Quantitative analysis of paraquat in vegetation by stable isotope dilution and liquid chromatography/electrospray ionization-mass spectrometry

Abstract

The method presented is a suitable approach for routine testing of paraquat in difficult sample types, such as winter wheat and alfalfa plant tissue, typically found with accidental spray drift. Hydrophilic interaction chromatography and ultra-performance LC is utilized with tandem quadrupole MS in the positive electrospray ionization mode. Three precursor-product ion transitions are measured in the multiple reaction monitoring mode, and paraquat d8 is added as an internal standard at the beginning of the extraction procedure to correct for losses in recovery and/or matrix effects in instrument response. A 5 g portion is digested with 6 M HCl in a 100° C water bath for 1 h. An aliquot is removed and adjusted to pH 7-8 prior to loading on a mixed mode weak cation-exchange SPE cartridge, and paraquat is eluted with formic acid-acetonitrile (10 + 90, v/v). Average recoveries of paraquat fortified at 0.020-0.080 ppm in winter wheat and alfalfa ranged from 80 to 114% (RSD 12-30%). Result data from naturally incurred paraquat (0.027-0.51 ppm) in composite garden plants, potato leaves, tree leaves, and alfalfa are presented. The LOQ is 0.020 ppm.