*2.1. Experimental Materials*

All reagents and chemicals used in this study were of analytical or chromatography grade. The enrofloxacin and enrofloxacin-*d5* standards (>95% purity) were obtained from Dr. Ehrenstorfer (Augsburg, Germany). The ciprofloxacin (CIP) and 1-bromoethane-2,2,2-*d3* were purchased from Shanghai AcmecBiochemical Co., Ltd. (Shanghai, China). N,N-dimethylformamide (DMF) and triethylamine (Et3N) were of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Methanol (MeOH), acetonitrile (MeCN), formic acid, ethyl acetate (EA), dichloromethane, acetic acid, and ammonium acetate of HPLC-grade were from Merck (Darmstadt, Germany). The certified reference material (CRM) (GBW 10167, Batch No. 1672107) was purchased from the Institute of Quality Standards and Testing Products for Agro-products of CAAS. The aquatic product samples were obtained from the local farmers' market in Shanghai, China. Ultrapure water (18.2 MΩ) was prepared in a Milli-Q water purification system (Millipore Co., Bedford, MA, USA).
