Infrared spectroscopy response time

Instrumental Interface. Gc/fdr instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is dkected into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then dkected to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

Despite the considerable amount of information that has been garnered from more traditional methods of study it is clearly desirable to be able to generate, spectroscopically characterize and follow the reaction kinetics of coordinatively unsaturated species in real time. Since desired timescales for reaction will typically be in the microsecond to sub-microsecond range, a system with a rapid time response will be required. Transient absorption systems employing a visible or UV probe which meet this criterion have been developed and have provided valuable information for metal carbonyl systems [14,15,27]. However, since metal carbonyls are extremely photolabile and their UV-visible absorption spectra are not very structure sensitive, the preferred choice for a spectroscopic probe is time resolved infrared spectroscopy. Unfortunately, infrared detectors are enormously less sensitive and significantly slower... 

It is desirable to be able to continuously monitor all the reactants and products so that a time resolved response curve is obtained for the separate components in the pulse. The sequence in which the products appear and the shape of each individual peak can, in principle, give information on the reaction sequence and on adsorption properties that cannot easily be obtained in any other way. Infrared spectroscopy has been used with good results, but the number of systems suitable is limited and at best only a small number of components can be analyzed for, Figure 1. The... 

Tunable diode laser absorption spectroscopy (TDLAS) has been used to measure oxides of nitrogen during flight (71). By tuning the laser to specific infrared absorption bands, the technique can selectively measure each compound. Detection limits are higher (25-100 pptrv for a 3-min response time) than the best chemiluminescent methods, and the instrumentation is less amenable to aircraft operations than the chemiluminescence techniques because of weight and size. 

C02 in the gas phase can be determined by means of its significant infrared absorbance (Fig. 10) at wave lengths (A) < 15 pm, particularly at 4.3 pm [289], or by acoustic means. Integrated photoacoustic spectroscopy and magnetoacoustic (PAS/MA) technology for combined C02 and 02 analysis has rapid response time and a small sample volume is sufficient. The acoustic methods are accurate, stable over long periods and very simple to use. 

Multidimentional nonlinear infrared spectroscopy is used for identification of dynamic structures in liquids and conformational dynamics of molecules, peptides and, in principle, small proteins in solution (Asplund et al., 2000 and references herein). This spectroscopy incorporates the ability to control the responses of particular vibrational transitions depending on their couplings to one another. Two and three-pulse IR photon echo techniques were used to eliminate the inhomogeneous broadening in the IR spectrum. In the third-order IR echo methods, three phase-locked IR pulses with wave vectors kb k2, and k3 are focused on the sample at time intervals. The IR photon echo eventually emitted and the complex 2D IR spectrum is obtained with the use of Fourier transformation. The method was applied to the examination of vibrational properties of N-methyl acetamid and a dipeptide, acyl-proline-NH2.in D20. The 2D IR spectrum showed peaks at 1,610 and 1, 670 cm 1, the two frequencies ofthe acyl-proline dipeptide. Geometry and time-ordering of the incoming pulse sequence in fifth-order 2D spectroscopy is shown in Fig. 1.3. 

In order to actually cover 19 decades in frequency, dielectric spectroscopy makes use of different measurement techniques each working at its optimum in a particular frequency range. The techniques most commonly applied include time-domain spectroscopy, frequency response analysis, coaxial reflection and transmission methods, and at the highest frequencies quasi-optical and Fourier transform infrared spectroscopy (cf. Fig. 2). A detailed review of these techniques can be found in Kremer and Schonhals [37] and in Lunkenheimer [45], so that in the present context only a few aspects will be summarized. 

Among the optical analysis methods, near-infrared (NIR) spectroscopy is the most popular method because of its non-destructive nature, the low operating cost and the fast response times (Armenia et al., 2007), and it also has been successfully applied to quality control in food (Pi et al., 2009 Leroy et al., 2003 Subbiah et al., 2008), petrochemical, pharmaceutical, clinical and biomedical and environmental sectors (Ripoll et al., 2008). Near-infrared (0.7-2.5pm 12900-4000cm-i) spectroscopy is further classified into NIR reflectance spectroscopy and NIR transmission spectroscopy. NIR can be non-dispersive (filter-based instrumentation), dispersive and use Fourier transform-based instrumentation. Table 1 lists some NIR spectroscopic applications suitable for pesticides determination. All these researches have shown the possibility and reasonability for determination of pesticide concentration using NIR spectroscopy. 

The photoinitiated polymerizations were followed by real-time infrared spectroscopy on thin films, radiation. The rates of polymerization were reported by them to increase with the light intensity according to a nearly square root law, up to an upper limit. The upper limit or the saturation effect was attributed by them to a fast consumption of flie photoinitiator under intense illumination. A strong correlation was found to exist between flie rate at which the temperature increases and fire rate of polymerization. The temperature shows the same light intensity dependence as the reaction rate, and levels off to a maximum value under intense illumination. Photopolymerization experiments carried out at a constant temperature of 25"C show that thermal runaway is not responsible for the increase of the polymerization rate observed at the beginning oftheUV exposure. 

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. 

Unfortunately, DPC instruments do not have sufficient response time to follow the very fast rate of free radical photochemical reactions encountered in rapid processing environments. As shown in Fig. 2.80 the time to 90% cure of a UV curable resin at a UV intensity of 400mW/cm is about one second at room temperature. This fast kinetic measurement was carried out by a technique known as real time infrared spectroscopy (RT-IR). By this method one follows quantitatively the development of cure by tracking the disappearance of unsaturation as a decrease in the area of an absorbance band that was initially associated with the uncured resin (Decker and Moussa, 1988). The data in Fig. 2.80 indicates that a power compensation DSC begins to give accurate conversion data when UV intensities are of the order of lOmW/cm or less for this resin. Importantly, the data also show that the free radical polymerization of this UV curable resin can be fitted to a linear log-log plot of time versus intensity over a range of intensities of nearly five orders of magnitude. It is... 

The recycle composition (yaz) is typically measured by online gas chromatographs, which may have significant time delays. If a faster response time of the analyzer is required, an infrared or Raman spectroscopy probe may be used. As discussed in Section 12.2.8 flow q 2 is typically measured and controlled by manipulating the... 

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