Determination of theanine in commercial tea by liquid chromatography with fluorescence and diode array ultraviolet detection.

Two liquid chromatographic methods that involve precolumn derivatization with o-phthaladehyde (OPA) and phenylisothiocyanate (PITC) with fluorescence and diode array UV detection for the determination of theanine have been developed. The chromatographic separations were achieved by reverse-phase high-performance liquid chromatography using octadecyl columns and gradient elution. The methods were applied to evaluate the theanine content of commercial tea leaves. The coefficient of variation of the peak area repeatability for within day (n = 8) and between day (n = 8 over 10 days) was lower than 3% for both of the methods. The estimated limit of detection (LOD) and limit of quantitation (LOQ) for the OPA method was 0.12 and 0.35 microg theanine, respectively. The PITC method was 500-fold more sensitive with LOD and LOQ values of 0.25 and 0.75 ng, respectively. The theanine content of the commercial tea samples varied from 2-5 mg/g leaf. The overall % recoveries for these methods ranged from 93-99.3. The sensitivity and simplicity of the method render them suitable for use in quality control laboratories.