Recent developments in the field of monolithic stationary phases for capillary electrochromatography

Advances in chiral separation using capillary electromigration techniques

This review gives an overview of recent developments in CZE, EKC, and CEC covering the literature since the year 2004. Since there appeared a special issue on applications, this review focuses on the progress in electromigration techniques and new methodological developments. New techniques, new chiral selectors as well as new chiral stationary phases for CEC are discussed.

Tailoring the macroporous structure of monolithic silica-based capillary columns with potential for liquid chromatography

The present work aims at the optimisation of the synthesis of methyl-silsesquioxane monolithic capillary columns using a sol-gel based protocol. The influence of reaction conditions such as temperature, reaction mixture composition and catalyst concentration has been examined. The morphology of the products was studied by scanning electron microscopy and nitrogen adsorption. Monolithic capillary columns were obtained with a skeleton-like structure with open pores. Pore diameters vary from 0.8 to 15 microm, diameters of the xerogel network vary from 0.4 to 12 microm, respectively. Specific surface areas up to 334 m2/g have been observed, however, many materials did not possess areas above few m2/g which represents the limit of detection of the nitrogen porosimetry measurements. Excellent adhesion to the capillary wall was observed in all cases, and drying was possible at ambient conditions without the formation of cracks.

Controlling the surface chemistry and chromatographic properties of methacrylate-ester-based monolithic capillary columnsviaphotografting

Preparation of monolithic capillary columns for separations in the CEC mode using UV-initiated polymerization of the plain monolith followed by functionalization of its pore surface by photografting has been studied. The first step enabled the preparation of generic poly(butyl methacrylate-co-ethylene dimethacrylate) monoliths with optimized porous properties, controlled by the percentages of porogens 1-decanol and cyclohexanol in the polymerization mixture, irradiation time, and UV light intensity. Ionizable monomers [2-(methacryloyloxy)ethyl]trimethylammonium chloride or 2-acryloamido-2-methyl-1-propanesulfonic acid were then photografted onto the monolithic matrix, allowing us to control the direction of the EOF in CEC. Different strategies were applied to control the grafting density and, thereby, the magnitude of the EOF. To control the hydrophobic properties, two approaches were tested: (i) cografting of a mixture of the ionizable and hydrophobic monomers and (ii) sequential grafting of the ionizable and hydrophobic monomers. Cografting resulted in similar retention but higher EOF. With sequential grafting, more than 50% increase in retention factors was obtained and a slight decrease in EOF was observed due to shielding of the ionizable moieties.

Recent progress in polar stationary phases for CEC

This review summarizes most of the recent developments in the preparation and application of polar stationary phases for CEC covering the literature published since the year 2004. These polar stationary phases have been adopted for separation of analytes by the modes of packing column CEC, open-tubular CEC (o-CEC) and monolithic column CEC. Currently, development of o-CEC using biomolecules, such as protein and DNA, as the immobilized ligands is highlighted partly due to the simplicity of preparation. Furthermore, monolithic columns have been extended quickly, particularly inorganic materials-based monoliths, such as silica, zirconia, hafnium, etc., as an alternative to packed columns have been developed quickly.

Recent advances in the control of morphology and surface chemistry of porous polymer-based monolithic stationary phases and their application in CEC

This review focuses on developments in the field of polymer-based monolithic columns for CEC published in the literature since the beginning of the year 2005. The possibility of in-situ preparation as well as easy control over their porous properties and surface chemistries clearly make monolithic separation media an attractive alternative to capillary columns packed with particles. Different variables such as polymerization conditions, morphology, and surface chemistry are shown to directly affect performance of monolithic capillary columns in terms of efficiency, analysis time, and retention.

Preparation and application of monolithic beds in the separation of selected natural biologically important compounds

The importance of monolithic (continuous) beds is connected with their easy preparation and the far-reaching possibilities of modification of their surface and porous properties. These properties make them particularly attractive for the analysis of biologically important compounds characterized by a wide spectrum of physicochemical properties. This review summarizes their preparation methods as well as their application as continuous beds for determination of such biologically important compounds as catecholamines, vitamins, flavonoids, amino acids, peptides, and proteins.

