Gas lines for spectroscopy
Filling an IR cell
At this stage, the line should be set up ready so that the IR cell can be filled with a gas.
1 ) Transferring product to the IR cell
At this stage the IR cell and the manifold are under passive vacuum. This means that they are under low pressure, so that any gas released onto the line will distribute itself across the line in order to equalise the pressure.
At this stage, the gas can be allowed to pass onto the line. Depending on the source of the gas determines how this is carried out:
Cylinder
Generally a small quantity of gas is required on the line. Assuming a lecture bottle fitted with a regulator and needle valve:
Ensure that the needle valve (equivalent to tap 7) is closed.
Briefly open the regulator valve and reclose it. This will allow a small amount of gas to be held between the regulator and needle valve.
Whilst watching the pressure gauge, carefully open the needle valve to allow some gas onto the line. You may know the pressure required for the cell, or you might adjust the amount later.
Close the needle valve at the required pressure.
Reaction flask
This may take various different forms, but generally the reaction is initiated and the tap (7) opened to allow the gas onto the line.
Note the final pressure on the manometer/gauge, in both cases this is the pressure on the manifold and in the IR cell.
2 ) Removing the IR cell
In order to remove the IR cell, both tap 4 & 5 will need to be closed. This keeps gas in both the IR cell and in the manifold.
Carefully remove the IR cell from the line, note that the space between taps 4 & 5 is under passive vacuum (and contains a small quantity of gas), so the cell is harder to detach.
At this stage, the IR spectrum can be recorded.
3 ) Reducing the pressure in the IR cell
Often it is desirable to then reduce the pressure in the IR cell and record additional spectra at lower pressures. Usually this is carried out working from high pressure (step 2 above) to lower pressures gradually, in order to record spectra which are on scale. Some compounds may require a collection of different pressures in order for all bands to be appropriately recorded.
There are a number of ways to acheive the different required pressures.
Reducing the pressure
There are a couple of different ways to reduce the pressure in an IR cell. Prior to doing any gas work, you may wish to try these two alternatives by filling a gas cell with some air and testing your abilities to carefully remove some of the contents using each option.
1 ) Reevacuating the manifold
Return the IR cell to the manifold and open tap 5.
2 ) Diluting the gas
It is often desirable to aid gas removal by bleeding in air to the manifold to bring it to atmospheric pressure before evacuating. This dilution phase aids the removal of gas from the line. This is acheived by opening the manifold air bleed (6).
3 ) Active vacuum
The manifold air bleed (6) is then closed, and the manifold tap (3) opened in order to evacuate the line. This air bleed / evacuate cycle is sometimes repeated to ensure removal of the gas.
4 ) Equalising the IR cell pressure
The manifold tap (3) is closed again to return the line to passive vacuum. This manifold passive vacuum will be greater than the IR cell at this point. The IR cell is then opened (tap 4) and this equalises the pressure over the manifold, thus reducing the pressure in the cell. Record the pressure again. The cell can then be removed (closing taps 4 & 5) and further IR spectra recorded.
