Recent trends in open-tubular capillary electrochromatography

Capillary electrochromatography (CEC), which combines the advantages of the high efficiency of capillary electrophoresis (CE) and the high selectivity of liquid chromatography (LC), has recently received considerable attention. Most CEC experiments have been performed with capillary columns packed with small LC packing materials (1.5–5 μm particle diameter). However, problems such as difficulties in packing the small LC packing materials and fabricating the frits still exist in preparing the CEC column. The use of open-tubular columns in CEC is therefore an alternative approach that can eliminate the problems encountered in packed-column CEC. So far, several types of open-tubular columns have been proposed for CEC separations and in this article recent progress in this area is reviewed.     

Recent progress in adsorbed stationary phases for capillary electrochromatography

This review surveys the recent progress in the adsorbed stationary phases for capillary electrochromatography (CEC). Adsorption-based methods for preparation of stationary phase are novel approaches in CEC, which allow rapid and facile preparing stationary phases with desirable selectivity onto an open-tubular fused-silica capillary, a bare-silica or ion-exchange packed column or a monolithic silica or polymer column. A variety of adsorbing agents have been developed as adsorbed stationary phases, including ionic long-chain surfactant, protein, peptide, amino acid, charged cyclodextrin (CD), basic compound, aliphatic ionene, and ion-exchange latex particle. The adsorbed stationary phases have been applied to separation of neutral, basic and acidic organic compounds, inorganic anions and enantiomers. They have also been applied to on-line sample concentration, fast separation and study of the competitive binding of enantiomers with protein.     

Physically adsorbed chiral stationary phase of avidin on monolithic silica column for capillary electrochromatography and capillary liquid chromatography

The physical adsorption method proposed previously has been successfully applied to a monolithic silica column. By virtue of the physical adsorption, a chiral stationary phase of avidin was prepared onto the silica monolith. The phase ratio of resulting stationary phase was evaluated with frontal analysis. The method proved to be comparable in phase ratio to the chemical bonding methods used in high-performance liquid chromatography (HPLC). Enantiomer separations were carried out in capillary electrochromatography (CEC) and capillary liquid chromatography (CLC) modes. Due to its larger phase ratio, the resulting column showed more powerful separation capability as compared to open-tubular CEC (OTCEC). Twelve chiral compounds were baseline-resolved. The resulting column showed high separation efficiency, with average theoretical plate numbers of 66 000/m for CLC and 122 000/m for CEC. Good reproducibility was observed, with RSD value less than 1.3% for retention time, retention factor and separation factor, and less than 6.6% for plate counts and resolution (n = 40). Fast separations were achieved with a short column. The test enantiomers were baseline-resolved within 4 min under CLC and CEC modes. In addition, field-enhanced sample injection (FESI) was coupled to CLC as well as CEC to improve the detection sensitivity.     

Advances in sol-gel based columns for capillary electrochromatography: sol-gel open-tubular columns

The development of sol-gel open-tubular column technology in capillary electrochromatography (CEC) is reviewed. Sol-gel column technology offers a versatile means of creating organic-inorganic hybrid stationary phases. Sol-gel column technology provides a general approach to column fabrication for microseparation techniques including CEC, and is amenable to both open-tubular and monolithic columns. Direct chemical bonding of the stationary phase to the capillary inner walls provides enhanced thermal and solvent stability to sol-gel columns. Sol-gel stationary phases inherently possess higher surface area, and thus provide an effective one-step alternative to conventional open-tubular column technology. Sol-gel column technology is applicable to both silica-based and transition metal oxide-based hybrid stationary phases, and thus, provides a great opportunity to utilize advanced material properties of a wide range of nontraditional stationary phases to achieve enhanced selectivity in analytical microseparations. A wide variety of stationary phase ligands can be chemically immobilized on the capillary inner surface using a single-step sol-gel procedure. Sol-gel chemistry can be applied to design stationary phases with desired chromatographic characteristics, including the possibility of creating columns with either a positive or a negative charge on the stationary phase surface. This provides a new tool to control electroosmotic flow (EOF) in the column. Column efficiencies on the order of half a million theoretical plates per meter have been reported for sol-gel open-tubular CEC columns. The selectivity of sol-gel stationary phases can be easily fine-tuned by adjusting the composition of the coating sol solution. Open-tubular columns have significant advantages over their packed counterparts because of the simplicity in column making and hassle-free fritless operation. Open-tubular CEC columns possess low sample capacity and low detection sensitivity. Full utilization of the analytical potential of sol-gel open-tubular columns will require a concomitant development in the area of high-sensitivity detection technology.     

