SIMPLE AND EASY DISCUSSION ABOUT GAS CHROMATOGRAPHY

2.  Introduction
 It is one of the separation technique
 Gas is used as the mobile phase, solid or liquid are used as the stationary phase
 Solid is not mostly used, liquid stationary phase is used
 GSC is not widely used
 GSC: principle is adsorption
 GLC: principle is partition
PRINCIPLE:
 Partition, the liquid coated on solid support then used as stationary phase
 The components are separated according to the solubilities
 More soluble in stationary phase: elute latter
 Less soluble in stationary phase: elute first
 The components are separated according to partition coefficient

3. REQUIREMENT FOR COMPOUND ANALYSED BY GLC:
1. Volatility: the compound possible to convert volatile state then only it is mixed with mobile
phase
2. Thermo stability: All the compounds convert into vaporised state by using high temperature then
only used, so at high temperature the compound is stable
REQUIREMENTS:
 Carrier gas
 Flow regulator
 Injection device
 Column
 Temperature control device
 Detectors
 Recorders

5. CARRIER GAS:
 Mostly used inert gases for mobile phase
 Eg: hydrogen , helium, nitrogen and argon
 Hydrogen:
 Thermal conductivity is more, low density, very useful in case of thermal conductivity
and flame ionisation detector
 Disadvantage: it reacts with unsaturated compounds

6. HELIUM:
 Advantage: Thermal conductivity is more.
 Disadvantage: Expensive
REQUIREMENTS OF CARRIER GAS
 Inertness
 Suitable to detector
 High purity
 Easily available and cheap
 Less risk of explosion or fire hazards
FLOW REGULATORS AND FLOW METERS:
 The carrier gas supplied at high pressure so to use flow regulator, to regulate the pressure
 Flow meter to regulate the flow rate of the gas
 Eg: rotameter, soap bubble flow meter

7. Rotameter:
 It is appearing like glass tube containing spring
 Fixed before the column inlet
 To pass the gas
 The spring is floated then measure the flow rate
Soap bubble meter:
 Instead of spring to use soap solution
 To pass the gas solution soap bubble is form then it travels particular pressure
INJECTION DEVICE:
 To use gas, liquid or solid samples
 Gases are introduced by using valve devices
 Liquid are introduced by using septum device
 Solid: dissolve in particular solvent then introduced in to column

8. COLUMNS:
 Important part of the instrument
 Made of glass or stainless steel
 Two types
1. Depending on its use: Analytical column, Preparative column
2. Depending on its nature: Packed column, open tubular or capillary column
ANALYTICAL COLUMN:
 Length:1-1.5m, diameter: 3-6mm
 To use only small quantity of sample
PREPARATIVE COLUMN:
 Length: 3-6m, Diameter: 6-9 mm
 To use large amount of samples. Possible to loaded

9. PACKED COLUMN:
 Commercially available
 Available in polar and non polar nature
 Eg: polydimethyl.siloxane, poly siloxane, poly alkylene glycol, polyethylene glycol
 Temperature range 60-320ºC
CAPILLARY COLUMN:
 Made of capillary tubing of 30-90m in length
 Diameter(internal)0.025-0.075cm
 Made of stainless steel in the form of coil
 The inner wall is coated with stationary phase liquid of a thin film 0.5-1µ
 These have least resistance to the flow of carrier gas
SCOT COLUMN:
Support coated open tubular column

10.  It is improved type of capillary column
 The supporting material is micron size in porus layer
 The the lliquid stationary phase is coated so to get more sample holding
PREHEATERS:
 For GC to convert the sample in to vaporised state
 The preheaters are present along with injection device
THERMOSTATICALLY CONTROLLED OVEN:
 To keep the column in thermostatically controlled oven by maintaining the temperature only to get
equal distribution between the solutes in mobile phase
 Column is kept at high temperature oven
 The columns are highly accurate and it maintains temperature to 0.1ºC

11.  Two types of operations are possible:
1. Isothermal programming: to maintain the same temperature up to the process
2. Linear programming: the oven is heated linearly over a period of time. To fix the temperature
initially 150ºC but at the end of process it becomes 200ºC
DETECTORS:
 To detect the component after separation mainly to detect the component present in the carrier gas
 Applicable to wide range of sample
 High sensitivity
 Rapid response
 Linearity
 Not to destructive the sample
 Simple to maintain
 In expensive

12. THERMAL CONDUCTIVITY DETECTOR[KATHAROMETER]
Principle: to measure the thermal conductivity difference between carrier gas and that of
katharometer
Method:
 It consist of two platinum wirs
 Both connected to wheatstone bridge
 One side pass only carrier gas other side to pass eluent from column
 The two platinum wires heated electrically
 To maintain temperature and electrical resistance
 To measure the baseline
 The compound eluted from column to get electrical resistance
 It is amplified and recorded
 Hydrogen and helium is most widely used carrier gas good thermal conductivity
Advantage:
 It is used for wide range of compounds, good linearity and simple

13. KATHAROMETER

14. Disadvantage:
 Low sensitivity, affected by fluctuation in temperature and flow rateresponse is only relative
 Biological samples cant be analysed
FLAME IONISATION DETECTOR:
 The ionisation detectors are based upon the electrical conductivity of carrier gas
 At normal temperature and pressure gasses act as insulators, but become conductive if ions are
present
Carrier gas: Hydrogen if used any other carrier gas to mix with hydrogen then use it
Advantage:
 Extremly sensitive
 Stable, organic compounds mostly used, linearity is excellent
 To pass the carrier gas from the column
 If only hydrogen or carrier gas no ionisation
 Produce electrical pulse
 Compound is present ionisation takes place

16. ARGON IONISATION DETECTOR:
 Argon atoms are excited by using alpha , beta particles
 Meta stable state --------- pass carrier gas with effulent-------------argon are ionised------- then
produce electrical pulse
 Argon treated with Radium.D or tririum, it goes to metastable state-------pass the carrier gas---
ionised, electric current pulse
Applications:
 Qualitative: to identify the compounds, to check the purity, identification of impurities
 Quantitative: %purity,
Direct comparison method
Calibration curve method
Internal standard method