the detector. Mass spectrometry combined with gas chromatography has made possible the identification of components in complex mixtures. Emitted radiation is also measured in gas chromatography. These detectors are termed radiogas detectors [79] and they measure the number of radioactive particles or pulses [38] with proportional counters [17, 731 or scintillators [34]. Detectors utilizing neutron activation analysis [9] can also be included in this group. In addition to the above-mentioned detectors, gas chromatography has employed the monitoring of differences in the magnetic properties of substances (paramagnetism or diamagnetism) in the magnetic detector [21], changes in the velocity of sound in the ultrasonic detector [24, 60, 821 and changes in the temperature of a flame [70], in surface potential [26] and in a number of other physical properties. The chemical properties of the eluted substances are utilized less frequently [67, 72, 831. Coulometry and conductimetry are used, as are polarography [65] and potentiometry with ion-selective electrodes [44 - 461. In analyses of the atmosphere, microbiological detectors [15] have been used. In Martin’s opinion [54], the reactions that will find the greatest use in the future are those in which the eluted substances are converted into CO, or H,O. The calibration will then be simple and, moreover, chemical amplification will be possible. It follows from this brief introduction that a very wide variety of physical principles are utilized for measuring purposes in gas chromatography. The following chapters

17 are devoted to discussion of selected detectors which are the most frequently used and/or bear promise for future development. The main attention is centred on clarification of the detection mechanism and of the dependence of the signal on the experimental conditions. Familiarity with these principles is essential for' correct interpretation of the results obtained and for optimization of the design of the measuring device.

1.1

CONCENTRATION DISTRIBUTION OF THE ELUTED SUBSTANCE AT THE COLUMN OUTLET

A substance S passes through the column and, as a result of the establishment of multiple equilibria, its concentration in the gaseous phase varies. If N , is the number of moles of substance S, then its instantaneous concentration at the column outlet can be expressed by the equation

where :1' is the retention volume, V o is the volume of gas passed during the elution of concentration cs and n is the number of theoretical plates. The volume of the eluted gas can be replaced by an expression containing the gas volume flow-rate, u , the elution time, t , and the retention time, r:,

The number of theoretical plates, n, can be expressed as the ratio of the column length, L, to the height equivalent to a theoretical plate, H:

n

= L/H

(1.4)

On substitution of expressions (1.2) - (1.4) into equation (l.l), the relationship for the instantaneous concentration of the eluted substance assumes the form cs

=

3 JL exp u . t , 2nH

[& -

(1 -

3'1

For the maximum concentration of the eluted substance, t = t , and hence

L

(1.6)

18

The integral time function of the concentration distribution of the eluted substance fulfils the condition of Gaussian random distribution /')s.di

=

Y

and has a maximum, c;Ipx, at time t = t , and two inflection points at times tinf located symmetrically on both sides of t,: tinf = t

L + R - H

The time interval between the inflection points corresponds to the width of the elution curve, 2 x A t , and is given by

At

=

J(L/H)

It follows from the Gaussian distribution law that the value of function Y equals unity within the integrated time interval, (- co, 00). When the integration limits are given by multiples of A t , the value of integral Y is always smaller than the

+

-f FIG. 1.1. An ideal elution curve: t~ f 4 A? - elution time limits.

-

elution time, A?

-

the elution curve width,

value corresponding to the oveiall amount of eluted substance (Fig. 1.1). When the multiple, 4(+At), is used, the error is as little as + 3 x which is satisfactory when considering the experimental conditions. The actual separation of the eluted substance in the column is affected by the siinultaneous processes of dissolution and adsorption, by shifts in the equilibria in the presence of inert substances, by the amount of eluate [33] and especially by the amount of eluted substance, N , [31]. The above relationships describing the theoretical separation must then be modified by introducing correcting terms [29, 841 in order to satisfy better the experimental conditions. With a change in the concentration of the eluted substance, a change in the concentration of the carrier gas occurs. These

19 changes can be expressed in terms of changes in the partial pressures, as the overall pressure is constant in the open column separating system. If no other substance is present in the colun~n,then it contains only the carrier gas, C, the instantaneous concentration of which, c,, is constant along the whole column; its partial volume, V,, equals the overall column volume and its partial pressure, p , at the column outlet. On introduction of a substance S into the column, the partial pressure of the carrier gas changes: the overall pressure is unchanged, so that

+ Ps

P

=

Pc

p

=

(N,

As

+ N s + ...) RT V

-

where N,, N,, etc., are the numbers of moles of substances C, S, etc., respectively. For p s , the relationship

is obtained and the partial volume of the eluted substance is given by V,

=

N,RT ~

P TABLE 1 . 1 THE AVERAGE VALUES OF THE CARRIER GAS A N D THE ELUTED SUBSTANCE PARTIAL PRESSURES IN VARIOUS COLUMNS Column

V

diameter [mml

[mljmin]

0.25

0.5 1.o

0.5

2.7 4.6 8.0

1.5 5 10 20 50 100 75 150

N c / l 5 sec [mole/secl

5

Y 10-6

1 1 i: 1 0 - 5 2 8 Y 10-5 5 6 :< lo-’ 1 I >: 2.2 x 55 10-4 1.1 x 1 0 - ~ 83 x 1.6 X

pc [torr]

632 395 698 646 691 730 745 698 752 71 3

Ns/15 sec [mole/sec]

10-6 I O - ~ 10-6 lo-’ 10-~ 10-5

10-~ 10-4 10-5 10-4

PS [torr]

128 265 62 114 69 30 15 62 8 47

20 On introduction of substance S into the column, the volume of the gaseous phase changes. The overall volume of the gaseous mixture equals the sum of the partial volumes of the individual components. As the column volume cannot be increased in the gas chromatographic system, the volume changes occur as changes in the volume flow-rate of the gaseous mixture. With a constant column cross-section, nr2, the changes in the volume flow-rate appear as changes in the linear flow-rate, u. The dependence of the height equivalent to a theoretical plate on the linear flow-rate is generally hyperbolic; it is evident that the maximum concentration of the eluted substance is also dependent on the flow-rate (equation (1.6)). 2

4

5

J

I

wyn w -

FIG. 1.2. The flame ionization detector response to CS, as a function of the amount of sample; traces 1 , 2 , 3 , 4 and 5 corresponds to injection of 1,2, 3, 4 and 5 PI, respectively.

In order to determine the magnitude of these changes, the variations in the partial pressures of the carrier gas and the eluted substance were calculated for various amounts of sample, replacing the concentration distribution (equation 1.5) by the average concentration of the substance eluted for 15 sec; the values are given in Table 1.1. Under the actual conditions in a separating column, the changes in the partial pressures are even more pronounced because of the Gaussian distribution, being of the order of tens of a per cent. These facts can be illustrated by the response of the flame ionization detector (FID) to 1-5 P I of carbon disulphide (Fig. 1.2).

21 As CS, is only slightly ionized in the FID (see Chapter 5), the ionization current decreases with an increase in the partial pressure. The minimum ionization current corresponds to the maximum concentration of CS2; the greater the amount of sample injected, the more pronounced is the minimum in the ionization current.

-t FIG. 1.3. The dependence of the instantaneous concentration of the eluted substance on the overall amount, N,, the linear flow-rate, ri, and the elution time, t.

Therefore, the instantaneous concentration of the eluted substance is a function of the overall amount of substance, N , , the linear flow-rate of the gaseous mixture and the elution time, as depicted schematically in Fig. 1.3. From the general equations, (1.6) and (1.8), it follows that the maximum concentration of the eluted substance at the column outlet (the maximum on the elution curve): - increases with the square root of the number of theoretical plates at a constant

flow-rate and elution time; - decreases with increasing flow-rate at constant elution time and number

of theo-

retical plates; - decreases with increasing elution time at a constant number of theoretical plates

and flow-rate. The elution curve width: - increases with increasing retention time at a constant number of theoretical

plates;

- decreases with an increasing number of theoretical plates at a constant retention time. The separated eluted substance is monitored by a detector at the column outlet.

22

The basic requirement placed on the detector is that the recording should give a true picture of the concentration distribution in the column. Therefore, the detector should not affect the number of theoretical plates, i.e. should not participate in' the separation process; the retention time of the substance, i.e., should be placed at the last theoretical plate of the column: the gas flow-rate, i.e., its internal diameter and hydrodynamic resistance should be identical with those of the column. these theoretical requirements cannot be met completely in practice, the measured values deviate from the actual separation results.

1.2

DETECTOR SIGNAL

The detector measures variations in the magnitude of an analytical property, a', of the entering substance. The detector records a measurable change after a molecule (radical, atom, ion) of the eluted substance collides with the detector sensor. Therefore, it is evident that reactions which occur outside the range of the sensor do not contribute to the measured change. The space in which the measurement takes place is denoted as the detector effective space. The measured change dependes only on those molecules present in this space which possess the required analytical property. The measured change, denoted as the signal, is proportional to the magnitude of the given analytical property (thermal conductivity, ionization cross-section, etc.) and to the number of molecules capable of reacting with the sensor. The signal is thus proportional to the instantaneous concentration of the eluted substance and can be described by the equation St = k x at x N;'

(1.10)

where S: is the signal due to eluted substance S in a device measuring analytical property a, k and I are constant characteristic of the experimental arrangement and N ; is the number of moles of the eluted substance in the effective volume of the detector. Signal Sz follows the concentration distribution of the eluted substance at the column outlet. At a given time, other substances that possess the same analytical property as the eluted substance may be present in the effective volume of the detector. The overall measured signal is then given by the sum of the signals for the eluted substance, the carrier gas and impurities:

so = s.,

=

s; + s; + sp

(1.11)

23 Before the eluted substance enters the detector, only the molecules of the carrier gas and of impurities present in the carrier gas are present in the effective volume of the detector. Stationary conditions are established in the detector and the measured signal is given by the equation

bc

=

S:

+ Sp

(1.12)

The signal measured in the absence of an eluted substance (equation (1.12)) is called the detector background current. As already mentioned, the gas chromatographic system is an open one, i.e., the partial pressures in the effective volume of the detector change during the elution of the studies substance. It is evident that the measured signal, S", is positive if A S ; > d(S;

+ Sp)

(1.13a)

and negative when the opposite inequality holds, i.e.,

A s ; < d(s:

+ sp)

(1.13b)

From these two inequalities, it follows that the greater the absolute value of the difference / A S : - d(Sg + Sp)l, the more the measured signal corresponds to the change caused by the eluted substance. Therefore, for successful measurement, it is necessary that the magnitude of the analytical property of the eluted substance should be larger than that of the carrier gas and of any impurity which might be present: a:

< a: > a:

If these values are comparable, the measured signal is very small [42]. An ideal measuring arrangement would involve a carrier gas that does not possess the measured property; nitrogen is not ionized in the FID system. During elution of the studied substance, the partial pressures of the carrier gas and of the impurities present decrease. Therefore, the detector background current decreases during the elution, attains a minimum at the maximum concentration of the eluted substance, again increases on the further elution and finally reaches its original value. 1.2.1

Detector response

The integral of the signal, s",over the interval ( t l , t z > , where f I and t , correspond to the beginning and the end of the elution, respectively, is called the detector response. R": (1.14)

24

The detector response thus consists of the sum of the background current and the signal of the eluted substance, as shown in Fig. 1.4. It is thus obvious that the detector response is proportional to the overall number of moles of the substance introduced 0

1

P

2

sa

.A I

A

'.

R

0

'.

- \\ '\",/

#

/.

)r -f

bc

I ?

at the column inlet and that the agreement between the response time-dependence and the distribution in time of the concentration of the substance at the column outlet improves with increasing values of inequalities (1.13a) and (1.1 3)b.

1.3

EFFECT OF THE MEASURING DEVICE ON SIGNAL CHANGES

The effective volume of the detector in which the measurement takes place can be expressed in terms of the gas flow-rate and the time, fder, required for the passage of the gaseous mixture through a volume I/,,,:

v,,, = u . tdet = ii . TI . r2 . tder

(1.15)

The time tdercan be calculated from the values following from the time distribution of the concentration of the eluted substance; the substance concentration distribution is characterized by values t , and A t (see Fig. 1.1). If the limits, + 4 x A t , are applied to function Y, the following relationship can be written for tder: (tL

- tl)

td,,/dt =

= +4dr = z . tdet

(1.16)

8/z

(1.17)

Equation (1.17) expresses the relationship between the separation conditions and the effective volume of the detector; it is evident that the fraction equals zero for an ideal detector. If t,,,/At 4 0, then the concentration distribution in the detector changes in comparison with the conditions at the column outlet, described by equation (1.5).

25 The newly established distribution of function St (see Fig. 1.5) is given by the following equation [62]:

When expressions for Pax are compared, it is observed that the maximum on the concentration distribution shifts by fdec/2on passage of the gas through the detector.

L ! ?

tI

A I

1 1

'det

-

v[m[/min]

FIG. 1.6. The shift in the elution time as a function of the effective volume of the detector and the carrier gas flow-rate.

FIG. 1.5. Conversion of function cs into funkction S t . TABLE 1.2

THE TIME, tR[sec], OF PASSAGE OF GAS THROUGH THE EFFECTIVE VOLUME OF THE DETECTOR AT VARIOUS FLOW-RATES

0.5 1 .o 2.5 5 10 25 50 I00 I50

1.2 0.6 0.24 0.12 0.06

3 1.5 0.6 0.3 0.15 0.06

6 3 1.3 0.6 0.3 0.12 0.06

12 6 2.4 1.2 0.6 0.24 0.12 0.06

30 15 6 3 1.5 0.6 0.3 0.15

0.10

60 30 12 6 3 1.2 0.6 0.3 0.2

120 60 24 12 6 2.4 1.2 0.6 0.4

240 120 48 24 12 4.8 2.4 1.2 0.8

600 300 120 60 30 12 6 3 2

The shift in the elution time, A t , , is inversely proportional to the gas flow-rate under constant experimental conditions, i. e., at a constant effective volume of the detector: t R = -'dec+ A 21

26 where A is an experimental constant (see Fig. 1.6). The shift in the elution time increases with increasing detector volume; some values are given in Table 1.2. Because of the finite values of the effective volume of the detector and the gas flow-rate and because of the necessity of measuring the concentration using a sensor that yields an electric signal, the measured signal is distorted. This distortion is called

O

L a 5 4

-5-4-3

-2-7

0 1 2 3 4

5

Af FIG. 1.7. Time constant of the device.

FIG. 1.8. Distortion of the response of a detector with laminar flow as a function of changes in the tder/dt ratio [19].

the time constant of the device, T. When it is assumed that each signal value corresponding to instantaneous concentration, cs, is reached by a step, then the given signal is, in practice, reached only after a certain time (Fig. 1.7) which is approximately TABLE 1.3 DISTORTION OF THE DETECTOR SIGNAL AS A FUNCTION OF VARYING tder/dt RATIOS FOR LAMINAR AND TURBULENT FLOW CONDITIONS

tder =

z ,df

8 4

2 1 0.5 0.25 0

__

-

Laminar

Turbulent

7;

%

-~

Maximum shift

_ _ _ _ . ~ _ - _ _

area

width

height

area

width

height

62 74 89 97 99.2 99.8 100

270 150 106 101 100.4 100.2 100

20 50 84 96 99 99.7

100

154 124 110 107 100

24 40 60

100

100 100 100 100 100 100

~~

The shift of the maximum on the elution curve, d t = ~ y .dt

78 91 97.4 100

Y

1.66 I .35 1.o 0.5 0.25

27

equal to three times the time constant. Owing to this distortion. the maximum of the output function, S""", is shifted with respect to the maximum of the input function, c;lnx. When the time constant increases, not only is the output function maximum shifted to longer times along the descending branch of the ideal Gaussian curve, but it is generally distorted. The time integral of the output function, S", within limits ( t , . t 2 ) has been defined as the response. Therefore, distortion of the signal causes distortion of the detector response, which is thus not identical with the concentration distribution at the column outlet. Figs. 1.8 and 1.9 show changes in the detector response caused by changes in the t,,,/dt ratio. Mass transport also plays a role during the passage of the eluted substance through the detector. If there is a concentration gradient of the eluted substance in the direction of the flow, i.e. if the mixture is transported by laminar flow alone and is not stirred, then the probability that all the eluted substances reach the sensor decreases with increasing detector volume and hence the response decreases (Fig. 1.8 and Table 1.3). If there is no concentration gradient in the detector along the direction of flow, i.e., the mixture is stirred 'as a result of turbulent flow,

0.8

-t

0.51 0.4

0.3t 0.21 0.1

c (4

L

0

L

8

12

16

20

At FIG. 1.9. Distortion of the response of a detector with turbulent flow as a function of changes in the f&t/dt ratio [19].

-t FIG. 1.10. Changes in the response of a detector with a large effective volume as a function of the carrier gas flow-rate.

then there is always a certain average concentration of the eluted substance in the effective volume of the detector; when the effective volume of the detector is increased or the flow-rate decreased, the response is distorted, as is shown in Fig. 1.9. Much attention has been paid to the dependence of signal distortion on the flow-rate of the mobile phase. When the carrier gas flow-rate increases, tdet decreases

28 more rapidly than the elution peak width, A t , and therefore the distortion decreases. An increase in the flow-rate leads to a decrease in the influence of the effective volume of the detector and the time constant of the device is determined by the time constants of the other components of the apparatus, such as the recorder. Response distortion for these cases is shown in Fig. 1.9. The largest shift in elution times is exhibited by devices that employ diffusion processes and those with large effective volumes of the detector, operating at low gas flow-rates. Large cells were employed in older types of thermal conductivity detectors; the average concentration is then measured. An increase in the flow-rate results in an initial increase in the peak height, corresponding to the maximum number of moles, N ; , present in the detector. On increasing the flow-rate, tdetis increased and hence the elution curve area is decreased (Fig. 1.10). It is thus desirable that the measuring device should have as low a time constant as possible and thus also the smallest possible effective volume of the detector. In the measuring device the signal S"is converted into an electrical impulse. This impulse is usually a voltage formed on the amplifier resistor, R, through which the current corresponding to changes in the effective volume of the detector passes. The current is sometimes measured directly using a current follower. Therefore, the output signal can always be converted into a current and has the dimension of [A]. As the response is defined as the time integral of the signal, it has the dimension of electric charge, [ C ] . The separation of the eluted substance at the column output is described by its concentration distribution. Therefore, the gas chromatographic experiment requires evaluation of the relationship between the measured response (signal) and the concentration of the eluted substance. 1.4

SAMPLE INJECTION

For quantitative measurements, it is necessary to know the dependence of the response on the concentration of the eluted substance, i.e., the calibration curve. A number of methods have been used for testing detectors [7, 811. The preparation of standard mixtures using accurate control of the gas pressure and flow-rate [l] places very high demands on the control accuracy and constancy. Calibration of the detector by means of absolute measurement [85] is also not suitable. Single injection of the sample into the column is the method most frequently employed for the calibration of measuring devices; however, it should be pointed out that this method is the least suitable compared with those described below. The evaluation of a single injection is subject t o response distortion due to the time constant, T, to absorption processes and to the fact that relatively large sample volumes are iiijected (Table 1.4), occupying a volume larger than that corresponding to one theoretical plate. Calibration by a pulse method [ 5 8 ] is subject to similar drawbacks.

29 TABLE 1.4 THE VOLUME OF THE BENZENE GASEOUS PHASE AT VARIOUS TEMPERATURES AFTER INJECTION OF VARIOUS A M 0 UNTS OF LIQUID SAMPLE ____

Amount of liquid [PI1 0.1 0.5 1 .o 2.0 5.0 10.0

vfJ

___._

[Pll

100°C

200'C

300°C

29 143 281 574 1430 2870

0.040 0.195 0.392 0.784 1.95 3.92

0.051 0.248 0.497 0.996 2.48 4.97

0.061 0.300 0.607 1.209 3.00 6.07

Ns 1.3 X 6.4 x 1.3 10-5 2.6 10-5 5.4 x lo-' 1.3 X

V [mil

-

It is preferable to use continuous sampling methods for testing apparatus. The diffusion evaporator method and the logarithmic diluter method are employed. In the former method [78], a constant amount of sample is injected into the detector for

FIG. 1 . 1 1 . Scheme of the diffusion evaporator; a - permeable PTFE tube, b - capillary.

FIG. 1.12. Scheme of the logarithmic diluter; 1 - vessel with an accurately known volume; 2 - container with the studied substance; 3 - container lid lever; 4 - carrier gas inlet; 5 -carrier gas outlet.

a pre-selected time. The diffusion evaporator utilizes a constant vapour pressure through permeable membranes or capillaries at constant pressure and temperature. The diffusion evaporator may consist, for example, of a closed PTFE tube, filled with the studied substance and placed in the stream of carrier gas (Fig. 1.11). At

constant temperature and pressure, a constant amount of the studied substance diffuses through the walls of the tube. The loss in weight of the tube is determined and the concentration of the studied substance in the effective volume of the detector can then be calculated when the overall volume of the gas is known. This method has the advantage that equilibrium conditions are established and the effects of the time constant, absorption, etc., are eliminated. The logarithmic diluter method [4] is based on dilution of the sample by the carrier gas at a constant flow-rate. A container with the studied substance is placed in the closed vessel of the diluter (Fig. 1.12); a small amount of substance is introduced from the container into the vessel, the content of the vessel is homogenized and the carrier gas is introduced. The concentration in the vessel decreases logarithmically and the actual concentration can be calculated from the equation cs = c; x

,-(ct)/V

(1.19)

ci

where is the initial concentration in the diluter vessel with volume Vand c, is the actual concentration at time t and carrier gas flow-rate v . The measured signal values can be attributed to all the instantaneous concentration values and errors stemming from inaccurate reading of the response are avoided. The diffusion evaporator and logarithmic diluter methods are the only correct methods for testing detectors, as they exclude systematic and gross errors and, because of the slow variation in the concentration of the studied substance, invohe negligible distortion by the time constant of the device.

1.5

PARAMETERS CHARACTERIZING DETECTORS

The classification of detectors frequently encountered in the literature [20] divides detectors into concentration and mass detectors, destructive or non-destructive, integral or differential and universal or selective detectors. One of the most frequently used classifications involves separation into two groups, concentration and mass detectors; however, we feel that this classification is not justified. In support of this classification, dependences of the signal and response on the flow-rate and pressure of the mobile phase have been cited. However, a change in these parameters leads to a change in the concentration distribution of the eluted substance at the column outlet and in the time, fdet, during which the substance remains in the effective volume of the detector. The concentration distribution at the column outlet is described by equation (1.5). Concentration cs is determined by the number of moles, N , = = G / M , , where G is the mass of the eluted substance and M , is its molecular weight. Therefore, changes in the signal as a function of the experimental parameters must correspond to the concentration distribution at the end of the column. The measured signal change corresponds to the number of molecules, i.e., to their concentration rather than their mass. The dependence of this signal on the carrier gas flow-rate is

31

determined by the construction of the measuring device, chiefly by the V,,, and z values. Classification into concentration and mass detectors should therefore be discouraged and the signal or the response should always be related to the concentration of the eluted substance. Classification into destructive and non-destructive detectors is based on the evaluation of changes that take place i n the detector due to the detector mechanism. Two cases may occur: properties either of the eluted substances or of products of reactions occurring in the detector are monitored. This classification thus requires a thorough knowledge of the detection mechanism. It should be remembered that irreversible changes may occur even in the thermal conductivity detector, as a result of contact of the substance with the heated filament. If the substances entering and leaving the detector should be identical, for example in preparative chromatography, the experimental conditions must be carefully selected. Classification into integral and differential detectors IS groundless at 11ie present level of electronics development. Differentiating and integrating circuits are common and the form of the recording can be chosen from the point of view of maximum accuracy; differential recording is employed for qualitative analysis and integral recording for quantitative analysis. Detectors containing electronic circuits monitor the concentration distribution of the eluted substance at the column outlet, i.e., yield a chromatogram with the shape of a differential curve. The above classifications seem to us to be unsuitable as they do not supply the experimenter with information for selecting the experimental conditions. The classification of detectors according to the analytical property (physical quantity) measured is therefore more advantageous. Detectors that measure changes in the conductivity, either thermal or electric, monitor variations in the sensor resistance. Ionization detectors follow variations in the electric conductivity of a medium placed in an electric field. Chemical detectors monitor the courses of chemical reactions, etc. Although this classification of measuring devices yields more information, it is, of course, not indispensable for detector application. Gas chromatographic measuring devices have been compared according to a number of criteria which have, however, been formulated differently by different workers. Some relationships are defined in both linear and logarithmic coordinates, e.g., the linearity and linear dynamic range [6], the definitions of some concepts are not unified, so that units which are not interconvertible result, etc. The following arguments and relationships are based on the generally valid equation for the chromatcgraphic detector signal (equation (1 .lo)) as a universal criterion.

1.5.1

Sensitivity

The concept of the sensitivity is derived from equation (1.1 l), which describes the overall detector signal as the sum of the contributions from the carrier gas, impurities and the eluted substance. The sensitivity of the measuring device for the eluted sub-

32 stance is then defined as the change in the measured signal, AS", resulting from a change in the concentration, AN;', of the eluted substance: (1.20) For the condition

it holds that

AS" AN;'

AR AN;'

- = k . a: z __

(1.22)

It follows from equation (1.22) that the sensitivity is given by the product of the experimental constants of the measuring device and the value of the analytical property of the eluted substance. Therefore, the sensitivities of individual detectors differs depending on their design, and are different for different substances. It follows from equation (1.20) that the presence of a carrier gas and impurities with measurable values of the particular analytical property results in a marked decrease in the measuring sensitivity. For this reason, measurement with a flame ionization detector is generally more sensitive than with a thermal conductivity detector, as nitrogen as a carrier gas is not ionized in the FID. The above phenomena should be borne in mind when the experimental conditions are selected and a carrier gas that contains no impurities which yield a signal in the particular detection mechanism should be used. 1.5.2

Detector linearity

The linearity, I, is the proportionality constant in the relationship between the logarithm of the signal and the logarithm of the concentration of the eluted substance. It is determined from equation (l.ll), which, in logarithmic form and under the validity of condition (1.21), can be written in the form log So = log ( k x a;)

+I

x log N , ,

( I .23)

By plottingequation (1.23), a straight line is obtained with a slope of I and an intercept on the y-axis equal to log k x a:, i.e., to the logarithm of the sensitivity (Fig. 1.13). Evaluation of the detector linearity in linear coordinates is frequently encountered in the literature. The corresponding devices are then called linear detectors, for which the proportionality constant between the signal and the concentration, i.e., the exponent I in equation (l.lO), is unity. Detectors with slopes other than unity are called non-linear. This approach cannot be considered to be correct, by analogy

33 with a number of types of detectors that involve exponential dependences of the measured signal on the concentration, such as optical methods, and methods that employ radioactive decay. Changes in the signal-to-concentration proportionality can also be caused by handling of the signal in electronic circuits. Therefore, it is generally necessary to evaluate the detector linearity in logarithmic coordinates using equation (1.23).

~~

--logNs

FIG. 1.13. The log S, = f ( 1 . log Ns) plot; log k . a: - logarithm of the sensitivity; I

1.5.3

-

detector linearity; (a2 - a,) - detector linear dynamic range.

Linear dynamic range

The equation for the instantaneous concentration of the eluted substance and its dependence on the experimental parameters, especially on the carrier gas flow-rate, show that this function exhibits a maximum. The detector linearity thus changes in an analogous manner in the interval (+ 1, - 1). As quantitative evaluation can be performed only at a constant linearity, it is necessary to find the concentration range in which this parameter is constant. The concentration interval, expressed as the logarithm of the concentration, within which the linearity does not change is called the detector linear dynamic range. If the amount of eluted substance varies in the concentration range - lo-"', the corresponding signals are given by the relationships

s.,

(1.24)

= ka;(lO-"')'

and (1.25)

Sz = ka:(lO-"')'

The above concentration interval corresponds to the equation log

s; - logs;

= I(E, - E , )

(1.26)

The linear dynamic range is then expressed by the difference in the exponents of the concentrations (m2 -a1) (see Fig. 1.13).

34

The linear dynamic range is important for the quantitative evaluation of chromatograms. However, it does not yield sufficient information, as it is not related to absolute concentrations. In order the evaluate the applicability of a detector, it is necessary to know the absolute measurable amount of a substance. This is expressed in terms of the lowest detectable amount. 1.5.4

Lowest detectable amount

The measured signal fluctuates owing to the inconstancy of the experimental parameters (temperature, gas flow-rate, line voltage, thermal stability of the electronic circuitry, etc.). These variations can be considered to be random errors in discontinuous measurements performed at times t , to t, during the interval t, (see Fig. 1.14) and are called the noise.

1

2

~

-

~

f,

t2

tJ

t4

~

- --

t"

~

-t

FIG. 1.14. The change in the signal with time under equilibrium conditions; 1 2 - noise; t , to t , - discontinuous measuring times in the interval t,.

- drift;

The noise oscillates around the average signal and the variation in individual values can be calculated from the assessed standard deviation : s = k,(SOmnx

- somin)

(1.27)

where k, is a tabulated constant dependent on the number of measurements and the term in parentheses expresses the maximum and minimum deviation of the signal under equilibrium conditions. The noise level varies randomly and it is necessary to consider the length of the time interval during which the noise is to be monitored in order to obtain a statistically significant value. The measurement time interval can be divided into portions identical with multiples of & 4 A t (the elution time) which correspond to the number of discontinuous measurements, n. When it is taken into account that the amount of information virtually does not increase for n > 5 and that the precision of the results increases very little, two values can be selected as the number of measurements, n = 5 and n = 10; then the noise level, (Soma' - Somi"), is evaluated in time intervals of 40 A t and 80 A t , respectively.

35 During evaluation of the chromatogram, it is important to know the change in the measured signal that can be considered to be statistically significant. This value can be found statistically by comparing the average values, 5” and (s” - AS”), assuming the same variance. Then the values of the Student distribution, t, are given by the relationship (1.28)

TABLE 1.5 THE CONFIDENCE LIMITS A N D COEFFICIENT K EXPRESSED I N TERMS OF THE NOISE MAGNITUDE AT 95:4 A N D 99% PROBABILITY LEVELS

n

Interval of the noise measurement

Confidence limits 957;

99%

2.6 1.02 0.46

6.02 1.68 0.66

Coefficient K 95%

99%

~~

3

5 10

24. Af 40. A t 8 0 . A1

5.2 2.04 1

12.04 3.36 1.32

The statistically significant value of AS” can then be calculated from the tabulated values o f t (see Table 1.5). As the variations in the above average values are identical and their confidence limits must not overlap, a statistically significant AS“ value is

FIG. I , 15. Evaluation of the chromatographic curve; Samax, Saminand 5“ - maximum, minimum and average signal values, respectively.

greater than twice the noise level and depends on the interval within which the noise is monitored (Table 1.5). This consideration is related to the instantaneous signal. This condition can be fulfilled by the maximum signal corresponding to the apex of

36 the elution peak, the other values being less than twice the noise level. Then the elution curve cannot be quantitatively evaluated, as the signal corresponding to time t , f dr is not known. Therefore, the latter value must also be statistically significant, i.e., ASa 2 2K(SamaX- Samin)

(1.29)

must be valid, where K is a coefficient dependent on the required probability, given in Table 1.5. Equation (1.29) defines the lowest detectable amount. If 95% probability is satisfactory and the noise is evaluated within a time interval corresponding to ten elution times (80dt), then any signal greater than twice the noise level can be treated quantitatively (see Fig. 1.15). When the probability is increased to 99%, then signals greater than 2.65 times the noise level can be evaluated; when the noise is monitored over five elution times, then signals 6.72 times greater than the noise level are significant at the 99% level.

1.5.5

Detector selectivity

The ability of the device t o react to only a limited number of substances is denoted as the selectivity, while a device that reacts with only a single substance is called specific. The selectivity for substances J and K is expressed by comparing their signals based on an identical analytical property, a. The signals of the two substances are given by the relationships

Si

= k . uYN;

and SE = k . a:";,

As N; = N ; , the selectivity is given by the expression (1.30) The selectivity therefore corresponds to the ratio of the values of the analytical property. It is evident that selective devices will be those which utilize the optical properties of substances, chemical reactions, etc. On the other hand, devices that employ direct ionization with high-energy particles are universal as all the crosssection values for direct ionization are similar. The probability of various reactions taking place in the detector changes with changes in the experimental conditions. In this way, the value of the measured property can be significantly increased or decreased and the device can become more or less selective. A practical requirement for a selective detector is that I' 2 10.

37 1.6

LITERATURE

1. Angely L., Levart E., Guiochon G., Peslerbe G.: Anal. Chem. 41, 1446 (1969) 2. Arutyunov Yu. I., Zibrova L. P., Karpov E. F., Karbanov E. M., Kravchenko V. S., Korlyakov G. A.: Zavod. Lab. 38, 660 (1972) 3. Ballinger P. R., Whittemore I. M.: Gas Chrornatogr. Abstr. 1971, 1040 4. Beke R.: Ingenieurs 1970, 5 5. Bevan S. C., Thorburn S.: J. Chromatogr. 11, 301 (1963) 6. Bowman M. C., Beroza M.: J. Chromatogr. Sci 7, 484 (1969) 7. Brazhnikov V. V., Sakodynskii K. I.: Gazov. Khromatogr. 1969, 5 8. Burchfield H. P., Wheeler R. J., Bernos 3. B.: Anal. Chem. 43, 1976 (1971) 9. Cram S. P., Brownlee J. L., Jr.: J. Gas Chromatogr. 6, 313 (1968) 10. Crider W. L., Slater R. W., Jr.: Anal. Chem. 41, 531 (1969) 11. Cuderman J. F.: Rev. Sci. Instrum. 42, 583 (1971) 12. Dagnall R. M., Johnson D. J., West T. S.: Spectrosc. Lett. 6, 87 (1973) 13. Dayem A. H.: J. Phys. (Paris), Colloq. 1972, 15 14. Del Campo Maldonaldo J. L.: Nucl. Sci.Abstr. 22, 28 207 (1968) 15. Druett H. A., Packman L. P.: Nature 218, 699 (1968) 16. Dudenbostel B. F., Priestley W.: Ind. Eng. Chem. 49, 99 A (1957) 17. Dupuis M. C., Charrier G., Alba C., Massimino D.: Anal. Abstr. 20, 852 (1971) 18. Eden C., Margoninsky Y . :J. Gas Chromatogr. 6, 349 (1968) 19. Esser R. J. E.: 2.Anal. Chem. 236, 59 (1968) 20. Ettre L. S.: Theory Appl. Gas Chromatogr. Ind. Med., Hahnemann Symp. 1st 1966 (Pub. 1968), 23 21. Farzane N. G., Ilyasov L. V.: Zh. Fiz. Khim. 42, 3119 (1968) 22. Farzane N. G., Ilyasov L. V.: Zh. Fiz. Khim. 45, 921 (1971) 23. Gillen K. T., Eiernstein R. B.: Chem. Phys. Lett. 5, 275 (1970) 24. Grice H. W.,David D. J.: J. Chromatogr. Sci. 7, 239 (1969) 25. Griffiths J., James D., Phillips C. S. G.: Analyst 77, 901 (1952) 26. Griffiths J. H., Phillips C. S. G.: J. Chem. Soc. 1954, 3446 27. Guglya V. G., Dergunov V. V.: Zh. Anal. Khim. 17, 2239 (1972) 28. Guglya V. G.: Zaood. Lab. 39, 403 (1973) 29. Guiochon G., Jacob L., Valentin P.: J . Chim. Phys. Physicochitn. Biol. 66, 1097 (1969) 30. Harley J., Pretorius V.: Nature 181, 177 (1958) 31. Harris W. E.: J. Chromatogr. Sci. 11, 184 (1973) 32. Hey H.: 2.Anal. Chem. 256, 361 (1971) 33. Huber J. F. K., Gerritse R. G.: J . Chrotnafogr. 80, 25 (1973) 34. Itaya M.: Chem. Abstr. 68, 101 441d (1968) 35. James A. T., Martin A. J. P.: Biochern. J . 50, 679 (1952) 36. Janik J.: CON. Czech. Chem. Commun. 19, 684, 700, 917 (1954) 37. Janghorbani M., Freund H.: Anal. Chem. 45, 325 (1973) 38. Jelen K., Lasa J., Ostrowski K.: Chem. Anal. (Warsaw) 53, 1239 (1968) 39. Karasek F. W., Gibbins K. R.: J. Chromatogr. Sci. 9, 535 (1971) 40. Karasek F. W., Tatone 0. S., Kane D. M.: Anal. Chem. 45, 1210 (1973) 41. Karavaeva V. G., Revel’skii I. A,, Zhukhovitskii A. A,: Zaood Lab. 39, 275 (1973) 42. Karp S., Lowell S.: Anal. Chem. 43, 1910 (1971) 43. King W. H.: Anal. Chem. 36, 1735 (1964) 44. Kneebone B. M., Freiser H.: Anal. Chem. 45, 449 (1973) 45. Kojima T., Ichise M., Seo Y. K.: Bunseki Kagaku 20, 20 (1971) 46. Kojima T., Ichise M., Seo Y.: Bunseki Kugakrr 22, 208 (1973)

38 47. Krico-Elektronic K. G.: Ger. Offen. 1,958,751 (May 27, 1971) 48. Lacase P.: Chromarographia 6, 32 (1973) 49. Lakshtanov V. Z., Markevich A. V., Dobychin S. L.: GUZOC. Khromatogr. 1966, 46 50. Lakshtanov V. Z., Markevich A. V., Dobychin S. L.: Zh. Prikl. Khim. 40, 2492 (1967) 51. Lomashevich S. A., Strokan N. B., Fisnek N. I.: Fiz. Tekhn. Poluprou. 6, 2247 (1972) 52. Lovelock J. E.: Nature 187, 49 (1960) 53. Martin A. J. P., James A. T.: Biochern. J. 63, 138 (1956) 54. Martin A. J. P.: Pure Appl. Chem. 34, 83 (1973) 55. McCarthy W. K., Lazarus M. L.: Chem. Instrum. 1, 299 (1969) 56. MC Donald J. R.: J. Chem. Phys. 54, 2026 (1971) 57. McLean W. R., Stanton D. L., Penketh G. E.: Analyst 98, 432 (1973) 58. Meyer A. S., Knapp J. Z.: ,Anal. Biochern. 33, 429 (1970) 59. Mulyarskii Ya. V.: Zauod. Lab. 36, 1451 (1970) 60. Noble F. W., Abel K., Cook P. W.: Anal. Chem. 36, 1421 (1964) 61. Opregaarg M.: Norw. Pat. 120,095 (Apr. 26, 1969) 62. Oster H., Ecker E.: Chromarographia 3, 220 (1971) 63. Overfield C. V.: Chem. Abstr. 74, 106 792c (1971) 64. Patin C., Patin H.: C. R . Acad. Sci., Sec. C 276, 311 (1973) 65. Polesuk J., Howery D. G.: J. Chromatogr. Sci. 11, 226 (1973) 66. Pompeo D. V., Otvos J. W.: US Pat. 2,641,710 (1973) 67. Poziomek E. J., Crabtree E. V.: J. Assoc. OffAnal. Chem. 56, 56 (1973) 68. Roundy C. B., Byer R. L.: J. Appl. Phys. 44, 929 (1973) 69. Scolnick M. E.: Chem. Abstr. 72, 28 251t (1970) 70. Scott R. P. W.: Nature 176, 793 (1955) 71. Seiyama T., Kagawa S.: Anal. Chem. 34, 1502 (1962) 72. Shcherban A. N., Furman N. I., Belogolovin N. S., Primak A. V., Skrynnik P. M.: Inner. Tekh. 1971, 7 8 73. Simpson T. H.: J. Chromatogr. 38. 24 (1968) 74. Siuda J. F., Debernardis J. F., Cavestri R. C.: J . Chromatogr. 75, 298 (1973) 75. Slawiiiska D., Kruh I.: Chem. Anal. (Warsaw) 18, 923 (1973) 76. Soerensen 0.: Haus Tech., Essen, Vortragsveroeff. 283, 34 (1973) 77. Stedman D. H., Daby E. E.: J. Air Contr. Poll. Assoc. 22, 260 (1972) 78. Stevens R. K., O’Keefe A. E., Ortman G. C.: Erioiron. Sci. Technol. 3, 652 (1969) 79. Strong C., Dils R., Galliard T.: Column 1971, 2 80. Turner D. W.: Nature 181, 1265 (1958) 81. Vermont J., Guillemin C. L.: Anal. Cheni. 45, 775 (1973) 82. Volkov E. F., Rabinovich S. I., Breschenko V. Y.: Ref. Zh, Khim. 1969, Abstr. No. 5 G 14 83. Vtorov B. C., Kalmanovskii V. I., Chulpava B. V., Sheshenin V. A., Yashin Y. I.: Ref. Zh. Khim. 1972, Abstr. No. 11 N 404 84. Wold S., Andersson K.: J. Chromatogr. 80, 43 (1973) 85. Zalkin V. S . : Zauod. Lab. 36, 129 (1970)

2.

The Thermal Conductivity Detector (TCD)

2.1

DETECTION MECHANISM

The thermal conductivity detector is among the most commonly used measuring devices in gas chromatography for monitoring substances separated in the column. This detector measures changes in the thermal conductivity of the carrier gas, caused by the presence of the eluted substances. The thermal conductivity, I f , of component S is determined by the gas density, e, the mean molecular velocity, ij, the mean free path, 2, and the specific heat at constant volume, C v :

It follows from equation (2.1) that the thermal conductivity is a function of the size of the molecules, their mass and the temperature, as

where D is the diffusion coefficient. The dependence of the thermal conductivity on the molecular masses of substances S and C is given by the relationship

Thermal conductivity values for some compounds are given in Table 2.1. The thermal conductivity of binary mixtures is given by the thermal conductivities of the components and their molar fractions, x [46]:

where A and B are constants and .yS = 1 - sc. By solving equation (2.4) for a change in the conductivity of a binary mixture [22], an expression that quantitatively describes the non-linear variation of the conductivity

40 with the concentration is obtained (see Fig. 2.1)

TABLE 2.1 THE THERMAL CONDUCTIVITY OF SOME COMPOUNDS w

Compound

N

K S x lo3 [W/cm. K]

Compound

1710 1470 303 240 239 24s 23 I 216 164

KS x lo3 [W/cm. K]

145 86.2 154 173 152 111 190 170

7 0 FIG.2.1. Thermal conductivities of some binary mixtures.

