Recent applications and chiral separation developments based on stationary phases in open tubular capillary electrochromatography (2019–2022)

Fabrication and evaluation of stationary phases with different morphology from two imidazole-based ligands for nonchiral and chiral electrochromatographic separation

In this work, two metal-organic porous materials from the two imidazole-based ligands (4-methylimidazole-5-carbaldehyde (aImeIm) and histidine (His)) and the same metal source (Zn(CH3COO)2·2H2O), namely His-ZIF-93 with a crystal zeolite imidazolate framework and His/aImeIm@Zn with an amorphous networks, were first designed and synthesized by the one-pot method. They were used as the stationary phases (SPs) of open-tubular capillary electrochromatography (OT-CEC) to evaluate and compare their separation performance for the nonchiral and chiral compounds. His-ZIF-93-bonded OT-CEC column successfully separated five families of nonchiral analytes due to the synergistic effect of His and the crystal framework. The separation mechanism included electrostatic interaction, molecular sieve effect, π-π interaction, electrophoretic mobility, hydrogen bonding interaction, and so on. His/aImeIm@Zn-bonded OT-CEC column exhibited a more superior enantioseparation capability for seven chiral compounds with an ultrahigh column efficiency of 7.6 × 105 plates/m due to a large number of chiral sites and the porous networks. Enantioseparation were attributed to the difference of adsorption and selectivity between the SPs and the two enantiomers through the adsorption experiments and the binding constants tests. Thus, both His-ZIF-93-bonded OT-CEC column and His/aImeIm@Zn-bonded OT-CEC column demonstrated the universality for the analytes, as well as excellent reproducibility, satisfactory stability and long lifetime.

Enhanced field-amplified sample injection by mobility decrease in capillary electrophoresis for the analysis of trace enantiomeric impurities in palonosetron injection

Palonosetron hydrochloride [PALO (3aS, 2S)] is a highly selective second generation 5-HT3 receptor antagonist with two asymmetric carbon atoms in molecular structure. For chiral drugs, detection of the enantiomeric impurity is critical for drug safety and efficacy, because the undesired enantiomer can cause adverse effects or reduce the drug's therapeutic effect. In this work, the mechanism of online preconcentration in the analysis of PALO injection using micellar electrokinetic chromatography (MEKC) with bile salt sodium taurocholate (STC) as chiral selector was studied, based on the background difference between sample solution and background electrolyte (BGE). In addition to the field amplified sample stacking due to the conductivity difference between the sample plug and BGE, the disassociation of protonated solutes with positive charge caused by the pH increase and the distribution of solutes into STC micelles with negative charge result in a significant decrease in effective mobility, when they enter the BGE. This, combined with a reduction in field strength, causes the solutes slow down quickly and stack at the interface between sample plug and BGE. However, achieving the analysis of all three possible enantiomeric impurities in hydrodynamic injection mode is challenging due to poor separation. This is caused by the long effective capillary length consumed during online preconcentration and the wide solute band eventually focused if enough sample solution is injected. This issue was resolved by using electrokinetic injection mode, where solutes are driven into the capillary by both electric field and electroosmotic flow and stack at the interface meanwhile. As a result, a sufficient amount of solutes can be injected in a shorter sample plug compared to hydrodynamic injection mode. The method achieved a limit of detection of 0.014 μg mL-1 and a limit of quantification of approximately 0.05 μg mL-1 for enantiomeric impurities, without compromising the separation. Those are 0.03 % and 0.10 % of the content of main ingredient, respectively, representing nearly a ten-fold improvement in sensitivity over our previous works. Other quantitative data include a linear range of 0.05-0.50 μg mL-1, a coefficient of determination (R2) of 0.993-0.998, a spike recovery of 96.1-100.6 %, and a precision (RSD) of 2.2 % and 3.6 % for the intra-day and inter-day analysis, respectively. For the first time, the identification and quantification of trace amount of enantiomeric impurity in real sample of PALO drug was successfully achieved using MEKC with bile salt as chiral selector.

Recent Advances in the Construction and Applications of Monolithic and Open-tubular Capillary Electrochromatography (2022-2024)

Capillary electrochromatography (CEC) has attracted significant attention and gained considerable recognition in the field of separation science owing to its excellent separation efficiency. While numerous reviews on CEC have been published in recent years, a comprehensive and systematic summary of the typical synthesis strategies for electrochromatographic stationary phases and their state-of-the-art applications in CEC remains lacking. This review highlights recent advances (over the past 3 years) and representative applications (including chiral separation, microextraction-coupled analysis, metabolomics, enzyme analysis, and food analysis) of monolithic and open-tubular stationary phases in CEC. The advantages and limitations of each methodology are critically analyzed to present a balanced evaluation. Additionally, this work outlines future prospects regarding the development trends in electrochromatographic stationary phase preparation methods and the evolving applications of CEC.

