Critical assessment of electrolyte systems for the capillary electrophoresis analysis of phenolic compounds in herbal extracts

This work presents a comparative evaluation of electrolyte systems for the capillary electrophoresis analysis of the phenolic compounds apigenin and luteolin, and their corresponding 7-O-glucosides naringenin, rutin, quercetin, umbelliferone, herniarin, chlorogenic, and caffeic acids, in methanolic, glycolic, and hydroalcoholic extracts of Matricaria recutita L. (Asteraceae). The electrolytes included tetraborate buffer at different pHs and concentrations, containing varied amounts of sodium dodecyl sulfate, β-cyclodextrin, and acetonitrile. The electrophoretic profile of the extracts changes considerably from electrolyte to electrolyte. However, in a single electrolyte, the profile of different extracts seems to be very similar (identical elution order), varying only in relative composition. The best condition for quantitative work was 20 mmol/L tetraborate buffer, pH 10, using direct detection at 337 nm. Baseline resolution of 9 among 11 selected standards was achieved readily in this electrolyte, showing distinct migration patterns for flavonoid aglycones (flavones, dihydroflavonols, and flavonols), coumarins, and acidic phenylpropanoids. To assure reproducibility, several capillary conditioning procedures were tested. Electrokinetic rinses with tetraborate buffer prior to sample injection gave the best results. Precision of migration times and peak areas were better than 4 and 2%, respectively, for 10 consecutive injections of the methanolic extract. A few method validation parameters also are reported, such as linearity (r²<0.999, concentration range from 5.0 to 75.0 μg/mL), limit of detection (3.8 μg/mL), and limit of quantitation (11.5 μg/mL), referred to apigenin, a commonly used compound to standardize chamomile extracts for topical use. Flavonoid glucosides that have distinct sugar moieties were separated readily in free solution; however, those with similar sugar moieties were separated only in micellar medium, using β-cyclodextrin as the additive. The presence of reference compounds in the extracts was confirmed by spiking techniques and their identification was assisted by a UV-spectra library. The validity of the diode array detector as an identification tool for phenolic compounds also is discussed. © 2001 John Wiley & Sons, Inc. J Micro Sep 13: 227–235, 2001