Fundamentals of capillary electrochromatography: migration behavior of ionized sample components

A graphene oxide-molybdenum disulfide composite used as stationary phase for determination of sulfonamides in open-tubular capillary electrochromatography

A graphene oxide-molybdenum disulfide (GO-MoS₂) composite was synthesized and utilized as the highly efficient stationary phase of open-tubular capillary electrochromatography (OT-CEC). The characterization results indicated that GO-MoS₂ composite was successfully synthesized. The GO-MoS₂-coated capillary column was prepared by covalent immobilization method for the determination of seven sulfonamides. The baseline separation of seven sulfonamides was achieved by GO-MoS₂-coated capillary column. The linear range was 0.05-100 μg/mL for sulfisomidine, sulfathiazole, sulfamerazine, phthalylsulfathiazole and sulfacetamide, 0.1-100 μg/mL for sulfamonomethoxine and sulfachloropyridazine with a satisfactory correlation coefficients (R²) > 0.9994. This developed OT-CEC method was successfully applied to determinate of seven sulfonamides in environmental water and milk samples with good recoveries of 85.77% - 109.10% and 80.03% - 109.97%, respectively. These results indicated that GO-MoS₂-coated capillary column possessed good stability and repeatability.

Open-tubular nanoelectrochromatography (OT-NEC): gel-free separation of single stranded DNAs (ssDNAs) in thermoplastic nanochannels

Electrophoresis or electrochromatography carried out in nanometer columns (width and depth) offers some attractive benefits compared to microscale columns. These advantages include unique separation mechanisms that are scale dependent, fast separation times, and simpler workflow due to the lack of a need for column packing and/or wall coatings to create a stationary phase. We report the use of thermoplastics, in this case PMMA, as the substrate for separating single-stranded DNAs (ssDNAs). Electrophoresis nanochannels were created in PMMA using nanoimprint lithography (NIL), which can produce devices at lower cost and in a higher production mode compared to the fabrication techniques required for glass devices. The nanochannel column in PMMA was successful in separating ssDNAs in free solution that was not possible using microchip electrophoresis in PMMA. The separation could be performed in <1 s with resolution >1.5 when carried out using at an electric field strength of 280 V/cm and an effective column length of 60 μm (100 nm × 100 nm, depth and width). The ssDNAs transport through the PMMA column was driven electrokinetically under the influence of an EOF. The results indicated that the separation was dominated by chromatographic effects using an open tubular nano-electrochromatography (OT-NEC) mode of separation. Interesting to these separations was that no column packing was required nor a wall coating to create the stationary phase; the separation was affected using the native polymer that was UV/O₃ activated and an aqueous buffer mobile phase.

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.

Computer simulation of electrophoretic aspects of enantiomer migration and separation in capillary electrochromatography with a neutral selector

A computer simulation study describing the electrophoretic separation and migration of methadone enantiomers in presence of free and immobilized (2-hydroxypropyl)-β-CD is presented. The 1:1 interaction of methadone with the neutral CD was simulated by using experimentally determined mobilities and complexation constants for the complexes in a low-pH BGE comprising phosphoric acid and KOH. The use of complex mobilities represents free solution conditions with the chiral selector being a buffer additive, whereas complex mobilities set to zero provide data that mimic migration and separation with the chiral selector being immobilized, that is CEC conditions in absence of unspecific interaction between analytes and the chiral stationary phase. Simulation data reveal that separations are quicker, electrophoretic displacement rates are reduced, and sensitivity is enhanced in CEC with on-column detection in comparison to free solution conditions. Simulation is used to study electrophoretic analyte behavior at the interface between sample and the CEC column with the chiral selector (analyte stacking) and at the rear end when analytes leave the environment with complexation (analyte destacking). The latter aspect is relevant for off-column analyte detection in CEC and is described here for the first time via the dynamics of migrating analyte zones. Simulation provides insight into means to counteract analyte dilution at the column end via use of a BGE with higher conductivity. Furthermore, the impact of EOF on analyte migration, separation, and detection for configurations with the selector zone being displaced or remaining immobilized under buffer flow is simulated. In all cases, the data reveal that detection should occur within or immediately after the selector zone.

