Application Of Ionic Liquid In Capillary Electrophoresis And Chiral Separation By Capillary Electrophoresis

Capillary electrophoresis (CE) has emerged as a separation technique. Due to its high resolution, efficiency, short analysis time, simplicity and low sample and solvent consumption, CE has proved to be a highly efficient separation technique. In recent years, much attention has been devoted to the improvement the separation selectivity of CE. Room-temperature ionic liquids (RTILs), sometimes called molten salts, are those compounds which are liquids at room temperature or whose melting points are slightly higher than ambient temperature and consist entirely of ionic species. Generally, the ILs present a variety of desirable properties, such as low melting point, conductivity, nonflammability, air and water stable, thermal stability, and can be easily and economically prepared. Recent years, the application of ILs for the separation of various classes of compounds in CE, gas chromatography (GC) and high performance liquid chromatography (HPLC) has been recognized. Furthermore, CE has also gained increased importance as a powerful analytical tool in the separation of enantiomeric compounds. Base on the main research directions of our group and the project of Natural Science Foundation of China (grant No. 20275014), some original studies are carried out as followings:(1) Separation and determination active components in medical plants by CE using ionic liquids as background electrolyte (BGE) or modifier was described. The separation mechanism was also discussed..(2) A micellar electrokinetic chromatographic (MEKC) method using sodium cholate as chiral surfactant for the enantioseparation of four stereoisomers of palonosetron hydrochloride was developed for the first time(3) Separation and on-line preconcentration of two coumarins, imperatorin and isoimperatorin, in Angelica dahurica Benth and its medicinal preparation were investigated by sample stacking in reversed migration micellar electrokinetic chromatography.The dissertation consists of six chapters.Chapter 1: the application of ILs in CE and the implementation of CE as a tool for enantioseparation were reviewed and discussed in detail.(1) Introduced the application of ILs in CE, including the physical and chemical character of ILs and its application in CE.(2) CE for enantiomer separation. The mechanism and methods of CE for enantiomer separation are summarized with some typical applications.Chapter 2: A micellar electrokinetic chromatographic (MEKC) method using ionic liquid as modifier for the quantification of the active components of lignans found in the medicinal herbs Schisandra species was developed for the first time. Preliminary investigations employing sodium dodecyl sulfate (SDS) as surfactant did not lead to the necessary resolution of the studied compounds, the addition of ionic liquid 1-butyl-3-methylimidazoium tetrafluoroborate (BMIM-BF₄) to the SDS micellar system resulted in the complete separation of all the compounds. Under the optimal conditions, the method successfully applied to the determination of lignans in extracts of Schisandra chinensis (Turcz.) Baill. and Schisandra henryi C. B. Clarke in less than 13 min. The separation mechanism was also discussed. SDS micelle solutions with only sodium counterions present, these positively charged imidazolium cations could be electrostatically attracted to the negatively charged SDS micelle surface, which neutralized the effective head group charge and reduced electrostatic repulsion between the charged hydrophilic headgroups of the surfactant molecules, thus affecting the size and shape of the micelles formed. Meanwhile, some imidazolium cations would entry the space of SDS chains and thereby altering the separation.Chapter 3: This chapter demonstrated a novel capillary zone electrophoresis (CZE) approach for the simultaneous determination of five anthraquinone derivatives including aloe-emodin, emodin, chrysophanol, physcoin and rhein in Rheum species and Polygonum cuspidatum, which employed only 1-alkyl-3-methylimidazolium -based ionic liquids (ILs) as background electrolyte (BGE) in the absence modifiers usually used in the literature. The proposed method provided a different separation profile compared to that obtained by ordinary CZE using borate and phosphate buffers. Under the optimized conditions, the proposed method was validated and successfully used in the analysis of anthraquinones in ethanol extracts of two Rheum (R. palmatum and R. hotaoense ) and P. cuspidatum samples and the results obtained were satisfactory. The method proposed herein was simpler compared to previously reported methods. The mobility of anthraquinone analytes increased with increasing amounts of ILs added to the migrating solutions. This was explained by ion association between the electrolyte cations and the analyte anions. The change in electrophoretic mobility was analyzed by a non-linear least-squares method to give the ion association constant (K_ass) between the electrolyte cations and anthraquinone anions, it suggested that ion association in aqueous solution was governed by hydrophobicity of the pairing cation and anions and/or electrostatic interaction.Chapter 4: The enantioseparation of four stereoisomers of palonosetron hydrochloride by micellar electrokinetic chromatography using sodium cholate as chiral surfactant was described for the first time. Sodium cholate was shown to be effective in separating palonosetron hydrochloride stereoisomers. Under the optimum conditions baseline separation of palonosetron hydrochloride stereoisomers was achieved within 18 min. The method is simple and repeatable (RSD < 4.6%).Chapter 5: On-line preconcentration and separation of two coumarins, imperatorin and isoimperatorin, in Angelica dahurica Benth and its medicinal preparation were investigated by sample stacking in reversed migration micellar electrokinetic chromatography. An acidic buffer was used to suppress the electroosmotic flow. Under the stacking conditions, the limits of detection (S/N = 3), in terms of peak height, were 4.1×10^-2 fig/ml for imperatorin and 4.2×10^-2μg/ml for isoimperatorin. Compared to usual hydrodynamic sample injection (5 s), 16- and 13-fold improvement in detection sensitivity were obtained for imperatorin and isoimperatorin, respectively. The proposed method has been successfully applied to the determination of the two coumarins in extracts of Angelica dahurica Benth and its medicinal preparation.Chapter 6: A simple and rapid method for the simultaneous determination of five anthraquinone derivatives including aloe-emodin, emodin, chrysophanol, physcoin and rhein in Rheum species and Polygonum cuspidatum was established by capillary zone electrophoresis (CZE) usingβ-cyclodextrin (CD) as modifier and urea to enhance its solubility. Under the optimized conditions, all of the five anthraquinones were baseline-separated within a short analysis time of 12 min with symmetrical peaks. The limits of detection for the analytes (S/N = 3) were 0.33-0.62μg/ml. The recoveries were ranged from 93.37 to 107.69%. The proposed method was successfully applied to the determination of anthraquinones in ethanol extracts of two kinds of Rheum plants (R. palmatum and R. hotaoense) and P. cuspidatum. The apparent binding constants of these derivatives withβ-CD were evaluated.