Collecting, transporting and processing DNA on fired cartridge cases

Collection and packaging

One cause of poor DNA recovery in fired casing testing is improper casing handling, packaging, and transportation. It is common practice to pack casings in paper envelopes or small cardboard boxes for transport from the crime scene to the laboratory, but this type of packaging creates conditions in which the casing may rub or otherwise come into contact with internal surfaces. A test showed that 56% of the DNA was lost³.

“Overall, this study shows that DNA transfer (i.e., loss) within the package is not just a possibility, but a very likely event under certain conditions,” the study authors concluded.

Given these results, some manufacturers have redesigned case packaging and transportation to avoid the problem. For example, the patented Case Grabber tool provides crime scene investigators with a unique way to safely collect, package and transport spent cases. The housing is held and caught from the inside by specially developed stainless steel spring tweezers. A clear plastic container (tube) is then snapped onto the device handle, which protects the housing surface from contact. The housing is held securely and the tool with the protected housings is safely transported to the laboratory. Once ready for processing, the protective tube can be removed and the cover can then be wiped down, etc.

While the potential benefits are clear, there are several challenges and considerations to consider when wiping fired cartridge cases for DNA.

Best practices for DNA recovery from bullet casings

To maximize the chances of recovering usable DNA from fired cartridge cases, several best practices should be followed:

Shell casings should be collected as soon as possible after the shooting to minimize environmental damage to any DNA present.

• Proper handling and packaging

Crime scene technicians and evidence handlers should wear appropriate personal protective equipment and avoid touching the surfaces of the enclosures to prevent contamination. An effective method is to pick up the case from the inside. Packaging of evidence should be limited to methods and materials that prevent contact with the exterior surfaces of the cases during transport.

A double swab technique, using a wet and then a dry sterile swab, has been shown to be effective for sampling the entire surface of the enclosure.

Laboratories should use DNA extraction methods that are optimized for samples with low template content and may include additional steps such as the use of spin baskets to maximize DNA recovery. Highly sensitive PCR amplification kits for touch DNA samples could be used to increase the chances of obtaining actionable profiles.

The use of probabilistic genotyping software for interpretation can also help maximize the probative value of complex, low-level DNA profiles often obtained from bullet casings.

Future directions

As technology continues to advance, the potential for recovering DNA from fired cartridge cases is expected to improve. Promising areas of research include techniques that allow DNA amplification directly from the evidence, bypassing the extraction step – which could increase sensitivity and reduce the risk of sample loss.

With increasing success rates and technological advances, bullet casing evidence is expected to become an increasingly valuable tool in criminal investigations. By following best practices – including using collection, packaging and transport tools to protect external trace evidence – and taking new developments into account, forensic laboratories can maximize the chances of recovering important DNA from these small but significant pieces of evidence.

The ability to directly link a suspect to a crime scene through DNA on a fired cartridge case could be the key to solving otherwise difficult cases. Although not every cartridge case will provide a useful profile, the ability to uncover critical evidence in even a fraction of cases justifies the effort of routine DNA swab sampling of fired cartridge cases.

About the Author: Alexander Jason holds a Master of Forensic Sciences (Honours) from National University and is a long-time, board-certified Senior Crime Scene Analyst. He is also a Fellow of the American Academy of Forensic Sciences and a technical advisor to the Assoc of Firearm and Toolmark Examiners. Jason’s research has been published in many forensic science journals, including the American Journal of Forensic Medicine and Pathology and the FBI’s Forensic Science Communications.

References

1. Recovery of DNA from fired and unfired cartridge cases: Comparison of two DNA collection methods, Elwick, Kyleen et al., Forensic Science International: Genetics, Volume 59, 102726

2. Montpetit S, O’Donnell P. An optimized method for obtaining DNA from fired and unfired ammunition. Forensic Science Int Genet. 2015 July;17:70-74. doi: 10.1016/j.fsigen.2015.03.012. Epub March 25, 2015. PMID: 25828369.

3. An In-depth Analysis of the Root Causes of Interpretation Errors, Peter Gill, Misleading DNA Evidence, Academic Press, 2014,