Capillary electrochromatography for separation of peptides driven with electrophoretic mobility on monolithic column

Facile synthesis of bifunctional polymer monolithic column for tunable and specific capture of glycoproteins and phosphoproteins

Efficiently tunable capture of the glycosylated/phosphorylated proteins is critical to meet the need of in-depth glycoproteome and phosphoproteome studies. Reported here is a new bifunctional polymer monolithic column by introducing benzeneboronic acid and phosphonic acid onto monolithic column (denoted as poly (EDMA-co-VPBA-co-VPA) monolith) for tunable and specific enrichment of glycoproteins and phosphoproteins via switching different mobile phases. Based on boronate affinity and immobilized metal affinity, the as-prepared poly (EDMA-co-VPBA-co-VPA) monolith exhibited superior performance in selective separation of small molecules and biomacromolecules containing cis-diol/phosphate groups or not. And the frontal chromatography analysis showed that the binding capacity of the poly (EDMA-co-VPBA-co-VPA) monolith towards horseradish peroxidase (HRP, glycoprotein) or β-casein (phosphoprotein) is four-fold higher than that of bovine serum albumin (BSA, non-glycosylated/phosphorylated protein). Furthermore, combined with mass spectrometry identification, the successful application in specific enrichment of glycopeptides/phosphopeptides from tryptic digests of HRP/β-casein and direct capture of low abundant endogenous phosphopeptides from human serum proved great practicability in complex samples. This study provides a novel insight for fabricating the monolithic columns with multifunctionalization to facilitate further post-translational modification (PTM)-proteomics development.

Preparation of Benzyl Quinine-modified Monolithic Column for Reversed-phase Capillary Electrochromatography

N-Benzylquininium chloride is a versatile functional monomer with quinoline and benzyl groups, which is beneficial for reversed-phase chromatography. In this study, a novel monolithic column with reversed-phase mode was synthesized using N-benzylquininium chloride as the monomer and 3-(acryloyloxy)-2-hydroxypropyl methacrylate as the cross-linker in a binary porogenic solvent consisting of PEG 400 and a 0.05 M sodium hydroxide aqueous solution. The alkaline solution were found to be useful for the improvement of the mechanical stability of the porous monoliths. The monolithic column showed excellent reversed-phase selectivity and various compounds, such as alkylbenzenes, phenols and polycyclic aromatic hydrocarbons, were separated successfully. The highest column efficiency was 1.75 × 10⁵ N m^-1. The relative standard deviations of the migration time for thiourea and four alkylbenzenes were all less than 5.0%, which indicates the monolithic column has good stability. The application of the monolithic column for the analysis of polycyclic aromatic hydrocarbons in spiked lake water samples illustrated its great potential for practical application.

An innovative reversed-phase monolithic column modified with 4-vinylbiphenyl and ionic liquid stationary phases for capillary electrochromatography

A poly(VBP-co-EDMA-co-IL) monolithic column was used for electrochromatographic separation.

Ionic liquid-copolymerized monolith incorporated with zeolitic imidazolate framework-8 as stationary phases for enhancing reversed phase selectivity in capillary electrochromatography

A novel ionic liquid (1-allyl-methylimidazolium chloride, AlMeIm⁺Cl^-) polymer monolith poly(ionic liquid-co-ethylene dimethacrylate) incorporated with zeolitic imidazolate framework-8 (ZIF-8-poly(IL-co-EDMA)) was firstly synthesized as stationary phases of monolithic column for capillary electrochromatography by one-step copolymerization. Incorporation of ZIF-8 into ionic liquid polymer monolith evidently enhanced the separation selectivity for four alkylbenzenes in reversed phase capillary electrochromatography (CEC), due to the synergistic effect derived from the same imidazole ring structure of ionic liquid and organic ligands of ZIF-8. Meanwhile, electroosmotic flow (EOF) was generated by ionic liquid in a wide range of pH values from 2.0 to 12.0. The resultant monolithic columns were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR). The results indicated that the prepared monolithic columns had good permeability and mechanism stability. The resultant monolithic columns were applied for the separation of neutral compounds, anilines and phenols. The highest column efficiency was 2.07 × 10⁵ plates m^-1 (theoretical plates, N) for toluene. Under optimal conditions, reproducibility was obtained with relative standard deviations (RSDs) of the retention time for run-to-run, day-to-day, column-to-column and batch-to-batch were in the range of 1.58 - 3.19%, 1.92 - 3.87%, 3.84 - 4.96% and 2.63 - 4.33%, respectively. Incorporation ZIF-8 into ionic liquid polymer monolith was a promising way for the application of new materials in the fabrication of novel monolithic columns.

