Course no CHM -754.
COURSE SUPERVISOR: DR RAHAT
• Gas chromatography is one of the
most versatile and ubiquitous
analytical techniques in the
laboratory. It is widely used for the
determination of organic compounds.
• Complex mixtures can be separate by
• A gas chromatography is a chemical
analysis instrument for separating
chemicals in a complex sample.
A gas chromatograph uses a flow-through
narrow tube known as the column, through
which different chemical constituents of a
sample pass in a gas stream (carrier
gas, mobile phase) at different rates.
It depending on their various chemical and
physical properties and their interaction with
a specific column filling called the stationary
As the chemicals exit the end of the
column, they are detected and identified
electronically. The function of the stationary
phase in the column is to separate different
components, causing each one to exit the
column at a different time (retention time).
Other parameters that can be used to alter
the order or time of retention are the carrier
gas flow rate, column length and the
In a GC analysis, a known volume of
gaseous or liquid analyte is injected into the
"entrance" (head) of the column, usually
using a microsyringe (or, solid phase
microextraction fibers, or a gas source
As the carrier gas sweeps the analyte
molecules through the column, this motion
is inhibited by the adsorption of the
analyte molecules either onto the column
walls or onto packing materials in the
The rate at which the molecules progress
along the column depends on the strength
of adsorption, which in turn depends on the
type of molecule and on the stationary
Since each type of molecule has a different
rate of progression, the various components
of the analyte mixture are separated as they
progress along the column and reach the
end of the column at different times
(retention time). A detector is used to
monitor the outlet stream from the column.
The time at which each component reaches
the outlet and the amount of that component
can be determined.
• The auto sampler provides the means to
introduce a sample automatically into
• Manual insertion of the sample is
possible but is no longer common.
Automatic insertion provides better
reproducibility and time-optimization.
• Different kinds of auto samplers exist.
Auto samplers can be classified in
relation to sample capacity (auto-
injectors vs. auto samplers, where auto-
injectors can work a small number of
samples), to robotic technologies (XYZ
robot vs. rotating robot – the most
common), or to analysis:
Static head-space by syringe technology.
Dynamic head-space by transfer-line
Solid phase micro extraction (SPME).
• In gas chromatography Helium, nitrogen,
Hydrogen or argon gases are used as a carrier
• The choice of gas is depend upon availability,
purity required, consumption and the type of
• Helium is preferred, when thermal conductivity
detectors are used. Because of it high thermal
conductivity relative to that of the vapours of
most organic compounds.
• Free standing gas cylinder must always be
supported by means of clamps or chains.
• Waste gases, especially hydrogen, must be
vented through an extraction hood.
Packed Column Injectors
In general, the sample injected onto a packed
GC column ranges in volume from 0.5 micro
liters to 5 micro liter and usually contains the
materials of interest at concentrations ranging
from 5% v/v to 10% w/v.
A Pack column injector
The sample is injected by a hypodermic syringe,
through a silicone rubber septum directly into
the column packing or into a flash heater.
Although the latter tends to produce broader
peaks it also disperses the sample radially across
Direct injection into the packing constrains the
sample into a small volume, but can cool the
front of the packing.
The silicone septum is compressed between
metal surfaces in such a manner that a
hypodermic needle can pierce it, but when it is
withdrawn the hole is closed as a result of the
septum compression and there is no gas leak.
The glass liner prevents the sample coming in
contact with the heated metal wall and thus,
reduces the chance of thermal decomposition.
The glass liner can be fitted with a separate
heater and the volatalization temperature can,
thus, be controlled. This "flash heater" system is
available in most chromatographs.
By using a syringe with a long needle, the tip
can be made to penetrate past the liner and
discharge its contents directly into the column
This procedure is called 'on-column injection'
and, as it reduces peak dispersion on injection
and thus, provides higher column efficiencies, is
often the preferred procedure.
Open Tubular Column Injection Systems
Due to the very small sample size that must be
placed on narrow bore capillary columns; a split
injection system is necessary.
As the sample passes the column opening, a
small fraction is split off and flows directly into
the capillary column are called a split injector.
Regulating the portion of the carrier gas that
passes to waste changes the split ratio. This
achieved by an adjustable flow resistance
situated in the waste flow line. This device is
only used for small diameter capillary columns
where the charge size is critical.
The device has certain disadvantages due to
component differentiation and the sample placed
on the column may not be truly representative.
The solutes with the higher diffusivities (low
molecular weight) are lost preferentially to those
with lower diffusivities (higher molecular
weights). Consequently, quantitative analyses
carried out using the high efficiency small
diameter capillary columns may have limited
accuracy and precision, depending on the nature
of the sample.
The Split Injection System
This problem was partially solved by
using larger diameter columns that
would permit on-column injection.
