How to Tip-Off a Sample

It is a common practice to seal a sample in a closed glass tube for storage or for further study by means of EPR (electron paramagnetic resonance) or NMR (nuclear magnetic resonance). Although it is not difficult to tip off a sample, if done poorly, the NMR tube will spin very badly. If done badly, the sample may be destroyed along with hours, weeks, or months of work. 

Glass is a very poor conductor of heat, and therefore it is not possible to heat one side of a tube to be tipped off and expect to be finished. The only way to properly tip-off a sample tube is to heat it uniformly in the area to be tipped—that is, one side and then 180° to that area (see Fig. 8-6). The gradual heating is done by alternately heating opposite sides in a frequent and uniform manner. You must try to heat the entire area gradually and uniformly. Glass becomes soft before it becomes juicy. If one side becomes juicy too fast, there is a greater chance for the wall to suck in or blow out. 

Whenever possible, before work begins, the sample tube should have a constriction made, where the tip-off will be made. The smaller the internal diameter, the 

The proper sequence of steps for making a tip-off is shown in series by Fig. 8.7 and listed in the following sequence  

As the tube begins to collapse, the walls begin to thicken. 

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