Potential of on-line isotachophoresis-capillary zone electrophoresis with hydrodynamic counterflow in the analysis of various basic proteins and recombinant human interleukin-3

Biorecognition element-free electrochemical detection of recombinant glycoproteins using metal-organic frameworks as signal tags

Accurate and sensitive determination of recombinant glycoproteins is in great demand for the treatment of anemia-induced chronic kidney disease and the illegal use of doping agents in sports. In this study, an antibody and enzyme-free electrochemical method for the detection of recombinant glycoproteins was proposed via the sequential chemical recognition of hexahistidine (His₆) tag and glycan residue on the target protein under the cooperation interaction of nitrilotriacetic acid (NTA)-Ni²⁺complex and boronic acid, respectively. Specifically, NTA-Ni²⁺ complex-modified magnetic beads (MBs-NTA-Ni²⁺) are employed to selectively capture the recombinant glycoprotein through the coordination interaction between His₆ tag and NTA-Ni²⁺ complex. Then, boronic acid-modified Cu-based metal-organic frameworks (Cu-MOFs) were recruited by glycans on the glycoprotein via the formation of reversible boronate ester bonds. MOFs with abundant Cu²⁺ ions acted as efficient electroactive labels to directly produce amplified electrochemical signals. By using recombinant human erythropoietin as a model analyte, this method showed a wide linear detection range from 0.01 to 50 ng/mL and a low detection limit of 5.3 pg/mL. With the benefits from the simple operation and low cost, the stepwise chemical recognition-based method shows great promise in the determination of recombinant glycoproteins in the fields of biopharmaceutical research, anti-doping analysis and clinical diagnosis.

A rapid, low-cost quantitative diagnostic method for hepatitis C virus infection using capillary zone electrophoresis

Hepatitis C virus (HCV)-RNA amplification is a costly procedure in terms of time and reagents. Consequently, the search for more a cost-effective specific HCV diagnostic method is of great interest. Capillary zone electrophoresis (CZE) methods that detect HCV in serum, plasma, whole blood, and ascites without the need for sample pretreatment are not currently available. Here, a CZE method was developed that detects a larger specific peak in serum and other body fluids of HCV-infected patients than that found in healthy or hepatitis B virus (HBV)-infected individuals. The nature of the HCV peak was investigated using biochemical treatments, including RNase, DNase, and chymotrypsin enzymes. Electroeluted HCV peak was applied to transmission electron microscopy; electron micrographs showed that the HCV peak was attributed to virus-like particles with diameter and morphological properties similar to non-enveloped HCV nucleocapsids. The determination of CZE-HCV and HCV-RNA levels using quantitative real-time reverse transcriptase-polymerase chain reaction (qRT-PCR) in 258 subjects revealed that these two tests were highly correlated (r = 0.92, p < 0.0001). One important issue of HCV testing is the storage conditions of serum to obtain reliable results. Serum samples at -20 °C showed the best preservation of the HCV peak up to one year. In conclusion, we detected HCV using CZE in a microliters volume from different body fluids. Besides the stability of samples in maintaining their peak height, the HCV-CZE test is rapid (<15 min) and a well-suited and low-cost technique. Thus, a major improvement in the quantitative diagnosis of HCV infection was established.

Theoretical and experimental studies on isotachophoresis in multi-moving chelation boundary system formed with metal ions and EDTA

In this paper, a general mode and theory of moving chelation boundary based isotachophoresis (MCB-based ITP), together with the concept of decisive metal ion (DMI) having the maximum complexation constant (lg Kmax) with the chelator, were developed from a multi-MCB (mMCB) system. The theoretical deductions were: (i) the reaction boundary velocities in the mMCB system at steady state were equal to each other, resulting in a novel MCB-based ITP separation of metal ions; (ii) the boundary directions and velocities in the system were controlled by the fluxes of chelator and DMI, rather than other metal ions; and (iii) a controllable stacking of metal ions could be simultaneously achieved in the developed system. To demonstrate the deductions, a series of experiments were conducted by using model chelator of EDTA and metal ions of Cu(II) and Co(II) due to characteristic colors of blue [Cu-EDTA](2-) and pink [Co-EDTA](2-) complexes. The experiments demonstrated the correctness of theoretical deductions, indicating the validity of the developed model and theory of ITP. These findings provide guidance for the development of MRB-based ITP separation and stacking of metal ions in biological sample matrix and heavy metal ions in environmental samples.

