Effects of Convection on Isotachophoresis of Electrolytes

Abstract

In this study, the form of the analytes distribution in isotachophoresis (ITP) in the presence of a convective flow is analyzed in a wide rectangular microchannel. The imposed convection is considered due to a mismatch of electroosmotic (EO) slip velocity of electrolytes of different electrophoretic mobilities. We compute the two-dimensional (2D) Nernst–Planck equations coupled with the Navier–Stokes equations for fluid flow and an equation for electric field. We use a control volume method along with a higher-order upwind scheme to capture the sharp variation of variables in the transition zones. The convection of electrolytes produces a smearing effect on the steepness of the electric field and ion distribution in the interface between two adjacent electrolytes in the ITP process. The dispersion of the interface in plateau-mode and the sample in peak-mode is analyzed through the second- and third-order moments. The dispersion due to nonuniform EO flow (EOF) of electrolytes is found to be different from the case when the dispersion is considered only due to an external pressure driven Poiseuille flow. The nonuniform EOF of electrolytes produces less dispersion and skewness in the sample distribution when the molecular diffusivity of the sample ionic species is close to the harmonic mean of the diffusivity of adjacent electrolytes. We find that the EOF may become advantageous in separating two analytes of close diffusivity. Our results show that the one-dimensional (1D) Taylor–Aris model is suitable to predict the dispersed ITP when the average convection speed of electrolytes is in the order of the ITP speed.