Abstract
A fast and simple method for the enantioseparation of l- and d-theanine using Marfey's reagent was developed. The method was validated for linearity, specificity, accuracy, stability, detection and quantification limits. The separation was achieved on RP columns with a gradient of 1 % acetic acid and acetonitrile. d-Theanine was synthesized with a purity of 96.5 %, and the enantiomeric composition of theanine was determined in 56 tea samples including instant teas. No detectable d-theanine was found in the fresh leaves and in the most of white teas. It was observed that d-theanine arises initially at the wither process and mounts up according to the strength of this step and to further handling or storage procedures. A quantitative relationship between the smallness of tea grades and the enantiomeric composition was observed just in Darjeeling teas. Higher proportions of d-theanine were found in the five cold water-soluble instant black teas gained after alkaline treatment (5.56–51.80 % of total theanine).
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