2.2. Standard solutions
OTC, TC, CTC, DOX, MINO, MTC and DEMC stock standard solutions (1.0 mg/mL) were prepared every 2 months by dissolving each in methanol and stored at −20 ◦C. Working combined mixed standard solution (100 g/mL) was diluted to volume with mobile phase and the working standard was stable for at least 1 week when stored in amber tube at 4 ◦C.