Electron microprobe analysis methods

Electron Microprobe A.na.Iysis, Electron microprobe analysis (ema) is a technique based on x-ray fluorescence from atoms in the near-surface region of a material stimulated by a focused beam of high energy electrons (7—9,30). Essentially, this method is based on electron-induced x-ray emission as opposed to x-ray-induced x-ray emission, which forms the basis of conventional x-ray fluorescence (xrf) spectroscopy (31). The microprobe form of this x-ray fluorescence spectroscopy was first developed by Castaing in 1951 (32), and today is a mature technique. Primary beam electrons with energies of 10—30 keV are used and sample the material to a depth on the order of 1 pm. X-rays from all elements with the exception of H, He, and Li can be detected. 

Stoichiometric variations in compositions of a material and of surface layers can be revealed by AEM. Because a relatively small amount of scattering occurs through a thin HRTEM specimen, X-rays are generated from a volume that is considerably less than in the case of electron microprobe analysis (EPMA). For quantitative microanalysis, a ratio method for thin crystals (57) is used, given by the equation ... 

X-ray diffraction uses X-rays of known wavelengths to determine the lattice spacing in crystalline structures and therefore directly identify chemical compounds. This is in contrast to the other X-ray methods discussed in this chapter (XRF, electron microprobe analysis, PIXE) which determine concentrations of constituent elements in artifacts. Powder XRD, the simplest of the range of XRD methods, is the most widely applied method for structural identification of inorganic materials, and, in some cases, can also provide information about mechanical and thermal treatments during artifact manufacture. Cullity (1978) provides a detailed account of the method. 

The accuracy of some isothermal techniques, particularly those that rely on observation of phases, is limited by the number of different compositions that are prepared. For example, if two samples are separated by a composition of 2at%, and one is single-phase while the other two-phase, dien formally the phase boundary can only be defined to within an accuracy of 2at%. This makes isothermal techniques more labour intensive than some of the non-isothermal methods. However, because it is now possible to directly determine compositions of phases by techniques such as electron microprobe analysis (EPMA), a substantially more quantitative exposition of the phase equilibria is possible. 

If mixing in each site is not ideal, would differ from the real equilibrium constant by the quotient of activity coefficients and hence may depend on composition. The measurement of the site occupancy (the fraction of Fe and Mg in each of Ml and M2 sites) is not trivial. There are two methods to determine the intracrystalline site distribution. One is by Mossbauer spectroscopy (MS), in which there are a pair of outer and smaller peaks, which are due to Fe in Ml site, and a pair of inner and larger peaks, which are due to Fe in M2 site (Figure 2-3). The ratio of Fe in Ml site to Fe in M2 site is assumed to be the area ratio of the pair of Ml peaks to the pair of M2 peaks. Using total Fe content from electron microprobe analysis, and the ratio from Mossbauer spectroscopy, Fe(Ml) and Fe(M2) concentrations can be obtained. 

The partially oxidized mixed-valence complexes were generated by electrochemical methods. In the case of the BU4N salt (x = 0.29), the extent of oxidation was determined by elemental analysis, electron microprobe analysis, mass spectra and the X-ray structure analysis only elemental analysis was cited for the Et4N (x = —0.5) derivative. Upon reduction, the essentially planar anions exhibit the same Ni—S and S—C bond lengthening (see Table 4) as mentioned in Section 16.5.3.1. Based on the relative changes in the last two entries in the above table (then the only available data), Lindqvist et a/.184 185 suggested the redox process was centered on the metal. 

The thickness of the deposits was determined by the ball cratering method or SEM measurement of cross-sections. The elemental composition was determined by electron microprobe analysis with wavelength dispersive spectroscopy (EPMA-WDS) on a Camebax Cameca equipment and by X-ray photoelectron spectroscopy (XPS) on a VG Escalab MK2 apparatus... 

The data presented in Table V were obtained by energy-dispersive X-ray analysis using an electron microprobe. The method is based on monitoring the sulfur content of an organic maceral (in this case, vitrinite), which is not associated with any cations, as an index of the organic sulfur... 

The analysis of corrosion scale or product may be done by wet chemical methods such as spectrophotometry or atomic absorption spectrophotometry in cases where the removal of corrosion scale is permitted, or by surface analytical techniques such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, electron microprobe analysis, by energy dispersive X-ray analysis in the case of samples which need to be preserved. 

Electron microprobe analysis (AES) surface analytical methods... 

Surface analytical methods — Important ex situ methods for surface analysis are X-Ray Photoelectron Spectroscopy (XPS) UV-Photoelectron Spectroscopy (UPS), Auger Electron Spectroscopy (AES), Ion Scattering Spectroscopy (ISS), Rutherford Backscattering (RBS), Secondary Ion Mass Spectroscopy (SIMS), Scanning Electron Microscopy (SEM), Electron Microprobe Analysis (EMA), Low Energy Electron Diffraction (LEED), and High Energy Electron Diffraction (RHEED). 

Sawhney, B.L., Electron microprobe analysis, in Methods of Soil Analysis Part 1— Physical and Mineralogical Methods, 2nd ed., Klute, A., Ed., American Society of Agronomy, Madison, WI, 1986. 

The XAFS study of Zn in a contaminated dredged sediment by Isaure et al. (2002) used a combination of methods (EXAFS, pXAFS, pPIXE, powder XRD, electron microprobe analysis, ICP-AES analysis, and selective chemical extractions) to identify three primary Zn-containing minerals (sphalerite, willemite, zincite) plus zinc associated with Fe-oxyhydroxide and phyllosilicates that was released by chemical weathering. 

If a mineral is sufficiently old and rich in Th and U, Pb concentrations can be high enough to be detectable using an electron microprobe. Analysis of all three elements then provides an estimate of age. This method has been widely used in uranium-ore studies, as pitchblende produces high Pb contents in relatively short times. Recently this technique has been applied with success to monazite (Suzuki and Adachi 1991, 1994 Montel et al. 1994, 1996 Williams and Jercinovic 2002), and, with more difficulty, to zircon, and xenotime (Suzuki and Adachi 1991, Geisler and Schleicher 2000)... 

Electron microprobe analysis can be used for the elemental analysis of preceramic polymers and ceramics. However, since these materials are generally quite thermoxidatively stable and thus are not readily amenable to the traditional combustion approach to elemental determination, chemical analysis methods can be complemented by x-ray fluorescence techniques. 

Electron microprobe analysis is based on excitation of an electron beam concentrated on a very small area (a few square micrometers) of a sample and examination of the X-rays produced. The method was pioneered by Castaing in 1949. 

In the end, it is worth mentioning that another kind of approach could still be undertaken to determine copper and lead forms in soils polluted with metallurgical dust. This is namely the direct electron microprobe analysis (Hiller and Bruemmer 1989 Weber and Kowaliiiski 1987 Weber 1989). However, some attempts to apply this method to the soils from copper smelter neighbour-... 

If these methods fail to identify a gemstone, then other more destructive techniques such as X-ray dif-fiaction, electron microprobe analysis, and internally coupled plasma mass spectrometry might be used for identification. Often these techniques use material that has been scraped off parts of the gemstone that win not be exposed to a viewer. X-ray diffraction techniques can be especially useful in confirming the identity of a mineral because each mineral presents a unique pattern. 

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