The course of the gradient was as follows: first, solvent A was flowing for 1 min, after that, solvent B was added in three steps, up to 10% in 10 min, to 44% in 22 min and to 100% in 30 min. Subsequently, solvent B was decreasing down to 0% in 17 min to come back to the beginning conditions. Finally, the stationary phase was rinsed with solvent A for 2 min. Injection volumes of 10 μL and the elution flow-rate of 1 mL/min were used in all experiments