Selective reflection spectroscopy

In the case of total internal reflection, where ktz is pure imaginary and the transmitted wave is evanescent, the results obtained above are still valid if one sets ktz = IK with k given by Eq. (3.38) for fi2 = 1. 

As we have seen in the preceding section, the reflectivity at the gas-solid interface contains the contribution from atoms with Vz 0 determined by X- Eq. (7.25). At normal incidence the vector kj is parallel to the 2-axis and the polarizability a(at v) depends only on the Vz component. Accordingly, 

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In this chapter we shall consider the optical response of a gas in the close vicinity of a solid surface. We shall become aware of different optical techniques which allow us to study various aspects of interactions between gas atoms or molecules and a surface. They are based on reflection of light from a gas-solid interface. Depending on whether the incidence angle is less or greater than the critical angle, one distinguishes between selective reflection spectroscopy (SRS) and evanescent wave spectroscopy (EWS). 

W.R. Hruschka, Data analysis wavelength selection methods, pp. 35-55 in P.C. Williams and K. Norris, eds. Near-infrared Reflectance Spectroscopy. Am. Cereal Assoc., St. Paul MI, 1987. P. Geladi, D. McDougall and H. Martens, Linearization and scatter-correction for near-infrared reflectance spectra of meat. Appl. Spectrosc., 39 (1985) 491-500. 

Connors and Jozwiakowski have used diffuse reflectance spectroscopy to study the adsorption of spiropyrans onto pharmaceutically relevant solids [12]. The particular adsorbants studied were interesting in that the spectral characteristics of the binary system depended strongly on the amount of material bound. As an example of this behavior, selected reflectance spectra obtained for the adsorption of indolinonaphthospiropyran onto silica gel are shown in Fig. 1. At low concentrations, the pyran sorbant exhibited its main absorption band around 550 nm. As the degree of coverage was increased the 550 nm band was still observed, but a much more intense absorption band at 470 nm became prominent. This secondary effect is most likely due to the presence of pyran-pyran interactions, which become more important as the concentration of sorbant is increased. 

The interaction between drug compounds and excipients, as these influence drug dissolution, can be successfully studied by means of reflectance spectroscopy. In one study concerning probucol and indomethacin, it was deduced that hydrogen bonding and van der Waals forces determined the physisorption between the active and the excipients in several model formulations [36]. Chemisorption forces were found to play only minor roles in these interactions. These studies indicated that surface catalytic effects could be important during the selection of formulation excipients. 

KNN Example 2 The goal of the project described in this example is to determine whether NIR reflectance spectroscopy can be used for sorting recycled plastic containers. The KNN classification is selected because it is simple and does not make assumptions about the statistical distribution of the classes. This is important because the number of samples for most of the classes in the training set is small (3-5) and, therefore, it is not possible to check any distributional assumptions. 

To illustrate some commonly encountered classification methods, a data set obtained from a series of polyurethane rigid foams will be used.55 In this example, a series of 26 polyurethane foam samples were analyzed by NIR diffuse reflectance spectroscopy. The spectra of these foams are shown in Figure 8.25. Each of these foam samples belongs to one of four known classes, where each class is distinguished by different chemistry in the hard block parts of the polymer chain. Of the 26 samples, 24 are selected as calibration samples and 2 samples are selected as prediction samples. Prediction sample A is known to belong to class number 2, and prediction sample B is known to belong to class number 4. Table 8.8 provides a summary of the samples used to produce this data set. 

Fourier transform infrared microscopes are equipped with a reflection capability that can be used under these circumstances. External reflection spectroscopy (ERS) requires a flat, reflective surface, and the results are sensitive to the polarization of the incident beam as well as the angle of incidence. Additionally, the orientations of the electric dipoles in the films are important to the selection rules and the intensities of the reflected beam. In reflectance measurements, the spectra are a function of the dispersion in the refractive index and the spectra obtained are completely different from that obtained through a transmission measurement that is strongly influenced by the absorption index, k. However, a complex refractive index, n + ik can be determined through a well-known mathematical route, namely, the Kramers-Kronig analysis. 

A number of problems arise in connection with the use of emission IR spectroscopy (IRES). One of them arises from the existence of temperature gradients, which can cause self-absorption of the emitted radiation by the colder outer parts of the sample itself another is concerned with the selective reflection that occurs in the vicinity of strong absorption bands. This reduces the absorptance and hence the emittance. Moreover, perturbations can be created by reflections and emission by the cell elements. These problems, however, can in part be overcome so that IR emission spectra can be successfully recorded and are widely used, for example, in the fields of polymer and corrosion science and mineralogy. Some uses of IRES... 

Teresa Iwasita and F. C. Nart provide a valuable perspective on the foundations, capabilities, and limitations of in-situ infrared external reflection spectroscopy of electrode surfaces, with emphasis on Fourier Transform instruments. In addition to the description of underlying principles and instrumentation, selected examples are given of the monitoring and interpretation of spectra of various species adsorbed at electrochemical interfaces. 

In the present paper, the main objectives are (i) to prepare reactive peroxocomplexes in situ at the material s surface starting from a precursor material, and (ii) to control the catalytic properties (activity, selectivity, oxidant efficiency) via modification of the micro-environment of the catalytic center through variation of the anion population. The catalyst precursors and the in situ formed peroxocomplexes are characterized by means of XRD, IR, TGA/DTA and UV-Vis reflectance spectroscopy. 

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