Capillary gas chromatography — Fourier transform infrared spectroscopy: theory and applications

Abstract

Among the detection methods available for gas chromatography, the on-line molecular spectroscopic techniques of MS and IR are unique with respect to the quality and the amount of information yielded. Both provide detection and identification from a single experiment. They represent by far the most sophisticated and flexible but also the most complex type of GC detection. Infrared spectrometry is best known for its ability to provide unambiguous identification each molecule possessing its own characteristic "fingerprint". Modern Fourier transform infrared (FT-IR) spectrometers are fast enough to acquire several full IR spectra even over narrow capillary GC peaks. In combination with GC, the FT-IR detector simultaneously provides non-specific total IR information and highly specific multichannel analysis. The essential parts of a GC-IR apparatus are: GC equipment, GC-IR interface, FT-IR optics, and the on-line data system. Two different approaches of interfacing GC with FT-IR have been developed: the "light-pipe" technique as a true realtime technique, and cold trapping methods. The light-pipe interface has become most popular and can be regarded as the "standard" GC-IR interface. The light-pipe is a heated, very special IR measuring cell. The effluent from the GC is continuously passed through this cell. The IR spectra of the eluted compounds are measured "on-the-fly" i.e. in realtime during their passage down the light-pipe. Since the first experiments in 1966 the sensitivity of light-pipe GC-IR has been improved by a factor close to 1000 and is now in the low ng range. But "sensitivity" of GC-IR cannot be just a bare number. Compound specific absorptivities are involved and the resolved spectral features are of different significance. Unfortunately, the most discriminating features in a spectrum are often not identical with the strongest absorption bands. As an alternative to the light-pipe technique, cold trapping the compounds of interest offers a way of achieving a substantial gain in sensitivity. Because GC separation deals with volatile compounds, a suitable trap must be cold enough to freeze out even compounds of high volatility. The compounds can be deposited on a cold infrared transmitting window or an infrared reflecting surface either directly by spraying or by using the established technique of matrix isolation (MI). The range of applications of GC-IR is still expanding (for review see [1]). Compared to GC-MS, infrared detection is particularly advantageous for distinguishing between (aromatic) isomers. Thus environmental control and the study of pesticides is a major field of application for GC-IR. For insecticides, pesticides, fungicides, etc. which can exist at different isomers or conformers, the biochemical activity is often bound to one or a few of the possible isomers or conformers. Other "typical" fields of application are: petrochemistry, the analysis of solvent mixtures, essential oils, flavor and fragrances, and polymer analysis by pyrolysis-GC-IR. The tremendous amount of data produced frequently during GC-IR work requires fast data processing also for the task of spectral identification. The availability of reliable spectral libraries and appropriate software support become key points for efficient GC-IR work.