Transient infrared transmission

Transient infrared spectroscopy (TIRS) is a mid-infrared technique [82] that has been developed to obtain spectra of moving solids and viscous liquids. TIRS spectra are obtained from the generation of a thin, short-lived temperature differential that is introduced by means of either a hot or cold jet of gas. When a hot jet is used, an emission spectrum is obtained from the thin, heated surface layer. This technique is known as transient infrared emission spectroscopy (TIRES). When a cold jet is used, the blackbody-like thermal emission from the bulk of the sample is selectively absorbed as it passes through the thin, cooled surface layer. The result is a transmission spectrum convoluted with the observed thermal spectroscopy. This method is known as transient infrared transmission spectroscopy (TIRTS). TIRS is ideally suited for online analysis because it is a single-ended technique that requires no sample preparation. This technique has been applied to the lignin analysis of wood chips [83]. 

Transient infrared emission spectroscopy (TIRES) and transient infrared transmission spectroscopy (TIRTS) have both been applied to quantitative compositional analysis of butyl methacrylate methyl-methacrylate copolymers. Standard errors are less than 1%. 

As noted above, it is difficult to account for the effect of temperature gradients across the sample, which makes quantification by infrared emission spectrometry rather inaccurate. A clever way of not merely getting around the problem of temperature gradients but actually benefiting from them has been described in a series of papers by Jones and McClelland [5-10]. The technique developed by these two workers is known as transient infrared spectroscopy (TIRS) and can be subclassified into two techniques, known as transient infrared emission spectroscopy (TIRES) [5,7] and transient infrared transmission spectroscopy (TIRTS) [8]. In both of these two techniques, the deleterious effect of self-absorption is minimized by avoiding the condition of thermal equilibrium that has been assumed for previous sections of this chapter. 

The alternative technique to TIRES is transient infrared transmission spectrometry (TIRTS). This technique is analogous to TIRES, but instead of the sample being at ambient temperature and being heated by the gas jet, the sample is above the ambient temperature and is cooled by a narrow jet of cold helium. Were the sample... 

Jones, R.W. and McClelland, J.F. (1990) Transient infrared transmission spectroscopy. Anal. Chem., 62,2247-2251. 

Fig. 15. Schematic representation of the time-resolved infrared (TRIR) flash photolysis apparatus used at Nottingham. The UV pulse laser generates transient species the continuous IR laser monitors the change in transmission at a particular IR frequency, producing a trace showing the IR absorbance as a function of time. The experiment is repeated at different IR frequencies so that a complete IR spectrum of the transient can be built up [reproduced with permission from (97), p. 103],...

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