Trace evidence characteristics of DNA: A preliminary investigation of the persistence of DNA at crime scenes.

The successful recovery of trace or contact DNA is highly variable. It is seemingly dependent on a wide range of factors, from the characteristics of the donor, substrate and environment, to the delay between contact and recovery. There is limited research on the extent of the effect these factors have on trace DNA analysis. This study investigated the persistence of trace DNA on surfaces relevant to the investigation of burglary and robbery offences. The study aimed to limit the number of variables involved to solely determine the effect of time on DNA recovery. Given that it is difficult to control the quantity of DNA deposited during a hand contact, human buffy coat and DNA control solution were chosen as an alternative to give a more accurate measure of quantity. Set volumes of these solutions were deposited onto outdoor surfaces (window frames and vinyl material to mimic burglary and 'bag snatch' offences) and sterile glass slides stored in a closed environment in the laboratory, for use as a control. Trace DNA casework data was also scrutinised to assess the effect of time on DNA recovery from real samples. The amount of DNA recovered from buffy coat on the outdoor surfaces declined by approximately half over two weeks, to a negligible amount after six weeks. Profiles could not be obtained after two weeks. The samples stored in the laboratory were more robust, and full profiles were obtained after six weeks, the longest time period tested in these experiments. It is possible that profiles may be obtained from older samples when kept in similarly favourable conditions. The experimental results demonstrate that the ability to recover DNA from human cells on outdoor surfaces decreases significantly over two weeks. Conversely, no clear trends were identified in the casework data, indicating that many other factors are involved affecting the recovery of trace DNA. Nevertheless, to ensure that valuable trace evidence is not lost, it is recommended that crime scenes are processed expeditiously.