2. Gas Chromatography
GC used for separation and analysis of volatile, gaseous
substances.
Typical uses of GC include testing the purity of a particular
substance, or separating the different components of a
mixture.
In some situations, GC may help in identifying a compound.
In preparative chromatography, GC can be used to prepare
pure compounds from a mixture.
2
4. Detectors
Devices used to or
designed to respond
to a very small
quantity of sample
present in the
column gas
4
5. Types of detectors
Non specific
Thermal
conductivity
Atomic emission
photoionization
Destructive
FID, NPD,PID, CD
Non destructive
TCD,ECD
Specific
Flame ionization
Nitrogen-Phosphorous
Flame photometric
Helium/argon ionization
Electron capture
chemiluminiscence
5
6. Ideal characteristics of detectors
Sensitivity should lie in the range 10-8 to 10-15
g/ml
It should respond rapidly
It should respond to a wide variety of
compounds
Good stability and reproducibility
It should produce linear response to the
concentration of solutes in the eluate
A temperature range from room temperature to
at least 400 degree celsius
Non destructive to the sample
6
7. Flame Ionization detector
•The FID was invented by scientist
Harley and Pretorious and
separately by McWilliams and
Dewer.
•It makes use of an oven, wherein
a flame is produced by burning
hydrogen gas in presence of
oxygen or air.
•Effluent from the column is
directed into a air/hydrogen flame.
•A definite potential difference is
maintained between the two
electrodes with the help of a series
of batteries.
•Amplifier and recorder record
chromatograms. 7
8. Working
• A portion of eluate coming from
the column is directed into the
furnace through the wire loop.
• Solvent evaporates and organic
compounds pyrolyses and forms
ions.
• These ions are attracted towards
the respective electrodes.
• This changes the potential
difference between the
electrodes and hence the current
in the circuit.
• As electrical resistance of flame is
high and resulting current is
small, an electrometer is
employed.
8
10. Minute amount of solute can be
detected
Gives linear response
As it responds to the number of C-
atoms entering the detector per unit
time, it is mass sensitive rather than
concentration sensitive
Insensitive towards water, CO2, SO2,
NO2
More complicated and more
expensive
Destructive of the sample
10
Functionals groups like carbonyl,
alcohol, halogen, amine are not
detected
Advantages
Disadvantages
11. Flame photometric detector
11
• It is selective towards compounds containing
sulphur and phosphorous
• The eluent is passed into the flame, which converts
phosphorous to HPO and sulphur into S2
• These excited species emit light as they
luminescene in the flame.
• Compounds containing phosphorus are detectable
with the 526nm filter, which is yellow on one side.
• The 394nm filter (blue on one side) allows detection
of sulfur-containing compounds.
12. • a combustion chamber to house the flame,
• gas lines for hydrogen (fuel) and air (oxidant),
• an exhaust chimney to remove combustion products,
• thermal (bandpass) filter to isolate only the visible and UV
radiation emitted by the flame.
12
13. Thermal conductivity detector
• It works on the principle of
wheatstone’s bridge.
• Out of four resistances in the
circuit, the magnitude of three
resistances remains constant.
• But that of fourth resistance
varies as per change in the
temperature.
• This change is because of the
difference in the capacity of
the solute and the carrier gas
to absorb heat (thermal
conductivity differences).
• The change in the
temperature changes the
resistance and hence the
current in circuit.
13
14. • Mixture is passed over coil B,
whereas only carrier gas passes
through coil A.
• Both have different thermal
conductivities.
• Hence when solid is eluted out in
the carrier gas , this mixture
removes differential amount of
heat.
• The temperature of coil B now
depends on the thermal
conductivity of this mixture.
• So the temperature of coil B
changes and hence the resistance
R4.
• The change in R4 provides on
formation about concentration of
a solute in the eluate.
14
15. • TCD is simple, rugged,
inexpensive
• Non destructive to the
sample
• Gives accurate results
Advantages
• It has low sensitivity
• Detection of organic
compounds using N2 or Co2
as carrier gas is less
sensitive.
Disadvantages
15
16. Electron capture detector
•This was invented in year
1954
•Consist of beta emitter
such as nickel adsorbed on
platinum or titanium foil
•Two electrodes are on
either side of the emitter
•They are connected to an
ammeter in order to record
the current in the circuit
16
17. •A beam of electrons is produced
by the beta emitter
•When carrier gas passes over the
emitter, the gas ionises producing
electrons
•In absence of compound,
ionization of carrier gas produces
a constant standing current
•When solute is eluted out from
the column, it captures electron
towards it. Hence current
decreases
•This decrease gives idea about
the concentration of a solute in
the sample
17
18. Advantages:
18
• Highly sensitive towards compounds containing
electronegative functional groups such as halogens,
peroxides, nitro etc.
• Detection and determination of chlorinated
insecticides
• Does not alter the sample
Disadvantages:
• Non linear response unless potential across the
detector is pulsed
19. Nitrogen Phosphorous Detector
• NPD uses a Hydrogen/Air flame
through which the sample is
passed.
