Gas lines for spectroscopy

Setting up a gas line

Initial state

Generally when you reach a gas line, it will have been left in the following configuration:



The manometer should therefore show no column of mercury, as the manifold is at atmospheric pressure. This is the state in which you should return the line to at the end of your use.

Turning on the line

The following are simplified diagrams of the vacuum line apparatus and the state (atmospheric, active or passive) that each section will be in at various stages and take you through the setup process to evacuate the line ready for use.

1 ) Switching on the pump

Initially, the line will be entirely at atmospheric pressure (as shown on the manometer). Ensure the vacuum air bleed (1) and trap isolation (2) are open and that the manometer tap (3) is closed and switch on the pump.


The pump will start working, but as the line is open to the air via the vacuum air bleed (1) the vacuum pump will struggle to acheive much vacuum. For most pumps this is desirable at the start to flush the vacuum through with air in a process known as 'ballasting' the pump. Typically this will be carried out for a few minutes before proceeding.

A schematic diagram showing a gas line set up.

2 ) Exposing the trap to vacuum

Closing the vacuum air bleed (1) will now place the first section of the line under vacuum. If you have your own pump for the line (as oppose to a shared house vacuum), the pump will gradually change in tone as it decreases its efforts pumping as the pressure lowers. When the pump no longer sounds as if it is working to evacuate this section of the line (this should take less than a minute), proceed to the next step.


In all work with vacuum lines (Gas, Schlenk etc), the sounds of the pump can give you a lot of clues as to what is going on in the experiment.


At this point, the the first part of the line will be under an active vacuum, as far as the manifold tap (3). Beyond this, the line remains at atmospheric pressure.

A schematic diagram showing a gas line set up.

3 ) Exposing the manifold to vacuum

Next, the manifold needs preparing to be brought under active vacuum. Close the manifold air bleed (6) and the IR cell tap (5). It is unimportant whether the IR cell is connected at this point. The reaction flask connection needs to be closed, this may be acheived by connecting whatever is required to the gas line, but ensuring its tap is closed (7).


Once this is ready, slowly open the manifold tap (3) whilst watching the manometer column (or pressure gauge). The pump will change in sound as it beings to actively pump again. Watch the manometer to see the pressure it reaches. The pressure should be close to the atmospheric pressure (if you have a good pump). If your pressure is noticably lower, you may have a leak, or forgotten to close a tap. You may hear hissing which will help direct you. Record your active vacuum pressure


At this stage, the manifold and manometer are under active vacuum along with the trap.

A schematic diagram showing a gas line set up.

4 ) Checking for leaks

At this stage it is sensible to look for any leaks on the line. Close the manifold tap (3) which places the manifold under passive vacuum. If the line is leaking, the pressure gauge will show a drop in pressure. The severity of the leak will change the rate of this drop.


Keep the manifold under passive vacuum and record the rate of pressure drop over a minute. Check this rate is ok with a demonstrator. The drop should generally be less than 2 mmHg over 60 seconds.


Providing there is no liquid nitrogen trap fitted, you may wish to carefully open the manifold air bleed (6) in order to see the effects of a leak. Leaks will cause a change in the vacuum pump sounds, as the vacuum has to work harder to remove the air.

A schematic diagram showing a gas line set up.

5 ) Evacuating the IR cell

At this stage it is generally a good point to attach and evacuate the IR gas cell. Attach the IR cell to the line (note that it it generally prudent to support the cell to keep it safely in place). Once the cell is attached, return the manifold to active vacuum by opening the manifold tap (3). Then keeping an eye on the vacuum gauge, open the IR cell tap (5). The vacuum will briefly dip as the section between taps 4 & 5 is evacuated. It is a good idea to then close the manifold tap (3) and check again for leaks. If you now find a leak, then you know it is where the IR cell is attached.


Then reopen the manifold tap (3) and then whilst keeping an eye on the pressure gauge, open tap 4 to evacuate the IR cell. Once the cell has evacuated (the pressure should be more or less the same reading as when you just had the manifold under active vacuum), check again for leaks by placing the line under passive vacuum and monitoring for any pressure drop.

A schematic diagram showing a gas line set up.

6 ) Line under passive vacuum

The line should now be held in the position shown on the left. The manifold and IR cell are all under passive vacuum. The pressure should be recorded. Consult a demonstrator if you have any leaks on the line.


The cell is now ready to be filled with a gas.

A schematic diagram showing a gas line set up.