Semi-permanent surfactant coatings for inorganic anion analysis in capillary electrophoresis

Capillary electrophoretic separations of inorganic anions are performed using a capillary coated with a mixture of the cationic surfactant didodecyldimethylammonium bromide (DDAB) and the zwitterionic surfactant 1,2-dilauroyl-sn-phosphatidylcholine (DLPC). These double-chained surfactants form semi-permanent coatings on the capillary wall, which allows the excess surfactant to be removed from the buffer prior to separation. Interactions between surfactant aggregates in the buffer and analyte anions are thus eliminated. The electroosmotic flow (EOF) can be altered from fully reversed (100% DDAB) to near zero (100% DLPC) using different ratios of DDAB and DLPC. Controlling the EOF allows for improved resolution of the anions while maintaining a rapid, co-EOF separation, free from analyte-surfactant additive interactions.     

酸性药物的反向电渗流高效毛细管电泳分离分析研究

以水杨酸、乙酰水杨酸为代表药物,采用十六烷基三甲基溴化铵（ＣＴＡＢ）为电渗流改向剂,考察了酸性药物在反向电渗流高效毛细管电泳体系中的分离行为,并对其中影响迁移时间、峰形及柱效的诸多因素进行了系统研究。研究结果表明,以阳离子表面活性剂作为电渗流改向剂的反向电渗流高效毛细管电泳体系能显著加快酸性药物的分析速度。对于ＣＴＡＢ与酸性药物相互作用导致峰形展宽、柱效降低的现象,可通过加入合适的有机添加剂（如β－环糊精或乙腈）加以改善。     

Surface modification of poly(dimethylsiloxane) microchips using a double-chained cationic surfactant for efficiently resolving fluorescent dye adsorption

This paper described a double-chained cationic surfactant, didodecyldimethylammonium bromide (DDAB), for dynamic surface modification of poly(dimethylsiloxane) (PDMS) microchips to reduce the fluorescent dyes adsorption onto the microchannel. When DDAB with a high concentration was present as the dynamic modification reagent in the running and sample buffer, it not only reversed the direction of electroosmotic flow, but also efficiently suppressed fluorescent dyes pyronine Y (PY) or rhodamine B (RB) adsorption onto the chip surface. In addition, vesicles formed by DDAB in the buffer with higher surface charge density and electrophoretic mobility could provide wider migration window and potential for the separation of compounds with similar hydrophobicity. Factors affecting modification, such as pH and concentrations of the buffer, DDAB concentration in the buffer were investigated. Compared with commonly used single-chained cetyltrimethylammonium bromide, DDAB provided a better modification performance. Furthermore, PY and RB were separated successfully on a PDMS microchip at the appropriate conditions with DDAB.     

Electroosmotic flow reversal for the determination of inorganic anions by capillary electrophoresis with methanol-water buffers

Manipulation of the electroosmotic flow (EOF) is essential for achieving optimized separations of small anions by capillary electrophoresis (CE). In this work, efficient suppression or reversal of EOF is achieved upon addition of small amounts of the cationic surfactants, cetyltrimethylammonium bromide (CTAB) or didodecyldimethylammonium bromide (DDAB) to the electrophoretic buffer. Highly stable and reversed EOF are achieved using the surfactants in the presence of up to 50% MeOH. In aqueous and low methanol containing solutions (up to 30%, v/v) surface aggregation of the surfactants at the capillary wall occurs at a concentration below the critical micelle concentration (CMC). The impact of MeOH on reversed EOF is predominantly a function of the diminished zeta potential of the silica, and to a lesser extent on the CMC in the bulk solution of the surfactant. Fast baseline separation and selectivity changes for small inorganic anions are observed when mixed aqueous-organic buffers are employed. Changes in EOF, micellar properties of the surfactant and selectivity for inorganic anions upon addition of various percent of methanol are also discussed.     