• NPD uses a rubidium/cesium
bead which is heated by a coil,
over which the carrier gas
mixed with Hydrogen passes.
• The hot bead emits electrons by
which are collected at the
anode and provides the
background current.
• When a component that
contains N/P exits the column,
the partially combusted N/P
materials are adsorbed on the
surface of the bead.
• This then increases the
emission of electrons.
19
20. Advantages:
• The specific response of NPD to nitrogen and
phosphorous, with relatively high sensitivity, makes
it useful for analysis of pharmaceuticals
• In environmental analysis involving herbicides
Disadvantages:
• Performance deteriorates with time
• Burning hydrogen vapours converts alkali silicate to
alkali hydroxide which has significant vapour
pressure and causes loss of cesium/rubidium
20
21. Photo ionization electrode
• PID responds to all molecules whose ionization
potential is below 10.6eV.
• It is often used in series with the FID.
• The PID detector consists of a 10.6 electron volt
(eV) UV lamp mounted on a thermostatted, low-
volume (100µL), flow-through cell.
• The temperature is adjustable from ambient to
250oC.
• The PID lamp is held in place by a spring-loaded
plate, so that the lamp may be quickly removed for
cleaning and replaced without any special tools. 21
22. • PID design uses a 10.6eV lamp
with a high voltage power
supply.
• Sample laden carrier gas flows
from the analytical column
into the PID sample inlet.
• When sample molecules flow
into the cell, they are
bombarded by the UV light
beam.
• Molecules with an ionization
potential lower than 10.6eV
release an ion when struck by
the ultraviolet photons.
• These ions are attracted to a
collector electrode, then sent
to the amplifier to produce a
signal 22
23. Helium ionization detector
• HID advantage to use helium as both the carrier gas and
the ionization gas. HID uses β-emitters as radioactive
source to create metastable helium species.
• The radioactive source ionizes helium atoms by
bombarding them with emissions. The metastable helium
species have an energy of up to 19.8 eV.
• As components elute from the GC's column they collide
with the metastable helium ions, which then ionize the
components.
• The ions produce an electric current, which is
the signal output of the detector. The greater the
concentration of the component, the more ions are
produced, and the greater the current. 23
26. Atomic emission detector
• The strength of this detector
lies in its ability to
simultaneously determine
elements
• It uses microwave energy to
excite molecules
• This emits radiations which
breaks down molecules to
atoms such as S,N,P,Hg, etc.
• These excited molecules emit
distinctive wavelength which
can be separated by a grating
and send to photodiode array
which produces the electric
signal 26
27. Chemiluminescence detector
e
• Nitrogen- and sulfur-containing
compounds commonly occur as
trace-level analytes in complex
samples and highly selective
detectors have been developed.
• The nitrogen
chemiluminescence detector
and the sulfur
chemiluminescence detector
have emerged as powerful tool
in GC,HPLC
• Nitrogen/sulfur analysers can b
based on the same
chemiluminescence reactions.
• Detectors of either element are
based on the same ozone-
induced gas phase
chemiluminescence 27
28. • Chemiluminescence is preceded by high temperature pyrolysis which
oxidizes the nitrogen in the sample (RN) to nitric oxide (NO):
Oxidation: RN + O2 → NO + CO2 + H2O
• The sulfur in the sample (RS) is converted first into sulfur dioxide
(SO2), which is then reduced in the presence of hydrogen to sulfur
monoxide (SO):
Oxidation: RS + O2 → SO2 + CO2 + H2O
Reduction: SO2 + H2 → SO + H2O
Overall: RS + O2 + H2 → SO + CO2 + H2O
• These reactions produce the species that react with ozone, producing
excited nitrogen dioxide and excited sulfur dioxide respectively
NO + O3 → NO2* + O2
Chemiluminescence: NO2* → NO2 + light (~ 1200 nm)
Reaction with ozone: SO + O3 → SO2*
Chemiluminescence: SO2* → SO2 + light (~ 360 nm)
28
29. References
29
• Introduction to analytical gas chromatography by Raymond P
.W.
Scott, 2nd edition,171-199
• Fundamentals of Analytical Chemistry by Douglas A. Skoog,Donald
West,F.James Holler, Stanley R. Crouch, 8th edition, 947-972
• Instrumental Methods of Analysis by Dr.S.S.Mahajan, 288-290
• Gas Chromatography Mass Spectroscopy(GC-MS)
(http://www.bris.ac.uk/nerclsmsf/techniques/gcms.html)
• Electron Capture Detector
(http://www.bucksci.com/atomic-absorption-
spectrophotometers/details/3127/3127/electron-capture-
detector-ecd.html)
• Flame Ionization Detector
(http://en.wikipedia.org/wiki/Flame_ionization_detector)
• Flame Ionization Detector
(http://www.cambustion.com/products/hfr500/fast-fid-
principles)
