High performance capillary electrophoresis - T.E.A.M.

Principles
determined by the capillary dimensions, conductivity of
the buffer, and the applied voltage. Significantly elevated
temperatures will result when the power generation exceeds
dissipation. Typical power generation ranges from
0.5 to 5 W/m. Temperature increases of 10 °C are not
uncommon, although 70 °C and higher can occur.
Normalized zone
deformation
100
80
60
40
20
0
= 0.20 mm
0.10 mm
0.075 mm
0.05 mm
0.025 mm
500 1000 1500
Field strength [V/cm]
Figure 12
Effect of Joule heating and temperature
gradients on solute zone deformation 4
( = capillary id)
While the absolute rise in temperature is generally not
detrimental (except possibly for sample degradation, and
so on), temperature gradients are. Thermal dissipation of
the heat through the capillary walls can result in higher
temperatures in the center than at the walls. These temperature
gradients cause viscosity differences of the running
buffer and give rise to zone deformation. This is illustrated
in figure 12 for a variety of inner diameter capillaries.
Control of temperature differentials is critical since a one
degree change in temperature results in a 2 to 3 % change
in viscosity (and a 2 to 3 % change in mobility).
The thermal gradient between the center of the capillary
and the surroundings is illustrated in figure 13. As shown,
the temperature difference depends on the inner radius, the
thickness of the wall, the thickness of the polyimide coating,
and the heat transfer coefficient to the surroundings.
Analytically this can be expressed by
[ ( ) ( ) ( )]
2
Qr 1
1 r 2
1 r 3
1 1
DT T
= 1n + 1n +
2 k 1
r 1
k 2
r 2
r 3
h
(17)
where:
Q = power density
r = radius
k = thermal conductivity
h = thermal transfer rate from the capillary
to the surrounding
subscripts 1, 2, and 3 refer to the buffer,
32