Capillary Electrochromatography Method Research Articles

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.

[Recent developments in the application of covalent organic frameworks in capillary electrochromatography].

Capillary electrochromatography (CEC) has received increased attention from the academic community because it combines the excellent selectivity of high performance liquid chromatography (HPLC) and the high efficiency of capillary electrophoresis (CE). Selecting the most appropriate stationary phase material is crucial to achieve better separation effects in CEC. In recent years, a considerable number of materials, such as graphene oxide, proteins, metal organic frameworks, and covalent organic frameworks (COFs), have been widely used as stationary phases in CEC to further improve its separation performance and extend its scope of potential applications. Among these materials, COFs have shown great application prospects in CEC owing to their unique properties, which include high porosity, large surface area, excellent stability, tunable pore size, and high designability of the framework structure. This review systematically summarizes published papers on the development and application of COFs in CEC from 2016 to 2023. First, two COF-based capillary columns (i. e., open-tube CEC columns and monolithic CEC columns) and their preparation methods are introduced. Second, the applications of CEC based on COF stationary phases in the separation of environmental endocrine disruptors, pesticides, aromatic compounds, amino acids, and drugs, particularly chiral drugs, are systematically summarized. The separation mechanism of CEC based on COF stationary phases is also introduced. At present, the good separation ability of COF-based CEC is mainly attributed to two factors: 1) The size exclusion effect of the pores of the COF stationary phase. Because of differences in the sizes of their organic molecular building units and side chains, COFs have varying pore sizes and topological structures. Thus, target analytes smaller than the pores of the COFs can enter the frameworks and interact with them during separation. On the other hand, target analytes larger than the pores of the COFs cannot enter the frameworks and interact with them during separation; thus, they can be separated. 2) The interactions between the target analytes and side chains (e. g., hydrophobic interactions, hydrogen bonding, π-π interactions, etc.) of the COFs. Since COFs usually contain alkyl side chains, aromatic structures, and oxygen and/or nitrogen atoms with high electronegativity, various interactions could occur between the COFs and target analytes. Finally, directions for the future development and strategic application of CEC based on COF stationary phases are proposed. We believe that future research in CEC based on COF stationary phases should focus on the following aspects: 1) The use of cheminformatics to design and construct COFs to improve the efficiency of COF capillary column preparation; 2) the development of milder methods to synthesize COFs that can meet the requirements of high performance COF capillary columns; and 3) in-depth research to explore the separation mechanism of CEC based on COF stationary phases to provide theoretical guidance for developing CEC methods suitable for the separation and analysis of complex samples.

Cyclodextrin-NH-MIL-53 open tubular stationary phase for capillary electrochromatography enantioseparation.

A novel chiral group functionalized metal-organic framework, Cyclodextrin-NH-MIL-53, was synthesized and modified on the inner wall of a capillary column via a post-synthetic process. The prepared chiral metal-organic framework was utilized as a chiral capillary stationary phase and used in an open-tubular capillary electrochromatography method to enantioseparate several racemic amino acids. Excellent enantioseparation of five pairs of enantiomers was obtained in this chiral separation system (Resolutions of D/L-Alanine = 16.844, D/L-Cysteine = 3.617, D/L-Histidine = 9.513, D/L-Phenylalanine = 8.133, and D/L-Tryptophan = 2.778). The prepared Cyclodextrin-NH-MIL-53 and the Cyclodextrin-NH-MIL-53-based capillary columns were characterized by scanning electron microscopy, X-ray diffraction, Fourie-transform infrared spectroscopy, and circular dichroism. The chiral capillary electrochromatography conditions, such as separation conditions, amount of Cyclodextrin-NH-MIL-53, and electroosmotic flow, were optimized. This research is estimated to present a novel insight and method for the design and use of metal-organic framework-based capillaries for enantioseparation.

Determination of benzimidazoles in fruits by open-tubular capillary electrochromatography based on ionic liquids grafted covalent organic frameworks.

