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.