Implementing ATR-FTIR spectroscopy and chemometrics for the forensic examination of sexual lubricants and their traces

Sexual lubricants are likely to be encountered at crime scenes due to the wide availability and variability of the products. These lubricants found in sexual assault cases may help supplement biological evidence or be the primary evidence where there is a lack of DNA evidence. The discrimination of sexual lubricants will help assess the potential contact between the victim and the suspect. In this study, an analysis of a total of 43 products of condom lubricants, bottled sexual lubricants, and personal hygiene products was carried out to evaluate their variability and discrimination potential. ATR-FTIR spectroscopy was used to analyze the samples, and the spectra were then visually examined and interpreted using chemometrics. Visual analysis revealed three different groupings of the samples based on their chemical profile. Chemometric classification was done using LDA and SVM in a two-stage classification process: identification of the type of product and a brand-level classification. The combination of LDA and SVM helped to discriminate the samples further in an objective manner. In stage 1 classification, SVM showed 100% classification accuracy. In the second stage, 70.83% classification accuracy was achieved using SVM for condom samples. Whereas, 96.15% classification was achieved using LDA for bottled lubricants. Evaluation of how these products can be linked to their sources is also needed when they are encountered as traces in various substrates. A substrate study was conducted while considering two factors: environment and time. The findings indicated that the type of substrate, lubricant, and storage period may affect the discrimination.