Transient infrared emission spectroscopy

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. 

In TIRES, a narrow jet of hot gas is passed onto the surface of a moving sample, so that only the region near the surface is heated. The intensity of radiation emitted from below the surface increases with time by thermal diffusion as heat is conducted into the bulk. Thus, the spectrum must be measured from a region of the sample very close to the point at which it is heated. The faster the sample is moved, the farther from the point of heating can the field of view of the spectrometer be located. 

An approximate value of the emittance at wavenumber v, e (, may be obtained by calculating the ratio of the background-corrected emission from the sample at that wavenumber, Es(y), and the emission at that wavenumber from a blackbody 

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 

Many experimental parameters affect TIRTS the same way that they do TIRES. For example, the extent of self-absorption depends on how long the emission is observed after the surface layer is heated, since the chilled layer starts to thicken immediately after being formed. Thus, the longer the chilled layer is observed after 

---

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]. 

RW Jones and JF McClelland. Quantitative Analysis of Solids in Motion by Transient Infrared Emission Spectroscopy Using Hot-Gas Jet Excitation. Anal. Chem. 62 2074— 2079, 1990. 

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%. 

Jones, R.W. and McClelland, J.F. (1989) Transient infrared emission spectroscopy. 

Jones, R.W. and McClelland, JJ. (1990) Quantitative analysis of solids in motion by transient infrared emission spectroscopy using hot-gas jet excitation. Anal. Chem., 62,... 

Transient Infrared Absorption (TRISP) and laser-induced fluorescence. Because the CJ temperatures are only 2000-3000 K, most of the molecular products are in the ground electronic state. Emission spectroscopy looks selectively at only a few extraordinary molecules which are scarcely representative of most of the products. Infrared absorption, on the other hand is ideal for probing the vibrotational states of the ground state molecules, and the fast response time of TRISP makes it ideal for detonations. The technique has not been applied extensively and is difficult to implement, but our preliminary attempts have shown that we can do it with the proper laser apparatus. Broadband CARS is an alternative approach if the instrumental difficulties of TRISP cannot be overcome. 

Table 1 lists many of the short-lived species detected in the gas phase with Fourier transform infrared spectroscopy. Two prominent groups are those headed by Bernath, now at the University of Waterloo, and by Howard at the National Oceanic and Atmospheric Administration (NO AA). The former group has used IR emission to study unstable diatomics produced in discharge sources or furnaces. The molecules studied in this group tend to be of astrophysical interest. The research team at NOAA mainly studies short-lived molecules of atmospheric significance. They employ a long flow tube fitted with White cell optics and coupled to a Bomem DA3.002 spectrometer. They usually make the transient they are interested in by performing a carefully controlled series of chemical reactions. 

Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam at a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with an appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). 

---