A mixture of 80% solution A (aqueous solution of 8.205 g sodium acetate, 0.5 ml triethylamine, 0.7 ml acetic acid and 5.0 ml acetonitrile in 1000 ml) and 20% solution B (acetonitrile-water; 60:40), adjusted to pH 5.8, was used as the mobile phase with a flow rate at 0.6 ml/min and the injection volume was 20 μl.