Back in the late 60’s forensic laboratories in London started to use Scanning
Electron Microscopes (SEM) for the manual detection and analysis of Gun Shot Residue
(GSR) particles on a routine basis.
Forty years later, SEMs are still used in laboratories world wide for the automated detection of GSR™ particles. Technological advances, including, the integration of Energy Dispersive Spectrometers for elemental analysis and a software platform for the automated detection, relocation, confirmation and reporting, have made this the go-to technology for elemental analysis and imaging of samples collected from suspect shooters.
Traditionally, GSR™ particles are composed of lead (Pb), barium (Ba) and antimony (Sb) and commonly range in size from 1 to 10 microns. They have varying morphologies and surface textures but the vast majority is generally spherical in shape. Ammunition can also be manufactured without lead. Lead-free ammunition is becoming more prevalent as manufacturers use lead-free or low-lead propellants to avoid the toxicity of lead. Aluminum (Al) and strontium (Sr) are often used in lead free cartridges. In addition, Sintox® GSR particles are characterized by spheroidal particles mainly consisting of titanium (Ti) and zinc (Zn).
SEM images of GSR particles containing lead, barium and antimony.
The ASPEX PSEM with the GSR™ reporter Software is an integrated SEM-EDS system combined with a software suite designed specifically for high throughput GSR™ analysis. A GSR™ reporting template that complies with ASTM E1588 is available for fast and accurate reporting of confirmed GSR™ particles. In addition, additional templates can be created to meet different applications in forensic crime labs.
The Automated Process
A region of interest (sample stub) is defined by driving the motorized stage to the edges of a
sample region and focusing on the edges. The resulting configuration is saved as a stage
An analysis-list is filled, containing all the parameters (such as: magnification, electronic field size, scan speed, detection thresholds, number of particles, classification rule files) necessary for the automated analysis application. The Automated Feature Analysis (AFA) application is started. Particles are detected and characterized based on morphological and chemical information. As particles are found, the GSR™ Reporter receives particle information from the AFA application. When the AFA application is finished or halted, the found particles can be inspected in the GSR™ Reporter.