Microemulsion electrokinetic chromatography coupled with dispersive micro-solid phase extraction for determination of isoflavones in soybean products

Abstract

A new method based on microemulsion electrokinetic chromatography (MEEKC) coupled with dispersive micro solid phase extraction (D-µ-SPE) was developed for the determination of isoflavones in soy products. D-µ-SPEs of real samples were carried out using multi-walled carbon nanotubes (MWCNTs) as adsorbent prior to MEEKC. Separations of selected isoflavones namely daidzein, genistein and formononetin by MEEKC were carried out using fresh daily-prepared microemulsion background electrolyte (BGE). The optimized MEEKC conditions for the separation of isoflavones were 4 mM borate buffer pH 8.5, 6.6% (w/v) 1-butanol, 0.9% (w/v) sodium dodecyl sulphate, 0.75% (w/v) ethyl acetate, 3% (w/v) acetonitrile at 3 s injection time, 27 kV and 35 °C. Meanwhile, the optimum D-μ-SPE conditions were 5 mg of MWCNTs and 300 µL of methanol as desorption solvent. Under the optimized conditions, the developed D-µ-SPE-MEEKC method showed good linearity in the concentration range of 1-10 mg/L with coefficients of determination (r2) > 0.99 and limits of detection of 0.27-0.95 mg/L. The method was successfully applied to the determination of isoflavones in five soybean products namely soy supplement, tofu, tempeh, egg tofu and fujook and good recoveries were obtained in the range of 74.5-112.5% with RSDs of < 3%. The method has proved to be simple and offers low consumption of organic solvent and relatively short analysis time, thus it is a potentially viable green alternative method for extraction and determination of isoflavones in soybean products.