A novel capillary electrochromatography method has been developed for the simultaneous quantification of ten benzimidazole fungicides in fruits. Herein, covalent organic frameworks (COFs) and ionic liquids (ILs) were successfully introduced to prepare open-tubular capillary column to improve the loading capacity and separation performance. The parameters effecting the analytical performance including pH and concentration of running buffer, separation voltage and the addition of organic solvent were investigated systematically. Under the optimized conditions, the method allowed the baseline separation of ten benzimidazole fungicides, and showed a good linearity in the range of 3.5-200μgkg-1 with the detection limits between 1.0 and 2.8μgkg-1. The intraday and interday precisions for recoveries were lower than 7.9% and 12.2%, respectively. Intraday and interday precisions for their retention times were lower than 3.2% and 6.6%, respectively. Satisfactory recoveries for grape, pear and orange samples at two concentrations were obtained ranging from 85.0 to 95.9% with RSDs lower than 7.8%, demonstrating the potential applications of the open-tubular capillary electrochromatography method for trace benzimidazole fungicides analysis in fruits.

Preparation of a zeolite imidazole skeleton-silica hybrid monolithic column for amino acid analysis via capillary electrochromatography.

We developed a novel, convenient and low-cost one-pot strategy for preparing a zeolitic imidazolate framework-8 (ZIF-8)-silica hybrid monolithic column by adding ZIF-8 directly to a polymer solution of the silica matrix. The simulated stationary phase and monolithic column prepared under optimal conditions were characterized using X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis nitrogen physisorption and zeta potential. The results obtained confirmed the successful introduction of ZIF-8 into the silica monolithic column, and the prepared monolithic column exhibited good permeability and physicochemical stability. A capillary electrochromatography method was developed based on a ZIF-8-silica hybrid monolithic column through which 15 mixed amino acids, 4 neutral compounds, 4 nipagin esters and 2 chlorinated fungicides were separated in 14, 5, 7 and 6min, respectively, under optimal conditions. The relative standard deviations retention times and column efficiencies in run-to-run, day-to-day and column-to-column varied in the ranges of 1.90%-2.21%, 2.13%-2.51% and 3.08%-6.65%, respectively, which demonstrated that ZIF-8-silica hybrid monolithic column exhibited satisfactory reproducibility and stability. The incorporation of ZIF-8 into a silica monolithic column is a promising method for preparing novel monolithic columns composed of a metal-organic framework.

Determination of Trace Sulfonamides in Environmental Water and Milk Through Capillary Electrochromatography Using PEG-MoS2 as Stationary Phase

In this work, a composite of polyethylene glycol (PEG) modified MoS2 (PEG-MoS2) was synthesized and used as the stationary phase of open-tubular capillary column via a chemical bonding strategy. The PEG-MoS2 exhibited a sphere-like architecture through characterization of scanning electronic microscopy. p-phenylenediboronic acid was introduced as a linker to combine the PEG-MoS2 with capillary column. A capillary electrochromatographic method was established for the determination of nine sulfonamides by using PEG-MoS2-coated capillary column. The intra-day, inter-day, and column-to-column relative standard deviations (RSDs) of migration time and peak area were in the range of 0.80–4.38% and 1.61–9.22%, respectively. The limits of detection were in the range of 20.4–64.8 ng/mL. The recoveries in environmental water and milk were from 83.26 to 116.29% and 80.86 to 118.22%. This work is the first report that PEG-MoS2 material is utilized as stationary phase for capillary electrochromatography.

Determination of sulfamerazine in aquatic products by molecularly imprinted capillary electrochromatography.

