An analytic description of electrodynamic dispersion in free-flow zone electrophoresis

Abstract

The present work analyzes the electrodynamic dispersion of sample streams in a free-flow zone electrophoresis (FFZE) chamber resulting due to partial or complete blockage of electroosmotic flow (EOF) across the channel width by the sidewalls of the conduit. This blockage of EOF has been assumed to generate a pressure-driven backflow in the transverse direction for maintaining flow balance in the system. A parallel-plate based FFZE device with the analyte stream located far away from the channel side regions has been considered to simplify the current analysis. Applying a method-of-moments formulation, an analytic expression was derived for the variance of the sample zone at steady state as a function of its position in the separation chamber under these conditions. It has been shown that the increase in stream broadening due to the electrodynamic dispersion phenomenon is additive to the contributions from molecular diffusion and sample injection, and simply modifies the coefficient for the hydrodynamic dispersion term for a fixed lateral migration distance of the sample stream. Moreover, this dispersion mechanism can dominate the overall spatial variance of analyte zones when a significant fraction of the EOF is blocked by the channel sidewalls. The analysis also shows that analyte streams do not undergo any hydrodynamic broadening due to unwanted pressure-driven cross-flows in an FFZE chamber in the absence of a transverse electric field. The noted results have been validated using Monte Carlo simulations which further demonstrate that while the sample concentration profile at the channel outlet approaches a Gaussian distribution only in FFZE chambers substantially longer than the product of the axial pressure-driven velocity and the characteristic diffusion time in the system, the spatial variance of the exiting analyte stream is well described by the Taylor-Aris dispersion limit even in analysis ducts much shorter than this length scale.

Estimating Stream Broadening in Free-Flow Electrophoretic Systems Based on the Method-of-Moments Formulation

“…Such blockage of EOF then generates a pressure-driven backflow in the transverse direction for maintaining a flow balance in the system leading to additional stream broadening. In this situation, the analyte velocity in the transverse direction may be expressed as ux=μE[1−(3α/2)(1−4y2/d2)] in regions far away from the channel sidewalls [8]. Notice that the spatially averaged value of u x in this description equals μE (1 − α ) which determines the lateral migration distance for the analyte stream at the channel exit.…”

Micro free flow electrophoresis

“…Instead, EOF recirculates back through the center of the channel, introducing a crescent shaped flow profile. 53, 54…”

Enhancement in MS‐based peptide detection by microfluidic free‐flow zone electrophoresis

“…This approach however restricts the separation voltage to values on the order of 10 V in order to prevent any noticeable electrochemical gas generation in the system requiring analysis channels that are narrower than 1 mm in order to attain electric fields on the order of 100 V/cm. Although high‐resolution FFZE separations are theoretically achievable in such narrow conduits, their realization can be challenging in practice due to zone broadening arising from nonideal operating conditions [35‐37]. Our previous work has shown that for glass‐based microchip devices, zone broadening in such narrow FFZE chambers mostly arises as a result of the transverse fluid circulations produced from the blockage of EOF by the channel sidewalls [33, 38].…”