was used as mobile phase B. For the first 15 min, the linear gradient was applied from 0 to 20% isopropanol and then held at 20% isopropanol for 12 min. In the last 5 min of the sequence, the mobile phase mixture was changed back to pure ammonium acetate solution. The column temperature was maintained at 30 °C while the flow rate was 0.9 mL/min.