The mobile phase was distilled water (solvent A) and 100% methanol (solvent B). The gradient elution program, with a mixture of solvents A and B, was as follows: 60–90% B for 0–14.0 min (Waters convex curve no. 5), 90–95% B for 14.0–14.5 min (Waters linear curve no. 6), 95% B for 14.5–16.0 min, 95–60% B for 16.0–16.5 min (Waters convex curve no.5), and 60% B for 16.5–18.0 min.