Through this article, the authors hope to enhance the understanding of pGSR/OGSR mixtures and provide scientific support when presenting case-work results in the courtroom that deal with questions of activity level regarding involvement, bystander or shooter, or no involvement in the firing event.
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 (nine percent and < 25 percent loss at six hours, respectively), but inorganic particles were more prone to secondary transfer than OGSR (up to 35 percent vs. 0 percent transfer, respectively). High percentages of particles were lost during vigorous activities like washing hands (99 percent loss) or rubbing hands (55 percent loss). In comparison, less loss was observed during the same studies for OGSR (<21 percent). 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. (Published Abstract Provided)