Recent highlights in stationary phase design for open-tubular capillary electrochromatography

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 %.

In-situ growth of a spherical vinyl-functionalized covalent organic framework as stationary phase for capillary electrochromatography-mass spectrometry analysis

Column technology is an important part in capillary electrochromatographic science. Developing novel stationary phase with high separation efficiency and high loading capacity is an essential work. In this work, a novel spherical vinyl-functionalized covalent-organic framework (COF-V) was synthesized at room temperature and firstly employed as stationary phase for CEC-MS analysis. The COF-V based CEC column was characterized by scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The results proved the successful modification of COF-V. The COF-V based column possesses the advantages like strong electroosmotic flow, high separation efficiency and high loading capacity. The CEC column showed powerful separation selectivity to several kinds of compounds, and the highest column efficiency (theoretical plates, N) was over 1.4 × 10⁵ plates·m^-1 for methylbenzene. Besides, the COF-V modified column exhibited excellent repeatability and stability. The relative standard deviations (RSDs) of retention times for intra-day (n = 5), inter-day (n = 3) runs and column-to-column (n = 3) were all less than 2.1%. Hence, the COF-V modified column was successfully applied in CEC-MS for determination of antiepileptic drug, triazine herbicides and active ingredients in traditional Chinese medicine.

Metal organic framework ZIF-90 modified with lactobionic acid for use in improved open tubular capillary electrochromatographic enantioseparation of five basic drugs

An in situ zeolite imidazole metal organic framework-90 (ZIF-90) modified capillary was prepared via the method of solvothermal synthesis. The coating of ZIF-90 was characterized by scanning electron microscopy, energy-dispersive X-ray spectrometry, and EOF. Capillary electrochromatography-based enantioseparation of the basic drugs propranolol (PRO), metoprolol (MET), atenolol (ATE), bisoprolol (BIS), and sotalol (SOT) was performed using lactobionic acid (LA) as the chiral selector. Compared with an uncoated silica capillary, the resolutions are greatly improved (PRO 1.40 → 3.23; MET 1.07 → 3.19; ATE 1.07 → 3.15; BIS 1.16 → 3.41; SOT 1.00 → 2.79). Effects of buffer pH values, proportion of organic additives, concentration of lactobionic acid, and applied voltage were investigated. Graphical abstract Schematic presentation of the preparation of zeolitic imidazolate framework-90 (ZIF-90) modified capillary (ZIF-90@capillary) for enantioseparation of drug enantiomers. The capillary was applied to construct capillary electrochromatography system with lactobionic acid for enantioseparation of basic chiral drugs.

Fluoro-functionalized stationary phases for electrochromatographic separation of organic fluorides

Fluorous affinity means remarkably specific interaction between highly organic fluorides. This work aims to explore the potential of fluoro-functionalized stationary phase for the separation of organic fluorides by means of fluorous-fluorous interaction. Here, by using the Michael addition strategy between 1H,1H,2H,2H-perfluorodecanethiol (PFDT) and polydopamine (PD), a novel fluoro-functionalized stationary phase was synthesized for open-tubular capillary electrochromatography (OT-CEC). The PFDT@PD was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectrometer (XPS). The PFDT@PD@capillary exhibited outstanding separation performance towards neutral compounds (such as alkylbenzenes and chlorobenzenes) and organic fluorides (such as fluorobenzenes and perfluoroalkyl methacrylates etc.) with high resolution and high separation efficiency by hydrophobic interaction and fluorous-fluorous interaction. In addition, the column shows good stability and reproducibility. The relative standard deviations (RSDs) of the retention time for intra-day (n = 5) and inter-day (n = 3) runs and between columns (n = 3) are less than 0.39%, 1.22% and 3.87%, respectively. This novel type of fluoro-functionalized stationary phase represents a great application potential in organic fluorides separation field.

