*2.1. Materials*

Enrofloxacin (C19H22FN3O3, 98%) was obtained from Nanjing Kangmanlin Biomedical Technology Co. Ltd. (Nanjing, China). These organic acids (tartaric acid (C4H6O6,99.5%), nicotinic acid (C6H5NO2,99.5%), suberic acid (C6H10O4,99%)) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The chromatography grade organic solvents (methanol (MeOH), acetonitrile (ACN), triethylamine (TEA), phosphate) were purchased from Tianjin Kemio Chemical Reagent Co., Ltd. (Tianjin, China). All of the analytical-grade organic solvents (ethanol (EtOH), dichloromethane (DCM), N,N-Dimethylformamide (DMF)) were obtained from Tianjin Damao Chemical Reagent Factory (Tianjin, China) and these were used without further purification. Purified water was prepared using the Millipore Milli-Q system.
