Abstract: The tip has intermediate its end at least two axially spaced projections which are formed on the wall of its axial bore to extend radially and part way into the bore to define therebetween a reaction chamber in the bore of the tip. A spherical receptor element is mounted in the reaction chamber for limited pulsating movement therein in response to the movement of a fluid sample into and out of the tip. Additional projections may be formed in the bore to increase the turbulence created by the sample as it moves through the tip. The spherical element is coated with a ligand having a specific affinity for a target molecule in the sample.