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Filtration is the physical separation of a component or components (which may be solid, liquid, gel, etc.) from a matrix stream (which is usually a fluid), without causing any other changes such as chemical reactions. In the laboratory, filtration is most commonly used to separate a solid (such as a precipitate) from a liquid, either by gravity feed or under vacuum by use of semipermeable barrier placed perpendicular to the fluid flow direction. Laboratory filtration media most commonly used are filtration papers, filtration frits, and membranes.