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Chromatogmzfihic ReviewsElsevier Publishing Company, AmsterdamPrinted in The Netherlands

SELECTIVE DETECTORS IN GAS CHROMATOGRAPHY

M. KREJ CI AND M. DRESSLERInstitute of Instrumental Analytical Chemistry, Czechoslovak Academy of Sciences, Leninova 82,Brno (Czechoslovakia)(Received February 1201, 1970)

CONTENTS1. Theoretical ..................................

A. Trends in the development of chromatographic detectors and basic terms. .....B. Qualitative chromatographic analysis ....................a. Identification of substances from information derived from the chromatogram _ .b. Reaction chromatography and chemical multipliersC. Selective detectors. .... .. .... ......... : : : : : : : : : : : :a. Definition of selective detectors .......................b. Requirements following from the definition of a selective detector ........D. Quantitative characteristics of detector selectivity ................a. Detector characteristics ..........................b. Selectivity as the ratio of the time integrals of the responses of two detectors _ .c. Classification of selective detection systems .................d. Detector selectivity as relative response ...................E. F orecast of selectivity for some detectors ...................a. Electron capture detector .........................b. Alkali flame ionisation detector .......................F. Methods of connecting selective detectors. ...................a. Parallel coupling of detectors ........................b. Series coupling of detectors ........................c. Carrier gas separators ...........................z. Analytical part. .............A. The electron capture detector , ...........................................B. The alkali flame ionisation detector ......................C. The surface ionisation detector ........................D. The flame photometric detector ........................E. Emission spectrometry ...........................F. Spectrofluorometric detection .........................G. The ultraviolet spectrophotometer detector ...................H. The infrared spectrophotometer detector ....................

II2245567s7

IO1313131415161617171724293135383939I . Mass spectrometry. . . . . . . . . . . . . . . . . . . . . . . 41J . Galvano-coulometric detectors . . . . . . . . . . . . . . . . . . . . 45K. The electrolytic conductivity detector . . . . . . . . . . . . . . . . 48L. The polarographic detector . . . . . . . . . . . . . . . . . 50M. The piezoelectric sorption detector . . . . . . . . . . . . . . . . . 503.Conclusions . . . . . . . . . . . . . . . . . . . _ . _ . . . . . _ . 51References. . . . . . . . . . . . . . . . . . . . . . . . . 53

I. THEORETICAL

A. Trends in the develo$ment of chromatogra$hic detectors and basic termsIn the present review we propose to deal only with detection systems that are

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2 M. KREJ ti, M. DRESSLER

directly connected to a chromatographic column. We regard gas chromatographyas an analytical method which includes the separation, identification, and quantitativedetermination of the individual components in the mixture analysed. We shall notdiscuss analytical procedures which involve further independent processing of themixture of substances under analysis either before or after the chromatographic sepa-ration. We shall, however, discuss analytical methods often used as independentanalytical methods (e.g. mass spectrometry) when they are used in direct conjunctionwith chromatographic columns for detection purposes.

A detection system is considered selective to substance i when the analyticalproperty of substance i, ai, induces a signal and/or response sufficiently quantitativelydifferent to distinguish it from the response induced by the analytical property ofsubstance j, ai. An analytical property is called such property of mass that is inexactly defined relation to the quantity and quality of the mass analysed72. Signaland response are (cf. ref. 222) defined as the immediate effect of the mass on thedetecting element and the response of the detecting element to the signal.

The application of gas chromatography has reached the stage where informationon the character of the individual components cannot be based on the interpretationof retention characteristics alone. This is why a considerable changeover to the useof more selective rather than non-selective 271detectors can be seen. This phenomenoncan be explained without difficulty. While pioneer work dealt with simple mixturesof substances, the identification of which did not usually present any serious problem,today the main task in research applications of gas chromatography is the analysisof complex natural materials (e.g. biological materials, crude oil etc.) where theidentification of individual components by means of retention characteristics interpre-tation alone is practically impossible. From todays point of view the not very highprecision of the early quantitative work in gas chromatography led further to con-sidering many detectors as completely non-selective, even though more recent precisestudies have shown their responses to be dependent on the properties of the com-ponent analysed.

The increasing tendency of manufacturers and analysts to use automatic dataprocessing will probably also allow the use, in future, of those detectors where wayshave been found to record small differences in detector response to individual sub-stances205 as selective detectors.B. Qualitative chromatographic analysis

(a) IdentiJ ication of substances from information derived from the chromatogramRetention characteristics are the values most often used in chromatographic

qualitative analysis. Both absolute values of the retention characteristics such ase.g. retention times, retention volumes, or specific retention volumes4,233, and relativevalues of these quantities such as e.g. relative retention times (volumes)233, or Kovatsretention indices* are used.

Characteristic parameters of a chromatographic curve, such as e.g. the heightof the curve, the width of the curve, are reflections of the analytical characteristicsof the substance analysed. A chromatographic curve is in definable relation to thecharacter and quantity of the substance analysed. In general practise, however, i.e.when common stationary phases and non-selective detectors are used, this curve isChtOWXZtOg. Rev., 13 (1970) I-59

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SELECTIVE DETECTORS IN GC 3neither selective nor specific for the substance analysed. The retention volume VR ispartly due to the values characterising the chromatographic system (dead volume ofthe column V,, and volume of stationary phase V, or surface area of adsorbent for agas-solid system) and partly due to the partition coefficient K, whose value dependson the substance analysed and the stationary phase, i.e.

vn = v, + KV,As the partition coefficient, generally speaking, is a function of the intensive thermo-dynamic value-chemical potential-(+ refs. 108 and 232), the retention volumefor a given system of solute and stationary phase, at constant temperature and pres-sure, is constant, however it is not selective or even specific. From the theoreticalpoint of view a negative result only can be considered as conclusive in a chromato-graphic experiment, i.e. substance A is not identical with substance B in case thatI RA # VRB. I f we carry out a qualitative identification from the values of retentioncharacteristics only, we must be aware of the risk that the chromatographic curvemay not pertain to the substance selected for calibration although the latter has thesame characteristics, and even that the curve may not be induced by a single substancebut by two251or even more substances that have the same partition coefficients inthe given system.

Similarly with the retention volume, the shape of a chromatographic curve isagain a function of the partition coefficient and of a series of further values charac-teristic of the chromatographic column being used (e.g. the particle size of the packing,the thickness of the liquid film etc.) and of the system used, viz., solute-stationaryphase (e.g. the diffusion coefficient of the solute in the liquid phase) or solute-mobilephase (e.g. the diffusion coefficient of the solute in mobile phase). I t is obvious thatthe width (shape) of a chromatographic curve is influenced by further non-selectivevalues so that the probability of the existence of two substances which would, in thegiven system, have not only the same partition coefficients but also all other identicalparameters, considerably decrease. A procedure233has been suggested for verifyingthe identity of a substance by comparing the chromatographic zone widths (or bycomparing the heights of theoretical plate) of different substances. Because of thecomplexity of the dependence of above-mentioned values on carrier gas flow, temper-ature, and comparatively small differences in zone widths, this identification procedurehas not been used very much. We only mention it here as it is a procedure charac-teristic of the efforts to obtain from chromatographic experiments further informationthat would decrease the probability of confusion among the substances being analysed.

Methods by which identifications have been made by taking advantage of thechange in the partition coefficients in an homologous series, which are known or canbe forecasted from several experimental data, have been used extensively in chromato-graphy; it being possible to correlate74~144~2s4~250~305etention characteristics-reten-tion volumes, specific retention volumes, relative retention volumes, K ovats indices(retention indices) etc.-with the values characterizing the homologous series. Suchvalues are either those which cannot be in any way determined from the chromatogram(e.g. molecular weight, number of carbon atoms, boiling point, the ratio of criticaland experimental temperatures etc.), or those found by one or several chromatographicexperiments (e.g. the above-mentioned shape of curve, the ratio of the retentioncharacteristics on two stationary phases mostly differing in polarity, retention charac-

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4 M. KREJ ti, M. DRESSLER

teristics measured at different temperatures, etc.). By interpolation of these de-pendences, the retention characteristic of a particular member of an homologousseries can be obtained and compared with retention characteristics determined ex-perimentally. Agreement between the two sets of data increases the probability ofthe identity of the substance being predicted and the substance being analysed. Thisidentfication procedure is, of course experimentally exacting and timeconsuming.

That is why procedures have been suggested to speed up the identificationprocesP4. The simultaneous determination of the retention characteristics of asubstance on a system of several columns connected in parallel and having packingsof different polarity enables one to obtain a set of retention characteristics in onesingle experiment. These data can then be correlated in any of the above-mentionedways. I f known reference substances are used as standards in the system, the proba-bility of error in the qualitative analysis is essentially decreased, since the deductionsare made from several sorption characteristics independent of each other, i.e. fromseveral pieces of information independent of each other.

All the identification procedures which have been indicated employ non-selectivedetectors. Detector data, therefore, cannot be used in any way as an independentsource of information about the identity of the substance being analysed.