In situ synthesis of metal–organic frameworks in a porous polymer monolith as the stationary phase for capillary liquid chromatography

In this study, HKUST-1 was synthesized in situ on the porous polymer monolith as the stationary phase for capillary liquid chromatography (cLC).

Analysis of nitrites and nitrates in hams and sausages by open-tubular capillary electrochromatography with a nanolatex-coated capillary column

In this work, a new open-tubular capillary electrochromatography (OT-CEC) method with the nanolatex-coated column was proposed for the determination of nitrites and nitrates in foodstuffs. The method was simple and repeatable as a result of avoiding the introduction of an electroosmotic flow reverse additive (such as cetyltrimethylammonium chloride) in electrophoretic buffer. The limits of quantitation were 0.89 and 1.05 mg kg⁻¹ for nitrate and nitrite, respectively, whereas the overall recoveries ranged from 94 to 103%. The developed OT-CEC method was successfully applied for 12 samples, and the residue profiles of nitrites and nitrates in hams and sausages were obtained and evaluated.

Effect of ion adsorption on CEC separation of small molecules using hypercrosslinked porous polymer monolithic capillary columns

Both poly(styrene-co-vinylbenzyl chloride-co-divinylbenzene) and poly(4-methylstyrene-co-vinylbenzyl chloride-co-divinylbenzene) monolithic columns have been hypercrosslinked and for the first time used to achieve capillary electrochromatographic separations. Although these columns do not contain ionizable functionalities, electroosmotic flow was observed due to adsorption of ions from a buffer solution contained in the mobile phase on the surface of the hydrophobic polymer. An increase of more than one order of magnitude was observed with the use of both monolithic polymers. The hypercrosslinking reaction creates a large surface area thus enabling adsorption of a much larger number of ions. Alkylbenzenes were successfully separated using the hypercrosslinked monolithic columns.

Sodium desoxycholate-assisted capillary electrochromatography with methacrylate ester-based monolithic column on fast separation and determination of coumarin analogs in Angelica dahurica extract

A rapid and sensitive CEC method with methacrylate ester-based monolithic column has been developed for separation and determination of five coumarins (byakangelicin, oxypeucedanin hydrate, xanthotoxol, 5-hydroxy-8-methoxypsoralen and bergapten) in Angelica dahurica extract. Surfactant sodium desoxycholate (SDC) was introduced into the mobile phase as the pseudostationary to dynamically increase the selectivity of analytes instead of increasing the hydrophobicity of stationary phase. In addition, other factors, pH of phosphate buffer, ACN content and applied voltage, for instance, have also an obvious effect on the resolution but little on the retention time. Satisfactory separation of these five coumarins was achieved within 6 min under a 30:70 v/v ACN-buffer containing 20 mM sodium dihydrogen phosphate (NaH(2) PO(4) ) and 0.25 mM SDC at pH 2.51. The RSDs of intraday and interday for relative peak areas were less than 3.0% and 4.7%, respectively; and the recoveries were between 87.5% and 95.0%. The LODs were lower than 0.15 μg/mL and the LOQs were lower than 0.30 μg/mL, respectively, while calibration curves showed a good linearity (r(2) > 0.9979). Finally, five target coumarins from the crude extracts of A. dahurica were separated, purified, and concentrated by D-101 macroporous resin, and were successfully separated and quantitatively determined within 6 min.

