Voltage influence Joule heat

The rate of Joule heating increases with the square of the applied voltage, so doubling the voltage increases P L by a factor of 4 (see Equation 18.6). The voltage is usually optimized for maximum efficiency but a compromise between efficiency and heat generation can often lead to shorter analysis times if there is sufficient resolution and the analytes are not influenced by elevated temperatures [3]. 

Qualitative and quantitative analysis require high repeatability of the separation parameters. To meet these requirements, all experimental conditions should be kept constant. Because negligible change in the separation conditions such as buffer pH, ionic strength, temperature, and other factors can alter the mobility of the analytes, temperature influenced by Joule heating must be minimized by operating under constant current instead of constant voltage. Capillary should be preconditioned in the same manner. 

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