The sweaty, greasy residue that forms a fingerprint might one day reveal the identity of a criminal as surely as the pattern of whorls and ridges.

Whenever we touch something, we leave behind a minute residue of proteins, salts and fatty acids. Because the exact proportions of these components vary between individuals, some forensic scientists suspect that a chemical fingerprint could be as unique as a physical one.

Getting information from such tiny samples can be tricky. But a team led by Dale Perry of the Lawrence Berkeley National Laboratory in California told the American Chemical Society meeting in Chicago last week that they can analyse samples of sweat less than 10 micrometres across - smaller than a single fingerprint ridge.

While powerful, their technique has a practical hitch - it needs a synchrotron, a huge particle accelerator that produces extremely intense light beams. When a thin infrared beam is shone at the sample, the wavelengths that are absorbed reveal its chemical make-up.

Age and sex

No one yet knows whether the chemical profile of our sweat is truly unique. One 1999 study by Gary Mong of the Pacific Northwest National Laboratory in Richland, Washington, showed that each of 79 test subjects had different sweat profiles when their prints were examined using gas chromatography.

"We have a hunch that they can at least be segregated by age and by sex," Mong says. His method required a couple of fingerprints' worth of sweat and destroyed the prints in the process.

Perry says his method is hundreds of times as sensitive and does not affect the prints, leaving them for other forensic scientists to study. So far Perry and his team have analysed three prints from people in their lab. He says more work is needed to find out how much information can be extracted from a chemical print.

Sweat profiles could even reveal when the print was made, since the characteristic chemicals evaporate or break down at different rates when exposed to air. "That's just speculation for the moment, but it's very exciting," says team member Wayne McKinney.

Stephen Homeyer of the FBI's Forensic Science Research Unit in Quantico, Virginia, agrees that sweat profiles could have their uses. But he's not convinced that chemicals can pin down identity, especially since prints can be affected by what people have been touching. "I don't know if you'd ever get past the issue of environmental contamination," says Homeyer. In any event, he adds, "I don't know of any forensics lab that has a synchrotron in the backyard."