UC Davis

Traditionally, forensic evidence items have been sampled using methods such as the double swab technique and any resulting profiles have been deconvoluted by conventional methods. However, some evidence is difficult to sample using traditional forensic methods. For example, the double swab technique, a wet swabbing followed by a dry swabbing, may not effectively collect biological evidence from large surface areas such as a shirt or from grooves and crevices of rough surfaces. In addition, the ability of either the wet or dry swab to absorb liquid could potentially impact how much DNA is retrieved from either swab for later DNA testing. The M-Vac® Wet-Vacuum System is a sampling technique that can cover large, rough, and/or porous surface areas that could otherwise prove challenging from which to collect cells. However, as sampling methods continue to be improved upon so that they collect more material, complex mixtures are more likely to be obtained. Complex mixtures can be difficult for an analyst to interpret manually. A tool that can aid an analyst in deconvoluting mixtures is STRmix™, a probabilistic genotyping software program. Probabilistic genotyping utilizes algorithms which apply statistical theory, biological modeling, and probability ratios to generate likelihood ratios for genotypes in a single-source or mixed DNA sample. Hence, STRmix™ probabilistic genotyping software was used to evaluate these complex mixtures. Neat saliva from four donors was prepared in an equal mixture ratio by volume and diluted to 1:3, 1:10, 1:30, and 1:100. For each dilution, 1 mL was pipetted onto three 6” x 6” sterile cotton swatches, dried, and then sampled using the M-Vac® to evaluate the robustness of the M-Vac® collection system. Three additional 1:3 saliva sample swatches were prepared and sampled using the double swab technique. Control samples were used to estimate how much DNA is lost from sampling, and the amount of DNA recovered from the different sampling techniques was compared. In addition, six denim swatches were spotted with 1 mL of the 1:3 saliva dilution. Three swatches were sampled with the double swab technique and three with the M-Vac®. Finally, 1 mL of the 1:3 saliva dilution was spotted on six bricks. Three bricks each were sampled with the double swab technique and with the M-Vac®.

This study evaluated the performance and utility of the M-Vac® Pre-Filter system and the M-Vac® system in the collection of DNA from different substrates. The amount of DNA recovered by using the M-Vac® was significantly more than that obtained by the double swab technique from cotton fabric and denim fabric. However, the amount of DNA recovered by using the M-Vac® was similar to that obtained by the double swab technique from brick. In addition, some DNA loss was detected when using the M-Vac® Pre-Filter system. Furthermore, this research determined that mixture samples collected with the M-Vac® from cotton fabric, denim fabric, and brick can be deconvoluted with greater efficiency using STRmix™ than manual interpretation of DNA profiles. More contributors were identified and more mixtures were able to be analyzed when combining the M-Vac® collection technique with STRmix™ probabilistic genotyping.