For very low concentrations of the eluted substance, all the terms in the expanded Taylor series (equation (2.5)) can be neglected except for the first, thus giving a linear relationship between the concentration of the eluted substance and the change in the conductivity of the binary system. Measurement with the TCD is based on monitoring changes in the electric conductivity of the filament, caused by variations in its temperature during passage of

41

the gaseous mixture through the measuring cell. A temperature gradient is established due to transfer of thermal energy by the medium. Under stationary conditions, the amount of heat transferred, Q,is proportional to the thermal conductivity of the flowing gaseous mixture and to the difference in the temperatures of the filament, T,, and the cell walls, T,:

During the design of practical measuring devices, it was found that the overall amount of heat transferred is not given by the thermal conductivity of the medium alone, but that the molar heat capacity and other factors also play a role. The following processes contribute to the overall heat change, i.e., to the measured signal:

- the thermal conductivity of the medium; forced convection of the gaseous mixture; free convection and diffusion of the gas; - the thermal conductivity of leads and connections; - thermal radiation. -

-

The participation of the thermal conductivity and of forced convection of the gas in the overall heat transfer can be distinguished only with great difficulty. Forced convection represents heat transfer coupled with mass transport under the dynamic conditions present in the gas chromatograph and amounts to about 25% of the overall change. This part of the transferred heat is proportional to the volume velocity of the gas, u, and to its heat capacity, C,:

Hence the TCD signal depends on the flow-rate of the carrier gas. The contribution of free convection corresponds to energy transfer in the concentration gradient and is negligible compared with the other factors. Heat transfer by the leads and electrical connections is determined by their crosssection, length and thermal conductivity. Therefore, it is desirable that the leads to the sensor should be as short as possible and have as small a cross-section as possible. It has been found that this parameter does not have a large influence on the overall change in the TCD signal in devices with heated filements, but that its importance increases when thermistors and transistors are used as sensors:

Heat radiation depends on the surface area of the measuring element, on its temperature and on its material quality. The literature gives values of less than 4% for the contribution of thermal radiation to the overall heat transfer.

42 TCD SIGNAL

2.2

Measurement with the thermal conductivity detector is based on monitoring changes in the resistance of the sensor, R , . A current I passes through the sensor and the thermal equilibrium in the measuring cell, through which a gaseous mixture with a thermal conductivity of K passes, can be expressed by the equation 12Rf = J d ( T , - T,) where a is the geometric constant of the measuring cell and J is Joule’s constant. It follows from the experimental arrangement that part of the thermal energy is lost during the passage of the gaseous mixture through the measuring cell and is manifested by a decrease in the sensor temperature, T,, as the temperature of the cell walls, T,, can be considered to be constant because of their high heat capacity. The sensor resistance is a linear function of the temperature: R , = RP(1

+ XT,)

(2.10)

and therefore the measured changes in the resistance are given by the relationships AR, = a R P . A T ,

(2.11)

and

AT,

AK

= - (T, -

K

T,)

(2.12)

The signal of the thermal conductivity detector, SrcD, is proportional to the change in the voltage of the measuring bridge, AE:

srcD

N

AE

AR a(T, - T,) AR - R, K (1 aT,)

f-

+

(2.13)

From equation (2.13) it follows that the signal depends on a number of parameters, the most important being -

the voltage of the measuring bridge, E ; the geometric constant of the measuring cell, a ; the properties of the construction material, expressed by a; the temperatures of the sensor and the cell walls, T, and T‘,respectively; the thermal conductivity of the carrier gas, gc,and its flow-rate; the thermal conductivity of the eluted substance, A K .

As has already been mentioned, the T C D signal is not given by changes in the thermal conductivity of the gaseous mixture alone; forced convection of the material, which has a certain heat capacity, is also important. It is obvious that this contribu-

43 tion will become smaller with increasing participation of diffusion in the mass transport in the measuring cell. Therefore, the term containing heat capacities is most significant with flow-through cells. Bearing this in mind, the following expression has been derived for SJCD[28, 291: (2.14) where constants A and 6 are determined by the effective collision cross-sections and by the molecular weights of the eluted substances and of the carrier gas, where 0 < A < 1 < 6,and u, is the molar flow-rate. The signal of the thermal conductivity detector is therefore strongly dependent on the molecular weights of the eluted substance and the carrier gas. It generally holds that the thermal conductivity decreases with increasing molecular weight, whereas the heat capacity increases. If hydrogen is the carrier gas, then all eluted substances will have lower thermal conductivities and higher heat capacities. The difference in the thermal conductivity term in the proportionality (2.14) increases with increasing molecular weight of the eluted substance and is always negative; the difference in the thermal capacity term increases and is positive. Hence the two processes have opposite effects and under certain circumstances their sum is equal to zero. 2.2.1

TCD background current

The background current of the thermal conductivity detector is given by the composition of the carrier gas, by its flow rate and by the detector temperature. As all practical designs employ a compensation bridge circuit, it is pointless to specify background current values. The T C D noise is caused by fluctuations in the detector temperature and in the carrier gas flow-rate [12]. For this reason, the measuring and the reference branches of the thermal conductivity detector are usually connected in a bridge circuit. This circuit is indispensable with flow-through and semi-diffusion cells, while diffusion cells yield results that are independent of the flow-rate and thus they have the lowest noise level. 2.2.2

TCD response

The T C D response is the sum of the signals, SJCD,over the substance elution time. It follows from equations (2.13) and (2.14) that the relative magnitude of the signal depends on the character of the eluted substance; it decreases with increasing molecular weight. A criterion frequently applied during evaluation of the effect of the structure of the eluted substance on the signal of the thermal conductivity detector is the relative molar response (RMR) [2, 3, 471. The RMR values are proportional to the various measuring sensitivities for various substances caused by their different

44 thermal conductivites. The literature gives the dependences of STCDon the molar and weight percent content of a substance. When light carrier gases are employed, then the proportionality of the signal to the weight per cent of the substance is usually TABLE 2.2 THE RELATIVE MOLAR RESPONSE TO ORGANIC OXYGEN-CONTAINING COMPOUNDS AND TO ALKYL-AROMATIC HYDROCARBONS [16] M - molecular weight Organic compounds

n-Alkyl-aromatic hydrocarbons Primary alcohols Esters RCOOR’ Methyl ketones n-Aldehydes n-Ethers

Number of carbon atoms c6-c10

c1-c4

R = Co-C,; R’ = Ci-C4 C,-C2 c1-c6 c2-c4

Equation for RMR

+ 17.0 0.672.M + 25.4 0.741.M

+ 25.9 + 24.6 21.9 + 49.5

0.630.M 0.688.M 0.631.M+ 0.473.M

used. In any event, the use of relative responses is to be recommended for quantitative measurements, as they depend very little on experimental conditions such as the bridge voltage, cell temperature and flow-rate. The RMR values for homologous

FIG. 2.2. The concentration dependence of the T C D response to some hydrocarbons,

N, - carrier gas, 1 - pentane, 2 - heptane, 3 - octane [29].

series of hydrocarbons, alcohols, etc., are given in Table 2.2 [16]. These values are obtained using hydrogen as the carrier gas and only a narrow range of molecular masses is covered. When the values are compared with those given in the literature

45 [25, 341, substantial differences are found. The validity of the published relative response rates must be judged carefully, as data concerning the measuring cell geometry and the experimental conditions are sometimes not specified. RMR values differing by as much as 18% between flow-through and semi-diffusion cells have been found [40]. It should be emphasized that, when using nitrogen as the carrier gas, even the significance of empirical RMR values is doubtful. Under these conditions, direct calculation of the concentration from the temperature dependences of il in the carrier gas employed [SO] has only limited validity. The molecular weight of the eluted substance significantly affects the character and magnitude of the J C D response. As the thermal conductivity decreases with increasing molecular weight, while the heat capacity increases, these two parameters have opposite effects and lead to conversion of negative peaks into positive peaks. This conversion is important with high-molecular-weight carrier gases, e.g., nitrogen. Fig. 2.2 shows that the change in the signal polarity is attained earlier with increasing molecular weight. TABLE 2.3 PARAMETERS CHARACTERIZING THE THERMAL CONDUCTIVITY DETECTOR WITH VARIOUS SENSORS Sensor Parameters

~.

heated filament

Sensitivity [mV/mole] Linear dynamic range Noise [pV] The lower detectable amount [PPnlI Proportionality constant ImVIKI

thermistor

transistor

4 x 104 5 f 3 [I1

4 x 105 4.4 & 10

2.5 x 106

5

0.2

0.012

6

56

2900

The basic parameters of the thermal conductivity detector depend on the experimental conditions used and therefore they are discussed in greater detail in the following paragraphs. Generally, it can be stated that the thermal conductivity detector is a universal measuring device with a wide linear dynamic range. However, the minimum detectable amounts are sometimes large and then other detectors are preferable. Some parameters characterizing the thermal conductivity detector are listed in Table 2.3. The linearity and linear dynamic range must be given in log-log coordinates because of the exponential form of equation (1.23).

46

2.3

EFFECT OF EXPERIMENTAL PARAMETERS ON THE MAGNITUDE AND SHAPE OF THE TCD SIGNAL

2.3.1

Carrier gas

In measurements with the thermal conductivity detector, hydrogen, helium, nitrogen, argon [45], carbon dioxide and mixtures of various gases, e.g., air or nitrogen with 5-10% of hydrogen or helium [43], are employed as carrier gases. The suitability of various carrier gases can be evaluated with reference to the discussion related to equation (2.14). If hydrogen or helium is used as the carrier gas, the difference in the thermal conductivities is always large and always negative. When the eluted substance is an inert gas with a low molecular weight, then the term containing the heat capacities plays almost no role and the T C D value is given chiefly by the change in the thermal conductivity of the gaseous mixture. When nitrogen is used as the carrier gas, the changes in the conductivity caused by the presence of an eluted substance are small, but the difference in the heat capacities increases significantly. It is evident that the magnitude of the T C D signal is inversely proportional to the molecular weight of the eluted substance and generally S L , is greater than SLY, as the thermal conductivity and heat capacity terms in equation (2.14) are comparable for nitrogen. The purity of the carrier gas effects the magnitude of the T C D signal [20]. If a sample is injected into a carrier gas containing impurities, then, owing to changes in the partial pressures, the impurities are replaced by the eluted substance and the resulting change in the signal is small. For this reason, pure carrier gases should be employed, although identical sensitivities for measurement with pure and impure carrier gases have sometimes been reported [45]. The pronounced dependence of the signal of the thermal conductivity detector on the gas flow-rate follows from the detection mechanism, involving removal of heat by forced convection of gases in the measuring cell. As already mentioned, this effect decreases with increasing participation of diffusion processes in the measuring cell; the dependence of the T C D signal on the flow-rate is negligible with the diffusion type of cell, expecially when a light carrier gas is employed [38]. It follows from equation (2.14) that the thermal conductivity term is independent of the flow-rate. The change in the signal with increasing flow-rate will be greater the greater is the ( C P , - C P ~value, ) i.e., the greater is the molecular weight of the eluted substance. This change, leading to peak inversion, will naturally be greater and more frequent when nitrogen is used as a carrier gas because of the similarity of the conductivity and heat capacity terms in equation (2.14). A decrease in the signal is not always observed during a change in the carrier gas flow-rate; in fact, over a certain range of flow-rates, there is virtually no signal change. This is due to the fact that the elution peak becomes narrower with increasing flow-

47 rate and hence the fdet/d t ratio increases with increasing detector volume. Consequently, the mean concentration of the eluted substance in the effective volume of the detector increases and a maximum concentration is attained. The dependence of the response on the flow-rate, which is regulated either before or after the column, is depicted in Fig. 2.3, from which it follows that the two dependences have the same shapes, i.e., the products of Cs and of the integration time are identical.

t

'

t I-

I

FIG. 1.3.The dependence of the J C D response on the flow-rate of the gaseous mixture; A - flow-rate varied before the column, B - flow-rate varied after the column, before the detector inlet [18].

From these dependences, it follows that the type of carrier gas used is of basicimportance. It is advisable to avoid the use of nitrogen and to work with hydrogen or helium. When using heavier carrier gases, the relationship between the response and the concentration may be markedly non-linear i n the region of chromatographically significant concentrations, resulting in distortion and inversion of chromatographic peaks.

2.3.2

Construction of the J C D

The measurement of thermal conductivity is carried out with a sensor, the resistance of which is strongly dependent on the temperature of the medium (Fig. 2.4). The sensor itself is a t a temperature 7;, which is higher than the temperature of the walls

48

of the measuring cell T,. The sensor temperature is constant with stationary heating conditions and a constant flow-rate of an unchanging gaseous mixture. A change in the composition of the flowing gases is reflected in a change in the sensor temperature, causing a change in the sensor resistance, R,, thus providing an electrically treatable signal. At present, heated filaments, thermistors and transistors are employed as sensors.

FIG. 2.4. The dependence of the sensor resistance on temperature; a - heated filament, b - thermistor, c - transistor.

The heated filament was the first sensor to be used in the thermal conductivity detector. The dependence of its resistance on temperature is linear over wide temperature range (up to 400" C). This advantage is partly offset by its relatively low sensitivity to temperature changes (see Table 2.3.). In addition to heated filaments, thermistors with negative thermal coefficients are used as sensors. The thermistor resistance is an exponential function of temperature and the maximum temperature used is about 100 "C; above this value, the sensitivity of the thermistor towards ,temperature changes is very low. If the thermistor resistance equals RP at a particular standard temperature, T o ,(usually 25 'C), then the resistance at temperature T, is given by the relationship. R,

=

[

RP exp -

(A 31 -

(2.15)

A transistor was first used as a sensor in the thermal conductivity detector in 1972 [31], its use being based on the direct conversion of changes in the sensor temperature into an electric signal. As transistors cannot be heated directly, they are maintained at a temperature T, by an external source. The steady-state thermal equilibrium can be expressed in terms of the transistor collector current, I,: I, E =

0 .

R (T, -

T,)

(2.16)

49

The sensor temperature, T,, changes during the elution and the change is manifested in a change in the collector current, A I , , which is measured directly. A bridge circuit is not used and this method leads to a considerable increase in sensitivity.

2.3.2.1

Seiisor heating voltage

In contrast to the heated filament detector, the dependence of the SrcD obtained with a thermistor on the voltage exhibits a maximum. The optimum value of the heating voltage is given by the relationship

E,,,

-J

(RO . 6 . Rc . T,)

(2.17)

Under constant thermistor parameters, i.e., standard-state resistance, Ro, and material constant, 6, and with a given carrier gas characterized by a thermal conductivity Kc, the optimum temperature of the supply voltage depends solely on the temperature of the detector walls (see Fig. 2.5).

FIG. 2.5. The dependence of the relative magnitude of SrCD on the heating voltage and the detection block temperature; a, b - thermistor, c - heated filament.

Fig. 2.6. The dependence of S T C Don the carrier gas quality; 1 - nitrogen, 2 - hydrogen.

A change in the carrier gas results in a change in the optimum heating voltage. As follows from the introductory section, carrier gases with lower molecular weights absorb more heat and therefore a larger heat supply is necessary in order to maintain the same sensor temperature and a similar sensitivity. This dependence is depicted schematically in Fig. 2.6. 2.3.2.2

Sensor pararneters

The signal of the thermal conductivity detector with a heated filament depends on the properties of the filament, especially on its specific resistance, e, and its thermal

50

coefficient, a. The signal is related to these values by the relationship

-

(2.18)

STCD alJe

The values of a, e and STCD for some materials are summarized in Table 2.4. TABLE 2.4 THE SPECIFIC RESISTANCE, THERMAL COEFFICIENT A N D THE RELATIVE RESPONSE OF SOME MATERIALS USED AS TCD SENSORS Material Platinum Pt-Ir 90-10 80- 20

Tungsten Nickel

a.

103[K-'] 4.0 1.2 0.8

4.54 4.91

p [ Q . mm2/m]

sTCD

0.106 0.24

13 2.5 1.5 19 18

0.31 0.058 0.072

One of the most commonly used materials is platinum, which, however, has poor mechanical properies; the filament diameter is therefore generally rather large (about 0.02 mm). Alloys of platinum with iridium or rhodium have better mechanical properties but yield smaller signals then platinum sensors. After platinum, tungsten is the most commonly used filament material. Its relative response is comparable with that of platinum and it also has very satisfactory mechanical properties, so that very thin filaments (down to 0.006 mm) can be used. However, it is oxidizeh by atmospheric oxygen at temperatures above 500 "C. Nickel filaments are occasionally used in corrosive media, but their use is limited by their poor mechanical properties [8, 9, 13, 15, 36, 441. A number of workers have tried to prolong the life-time of sensors, decrease the noise level and prevent drift of the background current. In addition to passivation of the W-Re filament at 330 "C by formation of oxides [8], the surface of the filament was gilded [17] and treated with H F [15] or CH,Cl, [36] vapour. These modifications led to an increase in the detector stability. The measuring filament has also been covered with a palladium catalyst [13], increasing the sensitivity for both a- and p-olefins. Thermistors are characterized by the values of Ro and the material constant. The thermistor resistance varies within a wide range, from 1 to lo6 0.In general, the detector can be used at higher temperatures if a high-resistance thermistor is employed, but a recorder with a high input impedance must be used. As the sensitivity of the thermistor to temperature changes is highest at low temperatures, thermistors with low resistances (of the order of lo3 to lo4 62) can be employed. The thermistor material constant corresponds to the energy required to increase

51 the temperature by A T , . Hence it is evident that, with increasing material constant, the detector time constant will increase, thus rendering the results less reliable. 2.3.2.3

Cell geometric constant

The amount of heat removed from the surface of a heated filament is proportional to its surface area (see equation (2.6)). For a cylindrical body, this geometric constant is given by the relationship (2.19) where 1 is the length of the heated filament, rc is the internal radius of the cell and rf is the external radius of the filament. It follows from equations (2.6) and (2.19) that STCDincreases with increasing length and radius of the heated filament and is inversely proportional to the cell radius. These dependences must be evaluated correctly, as it is impossible simultaneously to lengthen the filament and make the cell volume smaller. If the filament is lengthened, the cell must be larger and the detector volume increases; this leads to an increase in the detector time constant. Existing commercial detectors represent a compromise among the above requirements. The effect of the geometric constant on the magnitude of the TCD signal is represented schematically in Fig. 2.7.

I

M

0

15 - a

FIG. 2.7. The dependence of the TCD signal on the geometric constant at various temperatures of the heated filament.

m

FIG. 2.8. The dependence of STCD on the temperature of the filament and of the cell walls; the difference (Tf- T , ) is plotted on the x-axis.

When a thermistor is employed as the sensor, it is assumed to be spherical and the geometric constant is expressed by the relationship a =

4xrrrc

-

- 4nrt r c - rf

(2.20)

Therefore, the signal of a detector employing a thermistor is directly proportional to its radius.

52

2.3.2.4

Temperatures of the sensor and the cell walls

From the principle of the thermal conductivity detector, it follows that T, must generally be larger than T, (when a heated filament is used), so that (T, - T,) 2 - 200 "C. The detector signal is directly proportional to the difference between the I temperatures of the heated filament and the cell walls, from which follow important conclusions concerning the adjustment of the experimental conditions. At a selected detection block temperature, SrCDis proportional to the temperature of the heated filament, i.e., to the heating intensity (Fig. 2.8); on the other hand, at a given temperature of the heated filament, i.e., with constant heating, STCD is inversely proportional to the detection block temperature. In practice, it is most advantageous to maintain the temperature of the heated filament as high as possible (taking care not to burn the filament) and the detection block temperature as low as possible (avoiding condensation of eluted substances in the cell). When working with columns with programmed temperatures, the possibility of a decrease in the TCD signal must be borne in mind if the detection block is connected to the column thermostat. During an increase in the temperature of the column thermostat, the temperature difference (T, - T,) decreases and consequently STCD also decreases. The above rules also hold for thermistor sensors. It has been found experimentally that the highest measuring sensitivity is attained for a small difference between the sensor and cell temperatures, (T, - T,) = 35 - 50 " C , while the sensor temperature should not exceed 100 "C. STCDis strongly dependent on the detection block temperature: STcD 8 1 . 5 Rf1 l 2 Tc- 2 (2.21) Therefore, it is preferable to maintain the detection block temperature as low as possible. 2.3.2.5

Time constant of the TCD

The magnitude of the time constant depends on the effective volume of the detector. The requirement that the time constant of the measuring device should be as low as possible, leading to the smallest possible distortion of the elution curves, is partially in opposition to the requirements regarding the magnitude of the detector signal concerning, for example, the length of the heated filament or a large thermistor radius. The cell time constant is expressed by the relationships

'

vdet

=

7 =

0.632td,,

*

'dst

and These relationships are valid, however, only for cells in which transport occurs

53 exclusively through convection. A number of thermal conductivity designs have been proposed in which both diffusion processes and mass convection towards the sensor surface participate. According to this criterion, cells are classified as flow-through, semi-diffusion and diffusion. The shapes of these cell types are depicted in Fig. 2.9.

FIG. 2.9. Various shapes of thermal conductivity cells; a - flow-through, b - diffusion, c - semi-diffusion.

The expression for the time constant of flow-through cells reflects the significant dependence of their signal on the gas flow-rate. Therefore, manostats are placed before the cells in order to stabilize the flow-rate and the pressure [12]. The smallest distortion of the shape of the elution curve is achieved with a low time constant; designers of thermal conductivity cells thus attempt to make the flow-through cell volume as small as possible. Cells with volumes of 20 pI [21], 2.6 ,d [32] and even 1 pL1[23] have been described. TABLE 2.5 TIME CONSTANTS OF VARIOUS CELL TYPES IN THERMAL CONDUCTIVITY DETECTORS Cell

Flow-through Diffusion Semi-diffusion o1 / r 2 > 1 C I / U 2 J. 1

7

[sec]

Notes

most frequently used, most sensitive to all changes 10-20 unsuitable for modern measuring requirements up to 10 properties of the flow-through cell up to 20 properties of the diffusion cell

0. I- 1

54

It is obvious that the time constant will increase with increasing participation of diffusion in the transport process. The time constant of a semi-diffusion cell can be expressed by the relationship

1 z = 0.632t,,, V 2-111

where trl is the gas flow-rate through the measuring branch (see Fig. 2.9). The time constants of the cell types discussed are listed in Table 2.5. In addition to the time constant of the measuring cell, the sensor time constant, determined by its mass and heat capacity, must also be considered. Thus the sensor requires a certain time to record a change. The time constant of a heated filament is given by the relationship

-

As the heat capacity of the filament is proportional to its volume, i.e., mC, $1, the requirement of high T C D sensitivity (see equations (2.9) and (2.19)) leads to an increase in the sensor time constant. The heat capacity of the thermistor approximately equals the material constant, 6, and varies around 1 sec for most sensors of this type. In a provisional arrangement, the transistor sensor had a large time constant of 10 sec [31], which the authors felt could be decreased to 0.6 sec. In some designs, the heated filament is sealed in glass in order to decrease the catalytic action of the heated filament on the thermal decomposition of substances and to prevent corrosion of the sensor. Similarly, materials other than glass have also been employed, e.g., fluorinated plastic [ 5 ] . These modifications lead to an increase in the lifetime of the sensor and make its use at higher temperatures possible, but the TABLE 2.6 THE TIME CONSTANT OF THE THERMAL CONDUCTIVITY DETECTOR Sensor Cell

Length of cell - sensor [cm] Diameter of cell - sensor [cm] Gas flow-rate [ml/min] Time constant [sec]

heated filament

2 0.5 30 0.45

filament in

thermistor

transistor

2

0.2

0.2

0.002

0.1

0.2

0.2

0.01

4

1

9.6

10

55

time constant of the sensor and consequently that of the whole detector are increased considerably due to an increase in the heat capacity of the measuring element. Cell time constants and those of individual sensor types are given in Table 2.6, from which it follows that a heated filament has the lowest time constant. It should be used in combination with a flow-through cell whenever theoretical or quantitative empirical conclusions are to be made on the basis of the measured results. 2.3.2.6 Measuring circuits

If a heated filament or thermistor is used as a sensor in the thermal conductivity detector, it is connected in a Wheatstone bridge for compensation measurements. When a voltage E is brought to a bridge consisting of the sensor resistance, R,, and a reference sensor resistance, R,, then, for the current passing through the circuit (Fig. 2.10)

E = I(R,

+ R,)

A signal corresponding to the voltage change due to a change in the sensor resistance can be measured at the bridge output.

_-

. I

U

f FIG. 2.10. Scheme of the T C D constant-current circuit.

f FIG. 2.1 1. Scheme of the T C D circuit with a constant sensor temperature.

The above principle is used in both constant-current and constant-voltage circuits, with either two or four sensors. A single thermistor in a voltage-powered bridge has also been employed [7]; however, the only advantage of this circuit was its simplicity, the J C D parameters not being improved. In addition to the measurement of d.c. voltage and current, square-wave J C D operation has also been proposed. Constantcurrent bridges yield a broader linear dynamic range than constant-voltage bridges. Recently, a thermal conductivity detector with a constant sensor temperature was introduced [49]. In this circuit, a single variable resistor is connected in the bridge (Fig. 2.11). The uncompensated output voltage is amplified and fed back to the bridge

56

through a transistor controlling the current passing through the bridge. In this way, the current is instantaneously adjusted so that the sensor resistance, R,, and consequently its temperature, T,, do not change. It has been found experimentally that a device with a constant-temperature sensor has a number of advantages over constant-voltage or constant-current circuits. The lowest detectable amount is decreased ten-fold, the linear dynamic range is broader by one order of magnitude and the detector time constant is decreased. Concentrations of 10%by volume were measured in this way without a change in the detector linearity. The pronounced improvement in the J C D parameters when a constant temperature is used can be explained by the fact that, owing to the constant sensor temperature during the elution, the entire elution curve is measured under constant conditions with constant sensitivity. With constant-voltage or constant current circuits, the sensor temperature gradient follows the concentration gradient of the eluted substance. With increasing amounts of eluted substances, the measuring sensitivity decreases owing to a decrease in the sensor temperature. 2.4

APPLICATIONS OF THE TCD

The thermal conductivity detector is one of the most commonly used detectors in gas and liquid-solid chromatography [41]. In addition to a number of reviews [17, 19, TABLE 2.1 EXAMPLES OF THE APPLICATION OF THE THERMAL CONDUCTlVITY DETECTOR Substance determined

Material analyzed

Permanent gases

in air

Permanent gases Permanent gases

in in in in in in

N2, 0 2 , c 0 2

NO, 0 N Metal halides PCI, Chlorinated hydrocarbons Alkanes, alcohols, aliphatic acids Olefins, branched olefins ‘BH18; C10H22;

chlorine high purity ethylene exhaled air air organic compounds organic compounds

Theable lowest detectamount

10, 23, 35,43 I1 17a 30

0.002 1.11

1 1.1g

1.9 x 8.7 X

Ref,

3a 27 39 mole 5 mole 5

15

1.4; 2.7; 3.5ppm

37 13 32

57

24, 33, 34, 42, 481, the excellent treatment of the T C D in the book by Jentzsch and Otte [18] should be mentioned. Some examples of T C D applications are given in Table 2.7. When the T C D is compared with other detectors, for example the discharge detector [l], E C D [5] or FP D [14], better parameters are usually found for the T C D . Comparison of the T C D with the FID [4, 35, 371 depends on the structures of the eluted substances. The T C D is, of course, more suitable for inorganic gases such as CS,, COS, H,S and SO, [35]. During measurement of the signal ratio, S"/SrCD,

n

FIG. 2.12. Analysis of air using a TCD with a transmodulator [23].

values greater than unity were obtained for alkanes, and values smaller than unity for oxygen-containing compounds [37]. It can be presumed that the thermal conductivity detector will find further use in analyses of inorganic gases, of silylated samples, etc. [26]. A thermal conductivity detector with a palladium transmodulator [23] gave excellent results in analyses for rare gases in air (Fig. 2.12).

2.5

LITERATURE

I . Arnikar H. J., Rao T. S., Karmarkar K. 13.: J. Chrornatogr. 38, 126 (1968) 2. Barry E. F., Rosie D. M.: J. Chrorwtogr. 59. 269 (1971) 3. Barry E. F., Jr.: Chenz. Abslr. 76, 50 486c (1972) 3a. Beskova G. S., Filipov V. S.: Zaaod. Lab. 38, 154 (1972) 4. Bogoslovskii Yu. N., Razin V. L.: Zlz. Fiz. Khim. 45, 2490 (1971) 5 . Brazhnikov V. V., Sakodynskii K. I.: Clzern. Absrr. 74, 94 013j (1971) 6. Castello G., DAmato G.: J. Chromatogr. 32, 625 (1968) 7. Chowdhury B., Karasek F. W.: J. Chroniatogr. Sci. 8, 199 (1970) 8. Cieplinski E. W., Spencer S. F., Illingsworth W. L.: US Pat. 3,537,914 (Nov. 3, 1970)

58 9. Delew R. B.: J. Chromatogr. Sci. 10, 600 (1972) 10. Ediz S. H., van Swaay M., McBride H. D.: Chem. Instruni. 3, 299 (1972) 11. Gerdes W. F.: US Pat. 3,474,661 (Oct. 28, 1969) 12. Goryunov Yu. A,, Zalkin V. S., Okhotnikov B. P., Rotin V. A., Rozanova L. I., Maksimov B. G.: USSR Pat. 371 511 (Feb. 22, 1973) 13. Guillot J., Bottazzi H., Gyuot A., Trambouze Y.: J. Gas Chromatogr. 6, 605 (1965) 14. Gutsche B., Herrmann R.: 2.Anal. Chem. 249, 168 (1970) 15. Hachenberg H., Gutberlet J.: Brensf. Chem. 49, 242 (1968) 16. Hara N., Katsuda M., Kato H., Hasegawa K., Ikebe K.: Chem. Abstr. 68, 65 5D4r (1968) 17. Hartmann C. H.: Anal. Chenz. 43, 113A (1971) 17a. Jadrijevic V.,Deur-Siftar D.: Chromafogruphia 7, 19 (1974) 18. Jentzsch D., Otto E.: Detektoren in der Gas Chromatographie, Akademische Verlaggeselschaft, Frankfurt a. M. 1970 19. Johns T., Stapp A. C.: J. Chromatogr. Sci. 11, 234 (1973) 20. Karp S., Lowell S.: Anal. Chern. 43, 1910 (1971) 21. Kiefer M. E.: Ger. Offen. 2.222 617 (Nov. 30, 1972) 22. Lindsay A. L., Bromley L. A,: Ind. Eng. Chem. 42, 1508 (1950) 23. Lovelock J. E., Simmonds P. C., Shoemake G. R.: Anal. Chem. 43, 1958 (1971) 24. McNair H. M., Chandler G. D.: J. Chromatogr. Sci.11, 454 (1972) 25. Monfort J. P.: Chim. Anal. (Paris) 53, 646 (1971) 26. Morrow R. W., Dean J. A., Shults W. D., Guerin M. R.: J. Chromatogr. Sci. 7, 572 (1969) 27. Nikolaeva N. A., Dolgopolskaya P. I., Rezler R. Yu.: Chem. Abstr. 70, 111 448s (1969) 28. Novik J., WiEar S . , Janak J.: Coll. Czech. Chem. Commun. 33, 3642 (1968) 29. Novak J., Janiik J.: Coll. Czech. Chem. Commun. 35, 212 (1970) 30. Noviik J., JaniEek M.: Chem. Listy 65, 739 (1971) 31. Otte E., Gut J.: Chromatographia 5, 246 (1972) 32. Pecsar R. E., De Lew R. B., Iwao K. R.:Ana. Chem. 45, 2191 (1973) 33. Richmond A. D.: J. Chrornatogr. Sci.9, 92 (1971) 34. Rosie D. M., Barry E. F.: J. Chromatogr. Sci. 11, 237 (1973) 35. Schaeffer B. A.: Anal. Chenz. 42, 448 (1970) 36. Seibel A. C., Johns T.: US Pat. 3,533,858 (Oct. 13, 1970) 37. Sokolov D. N., Golubeva L. K.: Zauod Lab. 35, 143 (1969) 38. Tamura H., Hozumi K.: Chem. Abstr. 72, 128 311f (1970) 39. Thuerauf W.: 2.Anal. Chem. 250, 11 1 (1970) 40. Vermont J., Guillemin C. L.: Anal. Chem. 45, 775 (1973) 41. Versino C., Fogliano L., Giaretti F.: Riu. Combust. 21, 389 (1967) 42. Villalobos R.: Chem. Eng. Progr. 64, 55 (1968) 43. Volkov E. F., Rabinovich S. I., Breshchenko V. Ya.: Ref. Zh. Khim. 1969, 5 G 14 44. Wade R. L., Cram S . P.: J. Chromatogr. Sci. 10, 622 (1972) 45. Walsa J. T., McCarthy K. J., Merritt C., Jr.: J. Gas Chromatogr. 6, 416 (1968) 46. Wassiljewa A.: Physik. 2.5, 737 (1904) 47. Watanabe S., Nakasato S., Kuwayama H., Sasamoto Y., Shihaishi S., Seino H., Nagai T., Negishi M., Hayano S.: Yukagaku 22, 102 (1973) 48. Winefordner J. D., Glenn T. H.: Aduan. Chromatogr. 5, 263 (1968) 49. Wittebrood R. T.: Chromatographia 5, 454 (1972) 50. Zalkin V. S.: Zauod. Lab. 36, 129 (1970)

3.

Ionization Detectors

3.1

PHYSICAL PRINCIPLES OF THE DETECTION

The most extensive group of measuring devices used in gas chromatography consists of detectors that measure the ionization current. The process is based on the collision of a moving particle of high energy P (denoted as the primary particle or the source of the ionization energy) with a target particle S, which is thus ionized: P + S

-,

P ’ + S + + e

The collision produces a number of positive ions and secondary electrons, which move toward poles in an external electric field, giving rise to a measurable current in the electrode circuit, denoted as the ionization current. Reaction scheme (3.1) describes only the ionization of the target particle. It must be borne in mind that the collision of particles may lead to other types of interaction, such as changes in electronic states or changes in the vibrational and rotational states of molecules, which are strongly dependent on the properties of the colliding particles. This diversity in the collision process leads to variations in the value of the measured ionization detector signal, depending on the experimental conditions. However, commercially produced measuring devices (detectors) are only occasionally constructed on the basis of optimization of experimental conditions considering the physical basis of the process, so that results which are difficult to compare are frequently obtained and incorrect interpretations sometimes occur. In the literature, considerable attention has been paid to ionization detectors, corresponding to their importance in experimental techniques. The detectors are intuitively classified on the basis of the energy of the ionization source. In some instances other criteria have been employed, for example, whether a radioactive source is used in the detector. Distorted pictures may thus arise, leading to unconvincing explanations of detection mechanisms and to an abundance of anomalies in the detector response. A unifying viewpoint for the classification of ionization detectors is a discussion of the conditions of reaction (3.1), the properties of colliding particles, P and S, and the intensity of the applied electric field. These factors are common to all types of ionization detectors and can therefore be discussed in general. Specific interactions in particular detection mechanisms will be discussed in the chapters on the individual detectors.

60 3.1.1

The collision

The ionization energy can be carried by particles of various weights, either charged, e.g., an electron, proton or heavy ion, or uncharged, e.g., a photon, metastable atom or excited molecule. These particles have energies in the range 0.01 - 100,000eV and will obviously react differently with an atom or molecule. During the collision, the energy of the primary particle decreases by an amount corresponding to the ionization potential and to energy changes in the electronic, rotational and vibrational states. It generally holds that a primary particle moving in a medium of substance S will lose its energy (or part of it) to this substance during collisions with particles of S, until the primary particle energy corresponds to that of thermal motion (0.01 eV). This rule is generally valid for any type of particle, i. e., both for a helium atom metastable state and for an electron, proton, etc. Various interactions can occur during collision, for example changes in electronic, rotational and vibrational states and in ionization, each of these interactions having a different probability, denoted as the cross-section for ionization, electron capture, etc. The sum of all of the interactions of a single particle is then expressed by the gross cross-section, which gives a picture of the probability of particle annihilation. proton energy [MeV] I

0.7

,

0.2

0.4 0.6 1

1.0

1

1

1

2%-&%7&n& electron energy [ e ~ ]

FIG. 3.1. Comparison of the apparent experimental ionization cross-sections for protons and electrons of equal velocity incident on molecular nitrogen and hydrogen [9].

IS0

306'

4.50

600

electron energy [e V ]

FIG. 3.2. The probability of ionization of C2H2, NO, N, and H,. The ordinate represents the number of positive charges perelectron per centimetre path at 1 torr pressure and 0" C [9].

The greater are the particle energy and mass, the greater is the value of its gross cross-section. With decreasing particle mass, the probability of non-elastic collisions decreases. This is true for both primary and target particles. Thus it is evident that,

61 in ionization with electrons, the number of non-elastic collisions causing ionization (i.e., the background ionization current) will be the highest in nitrogen and the lowest in helium, i.e., N, > H2 > He. The increase in the cross-sections of molecules compared with monatomic gases is caused by the greater number of possible energy states in the former. The cross-section dependence for electron collisions in nitrogen, hydrogen and helium is shown in Fig. 3.1. The same rules also hold for the primary particle, i.e., the proton ionization cross-section will be greater than that for electron ionization (see Fig. 3.1). The ionization probability is greatest for particles with energies close to the ionization potential of the target substance. Thus the primary electrons emitted by tritium, with an energy of 18.8 keV, or by a nickel isotope, 63Ni, with an energy of 67 keV, will produce fewer ion pairs per collision than electrons with an energy of only 100 eV (see Fig. 3.2). As mentioned above, the non-elastic cross-sections of electrons are strongly dependent on their energy. The ionization probability increases from zero for primary electrons with only thermal energy to a maximum (75 to 100 eV) and then decreases. In general, the electron ionization cross-section for an electron energy of 500 eV is roughly half of the maximum value and it decreases further with increasing electron energy. This phenomenon is caused by the high velocity of the primary electron compared with that of the target particle orbital electrons, leading to a very low collision probability. The cross-section for ionization with a heavy ion has its maximum shifted toward higher primary particle energies. The ion velocity is lower than that of electrons and thus the time interval corresponding to passage of the ion through the electronic sphere is comparable to the time interval corresponding to the occurence of an orbital electron in the collision path. The collision probability is thus higher than that for electrons. The above rule that the maximum ionization efficiency corresponds to particles having energies close to that of the ionization state of the target substance is most strongly applicable to photons, for which the probability of non-elastic collision is limited by the permitted energy states of the products formed. The photon energy is quantized by the ionization energy of the target substance ( E I P )and the energy of the products of reaction (3.1):

All particles change their energy states during their lifetimes, with energies decreasing to thermal energy, i.e., the character of the interactions is continuously changing, a certain type being preferred under certain conditions. Particles with energies higher than the ionization energy of the target substance cause its direct ionization with simultaneous changes in the vibrational and rotational states of the molecules. Particles with lower energy cause changes in rotational and

62

vibrational states and, with electron collision, ionization of the target substance by electron capture may occur. On the basis of this brief survey of the interactions that can occur, the measuring devices used in gas chromatography can be arranged according to the most probable processes caused by the primary particles (see Table 3.1). TABLE 3.1

A SURVEY O F IONIZATION DETECTORS USED I N G C Primary.~ particle Most probable process

Energy range

B

20eV to 10eV

2 eV to 0.01 eV

3.1.2

Type

100 keV to 20 eV

5 eV to 2 eV

Detector

direct ionization

cross-section detector

excitation of metastable states in the presence of helium

helium detector

hv

direct ionization

photoionization detector

B, e Hem, Arm A B*

direct ionization direct ionization chemiionization

cross-section detector helium (argon) detector flame ionization detector

e

capture ionization

electron capture detector

hv

surface ionization or cold emission and electron capture alkali metal ionization

thermionic detector TlDA

e

capture ionization

electron capture detector

acceleration in an applied field and charge transfer

electron mobility detect or

Effect of the electric field intensity

Charged particles are attracted towards poles in an external electric field. During the resultant movement, the particles are accelerated and their energy increases. This acceleration depends on the electric field intensity, E (Vlcm), and the particle free path, which is proportional to the pressure p (torr) and the particle mass. As electrons have a low mass, they are substantially accelerated in an electric field, even at low E / p values. On the other hand, ions with large masses are only slightly accelerated in the E / p range 1to 200 Vcm-' . torr-'. Ion energies vary around 1 eV.

63

In addition to the pressure of the gas through which the particle moves, the qualitative properties of this gas must also be considered. The average particle energy will be higher in monatomic than in diatomic gases, because of the lower number of possible energy states in the former (see Fig. 3.3). Thus, under gas chromatographic conditions when helium is used as the carrier gas, there is a greater probability of accelerating the secondary electrons to an energy sufficient for direct ionization of the target substance; when nitrogen is used as the carrier gas, the probability of successive ionization by an accelerated secondary electron is negligible.

FIG. 3.3. The characteristic energy of elcctrons in He, N, and 0, as a function of E / p 191.

FIG. 3.4. Calculated energy distributions for electrons in helium for E / p values of 3, 6 and 10 V/cm. torr 191.

The energy distribution also changes with a change in the electric field intensity. With increasing E / p values, a greater number of particles of higher energy is obtained, as shown in Fig. 3.4. Hence it is clear that, when a d.c. electric field is used for collecting charged particles formed by ionization, changes i n particle energies also occur and result in further reactions involved in the detection reaction mechanism. Accelerated electrons lose their energy by colliding with atoms of molecules of the medium through which they are moving and also by collision with solid parts of the detector. These interactions lead to (1) heating of the anode;

(2) heating of the carrier gas;

(3) ionization by electron capture; (4) excitation of atoms or molecules of the carrier gas; ( 5 ) direct ionization of atoms or molecules.

