Combined use of chiral ionic liquid and cyclodextrin for MEKC: Part I. Simultaneous enantioseparation of anionic profens

Emerging Developments in Separation Techniques and Analysis of Chiral Pharmaceuticals

Chiral separation, the process of isolating enantiomers from a racemic mixture, holds paramount importance in diverse scientific disciplines. Using chiral separation methods like chromatography and electrophoresis, enantiomers can be isolated and characterized. This study emphasizes the significance of chiral separation in drug development, quality control, environmental analysis, and chemical synthesis, facilitating improved therapeutic outcomes, regulatory compliance, and enhanced industrial processes. Capillary electrophoresis (CE) has emerged as a powerful technique for the analysis of chiral drugs. This review also highlights the significance of CE in chiral drug analysis, emphasizing its high separation efficiency, rapid analysis times, and compatibility with other detection techniques. High-performance liquid chromatography (HPLC) has become a vital technique for chiral drugs analysis. Through the utilization of a chiral stationary phase, HPLC separates enantiomers based on their differential interactions, allowing for the quantification of individual enantiomeric concentrations. This study also emphasizes the significance of HPLC in chiral drug analysis, highlighting its excellent resolution, sensitivity, and applicability. The resolution and enantiomeric analysis of nonsteroidal anti-inflammatory drugs (NSAIDs) hold great importance due to their chiral nature and potential variations in pharmacological effects. Several studies have emphasized the significance of resolving and analyzing the enantiomers of NSAIDs. Enantiomeric analysis provides critical insights into the pharmacokinetics, pharmacodynamics, and potential interactions of NSAIDs, aiding in drug design, optimization, and personalized medicine for improved therapeutic outcomes and patient safety. Microfluidics systems have revolutionized chiral separation, offering miniaturization, precise fluid control, and high throughput. Integration of microscale channels and techniques provides a promising platform for on-chip chiral analysis in pharmaceuticals and analytical chemistry. Their applications in techniques such as high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) offer improved resolution and faster analysis times, making them valuable tools for enantiomeric analysis in pharmaceutical, environmental, and biomedical research.

Effects of amino acid-derived chiral ionic liquids on cyclodextrin-mediated capillary electrophoresis enantioseparations of dipeptides

The synergistic effect of chiral ionic liquids composed of tetraalkylammonium ions and the amino acids Asn, Asp or Pro on the enantioseparations of dipeptides mediated by β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin in capillary electrophoresis was studied. Addition of a chiral ionic liquid resulted in a concentration-dependent increase in the enantioresolutions compared to the sole presence of a cyclodextrin in the background electrolyte. The extent varied with the tetraalkylammonium cation (tetramethylammonium versus tetrabutylammonium) as well as the amino acid component of the ionic liquid. The presence of a chiral ionic liquid did not counteract the pH-dependent reversal of the enantiomer migration order of the dipeptides Ala-Phe, Ala-Tyr and Phe-Phe when increasing the pH of the background electrolyte from 2.5 to 3.5. Comparing the effect of a chiral ionic liquid based on Asp with the addition of equimolar concentrations of the individual components of the ionic liquid, a diverse picture was observed. In some cases, higher resolution values were obtained with the chiral ionic liquid, while for other cases superior enantioseparations were obtained upon separate addition of the amino acid component and a tetraalkylammonium chloride. With regard to the stereochemistry of the amino acid, a superior effect was typically observed using the l-configured amino acid, but in some cases higher resolution values were found in the presence of d-Asp. The rationale for the diverse observations is not obvious and may be due to the zwitterionic nature of analytes as well as the amino acid component of the chiral ionic liquid.

Single isomer cyclodextrins as chiral selectors in capillary electrophoresis

Since decades, cyclodextrins are one of the most powerful selectors in chiral capillary electrophoresis for the enantioseparation of diverse organic compounds. This review concerns papers published over the last decade (from 2009 until nowadays), dealing with the capillary electrophoretic application of single isomer cyclodextrin derivatives in chiral separations. Following a brief overview of their synthetic approaches, the inventory of the neutral, negatively and positively charged (including both permanently ionic and pH-tunable ionizable substituents) and zwitterionic CD derivatives is presented, with insights to underlying structural aspects by NMR spectroscopy and molecular modeling. CE represents an ideal tool to study the weak, non-covalent supramolecular interactions. The published methods are reviewed in the light of enantioselectivity, enantiomer migration order and the fine-tuning of enantiodiscrimination by the substitution pattern of the single entity selector molecules, which is hardly possible for their randomly substituted counterparts. All the reviewed publications herein support that cyclodextrin-based chiral capillary electrophoresis seems to remain a popular choice in pharmaceutical and biomedical analysis.