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.

Ionic conductance of nanopores in microscale analysis systems: where microfluidics meets nanofluidics

In this tutorial review we illustrate the origin and dependence on various system parameters of the ionic conductance that exists in discrete nanochannels as well as in nanoporous separation and preconcentration units contained as hybrid configurations, membranes, packed beds, or monoliths in microscale liquid phase analysis systems. A particular complexity arises as external electrical fields are superimposed on internal chemical and electrical potential gradients for tailoring molecular transport. It is demonstrated that the variety of geometries in which the microfluidic/nanofluidic interfaces are realized share common, fundamental features of coupled mass and charge transport, but that phenomena behind the key steps in a particular application can be significantly tuned, depending on the morphology of a material. Thus, the understanding of morphology-related transport in internal and external electrical potential gradients is critical to the performance of a device. This addresses a variety of geometries (slits, channels, filters, membranes, random or regular networks of pores, etc.) and applications, e. g., the gating, sensing, preconcentration, and separation in multifunctional miniaturized devices. Inherently coupled mass and charge transport through ion-permselective (charge-selective) microfluidic/nanofluidic interfaces is analyzed with a stepwise-added complexity and discussed with respect to the morphology of the charge-selective spatial domains. Within this scenario, the electrostatics and electrokinetics in microfluidic and nanofluidic channels, as well as the electrohydrodynamics evolving at microfluidic/nanofluidic interfaces, where microfluidics meets nanofluidics, define the platform of central phenomena.

Theory and practical understanding of the migration behavior of proteins and peptides in CE and related techniques

CEC is defined as an analytical method, where the analytes are separated on a chromatographic column in the presence of an applied voltage. The separation of charged analytes in CEC is complex, since chromatographic interaction, electroosmosis and electrophoresis contribute to the experimentally observed behavior. The putative contribution of effects such as surface electrodiffusion has been suggested. A sound theoretical treatment incorporating all effects is currently not available. The question of whether the different effects contribute in an independent or an interdependent manner is still under discussion. In this contribution, the state-of-the-art in the theoretical description of the individual contributions as well as models for the retention behavior and in particular possible dimensionless 'retention factors' is discussed, together with the experimental database for the separation of charged analytes, in particular proteins and peptides, by CEC and related techniques.

Open-Tubular Capillary Electrochromatography Coupled with Electrospray Ionization Mass Spectrometry for Peptide Analysis

In this study, the open-tubular electrochromatographic (OT-CEC) migration behavior of various peptides has been characterized using etched and chemically (n-octadecyl- and cholesterol-) modified capillaries, interfaced to an electrospray ionization mass spectrometer through a sheath liquid configuration. The stationary phases were fabricated by etching the inner wall of the fused-silica capillary and then chemically modifying the new surface through a silanization/hydrosilation reaction. Unlike some other OT-CEC stationary-phase preparation methods, leaching of the immobilized stationary phase and subsequent contamination of the electrospray ion source was largely avoided with this novel surface modification technology. The influence of the immobilized organic phases and those of the buffer electrolytes (pH, the type and content of organic solvent) on the retention and separation of the selected peptides was investigated. Significant peptide retention was found even at very low pH with both types of stationary phases, under conditions whereby the electrophoretic migration dominated the separation process. Due to the effective coverage of the etched surface by a silanization/hydrosilation reaction, adverse adsorption of charged analytes onto the capillary wall was minimized. As a result, very efficient and highly reproducible peptide separations were achieved over a broad pH range. Moreover, peptide-specific multizoning effects were observed. The origin of this novel phenomenon was explored. Compared to capillary electrophoresis electrospray ionization mass spectrometry system, much higher detection sensitivity could be obtained, since a larger amount of sample could be injected and stacked at the head of the open-tubular capillary column without deteriorating the separation performance. On the basis of these observations, these procedures have been adapted to allow the analysis of tryptic peptides generated from proteins.