Synthesis and evaluation of polymeric continuous bed (monolithic) reversed-phase gradient stationary phases for capillary liquid chromatography and capillary electrochromatography

There is a demand of novel high resolution separation media for separation of complex mixtures, particularly biological samples. One of the most flexible techniques for development of new separation media currently is synthesis of the continuous bed (monolithic) stationary phases. In this study the capillary format gradient stationary phases were formed using continuous bed (monolith) polymerization in situ. Different reversed-phase stationary phase gradients were tailored and their resolution using capillary liquid chromatography and capillary electrochromatography at isocratic mobile phase conditions was evaluated. It is demonstrated, that efficiency and resolution of the gradient stationary phases can be substantially increased comparing to the common (isotropic) stationary phases. The proposed formation approach of the gradient stationary phase is reproducible and compatible with the capillary format or microchip format separations. It can be easily automated for the separation optimizations or mass production of the capillary columns or chips.

Selection of comparison criteria and experimental conditions to evaluate the kinetic performance of monolithic and packed-bed columns

The present study concerns the problem of finding appropriate experimental conditions and comparison criteria to assess the kinetic performance of LC supports with different sizes or morphologies. A general procedure, based on evaluating each support for its own optimal mobile-phase composition, is proposed. The practical elaboration of the procedure is illustrated using the specific case of a capillary LC separation of a series of polycyclic aromatic test compounds employing silica-monolith capillary columns and capillary columns packed with 6-microm porous particles. To compare the systems for their ability to yield the fastest critical-pair separation, plate-height measurements are transformed into an effective plate number kinetic plot, i.e., a plot of the extrapolated retention time divided by the square of the extrapolated effective plate number (t(R)/N(eff)(2)) versus N(eff). This type of data representation provides a direct and universal basis to compare the kinetic performance of different LC supports and it corrects for differences in retention strength arising from different phase ratios.

Less common applications of monoliths: Preconcentration and solid-phase extraction

Monolithic materials are finding their place in a variety of fields. While liquid chromatography is the most emphasized use of this new category of porous media, some other just as important applications are eclipsed by the success of monolithic columns. This review article describes all current facets of use of monoliths in preconcentration and solid-phase extraction. In addition to the typical off line use that does not seem to be the main stream application for the monolithic materials, in-line connection of the preconcentration with HPLC, electrochromatography, electrophoresis, enzymatic digestion, as well as its applications in microfluidics are presented.

Monolithic media in microfluidic devices for proteomics

Considerable effort has been invested in the development of integrated microfluidic devices for fast and highly efficient proteomic studies. Among various fabrication techniques for the preparation of analytical components (separation columns, reactors, extractors, valves, etc.) in integrated microchips, in situ fabrication of monolithic media is receiving increasing attention. This is mainly due to the ease and simplicity of preparation of monolithic media and the availability of various precursors and chemistries. In addition, UV-initiated photopolymerization technique enables the incorporation of multiple analytical components into specified parts of a single microchip using photomasks. This review summarizes preparation methods for monolithic media and their application as microfluidic analytical components in microchips.

Separation of microcystins by capillary electrochromatography in monolithic columns

Contribution on microcystin variant analysis by capillary electrochromatography (CEC) with easily affordable spectrophotometric detection is presented. Two types of reversed-phase capillary columns formed by inorganic or organic polymer monoliths were prepared for this purpose. The analyses were performed isocratically by means of tris(hydroxymethyl) aminomethane (TRIS) buffers of mildly alkaline pH containing 30% (v/v) acetonitrile as the mobile phases. The samples were injected electrokinetically and the analyses were done at the same separation field strength of 500 V/cm. Microcystins were detected at 238 nm. Although both column types differ not only in monolith quality (inorganic versus organic) but also in the length of the aliphatic moiety (C8 versus C12) similar results were achieved. The on-column preconcentration as the encouraging prospect of electrochromatographic technique was also tested. Consequently 5% of column volume was injected in contrast with 0.5% at standard injection scheme resulting in the six times enrichment of the low concentrated cyanobacterial extract at the top of the separation column. From these preliminary results can be seen that the CEC method is fully applicable for rapid microcystin screening.

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.

Octyl-functionalized hybrid silica monolithic column for reversed-phase capillary electrochromatography

Hybrid silica monolithic stationary phase functionalized with octyl groups was synthesized by a two-step acid/base-catalyzed hydrolysis/co-condensation of tetraethoxysilane (TEOS) and n-octyltriethoxysilane (C(8)-TEOS). The influences of determining factors in the sol-gel process such as the monomer ratio and water content on the monolith formation were systematically investigated. An increase in the TEOS/C(8)-TEOS ratio in the polymerization mixture shifted the pore size distribution towards smaller pore diameter with larger pore volume. The optimal TEOS/C(8)-TEOS volume ratio was found to be 90/50, under which condition the median pore diameter of the monolith was around 1.0 microm with pore volume of 3.25 cm(3)/g. The chromatographic characteristics of the monolithic column prepared with the optimized fabrication condition were studied. Some aromatic compounds including alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs) and phenols were successfully separated on the octyl-functionalized silica monolithic column with high column efficiency up to 180,000 plates/m.

