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

Principles
µ EOF × 10 -4 (cm 2 / Vs)
4
3
2
1
Pyrex
Silica
Teflon
pH
3 4 5 6 7 8
Figure 6
Effect of pH on electro-osmotic flow
mobility in various capillary materials 2
specific conditions, the EOF can vary by more than an order
of magnitude between pH 2 and 12. Figure 6 illustrates this
effect for fused silica and other materials.
The zeta potential is also dependent on the ionic strength
of the buffer, as described by double-layer theory. Increased
ionic strength results in double-layer compression,
decreased zeta potential, and reduced EOF (see figure 9).
A unique feature of EOF in the capillary is the flat profile of
the flow, as depicted in figure 5c. Since the driving force of
the flow is uniformly distributed along the capillary (that is,
at the walls) there is no pressure drop within the capillary,
and the flow is nearly uniform throughout. The flat flow
profile is beneficial since it does not directly contribute to
the dispersion of solute zones. This is in contrast to that
generated by an external pump which yields a laminar or
parabolic flow due to the shear force at the wall (figure 7).
Figure 7a shows that the flow rate drops off rapidly at the
EOF
Laminar flow
a) Flow b)
Figure 7a, b
Flow profile and corresponding solute zone
wall. This quiescent solution layer is caused by friction
against flow at the surface. Since this layer extends a short
way into the solution, it is relatively unimportant to the
overall separation process (that is, other dispersive processes
dominate). Further, the flow rate and profile are
generally independent of capillary diameter. The profile will
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