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Aaron, Roger W. Gunshot Primer Residue: The Invisible Clue. FBI Law Enforcement Bulletin, June 1991. p 19-22.

Abstract: This article offers suggestions for the effective collection and use of gunshot primer residue evidence. The history of techniques that tried to associate an individual with a firearm is reviewed, including the ìparaffin testî (which is no longer used and is a misnomer for current gunshot primer residue analysis) and neuron activation analysis. Neuron activation analysis (NAA) identified two noncombustible primer mixture components, barium and antimony, as detectable residues after a firearm is discharged.

The most common procedure for recovering gunshot primer residue is wiping cotton swabs moistened with diluted nitric acid over the web and palm areas of each hand. Then NAA or atomic absorption spectroscopy (AA) is used to determine the quantities of barium and antimony on the swabs. Both elements must be found because each alone is not unique to GSR. Another method uses adhesive disks to pick up microscopic particles of GSR from the hands. Then, a scanning electron microscope uses energy dispersive x-ray analysis to detect particles containing barium and antimony.

GSR evidence should be collected immediately upon making an arrest because the residue will wear off through normal hand activities. Care should be taken with handcuffing because this can greatly decrease the amount of GSR on the hands. Samples should be taken with GSR collection kits; drugstore supplies should not be used a substitution.

Although GSR can associate an individual with a firearm, it does not identify that person as the shooter because GSR can settle on any hand placed near a weapon when it fires. Even handling a dirty weapon can deposit it. In addition, failure to find GSR does not mean that person did not fire or handle the weapon. Sufficient quantities are not always deposited and can be removed by hand washing or normal hand activities. An inconclusive finding of barium and antimony amounts means that the analyst can offer no opinon of value associating a tested individual with a firearm. In addition, the finding of barium and antimony does not consistute an unequivocal identification of GSR; the substances can be found on the hands and not have been deposited by a firearm discharge. GSR has little value in a suicide-homicide investigation because deposits can be made by close contact, but not actual firing, of the weapon (such as grabbing for or shielding the weapon as it fires). Suspects at the crime scene should only be sampled if they do not admit to or cannot otherwise be associated with a weapon at the approximate time of the shooting. Other body parts and surfaces are generally not suitable for GSR examinations.

For more information, contact:
FBI Law Enforcement Bulletin
FBI Academy
Madison Building, Room 209
Quantico, VA 22135

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