Enantiomers separation by capillary electrochromatography

Colorimetric discrimination and spectroscopic detection of tyrosine enantiomers based on melamine induced aggregation of l-cysteine/Au nanoparticles

Au nanoparticles (AuNPs) are decorated by l-cysteine (L-Cys), and the resultant chiral L-Cys/AuNPs can be used for colorimetric discrimination and spectroscopic detection of the tyrosine (Tyr) enantiomers. Melamine (Mel) can induce the aggregation of the L-Cys/AuNPs through ligand exchange, leading to a distinct color change from wine red to purple. Owing to the same rotatory direction of L-Cys/AuNPs and L-Tyr, the L-Cys/AuNPs exhibit a significantly higher binding affinity toward L-Tyr than D-Tyr, and thus the Mel induced aggregation of the L-Cys/AuNPs is greatly alleviated by the protection from the L-Tyr protective layer. Therefore, the Tyr enantiomers can be simply discriminated by naked eyes. In addition, the absorbance of the aggregated L-Cys/AuNPs at ∼630 nm increases linearly with decreasing concentrations of L-Tyr ranging from 10 nM to 1 mM due to the weakened protection effect from L-Tyr, and thus spectroscopic detection of L-Tyr can also be accomplished by the developed L-Cys/AuNPs with a limit of detection (LOD) of 5.3 nM.

Chiral zeolite beta used as stationary phase for enantioseparation in miniaturized open tubular capillary electrochromatography with amperometric detection

BACKGROUND

With the rapid growth of the demand for optically pure compounds in the fields of biology, medicine and stereospecific synthesis, it is of great importance to develop efficient, economical, simple enantioseparation and analysis methods. Open tubular capillary electrochromatography (OT-CEC) has attracted much attention in the field of chiral separation, but its column capacity and the sensitivity of common-used optical detection are relatively low. Zeolite beta nanomaterial is both enantioselective and size-selective, providing suitable chiral microenvironment for chiral recognition, and amperometric detection (AD) avoids the low sensitivity caused by the short optical path in optical detection to some extent.

RESULTS

Zeolite beta nanomaterials with different particle sizes (25, 50 and 200 nm) were synthesized, and the morphology and structure were characterized by scanning electron microscopy and X-ray diffraction. Then, a novel chiral OT column was prepared by one-step method using zeolite beta nanomaterial as chiral stationary phase, and its separation performance was characterized by miniaturized CEC with AD (mini-CEC-AD) device. Under the optimum conditions, six groups of chiral drugs achieved baseline separation. Norepinephrine enantiomers were used for evaluating the inter-day, intra-day and inter-column reproducibility of the prepared open-tubular column. The relative standard deviations of migration time, peak area, resolution and selectivity factor were within 8.7 %. The limits of detection for norepinephrine enantiomers were 0.18 μg mL-1 (S/N = 3), and the average recoveries were in range of 96.7-105.0 %. This developed method has been successfully applied to the analysis of impurity enantiomer in potassium dichromate (+)-norepinephrine injection sample.

SIGNIFICANCE

Zeolite beta nanomaterial was used as the stationary phase to prepare chiral OT columns for the first time, and this one-step preparation method is simple and easy. The introduction of zeolite beta enriches the types of chiral stationary phase materials in electrochromatographic columns, and mini-OT-CEC-AD system provides an alternative for fast enantioseparation of chiral compounds.

Applications of electrophoresis for small enantiomeric drugs in real-world samples: Recent trends and future perspectives

Separation and identification of chiral molecules is a topic widely discussed in the literature and of fundamental importance, especially in the pharmaceutical and food fields, both from industrial and laboratory points of view. Several techniques are used to carry out these analyses, but high-performance liquid chromatography is often the "gold standard." The high costs of chiral columns, necessary for this technique, led researchers to look for an alternative, and capillary electrophoresis (CE) is a technique capable of overcoming some of the disadvantages of liquid chromatography, often providing comparable results in terms of sensitivity and robustness. We addressed this topic, already widely discussed in the literature, providing an overview of the last 6 years of the most frequent and recent applications of CE. To make the manuscript more effective, we decided to divide it into paragraphs that represent the main field of application, from enantioseparation in complex matrices (pharmacokinetic studies or toxicological dosage of drugs, analysis of environmental pollutants, and analyses of foods) to quality control analyses on pharmaceutical formulas. About these, which are the fields of most meaningful use, we mentioned some of the most innovative and performing methods, with a look to the future on the application of new materials used, such as chiral selectors, that can make these types of analyses accessible to all, reducing cost, time, and excessive use of toxic solvents.

