Ruthenium

Ruthenium, because of its high atomic number, enhances image contrast just as is the case for osmium when examining certain rubber modified polymer systems as well as certain life science specimens. Because of the higher reactivity of ruthenium relative to osmium tetroxide, there actually are a number of systems that won't stain hardly or at all with osmium tetroxide but will stain quite readily with ruthenium tetroxide. There are also a number of applications for the ruthenium tetroxide staining of certain multiphase polymer systems.