Gunshot residue

Gunshot residue (GSR), also known as cartridge discharge residue (CDR), gunfire residue (GFR), or firearm discharge residue (FDR), consists of all of the particles that are expelled from the muzzle of a gun following the discharge of a bullet. It is principally composed of burnt and unburnt particles from the explosive primer, the propellant (gunpowder), and vaporized lead. The act of firing a bullet incites a very violent explosive reaction that is contained within the barrel of the gun, which can cause the bullet, the barrel, or the cartridge to become chipped. Meaning gunshot residue may also included metal fragments from the cartridge casing, the bullets jacket, as well as any other dirt or residue contained within the barrel that could have become dislodged.

A graphic representation of the GSR left on a target when fired upon from varying ranges.
Gunshot residue from a pistol shot

Law enforcement investigators will swab people's hands to look for gunshot residue if they are suspected to have discharged a firearm themselves or were in close contact with one at the time discharge. In order to figure out if GSR is present in an area, chemical tests, such as the Modified Griess test and the sodium rhodizonate test are performed. Any GSR samples are collected by swabbing with 5% nitric acid solution and onto a SEM disk for a confirmatory test with scanning electron microscopy. Scanning electron microscopy dispersive X-ray spectrometry (SEM-EDX) is used to determine the presence of gunshot residue as a confirmatory test. There are both inorganic and organic components in GSR. Organic GSR primarily consists of compounds classified as explosives or additives based on their chemical composition and they are analyzed by analytical techniques.

History

Before the use of the scanning electron microscope, hot paraffin wax was used to take a cast of the suspect’s hand. The cast was then sprayed with a reagent giving a coloration with nitro-compounds from the partially burnt and unburnt propellant particles. This approach, introduced in 1933 by Teodoro Gonzalez of the Mexico City Police Laboratory, is called dermal nitrate or paraffin test and is no longer used in casework.

In 1971 John Boehm presented some micrographs of gunshot residue particles found during the examination of bullet entrance holes using a scanning electron microscope. If the scanning electron microscope is equipped with an energy-dispersive X-ray spectroscopy detector, the chemical elements present in such particles, mainly lead, antimony and barium, can be identified.

In 1979 Wolten et al. proposed a classification of gunshot residue based on composition, morphology, and size. Four compositions were considered characteristic:

The authors proposed some rules about chemical elements that could also be present in these particles.

Wallace and McQuillan published a new classification of the gunshot residue particles in 1984. They labeled as unique particles those that contain lead, antimony, and barium, or that contain antimony and barium. Wallace and McQuillan also maintained that these particles could contain only some chemical elements.

Current practice

The most definitive method to determine whether a particle is characteristic of or consistent with GSR is by its elemental profile. GSR mostly derives from its primer cap mixture of components, which includes an explosive, oxidizer, fuel, etc. Lead-based primers are commonly used and they contain shock sensitive explosive lead, oxidizer barium nitrate, and antimony sulfide fuel.[1] These particles can be detected through a combination of scanning electron microscopy/energy (SEM) and dispersive X-ray spectroscopy (EDS):[2] Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM-EDX). The SEM first locates the particulates by looking for the right size and morphology, then the composition is analyzed by EDS.[2]

An approach to the identification of particles characteristic of or consistent with GSR is to compare the elemental profile of the recovered particulate with that collected from case-specific known source items, such as the recovered weapon, Cartridge cases or victim-related items whenever necessary. This approach was called ‘‘case by case’’ by Romolo and Margot in an article published in 2001. In 2010 Dalby et al. published the latest review on the subject and concluded that the adoption of a "case by case" approach to GSR analysis must be seen as preferable, in agreement with Romolo and Margot.

In light of similar particles produced from extraneous sources, both Mosher et al. (1998) aima et al. (2012) presented evidence of pyrotechnic particles that can be mistakenly identified as GSR. Both publications highlight that certain markers of exclusion and reference to the general population of collected particulate can help the expert in designating GSR-similar particles as firework-sourced.

