The mobile phase was acetonitrile (A) and
0.05% phosphoric acid aqueous solution (B) with a gradient program
as follows: 0–8 min, linear gradient 5–8% A; 8–28 min, linear gradient
8–10% A; 28–40 min, 10% A isocratic elution; 40–55 min, linear
gradient 10–14% A; 55–80 min, linear gradient 14–24% A and
80–90 min, 24% A isocratic elution at a flow rate of 1 mL/min. All injection
volumes of samples and standard solutions were 20 μL.