Used as a post fix and stain of tissues in scanning and transmission electron microscopy. Osmium tetroxide reacts with lipids in tissue by oxidation of unsaturated bonds of fatty acids, which adds density and contrast to biological samples. In normal tissues, presence of osmium results in intense black staining. In addition, it is used in electron microscopy for enhancing staining.40 mg/ml in water. Used as a post fix and stain of tissues in scanning and transmission electron microscopy. Osmium tetroxide reacts with lipids in tissue by oxidation of unsaturated bonds of fatty acids, which adds density and contrast to biological samples. In normal tissues, presence of osmium results in intense black staining. In addition, it is used in electron microscopy for enhancing staining.40 mg/ml in water.