(b) Reaction chromatography and chemical multipliersReaction gas chromatography 23v24 as been used by a number of workers for

facilitating the identification of individual components of a chromatogram. In a non-selective cycle, for example, the conversion 121y200f the analysed substances intocarbon dioxide and water has been used. A hydrocarbon with n C atoms, e.g., providesn moles of carbon dioxide. The sensitivity of detection is considerably increased inthis way. I n a selective detection cycle, group or selective (mostly catalytic) reactions,can be used, whereby selected types of substance or one substance can be removedfrom the chromatographic spectrum174, or converted into different substances261.Thechromatograms of the unchanged substances are then compared with those obtainedafter the chemical changes of the substances. This type of reaction gas chromato-graphy can be considered as the predecessor of selective detectors. In Fig. I the reactor

kl) (bl3 4

1 HFig. I. Exploitation of reaction gas chromatography for selective determination of compounds.I = Sample chamber; 2 = column; 3 = reactor; 4 = detector.

is positioned in front of one of two non-selective detectors connected in parallel (a) orbetween two non-selective detectors connected in series (b) behind the column145.This identification method should have a revival when chemical (molecular) multi-pliers are introduced into chromatographic methods201.It is obvious that the sensitivity of detection can be controlled to a considerableextent by connecting n multiplier reaction units in a series, as the amplification factoris an exponential function of n. Even though chemical multipliers can be used in aChromatog.Rev., 13 (1970) I-59

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SELECTIVE ETECTORSN GC 5non-selective cycle as e.g. (see ref. 252)

CO2 C-+2co 2co cue+ 2 CO, etc.1st stepco, --% = q. 2 CO2

the multipliers employing selective reactions will remain especially significant forselective detection systems. E.g. for hydrogen sulphide in a mixture with hydro-carbons (see ref. 22)

I , + H,S = 2 HI + S5 HI + HIO, = 3 I , + 3 Hz0i.e. 5 I , H2S-+ IO HI HI036 I,

1st5 I2 -+ step 6 I ,.The use of reactive organo-metallic compounds permits extraordinarily high

sensitivities to be reached. The combination of these sensitivities with group or se-lective reactions gives rise to new possibilities of selective detection.

By introducing chemical reactions into the detection system, further infor-mation about the identity of the substances analysed is obtained from a chromatogramand the possibility of confusing the identity of the substances in the chromatogramis again reduced. In practice, the use of selective detectors combines the function ofreactor and non-selective detector in a number of cases.C. Selective detectors

Katharometers and emissive detectorP were first used for direct comparisonof the responses of two different detectors. The ratio of the responses of both detectorswas assumed to be constant for individual groups of substances. Thus e.g. the ratioof the response of the emissive detector to that of the katharometer233 was found 0.14for a-paraffins, 1.25 for aromatic compounds and 0.44 for naphthenes. AS will beshown later, the assumption of constant response ratios for individual groups ofsubstances was too optimistic. I t only holds for a limited number of members ofindividual groups of homologous series.GRANTSworkll* can, today, be considered as the first significant step in theutilisation of detector responses for the identification of substances. The significanceof this procedure was demonstrated later when the electron capture detector was usedfor identification purposesrg4.

(a) Definition of selective detectorsThe definition of detector selectivity is far from uniform. Two aspects can beconsidered as basic according to which the selectivity of the detection system canbe evaluated250. Firstly, we take the case where the response of two detectors to asignal from one substance only is compared; and in the second case we compare theresponse of one detector only to the signal of two different substances.

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6 M. KREJCf, M. DRESSLERI. A detection system is considered selective if the molecular response of a

detector S to a signal (or response to a unit of mass) from a substance (A) (or the groupof substances A) differs from the response of a detector A to a signal from the samesubstance and if the ratio of responses of both detectors is sufficiently (see Sectionr.D.a.) different from the ratio of both detectors to a signal from substance B.

2. Detector response can be considered selective if the molar response (orrelative molar response, or response to unit mass) of the detector to a signal fromsubstance A (or the group of substances A) differs sufficiently (see Section 1.D.c.) fromthe molecular response of substance B.

The suitability of the individual definitions depends considerably on the purposefor which the detector is to be used. The application of a detection system accordingto the first definition is obviously more suitable for practical qualitative analysis,especially for selective differentiation of groups of substances in mixtures to be ana-lysed. This system permits the quantitative determination of individual componentsin the mixture or, at least, the determination of their interrelationships. It is thereforesuitable to use as a reference such detector that is the least selective. The interpre-tation of the information then consists of merely comparing the detectors responseto the individual components of the mixture being separated in the column.

The characteristics derived from the second definition are mostly used for thedetermination of detector properties. Experimentally, less exacting attitude to de-tector characteristics acquires, of course, a knowledge of the proportions of the com-ponents in a mixture. It is necessary to note that, in accord with the second definition,it is necessary to set the limit between selective and non-selective detectors by meansof the determination of the value of the response ratio only. From todays point ofview, the value of the relative molar response for different substances also differsin the case of detectors which are generally considered to be representatives of thecategory of non-selective detectors (as e.g. the FID12*,205,264).

(b) Requirements following from the dejkition of a selective detectorFrom the above definitions of detector selectivity it follows that the mere de-

termination of the detector response is not, alone, sufficient for qualitative analysis.(The term selectivity should in this case be substituted by the term sensitivity.) Inthe simplest case it is essential to determine the response of a reference (non-selective)detector. These requirements can be met, essentially, in three different ways:I. Both responses are determined in the course of one analysis, two detectorsbeing coupled with a column outlet, in parallel or in series. Two chromatograms arethus obtained, the peaks of which can be compared. This solution entirely satisfiesthe conditions mentioned in the introduction to this work. The results of only oneanalysis provide information concerning three things, viz., the retention charac-teristics, the response of the selective detector, and the response of the referencedetector. The particular advantage of this method is that the responses of both de-tectors are recorded under the standard quantitative conditions, i.e. either the sameamount of substance enters both detectors (in the case of connection in series), or aknown proportion of the total amount passes through (in the case of parallel detector).Furthermore, any column effects (ratio of area and height of curve, shape of curve)are almost entirely eliminated.

2. The responses of selective and non-selective detectors can be determined inChronzato,o. Rev., 13 (1970) I-59

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SELECTIVE DETECTORS IN GC 7two analyses in one chromatographic column. This is possible with most commercialapparatus. One analysis is usually not sufficient. The calibration curve must be used,as far as possible, over only a small range to eliminate the errors caused by inaccuraciesin sampling. However, this method of determining the response does not agree, inthe main, with the concept of selectivity for a detection system, as defined in SectionI.A.

3. An entirely independent chromatographic system is used for the determinationof the response of a standard system. In this case, a qualitative analysis to determinethe ratio of individual responses is not, in general, sufficient but a complete quanti-tative analysis must be carried out. I f the column selected is such that the chromato-graphic spectrum (the sequence of eluted substances) is changed, then apart fromthe quantitative analysis, the qualitative composition of the mixture must also bedetermined. The selective detector is then used only for verification of our assumptions.This solution does not agree with the assumption in Section I .A, either.From the procedures introduced for taking advantage of detector selectivityit follows that only in the last case can the detector selectivity be characterised ac-cording to the second definition in Section 1.C.a. In all the other cases, i.e. when areally selective detection system is of interest, it is advantageous to use the firstdefinition.D. Quantitative characteristics of detector selectivity

(a) Detector characteristics (ref. 222)The value of the detector response Ra to a component i must inevitably bedetermined mathematically for the quantitative characterization of detector se-

lectivity. I t wil l be derived from relationships for detectors mostly in use in chromato-graphy and form the basis of a classification222 of them.

Instantaneous detector response is generally described asRc / = $, P, K , $, a, a,, aa,yip T /

where K = construction parameter, a = the analytical property, y = the molarfraction of the solute in the gaseous phase, T = temperature; indices: i = solute,o = carrier gas, a = scavenger gas. The dependence of the response on the first twovalues, viz., carrier gas flow rate dV/dt and pressure P, can be used with advantage forthe classification of the detectors into the following groups: destructive D, non-destructive N, mass M, concentration C; at the same time, each of the detectors usedcan be included into one of the groups: MD, DC, NM, NC for certain concentrationranges and working regimes of the detector.For a selective detection system it is inevitably so that at least one of the valuesof a, for a selective detector ai, will be different from the a value of a standard detectorat. All the values in eqn. I are, of course, assumed to be constant during the elution,except yr, which will change in both detectors in the same way or in a known way(see eqn. 8). For analytical purposes it is more important to know the quantitativerelationship between the detector response and the total quantity i?i than merely theresponse under given conditions. The relation222 hen holds

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8 M. KREJti, M. DRESSLER

(2)where t, to t, is the time interval during which yi increases above, or decreases below,the limit of detectorla sensitivity.

The area under the chromatographic curve S is given by:

SC = $- f, dttl (3)

where dbldt is the chart speed.For individual types of detector then holds222

CNSi =-& .$..$..?!%.NidV/ dt dYCCDSt =KbD .-.-.-.-b RT I da, . Ndt P dV/ dt dyf

S,MN K db . v I da,= MN *dt .-. dV/ dt - . AiidYtdasrD = KdD . $ . e * NiY,

(4)

If an additional gas is used in the detectors, which often occurs with selectivedetectors, the constants K acquire different values Kf and the sum of the flow ratesof carrier and the additional gases dV/ dt + dV,/dt must be substituted for the carriergas flow rate.

(b) Selectivity as the ratio of the time integrals of the responses of two detectorsIf linearity of response is assumed, the ratio of the time integrals of the responses

of detectors used will be identical with the ratio of the instantaneous responses ofthe two detectors

[%(t)lS = kbi(t)lA (8)The term response will from now on be substituted for the term time integral ofresponse.

