Recent progress in enantiomer separation by capillary electrochromatography

In situ synthesis of a spherical covalent organic framework as a stationary phase for capillary electrochromatography

Covalent organic frameworks (COFs) are a novel type of crystalline porous organic polymer materials recently developed. It has several advantages in chromatographic separation field, such as high thermal stability, porosity, structural regularity, and large specific surface area. Here, a novel spherical COF 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 2,5-bis(2-propyn-1-yloxy)-1,4-benzenedicarboxaldehyde (BPTA) was developed as an electrochromatographic stationary phase for capillary electrochromatography separation. The COF TAPB-BPTA modified capillary column was fabricated via a facile in situ growth method at room temperature. The characterization results of scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) confirmed that COF TAPB-BPTA were successfully modified onto the capillary inner surface. The electrochromatography separation performance of the COF TAPB-BPTA modified capillary was investigated. The prepared column demonstrated outstanding separation performance toward alkylbenzenes, phenols, and chlorobenzenes compounds. Furthermore, the baseline separations of non-steroidal anti-inflammatory drugs (NSAIDs) and parabens with good efficiency and high resolution were achieved. Also, the prepared column possessed satisfactory precision of the intra-day runs (n = 5), inter-day runs (n = 3), and parallel columns (n = 3), and the relative standard deviations (RSDs) of the retention times of tested alkylbenzenes were all less than 2.58%. Thus, this new COF-based stationary phase shows tremendous application potential in chromatographic separation field.

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COF TAPB–BPTA was studied as OT-CEC stationary phase.

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In situ, room-temperature growth method was quite facile and efficient.

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Excellent separation performances toward various hydrophobic compounds.

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The maximum column efficiency was 1.78 × 10⁵ plates/m.

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Reproducibility and stability were found to be satisfactory.

Chiral stationary phases and applications in gas chromatography

Chiral compounds are ubiquitous in nature and play a pivotal role in biochemical processes, in chiroptical materials and applications, and as chiral drugs. The analysis and determination of the enantiomeric ratio (er) of chiral compounds is of enormous scientific, industrial, and economic importance. Chiral separation techniques and methods have become indispensable tools to separate chiral compounds into their enantiomers on an analytical as well on a preparative level to obtain enantiopure compounds. Chiral gas chromatography and high-performance liquid chromatography have paved the way and fostered several research areas, that is, asymmetric synthesis and catalysis in organic, medicinal, pharmaceutical, and supramolecular chemistry. The development of highly enantioselective chiral stationary phases was essential. In particular, the elucidation and understanding of the underlying enantioselective supramolecular separation mechanisms led to the design of new chiral stationary phases. This review article focuses on the development of chiral stationary phases for gas chromatography. The fundamental mechanisms of the recognition and separation of enantiomers and the selectors and chiral stationary phases used in chiral gas chromatography are presented. An overview over syntheses and applications of these chiral stationary phases is presented as a practical guidance for enantioselective separation of chiral compound classes and substances by gas chromatography.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Advances in the enantioseparation of β-blocker drugs by capillary electromigration techniques

β-Blocker drugs or β-adrenergic blocking agents are an important class of drugs, prescribed with great frequency. They are used for various diseases, particularly for the treatment of cardiac arrhythmias, cardioprotection after myocardial infarction (heart attack), and hypertension. Almost all β-blocker drugs possess one or more stereogenic centers; however; only some of them are administered as single enantiomers. Since both enantiomers can differ in their pharmacological and toxicological properties, enantioselective analytical methods are required not only for pharmacodynamic and pharmacokinetic studies but also for quality control of pharmaceutical preparations with the determination of enantiomeric purity. In addition to the chromatographic tools, in recent years, capillary electromigration techniques (CE, CEC, and MEKC) have been widely used for enantioselective purposes employing a variety of chiral selectors, e.g. CDs, polysaccharides, macrocyclic antibiotics, proteins, chiral ion-paring agents, etc. The high separation efficiency, rapid analysi,s and low consumption of reagents of electromigration methods make them a very attractive alternative to the conventional chromatographic methods. In this review, the development and applications of electrodriven methods for the enantioseparation of β-blocker drugs are reported. The papers concerning this topic, published from January 2000 until December 2010, are summarised here. Particular attention is given to the coupling of chiral CE and CEC methods to MS, as this detector provides high sensitivity and selectivity.

