Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > Page 9


Figure 6. Chromatogram and Spectra Obtained from an Early GC/IR/Mass Spec. Combination.


It is seen that, although lacking in resolution compared with modern spectrometers, very useful spectra were obtained and (as will be discussed later) the sample collection and regeneration was completely automatic. IR spectra are still occasionally used for confirming solute identification by comparison with reference spectra and this can often be useful for forensic purposes. This is usually done by computer; the normalized spectra of the unknown is subtracted from the normalized spectra of the suspected reference compound and, if a match is obtained (the difference is close to zero) the unknown is considered identified. In a similar manner, for certain compounds, UV and fluorescence spectra can also be used to confirm solute identity and the association of the GC with a UV spectrometer or a fluorescence spectrometer has proven to be much easier due to the significantly greater sensitivity of these spectrometers compared with that of the IR spectrometer.


The use of UV spectra and fluorescence spectra, however, are far less useful than mass spectra or IR spectra for solute identification. Except for certain substances (e.g. those containing aromatic rings) the majority of compounds give very similar UV spectra with very little fine structure to allow confident spectra matching. This is due to a multiplicity of adsorption bands merging to produce a broad envelope the cause of which will be discussed later. Similarly fluorescence spectra have much less detail than IR spectra (although often more than the corresponding UV spectra) which also makes them less useful for confirming solute identity. Under certain circumstances, however, they can be helpful and their specific advantage lies in the sensitivity of UV and visible sensors that allow satisfactory UV and fluorescence spectra to be obtain from very small amounts of solute without concentration.