Characterisation of retention in micellar high-performance liquid chromatography, in micellar electrokinetic chromatography and in micellar electrokinetic chromatography with reduced flow

The retention (migration) behaviour of various barbiturates, phenylurea and triazine herbicides in micellar electrokinetic chromatography (MEKC) with uncoated fused-silica capillaries was compared with the behaviour in micellar electrokinetic chromatography with reduced electroosmotic flow (RF-MEKC) using capillaries modified with linear polyacrylamide. The error in the values of the retention factors caused by the neglection of the contribution of the electroosmotic flow in RF-MEKC was investigated and a method for correcting this error was suggested. The retention was characterised using the lipophilic and polar indices to characterise and to predict the retention as a function of the concentration of the surfactant (sodium dodecylsulphate) in the running buffer in MEKC and in RF-MEKC. Homologous series of n-alkylbenzenes and of n-alkan-2-ones were compared as the standard sets for the calibration of the retention (migration) index scale. The values of the lipophilic indices of a given solute measured in reversed-phase HPLC, MEKC and RF-MEKC are close to each other. Under ideal MEKC conditions, the values of the polarity indices are close to one for various sample solutes. However, for partially ionised compounds such as weakly acidic barbiturates, where the contribution of the electrophoretic migration is significant, the values of the polarity indices are significantly lower than one. Optimum conditions for separations of mixtures of triazine and phenylurea herbicides and of barbiturates using various techniques tested were compared.     

Analysis of organic components of smokeless gunpowders: high-performance liquid chromatogaphy vs. micellar electrokinetic capillary chromatography

The aim of the present study was to verify the analytical performances of high-performance liquid chromatography (HPLC) and micellar electrokinetic capillary chromatography (MEKC) for the separation and qualitative determination of a selected group of organic components of smokeless gunpowders. The HPLC method was based on a gradient reversed-phase elution with a mobile phase composed of 0.17 M H(3)PO(4)/methanol; detection was performed by UV absorption at the wavelengths of 220, 254, and 270 nm. The MEKC experiments were carried out by using uncoated fused-silica capillaries (50 microm inside diameter, 50 cm effective length) and a running buffer composed of 10 mM sodium tetraborate at pH 9.24 added with 25 mM sodium dodecyl sulfate (SDS); the applied voltage was 25 kV; detection was either at a fixed wavelength UV of 214 nm or with a diode-array detector operating in the wavelength range from 190 to 350 nm. Both reversed-phase HPLC and MEKC techniques succeeded in resolving the tested standard mixtures of organic components of smokeless powders. Although the sequence of elution of the different analytes was slightly different between HPLC and MEKC, a statistical analysis based on the Spearman's rank correlation test showed that the two separation patterns were highly correlated. HPLC and MEKC were comparable in terms of elution/migration time precision, whereas MEKC showed higher reproducibility of peak areas. The interfacing of capillary electrophoresis with diode array UV detection provided distinct UV spectra of the individual analytes, thus improving, on the detection side, the analytical selectivity and identification power of capillary electrophoresis.     

Separation of flavonol-2-O-glycosides from Calendula officinalis and Sambucus nigra by high-performance liquid and micellar electrokinetic capillary chromatography.

Calendula officinalis and Sambucus nigra flowers were analysed by reversed-phase high-performance liquid chromatography (RP-HPLC) and micellar electrokinetic capillary chromatography (MECC). RP-HPLC was performed on C8 Aquapore RP 300 columns with eluents containing 2-propanol and tetrahydrofuran. MECC was carried out on a 72-cm fused-silica capillary using sodium dodecyl sulphate and sodium borate (pH 8.3) as the running buffer. The results obtained by these techniques are compared.     

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).     

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.     

