Trace Analysis of Sulfonylurea Herbicides in Water:  Extraction and Purification by a Carbograph 4 Cartridge, Followed by Liquid Chromatography with UV Detection, and Confirmatory Analysis by an Electrospray/Mass Detector

Abstract

Seven commonly used sulfonylureas (SUs), i.e., thifensulfuron methyl, metsulfuron methyl, triasulfuron, chlorsulfuron, rimsulfuron, tribenuron methyl, and bensulfuron methyl, were extracted from water by off-line solid-phase extraction with a Carbograph 4 cartridge. SUs were then isolated from both humic acids and neutral contaminants by differential elution. Analyte fractionation and quantification were performed by liquid chromatography (LC) with UV detection. Recoveries of SUs extracted from 4 L of drinking water (10 ng/L spike level), 2 L of groundwater (50 ng/L spike level), and 0.2 L of river water sample (250 ng/L spike level) were not lower than 94%. Depending on the particular SU, method detection limits were 0.6−2 ng/L in drinking water, 2−9 ng/L in groundwater, and 13−40 ng/L in river water. A preservation study of SUs stored on the Carbograph 4 cartridge was conducted. Over 2 weeks of cartridge storage, no significant analyte loss was observed when the cartridge was kept frozen as a precaution. Comparing this method with one using a C-18 extraction cartridge, the former appeared to be superior to the latter in terms of sensitivity and, chiefly, of selectivity. This method involves confirmatory analysis by LC−electrospray/mass spectrometry (MS) instrumentation equipped with a single-quadrupole mass filter. MS data acquisition was performed by a time-scheduled three-ion selected ion monitoring (SIM) program. The necessary structure-significant fragment ions were obtained by controlled decomposition of SU adduct ions after suitably adjusting the electrical field in the desolvation chamber. Under three-ion SIM condition, limits of detection (S/N = 3) calculated from the ion current profiles of those fragment or parent ions giving the lowest S/N values ranged between 0.5 (tribenuron methyl) and 3 ng (metsulfuron methyl, thifensulfuron methyl) injected into the LC column.