A cooperation tale of biomolecules and nanomaterials in nanoscale chiral sensing and separation

The cooperative relationship between biomolecules and nanomaterials makes up a beautiful tale about nanoscale chiral sensing and separation. Biomolecules are considered a fabulous chirality 'donor' to develop chiral sensors and separation systems. Nature has endowed biomolecules with mysterious chirality. Various nanomaterials with specific physicochemical attributes can realize the transmission and amplification of this chirality. We focus on highlighting the advantages of combining biomolecules and nanomaterials in nanoscale chirality. To enhance the sensors' detection sensitivity, novel cooperation approaches between nanomaterials and biomolecules have attracted tremendous attention. Moreover, innovative biomolecule-based nanocomposites possess great importance in developing chiral separation systems with improved assay performance. This review describes the formation of a network based on nanomaterials and biomolecules mainly including DNA, proteins, peptides, amino acids, and polysaccharides. We hope this tale will record the perpetual relation between biomolecules and nanomaterials in nanoscale chirality.

Carboxymethylated maltodextrin as a chiral selector for the separation of some basic drug enantiomers using capillary electrophoresis

In this work, carboxymethylated maltodextrin (Cm-MD) was successfully synthesized as an efficient anionic chiral selector and applied for the enantiomer separation of some basic drugs including tramadol, venlafaxine, verapamil, hydroxyzine, citalopram, fluoxetine, and amlodipine by capillary electrophoresis (CE). The synthesized chiral selector was characterized by the nuclear magnetic resonance and Fourier transform infrared spectrophotometry. Under the optimized Cm-MD modified CE conditions (background electrolyte: phosphate buffer (pH 5.0, 50 mM) containing 5% (w/v) Cm-MD; applied voltage: 20 kV; and capillary column temperature: 25 °C), successful enantiomer separation of all studied chiral drugs were observed. By comparison of Cm-MD and MD for enantiomer separation of the model drugs, it was revealed that Cm-MD exhibits a higher resolution in comparison to the MD modified CE. This enhanced resolution could be attributed to the electrostatic interactions between the cationic drugs and anionic Cm-MD and opposite direction mobility of the host-guest complex relative to the chiral analyte. The optimized Cm-MD modified CE method was successfully used for the assay of the enantiomers of citalopram and venlafaxine in commercial tablets. The proposed method showed the linear range of 5.0-150.0 mg/L and 10.0-150.0 mg/L for both enantiomers of citalopram and venlafaxine, respectively. The limits of quantification were 5.0 and 10.0 mg/L for the enantiomers of citalopram and venlafaxine, respectively. The limit of detection for all enantiomers was found to be < 3.0 mg/L. Intra- and inter-day RSDs (n = 4) were less than 9.7%. The relative errors were less than 9.4% for all enantiomers. The obtained results in this research show that Cm-MD as a new, efficient and inexpensive chiral selector can be used for enantiomer separation of basic drugs using the CE technique.

Recent development of chiral ionic liquids for enantioseparation in liquid chromatography and capillary electrophoresis: A review

Ionic liquids (ILs), which are salts in a molten state below 100 °C, have become a hot topic of research in various fields because of their negligible vapour pressure, high thermal stability, and tunable viscosity. Chiral ionic liquids (CILs) can be applied in chromatography and capillary electrophoresis fields to improve the performance of enantiomeric separation, such as chiral stationary phases (CSPs) and mobile phase additives in high-performance liquid chromatography (HPLC); CSPs in gas chromatography (GC); and background electrolyte additives (BGE), chiral ligands and chiral selectors (CSs) in capillary electrophoresis (CE). This review focuses on the applications of CILs in HPLC and CE for the separation of enantiomers in the past five years. The mechanism for separating enantiomers was explained, and the prospect of the application of CILs in chiral liquid chromatography (LC) and CE analysis was also discussed.

