Gunshot Residue Collection and Forensic Examination

Primer Gunshot Residue Particles
Photo from forensicevidence.net


My agency worked a double murder last week, and I got involved in a discussion about Gun Shot Residue tests, or GSR tests, which made me realize that many officers have not the slightest clue about how one is done, or what it really involves, they just know “it would tell you if you shot a gun”

That’s a very bold statement, and one we aren’t going to dissect here, that’s a story in itself. We will look into some history of the test; an old test and method, a not so old method, and the current favorite method.

A GSR test is performed to try and locate the presence of certain materials deposited from a fired gun. Usually you are looking at the hands and clothing of a subject , but I have successfully obtained positive results from inside a suspect vehicle to show it was itself involved in an incident, and from where the gun was most likely used in relation to the rest of the interior of the car.

The idea of gun shot residue tests date back many years, and advancing science has made the examination much more specific than it was in its beginning.

Despite what logic tells us, that ‘boom’ isn’t an explosion, it’s the extremely fast burning of powder. The force of the powder burning creates the energy that propel a projectile (bullet) down through the barrel of the firearm. The cartridge contains chemicals and elements that get expelled in a gunshot, some of these elements are what we are searching for here.

Paraffin Test

Gunshot residue contains burned particles (potassium nitrite) and some unburned particles (potassium nitrate), and in the days of old, the test used melted paraffin wax to collect the deposited particles; this method was simply called a paraffin test. Once the wax dried, it was removed, and if successful, the wax picked up the trace chemicals from the discharge of the weapon. a reagent was then applied to the wax, and if you got the reaction in the form of blue specks, that was indicative of the presence of nitrates, and the assumption was that nitrates equaled you had fired a gun recently. This test was discontinued by the time I became a CSI in 1995, in fact, it’s use was discontinued in the 1970’s.

Atomic Absorption

The Atomic Absorption Test took the stage after the paraffin test was proved unreliable, (because it was presented that nitrates were too common in the environment to reliably say the presence of it equaled scientific proof a person had trusty fired a gun.) When a gun is fired, the gunshot residue particles are emitted from the cylinder area of a revolver, from the ejection port of an auto loader, and from the muzzle of both. These particles are expelled out onto surfaces around the gun, onto the hand and clothing of the person holding the gun, and the immediate surrounding area. Persons and items close to the blast, specifically the shooter, will likely have GSR from the discharge on them. Bystanders and gunshot victims can have GSR particles on them as well, if close enough to the discharge, or if they get contaminated by airborne particles by coming into the areas of the discharge; sometimes even minutes later, as these microscopic elements can stay airborne for up to ten minutes.

The person discharging the firearm is are more likely to have a greater number of particles than bystanders, and victims, but not always.

In this test, we are looking for the presence of elemental particles contained in most US made cartridge primers, for proof of a firearm discharge; those elements being antimony, barium, and lead. The collection of these samples are performed by swabbing the area to be checked with a cotton swab, moistened with a 5% solution of nitric acid; usually the back of the hand, thumb and back of the hand (web area) and the palm of the hand around the thumb are subject to collection. The swabs are then sent to a crime lab to be subjected to Atomic Absorption Analysis, or AAA.

As the science improved, a new, more reliable test method evolved: Scanning Electron Microscopy (SEM). Test samples for SEM are collected using contaminant-free adhesive tabs, or pin mounts. The SEM allows the examiner to see the actual microscopic samples collected, as well as gaining an analysis of the particles present. The person taking this sample will check the same area of the hand as in the AAA test, however instead of swabbing the area, they will use pin mounts with adhesive carbon discs to touch the areas to be checked, and the adhesive collects the sample. These will be sent to a forensic lab for testing under the SEM.

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