Column technology for capillary electrochromatography

Column technologies for capillary electrochromatography (CEC) are reviewed. To achieve high efficiency, the inner diameters of open-tubular and packed columns should be less than 25 and 200 μm, respectively. To obtain acceptable separation speed under typical CEC conditions (e.g. 30 kV, 1 mm s⁻¹ electroosmotic flow velocity, and 2–4×10⁻⁸ m² V⁻¹ s⁻¹ electroosmotic mobility) the column lengths for open-tubular and packed columns should be less than 120 and 60 cm, respectively. Capillary CEC columns are generally classified into three types: packed, open-tubular, and continuous-bed or monolithic. The various column preparation procedures and the advantages and disadvantages of each column type are discussed in detail.     

Preparation and characterization of mixed-mode monolithic silica column for capillary electrochromatography

A silica-based monolithic stationary phase with mixed-mode of reversed phase (RP) and weak anion-exchange (WAX) for capillary electrochromatography (CEC) has been prepared. The mixed-mode monolithic silica column was prepared using the sol–gel technique and followed by a post-modification with hexadecyltrimethoxysilane (HDTMS) and aminopropyltrimethoxysilane (APTMS). The amino groups on the surface of the stationary phase were used to generate a substantial anodic EOF as well as to provide electrostatic interaction sites for charged compounds at low pH. A cathodic EOF was observed at pH above 7.3 due to the full ionization of residual silanol groups and the suppression in the ionization of amino groups. A variety of analytes were used to evaluate the electrochromatographic characterization and column performance. The monolithic stationary phase exhibited RP chromatographic behavior toward neutral solutes. The model anionic solutes were separated by the mixed-mode mechanism, which comprised RP interaction, WAX, and electrophoresis. Symmetrical peaks can be obtained for basic solutes because positively charged amino groups can effectively minimize the adsorption of positively charged analytes to the stationary phase.     

Chitosan-silica hybrid-coated open tubular column for hydrophilic interaction capillary electrochromatography

A novel and convenient protocol for the preparation of an open-tubular column coated with chitosan-silica hybrid using chitosan and silane-coupling agent (γ-glycidoxy-propyltrimethoxysilane) was developed for CEC, in which, chitosan was covalently bonded to the inner wall of a fused-silica capillary using γ-glycidoxy-propyltrimethoxysilane as a cross-linking agent. The stationary phase was hydrophilic due to the chitosan-silica hybrid with abundant amine and hydroxyl functional groups. The chromatographic characteristics of the column were evaluated by the separation of some organic acids and inorganic anions. The column showed good selectivity for nucleotides, aromatic acids, and inorganic anions. The mechanism for the separation of these compounds was primarily based on the hydrophilic and electrostatic interactions combined with the electrophoretic mechanism. The CEC method on the column for the separation of these compounds was compared with CE method in a bare capillary.     

Enantioselective open-tubular capillary electrochromatography using cyclodextrin-modified gold nanoparticles as stationary phase

Enantioselective open-tubular CEC (OTCEC) with thiolated β-CD modified gold nanoparticles (CD-GNPs) as stationary phase was developed. The enantioselective OT capillary column was fabricated by electrostatic assembly of poly(diallydimethylammonium chloride) (PDDA) followed by self-adsorption of negatively charged CD-GNPs. The enantioselective capillary column has a steady EOF mobility over a wide pH range of 3.0 to 9.2 (RSD 4.8%), and is quite stable over 240 min with very good column to column reproducibility. Efficient enantioseparation of the presented method was demonstrated by analyzing three drug enantiomers. Our results show that the column exhibits good run-to-run repeatability for enantioseparations and can maintain the enantioselectivity for more than 1 month if the column was stored in CD-GNPs solution at 4 °C.     

