Pentanoic acid

Analytical standards (99.9% purity) for triclosan (TCS) and methyltriclosan (MTCS) were purchased from Sigma-Aldrich, Shanghai, China. The chemical structure and molecular weight of TCS and MTCS are shown in Supplementary Fig. 1. Five medium-chain fatty acids (pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid and nonanoic acid) were purchased from J&K Chemical, Shanghai, China. HPLC-grade methanol, ethanol, ethylene glycol, acetone, acetonitrile, and dimethyl sulfoxide were purchased from Merck, Shanghai, China.
Stock standard solutions (1000 μg mL -1 ) for TCS and MTCS were prepared by dissolving each chemical in methanol and stored at 4 °C until use. Working solutions were diluted with Milli-Q 18 MΩ ultrapure water (Millipore, Bedford, MA, USA) and methanol (v/v = 50:50) to prepare secondary mixed stock solutions. All working solutions were prepared fresh weekly and stored at 4 °C.

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.