Source: Bartlett Group SOPs, University of Michigan Department of Chemistry
Tags: Schlenk line, vacuum line, nitrogen line, air-free chemistry, glovebox, inert atmosphere, backfilling, liquid nitrogen, condensed oxygen, catalyst poisons
Difficulty: Intermediate to Advanced | Prerequisites: General chemistry (gas laws, pressure), basic lab safety
The Schlenk line and glovebox are the two main tools for conducting air-sensitive and water-sensitive chemistry. The Schlenk line lets you alternate between vacuum and inert gas (typically nitrogen) to purge glassware and maintain an oxygen-free atmosphere, while the glovebox provides a fully sealed, inert-atmosphere enclosure for manipulating reagents by hand. Both systems have serious hazards if used incorrectly: the Schlenk line can condense liquid oxygen (which is explosive), and the glovebox atmosphere can be ruined by improper entry procedures or solvent contamination.
Schlenk line
A dual-manifold glass apparatus with a vacuum line (top tube) and an inert gas line (bottom tube, usually nitrogen). Allows alternating exposure of glassware to vacuum or inert gas through ports along the line.
Backfilling
The process of closing the vacuum port to a flask and opening the nitrogen port to replace the vacuum with inert gas. Typically repeated three times (evacuate, then backfill) to ensure complete displacement of air.
Mineral oil bubbler
A gas barrier at the end of the nitrogen line that allows excess nitrogen to escape while preventing ambient air from entering. If no bubbles are visible when nitrogen is flowing, pressure is building in the line, which is a serious explosion risk.
Solvent traps
Glass cylinders cooled with liquid nitrogen, placed in the vacuum line to catch organic solvents before they reach the pump oil. Prevents pump degradation and keeps corrosive vapours out of the pump.
Liquid nitrogen (LN2)
Cryogenic coolant at -196 °C used to cool solvent traps. Causes burns on skin contact. Nitrile gloves and hair ties must be removed from wrists before handling, as they freeze to the skin and prolong exposure.
Condensed oxygen
The most dangerous hazard on the Schlenk line. If ambient air leaks into the vacuum line while traps are cooled with LN2, oxygen can condense as a pale blue liquid. On warming, it may react explosively with trapped organic solvents or expand and over-pressurise the line.
Glovebox
A sealed enclosure filled with inert gas (nitrogen) that allows manipulation of air- and water-sensitive materials through built-in gloves. Materials enter and exit through antechambers that are evacuated and refilled to maintain the atmosphere.
Antechamber (airlock)
A sealed compartment on the glovebox used to transfer materials in and out without exposing the interior to air. The small antechamber requires at least 5 evacuate/refill cycles; the large antechamber requires 3 cycles of 20 minutes each.
Catalyst poisons
Substances that degrade the glovebox atmosphere purifier catalyst. Reversible poisons (ammonia, ethers) require 5 to 10 minutes of purging. Irreversible poisons (sulfur compounds, halogens, alcohols, hydrazine, heavy metals, amines) require at least 15 minutes.
The nitrogen line runs from a wall source through a desiccating column (Drierite, blue when fresh, pink when spent) into the Schlenk line manifold, and out through flow-adjustment valves to a mineral oil bubbler. The bubbler is both a gas seal and a pressure indicator: visible bubbles mean the line is exhausting properly; no bubbles mean pressure is building dangerously.
The vacuum line runs from a mechanical vacuum pump through solvent traps (cooled by liquid nitrogen dewars), past access ports, and ends at a pressure meter. A well-maintained pump with no traps should reach 70 to 80 mtorr; with cooled traps installed, 30 to 40 mtorr.
Hoses (rubber or Nalgene) connect the line to reaction flasks. Check for gashes or holes before each use.
Vacuum grease (siloxane/silica) seals joints in glassware setups. Do not grease the Schlenk line itself.
Dewars hold liquid nitrogen for cooling the traps. Despite their metallic appearance, they are glass with an internal vacuum chamber and will shatter violently if dropped.
Pressure equilibrium
The nitrogen line maintains slight positive pressure relative to the ambient environment. If the bubbler is blocked or flow valves are closed with nitrogen running, pressure builds and can eject stoppers or blow apart glassware. Always verify bubbler output when nitrogen flow starts. If you reduce bubbler output, ensure the system has an alternative exhaust.
Do not run incompatible reactions on the same Schlenk line simultaneously, as the headspace of all connected flasks communicates.
Vacuum pump maintenance
Change pump oil when performance drops, the exhaust smells foul, or thick smoke appears. If oil changes are frequent, review your trap-cooling procedure.
Cracked glassware
Star cracks and hairline fractures from handling or cleaning make glass susceptible to explosion under positive pressure or implosion under vacuum. Inspect every piece before use.
