Schlenk safety

There are a number of potential hazards associated with the use of Schlenk lines, and care needs to be taken to ensure safe operation. This page outlines some of these hazards and vigilance required for safe operation. It is important to be aware of how to work safely using a Schlenk line, and to plan for actions to be taken in the event of things going wrong.

Familiarisation with the line

The design of Schlenk lines is hugely varied, and it is important that you are familiarised with the specific design being used before undertaking any work. Understanding the configuration of the line, though knowing how things are connected together, the expected pressures (for both nitrogen and vacuum) and how the taps work are crucial aspects of ensuring any Schlenk line work is carried out in a safe manner.

Experienced Schlenk line users may be able to inspect the line and self-familiarise with the operations, but less experienced users must be familiarised with a specific line by someone who understands the workings of that line.

Use of high vacuum (low pressures)

Glassware on the line is exposed to high vacuum, which has the potential for implosion. All glassware being subjected to vacuum on the line should be inspected for any chips or cracks which could cause a hazard.

Use of a liquid nitrogen trap

Schlenk lines make use of a liquid nitrogen trap, which condenses any vapours from the line before these reach the pump. The use of a trap improves the vacuum pressure on the line, as solvent vapour is minimised, and the pump is protected from potential damage from solvent vapours.

However, there are multiple potential safety issues to consider with liquid nitrogen.

Potential for burns or injury from disposable gloves

Nitrogen boils at −196 °C ( 77 K), and has the ability to cause severe burns. Most burns occur from prolonged contact with the liquid, particularly where nitrogen gets caught in contact with skin and care needs to be taken when using to avoid the liquid being trapped, for instance in clothing or shoes.

Cryogenic gloves can be used when handling liquid nitrogen, however there is a balance between the loss of dexterity and the gain in protection to be balanced. Generally brief splashes from liquid nitrogen do not lead to burns, with the nitrogen freely running off skin without any injury, and nitrogen can safely be poured without the use of cryogenic gloves.

Injury from disposable gloves

Disposable gloves should not be worn whilst handling liquid nitrogen. Disposable gloves can lead to nitrogen can getting caught inside these gloves, which can lead to burns, but potentially more serious injuries can arise from the gloves being cooled below their glass transition temperature. This can result in the gloves fracturing into shards which can cause cuts.

Condensing of oxygen

The temperature of liquid nitrogen is below that of the boiling point for oxygen (−196 °C,  90 K), meaning that the trap is capable of condensing oxygen from the atmosphere into the trap. Care needs to be taken during the use of Schlenk lines to ensure that the cryogens are not placed around the trap whilst not under vacuum, as this will result in the condensation of oxygen. Oxygen can also be condensed by drawing air through the trap to the vacuum pump with a trap in place.

Cryogens should not be placed around any trap which is not under vacuum, and cryotraps should be removed before opening vacuum lines to the atmosphere (eg during shutdown). Air should not be drawn through the line (eg by opening a tap to vacuum without a connected flask) with a cryotrap in place. 

During use, the vacuum levels should be regularly monitored to check that the line is continually at low pressure. Observing that the line is not under low vacuum can indicate the presence of taps not being fully closed, or other leaks on the line. These should be promptly investigated and experienced help sought if there is any possibility that oxygen may have condensed in the trap, no matter how small an amount.

One of the major risks with condensed oxygen arises if the oxygen ends up being contained as part of a sealed system, which can lead to explosion if the gas warms up with no route for the pressure to be alleviated.

Condensing of solvent

Part of the role of the vacuum trap is to condense any solvent vapours from the line. Whilst using the line it is important to consider how much solvent vapour is likely to be condensing during use, and managing this. Generally, the vacuum trap should only be used to condense small volumes of solvent, and secondary traps should be utilised where larger solvent volumes are being evaporated (see solvent removal). There are a number of potential hazards which can arise where solvent is being condensed into the trap.

If larger amounts of solvents are being removed and condensed, there is the potential for the trap to become blocked with frozen solvent. If the trap involves a narrow pipe leading into the trap, this can become blocked with relatively small amounts of solvent vapour. This can in turn lead to reduced vacuum on the line, which can result in oxygen condensing.

Periodic inspection of the solvent trap should be carried out to check the volumes of solvent present in the trap. If solvent is accumulating, steps should be taken to empty the trap.

A much more serious situation can also arise if solvent and liquid oxygen both condense. The combined presence of a fuel source and oxidant can lead to a significant explosion and fire risk. If there is any suspicion that solvent and oxygen may have condensed then experienced help should be sought immediately, and evacuation of the laboratory should be considered.

Pressurised inert gas

Over-pressurisation, needle exits

Expected pressure readings and potential leaks

Over-pressurisation, needle exits