Ion beam detective
An improved technique for detecting trace volumes of gunshot residues has been developed, say researchers at the University of Surrey, UK. Using particle-induced X-ray emission (PIXE) on an ion microprobe, scientists were able to pick up elements undetected by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), which has traditionally been used for this application.
Gunshot residues found on the clothing, skin or hair of a criminal investigation suspect can reveal if that person has fired a weapon. These particles are made up of materials from the primer, bullet, bullet jacket, cartridge casing and gun barrel (largely consisting of lead, barium and antimony), and are usually spherical and a few microns in diameter. In some cases, it is difficult to discriminate between gunshot residues and other materials.
The PIXE method uses ion beams to identify tin, aluminium, silicon, chlorine, calcium and zinc, which certain gunshot residues are known to contain (the existence of silicon helps differentiate a shotgun from a pistol shot particle). The SEM-EDS technique cannot pick up these elements.
‘Ion beam analysis is sensitive enough to detect trace elements, and has the unique advantage of not destroying the sample,’ explains Dr Melanie Webb, who has been developing this project for the past six months.
She is currently investigating whether the technique can be used to provenance the samples being analysed.