Forensic exspertise of gunshot residues

Abstract

Gunshot residues originate from the discharge of firearms. It comprises gunpowder and primer residues and metallic residues from projectiles. When the firing pin of a gun strikes the primer cap of the bullet, a rapid high-temperature reaction of the primer mix takes place. GSR particles originating from primers are considered in this paper. Gunshot residue particles are formed as a result of rapid cooling of the discharge gases and solid particles. The gases condense in the spheroid forms, but they also interact with solid residue materials to form complex mixtures and aggregate forms. GSR particles range in size from one micrometer (μm) to twenty micrometers in diameter. Aggregates of GSR particles are usually larger than 20 micrometers. There are variations in the primer composition, but the common elements found in GSR particles are lead, antimony and barium. GSR sampling is done at the crime scene as expeditiously as possible, in order to avoid contamination, as the major concern when performing GSR analyses. GSR particles can be transferred by contact, air movement, washing and abrasion. The Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM/EDS) is an accepted scientific technique for the analysis of GSR. SEM/EDS can be applied either manually or automatically to search and analyse potential GSR particles. The elemental analysis by SEM/EDS is based on the interaction of a highenergy electron beam with the sample. As a result of this interaction the generation of back scattered electrons (for imaging) and characteristic x-rays (for elemental analysis) is obtained. Speed and accuracy are very important for forensic investigation; by an automated SEM/EDS analysis this requirements are met, since both elemental and morphological properties of GSR particles can be determined in a rapid and accurate manner. Unique GSR particles usually consist of lead, antimony and barium. The presence of unique GSR particles indicates that the person discharged a firearm, handled a firearm or was in the vicinity when a firearm was discharged.