Gas Chromatography - Instrumental Methods of Analysis B. Pharma 7th Semester

Gas Chromatography

Contents

       Gas chromatography

       Principle involved

       Criteria for compounds

Practical requirements

       Carrier gas

       Flow regulators and flow meters

       Injection devices

       Columns

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

       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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