This study addresses the need to provide comprehensive, well-controlled studies that produce practical data on the basic properties of touch DNA evidence in real-world environments that indicate how long a touch DNA sample would be expected to persist in a particular environment.
As DNA detection methods have increased in sensitivity and sophistication over the past 20 years, it has become possible to obtain DNA profiles from samples with very low DNA concentrations, including samples left in transfers from an individual who touches an object at a crime scene, referred to as “touch” DNA. In the current study, touch DNA collected from donors and commercially purchased control DNA were both deposited on several representative surface types; and the impacts of temperature, humidity, ultraviolet light irradiation, and exposure time on the DNA quantity and quality in the touch evidence were determined. DNA persistence was assessed by several metrics, including 1) total DNA concentrations as measured by quantitative polymerase chain reaction, 2) degradation index as determined by the ability to amplify longer DNA targets, and 3) suitability for DNA typing analysis by short tandem repeat (STR). Five different environmental conditions were established over the course of each experiment. DNA samples were collected from the stainless steel bolts by swabbing the area of target deposition with BODE swabs. Descriptions are provided for DNA purification, sample concentration, and DNA quantification, as well as statistical analysis. Collectively, these studies provide the most comprehensive information to date regarding basic properties of touch DNA evidence and enable improved recommendations for the forensic science community regarding best practices for the interpretation and evaluation of touch DNA evidence. These studies also yielded some recommendations for best practices when working with touch DNA samples in a laboratory setting. 6 figures, 3 tables, and 31 references