Simultaneous extraction of tetracycline, macrolide and sulfonamide antibiotics from agricultural soils using pressurised liquid extraction, followed by solid-phase extraction and liquid chromatography–tandem mass spectrometry

The veterinary antibacterial agents chlortetracycline (CTC), oxytetracycline (OTC), sulfadiazine (SDZ), erythromycin (ERY) and tylosin (TYL A, B, C and D) were extracted from soil using pressurized liquid extraction (PLE). Citric acid (pH 4.7) and methanol was used as extraction buffer, followed by tandem-solid-phase extraction (SPE) clean-up (SAX+HLB) for all compounds. For quantification two slightly different methods were employed using LC–MS–MS with MRM detection. The soil extraction method was validated using a loamy sand soil and a sandy soil, representing two typical Danish agricultural soils. Recoveries were 50–80% for the tetracyclines (CTC and OTC) and sulfadiazine (SDZ) and 60–100% for the macrolides (TYL and ERY). Limits of detection for the soil extraction method (LODsoil) were 0.6–5.6 μg kg−1 soil for CTC and OTC, 0.9–2.9 μg kg−1 soil for SDZ and 2.4–5.5 μg kg−1 soil for TYL A and ERY. Furthermore, the method was applied to field samples taken from two agricultural fields fertilised with liquid manure containing CTC and TYL A. These results showed a decline in the content of antibacterial agents throughout the sampling period of 155 days from 10 to 15 μg CTC kg−1 soil and 20–55 μg TYL A kg−1 soil to below or near the LODsoil listed above. Finally, the method was applied to barley grains harvested from the fields. None of the antibacterial agents were measured in grain samples, but recoveries for spiked grain samples were similar to soil recoveries.