Electrochromatographic Separation Research Articles

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.

Construction of chiral crown ethers in robust covalent organic frameworks for electrochromatographic enantioseparation

ABSTRACT Capillary electrochromatography (CEC) is a rapidly emerging separation technique that merges the high separation efficiency of capillary electrophoresis with the exceptional selectivity of liquid chromatography. However, it remains a synthetic challenge to design functional chiral stationary phases (CSPs) with high chemical stability against acid and base in CEC enantioseparation. Here we demonstrate that incorporating chiral crown ethers into stable covalent organic frameworks (COFs) enables efficient and stable separations of racemates by CEC. This facilitates the crafting of two three-dimensional (3D) chiral COFs by polycondensation of a chiral 1,1'-binaphyl-20-crown-6-derived dialdehyde and tetraamines with diisopropyl substituents. Both feature an 11-fold interpenetrated diamond framework, characterized by tubular open channels decorated with chiral crown ethers serving as enantioselective recognition and binding sites. These frameworks demonstrate excellent stability in water, acid and base, thanks to the presence of bulky isopropyl groups that shield the dynamic imine linkages. Moreover, the precisely defined COF channels enhanced the accessibility of the enclosed crown ethers to the analytes while providing strong protection against harsh environments, rendering them suitable for CSPs in CEC separations. They can effectively separate some important enantiomers, including ketones, epoxides and alkaline substances, when utilized as coatings on chiral columns, particularly facilitating the chiral separation of drugs. This study advances the application of COFs in electrochromatographic separations, expanding the scope of porous materials design and engineering to create COFs with targeted enantioselective properties.

Efficient synthesis of high-purity carbon dots via self-polymerization driven bottom-up growth for high-selectivity sensing and high-efficiency separation

Carbon dots (CDs) have attracted extensive interest due to their exceptional optical properties and facile synthesis processes. However, achieving high-efficiency synthesis of high-purity CDs with high yield remains an unresolved critical issue. Herein, we report a facile and versatile self-polymerization-driven bottom-up growth strategy for efficient fabrication of high-purity CDs. As a proof-of-concept demonstration, dopamine and its four analogues were used as the self-polymerizable precursors for self-driven synthesis of fluorescent CDs with high-purity. By employing a mixed-solvent system and rationally regulating periodate oxidation process, the highly controllable self-polymerization processes and efficient solvothermal-assisted production of polydopamine and polyphenols derived CDs were readily achieved. Further integrating high molecular weight membrane dialysis, we successfully obtained the highly purified polydopamine-derived CDs with high yield of 21.1 % and quantum yield of 3.6 %, confirmed by nuclear magnetic resonance, capillary electrophoresis and other types of high-resolution characterization techniques. The formation and luminescence mechanisms of the self-polymerized CDs were also elucidated through comparative analysis of their morphological features and elemental compositions. We also demonstrated the broad universality of this self-polymerization driven bottom-up strategy by utilizing by employing four other phenolic compounds, including l-dopa, gallic acid, catechol, and pyrogallol, as readily accessible precursors for high-yield synthesizing highly purified fluorescent CDs. Moreover, the enormous application potential of the high-purity CDs for high-selectivity sensing and high-efficiency electrochromatographic separation was thoroughly demonstrated.

Exploring the performance of cellulose tris-3,5-dichlorophenylcarbamate as a stationary phase for the chiral electro-chromatographic separation of azole antifungals

In this study, the enantiorecognition and separation performance of cellulose tris-3,5-dichlorophenylcarbamate (CDCPC) as the chiral selector for the enantioseparation of some chiral antifungal compounds was investigated. The CDCPC was bonded to 5 µm porous silica particles and the stationary phase packed into fused silica capillaries (250 mm x 75 µm I.D.) using the "slurry method". The column was used for the separation of twelve selected azole compounds in their enantiomers by capillary electrochromatography (CEC). To study chiral recognition, the mobile phase was modified changing the organic solvent, water content, buffer type, concentration, and pH. Each change resulted in the enhancement of specific interactions at the expense of others, leading to chiral recognition. Their effects on retention factors, enantiomeric resolution, and peak broadening are critically discussed. The developed methodology was used as an appropriate test bench to evaluate the stability of the stationary phase under different mobile phase conditions, demonstrating a remarkable enantioselective capacity even after several tens of hours of continuous use. The optimized CEC-UV method was very effective in the enantiomeric separation of seven out of twelve azole fungicides, totaling fourteen separated peaks in a single 30 min run analysis.

