Sulfonylurea herbicides are in widespread use for controlling many grasses, broadleaf and brush weeds in crops, rangeland and forests. An important feature of the sulfonylurea herbicides is their high herbicidal activity, which results in very low application rates (10-40 g/ha). Residues as low as 0.3 µg/kg can significantly influence the growth rates in some crops.

Direct determination by GC is not possible because the sulfonylureas are thermally unstable. Also some sulfonylureas do not give stable derivatives that would make them applicable to GC. HPLC is generally applicable for the separation of sulfonylureas, but UV detectors have limited sensitivity and selectivity for the analysis soil or herbage samples and requires a rigorous clean-up. Electrospray-MS offers a robust and selective method of detection capable of providing low detection limits.

Soil samples were extracted by a weak basic aqueous extract. The herbicides were then extracted from solution with C18 disks. Water samples were acidified and extracted by the same C18 disk procedure. Herbage samples were homogenised with ethyl acetate and solid sodium sulphate, concentrated then cleaned-up by gel permeation chromatography.

The separation was achieved using a l50 mm x 2.1 mm ID Zorbax C18 column held at ambient temperature and fitted with a 10 µl injection loop. The mobile phase used was 60% methanol, 40% water containing 0.5% acetic acid. The flow rate was 0.1 ml/min at 600 psi. The column outlet was direct coupled to a Fisons Electrospray Mass Spectrometer operating in SIR mode.

The instrumental parameters were optimised to give the best response to the M⁺+1 ion. Protonation was readily achieved by the addition of less than 1% acetic acid. Fragmentation was minimal at cone voltages of less than 40 V. At higher cone voltages some structurally significant fragment ions were observed. Good full scan spectra were obtained for 1 ppm injections.

SIR parameter sets were established for 14 sulfonylureas and studies carried out on detection limits. Under optimum conditions 5 ppb injections gave a Signal:Noise ratio > 3. Soil extracts gave no significant interferences at detection limits of 0.2 µg/kg soil.