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.

The tutorial below is focused on the basic steps involved in making a successful seal under vacuum conditions. There are many variations of this technique applicable to the many samples and conditions the glassblower may be working with.

The Sealing Steps



1.

Slowly warm the area of the constriction with a soft bushy flame. 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.



2.

After the pre-heat step re-adjust the torch setting to a smaller  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.



3.

Go to the opposite side of the glass that was heated in step two and repeat the process. Do not try to complete the seal off yet.



4.

Heat the rear portion of the constriction and allow the wall to collapse.



5.

Heat the front portion of the constriction allowing the glass to collapse and complete the seal. Keep the torch flame on the glass and go to the next step.



6.

Keeping the glass hot, pull down on the sample bulb. This step will further reduce the diameter of the constriction (which should be solid glass at this point). You can now tip off (melt away) the sample bulb from the main tube.



7.

The finished tube will look similar to this. Ideally, the mass of glass where the seal has been made will not be overly thick.

Some sealing suggestions

• Keep the flame small. It is very easy to over-heat the glass.
• Control - If your glass is over-heated over a large area the risk of implosion is much higher.
• Most implosions will occur just above or below the constriction. The glass wall /id ratio is greatest at this point - allowing the glass wall to invade the inner diameter at a greater depth, which produces thinning glass walls leading to implosion.
• If safety permits - continue to pump on the sample tube during the seal off. Gases produced during the heating process will be drawn off.



Quartz sample tubes

Prepare the quartz sample tube by making  a small indent in the tube body. Insert a solid quartz plug (rests on indent) after loading sample. Evacuate. Heat and collapse the quartz sample tube onto the solid plug. With care a vacuum tight seal can be obtained. Remove the entire  sample tube from the vacuum system. The sample bulb  can be removed from the tube top by using a wet saw.