In situ growth of imine-based covalent organic framework as stationary phase for open-tubular capillary electrochromatographic separation.

Designing advanced stationary phases to improve separation efficiency is essential in capillary electrochromatography. Due to their outstanding performance, covalent organic frameworks have recently demonstrated considerable promise in the field of separation science. Herein, an open-tubular capillary electrochromatography method was reported using porous imine-based covalent organic framework with sufficiently available interaction sites as stationary phase. The imine-based covalent organic framework coated capillary was easily prepared via an in situ growth method at room temperature, and its separation performance was evaluated, indicating the high separation efficiency for three types of analytes, including herbicides, polybrominated dibenzofurans, and bisphenols. Moreover, the imine-based covalent organic framework coated capillary showed good reproducibility and stability, with intraday (n=3), interday (n=3), and column-to-column (n=3) relative standard deviations of retention time and peak areas of less than 5%. The separation efficiency of the coated capillary remained unchanged even after 200 runs and the maximum theoretical plates reached up to 85595 N/m for 4,4'-ethylidenebisphenol. It was predicted that the imine-based covalent organic framework stationary phase would be a strong contender for chromatographic separation with high efficiency.

Sulfonic acid functionalized monolithic column for high selectivity capillary electrochromatography separation.

A new nano-scale spherical vinyl-functionalized covalent organic polymer (TAPT-DVA-COP) with uniform sizes around 300nm was initially constructed using 2,5-divinyl-1,4-benzaldehyde (DVA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) as monomers. Then, a sulfonic acid (-SO3H) modified COP termed COP-SO3H was developed based on post-sythesis method employing TAPT-DVA-COP as precursor. Capillary electrochromatography (CEC) monolithic columns were fabricated using the physical doping technique to exhibit the application potential of TAPT-DVA-COP and COP-SO3H. Compared to the TAPT-DVA-COP monolithic column, the COP-SO3H monolithic column achieved a highly selective separation between analytes with different properties, including monosubstituted benzenes, alkylbenzenes, hydroxybenzoates, nucleoside bases, and biogenic amines. Non-covalent interaction (NCI) analysis and experimental data show that the synergism of the sulfonic acid group and aromatic moieties on COP-SO3H endows the new stationary phase with diverse interactions, including ion exchange, hydrophobic, π-π and hydrogen bonding. In addition, the COP-SO3H monolithic column exhibited good reproducibility and excellent potential for the determination of hydroxybenzoates in compact powders and alkylbenzenes in effluent samples.

Preparation of open tubular capillary column covalently coated with polystyrene sulfonate with 4,4'-azobis (4-cyanopentanoyl chloride) as polymerization initiator for electrochromatographic separation of alkaloids, sulfonamides and peptides.

An open tubular capillary electrochromatography column covalently bonded with polystyrene sulphonate was prepared via in situ polymerization using functionalized azo-initiator 4,4'-Azobis(4-cyanopentanoyl chloride). Scanning electron, fluorescence and atomic force microscopy techniques showed the formation of a relatively rough layer of polymer. In addition, -CN and C = O stretching vibrations from infrared spectroscopy proved the successful immobilization of the azo-initiator through covalent bonding and X-rayPhotoelectronSpectroscopy confirmed the elemental composition of the formed polymer layer. The prepared column was found to be appropriate for small and medium-sized molecules separation. Compared to bare fused silica capillary column higher selectivity and resolution were obtained for the separation of alkaloids, sulfonamides and peptides as a result of the electrostatic and pi-pi stacking interactions between the small organic molecules and the coated column without compromising the electroosmotic flow mobility. Separation efficiency was also increased compared to the bare capillary for the separation of alkaloids (about 1.5 times). Moreover, intraday, inter day, intra batch and inter batch relative standard deviation values of retention time and peak area of peptides were within 2% and 10%, respectively, indicating a good repeatability of the column preparation procedure. The developed method for the covalent bonding of polymers through a functionalized azo-initiator could represent a promising stable method for the preparation of open tubular column. This article is protected by copyright. All rights reserved.

