The Science Behind GSR: Separating Fact from Fiction
“Go ahead, make my day”—perhaps the most memorable quote from vigilante brute, Inspector “Dirty” Harry Callahan in 1983’s Sudden Impact. While his words alone were bold and brash, had Callahan not been carrying a Smith & Wesson Model 29 .44 Magnum, they may not have had such an enduring impact. Much like this Hollywood dramatization, firearms are often the center of real life plotlines. In 2010, firearms were used in 67.5% of murders, 41.4% of robberies, and 20.6% of aggravated assaults in the United States, according to the Federal Bureau of Investigation’s Uniform Crime Reporting Program.1 These statistics highlight the facts that inspire the fiction and justify the need for scientific analysis. It is the job of forensic professionals to search for the evidence in gun crimes, from recognizing the signature elements of gunshot residue (GSR) to determining the distance a suspect stood from his target. Here is a behind the scenes look at the science surrounding GSR investigations and the facts that make them dynamic.
The firing of a gun is an intricate series of events.
When a firearm is discharged, a high-pressure reaction occurs within its chamber. After the trigger is pulled, the weapon’s firing pin strikes the primer cap of the cartridge. Inside the primer cap is the primer mixture, which typically comprises lead styphnate (the initiator), antimony sulfide (the fuel), and barium nitrate (the oxidizer). The interaction of these three compounds begins and sustains the chain of action required to discharge a firearm. The firing pin strikes the primer cap sparking the chemical reaction that ignites the gunpowder. The burning gunpowder causes a build-up of pressure and high temperature gases within the cartridge. Energy from the accumulation of heat and pressure is released by projecting the bullet forward and out of the muzzle of the firearm. At the same time that the bullet leaves the firearm, there is an expulsion of discharge gases which include burnt and unburnt gunpowder particulate, metallic components of the ammunition and firearm, and the elements of the primer mixture, specifically lead, antimony, and barium—the signature metals of gunshot residue.2
Gun powder and gun primer are different types of evidence.
Once the gun is fired, the discharge gases erupt from any available opening in the firearm forming a gaseous cloud of particulate or a “plume” around the shooter (see Figure 1). The plume is so supersaturated with particles, that as it rapidly begins to cool and settle, the individual particles will unavoidably collide with one another and fuse together. When the individual primer elements of lead, antimony, and barium become fused together in one single particle within a plume, evidence is created. These three-element or three-component particles are categorized as “characteristic of gunshot residue.” These are the particles for which an expert searches when analyzing hand samples, clothing samples, or surface samples for the presence of gunshot residue (GSR).
Figure 1: The “plume”, a gaseous cloud of particulate surrounding the shooter.
Primer components are not the only particles being expelled when a firearm is discharged. Among other materials, burnt, unburnt, and partially burnt gunpowder is also being ejected. Residues originating from the gunpowder of cartridges are nitrogen-containing materials that could have an evidentiary significance depending on the circumstance of the incident. Chromophoric (color-changing) chemical tests can be performed on gunpowder patterns resulting on target surfaces (e.g. a victim’s clothing) in order to determine how far away a shooter is from a target. It should be clear that gunpowder analysis for distance determination is different than gunshot residue primer analysis. There is no color changing test for GSR primer that will instantly discern whether or not an individual has discharged a firearm. Manufacturers selling color-changing GSR kits which can be used in the field to produce immediate results are actually selling gunpowder kits. These samplers will change color in the presence of nitrogen. While there are nitrogen-containing materials that are expelled when a firearm is discharged, there are also unrelated nitrogen-containing materials that are commonly found in many environments. It cannot be said with any certainty that a color change resulted from particulate created from the discharge of a firearm, making it difficult for this evidence to be effective in a court of law. When used, color changing kits may even eliminate valuable, confirmatory evidence from a subject’s hands, such as primer GSR particles. Thus, when gunshot residue primer analysis is necessary, it is important to collect samples using only adhesive scanning electron microscopy collection stubs.
GSR analysis requires sophisticated tools.
The preferred method used to analyze gunshot residue samples throughout the forensic community is scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS). Since the 1970s, SEM-EDS has been used to identify and confirm the presence of GSR primer. The scanning electron microscope allows analysts to view particles at a high magnification (~5,000x or greater) and examine those particles through X-ray detection. The user is able to see and measure the interaction of the SEM’s beam electrons with the samples’ electrons. These measured energies are represented by peaks on a spectrum which correspond to different elements found on a sample. Thus, the elemental profile of a particle, as well as the particle’s shape is able to be examined using a scanning electron microscope (see Figure 2).
Figure 2: A particle of GSR as seen under a Scanning Electron Microscope.
Several methods were used prior to the popular acceptance of the SEM, most notably atomic absorption through inductively coupled plasma-mass spectroscopy (ICP-MS). This outdated technique (although still used today in some laboratories) requires a swab-like collection medium and extensive preparation. Of most concern when using ICP-MS to test GSR samples is the fact that it is a bulk analysis. Results might reveal traces of lead, antimony, and barium, but they cannot determine if all three elements are fused together in one particle. Without that vital piece of information, those individual elements could have come from any number of sources. It is only the amalgamation of the three elements combined as one particle that allows an analyst to confirm the presence of gunshot residue. Therefore, it is always preferable to analyze evidence using the scanning electron microscope.
