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August 8, 2008

Fingerprints' Chemical "Footprints"?

Today's New York Times reports a story that appears in this week's Science.  According to the Times, "With a new analytical technique, a fingerprint can now reveal much more than the identity of a person. It can now also identify what the person has been touching: drugs, explosives or poisons, for example."  See full story HERE. In short, scientists (Demian R. Ifa, et al.) have used mass spectrometry to identify fingerprints after subjects' fingers were applied with various solutions, including drugs and explosives residue.  The researchers suggest that this technology might have several uses, including identifying what substances particular people might have handled recently and being able to distinguish overlapping fingerprints, by tracing the chemical "footprint" of the individual fingerprints.

Although there may yet be much value in this research, this single report hardly demonstrates its value for forensic purposes.  The researchers essentially identified the true-positive rate for this technology, and, so far as either the Times or the original article in Science report, the researchers have provided no data on false positives, true negatives, or false negatives.  Moreover, this study was a highly controlled laboratory study, so we don't know whether the technology might confront excessive "noise" when applied to the general population.  Indeed, given the reported amount of drug residue on United States currency, mass spectrometry that is too sensitive is likely to produce large numbers of false positives.

Hence, while the research results reported here are interesting and noteworthy, without considerable more work in this area, they appear a long way from daily forensic use.

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When my colleague DLF was posting this note, I was writing about the same thing. I posted my version only to see he had beaten me to the punch. The account that follows has more detail:

Latent fingerprints are made of sweat from small glands (called eccrines) that release their watery secretions through pores in the skin. If the finger that leaves a print has been in contact with chemicals such as drugs or explosives, some of these molecules also could be part of the residue. Researchers at Purdue University now have described a method using mass spectroscopy to detect both the presence and spatial pattern of these chemicals. As Ifa et al. (2008) explain:

"[D]rugs of abuse such as cocaine and {Delta}9-tetrahydrocannabinol ({Delta}9-THC, psychoactive component from cannabis) and explosives such as trinitrohexahydro-1,3,5-triazine (RDX, high-energy explosive) were imaged ... after exposing the participant's fingers to solutions of these compounds ... The prints were recorded from ordinary surfaces such as glass, paper, and plastic. Figure 1A shows the distribution of cocaine ... in a LFP [latent fingerprint] on glass. The level of detail of the image, acquired with a pixel size of 150 µm by 150 µm, allows clear distinction of the ridges and minutiae. Figure 1C shows an ink fingerprint of the same individual blotted onto paper and optically scanned at a resolution of 600 points per inch (ppi). These images were used as input in fingerprint recognition software. The output images with automatically detected minutiae ... matched with use of the usual fingerprint identification tools. This experiment demonstrates that the mass spectrometry analysis allows physical identification of a subject. We recorded ... images ... for a wide range of chemicals and substrates, including tape-lifted fingerprints. Transfer of the original chemicals with retention of spatial relationships allowed the analytical mass spectrometer to be used to acquire chemical information on as well as the identification of a particular individual."

In other words, it is possible to tell what someone has touched recently and to disentangle overlapping prints from two people who have sharply different concentrations of chemicals in their latent prints. An article in the New York Times observes that "The equipment to perform such tests is already commercially available, although prohibitively expensive for all but the largest crime laboratories. Smaller, cheaper, portable versions of such analyzers are probably only a couple of years away."

This is not the first report of chemical information lurking in latent fingerprints. DNA has been recovered and analyzed as well. But here's a question: How sensitive is the mass-spec test? Lots of currency in circulation has traces of cocaine on it. Would handling a bill produce a detectable trace within a latent print?



Demian R. Ifa, Nicholas E. Manicke, Allison L. Dill, R. Graham Cooks, Latent Fingerprint Chemical Imaging by Mass Spectrometry, 321 Science 805 (2008)

Kenneth Chang, Fingerprint Test Tells What a Person Has Touched, N.Y. Times, Aug. 7, 2008.

Posted by: DHK | Aug 8, 2008 1:56:54 PM

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