A 400 mL aliquot of amino acid standard solution (or tea sample) was mixed with 100 mL of potassium borate buffer (0.5 M). Then, 500 mL of reagent solution (3 mM) was added, mixed on a vortex-mixer and allowed to proceed at ambient temperature for 10 min. The reaction was then stopped by addition of 100 mL acetic acid (1 M). The mixture was used for HPLC analysis after filtration through a 0.45 mm nylon membrane filter.