Comparison of monolithic approaches for enantioselective capillary electrochromatography involving cyclodextrins

The access to CD-modified monoliths for enantiomeric separation by CEC can be divided into two main approaches. (i) Silica-based monoliths, prepared by either a sol-gel process or by sintering of silica particles, are modified after fabrication by coating with a CD selector. Alternatively the fusion of CD functionalized silica particle via gluing is feasible. (ii) Rigid or homogeneous organic polymer-based monoliths, prepared by polymerization of organic monomers in the presence of a porogen, are modified with the CD selector either by copolymerization or by physical incorporation into the continuous bed.

Theoretical and nomenclatural considerations of capillary electrochromatography with monolithic stationary phases

During the past decade, CEC has been one of the few novel achievements in the field of separation science attracting a wide interest. The technology progress permitted the realization of the long-sought idea to employ an electroosmotically driven flow through the columns improving the separations in terms of both resolution and efficiency. The early practical obstacles related to the use of conventional bead-packed columns have been solved by the introduction of continuous beds, also known as monoliths. Hitherto, various synthesis approaches have been successfully developed producing monolithic beds in situ in capillary columns, sharing similar physical structure built up of tiny particles (in the sub-microm range) that are covalently linked together and to the capillary wall. Parallel with the practical column technology studies, the theory of electrochromatography has been continuously developed, focusing on such basic issues as EOF characterization, separation efficiency, and peak dispersion effects. This review provides a short introduction to the theory of CEC with special attention to monolithic separation beds. The paper also summarizes the latest achievements in CEC and discusses the nomenclature, EOF characteristics, and some specific advantages of monolithic column technology.

Recent applications in capillary electrochromatography

In this review research papers on the application of CEC are summarized that have been published between May 2003 and May 2005. First, a short overview is given of trends and developments in CEC that may increase the applicability of the separation technique. Next, application-oriented research using CEC is described in biochemical studies, including proteomics and genomics, in the analysis of food and natural products, and in pharmaceutical, industrial, and environmental analysis.

Nanoparticle-based pseudostationary phases in capillary electrochromatography

During the past decades, research has been performed to enhance selectivity in CE by introducing different types of additives into the electrolyte. Research concerning this has taken many directions, especially during the last 5 years. A promising technique, which benefits from no packing or frits, is to use nanoparticles as the pseudostationary phase (PSP) in CEC. PSPs have the advantage of introducing a novel interaction phase for every analysis, which greatly simplify column exchange and circumvent contamination inherited from complex mixtures, e.g., biological samples. The field of nanoparticle-based PSPs used in CEC is covered in this review. The term CEC will be used consequently throughout this review, although some authors used the term EKC to categorize their work. Important requirements for the nanoparticles used and possible reasons for band broadening will be discussed. Applications with silica nanoparticles, polymer nanoparticles, molecularly imprinted polymer nanoparticles, gold nanoparticles, dendrimers, and polymeric surfactants as PSP will also be discussed.

Pore size characterization of monolith for electrochromatography via atomic force microscopy studies in air and liquid phase

This paper investigates the use of scanning electron microscopy (SEM) and atomic force microscopy (AFM) for the characterization of monoliths used in capillary electrochromatography (CEC) while focusing on the nature of the information available from both techniques. SEM imaging revealed a compact structure of non-porous micrometer sized particles homogeneously agglomerated. With a simple AFM methodology, we found by direct observation that the same material exhibits mesopores in the nanometer range while SEM showed non-porous surfaces. These results obtained by AFM clearly showed that micrometer sized particles shrank and micrometer sized pores increased in the monolith when wetted. Thus, AFM was capable of demonstrating the morphological differences between wet and dried monolithic materials that are not possible by other imaging methods at micrometer resolution.

