The analytical column was a 5-μm reversed-phase column (Gemini
C18 110 A; 150 × 4.6 mm) protected with a Phenomenex security C18
guard cartridge (4 × 3.0 mm). Mobile phase A was ammonium acetate
(20 mmol/L, pH 7.5):acetonitrile (98:2, vol/vol); and mobile phase B
was pure acetonitrile.