Determination of trace cationic impurities in butylmethylimidazolium-based ionic liquids: From transient to comprehensive single-capillary counterflow isotachophoresis-zone electrophoresis

Determination of impurities in ionic liquids (ILs) remains a difficult task. In this work, the hyphenation of isotachophoretic (ITP) preconcentration to zone electrophoresis (ZE) has been explored for the trace analysis of the cationic impurities Na(+), Li(+), and methylimidazolium (MI(+)) in butylmethylimidazolium (BMI(+))-based ILs. Simultaneous detection of UV-transparent and UV-absorbing impurities was ensured by a BGE composed of creatinine-acetate buffer. To induce ITP, three different strategies were evaluated: (i) Sample self-stacking ensured by the addition of ammonium acetate (NH(4)Ac) to 25-50-fold diluted IL solution (transient ITP). (ii) Complete ITP-ZE separation performed in a single capillary: ITP was realized in discontinuous electrolytes comprising an 80 mM NH(4)Ac, 40 mM acetic acid, 30 mM alpha-CD, pH 5.05, leading electrolyte (LE) and a 10 mM creatinine, 10 mM acetic acid, pH 4.9, terminating electrolyte (TE). To create the ZE stage, the ITP stack of analytes was moved back toward the capillary inlet by pressure and simultaneously the capillary was filled with the BGE. This protocol made it possible to accommodate a 2.5-times diluted IL sample. (iii) Complete counterflow ITP-ZE with continuous electrokinetic sample supply: the ITP stage was performed in a capillary filled with a 150 mM NH(4)Ac, 75 mM acetic acid, 30 mM alpha-CD, pH 5.0 LE, with 40-times diluted IL at the capillary inlet. BMI(+) from IL acts as the terminating ion. The LODs reached in this latter case were at the 10 and 1 ppb levels for MI(+) and Li(+) in diluted IL matrix, respectively.

pH-mediated acid stacking with reverse pressure for the analysis of cationic pharmaceuticals in capillary electrophoresis

When using capillary electrophoresis (CE) for the analysis of biological samples, it is often necessary to employ techniques to overcome peak-broadening that results from having a high-conductivity sample matrix. To improve the concentration detection limits and separation efficiency of cationic pharmaceuticals in CE, pH-mediated acid stacking was performed to electrofocus the sample, improving separation sensitivity for the analyzed cations by 60-fold. However, this method introduces a large titrated acid plug into the capillary. To overcome the limitations this low-conductivity plug poses to stacking, the plug was removed prior to the separation step by applying reverse pressure to force it out of the anode of the capillary. Employing this technique allows for roughly twice the volume of sample to be injected. A maximum sample injection time of 240 s was attainable with baseline peak resolution compared to a maximum sample injection time of 120 s without reverse pressure, leading to a twofold decrease in the limits of detection of the analytes used. Separation efficiency overall is also improved when utilizing the reverse pressure step. For example, a 60 s sample injection time results in 94,000 theoretical plates as compared to 60,500 theoretical plates without reverse pressure. This reverse-pressure method was used for detection and quantitation of several cationic pharmaceuticals that were prepared in Ringer's solution to simulate microdialysis sampling conditions.

