**2. Materials and Methods**

#### *2.1. Samples*

Fresh raw livestock and poultry meat (pork, beef, lamb and chicken) were provided by the Supervision, Inspection and Testing Center for Quality of Meat-Products, Ministry of Agriculture and Rural Affairs (Nanjing, China). All samples tested negative for residues of amphenicols and metabolites before the experiment.

#### *2.2. Chemicals and Reagents*

Standards of CAP, TAP, FF, FFA and CAP-D5 (purity ≥99.50%) were obtained from Dr. Ehrenstorfer GmbH (Augsburg, Germany). FFA-D3 standard (purity ≥99.50%) was purchased from Toronto Research Chemicals (North York, ON, Canada). HPLC-grade solvents, including methanol, acetonitrile (ACN), ethyl acetate and n-hexane, were supplied by Merck Company (Darmstadt, Germany). HPLC-grade acetone, formic acid and guaranteed reagent grade ammonium hydroxide were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water was supplied by the Sartorius-Arium pro system (Sartorius AG, Goettingen, Germany).

Individual standard stock solutions (1 mg/mL) of CAP, TAP, FF, FFA, CAP-D5 and FFA-D3 were prepared in methanol and stored at −20 ◦C for six months. CAP-D5 and FFA-D3 were used as internal standards. The combined standard working solution at a concentration of 10 μg/mL for each analyte was prepared by mixing equal volumes of the individual standard stock solution of CAP, TAP, FF and FFA with a concentration of 1 mg/mL and diluting with methanol. The mixed internal standard working solution at the concentration of 1.0 μg/mL was prepared by diluting the internal standard stock solutions (1 mg/mL) with methanol. The abovementioned standard working solutions were stored at −20 ◦C for up to three months.
