computerized data processing system is essential. Quantitative analysis can be accomplished by
monitoring standards and samples at a selected mass fragment (m/z) value. The technique of GC-MS is
now well-established as one of the most powerful if somewhat costly analytical tools available for the
study of complex samples.
GC-Infrared Spectrometry
Effluent gas emerging from a gas chromatograph at atmospheric pressure can be led directly into a
heated infrared gas cell via a heated transfer line. Vapour-phase infrared spectra of eluting components
can be recorded as they pass through a cell by a Fourier transform (FT) infrared spectrometer enabling a
full-range spectrum to be collected and stored in a second or less.
For maximum sensitivity, the volume of the gas cell should be similar to the volume of the eluting GC
peaks, e.g. 50– 300 μl for capillary columns. This is achieved with a light pipe, a glass tube 50 cm by 2
mm i.d. and coated on the inside with gold to maximize transmission of the radiation. The ends are
sealed with an IR transparent material such as potassium bromide or caesium iodide (Figure 4.29(d)). If
the cell volume is appreciably less than the peak volumes, good spectra from partially resolved peaks
can be obtained by careful sampling, i.e. peak slicing. Conversely, if the cell volume is appreciably
more than the peak volumes, cross-contamination of one peak with another may occur. Wide-bore
capillary columns or packed columns having higher sample capacities but poorer resolution are used
where increased sensitivity is required. As in the case of GC-MS a stream-splitter facilitates the
simultaneous use of an FID detector. Alternatively, if a non-destructive thermal conductivity detector is
used no stream splitting is necessary.
Compared to GC-MS, GC-IR is much less sensitive. Whereas a mass spectrum can be recorded from as
little as 10–^10 g of sample, at least 10–^6 g is required for an IR spectrum. Care is required in the
interpretation of vapour phase IR spectra as they differ in certain respects from the corresponding liquid
or solid phase spectra. Rotational fine structure may appear, band positions may be shifted slightly and
hydrogen bonding effects are non-existent. The availability of libraries containing thousands of
digitized vapour-phase infrared spectra that can be searched in seconds by computer and compared with
those collected from a chromatographic run has increased the power of GC-IR for qualitative analysis.
The full range of computerized spectral enhancement facilities enables the quality of weak or noisy
spectra to be improved and the spectra of contaminants to be subtracted.
GC-IR is becoming more widely used because FT spectrometers (p. 281) have virtually replaced the
older dispersive types and even with computerized enhancements are much cheaper than mass
spectrometers.