Page 79 - Tandem Techniques

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of the molecule and both can be used for compound identification. IR spectra, however, because of the
relatively large number of possible absorption bands, show considerable differences between diverse
molecules and contain much fine structure. In contrast, and as already discussed, the majority of UV
spectra are very similar even though the structure of the molecules may differ considerably.

Figure 2.7
IR Spectra Presented in Wavenumbers and Microns

Consequently, 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 [6] and Alpert [7] are
recommended.

In tandem systems, the IR spectrometer is commonly associated with either a gas or a liquid
chromatograph. Consequently the spectra in a GC tandem system will be taken as a vapor in the carrier
gas, whereas in an LC tandem system the spectra would normally need to be taken as a solution in a
liquid (the mobile phase). In general, spectra of the same materials taken