A model of thermally generated pH gradients in tapered capillaries

Potential applied at the two ends of a tapered capillary can produce an electric field and a pH gradient at the same time. These fields will effect electrophoretic separation, if the diameters of capillaries are not uniform. In some cases these gradients can be used to our advantage. For example, this approach provides a simple way to perform isoelectric focusing without carrier ampholytes as Joule heat produced in the tapered capillary generates a temperature gradient, which in turn produces a pH gradient. A mathematical model is developed to analyze the temperature distribution along the tapered capillary axis. According to the model, the span of the temperature gradient increases with the increase in heat generated per volume of the capillary channel, and the slope of the gradient is determined by the steepness of the taper. The temperature profile predicted by the model is verified by measuring temperatures at the outer wall of the capillary. The calculated pI value of a sample protein, hemoglobin, which focuses in the thermally generated pH gradient, is near to its literature value. The model provides us with a better understanding of the heat transfer in a tapered capillary and theoretically demonstrates the potential of capillary isoelectric focusing separation with a tapered channel.