This study evaluated the use of Laser Induced Breakdown Spectroscopy (LIBS) as a fast spectrochemical detection method for gunshot-residues analysis around bullet orifices in complex substrates.
Two approaches were assessed; the first one focused on shooting distance determination on bloody fabrics, and the second one provided a fast bullet hole chemical identification. Fifty cotton-based clothing samples were fired from different distance intervals, with a 9 mm pistol, and then covered in human blood. Thirty of the clothing items were used as calibration samples and 20 samples were shot at distances unknown to the analysts and used as blind tests. LIBS ablations were used to develop elemental maps of the analytes present on each blood-stained sample. Following data acquisition, discriminant analysis was performed as a prediction tool for unknown distance samples. The LIBS method demonstrated superior performance when compared to physical examinations and color testing (100 percent, 50 percent, and 40 percent correct classification rates, respectively). Additionally, 21 glass, wood, and drywall substrates were shot from close range and these materials were chemically mapped, using an optimized LIBS protocol. The substrates were specifically chosen to represent typical surfaces found at crime scenes that have shown challenging identification issues following traditional methods for bullet hole testing. The proposed method presented in this project enabled easy transfer of gunshot residues (GSR) from the substrates and quick mapping of GSR markers around the bullet orifice. LIBS offered improved analysis over conventional methods by eliminating the need of chemical reagents for testing, limiting sample preparation, enhancing selectivity, and generating permanent spectrochemical heat maps, as well as more accurate estimations for shooting distance determination and bullet hole identification. (publisher abstract modified)