The concept of detection system selectivity as a ratio of the responses of twodetectors

(9)is very suitable for qualitative analysis, as it implies both the element of selectiveresponse and the element of quantitative analysis of the mixture of compoundsinvestigated.It is assumed that SA # o. Although the responses of both detectors can beeither positive or negative, both values are always considered positive in connectionwith selectivity; therefore o < s < 00.Chromatog. Rev., 13 (1970) 1-59

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SELECTIVE DETECTORS IN GC 9TABLE 1SELECTIVITY AS THE RATIO OF THE RESPONSES OF A THERMAL CONDUCTIVITY DETECTORala ANDA FLAME IONISATION DETECTOR

Compound RG( TC) & RG( FI $ sPentane 1.023 0.982 I.042Hexane 1.020 0.996 1.024Heptane I.000 I.000 I.0002,sDimethylbutane 0.953 1.021 0.933Cyclohexane 1.040 1.028 1.012Benzene 0.905 I.093 0.828

a RG is a response of a weight unit compared to the heptane response RG = I.

Only in cases where a change in selectivity is investigated with respect to someparameters of the system, is it necessary to use negative values of s for the calculations.The requirement that S # o holds, of course, even in this case.

From the theoretical point of view, a system whose s = I + 6, where 6 is theerror in the determination31 of the ratio Ss/S*, can be considered as non-selective. Theincreasing demands on the accuracy of the quantitative determination of the sub-stances lead to the situation that when 6 = 2%, such systems of detectors will alsosatisfy the criteria of selectivity that are considered to be typical of representativesof non-selective detectors, see e.g. the system of thermal conductivity-flame ionisationdetectors for hydrocarbons. The values of s according to eqn. 9 for hydrocarbons,relative to heptane = I, are given in Table I. I t is obvious that under these conditionss differs by more than 2% even for the simplest type of homologuos series: pentane,hexane, heptane. However, with the present state of instrumentation 6 can be madelower by an order of magnitude235.Equally significant deviations of s values from I can be obtained in case of asingle flame ionisation detector using different ratios of nitrogen to hydrogen in theburneG*. The ratios of the maximum and minimum relative molar responses whichcan be attained for different substances are shown in Table 2. The RMR value in thiscase is influenced by the working conditions of the detector.

I t could be expected, in future, that these very small changes in the values ofthe selectivity s might be used for quantitative interpretation. The increasing accuracyof interpretation of quantitative chromatographic data and the wider and moreuniversal application of automatic integrators and computers, which can process theresulting data further235, ndicate this development.TABLE 2RMR RATIOS FOR ONE FID USING DIFFERENT NITROGEN AND HYDROGEN RATIOSCompound

Benzene I.079Acetone 1.278Butyl acetate 1.194Tetrachloromethane 1.110Pyridine 1.116

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IO M. KREJ ei, M. DRESSLER

At present, detection systems are usually considered as selective when theyhave s > IO.

With regard to the quantitative values as a criterion of the selectivity of achromatographic detection system it must be emphasised once again that the sub-stance or group of substances to which the selectivity is related and the operatingconditions of the detection system (for each of the detectors) must always be con-sidered. For example, let us consider the system of two of the detectors alreadymentioned, viz. the thermal conductivity and flame ionisation detectors, they wererightly considered as non-selective; but to the group of permanent and noble gases,carbon disulphide etc., this system will show high selectivity regarding the imper-ceptible response of the flame ionisation detector to the substances mentioned above.This property of the flame ionisation detector is often made use of when either thevapours analysed are diluted with permanent gases or carbon disulphide is used assoIvent23g.Under certain conditionsa a mere change in the flow rate of the hydrogenintroduced into the flame ionisation detector can change the value of s considerably.In Table 3 this value changes more than twelve times. At a hydrogen flow rate of95 ml/min there is a molar response of the detector to octane and the response totetrachloromethane nearly equal, however, at a flow rate of 55 ml/min the molarresponse of tetrachloromethane is fifteen times smaller than that of octane. In thelatter case the detection system can be considered as selective for octane.TABLE 3RM RATIO FOR ONE FID WlTH DIFFERENT HYDROGEN FLOW RATESH, velocity RM(C,HI,) /(ml/min) RM ccl,)

15.016.4

(c) Classification of selective detection systemsAs the basis for the classification of selective detection systems the generaleqn. I has been used and a classification of detectors followed from it. The basis of our

classification of detectors depends on three variable elements: the analytical propertyof the substance, the detector type and the working parameters of the detector.

The analytical property of the substances employed in the detector usuallyallows one to categorise the selectivity of the detection system according to a groupof substances and/or an homologous series. Detector type shows the dependence ofthe detection system on the flow rate of the carrier or carrier and additional gases,temperature and pressure. A change in detector working parameters can cause atransference in the significance of the individual elements of the analytical property,a change in signal, response, and thus even in the selectivity of the system.Diferent analytical property, the same detector type. The basic characteristics ofa detection system of this type are given by eqn. 9. From eqn. 9, the selectivity isproportional to the ratio of the analytical properties of selective (index S) and non-Chromatog. Rev., 13 (1970) r-59

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SELECTIVE DETECTORS IN GC IIselective (index A) detectors. A linear range for both detectors is assumed and thatwhen y = o, ato = o also. If the above-mentioned dependence does not pass throughthe origin (the latter part of the assumption), then the value of s = f(y) and thecoefficient of selectivity have a limited significance. The constant k includes an appa-ratus constant. In the case when the values T and P (see eqns. 4 to 6) are not identicalin both detectors, then this constant also includes the terms characteristic of the ratioof these values. The MD type detector (eqn. 7), being independent of pressure andtemperature, is the exception.

The basic advantage of the detection system consisting of two detectors of thesame type is the fact that value s is independent of the gas flow through both de-tectors and thus, in case of connection in parallel, is even independent of the splittingratio of carrier gas flow. If the carrier gas flow at the column outlet is split in a certainratio between two detectors (&A), then

(dV/dt)s lNak k+(dV/dt)* = - =N~)A (10)

After substitution from selected eqns. (4 to 7) with respect to eqn. 9 it canclearly be seen that s is independent of the ratio of gas flow through both detectors.The validity of the basic eqns. 4 to 7 is assumed.

Diffeerent agzalytical property, diffeerent detector type. The basic characteristic ofthe detection system, similarly to the previous case, is given by the ratio of analyticalproperties of a selective and a non-selective detector (eqn. 9). In addition it is alsonecessary to consider the parameters (pressure, temperature, volume), thus followsfrom the requirements of the quantitative characteristics of detectorszz2 (eqns. 4 to 7).If equal temperature and pressure are assumed for both detectors, TA = Ts,PA = Ps, a condition which is often satisfied, then use of the expressions 4 to 7leads to the relations:

CNSt KCN daijdyts1 = yii- = z da,ldyt

CNSt KCN RTCN da,b.Sg=-=-SF KMN PCNVMN da,ldycSic* KCN RTCN k+

S3==T=- KMD PMD (dl/'/dt)cN

(11)w

dalldys-da,ldyc (13)MNSt KNIN VMN da,ldyt

s4 = "FD --= - '+ (dV/dt)Mx da,/dytMD (14)

.SfD KCD RTCD k+S5=F=- KMD PCD (dv/dt)cD

CDSt KCD RTCD da,/dyis, = F = G PCDVMN da,/dyi

(15)(16)

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12 M.KREJCi,M.DRESSLER

In contrast to selective systems consisting of pairs of detectors of the same type,selective systems consisting of pairs of detectors of different types are usually de-pendent on the flow rate or on ratio of carrier gas flow rates. In our case only sl, s2,and s6 are independent of the carrier gas flow rate. Detectors of the CD type are rela-tively rare. Thus only the value s2, ndependent of the ratio of the carrier gas flowthrough the detectors, has practical significance.

The same detectors under d#eerent working parameters. Detection systems con-sisting of the same detectors working in different working parameters in such a waythat one of the processes provides the selectivity of the system fall into the thirdcategory. The selective determination of carbon monoxide240 by a catalytic combus-tion detector is an example of this. By suitable temperature contro1171J72 of thedetector wire, the combustion of carbon monoxide, specifically, in a mixture of othergaseous hydrocarbons, can be achieved and thus a detector signal specific to carbonmonoxide also.Different detector characteristics can often be imparted by reaction chromato-graphy or chemical multipliers (see section 1.B.b.). Detector sensitivity can often bechanged selectively by conversion of the effluent before its entry into the detector23!24.A substance or a group of substances are converted in a reactor into compounds towhich the detector is sufficiently sensitive. In some cases (e.g. conversion275 CO -+ CH,or hydrogenation of sulphur compounds261), selectivity can be increased by severalorders of magnitude and thus the value of s is also proportionally increased. In thefuture, use of chemical multipliers201 may supersede a number of these reactions andexpand the use of this type of selective system.Use of another analytical method. The three groups of detection systems thathave been described up to now are all detectors of binary mixtures. That means thata qualitative change in the effluent composition in a chromatographic zone cannot beregistered by the detector. This fact, among others, has led to the use of a completeanalytical method as the detection system.

When gas chromatography is used in combination with another analyticalmethod several basic demands must be met :

I. The analyser being used must have a sufficiently low time constant so thatonly a fraction or part of a fraction leaving the chromatographic column is analysed.

2. The analyser being used must be sensitive enough so that the efficiency ofseparation on the chromatographic column need not be decreased by increasing volumeof the mixture being led in.

