*3.2. Stock Solutions*

The stock standard solutions of the reference standards were dissolved in methanol with a concentration of 1.0 mg/mL for each compound, respectively. A series of working standard solutions (concentrations ranged from 0.05 to 1.0 μg/mL) were prepared by the successive dilution of the mixture of standard solutions with 20% methanol. All the solutions were stored at 4 ◦C before use.
