Analysis of sulfonylurea herbicides in soil water at sub-part-per-billion levels by electrospray negative ionization mass spectrometry followed by confirmatory tandem mass spectrometry

Liquid chromatography coupled to mass spectrometry (MS) with an electrospray ionization interface was used to analyze 4 sulfonylurea herbicides : chlorsulfuron, metsulfuron-methyl, thifensulfuron-methyl, and tribenuron-methyl. Fortified soil water was acidified to pH 4.5 (the pH was maintained neutral for tribenuron-methyl because it rapidly hydrolyzes at low pH) and extracted on C 18 cartridges. Extracts were cleaned up on silica cartridges. MS analysis was accomplished by selected ion monitoring of the deprotonated molecular ion in negative-ion mode. The 4 compounds were quantitated with the external standard technique based on the integrated abundance of the molecular ion [M - H] - . Good reproducibility and high sensitivity (detection limits for the 4 compounds were between 0.002 and 0.005 μg/L) were achieved. Tandem MS (MS/MS) in selected reaction monitoring mode on a characteristic product ion (m/z 139 for chlorsulfuron, metsulfuron-methyl, and thifensulfuron-methyl; m/z 153 for tribenuron-methyl) was used as confirmatory method. High sensitivity was maintained also with MS/MS, making it possible to identify sulfonylurea herbicides at very low concentrations.