Magnetic Molecularly Imprinted Chitosan Combined with a Paper-Based Analytical Device for the Smartphone Discrimination of Tryptophan Enantiomers

The separation of enantiomers plays a critical role in pharmaceutical development, ensuring therapeutic efficacy, safety, and patent protection. It enables the production of enantiopure drugs and enhances our understanding of the properties of chiral compounds. In this study, a straightforward and effective chiral detection strategy was developed for distinguishing between tryptophan (TRP) enantiomers. The approach involved the preparation of a magnetic molecularly imprinted chitosan (MMIC) for preparation of the sample, which was combined with a nitrocellulose membrane (a paper-based analytical device, PAD) integrated with D-TRP covalently grafted with polymethacrylic acid (PAD-PMA_D-TRP). Discriminating between the TRP enantiomers was achieved using AuNPs as a colorimetric probe. Indeed, the presence of D-TRP rapidly induced the aggregation of AuNPs due to its strong affinity to PAD-PMA_D-TRP, resulting in a noticeable change in the color of the AuNPs from red to purple. On the other hand, L-TRP did not induce any color changes. The chiral analysis could be easily performed with the naked eye and/or a smartphone. The developed method exhibited a detection limit of 3.3 µM, and it was successfully applied to detect TRP in serum samples, demonstrating good recovery rates. The proposed procedure is characterized by its simplicity, cost-effectiveness, rapidity, and ease of operation.

Construction of chiral capillary electrochromatography microsystems based on Aspergillus sp. CM96

Novel chiral capillary electrochromatography (CEC) microsystems were constructed based on Aspergillus sp. CM96. As a newly discovered intrinsic characteristic of the cell, cell chirality occupies an essential position in life evolution. Aspergillus sp. CM96 spore (CM96s) was chosen as a proof of concept to develop chiral capillary columns. Interestingly, various types of amino acid (AA) enantiomers were baseline separated under the optimized conditions. Furthermore, the time-dependent chiral interactions between AAs and CM96s were explored in a wider space. Pectinases generated from Aspergillus sp. CM96 fermentation were immobilized onto graphene oxide-functionalized capillary silica monoliths for separating AA enantiomers. Molecular docking simulations were performed to explore chiral separation mechanisms of pectinase for AA enantiomers. These results indicated that Aspergillus sp. CM96-based CEC microsystems have a significant advantage for chiral separation.

Preparation of β-cyclodextrin covalent organic framework-immobilized poly(glycidyl methacrylate) nanoparticle-coated open tubular capillary electrochromatography column for chiral separation

A new enantioselective open-tubular capillary electrochromatography was developed employing poly(glycidyl methacrylate) nanoparticles/β-cyclodextrin covalent organic frameworks chemically immobilized on the inner wall of the capillary as a stationary phase. A pretreated silica-fused capillary reacted with 3-aminopropyl-trimethoxysilane followed by poly(glycidyl methacrylate) nanoparticles and β-cyclodextrin covalent organic frameworks through a ring-opening reaction. The resulting coating layer on the capillary was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The electroosmotic flow was studied to evaluate the variation of the immobilized columns. The chiral separation performance of the fabricated capillary columns was validated by the analysis of the four racemic proton pump inhibitors including lansoprazole, pantoprazole, tenatoprazole, and omeprazole. The influences of bonding concentration, bonding time, bonding temperature, buffer type and concentration, buffer pH, and applied voltage on the enantioseparation of four proton pump inhibitors were investigated. Good enantioseparation efficiencies were achieved for all enantiomers. In the optimum conditions, the enantiomers of four proton pump inhibitors were fully resolved within 10 min with high resolutions of 9.5-13.9. The column-to-column and inter- to intra-day repeatability of the fabricated capillary columns through relative standard deviation were found better than 9.54%, exhibiting satisfactory stability and repeatability of the fabricated capillary columns.

Poly(carboxyethyl acrylate-co-ethylene glycol dimethacrylate) precursor monolith with bonded (S)-(-)-1-(2-naphthyl) ethylamine ligands for use in chiral and achiral separations by capillary electrochromatography

In this research report, the previously developed poly(carboxyethyl acrylate-co-ethylene glycol dimethacrylate) precursor monolith (referred to as carboxy monolith) is further exploited in the preparation of a chiral stationary phase for enantiomeric separations. The carboxy monolith precursor was subjected to post polymerization functionalization (PPF) with the chiral selector (S)-(-)-1-(2-naphthyl) ethylamine (NAS) at room temperature in the presence of N, N´-dicyclohexylcarbodiimide (DCC) in chloroform. The DCC, which is an organic soluble carbodiimide, permits the linkage for the amine functionality of the chiral ligand NAS to the carboxy group of the monolithic surface forming a stable amide linkage. The NAS column thus obtained allowed not only enantiomeric separations in the RP mode via its chiral site but also the separation of nonpolar species via its achiral functionality offering both hydrophobic and π-π interactions for aromatic compounds such toluene derivatives and polyaromatic hydrocarbons. The dual interaction sites (e.g., chiral, and achiral) of the NAS present a convenient column for the separations of slightly polar and nonpolar chiral and achiral solutes in the RP mode.

