Influence of varying electroosmotic flow on the effective diffusion in electric field gradient separations

Abstract

Recently the use electric field gradient focusing (EFGF) to enhance focusing of proteins has been proposed and explored to provide significant improvement in separation resolution. The objective of EFGF is to focus proteins of specific electrophoretic mobilities at distinct stationary locations in a column or channel. This can be accomplished in a capillary by allowing the electric potential to vary in the streamwise direction. Because the electric field is varying, so also is the electrokinetic force exerted on the proteins and the electroosmotic velocity of the buffer solution. Due to the varying electric field, the Taylor diffusion characteristics will also vary along the column, causing a degradation of peak widths of some proteins, dependent on their equilibrium positions and local velocity distributions. The focus of this paper is an analysis that allows characterization of the local Taylor diffusion and resulting protein band peak width as a function of the local magnitude of the EOF relative to the average fluid velocity for both cylindrical and rectangular channels. In general the analysis shows that as the ratio of the local electroosmotic velocity to the average velocity deviates from unity, the effective diffusion increases significantly. The effectiveness of EFGF devices over a range of protein diffusivities, capillary diameters, flow velocities, and electric field gradient is discussed.

Electric field gradient focusing using a variable width polyaniline electrode

“…The approach here is similar to that of Humble et al. who used a change in cross‐section of the conducting region with a variable‐width acrylamide gel . Because the gel was permeable to ions and small molecules, it could only be used to concentrate large molecules, such as proteins.…”

“…Humble et al. used a conductive acrylic polymer with a varying cross‐section surrounding the channel as an alternative way to create the electric field gradient . Ivory et al.…”

Simultaneous Separation of Negatively and Positively Charged Species in Dynamic Field Gradient Focusing Using a Dual Polarity Electric Field

“…In addition, operating at sufficiently low pH has other advantages in that, at lower pH, the surface charges on the wall and in the chromatographic packing will decrease or turn off and the amount of electroosmotic flow (EOF) within the system will decline 25 . This is particularly important in EFGF because the gradient in the electric field produces an EOF profile that varies along the length of the separation channel 30 . This becomes problematic when eluting focused bands through the outlet of the separation channel because the peak concentration, peak shape and resolution will change with the flow profile due to variations in EOF along the length of the channel.…”

Bioanalytical separations using electric field gradient techniques

“…Taylor diffusion, or the combined effects of diffusion in the presence of a flow field, was modeled as a function of electroosmotic flow in EFGF by Maynes et al [83]. Electroosmotic flow in EFGF causes peak broadening, which in turn decreases resolution.…”