the sum of the volumetric fluid flow rates through the auxiliary channels be less than that through the separation channels since the driving potential across the separations and auxiliary channels as well as the properties of the fluid are the same the flow rate through the channels scales as a l thus one factor that can be considered in proportioning fluid flow rate through the separation channel and associated auxiliary channels is the cross sectional area of the channel a divided by the length l of the channel it has been found that successful application of the invention can be obtained if the ratio of flow rates through the sample channel and the auxiliary channels is u2248 10 1 thus the rate of fluid flow through the auxiliary channels can be controlled by changes in the cross sectional area of the channels or their length such as by configuring the auxiliary channels in a spiral or serpentine pattern it should be noted that this ratio is for the sum of the flow rates through the various channels which means that individual auxiliary channels can have different volumetric flow rates providing the sum of flow rates through the plurality of auxiliary channels falls within the proper value a second approach to adjusting fluid flow rate is to fill the auxiliary channels with an appropriate medium that can be a particulate or porous medium to provide.