Instrumentation for capillary electrochromatography

One of the reasons for the immense interest in capillary electrochromatography (CEC) is its feature to combine chromatographic selectivity with the high efficiency and the miniaturization potential of capillary electrophoresis (CE). The capability of commercial CE instruments to run CEC has enforced the readiness of users and researchers to work on this separation technique. Nevertheless, to fully exploit the potential of CEC, a routine CE device can certainly not fulfill all requirements. Two different approaches have been made to overcome this problem. The first was to modify commercial CE instruments for various demands. Pressurization of the packed capillary to prevent "air" bubble formation, gradient elution capabilities and thermostating devices allowing a greater flexibility in column designs have been implemented in CE instruments of several manufacturers. A completely different approach is the development of modular laboratory-made instrumentation dedicated to special CEC requirements. In order to increase mobile phase velocity and thus the speed of analysis the availability of voltages higher than 30 kV was accomplished in some of these devices. Gradient elution was achieved by either coupling of gradient LC systems or an electroosmotic generation of the changing eluent composition. When a pressure gradient is applied between both column ends in addition to the voltage gradient, a hybrid between capillary HPLC and CEC results. This chromatographic mode is named pressure-assisted electrochromatography (PEC). Either CE instruments equipped with additional HPLC pumps or modular laboratory-made devices are suitable for PEC. In CEC, sensitivity for UV detection is rather poor due to the short optical path length for on-column detection in capillary separation techniques. A special cell design with enhanced light path is presented and further principles like, e.g., fluorescence detection and coupling to mass spectrometry are discussed.     

毛细管电色谱和加压毛细管电色谱的进展与应用

毛细管电色谱(CEC)以内含色谱固定相的毛细管为分离柱,以电渗流为驱动力,既可以分离带电物质也可以分离中性物质。它结合了毛细管电泳和高效液相色谱两者的优点,兼具高柱效、高分辨率、高选择性和高峰容量的特点,同时具有色谱和电泳的双重分离机理。然而,“纯粹”的电色谱在实际应用中有着天然的弱点,即: 在电流通过毛细管柱中的流动相时容易产生气泡(焦耳热作用),从而使电流中断和电渗流停止,毛细管柱必须被重新用流动相润湿后方能再次使用。加压毛细管电色谱(pCEC)将液相色谱中的压力流引入CEC系统中,不仅解决了气泡、干柱等问题,而且实现了定量阀进样和二元梯度洗脱。CEC和pCEC作为微分离领域的两种前沿技术,满足了当前复杂样品分析和分析仪器微型化的需求,近年来获得了广泛的关注。本文综述了这两种技术近来的发展,包括仪器、色谱固定相的发展,总结了其在生命科学、药物分析、食品安全以及环保样品分析等方面的应用进展,评述了各方法的特点,并展望了CEC和pCEC今后的发展和应用前景。     

Separation of enantiomers by capillary electrochromatography

The current popularity of capillary electrochromatography (CEC) has led to an increasing number of studies on the development and evaluation of enantioselective CEC systems. These studies clearly demonstrate that the most prominent advantage of electrically driven separation methods, the vastly increased column efficiency as compared to pressure-driven chromatography, can also be experimentally achieved for the separations of enantiomers. In analogy to high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE), several approaches have been used. The addition of a chiral selector to the mobile phase is the simplest method. Less erroneous and more elegant approaches are those that use open-tubular, conventional packed, and monolithic columns containing chiral stationary phases that stereoselectively interact with enantiomers. This review evaluates the new techniques and compares them to enantioselective HPLC and CE. Further, it describes the various concepts of enantioselective CEC and focuses on the current ‘state-of-the-art' column technology.     

Polyacrylamide grafted on multi-walled carbon nanotubes for open-tubular capillary electrochromatography: comparison with silica hydride and polyacrylate phase matrices

A new nanoparticle-bound polymer stationary phase was prepared by in situ polymerization of methacrylamide (MAA), bis-acrylamide crosslinker, and carboxylated multi-walled carbon nanotubes (multi-walled CNTs; MWNTs), using the abundant double bonds in the cyclopentadienyl rings in MWNT structure, on a silanized capillary. Each intermediate capillary between the synthesis steps was characterized by SEM, by ATR-IR, and by EOF measurements varying the pH, concentration, and volumetric ratios of ACN in running buffers. The resulting EOF profile was comparable to those of two other capillaries with different phase matrices, silica hydride and polybutyl methacrylate (BMA) phases. With the complex functionality of MWNTs on the hydrophilic polyacrylamide network, the MAA-CNT capillary was capable of separating diverse samples with a wide range of polarity and dissociation properties using open-tubular CEC. Besides optimizing CEC conditions, the migration times of samples were analyzed with respect to velocity and retention factors to evaluate electrophoretic and chromatographic contributions to the CEC mechanism. The migration rates of benzoic acids were determined by the electrophoretic mobilities of the various phenolate ions, while phenolic aldehydes and ketones were additionally influenced by chromatographic interactions, such as π-π, electrostatic effects, hydrogen bonding, and hydrophobic interactions. The retention factors were greater for flavonoids, which are polyphenolic, than for simple phenols, but were smaller than those obtained from the hydrophobic BMA-CNT column. A complete well-resolved separation of the cationic forms of tetracyclines was acheived either by electrophoresis or by chromatography in the MAA-CNT capillary, but not in the BMA-CNT and silica hydride-CNT capillaries.     

