Frans_M_Everaerts_Isotachophoresis_378342.pdf

172 DETECTION SYSTEMS
of consideration, especially if one is interested only in the suppression of the electro-
endosmotic flow.
In isotachophoretic analyses, the electroendosmotic flow is not constant in all zones,
but increases in the direction of the terminating zone. Ths effect increases the turbulence
of the liquid in each zone, but it is beyond the scope of this book to go into great detail.
Because hydrodynamic flow in the narrow-bore tube is blocked at one side by the semi-
permeable membrane, a profile as shown in Fig.6.35 may be postulated for both the
electroendosmosis and the temperature differences in the various zones.
There are still some differences of opinion concerning the boundary conditions for
the movement of liquids, especially if this is compared with the movement of the zone
boundaries. As in all types of calculation, the potential at the wall is taken determinative
for the electroendosmosis. This potential is often called the { (zeta) potential. When an
A B
Fig.6.35. Profile of a zone boundary in a narrow-bore tube that is blocked at one side by asemipermeable
membrane. The arrows indicate the movement of the zone boundary; Xindicates the direction in which
the sampk zones move. In both (A) and (B), the parabolic profile due to the difference in temperature
between the centre and the wall of the narrow-bore tube is in the same direction as the movement of the
zone boundary (-.-.-). A correction must be made for the difference in temperature on both sides of
the parabolic profile. In (A), the electroendosmotic flow is chosen to be in the same direction as the move-
ment of the zone boundary, while in (B) this flow is in the opposite direction. The dotted line indicates
this electroendosmotic proffle. A correction must also be made for the difference in electroendosmotic
flow on both sides of the boundary, because the potential gradients in the two zones are not equal.
The final profiie (hypothetical) is indicated in both (A) and (B) by a full line.