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

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

Mixed micellar electrokinetic capillary chromatography separation of depolymerized grape procyanidins

Oligomeric procyanidins are potent antioxidant polyphenols of potential interest as disease-preventing agents. Their efficiency depends on the size and composition of their oligomeric structures. The mean degree of polymerization of these compounds is usually estimated by thiolysis with thiol-alpha-toluene followed by analysis using high-performance liquid chromatography (HPLC). We show the development of a mixed micellar electrokinetic chromatography (MEKC) method for the separation of the major components obtained after thiolysis with cysteamine (catechins and their cysteamine conjugates). MEKC studies using sodium dodecyl sulfate (SDS as pseudostationary phase led to long migration times, e.g., with 100 mM SDS, at pH 7, the solutes were separated in about 40 min), while the use of sodium cholate (SC) produced an elution window relatively short. Using a mixed micellar SC-SDS system (50 mM phosphate at pH 7 containing 40 mM SC and 10 mM SDS), it is possible to separate these compounds in less than 15 min. The proposed method is useful to separate the major components of the thiolysate in effluents from food processing (e.g., skins and seeds from grape and apple) considered as potential procyanidin sources.     

Complimentary Role of Micellar Electrokinetic Capillary Chromatography and High Performance Liquid Chromatography in the Separation of Plant Phenolics

Abstract

A comparison was made between the efficiency of micellar electrokinetic capillary chromatography (MECC) and reversed phase high performance liquid chromatography (RP-HPLC) to separate a mixture of plant phenolics. Of a range of buffers 6 mM borate/10 mM phosphate/100 mM SDS at pH 8.5 was the most effective in separating a complex mixture of phenolics using MECC. Using this buffer the elution order and resolution was different from that obtained by HPLC using a reversed phase C18 column. These results illustrate how MECC and RP-HPLC are complimentary when examining complex mixtures such as those obtained from plant extracts. MECC using these conditions was applied to the examination of phenolics from leaf tissue of Eucalyptus margmata (jarrah).     

Analysis of artichoke (Cynara cardunculus L.) extract by means of micellar electrokinetic capillary chromatography

The main constituents of artichoke extract were separated by micellar electrokinetic chromatography (MEKC), using a buffer consisting of 100 mM sodium dodecyl sulfate (SDS) in 20 mM sodium dihydrogen phosphate, 20 mM disodium tetraborate (pH 8.6) as background electrolyte. Optimum separation voltage of 28 kV (positive polarity) and a capillary temperature of 25 degrees C gave the best analysis. The UV detection was performed at 200 nm. The method was successfully used to analyze plant and drug samples as well as for the study of artichoke antioxidant activity. The quantitative MEKC results were in good agreement to those obtained previously by reversed-phase high-performance liquid chromatography (RP-HPLC).     

Separation of 4-dimethylamino-6-(4-methoxy-1-naphthyl)-1,3,5-triazine-2-hydrazine derivatives of carbonyl compounds by reversed-phase capillary electrochromatography

4-Dimethylamino-6-(4-methoxy-1-naphthyl)-1,3,5-triazine-2-hydrazine (DMNTH) is a novel derivatizing reagent specially designed for the determination of carbonyl compounds. In this work, we describe the separation of DMNTH-derivatized carbonyl compounds by reversed-phase capillary electrochromatography (CEC). After systematic investigations of the effects of experimental conditions viz. pH and concentration of buffer, type of stationary phase, injection volume of sample, organic modifier, and temperature, optimal conditions were found. The sample compounds, which were separated with gradient high performance liquid chromatography (HPLC), were separated by CEC under isocratic elution due to the high efficiency. Comparisons of separations by CEC and micellar electrokinetic chromatography (MEKC) were made.     

Micellar electrokinetic chromatography estimation of size and composition of procyanidins after thiolysis with cysteine

This paper describes the characterization of procyanidin mixtures by acid depolymerization in the presence of cysteine (thiolysis with cysteine) and micellar electrokinetic chromatography (MEKC). Reversed-phase liquid chromatography (RP-HPLC) and MEKC were investigated for the separation of the major components of the depolymerized mixtures (catechins and their cysteinyl derivatives). The solutes could only be effectively separated using MEKC. Two background electrolytes (BGEs) are recommended: (i) 50 mM phosphate at pH 7, containing 40 mM sodium cholate (SC) and 10 mM sodium dodecyl sulfate (SDS); (ii) a BGE with the same composition but containing only 50 mM SDS. The MEKC procedures here reported, are cheap, reliable and fast, and their potential in the determination of the size and composition in procyanidin mixtures has been shown. The proposed MEKC methods were validated by comparison with our intralaboratory reference RP-HPLC method using cysteamine as thiol donor.     

