Diffusion dependent focusing regimes in peak mode counterflow isotachophoresis

Abstract

We present an analytical, numerical, and experimental study of pressure driven counterflow isotachophoresis (ITP). We study solutions to the Nernst-Planck equations in the axi-symmetric and radially dependent case, in the leading order of negligible body forces. We provide a simple model that describes the ITP interface shape for Poiseuille-type counterflows, and an asymptotic model which captures two distinct sample focusing regimes of peak mode ITP. We validate the existence of these regimes using numerical simulations and map the conditions under which each of the focal regions dominates. In particular, we demonstrate numerically that a species diffusivity is a key parameter determining its focusing regime. We experimentally show that this allows spatial separation of co-focusing species having distinctly different diffusivities. We further demonstrate that while dispersion associated with counterflow is typically considered to reduce peak concentrations, certain focusing regimes allow a net gain in sample concentration over the non-dispersed case.