Enantiomeric separations by capillary electrochromatography using a macrocyclic antibiotic chiral stationary phase.

Racemic mixtures of tryptophan and dinitrobenzoyl leucine have been successfully resolved by capillary electrochromatography (CEC) using the macrocyclic antibiotic teicoplanin, covalently bonded to a 5 microns silica support. Modification of a previously published packing procedure was required to pack reliable capillaries, capable of performing enantiomeric separations. Good levels of enantioselectivity were obtained in all cases, with optimised separations being performed in less than 6 min. Retention times, resolution and reproducibility are discussed.     

分子印迹技术在毛细管电色谱中的应用

分子印迹技术是制备具有分子识别功能聚合物，即分子印迹聚合物（MIPs)的一种新技术；毛细管电色谱（CEC）是一个具有发展前途的色谱新技术。将分子印迹技术和毛细管电色谱两种新技术相结合，优势互补，具有极大的发展潜力。本文对分子印迹技术在毛细管电色谱中的应用，以及各类MIPs-CEC毛细管柱的制备方法进行了较为全面的综述，引用文献52篇。     

Enantiomer separation by nonaqueous and aqueous capillary electrochromatography on cyclodextrin stationary phases

Native beta- and gamma-cyclodextrin bound to silica (ChiraDex-beta and ChiraDex-gamma) were packed into capillaries and used for enantiomer separation by capillary electrochromatography (CEC) under aqueous and nonaqueous conditions. Negatively charged analytes (dansyl-amino acids) were resolved into their enantiomers by nonaqueous CEC (NA-CEC). The addition of a small amount of water to the nonaqueous mobile phase enhanced the enantioselectivity but increased the elution time. The choice of the background electrolyte (BGE) determined the direction of the electroosmotic flow (EOF). With 2-(N-morpholino) ethanesulfonic acid (MES) or triethylammonium acetate (TEAA) as BGE an inverse EOF (anodic EOF) was observed while with phosphate a cathodic EOF was found. The apparent pH (pH*), the concentration of the BGE, and the nature of the mobile phase strongly influenced the elution time, the theoretical plate number and the chiral separation factor of racemic analytes.     

Enantioseparations in non-aqueous capillary electrochromatography using polysaccharide type chiral stationary phases

Enantioseparations of chiral compounds with different structures were studied in non-aqueous capillary electrochromatography (NAQ CEC). Three different polysaccharide derivatives, cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel OD), amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak AD) and cellulose tris(4-methylbenzoate) (Chiralcel OJ) were used as chiral stationary phases (CSPs). Methanolic or ethanolic ammonium acetate solutions served as a mobile phase. The effect of the type of the CSP, the loading of the chiral selector on wide-pore aminopropyl derivatized silica gel and operational parameters such as apparent pH, applied voltage, etc. on the EOF and chromatographic characteristics (alpha, N, Rs) were studied. NAQ CEC represents a valuable alternative and an extension to chiral separations by HPLC with common-size columns as well as to capillary LC and CEC in aqueous buffers.     

Amylose-3,5-dimethylphenylcarbamate immobilized on monolithic silica stationary phases for chiral separations in capillary electrochromatography

Chiral monolithic silica capillary columns were prepared by immobilization of amylose-3,5-dimethylphenylcarbamate (ADMPC) bearing a small fraction of 3-(triethoxysilyl)propyl residues through intermolecular polycondensation of the triethoxysilyl groups. The obtained columns were used for chiral separations in capillary electrochromatography (CEC). The effects of the silica monolith nature and the used chiral selector concentration on the resulting enantiomeric separations were investigated. Fifteen chiral compounds, including acidic, neutral, and basic substances were evaluated and twelve showed partial or baseline separation at some of the different conditions tested. These results demonstrated the promising applicability of ADMPC-immobilized monolithic silica columns in CEC enantioseparations, but also revealed the need for further improvements on the level of baseline separations and efficiencies.     

