Gradient elution was performed as follows:
0–0.5 min, 0–80% phase B; 0.5–9.5 min, 80–10% phase B; 9.5–
10 min, 10–15% phase B; 10–15 min, 15–5% phase B; 15–20 min,
5–80% phase B; and a re-equilibration period of 1 min with 80%
phase B used between individual runs.