Fingerprinting the Unfingerprintable

Chris - Fingerprints have been used to catch criminals since at least 1858 and since then, they've been the key to solving countless crimes. But some surfaces are hard to lift fingerprints from especially if that surface has been cleaned or chemically altered in the interim. Now, researchers at Swansea University have developed a new technique to fingerprint some things that were previously unfingerprintable. Dr. Geraint Williams helped to develop the system and he's with us. Hello, Geraint.

Geraint - Good evening, Chris.

Chris - So first of all, what actually, chemically speaking, is a fingerprint?

杰伦特——好吧,我想如果你想定义what a fingerprint is, it would be the deposit that's left behind when the fingerprint ridges or the fingertip friction ridges make contact with the surface. But compositionally speaking, there's a whole chemical cocktail in a fingerprint deposit. There are two parts to a fingerprint. There's the eccrine part which is basically secretions produced by the eccrine glands in your fingertips - and that is basically sweat. But sweat is pretty complex on its own; principally water but it contains things like chloride salts of various metals, sodium, magnesium chloride for example, things like urea, lactic acid, amino acids, et cetera. But there's also a second component to a fingerprint. These are sebaceous deposits which are produced by sebaceous glands, not in your fingertips paradoxically, but other parts of the human body, for example, in your face. So a person might touch their foreheads or the side of their nose and pick up these sebaceous deposits which are basically a whole cocktail of long chain fatty acids and fats by themselves. So the fingerprint is made up of a real chemical cocktail which is difficult to actually characterise.

Chris - So normally, if you dusted for prints, you'd be dusting, applying a chemical that would stick to those residues and that's how you'd visualise it, but why then would some surfaces not be fingerprintable?

Geraint - I guess if a fingerprint has changed or if it's simply just made up of an eccrine deposit. So, lots of the developers that are used to give you a high visual contrast between a fingerprint pattern and the surface in which it's deposited rely on the interaction of a developer with that sort of organic part, the fatty part, of the fingerprint. So if it's missing, lots of these developers simply won't work.

Chris - So how does your approach differ?

Geraint - Our approach differs from most schematic techniques in that it relies on measuring the interaction that the fingerprint itself makes with the surface on which it's deposited.

Chris - So talk us actually about your method. What does it involve?

Geraint - It's an instrumental method. The instrument we use is called a scanning Kelvin probe and it relies on scanning a very fine probe over their surface of interest. So, we're mechanically scanning the probe over the surface, the probe doesn't actually touch the surface. And what it does is measure a quantity called the Volta potential difference between the probe and the area of the sample directly underneath the probe, and that is an indicator of the chemical nature of that surface. So, if for example, a fingerprint deposit is on that part of the surface, then we'll have a considerably different Volta potential difference to a part which isn't in contact with the fingerprint deposit. Usually our technique is good for metallic or conducting surfaces and basically metals, and the chloride salts in the fingerprints actually de-passivate, cause a small amount of corrosion to occur, underneath the fingerprint deposit and it's that small amount of corrosion that we actually pickup with our scanning Kelvin probe.

Chris - What happens when someone comes along and cleans the surface? Of course, if a corrosion event has occurred, that won't be wiped off then.

Geraint - No, exactly. For example, if a criminal has been trying to cover his tracks by wiping off the fingerprints with a cloth, there is sufficient interaction of the chloride salts in fingerprints with the oxide film covering the metal for that chloride to be retained in that particular region, and it won't be wiped off physically.

Chris - What sorts of things are you using this for or could this be used for where present techniques just wouldn't cut it?

Geraint - The major items of interest to us are things like spent cartridge cases.

Chris - From guns for example.

Geraint - Yes, from serious gun crimes and those pieces of evidence. So if for example there's a shooting and somebody has left some shell cases at the scene of the crime, typically, it's almost impossible to lift any fingerprint detail from those pieces of evidence, providing maybe that the criminal has actually put his fingerprints on them to start with before they're actually fired. And that's because obviously, when you fire a bullet, the casings are subjected to fairly high temperatures in the firing mechanism. There's also some contamination from blowback propellant for example. There's also friction from the ejection mechanism. All these phenomena can try to make it extremely difficult to actually give any kind of fingerprint visualisation on that type of surface. But with the Kelvin probe, because most of the organic part of the fingerprint will be lost for example at the high temperatures to which the casing will be exposed, the Kelvin probe gives you a fighting chance because there's always a good chance that the chloride-rich deposits will still be there because they won't be volatilised at those kind of temperatures.

Chris - Terrific, Geraint. That's Geraint Williams from the Department of Engineering at Swansea University.