A particular mode of isotachophoresis (ITP) employs a pressure-driven flow
opposite to the sample electromigration direction in order to anchor a sample
zone at a specific position along a channel or capillary. We investigate this
situation using a two-dimensional finite-volume model based on the
Nernst-Planck equation. The imposed Poiseuille flow profile leads to a
significant dispersion of the sample zone. This effect is detrimental for the
resolution in analytical applications of ITP. We investigate the impact of
convective dispersion, characterized by the area-averaged width of a sample
zone, for various values of the sample P\'{e}clet-number, as well as the
relative mobilities of the sample and the adjacent electrolytes. A
one-dimensional model for the area-averaged concentrations based on a
Taylor-Aris-type effective axial diffusivity is shown to yield good agreement
with the finite-volume calculations. This justifies the use of such simple
models and opens the door for the rapid simulation of ITP protocols with
Poiseuille counterflow
