Electron probe x-ray microanalysis of subcellular ion transport in situ
Electron probe x ray microanalysis [EPMA] provides quantitative information within a single spectrum about elements of biological interest with atomic number of 11 or greater. Therefore, the transport of ions and their accompanying co and counter ions across organelle membranes can be studied in situ by sampling within and adjacent to the intracellular organelle of interest under resting and stimulated conditions.EPMA is based on the fact that the ionization of atoms by fast electrons generates x rays having energies characteristic of the excited atoms. The interaction of incident fast electrons with atomic nuclei generates a background of continuum x rays. Elemental quantitation of ultra thin sections with EPMA is generally based on the linear relationship between elemental concentrations and the ratio of the number of characteristic/continuum. The use of this principle, together with the appropriate standards for calibration, has been the most successful approach for quantitative biological EPMA. The spatial resolution of EPMA at present is better than 10 nm and the practical limit of sensitivity for detecting calcium, (albeit with high electron dose), is approximately 0.3 mmol/kg dry wt. Two modes of data collection are utilized: fixed probe analysis of a region of interest or a scanning probe mode, where an x ray spectrum is collected at each picture point, to obtain quantitative elemental x ray maps. To preserve the morphology and the in vivo distribution of diffusible elements, we prepare specimens by rapid freezing in sub cooled Freon or, more recently with a Lifecell CF100 metal are mirror device; thin sections cut at -130 °C to -160 °C on a Reichert cryoultramicrotome. Msec time resolution of physiological are events can be achieved by freeze trapping.