Comparison of two sample preconcentration strategies for the sensitivity enhancement of flavonoids found in Chinese herbal medicine in micellar electrokinetic chromatography with UV detection

Two on-column preconcentration techniques named stacking with reverse migrating micelles (SRMM) and anion selective electrokinetic injection and a water plug-sweeping with reverse migrating micelles (ASIW–sweep–RMM) were used and compared for concentration and separation of flavonoids in Chinese herbs using reverse migration micellar electrokinetic chromatography (RM-MEKC). The optimal background electrolyte (BGE) used for separation and preconcentration was a solution composed of 20 mM phosphoric acid (H3PO4)–100 mM sodium dodecyl sulfate (SDS)–20% (v/v) acetonitrile (ACN) buffer (pH 2.0), the applied voltage was −15 kV. To achieve reasonable results of the two techniques, the conditions which affected preconcentration were examined. A comparison of used techniques with normal hydrodynamic injection (5 s), concerning enhancement factors and limits of detection (LODs) was presented. Under the optimum stacking conditions, about 27–37- and 45–194-fold improvement in the detection sensitivity was obtained for SRMM and ASIW–sweep–RMM, respectively, compared to usual hydrodynamic sample injection (5 s). The LODs (S/N = 3) for SRMM and ASIW–sweep–RMM in terms of peak height, can reach down to 1.15 × 10−2 μg/ml for hesperetin and 2.4 × 10−3 μg/ml for nobiletin, respectively. Finally, the amounts of the six flavonoids in extract of Fructus aurantii Immaturus were successfully determined using ASIW–sweep–RMM. The six analytes were baseline separated with sample matrix under the optimum ASIW–sweep–RMM conditions and the experimental results showed that preconcentration was well achieved after the dilution of sample solutions.