Gas Chromatography by mirola

GAS CHROMATOGRAPHY
By Mirola Afroze, SO, DRiCM, BCSIR

Prepared by Mirola Afroze, SO, DRiCM, BCSIR

INTRODUCTION
Chromatography is probably the most powerful
and versatile technique available to the modern
analyst. In a single step process it can separate a
mixture into its individual components and
simultaneously provide an quantitative estimate
of each constituent. Samples may be gaseous,
liquid or solid in nature and can range in
complexity from a simple blend of two
entantiomers to a multi component mixture
containing widely differing chemical species.
Furthermore, the analysis can be carried out, at
one extreme, on a very costly and complex
instrument, and at the other, on a simple,
inexpensive thin layer plate.
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DEFINITION OF CHROMATOGRAPHY
The term chromatography is difficult to define
rigorously because the name has been applied to
such a variety of systems and teclmiques.
All of these methods, however, have in common
the use of a stationary phase and a mobile
phase.
Components of a mixture are carried through
the stationary phase by the flow of a gaseous or
liquid mobile phase, separations being based on
differences in migration rates among the sample
components.

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Chromatography may be defined as a separation
process that is achieved by distributing the
components of a mixture between two phases, a
stationary phase and a mobile phase.
Those components held preferentially in the
stationary phase are retained longer in the
system than those that are distributed selectively
in the mobile phase.

As a consequence, solutes are eluted from the
system as local concentrations in the mobile
phase in the order of their increasing
distribution coefficients with respect to the
stationary phase; ipso facto a separation is
achieved.

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HISTORY OF CHROMATOGRAPHY
The first scientist to recognize chromatography
as an efficient method of separation was the
Russian botanist Tswett (1), who used a simple
form of liquid-solid chromatography to separate
a number of plant pigments.
The colored bands he produced on the adsorbent
bed evoked the term chromatography for this
type of separation (color writing).
Although color has little to do with modern
chromatography, the name has persisted and,
despite its irrelevance, is still used for all
separation techniques that employs the essential
requisites for a chromatographic separation, viz.
a mobile phase and a stationary phase.
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The technique, as described by Tswett was
largely ignored for a along time and it was not
until the late 1930s and early 1940s that Martin
and
Synge(2)
introduced
liquid-liquid
chromatography by supporting the stationary
phase, in this case water, on silica in a packed
bed and used it to separate some acetyl amino
acids.
In their paper, they recommended replacing the
liquid mobile phase by a suitable gas, as the
transfer of sample between the two phases
would be faster, and thus provide more efficient
separations.
In this manner, the concept ofgas
chromatography was created but again, little
notice was taken of the suggestion and it was
left to Martin himself and A. T. James to bring
the concept to practical reality nearly a decade
later
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BASIC CHROMATOGRAPHIC TERMINOLOGY

Chromatograph: Instrument employed for a
chromatography.
Stationary phase: Phase that stays in place
inside the column. Can be a particular solid or
gel-based packing (LC) or a highly viscous
liquid coated on the inside of the column
(GC).

Mobile

phase: Solvent

moving

through the column, either a liquid in
LC or gas in GC.
Eluent: Fluid entering a column.
Eluate: Fluid exiting the column.
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Elution: The process of passing the mobile
phase through the column.
Chromatogram:

Graph

showing detector

response as a function of a time.
Flow rate: How much mobile phase passed /
minute (ml/min).
Linear velocity: Distance passed by mobile
phase per 1min in the column (cm/min).
tm: time at which an unretained analyte or
mobile phase travels through the column.
Adjusted retention time : tr = tr-tm

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Relative retention (separation factor) α=
t’r2/t’ 1 a ratio of relative retention times α >
1,indicates quality of the separation; ↑α =
greater separation
Capacity factor k = (tr-tm)/tm ↑k = greater
retention used to monitor performance of the
column α = k2/k1
Retention time tr: Retention time is the time
elapsed between the point of injection of the
sample and the point of appearance of the
peak of the chromatogram.

Retention volume: Under a given set of
operating conditions a constant volume of gas
is required to elute a component from the
column. Thus, the volume measured from the
point of injection to the projection of the peak
of chromatogram is known as retention
volume
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GAS CHROMATOGRAPHY
Gas chromatography (GC) is a powerful and
widely used tool for the separation,
identification and quantitation of components in
a mixture.
In early 1900s, Gas chromatography was
discovered by Mikhail Semenovich Tsvett as a
separation technique to separate compounds.

