Investigating the use of a Vacuum Metal Deposition (VMD) chamber in locating DNA on evidentiary items

Vacuum metal deposition (VMD) is a fingerprint visualisation technique that involves the evaporation of minute metal particles onto a substrate, which bind preferentially to fingerprint residues, allowing visualisation of touch marks. Since touch DNA is invisible to the naked eye, and there is a current difficulty in DNA sampling clothing items for areas where humans have been contacted, VMD can be used as a targeted sampling technique. This is presumed off the basis of Locard’s exchange principle, whereby ‘every contact leaves a trace’. The aim of this study was to explore the results that a VMD chamber can produce that may allow targeted DNA sampling on clothing. A five-stage, sequential methodology was employed. Experiments 1, 2 and 3 used eight common clothing types, cut into squares with fingerprints deposited and processed by silver-zinc VMD (VMDAg/Zn) to test the effects of fabric composition, positional bias, time since deposition (TSD) and multiple deposits. Experiment 4 involved four participants wearing two clothing pieces and being contacted in five different ways each, to determine if VMDAg/Zn could differentiate between areas of wearer and contactor marks for subsequent DNA analysis. Experiment 5 involved storing the previous experiments’ exhibits for four weeks to explore the effects of storage and time on VMDAg/Zn degradation. Results showed that the best fabric for VMDAg/Zn contact mark visualisation were semi-synthetic, porous, tightly woven, white and preferably contained elastane or polyester. These results also confirm VMDAg/Zn to perform equally as well on light-coloured substrates as the previously reported gold-zinc VMD (VMDAu/Zn). Further, positional bias and TSD were not shown to effect VMD results, however multiple overlaps of deposits, wearer DNA, swabbing and storage times all contributed to both decreased fingerprint quality and DNA profile yield. Whilst some of the results found in this study are positive, the disadvantages show that although visual targeting of contact marks is viable via VMD, DNA extraction may not be a successful option. More research should focus on different sampling techniques to alleviate this issue.