Using 4-(Carbazole-9-yl)-benzyl Chloroformate as a Derivatization Reagent for the Measurement of Amino Acids in Tea Samples by High-Performance Liquid Chromatography with Fluorescence Detection

An effective method for the determination of 20 free amino acids in tea samples using 4-(carbazole-9-yl)-benzyl chloroformate (CBBC-Cl) as a derivatization reagent by high-performance liquid chromatography combined with fluorescence detection (HPLC-FLD) has been established. CBBC-Cl quickly derivatized amino acids within 6 min in basic solution (pH 9.0) at room temperature (ca. 25 °C). The maximum excitation and emission wavelengths of the stable derivatives were 290 nm and 370 nm, respectively. The separation of 20 amino acids, including the theanine (THE) derivative, was optimized on a Hypersil BDS-C18 column. Identification of these derivatives was performed via liquid chromatography/electrospray ionization mass spectrometry (LC/ESI/MS) in positive-ion detection mode. Under optimal conditions, excellent linearity for all the amino acids was obtained, with determination coefficients of r2 > 0.9986. The detection limits (LODs) and quantitation limits (LOQs) ranged from 0.13 to 1.5 μg·kg−1 and 0.43 to 4.95 μg·kg−1, respectively. Satisfactory recoveries, ranging from 86.7 to 106.4%, were obtained for all the amino acids in the tea samples. The developed method has been successfully applied to the quantification of 20 amino acids in tea samples.