Geoffrey Dent

Improved Chromatographic Resolution for Gas Chromatography/Matrix Isolation Infrared Spectroscopy

When gas chromatography/matrix isolation infrared spectroscopy (GCV MI-IR) is carried out with a fixed cryodisk rotation speed, chromatographic resolution may be substantially degraded. In this research, an approach allowing dynamic programming of disk rotation speed is shown to result in much improved resolution in reconstructed chromatograms. Use of up to 254 different speeds during a single chromatographic run is possible. Thus, the analyst can match the requirements of any specific separation problem with an appropriate set of cryocollector movement speeds. As a consequence, much improved analytical performance without loss of sensitivity is possible for mixture analysis by GC/MI-IR, as is up to 30% higher apparent chromatographic resolution than is observed with a flame ionization detector. In addition, careful matching of sample spot size and detector area with the appropriate cryodisk speed leads to the detection of as little as 40 picograms of hexachlorobenzene.

Introduction, basic theory, and principles

The main spectroscopies employed to detect vibrations in molecules are based on the processes of infrared absorption and Raman scattering. They are widely used to provide information on chemical structures and physical forms, to identify substances from the characteristic spectral patterns (‘fingerprinting’), and to determine quantitatively or semi-quantitatively the amount of a substance in a sample. Samples can be examined in a whole range of physical states; for example, as solids, liquids or vapours, in hot or cold states, in bulk, as microscopic particles, or as surface layers. The techniques are very wide ranging and provide solutions to a host of interesting and challenging analytical problems. Raman scattering is less widely used than infrared absorption, largely due to problems with sample degradation and fluorescence. However, recent advances in instrument technology have simplified the equipment and reduced the problems substantially. These advances, together with the ability of Raman spectroscopy to examine aqueous solutions, samples inside glass containers and samples without any preparation, have led to a rapid growth in the application of the technique. In practice, modern Raman spectroscopy is simple. Variable instrument parameters are few, spectral manipulation is minimal and a simple interpretation of the data may be sufficient. This chapter and Chapter 2 aim to set out the basic principles and experimental methods to give the reader a firm understanding of the basic theory and practical considerations so that the technique

Comparison of light pipe, matrix isolation, and cryotrapping GC-FTIR

An inter-laboratory comparison has been carried out of three types of GC-FTIR interfacing: light-pipe, matrix isolation, and cryotrapping. Representative samples of industrial and environmental origin have been analyzed in parallel. Instruments utilizing sample storage of the GC-eluent are found to yield considerably better sensitivity than the light-pipe system. The chromatographic resolution was hardly degraded. Spectra obtained often show features characteristic to the interface used. Synchronous use of an FID or MS detector was found to allow better location and identification of the separated components.