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In this technique, a sample is converted to the
vapor state and a flowing stream of carrier gas
(often helium or nitrogen) sweeps the sample
into a thermally-controlled column.
This technique uses a gas as the mobile phase,
and the stationary phase can either be a solid or
a non-volatile liquid in which case small inert
particles such as diatomaceous earth are coated
with the liquid so that a large surface area exists
for the solute to equilibrate with.
GC is divided into two major categories:
• Gas Solid Chromatography (GSC)
• Gas Liquid Chromatography (GLC)

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Gas Solid Chromatography (GSC): GSC
utilizes as the stationary phase a glass or metal
column filled with powder adsorbent where the
adsorbent is a solid of large surface area. In
GSC, retention of solutes is dependent largely
upon differences in adsorption properties of the
solutes for the powder adsorbent as they pass
through the stationary phase.
Gas Liquid Chromatography (GLC): GLC
where the adsorbent is a non-volatile liquid
coated on an inert solid support or capillary
column in which the inside wall will be coated
with nonvolatile liquid. In GLC, the retention of
solutes is dependent largely upon the partition
co-coefficient of the solutes for the nonvolatile
liquid of the stationary phase.
In both cases, the mobile phase or the carrier
gas is an inert gas which is made to flow at a
constant rate through a packed column that is a
small diameter tube containing the adsorbent.
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Figure: Schematic of a Gas Chromatography
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WORKING PRINCIPLES OF GAS
CHROMATOGRAPHY
For separation or identification the sample must
be either a gas or have an appreciable vapour
pressure at the temperature of the column – it
does not have to be room temperature. The
sample is injected through a self sealing disc (a
rubber septum) into a small heated chamber
where it is vaporised if necessary. Although the
sample must all go into the column as a gas,
once it is there the temperature can be below the
boiling point of the fractions as long as they
have appreciable vapour pressures inside the
column. This ensures that all the solutes pass
through the column over a reasonable time span.
The injector oven is usually 50–100 °C hotter
than the start of the column.

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The sample is then taken through the column by
an inert gas (known as the carrier gas) such as
helium or nitrogen which must be dry to avoid
interference from water molecules. It can be
dried by passing it through anhydrous copper(II)
sulphate or self indicating silica (silica
impregnated
with
cobalt(II)
chloride).
Unwanted organic solvent vapours can be
removed by passing the gas through activated
charcoal. The column is coiled so that it will fit
into the thermostatically controlled oven.
The temperature of the oven is kept constant for
a straightforward separation, but if there are a
large number of solutes, or they have similar
affinities for the stationary phase relative to the
mobile phase, then it is common for the
temperature of the column to be increased
gradually over a required range. This is done by
using computer control, and gives a better
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separation if solute boiling points are close and
a faster separation if some components are
relatively involatile.
The solutes progress to the end of the column, to
a detector. The sample components can be
detected by a suitable detector at the exit. With a
suitable differential detector at the exit of the
column, the signal obtained is proportional to
the instantaneous concentration of the dilute
component in the binary gas mixture
Once a mixture has been separated by GC its
components need to be identified. The material
to be identified by GC is run through the
column so that its retention time (the time for
the components to pass through the column) can
be determined.

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INSTRUMENTATION OF GAS
CHROMATOGRAPHY
The instrumentation of gas chromatography
includes
i. Carrier gas
ii. Sample port
iii. Column
iv. Detector
v.Monitor & Recorder

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CARRIER GAS
The carrier gas plays an important role, and
varies in the GC used. Carrier gas must be dry,
free of oxygen and chemically inert mobilephase employed in gas chromatography.

Typical carrier gases include helium, nitrogen,
argon, hydrogen and air. Which gas to use is
usually determined by the detector being used,
for example, a DID requires helium as the
carrier gas.
When analyzing gas samples, however, the
carrier is sometimes selected based on the
sample's matrix, for example, when analyzing a
mixture in argon, an argon carrier is preferred,
because the argon in the sample does not show
up on the chromatogram.
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Safety and availability can also influence carrier
selection. The purity of the carrier gas is also
frequently determined by the detector, though
the level of sensitivity needed can also play a
significant role.
Typically, purities of 99.995% or higher are
used.
Carrier

gases

that are

used

in

gas

chromatography:
Helium is generally used because of its
Excellent thermal conductivity
Inertness
Low density
Greater flow rate
Limitations
It is highly expensive
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Hydrogen
Better thermal conductivity
Lower density
Limitations
It
may
react
with
unsaturated
compounds
It creates fire and explosive hazards

