Gun Shot Residue (GSR) consists of particles from the explosive primer, both burnt and unburnt, as well as the propellant, bullet fragments, the casing and the firearm used. When the gun is fired, the pressure from the bullet leaving the muzzle forces the debris out of the barrel at a high velocity, and scatters the debris in a cone-shaped pattern. Since it is a mix of all parts involved, it is a highly-specific tool in aiding forensic laboratories piece together a crime. These fragments and debris can be analyzed, providing information such as bullet and gun type, distance to target, or shooter conformation by comparing to known evidence samples.
Modern forensic laboratories rely on Scanning Electron Microscopes (SEM) with Energy Dispersive X-ray Spectrometry (EDS) technology to analyze GSR. Most GSR contains lead, antimony and barium, though some newer primers are lead-free. Thus, the best way to assess if the debris was GSR remains is to be analyzing the elemental composition and comparing it to known GSR characteristics as well as other evidence found at the scene. SEMs are able to analyze these particles for morphology and elemental composition without destroying the sample, unlike some other methods of GSR analysis previously used, allowing the analyst to evaluate the same sample multiple times if necessary.
In order to present the forensic community with a fast, accurate, non-invasive method to analyze GSR, Aspex has created a GSR software package for its Personal Scanning Electron Microscope (PSEM), making it a complete SEM/EDS system with a software suite designed specifically for high throughput GSR™ analysis. The GSR software has templates compliant with the American Society for Testing and Materials (ASTM) E1588 – Standard Guide for Gunshot Residue Analysis by Scanning Electron Microscopy/ Energy Dispersive X-ray Spectrometry, or the analyst can create templates for specific analytical needs.
The GSR Reporter software works with the Automated Feature Analysis (AFA) platform to assess the chemical and morphological makeup of the given sample. Since the PSEM is a fully-automated SEM/EDS system, contamination during the analysis is virtually nonexistent, as long as proper attention is paid when collecting and transferring the sample prior to and after the analysis.
Today, GSR analysis is a vital tool for crime units, allowing them to literally place the gun in the shooter’s hand in some cases. By analyzing particles for morphology and elemental composition, and comparing with other evidence found at the crime scene, analysts can recreate the scene and present solid evidence in court. The Aspex PSEM with GSR Reporter software is a complete GSR trace evidence system, providing forensic analysts with accurate results with minimal sample handling.
Reference:
Aspex, http://www.aspexcorp.com/industries/forensics.html
Wikipedia, http://en.wikipedia.org/wiki/Gunshot_residue
ASTM, http://www.astm.org/Standards/E1588.htm
FirearmsID, http://www.firearmsid.com/A_distanceGSR.htm
