Mixed-mode solid-phase extraction coupled with liquid chromatography tandem mass spectrometry to determine phenoxy acid, sulfonylurea, triazine and other selected herbicides at nanogram per litre levels in environmental waters

• Herbicides with acidic–neutral–basic characteristics can be retained by mixed-mode anion exchange SPE.
• Elution of the acidic herbicides is dependent upon the amount of acid in the methanol eluant.
• Increasing the amount of acid increases elution efficiency, however will also cause ion suppression due to excess interference.
• pH of the mobile phase has a particularly significant effect on the retention time and sensitivity of clopyralid and picloram.

The method presented uses a mixed-mode anion exchange SPE and liquid chromatography tandem mass spectrometry to analyze 5 sulfonylurea, 8 phenoxy acid, 12 triazine and 6 other herbicides in environmental waters. The mixed-mode SPE cartridge is able to retain a wide range of herbicides with acidic–neutral–basic characteristics, particularly the highly polar and acidic compounds clopyralid, dicamba and picloram. The neutral and basic herbicides can be effectively eluted with methanol, after which the acidic herbicides can be eluted using acidified methanol. The method has achieved an LOD of 0.7–3 ng/L for the sulfonylureas, 4–12 ng/L for the phenoxy acids and 0.4–30 ng/L for the triazine and additional herbicides, with recoveries in the range 76–107%, 73–126%, and 65–104%, respectively. The precision of the method, calculated as relative standard deviation (RSD), was below 10% for both sulfonylurea and phenoxy acid herbicides, and less than 20% for the remaining herbicides. The developed method was used to determine the concentration of target herbicides in a range of environmental waters, and many of the target herbicides were detected at ng/L level.