Conventional Micellar Electrokinetic Chromatography Research Articles

Separation and analysis of cis-diol-containing compounds by boronate affinity-assisted micellar electrokinetic chromatography

Cis-diol-containing compounds (CDCCs) are usually highly hydrophilic compounds and are therefore difficult to separate by conventional reversed-phase-based micellar electrokinetic chromatography (MEKC) due to poor selectivity. Here, we report a new method, called boronate affinity-assisted micellar electrokinetic chromatography (BAA-MEKC), to solve this issue. A boronic acid with a hydrophobic alkyl chain was added to the background electrolyte, which acted as a modifier to adjust the selectivity. CDCCs can covalently react with the boronic acid to form negatively charged surfactant-like complexes, which can partition into micelles formed with a cationic surfactant. Thus, CDCCs can be separated according to the differential partition constants of their boronic acid complexes between the micellar phase and the surrounding aqueous phase. To verify this method, eight nucleosides were employed as the test compounds and their separation confirmed that the combination of boronate affinity interaction with MEKC can effectively enhance the separation of CDCCs. The effects of experimental conditions on the separation were investigated. Finally, the BAA-MEKC method was applied to the separation and analysis of nucleosides extracted from human urine. BAA-MEKC exhibited better selectivity and improved separation as compared with conventional MEKC and CZE. Successful quantitative analysis of urinary nucleosides by BAA-MEKC was demonstrated.

Determination of triazine herbicide residues in water samples by on-line sweeping concentration in micellar electrokinetic chromatography

A new method for the determination of atrazine, simazine and prometryn in water samples by on-line sweeping concentration technique in micellar electrokinetic chromatography (MEKC) was developed. Various parameters affecting sample enrichment and separation efficiency were systematically studied. Compared with the conventional MEKC method, up to 60–200-fold improvement in concentration sensitivity was achieved in terms of peak height by using this sweeping injection technique. The compound strychnine was used as the internal standard for the improvement of the experimental reproducibility. The limits of detection (S/N = 3:1) for atrazine, simazine and prometryn were 9, 10 and 0.5 ng mL −1, respectively. This method has been successfully applied to the analysis of atrazine, simazine and prometryn in lake, steam and ground water.

On-line concentration and determination of all- trans- and 13- cis- retinoic acids in rabbit serum by application of sweeping technique in micellar electrokinetic chromatography

A simple and rapid micellar electrokinetic chromatography (MEKC) method with UV detection was developed for the simultaneous separation and determination of all- trans- and 13- cis-retinoic acids in rabbit serum by on-line sweeping concentration technique. The serum sample was simply deproteinized and centrifuged. Various parameters affecting sample enrichment and separation were systematically investigated. Under optimal conditions, the analytes could be well separated within 17 min, and the relative standard deviations (RSD) of migration times and peak areas were less than 3.4%. Compared with the conventional MEKC injection method, the 18- and 19-fold improvements in sensitivity were achieved, respectively. The proposed method has been successfully applied to the determination of all- trans- and 13- cis-retinoic acids in serum samples from rabbits and could be feasible for the further pharmacokinetics study of all- trans-retinoic acid.

Analysis of Strychnos alkaloids in traditional Chinese medicines with improved sensitivity by sweeping micellar electrokinetic chromatography

The application of an on-line sweeping preconcentration method in micellar electrokinetic chromatography (MEKC) for the determination of Strychnos alkaloids, namely strychnine and brucine, has been investigated in this work. After experimental optimizations, the best separation was achieved in 50 mmol l −1 H 3PO 4 (pH 2.0) containing 100 mmol l −1 SDS and acetonitrile in a ratio of 4:1 (v/v), with an applied voltage of −20 kV at 20 °C. The sample matrix consisted of 100 mmol l −1 H 3PO 4 (pH 2.0), and sample introduction was performed at 0.5 psi for 270 s, with photodiode array detection at 203 nm. Compared with the conventional MEKC injection method, up to 100-fold improvement in concentration sensitivity was achieved in terms of peak height by using this sweeping injection technique. In the method, the compound berberine was used as the internal standard for the improvement of the experimental reproducibility. The calibration curve was linear over a range of 0.5–15 μg ml −1 for both strychnine and brucine, with a correlation coefficient of 0.998 and 0.997, respectively. The detection limits (S/N = 3:1) for strychnine and brucine were 0.05 and 0.07 μg ml −1, respectively. The sweeping-MEKC method has been successfully applied to the analysis of strychnine and brucine in Strychnos nux-vomica L. and its Chinese medicinal preparations.