Comparison of the Performance of Different Bile Salts in Enantioselective Separation of Palonosetron Stereoisomers by Micellar Electrokinetic Chromatography

Bile salts are a category of natural chiral surfactants which have ever been used as the surfactant and chiral selector for the separation of many chiral compounds by micellar electrokinetic chromatography (MEKC). In our previous works, the application of sodium cholate (SC) in the separation of four stereoisomers of palonosetron (PALO) by MEKC has been studied systematically. In this work, the parameters of other bile salts, including sodium taurocholate (STC), sodium deoxycholate (SDC), and sodium taurodeoxycholate (STDC) in the separation of PALO stereoisomers by MEKC were measured and compared with SC. It was found that all of four bile salts provide chiral recognition for both pairs of enantiomers, as well as achiral selectivity for diastereomers of different degrees. The structure of steroidal ring of bile salts has a greater impact on the separation than the structure of the side chain. The varying separation results by different bile salts were elucidated based on the measured parameters. A model to describe the contributions of the mobility difference of solutes in the aqueous phase and the selectivity of micelles to the chiral and achiral separation of stereoisomers was introduced. Additionally, a new approach to measure the mobility of micelles without enough solubility for hydrophobic markers was proposed, which is necessary for the calculation of separation parameters in MEKC. Under the guidance of derived equations, the separation by SDC and STDC was significantly improved by using lower surfactant concentrations. The complete separation of four stereoisomers was achieved in less than 3.5 min by using 4.0 mM of SDC. In addition, 30.0 mM of STC also provided the complete resolution of four stereoisomers due to the balance of different separation mechanisms. Its applicability for the analysis of a small amount of enantiomeric impurities in the presence of a high concentration of the effective ingredient was validated by a real sample.

β-Cyclodextrin covalent organic framework supported by polydopamine as stationary phases for electrochromatographic enantioseparation

In this work, a new open-tubular capillary electrochromatography (OT-CEC) column was prepared using β-cyclodextrin covalent organic framework (β-CD COF) as a stationary phase. Polydopamine was used to assist fabrication of β-CD COF on an inner wall of a fused-silica capillary. The coating layer on the capillary was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Electroosmotic flow (EOF) was also studied to evaluate the variation of the inner wall of immobilized columns. Furthermore, the chiral separation effectiveness of the fabricated capillary column was evaluated by CEC using enantiomers of several related proton pump inhibitors as model analytes, including omeprazole, lansoprazole, pantoprazole and tenatoprazole. The effects of bonding time and concentration of β-CD COF, the type, concentration and pH of buffer, applied voltage were investigated to obtain satisfactory enantioselectivity. In the optimum conditions, the enantiomers of four analytes were resolved within 15 min with resolutions of 1.63-2.62. The relative standard deviation values for migration times and resolutions of the analytes representing intraday and interday were less than 6.75% and 4.24%, respectively. The results reveal that β-CD COF has great potential as chiral-stationary phases for enantioseparation in CEC.

An overview of poly (amide-amine) dendrimers functionalized chromatographic separation materials

Chromatography is one of the most important separation techniques in analytical chemistry. In which, the separation materials are the core for good separation results. Poly (amide-amine) dendrimers with regular three-dimensional structure, abundant terminal groups, controllable molecule chains, and unique cavities appear to have a positive impact on chromatographic separation materials. In the past decades, poly (amide-amine) grafted adsorbents and stationary phases have presented high grafting efficiency, controllable surface structure, good dispersion, and wide practical applications. In this review, the prepared poly (amide-amine) functionalized separation materials and their applications are systematically summarized. Functions, significance, structure-actvity relationships and benefits of poly (amide-amine) dendrimers in the proposed separation materials are discussed in detail. And we hope to provide a useful reference for the future development of chromatographic separation materials and inspire new discoveries in the study of poly (amide-amine) functionalized materials.