Heating or freezing flasks on the line
Flasks undergoing temperature change must be open to either vacuum or nitrogen to prevent dangerous pressure swings. Heating or freezing a closed system is almost never necessary and extremely hazardous.
Liquid nitrogen handling
Use insulated gloves. Remove nitrile gloves and hair ties from wrists, as they freeze to skin and prolong burns.
Condensing oxygen: the most dangerous Schlenk line hazard
Oxygen condenses into the solvent traps when ambient air leaks into the vacuum line while traps are at LN2 temperature. This happens from a vacuum-line leak or an open port left unattended, not during normal use (evacuating a sealed flask, for instance). Watch the pressure meter: a sudden, unexplained rise means a leak, and oxygen may be condensing.
Liquid oxygen is pale blue. If you see it when removing dewars: replace the dewars immediately, turn off the vacuum, open the system to the atmosphere through the closest large port, close the sash, warn labmates, and evacuate. Notify the PI. After the traps reach room temperature, rinse them into a beaker, test for peroxides, and neutralise with sodium thiosulfate or sodium sulfite before disposal.
Clean and dry all glassware in the drying oven for at least 30 minutes.
Start the vacuum pump with all ports closed, vents sealed. Confirm the pressure meter drops below 80 mtorr.
Open nitrogen flow. Confirm bubbler output. Check the Drierite column colour.
Cool traps with liquid nitrogen (fill dewars about one-third, raise onto traps, secure with clamps, top off). Target pressure: under 40 mtorr.
Assemble glassware (grease joints, check seals), connect to line while still warm, expose to vacuum until cooled to room temperature (15 to 20 minutes) to remove surface water.
Backfill: close vacuum, open nitrogen. Evacuate and backfill three times, then leave nitrogen flowing.
Add reagents under nitrogen flow (solids through a stopper; liquids via cannula or syringe transfer for sensitive reactions).
Run the reaction. Monitor traps and pressure; refill LN2 as needed.
Shut down: close ports, disable nitrogen, close pump isolation valve, remove dewars (check for blue liquid oxygen), vent slowly, thaw and clean traps.
Small antechamber
Verify from the log that the chamber has been evacuated at least 15 minutes since the last item was removed.
Refill to atmospheric pressure, open, insert materials, close.
Evacuate and refill at least 5 times.
Large antechamber
If an item was recently removed (check the log), evacuate for 1 hour before new entry.
Use the touchscreen "Antechamber" menu: 3 automated evacuate/refill cycles of 20 minutes each.
Manually refill, bring items in, then immediately manually evacuate the empty chamber.
Critical entry rules
All chemicals must be degassed and rigorously dried. All glassware must be oven-dried.
Sealed vessels containing liquid or dried reagents must be evacuated on a Schlenk line before entering the antechamber. A vessel sealed at atmospheric pressure will explode when the antechamber is evacuated.
Unopened chemicals packaged under inert atmosphere may enter directly.
Never bring in: aqueous solutions, alcohols, acetone, sharp objects (syringe needles only if absolutely necessary, handled with extreme care).
Remove all jewellery. Wear white nylon liner gloves to preserve the glovebox gloves.
When working with solids: blower at 20, circulator open. Dispose of solid waste in the "WASTE" container.
When working with solvents: wear secondary nitrile gloves. Check whether the solvent is a reversible or irreversible catalyst poison and set the blower/circulator accordingly (0/N for irreversible poisons; 20/Y for reversible). Stopper all reactions.
Liquid waste goes in the capped "Liquid Waste" bottle; keep it open as briefly as possible, always with the box set to 0/N.
Label all chemicals with sticker labels.
Log all materials leaving through either antechamber.
To remove items: refill the chamber, open interior door, place items in, reseal, turn to "Off," open exterior, remove items, reclose, evacuate.
After solvent use: purge the glovebox at the same blower/circulator setting used during solvent work. Return to 20/Y after purging. Log the purge.
If a glove develops a large hole (broken glass, for example), the atmosphere is rapidly compromised. Seal the affected port using the emergency seal door (located on the ceiling of the box): pull it into the port, lock with the black knob, activate the vacuum from outside. Consult the manual for glove replacement. If the solenoid valve fails and pressure builds (gloves swell), turn off the high-pressure N2 valve on the right side of the box immediately.
"If the bubbler stops bubbling, it just means the gas flow is low." A stopped bubbler with nitrogen flowing means pressure is building in the line. This can cause stoppers to blow out or glassware to explode. It is an emergency requiring immediate action.
"Nitrile gloves are fine when handling liquid nitrogen." Nitrile freezes to skin on contact with LN2, prolonging the burn. Use insulated gloves only, and remove nitrile gloves and anything else that could freeze to your wrists.
