New tool reveals criminals' hidden chemical clues

Fingerprints can provide vital "handling" evidence in the form of tiny chemical traces left at the scene of a crime, say scientists.

For more than 100 years, the unique patterns of fingerprints have been used to identify suspects in investigations. But a new technique described in the journal Science means they may in future yield far more clues.

US researchers have developed a tool that reads and provides an image of a fingerprint's chemical signature. The technology can determine the kind of material a person has recently handled, including explosives or drugs. It also helps separate overlapping fingerprints from different people.

Graham Cooks, from Purdue University in West Lafayette, Indianapolis, who led the research, said: "The classic fingerprint is an ink imprint showing the unique swirls and loops used for identification, but fingerprints also leave behind a unique distribution of molecular compounds.

"Some residues are from naturally occurring compounds in the skin and some are from surfaces or materials a person has touched."

The technique uses mass spectrometry, a standard method of chemical analysis. It turns molecules into electrically-charged versions of themselves called ions which can be used to determine their mass. Normally mass spectrometry has to be done using special equipment in a laboratory.

The new hand-held forensic system can operate in the field and involves spraying a sample with ionised water. When the droplets hit the surface of the sample, they cause the sample's molecules to be ionised. The molecules are sucked into a device that allows them to be analysed.

Computer software maps the information and produces a fingerprint image showing the distribution of different chemicals.

The scientists examined fingerprints in situ or lifted them using clear plastic tape. A co-researcher, Demian Ifa, said: "Because the compounds in each fingerprint can be unique, this technology can pull one fingerprint out from beneath other fingerprints."