One of the most well-known forensic techniques used in criminology and crime scene investigation is fingerprint comparison. The inside of the fingers and the palm of the hand are covered in tiny ridges that form a unique pattern for each person — even identical twins have different fingerprints — and these patterns remain the same throughout a person’s life. Sweat glands in the fingers secrete a fluid containing water, salts like sodium chloride, lipids, and amino acids, which deposits in the ridge pattern on surfaces touched by a finger. While the water evaporates quickly in most cases, the other components can last for a long time, leaving a pattern that, if recovered and analyzed, can be used to uniquely identify the individual. Fingerprints discovered at a crime scene can be compared to those of a suspect or those stored in a database; however, the prints must first be recovered and rendered fit for analysis.
To obtain suitable images from prints left at a crime scene, fingerprinting employs a variety of techniques. A fingerprint may be clearly visible on a surface like glass, or it may be an impression on a soft material; in these cases, all that is required is that the fingerprint be photographed. In other cases, the fingerprint may be hazy or not visible at all — this is referred to as a latent fingerprint — and treatment is required to create a print that can be analyzed.
Chemical treatment is required if the print cannot be dusted with a powder that adheres to the print. Dusting can be done with a variety of different powders. Depending on the nature of the surface, these are usually black or white in color; for dark surfaces, a white powder such as chalk or titanium dioxide may be used, while for light surfaces, a black powder such as charcoal or graphite is the best choice.
Latent fingerprints can be treated using a variety of chemical methods. Iodine fuming has been used for a long time. Iodine readily sublimes when heated, forming a vapor that reacts with the lipids in the fingerprint to form a dark brown compound, so exposing the print to iodine vapor will often reveal it clearly.
Another common chemical is ninhydrin, which is applied in liquid form and reacts with amino acids in the print to form a purple compound when heated. Cyanoacrylate fuming, also known as “super glue fuming” because cyanoacrylates are the main components of super glues, is a more recently introduced technique. On contact with the print, cyanoacrylate vapor forms a sticky, whitish substance. Dusting or treating with fluorescent chemicals can improve this even more.
Prints on patterns and minutiae are matched using fingerprint comparison. The main structure of the ridges is described by patterns, which are divided into three main types: arches, loops, and whorls, each of which has sub-categories. The four types of minutiae are dots, which are small, isolated fragments of ridges; bifurcations, where a ridge splits in two; islands, where a ridge splits then rejoins; and ridge endings, where a ridge terminates. Fingerprints taken at the scene of a crime can be compared to those of a specific suspect or to a large database of fingerprints stored electronically. The fingerprint comparison process is largely automated in the latter case, thanks to programs that use print matching algorithms.