Atomic Absorption spectroscopy Principle, Instrumentation, Application
Atomic Absorption Spectroscopy is a very common
technique for detecting metals and metalloids in samples.
• It is very reliable and simple to use.
• It can analyze over 62 elements.
• It also measures the concentration of metals in the sample.
HISTORY:-
The first atomic
absorption spectrometer was built by CSIRO scientist Alan Walsh in 1954. Shown
in the picture Alan walsh(Ieft), with a spectrometer.
PRINCIPLE :-
• The technique uses basically the principle that free atoms
(gas) generated in an atomizer can absorb radiation at specific frequency.
• Atomic-absorption spectroscopy quantifies the
• absorption of ground state atoms in the gaseous states.
• The atoms absorb ultraviolet or visible light and make transitions to higher
electronic energy levels. The analyte concentration is determined from the
amount of absorption.
•Concentration
measurements are usually determined from a working curve after calibrating the
instrument with standards of known concentration.
•Atomic absorption is
a very common technique for detecting metals and metalloids in environmental
samples.
INSTRUMENTATION OF
ATOMIC ABSORPTION SPECTROSCOPY :-
- Flame
Atomizer
- Non
flame Atomizer
- a. Electrothermal atomizer
- b. Cold vapour atomizer
- Flame
Atomizers
- Flame used shall produced temp.
in excess of 2000k.
- This requirement can only be
meet by burning the fuel gas in oxidant gas which is usually air, nitrous
oxide or oxygen or oxygen diluted with either N2 or argon.
|
FUEL |
OXIDANT |
TEMPERATURE |
|
Acetylene |
Air |
2400-2700 |
|
Acetylene |
Oxygen |
3300-3400 |
|
Acetylene |
Nitrous oxide |
2900-3100 |
|
Hydrogen |
Air |
2300-2400 |
- Liquid
or dissolved sample :-
are typically used with flame atomizer. The sample solution is
aspirated by a pneumatic nebulizer transformed into an aerosol which is
introduced into a spray chamber where it is mixed with the flame gases and
conditioned in a way that only the finest aersols droplets (< 10 μm) enter
the flame.
- On top of the spray chamber is
a burner head that produced a flame that is laterally long (usually 5-10
cm) and only a few mm deep. The radiation beam passes through this flame
at its longest axis, and the flame gas flow rate may be adjusted to
produced the highest concentration of free atoms.
- The burnner height may also be
adjusted so that the radiation beam passes through the zone of highest
atom cloud density in the flame resulting in the highest sensitivity.
Dis
advantage of flame atomization technique :-
- Only 5-15% of the nebulised
sample reaches the flame
- A minimum sample volume of
0.5-1.0 ml is need to give a reliable reading.
- Sample which are viscous
require dilution with a solvent.
2.Non
flame Atomizer :-
Hollow
Cathode Lamp
•Cathode is in the
form of a hollow cylinder made of the metal which has to be analysed
• Anode is made of tungsten filament
• They are sealed in a tube filled with inert gas like Neon or Argon
• A large voltage across anode and cathode causes the inert gas to ionize and
form a plasma
• These ions are accelerated towards the cathode causing atoms to be sputtered
off
• The ions and metal
atoms are excited due to collisons
• They give off photons of a certain wavelength when they reach ground state
3.Nebuliser :-
The nebuliser forms a
mist or aerosol of the sample
• This is done by forcing the sample at high velocities through a narrow tube
• The sample is mixed with a fuel and oxidant
• Commonly used fuel-oxidant mixtures are acetylene-air and acetylene-nitrous
oxide.
4.Interferences :-
• Chemical interference:–
Presence of thermally stable compound that is not totally decomposed by the
energy of the flame
High flame temperature provides energy for breakdown on interference
.Addition of releasing agent which reacts with the
- Ionization
interferences :-
- Atoms of the samples are
ionized causing reduction in number of electrons and absorbance.Addition
of excess element like alkali elements, which gets ionized easily.
- Flame temperature may be
reduced.
- Matrix
interference
- Due to viscosity, burning
characteristics, surface tension of solvent Due to usage of different
solvents in calibration and sample Addition of diluents to reduce
viscosity
• Background absorptionLight scattering by particles in flame or absorption by undissociated molecules This must be measured and subtracted from final results Absorption of elements occurs as a narrow line whereas interference occurs over a broad range
Working :-
• The atoms of the solid are converted to gaseous state in the atomiser
• Radiation of specific wavelength is emitted by the hollow cathode lamp onto
the gaseous atoms in the atomiser
• The monochromator focuses the specific wavelengths into the detector
• The detector finds the amount of light absorbed
• The concentration of atoms in the sample is directly proportional to the
absorbance
APPLICATION OF ABS :-
•Atomic absorption spectroscopy is one of the most widely used
techniques for the determination of metals at trace levels in solution.
•Its popularity as compared with that of flame emission is due to its relative
freedom from interferences by inter element effect and its relative
insensitivity to various in flame temperature.
• Only bor the routine determination of alkali and alkaline earth metals
is flame photometry usually preferred.
• Widely used for metal analysis in enviromental sample(air,water,and soil) and
in biological fluid and tissues.