Infrared gas cells

Effluent gas emerging from a gas chromatograph at atmospheric pressure can be led directly into a heated infrared gas cell via a heated transfer line. Vapour-phase infrared spectra of eluting components can be recorded as they pass through a cell by a Fourier transform (FT) infrared spectrometer enabling a full-range spectrum to be collected and stored in a second or less. 

Evacuated bulbs (Figure 4.7C) are generally used for trapping volatile components. Since this technique does not concentrate the sample, additional sample preparation may be required. For substances with high infrared absorptivity, the sample may be trapped directly in an evacuated infrared gas cell and analyzed directly. For nonvolatile samples that may condense on the inside walls, the cell must be heated before analysis. 

Fig. 5.1. Simple vacuum line. A iineof this sort might be used to transfer volatile samples to infrared gas cells, NMR tubes, etc. It might also be used for loading tubes for sealed-tube reactions and for vacuum sublimations.

A. Typical Systems. A simple system for the transfer of samples to an infrared gas cell or to a NMR sample tube consists of a fore pump, diffusion pump, trap, and manifold (Fig. 5.1). At the other extreme is a general-purpose chemical vacuum line, which permits the separation of volatile compounds, transfer of noncondensable gases, and storage of reactive gases and solvents (Fig. 5.2). When attack of stopcock grease is a serious problem, grease-free de-... 

In order to measure cell opening more accurately, a simple infrared technique was adopted. The carrier gas was passed through an infrared gas cell in a Perkin-Elmer Model 281B Infrared Spectrophotometer set at 2320 cm T and absorbance measured as a function of time after the end-of-mixing. 

In this technique, the separated compound is swept by the carrier gas as it emerges from the gas chromatographic column through a cold trap, where it condenses. The material in the trap is then either transferred to an infrared gas cell for examination in the vapour phase, or is transferred as a liquid to a suitable micro cell or may be condensed on a cold surface as a solid for examination by conventional spectroscopic techniques. 

Figure 4.22 shows how the low-boiling component is transferred to the infrared gas cell. The design of the apparatus is different, but the procedure for transference is identical... 

Volatile constituents are, however, sometimes encountered, viz expanding agents, plasticizers, lubricants, adhesives, solvents, monomers and degradation products of additives or of the polymer itself and infrared gas cell techniques can be of value in the examination of gas chromatographic fractions containing these types of substances. 

The sample is heated in a trap device and the volatiles produced transferred to an infrared gas cell. Examination of the infrared spectrum provides information on the identity of volatile additives present in polymers which boil tween 0 and 250°C. 

Figure 7.43. Apparatus for transfer of volatile components from trap to infrared gas cell. From Anderson with permission. Royal Society of Chemistiy London.

Infrared spectroscopy and thermogravimetry have been used in polymer analysis for many years. By coupling the effluent of thermogravimetry to an infrared gas cell, TG/IR (sometimes known as evolved gas analysis) has been used to examine the thermally induced decomposition products a variety of polymers including of poly(vinyl chloride) (7), polyacrylamide (2), tetrafluoroethylene-propylene (3) and ethylene-vinyl acetate (4) copolymers, as well as styrene-butadiene composite (5). 

Combining gas chromatography with infrared spectrometry involves passing the solutes in the carrier gas stream through a heated infrared gas cell positioned in a rapid scanning Fourier transform spectrometer. 

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