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.     

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.     

Comparison of oil-in-water and water-in-oil microemulsion electrokinetic chromatography as methods for the analysis of eight phenolic acids and five diterpenoids

Oil-in-water (O/W) and water-in-oil (W/O) MEEKC were compared for their abilities to separate and detect eight phenolic acids and five diterpenoids in Radix et Rhizoma Salviae Miltiorrhizae (RRSM). The effects of oil type and concentration, organic modifier, SDS, and buffer concentration on separation were examined in order to optimize the two methods. Oil contents and organic modifier were found to markedly influence the separation selectivity for both O/W and W/O systems. SDS concentration rarely affected separation resolution for O/W MEEKC, and separation of eight phenolic acids and five diterpenoids could be improved by changing the buffer concentration for W/O MEEKC. A highly efficient O/W MEEKC separation method, where the 13 compounds were separated with baseline resolution, was achieved by using a microemulsion solution of pH 8.0 containing 0.6% cyclohexane, 3.0% SDS, 6.0% 1-butanol, and 3.0% ACN. The W/O MEEKC was unable to resolve all the components. In addition, the analytic time in O/W MEEKC was shorter than that in W/O MEEKC. Finally, the developed O/W MEEKC method was successfully applied to determine analytic compounds in RRSM samples.     

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.     

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 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.     

A comparative study of micellar and microemulsion EKC for the analysis of benzoylurea insecticides and their analogs

An investigation of the basic factors which govern the microemulsion EKC (MEEKC) and MEKC for the separation of four benzoylurea (BU) insecticides and their four analogs was carried out. In MEEKC, the separation of eight BU compounds was optimized by changing the microemulsion composition, such as concentration of SDS, octane, n-butanol, and isopropanol percentages, as well as capillary temperature. Separation optimization was also carried out for MEKC, showing that ACN and a high level of another additive gamma-CD were needed to achieve effective separation of these analytes. Although separation with baseline resolution was achieved by either MEEKC or MEKC methods, the separation selectivity resulting from the proposed MEEKC method was completely different from that of MEKC. In addition, analytical time in MEEKC was longer than that in MEKC, but in view of theoretical plate numbers, detection limits, and reproducibility, both methods were effective for the analysis of BU insecticides and their analogs.     

Separation of Six Pyridoncarboylxic Acid Derivatives by Micellar and Microemulsion Electrokinetic Chromatography

Micellar electrokinetic chromatography (MEKC) and microemulsion electrokinetic chromatography (MEEKC) are two kinds of electrokinetic capillary chromatography (EKC), which are characterized of high solubilization capacity and separation efficiency. In our previous work, some polar organic compounds and hydrophobic neutral compounds were separated successfully by EKC1-3. In this paper, these methods were used for separating six pyridoncarboylxic acid derivatives with similar structures. T…     

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.     

Comparing micellar electrokinetic chromatography and microemulsion electrokinetic chromatography for the analysis of preservatives in pharmaceutical and cosmetic products

In this study, separation and determination of nine preservatives ranging from hydrophilic to hydrophobic properties, which are commonly used as additives in various pharmaceutical and cosmetic products, by micellar electrokinetic chromatograpy (MEKC) and microemulsion electrokinetic chromatography (MEEKC) were compared. The effect of temperature, buffer pH, and concentration of surfactant on separation were examined. In MEKC, the separation resolution of preservatives improved markedly by changing the sodium dodecyl sulfate concentration. Temperature and pH of running buffers were used mainly to shorten the magnitude of separation time. However, in order to detect all preservatives in a single run in a MEEKC system, a microemulsion of higher pH was needed. The separation resolution was improved dramatically by changing temperature, and a higher concentration of SDS was necessary for maintaining a stable microemulsion solution, therefore the separation of the nine preservatives in MEEKC took longer than in MEKC. An optimum MEKC method for separation of the nine preservatives was obtained within 9.0 min with a running buffer of pH 9.0 containing 20 mM SDS at 25 degrees C. A separation with baseline resolution was also obtained within 16 min using a microemulsion of pH 9.5 which composed of SDS, 1-butanol, and octane, and a shorter capillary column at 34 degrees C. Finally, the developed MEKC and MEEKC methods determined successfully preservatives in various cosmetic and pharmaceutical products.     