Particles similar to gunshot residue are not gunshot residue.
Confusion about GSR sometimes arises because other sources can produce particles that have a similar shape and chemistry to those of gunshot residue. These sources include fireworks, brake dust, primer actuated nail guns, and airbag exhaust. Some research indicates the presence of spherical lead-antimony-barium particles (exactly like those of GSR) on the brake pad linings of vehicles.3 While the finding of these particles may call into question the “specificity” of primer gunshot residue particles, it also reiterates the importance of examining the entire population of particulate on a sample.
Samples coming from fireworks, brake dust, or airbag exhaust would most often also contain some indicator element or elements that would alert the analyst that these particles did not originate from gunshot residue. Samples coming from brake pads tend to be irregularly shaped and also contain a high amount of iron. A trained gunshot residue analyst is able to tell the difference between a sample population taken from a brake pad lining and a sample population taken from someone who has just discharged a firearm. It is essential that particles originating from the discharge of a firearm have a smooth, molten-like morphology (as a result of a high temperature burn) and the correct elemental profile: a collaboration of elements including lead, antimony, and barium. It is also essential to ensure that these particles do not contain exculpatory elements that would indicate a source other than a firearm discharge. Exceptions include particles that are created from some less common types of ammunition (e.g. nyclad or lead free, clean fire).
When GSR particles are present, there is always a reason why.
Whenever a population of primer GSR particles have been detected on an individual’s hands or clothing, there are three possible explanations as to how they arrived there: 1) the individual discharged a firearm, 2) the individual was in close proximity to someone who discharged a firearm, or 3) the individual came into contact with someone or some surface that contained primer residue. It cannot be said with absolute certainty which of these three scenarios occurred; any one is possible. However, given certain particulars of an incident, a GSR expert is permitted to give his or her opinion as to whether or not he or she believes that one of the possibilities is more likely to have occurred. It is also permissible for an expert to render what he or she would “expect” the results to be. For example, a GSR expert testifying in court might be asked, “Would you expect to find hundreds of gunshot residue particles on someone if they shook hands with another person who had recently discharged a firearm?” In this instance, it is reasonable for an expert to say “It’s possible, but not likely.” There may exist a situation in which hundreds of particles relocate from one person’s hand to a second person’s hand through physical contact between the two, but this would be considered a larger than usual amount of secondary transfer. Every incident is different and must be interpreted and dissected based on all of the influencing circumstances.
The absence of GSR does not indicate innocence.
There are factors that contribute to particulate loss that must also be taken into consideration. For instance, suppose a firearm is discharged in a static environment (no wind or airflow). Hand samples are then collected immediately after the discharge on clean, dry hands. It would not be unusual to detect hundreds to thousands of GSR particles on these samples. Alternately, suppose a firearm was discharged outside during a windy day with inclement weather. Samples are collected several hours after the discharge, giving the subjects ample time to move, shake off, wipe, or wash their hands. In this instance, it might not be possible to find any GSR particles on the samples. This absence of gunshot residue is not an indication that the subject is innocent of discharging a firearm. All factors concerning particulate loss must be taken into account when determining if results are consistent with the aspects of a specific scenario.
GSR is a piece to the investigative puzzle.
A gunshot residue analyst can confirm the presence of GSR particulate and interpret results as they accompany a specific incident. However, when GSR is found on an item of evidence, be it a hand sample or a clothing sample, it is up to a jury to decide just how those particles were acquired. Gunshot residue evidence is a piece to the forensic puzzle. When GSR is detected, its presence can have the probative value necessary to create investigative theories. Gunshot residue in conjunction with other evidence (DNA, fingerprints, eyewitness testimony, etc.) is what allows those theories to be rejected or confirmed. It is the job of forensic professionals to aid an investigation by supplying the facts that will link evidence together. The presence of gunshot residue is a dynamic fact in any evolving forensic investigation.
- Crime in the United States, 2010, U.S. Department of Justice – Federal Bureau of Investigation, Uniform Crime Report, Released September 2011, www.fbi.gov.
- Guide for Primer Gunshot Residue Analysis by Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry 11-29-11, Scientific Working Group for Gunshot Residue Analysis, http://www.swggsr.org.
- Brake Linings: A Source of Non-GSR Particles Containing Lead, Barium, and Antimony, Carlo Torre et al, Journal of Forensic Sciences 2002; 47 (3): 494-504.
Allison C. Murtha is a Forensic Scientist and the Manager of the Forensic Science Department at RJ Lee Group. She has been accepted as an expert in the field of gunshot residue analysis throughout the United States, U.S. Virgin Islands, and Bermuda. RJ Lee Group, 350 Hochberg Road, Monroeville, PA 15146; (724) 387-1876; firstname.lastname@example.org; www.rjlg.com.
Lixian Wu, Ph.D. is the Laboratory Director of the Forensic Science Department and the Biotechnology & Pharmaceutical Services Department at RJ Lee Group.