Assessment of Diamond™ Nucleic Acid Dye for the identification and targeted sampling of latent DNA in operational casework

Abstract

Recovery and DNA profiling of latent touch DNA deposits is a ubiquitous practice by operational forensic laboratories that provides critical evidence in many criminal investigations. Despite recent improvements in the sensitivity of contemporary DNA profiling kits, the inability to localise and visually quantify touch DNA deposits on an exhibit means that ineffective or unwarranted sampling is often performed leading to poor success. Diamond™ Nucleic Acid Dye (DD) is a fluorescent DNA binding dye which has recently been shown to bind to corneocytes enabling visualisation and targeted sampling of touch DNA deposits under controlled conditions. The ability to translate these findings to operational casework, where a diverse range of substrates is encountered and the amount and distribution of touch DNA is uncontrolled, is currently unknown. Here, we provide the first report on the use of DD in an operational context. Spraying items with DD was shown to have no impact on downstream immunological testing, DNA extraction, or DNA profiling with the GlobalFiler™ PCR amplification kit. DD was shown to effectively locate areas of touch DNA on select exhibits using the Polilight. Issues with background fluorescence, non-specific staining, interference from fingerprint enhancement reagents, or absorbance of the excitation light by black surfaces demonstrated that DD is not compatible with all exhibits. Background fluorescence also prevented the use of DD to screen for the presence of cellular material on IsoHelix swabs post-sampling but it was suitable for screening Lovell DNA tapelifts. A casework trial of 49 plastic bag and tape exhibits showed limited application of DD to triage out negative items as DNA was recovered from items where DD fluorescence was not detected. Where DD fluorescence was detected, its broad distribution prevented targeted sampling and any correlation to be made between the amount observed and DNA yield or profiling outcome. The DD procedure also increased the time taken to search exhibits and risk of inadvertent contamination. Our study suggests that DD is not suited as a generalised screening technique across all touch casework exhibits but further investigation is warranted to determine its applicability to specific exhibit types.