Dynamics of capillary electrochromatography experimental study on the electrosmotic flow and conductance in open and packed capillaries

Reversed-phase capillary electrochromatography of pre-column derivatized mono- and oligosaccharides with three different ultraviolet absorbing tags

In this research report, an in house developed octadecyl monolithic (ODM) column has been exploited in the reversed-phase capillary electrochromatography (RP-CEC) of precolumn derivatized mono- and oligosaccharides with three different tagging agents, namely 1-naphthylamine (1-NA), 2-aminoanthracene (2-AA) and 3-amino-2,7-naphthalenedisulfonic acid (ANDSA). These three derivatizing agents, which differed in their charges, nonpolar characters and optical absorption properties, led to different RP-CEC elution patterns and UV detection signals. In fact, the limit of detection of the derivatized sugars were 50 µM for the ANDSA- and 1-NA-sugar derivatives and 35 µM for the 2-AA-sugar derivatives due to the presence of three fused aromatic rings in 2-AA versus 2 fused rings in the 1-NA and ANDSA tags. Furthermore, while the longer ANDSA-oligosaccharides eluted later than the shorter ones and the ANDSA-monosaccharides, 1-NA- and 2-AA-sugar derivatives necessitated the presence of borate ions at alkaline pH in the mobile phase to form in situ charged derivatives to facilitate their separation by RP-CEC, and the elution order was the reversal of that observed with the ANDSA-sugar derivatives; that is the mono- eluted later than the larger size oligosaccharides. In addition, plots of log t_R vs. number of glucose residues (n_Glc) for derivatized glucose and maltooligosaccharides yielded straight lines with slopes representing log η where η is the retention time modulus (i.e., ratio of retention time of two neighboring derivatives differing in one glucosyl residue). In the case of 1-NA and 2-AA derivatives, η was smaller than unity while it was greater than unity in the case of ANDSA-sugar derivatives because the elution occurred in the order of decreasing size of the homologous sugar derivatives in the former than in the later derivatives. The prepared ODM column was stable for more than a month of continuous use, a fact that allowed a good repeatability for intraday and interday analyzes.

Progress toward Plug-and-Play Polymer Strings for Optical Tweezers Experiments: Concatenation of DNA Using Streptavidin Linkers

Streptavidin is a tetrameric protein that is renowned for its strong binding to biotin. The robustness and strength of this noncovalent coupling has led to multitudinous applications of the pairing. Within the streptavidin tetramer, each protein monomer has the potential to specifically bind one biotin-bearing moiety. Herein, by separating various streptavidin species that have had differing numbers of their four potential binding sites blocked, several different types of "linking hub" were obtained, each with a different valency. The identification of these species and the study of the plugging process used to block sites during their preparation were carried out using capillary electrophoresis. Subsequently, a specific species, namely, a trans-divalent linker, in which the two open biotin-binding pockets are approximately opposite one another, was used to concatenate two ∼5 kb pieces of biotin-terminated double-stranded DNA. Following the incubation of this DNA with the prepared linker, a fraction of ∼10 kb strings was identified using gel electrophoresis. Finally, these concatenated DNA strings were stretched in an optical tweezer experiment, demonstrating the potential of the methodology for coupling and extending molecules for use in single-molecule biophysical experiments.

Recent Advances in Capillary Electrochromatography of Proteins and Carbohydrates in the Biopharmaceutical and Biomedical Field

Capillary electrochromatography (CEC) is a powerful hybrid separation technique that combines capillary electrophoresis and capillary chromatography, capable to address the analytical challenges of proteomics and glycomics. The focus of this paper is to review the recent developments in capillary electrochromatography of proteins and carbohydrates. The different column types applied in capillary electrochromatography such as packed bed, open tubular and monoliths are conferred in detail with respective separation examples. A comprehensive comparison is also given listing the mostly utilized coating methods, stationary phase materials and column preparation methods. The choice of porogenic solvent combinations for monolithic column fabrication is thoroughly discussed, paying close attention to the fine tuning options for the separation driving electroosmotic flow. Application examples of CEC in process analytical technology for the biopharmaceutical and biomarker discovery in the biomedical fields are also given.