Characterization of binding affinities in a chromatographic system by suspended state HR/MAS NMR spectroscopy

In the current work a racemate of (R)- and (S)-benzylmandelate was separated with a stereoselective polysaccharide-based chiral stationary phase by HPLC. To elucidate the occurring chiral molecular recognition processes in the heterogeneous system used, NMR spectroscopy was chosen under high resolution/magic angle spinning (HR/MAS) NMR conditions in the suspended state. Therefore, and as a proof of concept, a combination of several NMR methods such as spin-lattice relaxation time (T(1)) measurements (T(1)), the saturation transfer difference, and the 2D experiment of the transferred nuclear overhauser enhancement spectroscopy technique were applied. With HR/MAS NMR it is feasible to combine NMR and chromatography to achieve further insights into the separation process.

Effect of the nature of the single-isomer anionic CD and the BGE composition on the enantiomeric separation of beta-blockers in NACE

The separation of ten beta-blockers has been investigated in NACE systems using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-CD (HDMS-beta-CD) and heptakis(2,3-di-O-acetyl-6-O-sulfo)-beta-CD (HDAS-beta-CD). The influence on enantioresolution, mobility difference and selectivity of the nature of both anionic CD and BGE anion as well as their concentrations were studied by means of a multivariate approach. A D-optimal design with 25 experimental points was applied. For all studied analytes, the enantiomeric resolution was shown to be significantly influenced by both CD nature and concentration. Except for two compounds, HDAS-beta-CD was found to give higher enantioresolution values than HDMS-beta-CD. The best enantioseparation for all compounds was achieved in the presence of a high chiral selector concentration and for most of them at a low BGE anion concentration. For each investigated compound, operating conditions leading to the best enantiomeric resolution were deduced. A generic NACE system was then recommended, namely 10 mM ammonium acetate and 40 mM HDAS-beta-CD in methanol acidified with 0.75 M formic acid. This generic system was able to completely resolve the enantiomers of all beta-blockers, with a R(s) value of at least 4. Finally, the optimal conditions obtained modelling resolution, mobility difference and selectivity were compared.

Recent accomplishments in the field of enantiomer separation by CEC

The present review intends to summarize recent developments in the field of enantioselective separations and analysis by CEC. It covers studies published in English language in common peer-reviewed journals within the period between 2003 and 2006. Both, methods making use of chiral mobile phase additives as well as chiral stationary phases for electrochromatographic enantiomer separations, are reviewed. Achievements that have been made on the various column technologies, such as open-tubular, particle-packed, inorganic, organic and particle-fixed (hybrid-type) monolithic as well as molecularly imprinted polymer phases, are discussed.

Preparation of fritless capillary using avidin immobilized magnetic particles for electrochromatographic chiral separation

In capillary electrochromatography (CEC), magnetic particles (MPs) were packed in a fused silica capillary by using the magnetic field to be retained without frits. For a chiral CEC separation, avidin was immobilized onto the surface of the MPs (AVI-MPs) as a stationary phase by using the physical adsorption technique. The injected AVI-MPs into the capillary were stably captured with the magnet (surface magnetic flux density, 250 mT) under the separation voltage of 10 kV (190 V/cm). By employing the fritless AVI-MPs packed capillary, the chiral separation of ketoprofen was successfully attained with the packing length of only 5 cm. Effects of the modification condition of avidin, pH of background solution, and the packing length on the enantioseparation were also investigated. Under the optimal condition, furthermore, the repeatability for the retention time of ketoprofen was better than 1.5% in the relative standard deviation and the capillary-to-capillary reproducibility was also acceptable in the prepared fritless capillaries.

Supramolecular systems-based extraction-separation techniques coupled to mass spectrometry

The combination of supramolecular chemistry and MS has not only been fruitful in the field of gas-phase fundamental studies of host-guest complexes and supramolecular assemblies. Mass spectrometric analysis has also benefited from the ability of supramolecular systems to behave as pseudophases in which solutes partition from the bulk solvent phase. Supramolecular systems-based extraction and concentration schemes and separation techniques have been widely used in different fields of analytical chemistry and are ideally suited for coupling with MS. This review describes the present status of the application of supramolecular chemistry in mass spectrometric analysis and includes topics such as the use of coacervative liquid-liquid extraction and hemimicelle/admicelle-based SPE of organic compounds prior to chromatography and electrophoresis. It also discusses the recent advances in enantioselective analysis using CD in electrophoresis- and chromatography-MS. The potential and analytical challenges of these approaches in environmental and bioanalytical chemistry, where one can expect significant developments in the future, are outlined.

