Some components that are separated from a mixture by paper chromatography are colourless. A locating agent is sprayed on the chromatograph to make these colourless components visible. An example of a locating agent is ninhydrin (2,2-dihydroxyindane-1,3-dione), which is used to identify colourless amino acids in the separation of an amino acid mixture.
Ninhydrin reacts with colourless amino acids (with primary amine groups) to form the ninhydrin chromophore, which is purplish blue.
Iodine and UV light are other examples of locating agents.
