As most crime scene experts know, dusting for fingerprints can sometimes destroy parts of the prints, erasing potentially valuable forensic clues.
Conventional 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 left on many substances such as fibrous papers, textiles, wood, leather, plastic, multi-colored backgrounds and human skin can sometimes be difficult to detect this way. Plus, children's fingerprints are often more difficult to detect than adult prints due to the absence of an oily substance called sebum and the presence of other fatty acid deposits unique only to children.
Any improvements in detection techniques that can be forensically useful are welcome, and a group of government chemists believe they have an answer.
Researchers at the Los Alamos National Laboratory in New Mexico say they have developed a novel means of detecting fingerprints using X-rays that don't disturb the print in any way. The technique also is able to reveal chemical markers that could give investigators new clues for tracking suspects and missing persons.
The technique uses a process called microbeam 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.
This method is important because it does not require using developing agents to treat the print.
"We collect an image of a print pattern intact without altering its composition," says Christopher Worley, an analytical chemist at Los Alamos.
The research, however, has stalled at an unforeseen snag. The researchers have run out of money.
"The research is currently at a standstill awaiting additional funding," Worley says. "So far, we have not been able to find anyone interested, willing and able to provide funding to pursue the idea further."
So far the research is proof-of-concept only, to demonstrate the possibility of detecting fingerprint patterns using MXRF, whereby the fingerprint pattern is determined by detecting inorganic elements present in the print residue.
"Thus, we both detect the print pattern digitally and collect chemical information from the print as well," Worley says.
Fingerprints contain detectable quantities of salts, such as sodium chloride and potassium chloride, excreted in sweat. The Los Alamos researchers have shown they could detect the sodium, potassium and chlorine from these salts. 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.
Worley says the technique is another tool to be used in an attempt to visualize a print that might be difficult to detect with current powder or chemical treatment methods. He stresses that MXRF work is proof-of-concept only.
"We are not claiming we have a method to replace current protocols," he says. Rather, Worley believes this method compliments current techniques.
"While this method currently requires a prior knowledge of the print location, it clearly has some advantages over contrast-based techniques for special cases," he says.
For example, a print left from a finger coated with a residue such as gunpowder might be detectable from the sulfur and potassium content. Other distinctive fingerprint examples, such as those containing lotion, sunscreen, saliva or certain food residue, can be detected by MXRF based on inorganic elemental constituents.
"Because MXRF is a spectroscopic method, the elemental composition of a fingerprint is examined, and visual contrast with the substrate is irrelevant," Worley says.
Thus, if sufficient detectable residue is present, the print can be identified regardless of the background color.
"It is also a non-invasive tool, so the sample remains intact for other analysis or archiving," he says. Volatilization of water, oils and other organic components over time should not hinder print identification since only inorganic elements are detected.
X-ray fluorescence itself is not a new technology. The phenomenon is widely used for chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in areas such as geochemistry and archaeology. But this is the first use of MXRF (XRF performed with micrometer-size beam) for fingerprint detection.
In the line of beauty
One of the nation's leading experts on scientific evidence greeted Worley's MXRF work with enthusiasm.
"The beauty of this new visualization technique is that it permits you to visualize the latent without altering it," says Edward Imwinkelried, law professor at the University of California -- Davis and former chair of the evidence section of the American Association of Law Schools.
Any alteration in the visualization stage can distort subsequent stages in the process.
"If the print is altered in visualizing it, it does not matter how accurately the visualized print is recorded -- that image will not be an accurate depiction of the fingerprint impression at the crime scene," he says.
Imwinkelried, coauthor of the third edition of "Scientific Evidence," one of the leading treatises in its field that has been cited on several occasions by the U.S. Supreme Court, recently warned that existing fingerprint matches key to fighting international terrorism and keeping criminals off the street are no longer foolproof.
"We can no longer naively assume the reliability of our current fingerprint standards," he writes in "How We Can Improve the Reliability of Fingerprint Identification," a paper published in a recent issue of "Judicature," co-authored by criminal defense attorney and biometrics expert Michael Cherry, president of Cherry Biometrics. "Given the stakes, not only justice in a particular case but national security itself, we must do better."
Calls for reform
Cherry and Imwinkelried urge reforms.
The first system for classifying and identifying fingerprints was developed in the late 19th century by Sir Francis Galton, known for his famous quote that the odds of two individual fingerprints being the same are one in 64 billion.
Cherry and Imwinkelried are concerned that since the current world population exceeds 6 billion persons -- each usually with 10 prints -- the world population of fingerprints now therefore exceeds Galton's odds.
They also worry that fingerprint matching techniques which once used cards and then analog photographs to compare up to 10 fingerprints have been taken over by computerized systems using less precise digital images, and pre-screen matchers sometimes use only a single index finger.
"If we're going to rely on computer technology for the watch list on terrorism and for background checks ... we've got to have some assurance the computer system is reliably accurate," says Imwinkelried.
He and Cherry call for the high-powered computer analysis of existing fingerprint databases, called data mining, to detect new patterns and develop new criteria for matching fingerprints.
They also recommend the return to the Henry Fingerprint Classification System, which used all 10 fingers to classify an individual. The Henry system, Imwinkelried and Cherry say, would better help identify suspects who use aliases and would prevent criminal suspects, like alleged serial killer Jeremy Jones, from being re-released after each arrest by technical glitches in the FBI system. Jones is accused of committing several murders after he was repeatedly freed following arrests for other minor offenses. Because only one print was used for matching, the fingerprint-matching system failed to detect that he was using an alias.
"If analyzed properly, fingerprints can be as accurate as DNA," the authors say.
In an earlier "Judicature" article, Cherry and Imwinkelried argued for greater skepticism of using computerized fingerprint analysis, especially for its reliance on digitized images of fingerprint patterns.
"The bottom line is that digital images are simple, incomplete approximations of the images they attempt to capture," they wrote. The authors encourage courts to take a more skeptical look at fingerprint testimony, recommend that computer systems check as many fingerprints as are available and advise greater scrutiny of the matching criteria embedded in the programs that match fingerprints.
Right on the money
The Cherry-Imwinkelried articles relate to later stages in fingerprint analysis than the Worley MXRF visualization method. More specifically, they address the question of how prints should be recorded after visualization, the limitations of digital images and the criteria that the computer or human examiner uses to make the match or no-match decision.
Imwinkelried believes the entire fingerprinting system is so fraught with unreliability, enhancements at any stage are welcome.
"I applaud an improvement at any stage in the fingerprint process," Imwinkelried says. "Law enforcement and national security depend vitally on the validity of fingerprint analysis, and this research promises an improvement in the earliest stage of the process."
A number of issues remain to be pursued with the Worley method before it's available, not the least of which is designing an X-ray instrument specifically for analyzing fingerprints in the field. The instrument Worley used in the lab for his concept work was built for a variety of material analysis applications and not specifically for fingerprints. It is therefore not optimized for detecting trace levels of chemicals found in some types of prints. Optimization is possible with additional funding.
Douglas Page ([email protected]) is a science and technology writer living in Pine Mountain, California.