Joule Heating Effect in Constant Voltage Mode Isotachophoresis in a Microchannel

Microchip ITP (isotachophoresis) is getting popularity as a preparative technique for preconcentra­ tion 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 discon­ tinuous buffer for faster and higher resolution separa­ tion. 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 electroneu­ trality condition in the system. A set of nonlinear govern­ ing equations are solved numerically for temperature dependent properties such as diffusion coefficient, effec­ tive electrophoretic mobility, and thermal conductivity using a finite volume based model. Numerical results sug­ gest that for temperature dependent properties of control parameters, the separation speed of analytes is signifi­ cantly 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 prop­ agates toward the direction of band movement as separa­ tion proceeds.