Examination of Gunshot Residue

Composition of gunshot residue

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Discharge of a firearm produces combustion of the primer and smokeless powder in the cartridge. The residue created by this process constitutes gunshot residue. This residue may be found on the skin or clothing of the person who fired the gun, on an entrance wound of a victim, or on other target materials at the scene. The discharge of a firearm can deposit residues on all persons at close proximity to the discharge, so interpretations as to who fired the weapon should be made with caution.

The major primer elements are lead (Pb), barium (Ba), or antimony (Sb). Particles with the proper morphology that contain all three elements are considered “unique” to gunshot residue. Particles with one or two of these elements are considered “consistent” with gunshot residue. (However, see Caveats below)

The cartridge case, bullet, bullet coating, and/or metal jacket also contain specific elements that can be detected. Virtually all cartridge cases are made of brass (70% copper and 30% zinc). Some have a nickel coating. Primer casings are similar in composition. Bullet cores are most often lead and antimony, with a few having an iron alloy core. Bullet jackets are usually brass (90% copper / 10% zinc), but some are a iron alloy and some are aluminum. Some bullet coatings may also contain nickel.

Modern gunpowder, or "smokeless" powder, can contain a number of organic compounds. Nitrocellulose is virtually always present. Gunpowder is described as "single-base" when the basic ingredient is nitrocellulose and as "double-base" when nitroglycerine is added to the mixture. 

Detection of Gunshot Residue

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The major method for detection of primer residues is scanning electron microscopy with energy dispersive analysis (SEM-EDX). For this method, samples are obtained from the skin surfaces or clothing. It is also possible to sample the inside of vehicles. In this case, horizontal surfaces are best.

The method of collection for residue is easily carried out in the field. Samples are collected onto the gummed surface of a stub, or holder, applied to the surface (skin or other material) to be tested. The stub, with the residue on the surface, can be directly prepared for examination in the scanning electron microscope.

A major advantage of this method is that SEM can reveal the actual surface details of the particles examined, for comparison with known examples of gunshot residue, and pictures can be taken. The large particles of partially burned powder and the spheres of residue can be distinguished from contaminant materials.

 

Scanning Electron Micrograph of GSR

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An X-ray analyzer can be beamed directly onto the particles, so that the energy dispersive pattern (EDX) can be generated, giving the elemental composition of the particles. An automated computer-based search technique is commonly used.

 

Diagram of the SEM-EDX pattern of GSR

 

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It should be remembered that any hand or body part that was close to the fired weapon may have residue appearing consistent with having fired the weapon. Clothing should always be retained in order to aid in interpretation of the scene.

Caveats:

Gunshot residue analysis requires careful evaluation:

For a detailed analysis on the current state of GSR analysis, see the summary of a symposium held by the FBI on this subject. The link is here.

Please also read this article from New Scientist magazine on the potential unreliability of GSR Testing.

False positives may be caused by contamination or transfer of GSR by mishandling, or transfer from contaminated surfaces or clothing.

Residue from some fireworks and some automobile brake pads can contain the same elements as "unique" GSR.

False negatives result from washing of the hands or by the wearing of gloves. A rifle or shotgun may not deposit GSR on the hands. For these types of weapons, sampling the face may be necessary.

GSR particles on clothing can remain in place for a long, or a short time. Laundering, obviously, can be a big factor.

Instrument calibration is important. Some very common elements have EDX peaks very near Pb, Ba and Sb.

Some automated particle analysis systems will misidentify GSR particles. Any final conclusion must be made after the analyst manually examines the particle in question. This, of course, then introduces sources of human error; including bias, a desire to please through a positive, or negative, report, etc.

The number of particles needed for a “positive” finding and the percent of the stub that¬† has been examined in order to allow a “negative” finding are also major concerns and the subject of debate among experts in the field.