Recent advances in the enantioseparation promoted by ionic liquids and their resolution mechanisms

More and more various chemical media are being applied in enantioseparation; among them, ionic liquids (ILs) have attracted the long-term attention in this decade as green designable solvents. This paper provides comprehensive overview for the applications of ILs in chiral extraction, gas chromatography, liquid chromatography, capillary electrophoresis and other techniques for enantioseparation. Additionally, the important resolution mechanisms based on ILs have also been summarized and discussed. This review focuses on the latest development of enantioseparation methods by using ILs in various modes, leading to meaningful and valuable information to related fields and thus promotes further research and application of reported methods.

The Influence of Ionic Liquids on the Effectiveness of Analytical Methods Used in the Monitoring of Human and Veterinary Pharmaceuticals in Biological and Environmental Samples-Trends and Perspectives

Recent years have seen the increased utilization of ionic liquids (ILs) in the development and optimization of analytical methods. Their unique and eco-friendly properties and the ability to modify their structure allows them to be useful both at the sample preparation stage and at the separation stage of the analytes. The use of ILs for the analysis of pharmaceuticals seems particularly interesting because of their systematic delivery to the environment. Nowadays, they are commonly detected in many countries at very low concentration levels. However, due to their specific physiological activity, pharmaceuticals are responsible for bioaccumulation and toxic effects in aquatic and terrestrial ecosystems as well as possibly upsetting the body's equilibrium, leading to the dangerous phenomenon of drug resistance. This review will provide a comprehensive summary of the use of ILs in various sample preparation procedures and separation methods for the determination of pharmaceuticals in environmental and biological matrices based on liquid-based chromatography (LC, SFC, TLC), gas chromatography (GC) and electromigration techniques (e.g., capillary electrophoresis (CE)). Moreover, the advantages and disadvantages of ILs, which can appear during extraction and separation, will be presented and attention will be given to the criteria to be followed during the selection of ILs for specific applications.

Enantiomeric separation of ivabradine by cyclodextrin-electrokinetic chromatography. Effect of amino acid chiral ionic liquids

A chiral methodology was developed for the first time to ensure the quality control of ivabradine, a novel anti-ischemic and heart rate lowering drug commercialized as a pure enantiomer. With this aim, electrokinetic chromatography (EKC) was employed and the enantiomeric separation of ivabradine was investigated using different anionic and neutral cyclodextrins (CDs) and amino acid-based chiral ionic liquids (CILs) as sole chiral selectors. Baseline separation was only achieved with sulfated CDs, and the best enantiomeric resolution was obtained with sulfated-γ-CD. Under the optimized conditions, ivabradine enantiomers were separated in 6 min with a resolution of 2.7. Nuclear magnetic resonance experiments showed a 1:1 stoichiometry for the enantiomer-CD complexes and apparent and averaged equilibrium constants were determined. The combined use of sulfated-γ-CD and different CILs as dual separation systems was investigated, resulting in a significant increase in the resolution. The use of 5 mM tetrabutylammonium-aspartic acid ([TBA][L-Asp]) in 50 mM formate buffer (pH 2.0) containing 4 mM sulfated-γ-CD were considered the best conditions in terms of resolution and migration times for ivabradine enantiomers. Nevertheless, as no inversion of the enantiomer migration order was observed when combining CILs and sulfated-γ-CD and a good enantiomeric resolution and efficiency were obtained using just sulfated-γ-CD as the sole chiral selector, the analytical characteristics of this method were evaluated, showing good recovery (98% and 103% for S- and R-ivabradine, respectively) and precision values (RSD < 5% for instrumental repeatability, < 6% for method repeatability and < 7% for intermediate precision). The limits of detection (LODs) were 0.22 and 0.28 μg mL^-1 for S- and R-ivabradine, respectively, and the method enabled to detect a 0.1% of the enantiomeric impurity, allowing to accomplish the requirements of the International Conference on Harmonisation (ICH) guidelines. Finally, the method was applied to the analysis of a pharmaceutical formulation of ivabradine. The content of R-ivabradine was below the LOD and the amount of S-ivabradine was in agreement to the labeled content.