Open-tubular Capillary Electrochromatography with Janus Structured Au-Fe3O4 Nanoparticles Coating as Stationary Phase

A novel Au-Fe₃O₄ nanoparticles-based capillary column was fabricated by magnetic approach for open-tubular capillary electrochromatography (OT-CEC). The bifunctional dumbbell-like Janus Au-Fe₃O₄ nanoparticles (Au-Fe₃O₄ NPs) were prepared through a hydrothermal synthesis strategy, and the morphology was characterized by transmission electron microscopy (TEM). Multilayers Au-Fe₃O₄ NPs were easily coated onto the inner surface of silica capillary by an external magnetic field to generate an Au-Fe₃O₄ NPs-based column. Compared with a bare capillary, the modified surface exhibited more stable and suppressed electroosmotic mobility. The column showed good separation efficiency for neutral analytes in the OT-CEC separation mode, with theoretical plate numbers of up to 79705 per meter for naphthalene. The successful separation of dihydroxy benzene isomers and proteins demonstrated that the column exhibits a reasonable separation performance. The reproducibility of the Au-Fe₃O₄ NPs capillary was studied, with relative standard deviations (RSD) for day-to-day and column-to-column less than 1 and 1.75%, respectively.

Metal organic framework HKUST-1 modified with carboxymethyl-β-cyclodextrin for use in improved open tubular capillary electrochromatographic enantioseparation of five basic drugs

This work shows that the metal organic framework (MOF) HKUST-1 of type Cu₃(BTC)₂ (also referred to as MOF-199; a face-centered-cubic MOF containing nanochannels) is a most viable coating for use in enantioseparation in capillary electrochromatography (CEC). A HKUST-1 modified capillary was prepared and characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectra, elemental analysis and thermogravimetric analysis. CEC-based enantioseparation of the basic drugs propranolol (PRO), esmolol (ESM), metoprolol (MET), amlodipine (AML) and sotalol (SOT) was performed by using carboxymethyl-β-cyclodextrin as the chiral selector. Compared with a fused-silica capillary, the resolutions are improved (ESM: 1.79; MET: 1.80; PRO: 4.35; SOT: 1.91; AML: 2.65). The concentration of chiral selector, buffer pH value, applied voltage and buffer concentration were optimized, and the reproducibilities of the migration times and R_s values were evaluated. Graphical abstract Schematic presentation of the preparation of a HKUST-1@capillary for enantioseparation of racemic drugs. Cu(NO₃)₂ and 1,3,5-benzenetricarboxylic acid (BTC) were utilized to prepare the HKUST-1@capillary. Then the capillary was applied to construct capillary electrochromatography system with carboxymethyl-β-cyclodextrin (CM-β-CD) for separation of basic racemic drugs.

Open-tubular capillary electrochromatography with β-cyclodextrin-functionalized magnetic nanoparticles as stationary phase for enantioseparation of dansylated amino acids

Magnetic nanoparticles (MNPs) modified with β-cyclodextrin and mono-6-deoxy-6-(1-methylimidazolium)-β-cyclodextrin tosylate (an ionic liquid), which called MNP-β-CD and MNP-β-CD-IL, were coated into the capillary inner wall. Compared to an uncoated capillary, the new systems show good reproducibility and durability. The systems based on the use of MNP-β-CD or MNP-β-CD-IL as stationary phases were established for enantioseparation of Dns-modified amino acids. Improved resolutions were obtained for both CEC systems. Primary parameters such as running buffer pH value and applied voltage were systematically optimized in order to obtain optimal enantioseparations. Under the optimized conditions, the capillaries exhibited excellent chiral recognition ability for six Dns-amino acids (the DL-forms of alanine, leucine, lsoleucine, valine, methionine, glutamic acid) and provided a promising way for the preparation of chiral column. Graphical Abstract Schematic presentation of the open-tubular capillary electrochromatography systems with MNP-β-CD and MNP-β-CD-IL as stationary phases for enantioseparation of dansylated amino acids.

