In forensic science, swabs are routinely used to recover biological material. However, previous studies have demonstrated that there is no universal swab type and wetting agent combination that maximises DNA recovery across all sample types. These findings emphasise that the type of biological material, surface characteristics, and extraction protocol must be considered when selecting a swab and wetting agent combination for a specific sample type. Touch DNA samples are commonly submitted for forensic DNA analysis and typically contain small quantities of DNA. Methods for recovering and processing DNA during casework can result in loss; therefore, optimising recovery methods for this type of biological material is especially important, as even slight improvements in DNA yield can influence detection success. 

This study aims to contribute to the development of a standardised touch DNA sampling method by proposing a standardised swab and wetting agent combination optimised for the recovery of touch DNA from commonly encountered polypropylene surfaces. To achieve this, we tested the efficiency of five swabs (cotton, viscose, nylon, foam, and polyester) and three wetting agents (water, 100% isopropanol (IPA) and 2% sodium dodecyl sulfate (SDS)), in combination, to collect samples of synthetically and naturally derived touch DNA. Samples were recovered using a standardised single swab method and extracted using the DNA IQTM system and quantified with QuantifilerTM Trio.

Results showed that extraction efficiencies can be influenced by the addition of wetting agents, with IPA and SDS increasing DNA yield compared to dry or water-moistened swabs. Interactions between the swabs and wetting agents had the greatest effect on DNA recovery, with polyester swabs moistened with SDS yielding the highest recovery overall from all sample source types. The worst-performing swabs differed by sample source DNA type. Polyester, viscose, and cotton swabs moistened with water yielded the lowest DNA recovery for the synthetic touch deposits, while foam swabs moistened with water and IPA yielded the lowest recovery for the natural touch deposits. These results also show that synthetic and natural deposits behaved differently, suggesting that improvements to synthetic solutions are needed to better mimic natural deposits. This study has provided valuable insight into the recovery of touch DNA from plastic surfaces, information applicable to crime scene surfaces and other relevant touch DNA collection objectives, such as investigations into DNA transfer and shedder testing.

Authors

• Hilary Arsenault (Flinders University, Australia)
• Roland van Oorschot (Office of the Chief Forensic Scientist, Victoria Police Forensic Services Department, Australia)
• Duncan Taylor (Forensic Science SA, Australia)
• Adrian Linacre (Flinders University, Australia)
• Mariya Goray (Flinders University, Australia)