Detection of vaginal fluid stains on common substrates via ATR FT-IR spectroscopy

The analysis of body fluids is of utmost importance in forensic casework since many biological fluids contain DNA. The ATR FT-IR spectroscopy is an emerging approach for the confirmatory, rapid, facile, non-destructive, and on-site identification and differentiation of body fluid stains. Notwithstanding the ATR FT-IR spectroscopy is showing a colossal promise towards the identification of body fluids, and further forensic enquiry related to substrate's interference is still in its infancy stage. Therefore, in the present proof-of-concept study, the ATR FT-IR spectroscopy has been utilized for the detection of vaginal fluid stains and to investigate the effect of different substrates on sample analysis. Simulated vaginal fluid samples were prepared on some selected substrates such as glass, plastic, floor tiles, polished wood, paper, and on various cloth substrates and analyzed without any prior sample preparation. Results suggested that vaginal fluid can be successfully detected on non-porous substrates, but it turned out to be a challenging task on porous substrates. However, on the basis of certain peaks, successful identification of vaginal fluid can be done directly on various case-related substrates. The best approach for the detection of vaginal fluid depends upon the nature of substrates and type of interference encountered. In addition, 10 non-vaginal fluid substances which look similar to vaginal fluid and which may lead to misclassification of vaginal fluid or can deliver false-positive results were also analyzed. The spectra of look-alike substances were classified using the chemometric tools such as PCA and PCA-LDA. The developed PCA model successfully classified all vaginal fluid samples from non-vaginal fluid substances with 100% accuracy, specificity, and sensitivity rate. In addition, the effects of other factors such as aging and mixing with other body fluids have also been studied and the results have been described.