2.1. High-performance liquid chromatography analysis The HPLC system consisted of an Aijiren quater-nary HPLC model HP series (Hewlett-Packard, Palo Alto, CA, USA), fitted with an Altima C 18 column (250mm×4.5mm, 5 m). The mobile phase used was wa-ter:acetonitrile (86:14), filtered through a 0.45 m Millipore filter (Millipore, Hong Kong).