Trace element deposits from firearm shots: modern approaches to detection and interpretation

Abstract

Rising levels of firearm violence have become a critical public health issue worldwide, and their systemic impact contributes substantially to the costs borne by health care systems and criminal justice systems due to firearm-related deaths and injuries. Widespread civilian access to guns and marked regional variation in firearm violence create a strong and persistent need for reliable forensic tools to characterise weapons use in both criminal and non-criminal incidents. In this context, microelement gunshot residue (GSR) deposits are crucial for reconstructing shooting events and linking specific individuals or weapons to firearm incidents. The purpose of this article was to analyse recent literature on microelement deposits formed during firearm discharge and to review current approaches to their detection and forensic interpretation. A narrative review of articles indexed in Google Scholar from 2015 to the present was conducted using keywords related to firearms, residual shot components, elemental composition, X-ray fluorescence and spectrometric analysis, with inclusion criteria focusing on studies relevant to GSR examination. This paper outlines current approaches to characterising GSR as a complex, multicomponent system comprising inorganic and organic micrometre-sized particles released during propellant combustion, primer initiation, projectile–barrel interaction and gas dynamics around the shooter–target system. Conventional inorganic GSR, typically lead-based and commonly enriched with antimony and barium, is increasingly supplemented or replaced by more heterogeneous elemental compositions as a result of the introduction of lead-free primers and novel ammunition formulations, making pattern recognition less straightforward. SEM-EDX remains the workhorse for detecting GSR and classifying inorganic particles, but it is now complemented by high-resolution spectrometric techniques such as LA-ICP-MS, single-particle ICP-TOF-MS, ICP-OES and lead isotopic analysis, which together enable more detailed elemental and isotopic profiling of residues. Laser- and X-ray-induced fluorescence imaging techniques (LIBS, micro-XRF and MA-XRF) allow non-destructive, large-area mapping of microelement deposits on clothing, surfaces and other substrates, improving visualisation of spatial patterns that inform on shot direction and shooting distance. Analysis of microelement signatures in combination with assessment of macroscopic and thermal damage to fabrics and biological substrates can greatly improve reconstruction of shooting events and differentiation between various firing distances and scenarios. However, the evidentiary value of GSR is limited by particle transfer and loss, environmental GSR-like backgrounds, variability in ammunition composition and differences in laboratory practice, all of which may affect the comparability and robustness of results used to interpret firearm discharge events. Overall, this review suggests that the effective forensic and medico-legal use of microelement GSR deposits is possible only when complex analytical workflows and multiple complementary methods are employed, underpinned by strict sampling, validation and quality control procedures. Further standardisation of analytical protocols, inter-laboratory comparison studies and development of clear interpretative guidelines are essential to strengthen the probative value of GSR evidence across diverse forensic applications.