Gas-liquid Chromatography often just referred to as gas chromatography is a powerful tool in analysis. It has all kinds of variations in how it is done – if you want full details, a Google search on gas chromatography will provide you scary amounts of advice if you want it! This page just appears in a simple introductory manner at how it can be carried out. All forms of chromatography involve a stationary phase and a mobile phase. In all the other kinds 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 Particular chemical travels through the system will depend on how much of its time is spent moving together with the gas instead of being connected to the liquid in some manner.
Very small quantities of the sample That you are attempting to analyse are injected into the machine with a small syringe. The syringe needle passes through a thick rubberized disk called a septum that reveals itself when the syringe is pulled out. The injector is contained in an oven whose temperature can be controlled. It is hot enough so that the sample boils and is transported into the column for a gas from the helium or other carrier gas. There are two main types of column in gas-liquid chromatography. One of them is a long thin tube packaged with the stationary phase; the other is thinner and contains the stationary phase bonded to its internal gas chromatography. To keep things simple, we are just Going to consider the packed column. The pillar is typically made of Stainless steel and is between 1 and 4 metres long with an inner diameter of around 4 mm. It is coiled up so it will fit to a thermostatically controlled oven.
The column is packed with finely Floor diatomaceous earth, which is a very porous stone. This is coated with a high boiling liquid – typically a waxy polymer. The temperature of the column can be varied from about 50°C to 250°C. It is cooler than the injector oven, so that some parts of this mixture may float in the start of the column. Sometimes, as you will see below, the column starts off in a low temperature and then is made hotter under computer control as the analysis continues. One of three things might happen to some particular molecule in the mixture injected into the column:
- It may condense on the stationary phase.
- It may dissolve in the liquid onto the surface of the stationary phase.
- It may stay in the gas phase.
- None of those things is always permanent.
A compound with a boiling point higher Than the temperature of the column will clearly tend to float at the beginning of the column. However, some of it will evaporate again in precisely the exact same manner that water disappears on a warm day – although the temperature is below 100°C. The odds are it will then float again a bit further along the column. Similarly, some molecules may dissolve from the liquid stationary phase. Some chemicals are more soluble in the liquid than others. The more soluble ones will invest more of their time absorbed into the stationary phase; the less soluble ones will invest more of their time at the gas.