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 .