Simultaneous Determination of Residues of 13 Nitroimidazole Antibiotics in Soil by Ultra-High Performance Liquid Chromatography- Tandem Mass Spectrometry

An aliquot (5.00 g) of soil sample was taken, and mixed with 4 mL of water by vortex. The extractant (10 mL of acetonitrile) was added, and the mixture was vortexed for 3 min. After adding 1 g of sodium chloride, the mixture was centrifuged for 10 min at room temperature, and 7.5 mL of the supernatant was taken, and mixed with 0.3 g of anhydrous magnesium sulfate. The mixture was vortexed for 30 s and centrifugated for 5 min at 25 ℃. An aliquot (5 mL) of the supernatant was taken and blown to dryness by nitrogen at 35 ℃. The residue was redissolved by 1 mL of methanol, and the mixed solution was passed through a 0.22 μm filtration membrane. Thirteen nitroimidazole antibiotics in the filtrate were separated on Phenomenex Kinetex F5 100Å column with mixed solutions composed of methanol and 0.1% (volume fraction) formic acid solution at different volume ratios by gradient elution, and ionized by AJS ESI⁺ mode, detected by MRM mode, quantified by matrix matched method. As shown by the results, linear relationships between values of the mass concentration and the peak area of each antibiotic were kept in the same range of 0.5-100.0 μg·L^-1, with lower limits of determination (10S/N) in the range of 0.01-0.07 μg·kg^-1. Test for recovery was made by standard addition method, giving recoveries in the range of 71.5%-116%, and RSDs (n=5) of the determined values ranged from 1.1% to 7.8%. The proposed method was applied to the analysis of 20 actual samples, dimetridazole was detected in 1 sample with detection amount of 8.39 μg·kg^-1.