Oxytetracycline

The 28 selected antibiotics used can be divided into 6 groups, including 4 Tetracylines (TCs), 4 Macrolides (MCs), 3 Chloramphenicols (CPs), 1 Penicillins (PNs), 13
Sulfonamides (SAs), 3 Quinolones (QNs) (Table 1). Physico-chemical properties of antibiotics are listed in Table 1. Oxytetracycline, Thiamphenicol, and Kitasamycin were obtained from the Institute of Biomedical Research (China), while
Chloramphenicol and Sulfapyridine derived from the Institute of Metrology (China).
Others were purchased from J&K Scientific (China). All the compounds were at least reagent grade (>95% purity).
The PAC used in this study was obtained from Shanxi Xinhua Active Carbon Factory (China) with an average pore size of 3.03 nm, specific surface area of 852.94 m 2 /g, iodine adsorption value of 903 mg/L, methylene blue adsorption of 142 mg/L and particle size of 200 mesh (75 μm) (more than 95% passing). Firstly, the PAC was washed with deionized water and then dried at 105C for 2 hours and cooled to room temperature (25±1C) in the dryer. A slurry of PAC (1000 mg/L) was prepared using deionized water and stored at 4C, and mixed by ultrasonic oscillation for 30 min prior to use. The 0.45 μm glass microfiber filters (Millipore, USA), were soaked for over 2 hours in 200 mL deionized water, and then kept at 4C in the refrigerator.

Analytical standards included six antibiotics, ofloxacin (OFL), norfloxacin (NOR), ciprofloxacin (CIP), enrofloxacin (ENR), oxytetracycline (OTC), and tetracycline (TC) with purity >99.0%, and five medium-chain fatty acids, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, and nonanoic acid, all purchased from J&K Chemical, Shanghai, China.
Stock standard solutions (1000 μg mL -1 ) for the six antibiotics were prepared every month by dissolving each compound in methanol and stored at 4 o C for further use. Working solutions were diluted with Milli-Q ultrapure water (Millipore, Bedford, MA, USA) and methanol (v/v = 50:50) to prepare a secondary mixed stock solution of 10 μg mL -1 . All of the working solutions were prepared every week and stored at 4 o C for further use.
summer of 2017. The seawater sample was filtered through a 0.45-μm membrane filter and stored at -4 o C until analysis. The sediment sample was air-dried at room temperature before grinding and sieving. Fish, shrimp, shellfish, and squid were purchased from local fishing boats (Weifang, China) and stored at -20 o C after grinding (JX-FSTPRP-24 grinder, Shanghai, China). All samples were analyzed within 1 wk of sample collection.