Evaluation of the sub-surface morphology and composition of gunshot residue using focussed ion beam analysis

Recent work in the forensic analysis of Gunshot residues (GSR) has suggested that the sub-surface or internal composition and morphology of these residues be explored. A particular area of interest is in heavy metal free, or non-toxic ammunition, which are becoming more frequently encountered in the marketplace. As the formulation of the primer compound changes the conditions of the firearm discharge, there is the possibility that different primer formulations may result in the formation of different GSR particles with distinct internal morphologies and compositions. To that end, the internal morphology and composition of GSR particles may provide additional information that could be useful in the investigation of firearms crime. This research investigated the internal morphology of GSR originating from a variety of different ammunition products. Both traditional three-component primed ammunition, and a selection of heavy metal free and non-toxic alternatives were considered. Particles were identified using SEM-EDS, before being cross-sectioned using a focussed ion beam (FIB) instrument. The FIB-sectioned particles were then re-acquired and mapped using SEM-EDS, to assess both internal morphology and composition. Particles observed in this study presented distinct morphological and compositional features at the sub-particle level that may provide an indication of the primer formulation from which they originated. That said, further investigation of a variety of samples should be undertaken to verify the consistency of these features, or any deviations that may be observed based on primer type. However, these results indicate that there may be promise in obtaining additional detail from sub-particle morphology and composition.