Light-pipe-based interface

There are many examples of components of mixtures that have identihed by GC/FT-IR, so just one example will be given here. Norton and Griffiths [16] separated a mixture of eight barbiturates by capillary GC and measured their GC/FT-IR spectra using a light-pipe-based interface. Barbiturates are very polar molecules, and the absorptivity of several of their bands is atypically high (and much high than the absorptivities of most bands in the spectra of most compounds of lower polarity.) As a result, the amount of each component required to yield an... 

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

LIGHT-PIPE-BASED GC/FT-IR INTERFACES 23.2.1. Instrumental Considerations... 

Detection by infrared spectrometry should also be relatively easy with a flow cell. CO2 is a small, symmetrical, nonpolar molecule thus, it has a very sparse infrared spectrum. Of the two infrared-active fundamental modes, the bend at 667 cm is below the cutoff of narrowband MCT detectors, and the antisymmetric stretch at 2348 cm is in a spectral region where few compounds have fundamental absorption bands. Thus, it may be thought that supercritical CO2 would be the ideal solvent for infrared spectrometry and that the sensitivity of SFC/FT-IR interfaces based on the use of flow cells would be close to that of light-pipe-based GC/FT-IR interfaces. Furthermore, since supercritical CO2 vaporizes as soon as the pressure is released, the DD-SFC/FT-IR interface should be far simpler than the corresponding DD-HPLC/FT-IR interface. In practice, however, several factors mitigate against the SFC/FT-IR interface. 

This interface is analogous to a light-pipe-based GC/FT-IR system but is somewhat simpler because the flow rate of the gas required for TGA is larger than that for GC, and the volume of the gas cell can be much larger than 80 pL. 

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... 

In general, the interface between chromatographs and FT-IR spectrometers can be divided into two types those based on flow cells (such as the light-pipe GC/FT-IR interface) and those based on elimination of the mobile phase (such as the DD-GC/ FT-IR interface). Although not without its limitations, as outlined earlier in this chapter, the flow-ceU (light-pipe) approach for GC/FT-IR has proved to be a useful tool for the solution of a number of important problems of chemical analysis. Unfortunately, the same cannot be said for HPLC/FT-IR measurements, in which the column effluent is simply passed continuously through a flow cell. Let us examine the reason for this situation. 

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