“Your Honor, the accused’s fingerprints were found at the scene of the crime.” In how many novels, movies and TV shows have those words, or variations on them, spelled doom for a criminal? Of all the tools available to criminal investigators, fingerprinting is probably the one most familiar to the public at large.
That’s not too surprising, considering how long people have recognized the value of fingerprints as a form of identification: even the ancient Assyrians and Chines used fingerprints to sign legal documents. However, the modern use of fingerprinting began in the 19th century. A Czech pyschologist, Johannes Evengelista Purkinje, proposed a system of fingerprint classification in 1823, but didn’t get much response. Later in the century, however, British scientist Sir Francis Galton wrote a detailed study of fingerprints and presented a new classification system, using prints of all 10 fingers, which is the basis of the systems still in use.
Fingerprinting and police work became linked in the 1890s, when the police in Bengal, India, under the British police officer Sir Edward Richard Henry, began using fingerprints to identify criminals. Henry went on to become assistant commissioner of metropolitan police in London, where he established the first British fingerprint files in 1901. The practice of using fingerprints to identify criminals quickly spread through the rest of Europe and into North America. (Before fingerprinting came along, criminals were identified using a variety of body measurements.)
So just what is a fingerprint? It’s an impression made by the tiny “friction ridges” found on the underside of the ends of our fingers. Of course, friction ridges didn’t evolve just to make it easier for criminals to be identified by police. As their name implies, they’re really there to increase the friction between our skin and objects we want to pick up, making them easier to get a grip on.
What makes them such a great way of identifying a person is that they don’t change as a person ages, and every person’s set of friction ridges is unique. Even identical twins have different fingerprints.
Fingerprints are collected from a person by the simple procedure of rolling each finger on an ink pad, then rolling the finger onto a piece of paper. This produces a clear image of the loops, whorls (spiral-like shapes) and arches that are the basic fingerprint patterns. Using a computer, the prints can easily be checked against millions of other fingerprints on file. (The FBI alone maintains a database of some 173 million sets.)
Collecting fingerprints from a crime scene, however, is harder. There are three classes of fingerprints: visible (which can be seen by the naked eye, such as an inked fingerprint on a piece of paper), latent (usually invisible to the naked eye without some sort of development) and molded (an indented print left in putty on a window frame, for example, or in blood on a surface). The most common type of print is the latent print, which are the result of the transfer of the thin, invisible film of sweat and body oils from the finger to the object touched.
Smooth, hard surfaces are the best places to find fingerprints. Fingerprints aren’t normally found on rough surfaces, undressed woods, grained leather, rusty or cold metal, cloth or any other absorbent or porous material. As well, surfaces covered with dirt, grease or dust are also less receptive to fingerprints (unless the grease is thick enough to hold a fingerprint itself). Glass, aluminum foil, polished metals, polished woods, plastic bags, china, smooth painted surfaces and hard plastic surfaces, on the other hand, are very good at collecting fingerprints. Excellent fingerprints can be taken from paper, too, provided it’s not too porous.
Investigators use a variety of techniques to make latent prints visible. The oldest, and most familiar to the public, is a process called “dusting.” The investigator dips a brush in a container filled with graphite shavings, then lightly brushes the area he’s searching for fingerprints. The graphite shavings adhere to the sweat and oil that form the fingerprint; a copy of the print can then be lifted from the surface by placing clear tape over it, peeling the tape off and then placing the tape on a white card. A similar method uses lead shavings, which are brushed onto the print, then lifted off with a magnet instead of with tape.
Several more high-tech methods are also used. Special chemicals applied to the surface react with amino acids in sweat to become fluorescent when bombarded with laser light. In other words, the fingerprints glow in the dark–and therefore can be photographed.
Another method is to make the fingerprint itself a kind of photographic emulsion, by making the grease in the fingerprint react with silver ions. The fingerprint literally develops into a clear image of itself.
Today, fingerprints can even be lifted off surfaces (such as plastic bags) where it would have been impossible to find them a few years ago with the “Crazy Glue” method. Crazy Glue contains a chemical called cyanoacrylate which, when heated, vaporizes and adheres to fingerprints. Fluorescent dye can then be added and, under ultraviolet or laser light, the fingerprint, again, glows in the dark.
One surface police would particularly like to be able to get clear prints from in homicide cases is human skin. The problem is the victim’s skin contains the same sweat and oils as the fingerprints are made of. Researchers have been grappling with this problem for years, and although a few prints have been successfully retrieved, the method used (involving coating the print with iodine and pressing a silver plate against it to transfer it) is time-consuming and usually results in a uselessly blurry print.
An RCMP chemist named Della Wilkinson has come up with a better method, currently being tested, that could make it much easier to get clear prints from murder victims’ bodies. The technique, called alpha-napthoflavone, has been used by the British to detect terrorist fingerprints on walls, but has never been used this way before. The body is exposed to iodine fumes, then sprayed with another chemical. Fingerprints on the body turn a dark, highly visible blue. Wilkinson’s work has already attracted a great deal of attention; she’s recently been collaborating with the FBI on a study to find out how long fingerprints last on a body and how effective and efficient her technique will prove to be.
Fingerprinting has become so synonymous in the public’s mind with irrefutable proof that the hottest new method of proving that someone was at a crime scene is called “DNA fingerprinting,” even though it has nothing to do with fingerprints.
Or, at least, it didn’t until now. Researchers at the Victoria Forensic Science Centre in Victoria, Australia, report last week that they have been able to obtain DNA from the surface of objects that people touched for as little as five seconds as long as a year ago. They’ve recovered it from gloves, glasses, mugs, pens, car keys, briefcases, knives, locker handles and telephone headsets. They’ve even managed to recover DNA that was transferred from one person to another by shaking hands.
What this means is that even a fingerprint too smudged for ordinary identification could provide sufficient DNA for a “DNA fingerprint” to be constructed, providing investigators with a powerful new tool in the search for evidence. In fact, the research has already been used to provide evidence in cases of extortion, attempted murder, rape, armed robbery and drug trafficking.
It just doesn’t get any simpler to be a criminal, does it? Even with Sherlock Holmes on his tail, Moriarty had it easy.
Can fingers prints be taken off plastic totes or cardboard.
I don’t know!