Improving the Recovery of DNA from Fired Cartridge Casings Using Chelation Filtration Method

Gun-related crimes are the cause of many homicides and injuries every year across the United States. According to the Federal Bureau of Investigation statistics, approximately 49,000 people died from gun violence in the United States in 2021. Firearms were used in 81% of murders and 31% of aggravated assaults in 2021, primarily affecting teens and young adults. Cartridges and casings left on the scene often link shootings to a firearm and many times are the only evidence. Traditional swabbing of fired cartridge casings (FCCs) has yielded usable profiles only approximately 1–2% of the time, causing many laboratories to refrain from processing FCCs. Studies have provided increasing evidence that the metallic surface of the casings releases metal ions that bind to the deoxyribonucleic acid (DNA), cause inhibition during the DNA amplification step and potentially degrade the DNA. More recent techniques for improving DNA recovery involve specialized methods for soaking casings during extraction, which can result in additional time and expense. Furthermore, since it has been shown that the presence of metal ions in a sample can result in gradually increasing DNA strand breakage, there is a need to develop a rapid and simplified method to increase DNA recovery. In this article we have developed a quick two-step cellular collection method using Chelex to isolate metal ions, followed by vacuum filtration. This method is designed to be used after crime scene collection of the casings and prior to submission to the DNA laboratory. It enhances the recovery of DNA from casings by up to 40% over the conventional swabbing techniques and can be relatively inexpensive to incorporate into crime scene units in many jurisdictions.