Enantioseparation of esbiothrin by cyclodextrin‐modified microemulsion and micellar electrokinetic chromatography

A comparison between chiral cyclodextrin‐modified microemulsion electrokinetic chromatography (CD‐MEEKC) and cyclodextrin‐modified micellar electrokinetic chromatography (CD‐MEKC) for the enantiomeric separation of esbiothrin was carried out. For both methods, the separation conditions were optimized by varying CD types and concentration, running buffer pH and compositions, organic modifiers, and temperature. The optimal CD‐MEEKC conditions were 0.8% n‐heptane, 2.3% SDS, 6.6% n‐butanol, 90.3% 10 mM sodium tetraborate containing 3% (w/v, the ratio of CD mass to microemulsion volume) methyl‐β‐cyclodextrin, pH 10, 25°C. The optimized CD‐MEKC conditions were 3.3% SDS, 96.7% 10 mM sodium tetraborate containing 5% (w/v) β‐CD, pH 10, 25°C. The difference in physicochemical properties of the buffer and CDs resulted in different optimal CD type. The competitive distribution between the microemulsion (or micelle) and chiral CD contributed to the chiral separation. Both methods provided excellent separation (R_s ～? 3) with similar migration time (ca. 15 min). CD‐MEEKC provided higher separation efficiencies (>300000) than CD‐MEKC (>200000). The LODs for CD‐MEEKC and CD‐MEKC were 4.7 μg/mL and 3.2 μg/mL, respectively. The RSDs of migration time and peak area for CD‐MEEKC were slightly higher than for CD‐MEKC. Both the demonstrated CD‐MEEKC and CD‐MEKC methods provided high efficiencies, low LODs, and reproducible enantioseparations of esbiothrin.     

毛细管微乳电动色谱应用的研究进展

近年来毛细管微乳电动色谱(MEEKC)研究的范围不断扩大,其分离分析的化合物类型也不断增多。该文综述了2002年以来MEEKC应用的研究进展,指出了目前MEEKC研究中存在的一些问题,对今后的研究发展方向进行了展望。     

Micellar and microemulsion electrokinetic chromatography of hop bitter acids

The elution order of the hop α- and β-acids has been studied under different modes of electrokinetic separation. A model is advanced to explain the shorter migration times of the more hydrophobic β-acids compared to the α-acids in micellar electrokinetic chromatography (MEKC). For quality control of the bitter principles in hops, the ruggedness of electrokinetic separation could be improved by replacing MEKC by microemulsion electrokinetic chromatography (MEEKC).     

Chiral separations in microemulsion electrokinetic chromatography. Use of micelle polymers and microemulsion polymers

In this study, microemulsions of the chiral surfactant polysodium N-undecenoyl-D-valinate (poly-D-SUV) was utilized for enantiomeric separation by investigating two approaches using polymeric chiral surfactant in microemulsion electrokinetic chromatography (MEEKC). In the first approach, poly-D-SUV was used as an emulsifier surfactant along with 1-butanol and n-heptane. Enantioseparation of anionic or partially anionic binaphthyl derivatives, anionic barbiturates, and cationic paveroline derivatives were achieved by varying the mass fraction of 1-butanol, n-heptane and poly-D-SUV. For anionic or partially anionic analytes, relatively lower mass fractions of n-heptane, and poly-D-SUV were found to give optimum chiral separations as compared to that for cationic solutes. In the second approach, the chiral microemulsion polymer was prepared by polymerizing mixtures of 3.50% (w/w) of sodium N-undecenoyl-D-valinate (D-SUV) and 0.82% (w/w) of n-heptane (core phase) at varying concentration of 1-butanol. After polymerization, the n-heptane and 1-butanol were removed to yield solvent free microemulsion polymers (MPs) which were then utilized for the separation of anionic binaphthyl derivatives and anionic barbiturates. When MPs of D-SUV were utilized for chiral separation, 1.00% (w/w) 1-butanol and 3.50% (w/w) 1-butanol was optimum for enantioseparation of (+/-)-BNP and (+/-)-BOH, respectively. On the other hand, for anionic (+/-)-barbiturates very low concentration of butanol (0.25%, w/w) provided optimum resolution. Compared with micellar electrokinetic chromatography (MEKC), the use of micelle polymers or microemulsion polymers in MEEKC showed dramatic enhancement for resolution of (+/-)-BNP, while this enhancement was less dramatic for other binaphthyls [(+/-)-BOH, (+/-)-BNA] as well as for (+/-)-barbiturates and (+/-)-paveroline derivatives. However, higher separation efficiency of the enantiomers was always observed with MEEKC than in MEKC.     