Chiral separation using cyclodextrins as mobile phase additives in open-tubular liquid chromatography with a pseudophase coating

The chiral separation of various analytes (dichlorprop, mecoprop, ibuprofen, and ketoprofen) was demonstrated with different cyclodextrins as mobile phase additives in open-tubular liquid chromatography using a stationary pseudophase semipermanent coating. The stable coating was prepared by a successive multiple ionic layer approach using poly(diallyldimethylammonium chloride), polystyrene sulfonate, and didodecyldimethyl ammonium bromide. Increasing concentrations (0-0.2 mM) of various native and derivatized cyclodextrins in 25 mM sodium tetraborate (pH 9.2) were investigated. Chiral separation was achieved for the four test analytes using 0.05-0.1 mM β-cyclodextrin (resolution between 1.11 and 1.34), γ-cyclodextrin (resolution between 0.78 and 1.27), carboxymethyl-β-cyclodextrin (resolution between 1.64 and 2.59), and 2-hydroxypropyl-β-cyclodextrin (resolution between 0.71 and 1.76) with the highest resolutions obtained with 0.1 mM carboxymethyl-β-cyclodextrin. %RSD values were <10%. This is the first demonstration of chiral open-tubular liquid chromatography using achiral chromatographic coatings and cyclodextrins as mobile phase additives.

The separation characteristics and performance evaluation of the silica-based poly(pentabromostyrene) stationary phase in capillary electrochromatography

A mixed-mode capillary column packed with silica-based poly(pentabromostyrene) particles (denoted as SiO₂@pPBS) was prepared and applied to capillary electrochromatography (CEC) separation. With the presence of benzene rings and bromine atoms in polymer chains, the SiO₂@pPBS column provides a reversed-phase/hydrophilic mixed-mode retention mechanism owing to hydrophilic, hydrophobic and π-π interactions between the stationary phase and various analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides, phenols and anilines. In CEC mode, the separation behavior of charged solutes is not only related to the interaction with the stationary phase, but also influenced by electrophoretic effects, which may lead to different selectivities compared to high performance liquid chromatography (HPLC). A column efficiency of up to 1.22 × 10⁵ N m^-1 was achieved for p-chloroaniline. Besides, the RSDs of retention time of anilines for run to run (n = 5), day to day (n = 5) and column to column (n = 3) were all less than 4.4%. Finally, the SiO₂@pPBS capillary column was applied to the separation of coking wastewater with satisfactory results. All the results demonstrated that the SiO₂@pPBS capillary packed column with RP/HILIC mixed-mode has great application potential.

One-pot method for the synthesis of β-cyclodextrin and covalent organic framework functionalized chiral stationary phase with mixed-mode retention mechanism

As a welcomed porous material, covalent organic frameworks (COFs) have many advantages and are widely used in various aspects. Particularly, COFs have aroused great attentions of scientists in chromatographic separation field due to their outstanding advantages, such as high stability, large specific surface area and multiple voids. However, endowing COFs with chirality to construct chiral stationary phase (CSP) function is still facing many challenges. Here, we firstly prepared a β-cyclodextrin (β-CD) and covalent organic framework functional silica CSP named as COF@CD@SiO₂ by one-pot method to perform high performance liquid chromatography (HPLC) chiral separation. The morphology and structure of the synthesized stationary phase were investigated by a variety of characterization methods including Fourier transform infrared spectrometry (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), N₂ adsorption experiment, powder X-ray diffraction (XRD) and elemental analysis (EA). The prepared stationary phase realized fast separation of six enantiomers in a short time. The separation mechanism was mainly ascribed to the inclusion complexation of β-cyclodextrin and the mutli-interaction sites from COFs material. In conclusion, the prepared chiral column can be used to achieve fast separation of enantiomers with good stability and reproducibility. These results can open new avenue for using chiral COFs in liquid chromatographic separation.

S-citalopram imprinted monolithic columns for capillary electrochromatography enantioseparations

