A Schlenk line is an essential piece of equipment used for the manipulation of air and moisture sensitive compounds. They consist of a vacuum manifold which is connected to a vacuum pump, and an inert gas manifold which is connected to a source of purified and dry inert gas (typically argon or nitrogen). The inert gas is vented through a bubbler (either oil or mercury), allowing the inert gas pressure to be monitored. Alternatively, a gas regulator can be used in tandem with a pressure relief bubbler to monitor and adjust the inert gas pressure within the Schlenk line. A liquid nitrogen trap is used to condense solvent vapours and protect the vacuum pump. The two manifolds are connected by greased double oblique glass stopcocks or Teflon (J. Young’s) taps allowing connected glassware to be easily placed under vacuum or inert gas. The vacuum manifold is typically connected to a manometer to measure the pressure – a good vacuum pump and Schlenk line should get down to 1 x 10⁻³ mbar with ease.
‘Firing up’ the Schlenk line
Step 1: Attach a clean and dry solvent trap to the Schlenk line ensuring that the male ground-glass joint is sufficiently greased. Twist the solvent trap to evenly coat the grease and then secure in place with a clip.
Step 2: Ensure that all of the double-oblique stopcocks are in the closed position (horizontal) and twist the vent valve clockwise until sealed (once you can feel resistance – be sure not to over-tighten teflon taps!). Turn on the vacuum pump.
Step 3: The vacuum pump typically takes around 5-10 minutes (possibly more) to warm up, but after this time the manometer may read around 2-3 x 10⁻² mbar. Submerge the solvent trap in a Dewar of liquid nitrogen – the easiest option is to slowly raise a half full Dewar of liquid nitrogen around the solvent trap and then top it up with a second Dewar. Wrapping a tea towel or putting some other insulating material around the top of the Dewar helps to minimise liquid nitrogen evaporation, however it is important to make sure the nitrogen level is topped up through the day.
Step 4: The pressure in the vacuum manifold should be around 1 x 10⁻³ mbar with the liquid nitrogen in place. The inert gas supply can now be opened and the Schlenk line is ready for use.
If the Schlenk line has just been assembled after cleaning, it is necessary to purge the inert gas manifold by passing inert gas through it for 15-30 minutes prior to use. Some Schlenk lines are designed to allow the inert gas manifold to be fully evacuated without any risk of oil suck-back or regulator damage.
‘Closing down’ the Schlenk Line
Step 1: Ensure that all Schlenk flasks connected to the line are under inert gas and that all taps and stopcocks are closed.
Step 2: Open the vent valve by twisting anti-clockwise and then immediately turn off the vacuum pump. You will hear a whistling noise as the vacuum manifold is re-pressurised with air. Some Schlenk lines have an additional Teflon tap or stopcock before the vacuum pump which can be closed before re-pressuring the line.
Step 3: Remove the Dewar of liquid nitrogen and then carefully remove the solvent trap.
Step 4: Allow any collected solvent to thaw before discarding into an appropriate waste container.
Double Oblique Stopcocks vs. Teflon Taps
Schlenk lines come in all shapes and sizes, but one major difference commonly encountered in the lab is the use of either glass double oblique stopcocks or Teflon taps. Both have their advantages and disadvantages which will be discussed in further detail below:
Double oblique stopcocks are considered safer than Teflon taps because it is not possible to simultaneously have a stopcock open to both vacuum and inert gas. They also tend to be quicker and easier to use than Teflon taps but may not hold vacuum as well. Double oblique stopcocks are often manufactured specifically for each port on the Schlenk line meaning that replacement parts are less accessible and also means that greater care is required during cleaning (i.e. avoid using base baths for cleaning Schlenk lines with ground glass joints). Double oblique stopcocks must be evenly greased to ensure a good seal – this may contaminate reactions or even be incompatible with certain reagents or conditions employed in a reaction.
Teflon taps typically hold better vacuum than double oblique stopcocks and also avoid the use of excess grease which may be incompatible for some chemistry. They also give finer control which may be useful for drying solids or removing solvent under vacuum. One major disadvantage of Teflon taps is that it is possible to accidentally have both the vacuum and inert gas taps open simultaneously which can result in suck back of the oil or mercury used in the bubbler. Teflon taps are also prone to being over-tightened and can result in a broken Schlenk line.
Tools of a Schlenk Line Chemist
- Schlenk flasks are reaction vessels used for air/moisture sensitive chemistry. They are typically fitted with a side arm with a Teflon tap or ground glass stopcock allowing the flask to be evacuated or filled with inert gas.
- Ampoules are vessels equipped with a Teflon tap and side arm gas inlet, primarily used for the storage of anhydrous solvents or air/moisture sensitive liquid reagents/solutions.
- Septa (singular: septum) are rubber stoppers that provide an airtight seal, preventing the ingress of air or moisture, that can be pierced by needles or cannulae allowing liquids to be added or removed.
- Cannulae (singular: cannula) are hollow flexible tubes used to transfer liquids between reaction vessels. They are commonly made out of stainless steel or Teflon for chemical resistance, and are available in a range of gauges depending on the quantity of liquid to be transferred.