Advantages and limitations of a new cationic coating inducing a slow electroosmotic flow for CE-MS peptide analysis: a comparative study with commercial coatings

Size-based characterization of dendrigraft poly(L-lysine) by free solution capillary electrophoresis using polyelectrolyte multilayer coatings

Dendrigraft poly(L-lysine) (DGL) constitutes a promising dendritic-like drug vehicle with high biocompatibility and straightforward access via ring-opening polymerization of N-carboxyanhydride in water. The characterization of the different generations of DGL is however challenging due to their heterogeneity in molar mass and branching ratio. In this work, free solution capillary electrophoresis was used to perform selective separation of the three first generations of DGL, and optimized conditions were developed to maximize inter-generation resolution. To reduce solute adsorption on the capillary wall, successive multiple ionic polymer layer coatings terminated with a polycation were deposited onto the inner wall surface. PEGylated polycation was also used as the last layer for the control of the electroosmotic flow (EOF), depending on the PEGylation degree and the methyl-polyethylene glycol (mPEG) chain length. 1 kDa mPEG chains and low grafting densities were found to be the best experimental conditions for a fine tuning of the EOF leading to high peak resolution. Molar mass polydispersity and polydispersity in effective electrophoretic mobility were successfully determined for the three first generations of DGL.

Modifying last layer in polyelectrolyte multilayer coatings for capillary electrophoresis of proteins

Protein adsorption on the inner wall of the fused silica capillary wall is an important concern for capillary electrophoresis (CE) analysis since it is mainly responsible for separation efficiency reduction. Successive Multiple Ionic-polymer Layers (SMIL) are used as capillary coatings to limit protein adsorption, but even low residual adsorption strongly impacts the separation efficiency, especially at high separation voltages. In this work, the influence of the chemical nature and the PEGylation of the polyelectrolyte deposited in the last layer of the SMIL coating was investigated on the separation performances of a mixture of four model intact proteins (myoglobin (Myo), trypsin inhibitor (TI), ribonuclease a (RNAse A) and lysozyme (Lyz)). Poly(allylamine hydrochloride) (PAH), polyethyleneimine (PEI), ε-poly(L-lysine) (εPLL) and α-poly(L-lysine) (αPLL) were compared before and after chemical modification with polyethyleneglycol (PEG) of different chain lengths. The experimental results obtained by performing electrophoretic separations at different separation voltages allowed determining the residual retention factor of the proteins onto the capillary wall via the determination of the plate height at different solute velocities and demonstrated a strong impact of the polycationic last layer on the electroosmotic mobility, the separation efficiency and the overall resolution. Properties of SMIL coatings were also characterized by quartz microbalance and atomic force microscopy, demonstrating a glassy structure of the films.

Polyelectrolyte Multilayers in Capillary Electrophoresis

Capillary electrophoresis (CE) has been proven to be a performant analytical method to analyze both small and macro molecules. Indeed, it is capable of separating compounds of the same nature according to differences in their charge to size ratios, particularly proteins, monoclonal antibodies and peptides. However, one of the major obstacles to reach high separation efficiency remains the adsorption of solutes on the capillary wall. Among the different coating approaches used to control and minimize solute adsorption, polyelectrolyte multilayers can be applied to CE as a versatile approach. These coatings are made up of alternating layers of polycations and polyanions, and may be used in acidic, neutral or basic conditions depending on the solutes to be analyzed. This Review provides an overview of Successive Multiple Ionic-polymer Layer (SMIL) coatings used in CE, looking at how different parameters induce variations on the electro-osmotic flow (EOF), separation efficiency and coating stability, as well as their promising applications in the biopharmaceutical field.

Successive Multiple Ionic-Polymer Layer Coatings for Intact Protein Analysis by Capillary Zone Electrophoresis-Mass Spectrometry: Application to Hemoglobin Analysis

Adsorption of analytes, e.g., proteins, often interfere with separation in CE, due to the relatively large surface of the narrow capillary. Coatings often are applied to prevent adsorption and to determine the electroosmotic flow (EOF), which is of major importance for the separation in CE. Successive multiple ionic-polymer layer (SMIL) coatings are frequently used for protein analysis in capillary electrophoresis resulting in high separation efficiency and repeatability. Here, the coating procedure of a five-layer SMIL coating is described using quaternized diethylaminoethyl dextran (DEAEDq) as polycation and poly(methacrylic acid) (PMA) as polyanion. Depending on the analyte, different polyions may be used to increase separation efficiency. However, the coating procedure remains the same.To demonstrate the applicability of SMIL coatings in CE-MS, human hemoglobin was measured in a BGE containing 2 M acetic acid. DEAEDq-PMA coating was found to be the most suitable for hemoglobin analysis due to relatively low reversed electroosmotic mobility leading to increased electrophoretic resolution of closely related proteoforms. Thereby, not only alpha and beta subunit of the hemoglobin could be separated, but also positional isoforms of glycated and carbamylated species were separated within 24 min.

