Forensic Science – GSR Chemical Analysis

It is the goal of gunshot residue (GSR) analysis to put the suspect near a firearm when it is fired. Unfortunately, being near the gun when it goes off or simply handling the gun afterward can leave behind GSR on an innocent individual. GSR testing must be conducted right away after a gun is fired because GSR tends to fade away quickly and usually disappears after approximately two hours.

Crime scene investigators (CSI’s) must inspect any suspect’s face, hands, and clothing and get samples. The archaic paraffin test, where melted wax is used to recover residues from the firearm user’s hands, is no longer used anymore. Instead, CSI’s obtain GSR residue by swabbing the suspect’s hands, arms, and clothing with a moist Q-tip or filter paper.

Chemical analysis looks closely for byproducts of the burning of primer and gun powder. Specifically, forensic scientists look for the metals lead, antimony, and barium. The Q-tip or filter paper is then treated with a solution of diphenylamine, a chemical that interacts with metals by producing a color change. The test is positive if the color blue is produced. However, this test could be a false-positive result. Nitrogen-containing compounds such as fertilizer, tobacco, cosmetics, and urine can also interact with the amino group of diphenylamine to contribute to this false-positive. Therefore, it is up to the forensic scientist to confirm the results with more definitive testing.

Sometimes GSR can be found on the shooter’s clothing or in his hair if not found on his hands. To make this determination, a scanning electron microscope (SEM) many times can locate these minute GSR particles. The extreme temperatures of an explosion can melt and deform these particles. This can identify products of a firearm’s combustion. SEM is many times used in conjunction with energy dispersive X-ray (EDX) to determine a chemical fingerprint of the substance being tested. The presence of lead, antimony, and barium in these tiny particles while using SEM/EDX technology highly indicates that the substance is in fact GSR.