Abstract:

A HPLC method was presented for the determination of theanine in tea. The sample preparation was achieved by extraction of samples with water (material/liquid ratio: 0.5 g/100 ml) in 80 ℃ water bath for 45 min, cleanup using solid phase extraction on a Sep-pak C18 column and derivatization with ortho-phthalaldehyde. The chromatographic conditions were as follows: XDB C18 column (4.6 × 250 mm, 5μm) used temperature, 40－50 ℃; mobile phase A, ammonium acetate solution containing 20 mmol and mobile phase B, ammonium acetate solution, methanol and acetonitrile (1:2:2, V/V, containing 20 mmol of ammonium acetate) for isocratic elution; flow rate, 1 ml/min; and detection wavelength, 338 nm. The limit of detection was 5 mg/kg. The average recoveries at two spike levels of 0.02 and 0.05 mg/ml (n = 6) were 98.6% and 101.1% and the relative standard deviations (RSDs) were 0.67% and 1.41%, respectively. Contents of theanine in various different varieties of tea such as green tea, flower petal tea, black tea, dark tea and oolong tea were determined by this method. Theanine content was the highest in green tea while was the lowest in red tea, however no obvious differences were observed among flower petal tea, black tea and oolong tea.

Key words: tea, theanine, amino acid