GAS CHROMATOGRAPHY
- MAVEENA PEREIRA
198218
WHAT IS GAS CHROMATOGRAPHY?
• GC is the separation of the components of a mixture in vapour
or gas phase by interacting with stationary phases in a column.
• Also called as gas liquid chromatography (GLC) , vapour phase
chromatography or vapour liquid partition chromatography.
• 2 types of GC: 1) GLC : The mobile phase is unreactive gas,
such as N2 stationary phase consists of small amount of liquid.
2) GSC: The mobile phase is gas , but the stationary phase
consists of solid adsorbent.
PRINCIPLE:
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GC is used for separating stable volatile liquids with B.P less than 3500C and gaseous mixtures.
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The separation mechanism is mostly partition of the components of the sample mixture between
the mobile phase gas and stationary phase.
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The partitioning of the components is brought due to the differences in their boiling points and
relative affinities for the stationary and mobile phase. The components gets separated due to their
differential migration through the column.
High B.P liquids cannot be introduced into the instrument hence they are chemically modified.
The liquid samples in mg are vaporised immediately after introduction, mixed with the mobile
phase or carrier gas stream and passed through the stationary phase column.
The various factors that influence the separation of the components are:
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•
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Nature of the stationary phase
Temperature of the column
Length of the column
Flow rate of the carrier gas
GAS CHROMATOGRAPHY INSTRUMENT
COMPONENTS OF GAS CHROMATOGRAPH:
• Carrier gas cylinder: Helium or hydrogen is preferred gas due to greater
chromatographic efficiency
• The sample injection port: Facilitates the introduction of the sample introduction
of sample into carrier gas stream
• Column oven : An air oven which houses the column. It maintains a desired
temperature upto 4000C
• Columns: There are two types of columns packed and capillary. Packed column
contain a finely divided ,inert, solid supportive material coated with liquid
stationary phase. Capillary columns have an internal diameter of a few tenths of a
millimetre.3 types of columns are used straight, shaped and coiled.
• Flow controller: The flow of the carrier gas is controlled by the flow controller
knob.
• Pressure gauges: They provide sign if something is wrong with the gas supply. 40
– 80 psi of gas.
• Vent: The opening that allows the gas or liquid to pass out or into the confined
space.
• Data processor: The data processor amplifies the electrical signal
from the detector and transfers the signal to display unit.
• Recorder: A device that draws the chromatogram that results from a
chromatographic process onto chart paper and provides the visual
representation of the separation.
INSTRUMENTATI
ON:
Fig: schematic diagram of Gas chromatograph
COMPONENTS OF
INSTRUMENTATION:
1. A carrier gas which is maintained at a high pressure and is
delivered to the instrument at a rapid and reproductive rate.
2. A sample injection system
3. The separation column
4. One or more detectors
5. Thermostated chambers for the temperature regulation of the
column and detectors
6. An amplification and recorder system
METHODOLOGY
:• The sample to be separated is mixed with an approximate volume of heptane, acetone, ethanol
• The separation is carried out in a tubular column made up of glass, metal or teflon, in this column a sorbent is
filled as the stationary phase.
• The adsorbents are packed in the form of fine gaze graded powder while liquids are coated as fine film on the
column wall.
• A gas serving as the mobile phase, known as the carrier gas flows continuously through the column. The
sample is introduced in the vapour form at the carrier gas entrance end of the column.
• In sample inlet the sample is introduced and then it will enter the column, in column elution takes place and the
temperature of column oven is increases to 4000C.
• Once the sample is volatilised it passes down the column where the separation occurs.
• Separation occurs based on the interaction of molecules between mobile and stationary phase.
• The less volatile interact more with stationary phase and move slowly where as the more volatile molecules
interact more with mobile phase and move fast down the column.
• If the size of the column is small we used straight column, slightly high- shaped, very long- coiled column.
• If the temperature is changed in between the analysis that method is known as temperature programming.
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Once the separation is completed the detection is done with the detector
attached at the end of the column.
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Flame ionisation detector has 3 inlets, one for carrier gas, other two for
hydrogen and oxygen
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Ignitor ignites hydrogen and oxygen to produce the flame. When the sample
molecule reaches the flame they get ionised and electrons re released.
•
Across the flame there are 2 electrodes that with +ve and –ve charge. They
detect the electrons generated by the ionisation of the sample.
•
Electrons are detected in the form of current which is then amplified by the
ADVANTAGES OF GAS CHROMATOGRAPHY:
• The technique has strong separating power and even the
complex can be resolved into constituents.
• The sensitivity of the method is high. It is a micromethod and
only a few mg of the sample is sufficient for analysis.
• It gives good precision and accuracy.
• It requires short time.
• The cost of the instrument is low.
• It is suitable for routine analysis as it does not require highly
skilled persons.
DIFFERENCES BETWEEN GSC AND
GLC
GSC
GLC
1.The stationary phase is in solid state.
1. The stationary phase is in liquid state
2.The principle is adsorption
chromatography technique
2. The principle is partition
chromatography technique.
3.The retention time is long
3. The retention time is sort
4. No liquid phase is involved.
4. At least one liquid phase is involved.
5. Very small concentrations of samples
can be used
5. Higher concentrations of samples can
be used
APPLICATIONS OF GAS CHROMATOGRAPHY:
• Qualitative Analysis:
By comparing the retention times or volumes of the unknown to
the retention time of a series of standards or
By collecting the individual components as they emerge from the
chromatograph identifying these components by other methods.
• Quantitative Analysis:
GC helps in the measurement of the quantities of particular
constituents present in the sample.
MISCELLANEOUS
APPLICATIONS:
• The detection of steroid drugs used by athletes in sports and steroids
given to animals are carried by GLC. Pollutants such as CO, benzene
are monitored by GLC.
• In separation and identification of lipids, protiens, carbohydrates,
preservatives, flavors etc.
• Analyse the dairy products by GLC for aldehydes and ketones , fatty
acids and milk sugars.
• Pyrolysis GC is used for separation and identification of volatile
materials like plastics, natural and synthetic polymers, paints and
microbiological samples.
• GC is used in separation and identification of polycyclic aromatic
hydrocarbons, chlorinated pesticides(eg : DDT), organophosphates,
phenols, amines etc.
REFERENCES:
• Instrumental methods of analysis- B.Sivasankar