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. 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.

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. 

• 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.