Electrochemical and mass spectrometry methods for identification of gunshot residues (GSR) in forensic investigations

Gun violence continues to be one of most significant challenges straining the USA society causing thousands of human lives lost every year. In 2020 alone, firearm-related incidents including homicide, accidents, and suicides, reached a staggering number of over 43,000.1,2 With the increase in these types of incidents, several service areas in crime laboratories are heavily impacted by the number of cases run on a yearly basis. These include firearm examinations, gunshot residue (GSR) analysis, bullet hole identification, and shooting distance determination, which are crucial to support a criminal investigation and, overall, the justice system in our country. These areas are very resourceful for reconstructing firearm-related inquiries and evaluating the evidence under source (GSR present or absent) or activity (fired a gun or in the vicinity of the firing) propositions. GSR particles are evaluated based on single-particle morphological and elemental analysis (e.g., lead, barium, and antimony) by Scanning Electron Microscopy Energy Dispersive Spectroscopy (SEM-EDS) following the ASTM 1588-20 method.3–6 In addition to SEM-EDS, color tests are currently used to evaluate distance determination as per the recommendations given by the Scientific Working Group for Firearms and Toolmarks (SWGGUN) for nitrites, lead, barium, and copper.7, 8,9 Our research group has focused its attention on the development of emerging analytical tools that facilitate the detection of both inorganic (IGSR) and organic gunshot residues (OGSR) using electrochemistry (EC) along with data mining tools to support more objective data interpretation. This research aims to fill some of the gaps observed in existing technologies like color tests by offering faster and complementary methods to decrease subjectivity, cost, analysis time, to aid with triage and more cost-effective workflows at the crime scene and laboratory. The complementary OGSR information is anticipated to cause a breakthrough in the GSR analysis paradigm and respond to the current OSAC recommendations for this specialized area of work. 10–14 To this end, the development of innovative sampling methods for distance determination and bullet hole identification were investigated to simultaneously gain spatial and chemical information via electrochemical detection. In the case of distance determination, a set of 30 calibrations and 45 unknown distance clothing samples on various light, dark, patterned, and bloodstained fabrics were assessed to compare the electrochemical performance against current techniques. Discriminant analysis statistical classification method was applied for the classification of the 45 unknowns resulting in an electrochemical method accuracy of 74% compared to color tests at 58%. Bullet hole identification were investigated on 59 fabrics and other alternative substrates commonly found at crime scenes, such as wood, and drywall to assess potential interference and electrochemical performance from unknown shooting distance. Electrochemical methods successfully provided simultaneous detection of IGSR and OGSR with overall 98% accuracy using calibration thresholds for positive identification. OGSR results were confirmed using our research group\u27s previously validated OGSR solvent extraction and LC-MS/MS method. Transitions toward using portable technology probed investigation to compare the performance of portable and benchtop instrumentation for GSR analysis. A comparison of figures of merit and performance metrics found comparable results on the limits of detection, precision, linear dynamic range, and error rates, with 95.7% and 96.5% accuracies for identifying GSR using critical threshold analysis for benchtop and portable potentiostats, respectively. Quick sample collection and screening allowed for fast electrochemical detection in 15 minutes for bullet hole and distance application. The advantage of this methodology is the developed analytical scheme can be easily incorporated within the current workflow to enhance reliability (i.e., physical measurements, color tests, or SEM-EDS) due to its non-destructive nature and highly selective and sensitive characteristics. The conclusions of this work demonstrate the fit-for-purpose of electrochemical detection expanding from GSR analysis to distance determination and bullet hole identification with fast detection using a low-cost platform for simultaneous IGSR and OGSR detection