Oxytetracycline

Tetracycline (TC), oxyTetracycline (OTC), doxycycline (DC), sulfamethazine (SMZ), tsumacide (TMC), methacrylic acid (MAA) and 2,2′-Azobis(2-methylpropionitrile) (AIBN) were purchased from Sigma-Aldrich (Shanghai, China). Iron trichloride (FeCl₃) and ferrous chloride (FeCl₂·4H₂O) were obtained from Guoyao Group Chemical Reagent Co., Ltd (Shanghai, China). Divinylbenzene (DVB) was obtained from Rhawn Reagent Co., Ltd (Shanghai, China). Sodium hydrate (NaOH), Na₂EDTA, methanol, ethanol, acetonitrile and formic acid were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The straw biochar pyrolyzed at 400–450 °C was purchased from Liao Ning Golden Future Agriculture Technology Co., Ltd. (Anshan, China). All other solvents and reagents used in this study were of the highest available purity and at least of analytical grade. Doubly deionized water (DDW), obtained using WaterPro Water System (Labconco Corp., Kansas City, MO, USA), was used throughout the experiments.

Oxytetracycline (OTC), diphenyleneiodonium (DPI), 2-phenyl-4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) and N^G-nitro-L-Arg methyl ester (L-NAME) used for seedling treatments were purchased from Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO, USA). The fluorescent dyes 2′,7′-dichlorodihydrofluorescein diacetate (H₂DCF-DA) and 4-amino-5-methyl-amino-2′,7′-difluorofluorescein diacetate (DAF-FM DA) used for staining of H₂O₂ and nitric oxide, respectively, were obtained from Molecular Probes (Thermo Fisher Scientific, Waltham, MA USA), and 3,3-diaminobenzidine (DAB) was obtained from Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO, USA). Hydrogen peroxide (H₂O₂) was purchased from Dingguo Biotech Co., Ltd (Shanghai, China).

For this study, Sigma-Aldrich (St. Louis, MO, USA) provided oxytetracycline of high purity (98%). Inorganic salt medium (ISM) was comprised of—(NH4)2SO4 (2.0 g), K₂HPO₄ (0.5 g), and NaH₂PO₄ (0.5 g) adjusted to pH 7.0. Growth media—nutrient broth and nutrient agar, Mueller–Hinton agar, and OTC (g/l) constituted of (NH4)2SO4 (2.0 g), K₂HPO₄ (0.5 g), and NaH₂PO₄ (0.5 g). The solid medium contained 20 grams of agar per litre. Hi-Media provided commercial-grade antibiotic susceptibility discs for six antibiotics: tetracycline, gentamicin, ceftriaxone, oxytetracycline, erythromycin, and vancomycin. Merck provided acetonitrile and methanol of high-performance liquid chromatography grade.

The analytical standards enrofloxacin, ciprofloxacin, sulfaquinoxaline and oxytetracycline that were used in the validation were purchased from Sigma-Aldrich and Fluka (St. Louis, MO, USA).
Acetonitrile (LC-MS grade) from Merck (Darmstadt, Germany), trichloroacetic acid (TCA) from J.T. Baker (Center Valley, PA, USA) and heptafluorobutyric acid (HFBA) from Fluka (St. Louis, MO, USA) were used.

Acetaminophen (ACT), acetylsulfamethoxazole (ASMX), azithromycin (AZI), caffeine (CAF), carbamazepine (CBZ), clarithromycin (CLA), enrofloxacin (ENRO), erythromycin (ERY), iopamidol (IOPA), norfloxacin (NOR), oxytetracycline (OTC), sarafloxacin (SARA), sulfachloropyridazine (SCP), sulfadiazine (SPD), sulfadimethoxine (SDM), sulfamerazine (SMR), sulfameter (SMT), sulfamethazine (SMZ), sulfamethizole (SMI), sulfamethoxazole (SMX), tetracycline (TC), and trimethoprim (TMP) were purchased from Sigma-Aldrich. Ciprofloxacin (CIP) and diclofenac (DIC) were obtained from Cambridge Isotope Laboratories, Inc. (Tewksbury, MA). Sulfathiazole was purchased from ICN Biomedicals, Inc. (Irvine, CA). carbamazepine-d10 (d10-CBZ) was purchased from CDN Isotopes (Quebec, Canada). Chlortetracycline (CTC) was obtained from Acros Organics (VWR International, Westchester, PA). Paroxetine maleate (PRX) and venlafaxine (VEN) were obtained from Cerilliant (Sigma-Aldrich, St Louis, MO). The Barnstead NANOpure™ DIamond (Waltham, MA) purification system was used to obtain 18.2 MΩ water. LC-MS grade methanol and acetonitrile were obtained from EMD Millipore Corporation (Billerica, MA), and formic acid (88%) was purchased from Fisher Chemical (Pittsburgh, PA). Oasis™ HLB solid-phase extraction (SPE) cartridges were purchased from Waters (Milford, MA).

Antibiotics and pesticide residues were analyzed using analytic grade reagents. Standards of high purity grades (>99%) for 26 antibiotics belonging to four families: Sulfonamides (sulfacetamide, sulfisomidine, SMT, sulfapyridine, sulfamerazine, sulfameter, sulfachloropyridazine, sulfamethoxazole, sulfabenzamide, sulfadimethoxine, sulfaquinoxaline, sulfamethoxypyridazine, and sulfadoxine), quinolones (nalidixic acid, oxolinic acid, CIP, norfloxacin, ofloxacin, flumequine, danofloxacin, enrofloxacin, lomefloxacin, and sarafloxacin), tetracyclines (tetracycline and oxytetracycline), and β-lactams (ampicillin) in addition to a hundred pesticide standards were procured from Sigma-Aldrich. Each pesticide stock solution was accurately prepared in acetonitrile at a concentration of 1 g/L. The working standards were freshly prepared by diluting the stock solution in acetonitrile to the desired concentration. Both stock and working solutions were stored at −20°C.
High-performance liquid chromatography (HPLC)-grade water, HPLC-grade acetonitrile, disodium ethylenediaminetetraacetate dihydrate (Na₂ EDTA), and magnesium sulfate (MgSO₄) were also supplied by Sigma-Aldrich.