top of a coaxial conical surface constituting the bottom of the cavity the other channel 13 extends perpendicular to the axis of the cavity 10 near the surface 11 the cavity 10 comprises near its orifice a bore stepping in which a seal 14 is engaged through which the piston 15 of the pumping module 9 slides leaktightly this piston 15 which consists of a plunger having a cylindrical shape has on one side a conical end 16 with a shape substantially complementary to that of the bottom of the cavity 10 and on the other side a flat terminal surface in which opens a diametric cylindrical groove 16 u2032 centered perpendicular to the axis of the piston 15 in this example the connecting and guiding module 18 consists here of a metallic block 17 for example in aluminum alloy with a parallelepiped shape whereof two opposite surfaces f 1 f 2 serve as assembly surface for the pump module 9 and for the actuating module 2 respectively and a lateral surface 19 which serves as a fixing surface of the pump unit on a support for example in an analysis machine the block 17 comprises a coaxial central bore 20 opening into the surfaces f 1 f 2 this bore 20 comprises on the side of the surface f 1 a bore stepping extending over approximately u00bc of its length in which a tubular section 21 in a.