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Gunshot residue (GSR) analysis can be a critical piece of evidence in criminal investigations, but over the last several years many government laboratories have discontinued GSR analysis due to a lack of funding or re-prioritization. By understanding the science behind GSR and what can be learned from its analysis, investigators can make informed decisions regarding the collection of gunshot residue evidence.Gunshot residue (GSR) analysis can be a critical piece of evidence in criminal investigations, but over the last several years many government laboratories have discontinued GSR analysis due to a lack of funding or re-prioritization. By understanding the science behind GSR and what can be learned from its analysis, investigators can make informed decisions regarding the collection of gunshot residue evidence.

When someone pulls the trigger of a gun, a chain reaction occurs causing the firing pin to strike the primer cap on the base of the cartridge. The compounds in the primer cap react and ignite gunpowder contained in the body of the cartridge. When the gunpowder ignites, a high temperature and pressure reaction occurs, propelling the bullet out of the firearm. During this reaction, typically elements of lead (Pb), barium (Ba), and antimony (Sb) from the primer compounds escape the firearm from any opening that is not airtight, forming a vaporous cloud called the plume. As the temperature decreases in the plume, the elements of lead, barium, and antimony can and will condense together to form a primer gunshot residue (GSR) population consisting of several particle types.2,4 (See Table 1.)

It should be noted that most primers found in ammunition in North America consist of lead styphnate, barium nitrate, and antimony sulfide. Therefore, a combination of the three elements of lead (Pb), barium, (Ba), and antimony (Sb) from these compounds is significant. However, there are rare ammunition types that may contain additional elements such as tin (Sn). Analysts are trained to look for these elements found in rare ammunitions and an analyst may ask to perform a comparison of the fired-cartridge casing from the investigation.1,5

Particle Type

Elemental Composition(s)

Designation

3 – Component Particle

Pb-Ba-Sb

Characteristic of GSR

2 – Component Particle

Pb-Ba; Pb-Sb; or Ba-Sb

Consistent with GSR

1 – Component Particle

Pb-rich; Ba-rich; or Sb-rich

Commonly associated with GSR

Table 1: GSR particle types formed from the discharge of a firearm.

These GSR particle types can land on the shooter and in the immediate area surrounding the shooter. The analysis of GSR can be key evidence in criminal investigations—evidence that the Federal Bureau of Investigations (FBI) Laboratory stopped analyzing in 2005.2,4

When a major crime laboratory like that of the FBI stops analyzing key evidence, law enforcement and the broader community cannot help but feel suspicious about the method’s scientific validity. Typically, the discontinuation is due to laboratories having to prioritize their funding. This may leave little room in the budget for GSR analysis. As such, the FBI stated that while GSR analysis is a valid method, they receive very few GSR cases yearly and decided to shift the funding toward anti-terrorism activities. Despite the FBI’s transparency about their decision, there are increasing misconceptions about the analysis of GSR fueled by misinformation and a lack of educational resources.2,3

In the high stakes environment of criminal investigations, even the smallest piece of evidence, or lack of evidence, can help to convey the story behind what actually occurred. GSR can be critical, probative evidence, in a criminal case. One misconception among police officers is that GSR is not a confirmatory analysis. While GSR cannot definitively point to one person, as DNA analysis can, it does confirm whether or not a GSR population is present on a person or an object.

When a GSR population is found (at least one three-component particle is identified), a gunshot residue expert can say that it landed there in one of three ways:

  1. The subject discharged a firearm.
  2. The subject/object was in close proximity to a discharged firearm.
  3. The subject/object came into contact with an object that already had GSR on it.2

Because GSR evidence implies that a subject has been exposed to a GSR population in one of three ways, it is a key piece of evidence in solving an investigation. Conversely, the absence of GSR does not mean an individual was not exposed. There are several reasons why a person may not have GSR on them even if they were assumed to have discharged a firearm. These reasons include:

  • Time elapsed
  • Activity
  • Weather
  • Washing hands, clothing, or surfaces
  • Masking from biological material
  • Firearm or ammunition were not good depositors
  • No discharge2,4

There are some scenarios where only one and two-component particles are found through the course of analysis. In these scenarios, the population of particles could have come from the discharge of a firearm, but they also could have come from other sources in the environment. Examples of sources include nail guns, battery terminals, and pyrotechnics.4,6

Even a lack of GSR aids in reconstructing or corroborating the incident in question. This is especially true when GSR evidence is used in conjunction with other types of evidence.

