Optimizing FEG-SEM combined with an SDD EDX system for automated GSR analysis

摘要

Gunshot residue (GSR) analysis by scanning electron microscopy/energy dispersive x-ray spectrometry (SEM/EDX) is routinely performed in forensic science laboratories. At the end of 2011, a field emission gun (FEG)-SEM was installed at the authors' laboratory for this type of examination. Following the installation of the FEG-SEM, an extensive process was conducted, in order to determine the optimal operating parameters of this instrument for GSR analysis. Because the nature of synthetic GSR samples (such as the European Network of Forensic Science Institutes GSR2011 proficiency test sample) is significantly different from casework samples, it was decided not to use them for this purpose. Instead, a GSR casework sample, previously analyzed by tungsten-filament SEM as part of the laboratory's routine examinations, was selected. This sample was repeatedly analyzed (more than 200 times), under different operating conditions. The main parameters that were examined for the FEG-SEM were the electron source accelerating voltage, the backscattered electrons image acquisition time, the magnification, the backscattered electrons image resolution, and the energy-dispersive X-ray spectrometry acquisition time. The smallest detected feature was calculated by the software, based on the set image resolution and magnification. For each set of parameters, the detected GSR particles were recorded, in addition to the total number of detected particles and the time taken for the run. It was found that for a given smallest detected feature, the effective search conditions were higher magnification and lower number of pixels per line. The present work includes a detailed description of the optimization process and its results. The process utilized in this work is applicable for other laboratories conducting similar type of GSR analysis as well.