If the probabilities of the individual interactions are expressed qualitatively, it is found that, with increasing electric field intensity, the probability of the loss of the electron energy by process (1) increases, by processes (2), (3) and (4) decreases and by process (5) again increases. For the probabilities of processes (4) and (5), it is

64 found that (4) 2 ( 5 ) i.e., excitation to various states is always more probable than direct ionization [lo]. Acceleration of electrons under normal pressure and low E / p values (about 1 to 10) does not generally yield sufficient energy for direct ionization. This acceleration leads to a pronounced increase in the production of electrons with an energy of about 3 eV and the electron capture ionization probability thus significantly increases. This process is common to all ionization detectors and is discussed below. Excitation of atoms or molecules may sometimes lead to the formation of so-called super-excitated states or, with monatomic gases such as argon and xenon, which are present as impurities in carrier gas containers, to the formation of metastable states. While most super-excited states disappear by emission of energy, metastable states participate in the ionization of substances in the detection space. Process (I), leading to ionization of substances through so-called surface ionization, also contributes to the overall increase in the ionization current. It generally holds that the cross-sections of individual ionization mechanisms are additive [4]. It follows from the above discussion that the electric field intensity significantly affects the electron history, especially that of secondary electrons. Various designs of collecting electrodes and various electric field intensities .are employed for gas chromatographic measurements. The applied d.c. voltages lie in the range 100 to 2000 V, and have a pronounced effect on the E / p value for electrode distances of 1 to 10 nim. All detector designs employ E / p values from 0.2 to 4,000 V/cm . torr. Lower electric field intensities ( E / p < 10) are chiefly applied under the conditions corresponding to the flame ionization detector, the thermionic detector using an alkali metal, the photoionization detector, etc. With these types of detector, there are no changes in the secondary electron distribution and ionization by secondary electrons does not contribute to the measured ionization current. The second group contains detectors employing high E / p values, usually above 500. The helium and argon detectors and some versions of the photoionization detector belong in this group. Electrons are accelerated and their energy is increased above 10 eV, so that the probability of collision leading to ionization of the target substance increases considerably. The measured current consists of several contributions, among which ionization with accelerated secondary electrons is also important. The marked dependence of the experimental results on the electric field intensity must be borne in mind. However, it is also true that commercial instruments d o not always specify the electrode distance and the collecting voltage, so that it is often very difficult to compare results obtained with different instruments. In some experiments, the dependence of the polarity of the electric field on the experimental arrangement of the detector was studied. In general, the collection of ions and electrons is almost instantaneous at suitable electric field intensities and is not affected by the geometric arrangement of the poles. It must be borne in mind that ions formed by the ionization of molecules move much more slowly than electrons

65

and that the velocity of ionic movement does not change with their free path, in contrast to the situation with electrons, An increase in the electron velocity leads to the formation of a space charge, which causes a decrease in the ionization detector signal and an increase in the noise. The formation of a space charge is probable with currents above 10-9A, i.e., in JlDA systems and in some helium detectors. I t occurs only very rarely in FID and E CD systems and not at all in a PID with separated spaces. Whether the measured current is negative or positive is determined by the design of the electronic part of the instrument and is decided by the manufacturer. In recent years, some ionization detectors have employed a pulse electric field for collecting ion pairs. There are devices with variable pulse width and with different pulse frequencies. It is evident that the greater the pulse frequency, the more the pulse arrangement will resemble that employing a constant d. c. field. With a lower pulse frequency, the effect of the electric field on the fate of secondary electrons is decreased.

3.2

IONIZATION ENERGY SOURCES

Metastable atomic states, photons, i(- and a-particles or electrons obtained by discharge are used as primary particles. Most widely used are photons and P-particles emitted by certain radioisotopes (see Table 3.2). Using radioisotopes, a source with a constant intensity of primary particles is obtained. The suitability of using radioisotopes is determined by the character of the emitted radiation, safety requirements, their mechanical properties and also by the cost of their production. These requirements are best met by a-sources, especially tritium and the nickel isotope 63Ni.Their suitability is also given by their disintegration times, which permit the preparation of sources with a relatively high specific activity and the absence of a-radiation ensures safe handling without excessive precautions. For this reason, sources of a-radiation are unsuitable, as they generally also emit high-energy y-radiation. A similar problem is encountered when using 90Sr, which is in equilibrium with its daughter isotope. 90Y. The latter has a very short half-life (64 days) and produces high-energy /I-radiation, which gives rise to y-radiation in the surrounding matter and therefore requires special safety measures. The first radioisotopes used contained tritium deposited on a suitable support. Titanium is most frequently employed as the support material and the tritium is adsorbed on it. Desorption of the tritium takes place at increased temperatures; it is evident from Table 3.3 that the loss in the activity decreases exponentially with increasing temperature. Hence it is recommended that the temperature should not be increased above 220 "C when using tritium sources on titanium suports. For this reason, other supports for tritium were sought. The thermal stability of the source improved considerably (up to 325 "C) when scandium was used [3]. It is assumed that the hydride Sc3H, is formed.

66 The activities of tritium sources vary from several millicuries to several curies. These are high values and can be used only with pure /3-sources. TABLE 3.2 SURVEY O F RADIOACTIVE SOURCES SUITABLE FOR IONIZATION DETECTORS [8]

Note

12.3 5570 265 5.29 120 10.6 28/64 d 2.12 x lo5 2.6 2.07 30.4 2.6 93 16 1.7 3.88 21.4 1620 21.6 6.7 13.3 458

0.01 8 0.158 0.57 0.31 0.067 0.67 0.5412.27 0.29 0.3 0.65 0.5 0.23 0.076 0.15- 1.84 0.15 0.76 0.017

0 0 0 1.33 0 0.5 010.55 0 0.4 0.8 0.6 0.12 0.02 0.12-1.28 0.086 0 0.046 0.188

0.04

0.1

0.05

0

0.02

0.1 0.06-0.03

4.78

5.48

B B D

C D B A

A D A

D

A

A - expensive preparation B - low specific activity due to long half-life C - technical difficulties in application D - danger of y - radiation or of Bremsstrahlung

For measurements at higher temperatures (up to 400°C), the 63Ni isotope is used most frequently [12], and I4'Pm [ 6 ] , 239Pu [2], 241Am [ 5 ] and 137Cs[ll] have also been employed. It follows from the principles of the ionization mechanism that, in the source of ionization energy, the number of electrons emitted in unit time, i.e., the source activity, and not the primary electron energy is the decisive factor. Primary electron energies are in any event so high that the probability of direct ionization is very low

67 and is thus comparable for electron energies of 18 and 67 keV, as electrons with these energies produce identical numbers of ion pairs. An increase in the source activity leads to an increase in the number of primary electrons emitted and consequently also in the number of ion pairs formed. Hence it is impossible to compare various sources at different activites [3, 71. Measurement of the ionization current with an 8 mCi TABLE 3.3 LOSS I N THE TRITIUM ACTIVITY BY BLEEDING AT VARIOUS TEMPERATURES The original activity was 100 mCi, nitrogen carrier gas was employed Temperature ["C]

130 I70 210 230 250 285

Loss in activity [u Ci / h l 0.01

0.3 9 30 120 1610

'47Pm source and a 15 mCi 63Nisource can never distinguish between promethium and nickel, but will reflect the greater activity of the latter source by an increase in the ionization current and also in the background current, and by the higher sensitivity of the device with the higher source activity. The sensitivity ratio, which is usually cited, has approximately the same value as the ratio of the activities employed in the detectors. Photons are also used as source of ionization energy, but in a substantially narrower energy range. Photons in the vacuum UV spectral region have energies around 11 eV; for example, the L, line of the hydrogen spectrum corresponds to 1215.7 A, i.e., 10.2 eV. The utilization of this ionization energy source has so far been very limited. A flame- or resistance-heated metallic body can also be used as a photon source. The energy of flame photons generally varies from 3100 to 4400 A, i.e., 4.0 to 2.8 eV. The energy of photons emitted by heated bodies is the lowest and varies around 2 eV. Flame arrangements are most frequently employed in gas chromatography. It should be pointed out that the thermal energy does not always cause the ionization directly, but rather represents a necessary step in the ionization mechanism, for example in the flame ionization dctector.

68

3.3

REACTIONS IN THE IONIZATION DETECTOR

3.3.1

The slow-down mechanism

Primary particles emitted by a suitable source collide with the target substance, S, which is the carrier gas, gradually losing their energy. This process of particle slowdown or thermalization proceeds differently depending on the kind of particle and carrier gas. Simultaneously, it is also necessary to consider whether the carrier gas is monatomic or diatomic. In monatomic gases, of which particularly helium and sometimes argon are used for gas chromatographic purposes, ionization (equation (3.2)), the formation of helium metastable states (equation (3.3)) or the formation of exited helium atom states (equation (3.4)) takes place during primary electron slow-down: ionization >

B

+ He

metastable

B

+ He

excited

hv+

He

state

+

state excited state

.

B'

+ He+ f e

p'

+ Hem

(3.3)

+

(3.4)

/I'

He*

He*

(3.5)

If photons are used as a source of ionization energy, only the formation of an excited state (equation (3.5)) can be expected, as photon energies are not sufficient for excitation to higher energy levels to occur. The secondary electron energy exhibits a Maxwell distribution. In the ionization of neon (ZP = 21.3 eV) by 1 keV electrons, the following energy distribution for the secondary electrons was found: 65% had energies higher than 13.6 eV, 40% higher than 27.2 eV and 25% higher than 54.4 eV. The secondary electron distribution is always of the Maxwell type, even if. for example, 75 eV primary electrons are used, only the maximum shifts to lower values. Hence the thermalization of primary electrons proceeds through several collisions, in which the ionization or excitation processes can always be repeated. Diatomic gases are much more efficient than monatomic gases in slowing down primary particles. It generally holds that the more complex the molecule, the lower is its ionization potential and the greater the choice of possible energy states. Thus the probability of non-elastic collision is increased. Of diatomic gases, nitrogen is the most frequently used in gas chromatography. In this type of carrier gas, ionization (equation (3.6)) or the formation of excited states (equation (3.7)) takes place. The formation of long-lived super-excited states of diatomic molecules is negligible

69 compared with the probability of ionization. The energies of these long-lived molecular states are lower than the energies of metastable states, e.g., N: 11.88 eV [ll] and He"' 20.6 eV, but are sufficient for the direct ionization of a number of substances. However, the chief difference lies in the de-exitation process. While the cross-section for ionization by a metastable state is the largest compared with other particles, the probability of ionization by an excited long-lived particle is small:

When using photons as the source of the ionization energy, the formation of excited states usually occurs (equation (3.8)). Polyatomic substances, occurring as impurities in the carrier gas or added to it deliberately, e.g., hydrocarbons or water vapour, lead to more efficient slowing down of primary particles owing to the large number of excitable electronic, vibrational and rotational states. 3.3.2

Recombination

The ion pairs formed by ionization fill the effective detector space and collide with the medium and with other ion pairs as they move under the influence of the electric field. Collisions with the medium decrease their velocity, but their number remains constant. Hence this type of collision does not cause a decrease in the measured ionization current. However, collisions among ion pairs lead to a decrease in the number of ions and thus also to a decrease in the measured ionization current. A secondary electron can recombine with a positive ion directly (equation (3.9) and (3.10)), or in the form of a negative ion after being captured by an electronegative molecule in the medium (equations (3.11) to (3.14)): X,+

3- e

X+

-t Y - -4-

XY+i-

z-

Xf

1 Y'

X+

4- Y -

Z

+ X 4- energy

+

(Xz)*

+

XY -1- Z

(3.11)

-+

x

(3.12)

--f

--+

-+

X*

- Y + Z

(3.9)

y"

(3.13)

XY 4- Irv

(3.14)

X"

-1

70 Electron - ion recombination without radiative transition (equation (3.9)) is more probable, the recombination coefficient having a value of while the radiative transition reaction (equation (3.10)) has a recombination coefficient of lo-’’. The values of the recombination coefficients for reactions that take place under gas chromatographic conditions are given in Table 3.4. TABLE 3.4 RECOMBINATION COEFFICIENTS OF SOME INTERACTIONS [ 9 ] Reaction

O+ H+ NT

+e

3

-t

--f

+

0; +

O* H* --f N* e + O*

e e

a [cm3/sec]

+ hv + hv -+ N*

3.4 x l o - ” 4.8 x

+ O*

(250 K); 0.8 X (250 K); 1.3 X 2.8 x (300 K) 1.7 x (300 K)

(2000 K) (2000 K)

Ion - ion recombination is far more probable than electron - ion recombination and attains a maximum value in a pressure region around 760 torr (see Fig. 3.5). Three-body recombination (equation (3.1 1)) or dissociative recombination (equation (3.12)) is most probable. I

0

t -

0

2432 [to.]

FIG. 3.5. Ion-ion recombination in air; a is the recombination coefficient [ 9 ] .

All recombination processes lead to a decrease in the measured ionization current and should be suppressed under the experimental conditions as much as possible by decreasing the dimensions of the measuring cell and by hastening the collection of charged particles.

71

The occurrence of a space charge is also often encountered in connection with the collection of charged particles. A cloud of heavy ions, which have a low mobility compared with that of electrons (less than l/lOOO), moves very slowly towards the electrode and recombines with electrons moving in the opposite direction. However, this phenomenon starts to be operative at higher ion concentrations (above 10' ions/cm3) and need not be considered under gas chromatographic conditions except for the TlDA and some helium detectors.

3.3.3

Background current of the ionization detector

By gradual slowing down of the primary particles in the carrier gas, a certain number of ion pairs is formed and a measurable current occurs after the application of an electric field. This ionization current, passing in the absence of an eluted substance, is known as the background current. The background current is caused by direct ionization of impurities present in the carrier gas (icIMp),by ionization of the carrier gas itself (icHe) and by de-excitation of metastable states of monatomic gases (icdeeex): bci = icHe

+ i~,,, + icde-ex

(3.15)

The magnitude of the background ionization current is determined by the properties of the carrier gas, i.e., by the collision cross-sections. From the above expression, it therefore follows that, as the nitrogen ionization cross-section is greater than that of helium, i.e., icN, > icHe, the background current will be higher when nitrogen is used as a carrier gas than when helium is used. The background ionization current increases with the amount of impurities present and is highest for artificially prepared mixtures enriched with hydrocarbons.

3.4

LITERATURE

1. cermak V., Ozenne J. B.: Znt. J. Mass Spectrom. ton Phys. 7 , 399 (1971) 2. Girenko D. B.: Ref. Zh. Khim. 1972, 12 N 419 3. Hartmann C. H.: Anal. Chem. 45, 733 (1973) 4.Karavaeva V. G., Revelskii I. A,, Zhukhovitskii A. A.: Zaood Lab. 39, 275 (1973) 5. Lovelock J. E., Maggs R. J., Adlard E. R.: Anal. Chem. 43, 1962 (1971) 6. Lubkowitz J. A,, Parker W. C.: J. Chromatogr. 62, 53 (1971) 7. Lubkowitz J. A., Montoloy D., Parker W. C.: J. Chromatogr. 76, 21 (1973) 8. LukaE S . , SevEik J.: Chromatoyraphia 5, 258 (1972) 9. McDaniel E. W.: Collision Phenomena in Ionized Gases, J. Wiley & Sons, Inc., New York, London, Sydney 1964 10. Penning F. M.: Electric Discharges in Gases, Russian transl., Moscow 1960 11. Simpson T. H.: J. Chromatogr. 38, 24 (1968) 12. Wisniewski J. V., Mikkelsen L.: Facls Methods 7, 3 (1966)

4.

The Electron Capture Detector (ECD)

The ionization of eluted substances by electron capture was first utilized in gas chromatography in 1960 [37]. The pronounced dependence of the response for some atoms and functional groups on the measuring conditions has led to extensive use of this principle.

4.1

DETECTION MECHANISM

The measurement is based on the electronegativity of the eluted substances, i.e., their ability to form negative ions by capturing electrons. Negative ions have various stabilities, which are expressed by their electron affinities related to the energy of the ionizing photons. Therefore, the electron affinity is dependent on the electronic configuration of the atom or molecule: the more occupied the valence orbital of the negative ion, the more stable is the ion (the higher the electron affinity). Generally, atoms with occupied orbitals do not form negative ions, while atoms with a single vacancy in their outer orbital bind the electron most strongly. This rule also holds for molecules. If E, is the bond energy of the captured electron, EY is the bond energy of the i-th electron before electron capture and Ei- is that after capture, then, for a stable anion,

E, - T ( E ; - E;) > 0 I

The expression in parentheses represents the difference between the energies of the ground state of the atom or molecule and that of the ion. The stability of the bond to the captured electron is expressed by this inequality; the larger the term on the left-hand side, the more stable is the negative ion formed (see Table 4.1). The electron capture mechanism is strongly dependent on the energy of the colliding electron. It follows from the mechanism of electron thermalization (p. 68) that at a given instant electrons of various energies are present in the effective volume of the detector, some of which are able to ionize the atoms or molecules directly and some of which cause ionization by electron capture. Because of electron capture, the velocity of charge transport in the electric field changes and the probability of recombination increases. These processes are manifested by an instantaneous decrease in the ionization current.

TABLE 4.1 ELECTRON AFFINITIES OF SOME ATOMS A N D MOLECULES

Ion

Electron affinity [eV]

HHeCN0PSci 1-

0.8 0 1.25 0.5 1.46 1.33 2.15 3.1 3.23

Electron affinity [eV]

Ion

,

H; 0;

OHNONO; CH CN SF; SH-

0.9 0.44 1.78 0.91 3.9

1.6 3.1 3.39 2.5

The following ionization reactions take place in the detector: Electron capture e + A e +

A +

B

+ NO,(ORG) e + CI(0RG) e

+ hv

+

A-

-+

A-+

(4.

B'

-+

NO,(ORG-)*

-+

CI(0RG-)*

'1

(44 Y --f

+

NO,(ORG-)

CI-

4- ORG

+ Y'

(4.3) (4.4)

Direct ionization

Ionization by a metastable state of a rare gas atom He"

+ ORG

+

e

+ He

-1-

ORG'

(4.61

It can be seen that ionization reactions (4.1) -(4.4) cause a negative ionization current, while reactions (4.5) and (4.6) lead to a positive ionization current, the net current being given by their sum. It is therefore desirable that only the electron capture ionization mechanism should play a role in the E C D mechanism (equations (4.1)-(4.4)). Reactions (4.1) and (4.2) assume electron capture by an atom and their probability is very low. Electron capture by a molecule is up to 10' times more probable; three-particle collisions lead to higher yields than dissociative capture (equation (4.4)) [43]. Some conclusions on the ECD mechanism will probably become

74 possible with application of the drift spectrometric method, enabling the products of electron capture ionization to be identified [24, 2.51. It has been verified experimentally that free thermal electrons are predominantly bound in (H,O),O; ions in carrier gases containing oxygen or water vapour, the positive charge being carried mainly by protons in the form of hydrated ions, (H,O),H+. The decrease in the free electron concentration after the formation of hydrated anions leads to a slowing down of the electron capture mechanism (equation (4.1) -(4.4)). Generally, the presence of any electronegative substance in the carrier gas always suppresses electron capture by the eluted substance. 4.2

ECD SIGNAL

It follows from the E C D mechanism that equilibrium conditions are established in the detector when no eluted substance is present and correspond to the background current. When an eluted substance is introduced into the detector, a number of ionization reactions take place, as described by equations (4.1) - (4.6)). The measured current, I, is determined by the number of primary and secondary electrons and of negative ions migrating to the electrode:

I

=

[B']

+ [el + [ s - ]

(4.7)

If Qc and Qs are the ionization cross-sections of the carrier gas and of the eluted substance, respectively, and Q E is the electron capture cross-section of the eluted substance, S, then the measured signal can be expressed by the equations

where k is a constant characteristic of the experimental arrangement, A is the activity of the radioactive source, F is the Faraday constant and a is the recombination coefficient. It can be assumed that the first term on the right-hand side of equation (4.9) is constant for small amounts of eluted substances and the ECD signal can then be expressed by the equation discussed below. 4.2.1

ECD background current

This parameter is relatively high in comparison with other types of ionization detector and is quantitatively described by equation (3.15). It follows from equation (4.9) that the ionization background current increases with increasing activity of the P-radiation

75 source and that it is large when gases with large ionization cross-sections, Qc, are used (see Table 4.2). Thus the E C D background current is lower when helium is used as the carrier gas than when nitrogen is used, and is greatest for argon. From the measuring conditions, it follows that the measurement should be performed at a relaTABLE 4.2 IONIZATION POTENTIALS AND RELATIVE IONIZATION CROSS-SECTIONS FOR VARIOUS COMPOUNDS (QH = 1)

Gas

Ionization potential [eV]*

24.48 21.56 15.76 15.43 15.58 12.06 14.01 13.77 12.6

Ref.

QC

0.694 1.75 10.9

3

7.68 6.58 7.45 10.74 4.16n

+y

48 48 48 52 52 52 52 52 52

3

* The ionization potential values are taken from the Handbook of chemistry and Physics, 51st ed., The Chemical Rubber Co., Cleveland, Ohio, 1970-71.

CAI

bci FIG. 4.1. The dependence of the ECD signal on the magnitude of the background current for N and Ar -t 57; CH, as carrier gases [41].

ti\ el4 high background current [6], corresponding to the highest concentration of free electrons and thus to the highest probability of electron capture. The background current usually varies around lO-’A [33]. Fig. 4.1 depicts the dependence of the signal on the magnitude of the background current for various carrier gases. The results confirm that the ECD signal depends on the thermal electron concentration.

76 The ECD noise is caused by statistical fluctuations in the number of P-particles emitted and by instability in the temperature of the detection block; typical values are lo-'' - 10-"A. 4.2.2

ECD response

The E C D response is given by the sum of all the processes that take place in the detector effective volume during elution of a substance. The partial pressure of the carrier gas decreases during the elution and thus the number of ion pairs formed also decreases, resulting in a decrease in the background current, bci. Simultaneously with a

R

1

I

---f

FIG. 4.2. The ECD response for carrier gases lighter and heavier than the eluted substance; ici - ionization current of the eluted substance, with direct ionization mechanism; bci - ionization current of the carrier gas, with direct ionization mechanism; icEcD - ionization current of the eluted substance, electron capture mechanism; RECD- the measured ECD response.

the decrease in the partial pressure of the carrier gas, the partial pressure of the eluted substance changes so that their sum remains constant (equal to unity). Thus electron capture and direct ionization of the eluted substance with primary electrons take place, as depicted in Fig. 4.2. The probability of direct ionization is expressed by the ionization cross-sections, which are proportional to the size of the molecules. In the systems employed at present, the molecules of the eluted substance are almost always larger than those of the carrier gas (Qs > Q,), and the response is linear only in a narrow concentration range. With increasing size of the molecules of the eluted substance, the linear dynamic range becomes narrower and the possibility of peak inversion increases. If Qs< Qc,as in the determination of oxygen using argon as the carrier gas, the ECD response is linear over a wide concentration range and peak inversion cannot occur.

77 4.2.2.1 Linearity and linear d y n a m i c range The linearity and linear dynamic range are the two most frequently discussed E CD parameters. From the expression for the ECD signal, it follows that a linear dependence of log SECDon log c can be assumed for low concentrations of the eluted substance. The ECD linearity is usually less than unity, varying in the interval (0.5; 1) for various types of substances. This proportionality constant depends on the electron capture cross-section and approaches unity for substances with relatively high electron affinities, such as CCI, and CHCl, [33]. The ECD linearity decreases with increasing electron affinity; it equals 0.8 for lindane [36] and 0.5 for CS2 [35]. The linearity may change by as much As 20% at higher concentrations of a given eluted substance owing to participation of direct ionization [59]. The reported values of the ECD linear dynamic range fall into two very different groups; values smaller than two orders of concentration and greater than four orders can be found in the literature. On closer inspection, it is found that the low values are evaluated in linear coordinates, i.e., SECD/c,,while the large values correspond to log SECD- log cs coordinates. It should be emphasized that only the values given in log-log coordinates are correct and correspond to equation (1.23) for the ECD signal. The linear dynamic range values also depend on the experimental arrangement. They are small when the ionization current is measured at a constant d. c. voltage or at a constant frequency of the pulse circuit. Under constant current conditions, the E CD signal always corresponds to the change in the frequency and the linear dynamic range is broader [60] (see p. 82).

4.2.2.2 Sensitivit?, and selectivity of the ECD The considerable selectivity of the electron capture detector aroused interest, as it is possible to analyze mixtures of substances only some of which yield E C D signals. The high sensitivity of the device determines its use in trace analysis. However, the ECD sensitivity varies as a function of the amount of eluted substance, as shown in Fig. 4.3. The relative sensitivity and the selectivity of the E C D are determined by the electron affinity of the substances monitored. Table 4.3 gives the relative values of the sensitivity for various types of substances. The € CD selectivity follows from the ratio of the sensitivities and attains values as high as lo6. The E CD measuring sensitivity exhibits a pronounced dependence on the electric field intensity at the collecting electrodes. It should be pointed out that, owing to the small distance between the electrodes, substantial changes in the electric field intensity occur and may result in direct ionization [32, 341. Although the measuring sensitivity can be increased in a certain range by increasing the voltage, the linear dynamic range is usually narrowed simultaneously.

78

7.5

75000

750

CP97 FIG. 4.3. The sensitivity of the determination of chlorinated pesticides and their metabolites as a function of their concentration. TABLE 4.3 RELATIVE SENSlTIVITIES OF THE ECD FOR SOME COMPOUNDS [23] Substance

Active component

Ethane Naphthalene

Relative sensitivity 1

Butanol Acetone Chlorobutane Chlorobenzene

-OH

1,2-Dichloroet hane Anthracene

- CI n-system

Keto-steroids

O=&

Chloroform Nitrobenzene

-c1

Carbon tetrachloride Dinitrophenol Diethyl fumarate Diethyl oxalate Dihydropyridine

-- CI -NO, -COCH=CHCO-

-C==O

1-102

-c1 10,-

lo4

,;\A/ lo4- lo5

-NO,

--

-N

coco-

lo5- lo6

79 4.3

EXPERIMENTAL CONDITIONS AFFECTING THE E CD SIGNAL

4.3.1

Carrier gas

The carrier gas has a substantial effect on the electron therrnalization reactions and thus directly determines the probability of other reactions occuring. A gas with a high ionization cross-section is most suitable for optimizing the measuring conditions (see Table 4.2). Nitrogen or argon with the addition of polyatomic gases (CH,, CO,, H,O, C,,HZnt2,etc.) are thus preferable. When rare gases are used as carrier gases, metastable atomic states are formed in the detector and ionize the eluted substance. When helium is employed, then the gases added to it cause de-excitation of metastable atomic states and are ionized instead. Consequently, metastable states cannot affect the E C D detection mechanism (equation (4.6)) and the linear dynamic range of the measurement is broadened. If argon is used as the carrier gas, the energy of its metastable states (EArm= 11.7 eV [28]) is insufficient for ionization of the added gases and direct ionization of eluted substance by these metastable atoms may occur i n the effective volume of the detector. It can be assumed that large additions (up to lo;/,) of CH, to argon [54] play a role in direct thermalization reactions of primary electrons, owing to their high ionization cross-section. Hence it is preferable to add higher hydrocarbons (e.g., C,H,, ZP = 11.1 eV, or C,H,,, I P = 10.6 eV) or generally gases with ionization potentials below 11.7 eV. These gases have high ionization cross-sections and thus are very effective during electron thermalization; low ionization poentials exclude the presence of metastable atomic states. Simultaneously, the very low electron affinity of hydrocarbons ensures that the free electron concentration in the effective volume of the detector is not lowered and therefore the detection mechanism is favourably affected. Trace amounts of oxygen and water vapour are also present in the carrier gas containers. It has been found that as little as 10 ppm of H,O in nitrogen, i.e., an amount which cannot be removed by using a molecular sieve, causes about 90% of the thermal electrons to be bound in ion-molecule species, i.e., (H,O),O;, where n = 0 - 3 depending on the water content of the gas. When an electronegative substance is eluted, competitive reactions of the eluted substance and impurities present in the carrier gas with liberated electrons may occur [25]. It is evident that oxygen and water in the carrier gas decrease the concentration of free electrons and thus also the probability of the formation of a negative ion of the eluted substance. Therefore, it is necessary to employ carrier gases that are free from oxygen and water vapour and, in general, of all electronegative substances, if the linear dynamic range of the electron capture detector is to be broadened. When hydrogen is employed as the carrier gas, it strongly affects the emission of tritium from the source [21]. It has been found that inactive hydrogen molecules are exchanged for active tritium molecules adsorbed on the support, the source activity

80 thus being decreased. Therefore, hydrogen cannot be recommended as a carrier gas; it also has a low ionization cross-section, which leads to ineffective thermalization. It follows from the discussion of equation (4.9) that the E C D signal depends on the type of eluted substance. As the concentration of eluted substances increases during the elution, the concentration of carrier gas simultaneously decreases and the sum of the molar fractions remains equal to unity. As the ionization cross-sections are proportional to the molecular size, the inequality Qs > Qc is almost always valid; thus, with increasing amounts of eluted substance, direct ionization and recombination will oppose one another, so that the signal may be negative, zero or positive, depending on the conditions. A considerable increase in the linear dynamic range would decrease and the device would be would be achieved for Qs < Qc. Then SECD more sensitive for substances with small molecules [17]. Maggs et al. [41] obtained various shapes for the dependence of the response on the carrier gas flow-rate. Under certain conditions a maximum was reached, but the response generally increased exponentially with the flow-rate. A similar dependence was found by Eisentraut et al. [12]. It can be assumed that this dependence is correlated with the magnitude of the effective volume of the detector, as follows from the discussion of the conditions for gas-phase coulometry by thermal electron attachment [38]. For the best results, the carrier gas flow-rale must be optimized. Data on the effect of the carrier gas flow-rate on the ECD background current and signal can be found in the literature [9, 12, 411.

Eflect of temperature on the ?nagnitude of the ECD signal The detector temperature affects the number of electrons emitted from the radioactive source, their energy and the electron capture mechanism. It follows from the experimental results that SECD= T3" and that the detector temperature must be maintained within AO.1 "C for accurate quantitative analysis [41]. The detector temperature is limited by the p-source. With tritium, temperatures exceeding 200 "C are not recommended, although a maximum temperature of 325 "C was reported using a scandium support [16, 211 (see Chapter 3). For higher temperatures, 63Ni [65] or "?Pm on a gold foil [39] is suitable; 239Pu[19], 241Am [38] and 137Cs [56] have also been used. These sources can be employed up to 400 "C without any loss in activity. 4.3.2

Construction of the ECD

The electron capture detector is basically an ionization chamber with a p-source providing primary electrons. This source is placed in a suitable manner on the internal cylindrical walls of the chamber, through which the collecting electrodes protrude (Fig. 4.4). p-Source activities vary from 10 to 1000 mCi; in general, an increase in the p-source activity leads to an increase in the sensitivity and to broadening of the linear dynamic range, as the electron capture reaction is enhanced by

81

a higher concentration of thermal electrons. Therefore, the detector sensitivity can be compared only for identical source activities [21,40] (see p. 66). Study of the effect of the electric field on the ion collection shows that a homogeneous field is most suitable (p. 62). Bearing this in mind, an ECD has been constructed with the electrodes located in parallel [9, 27, 671, for both d.c. and pulsating voltages.

t FIG. 4.4. Scheme of the ECD; 1 - anode, 2 - insulation, 3 - heated detector block, cathode, 5 - column holder.

-

63Ni layer,

4

However, these constructions require a relatively large detector volume so that detectors with coaxial electrodes are more frequently employed. Here the detector mantle is usually the cathode and the cylindrical anode occupies a certain part of the chamber volume; the effective volume of the detector is less than 1 cm3. It has been found that the best results with coaxial electrodes are obtained by using a homogeneous electric field [ 3 5 ] . The magnitude of the polarization voltage is a critical ECD parameter. The electrons are strongly accelerated in the electric field and hence their energy changes. A number of detector constructions have been reported in which the ECD detector was converted into a cross-section detector by a change in the position of the electrodes or in the voltage on the collecting electrodes. In order to achieve as high an electron capture yield as possible, it is necessary to make measurements at low electric field intensities and, if practical, in a pulse circuit. In a pulse circuit, the ionization current is measured at a constant frequency with pulse intervals of 5 to 300 psec [l, 151. The pulse width varies from 0.5 to 2.0 p e c with an amplitude of 10 to 60 V. During the pulse, the electrons migrate to the electrode surface and their concentration in the effective volume of the detector decreases. Between pulses, the electron concentration increases; the lower the pulse frequency, the higher is the mean electron concentration and the more probable the electron capture reaction becomes. Fig. 4.5 depicts the variation in the electron

82 concentration, the ECD background current and the ECD signal with varying pulse frequency. It is evident that the higher the pulse frequency, the closer the measurement approaches measurement with d.c. voltage. A

I

-

pulse per!od

bc

I

I

xc]

FIG.4.5.Thedependenceof the concentration of free electrons in the effective volume of the detector, of the background current and of the detector response on the pulse interval [I].

FIG.4.6. A - ThemagnitudeoftheECDsignal as a function of varying pulse interval [41]; B - The magnitude of the E C D signal for oxygen with constant-current and constantfrequency circuits.

Recently, increased attention has been paid to measurements with a constant ionization current, where the measured signal corresponds to variations in the pulse frequency [41,60]. This approach is marked by widening of the linear dynamic range of the detector, as the measured signal is directly proportional to the eluted substance concentration. The signals of ECDs using pulse circuits are compared in Fig. 4.6. 4.4

APPLICATIONS OF THE ECD

The ECD has drawn the interest of experimenters because of its high sensitivity and selectivity for certain compounds. It has been used not only in gas chromatography but also in liquid [46, 631 and thin-layer [54] chromatography.

83 The dependence of the magnitude and character of the signal on the experimental conditions has already been discussed. It should be pointed out that, with commercial devices, it is impossible to optimize the electrode geometry, the electronic circuit parameters, the internal volume of the detector and the source activity, although these parameters affect the magnitude of the ECD response [lo] and it is very difficult to obtain identical results with detectors from different manufacturers. The only adjustable parameters are usually the type of carrier gas and the detector temperature. The signal generally shows a logarithmic dependence on the concentration and the analytical results must be evaluated in log-log coordinates. ECD applications have been reviewed many times [l, 2, 7, 14, 18, 20, 23, 26, 29, 58, 61, 62, 661; Smith’s review [57] is devoted exclusively to detector applications and lists 130 original papers; 2 years later, Awe [l] reported 250 papers. The most common application of the ECD lies i n analyses of insecticides, pesticides and other agricultural chemicals. These substances have been determined not only in plant materials, but also in blood, fats, potable water. foodstuffs, etc. The ECD has been used

FIG. 4.7. Chromatogram of hexane extract of garlic; 12 - methyl allyl disulphide, 14 - dimethyl trisulphide, 15 - unknown, 16 - diallyl disulphide, 17 - methyl propyl trisulphide, 18 - methyl allyl trisulphide, 19 - unknown, 20 - diallyl trisulphide, 21 - C,,S2 (tentative) [47].

in analyses for drugs and their metabolites in blood, urine, etc. In addition to these substances, the molecules of which contain groups of atoms with sufficiently large electron affinities, the ECD has been used in the analysis of derivatized samples. Derivatization studies have been devoted principally to biologically and biochemically important substances. In addition to steroids, amines 1441,catecholamines

84 TABLE 4.4 EXAMPLES OF ECD APPLICATION Lowest detectable amount

Substance determined

Phosphorus pesticides Chlorinated pesticides Dieldrin Lindane S-Triazine herbicides Insecticides Pentazocine in human plasma Benzothiazepin in blood Melangestral acetate in bovine tissue Halogenated diesters Aromatic nitro-compounds Substituted nitroimidazoles in blood, urine Primary amines in biological material Catecholamines Phenols SiCI, GeCI4 PCI, POCI, PSCI, C6H6

Se in sea water Be in lunar samples Alkyl iodides H2S in air

cos CSZ so2

NO, in air Sulphides, polysulphides, mercaptans, etc., in garlic

4 x 1 0 - l ~g/ml 0.02-2 ppm 0.1--100 ppb

1 0 - ~ - - 1 0 - ~mole

Linear dynamic range

4.7 5.3

Ref.

49 13; 19,40,49, 59 41 33 50

39 5 55 30 8 22 54

2.9 x 6.5 x 1.7 x 1.6 x 5.9 x 1.3 x

4x

mole lo-' mole lo-" mole lo-" mole lo-" mole 10-5mole lo-"

10-14

500 ppm 100 ppm 0.02 ppm 5 PPm

44 51 42 4 4 4 4 4 4 53 12 6 27, 35 27, 35 27,35 27, 31, 35 64 41

[51], phenols [42] and a number of other substances have been analyzed. A comparison of derivatization reagents has shown that monofluoro- and monochloroacetates exhibit lower response values than substances derivatized with hepta- or pentafluorobutyrate [42] or pentafluorobenzamide [45]. However, care must be taken to avoid decomposition on polar stationary phases [ll].

85

The ECD is frequently combined with the FID [44, 47, 49), giving additional information on the eluted substance. Fig. 4.7 shows the ECD and H D recordings obtained in the analysis of garlic. In addition to organic substances, metals have also been determined using the ECD. For example, down to 4 x lO-"g of Be was determined as the trifluoroacetylacetate in meteorites and lunar rocks from the Apollo 11 and 12 missions [12]. Some examples of ECD applications are given in Table 4.4. The electron capture detector therefore belongs among the most sensitive measuring devices used in gas chromatography. Its narrow linear dynamic range, which has been frequently discussed, is caused by interfering reactions, especially by direct ionization of the eluted substance by primary electrons and, when argon is employed as the carrier gas, by metastable atomic states. Reported low values of the linear dynamic range (less than 2 orders) are also frequently caused by evaluating the dependence of the detector response on the concentration of the eluted substance in linear coordinates, while correct evaluation employs log-log coordinates; then values of the linear dynamic range as large as five orders have been found.

4.5

LITERATURE

1. Awe A. W., Kapila S.: J. Cliroinatogr. Sci. 11, 255 (1973) 2. Behrent S.: Clwomatographia 3, 556 (1970) 3. Boer H.: Vapour Phase Chromatography ( D . H. Desty, ed.), p. 169, Butterworths, London 1957 4. Brazhnikov V. V., Sakodynskii K. I.: Tr. Kliitn. Khitn. Teklznol. 1969, 110 5. Broetell H., Ehrsson H., Gyllenhaal 0.: J. Chromatogr. 78, 293 (1973) 6. Castello G., D'Amato G.: J. Chromatogr. 54, 157 (1971) 7. Cough T. A,, Walker E. A,: AnaIyst 95, 1 (1970) 8. Dehennin L., Reiffsteck A,, Scholler R.: J. Chromatogr. Sci.10, 224 (1972) 9. Devaux P., Guiochon G.: J . Chrotnatogr. Sci.7 , 561 (1969) 10. Devaux P., Guiochon G.: J. Chrotnatogr. Sci. 8, 502 (1970) 1 1 . Ehrsson H., Walle T., Broetell H.: Acta Phar. Suicica 8, 319 (1971) 12. Eisentraut K. J., Griest D . J., Sievers R. E.: Anal. Chem. 43, 2003 (1971) 13. Ermakov V. V.: Ref. Zh. Khim. 1972, 12 N 434 14. Ettre L. S.: Theor. Appl. Gas Chrotnatogr. I d . Med., Hahnemann Symp. 1st 1966 (Pub. 1968) 23 15. Fenimore D. C . , Davis C. M.: J . Chromatogr. Sci.8, 519 (1970) 16. Fenimore D. C., Loy P. R., Zlatkis A,: Anal. Chein. 43, 1972 (1971) 17. Freeman R. R.: Clrem. Abstr. 76, 117 748k (1972) 18. Gill J. M., Hartmann C. H.: J. Gas Chrornatogr. 5, 605 (1967) 19. Girenko D. B.: Ref. Zh. Khini. 1972, 12 N 419 20. Hartmann C. H.: Anal. Clzeni. 43, 113A (1971) 21. Hartmann C. H.: Anal. Chetii. 45, 733 (1973) 22. Hoffsomrner J. C.: J. Chroniatogr. 51, 243 (1970) 23. Jentzsch H., Otte E.: Detektoren in der Gaschromatographie, Akademische Verlaggesselschaft, Frankfurt a. Main 1970 24. Karasek F. W., Tatone 0. S., Kane D. M.: A m / . Cliem. 45, 1210 (1973)

25. Karasek F. W., Kane D. M.: Anal. Chem. 45, 576 (1973) 26. Keller R. A.: J. Chromatogr. Sci. 11, 223 (1973) 27. Kilarska M.: Chem. Anal. (Warsaw) 15, 953 (1970) 28. Knapp J. Z., Meyer A. S.: Anal. Chem. 36, 1430 (1964) 29. Krejfi M., Dressler M.: Chromatogr. Rev. 13, 1 (1970) 30. Krzeminski L. F., Cox B. L.: J. Assoc. Off. Anal. Chem. 56, 74 (1973) 31. Landowne R. A,: Anal. Chem. 42, 1468 (1970) 32. Lasa J., Owsiak T., Kostewicz D.: J. Chromatogr. 44, 46 (1969) 33. Lasa J., Owsiak T.: Inst. Nucl. Phys., Cracow, Rep. 1970 No. 700 (in Polish) 34. Lasa J.: Inst. Nucl. Phys., Cracow, Rep. 1970, No. 737 (in Polish) 35. Lasa J., Korus A., Kilarska M.: Inst. Nucl. Phys., Cracow, rep. 1970, No. 738 (in Polish) 36. Lasa J.: Radiochem. Radioanal. Lett. 3, 349 (1970) 37. Lovelock J. E., Lipsky S. R.: J. Am. Chem. SOC.82, 431 (1960) 38. Lovelock J. E., Maggs R. J., Adlard E. R.: Anal. Chem. 43, 1962 (1971) 39. Lubkowitz J. A., Parker W. C.: J. Chromatogr. 62, 53 (1971) 40. Lubkowitz J. A., Montoloy D., Parker W. C.: J. Chromatogr. 76, 21 (1973) 41. Maggs R. J., Joynes P. L., Davies A. J., Lovelock J. E.: Anal. Chem. 43, 1966 (1971) 42. McCallum N. K., Armstrong R. J.: J . Chromatogr. 78, 303 (1973) 43. McDaniel W. E.: Collision Phenomena in Ionized Gases, John Wiley and Sons, Inc., New York, Sydney 1964 44. Moffat A. C., Horning E. C.: Anal. Letters 3, 205 (1970) 45. Moffat A. C., Horning E. C., Matin S. B., Rowland M.: J. Chromatogr. 66, 255 (1972) 46. Nota G., Palombari R.: J. Chrornatogr. 62, 153 (1971) 47. Oaks D. M., Hartmann H., Dimick K. P.: Varian Aerograph, Technical Bulletin 114-64, (1966) 78, 546 (1956) 48. Otvos J. W., Stevenson D. P.: J . Am. Cheni. SOC. 49. Pivarov G. A,: Ref. Zh. Khim. 1972, I 3 N 442 50. Purkayastha R., Cochrane W. P.: J. Agric. Food Chem. 21, 93 (1973) 51. Sakauchi N., Kumaoka S., Hanawa Y.: Enciocrinol. Jpn. 19, 589 (1972) 52. Shamin M. M., Lipsky S. R.: Anal. Cliem. 35, 1562 (1963) 53. Shirnoishi Y.: A n d . Chim. Acta 64, 465 (1973) 54. deSilva J. A. F., Munno N., Stojny N.: J. Pharm. Sci. 59, 201 (1970) 55. deSilva J. A. F., Munno N., Weinfeld R. E.: J . Pharm. Sci.62, 449 (1973) 56. Simpson T. H.: J. Chromatogr. 38, 24 (19158) 57. Smith R. V.: Amer. Lab. 3, 58 (1971) 58. Soerensen 0.: Haus Tech., Essen, Vortragsveroeff. 283, 34 (1973) 59. Solon J. M., Sullivan J. J.: Hewlett-Packard Technical Paper No. 48 60. Sullivan J. J., Smith D. H.: Hewlett-Packard Technical Paper No. 49 61. Svojanovskf V., KrejEi M., TesaPik K., JanAk J.: Chromatogr. Rev. 8, 90 (1966) 62. Toudoire B.: Anal. Absrr. 19, 1880 (1970) 63. Van Dijk J. H.: Brit. Pat. 1,292,754 (Oct. 11, 1972) 64. Wentworth W. E., Freeman R. R.: J. Chromatogr. 79, 322 (1973) 65. Wisniewski J. V., Mikkelsen L.: Facts Methods 7, 3 (1966) 66. Wotiz H. H., Clark S. J.: Chem. Abstr. 72, 9733k (1970) 67. Zlatkis A., Fenimore D. C.: Wsp. Krornatogr. 1972, 236

5.

