Gunshot residue – GSR. Gunshot residue consists of tiny (microscopic, 1-10 micron) particles of the cartridge primer that are ejected from various orifices of a gun when a cartridge is fired. GSR has historically been identified by it’s elemental composition signature of lead, barium, and antimony. These elements vaporize when the primer ignites, and then condense into the particles known as gunshot residue – GSR.
The residue floats in the air, and lands on all kinds of things, not the least of which is the hand, and possibly the clothing, of the shooter – and bystanders. These pictures will give you an idea of how it happens.
Prior to 1995, gunshot residue was identified by swabbing the hands of a suspect with alcohol swabs which were then analyzed with atomic absorption spectrometry (AAS). In the AAS process, the sample being analyzed must be vaporized in a small lab furnace, making it a destructive test which could not be repeated. The process would determine if lead and/or barium and/or antimony were present in the sample, and to what level. Given the presence of one or more of these elements, one could say that it was possibly gunshot residue. The term that was universally used was “consistent with gunshot residue”. However, the test could not determine the actual source of the lead, barium, or antimony. For example, mechanics who worked on brakes would test positive for gunshot residue, and there are lots of other environmental sources for this stuff.
The sampling protocol at the time did recognize a need for timeliness in swabbing the suspect, since the more time that goes by, the greater the chances that GSR or something “consistent with” it could be picked up from somewhere else. The time between the firing of the gun and the swabbing of the suspect is called the ΔT, and protocol said it should not exceed four hours.
It wasn’t until 1995 that the American Society for Testing and Materials (ASTM) published the first edition of its standard ASTM E1588 Standard Guide for Gunshot Residue Analysis by Scanning Electron Microscopy/ Energy Dispersive X-ray Spectrometry. This standard specifies that GSR identification must include both an elemental signature determination and a morphological analysis performed by SEM/EDS (scanning electron microscope and energy dispersive spectrometry). The standard may be purchased from ASTM International on-line for $35. http://www.astm.org/Standards/E1588.htm
With the advent of ASTM E1588 and the use of SEM/EDS, GSR sampling is now done with “adhesive lifters” rather than swabs.
GSR particles “look” like this when viewed with a scanning electron microscope:
You can see that they have different characteristic shapes (called “morphologies”).
So analysis can reliably identify GSR, but to determine if that GSR could have come from a particular shooting, the suspect-sampled GSR must be compared with samples taken from other durable case-specific items – like the actual gun or the victim’s clothing. The elemental signatures and the particle morphologies would need to match, and even then, all that can be said is that the GSR on the suspect is “consistent with” the GSR found on the gun/victim. And it cannot be said whether the suspect was the shooter. This quote from a GSR “update” in the May, 2011 FBI Law Enforcement Bulletin: “Reputable scientists always have reported that the finding of GSR cannot indicate the shooter.” (emphasis added) Here is the full text of that article:
The major problem with GSR is the issue of possible contamination. As stated previously, there are many environmental sources of particles that can yield a GSR elemental signature. GSR is “environmentally persistent” – it doesn’t degrade, or breakdown, or just disappear, and it is easily transferred from one surface to another. Sources of contamination can be arresting officers themselves, the seats of police cars, and the furniture in interrogation rooms. And even worse, GSR contamination can come from the forensic lab doing the analysis. In 2006, the FBI ceased doing GSR analysis. The public position was that this was so they could devote more of their resources to fighting terrorism. However, this decision was made following an internal audit of the FBI’s Quantico, VA crime lab in which it was found that the lab was “lousy” (my word) with GSR contamination. The FBI had also expressed concern about receiving samples that had not been obtained within the the 4-hour ΔT limit.
Here is an article from the Baltimore Sun in 2006 reporting the FBI’s decision to cease GSR testing: FBI Scraps GSR Testing
So what can we expect from GSR?
1) Modern analytical methods can reliably detect GSR.
2) GSR can indicate that someone was in the vicinity of a fired weapon, or that they may have been in secondary, or even tertiary, contact with something that was in the vicinity of a fired weapon.
3) A GSR sample cannot be attributed to any specific shooting with any statistical validity. The data doesn’t exist.
4) GSR cannot determine who might have been a shooter.