Exploration of ATR FTIR spectroscopy assisted by multivariate analysis for estimating the time since deposition of human and canine oral fluid on porous substrates

Oral fluid is commonly found at crime scenes, as a liquid sample or more frequently as a stain (i.e., cigarette butts, bottles, chewing gum, cloths, etc.). The study of the time since deposition (TSD) is of great significance in forensics because it allows to collect and analyse only those samples directly related to the criminal case, excluding the occupational traces, and reducing laboratory costs and efforts. In the present study, we evaluated the suitability of attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) to establish the TSD of human and canine oral fluid stains on different porous substrates such as paper, cotton, denim, and polyester fabric. We used the ATR FTIR spectra and chemometrics to detect the presence of human and canine oral fluid as stains on these substrates. Our results showed that Orthogonal Partial Least Square-Discriminant Analysis (OPLS-DA) models were able to distinguish human and canine oral fluid stains according to their TSD independently of the substrate.