Reagents. All organic solvents were HPLCgrade and all chemicals were analytical grade. Acetonitrile, was from J.T. Baker (Deventer, the Netherlands). Trichloroacetic acid (TCA) and sodium acetate was from Sigma-Aldrich (St. Louis, MO, USA). Heptafluorobutyricacid (HFBA) was from Fluka (St. Louis, MO, USA). PVDF filters were from Restek (College, PA, USA). Strata X columns were form Phenomenex (Torrance, CA, USA). Water was deionised (>18 MΩ cm−1) in-house by the Millipore system.
Analytical standard and standard solutions. Amoxicillin (AMOX), ampicillin (AMPI), penicillin G (PEN G), penicillin V (PEN V), oxacillin (OXA), cloxacillin (CLOX), nafcillin (NAF), dicloxacillin (DICLOX), cephapirin (CFPI), ceftiofur (CFT), cefoperazone (CFPE), cephalexin (CFLE), cefquinome (CFQ), cefazolin (CFZ), cefalonium (CFLO), sulfaguanidine (SGU), sulfadiazine (SDZ), sulfathiazole (STZ), sulfamerazine (SME), sulfamethazine (SMT), sulfamethoxazole (SMA), sulfamethoxypyridazine (SMP), sulfamonomethoxine (SMM), sulfadoxine(SDX), sulfaquinoxaline (SQX), sulfadimethoxine(SDMX), tylosin (TYL), erythromycin (ERY), spiramycin (SPI), tilmicosin (TIL), josamycin (JOS), danofloxacin (DAN), difloxacin (DIF), enrofloxacin(ENR), ciprofloxacin (CIP), flumequine (FLU), sarafloxacin (SAR), marbofloxacin (MAR), norfloxacin(NOR), oxolinic acid (OXO), nalidixic acid (NAL), chlortetracycline (CTC), tetracycline (TC), doxycycline(DC), oxytetracycline (OTC), streptomycin (STRP), dihydrostrepromycin (DISTRP), gentamycin (GEN),paromomycin (PAR), spectinomycin (SPEC), kanamycin (KAN), neomycin (NEO), lincomycin (LIN) and sulfaphenazole (IS) were from Sigma-Aldrich.
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