Surfactant-bound monolithic columns for CEC

A novel anionic surfactant bound monolithic stationary phase based on 11-acrylaminoundecanoic acid is designed for CEC. The monolith possessing bonded undecanoyl groups (hydrophobic sites) and carboxyl groups (weak cationic ion-exchange sites) were evaluated as a mixed-mode stationary phase in CEC for the separation of neutral and polar solutes. Using a multivariate D-optimal design the composition of the polymerization mixture was modeled and optimized with five alkylbenzenes and seven alkyl phenyl ketones as test solutes. The D-optimal design indicates a strong dependence of electrochromatographic parameters on the concentration of 11-acrylaminoundecanoic acid monomer and porogen (water) in the polymerization mixture. A difference of 6, 8 and 13% RSD between the predicted and the experimental values in terms of efficiency, resolution and retention time, respectively, indeed confirmed that the proposed approach is practical. The physical (i.e. morphology, porosity and permeability) and chromatographic properties of the monolithic columns were thoroughly investigated. With the optimized monolithic column, high efficiency separation of N-methylcarbamates pesticides and positional isomers was successfully achieved. It appears that this type of mixed-mode monolith (containing both chargeable and hydrophobic sites) may have a great potential as a new generation of CEC stationary phase.

Integration of monolithic frit into the particulate capillary (IMFPC) column in shotgun proteome analysis

Capillary column plays an important role in nano-flow liquid chromatography coupled with tandem mass spectrometry for dealing with the high dynamic range and complexity of protein samples in shotgun proteome analysis. In this study, the integrated monolithic frit into the particulate capillary (IMFPC) column was prepared. By comparing the prepared IMFPC column with conventionally fritless capillary column, smaller size of packing materials could be easily packed into the capillary to achieve higher average peak capacity and proteome coverage. As the monolithic emitter was integrated onto this type of column, the void volume between packing particles and electrospray emitter was eliminated and the electrospray quality was improved. The prepared IMFPC column was applied to proteome analysis of mouse liver extracts, and it was observed that the number of identified proteins and peptides increased 14.9 and 12.9% as well as the peak capacity increased 11.6% by using IMFPC column over conventionally fritless capillary column.

Capillary electrochromatography and on-line concentration

Capillary electrochromatography (CEC) is a micro-separation technique that combines the advantages of capillary zone electrophoresis with those of high-performance liquid chromatography. Accordingly, it has attracted extensive attention over the last decade. Among the stationary phases for CEC, monolithic stationary phase has been regarded as the most suitable stationary phase for CEC because of its simple preparation, the elimination of frits, and its excellent performance. In this chapter, procedures for preparing CEC monolithic columns with an improved configuration, in which there are stationary phases at both sides of detection window and no stationary phase at detection window, are presented. The separation of acidic and basic compounds on such monolithic columns is used as an example to demonstrate CEC separation protocol. Additionally, an on-line concentration technique in CEC is presented. As a result of the coexistence of stationary phase and electric field in a CEC column, it is possible to employ chromatographic zone sharpening and field-amplified sample stacking effects simultaneously to improve CEC detection sensitivity.

In-line system containing porous polymer monoliths for protein digestion with immobilized pepsin, peptide preconcentration and nano-liquid chromatography separation coupled to electrospray ionization mass spectroscopy

The use of two different monoliths located in capillaries for on-line protein digestion, preconcentration of peptides and their separation has been demonstrated. The first monolith was used as support for covalent immobilization of pepsin. This monolith with well-defined porous properties was prepared by in situ copolymerization of 2-vinyl-4,4-dimethylazlactone and ethylene dimethacrylate. The second, poly(lauryl methacrylate-co-ethylene dimethacrylate) monolith with a different porous structure served for the preconcentration of peptides from the digest and their separation in reversed-phase liquid chromatography mode. The top of the separation capillary was used as a preconcentrator, thus enabling the digestion of very dilute solutions of proteins in the bioreactor and increasing the sensitivity of the mass spectrometric detection of the peptides using a time-of-flight mass spectrometer with electrospray ionization. Myoglobin, albumin, and hemoglobin were digested to demonstrate feasibility of the concept of using the two monoliths in-line. Successive protein injections confirmed both the repeatability of the results and the ability to reuse the bioreactor for at least 20 digestions.

