Polymerisation reveals hidden fingerprints

15 October 2010

Fingerprints leave their mark on surfaces that remain even if they are washed off allowing them to be detected using disulphur dinitride, claim UK scientists.

Fingerprint analysis of crime scenes has become a powerful weapon in the forensic scientist's arsenal. The ridges present on the tips of the fingers are unique to a person. By comparison of impressions left in materials - typically flat, regular surfaces, such as glass - to a sample taken from a suspect it is possible to identify the owner of the fingerprints.

But there are occasions where fingerprints are not easily viewable or are fragmented. Here, latent fingerprinting - where obscured fingerprints are enhanced - can be used to identify an individual. In 2008, Paul Kelly and co-workers at Loughborough University noticed that the strained four-membered ring system S2N2 quickly polymerises to (SN)x in the presence of fingerprints. Detecting this polymer produces a visual image of the fingerprint.

It could be possible to identify fingerprints that have been washed from surfaces

Now they have shown that this polymerisation still occurs when the chemicals in the fingerprint that were thought to trigger it are 'washed off' the surface, which could occur by simple wiping or an explosive blast. The polymerisation is triggered by an effect brought about by interaction of the chemicals with the surface before they are removed.

'As long as the print has been on the surface long enough to bring this about before washing off - and this only has to be hours or less - this signature will be present and we can image it,' explains Kelly. He goes on to say that the key to this process is that it is 'based on the interaction of S2N2 vapour with the surface'. The crucial point is that a vapour can reach areas of a surface that are not accessible to solids and liquids, such as the crumpled remains of an explosive device, he adds.

John Plater, an expert in the enhancement of latent fingerprints at the University of Aberdeen in the UK sees promise in the system: 'developing latent fingerprints on metal surfaces is difficult and a vapour phase method may offer added sensitivity to existing methods.'

Kelly is keen to stress that this research is still in its infancy and the team is working on further research to develop the method so that it could be used in forensics.

Jon Watson


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