Like any piece of lab equipment, several routine problems are encountered when using a Schlenk line. Common problems and ways to identify and address these issues are outlined below:
Poor vacuum pressure
If the vacuum pressure is failing to get as low as expected, then this primarily suggests that there is a leak somewhere within the Schlenk line. This is easily identified when using a manometer (vacuum gauge), but can also be diagnosed if solvents are not being removed under vacuum as readily as normal, or if you observe oil being sucked from the bubbler into the inert gas manifold. This generally occurs when there is a poor seal, which is a frequent problem when using greased double oblique stopcocks. It may be possible to identify which stopcock is the problem by individually twisting each stopcock to inert gas and seeing whether the manometer reading changes to a significant degree, or by blocking the tubing of each port separately (by connecting/inserting something appropriate). If necessary, the stopcocks can simply be cleaned and regreased to ensure a more uniform seal. For Teflon-tapped Schlenk lines, this can also be problematic if the seal has worn down over time on the piston (often due to over-tightening), or if some residue is trapped between the glass seating and Teflon tap. This can be fixed by cleaning the tap (with EtOH and a Kim Wipe), or replacing the tap completely. Isolating and testing individual parts of the Schlenk line (if possible) is generally the best way to identify the route of the problem.
The vacuum pressure can also fail to get as low as expected if the solvent trap is blocked in some way, or needs to be topped up with liquid nitrogen to prevent evaporation of the condensed solvent/gases. The former issue often occurs when removing solvents that freeze easily (such as benzene, cyclohexane and 1,4-dioxane) since these condense easily and therefore towards the top of the trap. It may be necessary to shut down the Schlenk line, then thaw and empty the solvent trap to overcome this problem. This issue can be avoided by using an external solvent trap when removing solvent.
If the vacuum pressure is still poor after addressing and/or eliminating these previous issues, it may point towards a more serious concern such as problems with the vacuum pump (which would warrant maintenance or a full service by a trained professional), or a more severe leak or damage to the Schlenk line which can be identified using a Tesla coil gun (see cool video here).
Note: If your Schlenk line is not equipped to have a manometer directly attached to the vacuum manifold, it is still possible to test the vacuum pressure by attaching the manometer to the tubing with a suitable hosing adapter.
Slow cannula transfers
Slow cannula transfers can occur for several simple reasons including leaky septa, clogged cannulae or blocked bleed needles. These issues are easily addressed by replacing the septa, or cleaning/unblocking the cannula and bleed needle. If the cannula transfer fails to initiate altogether, then it may be necessary to increase the inert gas pressure (by partially blocking the gas outlet before or after the bubbler) or raise the position of the transfer flask (or lower the position of the receiving flask). Cannula filtrations can be slow when attempting to remove or isolate very fine solids. In this case, it may be necessary to replace the cannula filter, and allowing the solids to first settle before slowly lowering the cannula filter into the solution/suspension to avoid further blockages.
I sucked something into my Schlenk line 😥
We’ve all been there, don’t worry! The first thing to do (after immediately closing the tap to vacuum to avoid it happening even more) is assess whether you can and/or should continue further work on the Schlenk line. If you were in the process of drying a fine solid under vacuum and anticipate that it may get sucked into the line again, then it is often best to continue drying it anyway and clean the Schlenk line afterwards. To avoid this in future (if it is a persistent problem with a particular compound or happens far too often), then an external trap can be used, or alternatively a hosing adapter with a glass frit can be attached between the Schlenk flask and vacuum manifold. The stopcock should also be opened very slowly to vacuum and then incrementally opened further over time.
Solvent can also get sucked into the Schlenk line. This often happens when it bumps, and again the use of an external trap, as well as slowly opening the stopcock to vacuum whilst ensuring adequate stirring are the key measures to avoiding this.
Seized stoppers and stopcocks
Greased ground glass stoppers and stopcocks can often seize, either if they have not been used for a long time or more commonly if inadequately greased when put under dynamic vacuum. Several options are available to separate seized ground glass joints, however it is strongly advised to remove the contents of the flask first, if this is possible and necessary. Gentle heating with a heatgun or blowtorch can be effective since this slightly expands the glass and also loosens the grease, but ensure that appropriate PPE (i.e. insulated/protected gloves) are used to avoid burns or cuts in the event of a breakage. A professional glassblower can often fix this problems in seconds so this may be the best and safest option.
See here for more advice and preventive measures.