Originally written in Forensic Magazine. I suggest you sign up for a subscription if you are interested in interesting forensic science articles.
The Controversy Concerning Gunshot Residues Examinations
Dennis L McGuire, M.S.
August/September 2008
Lawyers, judges, and juries can be seriously misled by crime laboratory findings of the presence of gunshot residues (GSR).
Gunshot residues are defined in the Association of Firearms and Toolmark Examiners (AFTE) Glossary1 as “ The total residues resulting from the discharge of a firearm, it includes both gunpowder and primer residues, plus metallic residues from projectiles, fouling etc.”
To most AFTE members, this definition is reasonably understood; however, gunshot residues are often confused by members of the Criminal Justice System. The metallic cartridge (one round of ammunition) that is loaded into the chamber of a firearm consists of a bullet (projectile), gunpowder (propellant), a primer (igniter), and a cartridge case which contains all of the components before firing. When the primer is struck by the firing pin, a flash of hot gases ignites the gunpowder causing it to deflagrate (“explode”). The bullet is propelled from the cartridge case and forced through the gun barrel by the expanding gases.
Attorneys and judges often confuse gunshot residue examinations of primer particles containing lead (Pb), barium (Ba), and antimony (Sb) with examinations of the remaining elements of gunshot residue. Although the examinations of primer particles and those of gunpowder residues are examinations of gunshot residues, they are examined in very different ways. Primer particle examinations utilize instrumental techniques while gunpowder residues utilize visual and chemical methods.
The examination of gunshot residue primer particles is currently a controversial issue. Beginning with the guidelines given by the Aerospace Report of 1977,2 various GSR examiners have developed techniques, methods, and most importantly, opinions concerning the significance of their findings. There is a wide range of opinions offered in cases around the country.
As GSR examiners have advanced over the past forty years, there is little to suggest they have advanced in concert. The standards for expressing an opinion in one crime laboratory may be very quantitatively different than the standards in another laboratory. Fundamentally, GSR examiners are seeking microscopic particles containing lead (Pb), barium (Ba), and antimony (Sb) which, for the purposes of this article, are found uniquely in combination only in the primers of firearms cartridges. A particle composed of PbBaSb, fused together in a single unit, which exhibits the correct morphology (shape and appearance) to an experienced examiner is termed a unique particle. The examination itself consists of computer-controlled scanning electron microscope examinations of samples from suspect areas (commonly suspect shooter’s hands) to locate the particles coupled with an X-ray analyzer and an EDS, for energy dispersive analysis of X-rays to determine the elemental composition of the particles. In their book Current Methods in Forensic Gunshot Residue Analysis, A.J. Schwoeble and David L. Exline3 offer a thorough explanation of the process.
In 2002, a paper was published by Torre et al.4 reporting the finding of rounded particles of PbBaSb in automotive brake lining and wear products. Prior to this finding, it was generally accepted that the three element particle was truly unique to GSR. This has caused most GSR examiners to downgrade the three parts particle from “unique” to “characteristic,” albeit the most decisive of the characteristic particles. For the purposes of this article, the term unique will continue to be used to discriminate the three parts particle from the others.
Other particles of two (fused together) or one of the three elements, with the correct morphology, are called characteristic particles because they can be found in other sources in the environment. The logic that follows this is that you must have unique particles to opine that the source of the elements is from gunshot residue and that characteristic particles support the unique. The controversy now begins: How many unique and characteristic particles are needed to opine that gunshot residue is identified? The answer to this question varies widely from lab to lab across the United States and in the United Kingdom.
Contamination is the major concern when reporting GSR results. GSR particles are more delicate than the finest talcum powder and they are invisible to the eye. GSR particles can be transferred from surface to surface by contact, air movement, abrasion, and washing. Numerous studies for the presence of GSR particles in police cars, police stations, police equipment, and occupational environments have found that sources of contamination are abundant. To report the finding of a single unique particle of GSR on a person as a positive finding for gunshot residue is akin to reporting that the finding of a single grain of sand upon a person’s shoe is a positive determination that that person had recently been on a beach.
The American Society of Crime Lab Directors/Laboratory Accreditation Board (ASCLD/LAB)5 is responsible for the inspection and accreditation of crime laboratories. ASCLD/LAB sends inspection teams to crime laboratories which are seeking accreditation to determine if the laboratory is in compliance with currently accepted standard operating procedures, methods, and reporting results. One important procedure that is not on ASCLD’S check list however, is the standard for issuing an opinion concerning gunshot residue based upon primer particles analysis.
