Sample preparation.The PgME was prepared to a concentration of 12 mg/ml,with injection volume of
10 l Before injection,PgME was centrifuged (4 min at 5000 rpm) and the centrifuged supernatant was filtered through a 0.45 m PTFE filter (Waters), and submitted to HPLC–DAD and HPLC–ESI–MS analysis.