The actual pH value of the reaction mixture was 9.5–10. FMOC concentration of 3 mM provided not only a sufficient molar ratio of reactants ([FMOC]/[AA] _ 4.5) but also a relative small FMOC-OH peak. Thus, the elimination of reagent’s excess was not requisite. A tea sample with high content of AA was diluted twice with water before analyzing. As well the impurity peaks were miniaturized by using ACN as a solvent for FMOC.