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Metal organic frameworks (MOFs), as a novel separation medium s , have in recent years attracted wide consideration in capillary electrochromatography (CEC). However, the instability of frameworks limited the application of the MOF-based chiral separation capillary. Herein, Iron-based γ-cyclodextrin metal-organic framework (Fe-CD-MOF) with mesoporous was developed as the chiral stationary phase in open tubular capillary electrochromatography column (OT-CEC) for the stereoisomer separation of fourteen chiral drugs and one chiral alcohol. The capillary column with MOF coating exhibited the best performance (resolution value: 17.07) for anisodamine in any MOF-based capillary column that has never been reported. Moreover, the effect of pH, buffer concentration, and methanol content has been investigated. The novel coated capillary was also applied for the quantitative analysis of a real sample. The Fe-CD-MOF coated capillary showed outstanding repeatability and stability, with the satisfactory relative standard deviations (RSDs) for intra-day, inter-day, and column-to-column. Finally, the enantioseparation mechanism of the Fe-CD-MOF coated capillary was evaluated by adsorption kinetic experiments. In summary, this work indicated the novel and renewable Fe-CD-MOF was a potential chiral stationary phase in CEC chiral separation. • Fe-based γ-CD-MOF was first utilized as the chiral stationary phase in CEC. • A novel homochiral Fe-CD-MOF coated column was developed. • The coated capillary exhibited outstanding stereoisomer separation ability. • The enantioselectivity of Fe-CD-MOF was proved by adsorption kinetic experiments. [ABSTRACT FROM AUTHOR]

A method of simultaneous separation of water- and fat-soluble vitamins using pressure-assisted CEC with a methacrylate-based capillary monolithic column was developed. In the proposed method, water-soluble vitamins were mainly separated electrophoretically, while fat soluble-ones were separated chromatographically by the interaction with a methacrylate-based monolith. A mixture of six water-soluble and four fat-soluble vitamins was separated simultaneously within 20 min with an isocratic elution using 1 M formic acid (pH 1.9)/acetonitrile (30:70, v/v) containing 10 mM ammonium formate as a mobile phase. When the method was applied to a commercial multivitamin tablet and a spiked one, the vitamins were successfully analyzed, and no influence of the matrix contained in the tablet was observed. [ABSTRACT FROM AUTHOR]

Abstract: Development of capillary electrochromatography (CEC) largely depends on column technology. The past ten years or so have seen a great number of CEC works performed on monolithic columns, due to simplicity and robustness in column fabrication. Monolithic columns eliminate the issue of column fritting, which conventionally made particle-packed capillary columns fragile and introduced nonuniformity to the chromatographic bed. The particulate packing material, however, is still a popular type of stationary phase widely used in CEC, as the rich library of HPLC packing material provides a wide range of choices for chromatographic separations performed in electrodriven mode. In this study, we investigated a purely physical fritting method, single particle fritting technology, to immobilize particulate chromatographic material inside capillary tube in a sinter-free manner to produce robust capillary columns. Single particle fritted columns present significantly improved column-to-column reproducibility (n =10) in peak efficiency, retention factor, peak area and asymmetry (%RSD=5.4, 7.7, 6.2 and 6.1, respectively, at 26kV), enabling their practical application in high throughput parallel analysis using multiple columns. [Copyright &y& Elsevier]

In this work, a new open-tubular capillary electrochromatography (OT-CEC) column was prepared using β-cyclodextrin covalent organic framework (β-CD COF) as a stationary phase. Polydopamine was used to assist fabrication of β-CD COF on an inner wall of a fused-silica capillary. The coating layer on the capillary was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Electroosmotic flow (EOF) was also studied to evaluate the variation of the inner wall of immobilized columns. Furthermore, the chiral separation effectiveness of the fabricated capillary column was evaluated by CEC using enantiomers of several related proton pump inhibitors as model analytes, including omeprazole, lansoprazole, pantoprazole and tenatoprazole. The effects of bonding time and concentration of β-CD COF, the type, concentration and pH of buffer, applied voltage were investigated to obtain satisfactory enantioselectivity. In the optimum conditions, the enantiomers of four analytes were resolved within 15 min with resolutions of 1.63-2.62. The relative standard deviation values for migration times and resolutions of the analytes representing intraday and interday were less than 6.75% and 4.24%, respectively. The results reveal that β-CD COF has great potential as chiral-stationary phases for enantioseparation in CEC., (© 2022 Wiley-VCH GmbH.)