In situ growth of imine-based covalent organic framework as stationary phase for high-efficiency electrochromatographic separation

Design of the smart stationary phases, which can improve the separation efficiency is an essential work in the capillary electrochromatography (CEC). Owing to good excellent properties, covalent organic frameworks (COFs) have showed promising potential in the area of separation science. Here, a micro- and mesoporous COF TAPB-BTCA with adequate available interaction sites and outstanding mass transfer performance was first exploited as a stationary phase for high-efficiency in capillary electrochromatography. Through in situ growth approach, the COF TAPB-BTCA coated capillary column was facilely prepared at room temperature. The separation ability of the COF TAPB-BTCA coated capillary column was studied. The fabricated column showed high efficiency for the separation of six types of small molecular compounds, including alkylbenzenes, chlorobenzenes, phenols, parabens, vanillin and related phenolic compounds, and non-steroidal anti-inflammatory drugs (NSAIDs). The maximum theoretical plates reached up to 293,363 N/m for phloroglucinol, showing significantly improved column efficiency in comparison to previous reported COFs-based columns. In addition, the mass loadability for methylbenzene was achieved to 1.44 mg/mL. Also, excellent reproducibility and stability were obtained on the COF TAPB-BTCA coated columns. The relative standard deviations of intra-day (n = 3), inter-day (n = 3) and three batch tubes were all less than 2%, and no obvious change was presented in separation performance after the column was used 120 runs. This COF TAPB-BTCA-based stationary phase would be a promising candidate for high-efficiency chromatographic separation.

A monolithic azacalix[4]pyridine column for high-resolution and high-efficiency pressurized capillary electrochromatographic separation

Herein, a highly stable (NAllyl)4-bridged alyzacalix[4]pyridine monolithic column (nAC4P@capillary) was fabricated by copolymerizing nAC4P with GMA and EDMA into a stationary phase through a simple olefin addition reaction. The physicochemical properties of the prepared materials were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), energy dispersive X-ray (EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and elemental analysis (EA). A wide range of analytes with various properties was well separated and demonstrated the synergistic effect of the various interaction sites. The maximum column efficiencies (Nmax) for all the analytes reached 62,368 plates/m, and the relative standard deviations (RSDs) of the daily retention times of alkylbenzenes, measured over six months were all lower than 5.63%. Non-covalent interaction analysis reveals that nAC4P@capillary can effectively combine the advantages of electronic configurations, π-π interactions, CH-π interactions, O–H…N hydrogen bonds and lone pair-π interactions, providing excellent prospects for high-performance chromatographic separation.

Homomesoporous Metal-Organic Framework for High-Performance Electrochromatographic Separation.

Metal-organic frameworks (MOFs) have exhibited tremendous potential in the area of separation science. However, most of the developed MOF-based stationary phases contained only microporous structures and suffer from limited separation performance. Herein, homomesoporous MOFs with excellent mass transfer capability and strong thermodynamic interactions are first explored as the novel stationary phase for high-performance capillary electrochromatographic separations. As a proof of concept, noninterpenetrated mesoMOF-1 with uniform mesopore sizes (22.5 × 26.1 Å) and good stability was facilely grown on the inner surface of capillaries and applied as a homomesoporous MOF coating-based stationary phase for high-efficiency electrochromatographic separation. Seven types of analytes with different molecular dimensions were all baseline separated on a mesoMOF-1 coated column with high theoretical plate numbers and excellent repeatability, exhibiting significantly improved separation selectivity and column efficiency in comparison to a microporous HKUST-1 coated column. The maximum column efficiencies of the mesoMOF-1 coated column for substituted benzenes and halobenzenes reached up to 1.4 × 105 plates/m, and its mass loadability was also much higher than that of the HKUST-1 coated column. In addition, based on the analysis of adsorption kinetics and chromatographic retention behaviors, the interaction and retention mechanisms of different molecular-weight analytes on mesoMOF-1 coated stationary phases were systematically explored and disclosed in detail. These results indicate that the homomesoporous MOF-based stationary phase can effectively balance the kinetic diffusion (mass transfer capability) and thermodynamic interactions (the strength of adsorption interaction), having great potential for high-performance chromatographic separation.

