The subject invention pertains to a method for determining the affinity between binding partners, or a property of one of the binding partners dependent on the affinity, comprising the steps of: (i) contacting the binding partners, one of which is immobilised on a surface; (ii) oscillating the surface at increasing amplitude; and (iii) detecting a dissociation event. An analogous method can be used to separate a target analyte from a composition. The subject invention also pertains to an apparatus for determining the affinity between binding partners, and comprises: a surface (10) having one binding partner (16) immobilised thereon; means for oscillating the surface at increasing amplitude; and means (14, 15) for detecting a dissociation event.