2. 2 types
1. GSC 2. GLC
GSC is not widely used b/cos limited no of
stationary phases available.
Adsorption is the principle
GSC is used only in case where there is less
solubility of solutes in stationary phase,
which are rare.
GLC only
3. Partition is the principle
Stationary phase: liquid which is coated on to a solid
support
Mobile phase: Gas
Components are separated according to their
partition coefficients
Partition coefficient is the ratio of solubility of a
substance distributed between two immiscible
liquids at a constant temp.
4. 2 important criteria are
1. Volatility: unless a compound is volatile, it cannot be
mixed with mobile phase.
2. Thermostability:
All the compounds will not be in the form of vapour.(solids &
liquids)
Hence to convert them to a vapour form, they have to be
heated.
At that temp the compounds have to be thermostable
If they are not thermostable, the compounds cannot be
analysed by GC, since they will be decomposed.
5. Carrier gas
Flow regulators and flow meters
Injection devices
Columns
Temparature control devices
Detectors
Recorders and integrators
6. The choice of carrier gas determines the efficiency of
chromatographic separation
Eg: H2, He, N2, Ar
H2: better thermal conductivity, low density
Used in thermal conductivity detector / FID
Demerits:
1. it reacts with unsaturated compounds
2. inflammable
He: excellent thermal conductivity, but expensive
Used in thermal conductivity detector.
N2: inexpensive but has reduced sensitivity
7. Inertness
Suitable to the detector used
High purity
Easily available
Cheap
Less risk of explosion or fire hazards
Should give best performance
Consistent with the required speed of analysis
Compressible, gases are stored under high pressure in
cylinders
N2, He are the most commonly used
8. As carrier gases are stored under high
pressure, flow regulators are used to deliver
the gas with uniform pressure or flow rate
Flow meters to measure the flow rate of
carrier gas
1. Rotameter:
2. Soad bubble meter:
9. Gases can be introduced by valve devices
Liquids can be injected through loop or
septum devices
Most GC instruments have a high quality
rubber septum
Solid samples are dissolved in a suitable
solvent and injected through a septum
10. Glass / stainless steel
Stainless steel columns long life & can be
easily handled without the fear of fragility
But some samples react with them
Hence in such cases, glass columns are used
Eg: steroids
Glass columns are inert but highly fragile and
are difficult to handle
11. A) depending on its use:
1. analytical column: 1-1.5m of length & outer
diameter of 3-6mm
They are packed columns & are made up of glass or
stainless steel
Demerit: Only small quantity of sample can be
loaded
2. Preparative columns: larger & large amount of
sample can be loaded
3-6m of lengthy, outer diameter 6-9mm
13. Long capillary tubing of 30-90m
0.025-0.075cm internal diameter
Stainless steel & coiled
The inner wall is coated with the st.
phase liquid (0.5-1µ thin film)
these columns offer least resistance to flow of
carrier gas
More efficient than packed columns ( offers
more resistance)
Demerit: more sample cannot be loaded
14. An improved version of Golay or capillary
columns
A support material is deposited (1µ) on the
inner wall & then coated with a thin film of
liquid phase
Have a low resistance to flow of carrier gas
Advantage: more sample load
15. Preheaters: converts the sample into its vapour form & mix
them with mobile phase or carrier gas
Preheaters are present along with injecting devices
Thermostatically controlled oven:
In GC partition is the principle
Since partition coefficient as well as solubility of a solute
depends upon temp, temp maintenance in a column is highly
essential for efficient separation
Hence column & injecting devices should be maintained at a
particular temp.
