Sodium borohydride ( 4 96%) was
obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Acetone, phosphoric acid and acetic acid (30%) were pur- chased from Rionlon Bohua (Tianjin) Pharmaceutical & Chemical Co.,
Ltd. (Tianjin, China).
Chromatographic grade methanol and acetonitrile were gained from Merck Co.
(Darmstadt, Germany). Ultrapure water used throughout the study was prepared by an OKP Purification
system (Shanghai Laikie Instrument Co., China). All reagents used in the experiment were of
analytical reagent grade unless otherwise mentioned.