Particle analysis by scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy detector is the most powerful forensic tool that investigators can use to determine a subject's proximity to a discharging firearm or contact with a surface exposed to GSR (firearm, spent cartridge case, target hole). Test accuracy requires procedures that avoid secondary gunshot residue transfer from police officers onto subjects or items to be tested, and that avoid contamination in the laboratory.

The two main groups of specialists currently active on gunshot residue analysis are the Scientific Working Group for Gunshot Residue (SWGGSR) based in USA and the ENFSI EWG Firearms/GSR Working Group based in Europe.

Results

A positive result for gunshot residue indicates the SEM-EDX analysis found traces of distinctive chemicals on the clothing, surface, or person’s skin.[3] This means the object or person was nearby when a gun was fired. Another possible indication of a positive result on a person’s skin could be from touching something that was around the gun when it was fired. (For example: When a person goes to the aid of a victim of a gunshot wound, some gunshot residue particles can transfer from the victim.)

A negative result on someone could mean they were near it but not close enough for gunshot residue to land on them, or it can mean that the gunshot residue deposited on them wore off.[lower-alpha 1] Gunshot residue can also be removed from surfaces by washing, wiping, or brushing it off, so a negative result cannot fully rule out a gun was not fired by the tested object or area.[3] There are many possible scenarios that could explain the results, but they all require more evidence to be proven. Gunshot residue does not travel very far because the particles produced are of a small size and small mass, causing them to lack momentum. Depending on the type of fire arm and ammunition used, it will typically travel no farther than 3–5 feet (0.9–1.5 meters) from the muzzle of the gun.

Matching gunshot residue to a specific source

If the ammunition used was specifically tagged in some way by special elements, it is possible to know the cartridge used to produce the gunshot residue. Inference about the source of gunshot residue can be based on the examination of the particles found on a suspect and the population of particles found on the victim, in the firearm or in the cartridge case, as suggested by the ASTM Standard Guide for gunshot residue analysis by scanning electron microscopy/energy dispersive X-ray spectrometry. Advanced analytical techniques such as ion beam analysis (IBA), carried out after scanning electron microscopy, can support further information allowing one to infer about the source of gunshot residue particles. Christopher et al. showed as the grouping behaviour of different makes of ammunition can be determined using multivariate analysis. Bullets can be matched back to a gun using comparative ballistics.

Organic gunshot residue

Organic gunshot residues (OGSR) are the organic components of a gunshot residue, such as diphenylamine (DPA, a stabilizer).[1] They are not open to secondary transfer, therefore analysis of OGSR provides additional information which could help support the GSR evidence.[2] Organic gunshot residue can be analysed by analytical techniques such as chromatography, capillary electrophoresis, and mass spectrometry. OGSR lays on the skin differently compared to the way inorganic particles of GSR deposits on the skin.[2] Organic compounds can be vaporized when the gun is fired, then re-condense back on the skin surface; these compounds cannot transfer to other surfaces and objects.[2] The compounds can still slowly disappear, but this takes several hours and varies by compound.[1] In general, OGSR analysis is less acceptable to environmental interference than GSR analysis.[2]

Chemical Tests

Gunshot residue can be detected with chemical tests. There are three steps to process an item for gunpowder particles. The first step is to visually examine the object or surface, such as a visual analysis on the appearance of a bullet hole would be documented.

The Modified Griess Test detects nitrite compounds, which are a by-product of the combustion of gunpowder. Forensic examiners use this test to determine the gun to target distance. This test is performed first because it does not interfere with the later sodium rhodizonate test.[4]

The Sodium Rhodizonate test can detect a presence of lead; it results in a red or purple color when lead is present in the tested area.[4] It is a extremely sensitive, specific, and efficient method as it can obtain information on the origin of particulate debris, and it can be done on surfaces or objects.[5] This test can’t determine the precise distance of gun to target, however, it is often used around holes to determine if it is consistent with the passage of a bullet.[5]

See also
  • Blowback, material drawn into the barrel of a firearm post discharge

Notes
  1. Gunshot residue is the consistency of flour and typically only stays on the hands of a living person for 4–6 hours. Wiping the hands on anything, even putting them in and out of pockets can transfer gunshot residue off the hands. Victims do not always get gunshot residue on them; even suicide victims can test negative for gunshot residue.

