means of heat bonding a compatible polymer between said metallic foil and said test plate another iteration of the invention is an apparatus comprising a film of elastomer having a top and bottom surface bonded to a polymer film on at least one of said surfaces and the polymer film is likewise thermally sealed to a multiwell test plate the elastomer and polymer film together also make a vapor resistant barrier in an alternative embodiment our invention is an apparatus comprising a planar sealing member composed of elastic material and disposed on a test plate having a plurality of fluid receiving wells a metallic foil or polymer film is adhered to the planar sealing member a rigid lid defined by a peripheral skirt and a top and bottom surface having a predetermined matrix of openings with extending through is attached to the test plate and the openings correspond to the wells in the plate alternatively the lid has a single large rectangular opening the sealing member and metallic foil or polymer film is either molded or welded to said bottom surface with the sealing member in direct contact with the bottom surface and the metallic foil or polymer film as a barrier layer exposed toward the open wells in the test plate the polymer film or metallic foil is heat sealed to the plate a polymer that is compatible with the plate 39 s chemical composition adheres the metallic foil to the plate by thermally bonding the apparatus is can also be characterized by having an elastomer made of a thermoplastic polymeric compound.