Determination of 32 Antibiotics in Surface Water by Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry after Tandem Extraction with Mixed Ion Exchange Reversed Phase Adsorption Solid Phase Extraction Columns

The surface water sample (500 mL) was passed through a 0.45 μm filtrate membrane, and the filtrate was mixed with disodium ethylene diamine tetraacetic acid (0.4 g). The mixed cation (MCX cartridge) and anion (MAX cartridge) ion exchange reversed phase adsorption solid phase extraction columns were coupled, and loaded with the above sample solution at sampling rate of about 2 mL·min^-1. The 2 cartridges were separated after sampling, and the MCX cartridge was rinsed with 10 mL of 2% (φ, the same below) formic acid solution, dried with nitrogen for 30 min, and eluted with 4 mL of methanol and 4 mL of methanol solution containing 5% (φ, the same below) aqueous ammonia. The MAX cartridge was rinsed with 10 mL of 5% aqueous ammonia solution, dried with nitrogen for 30 min, and eluted with 4 mL of methanol and 4 mL of methanol solution containing 2% formic acid. The eluates collected from the 2 cartridges were combined, blown to nearly dry with nitrogen, and diluted to 1.0 mL with 50% (φ) acetonitrile solution. The internal standards were added, to make their mass concentrations in the above solution reach to 20.0 μg·L^-1. After mixing evenly, the mixed solution was passed through a 0.22 μm filtrate membrane, and the filtrate was introduced into the ultra-high performance liquid chromatograph. Six kinds of antibiotics (including etracyclines, sulfonamides, macrolides, quinolones β-lactams, and linkolamides) were sepatated on BEH C₁₈ column with mixed solutions composed of 0.1% (φ) formic acid solution and acetonitrile at various volume ratios under the gradient elution procedure, ionized and detected by positive electrospray ionization ion source mode and multiple reaction monitoring mode in the tandem mass spectrometer, with internal standard method as quantification method. It was shown that linear ranges of standard curves of the 32 antibiotics were in the range of 1.0-50.0 μg·L^-1, with detection limits (3s) of 0.3-1.4 ng·L^-1. The spiked recovery test was made on the actual surface water samples at the 3 concentration levels, giving values of recovery in the range of 48.5%-93.4%, and RSDs (n=6) of the determined values ranged from 1.3% to 17%. The proposed method was applied to the analysis of the 8 surface water sources in the Taihu lake basin, in which sulfamethoxazole, sulfamethoxam, lincomycin, clindamycin, clarithromycin, and norfloxacin were detected, and the maximum detection amount was 33.9 ng·L^-1 (lincomycin).