Screen-printed electrode modified with bismuth film and chemometric techniques for on-site detection and classification of gunshot residues

The analysis of gunshot residues (GSR) is important for forensic science to help understand what happened in a crime scene. Many works on this have been published; however, there are a number of questions that remain. An analytical method should ideally respond to these questions: 1 Can the residue from a shooting or the analysis present a false positive? 2. Is it possible to identify the gun used in a shooting and information about it such as caliber and type of ammunition used? An analysis, as well, must be precise, easy to use, reproducible and allowable on-site. To achieve these requirements, voltammograms registered on a portable potentiostat, and chemometrics analysis were used. Electroanalytical techniques provide meaningful chemical information about a sample. Chemometric tools make it possible to build models to identify even small differences and respond to these questions with data. A total of 90 GSR samples were collected with the help of the Institute of Criminalistics in the state of Pernambuco, Brazil. The GSR were taken from 3 types of firearms, 2 of them with distinct ammunition. To assess the risk of false positives, 45 non-shooting samples were analyzed. The model constructed achieved a classification accuracy of 100 % to respond to the question of false positives. There was an accuracy rate of over 85 % for the second question, of identifying information about the weapon. A blind test was performed and used to prove that the electroanalytical-chemometrics methodology was useful and effective for GSR forensic investigators, enabling the emission of reliable reports, and contributing to faster processing of legal proceedings.