Heat production and dissipation in capillary electrophoresis

Joule heating is an unavoidable phenomenon in capillary electrophoresis (CE) and results from resistive heating that occurs when an electric current (I)flows through the electrolyte when a potential difference is applied. The increase in conductivity with temperature results in a positive feedback3329: “3329_c018” — 2007/10/18 — 17:17 — page 546 — #2Electrophoresis and Associatedeffect in which the current increases until a steady state is reached. This may even cause the electrolyte to boil or superheat: this is known as autothermal runaway [1]. Temperature control is usually employed in CE to aid heat dissipation and provide acceptable precision, but measurement of the electrolyte temperature is often overlooked. In some older systems, only ambient temperature operation was available, with or without fan-forced airflow. The temperature of the electrolyte affects its viscosity (η), its dielectric constant (εr), and the zeta potential (ζ) [2], which affect the precision of migration times through the effect of η, εr , and ζ on the electroosmotic mobility (µeof ) and the electrophoretic mobility (µep). Even small changes in temperature can cause significant deviations in migration times [3]. The electrolyte temperature also has a major influence on peak broadening [4-6], such that separation efficiency generally decreases with increasing temperature. Radial temperature differences in the electrolyte result in viscosity differences across the capillary with analytes traveling faster in the warmer, lower viscosity zone near the axis of the capillary than in the cooler zones near the capillary wall [5]. Axial temperature differences that result from the layout of the instrument [7], and differences caused by variations in conductivity as the sample migrates through the electrolyte, also increase dispersion effects [6]