Recent advances in microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) is an electrodriven separation technique. Separations are typically achieved using oil-in-water microemulsions, which are composed of nanometre-sized droplets of oil suspended in aqueous buffer. The oil droplets are coated in surfactant molecules and the system is stabilised by the addition of a short-chain alcohol cosurfactant. The novel use of water-in-oil microemulsions for MEEKC separations has also been investigated recently. This report summarises the different microemulsion types and compositions used to-date and their applications with a focus on recent papers (2002-2004). The effects of key operating variables (pH, surfactant, cosurfactant, oil phase, buffer, additives, temperature, organic modifier) and methodology techniques are described.     

Microemulsion electrokinetic chromatography separation by using hexane-in-water microemulsions without cosurfactant: comparison with MEKC

A new hexane-in-water microemulsion was investigated as buffer in microemulsion EKC (MEEKC). At difference with other microemulsions, the addition of cosurfactant was not necessary to stabilize the microemulsion. The proposed microemulsion was successfully used to achieve electrophoretic separation of seven antibiotics including nitroimidazoles, cephapirin and tetracyclines. Selectivity and separation efficiency achieved in MEEKC were compared with MEKC. MEEKC technique proved to be more efficient than MEKC for performing the separation of the analytes and the presence of microemulsions was found to be critical to achieve the separation of tetracyclines. The proposed microemulsion also points out that solvents with high volatility, such as hexane, can be stabilized and used as a microemulsion of SDS.     

Recent advances in the development and application of microemulsion EKC

Microemulsion EKC (MEEKC) is an electrodriven separation technique. Separations are typically achieved using oil-in-water microemulsions, which are composed of nanometre-sized oil droplets suspended in an aqueous buffer. The droplets are stabilised by a surfactant and a cosurfactant. The novel use of water-in-oil microemulsions has also been investigated. This review summarises the advances in the development of MEEKC separations and also the different areas of application including determination of log P values, pharmaceutical applications, chiral analysis, natural products and bioanalytical separations and the use of new methods such as multiplexed MEEKC and high speed MEEKC. Recent applications (2004-2006) are tabulated for each area with microemulsion composition details.     

Recent applications of microemulsion electrokinetic chromatography

Compared to MEKC, the presence of a water-immiscible oil phase in the microemulsion droplets of microemulsion EKC (MEEKC) gives rise to some special properties, such as enhanced solubilization capacity and enlarged migration window, which could allow for the improved separation of various hydrophobic and hydrophilic compounds, with reduced sample pretreatment steps, unique selectivities and/or higher efficiencies. Typically, stable and optically clear oil-in-water microemulsions containing a surfactant (SDS), oil (octane or heptane), and cosurfactant (1-butanol) in phosphate buffer are employed as separation media in conventional MEEKC. However, in recent years, the applicability of reverse MEEKC (water-in-oil microemulsions) has also been demonstrated, such as for the enhanced separation of highly hydrophobic substances. Also, during the past few years, the development and application of MEEKC for the separation of chiral molecules has been expanded, based on the use of enantioselective microemulsions that contained a chiral surfactant or chiral alcohol. On the other hand, the application of MEEKC for the characterization of the lipophilicity of chemical substances remains an active and important area of research, such as the use of multiplex MEEKC for the high-throughput determination of partition coefficients (log P values) of pharmaceutical compounds. In this review, recent applications of MEEKC (covering the period from 2003 to 2005) are reported. Emphases are placed on the discussion of MEEKC in the separation of chiral molecules and highly hydrophobic substances, as well as in the determination of partition coefficients, followed by a survey of recent applications of MEEKC in the analysis of pharmaceuticals, cosmetics and health-care products, biological and environmental compounds, plant materials, and foods.     

