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

These peak shape distortions are caused by the differences
in conductivity, and hence field, in each zone. When the
solute zone has a higher mobility (that is, higher conductivity
and lower resistance) than that of the running buffer, the
front edge of the solute, which diffuses in the direction of
migration, encounters a higher voltage drop when entering
the buffer zone. This causes the diffusing solute (that is,
anions when the EOF is toward the cathode) to accelerate
away from the sample zone and results in zone fronting. As
the solute at the trailing edge diffuses into the running
buffer it also encounters an increase in voltage drop, but, in
the same direction of migration, and accelerates back into
the solute zone, keeping the trailing edge sharp. Similar
reasoning accounts for the sharp leading and diffuse trailing
edges of the reverse situation and for ions of opposite
charge. Neutral species are unaffected by these conductivity
differences.
Principles
Although these distortions always occur, they may be small
relative to other dispersive effects, including diffusion. Distortions,
however, are particularly evident with samples containing
solutes with a wide range of mobilities. An example
of the separation of inorganic ions and organic acids is illustrated
in figure 20. Note the fronting of the high mobility,
Absorbance AU
0.001
0.000
-0.001
Figure 20
Fronting and tailing of solutes due to
electrodispersion 8
Peaks: 1) Chloride, 2) Chlorate, 3) Fluoride
4) Acetate, 5) Propionate, 6) MES
Conditions: Detection = Indirect UV,
buffer = 0.01 M benzoic acid adjusted
to pH 8 with Tris, detection
wavelength = 254 nm, V = 25 kV,
l = 50 cm, L = 57 cm, id = 75 mm,
-0.002
-0.003
-0.004
-0.005
EOF
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Time [min]
6
41