Synthesis of spherical three-dimensional covalent organic frameworks and in-situ preparation of capillaries coated with them for capillary electrochromatographic separation

Covalent organic frameworks (COFs) are divided into two-dimensional (2D) and three-dimensional (3D) structures according to the connection dimension of covalent bonds. 3D COFs have smaller pore size and larger surface area, which would facilitate the separation of small organic molecules with similar structures and properties in capillary electrochromatographic (CEC) separation. However, the application of 3D COFs in CEC is still in its early stages. Thus, 3D COFs (3D-IL-COF-1 and 3D-IL-COF-2) were synthesis by a new synthesis method in this work. The related characterization results confirmed that spherical 3D-IL-COFs formed by the accumulation of small polyhedral particles were successfully synthesized. Then, 3D-IL-COFs were introduced into the inner wall of capillary by in-situ growth method to prepare the 3D COFs coated capillaries for the first time. The electrochromatographic separation performance of the 3D-IL-COFs coated capillaries was investigated using the 3D-IL-COF-1 coated capillary as the model. The results indicate that the 3D-IL-COF-1 coated capillary with uniform coating exhibits a broad-spectrum separation capacity not only for acidic, basic and neutral analytes but also for neutral isomers, which shows that 3D-IL-COFs becomes an attractive potential stationary phase for electrochromatographic separation.

Study of the separation ability differences of three covalent organic frameworks as coated materials in capillary electrochromatography

Covalent organic frameworks (COFs) have great potential applications in chromatographic separation. So it is crucial to understand the relationship between the separation ability of COFs and their structures. Here we report a strategy to evaluate the separation ability of three 2D COFs and explore the relationship between separation ability and their molecular structures. The three 2D COFs (COF-LZU1, COF-42 and COF-LZU8) have one same building unit 1,3,5-triformylbenzene, while varied from the conjugated linking units and functional side-chains. They were used to construct coated capillary column for capillary electrochromatographic separation of same groups of phthalates. They exhibited different separation efficiencies. COF-42 and COF-LZU8 coated capillary columns provided good signal resolutions and high column efficiencies with high theoretical plate numbers. It is demonstrated that COFs with hydrazone unit and longer side-chains provided higher selectivity and resolutions for the phthalates separation. Molecular simulations and DFT calculations were further proceeded to explore the deep reason why the three COFs coated CEC displayed different separation ability based on the host−guest interactions on molecular level. This work highlights a new opportunity to select or design functional COFs and improve their efficiency in chromatographic separation based on host−guest chemistry.

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

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

High Performance (High Pressure) Layer Electrochromatography Separation Technique: Equipment and Preliminary Results.

We present a new prototype device and propose a new analytical technique: high performance (high pressure) layer electrochromatography (HPLEC). The equipment provides a combination of overpressured layer chromatography (OPLC) and pressurized planar electrochromatography (PPEC), yet it still enables researchers to perform each of these analyses separately. In comparison to PPEC, HPLEC provides hydrodynamic flow of the mobile phase, irrespective of the voltage used and the mobile phase composition. The advantages of HPLEC over OPLC include the possibility of the use of the electrophoretic effect to influence the selectivity of separation and the use of the electroosmotic effect to facilitate the mobile phase flow in order to decrease backpressure and increase the flow velocity. Many operational parameters can be freely adjusted and optimized independently. The equipment is fully automated and can work in various separation/operational modes, including combinations of online/offline sample application and detection. We present preliminary results of simultaneous, fully online, multichannel HPLEC separation of analgesic drugs (including acetaminophen, ibuprofen, and tramadol) as an example of increasing analysis throughput.

Metal-Organic Frameworks-Based Immobilized Enzyme Microreactors Integrated with Capillary Electrochromatography for High-Efficiency Enzyme Assay.

Enzyme assays are important for studying enzyme-mediated biochemical reactions and for clinical diagnosis and drug development. The technique of an immobilized enzyme microreactor (IMER) integrated with capillary electrophoresis (CE) has been frequently utilized in online enzyme assays. However, the traditional approaches for IMER-CE enzyme analysis have some defects such as low loading capacity and poor stability. Herein, metal-organic frameworks (MOFs), which have enormous potential in the fields of enzyme immobilization and capillary electrochromatographic (CEC) separation, were first explored as novel support materials with good enzyme immobilization performance and stationary phases with excellent separation abilities to construct an integrated MOFs-IMER-CEC microanalysis system for a high-efficiency online enzyme assay. As a proof-of-concept demonstration, acetylcholinesterase (AChE) was immobilized on a densely packed UiO-66-NH2 nanocrystal coating on a capillary inner surface with abundant intercrystalline mesoporosity and was employed as a highly effective and robust IMER for CEC-integrated online enzyme analysis. The excellent separation performance of the UiO-66-NH2-modified capillary was verified by high-efficiency separation of three types of neutral, acidic, and basic compounds. The Michaelis-Menten constant and enzyme inhibition kinetics of UiO-66-NH2-IMER were systematically assessed, exhibiting distinct advantages such as remarkably increased enzyme loadability, superior affinity for substrates, and greatly improved stability and repeatability compared to CE-integrated IMERs prepared by the traditional covalent bonding method. Furthermore, the developed method was successfully utilized for detecting organophosphorus pesticides in leguminous vegetable samples, demonstrating its strong practicality. The study not only proposed a novel support material and construction strategy for a high-performance microchannel-based IMER but also can be widely used in bioanalysis and biosensing research.

