Pipette tip extension for treating a sample with a liquid attachable to a pipette tip

An assembly comprising

• a pipette tip for aspirating and/or dispensing a liquid,
• a pipette tip extension attached to the pipette tip, the pipette tip extension comprising
• a proximal end, a distal end, and an exterior wall extending between the proximal end and the distal end, the exterior wall having an outer side and an inner side and forming at the proximal end a reception aperture for inserting the pipette tip, and at the distal end a dispense aperture,
• an inner cavity enclosed by the inner side of the exterior wall,
• a distance element connected to the inner side of the exterior wall, for positioning the pipette tip within the inner cavity and for establishing a fluid uptake area adjacent to the inner side of the exterior wall, the fluid uptake area extending from the dispense aperture towards the reception aperture and being in fluid connection with the surrounding atmosphere at the reception aperture, and
• a constriction element which is connected to the inner side of the exterior wall of the pipette tip extension for controlling an insertion depth of the pipette tip within the pipette tip extension, the constriction element defining a gap with a gap height (h) between a distal end of the inserted pipette tip and the distal end of the pipette tip extension, the constriction element comprises an end stop for the pipette tip and is configured as a fluid-permeable sieve-like structure.