Development of tailor-made inorganic gunshot residue (IGSR) microparticle standards and characterization with a multi-technique approach

Abstract

The forensic analysis of inorganic gunshot residues (IGSR) involves analytical measurements from samples taken from skin and other substrates. The standard practice for IGSR analysis recommends the use of Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy (SEM-EDS) to identify the gunshot residues using combined information of the particle's morphology and elemental composition. However, the current deficit on IGSR standard reference materials (SRM) limits the optimization of SEM-EDS for modern, lead-free ammunition and the development of emerging analytical techniques. This study aims to enhance existing capabilities by producing tailor-made microparticle suspensions that can be used for the quality control of GSR analysis, validation of existing and emerging methods, interlaboratory testing, and systematic transfer and persistence studies. To fill this gap, IGSR microparticle standards were developed by discharging various leaded and lead-free primers under controlled conditions and creating suspensions in an organic medium, then evaluated for homogeneity and stability of morphology and elemental composition. The IGSR microparticles suspensions were evaluated by three analytical techniques—SEM-EDS, Laser-Induced Breakdown Spectroscopy (LIBS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) —to characterize the elemental composition and particle morphology. The ICP-MS digestion method was validated for these novel IGSR microparticle suspensions, and figures of merit and ruggedness testing are reported. The standard demonstrated stability in its dry and suspension forms, providing versatility for use in multiple types of analytical methods and substrates. This research is anticipated to assist forensic and environmental scientists by providing IGSR standards that can strengthen research, expand access to new detection techniques, and enhance laboratories' cross-validation and quality assurance.