Ten microliter of each standard solution was analyzed
by HPLC. Calibration curves were made by plotting peak area (Y) and the amount of
each sample in lg (X) as functions. The calibration curves for compounds 1–4 were
Y = 8.67704X + 0.05921 (r2 = 0.99912), Y = 33.25598X + 2.91175 (r2 = 0.99957), Y =
34.91380X + 7.08344 (r2 = 0.98626) and Y = 57.27852X + 5.90689 (r2 = 0.99491),
respectively.