In this study, the molecular imprinting method was used to separate enantiomeric forms of chiral antidepressant drug, R,S-citalopram (R,S-CIT) in aqueous solution by CEC system combining the advantages of capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC). For that, an amino acid-based molecularly imprinted monolithic capillary column was designed and used as a stationary phase for selective separation of S-citalopram (S-CIT) for the first time. S-CIT was selectively separated from the aqueous solution containing the other enantiomeric form of R-CIT, which is the same in size and shape as the template molecule. Morphology of the molecularly imprinted (MIP S-CIT) and non-imprinted (NIP S-CIT) monolithic capillary columns was observed by scanning electron microscopy. Imprinting efficiency of MIP S-CIT monolithic capillary column used for selective S-CIT separation was verified by comparing with NIP S-CIT and calculated imprinting factor (I.F:1.81) proved the high selectivity of the MIP S-CIT for S-CIT. Cavities formed for S-CIT form enabled selective (α = 2.08) separation of the target molecule from the other enantiomeric R-CIT form. Separation was achieved in a short period of 10 min, with the electrophoretic mobility of 7.68 × 10^-6 m² /Vs for R,S-CIT at pH 7.0 10 mM PB and 50% ACN ratio. The performance of both MIP S-CIT and NIP S-CIT columns was estimated by repeating the R,S-CIT separations with intra-batch and inter-batch studies for reproducibility of retention times of R,S-CITs. Estimated RSD values that are lower than 2% suggest that the monolithic columns separate R,S-CIT enantiomers without losing separation efficiency.

Chiral magnetic hybrid materials constructed from macromolecules and their chiral applications

Chirality is a fundamental and ubiquitous feature of living organisms in nature. Magnetic materials, in particular magnetic nanoparticles (MNPs), show some interesting properties such as large specific surface area, easy surface modification, magnetic responsivity and separation ability. Integrating MNPs with chirality in a single material will undoubtedly create a large number of advanced multi-functional materials. Despite the great advancements made in this area, there have been no review articles to summarize the relevant studies. The present work reviews the major progress recently made in constructing chiral magnetic hybrid materials (CMHMs) using macromolecules, which are classified based on the primary chiral macromolecular organic components, namely, biological polymers and synthetic polymers, and the applications of the resulting chiral hybrids in chiral research fields, including asymmetric catalysis, enzymatic resolution, chromatographic separation, enantioselective crystallization and enantioselective adsorption, are also summarized. The challenges and prospects of related research fields are proposed in the last section.

Preparation of a polydopamine/β-cyclodextrin coated open tubular capillary electrochromatography column and application for enantioseparation of five proton pump inhibitors

An open tubular capillary electrochromatography column was prepared by immobilizing β-cyclodextrin on the inner wall of pretreated capillary via noncovalent adsorption of polydopamine. The resulting coating layer on the capillary was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. Electroosmotic flow was studied to evaluate the variation of the immobilized columns. The prepared columns showed good chiral separation performance toward five proton pump inhibitors including lansoprazole, pantoprazole, tenatoprazole, rabeprazole, and omeprazole. The influences of β-cyclodextrin concentration, coating time, buffer pH, buffer concentration, and applied voltage on separation were investigated. In the optimum conditions, the enantiomers of five analytes were fully resolved within 15 min with high resolutions of 4.57 to 8.13. The method was extensively validated in terms of accuracy, precision, and linearity and proved to be robust. The relative standard deviation values for migration times and peak areas of the analytes representing intraday and interday were less than 1.9 and 3.6%, respectively. Further, the polydopamine/β-cyclodextrin coated capillary column could be successively used over 100 runs without showing significant decrease in the separation efficiency.

Potentiality of miniaturized techniques for the analysis of drugs of abuse

Capillary electromigration (CE) and liquid chromatographic techniques (CLC/nano-LC) are miniaturized techniques offering distinct advantages over conventional ones in the field of separation science. Among these, high efficiency, high chromatographic resolution, and use of minute volumes of both mobile phase and sample volumes are the most important. CE and CLC/nano-LC have been applied to the analysis of many compounds including peptides, proteins, drugs, enantiomers, ions, etc. Over the years, the methods described here have also been used for the analysis of compounds of clinical, forensic, and toxicological interest. In this review article, the main features of the mentioned techniques are summarized. Their potentiality for the analysis of drugs of abuse are discussed. Some selected applications in this field in the period of 2015-present are also reported.

Anhydride-linked β-cyclodextrin-bonded silica stationary phases with enhanced chiral separation ability in liquid chromatography

β-Cyclodextrin can be functionalized by derivation of reactive hydroxyl on the ring due to its special chiral environment and structural characteristics, which can be used to identify or separate a variety of chiral substance. In this manuscript, a series of excellent chiral stationary phases for high-performance liquid chromatography were developed for enantioseparation by using anhydride modified β-cyclodextrin bearing chiral (R/S)-α-phenethylamine or (S)-(+)-2-amino-1-propanol. They were characterized by elemental analysis, Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and BET. These chiral stationary phases presented good resolution and repeatability, about 17 kinds of enantiomers were effectively separated. And most of enantiomers were separated better than those reported in the literature in the same both normal and reversed phase modes. The RSD values of R_s for repeatability and column-to-column were below 0.44% and 2.83%, respectively. All results revealed that these new CSPs show great prospect for chiral separation in actual applications.

