Fourier Transform Infrared Spectrometry Detection

M. W. Raynor, Matheson Gas Products, Advanced Technology Center, Longmont, CO, USA K. D. Bartle, University of Leeds, Leeds, UK

Copyright © 2000 Academic Press

The analytical potential of any separation method is greatly increased when it is combined with a detector that gives qualitative and quantitative information about the separated components. Fourier transform infrared spectrometry (FTIR) has therefore been increasingly used as a detector for supercritical fluid chromatography (SFC), ever since Shafer and Griffiths published the first paper on combined SFC/FTIR in 1983. These researchers recognized that SFC's main application area was the analysis of thermally labile compounds and those too involatile or polar to be analysed by gas chromatography (GC). Furthermore, they realized the potential compatibility of SFC with FTIR detection due to the use of a mobile phase such as carbon dioxide, which has extensive regions of transparency in the infrared (IR) region. Once demonstrated, various SFC/FTIR interfaces were rapidly developed, due to extensive interest from both academia and industry.

The development of FTIR spectrometry itself has also been key to the success of combined SFC/FTIR. In FTIR spectrometry, IR radiation which has been modulated is passed through the sampling area and is detected by a highly sensitive mercury cadmium telluride (MCT) detector. The interferograms, which are plots of IR intensity versus time, are signal-averaged at intervals of less than 1 s and stored on the hard disk of the spectrometer's computer system. The data system then executes a Fourier transform of the inter-ferograms, which are compared against a background to produce an IR spectrum of absorbance (or percentage transmittance) versus wave number. In comparison to a dispersive IR spectrophotometer, which only allows one frequency of radiation to be detected at any one monochromator setting, a FTIR allows all frequencies to be detected simultaneously. Thus, not only can IR absorbance spectra be measured rapidly, a large number of spectra can also be co-added to increase the sensitivity of the technique (Fellgett's advantage). FTIR is therefore fast enough to measure a number of complete IR spectra in real time during the elution of a chromatographic peak. The FTIR has several other advantages over dispersive instruments. The interferometer in an FTIR instrument uses a gas laser to reference the position of the moving mirror. As a result, these instruments are characterized by high resolution and highly accurate and reproducible frequency determinations, which is particularly useful when spectra have to be co-added or subtracted for background correction (Connes' advantage). FTIR optics also provide much large energy throughput (Jacquinot's advantage) and have minimal light scattering (stray light advantage) in comparison to dispersive instruments that incorporate gratings and narrow slits. In summary, FTIR spectrometry can be used as a highly informative, nondestructive and universal detector for SFC. Since absorption bands can be assigned to individual functional groups in organic molecules, the technique is especially useful for selective detection and the identification of unknown analytes.

When used as a detector for SFC, FTIR is however constrained by two major problems: mid IR absorption by most chromatographically compatible mobile phases and relatively low FTIR sensitivity compared to many of the other commonly used chromato-graphic detectors. To minimize these problems, various ingenious interfaces have been explored. These designs appear to vary greatly, but they can be classified by essentially two approaches: flow cell interfaces, where the column effluent and analytes are monitored, in real time, by the IR beam as it passes through a high pressure flow cell, and mobile-phase elimination interfaces where the analytes are collected on an IR transparent window (for subsequent IR analysis) as the mobile phase depressurizes at the column outlet. Table 1 documents the main advances in the development of SFC/FTIR, and the following discussion considers both interface types in more detail.

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