References
  1. Vachon, Crystina R.; Martinez, Michael V. (September 2019). "Understanding Gunshot Residue Evidence and Its Role in Forensic Science". The American Journal of Forensic Medicine and Pathology. 40 (3): 210–219. doi:10.1097/PAF.0000000000000483. ISSN 1533-404X. PMID 31233396.
  2. Forensic Technology Center of Excellence (FTCoE). (2015). In-Brief: Organic Gunshot Residue Analysis for Potential Shooter Determination. U.S. Department of Justice, National Institute of Justice, Office of Investigative and Forensic Sciences.
  3. "Gunshot Residue Test | NC PRO". ncpro.sog.unc.edu. Retrieved 2023-04-14.
  4. Carroll, James (2018), "The Medical Examiner-Coroner and the Firearms Examiner", Multidisciplinary Medico-Legal Death Investigation, Elsevier, pp. 245–264, retrieved 2023-04-14
  5. Bashinski, J.V., The Evaluation of Gunshot Residue Patterns, the Rhodizonate Test for Lead, 1974, University of California, Berkeley.

Further Information
  • ASTM E1588-10e1, Standard Guide for GSR analysis by Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry, American Society for Testing and Materials, West Conshohocken, PA, 2010.
  • E. Boehm, Application of the SEM in forensic medicine, Scanning Electron Microscopy (1971) 553-560.
  • M Christopher, J Warmenhoven, FS Romolo, M Donghi, R Webb, C Jeynes, NI Ward, A New Quantitative Method for Gunshot Residue Analysis by Ion Beam Analysis. Analyst, 2013, 138, 4649.
  • O. Dalby, D. Butler, J.W. Birkett, Analysis of Gunshot Residue and Associated Materials—A Review, J. Forens. Sci. 55 (2010) 924-943.
  • M. Grima, M. Butler, R. Hanson, A. Mohameden, Firework displays as sources of particles similar to gunshot residue, Science and Justice 52 (1) (2012) 49-57.
  • H.H. Meng, B. Caddy, Gunshot residue analysis - review, J. Forens. Sci. 42 (1997) 553-570.
  • P.V. Mosher, M.J. McVicar, E.D. Randall, E.H. Sild, Gunshot residue-similar particles produced by fireworks, Journal of the Canadian Society of Forens. Sci. 31 (3)(1998) 157–168.
  • F.S. Romolo, M.E. Christopher, M. Donghi, L. Ripani, C. Jeynes, R.P. Webb, N.I. Ward, Integrated Ion Beam Analysis (IBA) in Gunshot Residue (GSR) characterisation. Forensic Sci. Int. 231 (2013), 219-228.
  • F.S. Romolo. Advances in Analysis of Gunshot Residue. In Emerging Technologies for the analysis of forensic traces, Edited by Simona Francese, Springer Publishing Company, pagine 183-202, ISBN 978-3-030-20541-6.
  • A.J. Schwoeble, D.L. Exline, Current Methods in Forensic Gunshot Residue Analysis, (2000) CRC Press LLC.
  • J.S. Wallace, J. McQuillan, Discharge residues from cartridge-operated industrial tools, J. Forens. Sci. Soc. 24 (1984) 495-508.
  • J.S. Wallace, Chemical Analysis of Firearms, Ammunition, and Gunshot Residue, (2008) CRC Press LLC.
  • G.M. Wolten, R.S. Nesbitt, A.R. Calloway, G.L. Loper, P.F. Jones, Particle analysis for the detection of gunshot residue. I: Scanning electron microscopy/energy dispersive X-ray characterisation of hand deposits from firing, J. Forens. Sci. 24 (1979) 409-422.
  • G.M. Wolten, R.S. Nesbitt, A.R. Calloway, G.L. Loper, Particle analysis for the detection of gunshot residue. II: occupational and environmental particles, J. Forens. Sci. 24 (1979) 423-430.
  • G.M. Wolten, R.S. Nesbitt, A.R. Calloway, Particle analysis for the detection of gunshot residue. III: the case record, J. Forens. Sci. 24 (1979) 864-869.
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