Doxycycline

CYA, bi-deoxy Cyadox (Cy1), N4-deoxy cyadox (Cy2), N1-deoxy cyadox (Cy10), QCT, carbonyl reduction bi-deoxy quinocetone (Q2), MEQ, bi-deoxy Mequindox (M1), carbonyl reduction N1-deoxy mequindox (M4), carbonyl reduction bi-deoxy mequindox (M5) and carbonyl reduction mequindox (M6) (Table 1) were provided by the National Reference Laboratory of Veterinary Drug Residues, Huazhong Agricultural University (Wuhan, Hubei, China). Amphotericin B, tetracycline, doxycycline, ketoconazole, enrofloxacin, danofloxacin, rifampicin, tilmicosin and kitasamycin were purchased from Dr. Ehrenstorfer (Augsburg, Germany). Kanamycin, pyrazinamide, lincomycin, ethambutol, ribavirin and isoniazid were purchased from TRC (Toront, Canada). Amikacin, clindamycin, and tylosin were purchased from Sigma (St Louis, MO, USA). Stock solutions of the above compounds were prepared at a final concentration of 1280 μg/mL.Table 1

Chemical information of QdNOs and their metabolites

Standards (purity >98%) of sulfadiazine, sulfamethazine, sulfamethoxazole, sulfapyridine, sulfamerazine, sulfachloropyridazine, sulfadimethoxine, sulfadoxine, sulfachinoxaline, sulfathiazole, sulfamethizole, trimethoprim, florfenicol, doxycycline, amoxicillin, cefalexin, penicillin V, sarafloxacin d8 and ciprofloxacin were purchased (Dr. Ehrenstorfer, Augsburg, Germany) and stored at 4°C. HPLC water, acetonitrile, formic acid 98%, ammonium formate (Merck, Darmstadt, Germany) and acetic acid 100% were obtained from J.T.Baker, Mallinckrodt Inc, MO, USA. All antimicrobial stock solutions (1000 μg/mL) were prepared in methanol excepted AMX and LEX (1000 μg/mL) which were prepared in methanol and water (1:1). Stock solutions were then diluted up to 1 μg/mL for standard curve preparation.