Solid fluorescence analysis

X-RAY FLUORESCENCE ANALYSIS OE COAL CONCENTRATES USING QUASI-SOLID SPECIMENS... 

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

X-Ray Fluorescence analysis (XRF) is a well-established instrumental technique for quantitative analysis of the composition of solids. It is basically a bulk evaluation method, its analytical depth being determined by the penetration depth of the impinging X-ray radiation and the escape depth of the characteristic fluorescence quanta. Sensitivities in the ppma range are obtained, and the analysis of the emitted radiation is mosdy performed using crystal spectrometers, i.e., by wavelength-dispersive spectroscopy. XRF is applied to a wide range of materials, among them metals, alloys, minerals, and ceramics. 

We prepared thin film Pt alloy electrodes by Ar-sputtering Pt and the second metal targets simultaneously onto a disk substrate at room temperature (thickness approximately 200 nm). The resulting alloy composition was determined by gravimetry and X-ray fluorescent analysis (EDX). Grazing incidence (i7= 1°) X-ray diffraction patterns of these alloys indicated the formation of a solid solution with a face-centered cubic (fee) crystal stmeture. 

X-ray fluorescence analysis is a nondestructive method to analyze rubber materials qualitatively and quantitatively. It is used for the identification as well as for the determination of the concentration of all elements from fluorine through the remainder of the periodic table in their various combinations. X-rays of high intensity irradiate the solid, powder, or liquid specimen. Hence, the elements in the specimen emit X-ray fluorescence radiation of wavelengths characteristic to each element. By reflection from an analyzing crystal, this radiation is dispersed into characteristic spectral lines. The position and intensity of these lines are measured. 

In a laboratory generator, electrons are accelerated by a potential around 30 kV towards a solid target, where they are stopped by impact. The output contains the line spectmm superimposed upon a continuous spectrum. The line, or characteristic spectrum is characteristic of the element and is used in X-ray fluorescent analysis to identity the type and amount of an element present in a sample. The continuous radiation is also called the Bremsstrahlung, from the... 

For catalytic application it is necessary to incorporate hetero-atoms into the silica framework. Several samples have been synthesised using different aluminia precursors. The metal content was determined by X-ray fluorescence analysis, UV-VIS spectra, IR spectra and solid state NMR spectroscopy, respectively. X-ray fluorescence analysis provides information about the metal content of the samples. By variation of the metallic precursor concentration the metal content of the product could be enhanced up to 10 % w/w. 

The most popular and elegant specimen preparation technique introduced by Claisse 17] is based on fusion of solid specimens with lithium tetraborate. The method was used with great success in our laboratory for the rapid quantitative X-ray fluorescence analysis of silicates, bricks, refractories, limes, iron, and manganese ores. The use of lithium tetraborate and lithium fluoride flux systems was therefore examined first. 

In spite of such complications, magnetic sorting is used constantly in the field. Magnets can also be useful identification tools in conjunction with sophisticated quantitative instruments. For example, x-ray fluorescence analysis of a component showed it to be 100% chromium. This is because the technique utilizes only a very thin surface layer. A test with a magnet indicated attraction. It was concluded that the part was chrome plated and not solid chromium (which would have been very unlikely). 

In the preceding chapter it had already been discussed that it is less the synthesis itself which may be the bottleneck in high-throughput zeolite science but rather the analysis of the solids formed in a high-throughput program. There are several standard characterization techniques which are typically employed to characterize zeolitic materials. These include powder XRD for phase identification, X-ray fluorescence analysis (XRF) or atomic absorption spectrometry to analyze elemental composition, sorption analysis to study the pore system, IR-speclroscopy, typically using adsorbed probe molecules to characterize the acid sites, NMR spectroscopy and many others. For some of these techniques parallelized solutions have been developed and described in the literature, other properties are more difficult to assess in a parallelized or even a fast sequential fashion. 

The X-ray signal in STEM is usually collected by compact solid state detectors based on lithium-drifted silicon diodes. The X-ray photon is sorted by energy, henee the alternative names for X-ray fluorescence analysis Energy Dispersive X-ray Spectroseopy (EDS or EDX). The typical energy resolution for an X-ray photon is of on the order of 150 eV This is suffieient in most cases for resolving peaks of different elements, but is inadequate for deteeting... 

Reus U, Markeit B, Hoffmeister C, Spott D, Guhr H (1993) Determination of trace metals in river water and suspended solids by TXRF spectroscopy A methodical study on analytical performance and sample homgeneity. Fresenius J Anal Chem 347 430-435 Sanchez HJ. (2001) Detection limit calculations for the total reflection techniques of X-ray fluorescence analysis. Spectrochimica Acta 56 2027-2036... 

X-ray photons arc yet a third product of electron bombardment of a solid. Both characteristic line spectra and an X-ray continuum are produced. This radiation serves as the basis for the electron niicroprobe for X-ray fluorescence analysis of SHM images. 

Solid samples used in x-ray fluorescence analysis include metallurgical specimens, briquetted powders, and borax discs. For quantitative analysis, both standards and unknowns must be in the same matrix and subjected to the same preparation. Because of the limited escape depth of secondary x-rays, particularly in the long-wavelength region, the surface layers must be representative of the entire sample. 

The set of all elements detectable by an analytical method can be expressed by a set over the atomic number, x. According to classical set theory, detection of elements with atomic numbers greater than a particular value is feasible, for example. X-ray fluorescence analysis can be used for analyses of elements from sodium with atomic number 11. The membership function is given by the solid line in Figure 8.17. 

Figure 8.17 Membership function for the detectability of elements by X-ray fluorescence analysis. Solid line Classical (crisp) set, broken line fuzzy set.

X-ray fluorescence XRF is one of the longest established techniques for trace elemental analysis. While XRF is not a very sensitive technique, its main advantages are the capability for direct solid sample analysis combined with multielement determinations. While sample pretreatment of solids can be substantially reduced or even omitted in some cases, perfect matching between standards and samples is required for accurate results, because of severe matrix effects. The main application field of XRF is, therefore, the analysis of solid materials, such as metallurgical and geological samples, where solid standards are readily available. Liquid samples can be analyzed either directly in special cells or by using preconcentration techniques with solid sorbents, which can be directly analyzed after sample loading. More modern methods, like total-reflection X-ray fluorescence, which is a multielement technique mainly for solutions, or particle-induced X-ray emission, which is a micromethod with some spatial resolution, have found limited application in some special areas. For speciation purposes, species separation has to be carried out in front in an offline mode. 

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