Micellar Electrokinetic Chromatography System Research Articles

Open access capillary electrophoresis: A walk up capillary electrophoresis service for the synthetic chemist

A capillary electrophoresis (CE) system has been developed to provide routine purity information to support synthetic organic chemists. A single micellar electrokinetic chromatography system that produces good selectivity with short run times was developed. The instrument operating software has been modified to run separations in a custom open access mode. No expert knowledge of CE is required to run separations.

Potential of Capillary Electrophoresis for the Profiling of Propolis

The usefulness of capillary electrophoresis (CE) with diode array detection for the profiling of Propolis, a hive product, is investigated. Water extracts of Propolis were analyzed with both capillary zone electrophoresis (CZE) at pH 7.0 and 9.3, and micellar electrokinetic chromatography (MEKC) with sodium dodecyl sulfate at pH 9.3. Characteristic profiles were obtained and several organic acids and preservatives could be identified by means of library comparison of the recorded UV spectra combined with addition of reference compounds to the extracts. The selectivity of the CZE and MEKC system differed considerably but the information obtained with both methods was similar. The dry residues of the water extraction were extracted with ethanol-water (70 : 30, v/v) and analyzed with the MEKC system to enable the separation of the more hydrophobic constituents of the Propolis samples. Complex profiles containing various well separated peaks were obtained allowing the identification of some interesting flavonoids. On the basis of the recorded CZE and MEKC profiles, the Propolis samples could be divided into two clearly different groups which are probably from a different origin.

Rapid estimation of octanol-water partition coefficients of pesticides by micellar electrokinetic chromatography.

Micellar electrokinetic chromatography (MEKC) was evaluated as a new technique for the rapid estimation of octanol-water partition coefficient (logKow). Retention measurements for more than 40 reference pesticides with varied structural characteristics and hydrophobicity were carried out in two MEKC systems, based on sodium dodecyl sulfate (SDS) and sodium cholate (SC), respectively. To enable an accurate determination of capacity factors in the SC-MEKC system, cypermethrin (a synthetic pyrethroid insecticide) was utilized instead of Sudan III as the SC micelle tracer, since a few highly hydrophobic pesticides were found to elute after Sudan III. The linear correlation between logarithmic capacity factor (logk') and logKow in the two systems was examined. It was found that, under the typical buffer condition (10 mM sodium phosphate with 60 mM surfactant, pH 7.0), the SDS-MEKC system provided a somewhat wider dynamic range for hydrophobicity (logKow from -1.0 to 4.5). However, the correlation of logk' with logKow was not very high when all the reference pesticides were included in one single calibration set. For the SC-MEKC system, the dynamic range for logKow was in the range of 1.0-5.5, and a good linear correlation existed between logk' and logKow, even when all reference pesticides were incorporated into a single calibration group. By comparing the regression line of the reference pesticides with that of a group of simple aromatic derivatives, it was discovered that molecular size and functionality posed a less significant effect on the measurement of logKow in the SC-MEKC system than in the SDS-MEKC system. Thus, SC-MEKC shall be the system of choice for the estimation of logKow. The typical error on logKow determination using the current MEKC technique was within 0.5 units, suggesting that MEKC can be a valuable complement to reversed phase high performance liquid chromatography (RP-HPLC) for the indirect determination of logKow. Besides maintaining all the advantages of the HPLC approach, the MEKC technique showed some unique benefits, such as better inter-column reproducibility, higher throughput, and less handling of toxic pesticides and solvents.

Migration behavior and separation of tetracycline antibiotics by micellar electrokinetic chromatography

The migration behavior and separation of six tetracyclines (TCs) were investigated by micellar electrokinetic chromatography (MEKC) in the pH range 5.0–9.0 using ammonium acetate buffer with the addition of sodium dodecyl sulfate (SDS). Mixed SDS–Brij 35, sodium cholate (SC) and tetradecyltrimethylammonium bromide (TTAB) were also used as surfactants. The influences of surfactant concentration and buffer pH on the separation of TCs were examined and the separations of TCs were optimized. Complete separation of six TCs was achieved within 8 min with 15 m M ammonium acetate buffer containing 20 m M SDS, with or without the addition of Brij 35 (0.135%, w/v), at pH 6.5 using a fused-silica capillary (42 cm×75 μm I.D.) at 15 kV. In general, good linear correlations of the logarithm of migration factor (log k′) versus the logarithm of octanol–water partition coefficient (log P ow ) in these micellar systems, except for the TTAB–MEKC system, were obtained. The results indicate that the migration of TCs in MEKC is mainly based on hydrophobic interactions. However, hydrogen bonding interactions also play a significant role in influencing the chemical selectivity of TCs. In addition, the micelle–water partition coefficients ( P mw ) of TCs, which are pH-dependent in the SDS–MEKC micellar system, are reported.