Preparation and application of porous organic cage capillary electrochromatographic chiral column

多孔有机笼(POCs)是一种新型的具有稳定有序三维空腔结构的多孔材料。通过2-羟基-1,3,5-均苯三甲醛与1R,2R-1,2-二苯基乙二胺发生席夫碱的缩合反应,合成了一种具有羟基功能基团的单一手性POCs材料;将其均匀涂敷在毛细管壁上制成色谱柱,利用电色谱柱成功拆分了二氢黄酮、吡喹酮、萘普生和3,5-二硝基-N-(1-苯乙基)苯甲酰胺4种手性化合物。探究了分离电压、缓冲溶液浓度及其pH值等因素对手性拆分的影响,获得了4种手性物质在POCs色谱柱上的最佳拆分条件。实验研究表明,二氢黄酮、吡喹酮、萘普生和3,5-二硝基-N-(1-苯乙基)苯甲酰胺获得优化分离效果所需的工作电压分别为13、14、14和12 kV;二氢黄酮适宜Tris-H3PO4缓冲溶液浓度为0.075 mol/L,吡喹酮、萘普生和3,5-二硝基-N-(1-苯乙基)苯甲酰胺适宜Tris-H3PO4缓冲溶液浓度为0.100 mol/L; 4种手性物质得到最佳分离效果时的pH值均为3.51。二氢黄酮、吡喹酮、萘普生和3,5-二硝基-N-(1-苯乙基)苯甲酰胺均达到基线分离,分离度分别为2.99、2.10、2.58和3.59。该POCs色谱柱还成功拆分了o,m,p-碘苯胺、o,m,p-硝基苯胺两种位置异构体。该研究表明POCs手性电色谱柱具有良好的手性识别能力,是一种优秀的手性分离材料,具有很大的电色谱手性分离应用前景。

Application of Sol-Gels Modified with Natural Plants Extracts as Stationary Phases in Open-Tubular Capillary Electrochromatography.

Ethanol extracts of three widely growing plants were added to silica sol–gel solutions, which were subsequently applied as wall surface modifiers in inner quartz capillaries. Modified capillaries were used for open-tubular capillary electrochromatographic separation of nucleotides and amino groups containing biological compounds (neurotransmitters, amino acids and oligopeptides). The experiments were performed at physiological pH 7.40, and eventual changes of effective mobilities were calculated. Specific compounds characteristic for each plant were tested as sol–gel additives as well, and thus-modified capillaries were used for the separations of the same analytes under identical conditions. The aim of this study was to find out possible interactions between physiological compounds and extracts of freely available plants anchorded in the sol-gel stationary phase in the flowing system. Even though the amount of the modifier in each capillary is very small, basic statistical evaluation showed some not negligible changes in effective mobility of tested analytes. These changes were bigger than ±5% for separations of nucleotides in capillaries with curcuma, Moringa or the mixture of synthetic additives as the sol-gel aditive, and for separations of amino compounds where these changes varying by additive, analyte by analyte.

Metal-Organic Framework-Based Enzyme Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Metal-Organic Framework-Based Enzyme Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Enzyme Metal-Organic Framework Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

Enzyme Metal-Organic Framework Composites as Chiral Stationary Phase for High-Efficiency Capillary Electrochromatographic Enantioselective Separation

A Monolithic Azacalix[4]Pyridine Column for High-Resolution and High-Efficiency Capillary Electrochromatographic Separation

A Monolithic Azacalix[4]Pyridine Column for High-Resolution and High-Efficiency Capillary Electrochromatographic Separation