4.2.2.5: Joule Heating
Joule heat is generated by passing electric current through a conductor and is directly proportional to the amount of power used. Joule heating can result in non-uniform temperature gradients throughout the capillary tube. Temperature increases are often around 10ºC, although increases of up to 70ºC or higher can occur. While uniform increases in temperature generally do not affect zone broadening, uneven increases in temperature leading to temperature gradients can cause substantial peak broadening and/or tailing. Heat can relatively easily dissipate through capillary walls, which can result in a large difference in temperature between the center of the capillary and the internal edges of the capillary. These gradients can result in differences in solvent viscosity, and thus air or liquid cooling of the capillaries is often necessary. Using thicker capillary tubes with a large outer radius and a smaller inner radius can also help combat this problem. Smaller sample volumes and higher surface-to-volume ratios limit the Joule heating, even when hundreds of volts per centimeter are applied.
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