BIOLOGICAL APPLICATIONS OF A COMPUTED CONTROLLED ANALYTICAL ELECTRON MICROSCOPE

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Further advances have been made in the capabilities of the computer controlled analytical electron microscope, developed jointly by BEIB and DCRT, and the instrumentation has been applied to several biological problems in the past year. Two of these studies involved extensive electron energy loss spectroscopy. The newly installed Gatan 607 magnetic sector energy loss spectrometer has been shown to be fully corrected for second order aberations. This has permitted a significant improvement in collection efficiency and energy resolution while allowing concurrent acquisition of energy dispersive x-ray (EDXS) and electron energy loss spectra (EELS) as well as simultaneous elastic dark field imaging in the scanning transmission mode. A basic study has been carried out to characterize the peak shapes of the x-ray detectors, and this has led to modifications to the Tracor Northern detector in order to improve performance. EELS and EDXS elemental maps and spectra have been obtained from pancreas beta cells prepared by rapid freezing, freeze-drying (or freeze-substitution) followed by embedding in order to investigate the composition of the secretory granules. Spectra have been quantitated to determine ratios of elements in the insulin-containing granule cores. A feasibility study has been made for obtaining low dose EELS maps using averaging procedures in periodic crytalline arrays of membranes. Data have been obtained from thin samples of reconstituted porin membrane containing lipid and protein sub-units and transmembrane channels. Both the experimental data and theoretical signal/noise estimations suggest that low dose elemental mapping should be possible.