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Fingerprint comparison is one of the best-known forensic techniques employed in criminology and crime scene investigation. The skin surface on the inside of the fingers and the palm is covered by tiny ridges that form a unique pattern for every person — even identical twins have different fingerprints — and these patterns remain unchanged throughout a person’s life. Sweat glands in the fingers secrete fluid containing water, salts such as sodium chloride, lipids and amino acids that leave a deposit in the pattern of the ridges on surfaces touched by a finger. While the water evaporates fairly quickly under most circumstances, the other components can persist for long periods, leaving a pattern that, if it can be recovered and analyzed, will uniquely identify the person. Fingerprints found at a crime scene can be matched against those from a suspect or on a database; however, before fingerprint comparison can take place the prints must be recovered and rendered fit for analysis.
A number of techniques are used in fingerprinting to obtain suitable images from prints left at a crime scene. A fingerprint may be clearly visible on a surface such as glass, or it may be an impression on some soft material; in these cases, all that might be required is for the fingerprint to be photographed. In other cases, the fingerprint might be unclear or not visible at all — this is known as a latent fingerprint — and treatment is required to produce a print that can be analyzed.
Dusting with a powder that adheres to the print might be effective — otherwise, chemical treatment is required. A variety of different powders can be used for dusting. These are usually black or white in color, depending on the nature of the surface; for dark surfaces a white powder such as chalk or titanium dioxide might be used, while for light surfaces a black powder, such as charcoal or graphite, would be the best choice.
There are a number of chemical methods for treating latent fingerprints. A longstanding method is iodine fuming. Iodine, when heated, readily sublimes, forming a vapor which reacts with lipids in the fingerprint forming a strongly colored brown compound, so exposing the print to iodine vapor will often reveal it clearly.
Another chemical that is often used is ninhydrin; this is applied in liquid form and, when heated, reacts with amino acids in the print forming a purple compound. A more recently introduced technique is cyanoacrylate fuming, also known as “super glue fuming,” as cyanoacrylates are the main components of super glues. Cyanoacrylate vapor, on contact with the print, forms a sticky, whitish substance. This can be further enhanced by dusting or treatment with fluorescent chemicals.
Fingerprint comparison matches prints on patterns and minutiae. Patterns describe the main structure of the ridges and are categorized into three main fingerprint types: arches, loops and whorls, each of which can be divided into sub-categories. Minutiae are small irregularities within the main pattern and fall into four categories: dots, which are small, isolated fragments of ridges; bifurcations, where a ridge splits in two; islands, where a ridge splits then joins again; and ridge endings, where a ridge terminates. Fingerprints taken at a crime scene can be compared with those from a particular suspect or with a large database of fingerprints held electronically. In the latter case, the fingerprint comparison process is largely automated, using programs that employ print matching algorithms.