Enantioseparation of benzoins and enantiomer migration reversal of hydrobenzoin in capillary zone electrophoresis with dual cyclodextrin systems and borate complexation

Rapid, precise, accurate, and reproducible methodology using capillary electrophoresis (CE) with dynamically coated capillaries for the analysis of heroin and its basic impurities and adulterants is presented. Highly selective determination of the above solutes is obtained by analyzing the same sample preparation by two CE methods. For the determination of heroin, its basic impurities and basic adulterants, dynamic coating of the capillary surface is accomplished using a commercially available reagent kit with an added cyclodextrin ((CD) polycation coating followed by polyanion coating with dimethyl-beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin). The addition of a cyclodextrin to the run buffer significantly improves the separation of these solutes. Neutral, acidic, and weakly basic adulterants which migrate near or after t0 do not interfere with the more mobile basic solutes. The determination of neutral, acidic, and weakly basic adulterants in heroin is accomplished using a modification of the above commercially available reagent kit. After first coating with a polycation, a negative coating is obtained using a surfactant sodium dodecyl sulfate. Micellar electrokinetic chromatography (MEKC) with dynamically coated capillaries gives an excellent separation of the neutral, acidic, and weakly basic solutes, with considerably shorter run times compared to conventional MEKC. In addition for this system, most basic solutes in heroin have longer migration times than the uncharged and acidic compounds.     

Towards a general approach for the impurity profiling of drugs by micellar electrokinetic chromatography

A general micellar electrokinetic chromatographic (MEKC) strategy for the impurity profiling of drugs was developed involving a sodium dodecyl sulfate (SDS) and a cetyltrimethylammonium bromide (CTAB) MEKC system. With this combination, in principle, each sample component passes the detector in at least one of the two MEKC systems provided that separation buffers of the same pH are used in both systems. In order to select the proper MEKC systems, the electroosmotic flow (EOF) and micelle migration time (t(mc)) were determined for separation buffers of several pH values, containing various amounts of surfactant and organic modifier. The selectivity of the MEKC systems was studied using a mixture of compounds with a wide range of physico-chemical properties. The final selection of two adequate MEKC systems for this approach was based on the requirements that the t(mc) (i.e., analysis time) of both systems was below 20 min and that the t(mc)/t(eof) ratio was above 3 or 2 for the SDS and CTAB system, respectively. Furthermore, the systems should provide high efficiency, exhibit differences in selectivity and use moderate concentrations of modifier and surfactant, so that, if needed, further optimization is possible. The selected MEKC systems contained 60 mM SDS or 10 mM CTAB, respectively, in a phosphate buffer (pH 7.5) with 10% acetonitrile. Some test compounds with extreme mobilities were used to demonstrate the suitability of the MEKC approach to detect each component of a sample. The potential of the proposed MEKC combination for impurity profiling was demonstrated by the analysis of fluvoxamine with several impurities at the 0.1% level.     

Sample stacking by field-amplified sample injection and sweeping for simultaneous analysis of acidic and basic components in clinic application

In this study, online sample concentration method, which coupled field-amplified sample injection (FASI) and sweeping technology with micellar electrokinetic chromatography (MEKC), was used to detect and analyze acidic and basic components in a single run. In order to concentrate the acidic and basic components simultaneously in a single run sweeping step, a combination of successive anion- and cation-selective injections were used. Before sample loading, a rinse buffer containing 50 mM Tris buffer (pH 3) with 41% MeOH and 0.1% polyethylene oxide (PEO) was injected in order to suppress the electroosmotic flow (EOF). Sample loading of anionic components was achieved by electrokinetic injection at a negative voltage of -2.5 kV for 80 s, and then the cationic components were injected at a positive voltage of +5 kV for 120 s. Finally, sweeping with SDS micelles from the separation buffer (25 mM Tris buffer with 60 mM SDS, pH 3) was performed at a negative voltage of -20 kV. This capillary electrophoretic methodology was applied to the quantification of acidic and basic drugs in commercial tablets and in plasma samples. The precision and accuracy of the proposed method at different concentrations ranging from high, medium, to low were evaluated on spiked plasma samples. The intra and interday precision and accuracy values at three concentrations were all below 6.1%. The method was also successfully applied to monitor the tested drugs in the plasma of nine elderly cardiovascular and/or Alzheimer's disease patients after oral administration of the commercial products.     

