Comparative study of capillary electroendosmotic chromatography and electrically assisted gradient nano-liquid chromatography for the separation of peptides

Investigation of the ion-exchange properties of methacrylate-based mixed-mode monolithic stationary phases employed as stationary phases in capillary electrochromatography

The potential of methacrylate-based mixed-mode monolithic stationary phases bearing sulfonic acid groups for the separation of positively charged analytes (alkylanilines, amino acids, and peptides) by capillary electrochromatography (CEC) is investigated. The retention mechanism of protonated alkylanilines as positively charged model solutes on these negatively charged mixed-mode stationary phases is investigated by studying the influence of mobile phase and stationary phase parameters on the corrected retention factor which was calculated by taking the electrophoretic mobility of the solutes into consideration. It is shown that both solvophobic and ion-exchange interactions contribute to the retention of these analytes. The dependence of the corrected retention factor on (1) the concentration of the counter ion ammonium and (2) the number of methylene groups in the alkyl chain of the model analytes investigated shows clearly that a one-site model (solvophobic and ion-exchange interactions take place simultaneously at a single type of site) has to be taken to describe the retention behaviour observed. Comparison of the CEC separation of these charged analytes with electrophoretic mobilities determined by open-tubular capillary electrophoresis shows that mainly chromatographic interactions (solvophobic and ion-exchange interactions) are responsible for the selectivity observed in CEC, while the electrophoretic migration of these analytes plays only a minor role.

CEC separation of insect oostatic peptides using a strong-cation-exchange stationary phase

The separation of several insect oostatic peptides (IOPs) was achieved by using CEC with a strong-cation-exchange (SCX) stationary phase in the fused-silica capillary column of 75 microm id. The effect of organic modifier, ionic strength, buffer pH, applied voltage, and temperature on peptides' resolution was evaluated. Baseline separation of the studied IOPs was achieved using a mobile phase containing 100 mM pH 2.3 sodium phosphate buffer/water/ACN (10:20:70 v/v/v). In order to reduce the analysis time, experiments were performed in the short side mode where the stationary phase was packed for 7 cm only. The selection of the experimental parameters strongly influenced the retention time, resolution, and retention factor. An acidic pH was selected in order to positively charge the analyzed peptides, the pI's of which are about 3 in water buffer solutions. A good selectivity and resolution was achieved at pH <2.8; at higher pH the three parameters decreased due to reduced or even zero charge of peptides. The increase in the ionic strength of the buffer present in the mobile phase caused a decrease in retention factor for all the studied compounds due to the decreased interaction between analytes and stationary phase. Raising the ACN concentration in the mobile phase in the range 40-80% v/v caused an increase in both retention factor, retention time, and resolution due to the hydrophilic interactions of IOPs with free silanols and sulfonic groups of the stationary phase.

Chromatographic behaviour of peptides on a mixed-mode stationary phase with an embedded charged group by capillary electrochromatography and high-performance liquid chromatography

Retention behaviour of biological peptides was investigated on a stationary phase bearing an embedded quaternary ammonium group in a C21 alkyl chain by both high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC). In HPLC experiments, variation of acetonitrile (ACN) content in the mobile phase showed that peptides are mainly separated by RP mechanism. The weak or negative retention factors observed as compared to C18 silica stationary phase suggested the involvement of an electrostatic repulsion phenomenon in acidic conditions. Comparison of HPLC and CEC studies indicated that (i) ion-exclusion phenomenon is more pronounced in HPLC and (ii) higher ACN percentage in mobile phase induce for some peptides an increase of retention in CEC, pointing out the existence of mechanisms of retention other than partitioning mainly involved in chromatographic process. This comparative study demonstrated the critical role of electric field on peptide retention in CEC and supports the solvatation model of hydrolytic pillow proposed by Szumski and Buszewski for CEC using mixed mode stationary phase in CEC.

