Non-destructive spit test

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  • Published: Mar 1, 2010
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Non-destructive spit test

Raman spectroscopy can identify samples of an unknown substance at a crime scene as human saliva during forensic analysis, according to a US study, the technique would preserve DNA evidence.

Forensic analysis is an enormous growth area with identification of mere traces of bodily fluids recovered at crime scenes increasingly a crucial component of forensic evidence. Given advances in DNA techniques and the fact that the result of many crime investigations pivot on valid genetic samples, the team explains that collecting viable samples of bodily fluid as evidence requires non-destructive testing for identification and matching prior to DNA analysis.

"Fluids such as blood, semen, saliva, and vaginal fluid can be very useful in identifying a victim or suspect," chemists Kelly Virkler and Igor Lednev of the University at Albany, SUNY, New York explain, "and they can also help piece together the events of a crime." The development of a fast and accurate technique for identifying a particular bodily fluid at the crime scene could be key to a successful crime investigation.

Virkler and Lednev add that Raman spectroscopy has great potential as just such a non-destructive tool. In 2008, the team reported that it could be used to identify bodily fluids at a crime scene but those experiments were carried out with just a single sample of each type of fluid. Now, the team has extended the work significantly to investigate the potential for spectroscopic differences among different "donors" of the same fluid.

The researchers have used near-infrared (NIR) Raman spectroscopy to obtain spectra for pure dried human saliva samples from several donors in a controlled laboratory environment. By applying principal component analysis (PCA) on the spectra they demonstrated that dry saliva is a particularly heterogeneous substance. However, the Raman spectra can be described as being a linear combination of a fluorescent background and three spectroscopic components.

"The relative contribution of each of the three components varies with donor," explain the researchers, "so no single spectrum could effectively represent an experimental Raman spectrum of dry saliva in a quantitative way." Nevertheless, by combining these three spectroscopic components a signature for a given dried saliva sample can be obtained. "This proof of concept approach shows the potential for Raman spectroscopy to identify an unknown substance to be saliva during forensic analysis."

While blood and semen samples have been a keen focus of developments in forensic analysis, because of their sometimes obvious links to a crime, saliva samples can be just as important. Saliva samples may be present in a sexual assault case, they can be obtained from bite marks on food, and from licked envelopes. The team points out that an ultraviolet lamp will illuminate a saliva stain, but will not differentiate it from other bodily fluids. One of the problems with identifying saliva samples at a crime scene is the general lack of solid particles in saliva, which can be observed in other fluids. Moreover, the presence of the enzyme amylase, which can be identified using commercial test kits, is not a definitive method because the enzyme is present in other bodily fluids, albeit in smaller quantities than in saliva.

"No confirmatory test for saliva is currently accepted in forensic laboratories," the team asserts, "so the development of a new analytical technique for confirmatory saliva identification would be very beneficial to forensic investigators, especially if it was non-destructive and could preserve valuable DNA evidence."

The team further explains the benefits of Raman spectroscopy to forensic investigators. Raman, they add reveals a specific signature for the based on the energy of the scattered laser light, and works well with disordered and heterogeneous samples. Crucially, water does not interfere significantly with Raman spectroscopy. Portable Raman spectrometers are already available and software pertinent to identifying saliva at a crime scene could readily be written for such devices based on the work of Virkler and Lednev.

The current research tested the approach using pure materials rather than actual crime scene samples. The next step will be to extend this proof-of-concept study to real-life samples.



Raman spit test courtesy of Sikirzhytskaya/Lednev
Raman can identify saliva
(Credit: A. Sikirzhytskaya)

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