Principle and procedure of gas chromatography

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Know the principle, types, applications of gas chromatography and how gas chromatography works.

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What is Gas Chromatography?

In analytical chemistry, gas chromatography (GC) is used to separate and determine the content of various analytes that can be vaporized without decomposition. Separation in gas chromatography depends on the volatility of the compost. The more volatile compound is quickly isolated, while the less volatile compound is slowly released. In this type of chromatography, the carrier gas is used as the mobile phase, usually an inert gas such as nitrogen and helium, and as the stationary phase, a very thin layer of polymer or liquid on an inert solid support, at inside. A metal or glass tube, called a column, is used.

Principle of gas chromatography:

Gas chromatography follows the principle of separation of volatile compounds with the mobile phase (gas) and the stationary phase (liquid or solid). The rate of separation of molecules through the column is based on the affinity for the stationary phase; the molecules that spread out in the gas come out first, while the others escape later.

The sample solution of complex mixtures injected into the instrument enters a gas stream which crosses the column and in which the various components are separated. The detector (usually FID, GC-MS) determines the number of analytes leaving the column. For analysis of a sample with an unknown concentration, a standard sample with a known concentration is injected into the sample injector. To calculate the concentration, peak area and retention time of the standard sample, it is compared to the test sample. In this type of chromatography, gas is always used as the mobile phase and the stationary phase is either solid or liquid.

Gas Chromatography Experimental Procedure:

Before starting a gas chromatography experiment, we must recognize the various components that are essential to carry out the process.

It is made up of four main elements.

1. Carrier gas: Since the carrier gas (hydrogen, helium) is used as the mobile phase in GC, it plays an important role in the isolation.

2. Injector: The port is intended to inject samples into the GC by manual or automatic sampling.

3. Oven: The temperature of the GC column is controlled by an oven to manage the separation and the retention time of the analytes.

4. GC column: A column, in which molecules, depending on their affinity for the mobile phase (gas) and the stationary phase, are separated into individual analytes.

5. Detector: TDetector used to determine the composition and concentration of a sample.

Gas Chromatography Procedure:

  • Maintain inlet and outlet gas pressure with a regulator mounted on the control panel.
  • Install the required column (packed/capillary) and it should not leak.
  • Create and download the method and sequence from the software and before the injection starts the flame. Parameters such as injector temperature, detector temperature, oven temperature, gas flow/pressure and sample/vial sequence, etc.
  • Prepare samples as needed.
  • Download the software method and light the flame before injection.
  • Saturate the mobile phase GC column to baseline.
  • Inject the sample manually with a syringe or autosampler, filling the vial at least half full.
  • Depending on their affinity for the stationary phase, the components of the sample mixture are isolated from the
  • At different times, the separated analytes arrive at the detector and are registered by the computerized system.
  • From the chromatogram, the retention time (RT), peak area, column efficiency, tailing factor, peak height and number of theoretical plates can be calculated.

Gas Chromatography Applications:

  • Gas chromatography is commonly used as a routine analytical technique in pharmaceutical industries.
  • It is used for both medical and forensic applications for the quantification of drugs and their metabolites in blood and urine.
  • GC is used in the analysis of pesticides and volatiles.
  • Gas chromatography techniques are used in many areas of forensic medicine.
  • GC is used in the analysis of flavors, fragrances and food products.
  • The GC is used to analyze organic compounds in environmental samples.

Advantages of gas chromatography:

  • The GC provides high sensitivity when used with a thermal detector.
  • GC has additional column length compared to high performance liquid chromatography, allowing complex mixtures of analytes to be separated with high resolution.
  • Sample analysis is faster than HPLC.
  • Gas chromatography has high resolving power compared to other methods such as HPLC, TLC and column chromatography etc.
  • A small amount of sample is needed for separation.
  • This method offers relatively high accuracy and precision.

Disadvantages of gas chromatography:

  • A major drawback of GC is that only volatile samples can be analyzed.
  • With the exception of mass spectroscopy, most GC detectors are destructive.
  • It is not able to recover isolated samples.
  • In HPLC and TLC, the selectivity is better compared to GC because a moving process can be easily replaced by changing.

The most frequently asked questions about chromatography are as follows.

What is the basic principle of GC?

GC separation is based on different distributions of the constituent molecules which are separated between the stationary phase (column) and the mobile phase (gas).

What are the types of gas chromatography?

Depending on the stationary phase used, there are two types of gas chromatography, such as gas-liquid chromatography (GLC) and gas-solid chromatography (GSC).

What is the main difference between GC and HPLC?

The main difference between GC and HPLC is the mobile phase used. In GC, the compounds in the sample mixture are separated using gas as the mobile phase, and in HPLC, the liquid is used as the mobile phase.

What is the main advantage of gas chromatography?

The main advantage of GC is fast analysis with high efficiency.

How many types of GC detectors are there?

Flame ionization (FID), electron capture (ECD), thermal conductivity (TCD), flame photometry (FPD), nitrogen phosphorus (NPD) and mass spectrometry (MS) are detectors used in gas chromatography.

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