These stoppers can provide a solution for those difficult situations when a standard tube cap cannot be used, as, for instance, when space is a limiting factor.  This problem occurs most often in solid state NMR work, when unique, compact or experimental probe designs may be encountered. The stoppers seal within the inner surface of the sample tube, and therefore do not extend beyond the outer periphery of the sample tube as does a standard tube cap.  They are made of a soft, resilient silicone rubber that forms a very effective seal without applying excessive force to the glass sample tube, thereby minimizing tube breakage.  If desired, the stoppers can be easily trimmed to length with a knife blade or scissors.  Being made from silicone rubber, they have a high degree of inertness, solvent resistance and high temperature capability (up to 200˚C).