A common application in microfluidic devices is on-chip capillary electrophoresis (CE). In this process, sample species are transported by electroosmotic flow and separated based on their electrophoretic mobilities. Separated analytes are typically detected using laser-induced fluorescence. It has been found that the sample shape and size, which is critical to the later detection processes or the quality of other analytical techniques, depends on many parameters, such as the sample diffusion coefficient, the applied voltages, and the electrical conductivity difference between sample and buffer. The conductivity difference can alter the electric field strength, which is the driving force behind both the electroosmotic bulk flow and the electrophoretic velocity of individual species. Therefore, the manipulation technique is required to consider the transport processes with conductivity differences. A numerical model presented in this paper is used to simulate the sample transport process with the consideration of conductivity gradient in order to develop the sample manipulation techniques. There are two situations studied here, which are sample pumping (where bulk transport is increased and analyte separation is delayed using a relatively high conductivity sample), and sample stacking (where bulk transport is decreased and analyte separation is expedited using a relatively low conductivity sample). The effects of applied electrical potential, sample diffusion coefficient and the extent of conductivity difference on the sample control are investigated through the developed model. The simulation results show that the sample transport with the consideration of conductivity gradient differs significantly from that of uniform conductivity case.

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