Influence of solvent on temperature and thermal peak broadening in capillary zone electrophoresis

Abstract

The present paper deals with the role of the solvent on thermal peak broadening. One main solvent property that determines the magnitude of the temperature gradient due to the generation of Joule heat in capillary zone electrophoresis is the thermal conductivity. As organic solvents have lower thermal conductivity than water (methanol and acetonitrile, e.g., nearly by a factor of 3) it can be hypothesized that the temperature gradient inside the capillary is more pronounced in organic solvents compared to an aqueous solution. On the other hand, the temperature dependence of the ion mobility (which is responsible for the velocity profile and thus for thermal peak broadening) is smaller in organic solvents. To get insight into the thermal effect of the solvent, first the temperature of a solution in a cylindrical tube was calculated utilizing the heat balance equation. It was shown that the two theoretical models most common in the literature (based on the analytical solution or on an assumption of the parabolic temperature profile in the tube, respectively) give the same results. The latter model was chosen for the further calculations, adding a quadratic term to express the electric conductivity as a function of the temperature. The temperature at the inner capillary wall and center as function of the capillary dimensions and the electric power was computed for electrolytes with a given conductivity at 25.0 degrees C with water, methanol, and acetonitrile as solvents. Capillary cooling systems used were circulating liquid cooling, enforced air-cooling, and natural convection in still air. The mean temperature (averaged over the cross section) resulting from Joule heating was compared with experimentally determined temperatures established upon application of an electric field; the latter temperature was derived from the measurement of the electric conductance of the background electrolyte solution and its (measured) temperature dependence. All investigations were carried out with solutions of the same initial electric conductivity (about 0.5 S.m(-1) at 25.0 degrees C). Agreement is found for natural convection conditions, and the deviation between theoretical and experimental results for the forced air and circulated liquid cooling systems can be related to the poorly defined thermal conditions of the capillaries in commercial instrumentation (with a part in a thermostated cassette and a part outside). For given conditions the temperature gradients in the organic solvents exceed largely those in water, independent of the type of cooling. As a consequence, the thermal plate height is significantly larger in organic solvents, at least under conditions where the deviation from the Nernst-Einstein limiting case is not too high. However, even for the maximum applicable field strengths the thermal plate height contributions are negligible compared to longitudinal diffusion in all solvents.

Determination of the surface heat‐transfer coefficient in CE

“…This treatment agrees with the classical approach to calculating the rise in temperature of the electrolyte adopted by Knox [12], Kok [18] and Porras et al [3], who assumed that a steady state is reached in which the rate at which heat is generated in the electrolyte is equal to the rate at which it is dissipated from the outer wall so that a constant temperature profile is achieved. Based on Eq.…”

“…DT Air is the difference in temperature between the external surface of the capillary wall and the temperature of the coolant T Set . Equation (3) shows how the power per unit length is calculated…”

New advances in microchip fabrication for electrochromatography

“…The problem is not only the temperature increase itself but also the formation of an undesired temperature gradient. Several groups [119][120][121] proposed numerical solutions and models to alleviate Joule heat effects on EOF and mass transport. Sweedler's group [122] followed the change by nanoliter-volume NMR thermometry and concluded that the temperature change due to the Joule heating has a significant effect on highelectric-field and large-diameter channels.…”

Zone Electrophoresis in an Inner-cooling Wide-bore Electrophoresis System with UV Detection

“…It indicates that the WE system generates much more Joule heat, which is in direct ratio to the effective cross section area, A. If this Joule heat cannot be dispersed in time, excessive heat will result in a temperature gradient in the radial channel, and consequently cause serious problems, such as sample zone broadening [20][21][22] and, even worse, stopping the separation process. Therefore, the configuration of the shell-and-tube heat exchanger was introduced into the WE system to enhance the dispatching of heat in situ in real time.…”