When the evidence pickings are slim, the slightest chemical trace may be all that's available to an investigator. But identifying the chemical trace, especially when bound to other material, is very difficult.
Dyes tend to bind chemically to cotton, making sampling problematic. But a new technique—surface-enhanced resonance Raman scattering spectroscopy (SERRS)—may help. SERRS may prove to be a powerful forensic tool, says Peter C. White, a senior lecturer in forensic science at the University of Strathclyde (Glasgow, Scotland). The frequency shift in Raman scattering that allows the identification of chemicals is a very small effect, but White has found that painting the sample with a colloidal suspension of silver greatly enhances the scattering. Using a laser with an output that matches the color of the dye in question causes resonance, also enhancing the results. In the end, White gets readings ten orders of magnitude higher than with basic Raman spectroscopy.
A single strand of fiber, 2 to 5 mm in length, will give up the chemical composition of a dye easily under SERRS, White says. He`s also been able to get excellent results from stains on glass, he says. "We've been able to do it off any surface," he said. "That is, scientifically, quite an accomplishment."
The technique is useful in examining what investigators call questioned documents. If someone, for instance, has taken a signed check and written in a new, higher amount, White can compare the Raman signature of the original with the suspect portion to see if different inks were used, thus suggesting a forgery (see figure). The tool in this case was an argon-ion laser emitting at 514 nm, focused through a microscope and then back to a charge-coupled-device camera.
White, who spent 15 years at the Metropolitan Police Forensic Science Laboratory in London, holds several British patents on his technique and is forming a company to market his system to law-enforcement and other laboratories. "We're bringing not just to the forensic stage but to the analytical laboratory a very, very useful technique," he says.