Abstract: This paper details a method for the separation and identification of fourteen organic compounds commonly found as constituents in commercial smokeless powders using a hexyl acrylate-based porous monolith. Capillary electrochromatography (CEC) coupled to UV and time of flight-mass spectrometry (TOF-MS) methods were both explored. The CEC–UV method provides an effective and efficient method for the detection of all components in the additive package of the powder. The TOF-MS procedure provides better sensitivity and selectivity allowing an additional confirmation of the presence of the subset of those compounds, which are detectable via positive and/or negative ion electrospray ionization mass spectrometry. Both methods were used for the analysis of smokeless powder components in a mixed standard as well as in the determination of the composition of the additive package of individual powders. [Copyright &y& Elsevier]

A porous crosslinked organic polymer based on N-acryloxysuccinimide (NAS) and ethylene dimethacrylate (EDMA) was prepared inside 75 µm i.d. fused silica capillary as functionalizable monolithic stationary phase for electrochromatographic applications. Succinimide groups on the monolith surface provide reactive sites able to react readily through standard electrophile-nucleophile chemistry. Propargylamine was used to prepare alkyne functionalized poly(NAS-co-EDMA). Onto this thiol-reactive polymer surface was grafted adamantane units via a photochemically-driven addition reaction. Chemical characterization was performed in situ after each synthetic step by means of Raman spectroscopy and grafting kinetics was investigated to ensure quantitative grafting of 1-adamantanethiol. The as-designed monolithic stationary phase exhibited typical reversed-phase separation mechanism as evidenced by the linear increase of the logarithm of retention factor of neutral aromatic solutes with the increase of the aqueous buffer content in the mobile phase. [ABSTRACT FROM AUTHOR]

A hexyl acrylate monolith was prepared via photopolymerization into a cyclic olefin copolymer micro-device for reversed-phase electrochromatography purposes. This work deals with the influence of the irradiation conditions (irradiation time and light intensity) used during synthesis on the electrochromatographic performances of the monolithic column obtained. Up to day, the monolith structure was often controlled by means of irradiation time at constant light intensity, but the effect of light intensity (irradiance) on the monolith structure was scarcely considered. This study shows a strong effect of light intensity on electrochromatographic efficiency. Using an optimal light intensity of 4.7 m W/cm2 and an irradiation time of a few minutes allows obtaining heights equivalent to a theoretical plate reaching down to 5 μm for the reversed-phase separation of six polycyclic aromatic hydrocarbons. [ABSTRACT FROM AUTHOR]

Abstract: This work reports the preparation of monolithic zirconia chiral columns for separation of enantiomeric compounds by capillary electrochromatography (CEC). Using sol–gel technology, a porous monolith having interconnected globular-like structure with through-pores is synthesized in the capillary column as a first step in the synthesis of monolithic zirconia chiral capillary columns. In the second step, the surface of the monolith is modified by coating with cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) as the chiral stationary phase to obtain a chiral column (CDMPCZM). The process of the preparation of the zirconia monolithic capillary column was investigated by varying the concentrations of the components of the sol solution including polyethylene glycol, water and acetic acid. CDMPCZM is mechanically stable and no bubble formation was detected with the applied current of up to 30μA. The enantioseparation behavior of the CDMPCZM columns was investigated by separating a set of 10 representative chiral compounds by varying the applied voltage and pH and organic composition of the aqueous organic mobile phases. [Copyright &y& Elsevier]

Abstract: A chloropropyl-functionalized silica (CP-silica) hybrid monolithic column was synthesized within the confines of a capillary via the sol–gel process using tetramethoxysilane (TMOS) and (3-chloropropyl)-trimethoxysilane (CPTMS) as the precursors. The resulting CP-silica hybrid monolith inside the capillary showed homogeneous macroporous morphology and was well attached to the inner wall of the capillary. The obtained CP-silica hybrid monolithic capillary column demonstrated the inherent hydrophobic property and could be applied as a reversed-phase stationary phase in CEC directly. Due to the great chemical reactivity of the incorporated chloro groups on the hybrid silica monolithic matrix, the chloropropyl moieties on the surface of the hybrid silica monolith matrix could be conveniently further modified by a tertiary amine of N,N-dimethyl-N-dodecylamine (DMDA) via the nucleophilic substitution reaction at 70°C to introduce a dodecyl groups (C12) onto the CP-silica hybrid monolithic matrix. The resulting C12-silica hybrid monolithic column not only demonstrated the significantly enhanced hydrophobic property in the separation of alkylbenzenes in reversed-phase capillary electrochromatography (RP-CEC), but also the strong electroosmotic flow (EOF) in a wide pH range. Five alkylbenzenes could be baseline separated in 3min with column efficiency ranging from 189700 to 221000N/m with a 70% ACN running buffer in CEC. [Copyright &y& Elsevier]