Flow splitting at the inlet electrode as a method for decreasing the electric current in electric field assisted liquid chromatography

The combination of pressurized flow and electric field offers, with the use of capillary columns, several options for retention control. However, it has been shown that the utility of this technique is strongly limited by the high electric current that is generated at the high electric field strengths that are needed. We have earlier shown that the high current is a result of locally increased mobile phase ion concentration in the electric field, particularly around the inlet electrode. In this paper, we report that by splitting the mobile phase flow around the inlet electrode a relatively constant ion concentration around the electrode can be obtained and the high currents are there by reduced.

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.

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.

Stepwise gradient of buffer concentration for capillary electrochromatography of peptides on sulfonated naphthalimido-modified silyl silica gel

The advantage of using a stepwise gradient of buffer concentration in CEC was demonstrated with the mixed-mode stationary phase, 3-(4-sulfo-1,8-naphthalimido)propyl-modified silyl silica gel (SNAIP). Before the application of a stepwise gradient, the effect of buffer concentration on the separations of six peptides and tryptic digests was investigated. Bubble formation caused by Joule heating at currents up to 95 microA was successfully suppressed by using SNAIP column even without pressurization, which contributed to a stepwise gradient of buffer concentration. Utilizing the stepwise gradient improved and shortened the separation of six peptides as compared to the separation under an isocratic elution.

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.

Controlling the retention in capillary LC with solvents, temperature, and electric fields

Once a suitable stationary phase and column dimensions have been selected, the retention in liquid chromatography (LC) is traditionally adjusted by controlling the mobile phase composition. Solvent gradients enable achievement of good separation selectivity while decreasing the separation time as compared to isocratic elution. Capillary columns allow use of other programming parameters, i.e. temperature and applied electric fields, in addition to solvent gradient elution. This paper presents a review of programmed separation techniques in miniaturized LC, including retention modeling and method transfer from the conventional to micro- and capillary scales. The impact of miniaturized instrumentation on retention and the limitations of capillary LC are discussed. Special attention is focused on the gradient dwell volume effects, which are more important in micro-LC techniques than in conventional analytical LC and may cause significant increase in the time of analysis, unless special instrumentation and (or) pre-column flow-splitting is used. The influence of temperature upon retention is also discussed, and applications where the temperature has been actively used for retention control in capillary LC are included together with the instrumentation utilized. Finally the possibilities of additional selectivity control by applying an electric field over a packed capillary LC column are discussed.

Retention behaviour of peptides in capillary electrochromatography using an embedded ammonium in dodecacyl stationary phase

The potential of a silica stationary phase bearing an embedded cationic quaternary amine in dodecacyl chain, to separate peptides by capillary electrochromatography (CEC) has been evaluated. The ability of this stationary phase, to generate a consistent anodic electroosmotic flow was first evaluated. This flow was found to be independent of pH over a wide range (2-12), of the acetonitrile percentage in the electrolyte. The stability of the stationary phase evaluated through the electroosmotic flow variations was demonstrated at extreme pH values (2.5 and 9.1). A careful examination of the influence of mobile phase conditions (acetonitrile percentage, salt concentration and nature of buffer) on the electrochromatographic retention and electrophoretic migration behaviour of different standard peptides was carried out. In acidic conditions, the electrokinetic contribution appears to be predominant compared to the chromatographic one. Several types of chromatographic interactions, reversed-phase partitioning and anion exchange, were involved in the CEC of peptides, whereas repulsive electrostatic interaction could be considered as negligible. This stationary phase affords different selectivity compared to that observed on a C18 stationary phase. Finally, the method was applied to the peptide mapping of beta-lactoglobulin and human growth hormone under unpressurized and isocratic elution.

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.

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.

Study of flow rate in pressurized gradient capillary electrochromatography using splitter and separation of peptides using an Amide stationary phase

A pressurized gradient capillary electrochromatograph using a splitter was constructed. The variation in flow rate during gradient elution was investigated and separations of peptides using an Amide stationary phase were demonstrated. The flow rate, which is one of the important factors to control chromatographic behavior, was increased during the gradient elution, and the mismatching of mobile phase between the column and the resistance tubing derived three variation patterns in the flow rate. The electrophoretic migration in electrochromatography could enhance in separation of peptides. The separated peak number of tryptic digest of bovine serum albumin was increased from 30 to 40 by the application of +5 kV.