DIFFUSION MEASUREMENTS IN MICROEMULSIONS

We have carried out diffusion coefficient measurements in both aqueous micelles and microemulsions using the techniques of palaeography and quasielastic light scattering (QLS) The former method involves the determination of the diffusion coefficient of an electroactive oil soluble probe at a polarizable microelectrode. For high water content microemulsions, both methods yield the same diffusion coefficients, which can be identified as the self diffusion coefficient For cetyltrimethylammonium bromide (CTAB) micelles, both methods yield the same result at the salt (NaBr) concentration at which the QLS measurements are independent of CTAB concentration. In more concentrated microemulsions, QLS data gave diffusion coefficients in agreement with polarography only for a sodium cetyl sulfate (SCS) system at 65-75 wt % water. For the SCS microemulsions at 60% water, and CTAB microemulsion at 60-75% water, the QLS data yielded rapid, nonexponential decays. However, consistent polarographic diffusion coefficients could still be obtained, By using probes of varying chain length (oil solubility), it has been demonstrated that these CTAB and SCS microemulsions containing butanol and pentanol cosurfactants respectively, are not cosolubilized systems but do contain distinct hydrophilic and hydropobic regions.     

Microemulsion based chromatographic techniques: Past lessons and future directions

Microemulsion Liquid Chromatography (MELC) and Microemulsion Electrokinetic Chromatography (MEEKC) respectively have emerged as new forms of analytical techniques for the separation of drugs of different water solubilities, solving different separation challenges and offer a variety of applications in pharmaceutical industry and routine quality control analysis of drugs. A comprehensive review of the literatures on recent developments in the field of Microemulsion based techniques was made. Various literatures dealing with microemulsion based chromatographic techniques were collected and studied extensively. Major findings of all the important literatures were summarized and classified in an appropriate manner to help reader understand the types, methods and applications of the technique. This article covers basic concepts of microemulsion, their optimization parameters, advantages and disadvantages and a plethora of applications and research done over the last decade. Almost all the major researches done in the field were tried to cover. Microemulsion based chromatographic techniques have been proved to be a newer and interesting technique that is extensively studied nowadays. Due to their polyphasic structure, microemulsion as eluent in both MELC and MEEKC offers a large number of advantages, applications and capability in separating the mixture of components.     

微乳液相色谱法及其应用进展

微乳液相色谱法是使用普通的正相或反相色谱柱,分别以油包水或水包油型微乳为流动相,用常规检测器进行样品分析的液相色谱法。该法具有独特的选择性,能够同时分离极性范围很广的化合物,流动相中的有机溶剂用量少,调节参数多,以及血浆样品可直接进样,梯度洗脱节省再平衡时间等优点,在复杂组分分离方面具有显著的优势。本文对微乳液相色谱法中常用微乳的组成和结构,各组成成分对分离的影响,以及该法的应用进展进行了综述。     

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.     

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.     

Comparison of microemulsion electrokinetic chromatography and micellar electrokinetic chromatography methods for the analysis of phenolic compounds

In this study, microemulsion electrokinetic chromatography (MEEKC) and micellar electrokinetic chromatography (MEKC) were compared for their abilities to separate and detect thirteen phenolic compounds (syringic acid, p-coumaric acid, vanillic acid, caffeic acid, gallic acid, 3,4-dihydroxybenzoic acid, 4-hydroxybenzoic acid, (+)-catechin, (-)-epigallocatechin, (-)-epicatechin gallate, (-)-epigallocatechin gallate, (-)-epicatechin, and (-)-gallocatechin), and two other ingredients (caffeine and theophylline) in teas and grapes. Separation of phenolic compounds was improved by changing the SDS concentration for MEEKC, but the SDS concentration rarely affected the resolution for MEKC. Organic modifier (acetonitrile or methanol) was found to markedly influence the resolution and selectivity for both MEEKC and MEKC systems. In addition, a higher voltage and a higher column temperature improved the separation efficiency without any noticeable reduction in resolution for MEEKC whereas they caused a poor resolution for the MEKC system. Although separations with baseline resolution were achieved by the optimized MEEKC and MEKC methods, the separation selectivity resulting from the proposed MEEKC method was completely different from that of MEKC.     