While some labs have stopped analyzing GSR, others enforce limitations on specific types of collection kits and scenarios. When forming guidelines, a laboratory must take into consideration the demand for testing along with the instrumentation, time, and personnel required for the analysis.5 These limitations vary according to the laboratory and are most often enforced in order to reduce backlog and to increase the likelihood of finding a probative result. Common limitations on kits include: only analyzing hand kits, not analyzing victims’ kits, and/or using a threshold cut-off for the time that has passed between the incident and evidence collection. Depending on the case scenario, this evidence (that may not be submitted to a local or state crime laboratory due to the above mentioned reasons) can still provide probative evidence. For example, even after time has elapsed since the incident, GSR can be collected from a subject’s clothing because GSR particles will tend to adhere to the weave of the fabric. However, due to this fact it should be noted that a GSR population found on clothing could have come from previous exposures to GSR (i.e. shooting ranges, hunting). Investigators should collect as much information about the subject as possible to rule out these scenarios.

Despite some laboratories moving away from GSR testing and others imposing limits, law enforcement personnel should collect GSR samples when probative. These samples can always be analyzed at a non-federally regulated laboratory.In the courtroom, lawyers may ask a police officer why he or she did not collect GSR or other evidence. When testifying, it would be better for the officer to be able to say that GSR evidence collected in the case was not probative rather than saying it is not the policy of the department to collect.Luckily, collecting GSR is simple and low cost. Collection kits that contain SEM (scanning electron microscope) stubs are available from several manufacturers for $7-$15.

After collecting GSR evidence, it is up to the police department, the DA, or the defense attorney to decide if the evidence will be sent for analysis. Though GSR evidence is also fairly low-cost to analyze, there are some situations when GSR kits are not analyzed right away. These situations could include budget constraints or scenarios such as suspected suicides. In these scenarios, the GSR kit could be collected and stored for future analysis. GSR particles are composed of heavy metals that will not degrade on the SEM stub.4 As a result, analysis could occur years after collection. Similarly, clothing that has been bagged and stored properly can be tested years later and still be found to have GSR on it.

Despite many labs not having the funding or capacity to accept GSR samples, gunshot residue can still be probative evidence. With knowledge about the science behind GSR and a clear understanding of what is possible within the confines of a case, law enforcement should always take the time to collect critical evidence at the crime scene. The decision to collect GSR could make or break a case.

References

  1. “Guide for Primer Gunshot Residue Analysis by Scanning Electron Microscopy/ Energy Dispersive X-Ray Spectrometry.” 11-29-11. “SWG-GSR.” Date accessed 10 Apr 2014. http://www.swggsr.org/FINAL%20GUIDE11-29-11.pdf
  2. Schwoeble, A.J. Current Methods in Forensic Gunshot Residue Analysis. New York: CRC Press, 2000.
  3. ASTM1588-08 Standard Guide for Gunshot Residue Analysis by Scanning Electron Microscopy/ Energy Dispersive X-ray Spectrometry (SEM/EDS).
  4. Trimpe, Michael. “The Current Status of GSR Examinations.” FBI Law Enforcement Bulletin, May 2011.
  5. Bykowicz, Julie. “FBI lab scraps gunfire residue.” 01 May 2006, The Baltimore Sun. 10Apr 2014. http://articles.baltimoresun.com/2006-05-26/news/0605260327_1_gunshot-residue-forensic-evidence-analysis
  6. Trimpe, Michael and Wright, Diana. “Summary of the FBI Laboratory’s Gunshot Residue Symposium, May 31 – June 3, 2005.” Forensic Science Communications, July 2006.

Julia Patterson is a Forensic Scientist for RJ Lee Group, Inc. She is an expert in trace evidence and has qualified as an expert witness in gunshot residue analysis in four U.S. states. RJ Lee Group, Inc. 350 Hochberg Road, Monroeville, PA 15146; (724) 325-1776; www.rjleegroup.com

 

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