Table 1.

Gradient program of mobile phase.

Time (minutes)	0.002 M solution of tetrabutylammonium sulfate (%)	Methanol (%)	Elution mode
0.0–7.0	80	20	Initial hold time
7.0–12.5	80 ⟶ 50	20 ⟶ 50	Linear gradient
12.5–15.0	80	20	Final hold time