We conducted experiments to characterize the quantity of DNA recovered on surfaces using 6 donors with a view to help assigning probabilities to the observation of given quantities of DNA under different transfer scenarios. The donors were asked to conduct a total of 120 simulations involving primary transfer on a knife handle. With 2 selected donors, 60 associated experiments involving secondary transfer were also carried out. DNA recovered on COPAN’s FLOQSwab™ was extracted, quantified and profiled using standard commercial kits. DNA mixtures were subsequently deconvoluted using STRmix™ to obtain the proportion corresponding to the person of interest (POI). The transfer proportion between the quantity of DNA on the bare hands and the amounts recovered on the touched surfaces was also measured and studied.
For a given activity, each donor left varying amounts of DNA amounting to distributions that can be characterized by their means and standard deviations. The quantity of transferred DNA is dependent on the donor and on the type of transfer. Typically, our “best” donor left an average of 0.84ng (SD = 1.23) on a knife handle compared to a mean of 0.07ng (SD = 0.09) for the least “prone to leave DNA” donor. For secondary transfer, we recorded a mean of 0.04ng (SD = 0.11) for the first donor and of 0.002 (SD = 0.01, max = 0.04ng) for the second.
Linked to the above is the observation that the transfer proportion (i.e. the ratio of the quantity of DNA on an hand to the amount of DNA recovered following a transfer) depends also on the donor and on the type of the transfer. Hence the amount of DNA obtained on a given touched surface cannot simply be deduced from the quantity of DNA available on a donor’s hand.
Given these sources of variability, it is not advised to use a single and fixed label, such as “good “or “bad” regardless of the circumstances, to describe a donor’s ability to leave DNA. To properly evaluate the probability of finding a given quantity of DNA the whole variation of DNA quantity should be accounted for. This can be done by using or measuring empirically the appropriate underpinning distribution for that quantity. Note however that it will be conditioned upon the donor, the receiving surface and the transfer mechanism.
We also explored the potential benefit of deconvoluting mixtures to better characterize the quantity of DNA left by the POI as opposed to the total quantity of DNA measured by quantification. Our results show that such deconvolution is beneficial when low quantities of POI’s DNA may be mixed with larger quantities such as in secondary transfer scenarios. For primary transfers on clean surfaces, the touching person will dominate in the recovered DNA and the deconvolution is not critical.