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Electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) are closely related techniques for high-magnification imaging and spatially resolved chemical analysis of solid samples. Both employ a finely focussed electron beam exciting a variety of secondary signals: Secondary electrons (SE) show surface morphology, backscattered electrons (BSE) local differences in average atomic number, cathodoluminescence (CL) local variation in defect chemistry, x-rays are detected by wavelength and energy dispersive spectrometers (WDS, EDS) for chemical analysis. Whereas SEM is optimized for high-resolution microscopy, the strengths of EPMA are quantitative chemical analysis of micrometer-sized volumes, line profiles and 2-dimensional element maps.
Our electron microprobe is predominantly used to determine the chemical composition of minerals in geological rock sections. More specialised applications include monazite U/Th/Pb geochronology to determine the age of rocks, or trace element distribution in sulphide minerals or quartz to gain knowledge about their formation.
Our SEMs are used as imaging and microanalytical tools for a wide range of scientific disciplines. Typical applications range from determination of elemental distribution (x-ray mapping) or mineral associations and texture in rocks (MLA mode), counting and measuring phytoplankton to investigate the effects of global warming, monitoring the manufacturing of microcapillaries used in separation chemistry to imaging of insects for taxonomy.
For accurate quantitative chemical analysis on any of the instruments, the samples have to be flat and polished. In general, specimens have to be dry and vacuum stable. For EPMA and SEM, non-conductive samples are coated with carbon, gold or platinum. However, fresh plant specimens can be investigated uncoated in the environmental and low vacuum modes of the FEI instrument.
The Hitachi FESEM provides ultra high resolution particularly at low accelerating voltages, which enables imaging of nanometer-sized surface features of vacuum-stable specimens with very thin or completely without coating.
The x-ray analytical microscope or μXRF (micro x-ray fluorescence analyser) employs a high energy x-ray beam to excite secondary x-rays in the sample. The mono-capillary primary optics focus the x-ray beam to a diameter of 10 or 100 micrometers, thus allowing spatially resolved chemical analysis of the elements Na-U. The energy-dispersive x-ray spectrometer (EDS) can be used for qualitative, standardless semi-quantitative analysis, and full quantitative analysis, using calibration curves obtained on standard reference materials. It can operate in spot-mode, where individual sites of interest on a sample are analysed, or in mapping-mode, where 2-dimensional element distribution maps are created. A transmission x-ray detector captures primary x-rays after they passed through the sample to show local differences in density and chemical composition, thus creating an image much like a medical x-ray image. Samples not withstanding vacuum (such as fresh biological material) can optionally be analysed in air with reduced light element sensitivity. No coating or other sample preparation is necessary. However, for quantitative analysis a flat, polished surface is required.
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Authorised by the Deputy Vice-Chancellor (Research)
11 December, 2013