Gas chromatography principle is based on “partition” based separation of componenets. This chromatography is of two types like gas liquid chromatography (GLC) and gas solid chromatography (GSC). Of them GLC is widely used and so our entire discussion would be related to it.
Gas Chromatography & principle (principle of GC):
The principle in gas chromatography involves separation of volatile components of the sample based on their partition co-efficient. This is ratio of solubility of substance in between gaseous mobile phase and stationary liquid phase. The components of the sample that are partitioned into gas come out first while others come later.
Gas chromatography runs on the principle of partition chromatography for separation of components. The stationary phase is a liquid layer supported over a stationary phase while the mobile phase is an inert and stable gas. Hence the name as Gas-Liquid chromatography (GLC).
This type of chromatography was primarily designed to evaluate volatile compounds like fatty acids, essential oils, etc.
The system is quite expensive, cumbersome and also has delicate instrumentation. The instruments maintenance, and operating costs are very high.
How gas chromatography works:
The gas is set to flow at a constant rate from the cylinder on to the liquid layer impregnated on a solid support in a column. The sample is injected into the injection point and is carried by the mobile gas into the column. Inside the column, the components get separated by the differential partition in between the mobile phase gas and stationary phase liquid. The component that partitioned into gas comes out of the column first and is detected by the detector. The one partitioned into liquid phase comes out later and is also detected. The recordings are displayed onto a computer software. From these peaks, one can identify the components and also their concentration.
Must read article Gas Chromatography Theory for details on other important aspects of GC.
Gas chromatography method: Below is the video of the instrumentation and method simultaneously.
Gas chromatography instrumentation
The gas chromatography apparatus can be listed as
1. The mobile phase gas in a cylinder: The mobile phase is an inert gas (monoatomic element gases or non-reactive gases like nitrogen, helium & hydrogen. The carrier gas is kept in a metallic cylinder and outflow is controlled by a regulator. From the gas carrier cylinder, the gas is passed under a fixed rate through a pressure gauge which indicates the speed of flow of gas into the column. Most commonly used gas is helium.
2. The injection system: This is present before column yet inside the thermal chamber to load sample under analysis into the system.
3. The column for gas chromatography. The gas chromatography column is usually long (few meters like 3 to 6 meters) and coiled for accommodation into a small thermal chamber. The column is mostly made of steel or glass.
The GC columns are of three types viz.
♠ Packed column. This is a column into which solid beads are packed. This column has advantages like efficient separation and precise readings.
♠ Tubular column. Here into a stainless steel hollow tube a thin layer of liquid is coated to act as a stationary phase. This column offers the least resistance to the flow of gas.
♠ Support coated tubular column. Here into stainless steel column, a thin solid layer is coated on to which a thin layer of liquid stationary phase is present.
4. The Detector: is another vital component of the gas chromatography apparatus. GC detectors detect the isolated components and help in the identification and quantification of the sample. They are of 4 types of GC detectors like
♦ Thermal conductivity detector: Here there are two columns which have a conducting wire in between. The gas is allowed to pass through the two columns of detectors, i.e., to one the effluent from gas chromatography column and to other gas from the gas cylinder directly. Since the temperature of both gases is the same, the thermal conduction is constant.
When the sample is injected into the gas chromatography column. The effluent gas carries the sample components into the detector column. Since effluent gas is mixed with sample components there results in the difference in thermal conductivity from prior one recording. This difference in conductivity is specific for the component analyzed. This is recorded for further comparison and identification of the components and their quantity.
♦ Flame ionization detectors: Here the sample components from effluent are ionized by subjecting to flame in a chamber. These ions raise upwards and are attracted towards anode or cathode based on the charge on them. When they impinge on the electrodes, the current is passed which is recorded. The strength and intensity of current depending on the sample and is specific.
♦ Argon ionization detector; These detectors are similar to flame ionization detectors with the only difference that argon ion gas is used to ionize the sample molecules. The argon ions are obtained by reacting argon gas with radioactive elements. Once argon ionizes they try to get back to the stable state by either taking or giving electron from the sample components thus making sample molecules to ions for detection.
♦ Electron capture detector, etc.
5. The computer to record the analyzed readings. This is connected with the detector and hence records the detector changes in reference to the flow of separated components from the exit of the column. The record is called gas chromatograph.
5. The thermal chamber to fix or maintain a fixed temperature.
* Precolumn and post column treatment of sample (if necessary). This is done to modify the sample. The sample should be stable on heating and also be separated properly. For this precolumn derivatization is used and for the sample to be detected properly, post-column derivatization is done by making a suitable chemical change.
As a further improvement in GC, the gas chromatography apparatus is fixed with Mass spectroscopy system (GC-ms) for better analysis of components regarding their mass.