A Novel Open Tubular Capillary Electrochromatographic Method for Differentiating the DNA Interaction Affinity of Environmental Contaminants

The interaction of chemicals with DNA may lead to genotoxicity, mutation or carcinogenicity. A simple open tubular capillary electrochromatographic method is proposed to rapidly assess the interaction affinity of three environmental contaminants (1,4-phenylenediamine, pyridine and 2,4-diaminotoluene) to DNA by measuring their retention in the capillaries coated with DNA probes. DNA oligonucleotide probes were immobilized on the inner wall of a fused silica capillary that was first derivatized with 3-(aminopropyl)-triethoxysilane (APTES). The difference in retention times and factors was considered as the difference in interaction affinity of the contaminants to the DNA probes. The interaction of the contaminants with both double-stranded (dsDNA) and single-stranded DNA (ssDNA) coatings was compared. Retention factors of 1,4-phenylenediamine, pyridine and 2,4-diaminotoluene in the capillary coated with ssDNA probe were 0.29, 0.42, and 0.44, respectively. A similar trend was observed in the capillary coated with dsDNA, indicating that 2,4-diaminotoluene has the highest affinity among the three contaminants. The relative standard deviation (RSD) for the retention factors was in the range of 0.05-0.69% (n = 3). The results demonstrated that the developed technique could be applied for preliminary screening purpose to provide DNA interaction affinity information of various environmental contaminants.

Influence of the Modifier Type and its Concentration on Electroosmotic Flow of the Mobile Phase in Pressurized Planar Electrochromatography

The aim of this work was to find a relationship between electroosmotic flow (EOF) velocity of the mobile phase in pressurized planar electrochromatography (PPEC) and physicochemical properties like zeta potential, dielectric constant, and viscosity of the mobile phase as well as its composition. The study included different types of organic modifiers (acetonitrile, methanol, ethanol, acetone, formamide, N-methylformamide and N,N-dimethylformamide) in the full concentration range. In all experiments, chromatographic glass plates HPTLC RP-18 W from Merck (Darmstadt) were used as a stationary phase. During the study we found that there is no linear correlation between EOF velocity of the mobile phase and single variables such as zeta potential or dielectric constant or viscosity. However, there is quite strong linear correlation between EOF velocity of the mobile phase and variable obtained by multiplying zeta potential of the stationary phase-mobile phase interface, by dielectric constant of the mobile phase solution and dividing by viscosity of the mobile phase. Therefore, it could be concluded that the PPEC system fulfilled the Helmholtz-Smoluchowski equation.

Inherent peak compression of charged analytes in electrochromatography

This work resolves peak compression of charged analytes in CEC with strong cation-exchange stationary phase particles. By combining electrochromatographic peak shape analysis with the results of numerical simulations and confocal laser scanning microscopy in the packed capillaries, we identify electrical field-induced concentration polarization as the key physical phenomenon responsible for the inherent existence of local electrical field gradients on the scale of an individual support particle. Consequently, positive and negative field gradients exist between and inside the particles along the whole packing. Their intensity depends on the particles cation-selectivity (governed by the particles volume charge density and the mobile phase ionic strength) and the applied field strength. The interplay of these local field gradients with the analytes retention (intraparticle adsorption) determines whether fronting, tailing, or spiked analyte peaks are observed, and it provides a mechanism by which strongly retained analytes can be eluted over long distances with little zone dispersion. Our analysis explains the "anomalous" peak compression effects with strong cation-exchange particles, which have been reported more than a decade ago (Smith, N. W., Evans, M. B., Chromatographia 1995, 41, 197-203) and since then remained largely unresolved.

