The extraction process of tea leaves was performed by adding 100 mL of boiling water to 1 g finely ground sample (see above). The sample was allowed to brew for 5 min on a magnetic stirrer (500 rpm) and then filtered. After tea was cooled down, the volume was made up to 100 mL with water. The separation was performed on Aqua column (250 mm _ 4.6 mm, Phenomenex1, Aschaffen-burg, Germany) and the detection wavelength was set at 210 nm. Eluent A was water (HPLC grade) and eluent B was acetonitrile. Injection volume was 20 mL. Theanine standard solutions in the range of 5–200 mg/L were prepared and used to construct the calibration curve.