双动态吸附毛细管电色谱的手性分离——采用阳离子表面活性剂和阴离子手性选择剂作为假固定相

建立了以十六烷基三甲基溴化铵或1,5－二甲基－1,5－二氮杂十一烷亚甲基聚N－甲溴化物为阳离子表面活性剂,并以磺丁基β－环糊精为手性选择剂的双动态吸附毛细管电色谱。以碱性的丙比胺和酸性的华法林作为拆分对象,考察了双动态吸附毛细管电色谱的手性分离行为,以及动态吸附柱的重复性。在双动态吸附毛细管电色谱条件下,丙比胺和华法林的手性分离度较大,丙比胺的分离度可达3．21,丙比胺连续进样10次,迁移时间的相对标准偏差小于1．0％。     

Analysis of bacitracin by micellar electrokinetic capillary chromatography with mixed micelle in acidic solution

A method used for quantitative analysis of bacitracin with micellar electrokinetic capillary chromatography (MEKC) is described. As capillary zone electrophoresis gave poor separation selectivity, MEKC was preferable. It was found that a zwitterionic surfactant, 3-(N,N-dimethylhexadecylammonium)-propanesulfonate (PAPS) gave the best selectivity among the several surfactants studied. As the analytes tend to adsorb onto the capillary wall due to their positive charge, an acidic solution composed of Tris-phosphate buffer at pH 2.5 was necessary to diminish such adsorption. The peak tailing caused by relatively strong ion pair interaction between the analyte and PAPS micelle could be reduced by adding nonionic surfactant Brij 35 to the PAPS solution. This phenomenon is possibly explained by a mixed micelle mechanism. In order to obtain the optimal conditions and to test the method robustness, a central composite experimental design was performed. The optimal conditions are as follows: 44 cm length of fused-silica capillary with 50 microm inner diameter, 90 mM Tris-phosphate buffer (pH 2.5) containing 17 mM PAPS and 0.3% w/v-Brij 35, 18 kV applied voltage, UV detection at 192 nm and 25 degrees C column temperature. Under the optimal conditions, more than 50 peaks could be obtained in 30 min. The method had a linearity range from 1 to 0.05 mg/mL (concentration of bacitracin A). The limit of quantitation (LOQ) and limit of detection (LOD) were 0.005 and 0.0012 mg/mL, respectively.     

Rapid separation of pharmaceutical enantiomers using electrokinetic chromatography with a novel chiral microemulsion

A novel oil-in-water microemulsion incorporating the chiral surfactant dodecoxycarbonylvaline (DDCV) was used to achieve the rapid enantiomeric separation of pharmaceutical drugs by electrokinetic chromatography (EKC). Incorporation of DDCV into a microemulsion resulted in an elution range more than double that provided the micellar form of the surfactant aggregate. Interestingly, for the same compounds the enantioselectivity provided by the chiral DDCV microemulsions ranged from 1.06-1.30 for the neutral and cationic drugs, which was slightly higher than that provided by chiral DDCV micelles. The use of a low surface tension oil (ethyl acetate) permitted a much lower concentration of chiral surfactant to be employed; this, together with the use of a zwitterionic buffer (ACES) resulted in a very low conductivity microemulsion that allowed a higher separation voltage to be utilized, resulting in rapid enantiomeric separations (< 8 min.). Mobility matching of the buffer cation(s) was used to improve peak shape and efficiencies. In our limited survey of the phase diagram, the optimum composition of the microemulsion buffer was 1.0% (w/v) DDCV (30 mM), 0.5% (v/v) ethyl acetate, 1.2% (v/v) 1-butanol and 50 mM ACES buffer at pH 7.     