Recent developments and applications of solid membrane in chiral separation

Chirality is a fundamental property in nature, and chiral molecules are closely related to human health and the origin of life. Therefore, the exploration and preparation of optically active compounds of paramount importance. Membrane separation is a large-scale and continuous separation technique that has been developing quickly in recent years. It has many potential applications, particularly in chiral membrane separation technology, which is currently a hotspot for study. Depending on the types of membranes, chiral membranes can be divided into two categories: chiral solid membranes and chiral liquid membranes. Solid membranes outperform the others in terms of better mechanical performance and separation efficiency. This review presents in-depth summaries of chiral solid membranes made of different materials, and their applications in drug separation. It also providing insights into the potential for the future development of chiral solid membranes.

Enantioseparation system based on a novel nanomaterial synthesized from chiral molecularly imprinted polymers and achiral metal-organic frameworks by capillary electrochromatography

A novel nanomaterial synthesized by chiral molecularly imprinted polymers (CMIPs) and achiral metal-organic frameworks (MOFs) was designed as stationary phase to prepare L-TRP@MIP(APTES-TEOS)@UiO-66-NH2@capillary for tryptophan enantioseparation in open tubular capillary electrochromatography. Compared with the capillary column coated only with CMIPs or achiral MOFs, this column remarkably improved the enantioseparation ability of tryptophan (resolution, 0.92/0 → 3.68). The chromatographic conditions (buffer pH, applied voltage, organic additive content) were optimized. Additionally, through static adsorption experiments, a conclusion was reached that the materials of stationary phase had stronger adsorption capacity for L-TRP than that for D-TRP, which revealed chiral separation mechanism of CEC system. This study opens up creative ideas for coating the novel nanomaterial in CEC system, which has good application prospects in the field of chiral separation.

Proportionally controlled dual-chiral covalent organic frameworks via thiol-ene click reaction for efficient enantioseparation

Developing a universally applicable and commercially hopeful chiral material is a significant challenge for the separation and analysis field. In this study, innovative dual-chiral CCOFs ([SH-β-CD-NALC]x-COFs) modified with 6-deoxy-6-mercapto-β-cyclodextrin (SH-β-CD) and N-acetyl-l-cysteine (NALC) are presented. These CCOFs were synthesized using a single, one-step bottom-up approach at room temperature, specifically designed for enantiomer recognition and separation. We investigated the effect of varying ratios of multiple chiral selectors in CCOFs on chiral recognition abilities through adsorption experiments for the first time. The precisely engineered [SH-β-CD-NALC]1/6-COF, with excellent stability, crystallinity, abundant chiral sites, and a greater specific surface area, was well suited as a chiral stationary phase (CSP) in various racemates separation. The results showed satisfactory resolution, column efficiency, stability, and reproducibility. In addition, mechanism studies have revealed that the dual-chiral COF can offer triple selectivity and achieve the effect where 1 + 1 is greater than 2. This work emphasized the advantages of dual-chiral COF in achieving racemic drug separation, providing a new approach for the development of high-performance chiral separation platforms in the future.

Simultaneous stereoisomeric preconcentration and determination of alkaloids and stereoisomers in Uncariae ramulus cum uncis using field enhanced sample injection and dynamic pH junction sweeping method by cyclodextrin electrokinetic chromatography

A rapid and sensitive enrichment technique, field enhanced sample injection-dynamic pH junction-sweeping (FESI-DypH-sweeping) was successfully developed for the simultaneous separation and concentration of alkaloids and stereoisomers of Uncariae ramulus cum uncis (UR) by cyclodextrin electrokinetic chromatography (CDEKC) with diode array detection system. The sample was prepared in a low-conductivity (FESI), low-pH (DypH) sample matrix (4 mM phosphate buffer, 3% methanol, pH=3), and the background electrolyte (BGE) consisted of a high-conductivity, high-pH buffer (40 mM phosphate buffer, pH 7.0, 8 mg/mL carboxymethyl-β-cyclodextrin, and 30% methanol). The crucial parameters influencing separation and enrichment efficiency were systematically optimized using single variable method. Under the optimum conditions, the method provided satisfactory resolution for seven alkaloids within 18 minutes, achieving enrichment factors ranging from 25 to 78 folds. The limits of detections (LODs) ranged from 0.04 to 0.12 μg/mL and limits of quantifications (LOQs) ranged from 0.10 and 0.40 μg/mL respectively. Intra-day and inter-day precision, expressed as relative standard deviations, were all below 3.5%.

