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

Abstract

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