Perphenylcarbamoylated β-cyclodextrin bonded-silica particles as chiral stationary phase for enantioseparation by pressure-assisted capillary electrochromatography

Perphenylcarbamoylated beta-cyclodextrin bonded-silica particles (5 microm) were packed into 75-mum fused-silica capillaries, and used for the enantiomer separation of neutral and basic solutes by pressure-assisted capillary electrochromatography. Triethylammonium acetate and phosphate buffer were employed as the BGEs. A cathodic EOF was observed with these two BGEs. Seven chiral analytes were successfully resolved into their enantiomers under optimized conditions, and five of them could be baseline-separated within 12 min due to their high electrophoretic mobility. Better results were achieved with phosphate buffer as the BGE. The effects of organic content and pH on the enantioseparation were also investigated.

Sensitive chiral analysis by capillary electrophoresis

In this review, an updated view of the different strategies used up to now to enhance the sensitivity of detection in chiral analysis by CE will be provided to the readers. With this aim, it will include a brief description of the fundamentals and most of the recent applications performed in sensitive chiral analysis by CE using offline and online sample treatment techniques (SPE, liquid-liquid extraction, microdialysis, etc.), on-column preconcentration techniques based on electrophoretic principles (ITP, stacking, and sweeping), and alternative detection systems (spectroscopic, spectrometric, and electrochemical) to the widely used UV-Vis absorption detection.

Comparison of vancomycin-based stationary phases with different chiral selector coverage for enantioselective separation of selected drugs in high-performance liquid chromatography

Two vancomycin-based chiral stationary phases (CSPs) with different coverage of the chiral selector vancomycin (Chirobiotic V and Chirobiotic V2) were compared. beta-Blockers and profens, as structurally diverse groups of drugs, were chosen as analytes. Retention and enantioseparation of beta-blockers were studied in reversed-phase (RP) and polar-organic (PO) separation modes. Higher retention and better enantioresolution were obtained on the CSP with higher coverage of vancomycin in the both separation modes. Baseline separation of four beta-blockers (eight enantiomers) in the PO mode was achieved on the Chirobiotic V2 column within 15 min. The enantioseparation of profens did not bring so excellent and easy to interpret results. Higher retention of profens on the Chirobiotic V2 column was not always accompanied by an improvement of their chiral separation in the RP mode. The polar-organic mode was not suitable for these derivatives at all. The most interesting result was obtained with flobufen; its chiral center is further away from the rigid part of the molecule, which mostly causes difficulties in enantioselective recognition. Nevertheless, the enantiomers of flobufen were shown to be much better (baseline) resolved on the CSP with lower coverage of the chiral selector (Chirobiotic V).

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.

High-performance liquid chromatographic enantioseparations on monolithic silica columns containing a covalently attached 3,5-dimethylphenylcarbamate derivative of cellulose

Covalent immobilization of 3,5-dimethylphenylcarbamate derivative of cellulose was performed in situ onto native silica monoliths cladded in a 50 mm x 4.6 mm polyether ether ketone high-performance liquid chromatographic (HPLC) column. The covalent attachment of cellulose derivative in the range of 16-19% (w/w) was performed via an epoxide moiety. The column obtained by this technique combines the high enantiomer-resolving ability of the polysaccharide derivative with favourable dynamic properties of monolithic HPLC columns. The covalent attachment of the cellulose derivative enables this column to be used in combination with the mobile phases which are incompatible with coated-type polysaccharide columns due to solubility of chiral selector in some organic solvents.

NMR studies of chiral recognition by cyclodextrins

Chiral recognition by cyclodextrins is of considerable importance, especially for pharmaceutical industry, in view of the possible side effects of the second enantiometer of chiral drugs. In general, it manifests itself in all NMR parameters (chemical shifts, coupling constants, NOE and ROE effects, and relaxation rates) on one hand. On the other hand, it allows one to determine the thermodynamic parameters characterizing diastereomeric complexes formed by cyclodextrins with enantiomeric guests. After an introduction and a general discussion of NMR manifestations of chiral recognition by cyclodextrin, the existing literature data on this problem will be discussed herein. Chirality 16:90-105, 2004.

