Determining Time Since Deposition for Human Saliva Stains Using Non-Genetic Signatures

Sexual assault cases are common within the forensic community and are constantly backlogged within laboratories for examination. One biological fluid, saliva, can often be present in context with the sexual assault due to the nature of the crime. The question used to be who’s DNA is present, but now since DNA analysis has become much more advanced, the new question of how long has the stain been there, arises. Four donors were used to create four different time point environments including 3 days, 7 days, 1.5 months (41 days), and 3 months (84 days) which were then collected with cotton swabs, one dry and one wet, and run through the AMNIS Imaging Flow Cytometer. By utilizing Imaging Flow Cytometry of these human saliva samples, autofluorescence pictures of the saliva samples from different time intervals were taken and analyzed using IDEAS analysis software to generate feature values for examination. After analysis of the feature value data, 3 and 7 day groups were placed together, while 41 and 84 day groups were placed together as well due to overlapping similarities in feature values. The main feature value of note was Intensity, specifically in channels 1 and 2 of the IFC. Intensity increased between the two grouped time points, which may be resulting from degraded amino acids revealing more surface area for autofluorescence when excited by the light laser in the channel. Future research can hopefully examine the amino acid content variability between human saliva samples over time