Investigation of capillary electrochromatography with brush-type chiral stationary phases

Fused-silica capillaries (100 microm ID) were packed with the (3R, 4S)-Whelk-O chiral stationary phase (CSP) bonded on 3.0 microm silica particles. The enantiomers of 41 neutral analytes containing stereogenic centers, axes or planes were examined by packed capillary electrochromatography. More than 30 of these were cleanly resolved, owing to the selectivities and efficiencies afforded by this CSP. High reproducibility with no indication of diminished performance was observed using the same capillary for hundreds of runs (including intermediate change of the buffer system) over a period of several weeks. Acetate, 2-(N-morpholino)ethanesulfonic acid, or phosphate buffers, each modified with either acetonitrile or methanol, were used as mobile phases. The influence of buffer concentration, modifier amount, temperature, applied voltage, and pH on performance of the brush-type CSP was investigated.     

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography (CEC) is achieved with open-tubular capillaries (o-CEC), with packed capillaries (p-CEC) or with monolithic capillaries. In o-CEC, capillaries are coated with a thin film containing cyclodextrin derivatives, cellulose, proteins, poly-terguride or molecularly imprinted polymers as chiral selectors. In p-CEC, typical chiral HPLC stationary phases such as silica-bonded cyclodextrin or cellulose derivatives, proteins, glycoproteins, macrocyclic antibiotics, quinine-derived and 'Pirkle' selectors, polyacrylamides and molecularly imprinted polymers are used as chiral selectors. Chiral monolithic stationary phases prepared by in situ polymerization into the capillary were also developed for electrochromatographic enantiomer separation.     

Enantiomer separations on a vancomycin stationary phase and retention mechanism of pressurized capillary electrochromatography

Several chiral drugs, promethazine, carteolol, celiprolol, and albuterol, were resolved with vancomycin as the chiral stationary phase by pressurized capillary electrochromatography (pressurized CEC) and capillary HPLC. The effects of pressure and electrical field strength on efficiency, resolution, and capacity factor in pressurized CEC were investigated. A mathematical model describing the relationship of capacity factor in pressurized CEC with voltage, pressurized flow velocity, electroosmotic mobility, and electrophoretic mobility was established, which was in good agreement with the experimental data.     

Enantiomer separation of pharmaceuticals by capillary gas chromatography with novel chiral stationary phases

New chiral stationary phases of polydimethylsiloxane anchored with (S)-(-)-t-leucine derivatives were provided for use in enantiomer separation of pharmaceuticals by capillary gas chromatography. Fifteen pharmaceuticals were separated into their enantiomeric pairs by converting them into pentafluoropropionyl and heptafluorobutyryl derivatives. The separation factor and resolution obtained from the new phases were superior to those from the conventional Chirasil-Val capillary column.     

Enantiomeric separations of primary amino compounds by capillary electrochromatography with monolithic chiral stationary phases of chiral crown ether-bonded negatively charged polyacrylamide gels

A novel enantiomeric separation method by capillary electrochromatography with chiral crown ether-bonded negatively charged polyacrylamide gels is presented. Two kinds of chiral crown ether derivatives, (+)-tetraallyl 18-crown-6 carboxylate and (+)-18-crown-6 tetracarboxylic acid 2-allyl ester were synthesized and allowed to covalently bind to a negatively charged polyacrylamide gel, a so-called monolithic stationary phase, respectively. The gel was placed in fused-silica tubing, the walls of which had been activated with a bifunctional reagent to make the resulting gel bind covalently to the inner surface. Enantiomeric separations of 12 primary amino compounds were achieved using these columns and mobile phases of 200 mM triethanolamine-300 mM boric acid buffers with high efficiencies of up to 135000 plates m(-1). Both the within- and between-run reproducibilities of retention time and separation factor were good. The reproducibilities of retention time and separation factor for three different columns prepared from a different batch of monomers were acceptable. The gel-filled capillaries were stable for at least 13 months with intermittent use for 3 months followed by storage at room temperature for 10 months. The result of the optical purity test of alanine-2-naphthylamide is also described.     