Synthesis, complete characterization, and enantioselective electrokinetic separation of functionalized ruthenium complex enantiomers

Electrokinetic chromatography was employed to separate the enantiomers of two novel functionalized ruthenium(II) complexes with different polypyridyl coordination spheres. The use of anionic carboxymethyl-beta-cyclodextrin as chiral mobile phase additive resulted in maximum efficiency and resolution for the enantiomer separation of both transition metal complexes. The syntheses of the [4-(3-hydroxypropyl)-4'-methyl-2,2'-bipyridine]-bis(2,2'-bipyridine)rethenium(II)-bis(tetrafluoroborate) and [4-(3-hydroxypropyl)-4'-methyl-2,2'-bipyridine]-bis(4,4'-dimethyl-2,2'-bypyridine)ruthenium(II)-bis(tetrafluoroborate) complexes and their complete characterization by means of two-dimensional (1)H and (13)C[(1)H] NMR techniques ((1)H-(1)H COSY and (1)H-(13)C HMQC) as well as elemental analyses and MALDI-TOFMS are described in detail. The functionalized complexes can be used as building blocks for further reactions with polymers, biopolymers, surfaces and nanoparticles.

On-line coupling of packed capillary electrochromatography with coordination ion spray-mass spectrometry for the separation of enantiomers

Pressure-supported packed capillary electrochromatography (CEC) and packed capillary high-performance liquid chromatography (pHPLC) have been coupled on-line to electrospray ionization-mass spectrometry (ESI-MS) and coordination ion spray-mass spectrometry (CIS-MS). Separation of enantiomers of barbiturates and chlorinated alkyl phenoxypropanoates were performed on a permethylated beta-cyclodextrin stationary phase by pressure-supported CEC. For on-line detection with ESI- and CIS-MS, a modified sheath-liquid interface was used. CIS-MS is a universal, novel ionization technique which improves the selectivity as well as the sensitivity. Charged complexes were formed through the addition of central complexing ions such as silver(I), cobalt(II), copper(II), and lithium(I) to the sheath flow. Advantages of CIS-MS detection compared to the ESI-MS mode are discussed. In the CIS-MS mode, increased sensitivity and high selectivity was attained through different possibilities of complexation. The superiority of pressure-supported CEC compared to pHPLC in the hyphenation with CIS-MS is demonstrated.

Liquid-phase microextraction combined with capillary electrophoresis, a promising tool for the determination of chiral drugs in biological matrices

A disposable device for liquid-phase microextraction (LPME) based on porous polypropylene hollow fibres has recently been introduced. In the present paper, LPME was combined with capillary electrophoresis (CE) and the combination was for the first time evaluated for chiral determination of drugs in biological matrices. The chiral antidepressant drug mianserin was selected as model compound. The mianserin enantiomers were extracted from 0.5 ml of plasma added internal standard and made alkaline with 0.25 ml of 2 M NaOH. The unionised analytes were extracted into di-n-hexyl ether impregnated in the pores of the hollow fibre, and into an acidic solution inside the hollow fibre. This resulted in a three-phase system where the extracts were aqueous, and hence directly compatible with the CE system. Efficient sample clean-up was seen and the extraction recovery was 80% for both enantiomers. Discrimination between the enantiomers in the extraction system was not observed. The limit of quantitation (S/N= 10; 12.5 ng/ml for both enantiomers) and the limit of detection (S/N=3; 4 ng/ml for both enantiomers) were below the therapeutic range for mianserin. The method was validated and successfully applied to determine R- and S-mianserin in plasma samples from seven patients treated with mianserin, indicating that LPME-CE is a promising combination for analysis of racemic drugs present in low concentrations in biological matrices.

Selected fundamental aspects of chiral electromigration techniques and their application to pharmaceutical and biomedical analysis

While capillary electrophoresis has been established as a major enantioseparation technique within the last decade, the potential of capillary electrochromatography is still studied extensively. This review summarizes recent applications of electromigration techniques with regard to the enantioseparation of chiral drugs. The first part discusses the general aspects of migration models and the enantiomer migration order. The application of capillary electrophoresis to chiral pharmaceutical analysis considers recent literature on: (1) chiral resolutions of non-racemic mixtures of enantiomers for the development of assays and the determination of the stereochemical purity of the drugs, (2) chiral separations of compounds in pharmaceutical formulations and products, and (3) enantioseparations of drugs in biological samples. A shorter section devoted to chiral electrochromatography discusses some fundamental aspects as well as the application to the chiral analysis of drugs including bioanalysis.

