In vacuum applications, a cold trap is a device that condenses all vapors except the permanent gases into a liquid or solid. The most common Objective is to prevent vapors being evacuated from an experiment from entering a vacuum pump where they would condense and contaminate it. Particularly large cold traps are necessary when removing large amounts of liquid as in freeze drying. Pumps that use oil either as their working fluid (diffusion pumps), or as their lubricant (mechanical rotary pumps), are often the sources of contamination in vacuum systems. Placing a cold trap at the mouth of such a pump greatly lowers the risk that oil vapors will back stream into the vacuum cavity. Care should be taken when using a cold trap not to condense liquid oxygen (a light Blue liquid) into the cold trap. Liquid oxygen is potentially explosive, and this is especially true If the trap has been used to trap solvent. Liquid oxygen can be condensed into a cold trap If a pump has sucked air through the trap when the trap is very cold.
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