Pressurized CEC of neutral and charged solutes using silica monolithic stationary phases functionalized with 3-(2-aminoethylamino)propyl ligands

A novel silica monolithic stationary phase functionalized with 3-(2-aminoethylamino)propyl ligands for pressurized CEC has been presented. The monolithic capillary columns were prepared by a sol-gel process in 75 microm id fused-silica capillaries and followed by a chemical modification. The diamino groups on the surface of the stationary phase are meant to generate the chromatographic surface and a substantial anodic EOF as well as to provide electrostatic interaction sites for charged solutes. The electrochromatographic characterization and column performance were evaluated by a variety of neutral and charged solutes. It was observed that the anodic EOF for the diamine-bonded monolith was greatly affected by the reaction time with 3-(2-aminoethylamino)propyltrimethoxysilane and the PEG amount in the sol-gel reaction mixture in addition to the mobile phase conditions. The monolithic stationary phase exhibited hydrophilic interaction chromatographic behavior toward neutral solutes. Good separations of various solutes including phenols, nucleic acid bases, nucleosides and nucleotides were achieved under different experimental conditions. Fast and efficient separations were obtained with high plate counts reaching more than 130,000 plates/m.

Preparation of polymethacrylate monolithic stationary phases having bonded octadecyl ligands and sulfonate groups: electrochromatographic characterization and application to the separation of polar solutes for pressurized capillary electrochromatography

In this report, the preparation of porous polymethacrylate-based monolithic columns by in situ copolymerization of octadecyl methacrylate (OMA), 3-sulfopropyl methacrylate (SPMA) and ethylene dimethacrylate (EDMA) in a binary porogenic solvent consisting of cyclohexanol/1,4-butanediol are proposed. These monoliths possess in their structures bonded octadecyl ligands and sulfonate groups and are evaluated in pressurized capillary electrochromatography (pCEC) system using small neutral and charged solutes. While the sulfonate groups are meant to generate the electroosmotic flow (EOF) necessary for transporting the mobile phase through the monolithic capillary; the octadecyl ligands are introduced to provide the nonpolar sites for chromatographic retention for neutral solutes. However, incorporating the sulfonate groups in the monoliths does not only support the EOF but also exhibit hydrophilic interaction as well as electrostatic interaction/repulsion with the monoliths in addition to electrophoretic migration with polar charged solutes (e.g., nucleotides). The monolithic stationary phases at different EOF velocities are easily prepared by altering the amount of SPMA in the polymerization solution as well as the composition of the porogenic solvent. Optimum EOF velocity, the highest efficiency and adequate chromatographic retention are obtained when 0.6% SPMA is added to the reaction mixture. Under these conditions, rapid separation and high plate counts reaching greater than 170,000 plates/m are readily obtained.

Fluid dynamics in capillary and chip electrochromatography

This review is concerned with the phenomenological fluid dynamics in capillary and chip electrochromatography (EC) using high-surface-area random porous media as stationary phases. Specifically, the pore space morphology of packed beds and monoliths is analyzed with respect to the nonuniformity of local and macroscopic EOF, as well as the achievable separation efficiency. It is first pointed out that the pore-level velocity profile of EOF through packed beds and monoliths is generally nonuniform. This contrasts with the plug-like EOF profile in a single homogeneous channel and is caused by a nonuniform distribution of the local electrical field strength in porous media due to the continuously converging and diverging pores. Wall effects of geometrical and electrokinetic nature form another origin for EOF nonuniformities in packed beds which are caused by packing hard particles against a hard wall with different zeta potential. The influence of the resulting, systematic porosity fluctuations close to the confining wall over a distance of a few particle diameters becomes aggravated at low column-to-particle diameter ratio. Due to the hierarchical structure of the pore space in packed beds and silica-based monoliths which are characterized by discrete intraparticle (intraskeleton) mesoporous and interparticle (interskeleton) macroporous spatial domains, charge-selective transport prevails within the porous particles and the monolith skeleton under most general conditions. It forms the basis for electrical field-induced concentration polarization (CP). Simultaneously, a finite and -- depending on morphology -- often significant perfusive EOF is realized in these hierarchically structured materials. The data collected in this review show that the existence of CP and its relative intensity compared to perfusive EOF form fundamental ingredients which tune the fluid dynamics in EC employing monoliths and packed beds as stationary phases. This addresses the (electro)hydrodynamics, associated hydrodynamic dispersion, as well as the migration and retention of charged analytes.

