Keeping air and moisture out

High-vacuum lines are closely related to Schlenk lines; these generally avoid the use of hosing and rubber septa and instead rely on direct ground glass or greaseless ball-and-socket joint connections in tandem with vacuum transfer techniques to manipulate liquids and gases. The typical vacuum pressure range for high-vacuum lines is 10^-4 to 10^-7 mbar; this is unachievable using rotary vane or screw pumps which are limited by the mean free path of gas molecules. Oil-diffusion or turbomolecular pumps are therefore employed for high-vacuum applications, however these must still be used in tandem with diaphragm, rotary vane or screw pumps to serve as a fore (roughing) pump to establish backing pressures between the lower mbar range and 10^-3 mbar for safe and efficient operation. The VACUU·PURE 10C screw pump is particularly well suited for use as a fore pump with turbomolecular pumps since both are oil-free and clean technologies.

The primary use of vacuum on a Schlenk line is “cycling” glassware, the process of performing repeated (usually three) vacuum and inert gas cycles to remove air and moisture from oven dried glassware and/or the hosing connecting the Schlenk flask or a sealed reagent/solvent ampoule to the Schlenk line.

A 100 mL flask filled with air contains approximately 1 mmol of O₂ at atmospheric pressure (~1000 mbar) which would be detrimental to many oxygen-sensitive organometallic or main-group compounds. Evacuating the flask down to 0.1 mbar reduces the concentration of O₂ down to 1 x 10^-4 mmol. Backfilling with inert gas and re-evacuating the flask once again to 0.1 mbar lowers this to 1 x 10^-8 mmol, and a third cycle lowers this even further to 1 x 10^-12 mmol. Importantly, this is several orders of magnitude lower than performing a single evacuation down to 0.001 mbar (1 x 10^-3 mbar), the ultimate vacuum of a well-maintained rotary vane vacuum pump. This emphasises the importance of performing multiple vacuum/inert gas cycles and illustrates that even diaphragm pumps (ultimate vacuum around 1 mbar) may be effective at establishing inert atmosphere conditions in the laboratory.

The use of vacuum on a Schlenk line enables the evaporation of solvent and other volatiles whilst maintaining a strictly inert atmosphere. In contrast to rotary evaporators, which typically employ diaphragm pumps with an ultimate vacuum down to 1 mbar, Schlenk lines are commonly equipped with rotary vane pumps, with ultimate vacuum pressures in the 10^-3 mbar range. This allows high-boiling solvents such as toluene, DMF and DMSO to be evaporated at room temperature, or for low boiling solvents such as pentane and Et₂O to be evaporated at low temperatures, both of which may be necessary when handling and isolating temperature-sensitive compounds. Strong stirring coupled with controlled application of vacuum is necessary to avoid bumping and contamination of the vacuum manifold of the Schlenk line.

An important consideration when evaporating solvent under vacuum on a Schlenk line is the use of traps. Typically cooled using a Dewar of liquid nitrogen (-196 °C), traps condense evaporated solvents, volatile liquids and some gases, which helps to protect the vacuum pump by limiting exposure to aggressive chemicals. It also allows the collected condensate to be discarded in an appropriate waste container. The use of an external solvent trap should be applied when evaporating large quantities of solvent (>50 mL), when removing particularly aggressive/corrosive chemicals, or when removing volatiles that have relatively high (>0 °C) freezing points such as benzene, 1,4-dioxane and DMSO.

The vacuum (10^-2–10^-4 mbar range) offered by Schlenk lines allow solids to be thoroughly dried under dynamic vacuum. This is typically part of an isolation process after evaporating solvent, or following a filtration and crystallisation/precipitation step, and removes residual organic solvent trapped within the solid. Such solids (assuming it is sensitive to air and moisture) are typically isolated and stored within an inert atmosphere glovebox for further handling and analysis. Drying solids under vacuum is also commonly performed to remove residual water, for example from molecular sieves or Celite (diatomaceous earth or kieselgur) and is often coupled with sustained heating above 100 °C. A frequent oversight when drying solids under vacuum is protection of the vacuum pump and vacuum manifold of the Schlenk line due to bumping or dispersion of very fine solids. To avoid this, an external trap can be used (which can be easily separated and cleaned afterwards) or a hosing adaptor fitted with a glass frit or glass wool insert can be employed.

Another application of vacuum on a Schlenk line is the purification or air- and moisture-sensitive liquids and solids by vacuum distillation or sublimation. Dynamic vacuum distillations are employed for the purification of high-boiling (>150 °C) liquids, whilst static vacuum distillations are an advanced method coupled with the freeze-pump-thaw method which is used for the purification of low-boiling (<150 °C) liquids whilst minimising evaporative loss. Dynamic vacuum can also be employed for performing sublimations using a cold finger insert or a specially designed sublimation apparatus.

The vacuum of Schlenk lines is also commonly used to perform the freeze-pump-thaw method, which effectively degasses liquids and solutions so that they can subsequently be stored under an inert gas atmosphere, or exposed to a reactive gas. Reactions may also be performed under a static vacuum to facilitate the release of a volatile by-product into the headspace.