Recent advances in micellar electrokinetic chromatography

This review contains nearly 200 reference citations, and covers advances in electrokinetic capillary chromatography based on micelles, including stabilized micelle complexes, polymeric and mixed micelles from 2003-2004. Detection strategies, analyte determinations, and applications in micellar electrokinetic capillary chromatography (MEKC) are discussed. Information regarding methods of analyte concentration, analyte specific analyses, and nonstandard micelles has been summarized in tabular form to provide a means of rapid access to information pertinent to the reader.     

Pharmaceutical applications of micelles in chromatography and electrophoresis

This review surveys the use of micelles as separation media in chromatography and electrophoresis. Applications to pharmaceuticals whose molecular masses are relatively small are focused on in this review. In high-performance liquid chromatography (HPLC), chromatography using micelles and reversed-phase stationary phases such as octadecylsilylized silica gel (ODS) columns is known as micellar liquid chromatography (MLC). The main application of MLC to pharmaceutical analysis is the same as in ion-pair chromatography using alkylsulfonate or tetraalkylammonium. In most cases, selectivity is much improved compared with other short alkyl chain ion-pairing agents such as pentanesulfonate or octanesulfonate. Direct plasma/serum injection can be successful in MLC. Separation of small ions is also successful by using gel filtration columns and micellar solutions. In electrophoresis, especially capillary electrophoresis (CE), micelles are used as pseudo-stationary phases in capillary zone electrophoresis (CZE). This mode is called micellar electrokinetic chromatography (MEKC). Most of the drug analysis can be performed by using the MEKC mode because of its wide applicability. Enantiomer separation, separation of amino acids and closely related peptides, separation of very complex mixtures, determination of drugs in biological samples etc. as well as separation of electrically neutral drugs can be successfully achieved by MEKC. Microemulsion electrokinetic chromatography (MEEKC), in which surfactants are also used in forming the microemulsion, is successful for the separation of electrically neutral drugs as in MEKC. This review mainly describes the typical applications of MLC and MEKC for the analysis of pharmaceuticals.     

Effect of Micelle Composition on Acidic Drugs Separation Behavior by Micellar Electrokinetic Capillary Chromatography

Micellar electrokinetic capillary chromatography (MECC) is a branch of capillary electrophoretic techniques, in which surfactant micelles are added to the electrolyte solution as pseudostationary phase. Separation in MECC is based on electrophoretic mobilities of the analytes when partitioned into micelles1. In this work, four acidic drugs similar in structure with aryl carboxylic acid were separated by MECC. The effects of type of surfactant, such as anionic surfactant SDS, nonionic …     

Fundamental studies on electrokinetic chromatography with PEGylated phospholipid micelles.

In this study, micelles prepared from distearoylphosphatidylethanolamine with covalently attached poly(ethylene) glycol) (PEG) of molecular weight 2000 (DSPE-PEG-2000) were employed in micellar electrokinetic chromatography (MEKC) as pseudostationary phases. Since DSPE-PEG-2000 contains long hydrophobic alkyl chains, an anionic phosphate group, and hydrophilic PEG chains, the prepared micelles are expected to provide a characteristic retention behavior for both neutral and ionic compounds. As a typical example, a baseline separation of phenol and 2-naphthol was successfully achieved by using the DSPE-PEG-2000 micelles as a background electrolyte for MEKC; such success clearly shows that the micelles can retain electrically neutral compounds. The MEKC separations of anionic and cationic compounds with a DSPE-PEG-2000 micellar solution and the enantioseparation of binaphthyl compounds with mixed micelles containing bile salt are also discussed.     

