Validation of the Expert Methodology “Detection of Condensed Traces of Gunshot Residue Containing Heavy Metal Compounds on Various Objects by Scanning Electron Microscopy and X-Ray Microanalysis”

Abstract We present and discuss the results of validation of a forensic qualitative testing technique which consists in the detection of the condensed traces of gunshot residue (GSR) in the form of individual microparticles on the objects under study and their identification by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) using a Mira III scanning electron microscope (Tescan, Czech Republic) equipped with an INCA GSR 450 system of X-ray microanalysis (Oxford Nanoanalysis, Great Britain). The procedure for detecting GSR particles containing heavy metal compounds, systems of classification of GSR particles, and interpretation of test results are described. The suitability of the method for solving forensic problems is confirmed. The validation procedure consists in assessing the reliability by comparing the test results obtained by experts (A and B) in two laboratories at different times using the same device and in confirming the competence of the experts on the basis of the results of “blind” tests. An ENFSI GSR PT 2018 A-03-07 standard sample is used. The “blind” test consists in the determination by each of the experts of the presence and number of particles classified as GSR in control samples taken in a ballistic laboratory during a full-scale experiment: from the hands of a shooter, from the hands of a person who did not shoot, and without microparticles (clean stage of an electron microscope). The reliability of the technique is characterized by the index (probability) of correct results of detection of GSR particles in the standard sample of at least 95.8% and by a small proportion of false results (no more than 5.4%). The competence of the experts is proved by the consistent results of “blind” testing of control full-scale samples containing and not containing GSR particles obtained in different laboratories. The results of the validation indicate the suitability of the method for obtaining reliable and valid information about the presence of GSR particles on the objects under study.