Proof's in the print

A new system developed at Purdue University detects trace amounts of explosives, drugs or other materials left behind in fingerprints.

It can also distinguish between overlapping fingerprints left by different individuals - a difficult task for current optical forensic methods.

A team led by R Graham Cooks, Purdue's Henry Bohn Hass Distinguished Professor of Analytical Chemistry, created the system that reads and provides an image of a fingerprint's chemical signature.

Cooks said: 'The classic example of a fingerprint is an ink imprint showing the unique swirls and loops used for identification, but fingerprints also leave behind a unique distribution of molecular compounds. Some of the residues left behind are from naturally occurring compounds in the skin and some are from other surfaces or materials a person has touched.'

Demian R Ifa, a Purdue post-doctoral researcher, said the system can also easily uncover fingerprints buried beneath others.

He said: 'Because the distribution of compounds found in each fingerprint can be unique, we can also use the system to pull one fingerprint out from beneath layers of other fingerprints.

'By looking for compounds we know to be present in a certain fingerprint, we can separate it from the others and obtain a crystal clear image of that fingerprint. The image could then be used with fingerprint recognition software to identify an individual.'

Researchers examined fingerprints in situ or lifted them from different surfaces such as glass, metal and plastic using common clear plastic tape. They then analysed them with a mass spectrometer developed in Cooks' lab.

Mass spectrometry works by first turning molecules into ions, or electrically charged versions of themselves, so their masses can be analysed.

A conventional mass spectrometer requires chemical separations, manipulations of samples and containment in a vacuum chamber for ionisation and analysis. Cooks', however, performs the ionisation step in the air or directly on surfaces outside of the mass spectrometer's vacuum chamber, making the process much faster and more portable, Ifa said.

The Purdue system performs the ionisation step by first spraying a stream of water in the presence of an electric field to create positively charged water droplets. Water molecules in the droplets contain an extra proton and are called ions. When the charged water droplets hit the surface of the sample being tested, they transfer their extra proton to molecules in the sample, turning them into ions. The ionised molecules are then vacuumed into the mass spectrometer to be measured and analysed.

To test out the system, the researchers placed a section of tape containing a lifted fingerprint on a moving stage in front of the spectrometer. The spectrometer then sprayed small sections of the sample with the charged water droplets, obtaining data for each section and combining the data sets to create an analysis of the sample as a whole. Software was used to map the information and create an image of the fingerprint from the distribution and intensity of selected ions.

The system has now been commercialised by Indianapolis-based Prosolia.