Investigation of the ion-exchange properties of methacrylate-based mixed-mode monolithic stationary phases employed as stationary phases in capillary electrochromatography

The potential of methacrylate-based mixed-mode monolithic stationary phases bearing sulfonic acid groups for the separation of positively charged analytes (alkylanilines, amino acids, and peptides) by capillary electrochromatography (CEC) is investigated. The retention mechanism of protonated alkylanilines as positively charged model solutes on these negatively charged mixed-mode stationary phases is investigated by studying the influence of mobile phase and stationary phase parameters on the corrected retention factor which was calculated by taking the electrophoretic mobility of the solutes into consideration. It is shown that both solvophobic and ion-exchange interactions contribute to the retention of these analytes. The dependence of the corrected retention factor on (1) the concentration of the counter ion ammonium and (2) the number of methylene groups in the alkyl chain of the model analytes investigated shows clearly that a one-site model (solvophobic and ion-exchange interactions take place simultaneously at a single type of site) has to be taken to describe the retention behaviour observed. Comparison of the CEC separation of these charged analytes with electrophoretic mobilities determined by open-tubular capillary electrophoresis shows that mainly chromatographic interactions (solvophobic and ion-exchange interactions) are responsible for the selectivity observed in CEC, while the electrophoretic migration of these analytes plays only a minor role.     

Interplay of chromatographic and electrophoretic processes in capillary electrochromatography

The separation mechanism in capillary electrochromatography (CEC) is a hybrid differential migration process, which entails the features of both high-performance liquid chromatography and capillary zone electrophoresis, i.e., chromatographic retention and electrophoretic migration. The adsorption of the different sample components on the stationary phase can be modified by the presence of the electric field across the column. Here, we use our previously published approach to decouple chromatographic retention from electrophoretic migration that allows us to investigate the "modification" of the retention process in CEC. This paper presents a methodology for characterization of changes in the retention of neutral and charged sample components, under identical conditions of stationary and mobile phase.     

Sol-gel stationary phases for capillary electrochromatography

A review is presented on the current state of the art and future trends in the development of sol-gel stationary phases for capillary electrochromatography (CEC). The design and synthesis of stationary phases with prescribed chromatographic and surface charge properties represent challenging tasks in contemporary CEC research. Further developments in CEC as a high-efficiency liquid-phase separation technique will greatly depend on new breakthroughs in the area of stationary phase development. The requirements imposed on CEC stationary phase performance are significantly more demanding compared with those for HPLC. The design of CEC stationary phase must take into consideration the structural characteristics that will provide not only the selective solute/stationary phase interactions leading to chromatographic separations but also the surface charge properties that determine the magnitude and direction of the electroosmotic flow responsible for the mobile phase movement through the CEC column. Therefore, the stationary phase technology in CEC presents a more complex problem than in conventional chromatographic techniques. Different approaches to stationary phase development have been reported in contemporary CEC literature. The sol-gel approach represents a promising direction in this important research. It is applicable to the preparation of CEC stationary phases in different formats: surface coatings, micro/submicro particles, and monolithic beds. Besides, in the sol-gel approach, appropriate sol-gel precursors and other building blocks can be selected to create a stationary phase with desired structural and surface properties. One remarkable advantage of the sol-gel approach is the mild thermal conditions under which the stationary phase synthesis can be carried out (typically at room temperature). It also provides an effective pathway to integrating the advantageous properties of organic and inorganic material systems, and thereby enhancing and fine-tuning chromatographic selectivity of the created hybrid organic-inorganic stationary phases. This review focuses on recent developments in the design, synthesis, characterization, properties, and applications of sol-gel stationary phases in CEC.     