"You can just open a sealed flask and put it in the glovebox antechamber." A flask sealed at atmospheric pressure will explode when the antechamber is evacuated to near-zero torr. Sealed vessels must be evacuated on a Schlenk line before entering.
"The glovebox atmosphere recovers on its own after solvent use." The purifier catalyst can be poisoned by solvent vapours. You must actively purge the glovebox after using solvents, at the correct blower/circulator setting, and log it.
⚠️ Know the three-cycle backfilling procedure (evacuate then backfill with N2, repeated three times) and why it is necessary to ensure complete air displacement.
⚠️ Condensed oxygen recognition and response: pale blue liquid in traps, immediate actions (replace dewars, open system, evacuate, notify PI).
⚠️ The bubbler as a pressure indicator: no bubbles with gas flowing equals a pressure buildup heading for an explosion.
⚠️ Glovebox antechamber protocols: the different cycle requirements for the small chamber (5 evacuate/refill cycles) versus the large chamber (3 cycles of 20 minutes each).
⚠️ Catalyst poisons: know examples of reversible (ethers, ammonia) versus irreversible (halogens, sulfur compounds, alcohols, amines) and the corresponding purge times and blower settings.
The mineral oil bubbler on the Schlenk line serves two purposes: ______ and ______. (Gas barrier preventing air ingress, and visual indicator that pressure is balanced.)
True or false: A well-maintained vacuum pump with cooled traps should reach about 30 to 40 mtorr. (True.)
Liquid oxygen in the solvent traps has a distinctive ______ colour. (Pale blue.)
How many evacuate/refill cycles are required for the small glovebox antechamber? (At least 5.)
True or false: Ethers are irreversible catalyst poisons for the glovebox purifier. (False, ethers are reversible poisons.)
Q: Describe the complete backfilling procedure and explain why it is done three times.
A: Close the vacuum port to the flask and slowly open the nitrogen port to replace vacuum with inert gas. Then close the nitrogen port, reopen the vacuum port to evacuate, and repeat. Three cycles are needed because each cycle exponentially reduces the residual oxygen concentration, approaching a sufficiently pure nitrogen atmosphere.
Q: You notice the pressure reading on your vacuum line is rising unexpectedly while the pump is running. What should you do and why?
A: A rising pressure with a running pump suggests a leak. Ambient air may be entering and condensing as liquid oxygen in the LN2-cooled traps. Seal off the line immediately, check all ports and connections, and fix the leak before continuing.
Q: Why must a sealed flask of reagent be evacuated on a Schlenk line before entering the glovebox antechamber?
A: The antechamber is evacuated to near-zero torr. A flask sealed at atmospheric pressure would have an enormous positive pressure differential relative to the chamber, and the glass would likely explode.
Q: What is the difference between a reversible and an irreversible catalyst poison in the glovebox, and how does this affect the purging procedure?
A: Reversible poisons (ammonia, ethers such as THF and diethyl ether) bind to the purifier catalyst temporarily and require 5 to 10 minutes of purging. Irreversible poisons (sulfur and phosphine compounds, halogens, alcohols, hydrazine, heavy metals, amines) bind permanently and require at least 15 minutes. The blower/circulator must be turned to 0/N for irreversible poisons to prevent circulating contaminated atmosphere through the catalyst.
Q: You are removing the LN2 dewars from your solvent traps and notice a pale blue liquid inside one trap. What is it, and what is the correct emergency response?
A: The pale blue liquid is condensed oxygen. Replace the dewars immediately, turn off the vacuum, open the system to atmosphere through the closest large port, close the fume hood sash, warn labmates, and evacuate the lab. Notify the PI. The oxygen could react explosively with trapped organic solvents or expand violently on warming.
Schlenk line and glovebox techniques are foundational for any research involving air-sensitive synthesis, including work with the solvent purification system (the purified solvents are dispensed into Schlenk flasks and transferred into the glovebox). The gas-handling principles here also connect to the Compressed Gas Cylinders SOP and the Tube Furnace SOP, both of which involve controlled-atmosphere work.
The condensed-oxygen hazard is an applied example of the relationship between boiling points, vapour pressure, and cryogenic temperatures from general and physical chemistry.
Schlenk line, Schlenk technique, vacuum manifold, nitrogen manifold, dual manifold, air-free chemistry, inert atmosphere, backfilling, evacuate and backfill, mineral oil bubbler, solvent traps, cold traps, liquid nitrogen, LN2, condensed oxygen, liquid O2, glovebox, glove box, antechamber, airlock, catalyst poisons, reversible poisons, irreversible poisons, Drierite, vacuum grease, Schlenk glassware, dewar, cannula transfer, syringe transfer, Bartlett Group SOP, University of Michigan