"When a gun is fired, a wealth of chemical and physical evidence is produced. This residue is deposited on surfaces near the shooter including then hands. The chemical evidence is a complex combination ranging from tiny particulates containing heavy metal oxides and sulfides, particles of burnt and partially burnt propellant, and condensed gases. Each type of evidence can be characterized analytically and used to help answer key forensic questions such as who did (or did not) fire the weapon."