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

Abstract

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. Copyright © 2013 John Wiley & Sons, Ltd.

Automated Inclusion Microanalysis in Steel by Computer-Based Scanning Electron Microscopy: Accelerating Voltage, Backscattered Electron Image Quality, and Analysis Time

“…The total analysis time for a steel sample is mainly the sum of three steps: stage movement, BSE image acquisition, and EDX spectrum acquisition (sometimes together with recording of a higher-magnification BSE image of each feature) (Izraeli et al, 2014). The time for each step can be estimated from the instrument settings (BSE pixel dwell time, EDX analysis time, total area analyzed, magnification, and image size) and the expected (or actual) number of features per unit sample surface area analyzed.…”

Molybdenum in Gunshot Residue: Experimental Evidences and Detection Challenges in the Presence of Lead and Sulfur

“…To test our hypothesis, we had to overcome a number of difficulties. A GSR software routine, when the automatic search is performed with the commonly used accelerating voltage in the range between 20 and 25 kV (Izraeli et al, 2014), will hardly classify any Mo-containing particle. Molybdenum mass concentrations in residue were supposed to be low, as Mo is not contained in any primer mixture.…”

“…A minimum voltage of 20 kV for the primary beam is required by international standards (ASTM E1588, 2020) for automated GSR analysis. Common choices operated by forensic GSR experts are 25 or 20 kV, the latter preferred when seeking data from sub-micrometric particles (Izraeli et al, 2014). Stubs collected from the firearm, and the shooter's hands were analyzed with an accelerating voltage of 20 kV, as required by the adopted standard operative procedures (SOPs) of the forensic laboratory of the Carabinieri Corps in Parma, Italy.…”

Classification Improvements in Automated Gunshot Residue (GSR) Scans

“…The brightness and contrast of the BSE detector were adjusted using gold (Au), rhodium (Rd), and cobalt (Co) standard stub. An iron (Fe) particle of size 1 μm is expected to be detected . The samples for GSRs analysis in the Israel police laboratory are collected using the Semoptics Ltd. GSR kits, containing two 1‐inch SEM stubs.…”