Capillary electrochromatography with monolithic silica columns. IV. Electrochromatographic characterization of polar bonded monolithic stationary phases having surface-bound cyano functionalities

Two polar ligands, namely 3-hydroxypropionitrile and 1H-imidazole-4,5-dicarbonitrile (IDCN) were covalently attached to epoxy-activated silica-based monolithic capillary columns via an epoxide ring-opening reaction to yield CN-OH-Monolith and 2CN-OH-Monolith, respectively. The silica monolith was prepared by a sol-gel process, and the resulting "rod-like" stationary phase was subjected to pore tailoring with an alkaline solution to convert small pore domains to mesopore domains, thus yielding a monolith with bimodal pore structure consisting of flow through pores (i.e., flow channels for mobile-phase flow) and mesopores that provide most of the adsorption capacity of the monolith toward the separated solutes. The two polar monoliths, CN-OH-Monolith and 2CN-OH-Monolith, were evaluated in normal-phase CEC with organic-rich mobile phases less polar than the stationary phase. The 2CN-OH-Monolith bearing more polar functions than the CN-OH-Monolith exhibited more retention and improved selectivity toward model polar solutes.

Electrohydrodynamics in hierarchically structured monolithic and particulate fixed beds

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.

Hydrodynamic flow and electroosmotic flow in zirconia-packed capillaries

Fused-silica capillaries were packed with Zirchrom-PBD stationary phase for application in CEC, nanoLC and pseudoelectrochromatography (PEC). Acido-basic properties of zirconia can be used to control the EOF even if the zirconia particles were coated by polybutadiene. As for native zirconia, the EOF is pH-dependent and the pI is close to pH 5. The mixed-mode pressure-voltage technique induced a modulation of the mobile-phase velocity as well as an electrophoretic migration of the solutes in order to improve the resolution of the separation. A significant increase of the flow appeared when both hydrodynamic and EOFs were in the same direction. But an important reduction of the electroosmotic velocity was observed when the hydrodynamic flow and EOF were opposed in Zirchrom-PBD columns. This behaviour has been observed at high or low pH on several columns. Separations of neutral and charged compounds have been performed with these columns in PEC mode.

Capillary electrochromatography of therapeutic peptides on mixed-mode butylmethacrylate monoliths

In this study, a porous mixed-mode n-alkyl methacrylate-based monolith has been used in the separation of therapeutic peptides. While the sulfonic acid (SCX) moiety derived from 2-acrylamido-2-methyl-1-propanesulfonic acid supports the generation of a stable electroosmotic flow (EOF) at both acidic and basic pH values, the butyl ligands provide the nonpolar sites for chromatographic resolution. The performance of the monolith was evaluated regarding the influence of pH on chromatographic resolution of peptides. The suitability of the butylmethacrylate/SCX monolith for the analysis of therapeutic peptides containing basic centres, for example arginine, at moderately high pH 9.5 and the stability to repeat injections of a mixture of peptides was demonstrated. Separations with efficiencies as high as 5.0 x 10(5) plates/m were obtained and the migration behaviour of the peptides at both low (2.8) and high (9.5) pH values could be rationalised based on their charge, molecular mass/shape and relative hydrophobicities.