Zirconium(IV) coordination-mediated rapid and versatile post-modification of polydopamine coating as stationary phase for open-tubular capillary electrochromatography

In recent years, mussel-inspired polydopamine (PDA)-based materials have attracted significant attention in the field of open-tubular capillary electrochromatography (OT-CEC) owing to their diverse and appealing properties. However, previously established functionalized PDA coating-based CEC stationary phases predominantly relied on the latent reactivity of PDA with amine/thiol-containing molecules, limiting the types of applicable modifiers and requiring time-consuming reaction processes. Herein, we presented a versatile and efficient method for the facile and rapid fabrication of diverse functionalized PDA coatings as OT-CEC stationary phases through a Zr(IV) coordination-mediated post-modification strategy. Different kinds of modifiers, including octadecylamine (ODA), lauric acid (LA), and perfluorooctanoic acid (PFOA), were rapidly and robustly grafted onto the PDA coating, verified through multiple characterization techniques. The influences of preparation parameters on the grafting efficiency of the functionalized PDA coating were systematically investigated. Utilizing the Zr(IV)-mediated ODA-, LA- and PFOA-functionalized PDA-based OT-CEC columns, we achieved high-efficiency baseline separation of a series of neutral analytes with excellent repeatability, good stability, and long lifetime. Given the strong universality of the Zr(IV) coordination-mediated post-modification approach, our study provides an effective pathway for advancing the development of a wider range of functional PDA-based chromatographic stationary phases.

Chiral-induced covalent organic framework as novel chiral stationary phase for chiral separation using open-tubular capillary electrochromatography

As a novel class of chiral stationary phase (CPS) material, chiral covalent organic frameworks (CCOFs) have already shown great promise in open-tubular capillary electrochromatography (OT-CEC) for chiral separation. The synthesis methods of CCOFs used in OT-CEC mainly include bottom-up, post modification and chiral induction. The CCOFs synthesized by bottom-up and post modification strategies already have lots of applications in capillary electrochromatography, however, the chiral-induced synthesized via an asymmetric catalytic strategy has not yet been reported for using as the chiral stationary phase (CPS) in OT-CEC or even in chromatographic separation. Herein, the chiral-induced COF (Λ)-TpPa-1 was synthesized by asymmetric catalytic synthesis and coated on the inner surface of a capillary by an in-situ growth strategy as the CPS for chiral drug separation. The baseline separation of six enantiomers was achieved within 14 min, with a high-resolution (Rs) range from 1.85 to 6.75. Moreover, the resolution and migration time of the capillary keep stable within 160 runs, showing its superior stability and repeatability. This research provides a new idea for the development and application of novel CPS materials in the field of capillary electrochromatography separation, also shows the new application of chiral induced COFs. Furthermore, the chiral-induced CCOFs can be easily applied to other chromatographic separation fields, exhibiting its extensive application value in chiral analysis separation.

Magnetic 3D macroporous MOF oriented urinary exosome metabolomics for early diagnosis of bladder cancer

Bladder cancer (BCa) exhibits the escalating incidence and mortality due to the untimely and inaccurate early diagnosis. Urinary exosome metabolites, carrying critical tumor cell information and directly related to bladder, emerge as promising non-invasive diagnostic biomarkers of BCa. Herein, the magnetic 3D ordered macroporous zeolitic imidazolate framework-8 (magMZIF-8) is synthesized and used for efficient urinary exosome isolation. Notably, beyond retaining the single crystals and micropores of conventional ZIF-8, MZIF-8 is further enhanced with highly oriented and ordered macropores (150 nm) and the large specific surface area (973 m2·g-1), which could enable the high purity and yield separation of exosomes via leveraging the combination of size exclusion, affinity, and electrostatic interactions between magMZIF-8 and the surfaces of exosome. Furthermore, the magnetic and hydrophilic properties of magMZIF-8 will further simplify the process and enhance the efficiency of separation. After conditional optimization, a 50 mL of urine is sufficient for exosome metabolomics analysis, and the time for isolating exosomes from 42 urine samples was 2 hours only. Incorporating machine learning algorithms with LC-MS/MS analysis of the metabolic patterns obtained from isolated exosomes, early-stage BCa patients were differentiated from healthy controls, with area under the curve (AUC) value of 0.844-0.9970 in the training set and 0.875-1.00 in the test set, signifying its potential as a reliable diagnostic tool. This study offers a promising approach for the non-invasive and efficient diagnosis of BCa on a large scale via exosome metabolomics.

