solid-phase extraction was chosen for sample pretreatment prior to HPLC analysis. To evaluate the elution volume of 30% methanol (containing 50 g •mL1 IS), different elution volumes including 0.5, 1.0, 1.5, 2.0 and 3.0 mL were tried to perform elution, the results showed that 1.5 mL (3 0.5) 30% methanol (containing 50 g•mL1 IS) could elute out the three target compounds.