A refractive index sensor has been fabricated in silicon oxynitride by standard UV lithography and dry etching processes. The refractive index sensor consists of a 1D photonic crystal (PhC) embedded in a microfluidic channel addressed by fiber-terminated planar waveguides. Experimental demonstrations performed with several ethanol solutions ranging from a purity of 96.00% (n = 1.36356) to 95.04% (n = 1.36377) yielded a sensitivity ( / n) of 836 nm/RIU and a limit of detection (LOD) of 6 × 10-5 RIU, which is, however, still one order of magnitude higher than the theoretical lower limit of the limit of detection 1.3 × 10-6 RIU. [ABSTRACT FROM AUTHOR]

Anion-exchange superporous cellulose (DEAE-SC) and microporous cellulose (DEAE-MC) adsorbents were packed in an electrochromatographic column, and the effect of external electric field (eEF) on the dynamic adsorption was investigated. The column was designed to provide longitudinal, transverse or 2-dimensional (2D) eEF. It was found that the electro-kinetic effect caused by the introduction of an electric field played an important role in the dynamic adsorption of bovine serum albumin to the adsorbents. The dynamic binding capacity (DBC) in the presence of 2D eEF was higher than in the presence of a one-dimensional eEF. The effect of flow velocity on the DBC of the two adsorbents was also demonstrated. It was found that the effect of electric field on the DEAE-MC column was more remarkable than that on the DEAE-SC column at the same flow rate, whereas the DEAE-SC column showed higher DBC and adsorption efficiency (AE) than the DEAE-MC column. With increasing flow rate, the DEAE-SC column could still offer high DBC and AE in the presence of the 2D eEF. For example, a DBC of 21.4 mg/mL and an AE of 57.7% were obtained even at a flow rate as high as 900 cm/h. The results indicate that the 2D electrochromatography packed with the superporous cellulose adsorbent is promising for high-speed protein chromatography. [ABSTRACT FROM AUTHOR]

Abstract: Fabrication of porous frits to retain stationary phases is a critical issue in column preparation for capillary electrochromatography (CEC). In this work, porous frits were prepared by applying an external magnetic field to magnetically responsive particles placed inside a fused-silica capillary. Three batches of uniform magnetite spheres with particle diameters of 0.3, 0.4, and 0.6μm and saturation magnetization values of 73.03, 74.41, and 77.83emu/g, respectively, were used as frit particles and octadecyl- and phenyl-bonded silica gels were packed successfully into frit-containing capillaries. The performance of the resulting magnetically immobilized frits and packed columns was evaluated. The electroosmotic mobilities in capillaries containing outlet frit only were found to be reduced by 2–4% whereas the plate heights of an unretained marker increased by 30–50% as compared to those in open capillaries. These variations are believed to be associated with the inhomogeneities of the packed structure of the frits. The magnetically immobilized frits showed adequate mechanical strength to withstand the flow drag force, allowing separation in capillaries packed with 5-μm stationary phases up to 10–15cm, thus rendering column efficiency and reproducibility comparable with those obtained with sintered frits. Taken together, retaining frits made of uniform magnetite particles serves as a viable alternative to sintered frits for column preparation, which offers several distinct advantages such as ease of preparation, improved durability as compared to sintered frits where the removal of the polyimide coating makes the packed column susceptible to breakage, and use of large-bore capillaries for semipreparative separations. [Copyright &y& Elsevier]