There are also difficulties associated
with this type of injector. On injection,
the sample breaks up into separate
portions, and bubbles form at the
beginning of the column causing the
sample to be deposited at different
positions along the open tube as the
On starting to develop the separation,
each local concentration of sample acts
as a separate injection.
• The actual components is effected in the
• Where the nature of the solid support, type and
amount of liquid phase, method of packing
length and temperature are important factor in
obtaining the desired resolution.
• The Column is enclosed in a thermostatically
controlled oven so that its temperature is held
constant to within 0.5 C. Thus ensuring
• The operating temperature may range from
ambient to over 400 C and for isothermal
operation is kept constant during the
Type of Column
1. Packed columns
• The internal diameter of Column is 2-6 mm.
The outer diameter of Column is 3-10 mm and it
is coil for compactness
Qualities of the packed column
• Glass column must be used if any of the
sample components are decompose by
compact with metal.
• The material chosen as a inaet support should
be uniform granular size and capable of being
packed in to a uniform bed in a column.
• The surface area of the material should be
large so as to promote distribution of the
liquid phase as a film and ensure the rapid
attainment of equilibrium between the
stationary phase and mobile phase.
• The most commonly used supports(e.g
celite) are made from diatomaceous materials
which can hold liquid phases in amounts
exceeding 20% without becoming too sticky
to flow freely and can be easily packed.
• To be a good resolution the height is
equivalent of the theoretical plate is
proportion to the average particle diameter.
• Rapidly increase the pressure due to decrease
the particle size.
• It is necessary to increase the pressure for
achieving the flow rate through the column
• These capillary columns (i.d < 1mm) are
increasingly used in GLC.
• It is used for complex mixtures.
• This result from a highly theoretical plate
numbers which can be attain with long
column of this type for the relatively smaller
• In these capillary columns the stationary
phase s coated on the inner wall of the tube,
Here 2 types of capillaries are available.
1. Wall coated open tubular
• Here the stationary phase is directly coated
on the inner wall of tubing.
2. Supported coated open tubular
• It have finely divided layer of solid support
material deposited on the inner wall of the
stationary phase is then coated
• Supported coated open tubular is less
efficient then wall coated open tubular.
• Capillary column are made up of thin wall
• The dimension of the column, which is coiled,
is 25- 200 m long and 0.2-0.5mm i.d.
• The drawback of this column is that they have
low sample capacity than pack column.
The choice of detector will depend the
concentration level of the sample.
The important properties of a detector are as
• This is usually defined as the detector
response (mV) per unit concentration of
• It is closely related to the detection limit.
Since high sensitivity often give low limit of
• The limit of detection will be rises if the
detector produced noise.
• The sensitivity also determines the slop of
calibration graph and therefore influences the
precision of the analysis.
2 . Linearity
• The linear range of the detector refer to the
concentration range over which the signal is
directly proportional to the concentration of
• Linearity in detector response will give
linearity of the calibration graph and allow
later to be drawn with more certainty.
• With the convex calibration curved, the
precision is reduced at higher concentrations
where the slope of the curved is much lesser.
• The high linearity range has high advantage
but detector small range will be used because
of their other qualities, although they will
need to be recalibrated over a number of
difference concentration ranges.
• The important characteristic of a detector is
the extent to which the signal output remains
constant with time, assuming there is a
• Lack of the stability can be exhibited in to two
1. Baseline noise.
1 .Baseline noise.
• Baseline noise is caused by a rapid random
variation in detector output difficult to
measure small peaks against the fluctuation
• Drift is often due to factors external to the
detector and impose a more fundamental
limit on its performance, such as temperature
change or column bleed and so is
controllable, where as noise is usually due to
poor contacts within detector and impose a
more fundamental limit on its performance.
• It must be reproducible
TYPE OF DETECTOR
Thermal conductivity detector
• The most important of the bulk physical
property detector is a thermal conductivity
detector which is universal, nondestructive,
concentration – sensitive detector. It is still
widely used in gas chromatography.
• Helium and hydrogen are the best carrier
gases to use in conjunction with this type of
detector. Since their thermal conductivity are
much higher than the other gases.
Thermal Conductivity Detector (TCD)
• In the detector 2 pair of match filaments are
arranged in a Wheatstone bridge circuit, 2
filaments is opposite arm of the bridge is
surrounded by the carrier gas only.
• The other 2 are surrounded by the effluent
from the chromatographic column.
• There are 2 types of gas channels through the
1. Sample channels.
2. Reference channels.
• When the pure carrier gas is pass over both
the Sample and reference filaments the
bridge is balanced but when the vapours
emerges from the column, the rate of cooling
of the sample filament become changed and
the bridge become unbalanced.
