Electron Microanalysis Core Facility
The NAU Electron Microanalysis Core Facility is located in room 104 of Wettaw Hall and houses a Cameca SX-50 electron microprobe (EMP) and a JEOL scanning electron microscope (SEM).
What is Electron Microanalysis?
Electron microanalysis is a non-destructive technique used to determine chemical compositions of tiny amounts of solid materials. A focused beam of high-energy electrons is used to bombard the sample and generate characteristic x-rays from volumes as small as 3 micrometers (3 x 10-6 m) across. The resulting x-rays may be either (1) diffracted by analyzing crystals and counted using gas-flow and sealed proportional detectors (wavelength-dispersive spectrometry, WDS) or (2) detected directly using a solid-state semiconductor detector (energy-dispersive spectrometry, EDS). Elemental compositions is determined by comparing x-ray intensities from materials of known compositions (standards) with those from the unknown materials and correcting for the effects of absorption and fluorescence within the sample.
Electron microprobe analysis permits precise accurate quantitative WDS analyses of solid materials. Instrumentation and analytical methods were first developed by R. Castaing in Paris as his 1950 Ph.D. dissertation. An electron microprobe is designed specifically for detecting and measuring characteristic x-rays. Scanning electron microscopes, which were developed about a decade earlier, are often equipped with EDS systems, which permit qualitative and semiquantitative x-ray analysis, also using the methodology developed by Castaing. The electron beam current in an electron microprobes is typically 10 to 200 nanoamps, roughly 100 times greater than that used in most scanning electron microscopes. Higher beam currents produce more x-rays (higher count rates) from the sample, which improves both the detection limits (typically several 100 ppm) and precision of the resulting analysis.
The most important aspect of electron microanalysis is that it provides chemical analyses in a textural context. Small-scale variations in chemical composition within a material, such as a mineral grain or metal, can be rapidly determined. These variations result from number of phenomena including pressure and temperature changes during growth and subsolidus element diffusion. Although principally used for geological investigations, the microprobe and scanning electron microscope are available to all the University community and outside researchers. Past analytical projects have included studies in metamorphic, sedimentary and igneous petrology, meteoritics, and characterization of archaeological materials such as pottery (paste and temper, glazes), glass, and lithics.
The Cameca SX-50 electron microprobe has four WDS spectrometers (with PC0/TAP, TAP/PET, PET/LIF, and PET/LIF paired analyzing crystals), and secondary electron (SE), backscattered electron (BSE), and cathodoluminescence (CL) detectors for imaging. Software permits automated acquisition of point analyses along lines, from grids, and from multiple stored locations. Systematic x-ray mapping of areas as large as 1 cm across using high sample currents (250+ nA) is also automated. There is an extensive set of standard materials (metals, minerals, compounds, and glasses with well-established chemical compositions), permitting quantitative analysis of all elements from fluorine (Z=9) to uranium (Z=92). Analysis points may be easily located using backscattered electron imaging with fine positioning accomplished using a reflected-light optical microscope (400x); this yields positioning accurate to 1 micrometer.
The JSM-6480LV scanning electron microscope has a large vacuum chamber, which can accommodate specimens up to 20 cm (8 inches) in diameter. Imaging can be undertaken using SE, BSE and CL. Additionally, BSE images of relatively flat samples can be acquired in topographic mode, which emphasizes height variations in the surface. The low vacuum (LV) option permits imaging of specimens using BSE that cannot be viewed at high vacuum due to high water content or a non-conductive surface. The microscope is equipped with an Oxford EDS/WDS system to provide semiquantitative analytical capabilities and x-ray mapping. It is possible to montage multiple x-ray maps to create composite images of large areas. Unlike most scanning electron microscopes, the instrument is typically operated at high sample currents (1-10 nA), maximizing x-ray production at the expense of image resolution. Analytical locations are selected using backscattered electron imaging, yielding positioning accurate to about 1 micrometer.
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