Atomic Spectros
Atomic Spectros
Atomic Spectros
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Outline
Introduction
Instrumentation
Applications
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Introduction
Atomic Spectroscopy - interaction of an atom
in the gas phase with EMR.
Based upon the ability of atoms to absorb or
emit light.
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Atomic Absorption Spectroscopy
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Instrumentation (AAS)
1. Radiation source
2. Atomizer
3. Monochromator
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1. Radiation source
Ne /Ar
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AAS
HCL
motorized
Mirror
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HCL…
Both anode & cathode are sealed in
Glass cylinder
Filled with the carrier gas of Argon or Neon.
At its end is a window transparent to the emitted radiation.
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HCL emission process
When an electrical potential is applied between the anode &
cathode, some of the fill gas atoms are ionized.
The +vely charged fill gas ions accelerate through the electrical field
to collide with the -vely charged cathode
Dislodge individual metal atoms in a process called ‘‘sputtering’’.
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1. Sputtering 2. Excitation
3. Emission
Atomization
Process of breaking analyte into gaseous atoms
1. Flame
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Flame Atomization
The sample is first converted into a fine mist consisting of small droplets
of solution by nebulizer.
The sample is aspirated into a spray chamber by passing a high-pressure
stream consisting of one or more combustion gases.
The impact of the sample with the glass impact bead produces an aerosol
mist.
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Fig. Flame atomization assembly equipped
with spray chamber and slot burner.
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Processes occur during atomization:
Dissociation
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Thermal energy in flame atomization is provided
by the combustion of a fuel–oxidant mixture.
Common oxidants/fuels:
Air–acetylene for easily ionised elements
Used for elements that are easily atomised like cadmium, lead, nickel etc..
Reducing flame
A fuel rich flame which produces an excess of Carbon and Hydrogen atoms
Help to breakdown the oxide bonds which form with some element like chromium, tin etc
Stoichiometric flame
Used for elements that are not so easily atomised like iron etc. 20
Flame atomization
(HCL)
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Merits of flame atomization
Ease of use
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Demerits
The efficiency of atomization may be quite poor.
This may occur for two reasons.
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Fig. Electrothermal analyzer
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The graphite is heated to a temp which is high
enough to evaporate the solvent from the solution.
The current is then increased so that
The sample is ashed & then ultimately it vaporises &
dissociates into gaseous atoms.
The light from the source (HCL)
Passes via the furnace & absorption during the
atomization step is recorded over several seconds.
ETAAS or GFAAS
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Atomization is achieved in three stages
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Graphite furnace…
Merits Demerits
Low detection limit Poor precision
normally ppb Background absorption effects
Excellent sensitivity Analyte may be lost at the
Low sample volume ashing stage
Direct analysis of solid The analytical range is
samples relatively narrow
No combustible gasses Higher initial cost
required Higher running costs
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Merits of furnace over flame
1. Residence time of the analyte in the optical path
Only a fraction of a second it rises via the flame.
Several seconds (furnace)
↑ sensitivity
2. Sample volume
1 − 2ml minimum for flame
as little as 1μl for furnace
3. Sample types
Liquid samples (flame)
A direct solid analysis without any sample preparation
in addition to liquid samples analysis (furnace)
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Inductively Coupled Plasma (ICP)
Sample is aspirated into a spray chamber through a nebulizer
using a system very similar to that for flame atomizer.
However, instead of combustible gases, argon is used as a
transport gas for the sample.
The plasmas are formed by ionizing a flowing stream of
argon, producing argon ions & electrons.
The high temperatures in a plasma:
Result from resistive heating that develops due to the movement
of the electrons and argon ions.
Desolvate, vaporize & largely atomize the sample.
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3. Monochromator
It selects the specific λ of light which is absorbed by the
sample & transfers it to the detector, & excludes other λ.
The selection of the specific light allows the determination
of the selected element in the presence of others.
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4. Detector & Read out Devices
The light selected by the monochromator is directed
onto a detector that is typically a photomultiplier
tube,
Whose function is to convert the light signal into an
electrical signal proportional to the light intensity.
Temp determines:
The degree to which a sample breaks down to atoms
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Boltzmann Equation: relates excited state population/ground state
population ratios to energy, temperature and degeneracy.
N* g*
( ) e -(E/RT)
Where, No go
N* = No of atoms in the excited state, No= No of ground state atoms,
g* /go= Ratio of statistical weights for excited and ground states,
1
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Interferences
Chemical interferences
Ionization interferences
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Spectral Interferences
Overlap of analyte signal with signals due to:
Other elements or molecules in the sample, or
Flame or furnace
Remedy:
Using high resolution spectrometers
Resolve closely spaced spectral lines
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Chemical interferences can be avoided/reduced by:
Adding releasing agents
Form thermally stable cpds with the interferents.
eg. lanthanum releases calcium from interferences like phosphate
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Ionization interferences
Occur when the electrons are removed from the
atoms which will create an ion.
Thermal energy excites atom/removes e- from
atom.
Reduces the ground state atoms &
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Atomic Emission Spectroscopy
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The instrumentation of AES is the same as that of
AAS, but without the presence of a radiation
source .
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Atomization & Excitation
Plasmas
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Flame Sources
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Plasma Sources
A plasma consists of:
A hot, partially ionized gas, containing an abundant conc of
cations & electrons that make the plasma a conductor.
8000 K
Circulation of the electrons under
the effect of the induced field
within the heart of the plasma
Concentric
quartz tubes
Plasma flow
(Ar)
Auxiliary Ar flow
Conductive coil connected
to RF generator
Injector tube
Nebuliser flow
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(sample + Ar) Fig. ICP torch
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A radiofrequency current in the induction coils
Creates a fluctuating magnetic field that induces the argon ions & electrons to
move in a circular path.
Between 6000 & 8000 K at a height of 15–20 mm above the coil, where
emission is usually measured.
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This temp requires thermal isolation from the outer quartz tubes
By introducing a high-velocity flow of argon tangentially along the walls of
these tubes.
ICP-AES 49
Advantages of plasma
Simultaneous multi-element Analysis – saves sample amount
Plasmas operate at much higher temp than flames, they provide better
atomization & more highly populated excited states.
Excitation & emission zones are spatially separated; this results in a low
background.
Conc range of several decades (105 – 106)
Disadvantages of plasma
Very complex Spectra - hundreds to thousands of lines
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Application…
Monitoring levels of elements(toxic/essential) in
samples:
Pharmaceutical products
Standards
Cosmetics
Food supplements etc.
With these information, the products can be approved or
withdrawn from the markets.
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Table: Assay of Pharmaceutical Substances official in BP (1993) by AAS
Name of Substance Elements Assayed Measured at(nm) Limits Prescribed
Activated Charcoal Cu, Pb, Zn 325 Cu = NMT* 25 ppm
Pb = NMT 10 ppm
Zn = NMT 25 ppm
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