Cationic vesicles as chiral selector for enantioseparations of nonsteroidal antiinflammatory drugs by micellar electrokinetic chromatography

A vesicle-forming chiral cationic surfactant (1R,2S)-(-)-N-dodecyl-N-methyl-ephedrinium bromide was evaluated as a pseudo-stationary phase in micellar electrokinetic chromatography (MEKC) for enantioseparation of eight non-steroidal anti-inflammatory drugs e.g., carprofen, flurbiprofen, fenoprofen, ibuprofen, indoprofen, ketoprofen, naproxen and suprofen by capillary electrophoresis. The effects of varying experimental conditions such as pH and concentration of surfactant in the running buffer on the enantiomer separation of the drugs are reported. A mixture of five of the above drugs was separated and each enantiomeric pair was also separated simultaneously in a single run by use of the surfactant. The strong electrostatic interactions between the analytes and the vesicles seemed to have a major role in the enantiomeric separation of the profens.     

Simultaneous Separation of Four Types of Steroid Hormones by Micellar Electrokinetic Chromatography with Cetyltrimethylammonium Bromide

Micellar electrokinetic chromatography (MEKC) using a cationic surfactant, cetyltrimethylammonium bromide (CTAB), was applied to simultaneous separation of four types of hydrophobic steroid hormones including androgens, estrogens, progestins, and glucocorticoids. Various parameters, such as the concentration of CTAB, type and proportion of organic modifiers, and sample matrix have been demonstrated to be important with respect to the separation resolution, analysis time, and repeatability. Successful separation of cortisone, hydrocortisone, estriol, testosterone, estrone, progesterone, and estradiol was achieved at ?25 kV using 100 mM Tris‐boric acid buffer (pH 9.0) containing 40 mM CTAB and 20% 2‐propanol using ultraviolet (UV) absorption detection. For estrogen compounds which possess intrinsic fluorescence, detection limits at the sub μM level could be achieved with laser‐induced fluorescence (LIF) detection. Two kinds of pharmaceutical preparations were analyzed using the developed method, and the results were in good agreement with the label claims.     

手性化合物的毛细管胶束电动色谱分离研究

以4种不同的N-长链烷酰-L-氨基酸胶束为手性选择剂,对3种不同性质的手性化合物(α-氯代丙酰替苯胺,2-氨基-3-对硝基苯基-1,2-丙二醇和华法林)的毛细管胶束电动色谱分离进行研究.结果表明,手性表面活性剂中不同的氨基酸残基和烷基链的长度对分离影响较大;随手性表面活性剂浓度增加,溶质保留时间增大,分离度增加,不同溶质的最佳分离浓度在100～150mmol/L之间;pH对电中性手性化合物分离影响不大,但对酸性或碱性手性化合物的分离影响较大.在选定的条件下,3种样品均在20min内完全分离,分离柱效达1×10⁵理论板数/m.     

A new mixed micellar electrokinetic chromatography method for analysis of natural and synthetic anabolic steroids

A simple, rapid and low-costing new mixed surfactant MEKC method has been developed for the analysis of five neutral anabolic steroids in this paper. It was found that the bile salt coupling with Triton X-100 was a suitable bi-micellar surfactant for the separation of these anabolic steroids with similar structure. The separation conditions were optimized in detail. The five natural and synthetic anabolic steroids, such as androstenedione (AD), 19-norandrostenedione (NAD), 1,4-androstadiene-3,17-dione (ADD), methandrostenolone (MA) and methyltestosterone (MT) were separated and detected in an alkaline buffer system (pH 9.0) containing 15 mM Britton-Robinson (BR) buffer, 50 mM sodium cholate (SC) and 0.1% (v/v) Triton X-100 with detection wavelength at 241 nm and 18 kV of separation voltage. Under the optimal conditions, five coexistence neutral steroids were completely separated within 12 min with the detection limits ranged from 0.20 to 0.51 μg/mL. This method was successfully used for detection and confirmation of the anabolic steroid methandrostenolone in methandrostenolone tablets and in the real human urine, GC-MS method was applied to confirm the free methandrostenolone existence in the urine sample in order to validate the reliability of MEKC method.     

