Trace Element Profiling of Gunshot Residues by PIXE and SEM-EDS: A Comparative Study

We critically compare particle induced X-ray emission (PIXE) on the ion microprobe with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEMEDS) for the characterisation of gunshot residues (GSR). Samples of gunshot residue from several different firearms were collected. Individual particles of GSR were analysed by SEM-EDS using a 30keV electron beam focussed to ~10nm and PIXE using a 2.5MeV proton beam focussed to ~4 microns. PIXE revealed trace or minor elements undetectable by SEM-EDS, and could discriminate GSR indistinguishable by SEM-EDS. Published as: M.J.Bailey, K.J.Kirkby, C.Jeynes, X-Ray Spectrometry, 38 (2009) 190-194; http://dx.doi.org/10.1002/xrs.1142 Introduction In order to demonstrate that a suspect in a criminal investigation has fired a weapon, police authorities routinely take swabs from the clothing, skin and hair of the suspect and use electron microscopy to search for gunshot residue (GSR) particles on the swabs. These particles are condensation products of the high temperature, high pressure reactions that occur when a firearm is fired and are known to be deposited at the crime scene and on the shooter [1]. The particles are made up of material from the primer, bullet, bullet jacket, cartridge casing and the gun barrel, are spherical, and are of the order of a few microns in diameter. There are several types of particle that are characteristic of gunshot residue, including those containing Pb, Ba and Sb [2]. In case work, it is highly desirable to be able to discriminate GSR from different sources to rule out possible contamination and to strengthen the link between a suspect and a crime scene. It is known that GSR varies in composition and morphology in accordance with the type of firearm that has produced it. The UK Metropolitan Police have developed a categorisation system for gunshot residues, putting them in 35 different categories in accordance with their morphology and chemical composition. It is accepted that the chemical composition of GSR is a better indicator of its provenance than morphology [3], [5]. The technique of choice amongst police institutions for the analysis of GSR is scanning electron microscope energy dispersive X ray spectrometry (SEM-EDS) since the technique is widely available, relatively cheap and can be automated. EDS also offers the advantage of imaging the particles as well as determining their chemical composition, and is nondestructive.