Do you often realise that during your whole day, while doing whatever you are doing, you leave marks on everything you touch? Probably not. Luckily, most criminals do not either. Most people know about fingerprints, but many underestimate the technology that is available to handle and analyze them. Thinking that partial or overlapping prints cannot be traced back to them, criminals often make the mistake of leaving fingerprints at crime scenes or on objects, handing over important material to the police. How are those prints deciphered and translated into useful information?
 

What does a fingerprint consist of?

When thinking of a fingerprint, you probably think of an arrangement of lines and loops, left as an impression by the friction ridges of a finger. But what do these lines consist of and how can they remain on a surface for sometimes up to forty years? First of all, keep in mind that, just like the rest of our world, fingerprints consist of molecules. We can divide the molecules that make up fingerprints into three groups:
 

• The body’s own chemicals and substances 
  We all produce chemicals that leave our body in the form of sweat. However, everyone produces different amounts of those chemicals, making our sweat quite distinguishable.
• Molecules that we put into our own body
  Our sweat also consists of molecules that we introduce into our own body, for         example food, drinks and drugs. Therefore, fingerprints can tell what a person ate, drank and also if he or she did drugs in some prior period.
• Molecules that our fingertips came in contact with
  Of almost every substance we come in physical contact with, a tiny bit remains on our skin. When again touching some other object, an even tinier  bit is left behind there. In this way, it can be figured out whatever the person to who the prints belong has come in contact with.

 

 
Those three types of molecules make for a very specific composition of the fingerprint. This composition can give us a lot of clues about possible suspects of a crime. However, for true evidence, also the arrangement of the fingerprint itself has to be studied and compared to the suspect.
 

Uniqueness of a fingerprint

Compared from the substances that a fingerprint consists of, the arrangement of a fingerprint itself is even more distinguishable. When a fingerprint is read by a computer, it tries to recognize specific patterns in the composition, such as loops, arches or junctions. The placement of and the distance between these patterns in the print is translated into computer code and saved.

In fact, a fingertip is unique on its own. However, some person’s fingertips may look quite similar to another’s, and the prints may look almost identical. It depends just on how clear the print was left and how it is processed into a database. As fingerprints are read by a computer in the form of pixels, there will always be a resolution at which they are similar. Naturally, a partial or faint fingerprint is harder to match to a person correctly than a whole print. It is therefore important that clear, whole prints are gathered, before any verdict can be returned.
 

Interpreting fingerprints

When it gets to recognizing the pattern, it briefly comes down to a computer comparing the print that was found to fingerprints in a database. However, when identifying the molecules in the substances, it gets more complicated. Mass spectrometry imaging (MSI) is used to analyse the composition of the fingerprint. This form of mass spectrometry visualises the distribution of molecules by measuring the mass of one very small sample at a time. The distributions of molecules in all samples together provide data on the composition of the matter on one’s fingertip. This method allows for detection of all sorts of materials, such as paint, drugs or organic matters. Especially advantageous about this method, is that fingerprints only containing specific molecules can be made out. In this way, a fingerprint can be constructed for every type of molecule present in the print, granting many more fingerprints from a single mark. This technique also helps enormously when multiple prints are overlapping each other, as there are always substances present in one print and not in the other.
 

 
By simply selecting prints of molecules that are in one print and not in the other and vice versa, prints can be easily isolated.
 

Catching the bad guys

In the USA, the Integrated Automated Fingerprint Identification System (IAFIS), the fingerprint database used by the FBI, contains fingerprints of over 100 individuals, of which roughly 70 million are in criminal master files. Obviously, when a person has his fingerprints in such a database, any of his prints found can be traced back to him. In this case, MSI can help isolate and improve the print to simplify the matching process. It happens often, however, that prints are found that do not immediately form a match with any prints already in the database. Here, MSI comes in very handy to gather clues about the suspect. Information on where the suspect has been, what he has done and even what he has eaten and drunk can be obtained from a print. This has often led to quick arrests and with the improvement of the spectrometry technology, it will only get easier in the future.

Dit artikel is geschreven door Pieter Dilg