QUALITATIVE ANALYSIS

The chromatographic parameter used for qualitative analysis is the retention volume or some closely related parameter. However, since retention param-eters cannot confi rm peak identity, it is common to couple a mass spectrometer (MS) to the GC (GC – MS) for qualitative analysis. GC – MS is so widely used that it is discussed in detail in Chapter 10 .

Table 8.1 lists the most common methods used for qualitative analysis in GC. Reference 1 is a good summary of these and other methods.

Retention Parameters

The retention time for a given solute can be used for its identity if the follow-ing column variables are kept constant: length, stationary phase and its thick-ness (liquid loading), temperature, and pressure (carrier gas fl ow rate). As an example, consider that an unknown sample produced the chromatogram shown in Fig. 8.1 a . If one wished to know which of the components were n - alcohols, a series of n - alcohol standards could be run under identical condi-tions producing a chromatogram like Fig. 8.1 b . As shown in the fi gure, those

129 Basic Gas Chromatography, Second Edition, by Harold M. McNair and James M. Miller Copyright © 2009 John Wiley & Sons, Inc.

CHAPTER 8

peaks whose retention times exactly match those of the standards can be identifi ed as the n - alcohols. In this example, this identifi cation process will only work if the components of the unknown are alcohols.

This procedure will not be effective if the number of possible compounds is large — retention volumes are not that characteristic. Since there are over 30,000 organic compounds in common use, the gas chromatograph by itself cannot be used to identify a single compound from among this large group.

Retention times are characteristic of a GC system, but they are not unique, so GC retention times cannot be used for qualitative confi rmation.

Since relative retention volumes are more reproducible than individual retention volumes, qualitative data should be reported on a relative basis. The retention index attributed to Kovats (see Chapter 4 ) has become a reliable method for reporting retention data. If a retention parameter is to be used for qualitative identifi cation or classifi cation, the Kovats retention index is a good method to choose.

Even the use of Kovats indices and other relative retention parameters will not always result in constant values that can be used in computerized analyses. Therefore some manufacturers have developed software and meth-odology to facilitate constant retention parameters. Agilent makes available for its instruments a procedure called retention time locking (RTL). By adjusting the inlet pressures on different systems, one can closely match the retention times of analytes on two systems using the same liquid phase.

Discussion of this procedure and software can be downloaded from their web site, www.agilent.com/chem .

TABLE 8.1 GC Methods for Qualitative Analysis

QUALITATIVE ANALYSIS 131

Selective Detectors and Dual Detectors

Selective GC detectors can sometimes be used to help identify classes of compounds to which they show high sensitivity. The list of detectors in Chapter 7 can be consulted for further information and references.

Fig. 8.1. Identifi cation of unknown by retention times using standards. From Miller, J. M., Chromatography: Concepts and Contrasts , 2nd ed., John Wiley & Sons, Hoboken, NJ, 2005, p. 354. Reproduced courtesy of John Wiley & Sons, Inc.

More interesting is the use of two different detectors in parallel at the exit of a GC column — so - called dual channel detection. The detectors chosen should have major differences in sensitivity for different classes of compounds.

Both signals are recorded simultaneously, producing parallel chromatograms like those shown in Fig. 8.2 . Identifi cations can be made by inspection of the chromatograms (Figs. 8.2 and 8.3 ) or from the ratios of the detector responses.

The latter are often characteristic of classes of compounds. Figure 8.4 shows that the ratios from the data in Fig. 8.3 clearly differentiate between paraffi ns, olefi ns, and aromatics in this example. When combined with the retention index, the ratio can lead to an identifi cation of a particular homolog within a given class.

Off - Line Instruments and Tests

In principle, one could collect the effl uent from a GC column and identify it on any suitable instrument. A simple setup for collecting effl uents in a cold trap is shown in Fig. 8.5 . The trapped sample could be transferred to an instrument for identifi cation (MS, FTIR, NMR, UV), subjected to micro-analysis, or reacted with a chemical reagent to produce a characteristic deriva-tive. Commonly, the most useful instruments (MS and FTIR) are usually coupled on - line.

Other methods that can be used for identifi cation are pyrolysis, derivatiza-tion, and the molecular weight chromatograph. References to these methods are given in Table 8.1 .

Fig. 8.2. Dual channel presentation of GC analysis of gasoline sample on a packed DC - 200 column. Courtesy of Perkin - Elmer Corp. From Miller, J. M., Chromatography:

Concepts and Contrasts , 2nd ed., John Wiley & Sons, Hoboken, NJ, 2005, p. 259.

Reproduced courtesy of John Wiley & Sons, Inc.

QUALITATIVE ANALYSIS 133

On - Line Instruments

GC – MS has already been mentioned as the premier method for qualitative analysis (see Chapter 10 ). A complementary identifi cation technique is Fourier transform infrared coupled to gas chromatography (GC – FTIR). The increased sensitivity of the Fourier transform method of data handling has contributed greatly to its utility.

Fig. 8.3. Dual channel presentation of GC analysis of air contaminants in a parking lot. Reprinted with permission from reference 13. Copyright 1983, American Chemical Society. From Miller, J. M., Chromatography: Concepts and Contrasts , 2nd ed., John Wiley & Sons, Hoboken, NJ, 2005, p. 360. Reproduced courtesy of John Wiley & Sons, Inc.

The two IR interfaces in common use are the light pipe [8] and so - called matrix isolation [9] . In the former method, the column effl uent is passed through a heated IR gas cell (light pipe), and in the latter, it is condensed and frozen into a matrix suitable for analysis by IR [10] .

Since IR is nondestructive, it is possible to couple both the IR and the MS to the same gas chromatograph, producing GC – FIIR – MS. The special requirements and some applications have been described [8, 14] .

Fig. 8.4. Relative (PID/FID) response for 15 hydrocarbons. Reprinted with permission from reference 13. Copyright 1983, American Chemical Society. From Miller, J. M., Chromatography: Concepts and Contrasts , 2nd ed., John Wiley & Sons, Hoboken, NJ, 2005, p. 361. Reproduced courtesy of John Wiley & Sons, Inc.

TCD

Sample Cold Trap Carrier

Gas

Cold

Outlet

Fig. 8.5. Simple trapping device for qualitative analysis.