Principle, Instrumentation and applications of AAS and AES

1. Atomic Absorption
and
Atomic Emission Spectroscopy
Mr. Lokesh Thote
Asstt. Professor
Dept. of Pharmaceutical Chemistry
KAMLA NEHRU
COLLEGE OF PHARMACY,
Butibori, Nagpur - 441108

2. INTRODUCTION
 ATOMIC ABSORPTION SPECTROSCOPY (AAS) is an
analytical technique introduced by Alan Walsh in the mid of
1920’s.
 It is most powerful instrumental technique for quantitative
determination of trace metals in liquid independent of their
molecular form.
 Near about 60 – 70 elements can be determined by this method.
 This method is use for determination of trace elements like
cadmium, B, Zn, Mg, Si, Se etc. in water, soil, food products,
biological samples etc.

3. PRINCIPLE
• The absorption of energy by ground state atoms in gaseous state
forms the AA Spectroscopy.
• When a solution containing metallic species is introduced into a
flame, the vapour of metallic species will be obtained (molecule
convert into atom).
• Some of the metal atoms may be raised to an energy level
sufficient high to emit the characteristic radiation of the metal
(Atomic Emission Spectroscopy).
• But large percentage of metal atoms will remain in non-emitting
ground state.
• These ground state atoms of a particular elements are receptive of
light radiation of their own specific wavelength.

4. • Thus when a light of this wavelength is allowed to pass through a
flame having atoms of the metallic species, part of that light will
be absorbed and the absorption will be proportional to the density
of atoms in the flame.
• Mathematically the amount of absorption of light absorbed is
given as fallows,
Where,
e = charge on element of mass m.
c = speed of light
N = total number of atoms absorb light
f = oscillator strength or ability of atoms to absorb at
frequency v
e2
Total amount of light absorbed = --------- Nf
mc

5. Elements detectable by atomic absorption are highlighted in pink
in this periodic table

11. The Atomic Absorption Spectrometer
Atomic absorption spectrometers have 6
principal components.
1. A light source ( usually a hollow cathode
lamp HCL).
2. Chopper
3. An atom cell (atomizer).
4. A monochromator.
5. Nebulization of liquid sample
6. A detector , and read out device .

12. Schematic diagram of AA
Spectrophotometer.

13. Picture of Atomic Absorption
Spectrophotometer

14. 1. Light Source
 The radiation source for AAS should emit stable, intense radiation
of the element to be determined.
A. HOLLOW CATHODE LAMP
 The light source is usually a hollow cathode lamp of the element
that is being measured .
 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).
 The lamp window is constructed of either quartz, silica or glass
 Each element has its own unique lamp which must be used for
that analysis .

15. Hollow Cathode Lamp
cathode
Anode
Quartz window
Pyrex body
Anode
Cathode

16. Scheme of a hollow cathode lamp

17. How it works
 When a potential difference (300 to 500 V) is applied between
two electrodes, a current in milliampere range is arise.
 The inert gas is charged at anode, and the charged gas is
attracted at high velocity to the cathode and eject metal atoms
from the cathode in a process called sputtering.
 Some sputtered atoms are in excited states and emit radiation
characteristic of the metal as they fall back to the ground state .
 The shape of the cathode which is hollow cylindrical
concentrates the emitted radiation into a beam which passes
through a quartz window, all the way to the vaporized sample.

18. • Since atoms of different elements absorb characteristic
wavelengths of light. Analyzing a sample to see if it contains a
particular element means using light from that element .
• For example with lead, a lamp containing lead emits light from
excited lead atoms that produce the right mix of wavelengths to
be absorbed by any lead atoms from the sample .
• A beam of the electromagnetic radiation emitted from excited
lead atoms is passed through the vaporized sample. Some of the
radiation is absorbed by the lead atoms in the sample. The
greater the number of atoms there is in the vapor , the more
radiation is absorbed .

19. • Advantages
• The spectral lines produced by HCL are so narrow that they are
completely absorbed by the atoms.
B. Electrodeless Discharge Lamp (EDL)
• It consist of an evacuated tube in which metal of interest
is placed.
• The tube is filled with argon at low pressure and sealed
off. The sealed tube is then placed in microwave
discharge cavity.
• Under this condition the argon becomes a plasma and
cause excitation of metal sealed inside the tube.
• The emission from the metal is that of its spectrum.

20. 2. Chopper
• A rotating wheel is interposed between hallow cathode lamp
and flame called as chopper.
• It used to break the steady light from lamp into an pulsating
light.
• This give polarizing current in photocell.

21. 3. 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 .
• The role of the atom cell is to primarily dissolvate a liquid sample
and then the solid particles are vaporized into their free gaseous
ground state form .
• In this form atoms will be available to absorb radiation emitted
from the light source and thus generate a measurable signal
proportional to concentration .

22. There are two types of atomizer :
a. Flame atomizer and
b. Non –flame atomizer.
• Flame atomizer
• The most common way is to use a flame which is used for
converting the liquid sample into gaseous form and also for
conversion of molecular entities into atomic vapour
• Commonly two type of burner are used.
 Total Consumption Burner, and
 Premix Burner

24. Total Consumption Burner
• In this the sample solution, fuel and oxidizing gases are passed
through separate passage to meet at the opening of the base of the
flame.
• The flame break up liquid sample containing metallic species into
droplet which are then evaporate to burnt, leaving the residues
which ie reduced to atoms
• It use oxygen with hydrogen and
Acetylene and give very hot flame.
Disadvantages:-
• This burner is noisy and hard to use.

25. Premix Burner
This burner is very suitable for AA studies of metals of group
IA, IB and IIB together with Ca, Pb, Ni and Pd

26. Temperatureof some flames
Temperature (K)
2000-2100
2100-2400
2600-2700
Fuel oxidant
H2 Air
C2H2 Air
H2 O2
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.

27. 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 p
rovides 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.

28. Non flame atomiser

29. Nebulisation of liquid sample:-
• Before the liquid sample enters into the burner, it is first of all
converted into small droplets, this method of formation of small
droplets form of the liquid sample is called nebulisation.
• Pneumatic Nebulizer:- gas moving at high speed

30. -- MONOCHROMATOR
Monochromators are used to disperse the radiation according to t
he 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.

31. -
DETECTORS
The role of the detector is to convert a light signal into an electrical
signal.
PHOTO MULTIPLIER TUBE
Generally in most of the instrument Photo Multiplier Tube (PMT) is
UseThis detector contains a photoemissive cathode and a series of
anode called as dynodes.
The number of electrons emitted from the cathode is directly proporti
- onal 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

32. Cont . . .
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
milliontimes.

33. ‘

34. Comparison Between Atomic Absorption and Emission
Spectroscopy
Absorption
• Atomic absorption
depends upon the
number of ground state
atoms .
• It measures the
radiation absorbed by
the ground state atoms.
• Presence of a light
source ( HCL ) .
Emission
• Atomic emission depends
upon the number of
excited atoms .
• It measures the
radiation emitted by the
excited atoms .
• Absence of the light
source .

35. Absorption
• The temperature in
the atomizer is adjusted
to atomize the analyte
atoms in the ground
state only.
Emission
• The temperature in the
atomizer is big enough to
atomize the analyte
atoms and excite them to
a higher energy level.

36. APPLICATIONSOFAAS andAES
 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)