On-line concentration sample stacking coupled with water-in-oil microemulsion electrokinetic chromatography

This study describes for the first time, the ability of a normal stacking mode (NSM) on-line concentration step coupled with water-in-oil (W/O) microemulsion electrokinetic chromatography (MEEKC), using six common penicillin antibiotics (oxacillin, penicillin V, penicillin G, nafcillin, ampicillin, and amoxicillin) as test analytes. Optimization of penicillin separation in the conventional W/O MEEKC system demonstrated that change in the type and concentration of the oil phase (1-butanol) and column temperature had a pronounced effect on the separation. With the subsequent development of the NSM coupled with W/O MEEKC, improved separation and detection sensitivities were observed when an organic solvent plug (1-propanol; 1.04 cm) was placed between the W/O microemulsion and the sample solutions. This could be attributed to the solution viscosity difference between the aqueous sample zone and the organic solvent plug causing the penicillin to be stacked in this 1-propanol plug. The optimal NSM W/O MEEKC provided about 12-fold increase in detection sensitivity compared with conventional sample injection (50 mbar, 3 s). Finally, this proposed method was successfully applied in the analyses of several food samples (porcine organs) spiked with penicillin.     

Capillary electrokinetic chromatography of insulin and related synthetic analogues

With the implementation of recombinant DNA technology in the pharmaceutical industry, some synthetic insulins have been developed in order to improve the therapy of diabetes. These analogues differ only slightly in the amino acid sequence, therefore displaying a great challenge for analytical chemistry. Within the work presented in this paper, capillary zone electrophoresis (CZE), micellar electrokinetic chromatography (MEKC) with sodium dodecylsulphate (SDS) as micelle-forming agent, and microemulsion electrokinetic chromatography (MEEKC) with microemulsions consisting of SDS, n-octane and 1-butanol were investigated for the separation of human insulin and five synthetic analogues. Best results were achieved with a solvent-modified MEKC system consisting of 100 mM sodium dodecyl sulphate and 15% acetonitrile in 10 mM borate buffer (pH 9.2). A similar system based on perfluorooctanoic acid as micelle-forming agent in ammonium acetate (pH 9.2) was successfully employed for the hyphenation with a quadrupole/time-of-flight mass spectrometer via a sheath-flow interface. In this case, detection limits at 10 mg/L could be achieved.     

Separation and determination of two sesquiterpene lactones in Radix inulae and Liuwei Anxian San by microemulsion electrokinetic chromatography

A novel microemulsion electrokinetic chromatography (MEEKC) method for separating and determining two sesquoterpene lactones, alantolactone (AL) and isoalantolactone (IAL), in Radix inulae and Liuwei Anxian San has been developed. The effects of several important factors such as internal organic phases, concentration of microemulsion, concentration of acetonitrile, injection time and running voltage were systematically investigated to determine the optimum conditions. The optimum microemulsion system was composed of n-hexane (0.32% w/w), SDS (1.24% w/w), 1-butanol (2.64% w/w), acetonitrile (10% w/w) and 10 mm sodium tetraborate buffer (85.80% w/w, pH 9.2). The applied voltage was 20 kV. The analytes were detected at 214 nm. Regression equations revealed linear relationships (correlation coefficients 0.9950 for AL and 0.9946 for IAL) between the peak area of each analyte and the concentration. The limits of detection (defined as a signal-to-noise ratio of about 3) were approximately 0.45 microg/mL for AL and 0.56 microg/mL for IAL. The levels of the analytes were successfully determined with recoveries ranging from 98.2 to 104.3%. Furthermore, a simple and effective extraction method, with methanol in an ultrasonic water bath for 60 min, was used for sample preparing. Also, MEEKC was compared with micellar electrokinetic chromatography (MEKC) and shown better separation results.     

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.     