Numerical analysis of electroosmotic flow in dense regular and random arrays of impermeable, nonconducting spheres

We present a numerical scheme for analyzing steady-state isothermal electroosmotic flow (EOF) in three-dimensional random porous media, involving solution of the coupled Poisson, Nernst-Planck, and Navier-Stokes equations. While traditional finite-difference methods were used to resolve the Poisson-Nernst-Planck problem, the (electro)hydrodynamics has been addressed with high efficiency using the lattice-Boltzmann method. The developed model allows simulation of electrokinetic transport under most general conditions, including arbitrary value and distribution of electrokinetic potential at the solid-liquid interface, electrolyte composition, and pore space morphology. The approach provides quantitative information on a spatial distribution of simulated velocities. This feature was utilized to characterize EOF fields in regular and random, confined and bulk packings of hard (i.e., impermeable, nonconducting) spheres. Important aspects of pore space morphology (sphere size distribution), surface heterogeneity (mismatch in electrokinetic potentials at confining wall and sphere surface), and fluid phase properties (electrical double layer thickness) were investigated with respect to their influence on the EOF dynamics over microscopic and macroscopic spatial domains. Most important is the observation of a generally nonuniform pore-level EOF velocity profile in the sphere packings (even in the thin double layer limit) which is caused by pore space morphology and which is in contrast to the pluglike velocity distribution in a single, straight capillary under the same conditions.

Buffer system for the separation of neutral and charged small molecules using micellar electrokinetic chromatography with mass spectrometric detection

An organic buffer system will be discussed that is suitable for the separation of neutral as well as charged molecules be means of micellar electrokinetic chromatography (MEKC). The buffers are based on the combination of a long chain alkyl acid, such as lauric acid with ammonium hydroxide or an organic base such as tris-hydroxymethylaminomethane (Tris). The resulting buffer system is able to separate neutral compounds based on its micellar properties. These buffers exhibit much reduced conductivity compared to traditional MEKC buffers, such as sodium dodecylsulfate (SDS), which contain inorganic salts. They also have inherent buffer capacity at high pH resulting from the basic buffer component, which in our studies had pK values from about 8-11. The separations that were observed showed high efficiency with plate counts in many cases above 500,000 plates per meter. The reduced conductivity allowed for the application of much higher electric fields, resulting in very fast analysis times. Alternatively, an increase in detection sensitivity could be achieved, as the reduced conductivity allowed for the use of capillaries with lager internal diameters. Combinations of different alkyl acids and organic bases provided for significant flexibility in selectivity tuning. Finally, the fact that the organic micellar buffer systems discussed here do not contain inorganic ions, allows for coupling with mass spectrometric (MS) detection. The possibility of MS detection combined with the high speed in analysis that can be obtained using these organic buffer systems, could make this approach an interesting option for high throughput analysis of combinatorial libraries.

Polar stationary phases for capillary electrochromatography

This review article summarizes the variety of polar stationary phases that have been employed for capillary electrochromatographic separations. Compared with reversed-phase stationary phases, the polar alternatives provide a completely different retention selectivity towards polar and charged analytes. Different types of polar stationary phases are reviewed, including the possible retention mechanisms. Electrochromatographic separations of polar solutes, peptides, and basic pharmaceuticals on polar stationary phases are presented.

Separation of small peptides by electrochromatography on silica-based reversed phases and hydrophobic anion exchange phases

Peptide separations are regarded as a promising application of capillary electrochromatography (CEC) and, at the same time, a suitable model to elucidate its mixed separation mechanism when charged analytes are involved. In this paper, studies on the separation of small peptides (2-4 amino acids) on a Spherisorb octadecyl silane (ODS) phase at acidic pH and on a strong anion exchange (SAX)/C18 mixed mode phase at weakly basic pH are reported. For the ODS phase a comparison of CEC, capillary zone electrophoresis (CZE) and high-performance liquid chromatography (HPLC) under identical buffer/eluent conditions is presented. The predicted retention factors for CEC under the assumption of simple superposition of HPLC retention and CZE migration matched the measured results for the peptides that had small retention factors in HPLC. For both types of stationary phases, a variation of the acetonitrile content in the mobile phase led to a wide range of retention factors, including negative values when co-electroosmotic migration was dominant. Though both the ODS and the SAX/C18 phase offer unique advantages, the SCX/C18 phase at pH 9 provides more flexibility to alter separation selectivity for the selected peptides.