Determining organic impurities in mother liquors from oxidative terephthalic acid synthesis by microemulsion electrokinetic chromatography

In this study, a microemulsion electrokinetic chromatography (MEEKC) method was developed to analyze and detect several aromatic acids (benzoic acid (BA), isophthalic acid (IPA), terephthalic acid (TPA), p-toluic acid (p-TA), 4-carboxylbenzaldehyde (4-CBA), trimesic acid (TSA), trimellitic acid (TMA), o-phthalic acid (OPA), and hemimellitic acid (HMA)), which are common organic impurities produced by liquid-phase catalytic oxidation of p-xylene to TPA. The effects of microemulsion composition, column temperature, column length and applied voltage were examined in order to optimize the aromatic acid separations. This work demonstrated that variation in the concentration of surfactant (sodium dodecyl sulfate (SDS)) and oil phase (octane) had a pronounced effect on separation of the nine aromatic acids. It was also found that a decrease in column length had the greatest effect on shortening separation time and improving separation resolution for these aromatic acids when compared to that of an increase in column temperature or applied voltage. However, the nature and concentration of cosurfactants and organic modifiers were found to play only minor roles in the separation mechanism. Thus, a separation with baseline resolution was achieved within 14 min by using a microemulsion solution of pH 2.0 containing 3.7% SDS, 0.975% octane, and 5.0% cyclohexanol; and a 50-cm capillary column (effective length of 40-cm) at 26 °C. As a result, the developed MEEKC method successfully determined eight impurities of aromatic acids in the mother liquors produced from the oxidation synthesis of TPA.     

Comparison of microemulsion electrokinetic chromatography and micellar electrokinetic chromatography as methods for the analysis of ten benzophenones

Microemulsion electrokinetic chromatography (MEEKC) and micellar electrokinetic chromatograpy (MEKC) were compared for their abilities to separate and detect ten similar benzophenones, which are commonly used as UV filters in various plastic and cosmetic products. Sodium dodecyl sulfate (SDS) concentration and column temperature rarely affected separation resolution for MEEKC, but separation of benzophenones could be improved by changing the SDS concentration and column temperature for MEKC. Buffer pH and ethanol (organic modifier) were found to markedly influence the separation selectivity for both MEEKC and MEKC systems. In addition, a higher electric voltage improved the separation efficiency without a noticeable reduction in separation resolution for MEEKC, whereas it caused a poor separation resolution for the MEKC system.     

Phase-transfer catalytic determination of phenols as methylated derivatives by gas chromatography with flame ionization and mass-selective detection

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 mM 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.     

Oil-In-Water Microemulsion LC Determination of Pharmaceuticals Using Gradient Elution

Efficient and novel oil-in-water microemulsion HPLC (MELC) separations of a range of solutes have been performed on conventional reversed-phase HPLC columns using gradient elution. This work follows previous successful separations using isocratic oil-in-water MELC [1]. It was found that by changing certain variables, peak-peak resolution, separation selectivity, efficiency and solute retention could be manipulated. The method was compatible with very low UV detection wavelengths. A robust separation method was developed for the quantitative analysis of 2 steroids in a combination-inhaled product for asthma. The method offered similar chromatography and run time when compared with conventional HPLC modes, thus demonstrating its potential for routine use. Stability-indicating methods were developed to separate synthetic and degradative impurities from the main component peaks in 4 pharmaceutical products. The methods offered quicker analysis times and equivalent selectivity to conventional HPLC modes. In developing the separations the effect on the chromatography of varying the operating parameters was studied.