On-line isotachophoretic preconcentration and gel electrophoretic separation of sodium dodecyl sulfate-proteins on a microchip

A microchip for integrated isotachophoretic (ITP) preconcentration with gel electrophoretic (GE) separation to decrease the detectable concentration of sodium dodecyl sulfate (SDS)-proteins was developed. Each channel of the chip was designed with a long sample injection channel to increase the sample loading and allow stacking the sample into a narrow zone using discontinuous ITP buffers. The pre-concentrated sample was separated in GE mode in sieving polymer solutions. All the analysis steps including injection, preconcentration, and separation of the ITP-GE process were performed continuously, controlled by a high-voltage power source with sequential voltage switching between the analysis steps. Without deteriorating the peak resolution, four SDS-protein analyses with integrated ITP-GE system resulted in a decreased detectable concentration of approximately 40-fold compared to the GE mode only. A good calibration curve for molecular weights of SDS-proteins indicated that the integrated ITP-GE system can be used for qualitative analysis of unknown protein samples.

In-capillary preconcentration of proteins for capillary electrophoresis using a cellulose acetate-coated porous joint

This work describes the in-capillary preconcentration of proteins using a cellulose acetate-coated porous joint. The capillary wall near the inlet end of a capillary was made porous by HF etching. During the etching process, a voltage was applied across the capillary wall and the electric current across it was monitored. As the current passed through the capillary wall, it became porous. A solution of cellulose acetate in acetone was added to the etched porous joint. After the acetone was evaporated off, a cellulose acetate-coated porous joint was formed. To preconcentrate the protein ions, an electric voltage was applied between the inlet end of the capillary and the coated porous joint; the protein ions electromigrated to the porous joint but could not pass through it, while the buffer ions could pass easily through the joint. After allowing a certain amount of time for protein preconcentration, a separation voltage was applied across the two ends of the capillary, and normal capillary electrophoresis was carried out. The preconcentration factors for cytochrome c, lysozyme, ribonuclease, and chymotrypsinogen were 65, 155, 705, and 800, respectively. The cellulose acetate-coated porous joint was shown to be strong and stable over time, and was used to analyze trace proteins and macromolecules in biological samples.

Isoelectric focusing sample injection for capillary electrophoresis of proteins

Carrier ampholyte-free isoelectric focusing (IEF) sample injection (concentration) for capillary electrophoresis (CE) is realized in a single capillary. A short section of porous capillary wall was made near the injection end of a capillary by HF etching. In the etching process, an electric voltage was applied across the etching capillary wall and electric current was monitored. When an electric current through the etching capillary was observed, the capillary wall became porous. The etched part was fixed in a vial, where NaOH solution with a certain concentration was added during the sample injection. The whole capillary was filled with pH 3.0 running buffer. The inlet end vial was filled with protein sample dissolved in the running buffer. An electric voltage was applied across the inlet end vial and etched porous wall. A neutralization reaction occurs at the boundary (interface) of the fronts of H+ and OH-. A pH step or sharp pH gradient exists across the boundary. When positive protein ions electromigrate to the boundary from the sample vial, they are isoelectricelly focused at points corresponding to their pH. After a certain period of concentration, a high voltage is applied across the whole capillary and a conventional CE is followed. An over 100-fold concentration factor has been easily obtained for three model proteins (bovine serum albumin, lysozyme, ribonuclease A). Furthermore, the IEF sample concentration and its dynamics have been visually observed with the whole-column imaging technique. Its merits and remaining problem have been discussed, too.

Coupling continuous separation techniques to capillary electrophoresis

One of the weak points of capillary electrophoresis is the need to implement rigorously sample pretreatment because its great impact on the quality of the qualitative and quantitative results provided. One of the approaches to solve this problem is through the symbiosis of automatic continuous flow systems (CFSs) and capillary electrophoresis (CE). In this review a systematic approach to CFS-CE coupling is presented and discussed. The design of the corresponding interface depends on three factors, namely: (a) the characteristics of the CFS involved which can be non-chromatographic and chromatographic; (b) the type of CE equipment: laboratory-made or commercially available; and (c) the type of connection which can be in-line (on-capillary), on-line or mixed off/on-line. These are the basic criteria to qualify the hyphenation of CFS (solid-phase extraction, dialysis, gas diffusion, evaporation, direct leaching) with CE described so far and applied to determine a variety of analytes in many different types of samples. A critical discussion allows one to demonstrate that this symbiosis is an important topic in research and development, besides separation and detection, to consolidate CE as a routine analytical tool.

