A reverse-phase Agilent Zorbax SB-C18 analytical column of 250 × 4.6 mm internal diameter (ID) and 5 μm particle size. Deionized water containing 0.1% formic acid (mobile phase component A) and acetonitrile (component B) were employed for the gradient program, which started with 20% B for 3 m and was linearly increased to 100% B in 27 m (held for 3 m). The column was then re-equilibrated for 12 m back to 20% B.