Conformational effects on the performance and selectivity of a polymeric pseudostationary phase in electrokinetic chromatography

The effect of the conformation of a polymeric pseudostationary phase on performance and selectivity in electrokinetic chromatography was studied using an amphiphilic pH-responsive polymer that forms compact intramolecular aggregates (unimer micelles) at low pH and a more open conformation at high pH. The change in conformation was found to affect the electrophoretic mobility, retention, selectivity, and separation efficiency. The low-pH conformer has higher electrophoretic mobility and greater affinity for most solutes. The unimer micelle conformation was also found to provide a solvation environment more like that of micelles and other amphiphilic self-associative polymers studied previously. It was not possible to fully characterize the effect of conformation on efficiency, but very hydrophobic solutes with long alkyl chains appeared to migrate with better efficiency when the unimer micelle conformation was employed. The results imply that polymers with a carefully optimized lipophilic-hydrophilic balance that allow self-association will perform better as pseudostationary phases. In addition, the results show that electrokinetic chromatography is a useful method for determining the changes in solvation environment provided by stimuli-responsive polymers with changes in the conditions.     

Micelle polymers, polymer surfactants and dendrimers as pseudo-stationary phases in micellar electrokinetic chromatography

Several authors have recently reported the use of micelle polymers, polymer surfactants and dendrimers as pseudo-stationary phases in electrokinetic chromatography. These reports have demonstrated the effectiveness of these phases for a variety of applications, including the separation and analysis of hydrophobic compounds and chiral compounds and the application of mass spectrometric detection. This review covers developments in this area since the first introduction of polymeric pseudo-stationary phases in 1992. The use of polymeric micelles in electrokinetic chromatography is compared briefly with capillary electrochromatography. Some thoughts on future directions in this area are presented.     

Separation and selectivity of benzophenones in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or sodium cholate modified mixed micelles

The separation and selectivity of nine benzophenones in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or sodium cholate (SC) modified mixed micelles were investigated in the pH range 6.5-8.0. The results indicate that the combined effects of buffer pH and SC concentration can greatly affect the separation and selectivity of benzophenones, particularly for benzophenones possessing a hydroxyl substituent at the 4-position of the aromatic ring with respect to the carbonyl moiety when using SDS-SC mixed micelles. Better separability can be obtained with SDS-SC mixed micelles than with SDS micelles. Complete separation of nine benzophenones in MEKC can be achieved with an appropriate choice of buffer pH and the concentration of SDS micelles or SC modified mixed micelles. The dependence of the migration order of those benzophenones based on their structures and solute-micelle interactions is discussed.     

Review of aqueous chiral electrokinetic chromatography (EKC) with an emphasis on chiral microemulsion EKC

The separation of enantiomers using electrokinetic chromatography (EKC) with chiral microemulsions is comprehensively reviewed through December 1, 2006. Aqueous chiral EKC separations based on other pseudostationary phases such as micelles and vesicles or on other chiral selectors such as CDs, crown ethers, glycopeptides, ligand exchange moeities are also reviewed from both mechanistic and applications perspective for the period of January 2005 to December 1, 2006.     

Enantiomer separation of drugs by micellar electrokinetic chromatography using chiral surfactants

A review surveying enantiomer separations by micellar electrokinetic chromatography (MEKC) using chiral surfactants is described. MEKC is one of the most popular techniques in capillary electrophoresis, where neutral compounds can be analyzed as well as charged ones, and the use of chiral micelles enable one to achieve the enantioseparation. The chiral MEKC systems are briefly reviewed according to the types of chiral surfactants along with typical applications. As chiral micelles or pseudostationary phases in MEKC, various natural and synthetic chiral surfactants are used, including several low-molecular-mass surfactants and polymerized surfactants or high-molecular-mass surfactants. Cyclodextrin modified MEKC using chiral micelles is also considered.     

Evaluation of polymers based on a silicone backbone as pseudostationary phases for electrokinetic chromatography.

