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

Principles
Current (uA)
300
250
200
150
100
50
Liquid thermostating
10 m/s air thermostating
No thermostating
0 5 10 15 20 25 30
Voltage (kV)
Figure 14
Ohm’s law plots to monitor Joule heating
voltage is increased. Another indication is the disproportionate
increase in EOF or solute mobility with increasing
voltage. Similarly, a disproportionate increase in current
with voltage (Ohm’s law) indicates a temperature increase
(figure 14).
A variety of methods to limit Joule heating are described in
table 5. Equation (17) indicates that temperature gradients
will be reduced linearly with a reduction in power. This can
be accomplished by lowering either the applied voltage or
decreasing the buffer conductivity by lowering the ionic
strength or decreasing buffer ion mobility. The latter methods
may be useful but have practical limitations. Reduced
buffer concentration may decrease buffering capacity and
also may lead to increased solute-wall interactions (see
below).
Variable
Effect
Decrease electric field
● Proportional decrease in heat generated
● Reduces efficiency and resolution
Reduce capillary inner ● Dramatic decrease in current (i a r 2 )
diameter
● Decreases sensitivity
● May cause increased sample adsorption
Table 5
Methods to control Joule heating and
temperature gradients
Decrease buffer ionic
strength or concentration
Active temperature control
● Proportional decrease in current
● May cause increased sample adsorption
● Thermostats and removes heat from capillary
An alternative is the use of low mobility buffers which contain
large, minimally charged ions, such as tris, borate, histidine,
and CAPS. Additional comments regarding buffer
selection can be found in section 3.1.1.1.
A dramatic decrease in temperature differences can also be
realized by reducing the capillary internal diameter due to
the squared dependence of area (and thus electrical cur-
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