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A septum or rubber septum, sometimes referred to as sleeve septum or seal septum (plural: septa) is a chemically resistant rubber membrane used in labs often in air-free techniques for the transfer of a substance (usually liquid or gas) without contact with air, usually using a syringe with needle. They can also be employed when working with toxic gasses, though this is less common.
The rubber septum is a cup-shaped silicone or butyl rubber lab item, with a serrated or non-serrated plug that fits in a ground glass joint and the upper rubber cup gets folded over outer glass joint head, to assure a good hermetic seal. Using a needle, a hole in punctured in the middle of the seal, from where the syringe or a cannula can be used to insert or extract reagents or gasses. To prevent wearing, try to use the same hole every time you operate any experiments.
Other forms of rubber septa are commonly used in air-sensitive reagent bottles or as lids for medical injections. These appear like a typical rubber stopper.
Rubber septa can be bought from lab suppliers or online.
If you want to reuse septa from old medical bottles, make sure to wash them properly and check to see if the hole is not widened.