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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
Some Causes of Errors in Quantitative Gas-Liquid Chromatography Sir: Gas-liquid chromatography (GLC) is primarily a separation technique. In speed, capacity, and efficiency of separation there are few techniques which can match chromatography. However, the quantitative aspects of GLC leave something to be desired. Shatkay and Flavian ( I ) recently reported the results of an experimental examination of the quantitative aspects of this technique. They found that even the internal standard method is not always as accurate or precise as has often been assumed or implied. The tendency of many textbooks to gloss over the quantitative weakness of GLC ( I ) is probably, a t least in part, due to the ease with which this technique performs certain analyses difficult to perform by the classical techniques, particularly quantitative analysis of complex mixtures. For example, although wet chemical methods can readily and with high accuracy determine the total concentration of alcohols in an aqueous solution, determination of the individual alcohols is a far more difficult problem for these techniques. Under many circumstances, the sacrifice in accuracy involved in performing such an analysis by GLC would therefore be considered well worth the results. In the case of highly complex mixtures such as petroleum fractions, there is really no alternative to GLC. The work of Shatkay and Flavian is timely. It focuses attention on a weakness which has far too long been ignored. However, they only cursorily touched on the causes of the effects they reported. I t is worth examining these in more detail, as improved control over the parameters concerned can often lead to improvements in accuracy and precision. All sections of the instrument can affect the results; however, the two most important are the detector and the column. The detector plays a vital role in determining the overall quantitative efficiency of the instrument. The most popular general purpose detector is the flame ionization detector (FID) which was also used by Shatkay and Flavian. This detector has a very wide linear dynamic range of response, generally claimed to be lo7 by the instrument manufacturers. This would appear to satisfy most requirements; however, as only the span of the range is generally given and rarely, if ever, the actual values, it is quite possible that only part of this range has practical significance; part of the claimed range could be outside normal operating concentrations. Claims for dynamic range are usually made as a general statement; if a material is quoted at all, it is a low molecular weight hydrocarbon such as propane. The response of the FID to a particular material is closely related to the proportion of carbon in the molecule; the presence of many other atoms, such as oxygen, nitrogen, halogens, and phosphorus, reduces the response. A material such as trimethylphosphate can be expected to have a lower response than a hydrocarbon such as undecane. This is confirmed by the work of Shatkay and Flavian. It would not be surprising if materials of low response were found to have narrower linear detector response ranges than the hydrocarbons. I t is also possible that two chemically different materials, although of similar linear response ranges, do not give a constant RA (area ratio(I)), because the spans over which
the responses are linear either do not overlap or do so only partially, the concentrations involved being outside the overlap area. Either of these proposals could explain some of the deviations found by Shatkay and Flavian. Figures 3 and 5 appear to indicate a case of detector overloading (response in nonlinear region) by the trimethylphosphate. Detector geometry plays an important part in determining the linear response range. The upper limit can often be raised by increasing the jet diameter. Column characteristics can also affect quantification. Materials susceptible to adsorption, such as polar materials, may prove problematic as they can and often do give rise to tailing peaks, the area of which is difficult to measure accurately. Part of the material may be absorbed so strongly that it comes off the column too slowly to be detected accurately, that is, it may be outside the linear range of the detector. The support, its history, as well as surfaces with which the sample comes into contact, all play an important role in adsorption. The solvent can also have an appreciable effect. Some solvents such as chloroform and benzene can produce soot in an FID if injected in large volumes. The deposition of soot in the detector can affect its response. Soot formation can be avoided by reducing the sample size or by optimizing the gas flows. Large volumes of solvents can interfere with separation, there being an increasing tendency to tailing, particularly with polar solvents. An example can be seen in Figure 3 of reference 1. As the sample size is increased, the tail of the solvent (methanol) peak reaches one of the analytes (trimethylphosphate) and in the final example the latter visibly sits on the tail of the solvent, this in spite of the fact that the sensitivity (range) has been reduced. Such a situation can readily lead to errors in quantitative analysis. Recorder limitations should be kept in mind when working with narrow peaks. The peak width at base should be at least twice the time required for a full scale deflection. Some of the older models of recorders require 1 and even 2 s for a full traverse. This point is however not important with electronic integration. This is not meant to be a detailed treatment of this subject. The few points that have been brought up are intended to show that, although for many analyses quantitative GLC may not be able to match the precision and accuracy of the classical techniques, an awareness of the limitations combined with reasonable care often leads to good quantitative results.
LITERATURE CITED (1) A. Shatkay and
S.Flavian, Anal. Chem., 49, 2222 (1977).
M. Ravey IMI Institute for Research & Development P.O. Box 313 Haifa, Israel RECEIVED for review February 22, 1978. Accepted March 22, 1978.
Quantitative Analysis for Hydrogen with a Microwave Plasma Detector Sir: Analysis of hydrocarbon pollutants at trace concentrations in the environment can be achieved by means of hydrogen isotope dilution analysis. This technique involves
adding a known amount of the perdeuterated isotope of the hydrocarbon to the sample and then measuring the hydrogen to deuterium ratio. The concentration of the hydrocarbon
This article is not subject to U S . Copyright. Published 1978 by the American Chemical Society
