Preparation of standard solutions
Stock solutions of 1 and 3–5 were prepared in methanol, 2 in methanolwater
(80:20), and 6 and 7 in chloroform-methanol (20:80) at a concentration
of 1.0 mg/mL. Standard solutions were prepared by diluting the
stock solutions with methanol to get the working concentrations of 1–7
separately.