Sample stacking for determination of aromatic acid impurities by microemulsion electrokinetic chromatography

In this study, a sample stacking step coupled with microemulsion electrokinetic chromatography (MEEKC) was used to detect and analyze nine 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 impurities produced during aromatic acid synthesis. First, the presence of both acid and water plugs at the front of the capillary improved the reproducibility in retention time and peak intensity of the tested analytes in the stacking method. Second, the pH and the electrolyte type of acidic plug and sample matrix were found to be the predominant influences on the aromatic acid stacking. The detection limits of these aromatic acids were reduced to the range of 0.00007-0.00032 μg mL⁻¹ by this optimal sample stacking step. This proposed on-line concentration MEEKC method was able to detect trace levels of aromatic acid impurities in commercial aromatic acid products that were not previously possible by the normal MEEKC method. Furthermore, these results in comparison with our previous studies on sample stacking MEEKC method indicated that all acidic species were concentrated by this simple stacking procedure. The sensitivity enhancement, however, was highly dependent on the types of functional groups present in the structures of analytes, and the enhancement was in the order of first the compounds carrying both carboxy and hydroxy groups (e.g. phenolic acid), followed by carboxylic acid compounds (e.g. aromatic acid), and then phenol compounds (e.g. polyphenol).     

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).     

Quantitative determination of UV filters in sunscreen lotions using microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) has been applied to the separation of some UV filters (Eusolex 4360, Eusolex 6300, Eusolex OCR, Eusolex 2292, Eusolex 6007, Eusolex 9020, Eusolex HMS, Eusolex OS, and Eusolex 232) commonly used in sunscreen lotions. Use of a MEEKC buffer with a mixed surfactant system to stabilize the oil droplets and an organic modifier in the aqueous phase allowed separation of most of the selected analytes in a single run in a system fitted with a diode array detector recording three wavelengths (240 nm, 300 nm, and 380 nm) simultaneously. The microemulsion employed consisted of 2.25 g of SDS, 0.75 g of Brij 35, 6.6 g 1‐butanol, 0.8 g n‐octane, 17.5 g 2‐propanol, and 72.1 g of 10 mM borate buffer (pH 9.2). Detection limits from 0.65 to 6.0 μg/mL were obtained and the calibration plots were linear over at least one order of magnitude for all analytes. The developed method could be applied to the determination of UV filters in several sun protection products including lotions, milks, and oils. Comparison of the obtained data with those from an HPLC method described in the literature showed acceptable agreement.     

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.     

Determination of aromatic amines in environmental water sample by hollow fiber-liquid phase microextraction and microemulsion electrokinetic chromatography

A novel method of microemulsion electrokinetic chromatography (MEEKC) coupled with hollow fiber-liquid phase microextraction (HF-LPME) was developed for determination of six aromatic amines including 4-methylaniline, 3-nitroaniline, 2,4-dimethylaniline, 4-chloroaniline, 3,4-dichloraniline and 4-aminobiphenyl. Baseline separation of six aromatic amines was achieved within 8 min by using the microemulsion buffer containing a 10 mM borate buffer at pH 9.0, 0.8% (v/v) ethyl acetate as oil droplets, 60 mM sodium cholate as surfactant, 5.0% (v/v) 1-butanol as co-surfactant. The influence factors relevant to the HF-LPME process were systemically investigated. The obtained enrichment factors were ranged between 70 and 157 in a 30 min extraction time, and the limits of detection ranged between 0.0021 and 0.0048 μg/mL. This purposed method was successfully applied for the analysis of aromatic amines in water sample and the recoveries were ranged from 87.2% to 99.8%.     

Analyses of phenolic compounds by microemulsion electrokinetic chromatography

In this study, a microemulsion electrokinetic chromatography (MEEKC) method was developed to analyze and detect 13 phenolic compounds (syringic acid, p-cumaric acid, vanillic acid, caffeic acid, gallic acid, 3,4-dihydroxybenzoic acid, 4-hydroxybenzoic acid, (+)-catechin, (-)-epigallocatechin, (-)-epicatechin gallate, (-)-epigallocatechin gallate, (-)-epicatechin, and (-)-gallocatechin), which are present in many plant-derived foods. The effects of cosurfactant, organic modifier, and oil were examined in order to optimize the separation of these phenolic compounds. The amounts of cosurfactant (cyclohexanol) and organic modifier (acetonitrile) were determined as the major influence on the separation selectivity, while the type of oil partially affected the separation resolution of the phenolic compounds. A highly efficient MEEKC separation method was achieved within 14 min by using a microemulsion solution of pH 2.0 containing 2.89% w/v SDS, 1.36% w/v heptane, 7.66% w/v cyclohexanol, and 2% w/v ACN. Furthermore, the present work could demonstrate that the nature of the oil phase has a significant influence on the separation selectivity of phenolic compounds.     