Characterization of anacardic acids by micellar electrokinetic chromatography and mass spectrometry

A possibility of using capillary electrophoresis for separation of anacardic acids (6-alkylsalicylic acids) has been studied. Conventional micellar electrokinetic chromatography (MEKC) in non-coated fused silica capillaries and reversed-flow micellar electrokinetic chromatography (RF-MEKC) in capillaries coated with polydimethylacrylamide was applied for separation of anacardic acids extracted from cashew nuts. Influence of the composition of background electrolyte on the resolution of anacardic acid isomers was evaluated. Separations were performed using sodium dodecyl sulphate (SDS) micelles and mixed micelles of SDS and polyoxyethylene lauryl ether as a pseudostationary phase. To further improve the separation in RF-MEKC, β-cyclodextrin and a dual cyclodextrin system of β-cyclodextrin with heptakis-6-sulphato-β-cyclodextrin was added to the working electrolyte. Best separation of anacardic acids were achieved in the polydimethylacrylamide-coated capillary using 10 mM phosphate background electrolyte pH 6.5 with addition of 1 M urea, 20% acetonitrile, 10 mM of β-cyclodextrin and 1 mM of heptakis-6-sulfo-β-cyclodextrin. Mass spectrometry was used for the identification of anacardic acids in the extract from cashew nuts in single and tandem mode using Q-TOF instrument. Nine anacardic acids were identified in the extract form the cashew nuts.

Analysis of catechins in Theobromacacao beans by cyclodextrin-modified micellar electrokinetic chromatography

A micellar electrokinetic chromatography (MEKC) method was developed for the quantitation of polyphenols (+)-catechin and (−)-epicatechin (catechin monomers) and the methylxanthine theobromine in Theobroma cacao beans. Owing to the poor stability of catechin monomers in alkaline conditions, a 50 mM Britton–Robinson buffer at a pH 2.50 was preferred as the background electrolyte. Under these conditions, the addition of hydroxypropyl-β-cyclodextrin (HP-β-CD) at a concentration of 12 mM to the SDS micellar solution (90 mM), resulted in a cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) endowed with two peculiar advantages compare to the conventional MEKC: (i) strong improvement of separation of the most important phytomarkers of T. cacao and (ii) enantioselectivity toward (±)-catechin. In particular, separation of methylxanthines (theobromine and caffeine), procyanidin dimers B1 and B2, and catechins (epicatechin and catechin) was obtained simultaneously to the enantioseparation of racemic catechin within 10 min. The enantioselectivity of the method makes it suitable in evaluation of possible epimerisation at the C-2 position of epicatechin monomer potentially occurring during heat processing and storage of T. cacao beans. The extraction procedure of the phytomarkers from the beans was approached using ultrasonic bath under mild conditions optimized by a multivariate strategy. The method was validated for robustness, selectivity, sensitivity, linearity, range, accuracy and precision and it was applied to T. cacao beans from different countries; interestingly, the native enantiomer (+)-catechin was found in the beans whereas, for the first time we reported that in chocolate, predominantly (−)-catechin is present, probably yielded by epimerisation of (−)-epicatechin occurred during the manufacture of chocolate.

