Investigating handheld near-infrared spectroscopy for forensic body fluid analysis

Forensic casework and crime scene examination will often involve the identification and analysis of biological evidence found on a wide variety of surfaces. One type of biological evidence most commonly encountered at the crime scene is body fluids, such as blood, semen and saliva. During an investigation, it is crucial to confirm the nature of a stain, determine if it is a body fluid and identify which specific body fluid it is. Common chemical tests for the identification of body fluid stains, like Kastle-Meyer or Phadebas test, are presumptive in nature, meaning that they present limited specificity and a certain risk of cross-reactivity. Due to their drawbacks, current literature focuses on developing confirmatory tests that target fluid-specific biomarkers or chemical signatures. The study presented explores the capability of a handheld commercial near-infrared spectrometer for the identification of body fluid stains, as well as obtaining relevant information concerning the donor source of the evidence, such as their biological sex. For this purpose, samples of blood, semen and saliva from healthy human male and female volunteers were gathered, deposited on glass substrates, and analysed at different points in time up to 4 weeks. The spectral data collected was used to build a training library from which chemometric models were designed for the discrimination of body fluids and the identification of the biological sex of the individual source of the evidence. Once suitable models were designed, these were tested against new samples of blood, semen, and saliva, along with samples of household products—fake blood, toothpaste, apple juice— known to appear similar to a body fluid stain or produce false positive results with routine presumptive tests. Although additional data gathering and modelling are still required for the identification of other body fluids, as well as the biological sex of the donor, the results in this study further confirm the capability of this technique to identify blood stains with a low incidence of false positives, presenting itself as an alternative to current presumptive tests that is fast, affordable, and non-destructive.