DNA, fingerprints, hair—individuals leave these little pieces of themselves wherever they go. This is why investigators work so thoroughly to collect every trace at a crime scene, in the hopes of piecing together how many people were there—and ultimately identify who those individuals are.

But without sufficient DNA samples or other unique identifiers left behind at a scene, it can be difficult to determine whether it was one person at a scene, or two, or a larger group. However, a team of researchers at the University of Albany Department of Chemistry have turned to another trace of the human body—that is constantly being produced and deposited—to potentially help investigators distinguish between individuals and thus better reconstruct a crime scene: sweat.

Jan Halámek, chemist and co-author of the paper “Metabolite Biometrics for the Differentiation of Individuals,” which was recently published in the journal Analytical Chemistry, told Forensic Magazine that by measuring levels of three metabolites—lactate, urea and glutamate—in the skin secretions of multiple individuals, his team was able to show that no two individuals had the same levels.

“None of them have the same analyte concentration,” Halámek said. “Those patterns, all those concentrations, are very unique to the person. This way you can distinguish between two different individuals from their sweat composition.”

Halámek explained that the human body is constantly producing sweat, and that people are constantly depositing small amounts of sweat on everything they touch. Sweat is what makes up the nearly invisible latent fingerprints found on the surfaces of walls, furniture and objects, etc., and contains a multitude of chemicals including 23 different amino acids and many other metabolites.

Lactate was one of the metabolites chosen for the experiment because it appears in high concentrations in sweat, varies greatly, and changes depending on a person’s lifestyle. The team also chose urea and glutamate in part because they are involved in separate parts of the metabolism, and are relatively independent from one another.

The research team, in their study funded by the National Institute of Justice, tested their assays to measure the metabolites on mimicked sweat samples containing the three targeted analytes, and collected authentic sweat samples from the forearms of 25 volunteers. The results of their tests revealed that the 25 individuals could be distinguished from one another due to their unique metabolite levels. A 3-D graph included in the paper plots the 25 measurements, with no two points overlapping.

3-D scatter plot with the average (N=3) change in absorbance value of lactate, urea and glutamate at their respective time points for the (A) 50 mimicked samples and (B) 25 authentic samples in the study. Reprinted with permission from Mindy E. Hair, Adrianna I. Mathis, Erica K. Brunelle, Lenka Halámková and Jan Halámek, Analytical Chemistry, p. E (Figure 4), 2018. Copyright 2018 American Chemical Society.

“If you have two fingerprints or sweat spots on the crime scene, then we can distinguish is it the same person or two different people,” Halámek explained. With this information, investigators could potentially determine how many separate individuals were present at a particular scene.

However, due to the dynamic nature of metabolite levels as a person goes throughout their day, Halámek said the ability to match the characteristics of sweat back to a specific person is currently limited, and the subject of future research.

“As we wake up, as we eat, as we walk—that changes,” he said. “That’s ongoing research, to see what is the time between (when) somebody commits a crime or deposits the sweat sample on the crime scene (…) how many hours after, if I catch you, can I still say that was this person?”

Research on sweat at the University of Albany has already shown that scientists can estimate a person’s sex from the metabolites in sweat, and Halámek hopes to expand this research to characteristics such as diet, lifestyle, medical conditions and more. In a spinoff of this forensics-related work, Halámek has also worked on developing a sweat-based system of biometric security and authentication for mobile devices, Forensic Magazine reported last year.