The Flame Ionization Detector (FID)

5.1

DETECTION MECHANISM

Since its introduction in 1958 [42, 711, the flame ionization detector has become one of the most popular measuring devices used in gas chromatography. Use of the FID is based on the niesaurement of variations in the ionization current in a hydrogen - oxygen flame due to the presence of eluted substances. During a study of the flame of a Bunsen burner [21], it was discovered that the electric conductivity of the internal cone is about 100 times greater than that of the outer mantle, although latter is hotter. Similarly, it has been found that the lower part of a diffuse flame is more conductive, i.e., contains more ions, than the upper radiating part. Hence it is clear that the thermionization process is not the source of ions present in a flame; this is also reflected in the fact that the energy of photons produced in the flame is much lower than the ionization potentials of organic substances. For these reasons, the concept of a chemionization mechanism for the formation of ions in flames was introduced in 1955, assuming collisions of neutral atomic excited states with molecules in their ground state. The FID utilizes the fact that pure hydrogen - oxygen flames contain very few ions (lo7 ions/cm3), while the ionization current increases when trace amounts of hydrocarbons are added (to 10" ions/cm3). The basic process taking place in a hydrogen - oxygen flame can be described by the equations

H

f0,

0

+

OH

+ H2

OH + O

(5.1)

H2 + OH $- H

(5.2)

+

-+

HZO

-t-

H

(5.3)

In the flame reaction zone formed by the outer mantle, which is about 0.1 nim wide under atmospheric pressure, the radicals formed react exothermically [ 9 5 ] : H

-}-

H $- Z

-+

H,

-1 Z

(5.4)

OH

+

H

-1 Z

4

H2O-j- Z

(5.5)

88

The thermal energy released splits the organic compound and it is assumed that a chemionization reaction occurs:

+0

CH

4

CHO'

+e

(54

Reaction (5.6) is the principle reaction in the flame. The FID yields a small response even for substances that do not contain hydrogen atoms, such as CCI,, CS, and C,O,CN. Hence it is probable that hydrogenation precedes ionization of the cracking products, leading to the formation of CH, or CH, [12], which undergo ionization according to the scheme [31]

CH,

+ OH +H

CH,

+0

CH,

+ CH20 + 0

CH,

0 2

-+

CH,

---t

CH,

-+

--t

+

+ H20

(5.7)

+ H, CH,O + H CH2O + OH

(5.10)

CHO++ OH-

(5.1 1)

(5.8) (5.9)

The assumption that heat is conducted by hydrogen from the reaction zone back into the stream of carrier gas and that hydrogenation occurs in this pre-reaction zone is experimentally verified when the composition of the combustion gases is varied (see Table 5.1). When the hydrocarbon was introduced into the detector in any way TABLE 5.1 THE RELATIVE MOLAR RESPONSES OF SOME FID DESIGNS [12]

Hydrocarbon

CH,

1

C2H6

2 3 2

C3H8 c2 H4

7

CZH, a) b) c) d)

Number of carbon atoms

RMR a

b

C

1 .oo 2.04 3.12 2.04 2.06

1 .oo 0.70 0.81 0.59 2.37

1 .oo 0.98 1.26 0.67 2.14

H, - air diffuse flame, hydrocarbons in H,. H2 - air diffuse flame, hydrocarbons in the air. H, - 0, N, diffuse flame, hydrocarbons in 0, Pre-mixed flame, H, - 0, N, hydrocarbons.

+

+ +

+ N,.

d 1 .oo

1.48 2.17 1.08 2.00

89 other than in the hydrogen stream, the response decreased considerably and was not proportional to the number of carbon atoms in the molecule [12]. It follows from the FID mechanism that each carbon atom capable of hydrogenation yields the same signal and that the overall FID response to the analyzed substance is proportional to the sum of these “effective” carbon atoms. The above ionization mechanism is probably also operative druring the monitoring of substituted picolines [61] CI CI

N1‘

)-a

CCI,

CI

>=,

or

Ny-)\=

I

CCI,

or fluoroalkanes. It has been found that the matnitude of the FID signal in the presence of fluoroalkanes, CnFzn, -,H,, varies with changes in their composition. The C/mole [5] to 7.6 x ionization efficiency is very small for CF,, from 1.1 x C/mole [13], compared with that for hydrocarbons (0.38 C/mole). However, the ionization efficiency for higher fluoroalkanes or partially fluorinated alkanes is comparable with that for alkanes alone. These results demonstrate that ionization occurs only after bond splitting [37] and after hydrogenation, forming CH, radicals which are subsequently ionized. The importance of the formation of the methyl radical in the ionization of organic substances is also reflected in the operation of a device called the catalytic detector [99]. Here the eluted substances are led over a heated platinum wire in an oxygen atmosphere. The substance are burned and the corresponding ionization current is measured. The response of this device is strongly dependent on the hydrocarbon structure. The highest ionization efficiency, 1 C/mole, was obtained for 2,2-dimethylpropane and 2,2-dimethylbutane, while values of 0.1 C/mole for branched hydrocarbons 0.05 C/mole for n-hydrocarbons were found. In addition to the ionization process, recombination of the ions formed and electron capture occur in the FID. It follows from the results of plasma chromatography [52, 531 that in the flame most electrons will be bound as hydrated electrons or in hydroxyl ions. The negative charge will also be centred on halogen, sulphur and oxygen atoms and on the C N molecule. The positive charge formed in reactions (5.6) and (5.11) will be centred on polar molecules such as N H f , HzCN’, and (CH,OH)H+ [loll, and mainly on water molecules: CHO’

+ H,O

H30’

+ CO

(5.12)

Reaction (5.12) is significantly shifted in favour of hydrated protons, the ratio of the cations formed being [H,O’]/[CHO+] = lo5. Hydrocarbon cations with large molecular weights, which have been detected by mass spectrometry, are not products of primary ionization but are formed as secondary ions in the mass spectrometer [35].

90 Three-body recombination between hydrated protons and hydroxyl anions and dissociative recombination [57] : H,O++

OH-

+ OH + H

H,O

-+

(5.13)

are the chief processes that take place in the FID. In both instances the number of ions and hence the ionization current are decreased. It has been found experimentally that trace amounts of metals with ionization potentials of 7 to 8 eV (especially lead) present in the flame cause an increase in the ionization current of as much as two orders of magnitude. This effect can be explained by three-body recombination of the metal cation formed in the flame, which is slower than dissociative recombination (5.13) [95]: H30+

+M

-+

H,O

+ H + M+

(5.14)

M+

+ e + Z

+

M

+ Z

(5.15)

Participation of recombination reactions in the FID mechanism is evident from the dependence of the magnitude of the FID signal on the pressure. The probability of recombination is small at low pressures and the FID signal is greatest [14, 681. With increasing pressure, from 3 torr, recombination increases [16, 171 and fluctuations in the atmospheric pressure by 10 torr cause variations in the FID response of more than 5% [15,44]. As the FID is an open system, the magnitude of the response is strongly dependent on fluctuations in the atmospheric pressure. When interpreting the FID mechanism, the marked dependence of this response on the experimental conditions and its proportionality to the number of effective carbon atoms in the molecule of the eluted substance must be borne in mind. Heteroatoms in organic molecules result in specific reactions, which generally leading to a decrease in the FID response. It is assumed that the cyanide group is ionized [Il, 551: CN

+ o -k 0

+

CO

+ NO'

$- e

(5.16)

During the analysis of amines [3], the magnitude of the response for hydrocarbons, amines and hydroxy compounds with equally long carbon chains were compared. It was found that the response for hydrocarbons is greater than that for the other substances. Primary and secondary aliphatic amines as well as alcohols and ethers decrease the FID signal by a value corresponding to one effective carbon atom, while tertiary amines decrease it by a value corresponding to two effective carbon atoms. It has also been observed that an amino or hydroxy group in the para-position causes a greater decrease in the signal than substituents in meta- or ortho-positions. Hence it can be assumed that cracking of the ring at a carbon atom with a group bound to it cannot occur because of steric hindrance and therefore the number of ionizable particles is decreased.

91

Oxygen present in organic substances decreases the FID signal. The original assumptions concerning the FI D mechanism maintained that the carbon-containing fragment of the substance must not contain a double-bonded oxygen. The formation of C=O (equation (5.6)) is a strongly exothermic reaction leading to ionization. When this bond has been formed, ionization cannot occur [loll. An electron capture mechanism must be assumed for other oxygen-containing groups, e.g., OH and H,O. It is then possible to explain the pronounced decrease in the relative molar response of oxygen-containing substances compared with nitrogen or sulphur [40, 41, 88, 89, 1031. Special attention has been paid to the CS2 response. Carbon disulphide, in a similar manner to hydrogen, yields very few ions on combustion with oxygen (about lo7 ions/cm3) [57, 1011: As the number of ions in the flame is three or four orders of magnitude greater in the presence of hydrocarbons, it is evident that the FID response to CS,, COS and H,S is very small. It has been found that the response to these compounds is always directly proportional to the increase in the hydrogen flow-rate [lo, 25, 26, 871, and it has been found that the highest response is obtained when the flame temperature is increased until the burner becomes hot. These experiments support a theory assuming that the eluted substances are cracked before combustion and ionization. The overall degree of ionization is decreased by electron capture by the electronegative sulphur atom. The electron capture mechanism is probably the predominating effect decreasing the FID response to compounds that contain halogen atoms [13, 541.

5.2

FID SIGNAL

From the equations describing the FID detection mechanism, it can be concluded that the measured FID signal corresponds to the ionization of the eluted substances by reaction (5.11). The magnitude of the ionization signal is proportional to the number of ionizable carbon atoms, decreased by the electron capture process involving electronegative groups formed as combustion products. If C, is the overall number of carbon atoms in the molecule of the eluted substance and C=O is the number of non-ionizable carbon atoms, the FID signal can be expressed by the equation

S‘”’ = bci

+ icC“ - ic,_.

- ic.,,,

(5.17)

The course of the reactions involved in the detection mechanism depends on the reaction conditions, particularly on the hydrogen and carrier gas flow-rates, determining the temperature of the space in which the eluted substances are cracked. The electric field intensity is important for the electron capture mechanism and is discussed below.

92 5.2.1

FID background current

A small hydrogen-oxygen flame (air is usually employed) in which the eluted substances are burned is used in the FID system. The FID background current is very small because very few ions are formed during the combustion of pure hydrogen. However, if trace amounts of hydrocarbons or of any carbonaceous material (e.g., from the stationary phase) are present in any of gases employed, the ionization current increases owing t o ionization of the impurity. The FID background current usually does not exceed 10-IOA. The noise of the FID background current corresponds to statistical fluctuations in the rate of formation of ions, chiefly as a result of changes in the pressure and flow-rate of hydrogen and hence in the flame temperature [56]. The amount of ionizable impurity in the gas containers is negligible. In properly constructed detectors, the noise is very low (about 10-13A). Noise in the background current may also be caused by thermo-emission from the electrode or burner material. This secondary emission and the resulting increase in the detector noise is greater when a wire collecting electrode placed above the flame and along its axis is employed. 5.2.2

FID response

The FID response is the sum of all of the ionization processes taking place in the flame during the elution of an organic substance. As the FID background ionization current is very small, changes in it as a result of a decrease in the partial pressure of

I

-t

FIG. 5.1. Schematic FID response to hydrocarbons (curve 1) and to oxygen- and chlorine-containing substances (curve 2); bci - ionization background current; icECD ionization current due t o electron capture; k O R G - ionization current of carbonaceous substances due to the FID mechanism; RFID - the detector response.

hydrogen during the elution of the analyzed substance are also very small. An increase in the ionization current corresponds to the ionization of carbonaceous fragments, the overall response being dependent on specific interactions of the

93 hetero-atoms present (Fig. 5.1). A comparison of schematic responses demonstrates that the FID response is the highest for hydrocarbons, being proportional to the number of carbon atoms, while substances that contain oxygen, sulphur or a halogen yield smaller responses depending on the hetero-atom - carbon character and on the electron affinity of the combustion products. The effects of substance structure on the magnitude of the FID response has been studied since the introduction of the detector. The theoretical signal (equation (5.17)) depends, in practice, on the experimental conditions and the hetero-atom bond TABLE 5.2 EFFECTIVE CARBON NUMBERS FOR SOME COMPOUNDS

Compound

1. CH4 2. CZH, 3. C2H4 4.C2H2 5 . C4H10 6. C-C-C

Number of carbon atoms

Reference compound

E CN

Ref.

2 2 2 4 4

5. 5. 5. 5. 5. 5.

0.95 2.0 2.0 2.2 4.0 4.0

2 2 2 2 2 2

2 2 2

1

I C 7. C=c-c-c 8. C-C=C-C 9. c=c-c

4

5.

4.0

4 4

5.

3.9 3.9

4 6 7

5. 5. 5. 5. 5.

5.

I C 10. c=c-c=c 11. C6H6 12. C7H16 13. CH,OH 14. C2HSOH 15. CCI, 16. CIOH22 17. C,, 18. C,, 19. Thiophene 20. Selenophene 21. Pyrrole 22. Acetophenone 23. 2-Methyl-5-trifluoroacetylpyrrole 24. 2-tert-butyl-5-trifluoroacetylpyrrole

21.

7

2 2 2 2 2 2 82 82 82 24 24 24 24

7

22.

5

24

10

22.

7

24

1

2 1 10 16 18 4

4 4 8

5. 16. 16. 16. 11. 19. 20.

4.0 5.8 7.4 0.83 2.0 0.64 10.0 18.7 19.3 3 3 3

94 character. This complex relationship between the response and the experimental parameters is difficult to express quantitatively. Multi-regression analysis as described in the literature [33] does not comply with the physical principle of the measurement, because it erroneously follows that the FI D response increases with decreasing polarization voltage and with decreasing flow-rates of hydrogen and air. Comparison of the magnitudes of the FID response for various substances is based on the evaluation of the effective number of carbon atoms in the substance molecule [61], i.e., the equivalent weight of the substance;

M . 12

ECN = __ I1

where ECN is the effective number of carbon atoms in a molecule of weight M and containing n carbon atoms. Although these values are strongly dependent on the experimental conditions, they must be known if FID recordings are to be quantitatively evaluated; selected values are given in Table 5.2. It has been found experimentally [l, 201 that the response to each of the carbon atoms present in the molecule follows the relative series; aliphatic, 1.0; olefins, 0.95; acetylene, 1.3; carbonyl, 0.0; nitrile, 0.13. Each oxygen atom decreases the response; for ethereal oxygen, the value is - 1.0 [92]. 5.2.2.1

Linear dynumic range and linearity of the FID

The FID has the broadest linear dynamic range of all detectors; values of six to nine orders of concentrtaion have been reported in the literature [73]. However, the reaI linear dynamic range of the FID is limited by the parameters of the electronic circuit, especially the amplifier linearity. Thus a linear dynamic range of four orders of concentration may be considered acceptable, and a value of six orders is difficult t o obtain [60]. An FID linearity equal to unity is generally reported. However, it follows from the detection mechanism that the FID signal is dependent on the experimental conditions and various values are quoted in the literature in the interval (0.5; 1> [72]. A number of papers have described “non-linearity” for the FID when the slopes of the straight line dependence of log SFID/logc differed from unity. These conclusions were drawn because of various approaches to the evaluation of the FID response, e.g., plotting the peak height versus the sample volume or the peak area versus the sample mass or concentration. Kaiser et al. [SO] notes that the FID response is proportional to the flow-rate of the eluted substance and recommended the following equation for the evaluation of the linearity of the detector:

where I (A) is the ionization current, q (C/g) is a conversion factor and W / t (g/sec) is the flow-rate of the substance. They considered a detector to be linear if q is con-

95

stant and evaluated this dependence in q versus log( W i t ) coordinates. As the magnitude of the signal is proportional to the number of ionizable carbon atoms, it should not be related to the flow-rate expressed by the mass of the eluted substance, but rather to its concentration. Evaluation of this dependence in semi-logarithmic coordinates is also not justified. The factor q is identical with the sensitivity of the detector or, after dividing by the Faraday constant, with the ionization efficiency, q [ 3 6 ] , and thus it is obvious that its value differs for various substances. The linearity of the detector should be evaluated by employing the general relationship

z=q.-

C'

t.n

where c is the concentration of the eluted substance, n is the number of effective carbon atoms and I is the linearity of the detector. By taking logarithms and plotting the relationship in log ( I . t ) versus log c coordinates, a straight line is obtained with a slope equal to the linearity of the detector. Measurement of the peak area as a function of the amount of sample injected is subject to a number of gross errors. It is therefore better to test the detector using a logarithmic evaporator [58, 841, as mentioned in the general discussion, or by the frontal method [81]. Even then it must be remembered that the FID signal depends strongly on the experimental conditions, particularly the gas flow-rates [72].

5.2.2.2

Sensitivity and selectivity of the FID

The FID is a very sensitive measuring device because of the value of the ionization efficiency, which is about lo-' C/mole, i.e., a change of mole of the eluted substance leads to a change of 10-'A in the current. The ionization efficiency of the FID varies depending on the experimental conditions. It follows from the detection mechanism that the more complete the cracking process, the greater is the signal and hence the sensitivity of the FID. The FID is a universal ionization detector for carbonaceous substances and therefore its selectivity cannot be specified. 5.3

EXPERIMENTAL CONDITIONS AFFECTING THE MAGNITUDE AND CHARACTER OF THE FID SIGNAL

5.3.1

Gas flow-rate

The effect of the hydrogen flow-rate is frequently discussed. It is generally concluded that there is a certain optimum flow-rate for a given system, corresponding to the maximum detector response. This can be explained on the basis of the detection mechanism: with increasing hydrogen flow-rate, the temperature of the space where

96 eluted substances are cracked increases, leading to an increase in the response [30]; the subsequent decrease in the response with a further increase in the flow-rate is probably caused by the fact that the flame becomes too long and thus heats the burner less effectively. Increasing the hydrogen flow rate also causes cooling of the burner owing to the high thermal conductivity of hydrogen. This concept has been verified by measurement of the dependence of the FID response on the burner temperature; it has been demonstrated that the ionization current increases with increasing burner temperatures [72]. In order to achieve the maximum degree of cracking, the use of an H, - N2 mixture is recommended [64]. Under FID conditions, argon, helium or helium with 0.03%of ammonia, in addition to nitrogen, are used as carrier gases [5l]. When the magnitudes of the signal are compared for various carrier gases under identical experimental condititions, it is found that the signal for helium is always smaller than that for nitrogen [22, 38, 391. These observations are in agreement with the theory of the ionization mechanism described above.

FIG. 5.2. The dependence of the FID response on the carrier gas flow-rate; A - the flow-rate through the detector is varied; B - the flow-rate through the detector is maintained constant by adding pure carrier gas before the detector inlet.

FIG. 5.3. Scheme of the FID.

Although it is frequently maintained that the FID response is independent of the flow-rate of the carrier gas, some workers have demonstrated experimentally that the contrary is true. The flow-rate of the carrier gas is adjusted, considering the optimum separation of the analyzed mixture. The FID response, however, increases with increasing flow-rate, reaches a maximum and then decreases [33,64]. This dependence has been observed with various designs of FID and can be explained by the fact that

97 the shape of the flame changes with increasing flow-rate of the carrier gas; the flame becomes drawn out and its surface area increases. As diffuse flames are employed in the FID system, their temperature increases, leading to an increase in the FID response. If additional carrier gas is fed into the system after the column with a flow-rate adjusted so that the flow-rate through the detector is constant, a constant FID response is obtained (Fig. 5.2). An increase in the FID response by adjustment of the flow-rate of carrier gas or hydrogen generally causes an increase in the noise level because the burner becomes hot and metal ions are emitted. It can be assumed that the cracking reaction time would be lengthened and consequently a greater FID response obtained if an external thermostat were connected to the part of the detector through which hydrogen and the eluted substances are led. The flow-rate of air has only a very small effect on the FID response. As diffuse flames are employed, it is necessary to use excess of oxygen (several times the stoichiometric amount), up to 600 cm3/min. The measurement is subject to a higher noise level when low flow-rates of air are employed. In addition to air, oxygen or an oxygennitrogen mixture is sometimes used; the flow-rate is then smaller as the stoichiometry of the burned mixture remains the same.

5.3.2

Geometry of the FID

The FID contains a small diffuse flame into which the eluted substance is fed. The ions formed migrate towards the electrodes, to which a small voltage is applied. An FID system is shown in Fig. 5.3. The burner is either a metallic jet or a quartz capillary with an internal diameter of about 0.5 mm [74]. Quartz is advantageous because of the suppresion of the emission of secondary ions from the burner, i.e., it leads to a decrease in the detector noise. On the other hand, the response is usually lower than with metallic burners [90] as there is no substantial increase in the temperature of the pre-reaction space due to the poor heat conduction and therefore the cracking process is suppressed. A system employing a burner with two separate jets has also been described [91]. The carrier gas and the eluted substance were introduced using one jet and a mixture of hydrogen and oxygen with the other. The detector dead volume was thus reduced and danger of explosion reduced but, due to the above suppresion of the cracking process, the FID signal was small. It is generally preferable that the eluted substance be introduced with the hydrogen flow and be heated to as high a temperature as possible before it enters the flame. The FID noise increases during prolonged measurements, usually owing to the deposition of high-boiling compounds on the surface of the collecting electrode, insulating layers on the electrode, etc. The detector must then be cleaned mechanically in order to obtain the initial conditions. In addition to mechanical cleaning of elec-

98 trodes, fluorinated compounds such as alcohols and ethers have been injected into the system [4S]. Presumably the hydrogen fluoride formed reacts with the deposited compounds to produce volatile fluorides, which are the evaporated. The electric field is used for collection of the charged particles. In most FID systems, a d.c. field is employed, the burner being one electrode with other electrode placed above it. The burner is usually the cathode and is insulated from the body of the detector. When a quartz capillary is employed, a metallic ring placed a t its orifice, or at or below the tip of the burner, serves as the cathode in order to suppress secondary ion emission and thus the noise as much as possible [97]. An a.c. electric field has also been tested [32] but has not enjoyed wider use. The shape of the electrodes and their geometry and polarity are the most frequently discussed detector parameters. Several shapes of collecting electrode are depicted schematically in Fig. 5.4. It has been found that detectors with cylindrical collecting electrodes, i.e., systems b, d and e in Fig. 5.4, exhibit the lowest noise [83], while systems a and c have a high noise level due to heating of the collecting electrode [22]. The latter systems have a further disadvantage in the deposition of solid particles of high-boiling substances [34] on the electrode surface; the noise is consequently ncreased and the signal decreased. When silylated samples are treated, SiO, or 2 Si02.P,0, is deposited on the surface of the electrode, as has been shown by using a

b

C

d

e

FIG. 5.4. Some FID collecting electrode shapes and positions with respect to the burner; for a description, see text.

Debye-Scherrer X-ray diffraction spectroscopy and emission spectroscopy [62]. The shape of the electrode determines the electric field gradient. The parallel plate arrangement is not suitable in the FID system, as the lines of force of the electric field are poorly developed and defined, because of the large voltage gradient in the flame space. This arrangement also requires that the positive and negative ions travel long distances, making the FID signal very variable [78]. Similarly, a very inhomogeneous electric field is obtained when symetrically placed semi-cylindrical electrodes are employed (see Fig. 5.4e) [SO]. The highest field intensity lies in the gap between the

99 edges of the two senti-cylindrical electrodes, while it is very low in the flame because of its location in the centre of the space enclosed by the electrodes. In Fig. 5.5 is depicted the electric field between the burner and the electrode formed by a coarse grid (A) when a third ring electrode with a potential equal to that of the anode (B, C) is placed in the system. It can be seen that the lines of force of the electric field are changed in the presence of the third electrode and hence the collection of charged particles was significantly affected. This phenomenon simulates changes in the flame shape due to an increase in the gas flow-rate. For this reason, contemporary FID designs contain an anode with an adjustable height [46, 691.

A

vir

A

FIG. 5.5. The electric field character in the FID system; A - collecting electrode burner; B - collecting electrode, auxiliary electrode,burner; C- collectingelectrode, auxiliary electrode, burner.

FIG. 5.6. The volt-ampere curve for the FID with a cylindrical collector (1, 15 mm; 2, 25 mm) and with parallel plates (3, 8 mm; 4, 12 mm).

The magnitude of the polarization voltage is selected from the saturation regions of the volt-ampere curve. The voltage usually does not exceed 300 V and depends on the construction of the detector [18]. When parallel electrodes are employed, a higher voltage must be applied in order to attain the saturatiori current region (Fig. 5.6). Although measurement employing a voltage between the saturated and multiplication regions of the Geiger-Muller curve has also been proposed [23], the absolute increase

100

in the ionization current is unfavourably affected by a considerable increase in the noise level and therefore it is probably unsuitable for the analysis of unknown samples. The collecting electrode has frequently been a subject for discussion. In general, it is preferable that the FID burner be the cathode and the collecting electrode the anode. As noted above, the highest charge concentration lies in the lower part of the flame and cations with low mobilities must thus travel only a short distance, while mobile ions travel further. It is obvious that the magnitude of the FID signal is independent of the polarity of the electrode in the parallel-plate arrangement [74], as the path lengths of all ions are equal in a symmetrical system. From a practical point of view, the electronic circuity is simpler when the polarization voltage is brought to the cathode (burner) and the anode is grounded. The detector noise is then substantially decreased, as capacitative effects in the cable between the anode and the amplifier are minimal, the device is not too sensitive to changes in the resistance of the girder insulation and higher cathode polarization voltages can be employed [ 5 9 ] .

FIG. 5.7. Schemes of the voltage and current amplifiers.

The ionization current is measured with a voltage or current amplifier (Fig. 5.7) [59]. In both instances, very low ionization currents can be measured (down to

10-13A). However, voltage amplifiers require a high input resistance, R , , of the order of 1014 R. The resistance between the anode and cathode and the connecting cables, R,, which is depicted by dashed lines in the scheme, must have approximately the same value. The maximum amplification of the voltage drop on the measuring resistor, R,, is thus utilized and the output voltage is then given by the equation

101

The gain of a current amplifier is determined by the magnitude of the feedback resistor, R,. For the magnitude of the output voltage, it then holds that

When the two expressions for the output voltage are compared, it is evident that the functioning of the current amplifier is not significantly dependent on the input resistance, insulating properties of the connecting cables, insulation of the girder, etc. For these reasons, current amplifiers are more advantageous and are employed more frequently.

5.4

F ID APPLICATIONS

The FID is probably the most widely used measuring device in gas chromatrography. Its application is also frequent in liquid chromatography [27 -29,45, 63, 93, 94, 1001, where the sample is usually transported on a moving wire. The evalutation of thinlayer chromatographic separations is based on a similar principle [76]: the plate is placed in a heated mantle into which a carrier gas is fed. With changing temperature gradient, the separated substances are gradually evaporated and introduced into the

FID. The simplicity of the design and the reliability of the FID have resulted in its use for teaching demonstration purposes; the elution of substances can also be observed visually on the basis of changes in the shape or colour of the flame [19,79]. A number of analyzers are also based on the application of the FID [43, 49, 70, 861. These analyzers have been designed not only for analyses in the earth’s atmosphere but also for measurements in extraterrestrial space [66, 1041. The FID is employed in almost all chemical and biochemical fields. Its universality and high sensitivity for organic substances permitted trace determinations from its early stages [65]. The FID is most sensitive for hydrocarbons. As discussed above, the response of the FID strongly depends on the structure of the eluted substances and on the presence of hetero-atoms in their molecules. These dependences have been utilized, for example for following the degree of polymerization of styrene [77]. The presence of oxygen or sulphur in the molecules of the eluted substance always decreases the response of the FID. Nevertheless, this detector can’ still be used for the analysis of such substances, e g., organic acids [I031 present in tobacco [lo21 and esters of fatty [47] and dicarboxylic acids [9]. Amines [75] and pyridine derivatives [51] have also been analyzed using the FID. The detector has made possible the analyses of various bichemically important substances, such as steroids [85, 1051. These substances, which usually have large molecules, must be pre-treated prior to the analysis, for example by silylation, preventing the use of some detectors, such as the ECD. The FID is thus also employed in pesicide analysis [6, 8, 961.

102

An FID has not been specially constructed for the analysis of inorganic substances. It follows from the detection mechanism that the eluted substance must form a methyl radical in order to be detectable by the FID. However, the use of the FID for the analysis of various inorganic substances has been repeatedly reported in the literature. For example, the FID is sometimes used for the determination of metals [4, 7, 8, 901. The measurement is then performed at high flame temperatures, using, for example, pre-mixed flames or multiple burners. Direct ionization of the metals by the TIDA mechanism is employed. The FID also responds to water [67, 981, rare gases [98], oxygen [40, 88, 981, nitrogen [40], nitrogen oxides [88] and carbon dioxide [98]. In general, these responses are not the result of the FID mechanism described above, but are caused by a decrease in the flame temperature and an increase in the electron capture probability, as reflected in the negative signals which are usually obtained.

5.5

LITERATURE

1. Ackman R. G.: J. Gas Chromatogr. 6, 497 (1968) 2. Andeatch J. A,, Feinland R.: Anal. Chem. 32, 1021 (1960) 3. Anderson A., Jerele S., Shimanskaya M. V.: Latv. PSR Zinat. Akad. Vestis, Kim. Ser. 1971, 480 4. Araki S., Suzuki S., Hobo T., Yamada M.: Bunseki Kagaku 19, 493 (1970) 5. Askew W. C., Maduskar K. D.: J. Chromatogr. Sci. 9, 702 (1971) 6. Aue W. A.: Aduances in Chemistry, Series No. 104, Pesticides Identification at the Residue Level, 4, 39 (1971) 7. Aue W. A., Hill H. H.: J. Chromatogr. 74, 319 (1972) 8. Aue W. A., Hill H. H.: Anal. Chem. 45, 729 (1973) 9. Binder H., Lindner W.: J. Chromatogr. 77, 175 (1973) 10. Blades A. T.: J . Chromatogr. Sci. 8, 414 (1970) I I . Blades A. T.: J. Chromatogr. Sci. 10, 693 (1972) 12. Blades A. T.: J. Chromatogr. Sci. 11, 251 (1973) 13. Blades A. T.: J. Chromatogr. Sci. 11, 267 (1973) 14. Blu G., Lazarre F., Guiochon G.: Anal. Chem. 45, 1375 (1973) IS. BoEek P., Novak J., Janik J.: J. Chromatogr. 43, 431 (1969) 16. B o k k P., Novak J., Janak J.: J. Chromatogr. Sci. 8, 226 (1970) 17. BoEek P., Novak J., Janik J.: J. Chromatogr. 48, 412 (1970) 18. Bolton H. C., McWilliam I. G.: Proc. R. SOC.London, Ser. A 321, 361 (1971) 19. Brabson G. D.: J. Chem. Educ. 49, 71 (1972) 20. Bruderreck H., Schneider W., HalBsz I.: Anal. Chem. 36, 461 (1964) 21. Calcote H. F.: Ionization in High-Temperature Gases, Shuler K. E., Ed., Academic Press, New York, London 1963, p. 107 22. Chizhov L. V., Shorygin A. P., Petukhova E. A,: Gazoc. Khromatogr. 1969, 66 23. Clardy E. K.: US Pat. 3,542,516 24. Clementi S., Savelli G., Vergoni M.: Chromatographia 5, 413 (1972) 25. Douglas D. M., Schaeffer B. A.: J. Chrornatogr. Sci. 7, 433 (1969) 26. Dressler M.: J. Chromatogr. 42, 408 (1969) 27. Dubsky H., Pajurek J., KrejEi M.: Chem. Listy 67, 93 (1973) 28. Dubsky H.: Chem. Listy 67, 533 (1973)

103 39. Dutta J., Ghosh A., Hoque M., Ghosh A,: Chem. Ahstr. 69, 32 820h (1968) 30. Eggertsen F. T., Stross F. H.: Thermochim. Acta 1, 451 (1970) 31. Fenimore C. P.: The International Encyclopedia of Physical Chemistry and Chemical Physics, Vol. 5., Chemistry in Premixed Flames (Trotman-Dickenson A. F., Ed.), Pergamon Press, Oxford 1964 32. Fertig G. H.: Ger. Offen. 1,803,616 (Jul. 12, 1969) 33. Folmer 0. F., Jr., Haase D. J.: Anal. Chirn. Acta 48,63 (1969) 34. Forster E. P., Weiss A. H.: J. Chromatogr. Sci. 9, 266 (1971) 35. Franklin J. L., Munson M. S. B., Field F. H.: Ionization in High-Temperature Gases (Shuler K. E., Ed.), Academic Press, New York, London 1963, p. 67 36. Gaspar G.: Meres. Automat. 18, 17 (1970) 37. Gray P., Herod A. A,, Jones A,: Chern. Rec. 71, 247 (1971) 38. Green L.: Hewlett-Packard Application Note GC-2-73, Operating Conditions for Optimum Performance of the Model 571 1 A Flame Ionization Detector 39. Green L. E., Mikkelsen L.: Hewlett-Packard Notes, Factors Affecting Linearity of the Flame Ionization Detector 40. Gupta M. C., Mathur M., Chandra K., Bhattacharya S . N.: J. Chromatogr. Sci. 11, 373 ( I 973)

J. Gas Clzromatogr. 5, 401 (1967) 41. Hainova O., BoEek P., Novak J., Janak .I.: 42. Harley J., Nel W., Pretorius V.: Nature 181, 177 (1958) 43. Hartmann K.: Wasser, Luft Betr. 15,21 I (1971) 44. Hill D. W.: Chem. Abstr. 73, 115 954p (1971) 45. Huber J. F. K.: J. Chromatogr. Sci. 7 , 172 (1969) 46. Ibragimov I. A., Farzane N. G., Feldleifer M. B.: Chem. Abstr. 70, 64 093k (1969) 47. Iverson J. L.: J. Assoc. OffAnal. Chem. 53, 1214 (1970) 48. Kaczaj J.: US Pat. 3,531,256 (Sept. 29, (1970) 49. Kadlec K.: Vod. Hospod. B21, 17 (1971) 50. Kaiser R., Stoll W., Fischer K.: Chromatographia 2, 20 (1969) 51. Kan I. I., Sembaev D. K., Suvorov B. V.: Zh. Anal. Khim. 25,374 (1970) 5’. Karasek F. W., Kane D. M.: J. Chromarogr. Sci. 10, 673 (1972) 53. Karasek F. W., Kane D. M.: Anal. Chem. 45,576 (1973) 54. Karmen A,, Kelly E. L.: Anal. Chem. 43, 1912 (1971) 55. Katachevtsev G. J., Tal’rose V. L.: Chem. Absrr. 78, 149 425k (1973) 56. Kalmanovskii V. I.: Anal. Abstr. 20, 4491 (1971) 57. King I. R.: Ionization in High-Temperature Gases (Shuler K. E., Ed.) Academic Press, New York, London 1963, p. 197 5 s . King W. A,, Dupre G. D.: Anal. Cheni. 41, 1936 (1969) 59. Knapp 0.: Chromatographia 2, 67 (1969) 69, Knapp 0.: Chromatographia 2, 1 I I (1 969) 61. Kozeiko T. A,, Mashkevich A. I.: Zacod. Lab. 39, 25 (1973) 62. Lakeland B. R., McDermontt I. T.: J. Chromatogr. 38, 392 (1962) 63. Lapidus B. M., Karmen A,: J. Chromatogr. Sci. 10, 103 (1972) . 41, 1919 (1969) 64. Levy R. L., Walker J. Q., Wolf C. J.: A t ~ a l Chem. 65. Lovelock J. E.:Anal. Chem. 33,162 (1961) 66. Lucero D. P., Smith P. H., Johnson R. D.: ISA Trans. 10, 58 (1971) 67. Lucero D. P.: J. Chromatogr. Sci. 10, 463 (1972) 68. Lucero D. P., Smith P. H.; J. Chromatogr. Sci. 10, 544 (1972) 69. McCoy R. W., Cram S . P.: J. Chromatogr. Sci. 7, 17 (1969) 70. McNair H. M., Chandler C. D.: J. Chroniatogr. Sci. 11, 454 (1973) 71. McWilliam I. G., Dewar R. A,: Narirre 181, 760 (1958)

104 72. McWilliam I. G.: J. Chromatogr. 51, 391 (1970) 73. Mead A. S., Speakman F. P.: Chromatographia 5 , 21 1 (1969) 74. Micheletti S. F., Bryan G. T.: Anal. Chim. Acta 48, 51 (1969) 75. Moffat A. C., Horning E. C.: Anal. Letters 3, 205 (1970) 76. Mukherjee K. H., Spaans H., Haahti E.: J. Chrornatogr. 61, 317 (1971) 77. Nikolaev A. F., Belogorodskaya K. V., Andreev A. I., Rumyantsev J. G.: Vysokomol. Soedin.,Ser. A15, 436 (1973) 78. NovAk J., BoEek P., Reprt L., Janhk J.: J. Chrornutogr. 51, 385 (1970) 79. Nowak A. V., Malmstadt H. V.: J. Chem. Educ. 45, 519 (1968) 80. Nunnikhoven R.: Fresenius’ Z. Anal. Chem. 236, 79 (1968) 81. Oster H., Oppermann F.: Chromatogruphia 2, 251 (1969) 82. Perkins G., Jr., Laramy R. E., Lively L. D.: Anal. Chem. 35, 360 (1963) 83. Prescott B. O., Wise H. L., Chesnut D. A.: US Pat. 3,451,780 (Jun. 24, 1969) 84. Rossiter V.: J. Chrornatogr. Sci. 8, 164 (1970) 85. Rutten G.A.F.M., Luyten J.A.: J. Chrornatogr. 74, 177 (1972) 86. Sawicki E., Corey R. C., Dooley A. E., Monkman J. L., Ripperton L. A,, Sigsby J. E., White L. D.: Health Lab. Sci. 8, 248 (1971) 87. Schaeffer B. A.: Anal. Chem. 43, 448 (1970) 88. Schaeffer B. A,: J. Chromatogr. Sci. 10, 110 (1972) 89. Sokolov D. N., Golubeva L. K.: Zacod. Lab. 35, 143 (1969) 90. Sokolov D. N., Vauin N. A.: Zucod. Lab. 36, 513 (1970) 91. Spencer S. F., Mikkelsen L.: US Pat. 3,399,974 (Sept. 3, 1968) 92. Sternberg J. C., Gallaway W. S., Jones D. T. L.: Gas Chromatography (Brumer N., Callen J. E.. Weiss M. D., eds.), Academic Press 1962 93. Stevens R. H.: J. Gas Chromatogr. 6, 375 (1968) 94. Stolyhwo A., Privett 0. S., Erdahl W. L.: J. Chrornatogr. Sci. 11, 263 (1973) 95. Sugden T. M.: Ionization in High-Temperature Gases (Shuler K. E., ed.), Academic Press, New York, London 1963, p. 145 96. Svojanovskq J., Nebola R.: Chem. Listy 67, 295 (1973) 97. Taft E. M.: US Pat. 3,399,339 (Aug. 27, 1968) 98. Thombs D. A,: Chromatographia 6, 111 (1973) 99. Umstead M. E., Woods F. J., Johnson J. E.: J . Chrornatogr. Sci. 8, 375 (1970) 100. Van Dijk J. H.: Brit. Pat. 1,292,754 (Oct. 11, 1972) 101. Van Tiggelen A.: Ionization in High-Temperature Gases, (Shuler K. E., ed.), Academic Press, New York, London 1963, p. 165 102. Van der Wal S. Sj.: Column 3, 5 (1970) 103. Watanabe S., Nakasato S., Kuwayama H., Sasamoto Y., Shiraishi S., Seino H., Nagai T., Negishi M., Hayano S.: Yukuguku 22, 95 (1973) 104. Wilhite W. F., Burnell M. R.: Proceedings of the Ninth National Instrument Society Of America, Analysis Instrumentation Symposium, April 29 - May I , Houston, Texas 1963 105. Yannone M. E., Mueller J. R., Osborn R. H.: Chromatogruphia 3, 13 (1970)

6.