Deviation from Ohm’s law in electric field assisted capillary liquid chromatography

Earlier studies of electric field assisted LC (EF-LC) have shown that the effect on charged analytes of the application of an electric field over a capillary LC column is relatively small. Charged analytes can only be affected by the electric field while present in the mobile phase, which makes the effective time for influence of the electric field t(0) independent of retention time. Because the charged analytes only can be affected for a short time the electric field strength ought to be high in order to increase the impact of the electric field on the separation. We have, however, found that only a relatively low electric field strength can be used in EF-LC when pressure is used as main driving force. The useful field strength was limited by a dramatic increase in the current. This increase in current was found to origin from an increased concentration of buffer ions that have an electrophoretic mobility towards the pumped flow.

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.

Peptide separation in hydrophilic interaction capillary electrochromatography

Separation of small peptides by hydrophilic interaction capillary electrochromatography (HI-CEC) has been investigated. The negative surface charge of a hydrophilic, strong-cation-exchange stationary phase (PolySULFOETHYL A) provided a substantial cathodic electroosmotic flow (EOF). The influence of acetonitrile content, ionic strength, mobile phase pH as well as applied voltage on the migration of the peptides was studied. Possible retention mechanisms of the peptides in HI-CEC were discussed. It was found that hydrophilic interaction between the solutes and the stationary phase played a major role in this system, especially when mobile phases with high acetonitrile content were used. However, an ion-exchange mechanism and electrophoretic mobility also affect the migration of the peptides in HI-CEC. Elution order and selectivity was proved to be different in HI-CEC and capillary zone electrophoresis (CZE), thus revealing the potential of HI-CEC as a complementary technique to CZE for the separation of peptides. Efficiency and selectivity of HI-CEC for the separation of peptides were demonstrated by baseline separating nine peptides in 6 min.

Separation of peptides by pressurized capillary electrochromatography

A pressurized electrochromatography (pCEC) instrument with gradient capability was used in this work for separation of peptides. Three separation modes, namely, pCEC, high-performance liquid chromatography and capillary electrophoresiscan be carried out with the instrument. In pCEC mode, the mobile phase is driven by both electroosmotic flow and pressurized flow, facilitating fine-tuning in selectivity of neutral and charged species. A continuous gradient elution can be carried out conveniently on this instrument, which demonstrates that it is more powerful than isocratic pCEC for separation of complicated samples. The effects of applied voltage, supplementary pressure and ion-pairing agents on separation of peptides in gradient pCEC were investigated. The effects of flow-rate of the pump and the volume of the mixer on resolution were also evaluated.

Recent applications of capillary electrochromatography

A review is presented of the most important recent applications of capillary electrochromatography (CEC) for the analysis of acidic, basic, and neutral compounds, of biomolecules, environmental substances, natural products, pharmaceuticals, and chiral compounds. Packed-column CEC (packed-CEC), open-tubular (OT-CEC), as well as pressure-assisted CEC (pseudo-CEC) are hereby considered. Papers published between July 1999 and April 2001 were taken into account. Applications before July 1999 have been reviewed in Electrophoresis 1999, 20, 3027-3065.

Enhancement of electroosmotic flow in capillary electrochromatography

A major impediment to enhancing the speed of separation in capillary electrochromatography (CEC) is the upper limit on the electroosmotic flow (EOF) velocity by the maximal zeta potential of the chromatographic surface. Here, a new approach to speeding up EOF, suggested by Yang and El Rassi (Electrophoresis 1999, 20,18-23), is examined critically. It entails the use of a tandem arrangement of a separating column and an auxiliary column, the sole function of which is to boost EOF velocity in the separating column and thus facilitate faster analysis by CEC. Based on the principle of conservation of mass and current and using experimental data obtained in a wide range of conditions, the flow velocities in the separating and auxiliary columns were evaluated. The results show that an equidiameter open tubular auxiliary column offers a greater enhancement of EOF velocity than a packed column. Nevertheless, within the scope of the experiments the enhancement of EOF velocity by as much as 50% by using open tubular auxiliary columns has been obtained.