Routine determination of sulfonylurea, imidazolinone, and sulfonamide herbicides at nanogram-per-liter concentrations by solid-phase extraction and liquid chromatographyrmass spectrometry

Sulfonylurea SU., imidazolinone IMI., and sulfonamide SA. herbicides are new classes of low-application-rate herbicides increasingly used by farmers. Some of these herbicides affect both weed and crop species at low dosages and must be carefully used. Less is known about the effect of these compounds on non-crop plant species, but a concentration of 100 ngrl in water has been proposed as the threshold for possible plant toxicity for most of these herbicides. Hence, analytical methods must be capable of detecting SUs, IMIs, and SAs at concentrations less than 100 ngrl in ambient water samples. The authors developed a two-cartridge, solid-phase extraction method for isolating 12 SU, 3 IMI, and 1 SA herbicides by using high-performance liquid chromatographyrelectrospray ionization-mass spectrometry HPLCrESI-MS. to identify and quantify these herbicides to 10 ngrl. This method was used to analyze 196 surface- and ground-water samples collected from May to August 1998 throughout the Midwestern United States, and more than 100 quality-assurance and quality-control samples. During the 16 weeks of the study, the HPLCrESI-MS maintained excellent calibration linearity across the calibration range from 5 to 500 ngrl, with correlation coefficients of 0.9975 or greater. Continuing calibration verification standards at 100-ngrl concentration were analyzed throughout the study, and the average measured concentrations for individual herbicides ranged from 93 to 100 ngrl. Recovery of herbicides from 27 reagent-water samples spiked at 50 and 100 ngrl ranged from 39 to 92%, and averaged 73%. The standard deviation of recoveries ranged from 14 to 26%, and averaged 20%. This variability reflects multiple instruments, operators, and the use of automated and manual sample preparation. Spiked environmental water samples had similar recoveries, although for some herbicides, the sample matrix enhanced recoveries by as much as 200% greater than the spiked concentration. This matrix enhancement was sample- and compound-dependent. Concentrations of herbicides in unspiked duplicate environmental samples were typically within 25% of each other. The results demonstrate the usefulness of HPLCrESI-MS for determining low-application-rate herbicides at ambient concentrations.