The Thermionic Detector Using an Alkali Metal Salt (TIDA)

Detectors denoted in the literature as thermionic detectors (JID), alkali flame ionization detectors (AFID), chemi-ionization detectors (CID), nitrogen detectors (NFID) and phosphorus detectors belong to the group of ioi ization detectors in which thermal energy is used as a source of ionization energy. However, the detection mechanism does not involve measurement of the ionization current of the eluted substance as with other ionization detectors (ECD, FID, PID, HeD); instead, changes in the ionization of an alkali metal present in the detector effective space are monitored. A detector using an alkali metal salt was first described in 1964 [38]. Since then, this detector has been one of the most frequently employed devices, especially in the analysis of phosphorus- and nitrogen-containing compounds. Its high sensitivity for these hetero-atoms led to its being called the phosphorus or nitrogen detector. The measurement of other atoms, such as sulphur, the halogens, arsenic, antimony, tin and lead, is not very sensitive and the varying magnitude and character of the detector response for these substances have often been studied. A common feature of all TIDA constructions is the measurement of the ionization current as the sum of all of the ionization processes that take place in the detector, alkali metal ionization predominating. Thermal energy for the ionization process is liberated by combustion or by electrical heating. The detection mechanism has been explained from several points of view. It has been assumed that the combustion products of phosphorus-containing substances react with the alkali metal or with its salt [38], the detector signal corresponding to the ionization of the evaporating salt. It has also been suggested that the ionization results from collisions of the alkali metal atoms with intermediates (e.g., radicals) formed in the flame [50]. Another mechanism involves evaporation of the alkali metal salt in the flame space as a result of interaction with photons [38, 471. Brezhnikov et al. [ 9 ] , who have contributed significantly to TIDA research, did not propose an unambiguous detection mechanism and pointed out that clarification of the mechanism is a condition for further TIDA development [8]. The mechanism has been partially elucidated by experiments with a flameless detector, called the chemi-ionization detector [59]. The results demonstrated unambiguously that ionization occurs in the gaseous phase and that neither hydrogen nor combustion products need be present. Ionization depends on the amount of alkali metal present in the gaseous phase, which in turn depends on the temperature of the surroundings. TIDA anomalies have been explained on the basis of evaluation of hetero-atom-selective interactions [61].

106 6.1

DETECTION MECHANISM

The TlDA mechanism theory assumes that, if the thermal energy is sufficient, the alkali metal salt evaporates and is atomized in the gaseous phase (equation (6.1)). The alkali metal atoms are excited and return to the ground state by emitting a photon (equation (6.2)) or by being ionized (equation (6.3)). If the eluted substance is introduced into this system, then it may react, e.g., by combustion (equation (6.4)) or ionization (equation (6.5)), or the products formed may be ionized. The ionic species formed by reaction (6.5) may recombine (equation (6.6)), as the probability of recombination of CHO' and H 3 0 + is considerably higher than that of recombination of the alkali metal cation with an electron [9]. In the TlDA system and for small amounts of eluted substances, most positive particles are alkali metal ions [lo]. Hetero-atoms in organic molecules undergo specific reactions (equation (6.7)), leading to significant differences in the detector response:

0RG ORG'

+e

P(N, S,CI) O R G

+

ORG++

e

-+

ORG

-+

specific interactions

(6.7)

The equilibrium of reaction (6.3) depends on the charge concentration in the effective space of the detector. If the ionization potential (IP,) of the alkali metal is greater than that of the eluted substance (ZPoRG),then the amount of charge in the reaction space increases, the equilibrium of reaction (6.3) is shifted in favour of the non-ionized alkali metal atoms and the ionization current corresponding to this process decreases. This is rather probable, as the ionization potentials of eluted substances are generally higher then those of the alkali metals (see Table 6.1) and the thermal energy is not sufficient for direct ionization of the eluted substance. If I P , < IP,,,, the ionization of the alkali metal is preferred and the ionization current increases. The ionization of the alkali metal atoms involves not only collisions of the atoms with photons but also chemi-ionization, with a mechanism assuming collision of the atoms with ions or radicals [lo].

107 Thermionic detectors using alkali metal salts exhibit various sensitivities and selectivities towards hetero-atom-containing substances. These differences stem from specific interactions of various hetero-atoms. In general, the formation of heteroatom compounds with low ionization potentials, the formation of thermally stable compounds by reactions of hetero-atom combustion products with the alkali metal and electron capture by the products formed are important. The contributions of these processes to the TIDA mechanism vary, depending on the kind of hetero-atom involved. TABLE 6.1 THE IONIZATION POTENTIALS OF THE ALKALI METALS A N D OF SOME ORGANIC SUBSTANCES [65]

Na K Rb

cs Ca Ba

5.14 4.34 4.17 3.89 6.1 1 5.21

CH4 C6H~

CCI, CH30H (CH3)zCO HCOOH CSZ

13.04 9.2 11.0 10.8 10.1 11.3 10.5

For the determination of chlorine using the TIDA, a surface ionization mechanism has been assumed [20, 481, involving capture of electrons by the electronegative halogen. This mechanism considers the ionization to take place directly on the electrode surface. However, it has been proved experimentally that the ionization occurs in the gaseous phase inside the flame and that the measuring sensitivity decreases during prolonged use of the TIDA owing to deposition of the alkali metal on the surface of the collecting electrode. Hence the surface ionization mechanism does not play a decisive role in the determination of halogens using the TIDA. The determination of fluorine in organic substances is based on the reaction of halogen fluoride, formed by combustion of the eluted substance with solid CaCI,. Gaseous hydrogen chloride is formed and is led over a heated container of the alkali metal salt; variations in the ionization or emission are then measured [40a]. The specific interaction of chlorine lies chiefly in electron capture and is manifested by an increase in the ionization current due to increased ion-ion recombination. This specific interaction mechanism has been verified by measurement of the response to halogenated benzenes. Chlorobenzene causes the largest decrease in the signal, corresponding to the electron affinity values in the series, CI > Br > I:

108

The magnitude of the JlDA signal increases in the reverse order [35]. When the responses for toluene and chlorobenzene are compared, a smaller response is again obtained for chlorobenzene [16], due to the above mechanism. The electron affinities of some atoms, molecules and radicals which may be formed as reaction products are given in Table 4.1. Combustion of sulphur-containing organic substances leads to the formation of a mixture of SO, and SO,. The electron affinity of these oxides is lower than that of the halogens and the specific interaction of sulphur under J l D A conditions consists of a reaction of the oxides with the alkali metal atoms, leading to formation of thermally stable compounds. This reaction causes a decrease in the alkali metal pressure (Table 6.5) in the effective volume of the detector and consequently a decrease in the ionization current. The decrease in the ionization current is most pronounced when an alkali metal iodide is employed and is smallest for an alkali metal sulphate. TABLE 6.2 THE IONIZATION POTENTIALS OF SOME COMPOUNDS [57,65] Compound

IP[eV]

16.8 9.25 12.9 9.75 11.5 6.86 ~

Compound

--_

10.9

PH3 PO PCI 3 PH,CI PN -~ (C6H513p

ASH,

~

9.98 0.42 3.18 1.57 0.88 7.34 ~ 10.03

cs'___ C'2 HCI CH3CI C,H,CI

_

ZP[eV]

9 5s 7.3 _ _ 7.3 10.7 10.7 12.5 10.45 10.05 13.2 12.8 11.5 8.5

_

The electron affinity of phosphorous compounds is the smallest compared with sulphur and chlorine, as follows from the position of phosphorus in the periodic system of the elements. The combustion products of phosphorus-containing compounds d o not form thermally stable compounds (see Table 6.5) but usually have very low melting points and thus efficiently increase the concentration of alkali metal in the flame. The formation of phosphorus compounds melting at low temperatures

109 and having low ionization potentials (Table 6.2) is the basis of the phosphorus-specific interaction in the TlDA mechanism and is the reason for the very high response to phosphorus-containing compounds. 6.2

TlDA SIGNAL

From the detection mechanism, it follows that either the emitted light intensity (equation (6.2)) or the ionization current (equation (6.3), (6.5)) can be measured in the TlDA system; the latter is more frequent. TABLE 6.3 THE HEAT LIBERATED BY COMBUSTION OF HYDROGEN A N D THE CORRESPONDING PARTIAL PRESSURE O F CsBr [61] HZ [cm3/min1 14.1

33.9 46.1 63.8

AH [ca~/sec] 0.45 1.12 1S O 2.01

PCsBr

NCsBr /SeC

[torr]

5 x low5 5.5 x I O - ~ 4 x 1013 2 x 10-1 1.o 7 x 1015

TABLE 6.4 COMPARISON OF THE CsBr PARTIAL PRESSURE IN FLAME [8] A N D FLAMELESS [59] TlDA SYSTEMS

Temperature of the alkali metal salt container ["C] 530 500

PCsBr

[torr] 5.5 x 3 x

[atml

Ref.

8 4 x 10-6

59

The TlDA ionization signal is determined by ionization reactions (6.3), (6.5), (6.6). Reaction (6.3) corresponds to the ionization of the alkali metal, which depends on the temperature of the medium. A hydrogen-air flame is employed as the source of the ionization energy; its temperature is sufficient for ionization of the alkali metal. Table 6.3 gives the amount of heat liberated by the combustion of various amounts of hydrogen. In flameless devices, the container for the alkali metal salt is electrically heated. From comparison of the salt partial pressures at given temperatures, it follows that thermal energy is the source of the ionization energy in both instances (Table 6.4).

110 It must be remembered that, when a flame is employed, the eluted substance is burned in the detector, thus liberating an amount of heat dependent on the composition of the eluted substance. For example, burning 0.3 mole of CH,, 0.165 mole of CzH6,0.07 mole of C6Hs, 0.09 mole of C5H5N, 2.1 mole of HCOOH, 0.345 mole of CH,COOH, 0.2 mole of C,H,OH or 0.13 mole of (CH,)S is equivalent to the combustion of 1 mole of hydrogen. An increase in the temperature occurs due to the combustion and hence reactions (6.1)-(6.3) are shifted in favour of the products, i.e., the ionization current increases; this increase is denoted as ic,. Reaction (6.5) expresses the ionization of the eluted substance by the FID mechawhile recombinism and contributes to an increase in the ionization current, ic,,,, nation (equation (6.6)) and electron capture (equation (6.8)) decrease the ionization current. The resultant ionization current is dependent on the specific interactions of hetero-atoms and on the relative values of the ionization potentials of the alkali metal and the eluted substance. If ZP, < ZPORG,which occurs most frequently, the TlDA ionization signal can be described by the equation S y

6.2.1

=

1i ic, = bci + ic, + ic,,,

(6.9)

TlDA background current

The TlDA background ionization current corresponds to reaction (6.3) caused by the heat from burning hydrogen. The contributions from impurities in gas containers and from bleeding of the stationary phase in the column are constant during the experiment and can be suppressed. The equilibrium of reaction (6.3) depends on the ionization potential of the alkali metal, on the melting point of the salt used (MP,,) and on the total amount of heat supplied within the given time interval ( A H ) . The dependence of the TlDA background current on these parameters is depicted in Fig. 6.1. The background ionization current is caused by thermal energy, i.e., by the heat liberated by hydrogen combustion, only in the absence of eluted substances. Therefore, the TlDA background current varies markedly with varying hydrogen flow-rate, by as much as several orders of magnitude (6 cm3/min - 10-"A, 30 cm3/min -10-8A) [ 2 2 ] , and consequently a highly stable gas flow-rate ( f O . l cm3/min) is required for work with the TlDA [ 2 8 ] . The melting point of the alkali metal salt and the ionization potential of the alkali metal affect the TIDA background current strongly. At constant pressure and temperature, the atomization of the alkali metal salt (equation (6.1)) is dependent on the composition of the salt used alone. In general, the lower the percentage of ionic bonds, the lower is the pressure of the alkali metal at a constant temperature. Examples of sodium salts and chlorides are given in Table 6.5. At constant temperature and pressure, the ionization of the alkali metal atoms (equation (6.3)) is a function of the ionization potential alone. It is generally true that the lower the ionization potential, the greater is the degree of ionization of the alkali metal. The value of

111 the TlDA background current increases from Na to Cs and from PO:- t o OH-. In a number of measurements, the experimental conditions are adjusted so that the background current has a constant value. Various temperatures must be employed by variation of the hydrogen flow-rate, depending on the salt composition (see Table 6.3). The pressure of the alkali metal in the effective volume of the detector then varies. In order t o achieve the required magnitude of the background current with somz salts (e.g., SO:- and F-), high hydrogen flow-rates must be used so that

AH

MPAX

FIG. 6.1. The dependence of the TlDA background ionization current on the reaction , the ionization potential heat, A H , the melting point of the alkali metal salt, M P A ~and of the alkali metal, IPA [61].

the limiting temperature of the flame can bc approached. A further increase in the flame temperature (e.g., by combustion of the eluted substance) is then impossible and therefore the ic, term in equation (6.9) cannot be increased. This situation leads to a negative response for the JIDA, which is more probable a t high background currents. F o r purposes of comparison, the pressure of the alkali metal in the flame rather than the magnitude of the background current should be used as a criterion. In general, the TlDA background ionization current is very high (up t o 10-8A). The TlDA noise is relatively high, varying around 10-"A, and is caused chiefly by non-constant gas flow-rates and by changes in the concentration of alkali metal salt in the flame. The latter frequently causes noise and drift with the TIDA. For the suppression of noise, an RC circuit has also been used [IS], but this technique is practical only with substances with long clution times, as the overall time constant of the device then increases.

112

It has already been pointed out that the emission signal can also be measured. The magnitude of the background current of the photoeniission sensor is about 10-'A [49]. However, this value is determinated by the design of the device and especially by the photomultiplier employed. 6.2.2

TlDA response

The TlDA response is given by the time integral of the TlDA signal during the elution of a substance from the column. From equation (6.9), it follows that the signal depends on the magnitude of the background current and on all of the ionization processes. In the absence of eluted substances, the background current corresponds to ionization by thermal energy alone, i.e., it is given by the combustion of hydrogen. As the partial pressure of hydrogen and thus its amount decrease during the elution, the amount of heat liberated and hence the magnitude of the background current decrease. The lowest value of the background ionization current is attained at the elution maximum of substance ORG. The ionization current, ic,, corresponds to the ionization of alkali metal due to thermal changes during the elution of ORG, e.g., due to its combustion (equation (6.4)). This term is zero before the elution and reaches a maximum at the elution maximum. As follows from a comparison of the heats of combustion of organic substances and of hydrogen, the term ic, is larger than bci, i.e., the ionization current increases (Fig. 6.3) due to reactions (6.1)-(6.3) shifting in favour of the products.

I

0

I

2m

I

m

[kJ/mole] FIG. 6.2. The dependence of the ionization efficiency of some mercaptans on the heat of combustion in the TlDA system [61].

These changes decrease in magnitude with increasing ionization potential of the alkali metal and with increasing melting point of the alkali metal salt. The contribution of the heat of combustion to the TlDA response is depicted in Fig. 6.2 for substances that contain an - SH group. This example also demonstrates that the whole molecule of an organic substance and not only the hetero-atom contributes to the TlDAresponse.

113

The ionization current, icORG,corresponds to the ionization of the eluted substance by the FID mechanism. The magnitudes of the ionization currents fall in the order icORG < bci < ic,, as is reflected in the absolute magnitudes of the ionization currents in the JlDA and FID systems. The mutual relationship among the individual ionization contributions determines whether the JlDA response is positive or negative. For a positive JlDA signal Abc, < (Aic,,,

+ Aic,)

while for a negative signal Abc, > (Aic,,,

+ Aic,).

The TlDA response is significantly affected by specific interactions involved in the detection mechanism. During the detection of sulphur, the magnitudes of ionization currents bc, and ic, are affected; they generally decrease with increasing amounts of the eluted substance. All ionization processes are affected during the monitoring

. '\,

--t

. !

0

'. ' ~ ~ ~

FIG. 6.3. Schematic dzpendence of the response, RTtDA, for some heteroatoms; bci - background ionization current, icA - ionization current of the alkali metal, caused by the heat liberated during combustion of the eluted substance, icoRG ionization current of the eluted substance due t o the FID mechanism, ~ C E C D- ionization current of the eluted substance due t o the ECD mechanism.

of chlorine, as the electrons liberated are captured by electronegative groups, resulting in a decrease in the ionization current. Nitrogen dioxide behaves in a similar manner to chlorine as it has a rather high electron affinity. During the measurement of phosphorus, the ic, term is not decreased as no thermally stable compounds are formed; ic,,, increases with increasing amounts of eluted substance because of the low ionization potentials of the products formed. The TlDA response is represented schematically in Fig. 6.3.

114 Depending on the amount of eluted substance involved, a maximum flame temperature is reached, corresponding to the maximum concentration of the alkali metal in the flame and thus to the maximum ic, value. As a result of cooling of the alkali metal salt reservoir and to distortion (lengthening) of the flame caused by the increased gas flow-rate, a decrease in ic, occurs, leading to a decrease in STID* with increasing concentrations of the eluted substance (Fig. 6.4) [14- 161. This effect, called peak inversion, has often been verified experimentally [3, 14, 551.

FIG. 6.4. Schematic dependence of the TlDA response and of the individual ionization current on the amount of eluted substance [61].

The T/DA response varies with the background current. The maximum response is achieved sooner with salts that have lower melting points [15, 661. These results confirm the theory given for the JlDA detection mechanism, which emphasizes the contribution from ionization current of the alkali metal caused by the combustion of the eluted substance (ic,). The less volatile the salt employed, the higher is the flame temperature required to reach the same bci value and hence the TlDA response is higher, or more negative, when salts with higher vapour pressures are used. Generally, the higher the background current, the sooner a JlDA response maximum is reached, depending on the amount of eluted substance, as a high background current corresponds to a high flame temperature and thus to the maximum pressure of an alkali metal salt. Temperature changes caused by combustion of the eluted substance are then minimal and the ic, term is very small. When large amounts are analyzed, it is preferable to decrease the hydrogen flow-rate, thus decreasing the TlDA background current and obtaining a response that increases as a function of the amount of sample. A negative TlDA response can then be converted into a positive response. Measurement of the emitted light is not frequently employed in the TlDA system. It has been reported [49] that, with simultaneous measurement of the ionization and emission signals, the alkali metal emission increased during the elution of chlorineor phosphorus-containing substances [39,49]. The emission of sodium salts increases

in the order PO:- < C1- < Br- < I-, the increase in the emission of the phosphate being 100 times less than that for the chloride. In the analysis of a mixture of halogenated benzenes, an increase in the sodium emission was observed only for iodobenzene, while for bromobenzene [6,43] and chlorobenzene the emission decreased, i.e., the ionization increased. Solid sodium chloride was used and reactions (6.1) and (6.3) are shifted in favour of the products during the elution of iodobenzene, as follows from consideration of the effects of bonding on the vapour pressure of the alkali metal (see Table 6.5). However, a decrease in the sodium or potassium emission during the elution of organic substances from the column has also been reported in the literature [5]. The results of these experiments confirm the effect of the heat of combustion on reactions (6.1) -(6.3), as the heat liberated causes ionization of the alkali metal. In general, a decrease in the emission will always occur in systems with high or medium temperatures. However, individual cases must be considered from the point of view of their mechanisms. TABLE 6.5 MELTING POINTS A N D PARTIAL PRESSURES OF SOME SALTS AT DIFFERENT TEMPERATURES [65] Partial pressure Salt

MP[‘ C]

[torr] ~

Na,HPO, NaOH NaP03 NaBr NaCl Na,S04 Na,PO, KCI RbCl CSCl

200- 250 320 625 750 800 884 1340 768 71 5 646

[“CI

.-

1

739

1

1

800 865

-10-5

1000 1000

1 1 1

821 792 744

~

6.2.2.1

Linearity and linear d y n a m i c range of the TlDA

As follows from the equation for the TlDA signal, a number of processes are involved in determining the measured value. For this reason, a number of so-called anomalies have been described in the literature, which can be explained, however, on the basis of the detection mechanism. The marked dependence of the magnitude of the response on the experimental conditions results in a TlDA linearity in the interval (1, -1). It is usually positive for phosphorus- and nitrogen-containing compounds. The response gradually assumes a negative value under varying conditions and the response for the halogens and sulphur is usually negative.

The TIDA linear dynamic range varies depending on the kind of hetero-atom involved. The literature data indicate that the linear dynamic range is about three orders of concentration for phosphorus and nitrogen-containing substances, while for chlorine-containing substances values up to six orders have been reported [ll, 121. 6.2.2.2 Sensitivity and selectivity of the TlDA

The TlDA exhibits high sensitivity and selectivity for some hetero-atoms. The highest sensitivity has been reported for phosphorus-containing substances [12], which have been determined in amounts down to 10-14g [46]. However, for evaluation of the TIDA sensitivity, it is necessary to know the ratio of the FID and TIDA signals, which has only very rarely been measured. The authors of original papers usually simply state the amounts of eluted substances determined. The value for the entire molecule is then evaluated without taking the hetero-atom into consideration and erroneous conclusions can be drawn. The measuring sensitivity must not be related merely to the hetero-atom in the molecule and the ideal approach involves consideration of the organic molecule as a whole, with due regard being paid to the hetero-atom. The low dependence of the response on the hetero-atom is reflected in identical ratios of the FID and TIDA signals for the pesticides Prometon, Propasin and Prometrin, which contain -OCH,, -SCH, and-C1 groups, respectively [19]. The dependence of the TIDA response on the composition of the entire molecule and not on the hetero-atom alone can, on the other hand, be documented,for example, by the fact that (C,H,),PO, yields a smaller response than (C,H,), P0,[40], trimethyl phosphate gives a smaller TABLE 6.6 RELATIVE MAGNITUDES OF THE TlDA RESPONSE TO SOME HETEROATOMS

Signal

TIDAIFID TlDA TlDA

N

P

As

Sb

Bi

60 100 5,000

11,000 1,000

10

10

10

20

S

CI

2

1 1

Ref.

32

9 30

response than o,o’-diisopropylmethyl phosphate [34], Methylparathion gives a smaller response than Parathion containing ethyl groups [9,28], and Trimet has a smaller response than Disyston [23]. However, it also follows from these results that there are relative differences among hetero-atoms, the magnitude of which depends on the experimental arrangement (Table 6.6). The experimental results confirm the given reaction scheme and the effect of hetero-atom-specific interactions.

117 Although this type of detector is often classified among selective detectors, it is actually selective only under certain experimental conditions [121. From the above discussion of the TIDA sensitivity, if follows that the TlDA response is significantly increased only in the presence of phosphorus and nitrogen and hence the detector TABLE 6.7 THE TlDA SELECTIVITY FOR NITROGEN-CONTAINING SUBSTANCES [30] Substance

Selectivity ~-~

~~

~-

C S CI Br I

5,000 2,000 5,000 5,000 200

--

_ ~ . _ _ _ _ _ _ - __.

may be considered to be selective for these hetero-atoms. The selectivities for nitrogen in organic molecules are given in Table 6.7. However, it should be pointed out that the TlDA selectivity varies with changing experimental conditions, such as the gas flow-rate and electrode geometry [30].

6.3

EFFECT OF THE EXPERIMENTAL CONDITIONS ON THE MAGNITUDE AND CHARACTER OF THE TlDA SIGNAL

6.3.1

Gas flow-rate

The magnitude of the background ionization current exerts a marked effect on the magnitude and character of the TlDA response. It has been verified experimentally that the background ionization current depends on the flow-rate and the quality of the carrier gas. The background current usually decreases with increasing carrier gas flow-rate, owing to increased cooling of the container for the alkali metal salt and consequently to a decrease in the partial pressure of the metal in the flame space [36]. The hydrogen flow-rate has a decisive effect on the TlDA response; for example, a change in the hydrogen flow-rate from 30 to 18 cm3/min causes a ten-fold decrease in the response [64]. It has been reported [27] that a change in the hydrogen flow-rate by 0.05% leads to a change in the ionization current by 1%. In addition to its dependence on the hydrogen flow-rate, the TlDA response also depends on the air flow-rate [26]. It has been found that, with the diffusion flames used in all TlDA systems, the background current and response decrease when the air flow-rate is decreased below 200 cm3/min, while they are constant at flow-rates above this value. This decrease can be explained by incomplete combustion causii:g a decrease in the flame temperature.

118 When helium rather than nitrogen was used as the carrier gas, the JlDA response decreased to one tenth of its initial value [19, 321. These changes result from the increased cooling of the alkali metal salt reservoir, so that reaction (6.1) is suppressed. The mutual relation of the carrier gas and hydrogen flow-rates exerts a great influence on the character of the JlDA response [21]. If the ratio of the flow-rates of the two gases is close to unity or less [37], then the response is negative; the higher the carrier gas flow rate, the less frequently negative responses are encountered. Thus a negative response can be converted to a positive one by decreasing the hydrogen flow-rate. 6.3.2

Detector geometry

The basic geometry of the TlDA is depicted in Fig. 6.5. The JlDA consists basically of a flame ionization detector in the measuring space of which an alkali metal salt reservoir is placed. The position and shape of this reservoir have been discussed in many papers; recently, pressed salt pellets with drilled holes have been employed most frequently [29] and are usually placed on top of the FID burner.

FIG. 6 . 5 . Scheme of

3

TIDA.

The salt container has been electrically heated to temperatures of 400 to 1000 "C [41]. The atomized alkali metal is then transported by the carrier gas into the measuring space [60]. In these systems, the partial pressure of the alkali metal is not as dependent on the flame temperature, i.e., on the combustion of eluted substances. Because of suppression of the effect of the ic, term in equation (6.9), this JlDA system is much more selective and the solvent response is relatively low. A similar effect on the TlDA response is exerted by the temperature of the detection block. With decreasing temperature of the JlDA thermostat, the background current and the detector signal both decrease [30].

119 Siting of the alkali metal salt reservoir elsewhere than below the flame has a number of drawbacks. The container content is very rapidly depleted, the time required for the establishment of equilibrium conditions is long, the detector sensitivity decreases with decreasing amounts of the alkali metal salt [I] and, during analyses of silylated samples, silicon dioxide is deposited on the surface of the container or on the salt. Placing the salt reservoir under the flame, as shown in Fig. 6.5, is advantageous because of the prolonged life of its contents, shortened preparatory time before the measurement and constant sensitivity. However, these designs are also not ideal. The salt evaporates from the surface layer during the measurement [8] and, when the alkali metal salt is mixed with an inactive substance, the latter remains on the reservoir surface so that a substantially higher temperature must be used for further evaporation [141. On an increase in the temperature, e.g., on an increase in the hydrogen flow-rate, the alkali metal salt diffuses from inside the reservoir to the surface and the initial conditions are re-restablished, When the salt pellet is placed under the flame, only one sixth of the coldest part of the flame is utilized. It is evident that an increase in the flame radiation then leads to an increase in the concentration of alkali metal in the flame and thus to an increase in the TlDA response. It was found experimentally that the TlDA response increased five times when the hole in the pellet was widened from 0.8 to 1.6 mm. Similar results were obtained during the measurement of pyridine using burners with diameters of 0.6 to 3.0 mm “41. The shape of the collecting electrodes and their distance determine the magnitude of the applied polarization voltage. Low voltages (about 100 V) are usually selected in order to obtain as low an electric field intensity as possible, which must, of course, be high enough to permit work in the saturation region of the Geiger-Muller curve [41, 461. If this condition is not met, the TlDA response decreases to as little as one tenth of its original value [28], owing to ion recombination. A three-electrode system has been proposed for suppression of ion recombination [63]. Older JlDA designs usually did not permit changes in the electrode distance and thus optimalization of the position of the collecting electrode was impossible. It must be borne in mind that the shape of the flame changes during optimalization of gas flow-rates [45] and lengthens with increasing flame temperature. Therefore, it is also necessary to optimize the position of the electrode in order to increase the detector response [30, 441. Much attention has been paid to the polarity of the collecting electrode [21, 641. When the collecting electrode is negative, i.e., a cationic current is measured, the detector noise is very low but the response is also low and distorted. The collecting electrode is usually maintained positive; the elution curves are then undistorted, as fast electrons are monitored. In the TlDA system, the maximum ionization current value is measured. As these currents are always higher than 10-9A, space charge also plays a role. Therefore, it is always preferable to measure the electron current and to introduce further electrodes in order to suppress the space charge effect.

120 6.4

TlDA APPLICATIONS

The TlDA is one of the most extensively used gas chromatographic detectors. The dependence of its response on the experimental conditions has led to an abundance of “anomalies” and “selective” measurements, resulting from insufficient consideration of the detection mechanism. The significant sensitivity of the JIDA for phosphorus- and nitrogen-containing substances makes it especially suitable for application

77min FIG. 6.6. Chromatogram of the TAB-esters of amino acids 1531; 1 - alanine, 2 valine, 3 - glycine, 4 - isoleucine, 5 - leucine, 6 - proline, 7 - theonine, 8 serine, 9 - cysteine, 10 - methione, 11 - hydroxyproline, 12 - phenylalanine, 13 - asparaginic acid, 14 - glutaminic acid, 15 - thyrosine, 16 - lysine, 17 tryptophan, 1 8 - arginine. TABLE 6.8 EXAMPLES OF TlDA APPLICATIONS

Material analyzed

Pesticides in grain in water Pesticides P (parathione)

Lowest detectable amount

Linear dynamic range

t 0 . 0 2 ppm

4 x 10-l2g

3

2.4 ng

1.7

1 ng

3 2.3

N C1 PH, in foodstuffs, water a-oxoacids Acetylcholine in the heart Phenylbarbitone, primidone, phenytoin in serum Pentazocine in blood N-drugs in urine N-nitrosamines in foodstuffs Amines, hydrazines Chlorinated hydrocarbons in soils Methane arsonic acid Alkali metals and the alkaline earths

Ref

58 62, 63 35, 35, 44, 52, 63 4, 25, 56 5, 43 7 31 42 24 33 13, 54 51 67 5, 17, 43 37 2

121

in biochemistry and clinical chemistry. Application of the TlDA permitted identification and classification of some microorganisms [53] and the analysis of amino acids(Fig. 6.6). The high sensitivity of the TlDA is also utilized in anti-doping tests on athletes and in analyses of hashish. marijuana, opium and LSD [54]. Some examples of TlDA applications are given in Table 6.8. Combined FID and TlDA responses were used for the determination of molecular hetero-atom contents [6]. Standard deviations of 5 5% represent good agreement in TlDA systems. The use of flameless systems is less frequent, although the signal measured under these conditions is more dependent on the hetero-atom than in devices employing flames. For this reason, the signal is lower in flame systems, e.g., 63 times for malathion, 59 times for methylparathion and 21 times for parathion.

6.5

LITERATURE

1. Abel K., Lanneau K., Stevens R. K.: J . Assoc. Off, Anal. Chem. 49, 1022 (1966) 2. Araki S., Suzuki S., Hobo T., Yoshida T., Yoshizaki K., Yamada M.: Bunseki Kagakic 17, 847 (1968) 3. Aue W. A., Gehrke C. W., Tindle R. C . , Stalling D. L., Ruyle C. D.: J. Gas Chromatogr. 5, 381 (1967) 4 . Aue W. A,: Chem. Abstr. 72, 18 2982 (1970) 5. Aue W. A,, Moseman R..F.: J. Cliromatogr. 61, 35 (1971) 6. Aue W. A,, Gerhardt K. O., Lakota S . : J. Chromatogr. 63, 237 (1971) 7. Berek B., Westlake W. E., Gunther F. A,: J . Agric. Food Chen7. 18, 143 (1970) 8. Brazhnikov V. V., Gur'ev M. V., Sakodynsky K. I.: Chromarographia 3, 53 (1970) 9. Brazhnikov V. V., Gur'ev M. V., Sakodynsky K. 1.: Cl~romatogr.Re". 12, 1 (1970) 10. Calcote H. F.: Ionization in High-Temperature Gases (Shuler K. E., ed.) Academic Press, New York, London 1963 11. Cremer E., Krans T., Bechtold E.: Chem. fug.-Teck. 33, 632 (1961) 12. Cremer E.: J. Gas Chromatogr. 5, 329 (1967) 13. Donike M., Jaenicke L., Stratmann D., Hollmann W.: J . C/zron?atogr.5 2 , 237 (1970) 14. Dressler M., Janak J.: Coll. Czech. Chem. Commun. 33, 3960 (1968) 15. Dressler M., Janak J.: Coll. Czech. C/iem, Commroi. 33, 3970 (1968) 16. Dressler M., Jan ik J.: J. Chromatogr. Sci. 7, 451 (1969) 17. Dressler M., Janak J.: J . Chromatogr. 44,40 (1969) 18. Dressler M., Drml M.: J. Chromatogr. 56, 23 (1971) 19. Ebing W.: Chromatographia 1, 382 (1968) 20. Fomin 0. K., Tikhomirov M. V.: Z/z. Fiz. Khim. 38, 813 (1964) 21. Gerhardt K. O., Aue W. A,: J. Cliromatoyr. 52, 47 (1970) 22. Giuffrida L.: J. Assoc. Off, Ayr. Clieni. 47, 293 (1964) Agr. Cltem. 47, I112 (1964) 23. Giuffrida L., Ives F.: J. Assoc. Off. 24. Goudie J. H., Burnett D.: Clin. Chiin. Acta 43, 423 (1973) 25. Greenhalgh R., Wilson M . : Column 1972, 10 26. Halasz I.: Anal. Chem. 36, 1428 (1964) 27. Hartmann C. H.: Bull. Enrit-on. Contanr. Toxicol. 1, 159 (1966) 28. Hartmann C. H.: Aerograph Phosphorus Detector, Research Notes, Summer 1966

122 29. Hartmann C. H.: US Pat. 3,607,096 (Sept. 21, 1971) 30. Hewlett-Packard Operating Note (1972), High Sensitivity Nitrogen Detector, Model 15 161 B 31. Hoffmann N. E., Gooding K. M.: Anal. Letters 2, 479 (1969) 32. Ives N. F., Giuffrida L.: J. Assoc. Of. Anal. Chern. 50, 1 (1967) 33. James S. P., Waring R. H.: J. Chromatogr. 78, 417 (1973) 34. Janak J., Svojanovski V., Dressler M.: Coll. Czech. Chern. Commrm. 33, 740 (1968) 35. Jentzsch D., Zimmermann H. G., Wehling I.: Z . Anal. Chem. 221, 377 (1966) 36. Johnson R. E.: L e d . Gas Chromatogr. 1966, 1967, 1 37. Johnson L. D.; Gerhardt K. O . , Aue W. A,: Sci. Total Enriron. 1, 108 (1972) 38. Karmen A,, Giuffrida L.: Nature 201, 1204 (1964) 39. Karmen A,: Anal. Chem. 36, 1416 (1964) 40. Karmen A.: J. Gas Chromarogr. 3, 336 (1965) 40a. Karmen A., Kelly E. L.: Anal. Chenz. 43, 1992 (1971) 41. Karmen A., Haut H.: Anal. Clieni. 45, 822 (1973) 42. Kilbinger H.: J. Neurochem. 21, 421 (1973) 43. Lakota S., Aue W. A,: J. Chroniatogr. 44, 472 (1969) Anal. Chem. 52, 533 (1969) 44. DeLoach H. K., Hemphill D. D.: J . Assoc. Off. 45. DeLoach H. K., Hemphill D. D.: J . Assoc. OffAnal. Cliem. 53, 1129 (1970) 46. Mees R. A., Spaans J.: Z. Anal. Cheni. 247, 252 (1969) 47. Moesta H.: Surface Sci. 2, 267 (1964) 48. Moesta H., Schuff P.: Ber. Bunsenges. Phys. Cheni. 69, 895 (1965) 49. Nowak A. V., Malmstadt H. V.: Anal. Cheni. 40, 1108 (1968) 50. Page F. M., Woolley D. E.: Anal. Chem. 40, 210 (1968) 51. Palframan J. F., McNab J., Crosby N. T.: J. Chromatogr. 76, 307 (1973) 52. Pivarov G. A,: Ref. Zh. Khitn. 1972, 13 N 442 53. Riedmann M.: Chemiker Zeitung 96, 618 (1972) 54. Riedmann M.: Naturwiss. 59, 306 (1972) 55. Riva M., Carisano A,: J. Chromatogr. 36, 269 (1968) 56. Riva M., Carisano A.: Crorr. Chini. 1969, 3 57. Rossini F. D., Wagman D.D., Evans W. H., Levine S., Jaffe L.: Selected Values of Chemical Thermodynamic Properties, Part 1 ., Tables, National Bureau of Standards, Circular 500, Part 1, Reprint July 20, 1961 58. Schulz D. R.: Bull. Environ. Confain. Toxicol. 5, 6 (1970) 59. Scolnick M.: J. Chromatogr. Sci. 8, 462 (1970) 60. Scolnick M. E.: US Pat. 3,589,869 (June 29, 1971) 61. SevEik J.: Chromatographia 6, 139 (91 73) 62. Soerensen 0.: Haus Tech., Essen, Vortragsveroeff. 283, 34 (1973) 63. Speakman F. P., Waring C.: Colunin 2, 2 (1968) 64. Svojanovskg V., Janak J., Dressler M.: Coll. C-iech. Chern. Comrnun. 31, 3925 (1966) 65. Weast R. C.: Handbook of Chemistry and Physics, 51st ed., The Chemical Rubber Co., 1970-1971 66. Yamada M., Suzuki S . , Akaki S . : Bull. Chem. SOC.Japan 46, 830 (1973) 67. Zandberg E. Ya., Rasulev U. Kh.: Zh. Anal. Khirn. 27, 2459 (1972)

7.

The Photoionization Detector (PID)

Progress in the application of gas chromatography is largely determined by the development of sensitive and realiable detectors. Among ionization detectors, the continuing development of the photoionization detector is noteworthy. Although this term appeared in the literature as early as 1960 [23, 301, the photoionization mechanism cannot properly be ascribed to detectors developed before 1972 [9]. 7.1

DETECTION MECHANISM

The ionization energy employed in this detector is carried by photons with an energy of 10 to 20 eV, produced by discharge in argon, helium or hydrogen. The original designs did not have separated discharge and detection spaces and so a number of reactions took place simultaneously. These detectors were denoted in the literature as photoionization detectors when the ionization current was measured [12, 13, 15- 17, 19. 92, 23, 29-31, 38, 41 -431, discharge detectors when the discharge current was measured [l, 2, 14, 20, 26, 32, 37, 391 and microwave emission detectors when the intensity of the emitted light, usually of very short wavelength, was measured [3 -81. The original PID design with directly connected discharge and ionization spaces has a number of drawbacks: ( I ) the separating column is connected to a device in which the pressure is less than 760 torr; (2) the partial pressure of the discharge gas varies and hence theenergydistribution among the emitted photons is not constant; (3) the ionization energy is carried by both photons and ions [20], which are strongly accelerated in the intense electric field and cause direct ionization of the eluted substance; moreover, metastable atoms or excited molecules with long lifetimes and large ionization cross-sections are also formed in the discharge space. Because of these complications, it was not possible to formulate a detection mechanism for this device and it was thus difficult to explain the character of the PID response. The use of the PID was further limited by the necessity of employing evacuated systems. The present PID designs [27,35] eliminate the above drawbacks and make possible the use of pure photoionization processes. These designs are based on mechanical

124

separation of the discharge compartment, filled with a suitable gas at a pressure of 0.1 to 1 torr, from the detection compartment, operating at atmospheric pressure, using a suitable optically transparent material (Fig. 7.1). Under these conditions, only photons with known energy enter the detection compartment and ionize the eluted substances. In addition to the eluted substance a carrier gas, C, is present in the detec-

G I ,DTFE E 3 olass FIG. 7.1. A scheme of a PID with separated discharge and detection compartments; 1 - photon source, 2 - optically transparent crystal, 3 - detection compartment, 4 - outlet of the capillary column, 5 - collecting electrode [35].

tor; for the ionization potentials of these substances, I P , > k v > ZPoRG.Then the processes taking place in the PID can be described by the following scheme:

+ kv

-+

ORG*

ORG*

4

ORG++ e

ORG*

-+

OR’

-+

current

-+

ORG’

-+

ORG’

3

ORG’

ORG

+ anode ORG’ + cathode ORG+ + e + C ORG* + C e

+ G’

+ C’ + C’

Equation (7.1) describes the excitation of eluted substances to super-excited states and de-excitation by either ionization (equation (7.2)) or dissociation (equation (7.3)). The probability of direct ionization of the eluted substance by a photon

125 without the formation of an excited state is negligible [ll]. The ionization probability is expressed by the photoionization efficiency, q,,:

where u2 and u3 are the rates of reactions (7.2) and (7.3), respectively.

7.2

PID SIGNAL

From the scheme of the PID reaction mechanism, it follows that only the eluted substance and not the carrier gas is ionized by photons. The ionization probability is determined by the photoionization efficiency, which, in turn, is given by the rates of the pertinent reactions, i.e., by the concentrations of the colliding substances. The reaction mechanism also assumes a decrease in the measured ionization current due to three-body recombination (equation (7.6)), the rate of which can be expressed by the relationship [36] Vg

=

pCv~M1I2 k

(7.9)

. urn

where p c and M are the partial pressure and molecular weight of the carrier gas, respectively, U is the voltage at the collecting electrodes and k and rn are constants. The measured ionization current is given by the rate of reaction (7.2), less the rate of ion recombination, and can be expressed by the equation [35]

\z

I

1: - I v

I = F

1

-

-~

1: - I v

k,

(7.10)

+ - [C]

?v

k2

where k , and k , are the rate constants of reactions (7.2) and (7.7), respectively, F is the Faraday constant and Z: and I , are the intensities of the photon flux before and after the detector, respectively. The second term on the right-hand side of equation (7.10) can be neglected at sufficiently high voltages, i.e., it can be assumed that recombination is constant and very low. The number of photons absorbed can be expressed by the equation 1:

-I,

=

I:[l

- exp (-o,N

. 1 . V,[ORG])]

(7.11)

where o, is the absorption cross-section, N is the Loschmidt number, 1 is the thickness of the absorption layer and V is the volume of 1 mole of a gas under standard condi-

126 tions. When equation (7.11) is expanded in a Taylor series and the first term is substituted in equation (7.10), an expression for the measured ionization current is obtained: I =

I;crvN. 1 . VNIORG] 1

(7.12)

- + k"C] v v

k2

It follows from the above expression that, under constant experimental conditions and for small concentrations of eluted substances, the FID signal is directly proportional to the concentration of the eluted substance:

With higher concentrations of eluted substances, the exponential dependence of the signal on the concentration becomes important, as has been verified experimentally (Fig. 7.2) and the log S,,, versus log c dependence must be employed for evaluation of the results.

