Spatial resolution limits of EPMA

Abstract

Abstract The development of field emission EPMA, has significantly improved the lateral resolution of EPMA. Two strategies are available for achieving high spatial resolution, either low overvoltage or low voltage analysis. Determining the spatial resolution for a particular analysis is complex and depends on the voltage, spot size, beam current, density of the sample, X-rays analysed and the precision and sensitivity required. Monte carlo simulations can be used to evaluate the spatial resolution for different analytical conditions and samples, provided the minimum spot size achievable at the conditions is known. Spot size is important in determining lateral resolution, which initially improves with decreasing accelerating voltages, and then increases as the minimum spot size becomes more significant than the distance electrons travel. For trace elements, the requirement for precision and sensitivity demands either higher overvoltages increasing the volume from which X-rays are generated, and/or higher beam currents increasing the spot size. At low overvoltage or low voltage conditions many additional factors must be considered: carbon contamination, coat thickness, erosion of the carbon coat, the stability of the sample and the problems surrounding the measurement of soft X-ray lines including L-lines for first row transition metals. When analysing L-lines for quantifying first row transition metals, either calibration curves must be used, or the anomalous mass absorption and variations in the fluorescence yields from partial shell occupancy must be accounted for, by using fits to experimentally determined values. By taking these factors into consideration, high quality measurements can be performed.