Abstract: Four anion-exchangers of different ion-exchange capacities (21–129μmol/mL) were prepared by coupling diethylaminoethyl to Sepharose 6FF. The adsorbents were used to study the dynamic protein adsorption in ion-exchange electrochromatography with an oscillatory transverse electric field perpendicular to the mobile-phase flow (IEEC). The static adsorption capacity of bovine serum albumin (BSA) increased from 67 to 186mg/mL in the ionic capacity range, but the effective pore diffusion coefficient of the protein decreased from 10.9×10−12 to 1.85×10−12 m2/s with increasing the ionic capacity due to the hindrance effect of the bound protein molecules at the pore entrance. So, the dynamic binding capacity (DBC) of protein in ion-exchange chromatography decreased with increasing the ionic capacity. By applying an electric field of 30mA, the DBC in IEEC packed with ion-exchangers of high ionic capacities (53 and 129μmol/mL) increased significantly (over 30% and 100%, respectively). This was because the high surface charge density led to high electroosmotic flow that enhanced intraparticle mass transfer in IEEC. In comparison, the DBC in IEEC packed with ion-exchangers of low ionic capacities (21 and 35μmol/mL) increased only slightly (ca. 10%) under the same condition. The results indicated the minor effect of electrophoretic mobility on the intraparticle mass transfer. Hence, it is beneficial to use ion-exchangers of high ionic capacity for high-capacity purification of proteins by IEEC. [Copyright &y& Elsevier]

• Lateral electrochromatography (LEC) allows for continuous enantio-separation. • Derived algebraic expressions determine the slopes of concentration trajectories. • Algebraic expressions may assist to optimize LEC for continuous separation. • Separation efficiency depends on the binding selectivity of the stationary phase. • Results of the simulations offer basic designing rules for LEC systems. The separation of enantiomers in a continuous manner represents a nontrivial task. Identical physical properties of an enantiomeric pair render the standard separation methods unusable, and its separation traditionally relies on the use of a chiral environment. Here, we theoretically analyze lateral electrochromatography as a potential method for continuous steady-state enantiomer separation. The developed mathematical model of a lateral electrochromatography device (LEC) treats the stationary (selective) phase as a pseudo-homogeneous environment. The intensity of the solute transport in that phase is given by an effective diffusion coefficient which depends on the degree of solute-stationary phase interactions. Under the assumptions of phase equilibrium and neglected diffusion/dispersion transport, we derived simplified algebraic expressions determining the slopes (deflections) of solute concentration trajectories allowing the solute separation. These expressions may assist in optimizing the LEC geometry and setting the control parameters such as the applied voltage or pressure difference. Numerical simulations on spatially two- and three-dimensional domains are in good agreement with both simplified algebraic predictions and available experimental observations. We show that LEC can separate enantiomers continuously in a steady-state regime under optimized operating conditions. However, the separation efficiency strongly depends on the enantioselectivity of the stationary phase. The results of the simulations also offer basic designing rules for a LEC that is compatible with continuous synthesis and separation of pharmaceuticals and other special chemicals. [ABSTRACT FROM AUTHOR]

Joule heating caused by electric currents is a major problem in scale-up electrochromatography. Here, to predict the temperature effects of lateral electric field electrochromatography and the process conditions required, a mathematical heat transfer model was established, which took into account the physical characteristics, including the electrical conductivity, of the porous medium and the ceramic plate in the gel chamber. The temperatures calculated using this model were within 4 °C of the experimentally measured results. For the lateral enlargement simulation of short columns (4 cm), it was found that when the width was enlarged to > 3 cm, the outlet temperature on the center line did not increase with increasing width. The electric field strength of 1700 V/m was maintained by increasing current strength according to the column size; further, when the gel chamber was enlarged to 0.075 × 0.075 × 0.1 m3, its internal temperature can be controlled within 16 °C under certain operating conditions, and an electric field strength of 1700 V/m can be obtained in the gel chamber. • Electrochromatography in a transverse electric field was mathematically modelled. • Use of a transverse electric field leads to less Joule heating than an axial field. • Scale-up electrochromatography was successfully performed using the model. [ABSTRACT FROM AUTHOR]

A porous monolithic sol-gel column with the solution of methacryloxypropyltrimethoxysilane in toluene with an acid catalyst was quickly prepared using microwave irradiation. Three typical silica based capillary monolithic columns prepared were used to evaluate the separation of some typical model compounds in the modes of CEC, pressure assisted CEC, and low pressure driven separation. Baseline separation of the analytes including thiourea, benzene, toluene, ethyl benzene, biphenyl, and naphthalene could be obtained by the polymer with a length of 10 cm. A scanning electron micrograph of a cross section of the capillary column showed that the gel took the form of a spherical particle aggregate and adhered to the column inner wall. [ABSTRACT FROM AUTHOR]

