Effect of manganese on ninhydrin color development by amino acids

Addition of microgram quantities (5–10 μg per 2 ml) of Mn2+, Fe2+, and Mo2+ increases the intensity of color developed by amino acids two- to three-fold when reacting with ninhydrin. The lowest limit detected by the increased sensitivity of the reaction is 3 nmol of an amino acid. Cd2+, Zn2+, and Al3+ depress the color formation due to reaction between amino acids and ninhydrin. When chromatograms of amino acids are first sprayed with aqueous manganese chloride and later with ninhydrin or with ninhydrin solution containing manganese, not only is the sensitivity increased, but the stained spots retain their color for longer periods.