Lead Isotope Measurement of Gunshot Residues for Cartridge Discrimination and Individualization

Gunshot residue (GSR) is the most common evidence in gun related cases. It plays an important role in linking the suspects, the guns and the victims in cases. Metallic GSR particles mainly come from the primers of cartridges. The formula of primers is identical for cartridges from different manufacturers in China, which makes them difficult to be discriminated by GSR with the commonplace method of scanning electron microscope scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS). Lead styphnate is one important component of primers. Measurement of lead isotope ratios of GSR seems reasonable to further discriminate cartridges since the abundance of lead isotopes varies significantly in nature. To make it valid for forensic purposes, it is important to have a fundamental understanding of the difference of Pb isotope ratio between cartridges from different manufacturers and batches. 44 GSR samples were subjected to laser ablation multicollector inductively coupled plasma mass spectrometry analysis and were described by Pb isotope ratios of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb. Univariate likelihood ratio computations were performed by kernel density estimation method for data distribution estimating. It was found that with these three Pb isotope ratios, 0.53 percentages of false positive and zero false negative rate were obtained. The results demonstrated that lead isotope ratio analysis and LR evaluation method can provide a practicable method to do cartridge discrimination and individualization. In addition, the change test of Pb isotope ratio proved that there was no obvious mass fractionation in the process of shooting.