Enantioseparation of propranolol, amlodipine and metoprolol by electrochromatography using an open tubular capillary modified with β-cyclodextrin and poly(glycidyl methacrylate) nanoparticles

The inner wall of a capillary was coated with glycidyl methacrylate (GMA) to form tentacle-type coating, and poly(glycidyl methacrylate) nanoparticles (PGMA NPs) were then immobilized on the film. Ethanediamine-β-cyclodextrin as chiral selector was covalently bonded into the PGMA NPs through the ring-open reaction. The materials were characterized by SEM, TEM and FT-IR. The modified column was applied to the enantioseparation of the racemates of propranolol, amlodipine and metoprolol. Compared to a capillary with a single layer of CD-PGMA (without GMA coating) and to a CD-GMA system (without PGMA nanoparticles), the performance of the capillary is strongly improved. The effects of buffer pH value and applied voltage were optimized. Best resolutions (propranolol: 1.27, metoprolol: 1.01 and amlodipine: 2.93) were obtained when using the PGMA-coated capillary system. The run-to-run, day-to-day and column-to-column reproducibility were tested and found to be highly attractive. The new stationary phase is likely to have a large potential and scope in that it may also be applied to chiral separations of other enantiomers, such as amino acids and biogenic amines. Graphical abstract Schematic presentation of the preparation of a capillary column with glycidyl methacrylate (GMA) coating which was then immobilized with poly(glycidyl methacrylate) nanoparticles and ethanediamine-β-cyclodextrin. This novel open tubular column was applied to construct capillary electrochromatography system for separation of basic racemic drugs.

In situ growth of Zr-based metal-organic framework UiO-66-NH2 for open-tubular capillary electrochromatography

The high stability and other properties of Zr(IV)-based metal organic frameworks(MOFs) make it a promising choice for chromatographic separation, while the application in open-tubular capillary electrochromatography (OT-CEC) separation has not been explored yet. Herein, we report the first example of the in-situ growth of UiO-66-NH₂ onto the capillary for open-tubular capillary electrochromatography. UiO-66-NH₂ consists of ZrCl₄ and 2-amino-1,4-benzenedicarboxylic acid, which is highly porous and stable in a variety of solvents. The prepared UiO-66-NH₂ modified capillary was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectra (FT-IR), and the results confirmed the successful growth of the UiO-66-NH₂ . The baseline separation of chlorobenzenes, phenoxyacids and two groups of phenols was achieved owing to the combined interaction of π-π interaction, hydrophobic interaction, molecular sieve effect, electrophoretic migration and hydrogen-bonding interaction etc. Besides, the prepared capillaries showed good reproducibility, with relative standard deviations (RSDs) for intra-day, inter-day and column-to-column runs in the range of 1.38-2.60%, 3.39-4.05%, and 3.47-5.03%, respectively. Our work indicates Zr(IV)-based MOFs are promising materials as stationary phase in CEC separation.

Advances in covalent organic frameworks in separation science

Covalent organic frameworks (COFs) are a new class of multifunctional crystalline organic polymer constructed with organic monomers via robust covalent bonds. The unique properties such as convenient modification, low densities, large specific surface areas, good stability and permanent porosity make COFs great potential in separation science. This review shows the state-of-the art for the application of COFs and their composites in analytical separation science. COFs and their composites have been explored as promising sorbents for solid phase extraction, potential coatings for solid phase microextraction, and novel stationary phases for gas chromatography, high-performance liquid chromatography and capillary electrochromatography. The prospects of COFs for separation science are also presented, which can offer an outlook and reference for further study on the applications of COFs.

Miniaturized high-performance liquid chromatography instrumentation

Miniaturized high performance liquid chromatography (HPLC) has attracted increasing attention for its potential in high-throughput analyses and point-of-care applications. In this review we highlight the recent advancements in HPLC system miniaturization. We focus on the major components that constitute these instruments along with their respective advantages and drawbacks as well as present a few representative miniaturized HPLC systems. We discuss briefly some of the applications and also anticipate the future development trends of these instrumental platforms.

