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Richard A. Pacer Indiana-Purdue University Fort Wayne, Indiana 46805
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Quantitative Gas Chromatography Using Peak Heights and Relative Response Factors An undergraduate student experiment
Quantitative measurements in gas chromatography depend upon the determination of the recorded peak areas or peak heights and the relationship of these quantities to the amount of solute in the sample. With the advent of electronic, ball and disc, analog, and voltage integrating devices, which may be directly attached or built into the recorder, area integration is normally performed simultaneouslv with the recordine of the chromatoeram. In the absence of an integrator, a common technique for measuring peak areas is "height times width at half height." But a very narrow peak whose width, for example, might be measured as 0.10 f 0.01 cm mav introduce a 10%error into the calculations. When small peaks must he measured or when the hand width is narrow. as is the case for earlv Dal . oeaks. . Nogare and Juvetl recommend the use of peak height measurements to obtain ouantitative data. Dimhat, et al.hep&t that peak height is virtually insensiti\.e to flow rate, but sensiti\,e to flurruntions of rolumn operating temperature. The reverse is true for peak areas. However, where two or more components of a multi-component sample are only partially separated, peak areas are preferred. Thus for sharp, narrow peaks which are well-re-
solved, and in the absence of an integrator, quantitative data based on peak heights are preferable to data hased on peak areas. The use of relative resoonse factors in an exneriment illustrates the fact that components of a mixturi, present in equal amounts, need not respond equally in order for that response t o he useful for quantitative purposes. As long as the resDonse is reoroducible.. a auantitative method is fea. sible. Relative resoonse factors have a varietv of names. depending on the analytical method. Thus molar absorptivities in spectrophotometric measurements, equivalent ionic conductances a t infinite dilution in conductance measurements, and diffusion current constants in polarography are all examples of relative response factors. In some methods these factors have no s ~ e c i aname. l but thev are iust as important. Pardue, et al.ipoint out that the ube of ielative resoonse factors to correct for detector resoonse in eas chromatography is recommended for all t h y qu&titative work. The eas chromatomaohic resnonse factors of Pardue. et aL3 and i f Karasek, eialh are based on peak areas.
Relative Rewonre Factors far p-Xvleneand Toluene in a Mixture of Benzene. D-Xvlene. and Toluene
In this experiment, equal-volume mixtures of benzene, p-xylene, and toluene are prepared and chromatographed under conditions which give sharp, well-defined peaks.
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Table 1.
r~denr
p-xylene
The Experiment
toluene
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Dal Nogare, and Juvet, "Gas-Liquid Chromatography," Wiley Interscience, New York, 1962, p. 254. Dimbat, M., Porter, P. E., and Stross, F. H., Anal. Ckem., 28,
-mn - -11~PSfil
3Pardue, H. L., Burke, M. F., and Barnes, J. R., J. CHEM. EDUC., 44,695 (1967). 'Karasek, F. W., DeDecker, E. H., and Tiernay, J. M., J. CHEM. EDUC., 51,816 (1974). Table 2.
Srudent
Results Obtained by Students on Unknowns Using the Peak HeightIRelative Response Factor Technique
%
%
%
%
%
%
Benzene.
Benzene.
Actual
Reported
p-xylene, Actual
p-xylene Reported
Toluene. AcNal
Toluene. Reported
592 / Journal of Chemical Education
% Error
% Error
Ave %
%Error
Error
Peak heights are measured by hand. The component which emerges first (benzene) is arbitrarily assigned a response factor of exactly one. Response factors for the other two components may be found by taking the ratio of the peak height for the component in question t o the peak height for the first component. The known (equal-volume) mixture may be run several times and random error minimized by averaging the results. Students are advised t o run both the known and the unknown three times each. The method appears to be free of'determinate error. When the unknown is run, the total adjusted response may he calculated in the following way Peak height for 1st component Total adjusted response = 1 . m . .. Peak height for 2nd component + Response factor for 2nd component Peak height for 3rd component (1) Response factor for 3rd component +
The % of each component (hy volume) can he found by relatine each individual adiusted resoonse to the total adjusted response. Thus, the % by volume of the 2nd component is equal t o Peak height for 2nd component Response factor for 2nd component X 100 Total adjusted response
(2)
Experimental Conditions
Instrument: Varian Aerograph Model 920 Column: SE 30,-190 r m long Sample size: 4 . 5 pl Column temperature: 95'C Injector temperature: 220°C Attenuation factor: 128 Filament current: 150 mA Results In this experiment, benzene, by definition, has a relative response factor of exactly one. Table 1 shows the relative response factors for the &her two components. These factors were obtained by seven students in an introductory analytical course, normally taken by chemistry majors in their junior year. The results in Table 2 were obtained on unknown mixtures of benzene, p-xylene, and toluene. Each student received a separate unknown, although two students may have received (unknown t o them) an unknown sample from the same batch.
Conclusions Using peak heights and relative response factors, in a gas chromatographic procedure, i t is possihle to determine benzene, p-xylene, and toluene in a mixture t o 1%accuracy. This approach should he applicable to any mixture of components which exhibits essentially ideal solution behavior, provided conditions can be established t o produce well-defined, sharp peaks. The wide variation in relative response factors for either component may he attributable to (1)slight variations in column temperature and/or detector current from one set of runs to another and (2) uncertainty in determining base lines, particularly in cases where the pen did not return to the same arbitrarv zero line after anoearance of a oeak. . However, the small overall percent error shows that this variation has little effect on accuracv as lone as (1) column temperature remains constant for n related set of runs and (2) the student is consistent in hislher) method of mensuring peak heights. Although the above procedure was set up in terms of percent hy volume, the experiment could have been set up just as easily in terms of percent by weight. Relative response factors would be different from the above, of course, depending on the differences in densities of the components. The use of relative response factors and calculations based on a "total adiusted response" should he possible in any analytical procedure in dhich (1)each component of the mixture eives a reproducible response, (2) the components do not-react ch~mically,and (3) there are no synergistic effects'(i.e., the components exhibit ideal solution behavior). In a mixture in which one (or more) of the components gives little or no response, this procedure would indicate onlv the relative amounts of the comnonents which do respond well. However, by combining the peak heighthelative response factor approach with the standard addition or internal standard technique, the absolute amounts of the comoonents in a eiven samnle which resoond well could be determined. If tge cornPodents do not kxhibit ideal solutionbehavior or if thev tend to interact chemicallv, the known mixture (from which relative response factors are calculated) should aporoximate the com~ositionof the unknown as closely as &sible. A two- or three-stage iterative orocess mav be necessarv to establish aopropriate response .. . factors for the sample in question. That is, after the approximate composition of the unknown is established, a known mixture-of this composition could be prepared and relative response factors redetermined. Note: A "handout" of the student experiment will be sent upon request.
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Volume 53, Number 9, September 1976 / S93
