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Sulfonamides are antimicrobials used widely as veterinary-drugs, and their residues have been detected in environmental matrices. An analytical method for determining sulfadiazine, sulfathiazole, sulfamethazine, sulfamethoxazole, sulfadimethoxine and sulfaquinoxaline residues in soils employing a solid phase extraction on-line technique coupled with ultra-high performance liquid chromatography and tandem mass spectrometry (SPE-UHPLC-MS/MS) was developed and validated in this study. SPE and chromatographic separation were performed using an Oasis HLB column and an Acquity UPLC BEH C18 analytical column, respectively, at 40 degrees C. Samples were prepared by extracting sulfonamides from soil using a solid-liquid extraction method with water:acetonitrile, 1:1 v/v (recovery of 70.2-99.9%). The following parameters were evaluated to optimize the on-line SPE process: sorbent type (Oasis and C8), sample volume (100-400 mu l), loading solvent (water and different proportions of water:methanol) and washing volume (0.19-0.66 mL). The method produced linear results for all sulfonamides from 0.5 to 12.5 ngg(-1) with a linearity greater than 0.99. The precision of the method was less than 15%, and the matrix effect was -27% to -87%. The accuracy was in the range of 77-112% for all sulfonamides. The limit of quantitation in the two soils (clay and sand) was 0.5 ngg(-1). The SPE column allowed for the analysis of many (more than 2000) samples without decreasing the efficiency. (C) 2016 Elsevier B.V. All rights reserved.