Diffuse reflection techniques

The hexafluoroargentate(III) anion was first obtained as the CS2K salt by Hoppe and Homann [26), who fluorinated a 2 1 1 mixture of CsCl, KC1, and AgNC>3 at 300 °C. A moment of 2.6 B.M. was reported for the product, and the electronic spectrum was studied by Allen and Warren (9), using the diffuse reflectance technique. 

With the exception of single-crystal transmission work, most solids are too opaque to permit the conventional use of ultraviolet/visible (UV/VIS) electronic spectroscopy. As a result, such work must be performed through the use of diffuse reflection techniques [8-10]. Important work has been conducted in which UV/VIS spectroscopy has been used to study the reaction pathways of various solid state reactions. Other applications have been made in the fields of color measurement and color matching, areas which can be of considerable importance when applied to the coloring agents used in formulations. 

In an extension of the diffuse reflectance technique, DR has been used to... 

In the diffuse reflectance technique (Fig. 14.2d), light scattered by a thick layer of particles is directed by the integrating sphere to a detector. Absorption... 

Infrared spectra of proteins may be obtained either in the solid state (KBr pellets, crystals, or by diffuse reflectance techniques (Yang et al., 1985)) or in solution (D2O solutions, ATR techniques, as in the case of protein adsorption studies (Gendreau et al., 1982 Sec. 6.4). Raman spectra of proteins are usually obtained in solution. 

ATR or diffuse reflection techniques are widely used for materials which are difficult to analyze by absorption methods, such as thin layers on nontransparent substrates, substances with very high absorption which are difficult to prepare in thin layers, or substances with a special consistency. Some basic considerations concerning quantitative ATR spectroscopy have been described by Muller et al. (1981). This publication emphasizes the fact that the functional behavior of the ATR spectrum of an absorbing sample must be evaluated with regard to the refractive index as well as to the absorption index of the sample. It is shown that, as a consequence, reflection measurements can be used to determine concentrations of nonabsorbing samples. Further information on reflection spectroscopy is presented in Sec. 6.4. 

The application of NIR spectroscopy has been further stimulated by the development of NIR diffuse reflectance techniques which are widely used in the analysis of agricultural, pharmaceutical, biochemical and synthetic polymer materials (Siesler, 1991). The rapidly increasing use of NIR spectroscopy is illustrated in the book Making Light Work Advances in Near infrared spectroscopy , edited by Murray and Cowe (1992) as well as in the Handbook of Near-Infrared Analysis by Bums and Ciurcak (1992). 

The advantages of NIR diffuse reflectance techniques which rapidly develop are due to the direct analysis of solids without any necessity for special sample preparation (Weyer, 1985 Stark et al., 1986 Murray and Cowe, 1992). In NIR absorption and reflectance techniques fibre optics may be used which allow analysis remote from the spectrometer. 

Diffuse reflectance techniques have been used to measure UV-Visible spectra of transition metal ions in zeolites and of adsorbed molecules for nearly 30 years. There have been no dramatic changes in experimental technique in recent times, although the theoretical basis for interpretation of the electronic spectra of transition metal ions in zeolites has received considerable attention in the past 10 years. There has also been a growing realisation of the importance of combining UV-VIS data with that from other spectroscopic and structural methods to fully characterise zeolite systems. 

For qualitative and quantitative mid-infrared studies, a dilution of a sample by a scattering, but transparent matrix is highly recommended. The diffuse reflection technique can be used for low concentrations because of enhancement effects for weak absorptions, which do not exist for transmission measurements. On the other hand, the dilution by KBr powder can... 

In conclusion, it can be stated that spectroscopic techniques will further dominate the analytical tools of the future with respect to qualitative and quantitative assays. This is because of their speed and the enormous information content of the spectra, especially in the infrared, and the fact that reagent-free multicomponent methodologies are available. The widespread diffuse reflection technique certainly has to compete with others in the laboratory and at the production site. However, for the study of bulk and dispersed systems, it will often be the method of choice. There are additional developments concerned with dedicated instruments and user-friendly interfaces, in which che-mometrics play an important role. It is hoped that the sophisticated algorithms presented in the literature will... 