Chiral separation in the class of proton pump inhibitors by chromatographic and electromigration techniques: An overview

Proton pump inhibitors (PPIs) are benzimidazole-derivative chiral sulfoxides, frequently used in the treatment of gastric hyperacidity-related disorders. Due to their stereoselective metabolism, the eutomeric forms of PPIs can present a more advantageous pharmacokinetic profile by comparison with the distomers or racemates. Moreover, two representatives of the class are used in therapy both as racemates and as pure enantiomers (esomeprazole, dexlansoprazole). A relatively large number of enantioseparation methods employed for the stereoselective determination of PPIs from pharmaceutical, biological, and environmental matrices were published in the past three decades. The purpose of the current overview is to provide a systematic survey of the available chiral separation methods published since the introduction of PPIs in the therapy up to the present. Analytical and bioanalytical methods using different chromatographic and electromigration techniques reported for the enantioseparation of omeprazole, lansoprazole, pantoprazole, rabeprazole, ilaprazole, and tenatoprazole are included. The analytical conditions of the presented methods are summarized in three comprehensive tables, while a critical discussion of the applied techniques, possible mechanism of enantiorecognition, and future perspectives on the topic are also presented.

A lipase-based chiral stationary phase for direct chiral separation in capillary electrochromatography

Candida antarctica lipase B (CALB) is a natural biocatalyst with an intrinsically strong chiral environment and a high degree of enantio-selectivity, which is widely used in the separation of racemates. Here, a facile and efficient covalent immobilization approach was utilized to immobilize CALB onto the capillary inner wall as a novel chiral stationary phase to explore and broaden its application in the direct chiral separation by electrochromatography. The obtained CALB immobilized capillary column was characterized by scanning electron microscopy (SEM), fluorescence imaging and Fourier transform infrared spectroscopy (FT-IR). The enantioseparation property of the CALB immobilized capillary column was confirmed by direct chiral separation of several pairs of monoamine neurotransmitter enantiomers in OT-CEC mode. Outstanding enantioseparation performance for three types of monoamine neurotransmitter enantiomers including epinephrine, norepinephrine and phenylephrine was obtained by the CALB immobilized column. Thanks to the effectiveness of covalent bonding method and the intrinsic stability of CALB, the prepared CALB immobilized capillary columns were quite steady and reproducible. The relative standard deviations for retention times of the enantiomers were as follows: for intra-day (n = 5) runs (≤0.25%), inter-day (n = 3) runs (≤0.72%) and between-columns (n = 3) (≤2.42%). After 90 consecutive runs in CEC mode, the CALB immobilized column still exhibited desirable enantionseparation performance.

Chiral Graphene Hybrid Materials: Structures, Properties, and Chiral Applications

Chirality has become an important research subject. The research areas associated with chirality are under substantial development. Meanwhile, graphene is a rapidly growing star material and has hard-wired into diverse disciplines. Rational combination of graphene and chirality undoubtedly creates unprecedented functional materials and may also lead to great findings. This hypothesis has been clearly justified by the sizable number of studies. Unfortunately, there has not been any previous review paper summarizing the scattered studies and advancements on this topic so far. This overview paper attempts to review the progress made in chiral materials developed from graphene and their derivatives, with the hope of providing a systemic knowledge about the construction of chiral graphenes and chiral applications thereof. Recently emerging directions, existing challenges, and future perspectives are also presented. It is hoped this paper will arouse more interest and promote further faster progress in these significant research areas.

