A study to investigate the persistence of polydimethylsiloxane and to determine the detection of polydimethylsiloxane in a body fluid matrix following Cellmark’s Sperm Elution©

Townsend, Sarah Elizabeth (2018) A study to investigate the persistence of polydimethylsiloxane and to determine the detection of polydimethylsiloxane in a body fluid matrix following Cellmark’s Sperm Elution©. Masters thesis, University of Central Lancashire.

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Lubricant trace evidence stands as associative evidence in sexual assault cases when there is an absence of semen. Polydimethylsiloxane (PDMS) is a substance applied as a lubricant by the majority of condom manufacturers worldwide. As the awareness of sexually transmitted infections has amplified immensely over recent years, the number of perpetrators wearing condoms during sexual assaults has risen also.
Accounts of false positive results arising from lubricant analysis in alleged sexual assault cases are becoming progressively common. This led to suggestions that PDMS may be unintentionally transferred onto blank cotton swabs during manufacturing. This project accumulated a selection of blank swabs from vaginal swab kits and sexual assault early evidence kits. All swabs were extracted with isooctane then analysed using an FTIR microscope and u-ATR accessory. No PDMS was detected on any swabs examined.
Recently, the lubricating jelly used in sexual assault kits has been replaced with a more cost-effective option. Aquagel and OptiLube are the lubricants now found in sexual assault kits. However, manufacturers have failed to report the ingredients incorporated within these lubricants. If PDMS was to be present, this could lead to false positive results in sexual assault cases. A sample of Aquagel and OptiLube were examined using the u-ATR, and both substances were revealed to be water-based lubricants containing no PDMS.
Previous research has indicated that 0.125µl is currently the smallest volume of PDMS detected using FTIR analysis. However, the quantity of PDMS expected on intimate swabs following sexual intercourse with a silicone lubricated condom is unknown. Extracting various volumes of PDMS from swab heads, and analysing using FTIR could potentially produce a quantitative curve. This could allow the volume of an unknown sample of PDMS remaining on intimate swabs to be identified. Results indicated that there is a positive correlation between the amount of PDMS detected and the intensity of the peaks on the spectrum. However, examining samples using a u-ATR and FTIR microscope is not a fully quantitative method due to multiple external factors. For example, the technique is unable to include the whole sample to allow complete analysis.
In current forensic casework, forensic experts don’t usually suggest lubricant analysis if the time that the intimate swabs were taken exceeded 24 hours since the alleged sexual assault. Previous research implied that PDMS was able to be detected in the vagina 48 hours after intercourse; however this research is very outdated. This was explored by participants undertaking sexual intercourse with their partner using a PDMS lubricated condom. Volunteers were then asked to take blind vaginal swabs at specific time intervals. Results indicated that PDMS was able to be detected on all swabs after 6 hours, and only 33% of swabs taken at 12 hours. No PDMS was recovered after 12 hours.
Up to this stage in the project, it can be confidently stated that volumes as low as 0.125µl are able to be detected on the swab head extracts using u-ATR after Sperm Elution. However, it is currently unknown whether the addition of multiple body fluids, in addition to semen, affects whether PDMS is able to persist on the swab head. Eight different body fluid combinations of semen, blood, saliva, vaginal material and faecal material were seeded onto blank cotton swabs alongside 2µl of PDMS. Swabs were then sperm eluted and extracted with isooctane. PDMS was able to be detected using the u-ATR on all of the swabs examined. This information will aid in the interpretation of casework results involving scenarios whereby multiple body fluids may be present.

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