“…The samples were immediately agitated for 30 s followed by centrifugation at 4500 × g for 10 min. The samples were extracted by solid-phase extraction as reported previously [6]. Briefly, Clean Screen extraction cartridges (C8 with a cationic exchanger; Clean Screen, Worldwide Monitoring) were conditioned sequentially with methanol (5 mL), water (5 mL) and 0.1 M phosphate buffer (pH 6.0, 3 mL).…”