Recent applications of capillary electrochromatography

A review is presented of the most important recent applications of capillary electrochromatography (CEC) for the analysis of acidic, basic, and neutral compounds, of biomolecules, environmental substances, natural products, pharmaceuticals, and chiral compounds. Packed-column CEC (packed-CEC), open-tubular (OT-CEC), as well as pressure-assisted CEC (pseudo-CEC) are hereby considered. Papers published between July 1999 and April 2001 were taken into account. Applications before July 1999 have been reviewed in Electrophoresis 1999, 20, 3027-3065.

Chiral separation by chromatographic and electromigration techniques. A review

This review gives a survey of different chiral separation principles and their use in high-performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), thin-layer chromatography (TLC), capillary electrophoresis (CE) and capillary electrochromatography (CEC) highlighting new developments and innovative techniques. The mechanisms of the different separation principles are briefly discussed and some selected applications are shown.

Enantioseparation of phenothiazines in capillary zone electrophoresis using cyclodextrins as chiral selectors

In this study, enantioseparations of five phenothiazines, including promethazine, ethopropazine, trimeprazine, methotrimeprazine, and thioridazine, in cyclodextrin (CD)-modified capillary zone electrophoresis were investigated using a phosphate buffer (40 mM) at pH 3.0. We focussed on the separation of phenothiazines with the use of CDs at low concentrations. Three different CDs, including beta-CD, hydroxypropyl-beta-CD (HP-beta-CD) and gamma-CD, were chosen as chiral selectors. The results indicate that effective enantioseparation of phenothiazines, except for methotrimeprazine, is simultaneously achievable with addition of gamma-CD at a concentration of 2.5-6.0 mM. The enantiomers of ethopropazine and trimeprazine are effectively separated with addition of HP-beta-CD at low concentrations, in the range 0.4-6.0 mM, whereas those of promethazine and trimeprazine are baseline resolved with beta-CD at much lower concentrations (0.02-3.0 mM) than with HP-beta-CD. The results also confirm that the separation window is greatly enlarged at low CD concentrations. Moreover, the drastic variations of the electrophoretic mobility of phenothiazines as a function of CD concentration reveal that phenothiazines interact very strongly with CDs in the order gamma-CD<HP-beta-CD<beta-CD.

Enantioseparations by capillary electrochromatography

The review summarizes recent developments in enantioseparations by capillary electrochromatography (CEC). Selected fundamental aspects of CEC are discussed in order to stress those features which may allow the success of this technique in the competitive field of enantioseparations. In addition, the comparative characteristics of the different modes of chiral CEC and the stationary phases are presented. The effects of the characteristics of the stationary and liquid phases and operational conditions on the separation results are discussed. Finally, some future trends are briefly addressed.

Effect of organic solvent, electrolyte salt and a loading of cellulose tris (3,5-dichlorophenyl-carbamate) on silica gel on enantioseparation characteristics in capillary electrochromatography

The effect of the amount of the chiral selector, cellulose tris(3,5-dichlorophenylcarbamate) (CDCPC) on the separation characteristics of enantiomers of some charged and neutral analytes was studied in capillary electrochromatography (CEC). For better understanding of the effect of the loading of the chiral selector on the particles and pore size of the packing material, laser-beam particle size analyzer and scanning electron microscopy (SEM) were used. As shown in this study, CDCPC can be used for CEC enantioseparations of a wide range of chiral charged and neutral analytes with high efficiency. The loading of the polysaccharide derivative on the surface of silica materials even in high amounts does not markedly affect the particle size and porous structure of the packing material. The separation characteristics are strongly affected by the loading of CDCPC onto silica gel in both CEC and capillary liquid chromatography (CLC).

Mechanistic study on the opposite migration order of clenbuterol enantiomers in capillary electrophoresis with beta-cyclodextrin and single-isomer heptakis(2,3-diacetyl-6-sulfo)-beta-cyclodextrin

Opposite migration order was observed for the enantiomers of the chiral beta2-adrenergic drug clenbuterol (CL) in capillary electrophoresis (CE) when resolved with native beta-cyclodextrin (beta-CD) and heptakis (2,3-diacetyl-6-sulfo)-beta-CD (HDAS-beta-CD). The possible mechanisms of the affinity reversal of the CL enantiomers depending on the structure of the CD were studied using 1H-nuclear magnetic resonance (1H-NMR) spectrometry and one-dimensional rotating frame nuclear Overhauser and exchange spectrometry (1-D ROESY). Significant differences were observed between the structure of the (+/-)-CL complexes with beta-CD and HDAS-beta-CD.