Abstract: An apparatus for sealing fluid containing vessels comprising a sheet of elastomer forming a sealing member having a top and bottom surface, and disposed on a plate containing a plurality of wells for storing fluids. A product that can seal the wells of a multiwell plate, and has the aspects of being vapor resistant, heat sealable, and able to be manipulated by automated analytical equipment. The sealing material is constituted of an elastomer made of synthetic rubber and a layer, comprising a polymer film or a foil attached to one side of the elastomer to form a vapor barrier. The combination of these elements produces a sealing mat that couples the barrier properties of the polymer film or foil with the sealing properties of the elastomer septum.