Microemulsion and micellar electrokinetic chromatography of Hematoporphyrin D: a starting material of hematoporphyrin derivative

An investigation of the basic factors which govern the microemulsion electrokinetic chromatography (MEEKC) and micellar electrokinetic chromatography (MEKC) separation of Hematoporphyrin D and its base hydrolysis product, hematoporphyrin derivative (HpD), was performed. These model compounds contain a complex mixture of porphyrin monomers, dimers and/or oligomers, and were utilized to gain insights into the MEEKC/micellar electrokinetic chromatography (MEKC) separation of samples containing highly lipophilic substances. For example, the organic modifier/cosurfactant (1-butanol) and/or oil phase (e.g., 1-octanol in comparison to ethyl acetate) were found to have an apparent influence on the separation selectivity of Hematoporphyrin D, the extent of which was dependent on the chemical nature of the surfactant employed (e.g., sodium dodecyl sulfate vs. sodium cholate). An interesting and important finding was that the presence of an organic modifier (methanol or acetonitrile at a concentration of 20% or higher) in the sample matrix as well as in the run buffer was essential for the optimal MEEKC or MEKC separation of a number of porphyrin monomers (including hematoporphyrin IX and its acetates, most likely hydroxyacetate, diacetate, and vinyl acetate, as well as its dehydration products, hydroxyethylvinyldeuteroporphyrin and protoporphyrin) contained in Hematoporphyrin D. On the other hand, the use of these optimized conditions for the MEEKC or MEKC separation of various oligomeric porphyrin species in HpD were unsatisfactory. As HpD is a well-known and effective photosensitizing agent in photodynamic therapy (a new approach for cancer treatment), the improved separation and characterization of various monomeric and oligomeric porphyrin species in HpD and its starting material, such as Hematoporphyrin D, is a challenging and important task.     

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.     

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.     

蛋白质的微乳液毛细管电动色谱分离研究

考察了用微乳液毛细管电动色谱(MEEKC)分离蛋白质时微乳液组成等不同因素对分离的影响,并与胶束电动色谱进行对比,探讨了其分离机理,为蛋白质的分离鉴定提供了一种有力的工具.     

微乳液毛细管电动色谱-场放大富集法灵敏分析尿液中的9种麻醉剂类药物

建立了一种微乳液毛细管电动色谱(MEEKC)-场放大富集(FASI)分析尿液中多种麻醉剂(吗啡、可待因、纳洛酮、海洛因、蒂巴因、可卡因、哌替啶、芬太尼、美沙酮)的方法。考察了微乳液组成、分离电压等因素的影响,得到的最佳微乳液组成(质量分数)为0.6%十二烷基硫酸钠、1.2%正丁醇、0.6%乙酸乙酯和97.6% 10 mmol/L硼砂缓冲液(pH 9.5);分离电压为25 kV。在上述微乳体系中,9种化合物在15 min内得到了基线分离。采用场放大在线富集技术提高了分析灵敏度,检出限(S/N=3)低至0.3 μg/L。模拟尿样样品中9种麻醉剂的加标回收率介于79.4%～119.9%之间,日内相对标准偏差小于5.5%。将该方法应用于美沙酮大鼠体外代谢样品的测定,结果令人满意。     

微乳液毛细管电动色谱快速分析解热镇痛药中的有效成分

建立了微乳液毛细管电动色谱快速测定解热镇痛药中非那西丁、氨基比林和咖啡因的新方法。采用由乙酸乙酯-十二烷基硫酸钠(SDS)-正丁醇-硼砂缓冲液组成的微乳液体系,以氯霉素为内标,3种有效成分在2.5 min内完成分离,峰面积相对标准偏差(RSD)在1.2%～1.6%之间,回收率在95.6%～104.0%之间。实验考察了缓冲溶液的浓度、pH值、SDS浓度以及助表面活性剂的种类、含量对分离测定的影响。该法可用于实际样品分析。     

Simultaneous Determination of Heroin, Amphetamine and their Basic Impurities and Adulterants Using Microemulsion Electrokinetic Chromatography

