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BENGAL SCHOOL OF TECHNOLOGY
The suggestion that separation of components of a
mixture in the gaseous state could be achieved using a
gaseous mobile phase was first Martin and Synge in
The first description of instrumentation and
application was made by James and Martin in 1952.
Gas chromatography is a technique used for
separation of volatile substances, or substances that
can be made volatile, from one another in a gaseous
mixture at high temperatures.
Types of GC
Gas Solid Chromatography(GSC)
The stationary phase, in this case, is a solid. It is the affinity
of solutes towards adsorption onto the stationary phase
which determines, in part, the retention time. The mobile
phase is, of course, a suitable carrier gas. This gas
chromatographic technique is most useful for the
separation and analysis of gases like CH4, CO2, CO, ... etc.
Gas Liquid Chromatography(GLC)
The stationary phase is a liquid with very low volatility
while the mobile phase is a suitable carrier gas. GLC is the
most widely used technique for separation of volatile
A carrier gas should have the following
properties:Highly pure (> 99.9%): inert: higher density:
compatible with the detector: cheap and available.
The carrier gas pressure ranges from 10-50 psi.
Depending on the column dimensions, flow rates from
1-150 mL/min are reported. Conventional analytical
columns usually use flow rates in the range from 20-50
mL/min while capillary columns use flow rates from 1-5
Commonly used gases include nitrogen, helium,
argon, and carbon dioxide.
Sample Injection Systems
Septum type injectors are the most common. These are
composed of a glass tube where vaporization of the sample
The sample is introduced into the injector through a self-
sealing silicone rubber septum.
The carrier gas flows through the injector carrying
The temperature of the injector should be adjusted so that
flash vaporization of all solutes occurs. If the temperature
of the injector is not high enough (at least 50 degrees above
highest boiling component), band broadening will take
Sample vials are glass, throw away type with vapor-
tight septum caps.
The sampler flushes the syringe with new sample to
remove traces of previous sample.
Pumps new sample to wet the syringe to remove any
bubbles, takes in a precisely measured sample and
pumps in to the gas chromatograph.
Purge and Trap sampling
Volatile organic samples can be purged from the sample
and trapped on Tenax GC contained in an 11 cm tube.
Tenax-GC is a porous polymer based on 2,6-diphenyl-p-
Trappe samples can be easily stored and sent to another site
Desorption fro Tenax occurs with helium flow at 300o C.
The desorped volatiles are then collected in a precolumn
cooled by dry ice.
The per column is then connected to the GC column , the
dry ice is removed and the analysis is started at room
Head Space Technique.
It involves analysis of volatile components in a complex and viscous
mixture containing high proportion of non-volatile components.
For quantitative analysis calibration of the volatiles
in the vapour is necessary. To reach this state the sample is placed in a
glass vial and thermostatted. When equilibrium is achieved, an aliquot
of the gas phase above the sample is rapidly transferred onto the GC
All the devices that are commonly used for gas sampling may be
applied to headspace analysis, including gas-tight syringes and gas-
Principle of headspace sampling by either direct on-column sampling or by
pressure/loop-filling with previous pressurization of the headspace vial.
Principle of cryogenic headspace trapping with splitless on-
column headspace sampling.
The column in chromatography is undoubtedly the heart of the
A column can either be a packed or open tubular.
These columns are fabricated from glass, stainless steel, copper, or
other suitable tubes.
Stainless steel is the most widely used because it is most inert and easy
to work with.
The column diameters currently in use are ordinarily 1/16" to 1/4" 0.D.
Columns exceeding 1/8" are usually used for preparative work while
the 1/8" or narrower columns have excellent working properties and
yield excellent results in the analytical range.
Column length can be from few feet for packed columns to more than
100 ft for capillary columns.
OPEN TUBULAR/CAPILLARY COLUMNS
Open tubular or capillary columns are finding broad
applications. These are mainly of two types:
• Wall-coated open tubular (WCOT) <1 mm thick liquid
coating on inside of silica tube
• Support-coated open tubular (SCOT) 30 mm thick coating of
liquid coated support on inside of silica tube.
The most frequently used capillary column, nowadays, is the
fused silica open tubular column (FSOT), which is a
The external surface of the fused silica columns is coated
with a polyimide film to increase their strength.
