The process where a substance divides itself between two immiscible solvents because it is more soluble in one than the other is known as partition. The more soluble ones will spend more of their time absorbed into the stationary phase the less soluble ones will spend more of their time in the gas. Some compounds will be more soluble in the liquid than others. Similarly, some molecules may dissolve in the liquid stationary phase. The chances are that it will then condense again a little further along the column. However, some of it will evaporate again in the same way that water evaporates on a warm day - even though the temperature is well below 100☌.
None of these things is necessarily permanent.Ī compound with a boiling point higher than the temperature of the column will obviously tend to condense at the start of the column. It may dissolve in the liquid on the surface of the stationary phase. One of three things might happen to a particular molecule in the mixture injected into the column: In some cases, as you will see below, the column starts off at a low temperature and then is made steadily hotter under computer control as the analysis proceeds. It is cooler than the injector oven, so that some components of the mixture may condense at the beginning of the column. The temperature of the column can be varied from about 50☌ to 250☌. This is coated with a high boiling liquid - typically a waxy polymer. The column is packed with finely ground diatomaceous earth, which is a very porous rock. It is coiled up so that it will fit into a thermostatically controlled oven. The column is typically made of stainless steel and is between 1 and 4 metres long with an internal diameter of up to 4 mm. To keep things simple, we are just going to look at the packed column. One of these is a long thin tube packed with the stationary phase the other is even thinner and has the stationary phase bonded to its inner surface. There are two main types of column in gas-liquid chromatography.
It is hot enough so that all the sample boils and is carried into the column as a gas by the helium (or other carrier gas). The injector is contained in an oven whose temperature can be controlled.
The syringe needle passes through a thick rubber disc (known as a septum) which reseals itself again when the syringe is pulled out. Very small quantities of the sample that you are trying to analyse are injected into the machine using a small syringe. Drawing a convincing and tidy coil defeated me completely! Note: You will have to imagine the coiled column in its oven. How fast a particular compound travels through the machine will depend on how much of its time is spent moving with the gas as opposed to being attached to the liquid in some way.Ī flow scheme for gas-liquid chromatography In gas-liquid chromatography, the mobile phase is a gas such as helium and the stationary phase is a high boiling point liquid adsorbed onto a solid. In all the other forms of chromatography you will meet at this level, the mobile phase is a liquid.
It has all sorts of variations in the way it is done - if you want full details, a Google search on gas chromatography will give you scary amounts of information if you need it! This page just looks in a simple introductory way at how it can be carried out.Īll forms of chromatography involve a stationary phase and a mobile phase. Gas-liquid chromatography (often just called gas chromatography) is a powerful tool in analysis.