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.     

Background and operating parameters in microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) is an electrodriven separation technique. Separations are generally achieved using microemulsions consisting of surfactant-coated nanometer-sized oil droplets suspended in aqueous buffer. A cosurfactant such as a short-chain alcohol is generally used to stabilize the microemulsion. This review summarizes the various microemulsion types and compositions that have been used in MEEKC. The effects of key-operating variables such as surfactant type and concentration, cosurfactant type and concentration, buffer pH and type, oil type and concentration, use of organic solvent and cyclodextrin additions, and temperature are described. Specific examples of water-in-oil microemulsions and chirally selective separations are also covered.     

Recent developments in the methodology and application of MEEKC

MEEKC is an electrodriven separation technique that utilises the unique properties of a microemulsion (ME) as a background electrolyte to achieve separation of a diverse range of solutes. MEs are composed of nanometre-sized oil droplets suspended in aqueous buffer, which is commonly referred to as oil-in-water ME. The droplets are stabilised by the presence of both a surfactant and co-surfactant. The use of water-in-oil MEs in MEEKC has also been investigated. This review details the advances in MEEKC-based separations from the period June 2008 - June 2010. Areas covered include online sample concentration, suppressed electroosmosis MEEKC, chiral separations, MEEKC-MS, MEEKC-ICP-MS and ME structure characterisation. The review also includes a fundamental introduction to MEEKC, along with a review of recent applications.     

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.     

Recent applications of microemulsion electrokinetic chromatography

Microemulsions are used in a similar way as micellar solutions are used for separations in capillary electrophoresis. Within the last approximately six years, a number of papers have appeared in the literature in which the separation characteristics of microemulsion electrokinetic chromatography (MEEKC) have been investigated. The total number of papers dealing with MEEKC is now close to 100. One of the major fields of application for MEEKC has been the characterization of the lipophilicity of chemical substances but the technique has also been applied for the analysis of substances within the fields of natural products, pharmaceuticals, vitamins, peptides, proteins, and nucleic acid bases as well as nucleosides. An overview of the applications is given together with the microemulsions used for each application.     

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.     

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.     

Application of microemulsion electrokinetic chromatography to the analysis of a wide range of pharmaceuticals and excipients

Microemulsion electrokinetic chromatography (MEEKC) is a capillary electrophoresis (CE) technique in which solutes partition with moving oil droplets present in a microemulsion buffer. Ionised species will also separate by electrophoresis. In this paper MEEKC is shown to give highly efficient and relatively rapid separations for a wide range of pharmaceuticals, vitamins and excipients. A single set of operating conditions was used to resolve both water-soluble and insoluble compounds. The method was also used to separate both ionic and neutral compounds. The method was especially useful in the analysis of water-insoluble neutral compounds such as steroids and lecithin, which are difficult to analyse by CE. The method was found to be both quantitative and highly repeatable. The quality of the separation was found to be dependent upon the sample diluent used if large injection volumes are employed. The use of MEEKC for the determination of complex mixtures such as multi-ingredient formulations and drug-related impurities was successfully demonstrated. MEEKC offers significant advantages over many forms of CE and capillary electrochromatography (CEC) and should be considered as an extremely useful option in pharmaceutical analysis.     

Comparison of microemulsion electrokinetic chromatography and solvent modified micellar electrokinetic chromatography on rapid separation of heroin, amphetamine and their basic impurities

Microemulsion electrokinetic chromatography (MEEKC) and solvent modified micellar electrokinetic chromatography (MEKC) were investigated with the goal of the rapid separation of complex heroin and amphetamine samples. The rapid simultaneous separation of 17 species of heroin, amphetamine and their basic impurities and adulterants was performed within about 10 min using MEEKC for the first time, whereas solvent modified MEKCs were unable to resolve all the components. The comparisons between MEEKC and solvent modified MEKC proved internal lipophilic organic phase in microemulsions played an important role in improving the separation performance with respect to efficiency. However, the role of internal lipophilic organic phase in MEEKC was disgusted at high concentrations of cosurfactant, and the separations of MEEKC and 1-butanol modified MEKC became similar at high concentrations of 1-butanol. The evaluation of reproducibility, linearity and detection limit of optimized MEEKC method provided good results for all the analytes investigated, thus allowing its application to real controlled drug preparation analysis.     

