Try something for me. Touch any surface, and unless you’re wearing gloves you’ll deposit a spectacular array of chemicals we call sweat. There are fats, amino acids, sugars, chlorides, sulfates, phosphates along with urea, lactic acid, metal ions, creatinine, choline, ammonia and of course water. These chemicals collect in the ridges on our skin and when deposited on a surface leave behind a fingerprint. But the problem, as far as forensic scientists are concerned is that the chemicals that make up sweat, and therefore fingerprints, are colorless.
The earliest technique for visualizing latent prints involved dusting with a powder such as graphite that adheres to sweat. The print that appears can then be photographed or lifted from the surface with tape. A number of chemical visualization techniques that can bring out latent fingerprints were also developed, with iodine fuming being the oldest. Iodine is a brown solid at room temperature but readily sublimes when warmed. Sublimation is the direct transformation of a solid to a vapour without going through a liquid phase. Iodine vapours dissolve in the fatty components of the fingerprint forming a brown image. The image will slowly disappear as the iodine evaporates but can be fixed if sprayed with a dilute starch solution. Starch and iodine form a dark blue stable complex.
Treatment with silver nitrate is another classic method. This time it is the salt content of the fingerprint that makes the technique possible. Silver nitrate reacts with sodium chloride to form a deposit of silver chloride which turns dark brown or black when exposed to ultraviolet light. This is the classic reaction that makes photography possible. Then there is ninhydrin, a compound that reacts specifically with amino acids, which are always present in sweat, to form a purple-blue product known as Ruhemann’s purple. The suspect object is sprayed with a ninhydrin solution and the latent print appears after several hours.
Cyanoacrylate visualization is the latest technique to reveal latent fingerprints. It uses, of all things, “Krazy Glue.” The amazing adhesive power of this glue is achieved by a polymerization process. A container of the glue contains small molecules known as cyanoacrylates which can link together to form a long molecular chain when exposed to moisture. This polymerization reaction takes place on the surface to which the glue is applied, triggered by atmospheric moisture. In the case of fingerprint visualization, the object in question is sealed in a container with a sample of the glue. The molecules of cyanoacrylate evaporate, fill the chamber, and then polymerize in the moist fingerprints, forming a white image. Unfortunately, all these clever chemical techniques can be nullified by the wearing of gloves.
