Evaluation of the enantioselectivity of carbon nanoparticles‐modified chiral separation systems using dextrin as chiral selector by capillary electrokinetic chromatography

Nanoparticles (NPs) can be used as pseudostationary phases (PSPs) in EKC, which is similar to the use of micelle additives as applied in MEKC. To date, the use of NPs to enhance enantiomeric separation by EKC with β‐CD or its derivative as chiral selector has been reported only in two papers. However, to the best of our knowledge, there has been no prior effort to use NPs for achieving enantioseparation with polysaccharides as chiral selector. This paper describes for the first time the use of carbon nanoparticles (CNPs) as PSPs to modify chiral separation system employing dextrin as chiral selector for the enantioseparations of several basic drugs in capillary EKC. Three different types of CNPs, including carbogenic nanoparticles (NPs), carboxylated single‐walled carbon nanotubes, and carboxylated multiwalled carbon nanotubes, were used as running buffer additives, respectively. The potential of the PSPs and the effects of dextrin concentration, buffer pH, and buffer concentration on the enantioseparations were evaluated. Four pairs of tested enantiomers were successfully resolved in less than 15 min with the resolution values in the range of 1.41–4.52 under optimized conditions. Compared to the buffer without NPs, the introduction of NPs into the buffer enhanced the separation of the enantiomers.     

Surface‐Assisted Laser Desorption/Ionization Mass Spectrometric Detection of Biomolecules by Using Functional Single‐Walled Carbon Nanohorns as the Matrix

A surface‐assisted laser desorption/ionization time‐of‐flight mass spectrometric (SALDI‐TOF MS) method was developed for the analysis of small biomolecules by using functional single‐walled carbon nanohorns (SWNHs) as matrix. The functional SWNHs could transfer energy to the analyte under laser irradiation for accelerating its desorption and ionization, which led to low matrix effect, avoided fragmentation of the analyte, and provided high salt tolerance. Biomolecules including amino acids, peptides, and fatty acids could successfully be analyzed with about 3‐ and 5‐fold higher signals than those obtained using conventional matrix. By integrating the advantages of SWNHs and the recognition ability of aptamers, a selective approach was proposed for simultaneous capture, enrichment, ionization, and MS detection of adenosine triphosphate (ATP). This method showed a greatly improved detection limit (1.0 μM ) for the analysis of ATP in complex biological samples. This newly designed protocol not only opened a new application of SWNHs, but also offered a new technique for selective MS analysis of biomolecules based on aptamer recognition systems.     

分子印迹聚合物颗粒在毛细管电色谱中的应用

依据分子印迹技术(MIT)制备的分子印迹聚合物(MIP)颗粒对模板分子及其结构类似物具有特异性识别和选择性吸附作用,同时具有较大的比表面积和快速的传质动力学特性,因而被广泛用作液相色谱固定相和固相萃取材料。将MIP颗粒作为固定相应用于毛细管电色谱(CEC),结合了CEC的快速、高效和MIP的高亲和性、高选择性的特点,成为分析科学领域最具有发展前景的分离技术之一。MIP颗粒在CEC领域有几种不同的应用形式: 作为填充材料填充到毛细管柱中;作为嵌入材料嵌入到毛细管柱内部不同基质的骨架中;作为准固定相添加到CEC运行缓冲溶液中。本文综述了近几年MIP颗粒在CEC领域应用的发展,对该领域今后的发展前景进行了展望。     

Use of nanoparticles in capillary and microchip electrochromatography

Applications of nanoparticles are of rising interest in separation science, due to their favorable surface-to-volume ratio as well as their applicability in miniaturization. A stationary phase with large surface area in combination with an electroosmotic flow-driven system has great potential in a highly efficient separation system. This review covers the use of various nanoparticles as stationary or pseudostationary phase in capillary and microchip electrochromatography. The use of nanoparticles in pseudostationary phase capillary electrochromatography and open-tubular capillary electrochromatography are thoroughly discussed. The stationary and pseudostationary phases that are described include polymer nanoparticles, gold nanoparticles, silica nanoparticles, fullerenes and carbon nanotubes.     

Past,present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.     

Marker compounds for the determination of retention factors in EKC

EKC and its sub‐techniques, such as MEKC and microemulsion EKC, have attracted wide interest in recent years. Investigations on this topic have covered several analytical applications, but attention has also been paid more and more to basic studies. This review provides an overview of the different approaches to calculating retention factors, which express the ratio of the amount of sample component in the pseudostationary and mobile phases. Special attention is given to the selection of markers for the determination of the electrophoretic mobility or migration time of a marker describing the behavior of the pseudostationary phase in EKC. Introduction of a hydrophobic marker is by far the most common approach, but the use of a homologous series of compounds is also quite popular. In addition, other possible approaches found in the literature will be described.     

CEC and EKC of natural compounds

In this review, an overview of CEC and EKC methods with their developments are summarized for different natural compounds. It is divided into three main parts. The first part elaborates the separation of lipophilic compounds without any charged groups. The second part constitutes CEC and EKC of lipophilic compounds containing ionizable functional groups whereas the third part contains hydrophilic compounds. Packed, monolithic, coated, or raw fused-silica (FS) capillaries are among the choice for stationary phases. Applications of these phases on the above-mentioned three classes of compounds, coupled with different detection methods, e.g. MS or LIF, are explored and their advantages and disadvantages are discussed.     

