BIOLOGICAL APPLICATIONS OF A COMPUTER CONTROLLED ANALYTICAL ELECTRON MICROSCOPE

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Tests have been performed to determine limits of elemental detectability for the Gatan parallel-detection electron energy loss spectrometer (EELS) fitted on the BEIB Hitachi 200-keV analytical electron microscope. Data were recorded from a series of standard samples and also from freeze-dried cryosections obtained with a Reichert F4C cryoultramicrotome. Spectra were obtained using a "second difference" method that eliminates channel-to-channel gain fluctuations in the 1024-channel photodiode array. It was thus possible to detect characteristic core-edges with extremely small peak/background (e.g. 0.001). Comparisons were made with energy- dispersive x-ray spectra recorded concurrently. It was shown that both calcium and iron could be detected with comparable sensitivity using the two techniques. We therefore expect EELS to have superior sensitivity for these elements in high resolution applications with the Vacuum Generators Field emission STEM. Further studies have been made on radiation induced mass loss of fluorine in labelled neurotransmitters and neurohormones with the eventual aim of localizing these compounds in cells. Parallel- recorded EELS provides a great improvement in sensitivity over conventional serial-recorded EELS and may make such experiments feasible.