The revelation of latent fingerprints with ninhydrin is a delight to organic chemists. In so far as the mechanism of development is a very plausible sequence of chemical reactions. According to an article entitled “The Development of Latent Fingerprints with Ninhydrin” by David Crown ninhydrin has been in chemical literature since it was first described in 1910 by Ruhemann. Staining latent prints with ninhydrin has been credited to Oden and von Hefsten in a 1955 British patent. Ninhydrin is the method of choice when attempting to reveal latent prints on paper. Typically, ninhydrin is mixed with a volatile solvent such as acetone and sprayed on the paper. The paper is then heated with a hair dryer or a flat iron to accelerate the reaction of ninhydrin with the amino acid residues contained in the latent print. The result is a blue or purple print. In order to generate a chromophore, ninhydrin must first condense with the primary amine of an amino acid to form an imine. The resulting imine can decarboxylate and hydrolyze in the presence of water to form a primary amine. This primary amine can condense with a second molecule of ninhydrin to form diketohydrindylidene-diketohydrindamine or “Ruhemann’s purple.” Evidently, the resulting prints can be enhanced with a 3% aqueous solution of zinc chloride. As the prints have a tendency to fade with time, it is best to take a photograph. It is reported that ninhydrin stained fingerprints can be readily subjected to other development techniques as well.
