Tandem detection of organic and inorganic gunshot residues using LC–MS and SEM-EDS

Gunshot residue (GSR) is a valuable form of forensic trace evidence in the investigation of firearms crime. The current gold-standard approach does not include the analysis of organic components of the residues, which may be a deficiency, particularly in cases where there is little to no inorganic gunshot residue (IGSR) present or its attribution to a firearm source is ambiguous. A solvent extraction method was used for the extraction of organic GSR (OGSR) from the most common sampling device used to collect IGSR (i.e., SEM stubs with double-sided carbon adhesive tape). It was found that extraction did not significantly disturb inorganic GSR present on stubs, which raises the possibility that a valuable, comprehensive tandem analysis of both organic and inorganic GSR may be implemented using a single and commonly used residue collection device. The organic extract was analysed using Ultra High Performance Liquid Chromatography coupled to an Ion Trap Mass Spectrometer using Electrospray Ionisation (UHPLC-ESI-MS/MS), with preliminary results indicating that organic components can be extracted and detected at levels appropriate to casework GSR analysis. Testing of traces collected from the hands of recent shooters showed detection of stabiliser compounds typical of OGSR, which were confirmed to be present in the test ammunition's propellant. Total analysis time is approximately 30 min per specimen, including preparation, instrumental analysis and data review. As the first step in the examination of GSR stubs in relation to a shooting case, extraction of organics and analysing them for the presence of OGSR may bring two operational benefits. First, that approach may be a useful way to determine which stubs warrant priority examination for IGSR, and second, it offers the possibility of providing relatively rapid case information to investigators.