Temperature profiles and heat dissipation in capillary electrophoresis

While temperature control is usually employed in capillary electrophoresis (CE) to aid heat dissipation and provide acceptable precision, internal electrolyte temperatures are almost never measured. In principle, this limits the accuracy, repeatability, and method robustness. This work presents a fundamental study that combines the development of new equations characterizing temperature profiles in CE with a new method of temperature determination. New equations were derived from first principles relating the mean, axial, and inner wall electrolyte temperatures (TMean, TAxis, TWall). TMean was shown to occur at a distance 1/3 times the internal radius of the capillary from the center of the capillary and to be a weighted average of 2/3TAxis and 1/3TWall. Conductance (G) and electroosmotic mobility (EOF) can be used to determine TMean and TWall, respectively. Extrapolation of curves of EOF versus power per unit length (P/L) at different temperatures was used to calibrate the variation of EOF with temperature (T), free from Joule heating effects. EOF increased at 2.22%/C. The experimentally determined temperatures using EOF agreed to within 0.2 C with those determined using G. The accuracy of G measurements was confirmed independently by measuring the electrical conductivity () of the bulk electrolyte over a range of temperatures and by calculating the variation of G with T from the Debye-Hückel-Onsager equation. TMean was found to be up to 20 C higher than the external temperature under typical conditions using active air-cooling and a 74.0-m-internal diameter (di) fused-silica capillary. A combination of experimentally determined and calculated temperatures enables a complete temperature profile for a fused-silica capillary to be drawn and the thickness of the stationary air layer to be determined. As an example, at P/L = 1.00 Wm-1, the determined radial temperature difference across the electrolyte was 0.14 C; the temperature difference across the fused-silica wall was 0.17 C, across the poly(imide) coating was 0.13 C, and across the stationary air layer was 2.33 C