Heroin and amphetamine are two most abused drugs that can be highly complex, containing various impurities, byproducts, adulterants and diluents due to differences in the agricultural and manufacturing procedures. Therefore, the comprehensive analysis of above illicit drugs seized or purchased undercover by law enforcement authorities is important for legal and intelligence gathering purposes, and clinical and pharmaceutical purposes as well1. In last decade, micellar electrokinetic chromatogr…     

Determination of itraconazole and hydroxyitraconazole in human serum and plasma by micellar electrokinetic chromatography

The electrokinetic separation of the hydrophobic antimycotic drug itraconazole (ITC) and its major metabolite, hydroxyitraconazole (HITC), by a binary aqueous-organic solvent medium containing sodium dodecylsulfate, by microemulsion electrokinetic chromatography (MEEKC) and by micellar electrokinetic chromatography (MEKC) was studied. The results suggest that the first approach is difficult to apply and that there is no substantial difference between separations performed using MEEKC and MEKC modified with n-butanol. The simpler MEKC method is more than adequate and was thus employed for the analysis of ITC and HITC in human serum and plasma. Separation was achieved in plain fused-silica capillaries having a low-pH buffer (pH 2.2) with sodium dodecyl sulfate micelles and reversed polarity. The addition of 2-propanol and n-butanol enhanced analyte solubility and altered the selectivity of the separation by influencing the magnitude of the electrophoretic component in the separation mechanism. Under optimised conditions and using head-column field-amplified sample stacking, an internal standard, ITC and two forms of HITC could be separated in under 9 min, with detection limits less than 0.01 microg/mL. Analysis of samples from patients currently prescribed ITC revealed a different HITC peak area ratio to that of the standards, suggesting a stereoselective component of ITC metabolisation. Comparison of MEKC data with those of a HPLC method employed on a routine basis showed excellent agreement, indicating the potential of this approach for therapeutic drug monitoring of ITC.     

Separation of Biphenyl Nitrile Compounds by Microemulsion Electrokinetic Chromatography with Mixed Surfactants

A mixture of nine biphenyl nitrile compounds with high hydrophobicity and similar structures was successfully separated by microemulsion electrokinetic chromatography (MEEKC) within 30 rain. The buffer system contained 100 mmol/L sodium dodecyl sulfate (SDS), 80 mlnol/L sodium cholate (SC), 0.81% heptane, 7.5% n-butanol, 10% acetonitrile and 10 mmol/L borate. The addition of SC, organic modifiers, sample preparation and temperature all showedremarkable effect on the separation. Meanwhile, the MEEKC method was briefly compared with micellar electrokinetic chromatography (MEKC) method.     

Separation of hydrophobic solutes by organic-solvent-based micellar electrokinetic chromatography using cation surfactants

In this study, the separation of 13 homologous stick-like hydrophobic solutes, i.e., biphenyl nitrile derivatives, by organic-solvent-based micellar electrokinetic chromatography (MEKC) was investigated in terms of separation medium composition, species and concentration of surfactant, other additives, separation voltage and temperature. The results showed that the 13 strong hydrophobic compounds were baseline separated in 25 min with a repeatability of less than 1.3% (RSD) for migration time. The separation medium was a mixture of methanol, 2-propanol and water (58.5:10:31.5), containing 150 mM cetyltrimethylammonium bromide (CTAB) and 20 mM sodium borate. Variety of solvent composition, temperature and applied voltage all showed remarkable effect on the separation. The organic-solvent-based MEKC method proved to be superior to the aqueous MEKC and microemulsion electrokinetic chromatography (MEEKC) methods for the separation of strongly hydrophobic compounds.     

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.     

Selectivity in microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) is a most promising separation technique providing good selectivity and high separation efficiency of anionic, cationic as well as neutral solutes. In MEEKC lipophilic organic solvents dispersed as tiny droplets in an aqueous buffer by the use of surfactants provide a pseudo-stationary phase to which the solutes may have an affinity either to the surface or they may even partition into the droplets. When the droplets are charged, typically negatively, they will migrate opposite to the electroosmotic flow and hence separation of neutral solutes may take place. In the present paper focus has been set on how to change selectivity in MEEKC. Changes in the nature of surfactant as well as in pH have been shown to be powerful tools in changing the selectivity. The type of lipophilic organic phase is of less importance for the separation of fairly lipophilic solutes. Also changes in the temperature surrounding the capillary may alter the selectivity.