Determination of quaternary ammonium herbicides in soils: Comparison of digestion, shaking and microwave-assisted extractions

Very challenging analytical problems arise from the continuous introduction in agriculture of chemical pesticides. Particularly, diquat (DQ), paraquat (PQ) and difenzoquat (DF) are a difficult group of quaternary ammonium herbicides to analyze. This article reviews and addresses the most relevant analytical methods for determining the selected herbicides in soil. We discuss and critically evaluate procedures, such as digestion-based methods, shaking extraction and microwave-assisted extraction (MAE). Clean-up of extracts was performed by solid-phase extraction (SPE) using silica cartridges. Detection of these herbicides was carried out by liquid chromatography (LC) coupled to UV detection and mass spectrometry (MS) as confirmatory technique. Recoveries ranged from 98% to 100% by digestion, from no recovered to 61% by shaking, and from 102% to 109% by MAE with estimated quantification limits between 1.0 μg/kg and 2.0 μg/kg by digestion and 5.0 μg/kg and 7.5 μg/kg by MAE using LC/MS–MS as detection technique. The recoveries obtained under the optimum conditions are compared and discussed with those obtained from digestion extraction and MAE.