3. The effective volume of analyser must be sufficiently small otherwise it couldcause a decrease in the efficiency 236of the chromatographic column.

The price of these detection systems is such that it often leads to the use ofsimplified versions of these analysers. Thus, e.g. a mass spectrometer with an ad-justable value of the mass being detected permits the selective detection of substanceshaving selected groups or elements 251. Even so, although this simplified and cheapermethod can solve some interesting analytical problems, an increasing number ofproducers and users incline to equipment which gives a complete and more or lessconcIusive analysis of the chromatographic fractions. But under neither of thesecircumstances chromatography can be considered as a separation method only. Theconclusive analysis, without previous orientational identification resulting fromretention characteristics of individual fractions, is very difficult in most complicatedChro~2atog. Rev., 13 (1970) 1-59

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SELECTIVE DETECTORS IN GC I3analyses (natural mixtures of substances) and, in many cases, even questionable.

The use of complete analysers as detection systems in gas chromatography is atypical example of systems combining a selective analyser (e.g. a mass spectrometer)and a non-selective chromatographic detector (a flame ionisation detector connectedin parallel or a record of the response of the ion source of the mass spectrometer).This illustrates that chromatography can be used here as an analytical (not onlyseparation) method permitting quantitative analysis of the mixture and also makesqualitative analysis easier. (I n many cases a selective analytical method is used as aconclusive method for the identification of substances the composition of which wasforecast from the elution characteristics.)

(d) Detector selectivity as relative responseWhile for qualitative analysis it is necessary to use a two-detector system, which

gives some information on the quantitative composition of the mixture being analysed,the use of one detector only is sufficient to determine the detector selectivity charac-teristics-and generally experimentally easier. Under these conditions, the detectorresponse to the signal of the substances being studied is then compared with that ofthe standard substance. The result is then expressed either in the form of relativemolar response222~250~303RMR)s, of the substance relative to the standard Y

RMRz, = darddyrda&b+-or in the form of relative weight response66r250RWR)t,)

RWRz, = daddctdadb

(17)

(1s)where c is weight concentration of the solute in the carrier gas. Hydrocarbon isusually selected as the reference substance. Detector response to it is considered asnon-selective. Under these conditions, as in the case of the two-detector system, itis suggested66 that a detector is considered as selective to a substance if the RWR orRMR is higher than IO.In certain special cases of the study of the characteristics of detector selectivity303even the substances to which the detector was just selective were used as standards.In this case, unlike that above, such a detector can be considered selective whenRMR -+ 1.0.This way of expressing detector selectivity accords better with physico-chemical studies of detector performance than with analytical practice.E. Forecast of selectivity for some detectors

Regarding the complexity of the processes taking place in some detectors, it isvery difficult to apply the general relations from Section I.D. to concrete cases. Theimport of selective detector information, however, frequently leads one to try toforecast the response or relative response of the detector.

(a) Electron capture detectorThe electron capture detector is considered to be mass non-destructive. I ts

response222 o a substance i can be expressed as follows:Chromatog. Rev., 13 (1970) I-59

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I4 M.KREJti,M.DRESSLER

Ri = KMN g (-3 0) yi (18)where KMN is a constant of the detector, and ei and cc, are the molar extinction coef-ficients of effluent and carrier gas, respectively. The molar extinction coefficient isan analytical property of the substance i and, therefore, according to eqn. 17 in thepreceding section, we can write for the relative molar response :

(&a 80)RMRi (Br_-80) (19)Over a small range, the contributions of the individual substituents in a mole-

cule to the molar extinction coefficient are additive (for the relative molar responserelated to the substance to which the detector is selective301y303, this range is esti-mated30a as having a value of RWR = I to 2). Then

&=ZI&s (20)where es is the contribution of a substituent to the molar extinction coefficient.

The values for the individual substituents have been tabulated301-303 in therange of additivity, and the value of the relative response can be estimated from these.The estimations, of course, suffer from a considerable error (usually up to 30%).

(b) Alkali jame ionisation detectorThe response of the flame ionisation detectoP2 is proportional to the number

of effective carbon atoms C,r and to the ionisation efficiency a.Ri = Km .a (Z Cer) . (dN/dt) (21)

where K I \ ID s a constant. One can also write for the alkali flame ionisation detector(AFID)

RP = [KM~* (Z&f) + KMD a (ZCef)l (dN/dt) (22)where KM is a constant and a* is the ionisation efficiency. In addition to the ionisationin the flame ionisation also occurs which is caused by the presence of alkali metalsand effective elements E,f (e.g. halogens, phosphorus) in the molecules of the effluent.I f a hydrocarbon is taken as a reference substance (i.e. the substance for whichE,r = 0), hen the relative molar response for an alkali flame ionisation detector canbe written as

RMRi = KMD a (ZC& + KM a* (ATE&KMD a (2:Cef)r (23)

Eqn. 23 will have the same form for the selectivity s as that according toeqn. 9 if the value s is taken as the ratio of the response of a flame ionisation detectorto that ofan alkali flame ionisation detector. Of course, the assumption (dN(dt)Frn =(dN/dt)AFrn must be fulfilled otherwise eqn. IO would have to be included in eqn. 23.

If the ionisation efficiency of both detectors is compared, it is found thatKMD a < KM a*

Chronzatog. Reu., 13 (1970) 1-59(24)

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SELECTIVE DETECTORS IN GC 15If a substance containing effective elements E,f in its molecule is used as referencesubstance, the expression for relative molar response can be written approximatelyin the form

RMRt = (Z EeM(Z Eef)rIn the cases where Cer is a constant the response (relative molar response)

increases relative to the number of effective elements in the molecule E,f in accordancewith eqn. 22, 23 or 25. An example g2 is shown in the first part of Table 4 in the caseof the first four substances. On the other hand, the second part of Table 4 (substances5 to 9) shows the cases when E,f is constant while the number of effective carbon atomsis different. Taking condition 24 into account, the ionisation efficiency calculated fora mole of effluent is constant.

TABLE 4NUMBER OF EFFECTIVE ATOMS E AND IONISATION FFICIENCY OF THE AFID=No. Compound E Zonisationeficienc?,(C/mole)I Chlorobenzene2 I, q-Dlchlorobenzene3 r,3,5-Trichlorobenzene4 r,z,4,5-Tetrachlorobenzene

I 1.382 2.793 4.104 5.53

2 Bromocyclohexane I 2.4q-Bromotoluene 2.5; /7-Bromostyrene I 2.4Bromobenzene I 2.49 2-Bromocymene I 2.5

-

In the. case shown in Table 4, a simple dependence of the retention charac-teristics (relative elution volumes) on the ionisation efficiency of the detector can befound. The dependence of the relative elution volumes on ionisation efficiency of alkaliflame ionisation detector for the substances, marked in the same way as in Table 4,is shown in Fig. 2. The apparent linearity of log Y with respect to ionisation efficiencyis in this case caused essentially by the lower ionisation efficiency of the flame ionisationdetector in comparison with the alkali flame ionisation detector (condition 24).F. Methods of connecting selective detectors

From the requirements following from the characteristics of detection systemsit is obvious that there are three basic possibilities with respect to the pneumaticcoupling of detectors. The coupling whereby a change in the ratio of effluent to carriergas does not occur (resulting molar fraction remains constant) is realised by eitherin parallel or in series connections. In some special cases (mass spectrometry) the

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161 Cl IBr 20 30 4CI

hl. KREJCf, M. DRESSLER

Fig. 2. Relation between relative retention volumes v and ionisation efficiency or the number ofhalogen atoms in solute molecule using an AF I J ).

concentration of effluent in the carrier gas beyond the column outlet must be increased.In this case so-called carrier gas separators are used.

(a) Parallel coupling of detectorsA series of separators used in preparative chromatography behind the columnoutlet for separating the effluent into two unequal portions had already been described

in chromatography. At the same time, approximately, carrier gas splitters were alsoused in front of a column. These devices both have the same aim: to permit the samplingof accurately measured-and thus also bigger-amounts of the mixture of substancesbeing analysed, as well as maintaining optimum performance (i.e. small loading) inthe former case of the detector, and in latter case of the capillary column. In boththese cases, carrier gas flow is usually split in ratios I/IO to I/IO.

The carrier gas flow is usually only split into two equal or only slightly differingportions in a system where the columns are connected in parallel, i.e. in equipmentfor temperature programming with a compensating column connection.It is evident that the instrumental introduction of selective detectors in paralleldoes not involve many complicated problems.

The detectors can be connected in parallel regardless of their type and sensi-tivity. Even when detector systems, the sensitivity of which does not differ much onefrom another, are preferred, the differences in detection sensitivity can be compensatedby the splitting ratio of the carrier gas flow. At present, the most frequently usedreference detector is the flame ionisation detector which is used in parellel with se-lective detectors, usually an electron capture or alkali flame ionisation detector. Thedependence of the detector signal on the ratio of the gas flow rate is discussed inChapter I.D.

(b) Series cou$ing of detectorsSeries connection of detectors

Chromatog. Rev., 13 (1970) r-59poses, much oftener than the case of parallel

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SELECTIVE DETECTORS IN GC I7coupling, problems concerning the separation of the components and the maintenanceof column efficiency. Although detectors with very small working volumes are usedat present, spreading of the chromatographic zones always occurs in the first detectorand with a consequent decrease in the separation quality of individual componentsin the second detector.

