Electron-probe microanalysis

This technique is associated with electron microscopy (below) and allows small regions containing 100 mg of P/g sample to be examined. The two-dimensional distribution of P over small sample 

A beam of electrons striking a target results in the emission of characteristic X-rays. This is the basis of the X-ray tube, as was discussed earlier in this chapter. A beam of electrons striking a sample will also generate characteristic X-rays from the sample. The use of a small diameter electron beam, on the order of O.l-l.O pm, to excite a sample is the basis of electron probe microanalysis. An electron probe microanalyzer is an X-ray emission spectrometer. The small diameter electron beam excites an area of the surface of the sample that is about 1 pm in diameter. Elemental composition and variation of composition on a microscopic scale can be obtained. 

Two different instruments are available for microanalysis. The electron microprobe analyzer (EMA) uses high electron beam currents to provide elemental analysis of samples, with moderate spatial resolution and low magnification of the sample. The intensity of emitted X-rays from the 

Both systems operate on similar principles. A tungsten filament emits electrons, which are focused by an electron optical system. The electron beam can scan the sample surface and can provide composition at a point, along a line or over a rectangular area, by rastering (moving) the beam across the surface in a series of parallel lines. The sample is mounted on a stage that can be accurately moved in the x- and y-direction and in the z-direction, normal to the plane of the sample. The system has an optical microscope, to permit alignment of the sample and selection of the area or feature of the sample to be analyzed. 

The X-ray analysis system for the EMA is a WD spectrometer with gas-proportional counter detectors. In the SEM, an ED X-ray spectrometer with a Si(Li) detector is used. The entire electron and X-ray optical systems are operated under a vacuum of about 10 torr. Modem systems are completely automated with computer control of the instrument parameters, specimen stage movanent, 

Samples must be solid and may be in almost any form. Thin films, bulk solids, particles, powders, machined pieces, and small objects (including biological specimens) can be analyzed. AU elements from beryllium (Z = 4) to uranium can be determined at concentrations of about 100 ppm or greater. For qualitative analysis, the surface finish of the sample is not important. For quantitative analysis, the surface of the specimen must be flat. A common method for achieving a flat surface for an SEM sample is to embed the sample in epoxy and then carefully polish the hardened epoxy to expose a flat surface of the sample. Calibration standards should have flat surfaces as well, and the composition of the standards should be similar to that of the samples. Alternatively, an FP approach using pure element standards can be used. For nonconductive samples, a thin coating of osmium is deposited on the surface of the sample. 

The distribution of elements in biological samples, mineral samples, soils, and other heterogeneous materials can be determined. Many other applications can be found in the literature. This is one of the few techniques that can provide spatial variation of composition at the micrometer scale. Other related techniques are discussed in Chapter 14. 

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The elemental composition of the fish otoliths is a potential source of the useful information to recreate environment history of the individual fish in some of the species. In-depth study of the chemical composition of the otolith center (formed eaidy in fish life) and otolith edge (formed later in fish life) ensures chronological and environmental information stored in the otoliths [1]. This infoiTnation may be achieved by X-ray electron probe microanalysis (EPMA). EPMA is the analytical method to determine the elemental composition of different otolith s parts, their sizes varying from ten up to some tens of microns. 

The complex of the following destmctive and nondestmctive analytical methods was used for studying the composition of sponges inductively coupled plasma mass-spectrometry (ICP-MS), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and atomic absorption spectrometry (AAS). Techniques of sample preparation were developed for each method and their metrological characteristics were defined. Relative standard deviations for all the elements did not exceed 0.25 within detection limit. The accuracy of techniques elaborated was checked with the method of additions and control methods of analysis. 

INVESTIGATION OF INDIVIDUAL PARTICLES OF ZEOLITE POWDER BY X-RAY ELECTRON PROBE MICROANALYSIS... 

Electron Probe Microanalysis, EPMA, as performed in an electron microprobe combines EDS and WDX to give quantitative compositional analysis in the reflection mode from solid surfaces together with the morphological imaging of SEM. The spatial resolution is restricted by the interaction volume below the surface, varying from about 0.2 pm to 5 pm. Flat samples are needed for the best quantitative accuracy. Compositional mapping over a 100 x 100 micron area can be done in 15 minutes for major components Z> 11), several hours for minor components, and about 10 hours for trace elements. 

