New Delhi (PTI): A new technique, which promises to lift fingerprints from a metal weapon even if it is washed, rubbed or wiped off, can become a new armour in the hands of forensic experts.

The technique would help the investigators crack important cases such as the Arushi murder and other high profile cases where the perpetrators of the crime had destroyed vital evidence to escape the clutches of law.

"In a research, forensic experts have found that the fingerprint on a metal never dies. And all of this can be attributed to the high content of ionic salt present in sweat in the hands that corrodes the metal," Director of Directorate of Forensic Sciences M S Rao said.

Dr John Bond, a researcher at the University of Leicester in England, developed this novel technique.

"To visualise a fingerprint after it is rubbed off, scientists measure the corrosion of the surface by deposits from the fingerprints," A Shweta Murthy, a forensic science student of Amity University, explained.

When one consumes processed food that tends to be high in salts as a preservative, the body excretes this excess salt as sweat. This sweat also comes out through pores in our fingers.

"So the fingerprint impression one leaves when one touches a surface will be high in salt. If one eats a lot of processed foods, the salt content will be higher and there will be higher chances of corrosion of the metal," Murthy said.

The ionic salts present in the fingerprint residue (mainly chloride ions) corrode the metal surface held.

Rao said the hidden fingerprints can be visualised even if wiped off when subjected to electrostatic charging and then followed by introduction of conducting powder, similar to the one used in the photocopy machines.

"The corroded parts having lower potential than the surface of metal attracts the conducting powder and helps visualise the hidden fingerprints," he said.

The degree of enhancement depend on the composition of the metal and level of salts secretion by fingerprint donor. Wiping the fingerprint downwards or washing it in hot soapy water makes no difference and heating of the metal produces a more durable sample.

This finding can help get prints from people who handled a cartridge before it was fired. This technique works on bullet casing to machine guns. Even if heat vaporises normal clues, police will be able to prove who handled a particular gun, Rao said.