Efficiencies of different methods for determination of organophosphate pesticide residues in fermented wheat substrate

In the present work, the efficiencies of three different sample preparation methods for GC/ MS determination of pirimiphos-methyl and chlorpyrifos-methyl residues in wheat (Triticum spelta) samples fermented by Lactobacillus plantarum were compared. All three methods were based on methanol:acetone=1:1 extraction, while further purification of the obtained sam- ples was altered. First purification was through a column containing a mixture of aluminium oxide and activated charcoal slurry-packed and eluted with dichlormethane, the second was purification on a florisil column slurry-packed and eluted with ethyl acetate:acetone=4:1, while the third was based on a combination of the former two methods, i.e. clean-up through col- umns filled with a mixture of aluminum oxide and activated charcoal slurry-packed and eluted with ethyl acetate:acetone=4:1. The second method was found the most effective for obtain- ing satisfactory recoveries at four fortification levels. For pirimiphos-methyl, recoveries were in the range of 91.3-96.0% and had good reproducibility, i.e. RSD ranging from 2.2-4.1%, while the corresponding range for chlorpyrifos-methyl was 81.6-88.2%, and the RSD range 2.5-5.4%. The chosen method was further optimized in order to establish the optimum volume of elu- tion solvent used during the clean-up procedures. The highest recoveries of 93.7±3.5% for pir- imiphos-methyl and 85.3±2.5% for chlorpyrifos-methyl were obtained after elution with 25 ml volume of solvent. Considering all, simple, efficient and reliable GC/MS detection of pirimiphos- methyl and chlorpyrifos-methyl residues in wheat grain substrate altered by fermentation with L. plantarum was achieved by the two-steps extraction with 25 ml of methanol:acetone=1:1 solvent mix for 30 min, followed by a clean-up procedure through a glass column with florisil coupled with elution by 25 ml of ethyl acetate:acetone=4:1.