X-ray analytical methods

X-RAY ANALYSIS. X-rays occupy that portion of the electromagnetic spectrum between 0.01 and 100 angstroms (A). Their range of approximate quantum energy is from 2 x 10-6 to 2 x 10 10 erg, or from 106 to 100 eV. Important X-ray analytical methods are based upon (1) fluorescence (2) emission (3) absorption and (4) diffraction. These methods are used qualitatively and quantitatively to determine the element content of complex mixtures and to determine exactly the atomic arrangement and spacings of crystalline materials. See also Ion Microprobc Mass Analyzer. 

The system Te—TeCl has been investigated by thermoanalytical and X-ray analytical methods, , i,2o, 3) yjjg result is given in Fig. 1. The system is... 

Chemical Properties. Elemental analysis, impurity content, and stoichiometry are determined by chemical or iastmmental analysis. The use of iastmmental analytical methods (qv) is increasing because these ate usually faster, can be automated, and can be used to determine very small concentrations of elements (see Trace AND RESIDUE ANALYSIS). Atomic absorption spectroscopy and x-ray fluorescence methods are the most useful iastmmental techniques ia determining chemical compositions of inorganic pigments. Chemical analysis of principal components is carried out to determine pigment stoichiometry. Analysis of trace elements is important. The presence of undesirable elements, such as heavy metals, even in small amounts, can make the pigment unusable for environmental reasons. 

The most fundamental limitation of the method is implicit in Equation 1-9. As all atoms absorb x-rays, the method cannot be specific it cannot ordinarily identify unknown elements in a sample, nor can it ordinarily give reliable analytical results on a sample containing unknown elements. On the other hand, it can usually show whether or not such a sample has an assumed ultimate composition.7... 

Electron probe and X-ray fluorescence methods of analysis are used for rather different but complementary purposes. The ability to provide an elemental spot analysis is the important characteristic of electron probe methods, which thus find use in analytical problems where the composition of the specimen changes over short distances. The examination of the distribution of heavy metals within the cellular structure of biological specimens, the distribution of metal crystallites on the surface of heterogeneous catalysts, or the differences in composition in the region of surface irregularities and faults in alloys, are all important examples of this application. Figure 8.45 illustrates the analysis of parts of a biological cell just 1 pm apart. Combination of electron probe analysis with electron microscopy enables visual examination to be used to identify the areas of interest prior to the analytical measurement. 

The chemical composition of the compounds, until now characterized predominantly by X-ray diffraction methods in analogy to lanthanide or actinide compounds, is increasingly being determined by standard analytical procedures. These methods (e.g. coulometry, spectrophotometry) have to be adjusted to the quantity of material available and effects of radioactive decay have to be taken into account. As with metals, chemical... 

TJecent interest in the trace element content of coal has increased the need for rapid and accurate analytical methods for their determination. Because x-ray fluorescence analysis has demonstrated its usefulness in determining major, minor, and trace elements in numerous other types of materials, it was felt that this method could be extended to trace element determinations in whole coal. In the past, such analyses were seriously hampered by the lack of standard samples. However, research being conducted in our laboratories under the sponsorship of the U. S. Environmental Protection Agency produced a large number of coal samples for which trace elements had been determined by two or more independent analytical procedures, for example, optical emission, neutron activation, atomic absorption, and wet chemical methods. These coals were used as standards to develop an x-ray fluorescence method that would determine many trace and minor elements in pressed whole coal samples. 

Many alternative techniques, both qualitative and quantitative, have been investigated either for screening purposes or as primary methods. Such techniques include atomic absorption spectrophotometry, molecular luminescence, electron spin resonance spectrometry, X-ray analysis methods, and electro analytical methods. Flameless atomic absorption spectrophotometry (FAAS) is the technique that has almost completely replaced NAA. 

The failure analysis can be done using a judicious combination of several methods such as visual examination, metallography, microscopy, electron microprobe, energy dispersive X-ray analysis, X-ray diffraction methods for determining residual stress in the sample, surface analytical techniques to determine the nature and composition of surface deposits and finite element analysis modeling. 

The study of the dyes, mordants, and other coloring materials used in pre-Columbian textile artifacts remains an inadequately explored area of analytical endeavor. Thin layer chromatography and related techniques should prove productive in the identification of vegetable dyes (7). Inorganic materials can be readily identified by x-ray diffraction methods. 

Three analytical tools were used to characterize the compounds. The first is powder x-ray diffraction methods using a 114.6-mm. diameter camera. The photographs were in general poor for germanium telluride and, as a result, the parameters were determined from low-angle reflections only. The second procedure involved room temperature Seebeck coefficient data (taken versus copper and converted to absolute values) which qualitatively vary inversely as the log of the carrier concentration. Finally, Hall measurements were taken on 1.6 X 0.5 X 0.1 cm. plates in a manner already described (6). 

Despite many advances in analytical methods in recent years, the structural characterization of materials that only occur as microcrystals less than about 30 l in diameter remains difficult and laborious. High resolution electron microscopy in the lattice imaging mode is by far the most powerful tool in giving the direct evidence of structural details essential for modelling clues, as has been demonstrated in the cases of recent zeolite structure solutions of theta-l/ZSM-23 (26) and beta (27), in addition to ECR-1. X-ray diffraction methods provide the essential confirmatory data, and sorption molecular probing and various well established spectroscopic methods are useful ancillary tools. 

X-ray crystallographic methods, which reflect differences in crystal structure, in most cases can be definitive in the identification and characterization of polymorphs, and whenever possible should be included in the analytical methods utilized to define a polymorphic system. 

Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and infrared spectroscopy are the common techniques used in the characterization of the structure of the congealed solid. Thermal analytic methods, such as DSC and differential microcalorimetric analysis (DMA), are routinely used to determine the effect of solutes, solvents, and other additives on the thermomechanical properties of polymers such as glass transition temperature (Tg) and melting point. The X-ray diffraction method is used to detect the crystalline structure of solids. The infrared technique is powerful in detecting interactions, such as complexation, reaction, and hydrogen bonding, in both the solid and solution states. 

In many cases, however, the costs arising from sample preparation will become decisive, which favors x-ray spectrometric methods, provided the earlier mentioned limitations are not encountered. Future progress will certainly depend on the avail-abilty of on-line sample treatment using, for example, flow injection and eventually on-line sample dissolution as is possible in some cases with microwave-assisted heating. Also the realization of separations in miniaturized systems and with minute amounts of reagents is very promising. In each instance the question of which method should be selected will have to be discussed for each type of analytical task to be solved. 

Generally, the sensitivity of X-ray spectrometrie methods is not suffieient for direct measurement of PGE in environmental samples. However, adequate total reflection X-ray fluorescence (TXRF) methods have been developed to enable, after adequate sample preparation and analyte enrichment via Hg-coprecipitation, PGE-determination in various samples like road dust, airborne particles or liver and kidney tissue of exposed European eels (Anguilla anguilla) (Messerschmidt et al. 2000 Sures et al. 2001). 

The system Te—TeBr has been investigated by thermoanalytical. X-ray analytical and metallographic methods The final result is given in Fig. 2. The system is quasibinary and contains one intermediate compound of composition Te2Br which melts incongruently at 225 °C. The existence and composition of this subbromide have been confirmed by determination of its crystal structure... 

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