Vibrational spectroscopy is the method of choice for the characterizing functional groups in complex organic molecules. Infrared transmission spectroscopy has been used on dried humics pressed into KBr pellets to determine the relative carboxylate content of humic materials (14-16). However, interferences arise from the presence of water bands and possible alterations of the samples under the high pressures used to form the pellets. Diffuse-reflectance techniques can avoid some of the difficulties associated with the KBr pressed-pellet method (9,17-18). To obtain a spectrum analogous to an absorption spectrum, the data are transformed from reflectance units to Kebulka-Munk (K-M) units. However, K-M units are related to... 

The Fourier transform mid-infrared (FTIR) spectra of the talc materials from the various vendors were measured by the neat, diffuse reflectance technique. Spectra of the three materials, acquired at a spectral resolution of 1 cm1, are presented in Figure 5. Identical spectra, including peak frequencies, peak width at half height, and peak shape, were measured for the three sources of talc. The major absorption bands and assignments are detailed in Table II [23]. 

Berthelot M, Cornu G, Daudon M, Helbert M, Laurence C. Diffuse reflectance technique for infrared analysis of urinary calculi. Clin Chem 1987 33 780-3. 

Determination of infra-red crystallinity index at different wavelengths (1429 cm V893 cm ) by the usual base line technique can be used [38]. When cellulose-II samples are considered, the 1372 cm 72900 cm" ratio is more reliable index [39, 40]. A rapid method for estimation of the degree of mercerization has been developed using a near IR diffuse reflection technique [41]. 

The techniques of Cylindrical Internal Reflectance (CIR) and Diffuse Reflectance spectroscopies are described herein. The CIR phenomenon was employed in three different apparatus. Two different high pressure CIR cells were used to study reactions homogeneously catalyzed by [y-HW2(CO)10]-. Low temperature reactions of Mo and W complexes were studied using an ambient pressure CIR cell. The diffuse reflectance technique was employed to study powdered samples of Ru carbonyl complexes supported on A1203. 

Experimentally, IR spectroscopy can be accomplished in a number of ways by Nujol mull, KBr disc or the diffuse reflectance technique. In the KBr disc technique, the compound is mixed with KBr and compressed into a disc using a press and die. This can be a disadvantage if the compound undergoes a polymorphic transformation under pressure (see Chan and Doelker, 1985). One way to overcome this problem is to use the diffuse reflectance Fourier transform (DRIFT) technique, whereby a few milligrams of compound is dispersed in approximately 250 mg of KBr and the spectrum obtained by reflection from the surface. 

The Uv-vis analysis was performed on a Shimadzu UV 210IPC by diffuse reflectance technique in the range 200-600 nm and absorbance between (-0.05) to (-K).75). 

For a discussion of reflectance spectroscopy, two types of reflectance must be defined, specular and diffuse. Specular reflectance is simply mirrorlike reflectance from a surface and is sometimes called regular reflectance it has a well-defined reflectance angle. Diffuse reflectance is defined as reflected radiant energy that has been partially absorbed and partially scattered by a surface with no defined angle of reflectance. The diffuse reflectance technique is widely used today for industrial applications involving textiles, plastics, paints, dyestuffs, inks, paper, food, and building materials. In the area of basic research, diffuse reflectance spectroscopy has been used in studies of solid-solid reactions, of species absorbed on metal surfaces, of radiation transfer, and of slightly soluble species. 

UV-Vis spectroscopy in solution is probably one of the most frequently applied spectroscopic methods in the quantitative analysis of pharmaceuticals (see other chapters of this book). In solid-state analysis, this situation is quite the opposite since most solids are too opaque to permit the use of this technique in the conventional transmission mode. UV-Vis spectroscopy on solids can only be realized via diffuse-reflection techniques connected with mathematical corrections (e.g. Kubelka-Munk function) and lacking the high reproducibility of UV-Vis spectroscopy in solution owing to particle dispersion effects. The number of published papers on the application of UV-Vis spectroscopy to solid pharmaceuticals is very small and these papers include topics such as photo-stabihty of dyes and active ingredients in tablets, drug-excipient interactions in dmg products, quantitative measurements on discolouration in dmg products, and others. For further reading we refer to Brittain [26] and the literature cited therein. 

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