A separation brought about by an exchange between a mobile gas phase and a liquid or solid stationary phase is known as Gas chromatography. They are of two types,
1. Gas Liquid Chromatography (GLC)
2. Gas Solid Chromatography (GSC)
The main components of Gas chromatography are:
Carrier Gas (Mobile phase): The mobile phase is a gas which is selected for its chemical inertness. Helium, nitrogen, argon and hydrogen are the most commonly used gases. Hydrogen, unless unavoidable, is not used as it is explosive. The gas used as the mobile phase is called a carrier gas.
Flow regulators: The flow of carrier gas is controlled by flow regulator.
Sample Injection system: A small quantity of sample (0.2µl) is inserted in the column through a rubber septum.
Column: The column is generally made of glass or stainless steel; depending upon the chemical reactivity of the substances to be separated. Columns may be 1 to 20 meters long and are usually coiled. The internal diameter of the column is generally about 4 mm, The column is packed with an inert support material such as celite. Alumina and micro glass beads have also been used.
Thermostat: Sample injection system, column and detectors must be kept at higher temperature. The column is operated at a known constant temperature which may vary from room temperature to 623K and this depends upon the volatility of the liquid phase.
Detector: The carrier gas, emerging from the column passes into a detector which gives an electric signal. The signal response is proportional to the concentration of the substance present in the carrier gas.
Working of Gas Chromatography
Consider a mixture containing three components A, B and C is to be separated by GLC. A very small quantity, 0.2 µl is injected into the front end of the column through a rubber septum by means of a syringe. The heating unit or the oven vaporizes the liquid and the vapor mixes with the carrier gas and the mixture sweeps through the column at a definite rate. Each component of the mixture in its passage through the column is retarded by the stationary liquid phase. The degree of retardation of a component depends upon the partition coefficient of that component in the liquid phase, i.e., the tendency of the component to dissolve in the liquid. Component (A) dissolves to the maximum extent, while component (C) dissolves to the least extent Component (C) emerges first along with the carrier gas followed by (B) and (A). The signals given by the detector are recorded. The development of the chromatogram can be obtained from grapher.
As the pure carrier gas flows through the column and passes in to the detector, a straight line is drawn by the pen recorder on the chart. This is the base line or the zero line. At the moment M, the sample is injected into the column. Peak C corresponds to the emergence of C, the most volatile or the least retained component in the mixture. When this component has passed, the properties of the gas mixture flowing out correspond approximately to those of the carrier gas and the curve drops to the base line. In the same manner B and A emerge one after another and peaks B and A are recorded. The distance from point M to L is the time during which the component C passes through the column. The rate of gas flow through the column being constant, a certain volume of gas flow through the column being constant, a certain volume of gas would have passed through the column in this time. This time is the retention time and the volume of the gas, the retention volume.