• It is used for the detection limit of permanent
gases light hydrocarbon and compound
which response poorly from the flame
• It is used for the gas chromatography studies
of metal chelates, e.g mixture of beryllium,
aluminium, gallium and indium
trifluoroacethyacetonates. For many general
applications it is replaced by Flame ionization
• An important characteristic of the
common carrier gases is that they behave
as perfect insulators at normal
temperature and pressures .The increase
conductivity due to the presence of a few
charge molecules in the effluent from the
column thus provide the high sensitivity
which is a feather of the ionisation based
• Ionisation detector in current used include
the flame ionisation detector, thermionic
ionisation detector, photo ionisation
detector and election capture detector
each of course employing a different
method to generate an ion current.
• The basis of this detector is that the
effluent from the column is mixed with
hydrogen and burned in air to produce a
flame which has sufficient energy to ionize
solute molecules having low Ionisation
Flame Ionisation detector
• The ion produced are collect at
electrodes and a result ion current
• The burner jet is the negative
electrode whilst the anode is usually
wire or grid extending into the tip of
the flame. Because the sample is
destroy in the flame.
• A stream-splitting device is inserted
between the column and detector
and allows the bulk of the sample to
by-pass the detector.
• It is widely applicable for gas
chromatography of organic
• It has high sensitivity, stability, fast
response and linearly response with
Electron capture detector
• Most ionisation detectors are based
on measurement of the increase in
current which occur when a more
readily ionised molecule appears in
the gas stream.
• the Electron capture detector is
differ from the other ionisation
detectors .it exploits the
recombination phenomenon, based
on Electron capture by compound
having affinity for free electron .it
only measure the decrease in
Electron capture detector
• A beta rays is used to generate slow
electron by ionisation of the carrier
gas (nitrogen preferred) flowing
through the detector.
• The slowly electron migrate to the
anode under a fixed potential and
give rise to a steady baseline current
• When the electron capturing gas is
emerges from the column. It will react
with electron by a negative ion of
much higher mass with a
corresponding reduction in current
• It response due to electron affinity of
the eluate molecules. Being sensitive
to compound containing halogens,
sulphur, anhydrides, nitrites,
conjugated carbonyls, nitrates and
organo metallic compounds.
• It has high sensitivity.
Element sensitive detector
• Element sensitive detector in current
used includes “Thermionic Ionisation
detector, Flame photometric detector and
atomic absorption detector”.
Thermionic Ionisation detector
• It only response to nitrogen
containing compound and
• It contains an electrically heated
rubidium silicate bead situated a few
millimeters above the detector jet tip
and below the collector electrode.
• The temperature of beat is
maintained at 600-800 degree
Flame photometric detector
• It only response to sulphur and
• Hydrogen – rich flame results in the
formation of luminescent species that
emit light character of the
heteroatom introduced in to the
• When nitrogen is carrier gas, it can be
• Hydrogen is introduced at the burner
to initiate combustion.
Atomic absorption spectroscopy
• It is used for the separation of
organo metallic compound.
TYPES OF CHROMATOGRAPHY
• Gas solid (adsorption) chromatography.
• Gas liquid (partition) chromatography.
The most important one of the two is gas
liquid chromatography, used in the foam of
• In gas-liquid chromatography, the mobile
phase is a gas such as helium and the
stationary phase is a high boiling point
liquid absorbed onto a solid.
A flow scheme for gas-liquid
Injection of the sample
• Very small quantities of the sample that
you are trying to analyses are injected into
the machine using a small syringe. The
syringe needle passes through a thick
rubber disc (known as a septum) which
reveals itself again when the syringe is
• The injector is contained in an oven whose
temperature can be controlled. It is hot
enough so that all the sample boils and is
carried into the column as a gas by the
helium (or other carrier gas).
Column: packed Column.
• The time taken for a particular compound
to travel through the column to the
detector is known as its retention time.
This time is measured from the time at
which the sample is injected to the point at
which the display shows a maximum peak
height for that compound.
• Different compounds have different
retention times. For a particular
compound, the retention time will vary
depending on the boiling point of the
• A compound which boils at a temperature
higher than the column temperature is
going to spend nearly all of its time
condensed as a liquid at the beginning of
the column. So high boiling point means a
long retention time.
• The solubility in the liquid phase. The more
soluble a compound is in the liquid phase,
the less time it will spend being carried.
along by the gas. High solubility in the
liquid phase means a high retention time.
• The temperature of the column. A higher
temperature will tend to excite molecules
into the gas phase - either because they
evaporate more readily, or because they
are so energetic that the attractions of the
liquid no longer hold them.
• A high column temperature shortens
retention times for everything in the
• The lower the temperature of the column,
the better the separation.
• There are several different types of
detector is use.
• The flame ionisation detector described
below is commonly used and is easier to
describe and explain than the alternatives.
GAS SOLID CHOROMATOGRAPHY
Comprises all gas
chromatographic methods in
which the stationary is an active
solid (e.g. charcoal, molecular
sieves).Separation is achieved by
adsorption of the components of a
The retention time is long, due to
which it is less used then gas