Steady-state heat transfer and thermal zone spreading in gel isoelectric focusing

The zone spreading caused by a transverse pH profile due to a temperature gradient through the thickness of a gel slab is estimated. The temperature difference (delta T) between the upper and lower gel surfaces can be calculated as a function of the electric power dissipated in the gel and the gel dimensions. It is found that when delta T is only 1 degree C the zone spreading due to this thermal excursion is as high as 0.5 mm. Thus, an admissible delta T is found to be equal to 0.2 degrees C, since this corresponds to a thermal zone spreading of only 0.1 mm, i.e. the same order of magnitude as the spatial resolution of a laser scanner. A thermal zone spreading of 0.1 mm is compatible with a resolving power of 0.01 pH unit, the current limit of conventional isoelectric focusing in amphoteric buffers. A requirement for the thickness of a gel slab is formulated: e.g., at 40 W applied power (over a gel surface area of 25 x 11 cm), a thermal zone spreading limited to 0.1 mm can only be obtained with a gel thickness of approximately 170 microns.