Use of dynamically coated capillaries for the determination of heroin, basic impurities and adulterants with capillary electrophoresis

Rapid, precise, accurate, and reproducible methodology using capillary electrophoresis (CE) with dynamically coated capillaries for the analysis of heroin and its basic impurities and adulterants is presented. Highly selective determination of the above solutes is obtained by analyzing the same sample preparation by two CE methods. For the determination of heroin, its basic impurities and basic adulterants, dynamic coating of the capillary surface is accomplished using a commercially available reagent kit with an added cyclodextrin ((CD) polycation coating followed by polyanion coating with dimethyl-β-cyclodextrin or hydroxypropyl-β-cyclodextrin). The addition of a cyclodextrin to the run buffer significantly improves the separation of these solutes. Neutral, acidic, and weakly basic adulterants which migrate near or after t 0 do not interfere with the more mobile basic solutes. The determination of neutral, acidic, and weakly basic adulterants in heroin is accomplished using a modification of the above commercially available reagent kit. After first coating with a polycation, a negative coating is obtained using a surfactant sodium dodecyl sulfate. Micellar electrokinetic chromatography (MEKC) with dynamically coated capillaries gives an excellent separation of the neutral, acidic, and weakly basic solutes, with considerably shorter run times compared to conventional MEKC. In addition for this system, most basic solutes in heroin have longer migration times than the uncharged and acidic compounds.

On-line micellar electrokinetic chromatography–mass spectrometry: feasibility of direct introduction of non-volatile buffer and surfactant into the electrospray interface

An on-line method for the coupling of micellar electrokinetic chromatography (MEKC) and mass spectrometry (MS) is presented which allows conventional MEKC conditions to be employed without further modification. The MEKC system is coupled directly to electrospray ionization (ESI) MS using a triaxial interface. A systematic study of the influence of the surfactant concentration, the nature and concentration of buffer salts and presence of organic modifier on the interface performance indicated the feasibility of the MEKC–MS approach. Effective interfacing of MEKC was achieved with both single quadrupole and ion-trap MS instruments. Using a background electrolyte containing 20 m M sodium dodecyl sulfate (SDS) and 10 m M sodium phosphate buffer, it is demonstrated that full MEKC runs of test mixtures of mebeverine and related compounds can be monitored by ESI-MS with satisfactory sensitivity. Sub-μg/ml levels of the analytes can still be detected in full scan mode, while detection limits are in the 10–50 ng/ml range when selected ion monitoring is applied. It is shown that such sensitivity would allow full-scan MS detection of 0.1% (w/w) levels of potential impurities in mebeverine. With the ion-trap instrument successful MEKC–MS/MS experiments were carried out providing information-rich MS spectra of the related compounds. Repeated MEKC–MS analyses proved that in the course of 1 day the migration time of mebeverine remained fairly constant while the MS-signal intensity only gradually decreased to approximately 65% of its original value. Once-a-day cleaning of the first part of the ion source, which takes only 5 min, suffices to preserve an optimal interface performance for a prolonged period of time.

Analysis of catechins in extracts of Cistus species by microemulsion electrokinetic chromatography

A microemulsion electrokinetic chromatographic (MEEKC) method was developed for the separation of six catechins, specific marker phytochemicals of Cistus species. The MEEKC method involved the use of sodium dodecyl sulfate (SDS) as surfactant, heptane as organic solvent and butan-1-ol as co-solvent. In order to have a better stability of the studied catechins, the separation was performed under acidic conditions (pH 2.5 phosphate buffer). The effects of SDS concentration and of the amount of organic solvent and co-solvent on the analyte resolution were evaluated. The optimized conditions (heptane 1.36% (w/v), SDS 2.31% (w/v), butan-1-ol 9.72% (w/v) and 50 m M sodium phosphate buffer (pH 2.5) 86.61% (w/v)) allowed a useful and reproducible separation of the studied analytes to be achieved. These conditions provided a different separation profile compared to that obtained under conventional micellar electrokinetic chromatography (MECK) using SDS. The method was validated and applied to the determination of catechin and gallocatechin in lyophilized extracts of Cistus incanus and Cistus monspeliensis.

On-line partial filling micelllar electrokinetic chromatography-electrospray ionization mass spectrometry

On-line combination of partial filling micellar electrokinetic chromatography (PF-MEKC) and electrospray ionization mass spectrometry (ESI-MS) is demonstrated for the analysis of triazine herbicides including atrazine, propazine, ametryne and prometryne. In comparison with conventional micellar electrokinetic chromatography (MEKC), PF-MEKC involves filling a small portion of the capillary with a sodium dodecyl sulfate (SDS) micellar solution for achieving the separation. In PF-MEKC, the triazine analytes first migrate into the micellar plug where the separation occurs and then into the electrophoresis buffer which is free of surfactant. Consequently, the electroosmotic transfer of neutral triazine herbicides to ESI-MS at the end of PF-MEKC capillary is comparable to conventional capillary zone electrophoresis ESI-MS. Therefore, PF-MEKC-ESI-MS provides a mechanism for the separation and mass detection of neutral molecules without the interference of surfactant.

