Amino Acids Could Help Determine Sex Of Fingerprint Originators

For more than a century, forensic scientists have scoured crime scenes for fingerprints to help identify who was present during a crime. New research goes beyond the swirls and whorls of fingerprint patterns to demonstrate that the amino acids in the sweat within fingerprints can identify the sex of the person who made them (Anal. Chem. 2015, DOI: 10.1021/acs.analchem.5b03323).

Researchers have used analytical chemistry before to try to glean information about the makers of fingerprints by measuring prints’ chemical composition in the lab. But Jan Halámek, an assistant professor of chemistry at the University at Albany, SUNY, wants to devise a portable, easy-to-read testing method that could determine attributes such as the sex, age, or ethnic origin of someone who left fingerprints behind. As a starting point, he tackled sex determination, based on hormone differences between men and women that cause women to have higher levels, and a slightly different distribution, of amino acids in their sweat, which is deposited in fingerprints along with lipids and other chemical traces. Amino acid levels in the sweat of females are about twice as high as in males, Halámek says.

First the team devised a method to extract the amino acids from a fingerprint by transferring it onto a polyethylene film, placing a drop of diluted hydrochloric acid onto the print, and then heating it. The water-soluble amino acids migrate into the solution, leaving the lipids behind.

To measure the levels of extracted amino acids, the team devised a colorimetric test involving two enzymes. L-Amino acid oxidase produces hydrogen peroxide from the amino acids, and then horseradish peroxidase catalyzes a reaction between hydrogen peroxide and a dye to make a colored compound that can be measured with simple light spectroscopy.

Halámek’s group first tested their procedure on mimicked fingerprint samples—50 amino acid mixtures randomly generated to represent compositions characteristic of male and female fingerprints—and found that the test had a 99% chance of correctly classifying a given mixture as male or female. Then the research team tested the method on real fingerprints. They collected three male and three female fingerprints on polyethylene film and found that they could successfully distinguish between the two sexes in a blind test. He and his lab members also successfully tested the method on real fingerprints left on five different surfaces, including a doorknob and a desktop.

Halámek has since transferred the reaction components onto paper to make a quick, easy-to-read test similar to pregnancy strips or glucometers, which can be used and understood by people without scientific backgrounds, he said.

He would like to improve the test so it would not require extraction, and wouldn’t result in the fingerprint being destroyed. “We’d like to make it simpler and more robust,” he adds, “as well as move the test onto the fingerprint itself.”

The research is first step toward an implementable methodology, said Simona Francese, who heads the Fingermark Research Group at Sheffield Hallam University. “The method shows to have an astonishing accuracy of prediction,” she says. It is certainly worth following this up with more in depth and comprehensive investigations.” Francese points out that the test sample was small, and it is not possible to predict whether this would work on people with different ethnicity, age, health status, and diet. In addition, fingerprints in wet or humid conditions—for example, a knife thrown into a river—would lose their amino acids, so the method wouldn’t work.

Stephen L. Morgan, a chemist at the University of South Carolina, said the method seemed simple and cheap—a big benefit compared with other methods, such as mass spectrometry—used to analyze chemical compounds in fingerprints. He wondered how much of a fingerprint is needed to do the analysis. “Sometimes only a partial print is available,” he says. “This assay could be done after the normal imaging and photography, but it might require that fingerprint enhancement reagents not be applied to the print.”