Development of capillary electrochromatography with poly(styrene-divinylbenzene-vinylbenzenesulfonic acid) monolith as the stationary phase

A new polystyrene-based monolithic stationary phase, which was prepared by single step in situ copolymerization of styrene, divinylbenzene and vinylbenzenesulfonic acid (VBSA), was developed as a separation column for capillary electrochromatography, in which VBSA was employed as the charge-bearing monomer. Polymerization time of the polystyrene-based monolith had slightly influenced the separation time of the tested analytes, but it effectively altered their separation resolutions. Furthermore, baseline separation for a wider range of acetonitrile levels of mobile phase was achieved when a monolithic column prepared by a longer polymerization time was used. This novel polystyrene-based monolithic column provided an adequate electroosmotic flow either in basic or acidic mobile phase when VBSA level was maintained at 2.6% (w/w). Finally, this proposed polystyrene-based column allowed seven tested analytes to achieve a reproducible baseline separation within 2.2 min with theoretical plate numbers higher than 164 000 plates/m.

Single-step preparation and characterization of polymeric monolith for pressurized capillary electrochromatography of typical homologs

A monolithic stationary phase was prepared in a single step by in situ copolymerization of iso-butyl methacrylate (IBMA), ethylene dimethacrylate (EDMA), and N,N-dimethylallylamine (DMAA) in a binary porogenic solvent consisting of N,N-dimethylformamide (DMF) and 1,4-butanediol. As the frame structures of monoliths, the amino groups are linked to support the EOF necessary for driving the mobile phase through the monolithic capillary, while the hydrophobic groups are introduced to provide the nonpolar sites for the chromatographic retention. To evaluate the column performance, separations of typical kinds of neutral or charged homologs, such as alkylbenzenes, phenols (including isomeric compounds of hydroquinone, resorcin, and catechol), and anilines (including isomeric compounds of o-phenylenediamine and 1,4-phenylenediamine), were performed, respectively on the prepared column under the mode of pressurized pCEC. Effects of the buffer pH and the mobile phase composition on the linear velocity of mobile phase and the retention factors of these compounds were investigated. It was found that the retention mechanism of charged solutes could be attributed to a mixed mode of hydrophobic interaction and electrophoresis, while an RP chromatographic behavior on the monolithic stationary phases was exhibited for neutral solutes. Especially, basic compounds such as anilines were well separated on the monolithic columns in the "counterdirectional mode," which effectively eliminated the electrostatic adsorption of basic analytes on the charged surface of the stationary phases.

Voltage-assisted capillary LC of peptides using monolithic capillary columns prepared by ring-opening metathesis polymerization

We examined the use of monolithic capillary columns prepared via ring-opening metathesis polymerization (ROMP) for peptide separation in voltage-assisted capillary LC (voltage-assisted CLC). In order to demonstrate their potential for peptide separation, ROMP-derived monoliths with RP properties were prepared. The preparation procedure of monoliths was transferred from ROMP monoliths optimized for CLC. ROMP monoliths were synthesized within the confines of 200 microm id fused-silica capillaries with a length of 37 cm. After optimization of the chromatographic conditions, the separation performance was tested using a well-defined set of artificial peptides as well as two peptidic mixtures resulting from a tryptic digest of BSA as well as a collagenase digest of collagen. ROMP monoliths showed comparable performance to other monolithic separation media in voltage-assisted CLC published so far. Therefore, we conclude that by optimizing the composition of the ROMP monoliths as well as by using the controlled manner of their functionalization, ROMP monoliths bear a great potential in CLC and CEC.

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.

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.

Capillary electrochromatography of proteins and peptides

This review summarizes applications of CEC for the analysis of proteins and peptides. This "hybrid" technique is useful for the analysis of a broad spectrum of proteins and peptides and is a complementary approach to liquid chromatographic and capillary electrophoretic analysis. All modes of CEC are described--granular packed columns, monolithic stationary phases as well as open-tubular CEC. Attention is also paid to pressurized CEC and the chip-based platform.

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.