Mechanistic Studies of Partial-Filling Micellar Electrokinetic Chromatography

The need for coupling micellar electrokinetic chromatography (MEKC) with electrospray mass spectrometry initiates the development of partial-filling MEKC. In comparison with conventional MEKC, only a small portion of the capillary is filled with a micellar solution for performing the separation in partial-filling MEKC. Analytes first migrate into the micellar plug, where the separation occurs, and then into the leading electrophoresis buffer, which is free of surfactants. A theoretical model is proposed for predicting the separation behavior of triazine herbicides in partial-filling MEKC. The comparisons between conventional and partial-filling MEKC in terms of separation efficiency and resolution of triazine herbicides are presented and discussed. The optimization techniques, possible applications, and advantages of partial-filling MEKC are similarly addressed.

Separation of closely related large peptides by micellar electrokinetic chromatography with organic modifiers.

Large peptides with similar electrophoretic mobilities were separated by micellar electrokinetic chromatography (MEKC) with organic modifiers. [Leu13]motilin and [Met13]motilin differ by only one neutral amino acid residue. Because the electrophoretic mobilities of these peptides are almost identical, these peptides were not separated by capillary zone electrophoresis (CZE). Such large peptides have not been separated by conventional MEKC either, because they interacted strongly with the micelle. However, they were completely separated by MEKC when an organic solvent was added to the micellar solution. Some insulins, larger peptides than motilin, from different origins, which have very similar electrophoretic mobilities, were also successfully separated by the same technique. The size of peptides which were separated without organic modifiers was examined.

A monomolecular pseudostationary phase for micellar electrokinetic capillary chromatography

AbstractA highly stable pseudostationary phase has been synthesized and evaluated for use in micellar electrokinetic capillary chromatography (MECC). Sodium 10‐undecylenate has been oligomerized to form a micelle‐like structure following the procedure of Durairaj et al. [1].The stability of this structure renders it suitable for use with mobile phases containing relatively high percentages of organic modifiers, enabling the separation and detection of hydrophobic analytes such as alkyl phthalates and polynuclear aromatic hydrocarbons (PAHs) which are difficult to analyze by conventional micellar electrokinetic chromatography. In addition to demonstrating the utility of this structure for the analysis of hydrophobic analytes, the electrophoretic and chromatographic properties of the new phase have been investigated.

Novel pseudostationary phase for micellar electrokinetic capillary chromatography

AbstractA novel, highly stable pseudostationary phase for use in micellar electrokinetic capillary chromatography has been synthesized and evaluated. Sodium‐10‐undecylenate has been oligomerized to form a micelle‐like structure following the procedure of Durairaj et al. [1]. The effective critial micelle concentration of this new pseudostationary phase is zero, permitting electrokinetic chromatography to be performed at lower ionic strength than is possible with the corresponding free surfactant. The structural stability of this pseudostationary phase permits its use with relatively high percentages of organic modifiers in the buffer medium, allowing the separation and detection of hydrophobic analytes such as alkyl phthalates which are difficult to analyze by conventional micellar electrokinetic chromatography.

Effect of urea addition in micellar electrokinetic chromatography

The addition of urea to a micellar solution was developed for the separation of hydrophobic compounds by micellar electrokinetic chromatography (MEKC). The logarithm of the capacity factor (k') decreased linearly with increasing concentration of urea. The use of a high concentration of urea allowed the MEKC separation of hydrophobic compounds, which were mostly included in the micelle and could not be resolved by conventional MEKC. The addition of urea also expanded the migration-time window and hence enhanced the resolution. The effect of urea is discussed from the standpoint of the contribution of urea to a diminished water structure around a hydrophobic solute. Two succesful examples of the application of the technique are given for the MEKC separation of a complex mixture and hydrophobic compounds.