Determination of 21 free amino acids in 5 types of tea by ultra-high performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS) using a modified 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) method

6-Aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) pre-column derivatization coupled with reversed-phase liquid chromatography and diode array detection (RPLC-DAD) is one of the conventional methods for analysis of free amino acids in tea. In this article, we improved method detection sensitivity and resolution by replacing the optical detector with a mass spectrometer (MS). We also optimized the extraction process and mobile phases. Application of mass spectrometry makes co-eluting compounds easier to separate. The improved method was then successfully applied in the analysis of twenty-eight commercial tea samples with various degrees of fermentation. Umami-tasting AAs were higher in green tea than in white tea, while sweet-tasting AAs were more elevated in white tea. Principal component analysis indicated that theanine, arginine, aspartic acid and glutamic acid could be potent discriminators for tea fermentation degree.