Determination and regulation of the migration window in electrokinetic chromatography

The determination of the velocities of the mobile and the pseudostationary phases (the migration (time) window) is mandatory for the determination of physicochemical properties by electrokinetic chromatography (EKC). This review offers a detailed discussion on the definition, the importance, the determination and the regulation of the migration (time) window in EKC. An overview on the theoretical treatment of chromatographic processes in EKC is given defining EKC in comparison to the term capillary electrophoresis. Methods to determine and influence the migration window are discussed with emphasis on measures that have been taken to modify the electroosmotic flow velocity. Pseudostationary phases (or separation carriers) that are taken into consideration are anionic and cationic micelles, mixed micelles, microdroplets (microemulsions), polymeric pseudostationary phases and dendrimers.     

分子印迹聚合物在毛细管电色谱拆分手性药物中的研究进展

手性药物通过与生物体内生物大分子之间的手性匹配与分子识别来发挥药理作用。两个对映体与体内手性环境相互作用的不同导致每个对映体表现出不同的药理活性、代谢过程、代谢速率及毒性等药代动力学特征。因此发展手性药物的拆分方法,对于手性药物的开发和生产过程的质量监控具有重要意义。分子印迹聚合物（MIPs）是以目标分子作为模板而制备的高分子聚合物,它具有特定的空间分子结构和官能团,对目标分子具有高度的特异性识别能力。基于该特点,MIPs非常适合于手性药物的拆分和纯化。毛细管电色谱（CEC）可同时基于毛细管电泳和液相色谱的分离机理对目标物进行分离,因此具有高分离效率和高选择性的特点。将MIPs材料作为CEC的固定相,可将这两种技术的优势结合,从而实现对手性药物的高效拆分。MIPs材料在1994年首次应用于CEC手性拆分,此后该研究领域开始获得关注和发展。MIPs材料主要通过4种模式在CEC中实现手性拆分,分别是作为开管柱、填充柱和整体柱的固定相以及分离介质中的准固定相。该综述以这4种模式作为分类基准,根据MIPs制备所需的材料和分离对象对其在CEC手性拆分中的应用进行了总结,揭示了MIPs在CEC手性拆分中的潜力,同时评述了这4种模式各自的优势与不足,并对将来MIPs在CEC手性拆分中的发展进行了展望。     

Nanoparticle-based pseudostationary phases in capillary electrochromatography

During the past decades, research has been performed to enhance selectivity in CE by introducing different types of additives into the electrolyte. Research concerning this has taken many directions, especially during the last 5 years. A promising technique, which benefits from no packing or frits, is to use nanoparticles as the pseudostationary phase (PSP) in CEC. PSPs have the advantage of introducing a novel interaction phase for every analysis, which greatly simplify column exchange and circumvent contamination inherited from complex mixtures, e.g., biological samples. The field of nanoparticle-based PSPs used in CEC is covered in this review. The term CEC will be used consequently throughout this review, although some authors used the term EKC to categorize their work. Important requirements for the nanoparticles used and possible reasons for band broadening will be discussed. Applications with silica nanoparticles, polymer nanoparticles, molecularly imprinted polymer nanoparticles, gold nanoparticles, dendrimers, and polymeric surfactants as PSP will also be discussed.     

基于碳纳米材料的毛细管电色谱固定相研究进展

碳纳米材料由于其具有独特的纳米结构、大的比表面积、较强的热稳定性、良好的导电性以及较好的吸附性能等物理化学性质,因而在分析科学、生命科学、材料科学及环境科学等领域得广泛的应用.结合国内外最新文献,对近5年来碳纳米材料在毛细管电色谱新型固定相的制备研究方面进展进行了评述,包括毛细管电色谱的分类及分离机理、毛细管电色谱柱的制备方法和优缺点,碳纳米材料(石墨烯、碳纳米管、氧化石墨烯、还原氧化石墨烯、富勒烯)的结构性质及制备方法、碳纳米材料在毛细管电色谱柱固定相中的应用及作用机理等,并对其在色谱应用领域的方向进行了展望.     

Fabrication of single‐walled carbon nanohorns incorporated a monolithic column for capillary electrochromatography

Single‐walled carbon nanohorns have received great interest for their unique properties and diverse potential applications. Herein, we demonstrated the feasibility of single‐walled carbon nanohorns incorporated poly(styrene‐divinylbenzene) monolith as the stationary phase for capillary electrochromatography, which were prepared by one‐step in situ copolymerization. Single‐walled carbon nanohorns were dispersed in styrene to give a stable and homogeneous suspension. The monolithic column gave effective separation for a wide range of aromatic compounds, which was based on hydrophobicity and π–π electrostatic stacking of single‐walled carbon nanohorns. The precisions of migration time and peak area varied in the ranges of 1.4–1.9% for intraday trials and 1.7–3.5% for interday trials, and 3.2–6.7% for intraday trials and 4.1–7.4% for interday trials, and 3.6–7.2% for inter‐column trials and 5.2–21.3% for inter‐column trials, respectively, indicating the good reproducibility of single‐walled carbon nanohorns embedded monolithic columns.     

