Forensic body fluid identification: the Raman spectroscopic signature of saliva

The potential use of Raman spectroscopy for nondestructive, confirmatory identification of body fluids at the crime scene has been reported recently (Virkler and Lednev, Forensic Sci.,Int., 2008, 181, e1-e5). However, those experiments were performed using only one sample of each body fluid and did not investigate the potential for spectral variations among different donors of the same fluid. This paper reports on the role of heterogeneity within a single human saliva sample as well as among samples from multiple donors. Near-infrared (NIR) Raman spectroscopy was used to measure spectra of pure dried human saliva samples from multiple donors in a controlled laboratory environment. Principal component analysis of spectra obtained from multiple spots on dry samples showed that dry saliva is heterogeneous and its Raman spectra could be presented as a linear combination of a fluorescent background and three spectral components. The major chemical components known to be present in saliva were used to tentatively identify the principal spectral components. The issue of potential spectral variations that could arise between different donors of saliva was also addressed. The relative contribution of each of the three components varies with donor, so no single spectrum could effectively represent an experimental Raman spectrum of dry saliva in a quantitative way. The combination of these three spectral components could be considered to be a spectroscopic signature for saliva. This proof of concept approach shows the potential for Raman spectroscopy to identify an unknown substance to be saliva during forensic analysis.