Specimen thickness is determined using X-ray microanalysis in conjunction with atomic absorption spectrophotometry. Specimens of evaporated metal films and standards comprising fixed concentrations of element (such as potassium) in organic media are used to determine elemental concentrations of endogenous elements in embedding resin, from which section thickness can also be determined. The method finally depends only on the single measurement of X-ray emission of such an element in the specimen. Calibration of the X-ray detector over the energy range with mineral salts enables the standard to be used for the quantitative analysis of any element under investigation.

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Electron microscopic x-ray microprobe analysis of pyroantimonate precipitates in platelets fixed in osmium tetroxide-pyroantimonate revealed calcium localization in the nucleoids of a-granules. This pool of calcium had largely disappeared within 10 sec after stimulation of platelets by thrombin. Such a rapid change suggests that this calcium pool may have a regulatory role in stimulus-response coupling.

X-ray microanalysis using the analytical electron microscope (EMMA 4) was performed on resting frog sartorius muscles fixed by the pyroantimonate technique. Calcium was the main element of interest analyzed. Pretreatment was with ethyleneglycoltetraacetate to chelate calcium and with Tris chloride to replace sodium. Osmium-fixed controls were also analyzed. Both crystal-diffracting and energy-dispersive analyzers were employed, the former for accurate quantitation of calcium content and the latter for qualitative assessment of other elements present. Significant concentrations of calcium occurred in the dense parts of the nucleus, the triads, the thin filaments and the sarcolemma. The amount of calcium was greatly decreased by ethyleneglycoltetraacetate pretreatment and slightly increased by sodium removal. After osmium fixation alone, little calcium was detected with no localization. frem the Joint Research Fund of the Hebrew University-Hadassah Medical School and by the Tenovus Organization.

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