Interfaces GC-FTIR

Norton KL, Lange AJ, Griffiths PR. A unified approach to the chromatography-FTIR interface GC-FTIR, SFC-FTIR and HPLC-FTIR with subnanogram detection limits. HRC-J. High Resolut. Chromatogr. 1991 14 225-229. 

Instrumental Interface. Gc/ftir instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is directed into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then directed to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

Figure 9.8 Schematic diagram of an integrated GC/FTIR system with a lightpipe interface.

In chromatography-FTIR applications, in most instances, IR spectroscopy alone cannot provide unequivocal mixture-component identification. For this reason, chromatography-FTIR results are often combined with retention indices or mass-spectral analysis to improve structure assignments. In GC-FTIR instrumentation the capillary column terminates directly at the light-pipe entrance, and the flow is returned to the GC oven to allow in-line detection by FID or MS. Recently, a multihyphenated system consisting of a GC, combined with a cryostatic interfaced FT1R spectrometer and FID detector, and a mass spectrometer, has been described [197]. Obviously, GC-FTIR-MS is a versatile complex mixture analysis technique that can provide unequivocal and unambiguous compound identification [198,199]. Actually, on-line GC-IR, with... 

In addition to standard liquid injection there are many GC accessories which can provide different methods of sample introduction to the column, such as HS, SPE, SFE, TD, TG, Py, etc. Examples of such GC-FTIR devices are TD-GC-FT1R (with a cryostat interface) and PyGC-FTIR. 

In the various GC-FTIR systems that are commercially available, three essentially different types of GC-FTIR interfaces can be identified (137). With the most commonly used interface, the GC column effluent flows through a heated light-pipe, and vapor-phase spectra are collected in real time at 1 s intervals. This... 

There are three different types of GC/FTIR interfaces light-pipe, matrix isolation (MI), and cryo-deposition (also direct-deposition, cryotrapping). In the two latter techniques, the sample is deposited on a surface before measurement of spectra. All three techniques have been used for the analysis of CWC-related chemicals. Light-pipe interface has been the most popular, even though the usage of cryodeposi-tion in this type of analysis has been increasing over the years. 

One important factor to consider when selecting the type of GC/FTIR instrument is the availability of spectral libraries for each interface type. This issue is discussed in more detail below in Section 3.3. 

In the field of FTIR, the availability of the instruments has a dualistic nature. Basic instruments, spectrometers, and basic sampling accessories, are available from multiple sources. The problem lies in selecting the right model from all available possibilities. In GC/FTIR instruments, the case is different. While the major FTIR manufacturers produce lightpipe interfaces, only few deposition-type instruments have been available. Erickson reviewed the situation of the GC/FTIR market in 1998 (12). The article discusses general developments and future prospects of GC/FTIR. 

Cryodeposition is the newest interface type for a GC/FTIR instrument. In this system, the eluents in the GC effluent are frozen on an IR transparent slide, which is cooled using liquid nitrogen. The carrier gas evaporates in the process so that the chemicals are directly deposited on the slide surface. Transmission spectra are then measured through the slide. These spectra are like normal condensed phase spectra, with rare exceptions. The sensitivity is five times better than in light-pipe, and the same or even slightly better than in GC/MI/FTIR. 

GC resolution degrades slightly in all of the available GC/FTIR interfaces. In the light-pipe type interface, some broadening of the chromatographic peaks occur because the diameter of the light-pipe... 

As already mentioned above, it is possible to hyphenate GC/MS and GC/FTIR instruments so that two analysis results can be obtained with one injection. This type of system is cost-effective, but from the analytical point of view is not always recommendable. Neither of the instruments can be used fully. If the GC/FTIR interface is of the lightpipe type, the concentration requirements are not met with most of the chemicals. In some instruments, the GC effluent is split 1 to 10 between MS and FUR. The result of the hyphenation can be that the sensitivity of the MS is lowered 10 times, but the concentration of the chemical is still too low for FUR identification. 

Furnace, Curie-point or heated filament pyrolysers linked to packed column or capillary column gas chromatograph. GC-MS or GC-FTIR interfaces.. ... 

Chromatographic interfaces are based on three common approaches the flow-through cell (light pipe) and solvent elimination with either matrix isolation or cold trapping [2,198,201]. Flow-through cells provide a simple and convenient interface for GC-FTIR, since typical mobile phases are transparent in the mid-infrared region. Mobile phase elimination interfaces are used primarily to increase sensitivity, and to obtain high-resolution or condensed phase spectra, for improved confidence of identification by library search techniques. Vapor phase spectra have characteristic broad absorption... 

Figure 9.17. Schematic diagram of a GC-FTIR interface based on matrix isolation. (From ref. [193] Marcel...

Economic Aspects. Costs for gc/ir/ms instruments vary widely depending on the sophistication of the components. At the lower end of the scale is the flow cell type gc/ftirs connected to a benchtop mass spectrometer. These are available for about 115,000—150,000. The isolation type gc/ftir can also be interfaced with a benchtop mass spectrometer. The prices range from about 200,000—300,000. 

There are two main types of FTIR detection for GCs, in the gas-phase using an in-stream optical system and through vapor deposition with detection being away from the GG flow stream. In the first, a light pipe that can transmit IR radiation is positioned on either side of a detection cell. Transparent windows pass the IR radiation into the flow ceU. The whole assembly is maintained at temperatures of 250 °C to 350 °C to prevent deposition of sample molecules. Most interfaces for this type of GC-FTIR also have heated transfer lines to and from the flow cell to ensure that no deposition occurs before introduction into the spectrometer. 

Figure 1 Schematic of typical light-pipe-based GC-FTIR interface (based on Hewlett Packard IRD).

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