Abstract: Photopolymerized sol–gel monolithic columns for use in capillary electrochromatography were prepared in 125μm i.d. polyacrylate-coated fused-silica capillaries. The polyacrylate-coating, unlike the polyimide one, is transparent to the radiation used (≈370nm), and thus, no coating removal is necessary. This is a very important particularity since intrinsic capillary column characteristics, such as flexibility and mechanical resistance, are unchanged. A mixture containing metacryloxypropyltrimethoxysilane (MPTMS) as the polymeric precursor, hydrochloric acid as the catalyst, toluene as the porogen and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (Irgacure 819) as the photoinitiator was irradiated at 370nm for 20min inside the capillaries to prepare the columns through sol–gel approach. The versatility and viability of the use of polyacrilate as a new capillary external coating were shown through preparation of two columns under different conditions, which were tested in electrochromatography for separation of standard mixture containing thiourea (marker compound), propylbenzene, phenanthrene and pyrene. [Copyright &y& Elsevier]

Photopolymerized sol-gel(PSG) columns were prepared using methacryloxypropyltrimethoxysilane as the monomer, toluene as the porogen and hydrochloric acid as the catalyst. Four different photoinitiators such as benzoin methyl ether, Irgacure 819, Irgacure 1700 and Irgacure 1800 were comparatively used in the reaction solution in the presence and absence of sodium dodecyl sulfate. The above eight solutions were respectively irradiated at 365 nm for 5–10 min in each capillary (75 μm inside diameter) to prepare the porous monolithic sol-gel column by a one-step, in situ, process. The chromatographic behavior of the eight PSG columns were comparatively studied, all of which exhibit reversed-phase character. Using these columns, several neutral compounds, namely thiourea, benzene, toluene, ethyl benzene, biphenyl and naphthalene can be separated from mixtures with a largest column efficiency of 74 470 plate/column for thiourea. Addition of sodium dodecyl sulfate in the polymerization process has a significant influence on the morphology and migration time. [ABSTRACT FROM AUTHOR]

Abstract: The use of a phenylalanine (Phe) functionalized tentacle-type polymer coated capillary column for protein separation by open tubular capillary electrochromatography (OTCEC) was demonstrated in this work. The tentacle-type stationary phase was prepared from silanized fused-silica capillaries of 50μm I.D. by glycidyl methacrylate graft polymerization and subsequent Phe functionalization. Due to the amphoteric functional groups of the Phe bonded on the tentacle-type polymer stationary phase, protein separation in the prepared column can be performed under both cathodic and anodic electroosmotic flow (EOF) by varying the pH values of the mobile phase. Model proteins including ribonuclease A (RNase A), myoglobin, transferrin, insulin were baseline separated under cathodic EOF with a mobile phase of pH 8.8. Comparison between the separation result of the four proteins under conditions of OTCEC and capillary zone electrophoresis indicates that the migration behavior of the four proteins in the prepared column was the result of the interplay of chromatographic retention and electrophoretic migration. Besides, three basic proteins including RNase A, cytochrome c (Cyt-c) and lysozyme (Lys) were fully resolved under anodic EOF with an acidic running buffer (pH 2.5). The elution order was the same as the isoelectric point values of the proteins (RNase A

Abstract: A novel preparative electrochromatographic column with seven compartments was developed. The design gave promise to provide transverse, longitudinal or two-dimensional (2D) electric field on the column. The central gel compartment of 2mL was packed with dye–ligand affinity adsorbent. Bovine serum albumin (BSA) was used as a model protein to investigate the dynamic adsorption behavior in the electrochromatography. The effects of various operating variables indicated that the electroosmotic flow on the charged adsorbent surface could promote the intraparticle mass transport of the protein, leading to the increase of its dynamic binding capacity (DBC). Moreover, it was confirmed that the 2D electric field promoted the mass transport more efficiently than a 1D electric filed. Hence, the DBC of BSA was 4.5 times increased with the 2D electric filed as compared to the 2.4 times increase in a 1D electric field. The results have proven that the 2D electrochromatography is promising for high capacity protein adsorption. [Copyright &y& Elsevier]