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.     

Polar stationary phases for capillary electrochromatography

This review article summarizes the variety of polar stationary phases that have been employed for capillary electrochromatographic separations. Compared with reversed-phase stationary phases, the polar alternatives provide a completely different retention selectivity towards polar and charged analytes. Different types of polar stationary phases are reviewed, including the possible retention mechanisms. Electrochromatographic separations of polar solutes, peptides, and basic pharmaceuticals on polar stationary phases are presented.     

Enantiomeric separations of cationic and neutral compounds by capillary electrochromatography with monolithic chiral stationary phases of beta-cyclodextrin-bonded negatively charged polyacrylamide gels

Enantiomeric separation by capillary electrochromatography with beta-cyclodextrin-bonded negatively charged polyacrylamide gels was examined. The columns used are capillaries filled with a negatively charged polyacrylamide gel, a so-called monolithic stationary phase, to which allyl carbamoylated beta-CD (AC-beta-CD) derivatives covalently bind. The capillary wall is activated first with a bifunctional reagent to make the resulting gel bind covalently to the inner surface of the fused-silica tubing. Enantiomeric separations of 15 cationic compounds were achieved using the above-mentioned columns and mobile phases of 200 mmol l(-1) Tris-300 mmol I(-1) boric acid buffer (pH 7.0 or 9.0) or 200 mmol l(-1) Tris-300 mmol l(-1) boric acid buffer (pH 7.0) containing an achiral crown ether (18-crown-6). Enantiomeric separations of two neutral compounds were also achieved using 200 mmol l(-1) Tris-300 mmol l(-1) boric acid buffer (pH 9.0) as a mobile phase. High efficiencies of up to 150,000 plates m(-1) were obtained. Both the within- and between-run reproducibilities of retention time and separation factor were good. The reproducibilities of retention time and separation factor for three different columns prepared from a different batch of monomers were acceptable. The gel-filled capillaries were stable for at least 3 months with intermittent use, utilizing the mobile phase of 200 mmol I(-1) Tris-300 mmol I(-1) boric acid buffer (pH 9.0).     

Enantiomer resolution screening strategy using multiple immobilised polysaccharide-based chiral stationary phases

CHIRALPAK IA, CHIRALPAK IB and CHIRALPAK IC are a new generation of chiral packing materials for resolution of enantiomers by chromatography. They are all prepared by chemical immobilisation of the respective polysaccharide derivatives onto a supporting silica matrix. The present article aims to establish an understanding of the global enantioselective profiles of this series of chiral stationary phases (CSPs). When used in combination, high success rates for enantiomer resolution could be achieved. These three CSPs exhibit an exceptional level of complementary chiral recognition characteristics by applying common screening and method optimisation strategies.     

Enantiomers separation by capillary electrochromatography using polysaccharide‐based stationary phases

The separation of chiral compounds is an interesting and important topic of research because these compounds are involved in some biological processes, fundamentally in human health. Among the various application fields where enantiomers are remarkable, drug analysis has to be considered. Most of the drugs contain enantiomers and very often one of the two isomers could be pharmacologically more active or even dangerous. Therefore, the separation of these compounds is very important. Among the different analytical techniques usually employed, capillary electrochromatography has demonstrated great capability in enantiomers resolution. The great potential of this electromigration technique stands mainly in its high efficiency due to the use of an electrosmotic flow (flat flow profile) and on the high selectivity because of the use of a stationary phase. Chiral separation can be obtained utilizing several chiral stationary phases including a polysaccharide derivative. The aim of this review paper is to summarize the main features of capillary electrochromatography and polysaccharide derivatives of chiral stationary phase. It also report examples of practical applications utilizing this approach.     

