Comparison of different mass spectrometric detection techniques in the gas chromatographic analysis of pyrethroid insecticide residues in soil after microwave-assisted extraction

A comparative study was carried out for the simultaneous determination of 11 pyrethroid insecticides in soil by gas chromatography (GC) - ion trap (IT)- mass spectrometry (MS), by means of two different ionization modes; electron impact and negative chemical ionization and three data acquisition procedures; full scan, selected ion monitoring and MS/MS. Pyrethroids investigated were tetramethrin, bifenthrin, phenothrin, lambda-cyhalothrin, permethrin, cyfluthrin, cypermethrin, flucythrinate, esfenvalerate, fluvalinate and deltamethrin. Soil samples were treated with toluene/water by microwave-assisted extraction for 9 min at 700 W and a cleaning up with florisil was performed. Clean soil samples were spiked with pyrethroids at a spiking level of 10, 25 and 50 ng/g. The method employed provides a concentration factor of 10. The ionization gas employed in the negative chemical ionization mode was methane. The use of MS/MS acquisition, in electron impact ionization, provided the best results, due to its high selectivity and sensitivity, giving very low limits of detection from 0.08 to 0.54 ng/g. In negative chemical ionization full scan and selected ion monitoring methods detection limits from 0.12 to 1.40 ng/g were found. The proposed methods were applied to several levels from 10 to 50 ng/g of spiked soils, being electron impact MS/MS method which minimizes matrix spectrum interferences and provided recovery average values from 84% to 120% with relative standard deviations which varied from 3.2 to 7.2%.