How does ammonia remove fingerprints?

Traces blur uselessly

Ank Tatort and CSI, every TV viewer knows them: magical powders that TV inspectors dust with bulky brushes at the crime scene in order to make invisible fingerprints clearly recognizable. Many a fictional villain has already been convicted. But the reality is sobering: "The success rate when analyzing fingerprints is only about ten percent," says Rob Hillman, professor of physical chemistry at the University of Leicester in the UK. What can be done to increase this meager yield - find a new wonder powder? Hillman's answer: "No, we need a whole new approach."

Most of the prints are invisible

Most of the fingerprints the police find during their investigations cannot be seen with the naked eye. Immediately visible fingerprints - the perpetrators ensure that - are usually blurred when the investigators arrive. The forensic technicians therefore have to "powder off" all suspicious objects and surfaces in order to find traces. The substances stick to the fingerprints, which are nothing more than deposits of sweat and fat.

The problem: before the investigators found them, the prints were often exposed to destructive environmental influences such as moisture or friction for a long time. There is not much left of the traces - often so little that the powder from the forensic technician hardly sticks to them. The result is an incomplete image of an imprint that cannot be used to identify the perpetrator.

Rob Hillman and his team have tackled the problem at the root: If the deposits from the grooves of the fingerprint are not enough, then the gaps in between have to be used. The researchers fill them with a fluorescent polymer. “This gives us a negative image of the fingerprint, so to speak,” explains Hillman.

Electric current that the researchers send through the examined sample ensures that the polymer is deposited. The prerequisite for this, however, is that the fingerprint is on a conductive surface. In the case of a crime, for example, it could be a metallic weapon such as a knife or a pistol. Because the fat in the impression is electrically insulated, the polymer only adheres to the gaps. Even residues a few nanometers thick prevent the polymer from being deposited and thus provide a clear image of the fingerprint.

The new procedure is uncomplicated and inexpensive - and therefore well suited for quick analyzes in the police laboratory. "Tatort" and "CSI" send their regards. •

by Stephanie Kappes

April 15, 2014