Model of isotachophoresis (displacement electrophoresis) in tapered capillaries.

The principle and theoretical description of boundary focusing under electrophoretic migration in a tapered capillary are presented. The model describes the gradual volume decrease of a moving boundary between weak electrolytes or ampholytes. The relations between the departure from the local steady state, the ratio of inlet to outlet capillary cross section, the effective mobilities and the selectivity are formulated. They allow the selection of operating conditions such that the departure from the local steady state can be decreased to an acceptable level. The relations predict that isotachophoresis (displacement electrophoresis) in a capillary with a shallow taper can maintain a constant ratio of actual boundary width to local steady state width, provided that the local capillary cross section decreases linearly with length. The significance of some assumptions needed for the model formulation is analyzed. Numerical examples are given indicating the method feasibility.