The flow rate was constant at 1.0 mL min
–1
and the column temperature was kept at 30 1C. The gradient condition of mobile phase was as follows: 45–83% B from 0 to 20 min and then held for 10 min; 83–92% B from 30 to 50 min; 92–100% B from 50 to 55 min and then held for 15 min. The column was equilibrated with the initial mobile phase for 5 min before the next injection. The injection volume was 10 mL. The relative standard deviations (RSDs) in retention times should be less than 1% during sample analysis process.