10%10- '0

I'

FIG. 7.2. The dependence of the PID signal on the amount of eluted benzene for the given carrier gas and the discharge tube input (351.

The signal of a photoionization detector with interconnected discharge and detection compartments is the sum of the ionization currents produced by photons, metastable states, electrons and ions. The degree of participation of individual ionization mechanisms depends on the experimental conditions, chiefly on the pressure of the discharge gas, the electric field intensity and the potential of the shielding electrode. The dependence of the signal on these conditions has not been quantitatively described.

I27

7.2.1

PID background current

The background current of a PID with separated compartments is among the lowest of all ionization detectors and is given by the ionization of impurities in the carrier gas and/or by the photo-effect. When the cathode is placed suitably, i.e., out of the path of the photon flux, background currents below 10-”A are obtained. Impurities in the carrier gas can play a role only when their ionization potentials are lower than the photon energy. In devices with separated discharge and detection compartments, the maximum photon energy is 11.5 eV; hence impurities which usually interfere, e.g., traces amounts of water or oxygen are without effect. For this reason, the detector is very stable against variations in the carrier gas flow-rate and the detector noise is very low (below 10-I3A).

7.2.2

PID response

From the equation for the PID signal, it follows that the ionization current is directly proportional to the intensity of the photon flux, the thickness of the absorption layer and the absorption and ionization cross-sections, i.e., to the quality and

I

A

-t FIG. 7.3. Schematic PID responsEs, RPID, for a pure carrier gas (A) and for a carrier gas containing an electronegative substance (B); bci - background ionization current, icplD - photoionization current, iCECD - electron capture ionization current.

amount of eluted substance. In addition to the ionization current, the intensity of the light, I,, can be measured. Each eluted substance absorbs light but ionization occurs only when IP,,, < Iiv. As the carrier gas plays a small role in the detection mechanism, the measured signal corresponds to the ionization of the eluted substance alone (Fig. 7.3). Because of the exponential dependence of the signal on the concentration, the dependence of the PID response on the concentration passes through a maximum.

128 The PID linear dynamic range is as high as four orders in the log SPJDversus log c coordinates. The linear dynamic range decreases in the presence of electronegative substances, as the electron capture mechanism becomes operative. The selectivity of the PID with separated compartments is determined by the relationship between the ionization potential of the eluted substance and the photon energy. It has been verified experimentally that substances with ionization potentials higher than the energy of incident photons are not ionized, in contrast to substances with lower ionization potentials. The PID yields a negative response for most organic substances. The PID selectivity can be improved in two ways: the energy of incident photons can be varied or the combined photoionization and photoabsorption signals can be measured. The ratio of the two signals is proportional to the photoionization efficiency of the eluted substance, qv. If a standard substance AB is selected, then the ratio of the ionization efficiencies is a qualitative characteristics of the eluted substance, ORG. 7.3

EFFECT OF THE EXPERIMENTAL CONDITIONS ON THE PID SIGNAL

7.3.1

Carrier gas

With the original designs of photoionization and discharge detectors, only argon and helium were employed as carrier gases, as they simultaneously acted as discharge

FIG. 7.4. The dependence of the PID signal on the discharge tube input for some carrier gases 1351.

gases. In contemporary devices with separated discharge and detection compartments, any carrier gas with an ionization potential higher than the photon energy can be used. Helium, hydrogen and nitrogen have been employed. During a study of the

129 eKe:t of impurities on the PID signal, air was also used as the carrier gas (Fig. 7.4). It was found that SPID is about 20 times smaller with air than with the other gases, which can be explained by the capture of the electrons formed by oxygen and water present in the carrier gas and thus by an increase in the recombination rate. It is thus evident that electronegative substances decrease the PID signal and their presence in the carrier gas is undesirable. 7.3.2

Geometric arrangement of the P/D

7.3.2.1 Discharge compartrnent

It has already been mentioned that older PID devices contained directly connected compartments. The eluted substances were then ionized by all the particles formed in the discharge. Responses to permanent gases, when argon was used as the discharge gas, have been reported in the literature. In argon, the maximum photon energy is 11.6 eV. Hence it is clear that the ionization of permanent gases was not caused by interaction with photons but by accelerated electrons and metastable atoms with energies higher than 11.6 eV. TABLE 7.1 THE OPTICAL TRANSPARENCE LIMITS FOR SOME SUBSTANCES I N THE UV SPECTRAL REGION

Substance -

Transparence limits

[nml

~

LiF MgF, CaFz Na F BaF, Sapphire

Ref.

[evl ~~

I04 112 122

I32 134 142.5

11.9 11.1 10.3 9.4

15, 28, 34

9.2

21 40

8.7

10

18 25

New PID designs are based on mechanical separation of the discharge and detection parts by an optically transparent substance. Glass and quartz are not transparent for high-energy photons and therefore crystals of alkali and alkaline earth metal fluorides are used as windows in these detectors (Table 7.1). This discharge tube is then an independent part of the PID and, after being filled with a discharge gas, yields a source of photons with a constant spectrum (Table 7.2). As the maximum energy of photons emitted from the discharge tube is determined by the crystal used, it is suitable to employ argon or hydrogen for filling the discharge tube. If a lower photon energy is

130 required, krypton or xenon can be used as discharge gases, or a different window can be placed across the photon flux. The transparence of lithium fluoride extends to very short wavelengths and is dependent on the temperature [18] and the humidity [34] and changes on prolonged use [40]. The LiF crystal has a transparency of about 60% for a wavelength of 1215.7 A. MgF, has better mechanical properties than LiF; its chief advantage is its insolubility in water and it is therefore more suitable for practical devices. TABLE 7.2 PHOTON SOURCES IN THE VACUUM UV SPECTRAL REGION Line spectrum

“41 ([eVI)

He Ne

Ar Kr

Xe HZ

60-110 (20.3- 11.3) 74- 100 (16.7-12.4) 106.7- 170 (11.6-7.3) 123- 180 (10.0- 6.9) 147-220 (8.45- 5.6) 180(6.9-)

584.3; 537.0 (21.3); (23.1) 735.9; 743.7 (16.8); (16.7) 919.8; 932.0; 1048.2; 1066.7 (13.5); (13.3); (11.8); (11.6) 1164.9; 1235.8 (10.7); (10.0) 1295.6; 1469.6 (9.6); (8.5) 850-1670; 1215.7 (7.4- 14.6); (10.2)

The shape of the discharge tube determines the intensity of the photon flux. Measurements have shown that the photon flux can be concentrated most effectively by leading it through a capillary (see Fig. 7.1). The distance between the electrodes affects the voltage at which discharge starts. It is therefore generally suitable to keep this distance as small as possible. 7.3.2.2 Detection compartment

Eluted substances pass through the detection compartment of the PID at atmospheric pressure. This part is mechanically connected to the discharge tube. The electrodes in the detector are outside the photon flux, thus giving a low background current and a low detector noise. The electric field intensity is very small in order to prevent electron acceleration. The effective volume of the detector requires the length of the absorption layer to be as large as possible. This requirement was taken into consideration [35] in the construction of a detector with an internal volume of 0.14 cm3.

131

The ionization currents in the PID are the lowest in the group of ionization detectors. Therefore, it is very important to employ perfectly shielded electric leads and to ground the whole measuring system property. If these requirements are not met, an increase in the detector noise occurs.

1.4

PID APPLICATIONS

In early work, a PID without separated compartments was frequently used for the analysis of permanent gases. Modern devices do not permit determinations of permanent gases and limit PID applications to substances with IP < 11.2 eV. This energy is sufficient to ionize most organic substances and some inorganic gases. The advantage of the PID lies in the fact that the carrier gas does not contribute to the ionization current, leading to a very small background current. When the PID is TABLE 7.3 PRINCIPAL PARAMETERS OF SOME IONIZATION DETECTORS FID

PID

ArD

Detector parameters

[241 ~

Background current [A] Noise [A] Linear dynamic range The lowest detectable amount [mole/sec] Substances detected

Ionization efficiency Carrier gas

5 10-12 3 10-14 6 7 \ 10-l~ propane organic

1 x 10-8 1 x 10-12 5.5

6x

4 x 10-11 4 i x 1 0 - l ~ 4 x lo-" propane oxygen organic and miscellansome eous inorganic 5 x 10-2 I x 10-3 Ar He

9 x 10-11 3 x 10-l~ 4

I x 10-j~ benzene organic and some inorganic I x H,, N,, He

compared with the FID, it is found that the FID exhibits a broader linear dynamic range, while the other parameters are comparable. Several ionization detectors are compared with the photoionization detector with unseparated and separated compartments in Table 7.3. The PID ranks among the most sensitive gas chromatographic detectors. Its simplicity, small volume and the possibility of employing very low carrier gas flow-rates make this detector suitable for use in very exacting gas chromatographic analyses.

132

7.5

LITERATURE

1. Arnikar H. J., Rao T. S., Karmarkar K. H.: Indian J. Chem. 5, 480 (1967) 2. Arnikar H. J., Rao T. S., Karmarkar K. H.: J. Chromatogr. 38, 126 (1968) 3. Bache C.A., Lisk D. J.: Biomed. Appl. Gas Chromatogr. 2, 165 (1968) 4. Braun W., Peterson N. C., Bass A. M., Kurylo M. J.: J. Chromatogr. 55, 237 (1971) 5. Dagnall R. M., Smith D. J., West T. S.: Anal. Lett. 3, 475 (1970) 6. Dagnall R. M., Deans D. R., Pratt S. J., West T. S . : Talanta 17, 1009 (1970) 7. Dagnall R. M., Deans D. R., Fleet B., Risby T. H.: Talanta 18, 155 (1971) 8. Dagnall R. M., West T. S . , Whitehead P.: Anal. Chem. 44, 2074 (1972) 9. Driscoll J. N.: Ger. Offn. 2,211,720 (Nov. 2, 1972) 10. Duncanson A., Stevenson R. W. H.: Proc. Phys. SOC.72, 1001 (1958) 1I . Ehrhardt H., Linder F., Tckkat T.: Advances in Mass Spectrometry (Kendricke, ed.), Vol. 4, p. 705, The Institute of Petroleum, London 1968 12. Freeman R. R., Wentworth W. E.: Anal. Chenz. 43, 1987 (1971) 13. Goldbaum L. R., Domanski T. J., Schloegel E. L.: J. Gas Chromatogr. 6, 394 (1968) 14. Goloskokov V. V., Kuzimina V. T., Levina L. E., Panyushkin V. V., Pimenov V. V.: Prib. Tekh. Eksp. 1973, 175 15. Karmen A., Giuffrida L., Bowman R. L.: Nature 191, 906 (1961) 16. Karmen A., Bowman R. L.: Nature 196, 62 (1962) 17. Karpov L. Ya., Kazakevich V. E., Dorbatenko V. D.: USSR Pat. 193,142 (March 2, 1967) 18. Knudson A. R., Kupperian J. E.: J. Opt. SOC.Am. 47, 440 (1957) 19. Krico-Electronic. K. G.: Ger. Offen. 1,958,751 (May 27, 1971) 20. Lakshtanov V. Z., Markevich A. V., Dobychin S. L.: Zh. Prikl. Khini. 40, 2492 (1967) 21. Laufer A. H., Pirog J. A., McNesby J. R.: J . Opt. SOC.Am. 55, 64 (1965) 22. Locke D. C.. Meloan C . E.: Anal. Cheni. 37, 389 (1965) 23. Lovelock J. E.: Nature 188, 401 (1960) . 33, 162 (1961) 24. Lovelock J. E.: A I ~Chem. 25. Melvin E. H.: Phys. Rev. 37, 1230 (1931) 26. Opregaarg M.: Norw. Pat. 120,095 (Apr. 26, 1969) 27. OstojiE N., Sternberg Z.: Chromatographia 7, 3 (1974) 28. Patterson D. A,, Vaughan W. H.: J. Opt. SOC.Am. 53, 851 (1963) 29. Price J. G. W., Fenimore D. C., Simmonds P. G., Zlatkis A.: Anal. Chem. 40, 541 (1968) 30. Robinson C. F., Brubaker W. M.: US Pat. 2,959,677 (Nov. 8, 1960) 31. Roesler J. F.: Anal. Chem. 36, 1900 (1964) 32. Schnell E., Platz T.: Microchim. Acta 1969, 1285 33. Schnell E., Fuchs H.: Microchim. Actn 1972, 97 34. Schneider E. G.: Phys. Rev. 49, 341 (1936) 35. Sevtik J., Kr);sl S.: Chromatographia 6, 375 (1973) 36. Sharpe J.: Nuclear Radiation Deteclors, Methum, London 1955, p. 130 37. Vree P. H., Fontion A,: US Pat. 3,540,851 (Nov. 17, 1970) 38. Watanabe K.: J. Chem. Phys. 26, 542 (1957) 39. Williams H. P., Winefordner J. D.: J. Gas Chromatogr. 6, 11 (1968) 40. Yakovlev S. A.: Pribory i Tekhn. Eksperim. 7, 175 (1962) 41. Yamane M.: J. Chromatogr. 9, 162 (1962) 42. Yamane M.: J. Chromatogr. 11, 158 (1963) 43. Yamane M.: J. Chromatogr. 14, 355 (1964)

8.

The Helium Detector (HeD)

The helium and argon detectors, utilizing metastable atomic states for the ionization of eluted substances, are also included among ionization detectors. Although only the helium detector is discussed below in more detail, all the conclusions drawn are TABLE 8.1 THE EXCITATION POTENTIALS OF THE METASTABLE STATES OF THE RARE GAS ATOMS Potential [eV] Gas

- -~

-

ionization He He2 Ne

Ar

N;

24.5

21.5

15.76

Configuration

Ref.

21s 23s

40 40

2s4 7s5 1 Pl 3 PO 3 p2

17 12 34 34 21 40 40

excitation 70.6 19.8 -1 5 17.6 16.6 16.55 11.83 11.72 11.55 11.88

7

equally valid for the argon detector, as the two detectors differ only in the energy of the metastable atomic states formed (see Table 8.1). This type of detector is the only ionization detector operating in the multiplication region of the volt-ampere characteristic.

8.1

DETECTION MECHANISM

Primary electrons, emitted by a suitable /J-suurce, collide with a monatomic gas during thermalization and ionize or excite its atoms. Only monatomic gases can be used as carriers; in addition to helium and argon, neon has also been employed [5, 171. The literature also contains data on the formation of excited molecular states

134 of nitrogen with long life-times and with an energy of 11.88 eV [7]. The metastable atomic states formed de-excite in collisions with the eluted substance and, if the ionization potential of the eluted substance is less than the excitation potential of the metastable state, cause ionization of the eluted substance. In addition, the primary electrons take part in the ionization of the eluted substance, as d o accelerated secondary electrons when intense electric fields are employed. The processes that take place in the HeD can be described by the followingreaction scheme: He"'

+ ORG +

He +ORG+ f e

He"

+

He++ He

He"' -+

+ ORG e + ORG

+

e

+ p' + e + e'

(8.1)

(8.3)

-+

ORGf+ e

(8.4)

-+

ORG'

(8.5)

Reaction (8.1) leads to ionization of eluted substances, which may be permanent gases with the HeD or, generally, to ionization of any substance with an ionization potential lower than the excitation potential of the metastable state. This reaction, together with reactions (8.4) and (8.5), leads to an increase in the ionization current in the presence of an eluted substance. Reaction (8.2) results in de-excitation of an excited atomic state without producing ionization of the eluted substance. In general, this reaction occurs when the excitation potential of the metastable state is lower than the ionization potential of the eluted substance. This reaction has a high probability when the eluted substance contains polyatomic molecules. In the HeD, this reaction occurs only when neon is eluted, while in the argon detector there are substantially more possibilities owing to the low energy of the excited state. This ionization mechanism, known as the Penning effect, depends directly on the number of metastable atomic states. The probability of the formation of metastable states depends on the primary electron energy and on the intensity of the applied electric field. The reaction exhibits the highest cross-section for electrons with an energy of about 20 eV [25]; the cross-section decreases at higher colliding electron energies. The effect of the intensity of the electric field on the formation of metastable states is also maximal a t about 500 V/cm . torr. However, with monatomic gases at any electric field intensity, metastable excitation is more probable than direct ionization [33]. It has been found during investigation of the dependence of the extent of ionization on the pressure that the highest ionization current is obtained for N,, CH4, Ar, H, and 0, at pressures of 14-16 torr; the current was up to 60 times higher than at normal pressures [32]. Although the authors suggest the possibility of

135

another ionization mechanism, it can be assumed that the probability of de-excitation of metastable states (equation (8.2)) and of recombination of ionic species (equation (8.3)) is substantially higher at normal pressures and hence the ionization current decreases. 8.2

HeD SIGNAL

The HeD signal i s given by current caused by the ionization of the eluted substance by metastable states of helium or argon. In addition, direct ionization with primary electrons takes place and the HeD ionization current is described by the equation [35] I

=

AF exp re exp

z , . I . cs

(k,/ki

+ cs) ( u e +

-7,

. a . cs)

where A is the activity of the p-source, F is the Faraday constant, z , is the number of non-elastic collisions leading to the formation of metastable states, u, is the electron velocity, I is the electron free path, I d and ki are the rate constants for the disappearance of metastable atomic states and secondary electrons, respectively, a is the ratio of the probabilities of non-elastic collisions in the carrier gas and in the presence of the eluted substance, cs is the concentration of the eluted substance and re = ze . ( I/ue),where z , is the number of non-elastic collisions leading to the formation of ion pairs. From equation (8.6), it follows that the dependence of the HeD signal on the concentration of the eluted substance passes through a maximum and may be positive or negative. The magnitude of the signal is directly proportional to the activity of the radioactive source and to the contribution of direct ionization (equation (8.4)), and depends strongly on the intensity of the electric field. In the saturation current region, the signal is independent of the field intensity, while it increases exponentially in the multiplication region [30, 371. When the argon and helium detectors are compared, it follows that the term J [ k d . ue)/(ki . z , . a)] is greater for argon owing to its larger collision cross-section and thus a maximum signal depending on the concentration will be reached at higher concentrations with argon than with helium. 8.2.1

HeD background current

The HeD background current is determined by the ionization of argon or helium by primary electrons, by de-excitation of metastable states (equation (8.3)) and by the ionization of impurities present in the carrier gas. The magnitude of the background current follows from equation (8.6), i.e., it is directly proportional to the activity of the radioactive source, the intensity of the applied electric field (due to changes in the electron velocity, u,) and the impurity concentration.

136 Ionizable impurities (IP < E P M m ) affect the background current most markedly, causing an increase over the value corresponding to the pure carrier gas (Fig. 8.1) owing to direct ionization of the impurity by metastable atomic states. An impurity that forms metastable states with higher energy than the ionization potential of the carrier gas has the same effect, for example, when trace amounts of helium are present in the argon detector. This phenomenon has been verified by a number of experiments in which the background current decreased with increasing purity of the helium carrier gas [3, 191. Some workers [31, 461 obtained the opposite results; it is probable that the carrier gas was actually contaminated during purification.

--[scJ FIG. 8.1. The relative magnitude of the HeD background current for various carrier gases; 1 - a carrier gas with an ionizable impurity or an impurity forming metastable atoms; 2 - pure carrier gas; 3 - a carrier gas with a non-ionizable impurity [28].

Non-ionizable impurities, such as neon in helium or permanent gases in argon, cause a decrease in the background current because the partial pressure of the rare gas is decreased and hence the number of metastable atomic states formed also decreases [44]. Therefore, it follows that any impurity affects the HeD background current. It should be pointed out that impurities need not necessarily come from the gas cylinder; the carrier gas can be contaminated during passage through the chromatographic system by bleeding of the stationary phase. From this point of view, sufficient attention must be paid to selection of material for connections and thorough degassing of the detector and of all leaks is of prime importance. 8.2.2

HeD response

The He D response equals the sum of the background current and the ionization current of the eluted substance. The magnitude of the response and its character are determined by all of the interactions of the eluted substance (Fig. 8.2). The ionization current increases as a result of the ionization of the eluted substance by metastable states and primary and accelerated secondary electrons, and decreases due to ionization by electron capture. Impurities present in the carrier gas exert a substantial effect on the magnitude and character of the response. The response for the eluted substance in a pure carrier gas depends on its ionization potential. If ZP,,, is higher than El',,, then the eluted substance is not ionized by metastable states of the carrier gas atoms. As the partial

137 pressure of the carrier gas decreases during the elution, as does the M" concentration, the measured response is negative: this is true, for example, of neon in the HeD [ 3 , 121, or oxygen, nitrogen and methane in the argon detector [14, 351. The dependence of the magnitude of the response on the concentration of the eluted substance is depicted schematically in curve C in Fig. 8.3.

I

d

6

c.

ic

-con~.

FIG. 8.2. Schematic HeD responses, RHeDfor (A) low and (B) high electric field intensities (bci - background ionization current; icMm, icSand ice- ionizationcurrentscorresponding t o ionization with metastable atoms, primary electrons and accelerated secondary electrons, respectively; icECD - electron capture ionization current.

FIG. 8.3. The dependences of the HeD response, background current and ionization current on the sample concentration [28].

If the ionization potential of the eluted substance is lower than the excitation potential of the metastable atomic state of the carrier gas, the eluted substance is ionized. In a pure carrier gas, the response is positive and reaches a maximum depending on the concentration, as follows from equation (8.6). The dependence of the response on the concentration of the eluted substance corresponds to curve D in Fig. 8.3. The HeD response in an impure carrier gas is dependent not only on the quality and amount of the eluted substance but also on the properties of the impurities. It is necessary to evaluate the mutual relations of ionization and excitation potentials and to consider competing processes. The HeD response for ionizable impurities (curves A and B) and non-ionizable impurities (curves E and F) are represented schematically in Fig. 8.3. The background

current always decreases during the elution, as the probability of direct ionization of the carrier gas and formation of metastable states decreases [2, 8, 18, 36, 471. With increasing concentration of the eluted substance, direct ionization by primary P-particles plays a progressively greater role, but the cross-section for this mechanism is substantially lower than that for the Penning ionization mechanism; therefore, the HeD response decreases with increasing concentration of the eluted substance. Exponential increase of the signal with increasing concentration of the eluted substance has TABLE 8.2 THE RESPONSE CHARACTER OF THE ARGON AND HELIUM DETECTORS AS A FUNCTION OF THE ELUTED SUBSTANCE AND THE CARRIER GAS IONIZATION POTENTIAL; LINE 1 - ArD, LINE 2 - HeD ~

Eluted substance

IP

10.4 H,S 11.1 C,H, 12.5 0,

Carrier gas containing the following impurity ~

pure

H,S

C3H8

10.4

11.1

2 '

12.5

CO

H,

Ne

He

14.1

15.6

21.5

24.5

+- +- + - +++- +- i+- +- +- ++- f - + - +-

+++++- ++ - +i+++- +- + - +- +0 0 0 0 0 t - -1 - f- +-

-

-

-

-

f -

14.1 CO

-

-

f -

15.6 H,

15.7 Ar

-1-

21.5 Ne

-

24.5 He

+ -

+ 0

-

0

-

-

4- -

-

-

-

-

-

-

-

-

-

-

-

-

0

0

0

0

+

++ ++-

+-

+0

+0

a positive response reaching a maximum depending on the concentration of the eluted substance a negative response attaining a limiting value depending on the concentration of the eluted substance a positive response changing to a negative response depending on the concentrations of the eluted substance and impurity no response for low concentrations

139

also been reported; this effect is caused by too high an electric field intensity, so that ionization by accelerated electrons occurs. If an ionizable impurity is contained in the carrier gas, the shape and magnitude of the response are determined by the concentration of this impurity. The ionizable impurity consumes the metastable atomic states of the carrier gas and thus a positive response can be expected only for low impurity concentrations, when a sufficient concentration of excited states, M", remains in the effective volume of the detector. If the concentration of an ionizable impurity is sufficiently large, no free metastable states remain in the effective volume of the detector and a negative response is obtained, as the cross-section for ionization by electrons is small compared with that for ionization by metastable states. In this way, the negative response of the HeD for permanent gases and the gradual appearance of positive responses with decreasing impurity concentration can be explained [2, 20, 32, 471. This inversion of response is more frequent with the helium detector, as the probability of contamination of argon with ionizable impurities is lower (only impurities with IP c 11.7 eV are important). A detailed discussion of various dependences of the response can be found in the literature [27, 281. Table 8.2 gives schematically the response character for the helium and argon detectors with pure and impure carrier gases. 8.2.2.1 Linearity and linear dynamic range of the HeD The linearity and linear dynamic range of the HeD depend strongly on the experimental parameters. It follows from equation ( 8 . 6 ) that, under constant experimental

I

\

\

FIG. 8 . 4 . The dependence of log SHeDon the logarithm of the concentration of the eluted substance at various electric field intensities [22].

conditions, the log SHeDversus c dependence is linear with a slope of k'r, ,,/[(kdue)/ li,z,a . log A], equal to the HeD linearity, where k' is an experimental constant. The HeD linearity is markedly dependent on the intensity of the applied electric field. At

140 intensities correponding to the saturation current voltage region, the linearity is less than unity, while linearities larger than unity are obtained in the multiplication voltage region. The HeD dynamic range is limited by competing processes that take place in the detector. When the concentration of the eluted substance increases, the direct ionization mechanism becomes important and the response decreases. However, when a high electric field intensity is employed, the number of ionizing particles in the effective volume of the detector increases exponentially due to acceleration of slow electrons and the response increases correspondingly. An example of the log SArD versus log cs dependence is given in Fig. 8.4. It follows from the discussion of the character of the HeD response that the ArD linear dynamic range is broader than that of the HeD and that the linear dynamic range for positive responses is narrower than that for negative responses, usually not exceeding four orders of concentration. 8.2.2.2 Sensitivity and selectivity of the HeD The sensitivity of the helium and argon detectors is the highest of all ionization detectors, as the response corresponds to the sum of all direct ionization mechanisms. The sensitivity varies around 1 x lo-' C/g. On the other hand, the HeD and ArD selectivities are among the lowest as the detection mechanism is based on direct ionization by almost all types of excited particles. The HeD is a universal ionization detector, as all substances can be ionized, In the ArD, only some substances are ionized; when a pure carrier gas is employed, a positive response is obtained for eluted substances with IP < 11.7 eV, while substances with IP > 11.7 eV yield a negative response.

8.3

EFFECT OF EXPERIMENTAL CONDITIONS ON THE HeD SIGNAL

8.3.1

Carrier gas

Only monatomic carrier gases can be employed in the HeD and ArD, as only these gases form metastable atomic states. The character of the carrier gas determines the energy of the metastable state and thus decides whether the eluted substance will be ionized. The impurity content in the carrier gas significantly affects the maghitude and character of the HeD response, as discussed above. For this reason, many workers have dealt with the purity of the carrier gas [2-4, 9, 10, 12, 19, 31, 38, 46, 471. We feel that the most effective purification line is that discussed by Bourke et al. [3, 41. Most of the H,O, CO, and easily condensed gases are removed by molecular sieve 5 A at room temperature and at the temperature of liquid nitrogen. Nitrogen is removed by passing the carrier gas over hopcalite (a mixture of copper, manganese and their oxides) at 350 "C. The last stage again contains a molecular sieve at the temperature of liquid nitrogen, to remove the remaining H,O and CO,. In this way,

141 the concentrations of impurities were decreased to the following values (ppm): COz < 0.02; CH, < 0.05; CO < 0.06; O 2 < 0.07; H,O < 0.1; N1 < 0.2; H , < 0.5; Ar < 0.5; Ne about 1 to 10. When the effect of impurities on the HeD and ArD response is evaluated, the maximum concentrations of impurities that do not significantly affect the SHeDvalue are given [9, 19, 471. The lowest concentrations are required for electronegative impurities, e. g., oxygen and water [45], which significantly decrease the HeD response due to electron capture. The HeD background current changes with changing carrier gas flow-rate [lo, 12, 461. Experiments with very pure helium [3] showed a decrease in the background current by 10% for a change in flow-rate from 50 to 120 cm3/min. When the carrier gas contains an impurity, the background current first increases with increasing flow-rate and then slowly decreases [lo]. The response then becomes negative. Independence of the ArD signal of the carrier gas flow-rate from 20 to 100 cm3/min has also been observed [24]. 8.3.2

Construction of the helium and argon detectors

The geometry of the HeD detection space strongly influences the measurement results and is indirectly reflected in the detector nomenc!ature. The literature contains references to macro-, micro- and triode-detectors, the schemes of which are depicted in Fig. 8.5. It should be remembered that the free path of /?-particles is very short in

3

F

,-

,I

13

t4

4

14

FIG. 8.5. Scheme of the argon detector; (a) micro-detector with homogeneous electric field; (b) macro-detector with heterogeneous electric field; (c) triode argon detector; 1 - radioactive source and cathode, 2 - anode, 3 - column, 4 - rinsing gas, 5 - gas outlet, 6 - triode [23].

HeD and ArD. About 90:/, of non-elastic collisions occur within 0.4 mm of the surface of the /J-source [15]. It is thus obvious that, in a macro-detector, the electron released on ionization must pass through large distances in order to reach the collecting electrode. During this transport, the probabilities of recombination, capture, etc., increase and the signal is lower than that of a micro-detector [39].

142 The electric field intensity exerts a decisive effect on the detector performance. It has already been mentioned in the introduction that the HeD and ArD are the only ionization detectors that operate in the multiplication region of the volt-ampere curve. The electric field intensity reaches values as high as 4000 V/cm [13, 301 and secondary electrons are thus accelerated to energies capable of directly ionizing eluted substances (equation (8.5)). When an argon micro-detector is employed, intense electric fields are attained at lower voltages and the detector sensitivity is thus increased. Either homogeneous electric fields (see, for example, ref. 12) or, more frequently, inhomogeneous fields are employed. In order to suppress the space charge effect and to decrease the probability of recombination in inhomogeneous electric fields, designs containing a third electrode with a potential more negative than that of the anode have been proposed. The collection of cations in the effective detector space is thus improved and the background current and noise are decreased.

6

7’ -1+-

L

50

100

l

-

150

200 250 300

PCI FIG. 8.6. The dependence of SAID on the detector temperature at various voltages.

As sources of primary particles, b-sources are most frequently employed, although an a-source, 226Ra, has also been used. At present, 63Ni and 3H are used most frequently. These cources have activities of about 100 mCi; as follows from equation (8.4), the HeD response increases with increasing activity. The use of radioactive sources is discussed in Chapter 3. Metastable atoms are also generated by an electric discharge in neon [26]. The detector temperature affects the number of particles emitted and thus changes the HeD signal. Experimental results with the ArD are shown in Fig. 8.6.

143

8.4

HeD APPLICATIONS

The use of the ArD as an ionization detector is limited to substances with ionization potentials lower than 11.7 eV. It should be pointed out that the application of the ArD in permanent gas analyses is not based on the ionization mechanism but on dilution of the carrier gas. The use of helium as a carrier gas broadens the scope of sensitive ionization measurements to substances with high ionization potentials, especially permanent gases. It follows from the detection mechanism that all substances can be monitored using these detectors; the measuring sensitivity should increase with decreasing ionization potential of the monitored substance. However, the HeD and ArD are not often employed for monitoring organic substances with low ionization potentials because of the high demands on the purity of the carrier gas and the marked dependence of the response on the experimental conditions. The HeD has enjoyed the most widespread application in the gas chromatography of permanent gases; it has also been used in preparative gas chromatography [43] and in liquid chromatography [16, 421. The HeD is the most sensitive device with a wide linear dynamic range used for the analysis of trace amounts of permanent gases. The lowest detectable amount is' 500 times less than that for the thermal TABLE 8.3 T H E LOWEST DETECTABLE AMOUNTS WITH THE HeD (ppb) Helium detector

Electron mobility detector

Gas

co2 co 0 2

CH4 N2

HZ

[I21

[3 1

[391

1411

0.8 3 3 3.5 15 20

20

500 1000

600 600 2000 1000 200 1000

60 70 50 200 500

700 300 3000 1600

conductivity detector [ll]. The lowest detectable amounts of some permanent gases with various HeD designs are given in Table 8.3. The HeD has been employed for the determination of nitrogen oxides [l, 10, 14, 291 at concentrations of 0.2 to 5 ppb, H,S, SO, and COS at concentrations of 0.0001% [48], carbon monoxide [6], chlorinated hydrocarbons [111, aldehydes, alcohols, ketones and esters [24] and propane, for which a linear dynamic range of six orders of concentration was obtained [37].

144 8.5

LITERATCTRE

1. Beke R.: Ingeriieurs 1970, 5 2. Berry R.: Narrrre 188, 578 (1960) 3. Bourke P. J., Dawson R. W., Denton W. H.: J . Chromatogr. 14, 1387 (1964) 4. Bourke P. J., Gray M. D., Denton W. H.: J . Chromatogr. 19, 189 (1965) 5. Bourke P. J., Dawson R. W.: Nature 211, 409 (1966) 6. Bruner F., Ciccioli P., Rastelli R.: J. Chroniatogr. 77, 125 (1973) 7. Cermik V., Ozenne J. B.: I n t . J . Mass. Spectroni. Ion Phys. 7, 399 (1971) 8. Collenson E., Tood J. F. J., Wilkinson F.: Nature 204, 394 (1965) 9. Ellis J. F., Forrest C. W.: Anal. Chim. Acta 24, 329 (1961) 10. Goldbaum L. R., Domanski T. J., Schloegel E. L.: J. Gas Chroniatogr. 6, 394 (1968) 11. Hachenberg H., Gutberlet J.: Brennst. Chern. 49, 242 (1968) 12. Hartmann C. H., Dimick K. P.: J . Gas Chroniatogr. 4, 163 (1966) 13. Hartmann C. H.: US Pat. 3,417,238 (Dec. 17, 1968) 14. Hill D. W., Newel1 H. A.: J. Chroniatogr. 32, 737 (1968) 15. Hill D. W.: Chem. Abstr. 73. 115 954p (1971) 16. Huber J. F. K.: J. Chromatogr. Sci. 7, 172 (1969) 17. Jesse W. P., Sadauskis J.: Phys. Re?. 100, 1755 (1955) 18. Karmen A,, Giuffrida L., Bowman R. L.: Nature 191, 906 (1961) 19. Karmen A,, Giuffrida, L., Bowman R. L.: J. Chromatogr. 9,13 (1962) 20. Karmen A.: Advance in Chromatography, M. Dekker, New York 1966, p. 317 21. Knapp J. Z., Meyer A. S . : Anal. Chern. 36, 1430 (1964) 22. Lasa J.: Chem. Anal. (Warsaw) 13, 1255 (1968) 23. Lasa J., BroS E.: Chern. Anal. (Warsaw) 18, 825 (1973) 24. Lasa J., BroS E., KoSE Z., Mendyka B.: Chern. Anal. (Warsaw) 18, 1033 (1973) 25. Lloyd C. R., Weigold E., Teubner P. J. O . , Hood S . T.: J. Phys. B5, 1712 (1972) 26. Lobov G. D.: Radiotekh. Elektron. 12, 1777 (1967) 27. LukiE S., SevEik J.: Chromatographia 5 , 258 (1972) 28. LukaE S., SevEik J.: Chromatographia 5, 311 (1972) 29. Lutz M.: Nucl. Tech. Environ. Pollut., Proc. Symp. 1970 (Pub. 1971) 203 30. Meyer A. S., Knapp J. Z . : Anal. Biocherir. 33, 429 (1970) 31. Parkinson R. T., Wilson R. E.: J. Chroniatogr. 24, 412 (1966) 32. Parkinson R. T., Wilson R. E.: J . Chrornatogr. 37, 310 (1968) 33. Penning F. M.: Physica 5, 286 (1938) 34. Penning F. M.: Electric Discharges in Gases, Russian Iransl., Moscow 1960 35. Rotin V. A,: Zavod. Lab. 33, 1446 (1967) 36. Rotin V. A.: Ref. Zh. Khim. 1969, 3 D 82 37. Rotin V. A.: Ref. Zh. Khini. 1969, 3 D 83 38. Serpinet J.: Anal. Chini. Acta 25, 505 (1961) 39. Shanin M. M., Lipsky S . R.: Anal. Chem. 35, 467 (1963) 40. Shearer-Izumi W.: J . Phys. B 5, L 253 (1972) 41. Smith V. N., Fidiam J. F.: Anal. Chenr. 36, 1739 (1964) 42. Van Dijk J. H.: Brit. Pat. 1,292,754 (Oct. 11, 1972) 43. Villalobos R.: Chem. B i g . Progr. 64, 55 (1968) 44. Walsa J. T., McCarthy K. J., Merritt C. H., Jr.: J. Gas Chromatogr. 6, 416 (1968) 45. Warren B. C. H., Dalzell M. G.: Chem. Abstr. 74, 18 647d (1971) 46. Wiseman W. A.: Nafure 190, 1187 (1961) 47. Wiseman W. A,: Nature 192, 364 (1961) 48. Zielinski E.: Chem. Anal. (Warsaw) 14, 521 (1969)

9.

The Flame Photometric Detector (FPD)

The principle of flame photometry was first used in gas chromatography in 1961 [16] for the visual detection of chlorine-containing substances. In 1964, Huyten and Rijnders [42] constructed a flame photometric detector with a photomultiplier for the determination of halogens. Flame photometry was used in 1965 [18] for the determination of sulphur dioxide in air and a year later this method was applied to the analysis

FID

FPO I

FIG. 9.1. Scheme of a dual flame photometric defector; 1 3

-

interference filter, 4

-

-

fibre rod, 2 - lens,

photomultiplier.

of sulphur-containing organic substances [141. During the following years, increasing attention was paid to the development of flame photometric detectors, leading to construction of one of the most selective measuring devices employed in gas chromatography (see Fig. 9.1). 9.1

DETECTION MECHANISM

The flame photometric detector is based on measurement of the intensity of molecular emission of the fluorescence of hetero-atoms in the molecules of organic substances. Under gas chromatographic conditions, low-temperature flames are used for excitation to suitable energy states. However, the intensity of the emission spectrum was also measured in the UV region of the spectrum where a s . sparks [34], d.c. sparks [131 or high-frequency discharges [23,44,74] were used for excitation. This detector construction is characterized by high excitation energy, corresponding to emission of shorter wavelengths, and is denoted in the literature as a microwave detector [23].

146 The principal processes that take place in the flame photometer can be represented schematically by the following set of reactions: P(S, N, X) ORG

+ hv

HPO*(S,, CN*, InX*)

+

ORG

(94

P(S, N, X) ORG’

-4-

e

(94

+

HPO(S,,, CN, InX)

-{-

hv’

(9.3)

+

HPO(S, CN, InX)

iZ’

(9.4)

+

HPO*(S:,

-+

CN*, InX*)

Reaction (9.1) leads to the formation of excited groups by collision with a photon (fluorescence) or with an excited atom (chemiluminescence). Excited groups return to their ground state by emission of radiative energy (reaction (9.3)) or by nonradiative de-excitation with another substance, the so-called “quenching” reaction (9.4). However, even higher excited states can be formed, which, in the limiting case, result in direct ionization of the eluted substance (reaction (9.2)). It is obvious that only reaction (9.3) is important during monitoring of the FPD signal; its yield is decreased when unsuitable flame conditions are employed (reaction (9.2)) and when other organic compounds are present in the flame (reaction (9.4)). Considerable attention has been devoted to the identity of the emitting groups. Phosphorus-containing substances can be monitored at a wavelength of 528 nm or 565 nm, corresponding to emission by the HPO group [22]. This group has a simple spectrum (see Fig. 9.2) but practical measurements are subject to interferences from the C-C group, which has an emission peak at 516.5 nm [71] (see Table 9.1), causing distortion of the phosphorus signal and a decrease in the selectivity of the measurement.

rnmi FIG. 9.2. The emission spectrum of the HPO molecule in cold flames [22].

Sulphur-containing compounds are usually monitored at 395 nm. The emission spectrum corresponds to the S, group and consists of a number of peaks of various intensities, located in the region 365-415 nm [21]. The most intense lines lie at 384 and 394 nm. Other workers [63] assume that the measured intensity corresponds

147

to the fluorescence of the SO, triplet, caused by absorption of light at 340-400 nm, or to de-excitation of the SO, singlet. Lines corresponding to the SO (320 nm) and SH (328 nm) groups have not been detected under FPD conditions. Among interfering groups, the CN group with an eniission peak at 385 nm [61] and the C-H group TABLE 9.1 THE EMISSION PEAK WAVELENGTHS OF HETERO-ATOMS A N D GROUPS CONTAINING HETERO-ATOMS

Hetero-atom

Emitting group

P S

HPO S?

F CI

CaF CUCl lnCl CuBr InBr CUI In1 1

cs

Br

I

C

c-c C-H C-N

B Si Fe Pb Sn Sn As Ti Zr

BO Si

SnCl As0

TiCl ZrCl

Wavelength (n m)

526 394, 384 251.6 529.9 433, 435, (484, 488, 526) 350, 353, 360, 364 426, 434, (488) 372.7 417, 436, 457, (507) 409.9 206.2 512.9, 516.5, 554.1 436.4, 563.5 390, 431.4 388.3 385 251.6 373.5 405.8 485 358 500 544.9 564

Ref.

22 21 23 38 56 28 56 36 56 37 24 71 15 15 24 61 65 47 2 2 2 77

43 43 43

with an emission peak at 390 nm [15] are noteworthy. The CN group is not formed under FPD conditions and can interfere only when present in the structure of the eluted substance. The emission of the C-H group at 390 nm is of low intensity and requires a high temperature. Hence it follows that monitoring of sulphur using an FP D is very selective. The determination of the halogens using a flame photometric detector, is based on the reaction of the halogens with indium or copper. InX or CuX is formed in the flame

148

and the intensity of its emission is measured. The InCl emission spectrum (see Fig. 9.3) is composed of a number of peaks, the most intense being that at 359.9 nm [28]. The copper halide spectra depend strongly on the flame temperature. At temperatures lower than 840°C, intense lines appear at about 430 nm [56] (see Table 9.1). It is assumed that the Cu,X, dimer is the emitting particle with copper halides [49].

340

350

360 In4

370

FIG. 9.3. The emission spectrum of InCl [28].

FIG. 9.5. The decrease in the S , emission with increasing concentration of organic substances [70].

FIG. 9.4. Thechemiluminescence spectrum of CS, under various conditions [19]. Hydrogen and air flow-rates, respectively: A - 206,604; C - 234,540; D - 264,470; E - 296,400; F - 328,400 ml/min.

