Transient infrared emission spectroscopy TIRES

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]. 

Becaii.se of the phenomenon of self-absorption the ideal sample for conventional emission studies is a thin layer (e.g.. a polymer film), on both metal and semiconductor. surfaces [81]. A sample is usually heated from below the emitting surface, the lower surface thus having a higher temperature than the upper one. Therefore, radiation emitted from below the upper surface is absorbed before it reaches the surface, and this self-absorption of previously emitted light severely truncates and alters features in the emission spectra of optically thick samples. This problem is overcome by using a laser for controlled heat generation within a thin surface layer of the sample, self-absorption of radiation thus being minimized. These methods, known as laser-induced thermal emission (LITE) spectroscopy [85], [86] and transient infrared emission spectroscopy (TIRES) [87], [88] can produce analytically useful emission spectra from optically thick samples. Quantitative applications of infrared emission spectroscopy are described in [89]-[91]. 

Jones and McClelland have demonstrated the application of transient infrared emission spectroscopy (TIRES) to quantitative compositional analysis of ethylene-vinyl acetate 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. 

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See also in sourсe #XX -- [ ]

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