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A cold trap is a device used in vacuum systems that condenses all vapors except the permanent gases into a liquid or (more often) a solid.
Cold traps are cylindrical glass tubes, made of a special glass capable of surviving repeated exposure to low temperatures. The glass tube is fixated to an air intake adapter, also made of glass, where one end is connected to a vacuum pump, while the other to the system. The glass trap is inserted in a cooling fluid, usually liquid nitrogen, but other cooling baths can be used if liq. N2 is not available, such as dry ice/acetone, though for safety reasons liq. nitrogen is more commonly used.
Cold traps are sold by lab suppliers, but good quality ones aren't cheap, with a price ranging between 500-1000 €.
DIY cold trap
A simple cold trap can be made by adding a gas wash bottle or a similar tube with an air intake adapter in a flask containing a cooling bath. Since borosilicate glass will not survive repeated uses, this type of makeshift cold trap cannot be used many times. It is also pretty terrible at removing all solvent vapors that are being sucked by the vacuum pump.
If improperly opened or handled, the condensed liquid/gas from the pump may rapidly dilate and cause a violent explosion.
NEVER USE COLD TRAPS THAT PRESENT CRACKS, NO MATTER HOW MINOR!