Porras Simo P, Marziali Ettore, Gas Bohuslav, Kenndler Ernst
Institute for Analytical Chemistry, University of Vienna, Vienna, Austria.
Electrophoresis. 2003 May;24(10):1553-64. doi: 10.1002/elps.200305437.
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.
本文探讨了溶剂对热峰展宽的作用。在毛细管区带电泳中，决定因焦耳热产生的温度梯度大小的一个主要溶剂性质是热导率。由于有机溶剂的热导率低于水（例如甲醇和乙腈，几乎低3倍），可以推测与水溶液相比，毛细管内的温度梯度在有机溶剂中更为明显。另一方面，离子迁移率（它决定速度分布，进而影响热峰展宽）的温度依赖性在有机溶剂中较小。为了深入了解溶剂的热效应，首先利用热平衡方程计算了圆柱形管中溶液的温度。结果表明，文献中最常见的两种理论模型（分别基于解析解或管内抛物线温度分布的假设）给出了相同的结果。选择后一种模型进行进一步计算，添加一个二次项来表示电导率随温度的变化。对于在25.0℃下具有给定电导率的电解质，以水、甲醇和乙腈为溶剂，计算了毛细管内壁和中心处的温度随毛细管尺寸和电功率的变化。所使用的毛细管冷却系统有循环液体冷却、强制风冷和静止空气中的自然对流。将焦耳热产生的平均温度（在横截面上平均）与施加电场时实验测定的温度进行比较；后者的温度是通过测量背景电解质溶液的电导率及其（测量的）温度依赖性得出的。所有研究均使用相同初始电导率（25.0℃时约为0.5 S·m⁻¹）的溶液进行。在自然对流条件下发现了一致性，强制风冷和循环液体冷却系统的理论结果与实验结果之间的偏差可能与商业仪器中毛细管定义不明确的热条件有关（一部分在恒温盒中，一部分在外部）。在给定条件下，有机溶剂中的温度梯度大大超过水中的温度梯度，与冷却类型无关。因此，至少在偏离能斯特 - 爱因斯坦极限情况不太高的条件下，有机溶剂中的热板高明显更大。然而，即使对于最大适用场强，与所有溶剂中的纵向扩散相比，热板高的贡献也可以忽略不计。