Capillary electrophoresis-mass spectrometry for the direct analysis of glyphosate: method development and application to beer beverages and environmental studies

In this study, we developed and validated a CE-TOF-MS method for the quantification of glyphosate (N-(phosphonomethyl)glycine) and its major degradation product aminomethylphosphonic acid (AMPA) in different samples including beer, media from toxicological analysis with Daphnia magna, and sorption experiments. Using a background electrolyte (BGE) of very low pH, where glyphosate is still negatively charged but many matrix components become neutral or protonated, a very high separation selectivity was reached. The presence of inorganic salts in the sample was advantageous with regard to preconcentration via transient isotachophoresis. The advantages of our new method are the following: no derivatization is needed, high separation selectivity and thus matrix tolerance, speed of analysis, limits of detection suitable for many applications in food and environmental science, negligible disturbance by metal chelation. LODs for glyphosate were < 5 μg/L for both aqueous and beer samples, the linear range in aqueous samples was 5-3000 μg/L, for beer samples 10-3000 μg/L. For AMPA, LODs were 3.3 and 30.6 μg/L, and the linear range 10-3000 μg/L and 50-3000 μg/L, for aqueous and beer samples, respectively. Recoveries in beer samples for glyphosate were 94.3-110.7% and for AMPA 80.2-100.4%. We analyzed 12 German and 2 Danish beer samples. Quantification of glyphosate and AMPA was possible using isotopically labeled standards without enrichment, purification, or dilution, only degassing and filtration were required for sample preparation. Finally, we demonstrate the applicability of the method for other strong acids, relevant in food and environmental sciences such as N-acetyl glyphosate, N-acetyl AMPA (present in some glyphosate resistant crop), trifluoroacetic acid, 2-methyl-4-chlorophenoxyacetic acid, glufosinate and its degradation product 3-(methylphosphinico)propionic acid, oxamic acid, and others.

Novel approach for the synthesis of a neutral and covalently bound capillary coating for capillary electrophoresis-mass spectrometry made from highly polar and pH-persistent N-acryloylamido ethoxyethanol

Statically adsorbed or covalently coupled capillary coatings are of crucial importance in capillary electrophoresis-mass spectrometry for the separation of peptides and proteins. So far, published coating strategies and commercially available coated capillaries have a limited pH-stability so that the analysis at strongly acidic pH is limited, or harsh rinsing procedures for biological sample analysis cannot be applied. We here present a capillary coating based on Si-C linkages to N-acryloylamido ethoxyethanol (AAEE) with a new synthetic strategy including LiAlH₄ surface reaction. We optimized the coating method with emphasis on stability and reproducibility applying harsh rinsing procedures (strong acid, strong base and organic solvent), using the electroosmotic mobility and separation efficiency of tryptic peptides as performance measure. Complete synthesis is performed in less than 2 days for up to 8 capillaries in parallel of more than 16 m total length. Intra- and inter-batch reproducibility were determined regarding electroosmotic mobility, separation efficiency and migration time precision in CE-MS separations of tryptically digested bovine serum albumin. Coating stability towards rinsing with strong acid (1 mol/L HCl), organic solvent (acetonitrile) and strong base (1 mol/L NaOH) was investigated. Outstanding performance was found for single capillaries. However, inter-capillary reproducibility is discussed critically. The new coating was successfully applied for reproducible CE-MS separation of large proteins in diluted serum, medium-sized peptides and small and highly charged polyamines in fish egg extracts using a very acidic background electrolyte containing 0.75 mol/L acetic acid and 0.25 mol/L formic acid (pH 2.2).

Improving repeatability of capillary electrophoresis-a critical comparison of ten different capillary inner surfaces and three criteria of peak identification

A poor repeatability of migration times caused by the fluctuations of electroosmotic flow (EOF) is an inherent weakness of capillary electrophoresis. Most researchers endeavor to prevent this problem using relative migration times or various capillary coatings which are expensive and not easy in comparison. Herein, we present an original approach to this problem: we apply a model sample designed to induce significant EOF instability, in order to critically compare ten capillary types with different physicochemical characteristics. Moreover, we accompany capillary modification with the evaluation of various criteria of peak identification: migration time, migration times ratio, and electrophoretic mobility. Our results show a great effectiveness of a dynamic coating in the stabilization of migration times, with the average RSD(%) value reduced from 3.5% (bare silica capillary) down to 0.5%. The good outcomes were also obtained for the surfactant-modified silica and amine capillaries. For the capillaries exhibiting significant instability of EOF, electrophoretic mobility turned out to be a more universal and reliable criterion of peak identification than relative migration time. It can be explained by an intrinsic dependency of migration times ratio on EOF change, which should always be considered during the selection of an internal standard.