Hydrogen-bonded organic frameworks as stationary phase for open-tubular capillary electrochromatography

BACKGROUND

Capillary electrochromatography (CEC) stationary phases have always been the focus of attention. The selection of excellent stationary phases are the key to realize separate of different compounds. Hydrogen-bonded organic frameworks (HOFs) are porous materials connected by hydrogen bonds between molecules, which have the advantages of renewable, high specific surface area and mild synthesis conditions. At present, HOFs are used in gas adsorption and storage, catalysis and drug delivery. Because of its unique advantages, HOFs have a bright future as CEC stationary phases.

RESULTS

Using melamine (MA) and 1,3,6,8-tetra (4-carboxylphenyl)pyrene (H4TBAPy) as reaction monomers, a HOFs named MA/PFC-1 was synthesized by solvent evaporation at room temperature. The inner wall of the capillary column was coated with MA/PFC-1 by chemical bonding. Sulfonamides were used as the target analytes. The effects of pH, phosphate buffer solution concentration, organic additive content and applied voltage on sulfonamides separation were investigated. The MA/PFC-1-coated capillary column had good resolution (>1.5) and reproducibility. The intra-day, inter-day, column-to-column, and inter-batch precision of the retention times were 0.03%-0.09%, 0.04%-0.09%, 0.03%-0.14% and 0.06%-0.09%, respectively. The intra-day, inter-day, column-to-column, and inter-batch precision of the peak areas were 0.11%-0.25%, 0.13%-0.20%, 0.12%-0.15% and 0.08%-0.15%, respectively. The MA/PFC-1-coated capillary column was run 150 consecutive times, and the results showed no noticeable change, which proved that this method had good stability.

SIGNIFICANCE

This work applied HOFs to CEC. The results show the that MA/PFC-1-coated capillary column has good separation performance. The MA/PFC-1-coated capillary column has been successfully applied to the determination of sulfamethoxazole in tablets, which has practical application value. To open up the application of HOFs in CEC and provide a new idea for developing new CEC stationary phases.

Enantioseparation of six profenoid drugs by capillary electrophoresis with bovine serum albumin-modified gold nanoparticles as quasi-stationary phases

Profenoid drugs are a kind of common non-steroidal anti-inflammatory drugs and their chiral enantiomers often have huge differences in pharmacological activities. In this work, a novel chiral separation system by capillary electrophoresis (CE) was constructed using gold nanoparticles (AuNPs) functionalized with bovine serum albumin (BSA) as a quasi-stationary phase (QSP), and the enantioseparation of six profenoid drugs was efficiently accomplished. Under optimal chromatographic conditions, the enantioseparation performance of the AuNP@BSA-based chiral separation system was greatly improved compared with that of free BSA (Resolutions, Ibuprofen: 0.89 → 8.15; Ketoprofen: 0 → 10.02; Flurbiprofen:0.56 → 9.83; Indoprofen: 0.88 → 13.83; Fenoprofen: 0 → 15.21; Pyranoprofen: 0.59 → 5.34). Such high Rs are exciting and satisfying and it is in the leading position in the reported papers. Finally, through molecular docking, it was also found that the difference in binding energy between BSA and enantiomers was closely related to the resolutions of CE systems, revealing the chiral selection mechanism of BSA. This work significantly improves the CE chiral separation performance through a simple strategy, providing a simple and efficient idea for the chiral separation method.

Determination of amino acids and peptides without their pre-column derivatization by capillary electrophoresis with ultraviolet and contactless conductivity detection. An overview

This review provides an overview of recent works focusing on the determination of amino acids (AAs) and peptides using capillary electrophoresis with contactless conductivity detection and ultraviolet (UV) detection, which is the most widespread detection in capillary electromigration techniques, without pre-capillary derivatization. Available options for the UV detection of these analytes, such as indirect detection, complexation with transition metal ions, and in-capillary derivatization are described. Developments in the field of direct detection of UV-absorbing AAs and peptides as well as progress in chiral separation are described. A separate section is dedicated to using on-line sample preconcentration methods combined with capillary electrophoresis-UV.

