Abstract: A microfluidic system for the concentration of particulate matter comprises a first reservoir (1) containing a buffer solution, a second reservoir (2) containing a analyte, and a third reservoir (3) containing beads suspended in a liquid. Microchannels link the reservoirs and an expanded portion (4). Hydrostatic pressure is applied to the reservoir (3) containing the beads while an electro-osmotic force (EOF) is applied between the reservoir (1) and the reservoir (3) to establish counter flows of liquids. A vortex forms in the flared portion (5) due to the counter flow of the liquids and the beads are concentrated in the vortex. By switching the EOF between reservoirs (1) and (2) the buffer solution can be replaced by the analyte to enable an analysis to be performed.