Trace evidence encompasses the multitude of materials left at or taken from a crime scene, such as blood droplets, clothing fragments, or gunshot residue. In order for these substances to be useful, they must carefully be preserved and collected at the crime scene, minimizing and controlling the further contact with them as not to expose them to additional contaminates. The trace evidence is placed into individual sterilized containers or paper bags, labeled, and taken to the forensic lab for analysis.
The first step in the forensic trace evidence analysis process is the visual examination. Each piece of evidence is visually inspected, with the notes written down. Detailed information must be kept about each inspection done on the sample, so that peers may be able to draw logical conclusions from the records. Once the visual information is recorded, the analyst can begin the microscopic analysis. This type of analysis can show details unavailable to the naked eye, such as composition and morphology of the sample. In the case of gunshot residue, it can lead to the vital information needed in solving a case.
Since the late 60’s, scanning electron microscopes have been used for gunshot residue analysis. Typically, these SEMs are equipped with an energy dispersive spectroscope, used for elemental analysis, and a software program capable of automated detection. Using the SEM to detect the components and morphology of the residue helps determine what type of material was left behind, which can lead to the reconstruction of the bullets and ultimately the crime scene.
The Aspex Personal Scanning Electron Microscope (PSEM), when coupled with the Aspex GSR™ reporter software, becomes an all-in-one GSR analysis machine. This fully integrated SEM-EDS system that can detect, relocate, and confirm the presence of gunshot residue is typically employed throughout modern-day forensic laboratories. Templates are available that comply with American Society for Testing and Materials (ASTM) E1588, or templates can be created for more precise information gathering in the forensic lab and customized for your specific lab. Since the PSEM is fully automated, the chance of false positives or contamination by handling remains low.
Reference:
Aspex, http://www.aspexcorp.com/products/gsr.html,http://www.aspexcorp.com/industries/forensics.html
ASTM, http://www.astm.org/JOURNALS/FORENSIC/PAGES/3938.htm
Wikipedia, http://en.wikipedia.org/wiki/Trace_evidence
