Gas Chromatography - Tandem Techniques - The Characteristics of Infrared Absorption > The Measurement of IR Absorption > Page 26


It is seen that both UV and IR absorption provide spectra that are characteristic of the molecule and both can be used for compound identification providing there is sufficient fine structure. IR spectra, however, in contrast to the majority of UV spectra, and because of the relatively large number of possible absorption bands, show considerable differences between diverse molecules and contain much fine structure. As a result, IR spectra can be far more useful for confirming compound identity than UV spectra. Unfortunately, the measurement of an IR spectrum requires considerably more sample than that required to obtain a UV spectrum and thus, although the IR spectrum is more informative, the technique is not as sensitive. For further details on IR spectroscopy the books by Conley (5) and Alpert (6) are recommended.


The Measurement of IR Absorption

The IR spectrometer has been frequently associated with the gas chromatograph but with the advent of higher resolution mass spectrometers, the need for this combination has been reduced. It is clear that in a GC/IR spectrometer combination, the spectra will be taken of a vapor sample contained in the carrier gas. In general, spectra of the same materials taken in a gas will be very similar to that taken as a vapor and Welti (7) suggests that vapor spectra might be considered as liquid spectra taken at infinite dilution. This, of course, will be only true if there is no interaction between the sample and solvent. The vapor and liquid film spectra of n-hexanol are shown in figure 16.





Figure 16. Vapor and Liquid Spectra of n–Hexanol