A comparsi between four different methods of sampling GSR from gunshot wounds in skin and bone: different applicability and reliability

Background and Aims: the sampling procedures of gunshot residues (GSR) on entry wounds is crucial and still a precise agreement on sampling procedures is yet lacking. Thus, an experimental study on GSR sampling was performed. Materials and Methods: a total of 20 samples (ten of pig skin free of fur and ten of bovine ribs without soft tissues) were shot once at a firing ground, with a semiautomatic pistol with 9x21 mm projectiles (full-metal-jacketed bullets). Moreover, two unshot samples (one pig skin and one bovine rib) were kept as controls. Samples were fired with a near-contact range and a range of about 40 cm, keeping the muzzle perpendicular to the surface of the sample. The periphery of the wound was divided into 4 wedges and collection of GSR was performed with graphite tape, white canvas tape, transparent adhesive tape and cotton swabs soaked in alcohol, one for each centimeter from the wound edges, up to a distance of 3 cm. The same samplings were performed on controls. Samples were treated with 1 ml 30% ultrapure nitric acid, diluted to 10 ml with MilliQ water, filtered with 0.45 micron Nylon filter and each sample was analyzed with ICP-OES (for Ba, Pb), ICP-MS (more sensitive for Sb, Pb, Cu) and GC-MS (for organic elements, usually present along with inorganic elements: i.e. ethyl centralite, phthalates), after the samples were treated with isopropyl alcohol (1 ml) and diluted to 5 ml with acetone. Moreover, two samples of shot bovine ribs (one with near-contact range, one with distant range) were also analyzed by SEM-EDX to identify GSR by drying the sample in vacuo. Results: First results showed the stub method with cotton swabs as best sampling procedure, since this technique enabled the highest amounts of residues to be gathered, and blank samplings showed higher purity with less interference by contaminants; the highest amount of metals were detected within the first 2-3 cm around the wound with near-contact ranges, while according to the results provided by the direct analysis with SEM in dried samples, constant concentration within 3-4-5 cm were detected with distant range. In all cases, stub and bones did not interfere with the element determination, as showed by blanks proofs. The determination of the element ratio (Cu/Pb, Sb/Pb) in most cases provided more reliable results and should thus be considered a better choice than the determination of single elements, since contaminations were evidenced and the elements' ratio in the bullets are quiet constant. The complementary combination of the methods used (ICP-OES, ICP-MS and GC-MS) provided an accurate assessment of the type of residues and concentrations, even though ICP-MS showed the highest sensitivity in the detection of inorganic metallic residues than ICP-OES. Conclusions: The study shows the importance of the sampling procedures in case of gunshot wounds: from a quali-quantitative point of view, cotton swabs appeared to be the best sampling procedure, and the combination between ICP-MS and GC-MS provided a reliable assessment of the type, amounts and distribution of GSR.