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

Principles
the exact pI of fused silica is difficult to determine, EOF
becomes significant above pH4. Nonionic materials such as
Teflon also exhibit EOF, presumably resulting from adsorption
of anions.
Counterions (cations, in most cases), which build up near
the surface to maintain charge balance, form the doublelayer
and create a potential difference very close to the
wall (figure 5b). This is known as the zeta potential. When
the voltage is applied across the capillary the cations
forming the diffuse double-layer are attracted toward the
cathode. Because they are solvated their movement drags
the bulk solution in the capillary toward the cathode. This
process is shown in schematic form in figure 5c.
The magnitude of the EOF can be expressed in terms of
velocity or mobility by
or
v EOF
= (e z / h) E (7)
m EOF
= (e z / h) (8)
where: v EOF
= velocity
m EOF
= EOF “mobility”
z = zeta potential
e = dielectric constant.
(note the independence of mobility on applied electric field)
The zeta potential is essentially determined by the surface
charge on the capillary wall. Since this charge is strongly
pH dependent, the magnitude of the EOF varies with pH.
At high pH, where the silanol groups are predominantly
deprotonated, the EOF is significantly greater than at low
pH where they become protonated. Depending on the
20