Sulfamethoxazole d4

All standard chemicals used for analyses were purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany), except for the internal standards (erythromycin-13C-d3, lincomycin-d3, sulfamethoxazole-d4, ciprofloxacin-d8, doxycycline-d3, and chloramphenicol-d5), which were purchased from Sigma-Aldrich (St. Louis, MO, USA). High performance liquid chromatography (HPLC)-grade solvents, including methanol, formic acid, and acetonitrile, were provided by Merck & Co., Inc. (Kenilworth, NJ, USA). Guaranteed reagents, including ammonium acetate, hydrochloric acid, sodium hydroxide, dipotassium hydrogen phosphate, and disodium ethylenediaminetetraacetic acid (EDTA), were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water was prepared using a water purification system. The internal standards were dissolved in acetonitrile as stock solutions and stored in a freezer at −40 °C. Working standard solutions were prepared immediately before measurements were taken by diluting the stock solutions with a mixture of menthol and water (9:1, v/v). For solid-phase extraction (SPE), Oasis HLB cartridges (6 mL/500 mg) were purchased from Water Oasis Co., Ltd. (Milford, MA, USA), glass microfiber filters (Whatman GF/F) were obtained from Sigma-Aldrich (St. Louis, MO, USA), and syringe-driven filters (PTFE) from Millipore Corp. (Bedford, MA, USA).