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

Atomic spectrometry update: Review of advances in the analysis of metals, chemicals and materials

There has been a large increase in the number of papers published that are relevant to this review over this review period. The growth in popularity of LIBS is rapid, with applications being published for most sample types. This is undoubtedly because of its capability to analyse in situ on a production line (hence saving time and money) and its minimally destructive nature meaning that both forensic and cultural heritage samples may be analysed. It also has a standoff analysis capability meaning that hazardous materials, e.g. explosives or nuclear materials, may be analysed from a safe distance. The use of mathematical algorithms in conjunction with LIBS to enable improved accuracy has proved a popular area of research. This is especially true for ferrous and non-ferrous samples. Similarly, chemometric techniques have been used with LIBS to aid in the sorting of polymers and other materials. An increase in the number of papers in the subject area of alternative fuels was noted. This was at the expense of papers describing methods for the analysis of crude oils. For nanomaterials, previous years have seen a huge number of single particle and field flow fractionation characterisations. Although several such papers are still being published, the focus seems to be switching to applications of the nanoparticles and the mechanistic aspects of how they retain or bind with other analytes. This is the latest review covering the topic of advances in the analysis of metals, chemicals and materials. It follows on from last year's review1-6 and is part of the Atomic Spectrometry Updates series

Expanding the Capabilities of Firearm Investigations: Novel Sampling and Analytical Methods for Gunshot Residue Evidence

During firearm-related incidents, the accurate collection and detection of gunshot residues (GSR) are crucial for reconstructing events and providing investigative leads. Regardless of the strong scientific foundations in the discipline, the standard practice for GSR can benefit from increased confidence in the results and the adoption of reference standard materials and screening tools that can lead to faster turnaround times and improved efficiency. Moreover, the implementation of comprehensive interpretation approaches requires strengthening the current body of knowledge on the transfer and persistence of inorganic and organic GSR compounds (IGSR, OGSR). As a result, the overall objective of this project was to expand the capabilities of gun-violence investigations by developing novel sampling and analytical strategies for GSR detection. We aimed to accomplish this goal through three main tasks: 1) Developing and validating analytical techniques and portable instrumentation for accurate GSR detection, 2) Developing a characterized in-house OGSR reference standard for quality control, method validation, and building knowledge on transfer and persistence, and 3) Performing systematic studies for the transfer and persistence of IGSR and OGSR on alternative matrices. This study investigated several spectrochemical methods that allowed the interrogation and detection of IGSR and OGSR. Development and validation studies were performed for emerging analytical techniques, including LIBS and LA-ICP/MS, and methods that use instrumentation widely available at crime laboratories, such as GC-MS, LC-MS/MS, and SEM-EDS. Over 500 specimens collected from known shooters and non-shooters were analyzed to assess the methods’ feasibility for trace GSR detection. Furthermore, a novel portable LIBS instrument was optimized and demonstrated reliable use for in-field screening of authentic gunshot residue specimens. The portable LIBS showed improved detection capabilities over benchtop systems due to enhanced CMOS detector technologies, an ablation cell custom-made for GSR examinations, microscopic visualization of particle morphology, and unique capabilities for single-particle analysis. Second, this study characterized an OGSR standard solution containing eight common analytes found in propellant formulations for its combined use with pGSR standards. A ruggedness test optimized storage conditions and sample preparation methods necessary for prolonged use of the standard, demonstrating stability for over 14 weeks. These one-of-a-kind pGSR/OGSR standards with known particle counts, chemical composition, and analyte concentrations, provided “ground truth” information in controlled transfer and persistence studies. Finally, this research conducted an extensive transfer and persistence study on IGSR and OGSR, consisting of over 800 samples collected from multiple matrices (e.g., authentic shooters, fabric samples, and synthetic skin membranes) and exposed to diverse activities and conditions. The utility of novel synthetic skin membranes (StratM®) models for understanding the post-deposition behavior of gunshot residues is demonstrated for the first time. The combined advantage of characterized GSR standards, synthetic skin substitutes, fast screening tools, and additional confirmatory methods, developed in this research, is anticipated to transform the current approaches used for analyzing and interpreting gunshot residues. Furthermore, implementing accurate portable LIBS instrumentation will open opportunities for in-field and triage screening testing otherwise unavailable to law enforcement