On-Column Coaxial Flow Chemiluminescence Detection for Underivatized Amino Acids by Pressurized Capillary Electrochromatography Using a Monolithic Column

A new method for pressurized capillary electrochromatography (pCEC) coupling with chemiluminescence (CL) detection using a modified on-column coaxial flow detection interface was developed. To evaluate the feasibility and reliability of the experimental setup, the typical CL compounds luminol and isoluminol were separated and detected by using this pCEC-CL system. A detailed investigation of CL detection interface and postcolumn CL reagent flow rate parameters was described. The excellent resolution and detection sensitivity was achieved by using 3-microm ODS-C18 packed column with 30% ACN (v/v), 5 mmol/L phosphate buffer (pH 8.0). Moreover, with the presence of Co(II) (1.0 x 10(-4) mol/L) in the mobile phase, the linear range of the concentration for luminol was 2.0 x 10(-9)-2.0 x 10(-6) mol/L with a detection limit (S/N = 3) of 2.0 x 10(-10) mol/L, and 2.5 x 10(4) theoretical plates was achieved. In addition, separation and detection of the underivatized amino acids (l-threonine and l-tyrosine) were accomplished by using a polymerized monolithic column based on the principle of the luminol-H2O2-Cu(II)-amino acid CL system. Under the optimum conditions, the mixture of amino acids was efficiently separated with satisfactory results.

Key to Analyte Migration and Retention in Electrochromatography

This work identifies electrical field-induced concentration polarization (CP) as a key physical mechanism influencing the retention behavior of charged analytes in electrochromatography with fixed beds of porous adsorbent particles. Due to an insufficient screening of intraparticle surface charge, under most general conditions the porous (permeable) particles become charge-selective. CP is caused by coupled mass and charge transport normal to the charge-selective external surface of the permeable particles, which leads to concentration gradients of ionic species in the adjoining interparticle electrolyte solution. Cation-exchange (cation-selective) particles were employed to investigate the influence of applied voltage on the retention factor of counterionic, i.e., positively charged, analytes. It is demonstrated by macroscopic retention data and microscopic studies resolving the CP phenomenon on a particle scale that the dependence of CP on electrical field and mobile-phase ionic strengths is directly reflected in concomitant changes of analyte retention. The CP zones that develop at the interface between interparticle and intraparticle pore space are recognized by charged, but not electroneutral analytes while entering or leaving the particles. The intensity of these convective-diffusion boundary layers (CP zones) depends on the applied field strength and charge selectivity of a particle. Thus, it is the charge-selective transport between the interparticle and intraparticle pore space in packed beds that prevails under typical experimental conditions in electrochromatography and that forms the physical basis for a general electrical field dependence of the retention factor of charged analytes.

Rapid separation and determination of carbamate insecticides using isocratic elution pressurized capillary electrochromatography

An isocratic elution pressurized CEC (pCEC) system was used to separate and determine ten carbamate insecticides. It was found that introduction of the electrical field, supplementary pressure, and SDS in the proposed method greatly improved the speed, column efficiency, selectivity, and repeatability for separation and determination of carbamates. On a capillary column of 75 microm ID packed with 3 microm octadecyl silica, baseline separation and detection of ten analytes was performed by using a mobile phase consisting of 30% v/v ACN and 70% v/v of 5 mmol/L ammonium acetate (pH 6.5) containing 1 mmol/L SDS and 0.01% triethylamine (TEA). Under the optimum conditions ten carbamate insecticides could be completely separated within 20 min. For the real vegetable samples, an SPE procedure for the cleanup of matrices was carried out prior to pCEC analysis. The detection limits of 0.05-1.6 mg/kg for ten carbamates and mean recoveries of 51.3-109.2% for eight kinds of vegetable samples at different concentrations of carbamates with RSD less than 11.4% were obtained, respectively. The proposed method has been proved to be effective in the rapid analysis of carbamate residues in vegetables.

