Spectroscopic element-specific

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

The application of atomic spectroscopic instruments as element-specific detectors in chromatography has been reviewed by van Loon More recently, Krull has extensively reviewed their use in high pressure liquid chromatography (HPLC). Atomic spectrometry has found wide acceptance in the field of liquid chromatography because, in most cases, the fractions can be directly analysed after elution from the column. However, it is possible to use the technique for the analysis of solid samples without first dissolving the matrix. This is particularly useful after electrophoresis, where the fractions are fixed either in a gel or on paper. Kamel et al. have shown that it is possible to cut the appropriate sections and insert them into the carbon furnace for analysis. The disadvantage of this approach is that the precision is usually poorer (about 10%) and it is difficult to calibrate the instrument. Nevertheless, this approach is very useful if it is used for qualitative speciation. 

Solid-state nuclear magnetic resonance (NMR), a canonical technique of chemistry and physics, possesses many versatile features such as, for example, elemental specificity and local structural, electronic, and motional sensitivity. In particular, NMR can characterize samples in most types of condensed matter, be it liquid or solid, single crystal or amorphous. Given adequate sensitivity it has, therefore, the unique ability of providing metal surface and adsorbate electronic and structural information on a molecular level and allows one to access motional information of adsorbate over a time range unattainable by any other single spectroscopic technique. In addition, solid-state NMR is nondestructive, technically versatile. 

Finally, reference is made to the element-specific detection via atom spectroscopic techniques, which include atomic absorption and atomic emission spectroscopy. 

The oldest of the spectroscopic radiation sources, a flame, has a low temperature (see Section 4.3.1) but therefore good spatial and temporal stability. It easily takes up wet aerosols produced by pneumatic nebulization. Flame atomic emission spectrometry [265] is still a most sensitive technique for the determination of the alkali elements, as eg. is applied for serum analysis. With the aid of hot flames such as the nitrous oxide-acetylene flame, a number of elements can be determined, however, not down to low concentrations [349]. Moreover, interferences arising from the formation of stable compounds are high. Further spectral interferences can also occur. They are due to the emission of intense rotation-vibration band spectra, including the OH (310-330 nm), NH (around 340 nm), N2 bands (around 390 nm), C2 bands (Swan bands around 450 nm, etc.) [20], Also analyte bands may occur. The S2 bands and the CS bands around 390 nm [350] can even be used for the determination of these elements while performing element-specific detection in gas chromatography. However, SiO and other bands may hamper analyses considerably. 

Some additional selective detectors have been described, but their use in biochemical GC has been minimal thus far. Among them, most notably, belong various optical spectroscopic detectors as well as various element-specific detectors based on the solute combustion and measurement of electrolytic conductivity [128]. While little has happened during the last decade with further development of the latter detector types, various gas-phase optical devices remain among the most interesting detectors for future studies. Element-specific plasma devices [129], UV absorption... 

Spectroscopic methods give struetural details complementary to those available from diffraction-based teehniques. The most generally applied methods continue to be NMR and vibrational spectroseopy (especially IR), but element specific techniques such as X-ray absorption spectroscopy are also of importance. As I will describe in Chapters 7 and 8, NMR and IR have also been used extensively for the study of the interactions of adsorbed moleeules with molecular sieves and to investigate the nature of acidity in these solids. 

Pharmaceuticals and drugs] ruggedness test, 848-864 system suitability test, 865 ralidation data elements specific to TLC, 848 mobile phase selection, 822 quality assurance of experimental data and documentation, 865-867 role of TLC in drug analysis, 868-873 chromatqgrapliic identification. 871 spectroscopic identification, 871-873 role of TLC in pharmaceutical analysis, 867-868 sensitivity to experimental conditions 821-822 stationary phase selection, 822 types of analytical aims in pharmaceutical analysis, 823... 

With the availability of intense tunable radiation in the range firom ultraviolet to hard X-rays from synchrotrons, powerful new experimental techniques have been developed to probe the structural and electronic properties of solids and surfaces. In particular, angle-resolved photoemission gives information about the electronic properties in the valence bands of solids while core level spectroscopy provides an element-specific spectroscopic tool. 

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

The focus is on the primary formation of bonds, not on subsequent reactions of the products to form other bonds. These latter reactions are covered at the places where the formation of those bonds is described. Reactions in which atoms merely change their oxidation states are not included, nor are reactions in which the same pairs of elements come together again in the product (for example, in metatheses or redistributions). Physical and spectroscopic properties or structural details of the products are not covered by the reaction volumes which are concerned with synthetic utility based on yield, economy of ingredients, purity of product, specificity, etc. The preparation of short-lived transient species is not described. 

Determination of protein secondary structure has long been a major application of optical spectroscopic studies of biopolymers (Fasman, 1996 Havel, 1996 Mantsch and Chapman, 1996). These efforts have primarily sought to determine the average fractional amount of overall secondary structure, typically represented as helix and sheet contributions, which comprise the extended, coherent structural elements in well-structured proteins. In some cases further interpretations in terms of turns and specific helix and sheet segment types have developed. Only more limited applications of optical spectra to determination of tertiary structure have appeared, and these normally have used fluorescence or near-UV electronic circular dichroism (ECD) of aromatic residues to sense a change in the fold (Haas, 1995 Woody and Dunker, 1996). 

The study of metals in biological systems requires techniques, some of them highly specific, some limited to certain aspects of the metal ion in question, some of more general applicability. Thus, Mossbauer spectroscopy in biological systems is restricted to iron-containing systems because the only element available with a Mossbauer nucleus is 57Fe. The EPR spectroscopic techniques will be of application only if the metal centre has an unpaired electron. In contrast, provided that crystals can be obtained, X-ray diffraction allows the determination of the 3-D structure of metalloproteins and their metal centres. 

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