Each embodiment includes a central sample reservoir and a plurality of satellite reservoirs. In a first embodiment, a first electrode in electrical contact with the central reservoir is charged and second electrodes in electrical contact with the satellite reservoirs are sequentially charged, thereby pI filtering molecules in the central reservoir into the satellite reservoirs. In a second embodiment, the central reservoir is configured to rotate so that molecules in a sample in the central reservoir are centrifugally pI-filtered into the satellite reservoirs. In a third embodiment, first and second electrodes proximate opposite first and second satellite reservoirs, respectively, are charged. Some molecules in a sample are pI filtered into the first and second satellite reservoirs. Third and fourth electrodes proximate opposite third and fourth satellite reservoirs, respectively, are then charged. Some molecules in a sample are pI filtered into the third and fourth satellite reservoirs.