The influence of contact force on forensic trace collection efficiency when sampling textiles with adhesive tape

DNA is a highly valuable lead to identify people who were possibly involved in a crime. Even by small contact events, minute amounts of DNA ('trace DNA') can be transferred from a DNA source to an evidentiary item, which can be enough for a successful DNA analysis. The focus of this research is to get more insight in the collection of trace DNA from textiles by 'stubbing', which is a tape-lifting method using double-sided tape placed on a stub. The relation between the 'stubbing force' (the normal force that is applied during stubbing) and the collection efficiency of microspheres is investigated.

Microspheres (Ø25 μm) were used as mock traces to mimic DNA-containing micro-traces. The particles were applied to textile substrates in a suspension of ethanol that was left to evaporate before sampling. Experiments were performed on three different polyester substrates. Traces were collected by stubbing while using 5 different stubbing forces. The number of microspheres placed on each substrate was counted before sampling and all stub-tapes were analysed after sampling to count how many of the microspheres were picked up, both by using stitched images from a digital light microscope. Custom-made image recognition software was used to automatically count the microspheres.

On all tested polyester substrates, the mean efficiency of the collection of microspheres increased with increasing stubbing force in a concave down increasing function. The increase of collection efficiency stagnated around 3-12 N, depending on the substrate material. The theoretical maximum collection efficiencies varied between 38% and 78%, depending on substrate material as well.

Stubbing with a force higher than 12 N does not notably influence the collection efficiency from the variety of textiles that were tested. However, because the theoretical maxima of the collection efficiencies were far from 100%, it is highly likely that stubbing multiple times on the same spot of a substrate increases the total collection efficiency. The gained knowledge will help to standardize and improve the effectiveness of stubbing.