NMR tubes must be well-balanced in order to spin stably during the experiment. The tube on the left will probably not even fit into the magnet. However, smaller labels should not be used unless the wrap completely and evenly around the tube; this is only practical for the larger, 10-mm tubes. The best approach to labeling 5-mm tubes is to simply write a sample ID in permanent marker within the top 2 inches of the tube.

The tube on the right has been wrapped with parafilm in an attempt to make a tight seal. For normal situations, the regular plastic cap provides a reasonably good seal, and will prevent volatile compounds from evaporating over the course of the experiment (although they are not sufficient for long-term storage). If your sample is particularly air- or moisture-sensitive, you can seal the tube with a small amount of parafilm (enough to encircle the tube only once or twice). Wrap the film around the tube, stretching slightly. Rub the area with your fingers, and the heat generated will soften the waxy film, creating a tight seal.