On-line coupling of pressurized capillary electrochromatography with end-column amperometric detection for analysis of estrogens

An improved technique, pressurized capillary electrochromatography (pCEC) coupling with end-column amperometric detection (AD), was developed and used for the separation and determination of estrogens. The effects of pH value, composition of mobile phase, concentration of the surfactant sodium dodecyl sulfate (SDS) and applied voltage on separation were investigated. The electrochemical oxidation of diethylstilbestrol (DES), dienestrol (DE), and hexestrol (HEX) could be reliably monitored with a carbon electrode at 0.9 V (vs. Ag/AgCl). The pCEC analyses were performed on a capillary separation column packed with 3 microm C18 particles with an acetonitrile/water (31%: 69%) mobile phase containing Tris buffer (5 mmol/L, pH 4.5) and 4 mmol/L SDS. High voltage up to 12 kV reduced the retention time dramatically and still provided a baseline resolution. In addition, supplementary pressure prevented bubble formation and provided reliability and reproducibility of the pCEC performance. The detection limits for the three estrogens ranged from 1.2 to 2.2x10(-7) mol/L, about 10 20-fold lower than those obtained with pCEC-UV detection. To evaluate the feasibility and reliability of this system, the proposed pCEC-AD method was further demonstrated with fish muscle samples spiked with estrogens.

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.

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.

Comparison of a three-peptide separation by capillary electrochromatography, voltage-assisted liquid chromatography and nano-high-performance liquid chromatography

A mixture of three peptides was separated by capillary electrochromatography (CEC), nano-HPLC and voltage-assisted LC. In the latter case the charged analytes migrate through a neutral stationary phase driven by electrophoresis while their interaction with the stationary phase provides the basis for a chromatographic separation. The stationary phases used were poly(glycidyl methacrylate-co-ethylene dimethacrylate)-based monoliths that could be used directly as neutral "C1"-type columns for voltage-assisted LC and nano-HPLC, while their application in CEC became possible after derivatization of the epoxy groups with ionogenic N-ethylbutylamine functions. The separation of the peptide mixture was possible in all three modes. Highest plate numbers and resolutions were obtained under voltage-assisted conditions. The elution order showed dependencies on the charge density but also on the hydrophobicity of the peptides and was different in the three investigated chromatographic modes. The effect of changes in the ionic strength and the organic solvent content of the mobile phase on the resolution and the migration behavior of the peptides was investigated and showed the expected behavior. Voltage-assisted LC is suggested as an alternative to CEC for the separation of charged analytes by electrochromatography.

Peak compression effects in capillary electrochromatography

Peak compression in CEC is a phenomenon that can generate very narrow peaks with extremely high efficiencies that defy current chromatographic theory. This review article summarises the content of publications in this area up to this date. Two main types of peak compression effects have been observed in the literature. First, an irreproducible and hard to control focusing effect of unclear origin, observed on strong cation exchangers. Second, a reproducible continuous stacking effect caused by sample composition induced system zones demonstrated on several types of stationary phases.

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.

Shielded Stationary Phases Based on Porous Polymer Monoliths for the Capillary Electrochromatography of Highly Basic Biomolecules

