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

occur. The primary causes of adsorption to the fused silica
walls are ionic interactions between cationic solutes and
the negatively charged wall, and hydrophobic interactions.
The large surface area-to-volume ratio of the capillary,
which is beneficial for heat transfer, in fact increases the
likelihood of adsorption. Significant absorptive effects have
been noted especially for large peptides and proteins
primarily because these species posses numerous charges
and hydrophobic moieties.
The variance due to adsorption can be given by
s 2 = k'v EOF
1 r 2 k' + 2
(19)
ads
(1 + k') 2 4D K d
where k' = capacity factor
v EOF
= electro-osmotic flow velocity
D = solute diffusion coefficient
l = capillary effective length
= first order dissociation constant.
K d
( )
Principles
The capacity factor is defined as
t
k' = r
– t 0
(20)
t 0
where t r
= elution time of a retained solute
t 0
= elution time of an unretained solute.
Equation (19) describes both axial diffusion (that is, across
the capillary) and adsorption-desorption kinetics (K d
).
The variance is strongly dependent on the magnitude of
the capacity factor. As shown in figure 16 and table 7,
small interactions can have dramatic effects on efficiency.
Capacity factors even less than 0.1 can be detrimental. In
37