A novel coating method for CE capillary using carboxymethyl-β-cyclodextrin-modified magnetic microparticles as stationary for electrochromatography enantioseparation

Magnetic microparticles (MMPs) have been extensively studied and aroused considerable interest in separation science owing to their superior characteristics. In this paper, a novel coated capillary with carboxymethyl-β-cyclodextrin-functionalized magnetic microparticles (CD-MMPs) as stationary phase was constructed and then applied to establish an open-tubular capillary electrochromatography enantioseparation system. The preparation of the CD-MMP-coated open-tubular column was very convenient because the coating of the magnetic microparticles onto the capillary column could be easily manipulated by an external magnetic field. The preparation conditions of the coated capillary such as magnetic field intensity and coating time are discussed in detail. The new constructed CD-MMP capillary system was applied to separate enantiomers of several racemic drugs. Compared to the uncoated capillary system, obviously preferable separations of tested enantiomers were obtained. Several important parameters affecting the enantioseparation, such as CM-β-CD concentration, running buffer pH, organic solvent, and applied voltage, were systematically optimized. Furthermore, satisfactory repeatability and chemical stability of this new CD-MMP capillary system were achieved in the experiment. Graphical abstract ᅟ.

The development of biphasic chiral recognition in chiral separation

In chiral separation, enantioseparation factor is an important parameter which influences the resolution of enantiomers. In this current overview, a biphasic chiral recognition method is introduced to the readers. This method can significantly improve the enantioseparation factor in two-phase solvent through adding lipophilic and hydrophilic chiral selectors which have opposite chiral recognition ability to organic and aqueous phases, respectively. This overview presents the development and applications of biphasic chiral recognition in liquid-liquid extraction and counter current chromatography. It mainly focuses on the topics of mechanism, advantages and limitations, applications, and key factors of biphasic chiral recognition. In addition, the future outlook on development of biphasic chiral recognition also has been discussed in this overview.

Enantioseparation by Capillary Electrophoresis Using Ionic Liquids as Chiral Selectors

Capillary electrophoresis (CE) is one of the most widely employed analytical techniques to achieve enantiomeric separations. In spite of the fact that there are many chiral selectors commercially available to perform enantioseparations by CE, one of the most relevant topics in this field is the search for new selectors capable of providing high enantiomeric resolutions. Chiral ionic liquids (CILs) have interesting characteristics conferring them a high potential in chiral separations although only some of them are commercially available. The aim of this article is to review all the works published on the use of CILs as chiral selectors in the development of enantioselective methodologies by CE, covering the period from 2006 (when the first research work on this topic was published) to 2017. The use of CILs as sole chiral selectors, as chiral selectors in dual systems or as chiral ligands will be considered. This review also provides detailed analytical information on the experimental conditions used to carry out enantioseparations in different fields as well as on the separation mechanism involved.

Renewable sources from plants as the starting material for designing new terpene chiral ionic liquids used for the chromatographic separation of acidic enantiomers

Synthesis of cheap and natural resources is an important topic in green chemistry.

Applications of nuclear magnetic resonance spectroscopy for the understanding of enantiomer separation mechanisms in capillary electrophoresis