CE-ESI-MS for bottom-up proteomics: Advances in separation, interfacing and applications

With the development of more sensitive hyphenation strategies for capillary electrophoresis-electrospray-mass spectrometry the technique has reemerged as technique with high separation power combined with high sensitivity in the analysis of peptides and protein digests. This review will discuss the newly developed hyphenation strategies for CE-ESI-MS and their application in bottom-up proteomics as well as the applications in the same time span, 2009 to present, using co-axial sheathliquid. Subsequently all separate aspects in the development of a CE-ESI-MS method for bottom-up proteomics shall be discussed, highlighting certain applications and discussing pros and cons of the various choices. The separation of peptides in a capillary electrophoresis system is discussed including the great potential for modeling of this migration of peptides due to the simple electrophoretic separation process. Furthermore, the technical aspects of method development are discussed, namely; background electrolyte choice, coating of the separation capillary and chosen loading method. Finally, conclusions and an outlook on future developments in the field of bottom-up proteomics by CE-ESI-MS will be provided.

Statically Adsorbed Coatings for High Separation Efficiency and Resolution in CE-MS Peptide Analysis: Strategies and Implementation

Coatings are necessary to prevent protein and peptide adsorption to the capillary surface and obtain high intermediate precision. In this protocol, we first present our basic strategy to address peptide separation using three different coatings: one neutral and two cationic coatings, the latter largely differing in their induced electroosmotic mobility. In detail, we will describe how we apply the statically adsorbed coatings to obtain very high plate numbers and high repeatability.With some model examples, we clearly describe the scope of the method for the analysis of peptide samples: tryptic digests are addressed as well as small glycoproteins and glycopeptides largely differing in their effective electrophoretic mobility. We also show that the method is suitable for a fast screening of peptide samples despite a high matrix load comprising of up to 500 mmol/L sodium chloride. We demonstrate that this basic CE-MS method is rather independent of the polarity of the analytes with a very fast near-baseline separation of very hydrophobic Aβ peptides related to the onset of Alzheimer's disease. These examples will give an impression, which coating is most suitable for a specific analytical application.Special attention is paid to difficult aspects of the coating procedure and the CE-MS method, e.g., the potential of cross-contamination when changing the coatings.

Chemical vapor deposition of aminopropyl silanes in microfluidic channels for highly efficient microchip capillary electrophoresis-electrospray ionization-mass spectrometry

We describe a chemical vapor deposition (CVD) method for the surface modification of glass microfluidic devices designed to perform electrophoretic separations of cationic species. The microfluidic channel surfaces were modified using aminopropyl silane reagents. Coating homogeneity was inferred by precise measurement of the separation efficiency and electroosmotic mobility for multiple microfluidic devices. Devices coated with (3-aminopropyl)di-isopropylethoxysilane (APDIPES) yielded near diffusion-limited separations and exhibited little change in electroosmotic mobility between pH 2.8 and pH 7.5. We further evaluated the temporal stability of both APDIPES and (3-aminopropyl)triethoxysilane (APTES) coatings when stored for a total of 1 week under vacuum at 4 °C or filled with pH 2.8 background electrolyte at room temperature. Measurements of electroosmotic flow (EOF) and separation efficiency during this time confirmed that both coatings were stable under both conditions. Microfluidic devices with a 23 cm long, serpentine electrophoretic separation channel and integrated nanoelectrospray ionization emitter were CVD coated with APDIPES and used for capillary electrophoresis (CE)-electrospray ionization (ESI)-mass spectrometry (MS) of peptides and proteins. Peptide separations were fast and highly efficient, yielding theoretical plate counts over 600,000 and a peak capacity of 64 in less than 90 s. Intact protein separations using these devices yielded Gaussian peak profiles with separation efficiencies between 100,000 and 400,000 theoretical plates.

Recent developments in capillary and microchip electroseparations of peptides (2011-2013)

The review presents a comprehensive survey of recent developments and applications of capillary and microchip electroseparation methods (zone electrophoresis, ITP, IEF, affinity electrophoresis, EKC, and electrochromatography) for analysis, isolation, purification, and physicochemical and biochemical characterization of peptides. Advances in the investigation of electromigration properties of peptides, in the methodology of their analysis, including sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, as well as in detection of peptides, are presented. New developments in particular CE and CEC modes are reported and several types of their applications to peptide analysis are described: conventional qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC techniques to provide relevant physicochemical characteristics of peptides are demonstrated.