Raman Spectroscopy: As Seen in CSI?

A recent editorial in Analytical Chemistry[1] alerted me to a paper that had initially slipped under the radar.

Kelly Virkler and Igor Lednev[2] were able to determine the difference between dried traces of human, cat and dog blood using a combination of lasers and computer analysis.  This is quite impressive.  The technique they used, Raman spectroscopy, is a notoriously weak technique.

In Raman spectroscopy, a sample is illuminated with a laser and miniscule changes in the colour of light that is scattered from the sample contains valuable information about molecules present in the sample.  These are very, very small changes.  Furthermore, most of the light that is scattered does not contain this information, only 1 in a million does.   Nonetheless, the use of lasers means that Raman spectroscopy lends itself to the analysis of microscopic samples as you can feed your laser beam down a standard microscope.  Furthermore, you don’t need to prepare or alter your samples in any way, although firing high powered lasers at something does tend to fry your samples if you are not careful – it can easily be a non-destructive technique.

What is most impressive about the work is the fact that they can not only determine that an unknown dried sample sitting at a crime scene is that of blood, but also that they can discriminate between different species that this blood might have come from.

If you look at the raw data you would normally get from Raman spectroscopy you might think that there is little difference between results.  To the human eye, the Raman spectra of human blood, feline blood or canine blood look very similar.  Use of computer analysis allowed the authors to tease out the subtle differences that are imperceptible to the eye.

If you dug deeper, you might even find consistent differences between different human samples.  Has the sample come from a man or a woman?  Were they healthy or did they have a particular illness?  Now that would be very useful for forensics.

[1] Gebel, E. (2009). Species in a snap: Raman analysis of blood Analytical Chemistry, 81 (19), 7862-7862 DOI: 10.1021/ac901827u
[2] Virkler, K., & Lednev, I. (2009). Blood Species Identification for Forensic Purposes Using Raman Spectroscopy Combined with Advanced Statistical Analysis Analytical Chemistry, 81 (18), 7773-7777 DOI: 10.1021/ac901350a