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

Principles
µ EOF
( × 10 -4 cm 2 /Vs)
8
7
6
5
4
3
2.0 2.5 3.0 3.5 4.0 4.5 5.0
In[conc(mM)]
8
7
6
5
4
3
-2 0 2 4 6
In[ionic strength (mM)]
Figure 9
Electro-osmotic flow mobility as a
function of buffer concentration and
ionic strength 3
circles = borate buffer
squares = phosphate buffer
triangles = carbonate buffer, all pH 8
both the capillary surface and the solute, while high pH
buffers deprotonate both. Knowledge of solute pI is often
useful in selecting the appropriate pH range of the running
buffer.
EOF can also be affected by adjusting the concentration and
ionic strength of the buffer. The magnitude of this effect is
illustrated in figure 9. High buffer concentrations are also
useful in limiting coulombic interactions of solute with the
walls by decreasing the effective charge at the wall. Heating
within the capillary, however, constrains the use of high
concentration buffers (the effects of heating are described
in 2.3.3. Analytical parameters). Typical buffer concentrations
range from 10 to 50 mM, although 100 to 500 mM and
higher have also been used.
Lastly, EOF can be controlled by modification of the capillary
wall by means of dynamic coatings (that is, buffer
additives) or covalent coatings. These coatings can increase,
decrease, or reverse the surface charge and thus the EOF.
Details regarding the nature of coatings are given in section
3.1.
2.3.3 Analytical parameters
The analytical parameters for capillary electrophoresis
can be described in similar terms to those for column chromatography.
Capillary zone electrophoresis (CZE) is the
simplest mode of CE and all subsequent discussions in this
section refer to it. Below, fundamental and practical descriptions
of time, mobility, solute zone dispersion, efficiency,
and resolution are presented.
2.3.3.1 Mobility and migration time
The time required for a solute to migrate to the point of
detection is called the “migration time”, and is given by the
quotient of migration distance and velocity. The migration
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