An aliquot (5 lL) was injected into the column and eluted at 45 C, with a constant flow-rate of 0.25 mL/min,using as solvents 5 mM ammonium formiate in water (solvent
A), 5 mM ammonium formiate in methanol (solvent B) and 0.5% formic acid (solvent C). Elution was performed with a gradient of 35% B (0–2 min), 35–95% B (2–4 min), 95% B (4–9 min), 95–35% B
(9–9.1 min) and 35% B (9.1–22 min), using a constant 5% C along all gradient program.