Introduction to gas chromatography
Gas chromatography is a chromatography technique that can separate and analyze volatile compounds in gas phase. Depending on stationary phase used in this analytical technique, there are two types of gas chromatography:
- Gas-liquid chromatography (GLC)
- Gas-solid chromatography (GSC).
Among these, GLC is most widely used method. A gas chromatography looks like:
Principle of gas chromatography
All chromatography have one stationary and one mobile phase. In this chromatography the mobile phase is always gas. But the stationary phase is either liquid or solid. If the stationary phase is solid, then that is called gas-solid chromatography or GSC. And if the stationary phase is liquid, then that is called gas-liquid chromatography or GLC. In GLC, the mobile gas phase is like helium and the stationary phase is high boiling point liquid adsorbed onto a solid. Like other chromatography, the mobile phase, for this case, is a chemically inert gas which carry the analyte through the heated column to separate to its individual compounds.
How does gas chromatography works?
Among various types of gas chromatography, GLC or gas-liquid chromatography is most popular method. This chromatography consists of an injection port, a column, a oven, a heater to control the temperature, a carrier gas flow control equipment and a detector.
An analyle in a very small quantity is injected into the machine through a rubber septum at injection port using a small syringe. The sample is then heated at 50 0C, above the boiling point of sample for vaporization. The sample vapor is then carried by mobile gas phase helium into the column.
There are two types of columns in gas chromatography: one is a long thin tube packed with stationary phase and the other is even thinner where the stationary phase bonded to the inner surface of the column. Here only the packed column is explained.
The column is normally made of stainless steel and coiled up that can easily fit inside an oven. The column is 1-4 meter long with a diameter of up to 4 mm. The column is packed with diatomaceous earth, a very porous rock. This finely grounded solid is coated with a high boiling liquid like waxy polymer.
How separation works
The separation in the column depends on three things:
- The components in the mixture can be carried along by the mobile phase, helium gas. It depends on the boiling point of certain components in the mixture and the temperature of the oven. If the boiling point is lower than the temperature of the oven the components will move along with the helium gas.
- The components can dissolved in the liquid stationary phase. The more soluble components will spend more time in liquid phase and less soluble components will spend less time in liquid phase. The less soluble compounds will have tendency to leave the liquid phase and move along with the gas phase.
- Some components can condense on the stationary phase. If the boiling point of some components are higher than the temperature of the oven, they may condense on the stationary phase. But it will eventually move on due to the long time exposure of heat like the water on the surface of the sea evaporates slowly in a hot day.
This way the molecules spend some time in the stationary phase and some time moves along with the mobile phase to reach to the detector.
The column stays inside a thermostatically controlled oven where temperature can be varied between 50-250 oC. This oven doesn’t allow heat transfer from or to the column. The temperature of the injection oven is higher than column oven. The high boiling components can be condensed in the beginning of the column. But as the analysis proceeds the temperature of the column oven rises which can be controlled if necessary.
The flame ionisation detector is most commonly used in Gas chromatography. In this type of detector organic molecules are burned by flame to get ions and electrons. The positive ions are attracted to cathode to gain electrons and they become neutralized. While the negative ions are attracted to anode to loss electrons electrons. So, The electron flows from electron rich anode to electron deficient cathode through the external circuit. This electric current is more if we have more organic compounds coming out of the column and it is less if there are less compounds. Depending on the amount of current passing through circuit the computer plots a graph and we can see that on display.