Quantitative Electron-Probe Microanalysis. (V. D. Scott and G. Love, eds.) John Wiley Sons, New York, 1983. Taken from a short course on the electron microprobe for scientists working in the field. A thorough discussion of EDS and WDS is given, including experimental conditions and specimen requirements. The ZAF correction factors are treated extensively, and statistics, computer programs and Monte Carlo methods are explained in detail. Generally, a very useftd book. 

The keystone of practical quantitative electron probe microanalysis is Castaing s first approximation, which relates the concentration for a constituent in the unknown to the concentration in a standard in terms of the ratio of X-ray intensities generated in the target ... 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

G. F. Bastin and H. J. M. Heijligers. Quantitative Electron Probe Microanalysis of Ultralight Elements (Boron—Oxygen). Scanning. 12, 225, 1990. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

P. Duncamb, S. j. B. Reed in K. F. J. Heinrich (ed.) The Calculation of Stopping Power and Backscattcr Effects in Electron Probe Microanalysis, NBS Special Publ. 298, Washington, 1968. 

Electron probe microanalysis (composition diffusion profiles local thickness)... 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

Van Zyl, J., Forrest, Q.G., Hocking, C. Pallaghy, C.K. (1976). Freeze-substitution of plant and animal tissue for the localisation of water-soluble compounds by electron probe microanalysis. Micron, 7, 213-24. 

The correlation of phosphate precipitation with decrease of conductivity (Wilson Kent, 1968), increase in pH (Kent Wilson, 1969) and hardness (Wilson et al, 1972) is shown in Figure 6.16. These results demonstrate the relationship between the development of physical properties and the underlying chemical changes, but there are no sharp changes at the gel point. Evidence from infrared spectroscopy (Wilson Mesley, 1968) and electron probe microanalysis (Kent, Fletcher Wilson, 1970 Wilson et al, 1972) indicates that the main reaction product is an amorphous aluminophosphate. Also formed in the matrix were fluorite (CaF ) and sodium acid phosphates. 

Halse, A. Hals, E. (1976). Electron probe microanalysis of secondary carious lesions adjacent to silicate fillings. Calciferous Tissues Research, 21, 183-93. 

A number of techniques have been employed that are capable of giving information about amorphous phases. These include infrared spectroscopy, especially the use of the attenuated total reflection (ATR) or Fourier transform (FT) techniques. They also include electron probe microanalysis, scanning electron microscopy, and nuclear magnetic resonance (NMR) spectroscopy. Nor are wet chemical methods to be neglected for they, too, form part of the armoury of methods that have been used to elucidate the chemistry and microstructure of these materials. 

DT-MS Direct temperature-resolved mass EPMA Electron-probe microanalysis... 

Electron Probe Microanalysis in the Scanning Electron Microscope 137... 

ELECTRON PROBE MICROANALYSIS IN THE SCANNING ELECTRON MICROSCOPE... 

Although conventional electron-probe microanalysis appears to be unsuitable for analysis of the exposed surface layer of atoms in an alloy catalyst, recent developments have shown that X-ray emission analysis can still be used for this purpose (89, 90). By bombarding the surface with high energy electrons at grazing incidence, characteristic Ka radiation from monolayer quantities of both carbon and oxygen on an iron surface was observed. Simultaneously, information about the structure of the surface layer was obtained from the electron diffraction pattern. 

Bi2S3 Bi2Te3 Bi(N03)3 Na2S Te02 XRD, SEM, electron probe microanalysis (EPMA), FESEM 162-166... 

Bismuth nitrate together with sodium sulphide and tellurium oxide have been used to grow bismuth chalcogenide films.162-166 The stoichiometric ratio 2 3 has been confirmed by XPS, EDX, and XRD.162-166 The Bi2Te3 films were rough and consisted of particles with a diameter of 30-100nm, and electron probe microanalysis showed a worm-like network structure.164-166... 

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