What is gas chromatography?
Gas chromatography (GC) in analytical chemistry is a common type of chromatography where the mobile phase is a gas and the stationary phase is separated as a vapour. The mobile phase which we usually used in gas chromatography is noble gas or an unreactive gas such as helium, argon, nitrogen, and hydrogen. A sample is injected into a heating block where the compound is vaporized. Gas chromatography got a boost when gas liquid chromatography was invented with good instrumentation. It is used for testing the purity of substances or the separation of a particular component from a mixture. The success and failure of gas chromatography analysis or separation depend mainly on the retention time.
Instrumentation of gas chromatography
The instrumentation of gas chromatography is given below the picture,
A good gas chromatography machine contains the following important components,
- Pressure regulator
- Sample injection port
- Gas chromatography column
- Stationary phase
- Signal recorder
Pressure is adjusted within the limits of 1 to 4 atmospheres while the flow control valve measures 1 to 1000 liters per minute of gas. Flow valves are adjustable by a needle valve mounted on the base. The preferred carrier gas may be helium, argon, nitrogen, and hydrogen due to their high thermal conductivity.
Sample injection port
Samples are injected by a microsyringe through a self-sealing silicon rubber septum in a heated metal. The metal bock is a heated electrical heater. We used different sizes of injection ports for the injection of a sample.
Gas chromatography column
The gas chromatography column can be made by tubing coiled into an open spiral. We used copper or stainless steel for high-temperature operation. The velocity of carrier gas flow rate depends on the inner diameter of the chromatographic column. The usual size of the column is 2 meters.
The detector can detect the arrival of components coming from the column to provide an electrical signal. Pressure and temperature detector are the two major groups of detectors used in gas chromatography. The detector in gas chromatography instrumentation is situated near the column to avoid the condensation of liquids or detect the sample before decomposition.
In a packed column gas chromatography instrumentation, we used mostly a thermal conductivity detector (TCD) or a flame ionization detector (FID). Among these TCD is the most popular. A flame ionization detector (FID) is most useful where the effluent is suitably attenuated by a stream splitter. TCD detector contains four heat sensing elements made by thermistors or resistance wires. The thermometers are electronic semiconductors of fused metal oxides whose electrical resistance varies with temperature.
Stationary phase in gas chromatography
Gas liquid chromatography can be available in almost an infinite variety of liquid partition materials. The liquid or stationary phase in gas chromatography can be divided into nonpolar, intermediate polarity, polar carbowaxes, and hydrogen bonding compounds like glycol. The maximum temperature of the stationary phase can be determined by its volatility. The excess volatility of the stationary phase can shorten the life of the column.
Loading of the column by stationary phase can be expressed by percentage of weight. For example, 15% means, a 100 gm column has 15 gm of stationary phase.
|Stationary phase in gas chromatography|
|GLC||Squalene, silicon oil, nonpolar polyethylene, glycol, glass Teflon beads, etc.|
|GSC (usual)||Silica gel, alumina, charcoal, molecular sieve inorganic salts, mineral, porous polymers.|
|GSC (reverse-phase)||Silica alumina coated with organic or inorganic compounds or complexes.|
The choice of recorder determines the ultimate accuracy of the gas chromatogram. The full-scale pass response should be 1 second. Sometimes amplification of signals is essential to give a gas chromatogram.
In the gas chromatogram, we have a Gaussian error function curve that is symmetrical. If a substance has no affinity for the stationary phase, the partition coefficient, K = 0. Therefore, it will not be retained by the column.
Principle and working of gas chromatography
Separation in gas chromatography is feasible by partitioning the sample between a mobile gas phase and a thin layer stationary phase of nonvolatile, high boiling liquid held on the solid support. The idea of fractioning gases by passage over solid or immobilized gases was first introduced in 1941. It is popular after 1955.
- A sample is injected into a heating block where the compound is readily vaporized. The sample vapour is carried by the carrier gas into the column inlet.
- The solute is absorbed in the head of the column by the stationary phase. It is traveled at its own rate through the column according to its partition coefficient value.
- Solutes are eluted according to their partition coefficient and enter into the detector.
- In the detector, solutes give a series of signals resulting from concentration changes and different rates of elution.
- The recorder provides a plot of the time ageist composition of the carrier gas stream. The peaks of the plot give the quantitative data in the gas chromatogram.
Applications of gas chromatography
Gas chromatography is usually used for the identification and analysis of organic molecules and polymers. In elemental analysis, the class of pyrolysis reactions where the organic products break down into carbon dioxide and water can be carried out by a gas chromatography instrument. It can be used in a wide number of industries with different types of applications.
- Food analysis: It is an important branch of analytical chemistry that can be used for the analysis and separation of food products. The gas chromatography technique is used widely for quantitative and qualitative analysis of food. The technique is used mainly for the analysis of food additives, flavors, and aroma. It can detect toxic elements such as pesticides, fumigants, and naturally occurring toxins in food. Therefore, this technique ensures the safety of food products.
- Quality control: The industries which produce cars, chemicals, and pharmaceuticals can be used gas chromatography instrumentation for quality control. Many chemical and pharmaceutical industries produce pure and large quantities of products by the application of gas chromatography.
- Research: Many new research papers can be published with the help of gas chromatography techniques. It is used mainly for researching and analyzing natural products and meteorites that fall to earth from space.
- Forensic analysis: The gas chromatography instrument is applicable in many forensic investigation and detection. For example, the causes and time of death can be investigated by this instrument.
- Air pollution: Air pollution is a major problem in today’s life. It can be negatively affected our health. Gas chromatography instrumentation is used for monitoring levels of air pollution caused by different types of air pollutants.
Advantages of gas chromatography
There are several advantages of gas chromatography. The most common advantages are,
- We used longer columns to give high separation efficiency. Gas chromatography columns are available in a wide range of diameter and length. Therefore, it offers a wide choice of applications for a wide range of separation.
- It has a wide range of freedom to change operational parameters such as chromatographic column, temperature programming, carrier gas flow rate, etc.
- Very convenient change of operational parameter during gas chromatographic process gives high resolution in the shortest possible time for the operation. It is not available in other chromatographic methods such as thin layer chromatography. Very good speed is obtained from the low viscosity of gases or vapor.
- It is ideal for the analysis of volatile compounds. Therefore, gas photography instrumentation is used for the analysis or separation of a wide range of compounds.