New crime scene tech can even find ‘hidden’ fingerprints

Crime scene investigation got a boost into the 21st century this past week, as a team of researchers at the University of Leicester, in the UK, announced the development of a new technique for gathering fingerprints that can even find 'hidden' prints.

Due to TV and movies, the techniques used up until now are pretty familiar to everyone — an investigator brushes a special powder onto the surfaces at a crime scene, and the powder sticks to the sweat and oils left behind when anyone at the scene touched something. The contrast between the colour of the powder and whatever the fingerprint is on lets the investigators see the print and get a record of it. Since the chance of two people having the same fingerprints is extremely slim (something like 1 in 64 billion), it gives them a good chance to identify who was there and narrow down who the criminal was.

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Criminals know about this though, and they've exploited one of the weaknesses of the technique, by literally wiping away the evidence. Even the most thorough criminal can miss something, though, and it's these missed or 'hidden' fingerprints that investigators have to rely upon. Quite often, though, the quality of the print isn't good enough for it to be used in court.

However, now steps in the research team from the University of Leicester, with a new, and incredibly accurate way of lifting and reading these prints.

This new method uses the fact that the residue of sweat and oils our skin leaves behind is insulating — that is, it doesn't conduct electricity. If the surface underneath the print is conductive, like a metal knife or bullet casing, a special coloured 'electro-active' film is applied that transfers the colour only to the conductive surface, and even the thinnest amount of residue will prevent the colour from being transferred. This reveals the fingerprint in negative, with extremely fine detail.

However, not to just leave it at that, the researchers actually took it a step further and put fluorescent 'marker' molecules in the film as well, that show up under different kinds of light. Thus, once they have the fingerprint highlighted with the film, they can hit it with different coloured light or even ultraviolet light, to distinguish the fingerprint even better.

"By using the insulating properties of the fingerprints to define their unique patterns and improving the visual resolution through these colour-controllable films, we can dramatically improve the accuracy of crime scene fingerprint forensics," Prof. Robert Hillman, a professor of chemistry at the University of Leicester, said in a statement.

The key to making this extra step work, though, was to ensure that the marker molecules inside the film stayed away from the metal surface, since contact with the surface would reduce their fluorescence. Really stepping into the modern era for this part, the researchers used a stream of neutrons — uncharged sub-atomic particles typically used in high-energy particle physics experiments — to see the positions of these molecules in the film and refine their technique of embedding them to get it just right.

"Neutrons are an ideal tool for understanding what is going on inside these complex systems," said Dr. Rob Barker, who works at the Institut Laue-Langevin (ILL), one of the labs that performed the neutron scans, according to the University of Leicester statement. "Whilst the mix of polymer and fluorescent molecules might look similar to x-rays and other surface-sensitive techniques, neutrons can easily distinguish between them."

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One added bonus of this new technique is that, because it deals specifically with the surface, rather than the fingerprint residue itself, it can be used along with the fingerprint powder, to make the contrast between the print and the surface even better.

These techniques have only been used on fingerprints taken in the lab, so they haven't been subjected to 'real-world' conditions just yet. That's the next step for the researchers, though, as they test the process on prints that have been exposed to water, or fire, or even on surfaces someone has used cleaning products on.

Still, Assistant Chief Constable Roger Bannister, of Leicestershire Police, had high praise for the University and the researchers in the statement, saying: "Fingerprints have been around in policing for over 100 years, but this technique opens up new avenues for the detection of crime in the modern era. This technique potentially offers opportunities for quick results for the more serious crimes in a way that may still permit other forensic analysis to be performed to maximize the opportunities to recover forensic evidence."

(Image courtesy: University of Leicester)

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