This review deals with the applications of nuclear magnetic resonance (NMR) spectroscopy to understand the mechanisms of chiral separation in capillary electrophoresis (CE). It is accepted that changes observed in the separation process, including the reversal of enantiomer migration order (EMO), can be caused by subtle modifications in the molecular recognition mechanisms between enantiomer and chiral selector. These modifications may imply minor structural differences in those selector-selectand complexes that arise from the above mentioned interactions. Therefore, it is mandatory to understand the fine intermolecular interactions between analytes and chiral selectors. In other words, it is necessary to know in detail the structures of the complexes formed by the enantiomer (selectand) and the selector. Any differences in the structures of these complexes arising from either enantiomer should be detected, so that enantiomeric bias in the separation process could be explained. As to the nature of these interactions, those have been extensively reviewed, and it is not intended to be discussed here. These interactions contemplate ionic, ion-dipole and dipole-dipole interactions, hydrogen bonding, van der Waals forces, π-π stacking, steric and hydrophobic interactions. The main subject of this review is to describe how NMR spectroscopy helps to gain insight into the non-covalent intermolecular interactions between selector and selectand that lead to enantiomer separation by CE. Examples in which diastereomeric species are created by covalent (irreversible) derivatization will not be considered here. This review is structured upon the different structural classes of chiral selectors employed in CE, in which NMR spectroscopy has made substantial contributions to rationalize the observed enantioseparations. Cases in which other techniques complement NMR spectroscopic data are also mentioned.

Ionic Liquids in Capillary Electrophoresis

Recently, a great interest was drawn toward ionic liquids (ILs) in analytical separation techniques. ILs possess many properties making them excellent additives in capillary electrophoresis (CE) background electrolytes (BGE). The most important property is the charge of the dissolved ions in BGE enabling the cations to interact with deprotonated silanol groups on the capillary surface and thereby modifying the electroosmotic flow (EOF). Ionic and/or proton donor-acceptor interactions between analyte and IL are possible interactions facilitating new kinds of separation mechanisms in CE. Further advantages of ILs are the high conductivity, the environmentally friendliness, and the good solubility for organic and inorganic compounds. The most commonly used ILs in capillary electrophoresis are dialkylimidazolium-based ILs, whereas for enantioseparation a lot of innovative chiral cations and anions were investigated.ILs are reported to be additives to a normal CE background electrolyte or the sole electrolyte in CE, nonaqueous CE (NACE), micellar electrokinetic chromatography (MEKC), and in enantioseparation. An overview of applications and separation mechanisms reported in the literature is given here, in addition to the enantioseparation of pseudoephedrine using tetrabutylammonium chloride (TBAC) as IL additive to an ammonium formate buffer containing β-cyclodextrin (β-CD).

Combined use of chiral ionic liquid surfactants and neutral cyclodextrins: evaluation of ionic liquid head groups for enantioseparation of neutral compounds in capillary electrophoresis

Cyclodextrins (CDs) are most commonly used chiral selectors in capillary electrophoresis (CE). Although the use of neutral CDs and its derivatives have shown to resolve plethora of charged enantiomers, they cannot resolve neutral enantiomers. The use of ionic liquids (ILs) surfactants forming successful complex with CDs present itself an opportunity to resolve neutral enantiomers. In this work, the effect of IL head groups and their complexation ability with heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) was studied for the separation of neutral enantiomers by CE. First, cationic IL type surfactants with different chiral head groups were synthesized. Physicochemical properties such as critical micelle concentration were determined by surface tension, whereas aggregation and polarity were determined by fluorescence spectroscopy. The complexation ability of ILs with TM-β-CD was characterized in the gas phase by CE-mass spectrometry. The influence of the type of ILs head group and its concentration on chiral resolution, resolution per unit time and selectivity were investigated for four structurally diverse neutral compounds. The binding constants of the neutral analytes to the IL-CD complex were estimated by y-reciprocal method. The hydrophobicity of the side chain of the IL head group displayed significant effect on the binding constants and enantioseparations.

Recent advances of ionic liquids and polymeric ionic liquids in capillary electrophoresis and capillary electrochromatography

Ionic liquids (ILs) and polymeric ionic liquids (PILs) with unique and fascinating properties have drawn considerable interest for their use in separation science, especially in chromatographic techniques. In this article, significant contributions of ILs and PILs in the improvement of capillary electrophoresis and capillary electrochromatography are described, and a specific overview of the most relevant examples of their applications in the last five years is also given. Accordingly, some general conclusions and future perspectives in these areas are discussed.

