This study describes the optimization and single-laboratory validation of a single residue method for determination of two bipyridylium herbicides, paraquat and diquat, in cowpeas by UPLC-MS/MS in a total run time of 9.3min. The method is based on extraction with an acidified methanol-water mixture. Different extraction parameters (extraction solvent composition, temperature, sample extract filtration, and pre-treatment of the laboratory sample) were evaluated in order to optimize the extraction method efficiency. Isotopically labeled internal standards, Paraquat-D6 and Diquat-D4, were used and added to the test portions prior to extraction. The method validation was performed by analyzing spiked samples at three concentrations (10, 20 and 50μgkg(-1)), with seven replicates (n=7) for each concentration. Linearity (r(2)) of analytical curves, accuracy (trueness as recovery % and precision as RSD%), instrument and method limits of detection and quantification (LOD and LOQ) and matrix effects were determined. Average recoveries obtained for diquat were between 77 and 85% with RSD values ⩽20%, for all spike levels studied. On the other hand, paraquat showed average recoveries between 68 and 103% with RSDs in the range 14.4-25.4%. The method LOQ was 10 and 20μgkg(-1) for diquat and paraquat, respectively. The matrix effect was significant for both pesticides. Consequently, matrix-matched calibration standards and using isotopically labeled (IL) analogues as internal standards for the target analytes are required for application in routine analysis. The validated method was successfully applied for cowpea samples obtained from various field studies.

The development and validation of an analytical method for the determination of 51 pesticides in tomato using UPLC-MS/MS (ultra performance liquid chromatography coupled to tandem mass spectrometry) was performed in order to fulfill the requirements of the NBR ISO/IEC 17025:2005. The goal of the validation was to evaluate all required parameters, such as linearity of analytical curves, instrument and method limits of detection and quantification, matrix effect and accuracy (trueness and precision). Stock solutions of the pesticides were prepared in organic solvent by two different analysts independently, in order to validate this preparation by comparing the detector response areas of injections (n = 7). Pesticides were extracted from tomatoes applying the Dutch mini-Luke method with 30 mL of each solvent as acetone, petroleum ether as well as dichloromethane. A small aliquot of the organic solvent extract was evaporated and reconstituted in methanol with 0.1% of acetic acid (v/v) for UPLC-MS/MS analysis. The recovery experiments were done by spiking blank samples at 10, 20 and 50 µg kg -1 , performed by two analysts, seven replicates for each concentration level plus the blank sample. From the 51 studied compounds, 46 showed recoveries within the acceptable range of 70-120% for all evaluated concentrations. For 82% of pesticides, the validated method limit of quantification was the lowest concentration studied (10 µg kg

Cowpea bean (Vigna unguiculata) is usually submitted to pre-harvest burndown with herbicides aiming to an early harvest. We aimed to quantify the residues of paraquat, diquat, glufosinate-ammonium and glyphosate in cowpea grains subjected to two timings of pre-harvest burndown. Three identical experiments were installed in Dourados (Lat. 22° S), Sinop (Lat. 11° S) and Teresina (Lat. 05° S), Brazil, comprising the pre-harvest burndown of cowpea bean with these herbicides, applied either with 50 or 75% of the pods dry and ready for harvest. As hotter and drier the location as slower the herbicide dissipation. Glyphosate should not be used for pre-harvest burndown of cowpea. Herbicide residues were above the maximum levels stated by the Brazilian legislation when applied with 50% dry pods. Burndown herbicides aiming to speed harvest should be applied with at least 75% dry pods in cowpea to ensure safe herbicide residue levels in grains.