The feasibility of polymeric phases based on a silicone polymer backbone as pseudostationary phases for electrokinetic chromatography has been investigated. Silicone phases were studied because of the range of chemistries that could be developed based on these backbones, and because successful development of silicone phases would make it possible to employ much of the stationary phase chemistry developed in the past thirty years. Three silicone polymer structures have been investigated, but only one had sufficient aqueous solubility to permit application in electrokinetic chromatography. This phase was characterized by a variety of methods and was shown to be a mixture of partially hydrolyzed poly(bis-(3-cyanopropyl) siloxanes. When employed as a pseudostationary phase, this material provided selective and efficient separations. The electrophoretic mobility of the silicone polymer is greater than that of sodium dodecyl sulfate (SDS) micelles and poly(sodium 10-undecenylsulfate), providing an extended migration time range. A striking characteristic of the polymer is that the electrophoretic mobility is greater than typical electroosmotic mobilities. The chemical selectivity of the phase is significantly different from that of SDS micelles or poly(sodium 10-undecenylsulfate). The silicone phase is a more cohesive, basic and polar phase than SDS micelles. In buffers modified with a high concentration of organic solvents, the chromatographic properties of the silicone polymer are inferior to those of the poly(sodium 10-undecenylsulfate). The greatest limitation of silicone polymers for this application appears to be limited aqueous solubility, which will make it difficult to realize a family of such polymers with different chemical selectivities.     

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, beta-cyclodextrin and a dual cyclodextrin system of beta-cyclodextrin with heptakis-6-sulphato-beta-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 beta-cyclodextrin and 1 mM of heptakis-6-sulfo-beta-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.     

Characterization of cephalosporin transfer between aqueous and colloidal phases by micellar electrokinetic chromatography

A new electrokinetic chromatographic method was applied to the determination of the partition coefficient between water and micelle for a group of cephalosporins (cefmetazol, cephradin, cefaclor, ceftazidim, cefodizim, cephapirin, cephalothin and ceftriaxon) using sodium dodecyl sulphate as an anionic surfactant in microemulsion and in micellar systems. In the new method, the running buffer contains both the micelles and the drug, and the injected solution contains the same concentration of micelles as the running buffer but not the drug. The mobility of the drug can be measured from a negative peak recorded the chromatogram. The required parameters for the determination of the capacity factor (mu(aq) and /mu(me) are the electrophoretic mobilities of the solutes in the aqueous and the micelle phases, mu(eff) is the effective mobility in the micellar system or in the microemulsion) were measured by the new micellar and microemulsion electrokinetic chromatography technique. Linear log-log relationships were found between both the micelle-water partition coefficient and the capacity factor and the n-octanol-water partition coefficient.     

电动胶束色谱中弱酸性化合物的柱上浓缩富集技术的研究

提出一种电动胶束色谱（ＭＥＫＣ）中弱酸性化合物的柱上浓缩富集技术,并得到实验证实。以七种弱酸性巴比妥类药物的ＭＥＫＣ分离为例,考察了样品溶剂中表面活性剂浓度、ｐＨ值和离子强度等对富集作用的影响。表面活性剂浓度,ｐＨ值对富集效应的影响较大。采用低浓度（略高于ＣＭＣ点）表面活性剂、低浓度缓冲液作样品溶剂,调节溶剂ｐＨ值小于待测化合物的ｐＫａ－１,就可以对弱酸性化合物进行柱上富集。采用这种富集技术,可以压缩样品带的宽度从而提高柱效,在此基础上可通过加大进样量提高化合物的检测灵敏度。     

Molecular recognition by chiral cationic micellar and micelle-like aggregates in electrokinetic capillary chromatography

We examined the enantiomer separation with micelles and a micelle-like polymer made with trimethylammonium-terminated surfactants all of whose hydrocarbon chains contain hydrogen bonding valinediamide moieties in electrokinetic chromatography (EKC). The surfactants used were 3-(N-dodecanoyl-L-valylamino)-propyltrimethylammonium bromide (surfactant 1) and 6-(N-nonanoyl-L-valylamino)hexyl-trimethylammonium bromide (surfactant 2); the micelle-like polymer was derived from 3-(N-10-undecenoyl-L-valyl)aminopropyltrimethylammonium bromide (surfactant 3). N-Acylamino acids and their isopropyl esters were separated with enantiomers with the same configuration as the chiral surfactant and which were retained to a greater extent than the counterparts in micelles. The micellar hydrophobic environment, in which amides function as hydrogen bonding sites with solutes, and ceased micellar kinetic association-dissociation with polymerization are discussed.     

Separation and selectivity in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or Tween 20-modified mixed micelles

The separation and selectivity of eight aromatic compounds ranging from hydrophilic to hydrophobic properties in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or Tween 20-modified mixed micelles were investigated. The effect of different operation conditions such as SDS and Tween 20 modifier surfactant concentration, buffer pH, and applied voltage was studied. The resolution and selectivity of analytes could be markedly affected by changing the SDS micelle concentration or Tween 20 content in the mixed micelles. Applied voltage and pH of running buffers were used mainly to shorten the separation time. Complete separation of eight analytes could be achieved with an appropriate choice of the concentration of SDS micelles or Tween 20-modified mixed micelles. Quicker elution and better precision could be obtained with SDS-Tween 20 mixed micelles than with SDS micelles. The mechanisms that migration order of those analytes was mainly based on their structures and solute-micelle interactions, including hydrophobic, electrostatic, and hydrogen bonding interactions, were discussed.     

The effect of injection procedures on efficiency in micellar electrokinetic capillary chromatography

Summary Micellar electrokinetic capillary chromatography (MECC) involves the application of a high voltage (10–30 KV) across a capillary column (75μm i.d.) which is filled with a solution containing micelles. The mobile phase in this work consisted of sodium dodecyl sulfate in an aqueous borate/phosphate buffer system. Injection parameters in MECC were investigated to minimize the contribution to band-broadening resulting from the injection process. Efficiencies as high as 240,000 theoretical plates/meter are reported.     

On-line sample preconcentration in micellar electrokinetic chromatography by sweeping with anionic-zwitterionic mixed micelles

On-line preconcentration by sweeping in micellar electrokinetic chromatography using mixed micelles of sodium dodecyl sulfate (SDS)-SB-12 is presented. Because of their large micelle radius, they permit increased partitioning of hydrophobic analytes into the core. In addition, they also possess lower negative surface charge relative to pure SDS micelles so anionic analytes can be retained better due to decreased electrostatic repulsion. As the efficiency of sweeping is predicated on the magnitude of retention factors, these advantages translated to better focusing. As much as a 370-fold improvement in detector response, in terms of peak height, was obtained for some neutral steroids, while about a 360-fold improvement was obtained for some phenol derivatives, which were previously not amenable to sweeping by pure SDS micelles.     

Systematic investigations of different capillary electrophoretic techniques for separation of methylquinolines

The migration behaviour of isoquinoline, quinoline, and methyl derivatives of quinoline in different capillary electrophoretic modes has been systematically investigated. Optimised separation conditions were established by varying the key parameters (solvent, pH, temperature, surfactant concentration, core phase) for aqueous and non‐aqueous capillary zone electrophoresis (NACE), micellar electrokinetic chromatography (MEKC) with anionic or non‐ionic micelles (SDS, Brij 35), and microemulsion electrokinetic chromatography (MEEKC) with charged or uncharged microemulsion droplets. A separation of all quinolines could be achieved by MEEKC with charged droplets, by MEKC or by formamide‐based NACE. Comparing the separations with respect to separation selectivity, substantial changes in migration order could be observed between the different techniques. Regarding separation efficiency, the number of theoretical plates and limits of detection (LOD) have been compared. The best LODs were achieved using SDS as surfactant in MEKC, followed by MEEKC.     

Capillary electrokinetic separation techniques for profiling of drugs and related products

Capillary electrokinetic separation techniques offer high efficiency and peak capacity, and can be very useful for the analysis of samples containing a large variety of (unknown) compounds. Such samples are frequently met in impurity profiling of drugs (detection of potential impurities in a pharmaceutical substance or product) and in general sample profiling (determination of differences or similarities between samples). In this paper, the potential, merits, and limitations of electrokinetic separation techniques for profiling purposes are evaluated using examples from literature. A distinction is made between impurity profiling, forensic profiling and profiling of natural products, and the application of capillary zone electrophoresis, micellar electrokinetic chromatography, and capillary electrochromatography in these fields is discussed. Attention is devoted to important aspects such as selectivity, resolution enhancement, applicability, detection, and compound confirmation and quantification. The specific properties of the various electrokinetic techniques are discussed and compared with more conventional techniques as liquid chromatography.