A presentation containing the Principle, shematic diagram, omponents of the instrument, working of the instrument, application, advantages and disadvantages of the instrument.

1. COIMBATORE INSTITUTE OF TECHNOLOGY
INSTRUMENTAL METHOD OF ANALYSIS (19MENE03)
PRESENTATION ON ATOMIC ABSORPTION SPECTROSCOPY (AAS)
by
JEBARAJ SAMUEL DORAI D
(19 62 006)
DEPARTMENT OF CIVIL ENGINEERING
M.E. Environmental Engineering and Management

2. INTRODUCTION
Atomic absorption spectroscopy (AAS) is a spectroanalytical procedure for the
quantitative determination of chemical elements using the absorption of optical radiation
(light) by free atoms in the gaseous state.
It is based on absorption of light by free metallic ions. The technique is used for determining
the concentration of a particular element (the analyte) in a sample to be analyzed.
Individual elements will absorb wavelengths differently, and these absorbance are measured
against standards.
The absorption of radiation by atoms in the sun’s atmosphere was first observed by
Wollaston in early 1800s. But, this technique was first introduced for analytical purpose by
Alan Walsh and Alkemade and Miltaz in the year 1956.

3. PRINCIPLE
AAS is the measurement of absorption of radiation by free atoms. The total
amount of absorption depends on the number of free atoms present and the
degree to which the free atoms absorb the radiation.
AAS quantitatively measures the concentrations of elements present in a
liquid sample. The elements in the gas phase absorb light at very specific
wavelengths which gives the technique excellent specificity and detection
limits.
At the high temperature of the AA flame, which may be either oxy-acetylene
as used here, or nitrous oxide/acetylene, the sample is broken down into
atoms and it is the concentration of these atoms that is measured.

4. SCHEMATIC DIAGRAM OF THE INSTRUMENT

5. COMPONENTS OF THE INSTRUMENT
Hollow cathode lamp
Flame Atomizer
Nebulizer
Monochromator
Photomultiplier tube
Readout device

6. WORKING OF THE INSTRUMENT
First, a reference sample solution is prepared which will be used to check or set the
wavelength of the element that is to be analysed.
The sample that is to be analysed is made sure that it is in liquid state. It is prepared with the
help of a standard procedure; it is purified by removing all the impurities that might cause
errors or interference in later stages of the analysis. This eases the working as the instrument
involves the aspiration of the samples.
A hollow cathode lamp acts as a source of radiation. During analysis, the lamp is switched
on and heated for five to ten minutes. Usually, the cathode in the lamp is made up of the
element that is to be analysed.
The atoms in the HCL are supplied with energy; atoms get ionized and this process is known
as Ionization. When the gaseous inert gas atoms have sufficient energy, the bombard the
cathode thereby vaporizing the metal atoms on the surface of it. This process is known as
Sputtering.

7. WORKING OF THE INSTRUMENT
When sputtering occurs, the atoms get excited and thus moves to the higher energy level.
The transition to the lower energy level emits a Resonance line. This is the first resonance
line which has the strongest absorptivity.
Meanwhile, the prepared sample is injected into the Flame Atomizer through a Nebulizer in
the form of aerosol with the help a pneumatic system. The sample is aspirated into the flame.
Here, the molecules of the sample are broken into atoms which are generally in the ground
state or lower energy level.
When the radiation from the HCL containing the ionized atoms hit the free atoms of the
sample, it gets excited to the higher energy level and then due to stability issues, the atoms
again start to decay to the lower energy level.
As seen earlier, the absorptivity decreases as the energy difference between the ground state
and the excited state increases. An important point to note that the wavelength of the first
resonance line for all metals and metalloids is greater than 200nm.

8. WORKING OF THE INSTRUMENT
The first resonance line for non-metals come into the ultraviolet region
below 185nm and thus it cannot be measured.
Hence, AAS can be used to measure the metals and metalloids.
The resonance line created is then directed towards a Monochromator which
concentrates it.
From there, it reaches the Photomultiplier Tube and then the results are
displayed on a Readout Device.

9. TYPICAL VIEW OF AAS

10. OVERVIEW OF THE COMPONENTS
1. Hollow Cathode Lamp
It is the light source that emits exactly the required wavelength for the
analysis; it also has an inert gas.
It is a cylindrical glass tube containing a Cathode made up of the analyte
metal or a coating of the metallic compound; an Anode made up of a tungsten
wire.

11. OVERVIEW OF THE COMPONENTS
2. Atomizer
The sample is drawn in through a small tube and taken to the nebulizer
where the solution is broken up into a fine mist.
The fine mist is carried to the atomizer, such as a Flame or Graphite. When
the mist reaches the flame, the intense heat breaks up the sample into its
individual atoms. This final process is called atomization.
Optimum flame conditions are different for different elements. So, it is
necessary to control the gas supplies and the height of the burner must be
adjusted to obtain maximum absorption.

12. OVERVIEW OF THE COMPONENTS
Flame and its profile:-
A mixture of different oxidants and fuels can be used to achieve a specific
temperature range. Because dissociation and breaking molecules down to
atoms is easier with more heat present, oxygen is the most common oxidant
used in flame atomization.

13. OVERVIEW OF THE COMPONENTS
Important regions in the flame are as follows:
• Base
• Primary combustion zone
• Interzonal region
• Secondary combustion zone

14. OVERVIEW OF THE COMPONENTS
3. Monochromator
Its purpose is to select a given line in the emission spectrum of the light and
isolate it from the other lines of spectrum.
It narrows down the bandwidth to separate the line chosen fir determination.

15. OVERVIEW OF THE COMPONENTS
4. Photomultiplier Tube (PMT)
The radiant energy signals can be converted into electrical signals using a Photomultiplier
tube.
The electronic system is capable of differentiating between the signal from the source and
the continuous signal from the flame. The PMT is useful when the lines lie in the ultraviolet
region of the spectrum.
The multiplication of the current from the PMT is carried out by an Amplifier.

16. APPLICATION OF THE INSTRUMENT
Generally, AAS is applicable in the fields of Environmental science and
Engineering, Food technology, Pharmaceuticals, Mining, Bio-monitoring,
Agriculture, Nanomaterials and Pathology.
In specific, the instrument can be used to analyse about 70 different metals.
The application of it in forensic sciences are for the Determination of trace
elements Elemental profiles of biological samples, Trace elements in artificial
fibres, Determination of the mode of poisoning, Hair analysis for heavy metal
poisons, Determinations of ammunition manufacturers and Differentiation of
Elements.

17. ADVANTAGES
The advantage of the technique is its degree of freedom from interference
from its environment (i.e.) by the presence of other elements. Minute amount
of an element can be determined in the presence of high concentration of
other elements.
The instrument is sensitive than emission flame photometry. More elements
can be quantitatively determined by this technique.
It is highly specific, because the atoms of a particular element can only
absorb radiation of their own characteristic wavelength.
More elements can be quantitatively determined by this method. The
sensitivity of this technique is higher for most elements.

18. DISADVANTAGES
The main disadvantage of this method is the need of a separate lamp source
for each element to be determined.
Some elements like Aluminium, Tungsten, Vanadium, Molybdenum cannot
be detected when a flame is used to produce the atomic state because these
elements raise the oxides in the flame.
This technique cannot be used to analyse non-metals as their resonance lines
in the ultra violet have difficulties to arise from the strong absorption of light
by the oxygen in the light path from the flame gases.
Seeing the limitations, the instrument can be coupled with different other
instruments to overcome the disadvantages. The different other instruments
include Cold vapour AAS, Hydride generating AAS, Graphite furnace AAS,
etc.