S 0342 (OTTO) Selenium shot, 2-6 mm, 99.999% Cas 7782-49-2 - used as a catalyst in some chemical reactions, but it is not widely used because of issues with toxicity. In X-ray crystallography, incorporation of one or more selenium atoms in place of sulfur helps with multiple-wavelength anomalous dispersion and single wavelength anomalous dispersion phasing.