Control Of Electroosmotic Flow Research Articles

Control of relative migration of small inorganic and organic anions with cyclodextrins in capillary electrophoresis (CE)

The capillary electrophoretic separation of iodide, nitrate, perchlorate, thiocyanate, bromate, iodate, and ethane-, butane-, pentane-, and octanesulphonate was examined in sodium chromate or potassium hydrogen phthalate electrolytes and in the presence of α -, γ -, (0-40 mmol/L) and β -cyclodextrin (0-10 mmol/L). Largest decreases in electrophoretic mobility were observed for iodide, perchlorate, and thiocyanate, probably due to inclusion of these anions in the cyclodextrin (CD) cavity. Changes in migration patterns and electroosmotic flow were observed, which depended on cyclodextrin type and concentration and on electrolyte-cyclodextrin interactions. Thus for larger cavity cyclodextrins ( β-or γ-cyclodextrin) or in the presence of chromate, relatively small decreases in electrophoretic mobilities for all the anions were observed, indicating that a good match between analyte and cavity size and minimal cyclodextrin affinity for electrolyte ions are essential for different migration patterns. Separation efficiencies were between 50 000 and 400 000 theoretical plates, and calibration plots for iodide and octanesulphonate were linear; R2 = 0.998 and 0.9999, respectively, in the concentration range (5 x 10-5)-(5 x 10-3) mol/L.Key words: capillary electrophoresis, inorganic and organic anions, cyclodextrins, inclusion complex, electroosmotic flow control.

Enhanced capillary zone electrophoretic separation of dinitrophenyl-amino acid derivatives through control of electroosmotic flow by the buffer cation

The effect of Mg2+, Cd2+ and Zn2+ are evaluated as buffer additives to reduce electroosmotic flow (EOF) to facilitate the capillary zone electrophoretic (CZE) separation of 2,4-dinitrophenyl (DNP) derivatives of amino acids. As the divalent cation concentration increases EOF decreases and reduction of EOF follows the order Zn2+>Cd2+>Mg2+. The electrophoretic mobility of the DNP-amino acid derivatives remains constant for a given buffer and pH over a change in divalent cation concentration. The effect of the divalent cation buffer additive is consistent with cation-exchange at the silanol sites on the fused-silica wall. Higher buffer pH favors greater cation-exchange and reduced EOF. Separation of multicomponent mixtures of DNP-l-amino acid derivatives in the presence of Mg2+, Cd2+ and Zn2+ are compared at pH 7.00. Derivative migration time, resolution and peak shape are enhanced and are best for Mg2+ as the buffer additive although Mg2+ must be used at a higher concentration than either Cd2+ or Zn2+. Raising the buffer pH to 9.25 in the presence of Mg2+ increases migration time and resolution but analysis time is also increased.

Capillary zone electrophoresis with external radial electric field control of electroosmotic flow and its application to the separation of synthetic oligopeptides

A new way of applying external radial electric field for the control of electroosmotic flow in capillary zone electrophoresis (CZE) has been developed. It is based on a new type of low-conductivity coating of the outer capillary surface by a polyaniline dispersion in hydroxypropylcellulose and on the application of a set of three high-voltage power supplies to form a constant radial electric field across the capillary wall as far as possible along the capillary length. Two power supplies are connected to the ends of the outer low-conductivity coating and the third one is applied to the ends of the inner capillary compartment filled with background electrolyte. The difference of electric potentials at the inner and outer capillary surface determines the voltage of radial electric field across the capillary wall and affects the electrokinetic potential at the solid–liquid interface inside the capillary. The effect of magnitude and polarity of external radial electric field on the flow-rate of electroosmotic flow, on the migration times of charged analytes (speed of analysis) and on the separation efficiency and resolution of CZE separations of synthetic oligopeptides, diglycine, triglycine, dalargin and dalargin–ethylamide has been evaluated. Application of external radial electric field has proved to be an efficient tool for regulation of electroosmotic flow in CZE and for optimization of migration times and resolution of oligopeptides in their CZE separations and analyses.