A novel stationary phase for capillary electrochromatography has been prepared via photoinitiated grafting of two layers of polymer chains onto the pore surface of a porous polymer monolith. To achieve the desired retention, the original monolith with optimized porous properties was grafted with an "interior" layer consisting of the ionizable monomer, 2-acrylamido-2-methyl-1-propanesulfonic acid, followed by a "covering" layer of hydrophobic polymer chains. This technique affords monolithic CEC columns that facilitate electroosmotic flow (EOF) while preventing ionized analytes from interacting with the charged surface functionalities. Grafting of the second layer does not adversely affect the EOF. Grafting times of 30 and 60 s for AMPS and butyl acrylate, respectively, enabled the preparation of a monolith with full shielding of the analytes from the ionizable functionalities and excellent chromatographic performance. This approach allows for the first time the independent optimization of both electroosmotic flow and retention properties in CEC columns. The efficient isocratic separations of mixtures of peptides, including some that are highly basic and would be affected by unshielded charges, were routinely achieved in 40-90 s using a simple MS compatible mobile phase consisting of 20 mmol/L ammonium acetate in a 1:1 water-acetonitrile mixture.

On the nature of the forces controlling selectivity in the high performance capillary electrochromatographic separation of peptides

In this minireview, the nature of the forces controlling selectivity in the high performance capillary electrochromatographic (HP-CEC) separation of peptides has been examined. For uncharged and charged peptides, a synergistic interplay occurs in HP-CEC systems between adsorptive/partitioning events and electrokinetically driven motion. Moreover, at high field strengths, both bulk electrophoretic migration and surface electrodiffusion occur. Thus, the migration behavior of peptides in different HP-CEC systems can be rationalized in terms of the combined consequences of these various processes. Moreover, in HP-CEC, the buffer electrolyte interacts with both the peptide analytes and the sorbent as bulk phenomena. These buffer-mediated processes control the solvational characteristics, ionization status and conformational behavior of the peptides as well as regulate the double-layer properties of the sorbent, and the ion flux and electro-osmotic flow characteristics of the HP-CEC system per se. These buffer electrolyte effects mediate mutual interactions between the peptide and the sorbent, irrespective of whether the interaction occurs at the surface of microparticles packed into a capillary, at the surface of a contiguous monolithic structure formed or inserted within the capillary or at the walls of the capillary as is the case with open tubular HP-CEC. Diverse molecular and submolecular forces thus coalesce to provide the basis for the different experimental modes under which HP-CEC can be carried out. As a consequence of this interplay, experimental parameters governing the separation of peptides in HP-CEC can be varied over a wide range of conditions, ensuring numerous options for enhanced selectivity, speed, and resolution of peptides. The focus of the peptide separation examples presented in this minireview has been deliberately restricted to the use of HP-CEC capillaries packed with n-alkyl-bonded silicas or mixed-mode strong ion exchange sorbents, although other types of sorbent chemistries can be employed. From these examples, several conclusions have been drawn related to the use of HP-CEC in the peptide sciences. These observations confirm that variation of a specific parameter, such as the pH or the content of the organic solvent modifier in the buffer electrolyte, simultaneously influences all other physicochemical aspects of the specific HP-CEC separation. Peptide selectivity in HP-CEC thus cannot be fine-tuned solely through the use of single parameter optimization methods. In this context, HP-CEC differs significantly from the analogous reverse phase high performance liquid chromatography (RP-HPLC) procedures with peptides. Rather, more sophisticated multiparameter optimization procedures, involving knowledge of (a) the field strength polarity, (b) its contour and flux characteristics, (c) effects of buffer electrolyte composition and pH, (e) the influence of the temperature, and (f) the impact of the sorbent characteristics, are required if the full capabilities offered by HP-CEC procedures are to be exploited. In this minireview, the HP-CEC migration behavior of several different sets of synthetic peptides has been examined, and general guidelines elaborated from these fundamental considerations to facilitate the interpretation and modulation of peptide selectivity in HP-CEC.

