Fingerprint Visualization Method Uses X-rays to Reveal Missing Clues

Newswise — Dusting for fingerprints can sometimes alter the prints, erasing valuable forensic clues. Now, chemists say they have developed a new fingerprint visualization technique using X-rays that leaves prints intact and, in addition, reveals chemical markers that could give investigators new clues for tracking criminals and missing persons. Their technique was described today at the 229th national meeting of the American Chemical Society, the world's largest scientific society.

The technique could be especially promising for tracking down missing or lost children, according to the researchers. Children's fingerprints are often more difficult to detect than adult's. The new method could detect prints based on chemical markers left behind in the child's fingerprints due to the presence of food, soil or saliva that can be used to track down evidence of the child's movements, the scientists say.

Traditional fingerprinting methods involve treating samples with powders, liquids or vapors to add color to the fingerprint so it can be easily photographed, a process called contrast enhancement. But fingerprints present on certain substances such as fibrous papers, textiles, wood, leather, plastic, multi-colored backgrounds and human skin can sometimes be difficult to detect by this method, according to study leader Chris Worley, Ph.D., an analytical chemist with Los Alamos National Laboratory in New Mexico. Besides permanently altering the prints, developing an effective visualization method can sometimes be time consuming, he adds.

The new technique uses a process called micro-X-ray fluorescence (MXRF), which rapidly reveals the elemental composition of a sample by irradiating it with a thin beam of X-rays without disturbing the sample. Salts such as sodium chloride and potassium chloride excreted in sweat are sometimes present in detectable quantities in fingerprints. Using MXRF, the researchers showed that they could detect the sodium, potassium and chlorine from such salts. And since these salts are deposited along the patterns present in a fingerprint, an image of the fingerprint can be visualized producing an elemental image for analysis.

In preliminary laboratory studies using the technique, Worley and his associates demonstrated that they could even detect fingerprints when lotion, soil, saliva or sunscreen was applied to the hands. Such prints might be difficult to detect using conventional screening methods.

"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," says Worley. "It won't replace traditional fingerprinting, but could provide a valuable complement to it."

Unlike traditional methods in which fingerprints can often be photographed at the crime scene, the new technique currently requires that samples be taken to the lab for analysis with the MXRF instrument and placed in an X-ray chamber, where a digital elemental image is collected on a computer and saved for analysis. If further testing and refinement prove successful, the experimental technique could be used commercially for fingerprint visualization in two to five years, the researchers predict.

In addition to revealing fingerprints, the MXRF technique also reveals chemical artifacts present in the prints themselves, providing new clues for crime scene analysis. Abnormally high levels of potassium, for instance, may suggest the presence of potassium nitrate, a component of explosives. High levels of sulfur and potassium may suggest gunpowder. Other elements could reveal environmental clues, such as soil type and food particles, that help track a suspect's movements. Even partial prints that cannot be used to identify a person might contain chemical artifacts that reveal useful crime clues, he says.

The technique does have limitations, Worley says. Some fingerprints will not contain enough detectable material to be "seen." In addition, MXRF fingerprint visualization can't detect every element. In general, the heavier the individual element, the more easily it is detectable by this method, Worley says. Lighter elements like carbon, nitrogen and oxygen can't be detected, but heavier elements like sodium, potassium and chlorine are more easily identified.

But in the future, the researchers hope to integrate other spectroscopic methods besides MXRF that can detect complex molecules in addition to elements, giving more complete forensic information. They also hope that their work will lead to a smaller, portable MXRF device that can be easily carried by forensic investigators to quickly test samples directly at the crime scene. More studies are planned, they say.

Besides Worley, other Los Alamos investigators involved in this study include Sara S. Wiltshire, Thomasin C. Miller, George J. Havrilla and Vahid Majidi.

The American Chemical Society is a nonprofit organization, chartered by the U.S. Congress, with a multidisciplinary membership of more than 159,000 chemists and chemical engineers. It publishes numerous scientific journals and databases, convenes major research conferences and provides educational, science policy and career programs in chemistry. Its main offices are in Washington, D.C., and Columbus, Ohio.