Open tubular-capillary electrochromatography: Developments and applications from 2013 to 2015

Open tubular CEC (OT-CEC) separates analyte mixtures by a combination of electrophoretic, electro-osmotic, and/or chromatographic effects. OT-CEC research is an active and growing field, with studies encompassing a wide range of investigations related to new strategies for chemical modification of the inner surface of the capillary, leading to the introduction of novel stationary phase coatings. This review has examined the literature on OT-CEC from 2013 to August 2015 and highlights the developments in the fabrication of highly selective stationary phases, based on materials that include cyclodextrin chiral selectors, graphene and graphene oxide, metal-organic frameworks, molecularly imprinted polymers, nanoparticles, nanolatex particles, nanocomposites, in situ generated polymers, block polymers, tentacle-type polymers, polyelectrolyte multilayers, polysaccharides, phospholipids, and proteins. This review, while considering the development of novel OT-CEC coating materials, specifically examines different immobilization or coating methodologies and approaches and also discusses the separation mechanisms that occur with these new materials. These OT-CEC coatings are intended mainly to separate low molecular weight molecules relevant to the pharmaceutical, agricultural, and food industries as well as for use in environmental monitoring.

Enantioseparations in open-tubular capillary electrochromatography: Recent advances and applications

This review highlights recent advances and applications in open-tubular capillary electrochromatography (OT-CEC) for enantioseparations during the last decade. Although extensive research has been conducted in the area of separations by use of CEC, and a big number of reviews have been published, there is not a review on exclusively the use of chiral stationary phases (CSPs) in OT-CEC for enantioseparations. Therefore, in this review, the design and synthesis of different CSPs are presented, and their potential applications in OT-CEC for enantioseparations are discussed. The different approaches to CSP development include chiral nanomaterials, porous layers, molecular imprinting, sol-gel technique, polyelectrolyte multilayer coating, polymeric coating and others.

Polydopamine-supported immobilization of covalent-organic framework-5 in capillary as stationary phase for electrochromatographic separation

Covalent-organic frameworks (COFs) are attractive materials for their fascinating properties, such as rigid structures, exceptional thermal stabilities, low densities, and permanent porosity with specific surface areas, which indicate potential for application in chromatography similar to related metal-organic frameworks (MOFs). However, the utilization of COFs in analytical chemistry is far behind as compared to that of the MOFs due to the challenging work of their immobilization. Here, we have successfully demonstrated the growth of the boron COF-5 on the inner wall of the fused silica capillary by a developed polydopamine-supported method. Combined with the layer-by-layer strategy, multilayer COF-5-coated capillary was obtained. The formation of COF-5 on polydopamine-coated substrate has been confirmed by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction. A novel stationary phase of COF-5 was developed on the basis of successful growth of COF-5 on polydopamine for open-tubular capillary electrochromatography (OT-CEC). Baseline separation of neutral, acidic and basic analytes was achieved on multilayer COF-5-coated capillary column. The fabricated capillary columns showed high column efficiency (154,060 plates/m for methylbenzene), excellent stability and repeatability. The precision (relative standard deviation (RSD), n=3) of retention time, peak height, and peak area for tested neutral compounds were in the range of 1.2-1.3%, 1.8-4.2%, and 0.9-2.4%, respectively. To the best of our knowledge, it was the first demonstration that COF-5 was developed as a novel stationary phase.

Separation of small organic molecules using covalent organic frameworks-LZU1 as stationary phase by open-tubular capillary electrochromatography