Packing structure and self-heating in capillary electrochromatography

The origin of bubble formation during operation of capillary electrochromatography (CEC) has been an issue of debate. Ohmic heating resulted from current passed through a packed column was proposed as the primary cause. However, this explanation has been questioned on the ground that the current measured in CEC is much lower than that measured with open-tubular separation systems where no bubble formation occurs. To resolve this issue, we carried out a theoretical study correlating self-heating of the electrolyte with packing structure of the column. We used a bundle of capillary tubes, a bundle of two types of capillary tubes and two bundles of capillary tubes connected serially to model, respectively, the flow channels in the column of non-porous particles, in the column of porous particles and in the column of various packing densities. The results from this study indicate that, for columns of homogeneous packing density, the heat output is indeed smaller than that in open-tubular columns of the same dimensions. In this case, the self-heating cannot be a key factor responsible for the bubble formation in CEC. However, for columns of heterogeneous packing density, a large excess of heat release may be produced in column sections of high packing density and, in turn, over-heating in such sections may become the primary cause for the formation of bubbles. It follows from this study that preparation of columns of homogeneous packing structure is essential to obtain reproducible and bubble-free CEC systems.     

磁场辅助毛细管电色谱

磁场辅助毛细管电色谱是液相色谱研究领域中出现的新技术．它利用外加磁场的引力将置于毛细管内的具有磁响应性的硅胶微球或四氧化三铁微球固定在管内任意位置．磁场固定微球聚集体既可用作填充柱,直接用于电色谱分离;也可用作柱筛,用于填装由商品色谱填料组成的色谱柱．这一技术的优势在于制备简便易行,柱管可以再生使用,适合于微流控芯片上柱筛或柱床的制作．本文简要评述磁场辅助毛细管电色谱的进展,包括磁性色谱填料的制备,磁场固定柱床电色谱,磁性柱筛电色谱及毛细管柱内柱结构参数的测定等方面．     

Metallomesogenic stationary phase for open-tubular capillary electrochromatography

A synthetic coppermesogenic polymer is prepared and then covalently bonded to the siloxane-based deactivated column as the stationary phases of open-tubular CEC with essentially high phase ratio. The EOF generated from the modified phase is surveyed through conventional aqueous buffers and hydroorganic mobile phases. Zeta potentials, which are computed from the EOF data and the ratio of dielectric constant to viscosity, are plotted as a function of pH, ionic molarity, and compositional range. These plots responsible for the electroosmotic characteristic of the bonded phases are found to be like those of bare fused-silica or deactivated columns through decreasing or increasing the ACN content in the mobile phase, respectively. This two-phase characteristic is basically derived from the polymeric configuration with carboxylato ligands attached onto the polysiloxane backbone. Phthalates and amino acids are suitable probes to examine the two phenomena, more-polar and less-polar mediums, respectively, and to judge whether the chromatographic retention is the major source of separation mechanism. With the mixing modes of Lewis acid-base interaction, dispersive force, and shape discrimination, the chromatographic partition adequately accomplishes the uneasily resolved separations by only CZE mode, although the electrophoretic migration is truly somewhat involved.     

Open-tubular capillary electrochromatography using a polymeric surfactant coating

A stable polyelectrolyte multilayer (PEM) coating was investigated for use in open-tubular capillary electrochromatography (o-CEC). In this approach, the PEM consisted of the cationic polymer of a quaternary ammonium salt, poly(diallyldimethylammonium chloride) and the anionic polymeric surfactant, poly(sodium undecylenic sulfate). Both the cationic and anionic polymers were physically adsorbed to the surface of a fused-silica capillary by use of a simple coating procedure. This procedure involved an alternate rinse of the positively and negatively charged polymers. The performance of the PEM coating as a dynamic stationary phase was evaluated by use of electrochromatographic experiments and showed good selectivity for both phenols and benzodiazepines. Reproducibility of the PEM coating was also evaluated by calculating the relative standard deviations (RSDs) of the electroosomotic flow (EOF). The run-to-run and capillary-to-capillary RSD values of the EOF were less than 1.5%. The endurance of the coating was more than 100 runs. The importance of the PEM coating was illustrated by comparing separations on a bare uncoated capillary with the coated capillary. In addition, the chromatographic performance using o-CEC and micellar electrokinetic chromatography (MEKC) was compared for the separation of benzodiazepines.     

亲水作用毛细管电色谱柱的研究进展

亲水作用毛细管电色谱是当前微分离技术的研究热点之一,其固定相的研究受到了广泛的关注。本文介绍了亲水作用毛细管电色谱开管柱、填充柱和整体柱的研究进展,重点对近年来发展的亲水作用电色谱整体柱的制备技术进行了系统阐述。引用文献68篇。