A novel approach is introduced and evaluated for the preparation of silica-based monoliths by a sol–gel technique where in situ polymerization was carried out by γ-ray irradiation within the capillary. The γ-radiation-initiated synthesis generated radicals directly on the monomer, thereby avoiding use of any initiator. The chromatographic behavior of the capillary monolithic columns was studied in the modes of CEC, p-CEC and low pressure-driven separation, all of which exhibited reversed-phase character. Various operational parameters, such as column temperature, separation voltage, acetonitrile content and buffer pH, were varied to assess their influence on column performance in the separation of a series of neutral compounds including thiourea, benzene, toluene, ethyl benzene, biphenyl and naphthalene. A scanning electron micrograph of a cross-section of the capillary column showed that the gel took the form of a spherical particle aggregate and adhered to the column inner wall. It provided a viable alternative to either thermally initiated or photo polymerization for the preparation of monolithic columns. [ABSTRACT FROM AUTHOR]

Abstract: This review briefly summarizes the present state of the preparation and use of capillary monolithic columns for liquid chromatography (LC) and electrochromatography (EC). Most important approaches to the preparation of monolithic stationary phases based on organic polymers are outlined and the properties of the monoliths obtained are compared with those of classical particulate phases. A few selected applications of monolithic columns are shown to demonstrate the most important advantages of monolithic capillary columns. It is concluded that both the monolithic and particulate capillary columns are important and that judicious choice of the type suitable for a particular application requires careful consideration of the purpose of the separation and the properties of the solutes to be separated. Monolithic columns are substantially younger than packed ones and thus will require further theoretical and experimental study to further improve their preparation and to enable reliable prediction of their properties and applicability; nevertheless, they are very promising for the future. [Copyright &y& Elsevier]

Abstract: Acrylate-based monolithic capillary columns were prepared from fused-silica capillaries using UV photopolymerization. The effect of the pretreatment of the capillary wall surface before polymerization was investigated and several procedures were compared. The columns were characterization by van Deemter curves and SEM imaging. The results indicated that a pre-silanization of the capillary wall in order to introduce methacrylate groups at the wall surface gave similar efficiencies but more homogeneous structures than when the silanization agent was introduced in the polymerization mixture. The conditioning of the capillary before silanization, especially the conditions of basic rinsing was also an important factor. The effect of the dose of UV light that was applied for the polymerization had also been investigated. The results demonstrated that the irradiation energy is a critical parameter. The minimum energy threshold required to obtain a suitable monolith was 3J/cm2 and the maximum was around 12J/cm2. A higher energy destroys the monolith. Within the convenient range of energy, the columns had the same efficiency and a good structure as seen by SEM imaging. Using the optimized procedure for the pretreatment and an adequate energy, the column-to-column repeatability was found good (n =12). The repeatability was obtained for the plate height at two velocity values, the retention factor and the electroosmotic mobility with RSD values below 10. [Copyright &y& Elsevier]

Abstract: The development of the systematic evolution of ligands by exponential enrichment (SELEX) process has allowed the isolation of oligonucleotide sequences (referred as aptamers) with the capacity to recognize various classes of target molecules with high affinity and specificity. At the present time, various analytical aptamer-based formats have been developed as suitable tools for the detection and quantification of numerous targets, possessing many potential advantages over the most popular antibody technology. In this review, the aptamer applicability to the separation, purification and quantification of various analytes by liquid chromatography, capillary electrophoresis and electrochromatography is addressed. [Copyright &y& Elsevier]

This review gives the basic principle of affinity capillary electrophoresis (ACE) and examines its utilization in bioscience on microchips. ACE on chip is used as a separation tool as well as for study of molecular interactions. MEKC and chiral separations on microfluidic systems are described. Applications for measuring bioaffinity are focused on enzyme assay and immunoassay that demonstrate a further development of classical ACE in capillaries. [ABSTRACT FROM AUTHOR]

Abstract: Monolithic supports are a new generation of stationary phases which are typically prepared using a simple molding process carried out within the confines of the capillary. They provide high rates of mass transfer at lower pressure drops, enable much faster separations and the nature of the pores allows easy permeability for large molecules. This review summarizes the current achievements and application of organic polymer-based monolithic columns, silica-based monolithic columns and their application in bioaffinity processing, modern biotechnology, determination of microorganisms and chiral separations. Special attention is paid to microfabricated devices with monolithic supports because their fabrication of particles directly in the channel eliminates the need for a frit and also creates a unique homogeneity of packing. [Copyright &y& Elsevier]

