Determination of amino acids content in tea species using liquid chromatography via pre-column fluorescence derivatization

A sensitive method for the simultaneous detection of eight amino acids (AAs), glycine, alanine, serine, glutamic acid, arginine, tyrosine, phenylalanine, and tryptophan, in different tea species has been developed and validated. This method was based on separation and detection of AAs using high performance liquid chromatography (HPLC), with a fluorescence detector that permits ultra-sensitive detection. Pre-column derivatization was performed to convert the non-fluorescent AAs into fluorescent species, using a specific reagent, o-phthaldialdehyde (OPA) and 3-mercaptopropionic (3MPA), in an optimized ratio of 3:1. A reversed-phase HPLC-column with two gradient phases, acetate buffer (pH 7.6) containing 3% tetrahydrofuran and a mixture of acetate buffer (pH 7.6): acetonitrile: methanol (1:2:2,v/v/v), were used for the separation and detection of AAs over the linear ranges of 0.5-50 nmol/L. Further, the method was validated according to the official guidelines and successfully applied to determine the AAs content in tea products (green, black, oolong, and white). Considerable variations were observed based on the tea quality and type. In summary, this study could be used for the on-line monitoring of the tea production process to determine the grade quality, adulterations and detect the AAs intake.