A molecularly imprinted monolith was prepared and evaluated for the special selective separation of sulfamerazine (SMR) by capillary electrochromatography (CEC). The single-step in situ polymerization method was applied through thermally immobilized vinyl groups of itaconic acid and a derivatization capillary column using SMR as the template. The monolith with optimal selectivity and permeability was performed at 45°C for 7 h according to the molar ratios of 1 : 4 : 10 (template/functional monomer/cross-linker). Under the optimized separation conditions of 75% acetonitrile in 20 mM phosphate buffer with pH 5.0, 15 kV applied voltage and 20°C column temperature, the imprinted monolith showed strong recognition ability for SMR and high column performance. Finally, the molecularly imprinted monolith coupled with the CEC method was successfully developed for the quantification of SMR in aquatic products, which was properly validated by a good linear relationship, recoveries and limit of detection. The coupling technique of the molecularly imprinted technology and CEC achieved pre-treatment enrichment and separation analysis in only one miniaturized chromatographic column.

Using open-tubular capillary electrochromatography with part-coating column for binding constants determination of β2 -adrenergic receptor with seven drugs.

An open-tubular capillary electrochromatography method has been developed for the determination of binding constants between β2 -adrenergic receptor (β2 -AR) and seven drugs. β2 -AR was oriented immobilized onto one part of inner surface of capillary via microwave-assisted technical synthesis. According to the linear relationship between coating length and the apparent mobility of analyte, the binding constant (Kb ) can be obtained by related theories and equations. The order of Kb values between drugs such as adrenaline hydrochloride, norepinephrine bitartrate, and propranolol hydrochloride with β2 -AR is well consistent with that reported in the literature. By the method, Kb values between four extracts of Radix Paeoniae Rubra and β2 -AR were also successfully obtained. Subsequently, computer models were applied to interpret the CEC experiments. And the results proved to be in good agreement with the method. The work, herein, demonstrates the potential of the method in drug-receptor affinity interactions evaluation and screening of lead compounds from natural sources.

Open-tubular capillary electrochromatographic determination of ten sulfonamides in tap water and milk by a metal-organic framework-coated capillary column.

In this study, a metal-organic framework (MOF), [Mn(cam)(bpy)], was synthesized and characterized by thermogravimetric analysis, scanning electron microscopy, and Fourier transform infrared spectrometry. An open-tubular capillary column was fabricated from [Mn(cam)(bpy)] via the amide coupling method. Ten types of sulfonamides were separated through the fabricated capillary column, which showed a good limits of detection (<0.07μg/mL) and linear ranges (1-100 or 5-100μg/mL) with a high correlation coefficients (R2 > 0.9987). The intra-day, inter-day and column-to-column relative standard deviations (RSDs) in the migration times ranged from 0.44 to 4.87%, and the peak area RSDs ranged from 0.80 to 7.28%. The developed capillary electrochromatography method can be successfully utilized for the determination of sulfonamides in tap water and milk samples.

A fast and accurate method for the pharmacokinetic research of four coumarin analogs in Fructus cnidii using capillary electro-chromatography with a methacrylate ester-based monolithic column.

In the present study, a monolithic capillary column with higher permeability was developed for the in vivo discrimination of four coumarin analogs (bergapten, 2'-acetylangelicin, imperatorin, and osthole) that typically require long separation times in HPLC. Instead of conventional methacrylate ester monolith (containing 19.5% porogen) with insufficient permeability (K=1.52 - 1.66×10-14 M2 ) for plasma sample, the proposed column (20.5% porogen) had better permeability (around 3.80×10-14 M2 ) while properties such as pore distribution, stability, and resolution changed slightly. As a result, due to the negatively charged electro-dynamic flow of the methacrylate ester groups in the monolith, the migration of targeted analytes was achieved within 6min (compared with 30min in HPLC) with acceptable resolution and improved sensitivity (0.005-0.02μg/mL vs. 0.04μg/mL). The proposed method was also applied to pharmacokinetic research: accelerated solvent extraction (ASE) was used to improve the extraction efficiency, which prepared extract much faster and more pure than conventional methods. As the pharmacokinetic parameters indicated, the monolithic capillary electro-chromatography method was efficient, sensitive, specific, and durable, guaranteeing its utility for the determination of multiple structure-related compounds in rat plasma.

Simultaneous separation of neutral and cationic analytes by one dimensional open tubular capillary electrochromatography using zeolitic imidazolate framework-8 as stationary phase

Developing simple methods for separation of analytes that belong to different classes is of great importance. Herein, we developed a simple one-dimensional (1-D) capillary electrochromatography method to demonstrate the simultaneous separation of target fractions that belong to two different classes (i.e., cationic and neutral analytes) without switching buffer solution by a zeolitic imidazolate framework ZIF-8 coated capillary column. Owing to the difference of charge-to-mass ratio of the cationic analytes, the interaction of the cationic analytes and coating material ZIF-8 and the hydrophobic interactions between the neutral analytes and the microporous framework of ZIF-8, six cationic analytes and four neutral analytes were simultaneously separated in a single run by 1-D capillary electrochromatography. The relative standard deviations (RSDs) of the analytes migration time for intra-day, inter-day and column-to-column were in the range of 0.11–0.87%, 0.54–2.04% and 2.00–6.89% and the RSDs of the analytes peak area for intra-day, inter-day and column-to-column were in the range of 0.70–4.45%, 1.33–6.20% and 2.27–11.88%. Additionally, the developed method was employed in the analysis of urine samples with satisfactory recoveries.

Simultaneous determination of nucleoside and purine compounds in human urine based on a hydrophobic monolithic column using capillary electrochromatography.

A novel capillary electrochromatography method was developed by using a polymethacrylate ester based monolithic column for the simultaneous separation and determination of five nucleoside and purine compounds in urine. The porous monolithic column was first designed by mean of in situ copolymerizing stearyl methacrylate, trihydroxymethylpropyl trimethylacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in a ternary porogenic solvent including cyclohexanol, 1,4-butanediol, and water. The performance and influence factors of the monolithic columns were investigated and evaluated by a CEC method. The five compounds including guanine, adenine, adenosine, cytidine, and N6 -methyladenosine were selected as analytes. Under the optimized condition of 6.0 mM phosphate buffer at pH 5.0 without ACN, five analytes were rapidly separated in 4 min with the high separation efficiency. The recoveries of spiked samples were ranged from 89.0 to 109.0% with RSDs less than 4.83%. Therefore, the proposed method could possibly provide for rapid separation and detection of the nucleoside and purine compounds in biomedicine sample.

Determination of dopamine, epinephrine, and norepinephrine by open-tubular capillary electrochromatography using graphene oxide molecularly imprinted polymers as the stationary phase

A novel capillary electrochromatography method was developed for the determination of dopamine (DA), epinephrine (EP), and norepinephrine (NE) by using a graphene oxide (GO) molecularly imprinted polymers (MIPs) coated capillary. In this article, GO was introduced as supporting matrix to synthesize MIPs in the presence of DA as template molecule. Then GO MIPs were used as the stationary phase in electrochromatography for the determination of DA, EP, and NE. The separation of these three analytes was achieved under the optimal conditions with a satisfactory correlation coefficients (R(2) ) > 0.9957 in the range of 5.0-200.0 μg/mL for EP and NE, and 20.0-200.0 μg/mL for DA, respectively. The RSDs for the determination of three analytes were <6.19%, and the detection limits were 1.25 μg/mL for EP and NE, and 10.0 μg/mL for DA, respectively. Finally, this method was used for the determination of DA, EP, and NE in human serum and DA hydrochloride injection.

Separation of aromatic amines by an open-tubular capillary electrochromatography method

A new and simple open-tubular CEC (OT-CEC) method with a novel diblock copolymer poly(butyl methacrylate)71 -block-poly(glycidyl methacrylate)9 as the coating based on its self-assembled properties has been developed. Compared with the bare capillary, this coating could act as a surfactant and improve the separation efficiency of aromatic amines. Meanwhile, the effects of pH value, buffer concentration, and the copolymer block ratio on the separation efficiencies were investigated in detail. It has been found that the three tested aromatic amines could be baseline separated by the OT-CEC method with the cooperation of SDS. The proposed OT-CEC method showed good repeatability with RSDs <3.2% for testing the EOF. Moreover, it was also well validated by satisfactory linearity and favorable recovery, which ensured its successful application in the separation of aromatic amines in nail polish samples. The results revealed the potential applicability of the OT-CEC method in cosmetic analysis.

Capillary electrochromatography–mass spectrometry determination of melamine and related triazine by-products using poly(divinyl benzene-alkene-vinylbenzyl trimethylammonium chloride) monolithic stationary phases

In this study, a capillary electrochromatography (CEC) method coupled either with UV or mass spectrometric detection was developed for the detection of trace-amounts of melamine and its related by-products (ammeline, ammelide, and cyanuric acid). A series of poly(divinyl benzene-alkene-vinylbenzyl trimethylammonium chloride) monolithic columns, which were prepared by a simple in situ polymerization with divinyl benzene (DVB), vinylbenzyl trimethylammonium chloride (VBTA) and different types of alkene monomers such as 1-octene, 1-dodecene or 1-octadecene were used as separation columns, with the poly(DVB-1-dodecene-VBTA) monolith as the optimal chromatographic material because it provided a better separation. The detection limits of four melamine derivatives were in the ranged of 0.6–2.18mgL−1 by the optimal CEC–UV mode, and were reduced from 2.2 to 19.4μgL−1 by the optimal CEC–MS mode. Finally, the proposed CEC methods successfully determined melamine contaminations (0.1mgL−1 per analyte) in several dairy products as test samples with analyte recovery range of 69–85% (intra-day) and 68–75% (inter-day), and with peak area reproducibility range of 4.3–8.6% and 8.7–15.6% for intra-day and inter-day, respectively. This is the first report for CEC separation coupled with MS detection applied in trace melamine residue analyses with a faster separation and comparable or even better detection ability than previous GC–MS, CE–MS, as well as LC–MS methods.

Development of a new open-tubular capillary electrochromatography method for in vitro monitoring of toxic aromatic amines distribution in rat blood

Exposure to aromatic amines from different industrial and agricultural activities entails a substantial risk of deleterious somatic effects, genetic damage and cancer development. Thus, a new and simple method for separation and analysis of aromatic amines has been developed by open-tubular capillary electrochromatography with a novel amphipathic block copolymer (poly(tert-butyl acrylate)(127)-block-poly(glycidyl methacrylate)(86)) coating based on its self-assembled property. Key factors affecting the separation efficiency of the test analytes, such as pH, buffer concentration and selective solvent, were studied in detail. Meanwhile, method validation was well evaluated by linearity (≥0.998), detection limit and recovery. Application of this developed protocol on in vitro monitoring of the target aromatic amines distribution in rat blood demonstrated its potential usage for separation and determination of aromatic amines in biological samples. Additionally, for assimilating more polymeric materials into analysis of aromatic amines, the effect of morphology changes of the amphipathic block copolymer coating on open-tubular capillary electrochromatography separation was also studied, and the result revealed that the block copolymer coating could play the same role as surfactant.

Electrochromatographic characterization of methacrylate ester-based monolith and capillary electrochromatography separation of flavonoids

A porous polymethacrylate ester-based monolithic column for capillary electrochromatography (CEC) was designed by mean of in situ co-polymerizing lauryl methacrylate (LMA), ethylene dimethacrylate (EDMA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in a ternary porogenic solvent including cyclohexanol, 1,4-butanediol and water. After investigating the influence factors of the CEC monolithic columns, four flavonoids (i.e., Rutin, Quercetin, Kaempferol, and Quercitrin) were separated and assayed to evaluate this monolithic column with CEC method. Under optimum conditions, the CEC method exhibited high separation efficiency, with rapid separation time of 3–4 min, for the four flavonoid samples using 10 mM phosphate buffer containing 70% acetonitrile (pH 9.0). Importantly, the proposed method could provide a promising approach for rapid separation and detection in biomedicine.

