Components

Gas lines usually include most of the following features.

Vacuum pump

Allows removal of air or produced gases to be removed from the line.

Vacuum air bleed tap

Used in combination with the closure of the vacuum trap tap, this tap allows air to be drawn through the vacuum pump. Usually this is carried out in order to 'ballast' the pump. Allowing the air to be drawn through helps to keep the pump in good working order. Although it should be noted that different pump designs have different instructions for routine pump maintainance. Labelled 1 in the above diagram.

Vacuum trap tap

Allows the vacuum pump to be isolated from the entire manifold, including the vacuum trap. Note the closure of this tap is generally not advisable if a liquid nitrogen trap is in place around the vacuum trap. Labelled 2 in the above diagram.

Vacuum trap

The vacuum trap can serve to protect the pump when used with a dewar of liquid nitrogen. This allows products to be cooled (usually liquifying or freezing them) in the trap and reducing the amounts reaching the pump. This is particularly desirable when working with compounds that could damage the pump. However, the presence of a liquid nitrogen trap also places additional safety considerations, as care needs to be taken to not draw much air through the line, as the liquid nitrogen (–196 °C; 77 K) is cold enough to condense oxygen (–183 °C; 90 K).

Manifold tap

This tap controls whether the manifold is connected to the vacuum pump or not. It is probably the most used tap on most gas lines. This tap is generally used to allow the air in the manifold to be removed, then the tap is closed to allow gases to then be manipulated under passive vacuum. Labelled 3 in the above diagram.

Manifold

The section of glass tubing connecting the various components (vacuum via manifold tap, manometer, IR cell and reaction flask). The manometer provides an indication of the working pressure here.

Manometer or pressure gauge

Allows the pressure in the manifold to be measured. These can be either traditional mercury manometers, or alternatively digital gauges with valves. A mercury manometer is the simplest design, whereby a tall glass tube (generally 1 m) is connected to the manifold at the top, and the bottom is placed into a container of mercury (mercury reservoir). When the manifold is placed under vacuum, the change in air pressure acting on the mercury (the air pressure acting on mercury in the reservoir) causes the mercury to be drawn up the manometer. This is the reason pressure is often measured in mmHg (historically this was also the same as Torr, although Torr now has a slighly different meaning), as this pressure reading is the amount the mercury column rises. The exact pressure difference depends on the atmospheric pressure (average atmospheric pressure is around 760 mmHg) and the capability of the vacuum. With a perfect vacuum, the difference would be a measure of the atmospheric pressure.

The manometer also serves as the pressure release. If gas is added to the line to a level above atmospheric pressure, any excess gas will 'bubble' through the mercury. With digital gauges in use this might be pressure valves which release. Some gas lines are also designed to run under positive pressure, where the lines can be taken to several atmospheres of pressure.

Manifold air bleed

Allows air to be bled into the manifold section of the line. Often used in order to 'dilute' a gas with air in order to remove it from the manifold using the pump. Care needs to be taken with the use of this tap if a liquid nitrogen trap is present around the vacuum trap. Labelled 6 in the above diagram.

IR cell attachment and tap

A connection on the manifold allowing the attachment and removal of an IR cell. The design of this varies, but push fittings are fairly common. Usually there is a tap to the cell itself (labelled 4 in the above diagram), and a tap onto the vacuum manifold (labelled 5 in the above diagram). When both taps are open, the cell is connected to the manifold.

IR Cell

The design of these varies a lot, in part depending on the spectrometer requirements. In most cases, the IR cell will consist of a cylinder of known length (the pathlength) with cell windows on each end. The cell windows need to be transparent across the IR range of interest, and not react with the compounds being handled on the line. There are various cells are available, with different usable ranges.

Reaction flask attachment

A means to connect whatever is being used to provide the gas to the line. This might be a cylinder attachment, or an attachement for a specialist flask or other apparatus for producing a gas. There may also be a tap fitted onto the gas line here, or taps may be provided by the apparatus. On the above diagram a reaction flask is shown connected, with the tap (labelled 7) being present on the attachement. In the photo the gas line here has a ground glass cone, allowing for a variety of different pieces of apparatus to be connected via a ground glass socket, giving flexibility to the line. In this photo the line is actually being sealed at this point with a round bottomed flask.

Key concepts in use

The most important concept when using a vacuum line is to have an idea of the contents in each section of the line. This is most commonly considered by thinking about the line in terms of three states parts of the the line may be in. It is important to be aware that different sections of the line are likely to be in different states.

Atmospheric pressure

Where the pressure in the section of line is the same as atmospheric pressure. This is usually where section is containing air (for instance before turning on the line), or it may be where air is mixed with the gases being added to the line.

Active vacuum

Where sections of the line are actively being evacuated by the vacuum pump. In most line designs it is only possible for sections to be opened to vacuum in a linear fashion. For instance, if you want to evacuate the IR gas cell, the manifold, vacuum trap and all the glassware leading to the pump would also have to be under active vacuum.

Passive vacuum

Where a section of line has been actively evacuated and then taps closed in order to keep the section of line under vacuum, but without any connection to the pump. The principle purpose of this is to allow the air to be removed from the line using active vacuum, the section isolated from the vacuum pump, then gases can be added to the line which will move into the lower pressure sections of line when taps are opened. The manometer is generally connected to the manifold, so that the 'passive pressures' can still be measured.

Diagrams

In the step by step guidance for using a gas line, the diagrams are coloured using the following conventions: