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Ninhydrin will react with a free alpha-amino group, NH2-C-COOH. This group is contained in all amino acids, peptides, or proteins. Whereas, the decarboxylation reaction will proceed for a free amino acid, it will not happen for peptides and proteins. Thus, theoretically only amino acids will lead to the color development. However, one should always check out the possible interference from peptides and proteins by performing blank tests especially when such solutions are readily available. For example, one can simply add the ninhydrin reagent to a solution of only proteins and see if there is any color development. There is no excuse for failing to perform such a vital test when the sample mixture contains both proteins and amino acids. There are also reports that chemical compounds other than amino acids also yield positive results.
This test can be used routinely for the detection of glycine in the absence of other interfering species. Although this is a fast and sensitive test for the presence of alpha-amino acids, because of the nonselectivity, it cannot be used to analyze the relative individual contents of a mixture of different amino acids. Furthermore, the color intensity developed is dependent on the type of amino acid. Finally, it does not react with tertiary or aromatic amines.
Note that since ninhydrin is a strong oxidizing agent, proper caution should be exercised in handling this compound. It is especially potent at the elevated temperature under which the reaction is carried out. The ninhydrin reagent will stain the skin blue and cannot be immediately washed off completely if it comes in contact with the skin. However, as in any other stain on the skin, the color will gradually rub off after about a day.