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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