Optimizing the Automated Analysis of Inorganic Gunshot Residue Particles by SEM-EDX: From Synthetic Particle Standards to More Time-Efficient Settings for Daily Casework

Gunshot residues deposited on all surfaces in the nearest vicinity of the shooting incident, when revealed, can contribute to the explanation of various aspects of such an incident for forensic purposes. Examinations of gunshot residue, mainly inorganic particles, at forensic laboratories are expected to be reliable and fast. This primarily depends on the performance of the used scanning electron microscope integrated with an energy dispersive X-ray spectrometer and the automatic program searching for particles of defined characteristics. Among the milestones on the pathway towards quality assurance in examinations of gunshot particles, the invention of the synthetic gunshot residue specimen ought to be named. Such a specimen with particles of known chemical content, size, and location is now used for proficiency testing, which is a condition for a forensic laboratory to obtain accreditation in this subject matter. In this publication, the need for optimization of the procedure for the examination of a synthetic specimen, in alignment with the necessary modifications for real gunshot particles, has been addressed. The presented process of validation resulted in two procedures. The first demonstrates the full capacity of the instrument for detecting all particles present in the synthetic specimen, including the 0.5 micrometer particle at the magnification of 250×. The other procedure is the modification of the first, however aiming at 1-micrometer particles or bigger (at the magnification of 120×) and allowing the necessary backscattered signal threshold changes depending on the actual composition of gunshot residue as well as the abundance of light element debris in the case of real gunshot particles.