Analysis of primer gunshot residue particles by laser induced breakdown spectroscopy and laser ablation inductively coupled plasma mass spectrometry

Abstract

This study reports novel approaches for the detection of gunshot residues (GSR) from the hands of individuals using Laser-Induced Breakdown Spectroscopy (LIBS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The methods’ performance was evaluated using 159 GSR standard and authentic samples. Forty specimens generated from characterized microparticles were used as matrix-matched primer gunshot residue (pGSR) standards to examine the elemental profiles of leaded and lead-free residues, compared to SEM-EDS and solution-ICP-MS. Also, 119 authentic skin samples were analyzed to estimate error rates. Shooter samples were correctly classified into three categories based on their elemental composition (leaded, lead-free, or mixed pGSR). A total of 60 non-shooter samples were used to establish background thresholds and estimate specificity (93.4% for LA-ICP-MS and 100% for LIBS). All the authentic leaded items resulted in the detection of particle(s) with composition characteristic of pGSR (Pb–Ba–Sb), as observed by simultaneous elemental identification of target analytes at the exact ablation times and locations. When considering the pre-characterized elemental composition of these primers as the “ground truth”, LA-ICP-MS resulted in 91.8% sensitivity (true positive rate), while LIBS resulted in 89.2% sensitivity. Particles containing Ba, Bi, Bi–Cu–K, and Cu–Ti–Zn were found in the lead-free residues. Identification of lead-free GSR proved more challenging as some of these elements are common in the environment, resulting in 85.2% sensitivity for LA-ICP-MS and 44.4% for LIBS. Overall accuracies of 94.9% and 88.2% were obtained for the LA-ICP-MS and LIBS sets, respectively. LA-ICP-MS provided an additional level of confidence in the results by its superior analytical capabilities, complementing the LIBS chemical profiles. The laser-based methods provide rapid chemical profiling and micro-spatial information of gunshot residue particles, with minimal destruction of the sample and high accuracy. Chemical mapping of 25 micro-regions per sample is possible in 2–10 minutes by LIBS and LA-ICP-MS, offering new tools for more comprehensive forensic case management and quick GSR screening in environmental and occupational sciences.