Facile preparation of ethanediamine-β-cyclodextrin modified capillary column for electrochromatographic enantioseparation of Dansyl amino acids

Herein, the fabrication of a fascinating multifunctional cyclodextrin (CD) chiral stationary phase and its chiral separation performance in capillary electrochromatography are proposed. A facile interfacial polymerization was used to anchor ethanediamine-β-cyclodextrin (EDA-β-CD) polymerized with trimesoyl chloride (TMC) and to form the chiral stationary phase (CSP) composite onto the surface wall of the capillary. The characters of prepared columns were confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray Photoelectron Spectrometer (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). This novel CSP offers multi-typical interactions including hydrogen bonding, π-interaction, hydrophobic and electrostatic interaction as well as steric effects which contribute to prominent chiral recognition for Dansyl-DL-amino acids in CEC modes. The EDA-β-CD modified column showed eminent enantioseparation performance towards five Dansyl-DL-amino acids (the DL-forms of valine, threonine, leucine, phenylalanine, serine). Besides, the prepared columns were perfectly reproducible and stable. The relative standard deviations of the enantiomer retention times for intra-day (n = 5), inter-day (n = 3) runs and column-to-columns (n = 3) are below 0.54%, 1.35% and 4.89%, individually. This innovative chiral stationary phase shows a broader application view and scope in chiral recognition domain.

Advances of capillary electrophoresis enantioseparations in pharmaceutical analysis (2017-2020)

Capillary electrophoresis is a powerful technique for the analysis of polar chiral compounds and has been widely accepted for analytical enantioseparations of drug compounds in pharmaceuticals and biological media. In addition, many mechanistic studies have been conducted in an attempt to rationalize enantioseparations in combination with spectroscopic and computational techniques. The present review will focus on recent examples of mechanistic aspects and summarize recent applications of stereoselective pharmaceutical and biomedical analysis published between January 2017 and November 2020. Various separation modes including electrokinetic chromatography in combination with several detection modes including laser-induced fluorescence, mass spectrometry and contactless conductivity detection will be discussed. A general trend also observed in other analytical techniques is the application of quality by design principles in method development and optimization.

History, advancement, bottlenecks, and future of chiral capillary electrochromatography

Capillary electrochromatography (CEC) represents a technique with less than 30 years of intense development and in this period this technique has seen huge promise, fast development, stagnation, and significant decline of innovative activity. The major goal of the present overview is not to present an extensive review of the literature on chiral CEC but to analyze the reasons for this dramatic development and attempting to answer questions such as: 1) Was the potential of CEC reasonably evaluated in 1990s before starting the explosive development in this field? 2) Did the development of this technique take the right track? 3) What other developments and competitive trends led to stagnation in the advancement of CEC? 4) Why is the activity in this field currently decreasing? 5) What are the current challenges and promises and what is the future of chiral CEC?

Chiral Separations in Preparative Scale: A Medicinal Chemistry Point of View

Enantiomeric separation is a key step in the development of a new chiral drug. Preparative liquid chromatography (LC) continues to be the technique of choice either during the drug discovery process, to achieve a few milligrams, or to a scale-up during the clinical trial, needing kilograms of material. However, in the last few years, instrumental and technical developments allowed an exponential increase of preparative enantioseparation using other techniques. Besides LC, supercritical fluid chromatography (SFC) and counter-current chromatography (CCC) have aroused interest for preparative chiral separation. This overview will highlight the importance to scale-up chiral separations in Medicinal Chemistry, especially in the early stages of the pipeline of drugs discovery and development. Few examples within different methodologies will be selected, emphasizing the trends in chiral preparative separation. The advantages and drawbacks will be critically discussed.

Use of Gamithromycin as a Chiral Selector in Capillary Electrophoresis

In this short communication, we report the use of a second-generation macrolide antibiotic, gamithromycin (Gam), as a novel chiral selector for enantioseparation in capillary electrophoresis (CE). A preliminary analysis of the experiment results shows that Gam is especially suitable for the separation of chiral primary amines. Factors influencing enantioseparations were systematically investigated including the composition of the background electrolyte (BGE), concentration of Gam, the type and proportion of organic solvents, applied voltage, etc. In particular, N-Methylformamide (NMF) was successfully used as a non-aqueous solvent for Gam, and shown to be extremely effective for the separation of primaquine (PMQ) and 1-aminoindan (AMI) when used alone or mixed with other commonly used non-aqueous solvents (e.g. methanol). To our knowledge this was also the first application of NMF as a non-aqueous solvent for antibiotic chiral selectors in CE. The best separations were obtained with 100 mM Tris, 125 mM H₃BO₃ and 80 mM Gam in methanol/NMF (25:75) solvent for PMQ and AMI, or 80-100 mM Gam in methanol for the other model analytes. Among the analytes, the resolution (Rs) of amlodipine (AML) reached up to 15.65, which is to our knowledge the highest value ever reported in CE studies for this compound (except for using molecularly imprinted polymers technique).