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

Instrumentation/Operation
However, if EOF is reduced, reversed, or if gels are used, it
may be necessary to reverse the polarity of the electrodes;
that is, to switch the cathode to the injection end. Since the
inlet and outlet ends of the capillary are usually predetermined
by the detector geometry, polarity switching must be
performed at the power supply. This can best be accomplished
by use of a dual polarity power supply. With such a
power supply it is important to realize that the high voltage
electrode and the ground electrode remain fixed. That is,
the high voltage electrode is driven either positive or
negative with respect to the ground electrode. It is also
beneficial if polarity switching is software controlled,
especially if switching is desired during an analysis.
While constant voltage analyses are most common, it is
often beneficial to use either constant current or constant
power modes. Constant current or power mode is particularly
useful for isotachophoretic experiments or when
capillary temperature is not adequately controlled. With
regard to the latter, temperature changes alter buffer
viscosity and migration time in constant voltage mode. In
constant current mode, these viscosity changes are compensated
by proportional changes in the applied voltage,
maintaining constant migration time.
Another power supply feature is the ability to run voltage,
current, or power gradients (also called field programming)
during an analysis. Field programming can be used to ramp
the voltage at the beginning of an analysis to avoid rapid
heating, thermal expansion of buffer, an expulsion of sample
from the capillary. Field programming is also particularly
useful for decreasing the analysis time of complex samples
and is often necessary for fraction collection. Since manipulation
of narrow, closely spaced solute zones (for example,
5 to10 s) is difficult under high field conditions, reduction of
the field immediately prior to collection increases the time
window and relaxes the stringent timing problems associated
with precise collection (see section 4.4.2).
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