Abstract: A microplate assembly comprising a multi-well microplate, a plurality of vent caps and a porous vent film. The microplate includes a frame that houses a plurality of open wells in a rectangular array. Vent caps mount on the microplate to seal and vent the wells. When the vent caps are coupled to the wells, an interior volume is formed in each well. The wells function as a vessel for liquid samples that occupy predetermined spaces within the interior volumes. The liquid samples remain within its predetermined space for all orientations of the microplate assembly. The vent cap comprises an array of well inserts. Each well insert comprises a sealing plug and a vent tube. The well inserts are interconnected by a perforated web. The sealing plugs form a seal at the mouth of the open wells. The vent tubes, which are fixed to the sealing plugs, terminate in a vent that communicates with the interior volumes outside the predetermined spaces occupied by the liquid samples.