Stacking in capillary zone electrophoresis

Due to the short light path of the capillaries, the CE detection limit based on concentration, is far less than that of HPLC and not sufficient for many practical applications. Several methods, based on different electrophoretic maneuvers, can concentrate the sample (stack) easily on the capillary before the separation step of capillary zone electrophoresis (CZE). These methods incorporate different types of discontinuous buffers as the means for invoking different velocities to the same analyte molecules to produce a sharpening of the band (stacking). In CZE, these buffers can be often very simple such as sample dilution or adding to the sample a high concentration of a fast mobility ion. However, in other applications these buffers can be as complicated as those required for isotachophoresis. Stacking can often yield a concentration factor of 5-30-fold, which can improve greatly in CZE the detection limits bringing them very close to those of HPLC. Different methods of stacking, the importance of discontinuous buffers and the different mechanism for concentration on the capillary are reviewed here. As there is a need for more practical applications, there will be more methods devised for stacking in CZE.

Improvement of capillary zone electrophoresis sensitivity with artificial seawater as the background electrolyte utilizing transient isotachophoresis for the determination of nitrate and nitrate ions in seawater

We describe capillary zone electrophoresis (CZE) with transient isotachophoresis (ITP) for the determination of low concentrations of nitrite and nitrate ions in seawater. Bromide-free artificial seawater was adopted as background electrolyte (BGE) to eliminate the interference of high concentrations of salts in seawater. To reverse the electroosmotic flow (EOF), 3 mM cetyltrimethylammonium chloride (CTAC) was added to the BGE. High concentrations of chlorate were added to sample solutions as the terminating ion to generate the ITP process before the CZE separation. In general, the stacking effect increased with increasing amounts of chlorate injected into the capillary. The limits of detection (LODs) for nitrite and nitrate were 0.063 and 0.033 mg/L when the chlorate concentration was 600 and 200 mM, respectively; these were half of those obtained by CZE without the transient ITP. The LODs were obtained at a signal to noise ratio (S/N) of 3. The relative standard deviations (RSD, n = 10) of the peak areas for these ions were 3.2 and 2.9%. The RSDs of peak heights for these ions were 1.6 and 2.1%. The RSDs of migration times for these ions were 0.67 and 0.46%.

Separation and detection techniques for peptides and proteins in stability research and bioanalysis

In this paper, a brief overview of the most commonly used methods for the separation and analysis of peptides and proteins in stability and bioanalysis studies is presented. To investigate the physical stability of peptides and proteins, size-exclusion chromatography and electrophoretic separation techniques are being used, apart from several other methods. To determine the chemical stability of these compounds, separation systems are also important, with informative detection modes, such as various spectroscopic detections, electrochemical detection and mass spectrometric detection. For the bioanalysis of peptides, separation is the most important factor, while the detection must be done at the highest possible level of sensitivity.

Enantiomeric separation of clenbuterol by transient isotachophoresis-capillary zone electrophoresis-UV detection new optimization technique for transient isotachophoresis

