Fingerprints

HISTORY

In 1880, Henry Faulds suggested that fingerprints were unique to an individual and could be used as the basis of identification by the police. The uniqueness of an individual’s fingerprints is now a long established fact and provides the basis of fingerprint evidence as a key-stone in many convictions at court.

In 1901 the British police adopted the Henry classification system as the basis for fingerprint identification. This system had been developed by Edward Henry and his assistant Azizul Haque a few years before.

A year later a man called Harry Jackson was convicted of a burglary by comparing his fingerprints with those from a crime scene, the first time a conviction had been achieved in a British courtroom using fingerprint evidence.

Since then fingerprinting has become established as the primary forensic tool in a crime scene examiner’s arsenal and, even though other techniques and disciplines have become essential, it has remained the one procedure which is used at every crime, from a simple criminal damage, to the most serious mass murder.

TECHNIQUES

The commonest fingerprinting technique is the one which most people will have seen being undertaken. Namely the application of some kind of powder with a fine filament brush to a smooth surface in an effort to develop latent fingerprint detail. Once developed the fingerprints are recorded and recovered onto a clear acetate sheet using a special clear lifting tape.

This technique works because nature has evolved our fingertips into many ridged grooves, to increase the friction that can be applied to an object to grasp and manipulate it. In addition the ridges are liberally supplied with pores which secrete moisture to further help the friction and thus aid our dexterity. The pores will naturally secrete more moisture when we are stressed or physically exerting ourselves, further helping us to securely manipulate objects. As a consequence, when we touch things the moisture on our fingers is transferred to the objects in the same pattern as our ridge detail, in much the same way as a child prints with paint, using a halved potato with a shape cut into the surface. In this case however, the pattern is invisible, as the moisture from our pores is clear. We say that this print is latent, i.e. that it is concealed, dormant and (literally) undeveloped.

Luckily the latent print has a different adhesive factor to the background surface and a gentle dusting with an opaque powder will cause the dust to stick to the ridge detail, whilst the powder on the background will fall or be dusted off, revealing the fingerprint.

Sometimes, a person’s fingers will be contaminated with substances such as oil, ink or blood. If they subsequently touch something a visible fingerprint may be deposited. This type of fingerprint is known as a patent fingerprint. The most obvious forensic application for this type of print would be for a suspect’s fingerprint to be created from a victim’s blood.

In this case it would also be essential for a sample to be taken of the blood from a contiguous (i.e. linked) area to the fingerprint to prove, using DNA profiling, that the blood which made up the ridge detail was indeed the blood of the victim.

A final type of fingerprint which might be encountered is a plastic one. In this case a person has handled an item in which a three dimensional copy of the print is created. This type of fingerprint can be seen if a finger is pressed into plasticine or clay. It can sometimes be seen in crime scenes in foodstuffs such as butter or margarine or in the thick, tacky paint on the side of paint tin.

This type of fingerprint is usually best recovered intact so that it can be recorded using specialist photographic techniques. Care must be taken that the fingerprint is photographically reproduced the correct way around so that a ‘negative’ fingerprint is not created. This is easily done and can be quite confusing. The correct final image should show the ridge detail in black and the background in white, as if it were a fingerprint on paper, taken with black ink. There are other items on which fingerprints can be deposited, which cannot be revealed using conventional fingerprinting techniques. In these cases the invaluable services of specialist equipment, operated by trained fingerprint technicians, is called upon. These experts are usually situated at a central fingerprint bureau. However they can be called on to attend the more serious crimes, bringing their equipment and expertise to where it is needed.

Paper is, surprisingly, a very good medium from which fingerprints can be recovered. A reagent called Ninhydrin is sprayed onto the paper and reacts with the amino acids in the fingerprint to reveal a purple stain corresponding to the ridge detail. This technique is easy to perform and is cost-effective. After drying, the fingerprints are photographed and the resultant images searched against the national fingerprint database or specific, named suspects.

After treating with Ninhydrin, the paper can also be treated with a silver based aqueous reagent called Physical Developer. This reacts with the sebaceous sweat in the fingerprint to form a grey deposit. Because this technique targets a different component of the fingerprint deposit, additional fingerprints and ridge detail can be revealed over and above that which the Ninhydrin alone can reveal.

Plastic items and polythene bags can be treated using a number of procedures. The two main techniques are called Metal Deposition and Superglue.

With Metal Deposition, the item to be examined is placed into a pressure vessel and a partial vacuum generated. A small quantity of gold is then heated to evaporation and the item is invisibly coated with gold. This process is then repeated with zinc. The zinc is attracted to the areas where fingerprints are located at a different ratio to the background level of the item, and the fingerprints are thus revealed.

The Superglue technique involves the item being placed into a fume cabinet at room temperature with a high humidity level. A quantity of Superglue (Ethyl Cyanoacrylate) is then vaporised at about 120o C and the vapour polymerises onto the fingerprints to form a white deposit, again revealing the fingerprint ridge detail.

FINGERPRINTING PROCESS

Fingerprints are developed, recorded and recovered at a crime scene and items which could, potentially, reveal fingerprints after chemical treatment are seized. These items must be treated carefully if the fingerprints are not to be damaged or lost. They are then transported to the fingerprint bureau along with any photographs and acetate lifts of the fingerprints recovered from the crime scene.

Once received the acetate fingerprint lifts are photographically printed and scanned onto the National Automated Fingerprint Identification System (NAFIS). They are then searched against the UK’s database of offender’s fingerprints and any specifically named suspects in the case.

The other items are assessed for the most appropriate treatments and a schedule established. This is important, as some techniques interfere with, or preclude others. Once they have been through the various processes, any fingerprints are recorded, photographed and the images scanned onto NAFIS for searching.

The fingerprints scanned onto NAFIS are coded by a fingerprint expert. The resultant code is searched using the NAFIS algorithms and any potential matches with a suspect returned to the fingerprint expert’s terminal. These returns are compared by the expert and a potential match made. These identifications are peer-checked by other experts to ensure that the match is valid and will bear scrutiny at any subsequent trial.

Once the identification has been verified an identification letter is sent to the C.S.I. department for circulation to the Officer in the case. This information is sent to the officer in the case (OIC) with an evaluation of the potential evidential value of the fingerprints.