Impurity profiling of dexamphetamine sulfate by cyclodextrin‐modified microemulsion electrokinetic chromatography

A CD‐modified microemulsion electrokinetic chromatography method has been developed and validated for dexamphetamine sulfate which allows the simultaneous determination of charged and uncharged impurities of the drug including the levorotary (R)‐enantiomer. The optimized background electrolyte consisted of 1.5% w/w SDS, 0.5% w/w ethyl acetate, 3.5% w/w 1‐butanol, 2.5% w/w 2‐propanol and 92% w/w 50 mM sodium phosphate buffer, pH 3.0, containing 5.5% w/w sulfated β‐CD. Separations were performed in a 50.2/40 cm, 50 μm id fused silica capillary at a temperature of 20°C and an applied voltage of ?14 kV. Carbamazepine was used as internal standard. The assay was validated in the range of 0.1–1.0% for the related substances and 0.1–5.0% for levoamphetamine based on a concentration of 3 mg/mL of dexamphetamine sulfate. The LOD of all analytes ranged between 0.05 and 0.2%. In commercial samples of dexamphetamine sulfate, levoamphetamine was found at concentrations between 3.2 and 3.8%, whereas none of the other impurities could be detected.     

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.     

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。     

Highly efficient and selective separations of a wide range of analytes obtained by an optimised microemulsion electrokinetic chromatography method

Summary Microemulsion Electrokinetic Chromatography (MEEKC) has previously been reported to be useful for the separation of a range of hydrophobic solutes. The previous reports had tended to show relatively long separation times. In this report the separation and operation conditions were optimized and a method was successfully employed in a range of novel applications. High efficiencies, rapid separations and high repeatability were demonstrated. The applications developed included a range of both neutral and charged compounds. Both water-soluble and water-insoluble compounds were also well separated. The novel applications developed included analysis of basic drugs, various aromatic acids, a range of neutral aromatics, profiling of ink components, direct injection of urine for profiling purposes and the analysis of a range of amino acids and their derivatives. The method could also be used for hydrophobicity measurements of the resolved solutes based upon their migration times. This report clearly shows that MEEKC can be widely applied to a range of compounds and is especially useful for complex mixtures containing solutes with varying charge and hydrophobicity.     

Determination of avermectins in commercial formulations using microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) was developed for quantitative analysis of avermectins, such as abamectin, doramectin and ivermectin, in commercial formulations, using the microemulsion buffer containing a 50 mM phosphate buffer at pH 2.5, 1.1% (v/v) n-octane as oil droplets, 180 mM sodium dodecylsulphate as surfactant, 890 mM 1-butanol as co-surfactant and 30% (v/v) ethanol as organic co-solvent. High accuracy and precision of the method were obtained. The contents of avermectins in commercial formulations determined by MEEKC were found to be insignificantly different with those determined by high performance liquid chromatography (HPLC). Therefore, MEEKC can be used an alternative method to HPLC for quantitative determination of avermectins.     

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.     

Evaluation of a multiarray system for pharmaceutical analysis by microemulsion electrokinetic chromatography

A multiplexed capillary electrophoresis (CE) system equipped with 96 channels was evaluated for high-throughput screening in drug discovery by microemulsion electrokinetic chromatography (MEEKC). Method transfer from a single channel to a multichannel CE system is described. Loss of efficiency and reduced migration times could be elucidated to the poor efficacy in Joule heat dissipation by forced air cooling in the multiarray system compared to liquid cooling in the single channel instrument. On the other hand, only 48 channels could actually be used because of the maximum total current of 3 mA. Precision data remained below 8% and 9% for migration times and peak areas, respectively. Some UV-detector cross-talk interference between neighboring capillary channels was noted. Impurities at 0.5% compared to the main peak (100%) could be detected with the multiplexed system which is 10 times lower compared to the single capillary system. Higher efficiency and improved figures of merit (absolute sensitivity and no cross-talk interferences) were obtained by using an array of only 24 capillaries.     

Utilisation of crown ethers in microemulsion electrokinetic chromatography for the separation of inorganic cations

A new method to influence the separation selectivity of inorganic cations in capillary electrophoresis is presented. This method combines the use of certain crown ethers to form complexes with a specific cation (changing its ionic radius/charge ratio and thereby its electrophoretic mobility) with partitioning of the crown ether/analyte complex between an aqueous phase and a pseudo-stationary phase, such as the oil droplet of a microemulsion. Several microemulsions, including uncharged oil droplets and oil droplets with different degrees of surface charge were tested to evaluate their ability to improve the separation of the selected analytes.     

