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Home Page --> Tutorial Lesson 14, Sealing Glass Ampules or Samples under Vacuum

Tutorial Lesson 14, Sealing Glass Ampules or Samples under Vacuum
Lesson 13
(Bending Glass)
Lesson 15

Sealing Glass Ampules or Samples under Vacuum

line drawing of a sample ampule

The Ampule or Sample Tube

Glass sample tubes may come in a variety of sizes and lengths. Your tubing 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. Likewise, if a hole is introduced into a cryogenically cooled tube during sealing, liquid oxygen can condense inside the tube; should the tube then be sealed, the tube will explode upon warming. This risk is not theoretical - a Nobel Laureate known personally by one of this site's editors lost an eye to an improperly sealed NMR tube. Consider all evacuated sample tubes to be potential "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. A generally accepted nomogram of allowable pressures for glass tubing may be found at this link: Glass Pressure Nomogram.

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.

Warning - As discussed above, there are many safety issues that need to be addressed before performing sample seal-offs under vacuum. High pressures, chemical properties/exposure and the process itself can be hazardous. A review of the sample material properties under vacuum and heat should be conducted prior to any attempts of a vacuum seal off. Consult with your safety and health professional before attempting any of these steps in the laboratory. Use appropriate precautions such as proper eye protection and/or safety/blast shields as needed, particularly when dealing with condensable gases, volatile materials, and/or cryogens.

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