Rapid and in-situ preparation COFs coated capillary by adsorption method for the separation and determination of phthalate ester using CEC

As a novel class of stationary phase materials, covalent organic frameworks (COFs) have shown great promise in open-tubular capillary electrochromatography. However, the current preparation of COFs coating capillaries heavily relies on tedious and time-consuming covalent bond methods. In this work, a novel, simple and rapid adsorption method was developed for fabrication of TPB-DMTP COF (fabricated from 1,3,5-tris(4-aminophenyl)benzene (TPB) and 2,5-dimethoxyterephthalaldehyde (DMTP)) coated capillary. Due to the crystallization process of the COF is greatly shortened because pre-modification capillary does not require silane coupling agent, this method enables the rapid preparation of COFs-coated capillaries. The organic molecular building units only need 25 min to form a stable COFs coating on the inner wall of a capillary by this method. To our knowledge, this is the shortest method for preparing COFs coated capillary up to now. The performance of the TPB-DMTP COF coated capillary was evaluated by using phthalate esters as model analytes. The results demonstrated that the TPB-DMTP COF coated capillary has excellent repeatability and stability. The relative standard deviations (RSDs) of the analyte's retention time of intra-day, inter-day and column-to-column were in the range of 0.05 %-0.27 %, 0.31 %-0.63 % and 0.31 %-0.88 %, respectively. And, no significant changes were observed in separation efficiency and retention time after over 200 runs. Finally, the TPB-DMTP COF coated capillary was applied for the determination of phthalates in marketed plastic bag and the recovery ranged from 88.0 % to 114.0 %.

Enantiomeric analysis of chiral phenyl aromatic compounds by coated capillary electrochromatography based on a MOF-on-MOF stationary phase

Chiral phenyl aromatic compounds (CPACs) are widely used in drug development, food/cosmetic production, and other organic synthesis processes, and their different enantiomers have distinct physiological activities and application differences. A double-layer metal-organic framework composite (MOF-on-MOF) was obtained by in situ synthesis of chiral metal-organic framework (CMOM-3S) on the surface of an iron-based metal-organic framework (NH2-MIL-101(Fe)). According to our investigation, MOF-on-MOF composite was for the first time applied to the stationary phase of capillary electrochromatography (CEC), and enantioseparations of eight CPACs were accomplished. Compared with single CMOM-3S, the enantioseparation performance of the coated capillary columns based on NH2-MIL-101(Fe)@CMOM-3S was improved by 34.07 ~ 720.0%. The R-/S-mandelic acid in actual sample (apricot leaves) was detected by the newly CEC system to be 0.0118 mg mL-1 and 0.0523 mg mL-1, respectively. The spike recoveries were 96.60 ~ 104.7%, indicating its good stability and accuracy. In addition, the selective adsorption capacity of MOF-on-MOF composites was verified by adsorption experiments.

Formation Process of SiF6@Cu2L4 Chiral Cage Pairs in a Glass Vessel: Catechol Oxidation Catalysis and Chiral Recognition

Self-assembly of CuX2 (X- = BF4-, PF6-, and SbF6-) with a pair of chiral bidentate ligands, (1R,2S)-(+)- and (1S,2R)-(-)-1-(nicotinamido)-2,3-dihydro-1H-inden-2-yl-nicotinate (r,s-L or s,r-L), in a mixture solvent including ethanol in a glass vessel gives rise to SiF62--encapsulated Cu2L4 chiral cage products. The SiF62- anion from the reaction of X- with SiO2 of the glass-vessel surface acts as a cage template or cage bridge. One of the products, [SiF6@Cu2(SiF6)(s,r-L)4]·3CHCl3·4EtOH, is one of the most effective heterogeneous catalysts for the oxidation of 3,5-di-tert-butylcatechol. Furthermore, an l-DOPA/d-DOPA pair is recognizable by the cyclic voltammetry (CV) signals of its combination with chiral cages [SiF6@Cu2(BF4)2(s,r- or r,s-L)4]·4CHCl3·2EtOH pair and [SiF6@Cu2(SiF6)(s,r- or r,s-L)4]·3CHCl3·4EtOH pair.

Potential of liposomes and lipid membranes for the separation of β-blockers by capillary electromigration and liquid chromatographic techniques

β-Blockers belong to a frequently used class of drugs primarily used to treat heart and circulatory conditions. Here we describe the use of lipid vesicles and liposomes as cell membrane biomimicking models in capillary electromigration (CE) and liquid chromatography (LC) techniques for the investigation of interactions between lipid membranes and β-blockers. In addition to liposomes, the use of commercial intravenous lipid emulsions, and their interactions with β-blockers are also discussed. Different CE and LC instrumental techniques designed for these purposes are introduced. Other methodologies for studying interactions between β-blockers and lipid membranes are also briefly discussed, and the different methodologies are compared. The aim is to give the reader a good overview on the status of the use of liposomes and lipids in CE and LC for studying β-blocker interactions.

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.