ESI contrast analysis: A new approach for element analysis with energy-filtering transmission electron microscopy (EFTEM)

Element microanalysis with energy-filtering transmission electron microscopy can be performed in different ways. Electron energy-loss spectroscopy (EELS) records the intensity as a function of the energy loss from selected regions. Elemental mapping with electron spectroscopic imaging (ESI) uses energy-filtered images at an element-specific energy loss from which a background image has to be subtracted. A combination of these two approaches is Image-EELS, which records a series of energy-filtered images, each at a different energy loss and measures the intensity of selected regions as a function of the energy loss. As an additional procedure ESI contrast analysis is introduced; with this we can investigate the image contrast as a function of the energy loss. The contrast can be measured for the total image or for selected regions as the standard deviation of the grey levels divided by the mean grey level. This procedure is added as a new feature to the existing Image-EELS program, so that EELS intensity spectra and the contrast can be directly compared. Alternatively, the contrast can be calculated step by step from individually recorded electron spectroscopic images, so that only simple equipment for image analysis is sufficient to realize this new method. Inner shell ionizations produce characteristic, but often weak element edges in energy-loss spectra which can be detected more sensitively and reliably as a rapid increase in contrast. Regional analysis of the contrast as a function of the energy loss, combination of data from different regions and the possibility to increase the intensity during recording of the images expand the application range of this new analytical method which bridges the gap between ESI and EELS analysis. With ESI contrast analysis it is possible to discriminate between faint element signals and artefacts in elemental mapping. This new approach for element detection is especially advantageous for biological objects which usually contain very small amounts of the interesting elements in heterogeneous and complex objects. As an example, nervous tissue of a fish was analysed after cytochemical precipitation of endogenous calcium.