Effects of alkylamines on electroosmotic flow and protein migration behaviour in capillary electrophoresis

This paper reports the use of four closely related alkylamines as running electrolyte additives in capillary electrophoresis that permit the control of electroosmotic flow and protein migration behaviour in uncoated capillaries. At pH 2.5 the direction of the electroosmotic flow was anodic with all additives and at constant ionic strength its magnitude increased with increasing alkylamine concentration. The observations are in qualitative agreement with a previous reported theoretical model that correlates the electroosmotic mobility with the charge density in the Stern region of the electric double layer, arising from the adsorption of the additive, and the charge density at the capillary wall due to dissociation of silanols.

Dispersion in capillary electrophoresis with external flow control methods

Recently, methods for controlling the bulk flow of the buffer which are independent of the electrophoretic migration have been demonstrated in capillary electrophoresis (CE) systems. These methods include controlling the electroosmosis by radially directed electric fields, and controlling the bulk flow by siphoning. This paper investigates basic dispersion mechanisms at work by examining the dispersion of dimethyl sulfoxide under various external control methods. It is found that the electroosmotic flow control methods exhibit the same dispersive behavior as the conventional CE system. In particular, the results for pH 7 buffers obey the theoretical expression for the plate height in all configurations. However, in all configurations, pH 2.7 buffers do not follow the theory, and counter to intuition, additional pressure can improve the plate height at pH 2.7. Taking all these methods together, one can develop a technique to appropriately adjust the electrophoretic and bulk flow to optimize resolution.

Correction. Electroosmotic Flow Control and Monitoring with an Applied Radial Voltage for Capillary Zone Electrophoresis

Correction. Electroosmotic Flow Control and Monitoring with an Applied Radial Voltage for Capillary Zone Electrophoresis

Correction. Effects of Buffer pH on Electroosmotic Flow Control by an Applied Radial Voltage for Capillary Zone Electrophoresis

Correction. Effects of Buffer pH on Electroosmotic Flow Control by an Applied Radial Voltage for Capillary Zone Electrophoresis

Electroosmotic flow control and surface conductance in capillary zone electrophoresis.

Electroosmotic flow within capillary zone electrophoresis can be altered electronically through a mechanism modeled as surface conductance. Experimentally, a small zone of conductive silver on the outer surface of the capillary is shown to permit good control of electroosmotic flow with an applied external voltage. This control is modeled as capacitance across the capillary wall and conductance along the double layer on the inner surface. Experimental results presented agree with theory developed from this model. Surface conductance in capillary zone electrophoresis may significantly simplify electronic control of electroosmotic flow and lead to a better understanding of the effects of chemical modifications to the inner surface of the capillary.

Charged surface coatings for capillary electrophoresis

AbstractFused‐silica capillaries with specially designed charged surface coatings were developed for the control of electroosmotic flow and for the separation of peptides and proteins in capillary electrophoresis. Cryptand moieties were immobilized onto the capillary surface through a crosslinking reaction of a cryptand‐containing polymer and copolymerization of a cryptand‐containing monomer. The resultant coatings provided switchable electroosmotic flow from low to high pH due to the complexing ability of the cryptand moieties with the metal ions from the buffer solution. A positively charged quaternary amine surface‐coated column was found to be very effective in reversing the electroosmotic flow compared to an untreated fused silica capillary, and to be suitable for separation of proteins whose pI values are greater than the buffer pH. Also, a negatively charged sulfonic acid surface‐coated column could be used to stabilize the electroosmotic flow from pH 3 to 9, and the magnitude of the electroosmotic flow could be controlled by varying the concentration of sulfonic acid groups in the coating solution. This negatively charged surface coating gave excellent reproducibility for the separation of a tryptic digest of β‐casein.

Manipulation of electroosmotic flow in capillary electophoresis

This paper reports the use of surfactant and polymer—C 18 coated capillaries that allow manipulation of electroosmotic flow (EOF). Although this approach to the control of EOF involves the preparation and use of multiple capillaries, all the coatings were prepared by a single procedure. It is shown that the ability to control EOF allows optimization of both separation time and resolution. In the case of proteins, low EOF maximizes resolution whereas high flow gives the shortest analysis time. It should be noted that proteins are a special case and this conclusion may not be true with other molecular species. Through selection of a specific coating, it is possible to complete a separation in the shortest time while maintaining sufficient resolution to give baseline resolution of proteins. The various coated capillaries were examined in capillary zone electrophoresis (CZE) and capillary isoelectric focusing (cIEF) separations of native protein standards and hemoglobin variants. Separation of glycosylated hemoglobin A 1 variants was achieved by cIEF within 10 min, including the focusing time. Good run-to-run reproducibility was obtained by flushing the capillary with the coating solution between analyses.

