An LC-ESI/MS method for determining theanine in green tea dietary supplements

Theanine is the major amino acid present in Camellia sinensis or green tea. A method for determining theanine in its native state using liquid chromatography with positive-mode electrospray ionization mass spectrometric detection was developed. Quantitation of theanine was achieved using theanine-²H₅] as an internal standard. This approach was utilized on different green tea matrix materials that are commonly used as dietary supplements including powdered plant leaves, a powdered plant leaf extract, and an oral dosage form that contains green tea. The theanine response was linear over several orders of magnitude, and excellent measurement precision was obtained for all three materials using the developed method.