A gradient of mobile phase
composed of methanol (solvent A) and 5 mmol/L KH2PO4, pH
2.65 (solvent B) was used according to the following program: linear
increment starting with 5–15%A in 1 min, from 15% to 35% for
the next 1 min and the return to the initial conditions within the
next 4 min with the flow rate of 0.2 mL/min.