Finding Evidence in Fingerprints

A technique reveals drugs and explosives on the scene

Next on CSI: This series of images shows that fingerprint images made using mass spectrometry are comparable to those made using traditional means. In (A), mass spectrometry is used to produce a fingerprint by imaging the presence of cocaine; the mass-spectrometry fingerprint can be employed as a starting point for a computerized image (B) generated using commercial fingerprint-analysis software. Below, (C) and (D) show a traditional ink print made with the same fingertip, and the corresponding computer image. (Red and blue circles in the computer-generated images correspond to features of interest, such as where ridges intersect.)

A new method for examining fingerprints provides detailed maps of their chemical composition while creating traditional images of their structural features. Instead of taking samples back to the lab, law-enforcement agents could use the technique, a variation on mass spectrometry, to reveal traces of cocaine, other drugs, and explosives on the scene.

Fingerprints are traditionally imaged after coating crime-scene surfaces with chemicals that make them visible. These techniques can be destructive, and different methods must be used, depending on the surface under study, says John Morgan, deputy director of science and technology at the National Institute of Justice, the research branch of the U.S. Department of Justice. "Mass-spectrometric imaging could be a useful tool to image prints nondestructively on a wide variety of surfaces," says Morgan.

Traditional mass spectrometry, the gold standard for identifying chemicals in the lab that uses mass and charge measurements to parse out the chemical components of a sample, typically involves intensive sample preparation. It must be done in a vacuum, and the sample is destroyed during the process, making further examination impossible and eliminating information about the spatial location of different molecules in the sample that are needed to create an image.

R. Graham Cooks, a professor of analytical chemistry at Purdue University, who led the fingerprint research, and his group used a sample-collection technique that he developed in 2004 and that can be used with any commercial mass spectrometer. Desorption spray ionization uses a stream of electrically charged solvent, usually water, to dissolve chemicals in a fingerprint or any other sample on a hard surface. "The compounds dissolve, secondary droplets splash up and are then sucked into the mass spectrometer," explains Cooks. As the instrument scans over a surface, it collects thousands of data points about the chemical composition, each of which serves as a pixel. The mass-spectrometry method can create images of the characteristic ridges of fingerprints that also serve as maps of their chemical composition.

In a paper published in the journal Science this week, the Purdue researchers describe using the method to image clean fingerprints and prints made after subjects dipped their fingers in cocaine, the explosive RDX, ink, and two components of marijuana. "We know in the old-fashioned way who it was" by providing information about the fingers' ridges and whorls, says Cooks of the fingerprint-imaging technique. The technique could also address the problem of overlapping fingerprints, which can be difficult to tell apart: fingerprints made by different individuals should have a different chemical composition. And "you also get information about what the person has been dealing with in terms of chemicals," says Nicholas Winograd, a chemist at Pennsylvania State University, who was not involved in the research.

Some of the chemicals found in fingerprints come from things people have handled; others are made by the body. The metabolites found in sweat are not well understood, but it's likely that they differ with age, gender, and other characteristics that would help identify suspects, says Cooks. Mass spectrometry could help uncover these variations. And Winograd says that the chemicals found in fingerprints might also provide information about drug metabolism and other medically interesting processes. Winograd, Cooks, and many others have recently begun using mass spectrometry to study the molecular workings of cancerous tissues and cells. Mass spectrometry might reveal that diagnostic information exists in sweat as well, says Winograd.

However, Morgan cautions that the work is preliminary and that the technology may prove too expensive for widespread adoption by law-enforcement agencies. Indeed, Cooks has not developed a commercial version of the fingerprint-analysis instrument.

"They have a long way to go," agrees Michael Cherry, vice chairman of the digital technology committee at the National Association of Criminal Defense Lawyers, who has extensive experience interpreting fingerprints. He says that Cooks's group has demonstrated the potential of the technology. However, after examining some fingerprint images made using mass spectrometry, Cherry says that the technology will require further development to be good enough to hold up in court.