Abstract: The straw comprises a tube (41) and a plug (42) consisting of a one-piece cylinder of microporous material, permeable to gases and liquid tight. The microporous material is a sintered self-sealing microporous material with at least a solid portion (42) having a diameter matching the inner diameter of said tube (41), said solid portion being in direct contact with said tube (41), said plug having in the tube a permeability to gases when it is dry such that if one end of the tube is placed in communication with a pressure source 150 mbar above atmospheric pressure and the other end of the tube is opened to atmospheric pressure, the air flow per unit area through the plug is in the range of 7.5 to 40 ml/min.mm2.