Abstract: We have investigated the basic dependence of electroosmotic flow (EOF) velocity and hydrodynamic dispersion in capillary electrochromatography (CEC) on the variation of applied field and mobile phase ionic strengths employing silica-based particulate and monolithic fixed beds. These porous media have a hierarchical structure characterized by discrete intraparticle (intraskeleton) mesoporous and interparticle (interskeleton) macroporous spatial domains. While the macroporous domains contain quasi-electroneutral electrolyte solution, the ion-permselectivity (charge-selectivity) of the mesoporous domains determines the co-ion exclusion and counter-ion enrichment at electrochemical equilibrium (without superimposed electrical field) which depends on mesopore-scale electrical double layer (EDL) overlap and surface charge density. This adjustable, locally charge-selective transport realized under most general conditions forms the basis for concentration polarization (CP) induced by electrical fields superimposed in CEC. CP characterizes the formation of convective diffusion boundary layers with reduced (depleted CP zone) and increased (enriched CP zone) electrolyte concentration, respectively, at the anodic and cathodic interfaces in fixed beds containing the cation-selective, silica-based particles (or monolith skeleton). CP originates in the electrical field-induced coupled mass and charge transport normal to the charge-selective interfaces and has consequences for the EOF dynamics, hydrodynamic dispersion, and analyte retention in CEC. A secondary EDL with mobile counter-ionic space charge can be induced in the depleted CP zone leading to induced-charge EOF in the macroporous domains. It is characterized by a nonlinear dependence of the average EOF velocities on applied field strength and strong local velocity components tangential to the surface which enhance lateral pore-scale dispersion, thereby decreasing (axial) zone spreading. Differences in the pore space morphology of random-close sphere packings and monoliths criticially affect the intensity of CP and induced-charge EOF in these materials. CP is identified as a key phenomenon in CEC which also influences effective migration and the retention of charged analytes because the local intensity of CP inherently depends on applied field and mobile phase ionic strengths. [Copyright &y& Elsevier]

Abstract: The application of voltage in micro-high performance liquid chromatography (micro-HPLC) creates a system where separation is governed by a hybrid differential migration process, which entails the features of both HPLC and capillary zone electrophoresis (CZE), i.e., chromatographic retention and electrophoretic migration. In this paper, we use our previously published approach to decouple these two mechanisms via analysis of the input data for estimation of electrokinetic parameters, such as conductivity, equivalent lengths, mobilities and velocities. Separation of weakly retained, charged analytes was performed via voltage-assisted micro-HPLC. Contrary to conclusions from data analysis using the conventional definitions of the retention factor, it is shown that our approach allows us to isolate the “chromatographic retention” component and thus, investigate the “modification” of the retention process upon application of voltage in micro-HPLC. It is shown that the traditional approaches of calculating retention factor would erroneously lead to conclusion that the retention behavior of these analytes changes upon application of voltage. However, the approach suggested here demonstrates that under the conditions investigated, most of the charged analytes do not show any significant retention on the columns and that all the changes in their retention times can be attributed to their electrophoretic migration. [Copyright &y& Elsevier]

We have demonstrated that the aminopropyl stationary phase (APS2) already used for analysis of polar compounds in hydrophilic interaction liquid chromatography can be successfully used in capillary electrochromatography. First, it was shown that the APS2 packing material affords a sustainable electroosmotic flow, the direction and magnitude of which depend on the nature of the electrolyte ions. A triple-layer model is proposed to describe this particular behaviour of the aminopropyl stationary phase. Finally, application to the separation of the tetracycline antibiotics used as test compounds in capillary electrochromatography is described and it is demonstrated that the final separation may be manipulated because of the electromigration and retention properties of the compounds, although migration times cannot be accurately predicted. [ABSTRACT FROM AUTHOR]