Study of the electroosmotic flow as a means to propel the mobile phase in capillary electrochromatography in view of further miniaturization of capillary electrochromatography systems

In this paper, we investigate the phenomenon of electroosmosis as a means to propel a mobile phase, in particular in view of an application in microfluidic systems, which are characterized by significantly smaller volumes of the reservoirs and the separation channels compared to conventional instrumentation. In the microfluidic chip, pH changes due to water electrolysis quickly showed an effect on the electroosmotic flow (EOF), which could be counteracted by either regularly exchanging or buffering the mobile phase. Surface treatment was of no effect in regard to EOF stabilization in empty channels but may have an influence in channels filled with a charged monolith. In fused-silica capillaries the EOF was generally found to decrease from 'naked' to surface-treated to monolith-filled capillaries. The EOF tended to be higher when an organic solvent (acetonitrile) was added to the mobile phase and could be further increased by substituting the water with equal amounts of methanol. In addition, the hydrostatic pressure exerted by the EOF was investigated. In a microfluidic chip with empty (cross-)channels such an effect could be responsible for a redirection of the flow. In capillaries partially filled with a noncharged (non-EOF-generating) monolith, a linear relationship could be established between the EOF created in the empty section of the capillary (apparent mobility) and the length of the monolith (backpressure). In capillaries partially filled with a charged (EOF-producing) monolith, flow inhomogeneities must be expected as a consequence of a superimposition of hydrodynamic pressure and EOF as mobile phase driving force.

Chiral recognition ability of an (S)-naproxen- imprinted monolith by capillary electrochromatography

The racemic naproxen was selectively recognized by capillary electrochromatography (CEC) on an (S)-naproxen-imprinted monolith, which was prepared by an in situ thermal-initiated polymerization. The recognition selectivity of a selected monolith strictly relied on the CEC conditions involved. The factors that influence the imprinting selectivity as well as the electroosmotic flow (EOF), including the applied voltage, organic solvent, salt concentration and pH value of the buffer, column temperature, and surfactant modifiers were systematically studied. Once the column was prepared, the experiment results showed that the successful chiral recognition was dependent on CEC variables. For example: the recognition could be observed in acetonitrile and ethanol electrolytes, while methanol and dimethyl sulfoxide (DMSO) electrolytes had no chiral recognition ability. The buffer with pH values of 2.6 or 3.0 at a higher salt concentration had chiral recognition ability. Column temperatures of 25-35 degrees C were optimal. Three surfactants, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and polyoxyethylene sorbitan monolaurate (Tween 20), can improve the recognition. Baseline resolution was obtained under optimized conditions and the column efficiency of the later eluent (S)-naproxen was 90 000 plates/m.

Perspective on concentration polarization effects in electrochromatographic separations

This work illustrates the appearance and electrohydrodynamic consequences of concentration polarization in the particulate and monolithic fixed beds used in capillary electrochromatography and related electrical-field assisted processes. Key property of most porous materials is the co-existence of bulk, quasi-electroneutral macroporous regions and mesoporous compartments which are ion-permselective (due to electrical double-layer overlap) causing different transport numbers for co-ionic and counterionic species, e.g., background electrolyte components, or the analytes. For a cathodic electroosmotic flow the (cation) permselectivity, together with diffusive and electrokinetic transport induces depleted and enriched concentration polarization zones at the anodic and cathodic interfaces, respectively, in dependence of the mobile phase ionic strength and applied electrical fields. At high field strength a secondary, nonequilibrium electrical double layer may be created in the depleted concentration polarization zones of a material stimulating electroosmosis of the second kind. The potential of this induced-charge electroosmosis with respect to nonlinear flow velocities and electrokinetic instability mixing (basically destroying the concentration polarization zones) is analyzed in view of the pore space morphology in random-close packings of spherical-shaped, porous particles and hierarchically structured monoliths. Possible applications based on a fine-tuning of the illustrated effects emerge for microfluidic pumping and mixing, or the intensification of sample recovery in adsorption processes. With this perspective we want to focus the attention on concentration polarization in electrochromatographic systems by presenting and discussing original data acquired on relevant microscopic as well as macroscopic scales, and point towards the importance of related effects in colloid and membrane science.*