Pore structural characterization of monolithic silica columns by inverse size-exclusion chromatography

In this work, a parallel pore model (PPM) and a pore network model (PNM) are developed to provide a state-of-art method for the calculation of several characteristic pore structural parameters from inverse size-exclusion chromatography (ISEC) experiments. The proposed PPM and PNM could be applicable to both monoliths and columns packed with porous particles. The PPM and PNM proposed in this work are able to predict the existence of the second inflection point in the experimental exclusion curve that has been observed for monolithic materials by accounting for volume partitioning of the polymer standards in the macropores of the column. The appearance and prominence of the second inflection point in the exclusion curve is determined to depend strongly on the void fraction of the macropores (flow-through pores), (b) the nominal diameter of the macropores, and (c) the radius of gyration of the largest polymer standard employed in the determination of the experimental ISEC exclusion curve. The conditions that dictate the appearance and prominence of the second inflection point in the exclusion curve are presented. The proposed models are applied to experimentally measured ISEC exclusion curves of six silica monoliths having different macropore and mesopore diameters. The PPM and PNM proposed in this work are able to determine the void fractions of the macropores and silica skeleton, the pore connectivity of the mesopores, as well as the pore number distribution (PND) and pore volume distribution (PVD) of the mesopores. The results indicate that the mesoporous structure of all materials studied is well connected as evidenced by the similarities between the PVDs calculated with the PPM and the PNM, and by the high pore connectivity values obtained from the PNM. Due to the fact that the proposed models can predict the existence of the second inflection point in the exclusion curves, the proposed models could be more applicable than other models for ISEC characterization of chromatographic columns with small diameter macropores (interstitial pores) and/or large macropore (interstitial pore) void fractions. It should be noted that the PNM can always be applied without the use of the PPM, since the PPM is an idealization that considers an infinitely connected porous medium and for materials having a low (<6) pore connectivity the PPM would force the PVD to a lower average diameter and larger distribution width as opposed to properly accounting for the network effects present in the real porous medium.

Preparation and characterization of long alkyl chain methacrylate-based monolithic column for capillary chromatography

This paper describes the fabrication of long alkyl chain methacrylate monolithic materials for using as stationary phases in capillary liquid chromatography. Following deactivation of the capillary surface with 3-(trimethoxysilyl)propyl methacrylate (gamma-MAPS), monoliths were formed by co-polymerisation of stearyl methacrylate (SMA) with ethylene glycol dimethacrylate (EDMA) in the presence of the initiator AIBN and a mixture of porogens including iso-amyl alcohol and 1,4-butanediol. The monoliths were prepared in 100 microm i.d. capillaries and the composition of the polymerisation mixtures were optimised in terms of the ratio of SMA/EDMA, the porogen composition and ratio of porogen to monomers. As the porogen weight fraction decreased, the microglobules became smaller and as expected, the total porosity decreased. In order to determine the usability of such materials, the column permeability K was measured by pumping water through the columns at different linear flow rates. Good results were obtained when these capillaries were used to separate mixtures of weak acids, neutral and basic compounds.

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.

Rapid separation and determination of structurally related anthraquinones in Rhubarb by pressurized capillary electrochromatography

A pressurized capillary electrochromatography (pCEC) with monolithic column has been developed for the rapid separation and determination of five structurally related anthraquinones in Rhubarb. The possibility of rapid separation resulted from the unique pore structure with high permeability and favorable mass transfer characteristics of the monolithic stationary phase. The effect factors such as organic modifier, acidity and concentration of running buffer, separation voltage were investigated to acquire the optimum condition. In the 220 nm wavelengths, the five anthraquinones could be baseline-separated rapidly within 5 min with the separation voltage of -20 kV in 10 mmol/L phosphate buffer (pH 6.2) containing 65% acetonitrile. The calibration graphs of rhein, aloe-emodin, emodin chrysophanol and physcion were linear by plotting the peak area against the analytes concentration over the range of 0.2-65, 0.1-30, 0.1-55, 0.5-30 and 0.5-55 microg/mL, respectively. The detection limits of five anthraquinones were ranged from 0.06 to 0.2 microg/mL and the recoveries of Rhubarb samples were about 81.3-86.4% (R.S.D.< or = 5.2%). This proposed method was successfully applied to determination of the five analytes in Rhubarb with satisfactory results.