Effect of microemulsion component purity on the chromatographic figures of merit in chiral microemulsion electrokinetic chromatography

Numerous combinations of one-, two-, and three-chiral-component microemulsions have been previously prepared in our group, using N-dodecoxycarbonylvaline (DDCV), 2-hexanol, and ethyl acetate, dibutyl tartrate, or diethyl tartrate. A few results of the various formulations investigated suggested the possible presence of minor impurities in one or more components of the microemulsion. In this study, the purity of the current lots of R- and S-surfactant were measured, as was the subsequent effect of minor impurities on the relevant chromatographic figures of merit (CFOMs) that describe a chiral separation, i.e., efficiency, enantioselectivity, retention, migration window (elution range), and resolution. Two related methods are proposed for correcting enantioselectivities measured in the presence of chiral impurities in the chiral microemulsion.     

Separation and sweeping of flavonoids by microemulsion electrokinetic chromatography using mixed anionic and cationic surfactants

In this report, a novel means for the separation and sweeping of flavonoids (quercetin, rutin, calycosin, ononin and calycosin-7-O-β-d-glucoside) by microemulsion electrokinetic chromatography using mixed anionic and cationic surfactants as modified pseudostationary phase was presented. The optimized background electrolyte consisted of 0.5% (w/v) ethyl acetate, 2.0% (w/v) SDS, 9 mM DTAC, 4.0% (w/v) 1-butanol and 10 mM sodium borate or 25 mM phosphoric acid. We systematically investigated the separation and preconcentration conditions, including the concentrations of surfactant, types of sweeping, sample matrix, the effect of high salt or acetonitrile, and sample injection volume. It was found that the use of mixed surfactants significantly enhanced the separation efficiency through the change of the efficient electrophoretic mobility of analytes. Compared with normal sample injection, 185-508-fold sensitivity enhancement in terms of limit of detection was achieved through effective sweeping of large sample volume at 50 mbar pressure (up to 45% capillary length). At last, the proposed method was suitable for the determination of Radix Astragali sample.     

On-line concentration of neutral analytes by complexation and acetonitrile sweeping in nonionic microemulsion electrokinetic chromatography with direct ultraviolet detection

To separate and detect neutral solutes in nonionic microemulsion electrokinetic chromatography (MEEKC), a novel method was developed, combining complex formation and acetonitrile (ACN) sweeping. In this report, dynamic borate complexation and on-line sweeping occurred simultaneously during a run. The operating parameters which affected the performance of analyte sweeping in nonionic MEEKC were examined in terms of borate complexation, ACN content, Brij-35 concentration and sample plug length. In addition, the validation of the method included tests of the limit of detection, reproducibility and sensitivity enhancement. 60–110-Fold of magnitude improvement in detection sensitivity for model compounds (ginsenoside Rf, ginsenoside Rb2, ginsenoside Re) using Brij-35 microemulsion was demonstrated. Furthermore, the method was applied to the determination of glucosides in the plant extract.     

胶束电动毛细管色谱法测定植物中的水杨酸

建立了以胶束电动色谱为分离模式测定植物中水杨酸的新方法。在一定范围内，随着硼酸和甲醇浓度的升高，苯甲酸内标和水杨酸的分离度以近似线性关系升高；随缓冲液pH的升高分离度呈非线性升高；随十六烷基三甲基溴化铵浓度的升高分离度呈非线性下降。在优化的条件下，两可在12min内分离。测定了苹果和梨样品，并做了回收率试验，回收率在97.1%-102%之间。     

Analysis of nucleic acid derivatives by micellar electrokinetic capillary chromatography

The use of micellar electrokinetic capillary chromatography (MECC) for the analysis of the major nucleobases, nucleosides, and nucleotides, and their chemically modified derivatives, has been developed and refined. The dimensions of the separating capillaries, the composition of the buffering systems, and the conditions used for electrophoresis were investigated in order to obtain the best performance. Particular emphasis was placed on the identification of the physiological constituents of nucleic acids and their chemically modified analogs: in vitro studies on calf thymus DNA exposed to genotoxic agents have demonstrated that adducted bases and nucleolides can be identified by MECC.     

在线扫集-胶束毛细管电动色谱法测定扛板归中的阿魏酸、咖啡酸和原儿茶酸

采用在线扫集-胶束毛细管电动色谱法(sweeping-MEKC)分离测定扛板归中的阿魏酸、咖啡酸和原儿茶酸。采用未涂层熔融石英毛细管(50 cm×50μm,有效柱长36 cm);环境温度25℃;缓冲体系为20 mmol/L NaH2PO4-80mmol/L十二烷基磺酸钠(SDS)-12.5%乙腈(V/V)(pH 2.20),紫外检测波长为216 nm,运行分离电压-20 kV,进样时间100 s。在优化条件下,3种有机酸均在20 min内出峰,峰面积RSD均小于5%。检出限分别达到98.52,118.73和27.27μg/L。