Analyses of synthetic antioxidants by capillary electrochromatography using poly(styrene–divinylbenzene–lauryl methacrylate) monolith

In this study, several organic polymer-based monoliths prepared by single step in situ copolymerization of styrene- and methacrylate ester-based monomers (styrene (S), divinylbenzene (DVB) and lauryl methacrylate (LMA)) were developed as stationary phases of capillary electrochromatography (CEC) for the analyses of synthetic antioxidants. These monoliths were characterized by examining the SEM image, IR spectrum, and measuring the pore size, surface area, conversion yield, and thermal decomposition temperature. The polymerization procedure was optimized by varying the reaction temperature, the reaction time, and the LMA–styrene ratio. The LMA–styrene ratio had the most significant influence on the peak symmetry of butylated hydroxyanisole (BHA) and 2, 6-di- tert-butyl-4-methyl phenol (BHT), the latter being greatly affected by excessive peak tailing in the poly(S–DVB) monolith. It showed that the interaction between the poly(S–DVB) monolith and the antioxidant (BHT or BHA) was significantly altered by the insertion of LMA. Compared with the best HPLC and CE methods previously reported, this proposed CEC method provides a comparable separation ability for the five antioxidants analyzed. This study demonstrates that the potentiality of poly(S–DVB–LMA) monolith as stationary phase, especially for CEC system, because of high thermal stability and good column reproducibility.

Separation of catechins and methylxanthines in tea samples by capillary electrochromatography

In this paper, the simultaneous separation of several polyphenols such as (+)-catechin, (-)-epicatechin, (-)-epigallocatechin, theophylline, caffeine in green and black teas by capillary electrochromatography (CEC) was developed. Several experimental parameters such as stationary phase type, mobile phase composition, buffer and pH, inner diameter of the columns, sample injection, were evaluated to obtain the complete separation of the analysed compounds. Baseline resolution of the studied polyphenols was achieved within 30 min by using a capillary column (id 100 microm) packed with bidentate C(18) particles for 24.5 cm and a mobile phase composed of 5 mM ammonium acetate buffer pH 4 with H(2)O/ACN (80:20, v/v). The applied voltage and the temperature were set at 30 kV and 20 degrees C. Precision, detection and quantification limits, linearity, and accuracy were investigated. A good linearity (R(2) > 0.9992) was achieved over a concentration working range of 2-100 microg/mL for all the analytes. LOD and LOQ were 1 and 2 microg/mL, respectively, for all studied compounds. The CEC method was applied to the analysis of those polyphenols in green and black tea samples after an extraction procedure. Good recovery data from accuracy studies ranged between 90% and 112% for all analytes.

Capillary electrochromatographic separation of illicit drugs employing a cyano stationary phase

In this study, we present a capillary electrochromatographic method for separation of basic compounds of interest in forensic science (amphetamine, methamphetamine, 3,4-methylenedioxyamphetamine, 3,4-methylenedioxymethamphetamine, 3,4-methylenedioxyethylamphetamine, cocaine, codeine, heroin, morphine, and 6-monoacethylmorphine). Several analytical conditions were taken into account to completely separate in the same run the 10 drugs of abuse analyzed. Chromatographic retention, selectivity and efficiency were evaluated in dependence of the type of stationary phase (CN and RP-C 18 derivatized silica particles), mobile phase composition, buffer type and pH, sample injection. The optimum separation parameters were set up using a mixture of aqueous sodium phosphate buffer (pH 2.5)/acetonitrile (80/20, v/v) as the mobile phase, 10 kV and 20 °C as applied voltage and capillary temperature, respectively. Under these conditions all the studied analytes were baseline resolved within 20 min. The method performance was investigated in terms of precision, linearity, sensitivity and accuracy to demonstrate the applicability of the developed capillary electrochromatographic system to forensic analysis. Calibration curves provided a good linearity over a working range of 100–1200 ng/mL for all analytes. Limits of detection and quantification were in the range 5–12 ng/mL and 10–30 ng/mL, respectively. Then the method was applied to the analysis of a human urine sample spiked with a basic compounds’ mixture. Urine samples’ pre-treatment was carried out through a solid phase extraction (SPE) procedure on strong cation exchange (SCX) cartridges.