Roland A.H. van Oorschot

1. Contact force (or pressure) is rarely a controlled variable in DNA transfer studies, including shedder tests, even though it is generally accepted that higher pressure increases DNA transfer1–4....

Swabs are commonly used in forensic contexts to recover biological material from surfaces. Previous investigations have shown that there is no single best swab and wetting agent combination for DNA recovery. Touch DNA is often present in low quality and quantity, and the recovery and processing methods used during casework can result in DNA loss, therefore even small improvements in recovery could influence successful detection. Therefore, a DNA recovery method tailored for a touch DNA workflow is fundamental to effective forensic casework. In this study, combinations of five swabs (viscose, cotton, nylon, foam and polyester) and three wetting agents (water, 100% isopropanol (IPA) and 2% sodium dodecyl sulphate (SDS)) were used to collect samples of synthetically and naturally derived touch DNA from a polypropylene surface. Surfaces were swabbed using a single systematic swabbing method and extracted using the DNA IQ™ system. Interactions between the swabs and wetting agents had the greatest effect on DNA recovery, and polyester swabs moistened with SDS were among the combinations with the highest DNA yield under these experimental conditions. The worst-performing swabs differed by DNA type sampled. Polyester, viscose and cotton swabs moistened with water were amongst the combinations yielding the lowest DNA recovery for the synthetic touch deposits, whereas foam and nylon swabs moistened with IPA had the lowest yield for natural touch deposits. This research aims to contribute towards the development of a standardised touch DNA recovery method applicable to the sampling of polypropylene surfaces and other relevant touch DNA collection related objectives, such as investigations into DNA transfer and shedder testing.