Large-Volume Sample Stacking with Polarity Switching in CE for Determination of Natural Polyphenols in Plant Extracts

摘要

A simple on-column preconcentration method for capillary electrophoretic determination of eight polyphenolic compounds (carnosic acid, cinnamic acid, caffeic acid and rosmarinic acid, quercetin, apigenin, luteolin and rutin) was devised. The method was applied for the assay of polyphenols in methanolic extract of the medicinal plant Orthosiphon Stamineus Benth. The analysis was carried out in fused silica capillaries (I.D. 50 μm, effective length 50 cm, total length 60 cm) with UV detection at 200 nm. The background electrolyte was 50 mM sodium tetraborate of pH 9.0 (adjusted with phosphoric acid). Large volume sample stacking with polarity switching was used for sensitivity enhancement. With sample injection representing 50% of capillary volume and polarity switching at 1.6 min, an average 90-fold enhancement of absorbance signal of the analytes was achieved. The calibration curves were linear (r = 0.9956–0.9994) in the range 0.2 to 1.8 μg mL−1 of an analyte. The repeatability of migration times and peak areas was characterized by RSD values 0.11–0.57 and 1.63–5.66%, respectively. The proposed method offers favourable limits of detection (9–16 ng mL−1) that compare well with those of LC.