Electron Microanalysis Core Facility

The NAU Electron Microanalysis Core Facility is located in room 104 of Wettaw Hall and houses a Cameca SX-50 microprobe and a JEOL scanning electron microscope (SEM).

The Cameca SX-50 microprobe has four spectrometers (PC0/TAP, TAP/PET, PET/LIF, PET/LIF crystals), and secondary electron (SE), backscattered electron (BSE), and cathodoluminescence (CL) detectors. Automation permits acquisition of point analyses along lines, from grids of points, and multiple stored locations. Systematic x-ray mapping of areas as large as 0.5 mm across using high sample currents (250+ nA) is also automated. There is an extensive set of standard materials (metals, minerals, compounds, and glasses), allowing quantitative analysis of all elements from fluorine (Z=9) to uranium (Z=92).

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 polished sections can be acquired in topographic mode, which emphasizes height variations in the surface. The "low vacuum" option permits imaging of specimens which cannot be viewed at high vacuum due to high water content or due to a non-conductive surface. The SEM is equipped with Oxford EDS and WDS systems to provide semi-quantitative analytical capabilities and x-ray mapping. It is possible to montage multiple x-ray maps to create large composite images. Unlike, most SEMs, the instrument is typically operated at high samples currents (1-10 nA), maximizing x-ray production at the expense of image resolution.

What is Electron Microanalysis?

Electron Microanalysis is a non-destructive technique used to determine chemical compositions of tiny amounts of solid materials using characteristic x-rays. This capability is most highly developed in electron microprobes, which are designed to optimize chemical analysis. Electron microprobe instrumentation and analytical methods were first developed by R. Castaing in Paris as his 1950 Ph.D. dissertation. Commercial electron microprobes became available in the 1960s, and have become standard analytical tools. Scanning electron microscopes may be equipped with semiconductor x-ray detector, which permits semiqualitative x-ray analysis.

Electron microanalytical methods use focused beams of high-energy electrons to generate characteristic x-rays from within a sample from volumes as small as 3 micrometers (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 using a solid-state semiconductor detector (energy-dispersive spectrometry, EDS). Chemical composition is determined by comparing the intensity of X-rays from materials with known compositions (standards) with those from the unknown materials and correcting for the effects of absorption and fluorescence in the sample.

The electron microprobe (EMP) is designed specifically for detecting and measuring characteristic x-rays. It uses an electron beam current from 10 to 200 nanoamps, roughly 100 times greater than that used in most SEMs. These higher beam currents produce more x-rays from the sample and improve both the detection limits (typically several 100 ppm) and precision of the resulting analysis. Analysis locations are selected using BSE imaging with fine positioning accomplished using a reflected-light optical microscope (400x), yielding positioning accurate to about 1 micrometer, a feature not available on an SEM. The resulting data yield quantitative chemical analyses in a textural context. Small-scale variations in chemical composition within a material, such as a mineral grain or metal, can be readily determined.

Although principally used for geological investigations, the microprobe and SEM are available to all in the University community and outside researchers. Projects have included research in metamorphic, sedimentary and igneous petrology, meteoritics, and studies of archaeological materials such as pottery (paste and temper, glazes), glass, and lithics.