Nitrogen
It is inexpensive
Limitations
It gives reduced sensitivity
Air
Air is used as carrier gas only when the
oxygen in the air is useful for the detector or
separations

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SAMPLE PORT /SAMPLE INJECTION
SYSTEM
The sample injection system is very important
because very small amount of sample is used.
The system must introduce the sample in a
reproducible manner and must vaporize
instantly so that the sample will enter the
column easily.
Liquid samples are generally introduced but
hypodermic syringe and solid sample must be
dissolved in volatile liquids (acetone) for direct
introduction
Syringe containing sample is introduced through
a septum, injection port oven temperature
heated to temperatures that ensures fast
volatilization of sample, i.e. above the b.p. of all
sample components, usually 275º C.

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Splitless Injection: (where the split vent is
closed) attempts to transfer all of the sample to
the column and is used for trace analysis.
Split Mode: only a small portion (maybe 1-10%
of the sample moves into the column, and the
rest is sent to waste. This is used when the
analytes are in high concentration and would
overload the column

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SEPARATION COLUMN
The columns can be constructed of metal or
glass tube. It can be any length from a few
centimers to over a hundred meters. It can be
coiled, straight or bent.
The choice of column depends on the sample
and the active measured. The main chemical
attribute regarded when choosing a column is
the polarity of the mixture, but functional
groups can play a large part in column selection.

The polarity of the sample must closely match
the polarity of the column stationary phase to
increase resolution and separation while
reducing run time. The separation and run time
also depends on the film thickness (of the
stationary phase), the column diameter and the
column length.
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Two types of columns are used in GC:
Packed columns are usually made of stainless
steel or glass and contain a packing of finely
divided, inert, solid support material that is
coated with a liquid or solid stationary phase. In
GSC, columns are packed with size graded
adsorbent or porus polymers while in GLC, the
packing is prepared by coating the liquid phase
over a size graded inert solid support. The
nature of the coating material determines what
type of materials will be most strongly
adsorbed. Thus numerous columns are available
that are designed to separate specific types of
compounds.

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Capillary columns have a very small internal
diameter, on the order of a few tenths of
millimeters, and lengths between 25-60 meters
are common. The inner column walls are coated
with the active materials (WCOT columns),
some columns are quasi solid filled with many
parallel micro-pores (PLOT columns) and solid
supported coated with stationary liquid phase
(SCOT). Most capillary columns are made of
fused-silica with a polyimide outer coating.
These columns are flexible, so a very long
column can be wound into a small coil.

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Figure: Capillary and Packed Columns

Figure: Capillary columns
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The Stationary Liquid Phase
The column is the heart of the gas
chromatograph, and the success or failure of a
particular separation depends to a large extent
on the choice of the stationary liquid phase. A
number of points should be considered when
selection of a liquid phase is made:
i. The solute solvent interaction forces that
contribute to the selectivity of the liquid phase.
Solution properties such as the polarity,
chemical interactions, and hydrogen bonding
and other cohesive forces will affect the
separation.
ii. Prediction of retention behavior Types of
solution, their concentration and use of
multistage columns, mixed columns determines
the prediction of retention times.
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iii. The temperature limitations of the liquid
phases The liquid phase should have low
volatility and no tendency to decompose at the
operating temperature.
iv. The possibility of irreversible reactions on
the column Irreversible reactions between the
solute and the liquid phase or its impurities or
degradation products may limit the usefulness of
certain solvents.
The solute solvent interaction
A. Cohesion forces:
Dipole-dipole interactions- results from the
interaction between two permanent dipoles.
Induction forces- results from an interaction
between a permanent dipole in either solute or
solvent and induced dipole in the other.
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Dispersion forces- arise from electric field
produced by the very rapidly varying dipoles
formed
between nuclei and
electrons.
Dispersion forces are always present in any
solute-solvent system.
B. Hydrogen bonding:
Hydrogen bonding is one type of orientation
force that is very important in GC. Hydrogen
bonds are intermediate in strength between the
strong chemical bonds. It can be classified into
five groups-Class (I) consists of compound
capable of forming networks of multiple
hydrogen bonds, Class (II) composed of
compounds containing both a donor atom
(ONF) and an active hydrogen atom, Class (III)
consists of compounds having donor atoms but
no active hydrogen, Class (IV) made up of
molecules containing active hydrogen atoms but
no donor atoms and Class (V) compounds of
this group show no hydrogen bonding at all.
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C. Polarity
In general, polar solutes will be retained to a
greater extent as the polarity of the solvent is
increased. Conversely, non-polar solutes are
retained to a greater extent as the polarity of the
solvent is decreased.
Thumb rule: Polar solvents dissolve polar
compounds and non-polar solvents dissolve
non-polar compounds