A method for the in-line preconcentration and enantioseparation of clenbuterol by transient isotachophoresis-capillary zone electrophoresis-UV absorbance detection (transient ITP-CZE-UV) has been developed. It implies the use of dimethyl-beta-cyclodextrin as chiral selector and the application of a hydrodynamic counterflow during the ITP step. ITP is used to focus the sample constituents prior to CE whereas a counterpressure counterbalances the electrophoretic migration of the compounds. The sample is then focused and kept stationary in the proximity of the capillary inlet before CZE separation, leading to an extended-volume ITP-CZE system. A new strategy for the fast optimization of the counterpressure has been developed which implies the measurement of the hydrodynamic and electrophoretic velocities of the analyte during ITP. The in-line preconcentration and enantioseparation of clenbuterol selected as model compound was optimized using this method. Salbutamol was chosen as internal reference in order to check the reproducibility of the method. A 173-nl volume of aqueous ample solution was injected which implies an improvement of the injection volume of about 16 and a resolution of 4.8 was obtained for the clenbuterol enantiomers. A concentration detection limit of 10(-6) mol/l was readily achieved for clenbuterol and salbutamol using only 3 min ITP preconcentration in in-line counterflow transient ITP-CZE-UV. Thanks to its fast optimization, the method is applicable to any enantioseparation by means of only five very short preliminary measurements.

Comparison between transient isotachophoretic capillary zone electrophoresis and reversed-phase liquid chromatography for the determination of peptides in plasma

Low levels of peptide drugs in human plasma can be determined employing off-line solid-phase extraction, followed by capillary zone electrophoresis with UV detection. A bioanalytical procedure is presented, using gonadorelin and angiotensin II in human plasma as model compounds. The solid-phase extraction method, based on a weak cation exchange mechanism, is able to remove interfering endogenous components from the plasma sample, extract the model peptides quantitatively, and give a possibility of concentrating the sample at the same time. Transient isotachophoretic conditions were kept to increase the sample loadability by about two orders of magnitude. Up to about 70% of the capillary was filled with the reconstituted extract, whereafter the peptides were selectively concentrated during the first 15 min. Subsequently, the concentrated sample zones were separated under capillary zone electrophoresis conditions, showing the technique's high resolution. For the model cationic peptides (gonadorelin, angiotensin II) good linearity and reproducibility was observed in the 20-100 ng/mL concentration range. A more extensive washing procedure permits quantitation of gonadorelin at the 5 ng/mL level. In comparison with a liquid chromatography analysis, superior mass sensitivity and separation are obtained with the transient isotachophoretic capillary zone electrophoresis method. Moreover, in this case equivalent sensitivity is achieved when it is directly compared with a liquid chromatography method with UV detection, keeping in mind that 60 times more sample is needed for the latter method. A further gain in sensitivity can be obtained when the analysis is combined with native fluorescence detection, as is demonstrated by combining liquid chromatography separation with fluorescence detection.

Coupling of biological sample handling and capillary electrophoresis

The analysis of biological samples (e.g., blood, urine, saliva, tissue homogenates) by capillary electrophoresis (CE) requires efficient sample preparation (i.e., concentration and clean-up) procedures to remove interfering solutes (endogenous/exogenous and/or low-/high-molecular-mass), (in)organic salts and particulate matter. The sample preparation modules can be coupled with CE either off-line (manual), at-line (robotic interface), on-line (coupling via a transfer line) or in-line (complete integration between sample preparation and separation system). Sample preparation systems reported in the literature are based on chromatographic, electrophoretic or membrane-based procedures. The combination of automated sample preparation and CE is especially useful if complex samples have to be analyzed and helps to improve both selectivity and sensitivity. In this review, the different modes of solid-phase (micro-) extraction will be discussed and an overview of the potential of chromatographic, electrophoretic (e.g., isotachophoresis, sample stacking) and membrane-based procedures will be given.

Development and validation of transient isotachophoretic capillary zone electrophoresis for determination of peptides

Although capillary zone electrophoresis (CZE) is known for its high resolution power and low mass detection limits, the concentration detection limits are rather poor when ultraviolet absorbance detection is used. To overcome this limitation, several on-column transient isotachophoresis (tITP) protocols have been developed and validated for the determination of both cationic and anionic model peptides, separately. Using this preconcentration method, up to 72% of the capillary can be filled with sample solution, without any loss in resolution. Thus, without any modification of the hardware set-up, the sensitivity is increased about two orders of magnitude. For the model cationic peptides (gonadorelin, angiotensin II) good linearity and reproducibility is observed in the 20 to 100 ng/mL concentration range. For the anionic peptides (N-t-Boc-Pentagastrin and two related peptides), a tITP method was developed using a dynamically coated capillary. The coating was prepared by adding Fluorad FC-135 to the leading electrolyte buffer. In this way a positively charged bilayer was formed on the inside of the capillary, producing an electroosmotic flow towards the outlet using reversed polarity conditions. In this way, acceptable analysis times were achieved. Using the developed tITP method, up to 72% of the capillary can be filled with sample solution as well. The anionic peptides are separated even better than when using CZE conditions. Linearity and reproducibility in the 20-100 ng/mL range proved to be excellent.

Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications

This review is in support of the development of selective, reproducible and validated capillary electrophoretis (CE) methods. Focusing on pharmaceutical and biological applications, the successful use of CE is demonstrated by more than 800 references, mainly from 1994 until 1998. Approximately 80 recent reviews have been catalogued. These articles sum up the existing strategies for method development in CE, especially in the search for generally accepted concepts, but also looking for new, promising reagents and ideas. General strategies for method development were derived not only with regard to selectivity and efficiency, but also with regard to precision, short analysis time, limit of detection, sample pretreatment requirements and validation. Standard buffer recipes, surfactants used in micellar electrokinetic capillary chromatography (MEKC), chiral selectors, useful buffer additives, polymeric separation media, electroosmotic flow (EOF) modifiers, dynamic and permanent coatings, actions to deal with complex matrices and aspects of validation are collected in 20 tables. Detailed schemes for the development of MEKC methods and chiral separations, for optimizing separation efficiency, means of troubleshooting, and other important information for key decisions during method development are given in 19 diagrams. Method development for peptide and protein separations, possibilities to influence the EOF and how to stabilize it, as well as indirect detection are considered in special sections.

Quantitative trace analysis of interleukin-3, interleukin-6, and basic model proteins using isotachophoresis-capillary zone electrophoresis with hydrodynamic counterflow

A quantitative analytical technique to determine trace concentrations of recombinant human interleukin-3 (rhIL-3), recombinant human IL-6 (rhIL-6), and various basic model proteins is described using isotachophoresis-capillary zone electrophoresis (ITP-CZE). Proteins were separated on coated fused-silica capillaries using a commercial capillary electrophoretic system modified for the application of isotachophoretic preconcentration with hydrodynamic counterflow. The effect of injection time and isotachophoretic focusing time was investigated and compared with predictions from existing mathematical models. Good linearity of the calibration graphs (r > 0.995) was observed for all investigated proteins. The limit of quantification was in the 10(-8) M range using UV detection at 200 nm. Within-day and between-day precision of peak area ranged between 1 and 6%. Precision was unaffected by isotachophoretic preconcentration. In conclusion, the described method is feasible to quantify trace concentrations of rhIL-3, rhIL-6, and basic proteins. Potential applications comprise issues of pharmaceutical quality control.

Recent application and developments of capillary isotachophoresis

Capillary isotachophoresis is a powerful electromigration separation method with a pronounced capability to concentrate trace components in diluted samples. At present, capillary isotachophoresis is utilized predominantly as the first step in on-line combination with capillary zone electrophoresis. This article is a continuation of previous reviews and summarizes the results published during 1993-1996.

Capillary zone electrophoresis of proteins

This review article with 237 references is focused on capillary zone electrophoresis (CZE) of proteins. It includes discussion of modeling electrophoretic migration of proteins, sample pretreatment before the analysis, methods reducing the sorptions of proteins on the capillary wall, and techniques for increasing selectivity by using electrolyte additives including the sieving matrices. Significant progress in detection techniques, namely in laser-induced fluorescence and mass spectrometry, is emphasized. Modifications of CZE using specific interactions, such as affinity capillary electrophoresis or capillary immunoelectrophoresis, are debated as well as combination of CZE with other separation methods such as high performance liquid chromatography (HPLC). A number of practical applications of CZE of proteins are described.