Sealing Glass Ampules or Samples under Vacuum
By Joe Walas, East Carolina University
Department of Chemistry Glassblowing Facility

The Ampule or Sample Tube

Glass sample tubes may come in a variety of sizes and lengths. Tube selection will depend on the type of sample and the properties it will have when under vacuum. Of particular importance are samples that are cryogenically cooled and sealed under vacuum. Samples that have been cooled, evacuated and sealed properly may contain very high positive pressures when returned to normal room temperatures. In essence, these evacuated sample tubes can become "mini-bombs". Great care should be used when handling these and all sample tubes. Potential sample pressure calculations should be determined before hand and these calculations should be used in the selection of the glass tubing. Most sample tubes or ampules will have a constriction at some point to aid in sealing off the lower bulb. The constriction will narrow the opening into the sample chamber, but will maintain or increase the wall thickness of the glass tube in the seal zone. This is an important feature for the novice glass worker.

Slowly warm the area of the constriction with a soft bushy flame. (If the heat is applied too quickly there is always the danger of thermal shock.) Moisture may be present and visible on the interior walls of the glass when this heat is applied. Allow the vacuum pump time to draw off this moisture. Depending on the sample properties, this may be a good time to drive off the moisture in the sample bulb as well.

After the pre-heat step reset the torch to produce a smaller compact flame. The flame will be focused on one side of the constriction in this step. Keep the flame moving back and forth - do not hold it on one spot - the glass will implode if you do! Watching the glass wall carefully, continue to apply the heat until the glass starts to move. The goal is to allow the glass to collapse to about 1/2 the diameter of the opening - but no more.

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