2. INTRODUCTION
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 analyse over 62 elements.
It also measures the concentration of metals in the sample.
3. DEFINITION
In analytical chemistry, Atomic absorption spectroscopy is a
technique for determining the concentration of a particular metal
element in a sample.
Atomic absorption spectroscopy can be used to analyse the
concentration of over 62 different metals in a solution.
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HISTORY
The first atomic absorption spectrometer was built by CSIRO scientist
Alan Walsh in 1954 (Australia). Shown in the picture Alan
Walsh(left), with a spectrometer.
Firstly used for mining, medical treatment & agriculture.
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PROPERTIES OF AAS
The most widely used method in analysis of elements
Based on the absorption of radiation
So sensitive (ppb)
Quantitative analysis
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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 state .
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.
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Continue..
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.
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TECHNIQUES & STEPS
The technique typically makes use of a flame to atomize the sample,
but other atomizers such as a graphite furnace are also used.
Three steps are involved in turning a liquid sample into an atomic
gas:
Three steps are involved in turning a liquid sample into an atomic
gas
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Continue..
Desolvation – the liquid solvent is evaporated, and the dry sample
remains.
Vaporisation – the solid sample vaporises to a gas.
Volatilization – the compounds making up the sample are broken
into free atoms.
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Schematic diagram of AAS
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Radiation Source
Beer’s Law only applies to monochromatic radiation.
In practice, monochromatic implies that the line width of the radiation being measured is less than the
bandwidth of the absorbing species.
Atomic absorption lines very sharp with an inherent line width of 0.0001 nm.
Due to Doppler effect and pressure broadening, line widths of atoms in a flame are typically 0.001 - 0.01 nm.
Therefore, we require a source having a line width of less than 0.01 nm.
Typical monochromator has a bandwidth of 1 nm i.e. x100 greater than the line width of the atom in a flame.
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LIGHT SOURCE
Hollow Cathode Lamp are the most common radiation source in AAS.
It contains a tungsten anode and a hollow cylindrical cathode made of
the element to be determined.
These are sealed in a glass tube filled with an inert gas (neon or argon ) .
Each element has its own unique lamp which must be used for that
analysis .
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Reactions in the Hollow Cathode Lamp
Apply sufficiently high voltage between the cathode and the anode:
(1) Ionization of the filler gas:
Ne + e- = Ne+ + 2e-
(2) Sputtering of the cathode element (M):
M(s) + Ne+ = M(g) + Ne
(3) Excitation of the cathode element (M)
M(g) + Ne+ = M*(g) + Ne
(4) Emission of radiation
M*(g) (M(g) + h
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NEBULIZER
Suck up liquid samples at controlled rate.
Create a fine aerosol spray for introduction into flame.
Mix the aerosol and fuel and oxidant thoroughly for introduction
into flame.
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ATOMIZER
Elements to be analyzed needs to be in atomic sate.
Atomization is separation of particles into individual molecules and
breaking molecules into atoms. This is done by exposing the analyte to
high temperatures in a flame or graphite furnace.
ATOMIZER
FLAME ATOMIZERS GRAPHITE TUBE
ATOMIZERS
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FLAME ATOMIZER
To create flame, we need to mix an oxidant gas and a fuel gas.
In most of the cases air-acetylene flame or nitrous oxide- acetylene
flame is used.
liquid or dissolved samples are typically used with flame atomizer.
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Electrothermal Atomisation - Graphite Furnace
Sample holder consists of a graphite tube.
Tube is heated electrically
Beam of light passes through the tube.
Offers greater sensitivity than flames.
Uses smaller volume of sample
◦ typically 5 - 50 l (0.005 - 0.05 ml).
All of sample is atomised in the graphite tube.
Atomised sample is confined to the optical path for several seconds (residence time in
flame is very short).
Uses a number of stages as shown on next slide.
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Advantages and Disadvantages of
Electrothermal Atomisation
Advantages
◦ very sensitive for many elements
◦ small sample size
Disadvantages
◦ poor precision
◦ long cycle times means a low sample throughput
◦ expensive to purchase and run (argon, tubes)
◦ requires background correction
◦ method development lengthy and complicated
◦ requires a high degree of operator skill (compared to flame AAS)
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Stages in a Graphite Furnace
Typical conditions for Fe:
◦ Drying stage: 125o for 20 sec
◦ Ashing stage 1200o for 60 sec
◦ Atomisation 2700o for 10 sec
◦ Removal of remaining inorganic matter at the maximum operating temperature
about 3000 OC
Requires high level of operator skill.
Method development difficult.
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GRAPHITE TUBE ATOMIZER
Uses a graphite coated furnace to vaporize the sample.
ln GFAAS sample, samples are deposited in a small graphite coated
tube which can then be heated to vaporize and atomize the analyte.
The graphite tubes are heated using a high current power supply.
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MONOCHROMATOR
This is a very important part in an AA spectrometer. It is used to
separate out all of the thousands of lines.
A monochromator is used to select the specific wavelength of light
which is absorbed by the sample, and to exclude other wavelengths.
The selection of the specific light allows the determination of the
selected element in the presence of others.
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DETECTOR
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.
The processing of electrical signal is fulfilled by a signal amplifier .
The signal could be displayed for readout , or further fed into a data
station for printout by the requested format.
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Advantages and Disadvantages of AAS
Advantages
◦ equipment relatively cheap
◦ easy to use (training easy compared to furnace)
◦ good precision
◦ high sample throughput
◦ relatively facile method development
◦ cheap to run
Disadvantages
◦ lack of sensitivity (compared to furnace)
◦ problems with refractory elements
◦ require large sample size
◦ sample must be in solution
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APPLICATIONS
Determination of even small amounts of metals (lead, mercury, calcium,
magnesium, etc) as follows:
Clinical analysis (blood samples: whole blood, Plasma, serum; Ca, Mg, Li, Na, K,
Fe)
Environmental studies: drinking water, ocean water, soil.
Food industry: Food analysis
Water analysis (e.g. Ca, Mg, Fe, Si, Al, Ba content)
Analysis of soils
Pharmaceutical industry.
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Applications of AAS
Agricultural analysis
◦ soils
◦ plants
Clinical and biochemistry
◦ whole blood, plasma and serum Ca, Mg, Li, Na, K, Cu, Zn, Fe etc.
Metallurgy
◦ ores, metals and alloys
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Applications of AAS
Water and effluents
◦ many elements e.g. Ca, Mg, Fe, Si, Al, Ba
Food
◦ wide range of elements
Animal feedstuffs
◦ Mn, Fe, Co, Cu, Zn, Cr, Se
Medicines
◦ range of elements
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CONCLUSION
One of the most important technique in quantitative analysis
It is based on the absorption of radiation
Measurements could be done at very sensitivity levels
It’s widely used method
The preparation of the sample is usually simple and rapid
Advantages
High sensitivity [10--10 g (flame), 10-14 g (non-flame)]
Good accuracy (Relative error 0.1 ~ 0.5 % )
High selectivity
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REFERENCES
Pharmaceutical analysis Instrumental methods Volume II by Dr. A.V.
kasture, Dr. S.G. Wadodkar, Nirali prakashan page no. 23.9 – 23.12
Instrumental methods of Chemical analysis by G.R Chatwal, S.K.
Anand, Himalaya publishing house, fifth edition page no. 2.340 – 2.360
