Sample-preparation methods diffuse-reflection measurement

These are GC-IR (5), LC-IR (6), and diffuse reflectance (7). On-the-fly GC-IR systems are commercially available, and lower detection limits are being continually reported. While GC-IR may not replace GC/MS in residue and metabolism work, it can provide valuable data in these areas. On-the-fly LC-IR systems have been developed and are also commercially available. The major problem in these systems is the strong infrared absorbence of many common LC solvents. However, with proper selection of solvents and the development of LC conditions specifically designed for the LC-IR experiment, these problems may be overcome. Recent reports on diffuse reflectance measurements by FTIR indicate the technique may provide a method of examining formulated material or TLC spots with no sample preparation. While this technique is still in the development stage, it may become quite significant in the future. 

M. R. A. Spragg, A Rapid Sample Preparation Technique for Diffuse Reflectance Measurements, Appl. Spectrosc. 38, 604-605 (1984). Describes his method of rubbing a hard polymer on SiC paper. 

Recently, Castiglioni, et al. have devised a new method, diffuse reflectance CD (DRCD), to measure the solid-state CD of a pure microcrystallite sample or one mixed with KBr [42,43] and checked their performance. An integrating sphere used for UV-Vis and IR spectroscopy was introduced (Fig. 6). The advantage of this technique is to remove possible artifact elements such as reactions with the KBr, the effect of pressure required for the pellet preparation, or sample dissolution in nujol. Further, samples can be recovered after the spectral measurement, in contrast to both KBr pellet and nujol mull methods. 

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. 

NIR spectroscopy has been used for more than 30 years in the x)d industry and agriculture. The main applications are determination of moisture and characterization of other compounds, e. g. protein content in grain and milk products. Usually diffuse reflection is measured because then the samples need not be prepared extensively. Compared to wet chemical analysis or other instrumental methods of analysis, NIR in particular allows rapid detection even under field conditions. Some basic characteristic wavelengths are fisted in Tab. 6.4. 

Fibrous materials may be analyzed by a number of different methods, including the standard method for sample preparation—compressed halide pellets. In this latter case, normally the fiber structure is destroyed during sample preparation. Alternatively, one can consider using diffuse reflectance, photoacoustic, or infrared microscopy. With due consideration to the preparation procedures, the fibers may be retained in their original form with these methods. In some cases, such as with IR microscopy, it is possible to study the orientation characterisitics of the fiber material and to correlate this information to certain mechanical properties of the fiber. The use of IR polarizers is imphed with this type of measurement. Diamond compression cells work well for single-filament fibers by both transmission (diamond anvil cell) and ATR. 

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