In the standard approach to lifting fingerprints from a crime scene, known as contrast enhancement, a sample is treated with a substance--either vapor, liquid or powder--that adds color to a fingerprint and allows it to stand out from its background. Prints left on such surfaces as leather, plastic or fibrous textiles, can sometimes be difficult to detect, however. The technique developed by Chris Worley of the Los Alamos National Laboratory and his colleagues is a noninvasive one that relies on a process known as micro-x-ray fluorescence (MXRF). When a surface is exposed to a thin beam of x-rays, the MXRF instrument detects elements such as sodium, potassium and chlorine, which are present as salts in human sweat. Because the salts are deposited along the ridges present in a fingerprint, the fluorescence can be used to assemble a digital image of a print. "This process represents a valuable new tool for forensic investigators that could allow them to nondestructively detect prints on surfaces that might otherwise be undetectable by conventional methods," Worley says. "It won't replace traditional fingerprinting, but could provide a valuable complement to it."
MXRF cannot detect all the prints that conventional techniques do, because some prints won't contain enough of the necessary elements. But it might find some prints that would otherwise be missed: the researchers' tests illustrated that MXRF successfully identified prints from subjects whose hands were exposed to sunscreen, lotion or saliva, which could interfere with contrast enhancement. Currently, this method can only test samples that can physically be transported to a laboratory that has an MXRF machine. If further testing and refinement of the technique are successful, the team predicts it could be used commercially in two to five years, perhaps as a portable device.