Chiral microemulsion electrokinetic chromatography with two chiral components: Improved separations via synergies between a chiral surfactant and a chiral cosurfactant

In this study, the combination of two chiral components in a microemulsion formulation for the separation of enantiomers via microemulsion EKC (MEEKC) was successfully accomplished. Previous publications of chiral microemulsions have utilized only one chiral entity; the surfactant, cosurfactant, or oil was chiral. This is the first study, to date, of the effects of using two chiral species in a single pseudostationary phase (PSP). The chiral surfactant dodecoxycarbonylvaline (DDCV) was used in conjunction with the chiral cosurfactant S-2-hexanol. Ethyl acetate was incorporated as the oil core of the microemulsion and the buffer was 50 mM phosphate at a pH of 7. Additionally, a microemulsion prepared with racemic 2-hexanol was used for comparison to a previous DDCV microemulsion and as a baseline for the newly formulated dual chiral microemulsion. The efficiencies, resolutions, and enantioselectivities for the S-2-hexanol, racemic 2-hexanol, and original 1-butanol DDCV microemulsions are compared. The hexanol-based PSPs provide improved efficiencies and resolutions. To evaluate the combination of each DDCV enantiomer (R and S) with S-2-hexanol, changes in Gibb's free energy were calculated. A synergistic effect was found when two chiral components were combined to form a microemulsion.     

Enhanced separation of Compound Xueshuantong capsule using functionalized carbon nanotubes with cationic surfactant solutions in MEEKC

A novel additive of multi‐walled carbon nanotubes (MWNTs) dispersed with cationic surfactants or mixed cationic/anionic surfactants was used for MEEKC separation of eight phenolic compounds, four glycosides, and one phenanthraquinone. In this context, several parameters affecting MEEKC separation were studied, including the dispersion agents of MWNTs, MWNTs content, oil type, SDS concentration, and the type and concentration of cosurfactant. Compared with conventional MEEKC, the addition of all types of MWNTs dispersions using single or mixed cationic surfactant solutions in running buffers was especially useful for improving the separation of solutes tested, as they influenced the partitioning between the oil droplets and aqueous phase due to the exceptional electrical properties and large surface areas of MWNTs. Use of cationic surfactant‐coated MWNTs (6.4 μg/mL) as the additive in a microemulsion buffer (0.5% octanol, 2.8% SDS, 5.8% isopropanol, and 5 mM borate buffer) yielded complete resolution of 13 analytes. The proposed method has been successfully applied for the detection and quantification of the studied compounds in a complex matrix sample (Compound Xueshuantong capsule).     

Twenty‐one years of microemulsion electrokinetic chromatography (1991–2012): A powerful analytical tool

Microemulsion electrokinetic chromatography (MEEKC) is a CE separation technique, which utilizes buffered microemulsions as the separation media. In the past two decades, MEEKC has blossomed into a powerful separation technique for the analysis of a wide range of compounds. Pseudostationary phase composition is so critical to successful resolution in EKC, and several variables could be optimized including surfactant/co‐surfactant/oil type and concentration, buffer content, and pH value. Additionally, MEEKC coupled with online sample preconcentration approaches could significantly improve the detection sensitivity. This review comprehensively describes the development of MEEKC from the period 1991 to 2012. Areas covered include basic theory, microemulsion composition, improving resolution and enhancing sensitivity methods, detection techniques, and applications of MEEKC.     

Background theory and applications of microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) is an electrodriven separation technique. Separations are achieved using microemulsions which are nanometre-sized oil droplets suspended in aqueous buffer. The surface tension between the oil and water components is reduced by covered the oil droplet with an anionic surfactant such as sodium dodecyl sulphate and a co-surfactant such as a short-chain alcohol. This review summarises the various microemulsion types and compositions that have been used in MEEKC. The effects of key operating variables such as pH and temperature are also described. The application areas of MEEKC are also described in some detail. MEEKC has been applied to a wide range of water-soluble and insoluble both charged and neutral compounds. Examples are described which include analysis of derivatised sugars, proteins, pesticides and a wide range of pharmaceuticals. At present there are only a limited number of publications describing the use of MEEKC but it is anticipated that this number will increase rapidly in the near future as more awareness of the separation possibilities that MEEKC presents increases.     

Sample stacking for the analysis of catechins by microemulsion EKC

In this study, an on-line concentration method, ASEI (anion-selective exhaustive injection)-sweeping technology which was coupled with microemulsion EKC (MEEKC), was used to analyze and detect six catechins ((-)-epicatechin, (+)-catechin, (-)-epigallocatechin gallate, (-)-epicatechin gallate, (-)-epigallocatechin, and (-)-gallocatechin). In addition to the effects of the buffer pH and electrolyte concentration on stacking, the compositions of microemulsion (types of oil phase, and types and levels of cosurfactant) also dominated the stacking effect of catechins. In MEEKC, the effect of the type of oil in microemulsion on separation mechanism is often unclear. This study had demonstrated that the oil type in microemulsion indeed altered the affinity of oil droplets with analytes. Finally, this proposed ASEI-sweeping MEEKC method was able to detect trace level of catechins in food products that was not previously possible by a normal MEEKC method.     

Chiral cyclodextrin-modified microemulsion electrokinetic chromatography

Cyclodextrin (CD)-modified microemulsion electrokinetic chromatography (MEEKC) or CD-MEEKC has not previously been applied to the area of chiral separations. Herein, the results of investigations of various microemulsions with CD additives are presented. Two different microemulsions are explored: an ethyl acetate sodium dodecyl sulfate microemulsion, and a chiral dodecoxycarbonylvaline (DDCV) microemulsion. Each microemulsion is paired separately with a neutral CD (hydroxypropyl-beta-CD) and an anionic CD (sulfated-beta-CD). In addition, the chiral DDCV microemulsion is investigated in both the R- and S- form. By varying simple parameters such as buffer system, applied voltage, surfactant enantiomer, and type of cyclodextrin, dramatic improvements in the chiral separations were noted. Resolution was found to be highly dependent on buffer identity and concentration, and somewhat dependent on whether the CDs used were randomly or highly sulfated. Under optimized conditions, the resolution ranged from 0.8 to 4.8, with plate counts ranging from 4000 to 26 000. Additionally, S- and R-levetiracetam, which had never before been enantioseparated via capillary electrophoresis (CE) methodologies, were separated in less than 8 min, with a resolution of 1.1.     

Microemulsion electrokinetic chromatography for separation and analysis of curcuminoids in turmeric samples

Microemulsion EKC (MEEKC) was developed for quantitative analysis of curcuminoids, such as curcumin (C), demethoxycurcumin (D), and bis-demethoxycurcumin (B). MEEKC separation of curcuminoids was optimized, and a change in resolution was explained using a modified equation for resolution in MEEKC without electroosmosis. The suitable MEEKC conditions for separation of curcuminoids were obtained to be the microemulsion buffer containing 50 mM phosphate buffer at pH 2.5, 1.1% v/v n-octane as oil droplets, 180 mM SDS as surfactant, 890 mM 1-butanol as cosurfactant, and 25% v/v 2-propanol as organic cosolvent; applied voltage of -15 kV; and separation temperature 25 degrees C. Achieved baseline resolution of C:D and D:B was obtained with R(s) -2.4 and analysis time within 18 min. In addition, high accuracy and precision of the method were obtained. This MEEKC method was used for quantitative determination of individual curcuminoids in medicinal turmeric capsules and powdered turmeric used as coloring additive in food, with simple sample preparation such as solvent extraction, dilution, and filtration, and without cleaning up by SPE.     

Solvent effects in microemulsion electrokinetic chromatography

The contribution of organic solvents to the mechanisms responsible for separation in microemulsion electrokinetic chromatography (MEEKC) is reviewed. Organic solvents are needed as constituents of microemulsions for a series of reasons. (i). A water-immiscible organic substance is used to form the actual oil phase of the microemulsion, (ii). a less hydrophobic solvent is commonly employed as a so-called co-surfactant, and (iii). in many cases an organic modifier is added to influence the solubility of the analytes in the aqueous phase of the microemulsion. All these organic solvents do not only participate in the separation in their actual function, but also interact with each other and the analytes. Variations in separation selectivities triggered by changes in the nature and/or concentration of these organic solvents present in microemulsions suitable for MEEKC are discussed in this work.     

Comparison of microstructures of microemulsion and swollen micelle in electrokinetic chromatography

Recently, 1-butanol modified MEKC was proven to be similar to MEEKC in separation performance. In the present work, typical microemulsion containing 0.8% n-octane/3.3% SDS/6.6% 1-butanol/20 mM borax buffer and corresponding swollen micelle without n-octane were used to compare their microdroplet structures including hydrodynamic radius, electrokinetic potential ζ and charge density at the hydrodynamic shear surface, as well as microenvironment polarity in the interior of the microdroplets. Three kinds of corticosteroids were separated with MEEKC and 1-butanol modified MEKC to assess their separation performances. The experiment results showed that both microstructure and separation performance in microemulsion and in swollen micelle systems were alike, no matter whether oil phase n-octane was present. The environment polarity in the core of swollen micelle was slightly higher than in the microemulsions, and both of them were higher than in n-octane medium. Furthermore, the influences of SDS and 1-butanol concentration on microstructures were measured in details. Increasing the amount of SDS, hydrodynamic radius decreased in microemulsion but increased in swollen micelle. On the contrary, ζ and shear surface charge density changed in the reverse trends. With increment of 1-butanol concentration, the hydrodynamic radius increased dramatically in microemulsions, whereas decreased slightly in swollen micelle. Even though using n-octane as oil core was not a key factor, microemulsions and swollen micelle as pseudostationary phase in EKC should not be exactly the same.     

The use of novel water-in-oil microemulsions in microemulsion electrokinetic chromatography

We describe the novel use of water-in-oil (W/O) microemulsions to achieve unique separations in microemulsion electrokinetic chromatography (MEEKC). The choice and concentration of the buffer type, surfactant and co-surfactant were all examined and optimized. Separations of a range of neutral and acidic analytes was shown to be markedly different to that obtained by (oil-in-water) O/W MEEKC. Neutral solutes are separated by virtue of their solubility (log P) values in O/W MEEKC with the more water-insoluble solutes migrating last. This separation process does not occur in W/O, as neutral solutes are not separated in order of log P.     

Mixture design in the optimization of a microemulsion system for the electrokinetic chromatographic determination of ketorolac and its impurities: method development and validation

Microemulsion EKC (MEEKC) was used for the determination of ketorolac and its three impurities. The microemulsion system was optimized, for the first time in the literature, using a multivariate strategy involving a mixture design. A 13-run experimental plan covering an experimental domain defined by the components aqueous phase (10 mM borate buffer pH 9.2), oil phase (n-heptane) and surfactant/cosurfactant (SDS/n-butanol) was carried out. Good results were obtained with all microemulsions tested considering as responses analysis time and resolution, and according to the desirability function the best microemulsion system was constituted by 90.0% 10 mM borate buffer, 2.0% n-heptane, 8.0% of SDS/n-butanol in 1:2 ratio. Finally, with the aim of reducing analysis time, a response surface study was carried out in the experimental domain defined by the process variables temperature and voltage and the best values were 17 degrees C and -17 kV, respectively. Applying the optimised conditions, a complete resolution among the analytes was obtained in about 3 min using the short-end injection method. The method was validated for both drug substances and drug product and was applied to the quality control of ketorolac in coated tablets. A comparison of MEEKC, MEKC and CEC for assaying ketorolac and its related substances has been made.     

Release of nortriptyline hydrochloride from oil-water microemulsions

The release of nortritptyline hydrochloride from oil-in-water (o/w) microemulsions (isopropyl myristate as oil, propylene glycol as cosurfactant, polysorbate 80 as surfactant and phosphate buffer, pH 7.4, as the continuous phase) containing increasing concentrations of polyethylene glycol 400, used to facilitate the diffusion of a drug from the inner oily phase of the microemulsion to the outer aqueous phase of such a dispersion system, was studied by determining the permeability constants of the drug through hydrophilic and lipophilic membranes separating the o/w microemulsions from the receiving aqueous phase (phosphate buffer pH 7.4). The permeability of nortriptyline hydrochloride from microemulsions through the lipophilic membrane increased as the concentration of polyethylene glycol 400 in the disperse system increased. The apparent permeability constant for nortriptyline hydrochloride, from the microemulsion without polyethylene glycol, was 1.36 x 10(-3) cm x h(-1), it increased up to 7.80 x 10(-3) cm x h(-1) in the presence of polyethylene glycol at a concentration of 50% (v/v) of the initial volume of the aqueous phase.     

