Hot metal

New fingerprint technology which analyses levels of salt and sweat could prove invaluable in the fight against serious crime and terrorism, reports Damian Small.

Professor Neil McMurray and his team at the University of Wales in Swansea have developed a technique that allows them to take prints from surfaces exposed to temperatures as high as 600C.

Conventional techniques have relied on establishing prints from fat and amino acid secreted through the skin, which dissolves when heated. Gun cartridges and bombs usually heat to around 500C – an obstacle temperature, which has long foxed US law enforcement agencies such as the CIA and FBI.

Speaking about the project, Professor McMurray said: “The basic aim is to visualise fingerprints on metallic and electrically conductive surfaces without the need to first ‘develop’ them – that is render them visible, using fingerprint powders or chemical reagents. Powders and reagents interact with the organic (fat and amino acid) fraction of the print deposit, but often do not work well with so-called ‘ecrine’ sweat prints which are composed mainly of water-soluble salts.

“This problem is compounded when organic portions of the print are lost through heating, as in the firing of a weapon or detonation of a bomb. Our approach is to use a machine called a Scanning Kelvin Probe (SKP), which takes a picture of the print-bearing surface in electrical potential and measures the electrical reactions generated by the sweat on the metallic surface. It visualises the differences in surface electrical potential induced by the inorganic salt component of the print.

“Prints may be visualised directly, without prior development, and because it is the involatile salt component which is responsible, prints may be recovered from ecrine deposits and from surfaces that have been heated up to 600C.

“Many metals spontaneously develop passive surface films, which may range from nanometers to several microns in thickness, when exposed to moist air. However, so-called aggressive ions, such as chloride, tend to break down passive films and activate metal surfaces through the formation of soluble metal chloride complexes. Consequently, the chloride rich inorganic salts present in the aqueous component of fingerprint secretions will tend to cause local depassivation of a metal surface in areas where secretions are deposited. So when a Kelvin microprobe is scanned over a latent fingerprint, it is to be expected that ridges will be disclosed as regions against the background of passive metal.

“In scientific terms, our work is demonstrating exactly how the inorganic (salt) component of fingerprint deposit interacts with surfaces. There has been much speculation about this topic, but it was very difficult to study effectively prior to the application of SKP. By finding out how different salts interact with different metals over different periods of time we will be able to say where and when the SKP is likely to be forensically useful.

“But there are spin-off gains as well. For example, our findings show that under the right conditions fingerprint deposits can induce a highly localised form of corrosion which could weaken critical components in, for example, aircraft structures.”

The team has received a grant of just over £200,000 from the Engineering and Physical Sciences Research Council (EPSRC) to conduct the work, and are currently collaborating with the Forensic Science Service and defence technology company QinetiQ to develop a version of the SKP which can be used by forensic scientists to tackle problems such as fingerprinting fired cartridge cases and bomb fragments.

“The FSS are providing us with information regarding the type of problem they face and with real weapon samples. QinetiQ will help with the production of the final SKP instrument,” said Professor McMurray. “We hope to have produced a forensically usable prototype SKP by the end of our EPSRC contract. This instrument could be used in the frontline, bu