Application of ionic liquids in liquid chromatography and electrodriven separation

Ionic liquids (ILs) are salts in the liquid state at ambient temperature, which are nonvolatile, nonflammable with high thermal stability and dissolve easily for a wide range of inorganic and organic materials. As a kind of potential green solvent, they show high efficiency and selectivity in the field of separation research, especially in instrumental analysis. Thus far, ILs have been successfully applied by many related researchers in high-performance liquid chromatography and capillary electrophoresis as chromatographic stationary phases, mobile phase additives or electroosmotic flow modifiers. This paper provides a detailed review of these applications in the study of natural products, foods, drugs and other fine chemicals. Furthermore, the prospects of ILs in liquid chromatographic and electrodriven techniques are discussed.

Evaluation of the enantioselectivity of glycogen-based synergistic system with amino acid chiral ionic liquids as additives in capillary electrophoresis

In this paper, two novel amino acid chiral ILs, tetramethylammonium-l-arginine (TMA-l-Arg) and tetramethylammonium-l-aspartic acid (TMA-l-Asp), were applied for the first time in CE enantioseparation to evaluate their potential synergistic effect with glycogen as chiral selector. As observed, significantly improved separation of tested enantiomers were obtained in the chiral ILs/glycogen synergistic systems compared to the single glycogen separation system. Several primary parameters affecting the enantioseparation, such as amino acid ILs (AAILs) concentration, glycogen concentration and buffer pH, were systematically investigated. An achiral tetramethylammonium hydroxide ionic liquid (TMA-OH) modified separation system was also evaluated to validate the superiority of the novel chiral ILs/glycogen synergistic systems. To further optimize the overall synergistic systems, the effect of three other parameters, including buffer concentration, applied voltage and capillary temperature were simultaneously analyzed by a central composite design (CCD), and excellent enantioseparations were achieved with the optimized parameters. The results indicate that the application of chiral ILs/glycogen synergistic systems is a promising way in chiral separation science.

Chiral selectors in CE: recent developments and applications

This review article provides an overview of the recent advances in enantioanalysis by use of electrophoretic techniques. Due to the big number of publications in the subject mentioned above, this article is focused on chiral method developments and applications published from 2008 until 2011, and it demonstrates chiral selectors used in CE. Numerous chiral selectors have been used over the years, and these include the cyclic and the linear oligo- and polysaccharides, the branched polysaccharides, the polymeric and monomeric surfactants, the macrocyclic and other antibiotics, and the crown ethers. Different dual-selector systems are also presented in this article, and the results are compared with those obtained by use of a single chiral selector. Finally, several pharmaceutical and biomedical applications based on chiral recognition are summarized.

Application of polymeric surfactants in chiral micellar electrokinetic chromatography (CMEKC) and CMEKC coupled to mass spectrometry

The use of amino acid-based polymeric surfactants (a.k.a. molecular micelles) in chiral micellar electrokinetic chromatography (CMEKC) has been shown to be a successful separation mode for capillary electrophoresis (CE). In this mode, chiral compounds can be enantioseparated with high efficiency, high chiral selectivity, and versatility. This chapter describes the state-of-the art studies published in the past 5 years in CMEKC using polymeric surfactants. Recent trends in the compatibility of chiral polymeric surfactants with mass spectrometric (MS) detection suggest that this type of chiral selector may be the most promising ones for chiral CE-MS applications. The synthesis of new anionic and cationic MS-compatible polymeric surfactants and their utility in CMEKC and CMEKC-MS are demonstrated. Examples of how to run a typical CMEKC-MS experiment using univariate and multivariate optimization of CMEKC and MS parameters are discussed.

A novel positively charged achiral co-monomer for β-cyclodextrin monolithic stationary phase: improved chiral separation of acidic compounds using capillary electrochromatography coupled to mass spectrometry

