To prepare NMR samples inside a glovebox, NMR tubes with a Young-type PTFE cap are recommended (Figure 1). They provide long-lasting leak-tightness and remain tight under vacuum. This is important for bringing samples back into the glovebox through the antechamber.

As with all PTFE caps and taps they are properly tight if a white ring is visible inside the glass. Tighten it more will not lead to better tightening but rather increase the risk of damaging the glass.