UC Davis

DNA analysis of touch evidence on cartridge cases has proven difficult despite numerous attempts to improve collection and extraction methods for this important type of evidence. Numerous factors influence the variability and often paucity of touch evidence such as shedder status, pressure and duration of contact, surface type, and even collection and storage. In addition to these factors, retrieval of biological information from cartridge cases is even more challenging since firing abrades the surface of the case, removing material, and metal in the case can react with DNA, resulting in oxidation and cleavage of the DNA backbone. Protein is also present in biological touch evidence. Protein is more robust chemically than DNA, and it also contains genetic and contextual information that could be useful to investigators. Extraction and analysis of both DNA and proteins from touch evidence can provide the maximum amount of information needed for probative results, especially when DNA is limited. To fully exploit all available biological information on a cartridge case, all types of information need to be transferred from a surface into a technical workflow. Additional methods need to be developed to maximize retrieval of material and to do so in a compatible manner. This study evaluated the efficiency of transferring artificial DNA and protein from a glass microscope slide surface into the workflow for seven different collection and transfer methods: the standard wet-dry cotton swab was compared with alternative methods, a ‘Copan microFLOQ® direct’ swab, with and without lysing agent, and with and without extraction buffer, a cell scraper, and adhesive silicone gel-film sheets. Based on the results obtained, it was determined the ‘Copan microFLOQ® direct’ swab was the overall most efficient and consistent with an average transfer of 58 ± 22% efficiency for DNA and 55 ± 32% for protein compared to 27 ± 24% and 2 ± 27% when using cotton swabs respectively. The evaluation was taken a step further to measure transfer of DNA and protein from real fingermarks on a glass microscope slide to the workflow. It was determined that the cotton swab and the ‘Copan microFLOQ® direct’ swab collected an average of 11.8 ± 20 ng and 15.7 ± 18 ng of total DNA, respectively, and 1200 ± 1200 relative densitometry units (RDU) and 2300 ± 1600 RDU of total relative protein density, respectively. This level of relative variation is high but consistent with previous studies for fingermark deposition. When normalized for individuals the ‘Copan microFLOQ® direct’ increased performance by 1.7-fold (p=0.23) for DNA and 1.9-fold (p=0.02) for protein. Finally, real-life scenarios were mimicked where the cotton swab and ‘Copan microFLOQ® direct’ were used to transfer DNA and protein from unfired and fired cartridge cases into the technical workflow. The cotton swab and the ‘Copan microFLOQ® direct’ swab collected an average of 1.1 ± 1.5 ng and 2.8 ± 2.4 ng of total DNA, respectively, and 660 ± 380 RDU and 1200 ± 1500 RDU of total relative protein density, respectively for unfired cases. When fired there was a significant reduction in the amount of DNA material, but the same trend was observed. The cotton swab and the ‘Copan microFLOQ® direct’ swab collected an average of 0.3 ± 0.5 ng and 0.5 ± 0.6 ng of total DNA, respectively, and 400 ± 340 RDU and 530 ± 450 RDU of total relative protein density. When normalized for individuals the ‘Copan microFLOQ® direct’ increased yields by 3.2-fold (p=0.03) for DNA and 2.0-fold (p=0.06) for protein when unfired and 3.7-fold (p=0.09) for DNA and 1.6-fold (p=0.24) for protein when fired. The firing process was harsher on DNA than on protein. Protein was 2.2-fold (p = 0.07) more persistent than the DNA on cartridge cases after firing. We conclude that transfer of residual biological material from fingermarks is improved when using ‘Copan microFLOQ® direct’ swabs compared to the standard wet-dry cotton swab method, and that the improvement applies to both idealized surfaces such as microscope slides, and challenging surfaces such as unfired and fired cartridge cases. We also observe that protein is more stable during the firing process and potentially can provide additional identifying and contextual information for investigators for this difficult type of evidence.