Cloud point sample clean-up and capillary zone electrophoresis with field enhanced sample injection and micelle to solvent stacking for the analysis of herbicides in milk

• Cloud point sample clean-up for complex milk samples.
• Direct injection in stacking CE of cleaned sample with nonionic surfactant.
• Two orders of magnitude sensitivity improvement in real samples.
• Fast sample clean-up and CE analysis within 30 min.

Sample clean-up by cloud point phase separation and analysis by capillary electrophoresis with stacking was developed for quaternary ammonium herbicides (i.e., paraquat and diquat) in milk. For sample clean-up, a mixture of 845 μL of milk sample, 5 μL of 100 mM phosphoric acid, and 150 μL of Triton X-114 was heated (60 °C for 2 min) and centrifugated (3000 rpm for 2 min) in 2-mL Eppendorf tube. The upper phase was directly analysed by capillary electrophoresis via electrokinetic injection at 10 kV for 150 s. The separation electrolyte was 100 mM phosphate buffer with 20% acetonitrile at pH 2.5. Before sample injection, a micellar solution (10 mM SDS in 80 mM phosphate buffer at pH 2.5) and an organic solvent rich solution (30% ACN) was hydrodynamically introduced into the capillary. These solutions provided the necessary conditions for stacking the cationic herbicides via the combination of field enhanced sample injection and micelle to solvent stacking. The LODs (S/N = 3) obtained from the entire strategy for paraquat and diquat in milk was 0.004 and 0.018 μg/mL, respectively. This is 1.5 to >2 orders of magnitude better than the corresponding LODs obtained from the electrophoretic analysis of herbicide standards prepared in the separation electrolyte. The strategy was also successfully applied to 5 milk samples available in the market.