Multiple effect of surfactants used as additives in background electrolytes in capillary zone electrophoresis: cetyltrimethylammonium bromide as example of model surfactant

Surfactants are frequently used in the preparation of background electrolytes (BGEs) in capillary zone elcetrophoresis (CZE) in order to affect and to optimize both the electroosmotic flow (EOF) and the separation process. Their effects are, however, always multiple, the resulting situation may be very complex and the separation process may even be destroyed. We use the surfactant cetyltrimethylammonium bromide (CTAB) as a model example and bring experimental results and related discussion which elucidate the multiple effect of surfactants in an integrated way. It is shown that even at concentration levels lower than 10(-4) M CTAB strongly reduces the cathodic EOF in bare fused-silica capillaries and converts it into anodic EOF. The magnitude and polarity of the EOF depends not only on the concentration of CTAB but also on the composition of BGEs used. The interactions of CTA cations with the bare capillary wall reduce sorption of cationic analytes and enables their analysis. CTA cations at levels below their critical micelles concentration (CMC) already interact with anionic analytes and reduce their mobilities. This association is strong with highly charged anions and by this, the reversal of the EOF, applying BGEs with highly charged anions is less effective. These interactions are competitive and also depend on the composition of the BGE used. At levels above its CMC, CTAB forms micelles and enables the application of the micellar electrokinetic capillary chromatography (MEKC) mode and the analysis of, e.g., neutral components. Simultaneously, it is shown that the presence of CTAB may increase the number of potentially formed system zones.     

Micellar electrophoresis using ionic liquids

In this study, ionic liquid based cationic surfactants were evaluated as pseudo-stationary phases in micellar electrokinetic chromatography (MEKC). The aggregation behaviour of long-chain (C(12) and C(14)) alkylimidazolium ionic liquids in water and aqueous phosphate buffer was investigated by spectrophotometry. The critical micelle concentrations of these salts were determined and compared to those of tetradecyl- and dodecyltrimethylammonium chloride, salts commonly used in capillary electrophoresis. The practical utilization of a new type of surfactant in MEKC was evaluated by introducing an ionic liquid into the running aqueous buffer to separate neutral analytes-methylresorcinol isomers and benzene derivatives.     

Investigation of solvent effects in capillary electrophoresis for the separation of biological porphyrin methyl esters

The effects of organic solvents on the capillary electrophoresis (CE) separation of a number of important biological porphyrin methyl esters - six weakly basic, hydrophobic cyclic tetrapyrroles possessing two and four to eight methyl ester groups around the periphery of the porphyrin ring - were investigated in the mode of micellar electrokinetic chromatography (MEKC), microemulsion electrokinetic chromatography (MEEKC), and nonaqueous CE. In aqueous MEKC, partial separation of the six neutral porphyrin methyl esters was obtained with an organic modifier (acetonitrile) in the concentration range between 20 and 40%, in which sodium dodecyl sulfate (SDS) molecules might be present in the form of SDS micelles and/or SDS micelle-like aggregates. Relatively stable SDS micelles can be formed in nonaqueous MEKC using formamide as the separation medium, but the separation of the target analytes remained unsatisfactory. Improved resolution of all six porphyrin methyl esters was obtained using MEEKC with the running buffer consisting of 0.8% w/w n-heptane (oil phase), 2.25% w/w SDS and 1.0% w/w Brij 35 (mixed surfactant), 6.6% w/w 1-butanol (cosurfactant), and 30% v/v 2-propanol (second cosurfactant), but reproducibility in terms of peak areas for certain porphyrins (especially uroporphyrin I octamethyl ester) was found to be very poor. Best separation performances were achieved with nonaqueous CE separations in which the weakly basic porphyrin methyl esters were protonated under strongly acidic conditions (e.g., using 10 mM perchloric acid) in mixed organic solvents. For example, using a 50:50 mixture of methanol and acetonitrile as the separation medium, baseline separation of all six (positively charged) porphyrin methyl esters can be obtained within 3 min and the average precision (RSD, N = 13) in terms of migration time and peak area were 0.55 and 2.16%, respectively.     

