Spectrochemical mapping using laser induced breakdown spectroscopy as a more objective approach to shooting distance determination

Abstract

This study proposes the application of Laser-induced breakdown spectroscopy (LIBS) as a novel and practical method to detect gunshot residues around bullet holes and to estimate shooting distances in firearm-related criminal events. Clothing samples fired at different distance intervals were analyzed by LIBS, and the spectral data collected from these experiments were used to construct 2D elemental maps. Spatial distributions of inorganic gunshot residues (IGSR), such as lead, barium, and antimony, were used to classify the shooting distance range. Seventy-three cotton textiles of different color and fabric patterns were shot using a 9 mm pistol and a Rossi revolver with different ammunition. Forty-five of the fabrics were shot at known distances to create a control training set, while the remaining twenty-eight samples were used as blind items, of unknown distances to the examiner, to test the accuracy of the method. The performance of the LIBS method was compared to visual and colorimetric tests currently used by crime laboratories. Results show that LIBS offers superior sensitivity, selectivity, reproducibility, and accuracy compared to color tests. Unlike the conventional assays, LIBS produces permanent chemical images that allow objective statistical treatment of the data. Principal Component Analysis and Discriminant Analysis of LIBS data resulted in 100% correct classification of the shooting distance ranges, while color tests resulted in 78.6% correct classification, 3.6% misclassification, and 17.8% inconclusive results. The LIBS method offered improvements over conventional tests such as simplicity, versatility, and reliability. First, the technique is virtually reagent-free, does not require sample preparation and the micro-beam can be scanned across a target in a matter of minutes. Second, the ablation platform can accommodate diverse substrates, and the laser settings can be controlled to offer different solutions, from ultra-fast screening for the identification of IGSR on bullet entrance holes to detailed chemical mappings for shooting distance determinations. Finally, LIBS affords a more reliable and objective presentation of the scientific results in the courtroom. The incorporation of LIBS in forensic laboratories will help to modernize current methodologies and increase the scientific validity of the detection of gunshot residues in crime scene reconstructions.