Evaluation of Different Swab Wetting Chemicals Affecting the Yield of DNA Obtained From Biological Evidence on Cartridge Casings

Abstract

Cartridge casings made from transition metals can be examined ballistically and also serve as significant evidence by containing touch DNA. However, the success rate of profiles obtained from this type of evidence is generally low. To enhance the success of DNA profiling from suspects' biological evidence, using swabs moistened with chemicals can be beneficial. Typically, swabs are moistened with water, whose hypotonic nature disrupts cell integrity, causing the release of DNA. However, water is not the only agent used for moistening swabs; various buffer solutions are also utilized. The ability of swabs to transfer touch DNA depends on the type of buffer solution used. Sodium dodecyl sulfate (SDS), a strong anionic detergent, denatures non-covalently linked secondary and tertiary structures increasing the release of bound DNA. Another buffer solution used for swab moistening is the Te+4 buffer, which contains EDTA and Tris. EDTA chelates metal ions, inactivating enzymes that could potentially damage DNA, while Tris adjusts the pH to an optimal level. This study aims to compare the effectiveness of microfiber and cotton swabs moistened with SDS, Te+4 buffer, and water in recovering genetic material from blood and epithelial cells deposited on brass cartridge casings. The study also evaluates the impact of firing on the quality of DNA profiles by analyzing the RFU difference obtained on cartridge case and cartridges. Although the number of complete profiles obtained from water- and SDS-wetted swabs are equal, the average RFU value of SDS-wetted swabs is approximately twice that of water-wetted swabs. The minimum number of complete profiles belongs to swabs wetted with Te+4 buffer. SDS is particularly advantageous over water when used on casings with epithelial cells. Microfiber swabs are more effective in eliminating degradative factors caused by firing, thus enhancing profiling success. Comparisons of the RFU values indicate that casings yield lower values compared to cartridges, supporting the negative impact of the high heat, pressure, and residues generated during firearm discharge.

Keywords

• Ballistics
• Swab wetting chemical
• DNA profiling
• Identification
• Touch DNA

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