**3.1 HPLC method validation**

Mean retention times of the standards were: GABA 14.05 min; theanine 13.15 min; caffeine 8.65 min; and EGCG 11.32 min. Consistent retention times were also observed for both regular oolong tea (GABA 14.08 min; theanine 13.19 min; caffeine 8.46 min; and EGCG 11.21 min) and GABA-enriched oolong tea (GABA 14.01 min; theanine 13.11 min; caffeine 8.39 min; and EGCG 11.13 min). The calibration curves for quantified constituents obtained from the duplicate and triplicate standards exhibited linear correlation coefficients (r2 ) of 0.991 for GABA, 0.996 for theanine, 0.992 for caffeine, and 0.998 for EGCG. Intraday CVs of peak area and retention time were GABA 1.61%; theanine 0.01%; caffeine 1.03%; and EGCG 0.06%. Interday CVs were GABA 5.41%; theanine 0.01%; caffeine 2.45%; and EGCG 0.01%. The spiked samples demonstrated mean recoveries of 100.33 ± 28.40% (range: 87–134%) for GABA and theanine, and 104.64 ± 19.07% for caffeine and EGCG.
