On-line solid-phase extraction-ultra high performance liquid chromatography-tandem mass spectrometry for the determination of avermectins and milbemycin in soils.

Avermectins and milbemycin are widely used as veterinary drugs and as agricultural pesticides, and their residues have been detected in soil. This study reports a simple and high-throughput method for determining ivermectin (IVER), abamectin (ABA), doramectin (DORA), eprinomectin (EPRI), and moxidectin (MOXI) residues in soils, employing an on-line solid-phase extraction technique coupled with ultra-high performance liquid chromatography and tandem mass spectrometry (SPE-UHPLC-MS/MS). The method was validated and applied for the determination of ABA in soils from an orange plantation treated with this pesticide. The sample preparation procedure consisted of extraction of the compounds from soil using methanol (with recoveries of 73-85%), and subsequent on-line SPE cleanup and concentration using a C8 sorbent coupled to the UHPLC-MS/MS system. The optimal conditions were: water:methanol (40:60, v/v) sample solvent; water:methanol (96:4, v/v) loading solvent; 2×250μL sample volume; and elution of the analytes retained on the SPE column in back flush with 5mmolL ammonium acetate:acetonitrile (10:90, v/v) chromatographic mobile phase. The method produced linear results in the ranges 0.1-10ngg (IVER, ABA, DORA, and MOXI) and 0.5-10ngg (EPRI), with linearity greater than 0.99. The precision of the method was better than 19% and accuracy was in the range 74-89%. The limits of quantitation were 0.2ngg for EPRI and 0.1ngg for the other compounds. The SPE column could be reused in more than 2000 analyses without loss of efficiency. The ABA concentration in the soil varied between 1.7 and 18ngg, and no dissipation was observed during five consecutive days after application of the pesticide to the orange plantation.