Exact electric field strength over the packed capillary electrochromatography column bed, theoretical considerations and practical determination

On the basis of the experiments, it was proved that Ohm's law holds true in a capillary electrochromatography (CEC) column. By using the additivity of the potential drop over the packed and unpacked sections, the exact values of the electric field strength over the packed CEC column bed were determined experimentally. The ratio of the resistance of packed CEC column sections to that of an open capillary of the same length (Rp/Ro,Lp) were calculated. Some theoretical calculations were made to show how some structural parameters of particle lattice affect the resulting Rp/Ro,Lp value of the column bed formed accordingly. It is suggested that to correctly elucidate the EOF phenomenon in a CEC column, the potential drop over the packed and unpacked column sections, Ep and Eo, should be accounted for respectively, rather than using the average field strength over the total column length.

Csaba Horváth's contribution to the theory and practice of capillary electrochromatography

This overview has been written as a tribute to a luminary of chromatography--Csaba Gyula Horváth, who passed away earlier this year. Since the scope of his work was enormous, the following pages focus only on one single aspect of his scientific activities, capillary electrochromatography. He was a visionary, recognizing the great potential of this method and devoted a large part of his efforts to studies of problems related to CEC, covering a huge variety of issues embracing the theoretical foundation, instrumentation, and column technology. During the period of time from 1996 to 2004, Csaba Horváth published almost 30 excellent papers concerning capillary electrochromatography, which are reviewed in this article.

The contributions of Csaba Horváth to liquid chromatography

This article is not a biographic description of the life of Csaba Horváth, but an attempt to describe his most important contributions to liquid chromatography, as viewed by an author who himself started with HPLC in 1973. With the introduction of columns packed with rigid pellicular particles, which could withstand high pressures, the combination of fast separations with high efficiency was designated high pressure liquid chromatography about 35 years ago. The contribution of Horváth to the birth of modern LC was to imagine the potential for high-speed separations in LC by transferring ideas and technology from gas chromatography. From this time, his strong participation in understanding and describing the processes of separation science made him a leading scientist in fundamental studies of separation principles until his passing away in April 2004. In addition to being one of the forefathers of modern liquid chromatography, Csaba Horváth described both the ionic interactions and the hydrophobic interactions on ion exchangers as well as on reversed phase materials and developed a theoretical basis for understanding these processes. If one person should be mentioned as the inventor of ion-pair chromatography of organic compounds, of hydrophobic interaction chromatography, and of the theoretical basis for reversed phase chromatography, this person is Csaba Horváth. Throughout the last 30 years he also had a strong interest in studying proteins in separation processes, and in the last decade he made valuable contributions to capillary electrochromatography, particularly for separation of proteins on monolithic columns. For this author is difficult to find another person who has had a greater impact on liquid chromatography than Csaba Horváth.

Planar electrochromatography

Recent developments in planar electrochromatography (PEC) in both the normal-phase and the reversed-phase modes, and at both atmospheric and elevated pressure, are reviewed. Other forced-flow techniques in planar chromatography are also briefly covered. Mobile phase migration in PEC is primarily due to electroosmotic flow, which is controlled by the applied electric field. Capillary mediated flow is an important secondary contributor to migration, and occurs because the layer is unsaturated as a consequence of liquid evaporating from the layer due to Joule heating. The magnitude of the electric field and the concentration of ions in solution are important variables that control both electroosmotic flow and Joule heating. Separations are faster and more efficient than those obtained by conventional planar chromatography, provided appropriate experimental conditions are selected. With inappropriate conditions, either mobile phase accumulates on the surface of the sorbent layer, or Joule heating causes excessive evaporation. The former results in poor spot shape, and the latter can cause the layer to dry. Good separations are obtained when there is a balance between these two effects. The problems associated with mobile phase accumulating on the surface of the sorbent layer, and with excessive evaporation of mobile phase, do not occur with pressurized planar electrochromatography. This technique is performed at high pressure, under conditions that allow heat to be removed form the sorbent layer. This allows the use of a substantially higher electric field than in PEC, and results in a high mobile phase flow rate.