Self-regulating dynamic control of electroosmotic flow in capillary electrophoresis

Capillary electrophoresis (CE), primarily because of the impressive separation power and the compatibility with large biomolecules, has established itself as an important separation tool over the past several years. One of the major barriers limiting the broad, routine application of the technique is the validation of the reliability of migration times and the quantitative accuracy when dealing with real samples. In particular, sample matrices can alter the nature of the surface of the inner wall of the capillary, leading to irreproducible results. Even when the temperature is constant and when adsorption of the analytes onto the capillary wall can be neglected, which is the case of pure zone electrophoresis (CZE), the [zeta] potential of the wall can still be altered by the injected ions. In this report, the authors demonstrate the utility of a concept for maintaining a constant electroosmotic flow rate for sample ions in CZE, independent of the changing sample matrix composition. Such an interactive control of electroosmotic flow has the advantage that the instantaneous flow rate need not be monitored. Once the stationary front is established, no further external intervention is needed. 23 refs., 2 tabs.

Fundamental and practical aspects of coupled capillaries for the control of electroosmotic flow in capillary zone electrophoresis of proteins

This article represents an extension to a new approach, which was introduced very recently by our laboratory, for the control of the magnitude of electroosmotic flow in capillary zone electrophoresis. In this new approach, short fused-silica capillaries having different ξ potentials were coupled in series, and the amount of the electroosmotic flow was conveniently varied by changing the lengths of the individual capillary segments. The different coupled capillary systems evaluated in this study comprised various combinations of untreated fused-silica capillaries and polyether-coated capillaries having various electroosmotic flow characteristics. A general equation relating the average electroosmotic flow velocity in the coupled capillaries to the intrinsic electroosmotic velocity of the connected segments and their corresponding lengths has been derived and verified experimentally. The rate of the electroosmotic flow in a given system of coupled capillaries could be tuned over a range bordered by the lowest and highest intrinsic flow-rates of the connected capillary segments. In addition, a system of coupled capillaries that permitted a stepwise change in the rate of the electroosmotic flow during analysis was introduced and evaluated. These elution schemes were useful in the rapid separation of oppositely charged proteins in a single electrophoretic run and in the rapid analytical determination of the various components of heterogeneous proteins.

Effects of buffer pH on electroosmotic flow control by an applied radial voltage for capillary zone electrophoresis.

Electroosmotic flow has been shown to be controlled via an applied radial voltage. Many factors determine the effectiveness of this control, and one major factor is buffer pH. In this study the effectiveness of the applied radial voltage for controlling electroosmotic flow while varying buffer pH is examined. Previously developed theory is applied and compared to experimental results for a pH range from 1.4 to 6.32. Analysis time is dramatically reduced by applying a radial voltage for separation of a peptide mixture at pH 1.4. Theory predicts laminar flow profiles under some conditions when applying this technique. However, experimental evidence at pH 6.32 and 1.4 shows no evidence of band broadening from a laminar flow profile. Theoretical and experimental results indicate the largest range of effective electroosmotic flow control via an applied radial voltage occurs at low ph. Furthermore, a sigmoidal relationship between electroosmotic flow and applied radial voltage is clearly apparent under these conditions. In contrast, at high buffer pH ( > 6) the relationship appears to be linear and is only over a limited range of flow velocities.

Electroosmotic flow control and monitoring with an applied radial voltage for capillary zone electrophoresis.

The control of electroosmotic flow by electronic means for capillary zone electrophoresis is presented. This is accomplished by the application of a radial voltage field with a rugged and flexible conductive polymer sheath. Fundamental theory for effect of the applied radial voltage on electroosmotic flow is developed. In addition, the effects of surface pretreatments are examined and compared to trends predicted by the theory.