Multi‐Residue Analysis of Corn and Soybean Pesticides in Brazilian Oxisols Using Gas Chromatography and Mass Selective Detection

Abstract

AbstractAvailable methods for measuring pesticide residues in soils usually have not been tested for tropical soils nor do they cope with the spectrum of pesticides used in the tropics. An accurate and efficient method for the simultaneous determination of 14 corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] pesticides in Oxisols is presented in this study. The quantification of the pesticides was achieved using gas chromatography coupled with mass selective detection. For the extraction of aged pesticide residues from soil samples, combinations of acetone, ethylacetate, hexane, methanol, water, and salt solutions were tested. Of the 20 different solvent systems evaluated, the mixture of acetone‐ethylacetate‐water 2:2:1 (v/v/v) was most efficient for the extraction of all pesticides except metribuzine [4‐amino‐6‐ (1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,3‐ triazine‐5(4H)‐one]; this herbicide was recovered best by ethylacetate‐water4:1 (v/v). Different sample aging‐times (5–30d) did not affect the recovery of pesticides from incubated samples. Extraction of pesticides from field‐soil samples also was best with acetone‐ethylacetate‐water. No differences in extraction efficiency were observed between a simple shake extraction (4h) and batch extractions with an additional rise in temperature (60°C) or a sonication (30 min). The elimination of the water from the soil extract was achieved by a liquid‐liquid extraction with dichloromethane, which recovered the whole polarity spectrum of pesticides quantitatively. The method recovery from soil samples of freshly spiked pesticides averaged 94.8 ± 2.4%. The method was validated for Oxisols of contrasting clay and organic C content.