The mobile phase consisted of a 0.4% aqueous solution of acetic acid (solution A) and acetonitrile (solution B), with the following gradient program: 0–40 min, solution B 5–25%; 40–45 min, solution B 25–35%; 45–50 min, solution B 35–50%, with a
flow rate of 1.0 mL/min, injection volume of 20 lL, and a run time