Simultaneous Determination of 23 Antibiotic Residues in Livestock Manure by UPLC-MS/MS Coupled with Solid Phase Extraction

10 mL of extractant composed of 0.1 mol·L^-1 Na₂EDTA-Mcllvaine buffer solution, acetonitrile and formic acid with the valume ratio of 19.5:80:0.5, and 50 μL of 100 μg·L^-1 mixed internal standard solution was added to 0.200 g of the freeze-dried, crushed sample. After being mixed well, the acidity of the mixture was adjusted to pH 4.0-5.0 with formic acid (1+10) solution, then 15 mL of n-hexane was added. After ultrasonic extraction for 15 min and oscillation for 15 min, the mixture was centrifuged at 8 000 r·min^-1 for 5 min at 4℃, and then 6 mL of the lower layer liquid was purified with a solid phase column composed with SAX column and HLB column in series, with 1 mL of acetonitrile (4+1) solution for elution. The eluent was dried at 60℃ by nitrogen blowing, and the residue was dissolved with 0.6 mL of formic acid (0.5+99.5) solution. After the solution was filtred through 0.22 μm microporous membrane, the filtrate was separated on a BEH C₁₈ column with acetonitrile-0.1% (φ) formic acid solution as the mobile phase for gradient elution. Electrospray ionization and multiple reaction monitoring were used for MS analysis. Linear relationships were found between peak areas and mass concentrations of 23 antibiotics within definite ranges, with detection limits (3S/N) in the range of 0.2-3.3 μg·kg^-1. Values of recovery obtained by standard addition method ranged from 51.7% to 102%, and RSDs (n=6) ranged from 3.5% to 13%. The proposed method was used to analyze the antibiotic residues in livestock manure of 21 large-scale livestock and poultry farms in the suburbs of Beijing, and many kinds of antibiotics were detected.