Possible uses of micellar electrokinetic capillary chromatography and high‐performance liquid chromatography for the chiral discrimination of some pyrethroids

Abstract

Micellar electrokinetic capillary chromatography (MECC) and high-performance liquid chromatography (HPLC) were used for the separation of stereoisomers of the lipophilic uncharged pyrethroids cypermethrin, alphamethrin, permethrin, and fenpropathrin. Different kinds of cyclodextrin (β-cyclodextrin, hydroxypropyl-β-cyclodextrin, dimethyl-β-cyclodextrin, and γ-cyclodextrin), surfactants (sodium dodecyl sulphate [SDS] and cetyltrimethylammonium bromide [CTAB]), and cations of background electrolyte (sodium, ammonium, TRIS, and Ammediol) were tested. Optimized conditions (background electrolyte: 50 mmol/l sodium phosphate, pH ≈ 2.5, 150 mmol/l SDS, 150 mg/ml γ-cyclodextrin) allowed the separation of alphamethrin, the eight cypermethrin stereoisomers being eluted in seven peaks and the separation of two enantiomers of fenpropathrin with resolution Rs = 10 and with n ≃ 500,000 theoretical plates. Different experimental conditions, e.g., mobile phase composition, temperature, injected amount, and flow rate, were also optimized in HPLC experiments. The optimal conditions (stationary phase: ChiraDex, 5 μm; mobile phase: 150 mmol triethylamine/l with H2SO4 in water (pH = 3.5) with methanol or acetonitrile; flow rate: 0.8 or 0.6 ml/min; temperature: ambient or 30°, 20°, or 10°C; experimental conditions were modified according to the type of analysis) allow chiral discrimination of alphamethrin enantiomers and analysis of permethrin stereoisomers. MECC offers higher efficiency and shorter analysis time than HPLC, but under tested conditions it was shown that the methods complement each other. Chirality 9:162–166, 1997. © 1997 Wiley-Liss, Inc.