What is Gas Chromatography?
It is a separation technique used to perform quantitative and qualitative analyses of different compounds present in a mixture. Typically, chromatography techniques require a stationary phase and a moving phase.
Image by Herney Gómez
The moving phase traverses over the stationary phase, and the mixture is separated based on the chemical interaction between the two phases. In gas chromatography, the moving phase used is a gaseous substance.
How does it work?
A gas chromatograph consists of the following elements:
- An inlet.
- A reservoir of mobile gas.
- A sampler.
- An analytical column in a column oven.
- A stationary phase.
- A detector.
- A computer to analyze results.
A sample of the mixture to be analyzed is taken by the sampler and inserted into the chromatograph’s inlet. If the sample is not in the vapor state, it is vaporized. It then mixes with the moving/carrier phase, which takes the sample into the analytical column.
Typically, inert gases are used as the carrier phase so that they do not react with the sample or the stationary phase. The analytical column is a thin, long, coiled pipe (typically 0.1 to 0.53 mm in diameter) with the stationary phase coated on its inside walls.
This setup is placed inside a column oven so that the sample can be heated to the desired temperature to separate the less volatile components. Through its journey in the analytical column, the sample interacts with the stationary phase, and its constituent compounds are separated from the mixture.
There’s a detector at the end of the column which detects the molecules that pass through it. As the mixture compounds are already separated in the analytical column, each compound is eluted out of the column at different times.
The time taken by the compound to come out of the column is called its retention time. A plot of the detector readings vs time, called a chromatogram, will help us analyze the sample constituents.
Types of chromatography
Based on the stationary phase
Based on the state of the stationary phase of the molecules or analytes, gas chromatography can be divided into two subtypes:
- Gas-Liquid Chromatography (GLC)
- Gas-Solid Chromatography (GSC).
Gas-Solid Chromatography (GSC) gets its name from having a solid stationary phase. Usually, it is the material with which the analytical column pipes are made. The separation of analytes occurs based on its adsorption properties with the solid.
These supports are usually made of aluminum or silica and sometimes contain a porous inside layer in the form of a fine mesh which improves the interaction with the analyte.
Gas-Liquid chromatography (GLC) uses a liquid stationary phase. There are many compounds available that can be used in GLC, making this method very versatile. The liquid creates a thin separating layer from the support material and interacts with the sample.
The separation happens because of the chemical bonding between the stationary phase and the sample. Choosing the correct stationary phase is critical. One should know what components might be present in the sample and its chemical bonding behavior.
2. Based on detectors or types of detectors
The detector is another critical component used in the system. There are two categories of detectors, general-purpose ones that analyze a wide range of compounds and special purpose detectors capable of detecting specific compounds with high sensitivity.
For example, atomic emission, photoionization, chemiluminescence, electron-capture, thermal conductivity, flame ionization, and mass spectrometer are some of the detectors used to measure the output sample.
The detectors are also classified based on their detection mechanism. Flame ionization detectors respond to carbon-hydrogen bonds and are the most widely used general-purpose detectors.
Specific element detectors sense particular elements like sulfur, nitrogen, and phosphorus, and they work on particular properties of the elements like their electron capturing ability, chemiluminescence, and flame photometry.
Gas Chromatography finds uses in many different kinds of fields due to its versatility and ease of use. Generally, it is used for quantitative and qualitative analysis of mixtures.
It can also determine the properties of specific elements found in a mixture (like specific heat, vapor pressure) without separating the mixture. It is used to extract pure compounds from a mixture in labs for chemical analysis.
Industries use gas chromatography to automate processes. It can be used to monitor the quantity of a particular component in a mixture so that a manual or automatic response can be planned to avoid undesirable variations leading to production inefficiencies.
The pharmaceutical and food industry uses gas chromatography to conduct quality control of its products. Environmentalists use chromatography to continuously monitor the quality of pollutants in the air.