In February 2008, I contacted an employee of ASCLD/LAB to determine their position on this issue6 and was informed that ASCLD inspects for generally accepted practices and validation studies concerning the methodology for the examination of the evidence. It is not their practice to set threshold standards for testing, but it is their approach to require labs to not report misleading results, or to properly qualify any results which could be misleading or misused by the laboratory’s customers. The ASCLD/LAB Board of Directors has appointed one of its members to evaluate the practice of reporting GSR test results in ASCLD/LAB accredited laboratories.7
The Federal Bureau of Investigation attempted to establish the criteria necessary for reporting a positive GSR result by conducting a Gunshot Residue Symposium from May 31 through June 3, 2005. A number of issues concerning GSR examinations were discussed including the FBI laboratory threshold for a conclusion of exposure to gunshot residue. In the summary of the symposium, issued in July 2006,8 it is written “The number of particles used to confirm a GSR population is a minimum of three PbBaSb particles. Additionally, other particles consistent with a GSR-type environment must also be present namely SbBa, BaPb, PbSb, and/or other elements or element composites routinely found in GSR particle populations.” Within this same summary, an examiner from the U.S. Army Crime Laboratory reported that their requirement was a minimum of four particles because of the logical propensity for the military environment to have higher GSR background levels.
In January 2006, I spoke with a representative of the Forensic Science Service in the United Kingdom, concerning their criteria for a conclusion.9 He stated that one to three particles (three parts) would be reported as “low levels” of GSR and would be termed as inconclusive. Two part particles with the correct morphology would not be reported as GSR.
The three above cited internationally known and recognized laboratories are in general agreement concerning the standards to be met. Other lesser known laboratories can choose to establish their own standards at lower levels and may even report one unique particle as the unqualified presence of gunshot residue.
In the conclusion of the Summary of the Gunshot Residue Symposium conducted by the FBI, it is noted “At the conclusion of the symposium several topics remained open for further discussion: namely the use of time limits in case-acceptance criteria and how to standardize the language used to report the presence of GSR particles. Topics such as these are often dictated by individual laboratory policies, as well as the circumstances of the particular case. Therefore it is unlikely that universal guidelines and terminology will evolve for the GSR community in these areas.” The editor’s note below the conclusion states “The FBI recently decided to stop conducting GSR examinations.” The reasons given were the number of increasing examination requests, the probative nature of the requests, and to use the resources previously dedicated to GSR for fighting terrorism.
Jurors, and sometimes judges, can be confused or misled when a crime laboratory reports an unqualified finding of the presence of gunshot residues. They cannot appreciate the significance of contamination if it is not mentioned in the results and without this information the case my go ahead without the gunshot residue findings being challenged.
Some crime laboratories, such as Boston Police Department10 and Broward County Sheriff’s Department11 (Fort Lauderdale, Florida), have chosen to avoid the controversy of GSR completely by simply choosing to not provide the examination service – perhaps the wisest decision. Until a uniform standard, based upon validated studies, for the reported positive determination for GSR is initiated, the findings of positive determinations of GSR should be seriously scrutinized.
The remaining elements of gunshot residue excluding primer particles – gunpowder patterns and gunpowder residues, often called powder burns or powder tattooing, continue to be a useful and reliable source of evidence. The density and dispersion of these residues are valuable in determining distances of close range gunshots. These residues can stand alone and should not be confused with primer particles.
References
1. AFTE Glossary, Fourth Edition, 2001
2. Wolten, G.M., Nesbitt, R.S., Calloway, A.R., Loper, G.L., and Jones, P.F., “Final Report on Particle Analysis for Gunshot Residue Detection.” The Aerospace Corporation, El Segundo, CA, 1977, ATR-77
3. Schwoeble, A.J.,and Exline, David L., Current Methods in Gunshot Residue Analysis, CRC Press, Boca Raton, London, New York, Washington, D.C., 2000
4. Torre, C., Mattutinjo, G., Vasino, V., and Robino, C., Brake Linings: “ A Source of Non-GSR Particles Containing Lead, Barium and Antimony,” Journal of Forensic Sciences (2002) 47:494–504
5. American Society of Crime Laboratory Directors, Laboratory Accreditation Board, 139 J Technology Drive, Garner, NC 27529
6. Neuner, John K., Program Manager, ASCLAD/LAB International. Personal communication, February 2008.
7. The appointment occurred at the February 2008 meeting of the ASCLD/LAB Board of Directors
8. Wright, Diana M. and Trimpe, Michael A. “Summary of the FBI Laboratory’s Gunshot Residue Symposium, May 31-June 3, 2005,” Forensic Science Communications, July 2006 – Volume 8 – Number 3
9. Bowden, Mark, Forensic Science Service, London, England, Personal communication, January 2006.
10. Hayes, Donald, Director, Boston Police Department Crime Laboratory. Personal communication, March 2008.
11. Greenspan, Alan, Criminalist III, Broward County Sheriff’s Office Crime Laboratory. Personal communication, March 2008.
Dennis L. McGuire, M.S. is a Forensic Examiner with Forensic Expert Services, Inc. where he consults in all types of firearms evidence cases, both criminal and civil. Dennis received both Bachelor of Science and Master of Science degrees from Florida Atlantic University. He spent 23 years as a police crime laboratory examiner, including nineteen years as a Police Laboratory Supervisor. Dennis is a Fellow of the American Academy of Forensic Sciences, Distinguished Member of the Association of Firearm and Toolmark Examiners, and a member of the International Association for Identification.
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