The work presented here demonstrates the incorporation of vinylbenzyl trimethylammonium (VBTA) as a novel positively charged achiral co-monomer to a glycidyl methacrylate-beta cyclodextrin (GMA/β-CD) based monolith, providing anion exchange sites with reversed electroosmotic flow (EOF) for capillary electrochromatography (CEC). The monolithic phases, GMA/β-CD-VBTA and GMA/β-CD (without co-monomer) were characterized by scanning electron microscopy, optical microscopy, pressure drop/flow-rate curves and nitrogen adsorption analysis. After optimizing the stationary phase and mobile phase parameters, chiral separations of 41 pairs of structurally diverse anionic chiral analytes were compared individually using the GMA/β-CD-VBTA and GMA/β-CD monolithic columns. The GMA/β-CD-VBTA monolith chiral stationary phase separated significantly more acidic compounds compared to the GMA/β-CD column. To-date there has been limited work in the development of chiral monolithic column for CEC-mass spectrometry (MS). Because of good electrodriven flow characteristics, which allow the column to maintain a stable current in the absence of outlet vial, GMA/β-CD-VBTA column was successfully coupled to single quadrupole mass spectrometer for CEC-MS of several chiral test compounds. In addition, the same monolithic CEC column when coupled to a triple quadrupole MS instrument, two orders of magnitude higher sensitivity was observed compared to a single quadrupole MS instrument.

Phosphonium-based ionic liquids in electrokinetic capillary chromatography for the separation of neutral analytes

In this work we elucidated the applicability of phosphonium-based ionic liquids (ILs) as pseudostationary phase in electrokinetic capillary chromatography (EKC) with UV-detection. The phosphonium ILs studied contain bromide, chlorine, or tosylate ions, as counter ions, and alkyl side chains of variable length on the phosphorous atom. The effects of the type and concentration of the IL, pH, ionic strength, and type of background electrolyte solution on the electroosmotic flow (EOF) and on the effective electrophoretic mobilities of some neutral model analytes were investigated and large variations in the migration times were observed. Especially the IL employed remarkably affected the strength and direction of the EOF Successful separations were obtained for neutral aromatic singly substituted analytes, namely benzene, toluene, phenol, and nitrobenzene. The results demonstrated the potential of capillary electromigration methods for rapid interaction studies between ILs and analytes, which is useful for the development of novel materials for sample preparation and separation purposes or for novel catalyst and chemical processing studies.

Chiral ionic liquids: a compendium of syntheses and applications (2005-2012)

In recent years, the field of chiral ionic liquids (CILs) has undergone exponential growth. As the technology has advanced, new ways of synthesizing stable and structurally diverse ionic liquids have been established. This has led to heretofore unknown applications of CILs as well as in improving efficiency of previously identified applications. In this review article we have compiled a comprehensive database containing structures and physical properties of notable CILs that have been synthesized during the last 6 years. Their applications in the fields of asymmetric organic synthesis, spectroscopy, and chromatography are also illustrated. This is an expansion of our previous review, which covered the literature before 2005.

Recent advances of enantioseparations in capillary electrophoresis and capillary electrochromatography

A comprehensive survey of recent developments and applications of capillary electromigration techniques for enantioseparations from January 2006 to June 2010 is presented. The techniques include capillary electrophoresis, chip capillary electrophoresis and capillary electrochromatography. The separation principles and the chiral recognition mechanisms are discussed. Additionally, on-line preconcentrations in chiral capillary electrophoresis are also reviewed.

MEKC as a powerful growing analytical technique

This review summarizes the principle and the developments in MEKC in terms of separation power, sensitivity, and detection approaches more than 25 years after its appearance. Newly used surfactants are mentioned. Classical and new sample concentration techniques in MEKC are described. The different detection approaches in MEKC with advantages, limitations, and future prospects are also discussed. This review highlights the wider application of MEKC in different analytical fields. Various recent selected applications of this technique in different analytical fields are reported.

Fundamental aspects of chiral electromigration techniques and application in pharmaceutical and biomedical analysis

Capillary electromigration techniques are often considered ideal methods for the analysis of chiral compounds due to the high resolution power and flexibility of the technique. Therefore, especially capillary electrophoresis using a chiral selector in the background electrolyte, also termed electrokinetic chromatography, has found widespread acceptance in analytical enantioseparations of drug compounds in pharmaceuticals and biological media. Moreover, mechanistic studies on analyte complexation by the chiral selectors have continuously been conducted in an effort to rationalize enantioseparation phenomena. These studies combined capillary electrophoresis with spectroscopic techniques such as nuclear magnetic resonance and/or molecular modeling. The present review focuses on recent examples of mechanistic aspects of capillary electromigration enantioseparations and summarizes recent applications of chiral pharmaceutical and biomedical analysis published between January 2009 and August 2010.