Transfer and persistence studies of inorganic and organic gunshot residues using synthetic skin membranes

Abstract

The complex nature of gunshot residue (GSR) transfer and persistence introduces challenges and skepticism in its evidential value. Therefore, this work evaluates the behavior and movement of inorganic and organic gunshot residues to assist in the evidence interpretation. The study encompassed over 650 samples, including 247 collections from human skin after firing a gun and 405 synthetic skin and fabric substrates after depositing a characterized pGSR/OGSR standard. Transfer and persistence experiments were evaluated on different substrates (hands, ears, nostrils, forehead, hair, fabrics, and synthetic skin), at different times after firing (0 to 6 h), and common post-shooting activities (rubbing hands, handshaking, running, washing hands and fabrics). Ground truth knowledge of particle counts and analyte concentrations was used to calculate the recovery for inorganic and organic constituents from clothing and a synthetic skin membrane (StratM®). Authentic shooter skin samples were compared to synthetic skin to establish the validity of skin-substitute models. During controlled experiments, inorganic particles persisted longer than OGSR on inactive samples (9% and < 25% loss at 6 hrs, respectively), but inorganic particles were more prone to secondary transfer than OGSR (up to 35% vs. 0% transfer, respectively). High percentages of particles were lost during vigorous activities like washing hands (99% loss) or rubbing hands (55% loss). In comparison, less loss was observed during the same studies for OGSR (<21%). This study offers a deeper understanding of gunshot residue transfer and persistence mechanisms that can assist analysts and investigators in improving sample collection and interpretation of evidence.