Synthesis of a polyrotaxane-based macroporous polymer as stationary phase for capillary electrochromatography via host-guest complexation of N,N '-ethylenedianilinediacrylamide with statistically methylated beta-cyclodextrin

A synthetic route to acrylamide-based monolithic stationary phases for CEC with rotaxane-type immobilized derivatized beta-CD was explored. N,N'-Ethylenedianilinediacrylamide was synthesized as the water-insoluble crosslinker forming water-soluble inclusion complexes with statistically methylated beta-CD. Mixed-mode stationary phases were synthesized by free radical copolymerization of the bisacrylamide-CD host-guest complex with water-soluble monomers and an additional water-soluble crosslinker in aqueous solution. Complex formation in solution and inclusion of the pseudorotaxane into the polymeric network (formation of a polyrotaxane architecture) were studied by means of (1)H-NMR chemical shift analysis, CD modified micellar EKC (CD-MEKC), 2D-NOESY spectroscopy, and solid state( 13)C-NMR spectroscopy. The presence of a mixed-mode selectivity of the stationary phase based on hydrophobic and hydrophilic interaction was confirmed by CEC with neutral polar and nonpolar solutes.     

Selectivity of single, mixed, and modified pseudostationary phases in electrokinetic chromatography

The selectivity of a compilation of single, mixed, and modified EKC pseudostationary phases, described in the literature and characterized through the solvation parameter model, is analyzed. Not only have micellar systems of different nature been included but also microemulsions, polymeric, and liposomial phases. In order to compare the systems, a principal component analysis of the coefficients of the solvation equation is performed. From this analysis, direct information of the system properties, differences in selectivity, as well as evidence of lack of accuracy in some system characterizations are obtained. These results become a very useful tool to perform separations with mixtures of surfactants, since it is possible to know which mixtures will provide a greater selectivity variation by changing only the composition of the pseudostationary phases. Furthermore, the variation of the selectivity of some mixtures, as well as the effect of the addition of organic solvents on selectivity, is also discussed.     

Advances in chiral separation using capillary electromigration techniques

This review gives an overview of recent developments in CZE, EKC, and CEC covering the literature since the year 2004. Since there appeared a special issue on applications, this review focuses on the progress in electromigration techniques and new methodological developments. New techniques, new chiral selectors as well as new chiral stationary phases for CEC are discussed.     

New Pseudostationary Phases for Electrokinetic Chromatography: A High-Molecular Surfactant and Proteins

To extend the applicability of electrokinetic chromatography (EKC), two new types of pseudostationary phases have been introduced. A high-molecular surfactant, butyl acrylate/butyl methacrylate/methacrylic acid copolymer (BBMA) is employed as a micellar forming surfactant for miccllar electrokinetic chromatography (MEKC). The critical micelle concentration of BBMA is essentially zero, which means the micellar concentration is constant irrespective of temperature and buffer. Some characteristic features of BBMA as the pseudostationary phase for MEKC is investigated in comparison with conventional ionic surfactants. Ovomucoid and avidin, which are proteins isolated from egg white, have been found to be useful chiral selectors in affinity EKC. A few examples of the separation of enantiomers with these proteins are shown.     

Surfactant-mediated capillary electrochromatography with octadecyl-silica- packed capillary columns for the separation of nonpolar compounds. Case of pyrethroid insecticides

Capillary electrochromatography (CEC) with octadecyl-silica-packed capillary columns was evaluated in the separation of nonpolar compounds, e.g., pyrethroid insecticides, using surfactant-rich mobile phases. This novel concept is referred to as surfactant-mediated capillary electrochromatography (SM-CEC), and is based on including a charged surfactant, namely sodium di-2-ethylhexyl sulfosuccinate (DOSS), in the mobile phase. Under these conditions, DOSS plays the role of a slowly moving pseudostationary phase so that solutes are partitioned between a mobile phase, a fixed stationary phase and a slowly moving pseudostationary phase. The SM-CEC system was investigated with pyrethroid insecticides over a wide range of DOSS and acetonitrile concentrations in the mobile phase. Pyrethroid insecticides, which are very hydrophobic solutes consisting of geometric isomers and diastereomers, were better resolved in SM-CEC than in straight CEC.     

Recent progress in the use of ionic polymers as pseudostationary phases for EKC

This review concerns the introduction, characterization, and application of polymeric pseudostationary phases (PSPs) for EKC since 2004. Achiral and chiral polymers and separations are reviewed, as is the application of polymeric PSPs for the combination of EKC with mass spectrometric detection.     

Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

This review summarizes recent developments in chiral separation in capillary zone electrophoresis (CZE), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC) covering literature published since the year 2000. New chiral selectors and innovative approaches for CE and CEC are introduced. Recent progress in column technology for CEC is highlighted and the development of new chiral stationary phases is discussed. This review is not dedicated to list applications but will focus on new developments.