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

Sulfonylurea (SU), imidazolinone (IMI), and sulfonamide (SA) herbicides are new classes of low-application-rate herbicides increasingly used by farmer s. Some of these herbicides affect both weed and crop species at low dosage s and must be carefully used. Less is known about the effect of these compo unds on non-crop plant species, bur a concentration of 100 ng/l in water ha s been proposed as the threshold for possible plant toxicity for most of th ese herbicides. Hence, analytical methods must be capable of detecting SUs, IMIs, and SAs at concentrations less than 100 ng/l in ambient water sample s. The authors developed a two-cartridge, solid-phase extraction method for isolating 12 SU, 3 IMI, and 1 SA herbicides by using high-performance liqu id chromatography/electrospray ionization-mass spectrometry (HPLC/ESI-MS) t o identify and quantify these herbicides to 10 ng/l. This method was used t o analyze 196 surface- and ground-water samples collected from May to Augus t 1998 throughout the Midwestern United States, and more than 100 quality-a ssurance and quality-control samples. During the 16 weeks of the study, the HPLC/ESI-MS maintained excellent calibration linearity across the calibrat ion range from 5 to 500 ng/l, with correlation coefficients of 0.9975 or gr eater. Continuing calibration verification standards at 100-ng/l concentrat ion were analyzed throughout the study, and the average measured concentrat ions for individual herbicides ranged from 93 to 100 ng/l. Recovery of herb icides from 27 reagent-water samples spiked at 50 and 100 ng/l ranged from 39 to 92%, and averaged 73%. The standard deviation of recoveries ranged fr om 14 to 26%, and averaged 20%. This variability reflects multiple instrume nts, operators, and the use of automated and manual sample preparation. Spi ked environmental water samples had similar recoveries, although for some h erbicides, the sample matrix enhanced recoveries by as much as 200% greater than the spiked concentration. This matrix enhancement was sample- and com pound-dependent. Concentrations of herbicides in unspiked duplicate environ mental samples were typically within 25% of each other. The results demonst rate the usefulness of HPLC/ESI-MS for determining low-application-rate her bicides at ambient concentrations. (C) 2000 Elsevier Science B.V. All right s reserved.