Forensic Trace Evidence - Tracing Gunshot Gases and Particles

Whenever a firearm is discharged, the primer and the powder detonate inside the cartridge, violently pushing the bullet down the barrel. Most if not all of the explosive gases and particles created by the explosion follow in the bullet's wake. Some of these particles pass through an opening in the firearm. This is very true for revolvers, which has a tendency to leak more gases than semiautomatic firearms.

The combination of chemicals and particles of these gases are collectively known as gunshot residue (GSR). Gunshot residue sticks to the firearm user's hand, arm, clothing, hair, and face. They also stick to nearby walls, curtains, or future and on the victim's clothing and skin in close-up shootings.

Weather such as wind and rain can alter the pattern of or reduce the spread of the cloud of GSR. In effect, the forensic firearms expert may find GSR in places he would least expect. On white clothing, GSR patterns are easily identifiable as smudges or smears, but on dark, multicolored, or blood-soaked clothing, the patterns are not too evident.

GSR residues can be identified by infrared photography under substandard circumstances. The Griess test may also identify the pattern. This test involves the use of filter paper soaked in vinegar that when pressed over the area in question and then immersed in a reagent that interacts with the nitrogen containing compounds found in GSR yields a GSR pattern. Photographic paper can also be used in place of filter paper to reveal evidence of a GSR pattern.

The next time you watch your favorite CSI TV show, you will have a better understanding of what the procedures are when criminal scientists run tests to disclose the presence of a GSR pattern made of gases and particles.

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