Gas Chromatography Theory:
Gas chromatography relies on the separation of components of a volatile compound. Under the influence of a gaseous solvent flow which acts as mobile phase, the compounds are separated over a liquid stationary phase.
The compounds having partition into gas elute first out of the column. While those with greater partition into stationary liquid phase elute out last. The carrier gas is inert and does not react with the sample and also a detector.
Though the principle of gas chromatography looks so simple, it is cumbersome. There is some reasonable amount of theory regarding its instrumentation and operation. The operation heavily relies on sophisticated instrumentation.
Gas chromatography theory of instrumentation design
1. Gas cylinder: The mobile gas used in gas chromatography is inert, inexpensive, non-explosive and very pure in its composition.
Some of the examples are nitrogen and helium.
2. The flow regulator lets the gas from the cylinder pass with a fixed rate. Any alteration in the rate of gas flow leads to improper measurement and analysis.
3. GC Columns: The GC columns are metallic column preferably. They are very long and hence coiled to reduce the space required. Theoretically, the rise in the length of column enhance the separation capacity and thereby better resolution column is so long unlike other chromatography columns.
This is because the gas flow rate is very high in compared to other mobile phase flow in other systems. In other systems liquid is the mobile phase and hence the flow rate is far behind that of gas. Therefore to provide sufficient separation, the column is more. Increase in column length increases the area of exposure of components to the mobile phase better and they are not just carried away under the speed of gas flow.
4. Thermal chamber: This encloses the column and maintains the column at a fixed temperature. Maintenance of column at high temperature helps the sample to remain in volatile and thereby easy flow and separation of the components under the influence of mobile gas.
5. Detector: the detector used is mostly thermo-conductive detector whose conductivity changes with the gas flow and also with gas mixed with components. Thus recordings of changes in conductivity are noted.
6. Pre-column treatment: this is a technique used to modify the constitution of the sample before its entry into the column. The low volatile compounds are chemically modified to the highly volatile compound. This helps in better separation and also the identification of the compounds.
7. Post-column treatment: This is intended to alter the components coming out of the column to be easily detected by the detector.
The inclusion of pre-column and post column treatment or derivitization helps to increase the range of the compounds that can be estimated by gas chromatography. That is the compounds which have low volatility can be determined by making them volatile by pre-column treatment and those compound with poor detection by the detector can be altered for easy detection by the detector.
The entire system should be placed inside a chromatography lab. This should be well equipped with proper lighting, space and also air conditioning.
See the applications of gas chromatography.