“…Certain amounts of reference compounds 1, 3,5,6,7,8,10,12,16,17,18,20,21,23,24,25,27,28,29,35, and 37 were dissolved in methanol to obtain reference compound stock solutions (about 1.0 mg/ml) and stored at 4 C. Reference compound stock solutions were mixed and diluted with methanol to prepare the reference compound mixture working solution (about 1.0 μg/ml for each compound), and the solution was filtered by a 0.22-μm PTEE syringe filter before analysis.…”