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

Principles
For the analysis of small ions (for example, sodium, potassium,
chloride) the magnitude of the EOF is usually not
greater than the solute mobilities. In addition, modification
of capillary wall charge can decrease EOF while leaving
solute mobility unaffected. In these circumstances, anions
and cations can migrate in opposite directions.
2.3.2.1 EOF control
While the EOF is usually beneficial, it often needs to be controlled.
At high pH, for example, the EOF may be too rapid,
resulting in elution of solute before separation has occurred.
Conversely, at low or moderate pH, the negatively
charged wall can cause adsorption of cationic solutes
through coulombic interactions. This latter phenomenon
has been especially problematic for basic protein separations.
In addition, electrophoretic separation modes such
as isoelectric focusing, isotachophoresis, and capillary gel
electrophoresis often require reduction of EOF.
Fundamentally, control of EOF requires alteration of the
capillary surface charge or buffer viscosity. There are
several methods to accomplish this, as detailed in table 1
and in the following paragraphs. Note that conditions that
affect the surface charge of the wall often affect the solute
(such as buffer pH). Successful separations are usually
obtained when the conditions optimize both EOF and solute
mobility properties.
The rate of EOF can most easily be decreased by lowering
the electric field, as described by equation (7). This action,
however, has numerous disadvantages with regard to
analysis time, efficiency, and resolution. From a practical
point of view, the most dramatic changes in EOF can be
made simply by altering the pH of the buffer, as described
above (figure 6). Adjusting the pH, however, can also affect
the solute charge and mobility. Low pH buffers will proto-
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