Support materials and Stationary
The solid support should ideally have large surface area (at least 1 m2/g), has
a good mechanical stability, thermally stable, inert surface in order to simplify
retention behavior and prevent solute adsorption, has a particle size in the
range from 100-400 mm.
Examples:- diatomaceous earth, glass beads with suitable mesh size( 80-100
mesh, 100-120 mesh).
A liquid stationary phase should be inert to the analyte, less volatile,
and thermally stable.
In general, the polarity of the stationary phase should match that of the
sample constituents ("like" dissolves "like"). Most stationary phases are
based on polydimethylsiloxane or polyethylene glycol (PEG) backbones:
Support materials and Stationary
The polarity of the
stationary phase can be
changed by derivatization
with different functional
groups such as a phenyl
group. Bleeding of the
column is cured by
bonding the stationary
phase to the column; or
crosslinking the stationary
Liquid Stationary Phases
should have the following
• Low volatility.
• High decomposition
• Chemically inert (reversible
interactions with solvent).
• Chemically attached to
support (to prevent
• Appropriate k' and a for
Gas chromatographs are usually capable of performing what is known
as temperature programming gas chromatography (TPGC).
TPGC is a very important procedure, which is used for the attainment
of excellent looking chromatograms in the least time possible.
Isothermal - Keep oven at one temp thru run. Not very useful.
Possibly useful for series of very similar compounds differing by boiling
points such as alcohols ( MeOH, EtOH, n-PrOH, i-PrOH, BuOH, i-
Gradient - temp profile: 40 deg hold for 10 min then 10deg/min to 240
deg and hold there for 20 min. Advantages: 1- resolution and 2-
0 10 20 30 40 50 60
Several detectors are
available for use in GC.
Each detector has its
own characteristics and
features as well as
drawbacks. Properties of
an ideal detector include:
1. High sensitivity.
2. Minimum drift.
3. Wide dynamic range.
4. Operational temperatures up
to 400 oC.
5. Fast response time.
6. Same response factor for all
7. Good reliability (no fooling).
9. Responds to all solutes
Types of Gas Chromatography
Responds to all compounds present in carrier gas stream except the
carrier gas itself
Responds to range of compounds with a common physical or chemical
Responds to a single specific compound only
Detectors can also be grouped into concentration or mass flow detectors
The response of such Gas Chromatography detectors is proportional to
the concentration of the solute in the detector such as TCD. Dilution of
sample with makeup gas will lower detector response.
Mass Flow Dependent
Signal is dependent on the rate at which solute molecules enter the
detector such as FID. Response of such detectors is not affected by
makeup gas flow rate changes.
Flame Ionization Detector (FID)
Mass sensitive detector
Response depends on conducting
power of ions or electrons produced
on burning of organic compounds in
Selective detector but sample
detected must be combustible
Large linear dynamic range (107)
No response to inorganic and
permanent gases such as CO, CO2,
NH3, CS2, N2, etc.
It is the most widely used detector
in Gas Chromatography
Response depends on the
difference between the
carrier gas and the eluted
Wide dynamic range (107 –
% to ppm levels)
Responds also to inorganic
gases such as CO, CO2, NH3,
CS2, N2, etc.
Sample is not wasted
Easy to operate
Electron Capture Detector (ECD)
The ECD ionizes the carrier gas by
means of a radioactive source. The
potential across two electrodes is
adjusted to collect all the ions and a
steady saturation current, is
Electrons from β-source ionize
carrier molecules capture electrons
and decrease current ; Simple and
reliable ; Sensitive (10-15 g/s) to
electronegative groups (halogens,
peroxides) ;Largely non-destructive
; Insensitive to amines, alcohols and
hydrocarbons ; Limited dynamic
Qualitative Analysis .
Separation of fatty acids derived from fixed oils
Miscellaneous-analysis of foods like carbohydrates, proteins, lipids, vitamins,
steroids, drug and pesticides residues, trace elements
Pollutants like formaldehyde, carbon monoxide, benzen, DDT etc
Dairy product analysis- rancidity
Separation and identification of volatile materials, plastics, natural and
synthetic polymers, paints, and microbiological samples
Inorganic compound analysis
Residual solvent analysis.