Schlenk lines and vacuum lines provide a convenient means of manipulating air and water-sensitive materials without the use of an inert atmosphere glove box. There is no strict definition that divides the term "Schlenk line" from "vacuum line", "chemical transfer line" or "vacuum transfer line", but in some general distinctions can be drawn:
Vacuum lines usually have a better vacuum (10-4 to 10-7torr) than Schlenk lines (10-2 to 10-4torr), because vacuum lines typically have a diffusion pump. A way of delineating these would be to use the terms "Schlenk line" and "High vacuum line".
Vacuum lines usually connect to the experimental apparatus with joints, which provide a better seal and vacuum than the simple rubber pressure tubing connections used on Schlenk lines.
Manipulations involving the measurement or condensation of gases are usually performed on a high vacuum line. Operations involving cannulas and counterflow techniques are usually performed on Schlenk lines. The truth of this statement varies from laboratory to laboratory!
A Schlenk or high vacuum line consists of a glass manifold that has several stopcocks, valves or openings in addition to a connection to a vacuum source (typically a mechanical and/or diffusion pump). Having several ports on the line is convenient because several different flasks or reaction vessels may be used simultaneously. For example, gases can be vacuum transferred from one flask to another or several reactions may be run at the same time.
These lines may be of a dual or single manifold design. In a single manifold design, the manifold's main purpose is for vacuum. Here is an example of a single manifold design which uses all-Teflon(tm) valves and O-ring joints to ensure a good vacuum seal. Each port has its own independent bubbler, so up to three reactions can be stirring under nitrogen at one time.
A dual manifold design provides one manifold for vacuum and another for nitrogen or a reactant gas. A connection between the two manifolds permits the nitrogen manifold to be easily evacuated.
Shown below is a diagram of a dual manifold high vacuum line. Note that the lower manifold is for vacuum and the upper one is for nitrogen.
Two features worth noting in this drawing are:
Main Trap. When the line is running, this trap is immersed in a liquid nitrogen dewar. This stops volatile or corrosive vapors that have escaped the pre-trap and prevents them from entering the pump. New pumps can cost one to two thousand dollars, so protecting the pump is its ONLY role. NEVER PULL SOLVENTS INTO THE MAIN TRAP!
Pre-Trap. This is an additional liquid nitrogen trap to condense vapors or gases from the vacuum line. It is also used to collect solvents that are removed from reaction flasks. If you are removing a lot of solvent from a reaction the nitrogen level will drop quickly, so be sure to keep an eye on it.
Click on any of these manifolds for more information or to order:
The use of high vacuum and Schlenk lines poses serious risks for injury or death. Some of the most important safety considerations include:
Explosion - Explosions can occur in a number of ways, including:
The use of pressurized gases. High vacuum manifolds are often connected to an inert or reactant gas supply line. One must ensure that the vacuum system is not closed when the gas supply is opened - there MUST be a source of pressure relief such as a bubbler. The pressure must be monitored with an electronic gauge, manometer or bubbler; make sure the valve to the pressure reading device is open to the manifold!
Always TRIPLE CHECK that the manifold and supply line are connected to pressure relief (and your pressure sensor) before opening the gas supply. ALWAYS use an appropriate pressure regulator to avoid opening the line to more than 1 atm of pressure at any time.
Condensed gases. Some gases, such as carbon monoxide and ethylene, are easily condensed into a liquid nitrogen-cooled trap. If the coolant level drops or you remove the nitrogen dewar without providing a means of pressure relief, the liquid may convert back to vapor. For example, just 10 mL of liquid CO (b.p. -191.5 °C) corresponds to 6.5 liters of gas. In a vacuum line with an internal volume of 500 ml the internal pressure would be 13 atm, more than enough to shatter the manifold with explosive force!
Runaway reactions. Some reactions can occur violently and evolve large quantities of gas. Always provide a source of pressure relief!
Heating a closed system. Never heat a vessel on a vacuum line without being open to a bubbler. Vacuum distillations must always have a pressure relief/regulator such as a manostat.
Explosions of glass vacuum lines have lead to death and serious injuries. Always wear your safety glasses/goggles to protect your eyes. Consider placing your vacuum manifold in a fume hood or behind a sliding blast shield to further protect yourself.
Implosion - An unseen star crack or stress in a glass manifold can give rise to a catastrophic failure of the line while under pressure. Likewise, hitting the line with apparatus can cause a failure. While not usually as serious as an explosion, implosions generally involve sharp pieces of flying glass. And if the materials you are using are flammable or pyrophoric - watch out!
Liquid oxygen - If a constant stream of air is pulled through a vacuum trap cooled with liquid nitrogen, liquid oxygen may condense in the trap (see item 1b above!). Liquid oxygen is exceedingly dangerous and reacts violently with most organic substances, including Teflon tape, vacuum grease, and organic solvents. Even without this consideration, the pressure generated when a small quantity of liquid oxygen vaporizes in a small space such as a vacuum manifold generates enough pressure to shatter the line.
Should you lower the trap on your line and find a pale blue liquid, immediately replace the trap and back away. Consult your supervisor IMMEDIATELY. Warn others of the danger, posting signs if necessary.
Disclaimer: This section discusses general dangers associated with high vacuum and Schlenk lines and is not meant to be a comprehensive reference to dealing with or assessing the hazards. There are many circumstances that have not been foreseen or discussed here and you should recognize the inherent danger in relying solely on this information!
This page was last updated Tuesday, November 16, 2010.
This document and associated figures* are copyright 1996-2013 by Rob Toreki. Send comments, kudos and suggestions to us via email. *Some of these figures are adapted from the Chemglass, Inc. catalog and have been reproduced with permission.