Deuterium filters were components of the matter-injection systems of NX class starships, responsible for ensuring that any impurities in the deuterium entering the warp core were eliminated. In 2161, while performing routine purging of the injectors, Commander Charles Tucker queried a member of his engineering team as to whether or not the deuterium filters had been cleared, to which the officer responded that all but three were completed. (ENT: "These Are the Voyages...")