Vacuum ramps are a tool used in chemistry. Invented by Wilhelm Schlenk, they are also referred to as 'Schlenk's line', and find their use in manipulating and synthesising air-sensitive compounds.
A ramp consists of a double glass duct, or line, equipped with a variable number of outlets, to which samples are connected. One of the two lines is connected to a source of purified inert gas, while the other is connected to a vacuum pump. The inert gas line has a vent outlet via an oil bubbler. In addition, special types of taps make it possible to select the inert gas line or the vacuum line without having to move the sample to the other line. Thanks to the number of outlets and lines, a different number of reactions or operations can be carried out simultaneously.
When reactants are sensitive to oxidation, the presence of traces of oxygen can be a problem. For this, the inert gas must be purified by passing it through a deoxygenation catalyst. This is usually a copper oxide column, which reacts with the traces of oxygen present in the inert gas.
Liquid nitrogen is usually inserted into the vacuum pump and the initial portion of the vacuum ramp. The purpose of this procedure is to condense and collect gases and liquids that have evaporated during the reaction, to prevent organic or corrosive compounds from damaging the mechanical pump. Liquid nitrogen is used as its boiling point is -196°. However, oxygen is also condensed at this temperature, which can easily explode. Consequently, care must be taken to ensure that no significant amount of air enters the pump.
Main operations that can be performed with ramps
Countercurrent additions. Reagents stable in air are added to the reaction vessel against a flow of inert gas.
Use of syringes and rubber septa. They are used for the purpose of transferring liquids and solutions.
Syringes and rubber septa are used.
Cannula transfers. Liquids or solutions of air-sensitive reagents are transferred between different vessels sealed by rubber septa, using a tube called a cannula. The flow of liquid is supported by vacuum or an inert gas pressure.