Some selected problems of nonaqueous microemulsions — phase diagrams and variation of the composition in the quaternary and ternary systems

A variety of quaternary and ternary systems of the type of dodecane/aliphatic alcohols/Na-dodecyl sulphate/water or a water-replacing component (formamide, ethylene glycol, propylene glycol, propylene carbonate, dimethylsulfoxide, acetonitrile) were subjected to a comparative analysis of microemulsification. The compositions of the systems of the type of oil/surfactant/cosurfactant/water or nonaqueous liquid were changed with respect to surfactant (Na-dodecyl sulfate and triton X 114) concentration, cosurfactant (homologous aliphatic alcohols) concentration and mixing ratio of water and water-replacing component. The appertaining phase diagrams were plotted and compared with those of aqueous systems.The experimental results suggest that the phase regions, which are designated as nonaqueous microemulsions, probably represent, not only microemulsions in the usual sense, but a separate kind of mixed phase whose microstructure is examined by special studies. Analogously to aqueous microemulsion systems, transparency and spontaneous formation of homogeneous multicomponent systems cannot serve as the sole criteria for waterless microemulsion formation. But they are important guiding properties of microemulsion formation in multicomponent systems.The variety of components involved in the chemical composition and the current insufficient knowledge do not permit to generally decide whether nonaqueous systems can be assigned to microemulsions or to molecular solutions. For clarifying this problem a detailed examination of the miscibility behavior, especially that of binary systems as a function of temperature, and the role that surfactant and cosurfactant play in the formation of homogeneous systems will be necessary.     

Determination of food colorants by microemulsion electrokinetic chromatography

A microemulsion electrokinetic chromatography (MEEKC) method was developed to analyze and detect eight food colorants (tartrazine, fast green FCF, brilliant blue FCF, allura red AC, indigo carmine, sunset yellow FCF, new coccine, and carminic acid), which are commonly used as food additives in various food products. The effects of sodium dodecyl sulfate (SDS) surfactant, organic modifier, cosurfactant, and oil were examined in order to optimize the separation. The amount of organic modifier (acetonitrile) and SDS surfactant were determined as apparent influences on the separation resolution while the type of oil and cosurfactant rarely affected the separation selectivity of the eight colorants. A highly efficient MEEKC separation method, where the eight colorants were separated with baseline resolution within 14 min, was achieved by using a microemulsion solution of pH 2.0 containing 3.31% SDS, 0.81% octane, 6.61% 1-butanol, and 10% acetonitrile. This optimal MEEKC method has a higher separation efficiency and similar detection limit when compared to conventional capillary electrophoresis (CE) method. Furthermore, a sample pretreatment is rarely needed when this MEEKC technique is used to analyze colorants in food products, whereas a suitable sample pretreatment (for example solid-phase extraction) has to be employed prior to CE separation in order to eliminate matrix interferences resulting from the constituents of the food sample.     

磷脂微乳电动色谱的定量结构保留关系研究

以大豆磷脂为主要的表面活性剂,制备适合毛细管电动色谱使用的不同构成比的微乳体系, 应用溶剂化参数模型研究了中性溶质在其中的定量结构保留关系.使用动态涂层毛细管, 以二甲基亚砜和十二烷基苯分别作为电渗流和微乳液滴迁移的标记物, 测定了26个具有不同结构小分子中性化合物在17种微乳电动色谱体系下的保留因子, 建立了线性溶剂化能量关系(LSER)方程.通过比较两体系的LSER方程系数比较体系相似性.结果表明, 本研究建立的磷脂微乳电动色谱体系在线性溶剂化特征上和其它构成的微乳电动色谱体系相似.对溶质保留贡献较大的是溶质体积和有效氢键碱度, 油相种类及浓度对溶质的保留选择性无明显影响.     

Formation of Microemulsions in Aqueous NaCl/Sodium (3-Dodecanoyloxy-2-Hydroxy-Propyl) Succinate/Glycerol Mono(2-Ethylhexyl) Ether/Oil Systems

Sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate (SLGMS) forms microemulsions by mixing with cosurfactants such as glycerol mono(2-ethylhexyl) ether (MEH), although the combination with ordinary cosurfactant such as hexanol does not form a microemulsion of large solubilization. The middle-phase microemulsion coexists with excess water and oil (octane) phases at an optimum-mixing fraction of SLGMS and MEH in the presence of salt. The monomeric solubility of MEH in oil is low and MEH is mainly combined with SLGMS at an oil—water interface inside microemulsions. With decreasing salinity, the three-phase body shrinks and eventually disappears. The three-phase body may be terminated at a tricritical point, at which three phases simultaneously coexist. The effect of type of oil on the solubilization capacity of the microemulsions is also discussed.