The present invention is based on the realization that the bonds between a target molecule, or a target molecule attached to a particle, and a surface, can be ruptured by mechanically oscillating the surface at increasing amplitude, leading to detachment of the target molecule or particle from the surface. The required acceleration, and hence force, will depend on a variety of factors, including the mass of the molecule or particle, the nature of the bond to the surface and the geometric shape or size of the target molecule or particle. The present invention may therefore be used to separate or to size different target molecules, or to detect their presence.