Furthermore, a destructive detector cannot be used as the first one in the series.The only exception to this is in the use of flame ionisation and alkali flame ionisationdetectors in series. This exceptional case follows from the properties of the alkaliflame ionisation detector, the effective component of which is not subjected to achange on passing through the ionisation detector. In addition the selectivity of thissystem is increased still further on account of the removal of the carbon skeleton ofthe substances.

In general, however, the use of a non-destructive type detector is an essentialrequirement.(c) Carrier gas separators

The combination of high separation efficiency, especially in the case of capillarycolumns coupled with the high selectivity of spectral methods, particularly massspectrometry, has been shown to be extraordinarily efficientzg6. Direct instrumentalcoupling brings about advantages in the form of time and labour saving, and alsopermits the transfer of extremely small amounts (approximately IO-* g) of the sub-stances, without any danger of their decomposition by moisture or air, from thechromatograph into the spectrometer.

To maintain the optimum conditions of chromatographic separation and thefunction of mass spectrometer, carrier gas separators (see Section 2.1) must be usedso that the carrier gas is enriched by solute. A maximum amount of 0.1 to 0.3 ml/minof carrier gas may enter the ion source of the spectrometer. This carrier gas flow is,of course, too low to maintain good efficiency of the chromatographic column.

Gas chromatography coupled with mass spectrometry is the most efficientsystem.of qualitative analysis of complex natural mixtures known at present. Flameionisation detectors in parallel are used as non-selective reference detectors or theionisation current from the source of ions is directly measured on the entry of thesolute into the mass spectrometer from the chromatographic column.

2. ANALYTICAL PART

A. The electron capture detectorThe physical basis of detection is the reaction of free electrons with moleculesof gases which results in the formation of stable negative ions1g011g3.AB + e- = AB- f energy (1)AB + e- = A + B- A energy (II)In the case of ionisation of an inert gas-nitrogen-only electrons are present

as far as negative particles are concerned. The probability of the recombination ofChromatog. Rev., 13 (1970) I-59

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18 M.KREJCi,M.DRESSLERthese electrons with positive ions is low because of their very different velocity (thevelocity of an electron is approximately 10~ times higher than that of positive ions)288.In this case, only a low potential is necessary for the collection of the electrons i.e. toattain the saturation current lg3,194. f an ionised inert gas contains compounds havinghigh electron affinities, some free electrons may be captured by the molecules of thesecompounds with the formation of negative ions proceeding according to reaction (I)or (II). The velocity of these anions is much lower than that of the free electrons, andthe probability of recombination between negative and positive ions is, therefore,IO~--IO~ times higherlag than that between electrons and positive ions. The presenceof a compound able to capture electrons will be indicated by a decrease in the ionisationcurrent in the detector.

In this case, the potential, necessary for the collection of the negative particlesand attaining saturation current, is higher and proportional to the electron affinityof the compound present in the detector. For a given compound, however, this po-tential does not depend on electron affinity alone but also depends on the pressure,construction, and detector temperatureXg4.

According to the type of potential applied, two detection methods are recog-nised, i.e. the so-called d.c. methodls4 and the pulse method lge. In the former case, aconstant d.c. potential is applied to the detector electrodes and the detector is exposedto this potential for the whole course of the working operation. With the pulse method,a potential is applied to the electrodes in short time intervals (approximately xoo,usec)for a short period (approximately 0.5 psec). Thus for most of the working period thedetector is not subjected to the potential. The scheme of the circuit connections forboth methods is shown in Fig. 3.

Fig. 3. Circuit connections for the electron capture detector193. (A) d.c. mode of operation:S = source of potential zoo V; R, = I ML?; D = detector; A = amplifier and recorder; V =voltmeter. (B) Pulse method of operation : P = pulse generator, source impedance 1000 f2 orl ess; D = detector; C, = IOOO u,uF; R, = I OMSZ; A = amplifier and recorder.

The probability of electron capture by different types of molecules spreadsover a range of ro6 and depends on the presence of the so-called electrophores in themolecules.

Electron affinities of some compounds expressed relative to chlorobenzene areshown in Table 5. In addition to the type of electrophore, the number of bonds andeIectrophores and structure of the molecular fragment also affect the detectionsensitivity.

In the case of halogen compounds the detection sensitivity increases in theChromatog. Rev., 13 (1970) I -59

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SELECTIVE DETECTORS IN GC 9TABLE 5RELATIVE ELECTRON ABSORPTION COEFFICIENTS OF VARIOUS COMPOUNDS AND OF CLASSES OFCOMPOUNDS FOR THERMAL ELECTRON+

ElectronabsorptioncoejicientCompounds and classes Electrophores

0.01

O.OI--0.xO.I--I.0r.o--IO

IO--IO0IOO--IO00

1000-10~

Aliphatic saturated, ethenoid, ethinoid and dienehydrocarbons, benzene and cyclopentadieneAliphatic ethers and esters and naphthaleneAliphatic alcohols, ketones, amines, aldehydes, nitri les,monofluoro and chloro-compoundsEnols, oxalate esters, stilbene, azobenzene, aceto-phenone, dichloro, hexafluoro and monobromo-compoundsAnthracene, anhydrides, benzaldehyde, tr ichloro-compounds, acyl chloridesAzulene, cyclooctatetrene, benzophenone, cinnam-aldehyde, monoiodo, dibromo, tri- and tetrachloro-compounds, mononitro-compoundsQuinones, r,e-diketones, fumarate esters, pyruvateesters, diodo, tribromo, polychloro, and polyfluoro-compounds, dinitro-compounds

NoneNone.OH, .NH,, .CO., .CN,halogens.CH:C.OH, .CO.CO.,halogens.CO.O.CO ., halogens,phenyl . CO.halogens, NO,,phenyl.CH:CH.CO*.co.co.,.CO.CH:CH.CO.,quinone structure,NO,, halogens

sequence Cl < Br < I ; for the butylhalides, e.g., the corresponding values for mini-mum detectable amounts are I x ~o-ll, 8 x 10-14, 2 x 10-1~ moles/set. In additionthe response is not proportional to the number of halogen atoms in the molecule, butit increases rapidly with an increasing number of atoms7*-80,1g3. ith sterol-halogeno-acetates, however, the response was found17 to increase in the inverted sequence, i.e.trichloro- < dichloro- < monochloro-compounds or Br < Cl. This anomaly is ex-plained by the increase in induced polarity of the double bond C=O and by ad-dition of an electron to the carbon atom of the CO group instead of to the carbonatom binding a halogen atom.TABLE 6EFFECT OF THE POSITION AND THE MULTIPLE SUBSTITUTION OF ELECTROPHORES ON ELECTROWABSORPTIONQS

Electrophore Compound or class AbsorptioncoeffccientCl

Cl*

Cl,

Cl.3

Vinylic 0.2Aromatic IAliphatic 0.3Allylic 55Benzylic 110Benzene, o 42Benzene, m 30Benzene, p I I.CHCl, IBenzene, 1,2,3 113Benzene, 1,2,4Benzene, 1.3,; 7560. ccl, 500Hexachlorobenzene 1100ChtOmUtOg. Rev., 13 (1970) I-59

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20 M.KREJCi,M.DRESSLER

In the d.c. method the effect of the molecular fragment on the response tochlorobutane can be seen as an increase resulting from the different stabilities andvelocities of the electron additions. The sequence is n < iso < ted. < sec., while forbromo and iodobutane the sequence is vz < iso < sec. < ted.In case of the pulse method the highest response is always given by the tertiaryhalogenobutane176.

The responses to acetyl and alkylchlorides are considerably higher than thoseto arylchlorides2s8, the response to halogen bound to a double bond of ethylene orbenzene is low in comparison with halogen in an allylic or benzylic position18s~1s3.Likewise herbicides, such as Diuron, Linuron etc., containing chlorine, do not showvery high sensitivities126, probably due to the influence of the chlorine bond on thearomatic ring.

A survey of the effect of the number and position of electrophores in a com-pound on electron absorption is shown in Table 6.With phosphorus pesticides, electron affinity is also dependent on the molecularstructure, and increases in the systox group in the sequence

0 S 0 SII II II IIP-O < P-O < P-S < P-SZIELINSKI et a1.301-303 tried to determine the mean relative response due to

individual structural groups, and from these individual contributions to calculatethe total relative response of a molecule. Their theoretical calculations are in verygood accordance with experimental results.

I t is interesting that the detector is sensitive not only to the compounds con-taining halogens1ss~1s0~1s3~1s4~2ss,hosphorus and sulphur 56,106,116,223, lead75,120,1S3,195,nitrogen dioxide218s21Qand nitrates4Q+, oxygen compounds, ozone and oxygen101J 8Q,but also to certain hydrocarbons such as e.g. azulene and stilbene18sJ s3, cyclooctate-traene, anthracene and other aromatic compounds lss. Some examples of the sensitivityto some compounds are as follows : Parathion -I x 10-l~ moles/sec5s, tetraethyllead-3.7 x 10-l~ moles/sec75, tetrachloromethane -I x 10-l~ g/seclQO,and chloroform-3 X IO-l3 g/seclQ3.

Detector response depends on detector temperature, the higher the temperaturethe higher the response. This increase in response is higher, however, with the d.c.method176 than with the pulse method 176,218. he higher the detector temperature, thelower is the potential necessary to attain the value of the saturation backgroundcurrent7sps0. This effect may be caused by changes in gas density; at higher temper-atures electron collection is easier as the number of collisions with gas molecules issmaller. The maximum value of the background current, however, does not change.Maximum response does not change either; with increasing temperature, however,the potential necessary for it to be attained again decreases78,80.

