with a linear gradient solvent system of 0.1%
aqueous formic acid (solvent A) and acetonitrile 0.1% formic acid
(solvent B) as follows: 90% solvent A until 4 min, followed by 90–
75% solvent A over 25 min, then 75–10% A over 35 min, then going
back to 90% solvent A until 45 min.