Covalent organic frameworks (COFs) have attracted much attention because of their permanent nanoscale porosity and higher surface area compared to zeolites as well as robustness. COFs have great potential in several fields such as hydrogen storage, gas separation, and catalysis. However, COFs have not yet been applied in capillary electrochromatography. Herein, covalent organic frameworks-LZU1 (COF-LZU1) was used as the stationary phase in open-tubular capillary electrochromatography for the first time. Compared to the monoliths used in electrochromatography, the preparation technique of a COF-LZU1-coated capillary was simple and practical. The baseline separation of model analytes including alkylbenzenes, polyaromatic hydrocarbons, and anilines by the COF-LZU1-coated capillary was achieved based on the size selectivity of COF-LZU1 porous structure and hydrophobic interactions between the model analytes and organic ligands of COF-LZU1. The load capacity of the COF-LZU1-coated capillary for naphthalene was 0.6mg/mL. For three consecutive runs, the intraday relative standard deviations (RSDs) were 1.4-2.6% for the migration time and 2.7-8.7% for the peak area. The interday RSDs were 1.3-3.9% for the migration time and 3.7-9.7% for the peak area. The column-to-column reproducibility of migration time was in the range 1.0-3.9%. Moreover, the coated capillary was used for >300 runs with no changes in the separation efficiency. Thus, COFs have great potential in capillary electrochromatography and may provide a new method for chromatographic separation.

Molecular imprinting science and technology: a survey of the literature for the years 2004-2011

Herein, we present a survey of the literature covering the development of molecular imprinting science and technology over the years 2004-2011. In total, 3779 references to the original papers, reviews, edited volumes and monographs from this period are included, along with recently identified uncited materials from prior to 2004, which were omitted in the first instalment of this series covering the years 1930-2003. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by sections describing fundamental aspects of molecular imprinting including the development of novel polymer formats. Thereafter, literature describing efforts to apply these polymeric materials to a range of application areas is presented. Current trends and areas of rapid development are discussed.

Electrical field-induced extraction and separation techniques: promising trends in analytical chemistry--a review

Sample preparation is an important issue in analytical chemistry, and is often a bottleneck in chemical analysis. So, the major incentive for the recent research has been to attain faster, simpler, less expensive, and more environmentally friendly sample preparation methods. The use of auxiliary energies, such as heat, ultrasound, and microwave, is one of the strategies that have been employed in sample preparation to reach the above purposes. Application of electrical driving force is the current state-of-the-art, which presents new possibilities for simplifying and shortening the sample preparation process as well as enhancing its selectivity. The electrical driving force has scarcely been utilized in comparison with other auxiliary energies. In this review, the different roles of electrical driving force (as a powerful auxiliary energy) in various extraction techniques, including liquid-, solid-, and membrane-based methods, have been taken into consideration. Also, the references have been made available, relevant to the developments in separation techniques and Lab-on-a-Chip (LOC) systems. All aspects of electrical driving force in extraction and separation methods are too specific to be treated in this contribution. However, the main aim of this review is to provide a brief knowledge about the different fields of analytical chemistry, with an emphasis on the latest efforts put into the electrically assisted membrane-based sample preparation systems. The advantages and disadvantages of these approaches as well as the new achievements in these areas have been discussed, which might be helpful for further progress in the future.

Preparation and characterization of lysine-immobilized poly(glycidyl methacrylate) nanoparticle-coated capillary for the separation of amino acids by open tubular capillary electrochromatography

In this study, poly(glycidyl methacrylate) (PGMA) nanoparticles (NPs) were prepared and chemically immobilized for the first time onto a capillary inner wall for open tubular capillary electrochromatography (OTCEC). The immobilization of PGMA NPs onto the capillary was attained by a ring-opening reaction between the NPs and an amino-silylated fused capillary inner surface. Scanning electron micrographs clearly demonstrated that the NPs were bound to the capillary inner surface in a dense monolayer. The PGMA NP-coated column was then functionalized by lysine (Lys). After fuctionalization, the capillary can afford strong anodic electroosmotic flow, especially in acidic running buffers. Separations of three amino acids (including tryptophan, tyrosine and phenylalanine) were performed in NP-modified, monolayer Lys-functionalized and bare uncoated capillaries. Results indicated that the NP-coated column can provide more retention and higher resolution for analytes due to the hydrophobic interaction between analytes and the NP-coating. Run-to-run and column-to-column reproducibilities in the separation of the amino acids using the NP-modified column were also demonstrated.