Capillary electrochromatography of peptides and proteins

This review surveys the accomplishments in the separation of peptides and proteins by capillary electrochromatography (CEC) over the last decade. A significant number of research articles have been published on this topic since the last review. Peptide and proteins separations have been carried out in all three formats of CEC, i.e., packed bed, continuous bed and open-tubular (OT) format. In addition to electrophoresis, different chromatographic modes have been successfully exploited with the most prevalent being reversed-phase mode followed by ion-exchange. Although many researchers continue to use model proteins and peptides primarily to evaluate the performance of novel stationary phases some researchers have also applied CEC to the analysis of real-life samples. The potential of CEC to yield complementary information and sometimes a superior separation with respect to established techniques, i.e., microbore HPLC and capillary electrophoresis has been demonstrated. Instrumental modifications in order to facilitate coupling of CEC to mass spectrometry have further upgraded the value of CEC for proteomic analysis. Capillaries are still the separation vehicle of choice for most researchers yet the microfluidic platform is gaining momentum, propelled particularly by its potential for multitasking, e.g., performing different chromatographic modes in series.

On-chip electrochromatography using sol–gel immobilized stationary phase with UV absorbance detection

A chromatography column on a chip was fabricated by immobilizing reversed-phase stationary phase particles (5 microm, C4) using sol-gel technology. Channels were fabricated in quartz using photolithography and wet etching. Localization of the stationary phase was achieved by immobilizing the stationary phase at the desired location in the separation channel prior to bonding of the cover plate. Cross channel design was employed for gated injection. An optical fiber setup was developed for carrying out on-chip UV absorbance detection. The effective optical path length was theoretically determined for the trapezoidal shaped channel and the result was shown to match closely with the experimentally determined value. The effect of applied voltage on velocity was evaluated using thiourea as an unretained marker. Separation performance of the stationary phase was demonstrated by separation of three peptides (Trp-Ala, Leu-Trp and Trp-Trp) under isocratic chromatographic conditions.

Development and application of polymeric monolithic stationary phases for capillary electrochromatography

Monolithic columns for capillary electrochromatography are receiving quite remarkable attention. This review summarizes results excerpted from numerous papers concerning this rapidly growing area with a focus on monoliths prepared from synthetic polymers. Both the simplicity of the in situ preparation and the large number of readily available chemistries make the monolithic separation media a vital alternative to capillary columns packed with particulate materials. Therefore, they are now a well-established stationary phase format in the field of capillary electrochromatography. A wide variety of synthetic approaches as well as materials used for the preparation of the monolithic stationary phases are presented in detail. The analytical potential of these columns is demonstrated with separations involving various families of compounds and different chromatographic modes.

An analysis of induced pressure fields in electroosmotic flows through microchannels

Induced pressure gradients are found to cause band-broadening effects which are important to the performance of microfluidic devices, such as capillary electrophoresis and capillary chromatography. An improved understanding of the underlying mechanisms causing an induced pressure gradient in electroosmotic flows is presented. The analysis shows that the induced pressure distribution is the key to understanding the experimentally observed phenomena of leakage flows. A novel way of determining the static pressures at the inlet and outlet of microchannels is also presented that takes account of the pressure losses due to flow contraction and expansion. These commonly neglected pressure losses at the channel entrance and outlet are shown to be important in accurately describing the flow. The important parameters that define the effect of induced pressure on the flows are discussed, which may facilitate the design of improved microfluidic devices. The present model clearly identifies the mechanism behind the experimentally observed leakage flows, which is further confirmed by numerical simulations. Not only can the leakage flow occur from the electric-field-free side channel to the main channel, but also the fluid in the main channel can be attracted into the side channel by the induced pressure gradient.