Less common applications of monoliths: I. Microscale protein mapping with proteolytic enzymes immobilized on monolithic supports

This review summarizes the recent contributions to the rapidly growing area of immobilized enzymes employing both silica and synthetic polymer-based monoliths as supports. Focus is mainly on immobilized proteolytic enzyme reactors designed for studies in proteomics. Porous monoliths emerged first as a new class of stationary phases for HPLC in the early 1990s. Soon thereafter, they were also used as supports for immobilization of proteins and preparation of both stationary phases for bioaffinity chromatography and enzymatic reactors. Organic polymer-based monoliths are typically prepared using a simple molding process carried out within the confines of a "mold" such as chromatographic column or capillary. Polymerization of a mixture comprising monomers, initiator, and porogenic solvent affords macroporous materials. In contrast, silica-based monoliths are first formed as a rigid rod from tetraalkoxysilane in the presence of PEG and subsequently encased with a plastic tube. Both types of monolith feature large through-pores that enable a rapid flow-through. Since all the solutions must flow through the monolith, the convection considerably accelerates mass transfer within the monolith. As a result, reactors including enzyme immobilized on monolithic support exhibit much higher activity compared to the reactions in solution.

Monolithic continuous beds as a new generation of stationary phase for chromatographic and electro-driven separations

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.

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.

Monolithic silica-based capillary column with strong chiral cation-exchange type surface modification for enantioselective non-aqueous capillary electrochromatography

A silica-based monolithic stationary phase prepared by the sol-gel process in a 100 microm I.D. fused-silica (FS) capillary has been modified chemically with 3-mercaptopropyl trimethoxysilane followed by immobilization of a strong cation-exchange (SCX) type chiral selector, (S)-N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutane phosphonic acid, by radical addition reaction onto the reactive sulfhydryl surface. After a fine-tuning of the mobile phase composition, the enantioselective capillary column was evaluated for the separation of various chiral basic drugs by enantioselective non-aqueous capillary electrochromatography (CEC), in comparison to capillary column analogs packed with 3.5 microm silica particles having attached the same selector. The performance of the monolithic silica column was further compared to corresponding polymethacrylate-based organic polymer monoliths. The study indicated that strong counter-ions such as 2-aminobutanol or N,N,N',N'-tetramethylethylenediamine are needed, although they reduce the electroosmotic flow velocity and separation factors in comparison to less efficient counter-ions, in order to allow the elution of the oppositely charged solutes in the ion-exchange retention mode within reasonable run time and as sharp zones. In contrast, weak counter-ions such as N,N-diisopropylethylamine (Huenig base) provided stronger electroosmotic flow and much better separation factors, but relatively poor peak efficiencies. Overall, with the chemically functionalized monolithic silica column the high quality separations of packed column analogs could be approximated, with regards to both separation factors and peak performances. On the other hand, the monolithic capillary column certainly outperformed the packed column in terms of system robustness under capillary electrochromatography conditions and showed excellent column longevity. The enantioselective strong cation-exchange-type monolithic silica column performed also well in comparison to the organic polymer monolith.

Capillary electrochromatography of amino acids with a protein-bonded porous-layer open-tubular column

A protein-bonded porous-layer open-tubular (PLOT) column has been synthesized and applied to the separation of amino acids by CEC. The porous layer was coated on the capillary inner wall by in situ polymerization of 2-hydroxyethyl methacrylate and 2-vinyl-4,4-dimethylazlactone in the presence of 1-decanol as a porogen inside a fused-silica capillary silanized with gamma-methacryloxypropryltrimethoxysilane. The azlactone functionalities at the surface of the porous polymeric support layer were allowed to react with BSA to yield a protein-bonded PLOT column. This porous layer was characterized by scanning electron microscopy and its thickness was about 1 microm. CEC on this column gave enhanced resolution of three amino acids (histidine, phenylalanine, and tryptophan), and baseline separation was achieved with 20 mM phosphate buffer, pH 8.0.

Extending the array of crosslinkers suitable for the preparation of polymethacrylate-based monoliths

The in situ preparation of monolithic capillary columns comprising copolymers of butyl methacrylate with ethylene dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, and pentaerythritol tetraacrylate using thermal polymerization within 250 microm ID capillaries and their application for micro-HPLC separations of proteins has been studied. For all crosslinkers, optimization of the porogenic mixture consisting of 1-propanol and 1,4-butanediol yielded monoliths with pore sizes above 1 microm suitable for rapid separations at low back pressure. Very good separations were achieved for a protein mixture consisting of ribonuclease A, cytochrome c, myoglobin, and ovalbumin with all tested columns.