Application of MELC and MEEKC for the Analysis of Paracetamol and Related Impurities in Suppositories

Rapid MELC and MEEKC methods were developed for paracetamol suppository assay. MELC methods for paracetamol analysis and for separation of paracetamol and its impurities were previously reported. In this study, further development of MEEKC methods and a MELC method using anionic and cationic microemulsions gave excellent validation results for paracetamol content in suppositories. SDS Microemulsion instability resulted in poor reproducibility for impurity separations using gradient elution. A novel isocratic CTAB MELC method achieved reproducible separation of paracetamol and its impurities at 0.1% levels. MEEKC methods using SDS and CTAB microemulsions resolved all of the impurities however detection at 0.1% levels was not possible. These methods gave significant benefits in terms of reduced sample pre-treatment requirements. CTAB microemulsions had greater solubilising power than their SDS equivalent and were more stable due to their longer alkyl chain length.     

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.     

微乳液毛细管电动色谱应用进展

微乳液毛细管电动色谱（MEEKC）是在胶束电动色谱基础上发展起来的一种电动色谱新技术。近几年来,MEEKC在各个领域的应用又有了长足的进步,尤其是在维生素、药物、天然产物、手性分离、生物分子、环境分析,以及药物疏水性评价上呈现出强势的发展,本文针对这些领域的MEEKC方法进行了评述。     

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.     

Determination of octanol-water partition coefficients for carbonate esters and other small organic molecules by microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) was assessed as a tool for determination of octanol-water partition coefficients using 34 solutes encompassing 8 carbonate esters. It was confirmed that microemulsions containing 1.44-2.88% w/w SDS, 6.49% w/w 1-butanol, and 0.82% w/w n-heptane constitute a good model of octanol-water partitioning in the pH range of 1.4-7.4. Use of the migration index concept led to improved repeatability of the MEEKC method compared to the use of retention factors. Using a dynamical coating, a high electroosmotic flow at pH 1.4 and 4.75 was achieved expanding the practical pH working range of the MEEKC system. The correlation obtained between the migration index and log P was unaffected by pH indicating that the properties of the microemulsion droplets and, thus, partitioning are independent of pH. No evidence for congeneric behavior was found for the sample set comprising solutes with different hydrogen bonding properties suggesting that simple reference compounds can be used as calibrators. Lipophilicity estimates for the series of carbonate esters were obtained. The increase in lipophilicity with chain length was smaller than expected from the Hansch substituent constant, pi.     

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.     

A comparative study of microemulsion electrokinetic capillary chromatography and micellar electrokinetic capillary chromatography for the analysis of UV-absorbing compounds in human urine

Separations of human urine by microemulsion electrokinetic chromatography (MEEKC) and micellar electrokinetic capillary chromatography (MEKC) with respect to resolution, migration times and efficiencies were optimized and compared. The optimised MEEKC and MEKC methods were simple and fast, both of which are excellent characteristics for the complex separations required in clinical and biomedical studies. However, resolution in MEKC was significantly greater than in MEEKC although migration times were 30% faster for the optimised MEEKC method. In addition, a faster analysis method (short-end injection) specifically for routine screening purposes was also investigated. With both MEEKC and MEKC modes, this provided short separations (less than 4 min for urine) with no major compromise in resolution. In conclusion, we found that MEEKC offered no real advantage over MEKC for urine analysis.     

Microemulsion electrokinetic chromatography for the analysis of green tea catechins: effect of the cosurfactant on the separation selectivity

Microemulsion electrokinetic chromatography (MEEKC) was applied to the separation of six catechins and caffeine, the major constituents of the green tea. The developed methods involved the use of sodium dodecyl sulfate (SDS) as surfactant, n-heptane as organic solvent and an alcohol as cosurfactant. The separations were performed under acidic conditions (pH 2.5 phosphate buffer, 50 mM) to ensure good stability of the catechins, with reversed polarity (anodic outlet). The effect of the alcohol nature on the MEEKC selectivity was evaluated; nine alcohols were used as cosurfactant: 1-butanol, tert-butanol, 1-pentanol, 2-pentanol, 3-pentanol, cyclopentanol, 1-hexanol, 2-hexanol, and cyclohexanol. The migration order of (+)-catechin (C), (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-gallocatechin (GC), (-)-epigallocatechin gallate (EGCG), (-)-epicatechin gallate (ECG), caffeine and theophylline was significantly affected by the alcohol used as cosurfactant. Using nine microemulsions, four different selectivities were achieved: A (cyclohexanol); B (2-pentanol, 3-pentanol, 1-hexanol, 2-hexanol); C (1-butanol, 1-pentanol, cyclopentanol); D (tert-butanol). MEEKC methods, based on 2-hexanol and cyclohexanol as cosurfactant were validated and successfully applied to the analysis of catechins and caffeine in commercial green tea products.     

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.     

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.     

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.