X-ray Vision Picks Up Clues to Crimes

New technology will soon help detectives get computer images of fingerprints and let them know more about the person who left them

8 June 2005--Television and the movies have always shown us technology's coming attractions--from the wireless communicators used by Captain Kirk and the crew on "Star Trek," to the endless array of toys that allowed James Bond to escape the clutches of the supervillain just in time to get the girl.

Continuing the tradition, in the episode of the TV crime drama "CSI: Miami" that aired on 16 May, crime scene investigators solved an old murder using X-ray fluorescence spectroscopy. The energy from an X-ray beam causes different elements to give off light in unique patterns. The conceit is that the equipment didn't exist years earlier when the crime was committed. But now the investigators are able to identify the perpetrator after a trace-element detector using X-ray fluorescence reveals small amounts of cobalt on a shirt found at the crime scene. This element-detection technique is not limited to the fictional world of television. It is used in real-life crime labs to detect the presence of chemicals that are, say, markers for explosives; at pharmaceutical companies to verify the breakdown and distribution of drugs in tablets; and by archaeologists to determine the origin of artifacts.

Now, researchers at the Los Alamos National Laboratory, in New Mexico, have made refinements to micro–X-ray–fluorescence devices that will soon let forensic analysts use trace-element detection to improve fingerprint collection. The newly enhanced technology produces images of fingerprints along with data on how much of several trace elements were in the sweat or other residue on the person's fingers.

The Los Alamos scientists were aware of the limitations of contrast enhancement, the standard method of collecting fingerprints. This technique--using powder, vapor, or liquid to make fingerprints stand out to be photographed--is not effective for prints left on wood, leather, human skin, or black or multicolored surfaces. Furthermore, treating a sample with chemicals to improve visibility alters the sample, making it difficult or impossible to do subsequent tests for, say, traces of DNA or elements that would indicate the presence of explosives.

Between work on their funded projects, the researchers sought an answer to a question that had intrigued them: could X-ray fluorescence be used to create computer images of fingerprints? Over time, the team developed control software for guiding the X-ray beam in a commercially available trace-chemical detector across a fingerprint. They also wrote algorithms for turning data gathered by the machine into three-dimensional images that reveal the swirl patterns of fingertips.

As with ordinary trace-element detection, when the X-ray beam passes over the specimen, the fingerprint is irradiated with enough energy to knock electrons off the innermost orbitals of the elements that are present. As electrons from higher energy levels fall down to replace those that have gone missing from lower-energy-level orbitals, each atom emits an X-ray photon that is unique to that element. Determining the identity of the element is simply a matter of measuring the wavelength of the photon it emits. This makes it possible to pick up traces of the sodium, potassium, and chlorine in the salts excreted in human sweat, says George Havrilla, who, in the midst of other related work, came up with the principles underlying the process.

The detector is able to pick up the position of the source of each photon emitted and tell not only whether these telltale elements are present, but in what amounts. Since these atoms are deposited mostly along the "friction ridges," which form fingerprints, researchers knew they could create software that would turn this information into what is essentially a topographical map of the chemicals that looks just like a fingerprint image produced using standard methods.

Christopher G. Worley, who presented a paper on the topic at the annual meeting of the American Chemical Society in mid-March, says the technique will complement traditional fingerprinting rather than replacing it. One reason is that trace-detection machines are currently too big to lug to a crime scene, so a specimen must be brought back to a crime lab for analysis. But this drawback takes on less importance when tracking the whereabouts of missing children, for instance. Worley said that micro–X-ray fluorescence is useful for telling if a child had touched a glass or picked up a book--while dusting or spraying to make a fingerprint ready to photograph isn't. That's because capturing fingerprints by conventional means often depends on the presence of sebum, the oil produced by glands under the skin that simultaneously keeps it from getting too dry and makes it water-resistant. These glands are not active before puberty, so a child's fingerprint is hard to pick up by dusting it with a powder or spray. But the sweat glands are active from birth, excreting the same salts that they do in adults.

Havrilla said that, to his knowledge, law enforcement agencies have yet to use micro–X-ray fluorescence to find any fingerprints at crime scenes. He noted that there is more work ahead if this fingerprint-detection method is to gain certification for use in crime labs. Thresholds need to be established for how much material needs to be present in a sample, how sensitive the detector should be, and how high the image resolution must be. He said that with sufficient interest (and funding) from law enforcement agencies or the U.S. Department of Justice, they'll make more refinements such as shortening the time needed to analyze a sample and making the system less expensive. Currently, it costs US $175 000, but the Los Alamos researchers believe the price will drop if many crime labs buy the equipment.

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