Principle and Working of Gas Chromatography

Gas chromatography is one of the most useful analytical tools in pharmaceutical and chemical laboratories. Because of its reliability and accuracy, this technique has been used for decades to analyze chemical and pharmaceutical products. 

Whether you are a student willing to enter the analytical field or an analytical professional, it is essential to understand the working of gas chromatography. In this article, we will understand the principle and working of gas chromatography.

What is Gas Chromatography?

The use of gas chromatography is similar to high-performance liquid chromatography. It is also used to identify and quantify components in a mixture. It is particularly used for determining the volatile substances present in pharmaceutical products, while in the chemical industry, it is used to determine the purity of organic compounds.

The Principle of Gas Chromatography

The separation of compounds using a column is the core principle of gas chromatography. The compounds from a mixture are separated based on their interaction with the gas mobile phase and the polymer stationary phase. Carrier gas, like helium, hydrogen or nitrogen, works as a mobile phase, while the stationary phase is a polymer coated inside the column.

Compounds in the sample travel through the column at a different speed that depends upon the vaporization of the compound and its interaction with the stationary phase. If a compound interacts more with the stationary phase takes a longer time to travel in the column, while other compounds that interact less with the stationary phase take a shorter time to come out of the column. This difference in time creates a separation of compounds.

Components of a Gas Chromatography System

To understand the working of gas chromatography, it is required to know its main parts.

1. Carrier Gas

In gas chromatography, gas works as the mobile phase. Usually, helium, nitrogen and hydrogen gases are used as carrier gases. It works as a vehicle for the sample to travel through the system.

2. Injector Port

The injector port is the part where the sample is injected. The sample is instantly vaporized due to the high temperature of the injector port (250-300°C). From here, the vaporized sample travels into the column with the carrier gas.

3. Column

The column is the heart of gas chromatography, where separation happens. GC columns are generally made up of stainless steel or glass. The column is coiled inside an oven that heats it up to the required temperature. The inner side of the column is coated with the stationary phase. Columns are of two types, packet columns, which are used for older systems or large samples and capillary columns are modern, more efficient and widely used in gas chromatography. Column length varies from 30 meters to 100 meters, depending on the nature of the compound to be analysed.

4. Oven

The oven maintains the temperature of the column as required for simple analysis. Depending on the sample’s nature, oven temperature can be kept constant for isothermal analysis or gradually increased for temperature programming.

5. Detector

The detector identifies the compounds when they exit from the column. Commonly used detectors in gas chromatography are:

A. Flame Ionization Detector (FID)

The FID detector is widely used in pharmaceutical and chemical industries for gas chromatography, especially for analysis of organic solvents. The organic compounds separated by column are burned in a hydrogen flame. Carbon atoms of hydrocarbons are ionized by the flame and these ions produce a current between two electrodes. This current is measured by the detector. The intensity of the current is directly proportional to the number of carbon atoms present in the sample, which helps to identify the compound.

B. Thermal Conductivity Detector (TCD)

This is a multipurpose detector that can detect any compound, whether it is organic or inorganic. The only limitation is that the thermal conductivity of the compound must be different from the carrier gas. When a compound exits from the column, it changes the thermal conductivity of the carrier gas. This change is detected and the compound is identified.

C. Mass Spectrometry (GC-MS)

In this detector, mass spectroscopy is coupled with gas chromatography. It not only detects the compound but also identifies its structure. After exiting the column, compounds enter the mass spectrometer, where they are bombarded with electrons that break them into charged fragments. These fragments are shortened on the basis of their mass-to-charge ratio using a mass analyzer. Here detector records the abundance of each fragment and creates a mass spectrum that is identical for different compounds.

Step-by-Step Working of Gas Chromatography

Let us understand the working of gas chromatography in four simple steps.

Step 1: Sample Injection

A very small quantity of liquid sample (generally 5-10 microliters) is injected into the injector port using a microliter syringe. The port remains heated, converting the liquid instantly into gas.

Step 2: Vaporization and Transportation

The paparazzi sample enters the column by the current of carrier gas. From this point, simple travels in the column with the help of carrier gas.

Step 3: Separation Inside the Column

Sample mixture travels in the column where each component interacts at a different level with the stationary phase. More volatile components interact less with the stationary phase and elute faster from the column, while others take longer. This causes a separation between the components of the mixture. Each component takes a specific time to pass the column and reach the detector.

Step 4: Detection

After exiting the column, each component hits the detector. The detector sends an electric signal to the computer, where it displays a chromatogram having peaks for each component. The time that each component takes to separate is known as your retention time. Each compound has a specific retention time. The size of a peak in the chromatogram shows the amount of compound.

Advantages of Gas Chromatography

Gas chromatography has many advantages, due to which it is still relevant to use in pharmaceutical analysis. Some important advantages are

• It is highly sensitive and detects the substance in minute quantities.
• It can analyze the samples in minutes.
• It is very useful for quantitative analysis because of its accuracy and reproducibility.
• It is useful in the pharma and petrochemical industry because it can analyze volatile substances.

Limitations

• It can analyze the volatile compounds that can vaporize without decomposition
• Simple and instrument preparation is time-consuming
• It cannot be used for analyzing large molecules like proteins

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