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 three sections: a distal section, a central section and a proximal section. The distal section has an inside diameter sized to accept the outside diameter of a preselected size NMR sample tube substantially without an interference, the central section is sized to provide a compliant interference fit with the outside diameter of the preselected size NMR sample tube, and the proximal section is sized to accept the outside diameter of the preselected size NMR sample tube without interference. As a user inserts the open end of the NMR sample tube axially into the closure, the distal section guides the tube into the closure, the central section provides a user perceptible resilient resistance to the passage of the tube and the movement of the tube open end into the proximal section allows the user to perceive a lessened resistance to the movement of the tube followed by seating the tube open end against the first portion of the closure.