Other fixatives used for electron microscopy are osmium tetroxide or uranyl acetate. Osmium powder has the characteristic smell of osmium tetroxide. SuperDraco engines use a storable propellant mixture of monomethylhydrazine fuel and dinitrogen tetroxide oxidizer. Hydrazine is hypergolic with oxidants like dinitrogen tetroxide or hydrogen peroxide, but not truly pyrophoric. It is fuelled by very toxic unsymmetrical dimethyl hydrazine and nitrogen tetroxide. Examples of such solutes are noble gases and osmium tetroxide. Some stains, such as osmium tetroxide and osmium ferricyanide, are very chemically active. Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO. The more common examples&colon; ruthenium tetroxide, osmium tetroxide, and xenon tetroxide. Osmium tetroxide forms red osmates upon reaction with a base. Ruthenium tetroxide (RuO) is highly volatile, as is ruthenium trioxide (RuO). Some osmium compounds are also converted to the tetroxide if oxygen is present. It involves the local administration of osmium tetroxide (OsO), which is a highly toxic compound. This dramatically reduces the amount of the highly toxic and very expensive osmium tetroxide needed. Four tanks stored of monomethylhydrazine fuel and nitrogen tetroxide oxidizer.