In the case of reaction (I), in which a molecular negative ion is formed, the energyofelectron capture (electron affinity) is released either as radiation or is divided amongother molecules during subsequent collisions. With this type of capture, the cross-section for electron capture decreases with temperature or with increasing electronenergy. Dissociation with the formation of a radical and atomic or molecular negativeChromatog. Rev., 13 (1970) 1-59

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SELECTIVE DETECTORS IN GC 21ion occurs with reaction (II) after electron absorption. The increase in temperature orenergylgl of the electrons may increase the probability of electron absorption.

The change in response with detector temperature thus depends on the reactionmechanism. In reaction (I) the response decreases, e.g. as for azulene; in reaction (II)it increases, e.g. as for dodecylbromide 25g. A detailed analysis of the influence of thereaction mechanism on the temperature dependences has been given by WENTWORTHAND CHEN~$~,and shown in examples by ZLATKIS et al.22%304a.

The maximum working temperature of the detector is, however, limited by thematerial of the radiation source. The most frequently used material, 3H, looses 4 ,uCidaily, but on heating above 188 almost five times as much76 3H is lost. In practiceit is not recommended to use a temperature above 2oo15Q57,25* with a 3H detector.Above this temperature 226Ra, 241Am or 63Ni are used 257,259@*. The decrease in back-ground current is very small with 6aNi; in the course of four months running at atemperature of 3oo-400 it decreased from a value 9.65 x IO-~ A by 0.8 x 10-l~ A(ref. 259). A comparison of 3H and 63Ni detectors was carried out by HARTMANNet a1.133,who found that the response of the 3H detector (detector temperature 20.5~)is higher than that of the 63Ni one (detector temperature 240~) ; the linear range of theresponse of the 3H detector was approximately higher by an order of magnitude.

Changes in response due to carrier gas flow rate depend on the method of de-tection being used. With the d.c. method response increases with a decrease in flowrate84~85J76~253.Below a certain flow rate, however, the response decreases78-80~84,85very rapidly. The carrier gas flow rate through the detector should be higher than100 ml/min1g3. In order to reach these velocities, a so-called scavenger gas is sometimesused; it is added to the carrier gas after the chromatographic co1umn7a--80~1g3. n thiscase, the chromatographic peak height is inversely proportional to total flow rate78-80.WASHBROOKE~~~ howed, however, that the response increases with increasing carriergas tlow rate. With the pulse method the tlow rate dependence is negligible54 andabove a certain value the flow rate has no effect at allr76.

Detector response is also dependent on detector construction58. It dependsvery much on the distance between the electrodes. For 8 mm and greater distancesthe response is constant, below 8 mm it decreases rapidly229. An inverse dependencewas also found by WASHBROOKE~~~,who, in addition, found that the response disappearsat a distance of 0.45 mm.

With the pulse method, the response is dependent on the pulse duration and onthe interval between individual pulses. With increasing pulse duration time theresponse increases up to a certain level and then remains constant78~80,254*25g. Thesame dependence holds also for the pulse period78~80~1s1,25D.

The background current is within the range of 2 x IO-~ to 2 x IO-~ A54,18g~218*21g,detector noise in the range of I x IO-i2 to 7 x IO-12 A5%75,218?9. Detector responseis linear over a range of three orders of magnitude 54,84,85,17%191nd it is the same forboth the pulse and d.c. methods54.

The cross-section for the capture of free electrons is rather small (approx. IO-l6cm2)lBg even with highly absorbing gases. However, in the course of electron transferthrough a layer of gas of I cm thickness at atmospheric pressure up to IO5 COlliSiOnswith gas molecules occur. Capture probability and effective ionisation efficiency arethus very high for strongly absorbing gases,

The ionisation efficiency for Parathion is approximately ro3 C/mole58; forChromatog. Rev., 13 (1970) r-59

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22 M. KREJCi, M.DRESSLER

compounds containing more halogen atoms it is still higher. This high molar responsemeans that minimum detectable amounts are very low even at high noise levels(approx. 5-10-l~ A), e.g. for butyliodide 176the level is 2 x 10-l~ moles/set.

The response attains a maximum at certain voltages 8%190,218,299_his optimumvoltage is different for different types of compounds and is thus characteristic of agiven compound and can serve as qualitative characteristic. With the pulse method,this means of identifying the compounds cannot be used unless the concentration ofthe vapours is known. Electron absorption is, in this case, determined by comparisonof signals of an electron capture detector and of a quantitative detectoP3.

Another method for the differentiation of compounds, which is a modificationof the pulse method, was used by LOVELOCK et al. lg2. It consists in changing the energyof the electrons by applying a high frequency electric field during the time intervalbetween individual pulses; electron energy influences the velocity of electron capture.With compounds where capture proceeds according to reaction (I), capture velocitydecreases with increasing electron energy. In case of reaction (II), the velocity in-creases to a maximum and then decreases again. Application of such a field mayheighten the response to compounds in reaction (II), e.g. to chlorobenzene, while theresponse to compounds absorbing electrons according to reaction (I) decreases, e.g.to benzaldehyde.

A simple ionisation cell with a low potential does not work as absorption de-TABLE7FALSE RESPONSE OF THE D.C. ION CHAMBER ELECTRON ABSORPTION DETECTOR WHEN USED WITHVARIOUS CARRIER GASES AND TEST SAMPLES-la = Low concentration of a strong electron absorber; b = high concentration of a weaklyabsorbing compound or a low concentration of a strong absorber mixed with a high concentrationof non-absorber; c = high concentration of a non-absorber.Unwanted mode ofdetection Carrier gas

Cross-section HeliumHydrogenNitrogenArgonArgon-methane

Metastable atomprocesses(argondetector)

HeliumArgon

NitrogenArgon-methaneElectron mobilityeffects ArgonHelium

Nitrogen

Argon-methane.._____ .____&O?lZUtOg. Rev., 13 (1970) I-59

Extent and nature of CYYOYYalse wsponse z&htestsanzplesaVery slightreductionNone

Slightreduction

None

Slightreduction

bSignalreducedSignalreduced

SignalreducedgreatlyNone

None

None

SignalincreasedSignalincreasedNone

cNegativeresponseSlightnegativeresponseLargenegativeresponseNone

Large falseresponseFalseresponseNone

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SELECTIVE DETECTORS IN GC 23tector onlylgl; it can also work as a cross-section detector, an argon detector, or adetector of electron mobility. The range of these detection processes depends on thegas composition, intensity of the radiation source, potential, and detector geometry,as is shown in Table 7.Traces of eluted components absorbed on the surfaces of electrodes may, undercertain circumstances, be the cause of a potential which is either complementary orinverse to the potential applied lgl. This constant potential is usually of the orderof several volts. If it is inverse, the chromatographic peak has an anomalously largearea and often tails; if it is complementary, the response decreases and the baseline,after passing the substance through the detector, can be depressed.

Electron energy, in noble gases and to lesser extent also in nitrogen, is consider-ably higher than the energy at thermal equilibrium for gas molecules, as the electronenergy obtained from the applied field only weakens slowly due to the elastic collisions.Polyatomic gases, however, reduce electron energy by non-elastic collisions. Thecross-section for electron capture changes considerably with electron energy in thecase of some compounds, especially if they contain halogen atoms. If more than onecompound is present in the detector, distortion of the results may occur as a result ofchanges in molar response.

The differences in mobilities of positive ions and electrons can result in a spacecharge being formed in the vicinity of the cathode in the detector, the potential ofthis charge being inverse to the potential applied. This charge changes the mobilityof the free electrons and as the probability of absorption depends on the length oftime the electron spends in the detector, the response also changes. At the same timethe linearity of response may change as well.By use of a carrier gas with the addition of non-absorbing material, the energyof the free electronslgl can be reduced due to non-elastic collisions. A suitable mixtureis argon with 5% of methane. The addition of methane also serves as a means ofremoving metastable argon atoms by deactivating collisions. Thus, under theseconditions the detector can neither work as an argon detector nor as a detector ofelectron mobility. Electron energy (and thus also the absorption cross-section of thesolute) can no longer change in the presence of other vapours as it is already sufficientlylow. Contact potential and space charge will, however, have a still greater effect underthese conditions as electron velocity in the mixture is higher, and the potential usedmust thus be lower.It is obvious that the use of the d.c. method cannot remove all the detectortroubles. However, they can be removed by use of a method in which recombination ismore effective as a result of shortlasting application of potential.

The detection sensitivity is dependent on the background current. Everystationary phase has a certain vapour pressure at a given column temperature. Aseven a small vapour pressure may cause change in the background current, andthus also in sensitivity, the temperature limit for use of the phases in the case of anelectron capture detector is far lower than that for other detectors78~80 (Table 8).

For some phases, especially non-polar ones, such as Apiezon L and methyl-silicones, this effect of column temperature may be decreased by increasing the de-tector temperature25g.