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DETECTORS
The detector is that device and associated
equipment used to sense and measure the small
amounts of the components present in the
carrier gas stream leaving the chromatographic
column. The impulse received from the elute of
the column in the form of solute vapor is sensed
by the detector. It in turn converts this impulse
into an electrical signal proportional to the
concentration of the solute in the carrier gas.
This signal is then amplified and recorded as a
peak in the chromatograph.
In all types of detectors, when the carrier gas
passing they give a zero signal but when a
component is eluted, it is detected and a signal
proportional to the concentration of that
component is produced.

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Classification of detectors:
Depending upon selectivity detectors are
classified as
Non-selective detectors: Responds to all
compounds except the carrier gas (e.g. TCD,
FID, (PID)
Selective detectors: Respond to a range of
compounds
with common
physical and
chemical properties (e.g. FID, ECD, PID, N and
P detector, Sulfur chemiluminiscence)
Specific detectors: Respond to a single chemical
compound
Detectors are also classified as
I. Integral and differential
II.
Destructive (FID, NPD, FPD)
nondestructive (e.g. TCD, ECD, PID)

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Characteristics of an ideal detector
Functional
• Sensitivity
• Stability
• Versatility
• Proportionality
• Reactivity
• Response Time
• Signal Recording
Non Functional
• Simplicity
• Cost and availability
• Robustness
• Safety

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Detectors that are used in GC
• Thermal conductivity detector (TCD)
• Flame ionization detector (FID)
• Electron capture detector (ECD)
• Discharge ionization detector (DID)
• Flame photometric detector (FPD)
• Hall electrolytic conductivity detector
(HECD)
• Helium ionization detector (HID)
• Nitrogen phosphorous detector (NPD)
• Photoionization Detector (PID)
• Mass selective detector (MSD)

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Flame Ionization Detection (FID)
The most commonly used detector is the flame
ionization detector (FID) it is a general carbon
detector. It does not detect compounds that do
not contain carbon such as nitrogen(N2),
oxygen(O2), or water. The presence of N, O, or
S in a carbon compound will tend to decrease
the response of the FID.
The Carbon atoms (C-C bonds) are burned in a
hydrogen flame. The hydrogen can be supplied
ether from a cylinder or from an electrolytic
hydrogen generator. The hydrogen must be
pure to avoid background noise. A charcoal
filter is often placed in the hydrogen supply line
to remove any organic contaminants.

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The response of the detector depends on the
flow of the hydrogen, air and the makeup gas (if
it is used).
A certain amount of inert gas is needed for
optimum response of the detector. Generally
the flow from a capillary is too low so a makeup
gas is used to provided the inert gas flow.
The makeup gas has other beneficial effects
such as stabilizing the detector, prolonging the
lifetime of the jet, and purging any unswept
areas of the detector.
It is also very important to adjust the air and
hydrogen gas flows for optimum response.

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Mechanism: Compounds are burned in a
hydrogen-air
flame.
Carbon
containing
compounds produce ions that are attracted to the
collector. The number of ions hitting the
collector is measured and a signal is generated.
Selectivity: Compounds with C-H bonds. A
poor response for
some
non-hydrogen
containing organics (e.g., hexachlorobenzene)
Sensitivity: 0.1-10 ng
Linear range: 105-107
Gases: Combustion - hydrogen and air; Makeup
- helium or nitrogen
Temperature: 250-300°C,and 400-450°C for
high temperature analyses
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Figure Flame ionisation detector

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The Nitrogen–Phosphorus Detector (NPD)
The NPD is fundamentally similar to the flame
ionisation detector. They both work by forming
ions and subsequently detecting them as a
minute electrical current.
However, a major difference arises in the way
the ions are formed. The eluate (exit gases) of
the GC is forced through a jet in the presence of
air and hydrogen gas.
The mixture passes over the surface of a heated
rubidium salt in the form of a bead. The excited
rubidium atoms (Rb*) selectively ionise
nitrogen and phosphorus.

