Improved strategies for sampling and mixture separation of trace biological materials

Thesis with publications.

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy by published work to the Department of Genetics, Institute for Molecular Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Effective collection of biological samples is paramount in forensic investigations as DNA profiles have become powerful tools in exculpating and inculpating individuals. Collecting mixed samples containing more than one profile decreases the probative value of evidence. There is a paucity of data informing selection of the best collection methods, especially for trace quantities of biological material. This thesis evaluated the efficacy of a range of commonly used collection devices and methods and assessed novel methods of collection and mixture separation prior to extraction. Swab types were compared for the collection of different biological fluids, plus touch DNA, from a variety of substrates. Tests were also performed to compare yields from frozen and air-dried swabs. Findings led to recommendations for the selection of swab types for specific sample/substrate combinations. The efficacy of two tapes for collection of biological evidence from various fabrics was compared, both to each other and to swabbing. Significant differences were found between the two tape types, and swabbing was more effective than taping in some instances. The data characterised a threshold, related to tape adhesion, beyond which taping will no longer be effective. Gluelifting with the Pinpoint DNA Isolation System was evaluated as a method for sampling DNA from non-porous surfaces. Its yield was comparable to, and less variable than, swabbing from rough surfaces. A novel strategy was designed and tested to differentially tapelift DNA from separate, layered deposits on either the same or reverse side of fabric. Although the study did not yield separated profiles, it highlighted the extent of migration of biological material through some porous substrates. Preliminary assessment of flow cytometric cell separation for mixtures of human biological fluids using cell-specific antibodies was conducted. This effectively recovered dropped-out profiles from mixed samples. Cumulatively, these findings provide significant opportunities for forensic practitioners to enhance the recovery and probative value of trace samples and thus better service the justice system.