How Chemicals Left Behind on Your Phone Could Identify You

Mass spectrometry is finding a new role in forensic science

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Your phone is uniquely yours in more ways than you realize. Chemicals found on your skin, or even expelled through your skin, land there and can be identified using mass spectrometry, according to research from the University of California, San Diego.

The paper, published in Proceedings of the National Academy of Sciences, examined mass spectrometry, which sorts chemicals by weight, as a tool for criminal profiling. Those chemicals that you leave behind can tell a lot about you, if you know what to look for.

“Most molecules that are found on our skin are coming from molecules from our lifestyle,” says Amina Bouslimani, a postdoc at UC San Diego and lead author of the paper. “Then we thought, OK, what if these molecules can be transferred to objects that we use daily, like phones, computers, keys or wallets? If this is possible, we might be able to learn about individual lifestyles and personal routine based on the chemical traces that we can recover from these objects.”

By lifestyle, Bouslimani means chemicals that you use or are exposed to—types of makeup, food, medications, deodorant, shampoo, even the places you’ve been, via carpet fibers, soil and other environmental clues. The stuff you absorb, as well as the stuff you consume, appears in trace amounts on the things you handle. Mass spectrometry is one way to see it.

Mass spectrometry is not new, even in the forensic field. Among law enforcement, it’s primarily used to identify drugs or explosives—specific, easy to isolate and identify substances. For this research, scientists swabbed participants’ phones, similar to how TSA agents sometimes swab for trace explosives. The sample from the swab goes into a mass spectrometer, and the results are matched to the properties of known chemicals using a complicated reference system with a large, computationally intensive dataset. 

Bouslimani sees this new technique as one tool, similar to DNA evidence or fingerprints, to help profile potential suspects. This isn’t a tool for identification the way DNA analysis is. Instead, it’s a way to examine the habits of an individual in hopes of narrowing down the pool of suspects and speeding up the work police and lawyers have to do.

Glen Jackson, a professor of forensic science at West Virginia University who also works in mass spectrometry and founded a journal called Forensic Chemistry, however, is skeptical. “This isn’t at all practical. Crime labs can’t do this. It’s too expensive. It’s too difficult. They wouldn’t know what to do with the data, and they wouldn’t know what to do with it in court,” he says.

That said, Jackson acknowledges that, once this technology becomes practical, the fiscal benefits of investigative efficiency will drive the practice forward quickly. “I think articles like this, although they’re not at all ready for a crime lab, they plant the seed for pushing the boundaries of what’s possible," he says.

“Even if there is DNA or fingerprints available, then the person who touched this object might simply not be in the database. So then, how can this criminal investigator narrow down the person who owns or touched these objects?” says Bouslimani. “The criminal investigator can learn about individual lifestyle and daily routine.”

A phone that shows trace elements of a particular medication was probably used by someone with a particular condition. Traces of caffeine, nicotine or sunscreen can point to other habits. One that has DEET on it may have belonged to someone who likes camping (DEET is the primary chemical in some mosquito repellents). Then again, that person could be a gardener, a fisherman or many other things. They might simply work in Lowe’s, stocking DEET-infused candles, points out Jackson.

What’s more important, then, is the pattern, or profile as a whole of many different chemicals taken together. Bouslimani’s research featured a small set, compared against existing databases from several sources. For it to be practical, says Jackson, this technique has to become cheaper and easier to use (mass spectrometers can run into the hundreds of thousands of dollars, and basically require a PhD to operate, he says), as well as defensible in court.

“It will be very important in the future to develop a database that will contain a lot of lifestyle molecules, and this is what we are missing,” says Bouslimani. “Molecules from food, molecules from beauty products, molecules from carpet, molecules from the environment.” A more complete database will enable a more complete profile.

Criminal profiling isn’t the only potential use for this technology. It could also be used to measure exposure to chemicals (Bouslimani and the other researchers found molecules from flame retardants on some volunteers’ skin). In the medical field, chemicals expelled from the skin could give clues to how effectively a patient metabolizes a therapeutic drug.

“I’ve seen an awful lot of overlap with human health,” says Jackson. “If we understand the transfer of chemicals between different surfaces, then that can be beneficial either for human health or forensic science.”

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