Gas Chromatography : Module 4

Role of Gases in Gas Chromatography

 “Formal education will make you a living, self education will make you a fortune “   — Jim Rohn

Gases play a crucial role in a Gas Chromatography system

• Transportation of injected sample to the column and subsequent transfer of separated components to the detector
• Act as fuel to support combustion in the detector
• Support combustion process in the flame when using flame ionization detection

Color coding of gases

Handling of Gases requires special precautionary measures due to their characteristics and hazard potential. Color codes have been adopted universally for easy identification and safety measures during handling.

Colour Coding of Gases

Carrier Gas

Carrier Gases

A carrier gas facilitates transport of the injected sample to the column for separation of components of the mixture. Typical purity levels should be 99.995% or higher. Desirable features of carrier gas are :

• Inertness towards sample and stationary phase
• Should not have a response on the detector

Commonly used gases are Nitrogen, Helium and Argon. Helium gas is preferred as carrier with TCD detector because of high explosion hazard of hydrogen. Traps are recommended in the gas lines to prevent moisture or other impurities from reaching the columns.

Combustion or Fuel Gas

Hydrogen is commonly used as fuel gas when using FID detection 

Hydrogen gas Oxidant Gas

Zero air which is atmospheric air free of atmospheric impurities and is used to support combustion of sample in FID detector.

Zero air Effect of flow rate of carrier gas :

• Increase of flow rate has same effect as increase of temperature i.e. analysis speed increases but at cost of resolution
• Flow rate can be measured in terms of volumetric flow with a soap bubble flow meter, rotameter or electronic flow controller
• Mass flow meter measures rate of mass flow of gas per unit time. Mass flow rate measurements are not affected by atmospheric temperature or pressure changes and by presence of water vapour in the gas stream.

Safe Handling of gases

Gas cylinders should be placed outside the instrument room protected from direct sunlight or rain. If placed inside the instrument room they should be secured properly to a lab bench or wall with chains. While transporting from one location to another the valve should be closed and cylinder cap fixed.

Tip of the day

Use of high purity gases along with on-line traps will ensure consistency of results.

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