A further disadvantage of the detector is the relatively high number of para-meters affecting the sensitivity of detection. The fact that the change in sensitivity

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94TABLE 8INFLUENCE OF PHASE BLEI?,,INGON DETECTOR RESPONSE

.__ _~__Phase Preconditioning

.._~ ._~~_~ ___ -~-Squalane 105 16Apiezon L 200 16Silicone SE-30 280 16Silicone SE-52 280 16Polyethylene glycol succinate 200 20Carbowax 20 M 200 16Di-n-decylphthalate 50 16Tri-z,+xylenylphosphate 115 16P,fi-Oxydipropionitrile 7o 16

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

M. KREJCi, M. DRESSLER

__.Maximum workingtewzperatuve (ClWith an With aEC conventionaldetector detectorIO0 100=5o 250210 350170 350150 225II0 20065 I7580 150< ambient 65

can differ for different compounds, depending on the conditions, is especially signifi-cant since changes in relative response may occur on account of this. Relative re-sponses were found to change with potential, temperature, and pulse period, but theeffect of a change in pulse time on the response was negligible.

The response is approximately the same with the pulse method as with thed.c. method54,2g9, however, it is relatively independent of voltage81~218J 5g and hasbetter reproducibility i6*. Hence the pulse method obviously has advantages over thed.c. method.

The sensitivity of a miniaturised detector coupled with capillary columns issomewhat higher82 than that of an ordinary detector for work with packed columns.Scavenger gas is used with a detector of this type; its minimum tlow rate is given asIO ml/min as at lower flow rates an increase in HETP82a occurs. The flow rate influencesthe dependence of the response on the potentialsza and on the distance between theelectrodes231a.

For qualitative determinations, an electron capture detector in parallel witha flame ionisation detectorgg,225,304 s often used.B. The alkali flame ionisation detector

The alkali flame ionisation detector, also called a thermionic detector, so-dium detector or phosphorus detector, is a modification of the flame ionisationdetector. I t makes use of differing ionisation processes in the flame in the presence ofan alkali metal. It was found that the response of this detector to some compoundscontaining certain heteroatoms is many times higher than that of the flame ionisationdetector ; e.g. for phosphorus compounds this increase is about 1,000 times. KARMEN~~,in one of the first papers on this detector, ascribes the enhancement of the response toaugmentation of alkali metal salt concentration in the flame as a result of the in-creased volati lity of the salt in the presence of phosphorus. Later, it was, however,found that response is also enhanced if the alkali metal is introduced into the flamein a stream of helium@?.Chronzatog. Rev., 13 (1970) x-59

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26 M. KREJ Ci, M. DRESSLERTABLE 9DEPENDENCE OF AFID RESPONSE ON THE ALKALI METALCompound Ratio40 A FIDIFID A FID responses (Clmole)88

K Rb CS NlZ K Rb cs

N compound 30 60 150P compound 10 000 II 000 12 000As compound 5 IO 30a Background current = 6.10-f A.

3.5 x 10-I 5.9 x 100 1.3 x 101 1.7 x 1012.1 x 102 6.4 x 10~ 8.1 x 102 I.2 x 103

148369,270, jet diam&+@J46, car&r gaslOO,l46, and the effect of the anion of the saltbeing used179as.

The alkali metal8,89,96,100,112,146,148,158nd the hydrogen flow rate8J 7J 0gJ 57~177J 77have the greatest influence on the response value. The response increases on increasingthe atomic number of cation89*146 (Table 9).

Small changes in the hydrogen flow rate affect the response value considerably.The amount of hydrogen determines the flame temperature and, therefore, also thetemperature of the alkali metal salt. This means that on increasing the hydrogenflow, the background current increases as a result of stronger emission and higherionisation of the alkali metal salt. It was founds8 that under constant conditions (air

Fig. 5. Plot of AFID ionisation efficiency against background currenta8. I = P compound; z =N compound; 3 = C compound; 4 = Cl compound; 5 = S compound.Chromatog. Rev., 13 (1970) I-59

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SELECTIVE DETECTORS IN GC 27and carrier gas flow rates, jet diameter and detector geometry) and if the backgroundcurrent is maintained at a constant value, the detector response is not dependent onsmall changes in the hydrogen flow rate which compensate the changes in backgroundcurrent caused by the leak of alkali metal salts with time and on the distance of thecollector electrode from the flame. The increase in response with increasing hydrogenilow rate, therefore, may be connected with the increase in background current,Changes in direction (polarity) of the responses9 are also connected with an increasingvalue of the background current. Such changes for compounds containing phosphorus,nitrogen, chlorine, sulphur and hydrocarbons, and a potassium salt are shown inFig. 5.

The response to all these compounds at first increases, but that to compoundscontaining chlorine and sulphur and to hydrocarbons first reaches a maximum andthen acquires negative valueP at a certain value of background current dependingon the heteroatom of the solute and on the cation of the salt used. The response curvesfor other cations are similar, only with chlorine-containing compounds and sodiumas the alkali salt the response increases continuously.

Table IO shows the ionisation efficiencies for compounds with different hetero-atoms, obtained under optimum operating conditions.

The alkali flame ionisation detector is selective to phosphorus compoundslos,111~157J60,267~276, alogen compounds74J0sJ57~267 (with the exception of fluorine),arsenic146J4s, nitrogen~~66~88~s8~132~146~2s2,in14s, and sulphur compoundsso~91.

The selectivity s (9) for phosphorus compounds, in the case of a flame ionisationdetector, increases up to a value of 103-104 if salts of any of the alkali metals (Na,K, Rb, Cs) are used.There is a selective increase in the response in the case of nitrogen compoundswith all alkali metals except sodium 89,146. Molar response is essentially independent

TABLE 10SENSITIVITY OF AN ALKALI FLAME IONISATION DETECTORCompound Ionisation efficiency (Clmole)

Double-jlame Single-flamedetector@ detector

O,O-Diisopropyl methyl-phosphonate 1.3 x IO* 2.2 x 102DDVP I.5 x 108 -Disyston I.9 x IO= -Triethylarsine 2.3 x 10-l 2.3 x 10-lTetrapropyltin -7.0 x 100 -Thiophene 4.5 x 10-z -6.0 x IOOThiophane -5.7 x 100Hexyl hydrosulphide - -6.1 x 10~Pyridine - 4.6 x IOa-Methylpyridine 4.8 x 100Acrylonitrile 4.8 x 10~Chlorobenzene I.6 x 100 I.4 x IdBromobenzene 3.4 x 100 2.4 x IO04-Bromotoluene 2.3 x 10~ 2.5 x 10~/3-Bromostyrene 7.0 x 10-l 2.2 x IO0

Referencefor shgle-&medetector

88

919191*7a878792929292

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28 M. KREJ Cf, M. DRESSLERof compound structure146 and it is approximately 40-100times higher than with anFID.

Data up to the present time show that the AFID response to halogen compoundsappears to be very much dependent on the detector construction. J ANAK AND SVOJ A-NOVSK.';'~~*and KARMEN~~~ ound that the detector is selectively sensitive to thesecompounds when salts of all alkali metals are used, while GI UFFRI DA et ~1.l~~ oundthat with the use of KC1 or KBr the response to halogen compounds can be depressed.The response of an alkali flame ionisation detector with a jet tip of pressed salt canbe even negative92,130,75,15a to halogen compounds under certain conditions. Increasein the selective response, compared with the FID, for chloro- and bromo-compoundsonly occurs in the case of sodium saltss2, with iodo-compounds it only occurs witha potassium salt.

Selectivity to sulphur compounds is manifested, not only in the increase in theresponse (s = IO to 20) but also in the direction (polarity) of the response. In the caseof potassium, rubidium and cesium salts, the response is negative, i.e. a decrease inionisation current occursoolsl in the detector.

The upper system of a double-flame detector is highly selective to compoundscontaining phosphorus and halogens (except for fluorine). I t is not possible to say,however often it may be the case, that it is sensitive to the above-mentioned com-pounds only. Sensitivity to other compounds depends on the function of lower systemand on the type of the cation used 14*1l~~. he solvent for example always gives aresponse.

Each of the systems of the double-flame detector has its own electric field.Experimental work has shown that these two fields influence one another and thatthey can distort the performance of both detectors148~26g~270.

The effect of column temperature on detector sensitivity was studied and itwas found that : (I) with decreasing temperature (and therefore with increasing timeof solute passage through the jet of the flame ionisation detector) the response valuedecreases, (2) the curves of the upper system are always narrower than those of thelower one, and (3) that narrowing of the elution curve and loss of sensitivity arecaused by the action controlled by the relation holding for diffusion of the gases. Thisfinding explains the different ionisation efficiencies of monobromo-compounds and thedifferent contributions of individual chlorine atoms in mono-, di-, tri-, and tetra-chlorobenzene, when ionisation efficiency always decreases in the sequence of in-creasing elution volumes140.

Ionisation efficiencies of monobromo-compounds and chlorobenzenes are equalor are a multiple of the number of heteroatomsg2 in the case of a single-flame detector.This detector is, however, sensitive to all compounds which can be detected in anordinary flame ionisation detector though the sensitivity in this is always lower.Selective enhancement of the response, however, occurs not only with halogen andphosphorus compounds but also with compounds containing nitrogen and sulphur.

With a single-flame detector with a jet tip of pressed alkali metal salt, the noiseis considerably lower than with a double-flame detector and the minimum detectableamount is, therefore, also lower at approximately the same ionisation efficiency. Withthis detector type the value and direction (polarity) of the response are stronglydependent on the type of alkali metal cation and on the background current. Bysuitable selection of the experimental conditions, it is possible to achieve a selectivityChromatog. Rev., 13 (1970) I-59

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SELECTIVE DETECTORS IN GC. 29of response to a given heteroatom not only by an increase in the selectivity s (9)but also by a change in the direction (polarity) of responsess~g1J 2J 75J 75a.