The monitoring of halogens in systems containing indium is not disturbed by interference from other groups. Measurements with copper-containing systems are affected by the emission of the C-C (436.4 nm) and C-H (431.4 nm) groups [15]. The flame photometric detector has been employed for the gas chromatographic determination of heavy metals; here the metal atom emission is measured rather than the molecular emission and thus requires a higher excitation energy. In general, emission by the test metal halides is monitored [2, 43, 771. As the emission peaks for individual metals differ considerably, interferences must be considered separately for

149 each case. The emission peak wavelengths of some metals are given in Table 9.1. The determination of nitrogen using the FPD makes use of the C N group emission at 385 nm [61]. This group is not formed at low temperatures, characteristic of the flame photometric detector, but its emission can be measured when it is present in the structure of the eluted substance, e.g., in acetonitrile, some pesticides etc. The magnitude of the excitation energy significantly affects the detection mechanism of the flame photometric detector. A characteristic property of the FPD mechanism is that molecular emission takes place a t low temperatures, in so-called cold flames [31, 321. The maximum emission intensity is reached in all instances a t temperatures below 850 "C, the maximum emission for the S, group being attained around 500 "C. This low flame temperature is obtained by using excess of hydrogen in the gaseous mixture. On increasing the molar fraction of oxygen in the burning mixture, the flame temperature increases, the emission spectrum intensity decreases and the emission peak positions change (see Fig. 9.4) [19]. The results of experimental studies indicate that the [H2] : [O,] molar ratio in the diffuse flame should be greater than 1.4 in order to obtain the niaximum emission intensity. Simultaneously, it is desirable that the overall gas flow-rate should be as low as possible. Under these conditions, the maximum emitted radiation intensity is obtained for sulphur, phosphorus and the halogens. Another process, which significantly affects the detection mechanism in the flame photometric detector, is non-radiative de-excitation, called quenching. This phenomenon is due to molecules of organic substances that absorb the energy of excited molecules during collision, with simultaneous changes in their vibrational and rotational states. The decrease in emission is an exponential function of the concentration of the organic substance and the resulting emission intensity is a mere fraction of the initial value (see Fig. 9.5) [70]. For example, the emission signal intensity is decreased by 25:4 in the presence of as little as 3;: of water [51]. Hydrocarbons substantially decrease the emission signal [20] and therefore in the determination of sulphurcontaining compounds in petrol it is preferable to determine sulphur after sample combustion, as C 0 2 exhibits a lower quenching intensity [58].

9.2

FPD SIGNAL

The magnitude of the FPD signal is determined by the intensity of the emitted light on the basis of the chemiluminescence process. The intensity of the emitted light I, is proportional to the concentraion c of the eluted substance:

I,

= @ . l0(l

- e-kbc)

(9.5)

where I , is the initial intensity of the excitation source, @ is the fluorescence yield, b is the optical path length and k is a proportionality constant.

150

O n the Taylor series expansion of the exponential term and neglecting all terms with exponents greater than 2, equation (9.5) can be written in the form

I,

=

2.3 @I,kbc

and log I ,

=

K

+ log c

where K = log 2.3 @Z,kb. However, emission as well as fluorescence processes take part in the FPD niechanism. The emitted light intensity, I , , can be expressed by the equations

I,

=

A . M , . hv . bkc 6

and log I ,

=

K

eE,,kT

+ K” log c

(9.9)

where A is the probability of radiative transfer, iM, is the molecular mass, hv is the energy of the emitted light, E, is the excitation energy, 6 is a constant; k is the Boltzmann constant and Tis the absolute temperature. It follows from the above equations for the measured intensity of the emitted light that the logarithm of the FPD signal is a linear function of the logarithm of the concentration. The flame photometric detector signal is given by the sum of the intensities of all of the emitting groups of the eluted substance and of interfering groups, less the quenched portion of the excited states. The overal signal for phosphorus, measured at 526 nm, can be described by the equation (9.10) A

It can be seen that the measured FPD signal is relatively strongly affected by the interfering groups. On the basis of the discussion of the detection mechanism, it can be concluded that the monitoring of phosphorus will be affected most by interfering groups, i.e., will be the least selective, but that the signal measured at 526 nm will be one of the largest. The monitoring of sulphur at 394 nm is not affected by interferences, i.e., it is selective, the signal measured being relatively small, as it corresponds only to emission by the S, group. During analysis of pesticides containing both phosphorus and sulphur, the corresponding intensities of the emitted light are subject to mutual interferences. Empirical equations representing this coincidence have been published [3 11, but they do not consider the interference from C-C groups. The overall response at 526 nm and 394 nm is given by the relationships

151

where K , - K, are constants dependent on the amount of sample injected and P and S are the numbers of atoms of phosphorus and sulphur per molecule, respectively. Bowman and Beroza [lo] determined 21 pesticides containing various numbers of phosphorus and sulphur atoms, using a dual FPD. They found that the signal measured at 526 nm (phosphorus) is invariably a number of times greater than the signal at 394 nm (sulphur). Among the pesticides studied, one did not contain sulphur (Gardona, C,,H,CI,O,P) and another did not contain phosphorus (Sulphanone, C,,H, . . CI02S). They found that, with identical amounts injected, the response signal for Cardona measured at 526 nm was 800 times grater than that measured at 394 nm, while the Sulphanone signal at 394 nm was 22 times greater than that at 526 nm. Thus, the emission of the interfering C-C group participates in the magnitude of the signal at 526 nni about 40 times more intensely than in the signal at 394 nm. From the values obtained, they calculated the ratio of the phosphorus and sulphur signals, (Rp/Rs)-1/2 . lo3. They found that this ratio is 5.2-6.0 for PS-type substances, 2.8 -3.3 for PS,-type substances and 1.7-2.3 for PS,-type substances. In our opinion, the variation of this ratio depends on the number of interfering atoms. We evaluated the dependence of the R,/R, signal ratio on the molecular weight of the pesticides studied and found that pesticides with approximately the same number of carbon and hydrogen atoms have approximately the same value of the Rp/Rssignal ratio (see Fig. 9.6). Similar conclusions were made for pesticides with other PS ratios.

30

Q?

1

1

200

250

I

300

I

1

350

400

molecular weight

FIG. 9.6. The dependence of the ratio of phosphorus and sulphur signals on the number of nitrogen atoms in the molecule.

9.2.1

FPD background current

The background current of the flame photometric detector is given by the chemiluminescence of the blue continuum of the oxygen - hydrogen flame, with a maximum

152 emission intensity around 450 nm, and by the emission of impurities present in the burning gases. The magnitude of the background current is determined by the photomultiplier voltage and temperature. Fluctuations in the latter parameter generally cause drifting of the background current and are encountered in long-term measurements. For this reason, the area around the photomultiplier is cooled by a gas stream or by water in a number of constructions [25], thus decreasing the FP D background current by up to ten times and enabling work to be carried out at higher detector temperatures. The background current varies within the range - lo-' A. FPD noise is caused primarily by instability of the high-voltage source and by fluctuactions in the voltages on individual photomultiplier dynodes. It should be borne in mind that, with a gain of l o 5 - lo', high noise (lo-'' - lO-'A) results, even if highly accurate resistors are employed in the photomultiplier voltage divider. Another source of noise is instability in the flow-rate of the burning gases, which causes variations in the continuum emission intensity. Hence, the FP D background current increases with increasing flow-rate of the combustion gases and with increasing flame temperature, i.e., with decreasing H, : 0, molar ratio. For measurements with the FP D it is suitable to employ low gas flow-rates with a small excess of hydrogen and to maintain the flame photometric detector at a constant low temperature. If these conditions are met, the FPD background current and noise are minimal. 9.2.2

FPD response

In agreement with equation (1.14), the response of the flame photometric detector equals the integral of the signal, S, over the elution time, A t : RFPD= JKS dt = S K ' . I , d t 360nrn

385nm

,

bc

525nm

-1

FIG, 9.7. The magnitude and character of the FPD response, RFPD,at 360, 385 and 525 nm; bc, background current; ec, emission current of InCI3, S,, HPO and C , molecules.

153 The intensity of the emitted light is directly proportional to the concentration of excited particles; thus, RFPD= K”

.c .At

From this equation, the dimension mol/sec follows for the response of the flame photometric detector. When describing the FPD response we can assume that the FPD background current does not vary during the elution, as variations in the hydrogen partial pressure during the elution time are negligible so that the intensity of the emission continuum does not change. When the detector is correctly designed, the signal measured at 360 and 396 nm is not affected by other emitting groups, while that at 526 nm is affected by the emission of the C-C group. The FPD response at 360, 396 and 526 nm is shown in Fig. 9.7.

9.2.2.1 Linearity and linear dj’naniic r a n g e of the FPD Various values were obtained from the slope of the linear log S versus log c plot, depending on the hetero-atom monitored. The FPD linearity for phosphorus is unity [73]; a predominant part of the intensity measured at 526 nm corresponds to the emission of interfering groups. The linear dynamic range at 526 nm is maintained over five orders of concentration. The linearity of the flame photometric detector for the halogens has also been found to be unity [l2, 731 at 360 nm, which is the wavelength corresponding to the InC1, emission. The linear dynamic range at 360 nm is equal to 3.7 orders of concentration. Measurements a t 394 nm on sulphur-containing substances yielded a value for the slope of the linear log S versus log c graph of 1.6-2 [39]. The higher value is due to the emission mechanism of the sulphur groups, S, or SO,. With both groups, an exponential increase in the number of emitting groups occurs, according to the schemes given below [63, 691. By three-body recombination with substance M, which is not a hydrogen atom, an excited state is formed [69]:

154

The molecule of SO, absorbs light with resultant excitation of a singlet and a triplet, which de-excite according to the following scheme [63]:

+M

'so,

3 S ~ ,

+ 3s0,

-+

-1- M

so, -t /2v

Hence it is clear that the intensity of the emitted light is given by the relationship

I,

= Kc'

where I is a constant representing the linearity of the flame photometric detector. The linear dynamic range values are given in Table 9.2. TABLE 9.2 LINEARITY AND THE LINEAR DYNAMIC RANGE OF THE FPD FOR SULPHUR-CONTAINING COMPOUNDS

Compound

SO, -S-,

- SH

Concentration range

ppm 0.1-100 ppm 0.1-1

S S - SH SO,

W

CH3SH

10- 1000 ppb 10-1000 ppb 30- 1000 ppb

Linearity

Linear dynamic range

1.6

I 03 1.5-2 1.69- 2 2 1.69 1.76 1.87

5 x 102

I o3 103 5 x 10,

Ref.

18 58 39 69 68 53,72 53 53

The value of I depends on the flame conditions [69]. Sugiyama et al. pointed out that the flame temperature increases and the emission intensity decreases with decreasing [H,] : [O,] ratio. The value of I is inversely proportional to the emission intensity, i.e., it decreases with increasing temperature. An increase in the flame temperature suppresses the chemiluminescence process, on which the S, group emission is based, and enhances interfering emission processes. It should be pointed out that the extent of interaction depends on the construction of the detector and that determined I values may differ considerably. Some workers have devoted attention to the structure of organic substances from the point of view of the bonding of the sulphur atom. They concluded that the sulphur emission signal in organic molecules decreases in the order EtSSEt > EtSEt > > EtSOEt > EtS0,Et > EtS0,Et [46]. The results of measurements on mercaptans

155 (EtSH, iso-PrSH, n-PrSH, tert-BUSH and n-BuSH) and sulphides (Me,S and MeEtS), for which the same magnitude of signal was found, to within 6% [68], agree with the above conclusion. Similar conclusions were made during calibration of the FPD for CS,, CH,SH, SO, and H,S, the signals of which did not differ substantially [67].

9.2.2.2 Sensitivity and selectivity of the FPD The sensitivity of a measuring device is defined as the change in the measured signal per unit concentration change, i.e., A S / A c . It follows from the discussion of the detection mechanism and the magnitude of the signal that the flame photometric detector is not equally sensitive for all monitored heteroatoms. The highest sensitivity is attained in measurements at 526 nm. It should be noted that other substances are also monitored at this wavelength. Interference from sulphur at 526 nm has been described in the literature. However, this interference is not caused by sulphur but by a carbon atom in the sulphur-containing organic molecule. Because of the nonselectivity of the emission signal at 526 nm, the investigator should always check whether a substance in an unknown sample emitting at 526 nm actually contains a phosphorus atom or should perform the measurement at 565 nm when the interference from the C-C group is very low. The FPD sensitivity at 394 nm (sulphur) and at 360 nm (halogens) is similar and about 40 times less than at 526 nm. The determinations at both these wavelengths are specific for compounds that contain sulphur and C N or a halogen. The selectivity of the flame photometric detector is greatly affected by the geometric arrangement of the optical section, and thus is different for various instruments. In the determination of sulphur it attains values as high as 10'.

9.3

EFFECT OF EXPERIMENTAL CONDITIONS ON THE MAGNITUDE OF THE FPD SIGNAL

9.3.1

Composition of the gases and their flow-rates

The composition of the combustion gases determines the flame temperature and thus also the character of the spectrum and the intensity of the emitted light. Molecular emission, excited at low temperatures, is utilized in the flame photometric detector. For this reason, a flame with excess of hydrogen is usually employed in the FPD. It is desirable that the [H2] : [O,] molar ratio should be greater than 1.4. Free hydrogen is partially consumed in the combustion of eluted substances, but a greater part cools the flame and hence optimizes the excitation. In the initial FPD constructions, large gaseous mixture flow-rates, of the order of litres per minute, were usually employed [18]. In general, large gas flow-rates are not suitable, as the flame tempera-

156 ture increases with increasing amounts of combustion gases, resulting in a decrease in the molecular emission. An increased amount of combustion gas, with a constant elution volume of the test substance, leads to a decrease in the concentration of the eluted substance in the effective volume of the detector and thus also to a decrease in the measured signal. Thus newer FPD designs use lower gas flow-rates, e.g., 80 nil/min of H, and 20 ml/min of air. The decrease in the air flow-rate is limited by the fact that a flame that contains excess of hydrogen is unstable and is easily extinguished. This frequently happens during elution of solvent from the column when the amount of oxygen drops below the nesessary level during solvent combustion. By decreasing the hydrogen and oxygen flow-rates, the continuum intensity is also decreased, the FPD background current decreases and a better signal-to-noise ratio is obtained. When oxidizing flames are used [61, 711, worse results are obtained than with reducing flames; on the other hand, oxidizing flames permit direct connection of the FID with the FPD. Nitrogen is generally used as the carrier gas. The FPD signal can be increased by using helium, owing to more efficient heat removal. However, Bowman [ l l ] did not observe any change in the FPD signal when using helium as the carrier gas, possibly due to too high hydrogen and air flow-rates. The carrier gas flow-rate is determined by the requirements placed on the separating part of the apparatus. In general the FPD signal increases with increasing carrier gas flow-rates, owing to a decrease in the flame temperature; this has been verified experimentally [331. Carrier gases have not been especially purified for use in the flame photometric detector, with the exception of so-called sensitized measurements [3, 201. This design led to substantial narrowing of the linear dynamic range of the detector [68]. Impurities in the carrier gas result in the quenching of excited states and thus may cause a decrease in the FPD signal. Water molecules are ten times more efficient for quenching than the permanent gases (N,, 0,, CO,, etc.). Saturated hydrocarbons and hydrocarbons with double bonds exhibit effects similar to that of water. Purification of the carrier gas is unimportant as much more water is formed in the flame space during combustion than is introduced in the carrier gas. 9.3.2

Detector temperature

The detector temperature exerts a decisive influence on the background current and noise in the flame photometric detector. As the photomultiplier requires a low temperature, it is necessary to keep only the part of the detector from the column outlet to the end of the burner at an elevated temperature and to maintain the other parts of the detector at as low a temperature as possible, using suitable insulation. Under these conditions, the most advantageous signal-to-noise ratio is obtained. With increasing temperature, the signal-to-noise ratio decreases and the FPD response thus also decreases [54, 551. On an incresase in the detector temperature from

157 100 to 160°C, the response to hydrogen sulphide and sulphur dioxide decreases by half [53]. A further increase in the detector temperature, e.g., to 250 "C [25], requires cooling of the detector body with water.

9.3.3

Construction

The flame photometric detector measures the intensity of emission spectra originating in the flame space. By placing an interference filter in the radiation path, the wavelength corresponding to the monitored hetero-atom is selected. The light passed through the filter is amplified by the photometer and the resulting current is recorded. Either diffusion or pre-mixed flames are employed in the flame photometric detector. Diffusion flames have lower temperatures and therefore are more suitable for monitoring the molecular emission of phosphorus, sulphur, the cyanide group or copper or indium halides. Under these conditions, the measurement is performed with a low background current and a low noise level. When a pre-mixed hydrogen air flame is used, the noise increases by as much as forty times [ 2 2 ] . Awe et al. [2] stated that, during the determination of heavy metal halides, they obtained identical results using flames with excess of hydrogen or excess of oxygen for the determination of Fe (373.5 nm), Pb (405.8 nm) and Sn (485 nm). The hydrogen flow-rate was 830 ml/min. Flames with excess of hydrogen are frequently extinguished during solvent elution. Thus burners [48] and control circuits have been modified for re-ignition of the flame "61. For the determination of halogens, a container with indium or copper must be placed in the flame reaction zone. These containers are either mounted directly on the FPD burner in a similar way to the TlDA construction, or two flames, placed one above the other, are used, the lower flame saturating the burning mixture with the metal and emission being measured only after combustion in the upper flame [28]. In addition to metal salts or amorphous metals in containers, gauges or wires made of these materials are also placed in the flame. The larger the surface area, the better the performance. The metallic parts of the detector must be situated so that their radiation does not contribute to the FPD background signal. A substantial portion of the FP D consists of the optical part before the interference filter. This part determines the selectivity of the device and increased attention should be paid to it. A necessary condition for proper functioning of the interference filter is that the Iight beam pass through the filter plane at right-angles. If this condition is not met, a broad wavelength interval is passed by the filter and costly filters with i.,,,~,, = 5 nm perform equally poorly as those with Arnaxl,* = 30 nm. Then particularly the determination of phosphorus suffers from considerable interference from the C-C group. The poorest measuring selectivity is obtained using mirrors. On the other hand, the highest signals are obtained owing to the low loss of light. The above difficulties

158 can be only partially removed by using a lens, as the flame is not a point light source. Efforts to achieve maximal selectivity have led to the use of fibre optics [61], using which the sperical image of the flame is converted into a radiating point (see Fig. 9.1). However, this design is less sensitive owing to loss of light through absorption. Successful design of the optical part is a necessary condition for the construction of a selective flame photometric detector. Some workers have employed devices with shielded flames [48]. A glass cylinder is placed around the flame, which absorbs wavelengths shorter than 370 nm, thus suppressing the detector background current. However, it should be pointed out that these wavelengths should not be passed by the interference filters if the detector is correctly designed. If the background current decreases due to shielding, then the interference filter passes a broad range of wavelengths, i.e., the device is not selective. Shielding decreases the emission intensity of the flame itself and of the hetero-atom monitored and the FPD signal is lower [53]. The choice of interference filter quality depends on the experimenter’s demands. If the flame photometric detector is to be used for the detection of hetero-atoms, then it must be as selective as possible and consequently an interference filter with a small Amnx,,2 value must be employed. On the other hand, if the FPD is to be used only as a single purpose instrument, e.g., for the determination of sulphur-containing value can substances in paper mills [27], an interference filter with a larger be used, as no interfering substances are present in the samples (the determination is not affected by substances that do not contain sulphur). By using a less selective device, an increase in the sensitivity is obtained.

FIG. 9.8. Scheme of the electric circuit of the flame photometric detector.

The photomultiplier is the measuring part of the FPD. Photomultipliers are selected according to their spectral characteristics, which determine the dependence of the gain on the wavelength of the incident radiation. The overall gain of the photomultiplier (up to lo’) is determined by the total voltage applied to the voltage divider. The gain is linear in wide voltage ranges and the value to be employed is selected according to the most advantageous signal-to-noise ratio, usually from 700 to 1200 V. The photometer circuit is shown in Fig. 9.8. It must be emphasized that the photomultiplier is

159 a very sensitive measuring device with a limited life-time. This life-time is determined by the magnitude of the current drawn and therefore it is necessary to prevent strong irradiation of the phot3niultiplier when it is switched on. The flame photometric detector can easily be connected with another detector. Dual FP Ds, one part measuring at 394 r.m (sulphur) and the other at 526 nm (phosphorus), have been described [lo, 311. Versino and Rossi [73] constructed a flame photometric detector that permits simultaneous measurement at three wavelengths corresponding to phosphorus (526 nm), sulphur (394nm)and chlorine (360 nm). The outputs are separated and yield two (three) recordings corresponding to phosphorus, sulphur and chlorine. This detector design is advantageous for pesticide analysis. The flame photometric detector ici also often connected with a universal detector, e.g., a J C D [36, 471 or an FID [3, 61, 711. Connection of the FPD with the FID is especially easy, as ionization of organic substances also occurs in flames. 9.4

USE OF THE FLAME PHOTOMETRIC DETECTOR

The importance of the flame photometric detector in contemporary gas chromatographic instrumentation is steadily growing, as shown by a number of reviews [39, 50, 45, 60, 64, 751. The FPD is used increasingly frequently in air pollution analysis and in the analysis of pesticides. In general, the FP D can be imployed for the analysis of substance5 that contain phosphorus, sulphur, a halogen or the cyanide group. In addition to these substances, the FP D is also used for the determination of heavy metals. The applications of the flame photometric detector are surveyed in Table 9.3. When the properties of the FP D are compared with those of other types of detector, it is found that the minimum detectable amounts are equal to [35, 591 or higher than those obtained with, for example, the TIDA or the ECD [ 7 3 ] . In all instances, a wider linear dynamic range and a better reproducibility is obtained with the FP D. The high selectivity of the flame photometric detector enables minimization or complete omission of purification procedures after the extraction of pesticides, thus considerably speeding up the analysis. The analytical results have excellent accuracy and precision (see Fig. 9.9). It can be seen from the example given in Fig. 9.9 that only halogen-containing pesticides are monitored in the unseparated mixture using the FP D, while the ECD monitors the sum of pesticides and interfering substances. It can be assumed that further development in the construction of flame photometric detectors will be directed towards improvement of their selectivity and that they will become important in qualitative determination of pesticides using tabulated response ratios, RPlR9 R P I R C b RSlRCI. The FPD is undoubtedly the most suitable detector for the determination of sulphur-containing compounds in air. No other detector permits the determination of sulphur dioxide, hydrogen sulphide, carbon disulphide, mercaptans, sulphides and

160 TABLE 9.3

SURVEY OF FPD APPLICATIONS AND CHARACTERISTICS Minimum detectable amount

Concentration range

Ref.

in air

10 PPb

10- 1000 ppb

53

in air

10 PPb 0.05 ppm 0.1 pprn

0-1 ppm

0.1 - 100 ppm

18 20 58

0.001 ppm

0.001- 10 ppm

67

10 PPm 0.003 pg 0.1 ppm

10- 1000 ppm

30, 72 59 29

Substance determined

SO29 H,S CH3SH SO2

cs2

Mercaptans sulphides

in petrol distillation fractions

SO,, H2S. CH3SH, CS, sulphides in air s o , , H2S in air so2 in exhaust Mercaptans, gases from sulphides thiophene petrol-burning engines in citrus oils Total sulphur in coal CS2 Pesticides SO2

in presence of 0 3 , H2S, NO,,

5 x lo-" 3 ppb ng level

1 - 6.4:: mole/sec

20 ppb

1-lo4 ng 0.1 - 500 ppm

26 44

5 , 7, 8, 9, 14,17, 3 1, 55,66, 62 51

co2

Parat hion, methylthion Malathion, ethion, fenthion, methyltrithion, phorate and a number of others PH3 in foodstuffs, air, water P in water P Pesticides

8 x lO-"g/sec 25 ng

5 Pg

0.2 ppb 40 Pg 0.05 ppb

48 25- 100 ng

10

4 1 31 74 5, 6. 7, 8, 9, 10, 14, 17, 62, 66

161 Table 9.3 (continued)

Substance determined

Disyston Malathion Tetraethyl pyrophosphate Methyl parathion DEPP DePPT Organo-halogens CCI, Chlorobenzene CCI, Organo-halogens, chlorobenzene, m-diiodobenzene, etc. Pesticides in milk, corn lindane, aldrin, D D T DDT, dieldrin, in butter etc. Lindane in foodstuffs Aldrin Insecticides Organobromides Organoiodides c6 F6

CH,CN Organometallics Fe, Pb, Sn B Si Cr in urine Ti, As, Zr Sn

Minimum detectable amount

Concentration range

Ref.

1.9 x 10-8g/sec 2 x IO-'g/sec 7.7 x 10-'Og/sec 1.1 x Io-"g/sec 2.3 x 10-'3g/sec 1.6 x 10-'3g/sec 5 Ncg 4. I 0 - 9mole/sec 10-6g 16 ppm

0.07 pg

71 lo5

50 50 16 44 61 20 106

11, 52

0.01-IOpg

11, 12, 52

0.23 pg

33

0.022 pg 1.1 x 10-"g/sec 0.0007 pg 0.01 pg 0.047 pg 10-6g

0.44 ng lo-"

73

mole

s x 103

0.01- 1.4 pg

2-20 ng 0-90 ng 1 o4

40 73 32, 34 36,S2 37 38 61 41 2 65 43 77

162 sulphoxides in concentrations of the order of 10 ppb after their separation. During analysis of such low concentrations in the atmosphere, the adsorption of gases on metallic parts of the instrument has been observed and these parts were thus replaced by PTFE [72]. Goretti and Possanzini [29] arrived at an interesting conclusion during Vegetable sample extmct

a) FPD halogen-mode

interfen? compoun s pesticides --_____

4A +.

I

4; J '.--_--_-__ J C

FIG.9.9. Analysis of chlorinated pesticides in a vegetable extract, performed with (a) the FPD and (b) the ECD [73].

determinations of sulphur-containing substances in exhaust gases from petrol engines; they found that the amount of sulphur-containing substances in the analyzed mixture is negligible in spite of the fact that these substances could be smelled. The flame photometric detector is also used for the determination of heavy metals. The method has been applied in clinical analysis for the determination of chromium in amounts of up to 90 ng in urine [57]. 9.5

LITERATURE

1 . Ackman R. G., Addison R. F.: Chem. Abstr. 75, 29 570v (1972) 2. Awe W. A., Hill H. H., Jr.: Anal. Chem. 45, 729 (1973) 3. Baldwin J. W.: Chem. Abstr. 69, 113 121h (1969) 4. Berek B., Westlake W. E., Gunther F. A.: J. Agric. Food Chem. 18, 143 (1970) 5. Beroza M., Bowman M. C.: J. Agric. Food Chem. 14, 625 (1966) 6. Beroza M., Bowman M. C.: Environ. Sci. Technol. 2, 450 (1968) 7. Bowman M. C., Beroza M.: J. Assoc. Off. Anal. Chem. 49, 1046, 1154 (1966) 8. Bowman M. C., Beroza M.: J. Agric. Food Chem. 15,465, 671 (1967) 9. Bowman M. C., Beroza M.: J. Assoc. Off. Anal. Chem. 50, 926, 940, 1228 (1967) 10. Bowman M. C., Beroza M.: Anal. Chem. 40, 1448 (1968)

163 11. Bowman M. C., Beroza M.: J. Chrornatogr. Sci. 7, 484 (1969) 12. Bowman M. C., Beroza M., Nickless G.: J. Chromatogr. Sci. 9, 44 (1971) 13. Braman R. S., Dynako A.: Pat. Ger. Offen. 1,900.309 (Sept. 4, 1969) 14. Brody S. S., Chaney J. E.: J. Gas Chromatogr. 4, 42 (1966) 15. Calcote H. F.: Ionization in High-Temperature Gases, Shuler K. E. (ed.), Academic Press, New York, London 1963, p. 107 16. Chovin P., Lebbe J., Moureu H. J.: J. Chromarogr. 6, 363 (1961) 17. Corley C., Beroza M.: J. Agric. Food Chem. 16, 361 (1968) 18. Crider W. L.: Anal. Chem. 37, 1770 (1965) 19. Crider W. L.: Anal. Chem. 41, 534 (1969) 20. Crider W. L., Slater R. W., Jr.: Anal. Chem. 41, 531 (1969) 21. Dagnall R. M., Thompson K. C., West T. S . : Analyst 92, 506 (1967) 22. Dagnall R. M., Thompson K. C., West T. S . : Analyst 93, 72 (1968) 23. Dagnall R. M., Pratt S. J., West T. S . , Deans D. R.: Talanta 16, 797 (1969) 24. Dagnall R. M., Smith D. J., West T. S . : Anal. Lett. 3, 475 (1970) 25. Dale W. E., Hughes C. C.: J. Gas Chromatogr. 6 , 603 (1968) 26. Darlage L. J., Block S. S., Weidner J. P.: J. Chromatogr. Sci. 11, 272 (1973) 27. Devonald B. H., Serenius R. S., Mchtyre A. D.: P u b . Pap. Mag. Can. 73, T 68 (1972) 28. Gilbert P. T.: Anal. Chem. 38, 1920 (1966) 29. Goretti G., Possanzini M.: J . Chromatogr. 77, 317 (1973) 30. Greer D. G., Bydalek T. J.: Environ. Sci. Techno(. 7, 153 (1973) 31. Grice H. W., Yates M. L., David D. J.: J. Chromatogr. Sci 8, 90 (1970) 32. Gunther F. A., Lopez-Roman A., Asai R. I., Westlake W. E.: Bull. Environ. Contam. Toxicol. 4, 202 (1969) 33. Gutsche B., Herrmann R. Z., Rudiger K.: Z . Anal. Chem. 241, 54 (1968) 34. Gutsche B., Herrmann R. Z.: Z . Anal. Chem. 242, 13 (1968) 35. Gutsche B., Herrmann R.: Z . Anal. Chem. 245, 274 (1969) 36. Gutsche B., Herrmann R.: Z . Anal. Chem. 249, 168 (1970) 37. Gutsche B., Herrmann R.: Z . Anal. Chem. 253, 257 (1971) 38. Gutsche B., Herrmann, R., Rudiger K.: Z . Anal. Chem. 258, 273 (1972) 39. Hartmann C. H.: Anal. Chem. 43, 113 A (1971) 40. Herrmann R., Gutsche B.: Analyst 94, 1033 (1969) 41. Hill H. H., Aue W. A,: J. Chromatogr. 74, 31 1 (1972) 42. Huyten F. H., Rijnders G. W. A.: 2. Anal. Chem. 205, 244 (1964) 43. Juvet R. S., Jr., Dubrin R. P.: Anal. Chem. 38, 565 (1966) 44. Kamada H., Kokubun N.: Bunseki Kagaku 17, 575 (1968) 45. McNair H. M., Chandler C . D.: J. Chromatogr. Sci. 11, 454 (1973) 46. Mizany A. I.: J. Chromatogr. Sci 8, 151 (1970) 47. Morrow R. W., Dean J. A., Shults W. D., Guerin M. R.: J . Chrornatogr. Sci. 7, 572 (1969) 48. Moye H. A.: Anal. Chem. 41, 1717 (1969) 49. Mulliken R. S.: Phys. Rev. 26, 1 (1925) 53. Natusch D. F. S . , Thorpe T. M.: Anal. Chem. 45, 1185 A (1973) 51. Okabe H., Splitstone P. L., Ball J. J.: J . Air Pollut. Contr. Ass. 23, 514 (1973) 52. Overfield C. V., Winefordner J. D.: J. Chrornatogr. Sci. 8, 233 (1970) 53. Pescar R. E., Hartmann C. H.: J. Chrornatogr. Sci. 11, 492 (1973) 54. Revel’skii I. A., Johnson V., Ilmoja K., Karavaeva V. G . , Loog E., Sirota T. S . : Ref. Zh. Khirn. 1972, 12 N 422 55. Revel’skii I. A., Jonson V., Ilmoja K., Bezov V. M., Karavaeva V. G., Loog E., Sovakova T. M.: Ref. Zh. Khim. 1972, 12 N 448 56. Ritschl R.: Z. Physik. 42, 172 (1927)

164 57. Ross R., Shafik T.: J. Chromatogr. Sci. 11, 46 (1973) 58. Rupprecht W. E., Phillips T. R.: Anal. Chim. Acta 47, 439 (1969) 59. Scaringelli F. P., Rehme K. H.: Anal. Chem. 41, 707 (1969) 60. Selucky M. L.: Chromatographia 4, 425 (1971) 6 1. SevCik J.: Chromatographia 4, 195 (197 1) 62. Shafik M. T.: Bull. Environ. Contam. Toxicol. 3, 309 (1968) 63. Sidebottom H. W., Badcock C. C., Jackson G. E., Calvert J. G., Reinhardt G. W., Damon E. K.: Environ. Sci. Technol. 6, 72 (1973) 64. Soerensen 0.: Haus Tech., Essen, Vortagsveroeff. 283, 34 (1973) 65. Sowinsky E. J., Suffet I. H.: J. Chrornatogr. Sci. 9, 632 (1971) 66. Stevens R. K.: J. Assoc. Of. Anal. Chem. 50, 1236 (1967) 67. Stevens R. K., O'Keeffe A. E., Ortman G. C.: Environ. Sci. Technol. 3, 652 (1969) 68. Stubbs R. C.: Chem. Abstr. 76. 94 3212 (1972) 69. Sugiyama T., Suzuki Y.,Takeuchi T.: J. Chromatogr. 77, 309 (1973) 70.Sugiyama T., Suzuki Y.,Takeuchi T.: J. Chromatogr. 80, 61 (1973) 71. Svojanovskf V., Nebola R.: Chern. Listy 67, 295 (1973) 72. Tourres D. A.: Chromatographiu 5 , 441 (1972) 73. Versino B., Rossi G.: Chromatographia 4, 331 (1971) 74. West C. D.: Anal. Chem. 42, 811 (1970) 75. Winefordner J. D., Glenn T. H.: Advan. Chromatogr. 5, 263 (1968) 76. Winnett G.: J. Chromatogr. Sci. 8, 554 (1970) 77. Zado F. M., Juvet R. S . , Jr.: Anal. Chem. 38, 569 (1966)

10.

The Coulometric Detector (CD)

Most of the measuring devices employed in gas chromatography are based on the monitoring of various physical properties of eluted substances, e.g., thermal or electric conductivity, ionization potentials and luminescence. The signal of the coulometric detector alone corresponds to changes due to a chemical reaction of the eluted substance. For monitoring the course of the chemical reaction, an electroanalytical method known as coulometric titration is employed. Using this technique, the reagent is continuously regenerated and the electrolytic current is then a measure of the chemical reaction kinetics.

10.1

DETECTION MECHANISM

The coulometric detector is based on monitoring a chemical reaction between an electrolyte, the composition of which is changed in the reaction space, and the eluted substance; this principle was first employed in gas chromatography in 1957 [26], when organic bases, acids, aldehydes and ketones were titrated with electrogenerated Hf or OH- ions. However, the properties of substances studied by gas chromatography are very variable and the substances often have low reactivity. Therefore,

air H2 I

I column

FIG. 10.1. Scheme of connection of the flame ionization and coulometric detectors.

pre-treatment is sometimes required before the determination; the eluted substance is burned forming definite products whose reactivity is known. This method was applied in 1960 for the determination of chlorine-containing substances [lo] and later for the determination of sulphur [2] and nitrogen [30] in organic substances. The combustion products enter the coulometric cell reaction space and react with the

166

electrolyte. The changes accompanying this chemical reaction are monitored by the detector indication system, which operates the generation system renewing the initial state in the system; the generating current is thus a measure of the changes being compensated (see Fig. 10.1). This mechanism of coulometric detector operation can be represented by the following scheme: Combustion space: CI(S, N) ORG

+ 0,

-+

CI,(SO,, NO,)

+ CO, +

HZO

(10.1)

Reaction space: CI,+ SO,

3Br-

-+

Br;

+ 2CI-

+ 2 [CuBr4l2- + 2 H,O

N O z + 3 Br - + 2 H +

(10.2) -+

+

2[CuBr,]-

+

Br;+

+ SO:- + 4 H f + 4Br+ H,O

NO

(10.3)

( 10.4)

Electrolytic generation on the working electrode: (10.5)

[CuBr,]’-

+ 2e

+

[CuBr,]-

+ 2 Br-

(1 0.6)

Re-establishment of the initial state in the reaction space: Br;

+ 2[CuBr,]- + Br-

+

(1 0.7)

2 [CuBr,lZ-

TABLE 10.1 COMBUSTION PRODUCTS I N OXIDATION A N D REDUCTION MODES Combustion products Hetero-atom

CI(Br, I)

S N a

reduction mode

oxidation mode

HCl(HBr, HI) H2S NH,

so,

CI2(Br2, I,) b,

a

so, a

N,, NO

The combustion product has oxidizing ability The combustion product has reducing ability

’, NO,

a

167 The first reaction in the coulometric detector mechanism can vary. If excess of hydrogen is used, the products have reducing properties [27, 401 (see Table 10.1). Cathodic oxygen reduction in an alkaline medium is sometimes utilized [18, 331; a silver electrode is employed as the cathode and a lead electrode as the anode. The reduction of oxygen can be described by the scheme: O + H z O + 2e + 2 0 H -

Pb

Pb2+

-+

Pb2+

(10.8)

+ 2e

+ 30H-

-+

(1 0.9)

PbO(0H)-

+ H,O

(1 0.10)

As follows from the described coulometric detector mechanism, it is possible to monitor reaction (10.1). A device known as a reaction coulometer, in which a constant concentration of oxygen is maintained, has been proposed [7]. The amount of oxygen consumed during combustion of eluted substances is electroanalytically regenerated to obtain the initial value. Similarly, hydrogen has been regenerated in a coulometric system working under reducing conditions [27]. In quantitative evaluations, it is assumed that the reaction products are CO,, H,O and oxides or hydroxides of hetero-atoms, depending on the conditions in the flame. The reaction stoichiometry can be expressed by the equation C,H,O,

+ x0,

=

aCO,

+ (b/2)H,O

where x

=

a

+ (1/4)b - (1/2)c

TABLE 10.2 THE EXTENT OF CONVERSION OF SO, UNDER DIFFERENT CONDITIONS [29] Temperatureof the combustion tube ["C] 550 600 650 700 750 850 950

SO, conversion

70 80 91 93 89 74 63

168 In addition to acid-base and complexing properties, redox properties are very frequently utilized and are employed in most coulometric detector constructions for the determination of sulphur and chlorine. However, combustion often does not yield a single product containing the particular hetero-atom, but a mixture of product with various properties (see Table 10.2). Under certain combustion conditions, sulphur dioxide and trioxide [6, 291, hydrogen chloride and chlorine or nitrogen and nitric oxide [ll, 341 will be present among the combustion products. As substances with various properties enter the reaction space, the results of the determination of these hetero-atoms are usually lower than the theoretical values. For the determination of nitrogen, the hydrogenation reaction on a nickel catalyst can be used advantageously [30]. Under these conditions, only NH, is formed and is titrated with generated H f ions. The combustion products react with the electrolyte in the reaction space of the coulometric cell. During analysis of unknown samples, the initial gaseous mixture contains products with both reducing (e.g., SO2 and NO) and oxidizing (e.g., CI, and NO,) properties. Therefore, proper coulometric cell function requires that the reagent be both reducible and oxidizable. Other reaction types, such as acid-base or precipitation reactions, are less advantageous for detector application, as they provide very limited information about the analyzed sample. So far, a halogen halide system has been most frequently used for the determination of sulphur, and the Ag' - Ag system for the determination of halogens [12]. The most commonly used commercial instrument, produced by Dohrman Instruments Company is constructed to employ these two systems [15]. Another parameter which strongly affects the operation of the coulometric detector is the course of the electrode reaction. The substance which re-establishes the initial electrolyte concentration in the reaction space must be formed with 100% current efficiency at the working electrode. If another reaction takes place on the electrode simultaneoiisly (a side-reaction), then the electrolysis current cannot be monitored quantitatively. From this point of view, the frequently employed halogen - halide system are not suitable, as the halogen (iodine, bromine) electrode reduction does not proceed with 100% current efficiency. It is therefore preferable to employ the copper (I) - copper (11) system in the presence of excess of bromide ions [37]. The [CUB~,]~-/[CUB~,]-+ Br;/Br- system is reduced and oxidized with 100% current efficiency at current densities up to 5 mA/cm2. The bromide complexes of copper ions are stable and hence this system is also stable even in an air stream [25]. Detectors containing solid reagents have been described [4, 9, 13, 141. They are based on the electrochemical cell studied by Wagner [22,44] and have been used for the determination of halogens and sulphur. Free oxygen interferes in the measurement. In general, coulometric detectors have not enjoyed wide application in gas chromatography instrumentation. On the other hand, they have been very important in analyses for sulphur dioxide and chlorine in the atmostphere and most commercial analyzers for these pollutants employ the coulometric detector principle.

169

In addition to constant-potential and constant-current coulometric methods, polarography with the dropping mercury electrode has also been employed in gas chromatography [17, 23, 471, as has pulse polarography in liquid chromatography [28]. The necessary condition for the application of these methods is the polarographic activity of the eluted substances, i. e., their electrode oxidizability or reducibility. However, among the large number of organic compounds, only certain groups, e.g., hydrazones and thiols, have this property. The usefulness of this method is also considerably affected by the rate of the chemical process; when the rate is slow, the separation efficiency is not adequately utilized. Therefore, it is more advantageous to employ sample combustion.

10.2

CD SIGNAL

The magnitude of the coulometric detector signal is determined, for example, by the magnitude of a change in the equilibrium potential of the reaction system, due to the chemical reaction. The coulometric detector compensates for these changes by electrolytic production of the reagent until the initial state is re-established. The electrolytic current measured is proportional to the chemical changes that take place. This proportionality is quantitatively described by the two Faraday laws, which can be expressed by the relationship QM w=-=nF

I.t .M nF

(10.11)

where W(g) is the mass of the converted substance of molecular weight M (g/mole), n is the number of electrons exchanged when current I (A) passes through the electrolyte for time t (sec), F being the Faraday constant (96, 500 C/mole). It follows from equation (10.11) that the signal of the coulometric detector is the measured electrolytic current that is proportional to the number of moles converted in time t :

(10.12) As modern technology readily permits the measurement of times of the order of mole/sec can be determined seconds and currents of the order of 10-’A, about using the coulometric detector. It follows from the mechanism of the operation of the coulometric detector that substances with identical properties yield identical signals. Thus the signal due to sulphur dioxide will be increased by that of nitrogen oxide and chlorine will be determined together with nitric oxide. The coulometric detector signal is affected by interfering substances and can be described by the equation (for the determination of

170 sulphur)

SF = K . cS + K ' . c,.