16. Isothermal programming: same temp is maintained
throughout the process of separation
Linear programming: in which the oven is heated
linearly over a period of time
This is required when a sample has a mixture of low
bp & high bp compounds
Separation of complex mixtures
17. Heart of the apparatus
Requirements of an ideal detector:
Applicability to wide range of samples
High sensitivity to even small conc
Rapidity of response
Linearity: i.e., less response to low conc & vice versa
Response should be unaffected by temp, flow rate or characteristics of
carrier gas
Non destructive to the sample in case of preparative work
Simple & easy to maintain
inexpensive
19. Principle: is based upon thermal conductivity difference b/n carrier gas &
that of component
TCD has 2 platinum wires of uniform size which form part of Wheatstone
bridge
Through one of them, pure carrier gas always flows & through the other
the effluents of the column passes
2 Pt wires are heated electrically
When pure carrier gas passes through both or them, there is no diff in
temp or resistance & hence baseline is recorded
When a component emerges from the column, it alters the thermal
conductivity & resistance of the wire
Hence this produces a diff in resistance
So conductivity b/n wires, which is amplified & recorded as a signal.
20. The thermal conductivities of some carrier gases:
H2=32.7 ; He=33.9 ; N2=5.2; CH4=6.5; C6H12=3.0
Advantages:
Applicable to most compounds
Linearity is good
Sample is not destroyed & used in preparative scale
Simple, easy to maintain & inexpensive
Disadvantages:
Low sensitivity
Affected by fluctuations in temp & flow rate
Response is only relative & not absolute
Biological samples cannot be analysed
21. Based upon the electrical conductivity of carrier gases
At normal temp & pressure, gases act as insulators, but become
conductive if ions are present
H2 is the carrier gas used in FID
If the carrier gas is either N2/Ar, it can be mixed with H2
Anode: Ag gauze placed over the burner tip
Cathode: burner tip made up of Pt capillary
When pure carrier gas alone passes, there is no ionization & no current
flows
When a component emerges, no. of ions are produced b/cos of ionization
by the thermal energy of the flame
This causes a potential diff & causes a flow of current which is amplified
& recorded as a signal
22. Extremely sensitive & background noise is
low
µg quantities can be detected
Stable & insensitive to small changes in the
flow rate of carrier gas & water vapour
Responds to most of the org compounds
Linearity is excellent
23. AID depends on the exitation of Ar atoms to a metastable
state, by using radioactive energy.
This is achieved by irradiating the carrier gas with either α- or
β- particles
α- particles can be obtained from radium-D
β- particles can be obtained from Sr90
/ H3
These high E particles ionize the Ar atoms & hence they are
exited to metastable state
These molecules collide with the effluent molecules and
ionizes them
These ions when reach the detector will cause an increase in
current
Thus the components are detected
24. Advantages:
Responds to most of the org compounds
Sensitivity is very high
Disadvantages:
Response is not absolute & it is relative
Linearity is poor
Sensitivity is affected by water & is much reduced for
halogenated compounds
The response varies with the temp of the detector
High temp like 2400
C, voltages of 1000V or less are usually
necessary
25. ECD has 2 electrodes
Column effluent passes b/n them
One of the electrode is treated with a radio active isotope which emits
electrons as it decays.
These emitted electrons produce 2o
electrons which are collected by the
anode, when a PD of 20V is applied b/n them
When carrier gas alone flows through, all the 2o
electrons are collected by
the +vely polarised electrode
Hence a steady baseline is recorded
Effluent molecules which have affinity for electrons, capture these e-
when they pass through the electrodes
Hence the amount of steady state current is reduced
This diff is amplified & recorded as output signal
26. The carrier gas used in this detector depends upon the e-
affinity of the compounds analysed
For compounds with high e- affinity, Ar is used
For low e- affinity , N2, H2, He or CO2 can be used
Advantages: highly sensitive (10-9
)
Disadvantage: ECD can be used only for compounds with e-
affinity
Halogenated compounds, pesticides etc can be detected by
ECD
28. Recorders : to record the responses
They record the baseline & all peaks obtained with respect to
time
Retension time for all the peaks can be found out from such
recordings, but the area of individual peaks cannot be known
Integrators: improved version of recorders with some data
processing capabilities
Can record the individual peaks with Rt, height & width of
peaks, peak area, % of area , etc
Int provide more information on peaks than recorders
