Chemical profiling of ballistic materials : analysis of organic gunshot residue

Abstract

Gunshot residue (GSR) is a complex chemical mixture that is created during the discharge of a firearm. Its detection and interpretation play a crucial role in the investigation of firearm incidents. Current GSR analysis is limited to inorganic GSR (IGSR), however, the evidential value could be strengthened by inclusion of organic GSR (OGSR). The present study aims to address this potential by proposing a categorisation system for relevant OGSR compounds and developing a methodology for the collection, extraction and analysis of both organic and inorganic GSR from a single sample. The organic composition of more than 50 propellant powders has been determined and compared against more than 200 propellant compositions reported in the literature. This work has resulted in a three-tier categorisation system for OGSR compounds, which together with the current IGSR classification will provide unequivocal identification of GSR materials with the possibility of discriminating between GSR from different ammunition types. Evaluation of MonoTrap extraction showed that this is an effective pre-concentration technique for the characterisation of propellants. Solid-phase microextraction (SPME), however, was the superior method for the extraction of OGSR compounds from various sampling media, including swabs and stubs. The optimised methodology involves GSR collection using carbon adhesive stubs followed by SPME gas chromatography – mass spectrometry (GC-MS) analysis of OGSR and subsequent scanning electron microscopy – energy-dispersive X-ray spectrometry (SEM-EDX) analysis for IGSR. This protocol has resulted in the detection of both characteristic IGSR and categorised organic compounds, demonstrating the ability to obtain a full chemical profile from a single sample. Detection of both first and second tier organic compounds provides complementary compositional information that could be used to discriminate between samples. Furthermore, this methodology requires no changes to the current sampling and IGSR analysis protocols and addresses the limited storage time of OGSR. Since GC-MS instruments are readily available in most analytical laboratories, implementation of the proposed protocol is feasible.