Abstract: The separation mechanism in capillary electrochromatography (CEC) is a hybrid differential migration process, which entails the features of both high-performance liquid chromatography (HPLC) and capillary zone electrophoresis (CZE), i.e., chromatographic retention and electrophoretic migration. The focus of this paper is on the use of electrokinetic data, such as current, electroosmotic flow (EOF) and column efficiency measurements, that are readily available, for an improved understanding of CEC separations. A framework is presented here for the use of this data for evaluation of a variety of performance parameters including, conductivity ratio, interstitial EOF mobility, porosity, and zeta potential. This framework is applied for characterization of two monolithic columns with different chemistry that were manufactured in-house. The above-mentioned performance parameters were calculated for the two columns and it is found that the poly(VBC–EGDMA–SWNT) monolithic column with the GPTMS–PEI coating offers a significantly improved flow distribution in comparison to the poly(VBC–EGDMA) monolithic column. This observation is confirmed by performing separation of peptides on the two columns and height equivalent of a theoretical plate (HETP) measurements on the resulting peaks. It is shown that following our approach leads to an improved understanding of the separations achieved with the columns and to better column design. [Copyright &y& Elsevier]

Abstract: Different techniques and methods of electrochromatography on “lab on a chip” devices are reviewed. Described approaches include open-channel microchip electrochromatography relying on C8, C18 and novel gold nanoparticle (GNP) coating of microchannel wall; packed-channel microchip electrochromatography with new ways of automated loading and unloading of conventional octadecylsilica beads; monolith-based microchip electrochromatography with tailored casting of stationary phase at the specific places of microfluidic network and novel photolitographically fabricated collocated monolithic structures. Specific issues related to the microchip electrochromatography, i.e. importance of high aspect ratio of the microchannels in the open-channel electrochromatography or approaches eliminating the wall effect in the monolith-based electrochromatography, are discussed. Various applications for environmental, pharmacological, genomic and proteomic analysis are described. The operation parameters of reviewed microsystems are summarized in easy-to-read tables. [Copyright &y& Elsevier]

This study investigates the impact of peptide primary structure and the characteristics of two different types of immobilised hydrophobic ligands on the open tubular electro-chromatographic migration behaviour of a family of related synthetic peptide analogues. The results further document the important role that mixed mode electrophoretic/chromatographic processes play in determining peptide selectivity in OTCEC systems. Procedures to fine-tune selectivity of closely related peptides in these systems have been established with buffer electrolytes of different composition in terms of the organic solvent content and different pH conditions. [ABSTRACT FROM AUTHOR]

Abstract: A capacitively coupled contactless conductivity detection (CCD) system has been applied for the detection of neutral synthetic polymers in capillary size-exclusion electrokinetic chromatography (SEEC). Polystyrene standards, that were used as a model compounds, were separated on a capillary column packed with porous 10μm silica particles with an electrokinetically driven mobile phase, and detected by CCD and UV detection simultaneously. Mass-calibration curves for polystyrene were constructed. Satisfactory results were obtained for the linearity, the run-to-run repeatability (<0.2% for the relative retention and <4% for the peak area) and the robustness of the detector. One of the major issues in this preliminary study was to investigate the origin of the peaks observed for the polystyrene standards. The effect of the molar mass of the polystyrenes on the sensitivity was small. Therefore, the signals obtained could not be explained as the result of an increased viscosity and a decreased solution conductivity of the solute zone. An alternative hypothesis is suggested, and recommendations for further research are given. [Copyright &y& Elsevier]

Hepta-Tyr antibiotic modified silica stationary phase was used for the chiral resolution of d,l-loxiglumide, a new drug under investigation proposed for the treatment of gastrointestinal diseases. The chiral stationary phase was packed into fused silica capillaries of 75μm i.d. for a length of only 7cm and used for both capillary electrochromatography (CEC) and nano-liquid chromatography (nano-LC) running the experiments with the same instrumentation; in order to increase the electroosmotic flow (EOF) the antibiotic stationary phase was mixed with amino-silica particles (3:1, w/w) generating a relatively high reversed EOF. The enantiomeric resolution of loxiglumide by CEC was strongly influenced by several experimental parameters such as applied electric field, mobile phase composition, capillary temperature, etc. Optimum experimental conditions were found applying 15kV at 20°C and eluting with acetonitrile–sodium phosphate buffer at pH 6 (1:1, v/v). The same capillary was tested for nano-LC experiments. Good chiral separation of loxiglumide was achieved selecting the appropriate mobile phase considering the type and concentration of organic modifier. The nano-LC optimised method was therefore validated and applied to the analysis of a pharmaceutical formulation declared to contain only d-loxiglumide. [Copyright &y& Elsevier]