Investigation of mixed-mode monolithic stationary phases for the analysis of charged amino acids and peptides by capillary electrochromatography

The potential of N,N-dimethylacrylamide-piperazine diacrylamide-based monolithic stationary phases bearing sulfonic acid groups for electroosmotic flow generation is investigated for the separation of positively charged amino acids and peptides. The capillary columns were used under electrochromatographic but also under purely chromatographic (nano-HPLC) conditions and the separations interpreted as the result of possible chromatographic and electrophoretic contributions. The stationary phases were found to be mechanically stable up to pressures of 190 bar and chemically stable towards a wide variety of organic and hydro-organic mobile phases. In order to investigate the retention mechanism, the salt concentration and the organic solvent content of the (hydro-)organic mobile phase were varied in a systematic manner, taking three aromatic amino acids (phenylalanine, tryptophan, histidine) as model analytes. The respective contributions of electrostatic and hydrophobic and/or hydrophilic interactions were further investigated by varying the charge density and the hydrophobicity of the standard stationary phase. The former was done by varying the amount of charged monomer (vinylsulfonic acid) added during synthesis, the latter by (partially) replacing the interactive monomer (N,N-dimethylacrylamide) by other more hydrophobic monomers. A mixed mode retention mechanism based primarily on electrostatic interactions modified in addition by "hydrophilic" ones seems most suited to interpret the behavior of the amino acids, which stands in contradistinction to the previously investigated case of the behavior of neutral analytes on similar stationary phases. Finally the separation of small peptides was investigated. While the separation of Gly-Phe and Gly-Val was not possible, the separation of Phe-Gly-Phe-Gly and Gly-Phe but also of the closely related Gly-His and Gly-Gly-His could be achieved.

Protein separation by monolithic capillary electrochromatography

This work presents the separation of model proteins by capillary electrochromatography involving a monolithic stationary phase with C4 functionality. The monolith was fabricated in UV-transparent capillaries by employing a slight modification of a recently published photopolymerization procedure. With the number of theoretical plates per column ranging between 11000 and 33000, the separation efficiency proved to be lower than capillary zone electrophoresis where plate numbers ranged between 18000 and 66000. However, higher resolution was obtained due to the additional chromatographic separation mechanism. Inter- and intra-column reproducibility were evaluated, the latter could be significantly improved when using a rinsing procedure that contained 0.05% sodium dodecylsulfate in the mobile phase. Plate heights became nearly independent of mobile phase velocities higher than 0.5 mm/s indicating that high velocities can be applied without sacrificing efficiency. Furthermore, peak heights showed a dependence on injection times. For proteins, an increase in capacity factors was found when increasing the percentage of organic solvent in the mobile phase.

Analysis of food proteins and peptides by chromatography and mass spectrometry

The research topics and the analytical strategies dealing with food proteins and peptides are summarized. Methods for the separation and purification of macromolecules of food concern by both high-performance liquid chromatography (HPLC) on conventional packings and perfusion HPLC are examined. Special attention is paid to novel methodologies such those based on multi-dimensional systems that comprise liquid-phase based protein separation, protein digestion and mass spectrometry (MS) analysis of food peptide and proteins. Recent applications of chromatography and MS-based techniques for the analysis of proteins and peptides in food are discussed.

Separation of negatively charged nonsteroidal anti-inflammatory drugs by reversed-phase capillary electrochromatography

Reversed-phase capillary electrochromatography in a 5-microm C18 fully packed capillary was employed to optimize the separation of negatively charged nonsteroidal anti-inflammatory drugs. The effect of the physico-chemical parameters and different analysis modes on the separation of 2-arylpropionic acids was studied and evaluated. The mobile phase composition, buffer type, concentration and pH differently influenced the peak efficiency and resolution, selectively modulating the analytes interaction with the stationary phase. The use of zwitterionic MES or acetate mobile phases strongly modulated the analytes migration order and peak efficiency. The optimum experimental conditions were found in MES buffer, pH 5.0, containing the 75% acetonitrile-methanol (1:1). All the analytes were baseline separated in a mixture in less than 13 min with peak efficiencies in the range of 78,500-84,200 N/m. Under these conditions the analytes were negatively charged and their effective electrophoretic mobilities played a role in the separation. The analysis of different pharmaceutical preparations containing anti-inflammatory drugs, e.g. drops and tablets, is also presented after a very simple sample pretreatment.