2. Need of Atomic Absorption Spectroscopy
Generally flame photometry is used for detection of metallic
elements but limited to only alkali metals and alkaline earth
metals, because they require less energy for excitation.
Other transition metals require high energy for their excitation
which is not provided by the flame temperature. So a specific
lamp of element to be determined is needed to provide a light of
particular wavelength to cause of excitation of transition
metallic elements.
Ex-Na+ lamp, K+ lamp, Co lamp, Cr lamp etc….
3. PRINCIPLE
Atomic absorption spectroscopy is a quantitative method
of analysis that is applicable to many metals and a few
nonmetals.
It is the absorption phenomenon.
When a sample solution is aspirated into the flame, the
solvent is vaporised leaving particles of the solid salt.
The salt burns and converted into the gasses form which
is dissociated into free neutral atoms and some of them
excited by the flame heat. Remaining atoms in the
ground state absorbs light of specific wavelength
emitted by the lamp of same element to be determined
and the intensity of the light absorption is measured.
4. BOLTZMANN DISTRUBUTION EQUATION
N*/No = e - ∆E/kT
N* – number of excited atoms
N
o – number of unexcited atoms
∆E – difference in energies of two levels
K – Boltzmann constant
T – temperature of the flame
8. When a potential (300-500 V) is applied between two
electrodes, The electric discharge ionizes the inert gas (Ne or Ar
usually) atoms filled in the hollow tube, which in turn, are
accelerated into the cathode with high velocity and sputter
metal atoms into the gas phase. Metal atoms become excited
and after returning to ground state give rise to metal emission
spectrum.
2. Electrodeless Discharge Lamp
Used for volatile elements
Consist of evacuated tube in which metal of interest is
placed.
Tube filled with argon gas at low pressure, sealed and placed
in a microwave discharge cavity & cause excitation of metal
sealed inside the tube.
9. CHOPPER
A rotating wheel is interposed between the hollow cathode
lamp and flame .This rotating wheel is known as chopper.
It is interposed to break the steady light coming from the lamp
into pulsating light which is used to measure the intensity of
light absorbed by elements without interference by radiation
from the flame itself.
ATOMISER
Atomiser converts the liquid into small droplets which are
easily vaporised.
Types of atomisers :-
1. Flame atomiser:-
a.) Total consumption burner
b.) premixed burner
2. Non-flame atomiser
10. Flame atomiser
a). Total consumption burner:- In this whole sample is atomised
into the flame, hence named as total consumption burner.
Fuel used – H2
Oxidant – O2
Disadvantages
noisy and hard to use.
11. b). Premixed burner :- It is most widely used because of
uniformity in flame intensity.
In this the sample solution ,fuel and oxidant are mixed before
they reach the burner tip.
Advantages
Non-turbulent
Noiseless
Stable
Disadvantages
Only 5% sample reaches
to the flame and rest
95% is wasted.
12. Temperature of some flames
Fuel oxidant Temperature (K)
H2 Air 2000-2100
C2H2 Air 2100-2400
H2 O2 2600-2700
C2H2 N2O 2600-2800
For some elements that form refractory oxides (molecules hard
to break down in the flame) nitrous oxide (N2O) needs to be
used instead of air (78% N2 + 21% O2) for the oxidant. In that
case, a slightly different burner head with a shorter burner
slot length is used.
13. Non flame atomiser
The graphite furnace is an electro thermal atomiser system that
can produce temperatures as high as 3,000°C. The heated
graphite furnace provides the thermal energy to break chemical
bonds within the sample held in a graphite tube, and produce
free ground state atoms.
The ground-state atoms are capable of absorbing energy, in the
form of light, and are elevated to an excited state. The amount of
light energy absorbed increases as the concentration of the
selected element increases.
15. MONOCHROMATOR
Monochromators are used to disperse the radiation according to
the wavelength.
They are of two types:
1) Prism
2) Grating
1). Prism monochromator :-
Quartz material is used for making
prism, as quartz is transparent over
entire region.
16. 2). Grating monochromator :- it consists of a series of parallel
straight lines cut into a plane surface.
Diffraction by grating follows a law of diffraction:-
nλ = d(sin I ± sin ø)
where n – order
λ – wavelength
d – distance between grooves
I – angle of incidence
ø – angle of dispersion
17. DETECTORS
The role of the detector is to convert a light signal into an electrical
signal.
Generally in most of the instruments PMT is used
The detector contains a photoemissive cathode and a series of
dynodes. The number of electrons emitted from the cathode is
directly proportional to the intensity of the light beam. Electrons
emitted from the cathode are accelerated to the first dynode by a 90
volt potential where the electron impact dislodges several additional
electrons which are accelerated to the next dynode by an additional
90V potential. After nine dynodes (each one at +90V with respect to
the one before it), the number of electrons finally reaching the anode
is in the order of ten million for each incident photon. The current
measured at the anode collector is still proportional to the intensity
of the light but it has been amplified over a million times.
19. INTERFERENCE
Interference is a phenomenon in which two waves
superimpose to form a resultant wave of greater or lower
amplitude. Interference decrease the intensity of absorption of
light . Interference usually refers to the interaction of waves
that are correlated or coherent with each other, either because
they come from the same source or because they have the same
or nearly the same frequency.
Types of interferences
1) Spectral interference
2) Chemical interference
3) Physical interference
20. Spectral interferences
Spectral interferences arise when the absorption or emission
of an interfering species either overlaps or lies so close to the
analyte absorption or emission that resolution by the
monochromator becomes impossible. ex:-Manganese triplet
(4031,4033,4035Å) overlapped by gallium line(4033Å).
This interference can be corrected by amplitude modulation
of the source.
Chemical interference
Occurs due to incomplete dissociation of compounds in the
flame when the concentration of compound is more.
Removed by use of higher flame temperature.
Chemically –by addition of more thermally stable compound.
ex-addition of lanthanum to the aluminium and magnesium
for detection of magnesium
21. Physical interference :-viscosity
-solvent
-ionization
1) Viscosity – viscosity is invertionaly proportional to the
intensity of absorption.
2) Solvent – organic solvent increases the intensity and
aqueous solvent decreases the intensity of absorption.
3) Ionization – occurs due to high flame temperature. A
number of vaporized atoms become ionized by the flame.
Resulting ions absorb at a different wavelength than the
vaporized atoms the new wavelength will not be selected by
the monochromator and low results occurs.
Na Na+ + e-
Overcome by addition of more easily ionizable element
Ex- ionization interference of Na is corrected by the addition
of Potassium to the Sodium .
22. APPLICATIONS OF AAS
Qualitative analysis of elements.
Quantitative analysis of elements.
Simultaneous elemental analysis.
Clinical analysis of biological fluids such as urine and blood
Environmental analysis – analysis of river water, industrial wastes,
seawater, drinking water, air etc….
Pharmaceutical analysis
Determination of metallic elements( Cu,Zn,Ni) in food industry.
Determination of lead in petrol (diethyl lead and tetraethyl lead)
23. Difference between AAS and Flame
Photometry
Flame photometry Atomic absorption spectroscopy
Emission phenomenon Absorption phenomenon
Limited to only alkali and alkaline earth
metals
used for approx. 81 metals
Dependent upon flame temperature Independent upon flame temperature
Dependent upon number of excited atoms Dependent upon unexcited atoms
