Gallic acid, catechin, epicatechin and syringin (Internal standard, IS) were purchased from the Chinese National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China). Methanol (HPLC grade) was purchased from VWR International Company (Darmstadt, Germany). Formic acid (analytical reagent) was purchased from the First Chemical Company of Nanjing (Jiangsu, China). LiChrospher SPE cartridges, 250 mg, were supplied from Hanbang (Huaiyin, China). Water was distillated twice before use.