Mobile phase B was composed of 10 mM ammonium formate in ethanol 96% with the same proportion of formic acid as mobile phase A to avoid an ionic concentration gradient. The gradient profile consisted in an initial hold of 1 min at 21% of organic phase B followed by a linear ramp from 21% to 36% of mobile phase B, and 6 min of equilibration. For all experiments, the sample injection volume was kept at 2 μl.