Sodium maleopimaric acid as pseudostationary phase for chiral separations of amino acid derivatives by capillary micellar electrokinetic chromatography

A new type of chiral surfactant, sodium maleopimaric acid (SMA), was synthesized, and employed for the enantioselective micellar electrokinetic chromatographic (MEKC) separation of amino acid enantiomers derivatized with naphthalene-2,3-dicarboxaldehyde (NDA-D/L-AAs). The effect of the surfactant concentration, type and concentration of the BGE, and buffer pH on the resolution was studied, and optimized conditions were used to evaluate the ability of this new surfactant to perform chiral separations toward NDA-D/L-AAs by MEKC. Enantiomeric separations of NDA-D/L-AAs were achieved with a running buffer consisting of 100 mM borate (pH 9.5) and 20 mM SMA in a 58.5 cm length x 50 microm id capillary. Under the conditions selected, two pairs of tested amino acid enantiomers including NDA-D/L-trptophan (Trp) and NDA-D/L-kynurenine (Kyn) were resolved.     

Analysis of ecdysteroids by micellar electrokinetic chromatography with on-line preconcentration

In order to enhance the UV detection sensitivity, an application study of an on-line preconcentration technique for micellar electrokinetic chromatographic (MEKC) was carried out. The simultaneous determination of four test ecdysteroids, 20-hydroxyecdysone, ajugasterone C, polypodine B and ponasterone A has been investigated by using the normal stacking mode in MEKC with UV detection. The effects of anionic surfactant composition and concentration, the applied voltage, the pH buffer, the kind and the amount of organic solvent and the injection time on the analyte resolution were evaluated. The optimised conditions for the separation involved the use of a 50 mM borate as the running buffer containing 50 mM of a mixture of sodium dodecyl sulphate (SDS) and sodium cholate (SC) in the ratio of 1:1 together with a concentration of 10% (v/v) of 2-PrOH at pH 9.0. Hydrodynamic injection of 12 s at 50 mbar and separation voltage of 20 kV at temperature of 20 degrees C were employed. These conditions allowed a repeatability separation within 21 min. Concentration detection limit for the neutral analytes studied improve about an order of magnitude. The method was also applied to the determination of ecdysteroids in a real sample.     

Separation of aromatic sulfonate compounds by coelectroosmotic micellar electrokinetic chromatography

Summary Coelectroosmotic micellar electrokinetic chromatography (coelectroosmotic MEKC) has been investigated for the separation of twelve aromatic sulfonate compounds. The advantage of this method is that it combines the efficient separation characteristic of MEKC and the short analysis time of the coelectroosmotic mode. MEKC was performed with either cetyltrimethylammonium bromide (CTAB) or polyethylene glycol dodecyl ether (Brij 35) surfactants as pseudostationary phases and 2-propanol as organic modifier. The electroosmotic Flow (EOF) was reversed by adding two types of EOF modifier, an alkylammonium salt (cetyltrimethylammonium bromide, CTAB) or a cationic polyelectrolyte (hexadimetrine bromide, HDB). The surfactant concentration, applied voltage, and temperature were optimized, the influence of 2-propanol on the MEKC resolution of the compounds was studied. The effect of the osmotic modifier on the separation was also investigated.