Sample focusing method and device for field-flow fractionation techniques that lead to improved detection, improved separation resolution, and a compressed sample plug while permitting a more straightforward quantitation of peaks and reliable large volume injections. The method and technique can be implemented in separations that are performed by a variety of field-flow fractionation techniques, including thermal FFF, electrical FFF, sedimentation FFF, gravitational FFF, dielectric FFF, photophoretic FFF, flow FFF, asymmetric flow FFF, and symmetric flow FFF. The sample focusing device can be integrally built into a separation channel or it can be manufactured as an attachable independent piece.