Pharmaceutical applications of micelles in chromatography and electrophoresis

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

Separation of binaphthyl enantiomers by capillary zone electrophoresis and electrokinetic chromatography

The capillary electrophoretic (CE) separation of the enantiomers of three binaphthyl compounds is investigated. Several CE modes such as cyclodextrin (CD) modified capillary zone electrophoresis (CZE) (CD-CZE), micellar electrokinetic chromatography (MEKC), cyclodextrin electrokinetic chromatography (CD-EKC), etc. are employed for the simultaneous enantiomer separation of the three solutes. The successful separation was achieved by combining two modes, in other words by using more than two chiral selectors. A development of the CE enantiomer separation is demonstrated for the binaphthyl compounds. The enantioselectivity of binaphthyl compounds is alo briefly discussed.     

Retention indices in micellar electrokinetic chromatography

The use of retention indices in micellar electrokinetic chromatography (MEKC) is evaluated both from a theoretical and a practical point of view. Fundamental equations for the determination of retention indices in MEKC are described, showing that retention indices are independent of the surfactant concentration. Possibilities as well as limitations of different homologous series as reference standards are described. In addition, the practical application of retention indices for identification, investigation of solute-micelle interactions, characterization and classification of pseudo-stationary phases and determination of solute lipophilicity are discussed.     

Micellar electrokinetic chromatography - From theoretical concepts to real samples (Review)

Micellar electrokinetic chromatography (MEKC) is an alternative to liquid chromatographic separations. It is a highly efficient separation technique that is performed with the same experimental set-up as is used for capillary electrophoresis (CE), thus extending the applicability of CE to neutral solutes. MEKC can be regarded as a separation technique with a similar scope to that of reversed-phase high-performance liquid chromatography (RP-HPLC), having advantages over HPLC with regard to the efficiency of the separation system, separation speed, cost, and tolerance to matrix constituents. This paper discusses the applicability of MEKC to real samples and also addresses developments widening the scope of this emerging technique: on-line concentration by stacking or sweeping and sensitive detection schemes.     

非水胶束电动色谱分离邻苯二甲酸酯类化合物

非水胶束电动色谱(NAMEKC)兼具非水毛细管电泳的优点和胶束电动色谱的分离机制,尤其适于对强疏水性化合物进行分离分析。在以甲酰胺为非水溶剂的电泳介质中,采用十二烷基硫酸钠（SDS）形成胶束相,开展NAMEKC方法的研究。通过添加水溶液、调节水溶液酸度、添加有机溶剂、改变SDS浓度等操作条件的考察,在15 min 内实现了3种美国环保局优先监测的污染物——邻苯二甲酸二甲酯、邻苯二甲酸二乙酯、邻苯二甲酸二丁酯的分离。分离度最小者为1.5,检测限优于3.04 mmol/L(以信噪比为3计)。3种典型的强疏水性物质的成功分离,显示出NAMEKC方法在分离疏水性物质方面的优势,扩展了NAMEKC在电中性有机物分析中的应用。     

Comparative study of capillary zone electrophoresis and micellar electrokinetic chromatography for the separation of twelve aromatic sulphonate compounds

Summary Two modes of capillary electrophoresis (CE), capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), were investigated for the separation of 12 aromatic sulphonate compounds. In CZE, although the voltage applied, the buffer concentration and the pH were optimized for effective separation of the compounds studied, under the best conditions four of the five amino compounds coeluted, as did naphthalene-1-sulphonic acid and naphthalene-2-sulphonic acid. In MEKC, sodium dodecyl sulphate (SDS) and Brij 35 were chosen as the anionic and nonionic surfactants and the effect of the concentration of micelles was examined. The effect of adding methanol as the organic modifier was also investigated with each of these micellar systems. All the analytes, including the isomers, were completely separated by use of MEKC with Brij 35 but when SDS was used only 11 compounds were separated because two amino compounds coeluted.