The ionisation efficiency of this detector is considerably higher than that ofother ionisation detectors. When a cesium salt is used, it reachesa a value of approxi-mately 2 x 10~ C/mole (2 %) for phosphorus compounds. This means that the absoluteresponse is 2-3 orders of magnitude higher than that with the flame ionisation detector.However, detector noise is also higher by 2-3 orders of magnitude. The minimumdetectable amount is, therefore, comparable with that of the flame ionisation detector.

The disadvantage of both types of alkali flame ionisation detector is the decreasein sensitivity with time resulting from loss of the alkali metal in flame. The velocityand value of this decrease depend considerably on the geometry and the way in whichthe salt is placed in the flame. The rate of this decrease can be considerably loweredwith some types of detector130,131,224,nd if at the same time the decrease in back-ground current is compensated by a change in hydrogen flow, it can, for a certainperiod of time, have a zero value*.C. The surface ionisation detector

The surface ionisation detector (thermionic) makes use of an effect describedby KINGDON AND LANGMUI@~P~~~ and was used as a leak detector by RICERS. Theeffect consists of emission of positive ions of an alkali metal from a heated electrode,the material of which has a higher electron affinity than is the value of the ionisationpotential of the alkali metal. In the presence of halogens and in oxygen media, theemission of positive ions increases. According to MOESTA AND SCHUFF~~~ the increasein positive ion emission is caused by the fact that a complex, Cl-K+-0, is formed onthe surface which changes considerably the work function.This detector was first used in gas chromatography by CREMER et a1.6g(Fig. 6)and it is usually called the thermionic detector6Q7.

3

From olumnFig. 6. Diagram of a surface ionisation detector. I= Cathode; 2 = anode; 3 = heater.

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30 M. KRE J Cf, M. DRESSLERIt has been found sensitive to halogen compounds 66,67,69,70,74,179,210nd to some

organophosphorus pesticides containing sulphur 210. Sensitivity to chlorine is giverPgas 3 x 10-10 g/set; the minimum detectable amount of tetrachloromethane is 1.5 xIO-lo g/secB3, sensitivity to mercaptans and hydrogen disulphide is approximatelyIO p.p.m.l*_ The response is independent of the type of chlorine atom bond and mole-cular structurejg. It was foundjhowever, that the solvents used in a large excess withrespect to the halogen compound- s caused changes in the baseline70 after havingpassed through the detector. CREMER et al. 70 introduced a so-called pre-combustionremoving all these effects and also improving the reproducibility of the response. Atthe same time this pre-combustion decreases sensitivity and increases noise. Molarresponses of mono-, di-, tri-, and tetrachloromethane are in the ratio I :z:3:4 (ref. 18).Ratios from I :z up to I :2.2 were found for mono- and trichlorobenzene70, but afterpre-combustion, the theoretical value of I :3 was reached.

Detection sensitivity depends on the temperature to which the wire is heated(Table I I ) and is thus connected with the background current210.TABLE 11CHANGE IN THE RESPONSE OF A PLATINUM THERMIONIC DIODE TO SOME PESTICIDE COMPOUNDSWITH INCREASING INCREMENTS OF FILAMENT HEATER CURRENT=l

Pesticide

LindaneHeptachlorAldrinTelodrinKelthaneHeptachlor epoxide+,fi-DDTDieldrinMethoxychlorParathionMalathionTrithion

Injected Filament current and re.sponse asCM?) measured by peak height in mm

1.7 A 1.8 A 1.9 A1.8 x 10- 4 I O 30 913.8 x IO-~ ' 5 32 1163.6 x I O- ~ 7 ' 7 723.6 x IO-~ II 26 971.1 x 10-4 7 I4 753.6 x 10-4 II 22 953.6 x IO-~ 4 9 403.6 x I O- ~ 4 9 399.0 x 10- s 13 30 904.0 x 10-S 0 0 08.0 x 10-a 0 0 08.0 x 10-3 -a 4 81.6 x IO-~ 134.0 x 10-3 7 ' 78.0 x IO-~ - - ::

a Dash indicates that no observations were made under these conditions.

With increasing background current (higher temperature) the response increasesbut at the same time the detector noise also increases so that there is no sense inincreasing the current above a certain value (differing with the age of anode)*. Asa result of strong dependence of the sensitivity on temperature, the detector must bethermostated with accuracy i 0.5%r*@.

The response depends on the type of carrier gaP, probably on account of thedifferent thermal conductivities, on carrier gas flow rates3, and on pressure18*6g. Atlow pressures, the sensitivity is 5-10 times higher.

Detector response is linear over a range of 4-5 orders of magnitude70~s3.ChVOWLatOg.TV., I3 (1970) I-59

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SELECTIVE DETECTORS IN GC 31As the molar response of this detector is independent of the type of halogen

atom bond and molecular structure, and as it is a multiple of the number of atomsin a molecule, the response of more compounds can be forecast from one calibration.Relatively high selectivity is a further advantage of the detector.The essential disadvantage is the time instability of the detectorll. Sensitivitydepends very much on the age of the anode18.The molar response ratios for compoundscontaining fluorine, chlorine, bromine, and iodine is 0.9:1 :o.8:0.1. These ratios,however, also change with time. I t is true that chlorine always has the highest valuebut the response can decrease to zero for fluorine, and can be nearly equal to that forbromine in the case of iodine. New anodes show non-linearity in response for a certainperiod of timen.

The detector also has a relatively high time constantl reaching values of 10-20seci8,69.D. The flame photometric detector

The principle on which the flame ionisation detector is based consists in thephotometric detection of the flame emission. Any flame photometer adapted to agiven chromatograph and placed behind the chromatographic column can serve asa detector.

The coupling of a chromatograph with a photometer has been described indetail by BRODY AND CHANEY~~ nd J UVET AND DURBI N~~.An oxygen-hydrogenflame is used for excitement, and nitrogen or helium as carrier gas.

The first use of flame photometry as a means of detection was the application ofBeilsteins test for halogen determination125~g3~217with the exception of fluorine).A copper wire is placed in the flame and in the presence of halogenides the flame turnsgreen. The sensitivity of this method was in the order of micrograms.

Later, this method of detection was used in gas chromatography for the de-termination of metal chelatesg4~154~300.he sensitivities to individual elements areshown in Table 12, together with the appropriate wavelengths. Sensitivities are

TABLE 12LIMITS OF DETECTION FOR THE FLAME PHOTOMETRIC DETECTOR156,300

Emission Limit ofwavelen$h detectionImpI (moles)

MOF~ 520.0WF.9 520.0 j ; ::I::SnCl* 358.0 7 x IO-*Al(tfa), 486.0 5 x 10-9C&b 431.0 I x 10-BCH,COCH, 431.9 2 x 10-6TiC14 544.9 4 x 10-nAsCl, 500.0 2 x 10-oZrCll 564.0 2 x 10-SRh(hfa), 369.2 1 x 10-11Cr(hfa), 425.4 I x 10-10CO, 431.5 7 x IO-

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32 M.KREJCi,M. DRESSLERexpressed as the number of moles giving a signal double that of the noise level onpassage of the compound through the detector. It can be increased by up to twoorders of magnitude by improvement of the optical system300. By suitable selectionof the slit width and the wavelength, the detection of emission of the element onlymay be obtainedi55. For example, in the case of volatile chromium chelates at 425.4 m,ueven a ten-fold excess of iron chelates having the same elution volume is not detected.Likewise, chromium compounds have no effect on the detection of iron compoundsat 372.0 rnp. As this detector is often 102-104 times more sensitive to metallic com-pounds than to organic ones, it is relatively insensitive to organic impurities and tobleeding of the stationary phase.

Phosphorus and sulphur compounds (mainly insecticides and pesti-cides26,36,37,107,129,140,260)rom organic compounds have been determined by means ofa flame photometric detector. In this case39 the flame is screened from direct view ofthe optical system and emission from compounds, which do not contain the hetero-atoms mentioned, occurs inside the screened space of the flame on their combustion.I f a compound contains phosphorus or sulphur atoms, the emission appears abovethe flame screen and this light is detected. This arrangement leads to high detectorselectivity in spite of the fact that carbon monoxide and hydrocarbons also show anemission at the wavelength used for phosphorus. The sensitivity to phosphorus com-pounds, e.g. Parathion, is 5 x ~o-~~g/sec, which is 5 x IO-r3 g/set of phosphorus at526 m,u, using an interference filter. The sensitivity to the same compounds is 0.6p.p.m. at the wavelength specific to sulphur, 394 m,u. Sulphur compounds also givea response at 526 m,u. The response under these conditions is 5510% of the sulphurresponse at 394 m,u. The phosphorus compounds response26 is 10~ to 10~ times higherthan that of compounds which do not contain phosphorus.

In the case of compounds containing both phosphorus and sulphur atoms, thesimultaneous detection of both the elements can be carried out by using two differentinterference filters and two optical pathways 36+22. Sensitivity to phosphorus is 800times higher at 526 rnp than at 394 m,u; sensitivity to sulphur is 22 times higher at394 rnp than at 526 m,u.

It was found that the ratio Rp/dRs, where Rp is response to phosphorus andRs response to sulphur, is with use of a dual system of FPD proportional to the con-tents of phosphorus and sulphur in a molecule 36. Thus amounts of the heteroatoms ina molecule can be directly estimated from the ratio of the res