(10.13)

where cs and cN are the concentrations of sulphur and nitrogen, respectively.

10.2.1

CD background current

The background current of the coulometric detector or analyzer is given by the content of impurities in the carrier gas which have oxidizing or reducing properties and by the reaction of atmospheric oxygen with the electrolyte. As coulometric detectors employ compensation circuits, the background current is zero. Under gas chromatographic conditions, the effect of impurities in the carrier gas can be eliminated either by using sufficiently pure gas or by introducing a purification line. Electrolytic reaction with oxygen occurs especially with the iodine - iodide system, iodide being oxidized to iodine by excess of oxygen from the combustion system or, with analyzers, by reaction with atmospheric oxygen. With other redox systems, an equilibrium is soon established and the system then appears to be stable. Noise occurs as a result of fluctuations in the indicating circuit of the coulometric detector. In all detector constructions, electroanalytical indication methods are employed. In indicator electrodes are placed in solutions through which large volumes of gases pass, in the vicinity of the gas inlet so that the time constant of the device is as low as possible. Concentration inhomogeneity around the electrode leads to coulometric detector noise. Noise in coulometric analyzers can be suppressed electronically by increasing the time constant; the modification is suitable as these analyzers monitor long-term variations in concentration. 10.2.2

CD response

The response of the coulometric detector corresponds to the signal SCDintegrated over the elution time, A t . As the signal is proportional to the concentration of the eluted substance, RCD= K " . c . A t

(10.14)

The response of the coulometric detector corresponds to changes in the concentration of the eluted substance. The background current of the coulometric detector does not change during elution, as the amount of free oxygen in the detector, which determines the equilibrium conditions, is in large excess compared with the eluted substance.

171

If the eluted substance contains more than one hetero-atom, then the response of the coulometric detector corresponds to their sum (see Fig. 10.2). 10.2.2.1 Linearity and linear dynamic range of the CD

The linearity of the coulometric detector is unity for all hetero-atoms. At higher concentrations, deviations from linearity occur owing to side-reactions of the generated reagent. The linear dynamic range of the coulometric detector is not very broad; a range of three orders of magnitude has been reported.

1

-t

FIG. 10.2. The magnitude and character of the response of the coulometric detector, RcD, for a substances containing sulphur and chlorine; bc - background current, gcs - generation current for sulphur, gccl - generation current for chlorine.

10.2.2.2 Sensitivity and selectivity of the CD

The coulometric detector is a very sensitive device, a change of lo-' mole corresponC, i.e., a change of 1 mA/sec. The high sensitivity of the ding to a change of coulometric detector permits the attainment of high precision. The selectivity of the coulometric detector depends on the detection reaction mechanism. If a redox reaction is employed, then all substances that have reducing properties give similar results, as do all substances that have oxidizing properties and the two groups are qualitatively distinguished by the polarity of the signal with respect to the background current. For the determination of nitrogen-containing substances, a reducing medium in the combustion tube is successfully employed in the presence of a catalyst. Under these conditions, all nitrogen-containing substances are converted into ammonia, which is then determined by electrolytically generated protons. When the correct conditions are selected, only nitrogen undergoes the reaction and the device operates specifically.

172

10.3

EFFECT OF EXPERIMENTAL CONDITIONS ON THE MAGNITUDE OF THE CD SIGNAL

10.3.1

Gas flow-rate

With the coulometric detector, it is necessary to burn the eluted substances before they enter the coulometric cell. The composition of the gaseous mixture is determined by the manufacturer, who selects the composition of the reaction solution considering the qualitative properties of the combustion products. The amount of gases passed does not affect the detection mechanism but does influence the detector operation. The gaseous mixture is damp after passing through the solution, i.e., moisture is gradually evaporated in the reaction space, with consequent changes in the composition of the electrolyte, leading to an increase in the noise. A continuous supply of electrolyte is especially necessary with continuous analyzers. The electrolyte must be added at daily to weekly intervals with flow-rates of 0.5 to 1.5 I/min.

10.3.2

Construction of the detector

The eluted substances are converted into products with known properties in the combustion space of the coulometric detector. Combustion usually takes place in combustion tubes of various volumes (up to 200 ml). It is evident that when the volume of the combustion space is larger than the difference in the elution volumes of two successive substances, a mixture is again formed in the combustion space and the separation process is negated. In addition to combustion tubes, a pyrolyzer has also been used [16, 311, for example, for the determination of toxic substances in the atmosphere at the parts per billion level, but the results were similar to those obtained with combustion tubes. These designs are not advantageous and therefore combustion tubes have been replaced by the FID burner [37], which permits simultaneous recordding of both universal and specific responses. In this way, the dead volume is considerably decreased and the risk of re-mixing the separated components is less. The same criteria relate to the reaction space of the coulometric cell. A small volume is desirable, as concentration changes in a small volume are larger than in a large volume and consequently the changes in the indicating circuit of the coulometric detector are greater. The original device, employing an electromagnetic stirrer, had a volume of about 40 ml [lo]. In later constructions, the electrolyte was stirred by the passing gas. The electrolyte circulates in the reaction space, the volume of which was decreased to 5 ml [37,41]. By excluding the combustion tube and using a coulometric cell with a small reaction space (see Fig. 10.3), a low time constant and undistorted elution curves are obtained; with a detector effective volume of 5 ml and a gaseous mixture flow-rate of 300 ml/min, the coulometric detector delay is 1 sec.

173 A number of methods can be employed for indicating changes in a coulometric analyzer [48]. Potentiometry is generally used because of its advantageous signalto-noise ratio. In addition, amperometry and biamperometry have been tested, but these methods invariably exhibited a lower signal-to-noise ratio than potentiometry.

j+% - -k A7

*

-_ _

+%,

--?+--

9

SECTION 7

B 6 3 5

A-A

2 4

9 1 SECTION

z

,8

FIG. 10.3. Scheme of a coulometric cell; 1 - body of the cell, 2 - reaction space., 3 - vent, 4 - inlet for the gaseous mixture, 5 - working electrode, 6 - indicating electrode, 7 - reference electrode, 8 - auxiliary electrode, 9 - frit.

The reaction taking place is followed using an indicator electrode whose potential changes correspond to concentration changes in the solution. Variations in potential in the indicating circuit are amplified and used for control of the generating current and consequently of the amount of substance added to the solution. The magnitude of the generating current is recorded and corresponds to the elution curve. The indication and generation spaces are separated by a capillary [15] or by glass frits and membranes [16, 371. The cathodic and anodic electrode processes need not necessarily affect one another; then it is unnecessary to separate the electrode spaces and it is possible to decrease the number of electrodes [18]. The electronic part of the coulometric detector (see Fig. 10.4) can be a source of distortion of elution curves. Most instruments operate with a proportional d.c. current source powered by a 15 V source. With an electrolyte conductivity of 10-3Q-'

174

the maximum generation current is 15 mA. Thus more than lo-' mole/sec cannot be eluted from the column. This condition is not always fulfilled and the elution curves are flat at the top (see Fig. 10.5). The beginning of the elution curve corresponds to the beginning of the elution of the substance from the column, but the end of the curve does not necessarily correspond to the end of the elution. + 15V

P

i I I I

I I

I

I

I b d L-----

-15V

-15V

FIG. 10.4. Scheme of the electronic part of the coulometric detector.

FIG. 10.5. Chromatogram of 2 p1 of the mixture of (CH,),SO (3) and C,H,CI (2) in acetone (1).

With continuous analyzers, the concentration level of, for example, sulphur dioxide is monitored, the concentration changes not exceeding one order of magnitude. Therefore, situations in which the generating current is insufficient for re-establishment of the initial conditions do not occur.

175 10.3.2.1 Bius

It is impossible to vary a large number of parameters with the coulometric detector, similar to analyzers of airborne pollutants. The composition of the reaction solution is determined by the manufacturer and cannot be changed. However, the composition of the solution determines the sensitivity of the detector towards oxygen in the gaseous mixture. After filling the detector reaction space and introducing the gaseous mixture, the potential of the system increases and attains an equilibrium value. This value is reached within 10 to 15 min. After attainment of the equilibrium state, a value is set by a compensation potentiometer so that no electrolytic current passes through the circuit. Thus the zero position is adjusted in the indicating circuit and changes caused by the chemical reaction are then either positive or negative with respect to this value. If the background current of the coulometric detector is large after adjustment of the compensation potentiometer, then the adjustment is incorrect and must be changed.

10.3.3

Temperature

As the reaction in the coulometric detector or analyzer proceeds in the liquid phase, the entering gases must be absorbed in the electrolyte. The solubility of gases, however, depends significantly on temperature, decreasing with increasing temperature. In the coulometric detector, the temperature of the entering gaseous mixture is approximately constant and relatively high. With continuous-measurement coulometric analyzers, the temperature of the entering gaseous mixture depends on the time of day. At night, the ambient temperature is lower than during the day and the results of the determination of SO, may vary accordingly. More recent devices, e.g., the Philips SO, Monitor contain a thermostat to remove this drawback.

10.4

APPLICATIONS OF THE CD

The coulometric detector was originally constructed for analyses of sulphur-containing compounds in oil distillation fractions. Later, it was applied in pesticide analysis. Examples of the use of the detector are given in Table 10.4. It should be noted that the coulometric detector is not employed very frequently in practice. For the analysis of sulphur and chlorine-containing substances, the flame photometric detector, which has parameters comparable with or better than those of the coulometric detector [5, 32, 361, has been used more frequently, also because of its advantage of easier operation, similar to the electrolytic conductance detector [12]. For example, the minimum detectable amount for the flame photometric detector is three orders of magnitude smaller.

176 The use of the coulometric detector is advantageous in determinations of nitrogencontaining substances because of the high selectivity of the determination of nitrogen as NH,. TABLE 10.3 MAXIMUM PERMITTED CONCENTRATIONS OF ATMOSPHERIC POLLUTANTS (ppm) [46]

Industrial Po'1utant

atmosphere (USA)

co

100 3.0

HF NO2 SO2 0 3

5.0 5.0 0.1

Non-industrial atmosphere (USSR) maximum instantaneous

5 0.04 0.15 0.20

maximum all-day

2 0.015 0.05 0.06

The coulometric detector has significant application in analyses for atmospheric pollutants. The maximum permitted concentrations of pollutants in the atmosphere are specified by national laws; these are divided into industrial and urban atmosphere categories. For airborne pollutants in cities, instantaneous maximum concentrations and maximum daily average limits are specified. It can be seen from Table 10.3 [46] that the analyzers must operate in an interval of several orders of pollutant concentrations. Single-purpose coulometric analyzers have been designed for the determination of sulphur dioxide and chlorine. The air or gaseous mixture analyzed is passed through the reaction solution and the output signal is stored at the measuring post or is transmitted to the control post by means of a telemetric system. Continuous methods of monitoring the concentrations of pollutants in the atmosphere are the only possible means of detecting critical events and will find progressively greater application. Determinations of sulphur dioxide have been carried out over a wide concentration range, 0.1 - 150 mg/m3. The determinations are not subject to a systematic error if the copper(1) - copper(I1) system in excess of bromide is employed. Values systematically lower by 1 to 10% were obtained with the iodineliodide system. Similar results were obtained with the Dohrmann cell, where the systematic error amounted to 15-20% ~361. The determination of chlorine with the coulometric detector has been carried out in gaseous mixtures with concentrations from 0.1 to 150 mg/m3 of C1 (see Fig. 10.6). The determination did not exhibit a systematic error. It was found during the applica-

177 tion of this method to atmospheric analysis that the amount of free chlorine is very small and that the intense smell observed was probably caused by chlorinated substances. Chlorine thus bound cannot be determined by the coulometric detector unless it is previously liberated as free chlorine. It has been found that the FPD is preferable for this type of analysis, as all chlorine-containing substances react in the same way in the FPD.

START

<@ REPRlDUCl6lLlN of REPEATED MEASUREPENTS 10 rng C~,/m’ introduced

START

I?--==@,

FIG. 10.6 Repeated determinations of 10 mg/m3 of chlorine with a coulometric analyzer.

In view of the considerable toxicity of nitrogen oxides, an attempt was made to determine them with a coulometric analyzer. It was found that the analytical results are strongly affected by the flow-rate of the gaseous mixture and that the reaction is not quantitative [38]. It can be assumed that the discrepancies are caused by the unclarified stoichiometry of the reaction of nitric oxide with the absorbent [35]. Moreover, the low solubility of NO and NO2 in water plays a role [8]; a great part of the studied oxide thus remains in the gaseous phase. The coulometric detector has also been applied in liquid chromatography [21, 28, 42, 431. The potentiostatic circuit is employed and minimum detectable amounts at the picogram level were found. For determinations of metal ions, halides, amino

178 TABLE 10.4 EXAMPLES O F THE APPLICATIONS OF THE COULOMETRIC DETECTOR

Substance determined

Lowest detectable amount

Material analyzed

Linear dynamic range

Ref. __

2 2 2 2 2

HZS SO, CH3SH CH3SCH3 CH3SzCH3 Benzothiophenes, petrol dibenzothiophenes, distillation one-ring thiophenes fractions (CH,),SO

9x

SO2

1.5 X lo-', 0.1 P d l

SO2

at mosphere nitrogen

cs2 Su1phur:containing substances Mercaptans, hydrocarbons sulphides, disulphides Sulphur 2-Chlorobenzoic acid Chlorobenzene gaseous mixtures CI2 atmosphere in Halogenated hydrocarbons submarines, bathyscaphes and satellites Chlorinated substances CH31, C,H,I CHBr, CCI, Chlorpicrin Trichloroethylene Pyridine, pyrrole, indole N-containing substances NO, NH, NO2

g/sec mole/sec

29

2

37

3

38 36 24,41 41

3

39 23

3

40

3

37 38 45

14

0.03 pg/l 10-6:< 2~ 1 0 - 5 %

10 ng 1 0 - l ~g. equiv/sec

g/sec 0.01 mg/m3 10 ppb

6 X lo-', atmosphere

3

mole/sec

13

31 31 31

2 ppb 7 PPb 300 ppb 3000 ppb 600 ppb

30

hydrocarbons nitrogen gaseous mixtures

-

23

1 0 - l ~g equiv./sec 10-6x

4

24,40

0.1 mg/m3

2

34,38

179 Table 10.4 (continued)

Substance determined

Acids, aldehydes, ketones H,, 0 2 , CH, He, Ar, Ne, N,, NO,, CO2 PH3 Pesticides Amino acids, carboxylic acids, phenols, carbohydrates

Material analyzed

Lowest detectable amount

Linear dynamic range

Ref.

26

5x 10-~m1

9 20

oxygen food, air, water liquid chromatography

5 ng pg level 5 x lo-''

5 3

mole

12 43

acids, carboxylic acids, phenols and sacharides, the column is packed with an ion to 5 x lo-'' mole. However, exchanger and the detection limits vary from 5 x with this instrument it is necessary to vary the composition of the reaction electrolyte with changes in the type of substance. 10.5

LITERATURE

I . Aavik H. E., Kabun A. V., Kallasorg R., Revel'skii LA.: Ref. Zh. Khim. 1972, 12 N 421 2. A d a m D. F., Jensen G. A,, Steadnian J. P., Koppe R. K., Robertson T. J.: Anal. Chem. 38, 1094 (1966) 3. Bailey P. L., Bishop E.: Analysr 97, 31 1 (1972) 4. Bechtold E.: Z . Anal. Cheni. 221, 262 (1966) 5. Berek B., Westlake W. E., Gunther F. A,: J. Agric. Food Chem. 18, 143 (1970) 6. Burchfield H. P., Wheeler R. J.: J . Assoc. Off. Anal. Chetn. 49, 651 (1966) 7. Burton G., Littlewood A. B., Wiseman W. A,: Gas Chromatography 1966, Butterworths, London 1966 8. Christian J. D.: Anal. Chetn. 45, 698 (1973) 9. Clegg J. B.: J. Chromatogr. 52, 367 (1970) 10. Coulson D. M., Cavanagh L. A,: Anal. Cheni. 32, 1245 (1960) 11. Coulson D. M.: J . Gus Chrojnarogr. 4, 1966, 285 12. Coulson D. M.: Amer. Lab. 1969, 22 13. Cremer E., Bechtold E.: Swiss Pat. 447,665 (March 29, 1968) 14. Cremer E., Bechtold E.: Swiss Pat. 450,013 (Apr. 30, 1968) 15. Dohrmann Instruments Company, San Carlos, Calif., Preliminary operation instructions for Dohrmann microcoulometric titrating system 16. Farzane N. G., Ilyasov L. V., Akhmerov A. S.: Zlz. Fiz. Khim. 44, 1363 (1970) 17. Fleet B., Risby T. H.: Tulanru 16, 839 (1969)

18. Hersch P.: 2nstr. Practice 11, 817, 937 (1957) 19. Hersch P.: Lect. Gas Chromatogr. 1966, 1967, 149 20. Ikels K. G., Neville J. R.: J. Gas Chromatogr. 6, 222 (1968) 21. Kissinger P. T.,Refshauge C., Dreiling R., Adams R. N.: Anal. Lett. 6, 465 (1973) 22. Kobayashi H., Wagner C.: J. Chem. Phys. 26, 1609 (1957) 23. Krichmar S . I., Stepanenko V. E.: Zh. Anal. Khim. 24, 1874 (1969) 24. Krichmar S. I., Stepanenko V. E., Galan T. M.: Zh. Anal. Khim. 26, 1340 (1971) 25. Lebenhart P., Sevtik J.: Coll. Czech. Chem. Commun. - in press 26. Liberti A.: Anal. Chim. Acta 17, 247 (1957) 21. Littlewood A. B., Wiseman W. A.: J. Gas Chromatogr. 5, 334 (1967) 28. MacDonald A,, Duke P. D.: J. Chromatogr. 83, 331 (1973) 29. Martin R. L., Grant J. A.: Anal. Chem. 37, 644 (1965) 30. Martin R. L.: Anal. Chem. 38, 1209 (1966) 31. McFee D. R., Bechtold R. R.: Amer. 2nd. Hyg. Ass. J. 32, 766 (1971) 32. Mizany A.: J. Chromatogr. Sci. 8, 151 (1970) 33. Phillips T. R., Johnson E. G., Woodward H.: Anal. Chem. 36,450 (1964) 34. Rostenbach R. E., Kling R. G.: J. Air Pollut. Contr. Ass. 12, 459 (1962) 35. Saltzmann B. E.: Anal. Chem. 26, 1949 (1954) 36. Scaringelli F. P., Rehme K. H.: Anal. Chem. 41, 707 (1969) 37. SevEik J.: Chromatographia 4, 102 (1971) 38. SevEik J., Lebenhart P.: Enuiron. Sci. Technol. - in press 39. Shibazaki Y . , Tamura F.:Chem. Abstr. 74, 150 673p (1971) 40. Stepanenko V. E., Krichmar S . I.: Zh. Anal. Khim. 26, 147 (1971) 41. Stepanenko V. E., Krichmar S . I.: Zuuod. Lab. 38, 1200 (1972) 42. Takata Y . , Arikawa Y . : Bunseki Kagaku 22, 312 (1973) 43. Takata Y . , Muto G.: Anal. Chem. 45, 1864 (1973) 44. Wagner C.: J. Chem. Phys. 21, 1819 (1953) 45. Williams F. W., Umstead M. E., Johnson J. E.: Chem. Abstr. 72, 93 107q (1970) 46. Zeedijk H.; Chem. Tech. (Amsterdam) 23, 721 (1968) 47. Zhantalai B. P.: USSR Pat. 199 493 (Jul 13, 1967)

11.

The Electrolytic Conductance Detector (ElCD)

11.1

DETECTION MECHANISM

The use of the electrolytic conductance detector is based on the measurement of variations in the low-frequency conductance of electrolytes. The electrolytes are formed by the chemical reactions with water of the products of the combustion of the eluted substances. It follows from the principle of this detector that only those substances will be detected whose solutions behave as electrolytes, i.e., contain ionic forms of the eluted substances. As most organic compounds are non-electrolytes, except for organic acids and bases which form weak electrolytes in water and give rise to a very low conductance, a combustion tube is placed at the outlet of the separating column. Oxides or hydrides are formed on combustion and react with water to form dissociated acids or bases. The principle of the electrolytic conductance detector was first utilized in 1955 [151 for the direct conductimetric determination of acids. The combustion process was introduced by Piringer et al. [lo, 111 in 1962 and was further developed by Coulson [2, 31 for the determination of halogens, sulphur and nitrogen in organic molecules. Combustion is carried out under either oxidizing or reducing conditions and the detection mechanism can be described by the following reaction scheme: Combustion:

S(CI, N)ORG

+ 0,

S(CI, N)ORG

+ H2

-+

S0,(C12, NO,)

+ CO, + H 2 0

(11.1)

-+

H2S(HCI,NH,)

+ CH, + H,O

(11.2)

In solution SO,

+ 3H20

-+

SO,

+ 3H20

-+

N,O, CO,

+ 3 H,O + 3H20

-+

-+

+ SO:2 H 3 0 + + SO:2 H 3 0 + + 2NO; 2H30+ + C0;2H30+

(11.3) (11.4)

(11.5) (1 1.6)

182

H30+

+ S2+ CI-

(11.8)

OH-

$. NHf

(11.9)

H2S

+ 3H20

-t

2H30+

HCI

+ H20

-+

NH,

+ H20

-t

(11.7)

From these equations, it can be seen that the change in the conductance is proportional to the concentrations of H 3 0 + and OH- ions. The concentrations of these ions in solution are proportional to the concentrations of the combustion products and hence to the concentration of the eluted substance. The dissociation constant appears as a proportionality constant in this relationship (see Table 11.1). TABLE 1 1 . 1 THE DISSOCIATION CONSTANTS OF SOME ACIDS A N D OF AMMONIA

Acid, base

H2S03

H2S04

HNO2 H2C03

H2S

HCI NH,OH

Dissociated form HSO,

so: HSO,

so;-

NO; HCO; co: HS S2 -

c1-

NH;

PK,, P K ~

1.54 X lo-' 1.02 x Completely 1.20 x 10-2 4.6 x 10-4 4.3 x lo-' 5.61 X l o - " 9.1 X l o - * 1.1

x

10-12

Completely 1.97 x 1 0 - 5 ,

1.81 6.91

Temperature ("C) 18 25

1.92 3.37 6.37 10.25 7.04 11.96 4.75

12.5 25 18

25

The reactions of the combustion products resemble one another and therefore interferences may occur during measurement of the electrolytic conductance. A column containing silver nitrate is therefore placed before the gaseous and liquid phase mixer in order to remove hydrogen chloride when sulphur is measured or sulphur oxides using calcium oxide when chlorine alone is to be monitored. When a reducing combustion atmosphere is employed, the acidic products are trapped in an Sr(OH), scrubber and the measured variations correspond to basic NH,. Interference from C 0 2 in the detector mechanism is very small. It is usually stated in the literature that its dissociation rate is low. The solubilities of the combustion products are given in Table 11.2 and show that, although the solubilities of COz or HzS are 25 times and 85 times that of SOz, respectively, their interference is nonethe-

183

less very small. Hence it is evident that the extent of dissociation rather then the solubility is decisive for determining the interference from individual hetero-atoms. TABLE 11.2

SOLUBILITY OF SOME GASES IN WATER AT 20 "C Litres of gas/litre of water

Gas

co2

0.859 0.0394 2.526 0.046 89.9* 82.3*

SO2 HZS NO NH3 HCI

* grams of 11.2

gas in 100 g of water.

ElCD SIGNAL

The signal of the electrolytic conductance detector corresponds to changes in the conductance of the solution due to the varying concentration of the eluted substance. If the resistance of the solution is R , then the conductance is proportional to the sum of the products of the concentrations, ci, charges, zi, and ionic mobilities ,Ii, of the ions present in the solution: .4

- _-- I & z , ~ , R

(11.10)

Q i

where Q is the vessel constant (cm). The concentrations of the ionic forms of weak electrolytes depend on the magnitude of the dissociation constant and on the overall concentration, c, of the hetero-atom eluted from the column. The dependence is S-shaped and can be described by the equation [H']

=

J[lO-'"

j-

( c . a)']

(11.11)

where a is the degree of dissociation, for which u =

where x

=

-x

K,,/c.

+ x2 + 4x 2

(11.12)

184

Hence the signal, e.g., for sulphur, will be given by the expression

1 sEJCD = - ,/[10-14 + ( c . q](I+,+ +

(1 I. 13)

Q

The ionic conductance of anions and cations are similar except for H" and OHions, whose conductances are several times higher (see Table 11.3). The detector TABLE 11.3 IONIC CONDUCTANCE VALUES AT 25 "C

Ion

i.j

H+

3 50

Ton

4

$so:-

79 71

NO;

c1OH-

N H ~

192

76 75

%:I

I

2

0

2

4 -log c

6

FIG. 1 1 . 1 . The relationship between the logarithm of the dissociation constant and the overall concentration of an acid or a base. The shaded area indicates the region of complete dissociation of the acid or base.

7

6

5

4

- loge

FIG. 11.2. The equivalent conductances of several electrolytes in water [15]; 1 - SO,, 2 - SO,, 3 - HCI, 4 - NH3, 5 - CO,.

signal is therefore determined by the proton concentration. The degree of dissociation, a,reaches a value of unity even for the weakest acids at infinite dilution; in Fig. 11.1 are given the concentration regions in which electrolytes are completely dissociated. It then follows that the sensitivity of the electrolytic conductance detector is identical for low hetero-atom concentrations.

185

11.2.1

ElCD background current

The detector background current is determined by the dissociation of water (the ion-product of water is lo-’, at 25 “C, see equation (11.11)) and by the presence of ionic impurities in water. If “conductance” water is employed, then the detector background current is very low. When ion exchangers are used, a certain concentration of ions is present in the water and the background current is correspondingly increased, As follows from equation (11.13), the conductance of water plays a role at low eluted substance concentrations and there is a change in the detector linearity. For this reason, the addition of 1 ppm of HCl during measurement has been proposed [ 5 ] ; however, this method cannot change the shape of the log SE‘CD/log c curve and, moreover, it increases the background current. 11.2.2

ElCD response

The detector response is the integral of over the elution time, A t . As follows from equation (11.11), the detector response is proportional to the concentration of the eluted substance, c, and depends on its properties, i.e., on the dissociation constant of the corresponding acid or base, when the measurement is carried out under otherwise constant conditions (the vessel constant, Q, must not change). The E l C D linearity depends on the amount of substance eluted. In general, it is less than unity for small amounts of substances, as the overall conductance is determined by the dissociation of water in the concentration range lo-’ M. At higher concentrations, the linearity is again lower than unity, as the degree of dissoc = 1 only in the concentraciation, CI, is less than unity. It holds that log SEICD/log tion range M. TABLE 11.4 SOLUBILITY PRODUCTS OF SEVERAL SILVER A N D CALCIUM SALTS Salt

Ks

AgCl Ag,CO, Ag,SO, Ag,SO,

Salt

1.1 x

10-10

6.1 x lo-’, 1.0 x lo-’’ 1.2 x

Ks ~-

~~~~

~~

CaCO, CaSO, CaSO,

4.8 x 1 . 0 1~ 0 - ~ Soluble

The linear dynamic range of the detector follows from equation (11.11) for 01 = 1. In general the linear dynamic range decreases in the order C1 > S > N (Fig. 11.2).

186 The selectivity of the ElCD is determined by interactions among the combustion products. In the nitrogen detector mode with a Ni catalyst, a single basic product is formed and therefore the measuring selectivity is high. In the oxidation detector mode, all the products formed are acidic and hence various salts are placed in the gaseous mixture stream in order to improve the selectivity. When AgNO, is employed to remove HCl from the gas stream, interaction of sulphur dioxide and trioxide with Ag' ions also occurs, as follows from the solubility products (see Table 11.4). The TABLE 11.5 THE SELECTIVITY VALUES OF THE ELECTROLYSIC CONDUCTANCE DETECTOR [I 51 Signal ratio

N IS NIP N/CI N/CH,OH

Selectivity

2.3 x

103

2.2 x 8.0 x

103 104 103

5-s.ox

Signal ratio

Selectivity

N/higher alkanes

5.0 X lo6 1.0 x 104 1.0 x lo4

a/s CP

sulphur signal is considerably decreased as the combustion does not yield sulphur dioxide with 100%efficiency [6] (see Table 10.2). Sulphur dioxide forms a sparingly soluble compound and the measuring selectivity is poor in the presence of a large excess of carbon dioxide. The selectivity for halogens is better as HCl is not trapped in CaO. The selectivity values of the electrolytic conductance detector are summarized in Table 11.5.

11.3

CONSTRUCTION OF THE ElCD

The substances eluted from the separating column pass through a combustion space or a pyrolyzer [l] and enter a gas-liquid contactor in which reaction with water takes place. The gaseous phase is separated in a separator and the electrolyte enters the measuring cell. The cell is very small, having a volume of about 50 p1 [lo, 161. The electrolyte then enters a reservoir and passes through an ion exchanger back to the contactor (see Fig. 11.3). As the measurement is based on a chemical reaction, the absorption surface and the flow-rate of reaction gas must be optimized. In addition to selection of the frequency, attention must be paid to the temperature of the measuring cell and of the circulating solution, as the ionic conductances increase considerably with increasing temperature.

187 The conductance measurements are carried out in a low-frequency circuit. When the frequency is increased to the MHz region, it is possible to perform impedance measurements on solutions of very low conductance [7,16]. On this principle are based high-frequency detectors used in liquid chromatography. Measurement of the dielectric constant, based on a similar principle, has not found broad application [18].

c

3

I

10

FIG. 11.3. Scheme of the electrolytic conductance detector 1151; 1 - column effluent, 2 - reactor tube, 3 - furnace, 4 - gas-liquid contactor, 5 - transfer line, 6 measuring cell, 7 - water reservoir, 8 - gas-liquid separator, 9 - pump, 10 - ion exchanger mixed bed, 11, 12 - needle valves, 13 - vent.

11.4

APPLICATIONS OF THE ElCD

The electrolytic conductance detector is not one of the most widely used measuring devices in gas chromatography. It is generally used for pesticide residue analysis, for pesticides containing sulphur, chlorine and nitrogen. The minimum detectable amounts are in the nanogram range. This detector has also been used for analyses of TABLE 11.6 EXAMPLES OF THE APPLICATION OF THE ELECTROLYTIC CONDUCTANCE DETECTOR Substance determined

Material analyzed

Lowest detectable amount

Linear dynamic range

6- 100 ng 0.1 ng 0.1 ng

2-1000 ng

Metal halides S, CI pesticides

N pesticides CI pesticides Herbicides S-Triazine N-Nitrosamines

0.02-2 ppm foodstuffs

Ref.

16, 17 1-3, 5 , 7 9, 14 4 13 8'

188

metal halides, separated gas chromatographically. A survey of the applications given in Table 11.6. It can be generally stated that the electrolytic conductance detector is a very simple measuring device, its design ranging from a universal type [12] to a highly specific nitrogen detector. 11.5

LITERATURE

1 . Cochrane W. P., Wilson B. P., Greenhalgh R.: J. Chromatogr. 75, 207 (1973) 2. Coulson D. M.: J. Gas Chromatogr. 3, 134 (1965)

Coulson D. M.: J. Gas Chromatogr. 4, 285 (1966) Dolan J. W., Hall R. C.: Anal. Chem. 45, 2198 (1973) Jones P., Nickless G.: J. Chromatogr. 73, 19 (1972) Martin R. L., Grant J. A,: Anal. Chem. 37, 644 (1965) McCarthy W. J., Lazarus M. L.: Chem. Instrum. 1, 299 (1969) 8. Palframan J. F., McNab J., Crosby N. T.: J. Chromatogr. 76, 307 (1973) 9. Patchett G. G.: J. Chromatogr. Sci. 8, 155 (1970) 10. Piringer O., Pascalau M.: J. Chromatogr. 8, 410 (1962) 1 1 . Piringer O., Tataru E., Pascalau M.: J. Gas Chromatogr. 2, 104 (1964) 12. Polesuk J., Howery D. G.: J. Chromatogr. Sci 11, 226 (1973) 13. Purkayastha R. Cochrane W. P.: J. Agric. Food Chem. 21, 93 (1973) 14. Rhoades J. W., Johnson D. E.: J. Chromafogr. Sci. 8, 616 (1970) 15. Selucky M. L.: Chromatographia 5, 359 (1972) 16. Tesatik K., Kaliib P.: J. Chromatogr. 78, 357 (1973) 17. Itiro T., Kiyoteru 0.: 2.Anal. Chem. 262, 346 (1972) 18. Winefordner J. D., Glenn T. H.: Advan. Chromatogr. 5, 263 (1968) 3. 4. 5. 6. 7.

INDEX

acetylene 60, 93, 94 acids 165, 179 aliphatic 56, 110 carboxylic 179 miscellaneous 101 affinity, electron 72,73,77,79,83,93, 108, 113 air 46, 88, 92, 97 pollution 159, 176 alcohols 44, 56, 90, 93, 98, 110, 143 aldehydes 44, 143, 165. 179 alkali metals 105ff. 107--110ff, 120 alkanes, fluoro 89 americium 66, 80 aniines 83, 90, 101 amino acids 120 animonia 40, 89, 96, 108 amount, lowest detectable 34, 45, 56, 143, 187 analysis, trace 77 antimony 105 applications 56, 57, 78, 82B, 93, I O l f f , 120, 131, 143, 187 (see also contents) argon 40, 45, 46, 64, 75, 76, 78, 79, 96 arsenic 105, 108 Awe 83, 157 bases 165 benzene 29, 110 substituted 107, 108, 115 Reroza 151 blood 83, 84, 120 Bowman 151 capacity, heat 42, 43, 55 carbon 73 dioxide 45, 46, 56, 75, 78, 102 disulphide 20, 21, 57, 77, 84, 88, 91, 159 monoxide 40, 75, 143 tetrachloride 77, 78, 88, 93 niisc. compounds S7, 84, 88, 93 (see also hydrocarbons)

carbonyl 94 cell 51, 52, 70, 80, 81 diffusion 45, 46, 53 flow-through 43, 53, 5 5 semi-diffusion 43, 53 cesium 66, 80 charge, space 65. 71, 119 chemiluminescence, intensit) of 149 chlorides 84, 1 1 5 chlorine 73, 107, 113. 114, 116, 168 containing substances 165, 175, 178 chloroforni 77, 78 chromatography, liquid 82, 101 plasma 89 preparative 31 thin-layer 82, 101 coefficient, recombination 70, 74 combustion, hest of 1 I5 conductance, of electrolytes 181 - I83 equivalent 184 ionic 184 conductivity, thernial 39. 40. 42-46, 49 constant, dissociation 182 time 26-28, 30, 3 1 , 47, 51-56, 111 convection, gaseous 41, 46. 53 coulometer, reaction 167 Coulson 181 cross-section, absorption 125. I27 collision 71 electron capture 60 excitation 134 ionization 22,36,60,61,64,75,78--80, 127 current, background 23,24,43,61,71,74- 76, 82, 92, 110-112, 114, 117, 118, 127, 135, 136, 141, 151. 156, 170. 185 electrolytic 165, 169 generating 173, 174 ionization 21, 59, 71, 72, 88ff, 92, 94, 96, 100, 105, 106, 108- 110. 112- 114. 117, 119, 125, 126, 134

190 curve, calibration 28 detectors (see also contents) catalytic 89 cross-section 62 discharge 57 electron capture 57, 62, 65, 72ff, 105, 159 electron mobility 62 flame ionization 21, 23, 32, 57, 64, 65, 84, 87ff, 105, 110, 113, 118, 159 flame photometric 57, 145ff helium (argon) 62, 64, 71, 105, 133ff ionization 59ff, 131 photo-emission 1 I 2 photo-ionization 62. 105, 123ff thermal conductivity 25, 39, 39ff, 159 therniionic 62, 64, 65, 71, 1@5R', 157, 159 dilutcr, logarithmic 29, 30 distribution, concentration 30 drugs 83, 121 efficiency, current 168 ionization 61, 95 photoionization 125 Eisentraut 80 electrode, collecting 64, 98, 100, 107, 119, 141 shape 98 esters 44, 143 ethane 88, 93, 110 ethers 44, 90, 94, 98 evaporator, diffusion 29 Faraday laws 169 field, electric, intensity of 142 filament 48, 50ff filter, interference 158 flame 157ff diffusion 157 pre-mixed 157 shielded I58

62, 126, 135. 139,

garlic 83 gas, carrier IS, 20,22,23,27, 30-32, 43-47, 49, 68. 71, 75, 79, 80, 83, 91, 96, 97, 117, 118, 128, 138, 140, 156 combustion 155, 156 Row-rate of 17, 20--22, 24-28, 30, 34, 41, 43-48, 53, 54, 80, 91, 94---98, 110, 111, 114, 117-119, 155, 156, 172, 186

permanent 56, 79, 102, 131, 142 solubility of, in water 183 Geiger-MLiller curve 99, 119 gold 80 Goretti 162 halides 56, 84, 98, 107, 111, 115, 187 halogens 89, 91, 105, 107, 108, 115. 147. I6S, 181 helium 40, 45-47, 61, 68, 73, 75, 79, 96 herbicides 84 Huyten 145 hydrocarbon5 71, 75, 78, 79, 87-94, 101, 109, 110, 120 chlorinated 56, 143 n-alkql-aromatic 44 hydrogen 40, 43-45, 47, 60, 61, 70, 73, 75, 79, 80, 87-89, 91, 92, 95, 105, 109-111, 114 sulphide 57, 84 hydroxyl 89, 90, 1 I 1 insecticides 83, 84 ionization 59ff, 64, 68, 74, 80, 82, 87, 83, 91, 105, 106, 109, 112 mechanism 64, 66, 63, 87, 88ff, 107 probability 125 Jrntzsch

57

ketones 78, 143, 165, 179 methyl 44 light, emitted, measurcment of 114 linearity 31, 32, 56, 77, 94, 115, 139, 153, 154, 171, 185 lindane 77 Maggs 80 mechanism, detector 59, 68, 72, 79, 80, 87, 90ff, 94, 95, 105, l06ff, 109, 113-115, 123, 145 melting point 115 metals, heavy 148, 159, 162 methane 40, 75, 78, 79, 88, 93, 110 methyl radical 89, 102 multi-regression 94 neon 68, 75 nickel 61, 65-67, 80, 81

191 nitrogen 23, 3 2 , 40, 45-47, 56, 60, 61. 63, 67, 70, 71, 73, 75, 78, 79. 88. 91, 96. 97, :02, 105, 108, 115-117, 119, I81 containing substances 176, I73 oxides 40, 56, 60, 103, 108. 113, 177 nitrile 94 noise 34-36, 43, 45, 50, 92, 97, IGO, 11 I , 119. 142, 152, 156. 170 olefins SO, 56, 9 1 Otte 57 oxygen 40, 56, 70, 73, 74- 76, 79, j i i 94,97, 101, 103

~

89, 9 I .

pair, ion 69 peak, elution, area of 28, 94, 95 elution, height or 21, 28 elution, inversion 45 -47. 76. 114, I IS elution, \ 4 i ; l t I i of IS, 21, 24, 27, 28, 47 Penning efl’ect 134, 138 pesticides 8 3 , 84, 101, 116, 120, 121, 150, 151. 159, 160, 175, 179, 187 chlorinated 78, 187 phenols 84, 179 phosphorus 105, 108, 1 1 1 , 113-117, 119 containing substances 146 photomultiplier I58 photon flur 130 picolines 89 I’iringer 18 I plastic, fluorinmxi 54 platinum 89 plutoniurn 66, 80 polarography 169 pulse 169 Possanzini I62 potcntial, ionization 60. 75, 79, 87. 106, 107, 109--111, 113 excitation 133 pressure 29, 87, 90, 52 partial 115, 117, 118 vapour 114, I 15 pronietheuni 66, 67, 80 p d s c frequency 6 5 , 81 quenching of radiation

149

range, linear dynamic ? I , 33, 45, 5 0 , 76, 77. 79, 80, 82, 85, 94, 115, 116, 128, 139, I S ? . 153,171, 185

rccombination 69R, 72, 80, 89, 90, 106, 107, 119, 125, 142, 153 response, relative molar 4 3 f , 91 Rijnders 145 Rossi 159 scandium 65, 80 sensitivity 31,43, 45, 46, 49, 56, 67, 77, 80. 81, 95, 107, 116, 119. 140, 155, 171 selectivity 36, 77, 95, 107, 116-1 18, 128, 140, 155, 171, 186 si!ylaied samples 98, 101, 119 Smith 83 source, ionization 6.5, 66ff photon 67, 130 radioactive 65, 80, 82 o! 65 /I 65. 66, 74, 80, 133, 142 y

65

spectrometry, mass 89 spectruni, emission, of heteroatonis 147 state, nietastahle 64, 69, 71, 79, 133, 134 steroids 83, 101 strontium 65 styrene 101 sulphur 73, 89. 91, 101, 105, 108, 1 1 3 , 115, 168,181 dioxklc 57, 108, 145, 159, 168 oxides 111 containing substances 146, 159, 160, 175, 178 sulphides 84, 112 I

tempernture 34, 39, 42, 43-501l’, 65, 67, 76, 80, 83, 91, 95, 101, 105, 108, 109, 111, 114, 115, 118, 156, 175, 186 flame 110ff, 117-119, 154 thermistor 41, 48, 50, 55 theoretical plate 28 height equivalent to 17, 20 number of 17, 21, 22 t j i w , elulion 17, 21, 26, 28, 34, 43 retentioli 17, 21, 22 trmsparence. optical 129, 130 transistor. 41, 48, 54. t r:i ns mod ti la t o I. 57 tritium 65, 65, 79, 80 urine

83, I20

192 Versino 159 volume, effective detector 22, 25-28, 31, 47, 51, 69, 72,79,80-83, 106, 108, 111, 172 dead 107 retention 17

water

74, 79, 83, 84, 89, 102, 120

xenon

64

yttrium

65