Enantiomer separation by gas chromatography on cyclodextrin chiral stationary phases

Fused silica capillary columns coated with several alkyl or acyl cyclodextrin derivatives, especially those of α- and β-cyclodextrins, are suitable for the enantiomer separation of a wide variety of volatile compounds of different molecular size and functionality. Positional isomers and more than 250 pairs of optical isomers have been resolved, including chiral hydrocarbons, acetals, ethers, epoxides, carbonates, lactones, esters, acids, ketones, aldehydes, alcohols, halocarbons, and also nitrogen-and sulfur-containing compounds. The physical properties of the cyclodextrin derivatives, even those obtained as viscous fluids, could be improved by dissolving them in polysiloxane liquid phases commonly used for GLC.     

Polymeric monolithic stationary phases for capillary electrochromatography

This review summarizes the contributions of a number of groups working in the rapidly growing area of monolithic columns for capillary electrochromatography (CEC), with a focus on those prepared from synthetic polymers. Monoliths have quickly become a well-established stationary phase format in the field of CEC. The simplicity of their in situ preparation method as well as the good control over their porous properties and surface chemistries make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials. A wide variety of approaches as well as materials used for the preparation of the monolithic stationary phases are detailed. Their excellent chromatographic performance is demonstrated by numerous separations of different analytes.     

Polymer-brush stationary phases for open-tubular capillary electrochromatography

Synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes from the inside of silica capillaries by surface-initiated atom transfer radical polymerization (ATRP) yields unique stationary phases for open-tubular capillary electrochromatography (OT-CEC). Although PHEMA brushes have only a small effect on the separation of a set of phenols and anilines, derivatization of PHEMA with ethylenediamine (en) allows baseline resolution of several anilines that co-elute from bare silica capillaries. Derivatization of PHEMA with octanoyl chloride (C8-PHEMA films) affords even better resolution in the separation of a series of phenols and anilines. Increasing the thickness of C8-PHEMA coatings by a factor of 2 enhances resolution for several solute pairs, presumably because of an increase in the effective stationary phase to mobile phase volume ratio. Thus, this work demonstrates that thick polymer brushes provide a tunable stationary phase with a much larger phase ratio than is available from monolayer wall coatings. Through appropriate choice of derivatizing reagents, these polymer brushes should allow separation of a wide range of neutral molecules as well as compounds with similar electrophoretic mobilities.     

Monolithic enantiomer-selective stationary phases for capillary electrochromatography

Monolithic materials have become a well-established format for stationary phases in the field of capillary electrochromatography. Four types of monoliths, namely particle-fixed, silica-based, polymer-based, and molecularly imprinted monoliths, have been utilized as enantiomer-selective stationary phases in CEC. This review summarizes recent developments in the area of monolithic enantiomer-selective stationary phases for CEC. The preparative procedure and the characterization of these columns are highlighted. In addition, the disadvantages and limitations of different monolithic enantiomer-selective stationary phases in CEC are briefly discussed.     

Separation strategy for acidic chiral pharmaceuticals with capillary electrochromatography on polysaccharide stationary phases

The effect of five factors on the capillary electrochromatographic enantioseparation of acidic compounds was studied using an experimental design. The studied factors were pH, acetonitrile content in the mobile phase, temperature, buffer concentration, and applied voltage. These experiments allowed defining a generic separation strategy applicable on acidic compounds with chemical and structural diversity. The starting screening conditions consist of a 45 mM ammonium formate electrolyte at pH 2.9 mixed with 65% acetonitrile, an applied voltage of 15 kV, and a temperature of 25 degrees C. The screening phase occasionally can be followed by an optimization procedure. Evaluation of the proposed strategy pointed out that it allows achieving baseline resolution within a relatively short time when a beginning of separation is obtained at the starting conditions. This strategy revealed enantioselectivity for 11 compounds out of 15, of which 10 could be baseline-separated after the proposed optimization steps.