Gas-liquid Chromatography often just Called gas chromatography is a powerful tool in analysis. It has all types of variations in how it is done – if you want complete details, a Google search on gas chromatography will supply your scary amounts of advice if you would like it! This page just looks in an easy introductory manner in how it can be carried out. All kinds of chromatography involve a stationary phase and a mobile phase. In the rest of the types of chromatography you may meet at this level, the mobile phase is a liquid. In gas-liquid chromatography, the mobile phase is a gas such as helium and the stationary phase is a high boiling point liquid adsorbed on a solid. How fast a specific chemical travels through the machine will depend on how much of its time is spent moving along with the gas rather than being on the liquid in some fashion.
Very small quantities of the sample That you are trying to analyse are injected into the machine with a small syringe. The syringe needle passes through a thick rubberized disc called a septum that shows itself when the syringe is pulled out. The injector is contained in an oven whose temperature can be controlled. It is hot enough so the sample boils and is transported into the pillar to get a gas out of the helium or other carrier gas. There are two primary types of column in gas-liquid chromatography. One of these is a long thin tube packed with the stationary phase; another is thinner and includes the static phase bonded to its inner gas chromatography. To keep things simple, we are just Going to think about the packed gas chromatography column. The pillar is typically made from stainless steel and is between 1 and 4 metres long with an inner diameter of approximately 4 mm. It is coiled up so it is going to match to a thermostatically controlled oven.
The column is packed with finely Floor Diatomaceous earth, which is a really porous stone. This is coated with a high boiling liquid – typically a waxy polymer. The temperature of the column could be varied from approximately 50°C to 250°C. It is cooler than the injector oven, so that some pieces of the mixture may float at the onset of the column. Occasionally, as you will see below, the column starts off at a low temperature and then is made hotter under computer control as the analysis continues. One of three things might happen to a particular molecule in the mixture injected into the column, it could condense on the stationary phase, it could dissolve in the liquid on the surface of the stationary phase. It may remain in the gas phase. None of these things is always irreversible.
A compound with a boiling point higher in relation to the warmth of the column will clearly tend to float in the start of the column. However, some of it will evaporate again in the exact same way that water disappears on a warm day – even though the temperature is below 100°C. The chances are it will float a little further along the column. Similarly, some molecules may dissolve from the liquid stationary phase. Some compounds are more soluble in the liquid than others. The more soluble ones will spend more of their time consumed into the stationary phase; the less soluble ones will spend more of their time in the gas.