Choice of capillary electrophoresis systems for the impurity profiling of drugs

In order to develop a strategy for the impurity profiling of drugs, the possibilities of some capillary electrophoresis systems were investigated. A mixture containing a drug and some of its possible impurities has been used as a model problem. The test compounds were investigated by capillary zone electrophoresis (CZE) and by micellar electrokinetic chromatography (MEKC). The pH of the CZE buffer was varied, but the two stereoisomers could not be separated. Moreover, CZE is not suitable for neutral compounds. In MEKC, two different types of surfactants, sodium dodecyl sulphate (SDS) and cetyltrimethylammonium bromide (CTAB), have been used and the effect of type and concentration modifier on the separation and the elution window was studied. In the SDS system, both the resolution and the elution window could be increased considerably by the addition of modifier. The use of two MEKC systems of different selectivity seems to be a combination with high potential for the impurity profiling of drugs.

Chemical selectivity in micellar electrokinetic chromatography II. Rationalization of elution patterns in different surfactant systems

Retention behavior in micellar electrokinetic chromatography (MEKC) is investigated using linear solvation energy relationships (LSERs) for two pseudo-stationary phases, one consisting of cationic micelles of tetradecyltrimethylammonium bromide (C 14TAB) and the other of an anionic triblock copolymer, poly(methyl methacrylate-ethyl acrylate-methacrylic acid) (Elvacite 2669). It was found that solutes' migration behaviors in these two MEKC systems are mainly influenced by their si/e ( V/100) and hydrogen bonding acceptor (HBA) strength (β). However, solutes' hydrogen bonding donor (HBD) strength (α) has minor effects on their migration in MEKC. The characteristics of these two systems were compared to three other previously reported anionic micellar systems of sodium dodecyl sulfate (SDS) (anionic hydrocarbon), sodium cholate (SC) (anionic bile salt) and lithium perfluorooctane sulfonate (LiPFOS) (anionic fluorocarbon). It was concluded that hydrogen bonding interactions play a major role in providing different chemical selectivity among these five MEKC systems. Both C 14TAB micelles and the ionic polymer of Elvacite 2669 provide hydrogen bonding acceptor (HBA) sites for solutes, which is similar to SC micelles. In fact, C 14TAB is the strongest HBA, while Elvacite 2669 has HBA strength similar to that of SC micelles. On the other hand, the fluorocarbon micelles of LiPFOS are the strongest hydrogen bond donor (HBD) micelles, followed by the weak HBD SDS micelles. In general, cavity formation has little or no effect on chemical selectivity among hydrocarbon surfactant MEKC systems (i.e., SDS, SC and C 14TAB). Information obtained from the LSER analysis is used to rationalize the elution patterns in MEKC with different types of pseudo-stationary phases.

Capillary zone electrophoresis and micellar electrokinetic chromatography, with taurodeoxycholate as micellar agent, of protein kinase A peptide substrates.

The separation of protein kinase A peptide substrates with the general formula -X-Arg-Arg-Ala-Ser-Y-, where X and Y may be the same or different amino acids, was studied by capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC). Taurodeoxycholate (TDC) was used as the micellar agent. CZE was effective in separating a peptide series differing in the number of amino acids, but not for a series with a difference in the terminating amino acid. For the latter series, MEKC generally gave a higher selectivity, but some of the peptide pairs were more easily separated by CZE, demonstrating the complementary character of the two techniques. The efficiency of the MEKC system was typically < 50% of that of CZE, but its higher selectivity generally outbalanced the lower efficiency regarding resolution. The distribution of the peptides to the micelles was studied by determination of retention factors. Electrostatic and hydrophobic forces were found to be determining factors in the distribution; the most highly charged basic peptides were most heavily distributed, and for peptides with the same charge those containing more hydrophobic amino acids were more strongly distributed. The contribution of some structural features to the distribution degree was also determined.

Deductive prediction of measurement precision and optimization of integration time and wavelength in capillary electrophoreses

This paper demonstrates that the relative standard deviation (RSD) of measurements in a capillary electrophoresis system can be predicted theoretically from the baseline and the signal shape at low sample concentration. The only requirements for prediction of the uncertainty are the Fourier transform of the baseline (here 2048 data points) and the observation of signal shape. The micellar electrokinetic chromatography (MEKC) of acetaminophen and caffeine is taken as an example. The optimum is defined here as the condition of the lowest RSD or highest precision. The optimum single wavelength is selected from between 220 and 350 nm for the MEKC system equipped with a photodiode array detector. The optimum time domain of signal integration is shown to be even narrower than the entire signal region, providing an RSD value about half that for integration of the entire region. The theory is in good agreement with observed RSD values.

Selectivity control in micellar electrokinetic chromatography of small peptides using mixed fluorocarbon-hydrocarbon anionic surfactants

Electrophoretic mobilities and capacity factors for a group of Trp-containing small peptides were determined by micellar electrokinetic chromatography (MEKC) using mixtures of a fluorocarbon anionic surfactant, lithium perfluorooctane sulfonate, and a hydrocarbon anionic surfactant, lithium dodecyl sulfate. Upon mixing these two surfactants, which have different microenvironments and interactive characteristics, greater control over migration of solutes is achieved. The changes in the composition of mixed micelles such as the mole fraction of the surfactants result in different solute-micelle binding as well as migration times of the micelles ( t mc). Consequently, capacity factor, selectivity and elution window ( t mc/ t 0) change with the composition of the mixed micellar system. Another characteristic of the mixtures of fluorocarbon-hydrocarbon surfactants is the possibility of forming two different types of micelles which offers an additional partitioning process for each solute in the MEKC system. Such a unique phenomenon offers a higher degree of selectivity control. This mixed MEKC system is quite effective for the separation of small peptides. It provides an alternative to the free-solution capillary zone electrophoresis system for the separation of charged solutes with nearly identical electrophoretic mobility.

Effect of organic modifier concentrations on electrokinetic migrations in micellar electrokinetic chromatography.

The effect of low concentrations of organic modifiers on the electroosmotic mobility, mueo, and electrophoretic mobility of the micelle, muep, mc, in micellar electrokinetic chromatography (MEKC) with sodium dodecyl sulfate (SDS) solutions was investigated. The results showed that mueo was mainly influenced by the viscosity of the running solution. A linear relationship between mueo and the concentration of the organic modifier was found in MEKC. A newly defined parameter, the mobility ratio, Rm, mc, which is the ratio of the electrophoretic mobility of the micelle to the electroosmotic mobility, and which characterizes the surface charge density of the micelle in MEKC system with organic modifiers, was introduced. A linear relationship between Rm, mc and the organic modifier concentration was observed. The parameter Rm, mc was found to be useful to investigate the changes in the micellar phase with the addition of organic modifier. The measured muep, mc values showed different and interesting characteristics among different organic modifiers.

Polymeric reagents for derivatizations in micellar electrokinetic chromatography.

A polymer immobilized o-nitrobenzophenone reagent was prepared for analysis of amine drugs in micellar electrokinetic chromatography (MEKC). A model compound, propylamine, was used to characterize the reagent's performance in MEKC. Derivatizations were performed on the CE instrument with reagent in the sample vial. The yielded derivative was directly sampled from the reaction mixture, and directly injected onto the MEKC system. The derivatization reagent was also applied to the derivatization of n-alkyl amine mixtures and amino acids. The method was validated for adamantanamine in urine and in plasma by single-blind spike analysis. Precisions and accuracies for all samples were less than 6.0% for urine samples and 10% for plasma samples. The procedure was a direct injection technique requiring minimal sample preparation for the analysis of drugs in biofluids.

Separation of phthalates by micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC) of priority phthalate esters was investigated. A commercial photodiode-array ultraviolet—visible detector was modified for on-column detection in an MEKC system. For the separation of phthalate esters, electrophoretic media with sodium dodecyl sulphate in phosphate—borate buffer were used. The retention behaviour of the phthalate esters at different concentrations of micellar solution and at different pH values of the electrophoretic media was investigated. In addition, the effect of different voltages across the capillary tubing was examined. The results successfully demonstrated the application of MEKC for the separation of a group of six phthalate esters, five of which are priority pollutants.