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 tightened when a white ring is visible inside the glass (Figure 2). Tightening it further will not lead to a better seal, but rather increase the risk of damaging the glass. Also, keep the sealing surfaces clean of any powder to ensure proper sealing.