The poster on this research, ANYL 246, will be presented at 7:00 p.m., Sunday, March 13, at the Convention Center, Room Sails Pavilion, during the "General Papers" symposium.

Christopher G. Worley, Ph.D., is an analytical chemist with Los Alamos National Laboratory in Los Alamos, N.M.

ANYL 246Detection of fingerprints based on elemental composition using micro-X-ray fluorescence

Christopher G. Worley1, Sara S. Wiltshire2, Thomasin C. Miller2, George J. Havrilla2, and Vahid Majidi3. (1) Actinide Analytical Chemistry, Los Alamos National Laboratory, MS G740, Los Alamos, NM 87545, (2) Analytical Chemistry Sciences, Los Alamos National Laboratory, (3) Center for Homeland Security, Los Alamos National Laboratory

A novel means of detecting fingerprints was examined in which they were imaged based on the elemental composition of the print residue rather than by visible contrast between the print and the background. Digital images were collected from both visible and latent fingerprints using Micro-X-ray fluorescence (MXRF). Because MXRF examines the elemental composition of a sample, a number of unique types of fingerprints were studied such as those consisting of lotion, soil, saliva, and sunscreen. MXRF is a nondestructive technique. Hence, a fingerprint image can be acquired without chemically or physically altering the print residue. Thus, MXRF offers a new and complementary method for detecting fingerprints which are left pristine for further analysis (including potential DNA extraction) or archiving purposes. Furthermore, determination of the elemental composition of fingerprints may provide ancillary information for more extensive forensic analysis.

Briefly explain in lay language what you have done, why it is significant and what are its implications (particularly to the general public)

A method was developed to detect fingerprints using a technique known as micro-X-ray fluorescence. The traditional method of detecting fingerprints involves treating the sample with certain powders, liquids, or vapors to add color to the fingerprint so that it can be easily seen and photographed for forensic purposes. This is known as contrast enhancement, and a multitude of chemical processing methods have been developed in the past century to render fingerprints visible. However, fingerprints present on certain substances such as fibrous papers and textiles, wood, leather, plastic, adhesives, and human skin can sometimes be difficult to detect by contrast enhancement. Children's fingerprints are also difficult to detect due to the absence of sebum on their skin, and detection of prints left on certain colored backgrounds can sometimes be problematic. Micro-X-ray fluorescence (MXRF) was studied here as a method to detect fingerprints based on chemical elements present in fingerprint residue. For example, salts such as sodium chloride and potassium chloride excreted in sweat are sometimes present in detectable quantities in fingerprints. We demonstrated that MXRF can be used to detect this sodium, potassium, and chlorine from such salts. Furthermore, using MXRF, each of these elements (and many other elements if present) can be detected as a function of location on a surface, so we were able to "see" a fingerprint because these salts are deposited mainly along the patterns present in a fingerprint (traditionally called friction ridges in forensic science). MXRF is not a panacea for detecting all fingerprints; some prints will not contain enough detectable material to be "seen" ; however, determining an effective means of coloring a fingerprint with traditional contrast enhancement methods can sometimes be an arduous process with limited success. Thus, MXRF offers a possible alternative for detecting fingerprints, and it does not require any additional chemical treatment steps which can be time consuming and permanently alter the sample. Additionally, MXRF is noninvasive, so a fingerprint analyzed by this method is left pristine for examination by other methods (eg. DNA extraction).

How new is this work and how does it differ from that of others who may be doing similar research?

To the best of the author's knowledge, no studies have been published to date concerning the detection of fingerprints by micro-X-ray fluorescence. Some studies have been published in which other spectroscopic methods were employed to examine the chemical composition of fingerprints (eg. IR, SEM/EDX, and Auger), but very few papers discuss the actual detection and imaging of a complete fingerprint by any spectroscopic method. Thus, this work is unique.