Efficiency improvements on ninhydrin method for amino acid quantification

The ninhydrin method was introduced for quantitative determination of amino acids in the late 1940s. During the past several decades, considerable modifications including different heating times, heating temperatures, buffer systems, pH values of buffer solutions and solvents for ninhydrin reagent have complicated this method. To simplify the diverse available protocols and to enhance efficiency, the effects of heating time and pH values on the commonly used lithium hydroxide/acetic acid buffer system were investigated. Furthermore, the suitability of using sodium hydroxide/acetic acid buffer to replace lithium hydroxide/acetic acid buffer was also evaluated. The results show that relatively inexpensive sodium hydroxide/acetic acid buffer system or as simple as sodium acetate solution could be used in the ninhydrin method. The results also showed that a shorter heating time of 10 min could achieve a comparable degree of color development as the traditionally suggested heating time of 30 min. The improvements of the ninhydrin method presented in the study make this method even more convenient, less expensive, and less time consuming for quantification of compounds containing amino group.