Science Interviews

Finding Forgotten Fingerprints

Chris - Now, forensic scientists at the University of Leicester, working with Northamptonshire police, have announced a major breakthrough in crime detection which could lead to hundreds of cold cases being reopened.  This is the work of Dr John Bond and he’s a scientific support officer for Northamptonshire police. He’s also a fellow of Leicester University. John, thank you for joining us. You’ve found a way of getting fingerprints from surfaces that couldn’t previously be fingerprinted. How does this actually work?

John - Basically, we’ve been looking at new ways of enhancing fingerprint deposit. That’s the secretion of sweat that you might leave on any surface that you might touch with your fingers. For many years the police in this country and worldwide had a range of conventional techniques but all require some sort of physical or chemical interaction with the deposit that you leave behind. They either stick to it and make it visible or they chemically react with it and maybe change its colour so you can see them.

Chris - So that must mean there are physical constraints over what sorts of surfaces can be fingerprinted?

John - Absolutely. Smooth, non-porous works very well for things like powder. A technique using superglue where the superglue actually polymerises: forms white strands on the fingerprint deposit. On things like paper where the fingerprint deposit might soak in there’s a range of chemicals that react with things like amino acids that are secreted in your sweat. All of those techniques require that deposit to still be there. If you remove the deposit all conventional techniques will fail.

Chris - This will be, for example washing the surface or wiping the surface – on the part of the criminal – to try and clean up the evidence?

John - Yes and that would be a very good example of that. It could also be extreme environment conditions that might, as you said, wash away or even vaporise the fingerprint deposit.

Chris - How does your new technique work?

John - What we’ve been looking at is a phenomenon we’ve found that fingerprint deposits will tend to corrode metal surfaces. There’s some constituents in the fingerprint deposit that on metals like brass and copper will corrode the metal to an extent that even when you’ve then got rid of the residue totally you can sometimes actually see an image of where the fingerprint was in the metal or, where that’s not possible, we’ve developed a technique to actually enhance that corrosion and make the fingerprint become visible again.

Chris - So how do you visualise the fingerprint in the form of its corrosion pattern on that surface?

John - We take the metal and apply an electrical potential to it at the order of 2500V.  We then apply a very fine conducting powder, very similar to photocopying toner powder. What we’ve discovered is that that will preferentially adhere to the metal at the points where the corrosions occurred which are coincident with the original fingerprint ridge pattern. You get an image of where the fingerprint was in this black powder.

Chris - Why does it stick just where the fingerprint is? Why does it preferentially adhere there?

John - What we’ve discovered is in the areas of corrosion the potential is a few volts less than the 2500V that you apply. When the conducting powder is streaming across the surface of the metal it takes on 2500V, it takes the potential but the bulk of the metal is at. With these points of lower potential it seems to sit in that area and take the lower potentials and not have enough energy to get back up out of that potential well. It resides in the areas of lower potential.

Chris - How do you translate the photocopier toner into a physical image you can see?

John - It just appears as a black image against the contrast of the copper or the brass metal. You can actually just see it sitting there.

Chris - what sorts of things do you can apply this to which will help to solve new cases?

John - A very good practical example of this – and it’s very fortunate – is that most bullet casings are made of brass. Already we’ve been able to show in some of our research that a fingerprint deposited on a gun cartridge case prior to being loaded into the gun which was then enhanced after the gun has been discharged can reveal the fingerprint with this black conducting powder. For the first time we can actually get a fingerprint of who was loading the gun.

Chris - Are the police actually using this activity now or will there be a trial period before it can be admitted as evidence into say, a court of law?

John - We have demonstrated the practical use of it with these gun casings and we’ve now been approached by a number of police forces in the UK and also a prosecuting attorney. In the US that have live and sometimes historical cases with gun cartridges. People say to us,

“Look this hasn’t worked conventionally. We haven’t got anything on it. We can’t do any more. Let’s have a go with your technique.”

Chris - John, thank you very much for joining us to talk about your work.

John - Thank you.

June 2008