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The ions formed allow a small electric current to
flow between two charged surfaces which,
under different operating conditions, gives a
response to either nitrogen or phosphorus
containing compounds, or both.
To differentiate between nitrogen and
phosphorus containing compounds the retention
time of each solute in the column is used.
Mechanism: Compounds are burned in a
plasma surrounding a rubidium bead supplied
with hydrogen and
air. Nitrogen and
phosphorous containing compounds produce
ions that are attracted to the collector.
The number of ions hitting the collector is
measured and a signal is generated.

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Selectivity:

Nitrogen

and

phosphorous

containing compounds
Sensitivity: 1-10 pg

Linear range: 104-10-6
Gases: Combustion - hydrogen and air; Makeup
- helium

Temperature: 250-300°C

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Figure The nitrogen–phosphorus detector

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Thermal Conductivity Detector (TCD)
Measures the changes of thermal conductivity
due to the sample. A detector cell contains a
heated filament with an applied current. As
carrier gas containing solutes passes through the
cell, a change in the filament current occurs.
The current change is compared against current
in a reference cell.
The difference is measured and a signal is
recorded. When a separated compound elutes
from the column , the thermal conductivity of
the mixture of carrier gas and compound gas is
lowered.
The filament in the sample column becomes
hotter than the control column. The imbalance
between
control
and
sample
filament
temperature is measured and a signal is
recorded.
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Mechanism: A detector cell contains a heated
filament with an applied current. As carrier gas
containing solutes passes through the cell, a
change in the filament current occurs. The
current change is compared against the current
in a reference cell. The difference is measured
and a signal is generated.

Selectivity: All compounds except for the carrier
gas
Sensitivity: 5-20 ng
Gases: Makeup: same as the carrier gas

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Figure: Thermal conductivity detector (TCD)

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Electron Capture Detector (ECD)
Another useful detector is the electron capture
detector (ECD) It is an excellent detector for
molecules containing an electronegative group
such as Cl or F etc. (or derivitized molecules) It
is probably the second most common detector
after the FID. It is most often used for the trace
measurement of halogen compounds in
environmental applications for
detecting
insecticide and herbicide residues.
The ECD uses a radioactive source such as Ni63
which produces Beta particles which react with
the carrier gas producing free electrons. These
electrons flow to the anode producing an
electrical signal .
When electrophillic
molecules are present, they capture the free
electrons, lowering the signal. The amount of
lowering is proportional to the amount of
analyte present. It is sensitive down to 10-15but
the dynamic range is only about 104.
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Mechanism: Electrons are supplied from a 63Ni
foil lining the detector cell. A current is
generated
in
the
cell.
Electronegative
compounds capture electrons resulting in a
reduction in the current. The amount of current
loss is indirectly measured and a signal is
generated.
Selectivity: Halogens, nitrates and conjugated
carbonyls
Sensitivity:

0.1-10

pg

(halogenated

compounds); 1-100 pg (nitrates); 0.1-1 ng
(carbonyls)
Gases: Nitrogen or argon/methane

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Figure: Electron Capture Detector (ECD)

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Photoionization Detector
The photoionization detector (PID) uses a UV
lamp (xenon, krypton or argon lamp, depending
on the ionization potential of the analytes) to
ionize compounds. The ionization produces a
current between the two electrodes in the
detector. The detector is non-destructive and
can be more sensitive than an FID for certain
compounds (substituted aromatics and cyclic
compounds for example).
Mechanism: Compounds eluting into a cell are
bombarded with high energy photons emitted
from a lamp. Compounds with ionization
potentials below the photon energy are ionized.
The resulting ions are attracted to an electrode,
measured, and a signal is generated.

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Selectivity: Depends on lamp energy. Usually
used for aromatics and olefins (10 eV lamp).
Sensitivity: 25-50 pg (aromatics); 50-200 pg
(olefins)
Gases: Makeup - same as the carrier gas
Temperature: 200°C

Figure: Photoionization Detector

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Atomic Emission Detector
One of the newest gas chromatography detectors
is the atomic emission detector (AED). The
AED is quite expensive compared to other
commercially available GC detectors, but can be
a powerful alternative. The strength of the AED
lies in the detector's ability to simultaneously
determine many of the elements in analytes that
elute from the column.
It uses microwave energy to excite helium
molecules (carrier gas) which emit radiation
which breaks down molecules to atoms such as
S, N, P, Hg, As, etc. These excited molecules
emit distinctive wavelengths which can be
separated by a grating and sent to the detector
(typically a photodiode array) which produces
the electrical signal. The atomic emission
detector is very sensitive (10-15) and has a
dynamic range of 104.
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Figure: Atomic Emission Detector

