The present invention provides devices and methods to separate and concentrate target species. In some embodiments, a punctuated continuous microchannel or parallel processing (array-based) separations are provided, the microchannel having a plurality of sequential, constrictive insulating features to form a plurality of reservoirs including a first, second and third reservoir, and a plurality of constricted passageways including a first constricted passageway that connects the first reservoir to the second reservoir and a second constricted passageway that connects the second reservoir to the third reservoir. A voltage is applied to the microchannel to create different electrical fields and/or different dielectrophoresis (DEP) gradients at each of the plurality of constricted passageways in order to separate species that have differing ratios of electrokinetic mobility to dielectrophoretic mobility.