Forensic analysis of biological fluid stains on substrates by ATR-FTIR spectroscopic approach and chemometrics

Body fluids are critical pieces of evidence at a crime scene. It is vital that they are correctly detected and analyzed to obtain as much information as possible. Additionally, establishing time since deposition (TSD) of a stain can help determine if a sample is relevant to the crime and verify the alibis of any individuals involved. Currently, identification of body fluids is made with presumptive and confirmatory techniques, which are nonspecific and destructive. For these reasons, selective, non-destructive, and fast techniques need to be investigated and validated for their use in forensic investigations. Attenuated total-reflection Fourier transform infrared spectroscopy (ATR FTIR), can analyze very small stains without destroying them, allowing further analysis with complementary techniques. It provides large amounts of data, which makes its processing essential to achieve a correct interpretation of the data. This can be accomplished through chemometrics. This work focuses on ATR-FTIR spectroscopic to study forensic scientific problems such as the detection of body fluid stains and their aging process, among others. The results show that Orthogonal Partial Least Square-Discriminant Analysis (OPLS-DA) of IR spectra of the studied samples is able to differentiate blood and oral fluid stains from other interfering substances on common substrates present at crime scenes; distinguish the source of body fluid stains as human or animal; and differentiated by their TSD up to 3 months after their deposition. In the European project Real-tIme on-site forenSic tracE qualificatioN (EUH2020-SUFCT020-883116), a laser-based infrared sensor is under development that allows for stand-off detection of explosives, drugs and precursors, but also samples of biological origin. In preliminary test measurements, it can differentiate between common sources for false positives and biological origin. Therefore, laser-based IR spectroscopy has a great potential for forensic purposes.