Chemical Analysis of Firearm Discharge Residues Using Laser Induced Breakdown Spectroscopy

According to the Gun Violence Archive, in 2017, firearms were involved in 57,134 criminal incidents in the United States. The detection of firearm discharge residues (FDR), including inorganic and organic gunshot residues, can provide essential information in such investigations. For instance, when the question of suicide or murder arises, estimating the barrel to victim distance plays a critical role in the outcome of the case. In addition, clothing, wounds and other target materials are often inspected to determine if a bullet has produced an entry or exit orifice. Currently, the most common method for distance determination and identification of bullet entrance holes is by chemical colorimetric tests that react in the presence of nitrites or lead. Although these color tests are widely used in forensic laboratories, the major pitfall is their variability and poor selectivity for gunshot residues. Dark or bloody items significantly diminish the efficacy of these assays, they are difficult to perform on non-movable or large objects, and false positives can be derived from oil, dirt, and other common contaminants. The chance of outside sources affecting the color distribution pattern has led to the exploration of alternative instrumental methods for estimating a shooting distance, including Atomic Absorption Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), X-ray fluorescence (XRF), and Atomic Force Microscopy (AFM). Nevertheless, these instrumental methods have limitations for GSR analysis, as well, such as destruction of the sample, long time of analysis, or complex sample preparation. Therefore, there is a critical need for the development of an analytical method that is sensitive to GSR, fast, practical, non-invasive, objective, and accurate. The primary goal of this study was to investigate the capabilities of LIBS for shooting distance determination and identification of FDR on substrates of interest. The central hypothesis of this research was that Laser-Induced Breakdown Spectroscopy would improve the scientific reliability of the detection and observation of gunshot residues. This assumption was based on the ability of LIBS to perform simultaneous multi-elemental detection at low ppm levels, LIBS’ superior selectivity, and the potential for confirmation of numerous emission species per analyte. To assess the forensic utility of LIBS, the method was developed and validated for the analysis of substrates commonly found during firearm-related crimes. Residues from different ammunitions and firearms were analyzed off 133 fabrics, glass, drywall, and wooden samples. Statistical methods, like principal component analysis and multivariate discriminant analysis were performed to estimate shooting distances and identify the presence of GSR residues. Color tests lead to misclassification of 9 out of 35 unknown shooting distances (26%), while the LIBS method correctly classified 100% of the unknown distance testing samples by Discriminant Analysis. Additionally, LIBS was able to correctly identify elemental profiles of gunshot residues from all standard ammunitions deposited on clothing, drywall, glass, and wood. LIBS allowed for rapid and accurate chemical mapping of GSR patterns on pieces of evidence typically found at a crime scene. Chemical imaging of lead, barium, and antimony provided more objective approaches to estimation of shooting distance and bullet hole identification, compared to color tests. Moreover, LIBS provided enhanced detection of standard ammunitions and partial detection of lead-free ammunitions. However, some challenges still exist with accurate detection and identification of non-toxic ammunitions. The proposed method is anticipated to aid in crime scene reconstruction of criminal events involving firearms. The superior capabilities of LIBS analysis, compared to current practice, increases the certainty on these examinations and enhances the reliability of the information used during the investigative stages and when the evidence is presented in a court of law