What is ninhydrin – in forensic science

A versatile compound used to visualize latent fingerprints on porous surfaces such as paper and cardboard. It can also serve as a developing agent for use in thin layer chromatography (TLC) in drug analysis and toxicology. Ninhydrin is also sometimes referred to as triketohydrinhene hydrate. Because of its versatility and sensitivity, it has become one of the primary tools in latent fingerprint visualization and has displaced iodine fuming as the method of choice for porous materials. The first synthesis of ninhydrin is attributed to Ruhemann in 1910, and the characteristic purple produced by ninhydrin reacting with latent prints is called Ruhemann purple. However, ninhydrin was not recognized as a developer for latent prints until the 1950s. Since then, use of and research into ninhydrin has expanded, leading to several breakthroughs.

Ninhydrin reacts with the amino acids and their degradation products that are part of any latent print. Amino acids are an especially attractive target for development since they adhere to cellulose, the primary ingredient in wood-derived products such as paper and cardboard. They are also fairly stable, allowing prints to be developed long after they were originally deposited. Ninhydrin can be swabbed or sprayed onto a surface, or the entire article can be dipped into a solution. NFN is a designation for a nonflammable spray containing ninhydrin and a propellant that will not burn such as a hydrochlorofluorocarbon. Once ninhydrin is applied to an article, development of the prints can take time (hours or even days), but increasing heat and moisture in the development environment can accelerate it. Ninhydrin can also be used in a sequence of several reagents, the most common being DFO (a ninhydrin derivative), followed by ninhydrin, and a mixture known as physical developer (PD). Ninhydrin and related compounds such as DFO can also be coupled with laser luminescence and metallic salts such as zinc chloride (ZnC^) used with alternate light sources and lasers. As a result, very faint fingerprints can often be developed. Research into ninhydrin and related compounds is continuing at a rapid pace, resulting in ever more sensitive techniques, reagents, and sequences.