Joule Heating Effect in Constant Voltage Mode Isotachophoresis in a Microchannel

Abstract

Microchip ITP (isotachophoresis) is getting popularity as a preparative technique for preconcentration and separation of chemical species and/or ions in liquid phase. In constant voltage mode ITP, generally a high electric potential difference is applied to a discontinuous buffer for faster and higher resolution separation. However, the higher current from the applied electric field induces Joule heating in the buffer which modifies the mobility and diffusion coefficient of analytes in the system. This change in mobility and diffusion coefficient strongly influences the transient separation process in ITP. In this study the effect of Joule heating on separation behavior of analyte compounds has been presented in a constant voltage mode ITP where two chemical species are separated from an initial mixture. The model is based on mass, energy, and charge conservation and electroneutrality condition in the system. A set of nonlinear governing equations are solved numerically for temperature dependent properties such as diffusion coefficient, effective electrophoretic mobility, and thermal conductivity using a finite volume based model. Numerical results suggest that for temperature dependent properties of control parameters, the separation speed of analytes is significantly different from that of constant temperature case. In constant voltage mode ITP, the temperature peak forms at the location of trailing electrolyte, and its influence propagates toward the direction of band movement as separation proceeds.