(8a) Advantages of Microgradients for Steady State Separations

Core to all separations science is resolution: causing important components to move in time or space away from each other. Although a description of core processes controlling resolution across all separations systems does not exist, the most successful techniques are considered to be electrophoretic in nature. Early demonstrations of capillary- and microchip- based electrophoresis have shown absolutely extraordinary resolution using high fields showing deuterated vs. hydrogenated and sub-millisecond separations. These results were obtained in systems operating in a linear mode—spreading and diluting a mixture out along a single axis. Our work uncovers a radically new approach enabled by the short length-scale of microdevices which exploits local field gradients. The core to this discovery is setting up punctuated microgradients separated by flat field zones. This uniquely minimizes the effects of diffusion by dramatically increasing the local restoring forces. As a result, not only does the resolution increase by a factor tied to the strength of the local gradients, but can be accomplished this in parallel while increasing concentration of the targets (focusing). This opens up several new venues for exploitation, including dynamic and programmable collection of specific analytes at higher resolution than is available from any traditional technique in an array format. Speeds of separations are also dramatically increased. We describe the underlying theoretical principles and provide examples of flow/electric field and electrokinetic/dielectrophoretic devices in a microfluidic format. The underlying models extend to any force which can be induced into dispersed microgradients in the presence of a static counter field.