Gas Chromatography

Objectives

By the end of this session, students will be able to:

Ø  Explain the principle involved in Gas Chromatography & outline the components of GC

Ø  State ideal characteristics of carrier gas

Gas Chromatography

•       Consists of gas solid chromatography (GSC) and

•       Gas liquid chromatography (GLC)

•       In both types gas is used as mobile phase

•       Either solid or liquid as stationary phase

Gas Solid Chromatography (GSC)

•       Not widely used because of limited number of stationary phases available

•       Principle of separation is adsorption

•       Used for low solubility of solutes in stationary phase  

•       It happens very rare

•       We discuss more about GLC

Gas Liquid Chromatography (GLC)

•       Principle involved is partition

•       Gas is used as mobile phase

•       Liquid which is coated on to a solid support is used as stationary phase

•       Mixture of components to be separated is converted to vapor and mixed with gaseous mobile phase

•       Component which is more soluble in stationary phase travels slower and eluted later

•       Component which is less soluble in stationary phase travels faster and eluted first

•       No two components have same partition coefficient for a fixed combination of stationary phase, mobile phase and other conditions

•       Components are separated according to their partition coefficients

Criteria for Compounds

•       Criteria for compounds to be analysed by gas chromatography

Volatility

•       Unless a compound is volatile, cannot be mixed with mobile phase

Thermostability

•       All the compounds will not be in the form of vapor

•       Will be solid or liquid samples

•       To convert them vapor, have to be heated to a higher temperature

•       At that temperature, compounds have to be thermostable

•       Only thermostable compounds can be analysed by GC

Practical Requirements for gas chromatography

•       Carrier gas

•       Flow regulators and flow meters

•       Injection devices

•       Columns

•       Temperature control devices

•       Detectors

•       Recorders and Integrators

Carrier Gas

•       Choice of carrier gas determines the efficiency of chromatographic separation

•       Most widely used carrier gas are hydrogen, helium, nitrogen and argon

Hydrogen

•       It has better thermal conductivity, low density

•       Useful in case of thermal conductivity detector and flame ionization detector

•       Drawback- reacts with unsaturated compounds

•       It is inflammable

Helium

•       Has excellent thermal conductivity

•       It is expensive

•       Good carrier gas when used with thermal conductivity detector

Nitrogen

•       Inexpensive but has reduced sensitivity

Requirements of carrier gas

•       Inertness

•       Suitable to the detector used

•       High purity

•       Easily available

•       Cheap

•       Less risk of explosion or fire hazards

•       Should give best column performance consistent with required speed of analysis

•       Considering the requirements nitrogen and helium are the most commonly used carrier gas

•       Carrier gases are compressible, stored under high pressure in cylinders

Flow regulators

•       Used to deliver the gas with uniform pressure or flow rate

Flow meters

•       Used to measure the flow rate of carrier of gas

•       Rotameter and soap bubble flow meter

Flow Meters

Rotameter

•       Placed conveniently before column inlet

•       Has an ordinary glass tube (like burette) with a float held on to a spring

•       Level of float is determined by the flow rate of carrier gas

•       It is precalibrated

Soap bubble meter

•       Similar to rotameter

•       Instead of float, soap bubble indicates the flow rate

•       It has a glass tube with a inlet tube at the bottom through which gas comes in

•       Rubber bulb is used to store soap solution

•       Bulb is gently pressed for release of soap bubble by pressure of carrier gas and travels up

•       Distance travelled upwards is a measure of flow rate of carrier gas

•       Graduations are precalibrated

Injection devices

•       Samples for introducing into the column can be of any type

•       Either gas, liquid or solid in nature

•       Gases can be introduced into column by valve devices

•       Liquids can be injected through loop or septum devices

•       Most GC instruments have high quality rubber septum through which sample solution is injected  

Injection Devices

•       Solid samples are dissolved in a suitable solvent and injected through a septum

Columns

•       Important part of GC which decides separation efficiency

•       Made up of glass or stainless steel

•       Stainless steel columns have advantage of long life

•       Can be easily handled without the fear of fragility

•       But some samples react with them

•       In such cases glass columns are used (Example: steroids)

•       Have the advantage of inert

•       But highly fragile and difficult to handle

Columns

•       Can be classified according to nature and use

•       Depending on its use

Analytical column

•       Have a length of 1–1.5 metres and outer diameter of 3-6 mm

•       Packed columns and are made up of glass or stainless steel

•       Only small quantity of samples can be loaded on to the column

Preparative column

•       Larger when compared to analytical columns

•       Large amount of samples can be loaded

•       Have a length of 3-6 metres and outside diameter of 6-9 mm

Summary

•       Consists of gas solid chromatography (GSC) and Gas liquid chromatography (GLC)

•       In both types gas is used as mobile phase

•       Liquid which is coated on to a solid support is used as stationary phase in GLC

•       Component which is more soluble in stationary phase travels slower and eluted later

•       No two components have same partition coefficient for a fixed combination of stationary phase, mobile phase and other conditions

•       Criteria for compounds to be analysed by gas chromatography- should be volatile and thermostable

•       Choice of carrier gas determines the efficiency of chromatographic separation

•       Most widely used carrier gas are hydrogen, helium, nitrogen and argon

•       Flow meters- Rotameter and Soap bubble meter

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