A selectively removable closure for closing the open end of an NMR sample tube having an open end and a closed end of the invention includes a cylindrical proximal first and a distal second portion, both portions substantially congruent to a central axis, the second portion has a hollow bore extending therethrough, the hollow bore has: a first and a second distal section, a central section and a proximal section. The first distal section has an inside diameter sized to accept the outside diameter of a preselected size NMR sample tube substantially without an interference, the second distal section has an interference that ends into the central section with a ramp with an angle to the central axis greater than 70 degrees, the central section corresponds in a locked position on the NMR sample tube with a locking ring at least partially surrounding the NMR locking tube to form a secondary locking seal.