These samples were kept in ultrasonic cleaning for 20 min. Then,
the mixture was centrifugated at 3000 rpm for 10 min, and the supernatant
fluid was filtered through a 0.45 μm membrane. After 5 mL of the filter
liquor had been treated as derivatization and CPE procedures, 20 μL of the
surfactant-rich phase was analyzed by a HPLC-Vis method.