Discharged firearm cartridges produce unique microscopic particles referred to as gunshot residue (GSR). GSR is the product of combustion of the primer material. Depending on the type and brand of ammunition, GSR particles typically contain varying amounts of lead, barium, and antimony, along with one or more other elements, such as aluminum, silicon, phosphorus, sulfur (trace), chlorine, potassium, iron (trace), nickel, copper, zinc, zirconium, and tin. If particles containing lead, antimony, and barium are detected on a crime suspect’s hands or clothing, it is taken as sufficient evidence that the person has either recently handled a discharged firearm, was present in the general vicinity of a weapon while it was discharged, or came in contact with a surface contaminated with GSR. Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS) is the most commonly used technique for detecting the presence of GSR particles. GSR samples are collected from a suspect’s hands and clothing using adhesives mounted on standard SEM sample stubs. SEM imaging is used to verify the characteristic size, shape, and morphology of GSR particles and EDS can identify the elemental composition of individual particles.

 

Determining the presence of lead, barium, and antimony fused together in a single particle having the correct morphology is all that is normally required for the positive identification of GSR. However, because of the high temperatures (~1,500-3,600 °C) and pressures (~14,000-65,000 psi) that result within 1 millisecond of discharging a firearm cartridge, highly complicated chemical interactions are likely to occur that will affect the chemical composition of the GSR particles. Because of its nanometer-scale sampling depth and the ability to provide detailed chemical state information, X-ray photoelectron spectroscopy (XPS) can provide important information regarding the surface chemistry of GSR; therefore, XPS offers potential as a complementary technique to SEM/EDS analysis. The aim of this investigation was to use XPS to gain further knowledge regarding the surface chemistry of GSR.