Enrofloxacin

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.

Analytical standards for fleroxacin (FLE), ofloxacin (OFL), norfloxacin (NOR), ciprofloxacin (CIP) and enrofloxacin (ENR) were purchased from J&K Chemical, China and used without further purification. HPLC-grade ethanol, methanol, ethyl acetate, acetonitrile and acetone were sourced from Merck (www.merck.com.cn). Salts (magnesium sulfate (MgSO 4 ), sodium sulfate (Na 2 SO4), sodium acetate (CH 3 COONa), ammonium sulfate ((NH 4 ) 2 SO 4 ) and ammonium acetate (CH 3 COONH 4 )) with purities ≥99% were obtained from Aladdin Industrial Co. Ltd.
Stock standard solutions (1000 g mL -1 ) for each FQ were prepared by dissolving each compound in methanol and stored at 4 • C. Stock solutions were diluted with methanol to prepare a secondary mixed stock solution of 10 g mL -1 . Mixtures of standard working solutions for extraction at different concentrations were prepared daily by dilution with Milli-Q ultrapure water (Millipore, Bedford, USA).