Fluorescein Isothiocyanate —Ninhydrin

Tible 1 Color reactions of amino acids with fluorescein isothiocyanate — ninhydrin (extract from original table). 

The amino acids probably react with the fluorescein isothiocyanate to yield fluorescein thiourea derivatives. These are hydrolyzed at elevated temperature in alkaline medium so that the amino groups that are produced can then react with ninhydrin. 

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