The standards of ENR, CIP, SAR, SMM, SM2, SQ, TIM, TYL, KIT, and TAM were supplied by the China Institute of Veterinary Drugs Control (Beijing, China). HPLC-grade ACN, MeOH, and formic acid were provided by Fisher Scientific (Fair Lawn, NJ, USA). Analytically pure ACN, MeOH, ethyl acetate, acetic acid, hexane, and TCA were obtained from the Guangzhou Chemical Reagent Company (Guangzhou, China). Ultrapure water was obtained by a Millipore system Milli Q (Molsheim, France).
Stock standard solutions (1 mg mL−1) were prepared by dissolving each compound in HPLC-grade methanol or 2% ammonia in methanol (v/v) and were then stored at −20 °C in the dark until use. The working standard solutions of compounds at appropriate concentrations were prepared by diluting the individual stock standard solutions in HPLC-grade methanol on the day of use.
Stock standard solutions (1 mg mL−1) were prepared by dissolving each compound in HPLC-grade methanol or 2% ammonia in methanol (v/v) and were then stored at −20 °C in the dark until use. The working standard solutions of compounds at appropriate concentrations were prepared by diluting the individual stock standard solutions in HPLC-grade methanol on the day of use.