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 fitted with 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 throughout 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.
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.