However, it was not until the mid 1950's that it was discovered ninhydrin could be used to develop latent prints on porous surfaces. When ninhydrin comes into contact withamino acids in fingerprint residue, the reaction yields a red to purple print. While effective on virtually any porous surface, ninhydrin is most appropriate for paper. On wood, silver nitrate may yield prints with darker ridges and better contrast because sometimes faint purple ridges of a ninhydrin print may get obscured by the wood grain and brown background color. This discussion, therefore, will address the use of ninhydrin on paper.
Several variables come into play.
First, the types of amino acids and their concentration in the print determine whether a reaction will occur. Second, the reaction is most effective in an acidic environment but many white papers have calcium carbonate fillers or whiteners. Such fillers result in a basic environment tin the paper unless a mild acid is added to the ninhydrin reagent. Third, temperature and humidity influence the development of prints. Fourth, the strength of the ninhydrin and the type of carrier affect the reaction.