Transition zone dynamics in combined isotachophoretic and electro-osmotic transport

The present study focuses on the interplay of isotachophoresis (ITP) and electro-osmotic flow (EOF). While EOF is commonly suppressed in ITP applications, we investigate scenarios of the combination of both EOF and ITP. Experimental results of ITP/EOF experiments within cross-patterned polymer chips show characteristic deformations of fluorescent sample zones sandwiched between leading and trailing electrolytes. A changing curvature of the deformation is observed during ITP/EOF runs, but overall a well defined sample segment is maintained after a transport over a few centimeters. By means of numerical modeling we study the deformation attributed to the mismatch of EOF between leading and trailing electrolytes. The model results are found to qualitatively agree with our experimental findings. We introduce the ratio of the EOF velocities in the leading and trailing electrolyte, expressed via the respective mobilities, as a dimensionless parameter gamma and show that in the case where electro-osmotically induced convection dominates over electromigration the deformation width scales as 1-gamma. In particular, we find that the EOF-induced dispersion virtually vanishes for the case gamma=1. Hence, in this particular case isotachophoretic self-sharpening and electro-osmotic pumping can be combined without any detrimental effects on sample transport even for large EOF velocities.