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Thermal Conductivity Detector
The thermal conductivity detector (TCD)
consists of an electrically-heated wire or
thermistor. The temperature of the sensing
element depends on the thermal conductivity of
the gas flowing around it. Changes in thermal
conductivity, such as when organic molecules
displace some of the carrier gas, cause a
temperature rise in the element which is sensed
as a change in resistance. Low molecular weight
gases have high conductivities so hydrogen and
helium are often used as carrier gases. Nitrogen
and argon have similar conductivities to many
organic volatiles and are often not used.
However if nitrogen is used as a carrier gas, the
detector can be used to measure hydrogen or
helium. TCD’s are often used to measure
lightweight gases or water (compounds for
which the FID does not respond).
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The TCD is not as sensitive as other detectors
but it is a universal detector and is nondestructive. However, modern detectors called
micro-TCD’s have very small cell volumes, and
new electronics that produce much higher
sensitivities and wider linear ranges. Due to its
increased sensitivity, and the fact that it is a
universal non-destructive detector, it is again
becoming more popular for certain applications.

Figure: Thermal Conductivity Detector

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MASS SPECTROMETER (MS)
The Mass spectrometer is the only detector
which does not require higher temperature than
is the temperature of GC column. The outlet of
capillary column of GC is placed directly to
ionization source. The MS employed consists of
EI or CI ionization and RF analyzer such as
quadrupole or ion trap.
Mechanism: The detector is maintained under
vacuum. Compounds are bombarded with
electrons (EI) or gas
molecules (CI).
Compounds fragment into characteristic charged
ions or fragments. The resulting ions are
focused and accelerated into a mass filter. The
mass filter selectively allows all ions of a
specific mass to pass through to the electron
multiplier. All of the ions of the specific mass
are detected. The mass filter then allows the
next mass to pass through while excluding all
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others. The mass filter scans stepwise through
the designated range of masses several times per
second. The total number of ions are counted for
each scan. The abundance or number of ions per
scan is plotted versus time to obtain the
chromatogram (called the TIC). A mass
spectrum is obtained for each scan which plots
the various ion masses versus their abundance
or number.
Selectivity: Any compound that produces
fragments within the selected mass range. May
be an inclusive range of masses (full scan) or
only select ions (SIM).
Sensitivity: 1-10 ng (full scan); 1-10 pg (SIM)
Gases: None
Temperature: 250-300°C (transfer line), 150250°C (source)
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Table: GC Detectors

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APPLICATION of GAS CHROMATOGRAPHY

Qualitative analysis
Generally chromatographic data is presented as
a graph of detector response (y-axis) against
retention time (x-axis).
This provides a spectrum of peaks for a sample
representing the analytes present in a sample
eluting from the column at different times.

Retention time can be used to identify analytes
if the method conditions are constant. Also, the
pattern of peaks will be constant for a sample
under constant conditions and can identify
complex mixtures of analytes.

Gas Chromatography

Page | 63


Qualitative analysis
The area under a peak is proportional to the
amount of analyte present. By calculating the
area of the peak using the mathematical function
of integration, the concentration of an analyte in
the original sample can be determined.
Concentration can be calculated using a
calibration curve created by finding the response
for a series of concentrations of analyte, or by
determining the response factor of an analyte.

Gas Chromatography

Page | 64


High Efficiency of separation
The technique has strong separation power and
even complex mixture can be resolved into
constituents
Speed of separation
The analysis is completed in a very short time
The sensitivity in detection of compounds
The sensitivity is quite high and it is
micromethod and only a few mg of sample is
sufficient for analysis.
Accuracy
It gives good precision and accuracy
Gas Chromatography

Page | 65


Application
It has wide application for most groups of
pharmaceutical agents
Personnel
The technique is suitable for routine analysis
because the operation of a gas chromatograph
and related calculations does not require highly
skilled persons.
Limitations of Gas Chromatography
The primary limitation of gas chromatography is
that the sample must be capable of
volatilized/vaporiaed
without
undergoing
decompositions. The samples must be thermally
stable to prevent degradation when heated. In
addition, sample cannot be used for further
analysis once separated.

Gas Chromatography

Page | 66

Gas Chromatography by mirola

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