detail chapter of AAS

1. By Saniya Shaikh

2.  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. In analytical
chemistry the technique is used for determining
the concentration of a particular element (the
analyte) in a sample to be analyzed. AAS can be
used to determine over 70 different elements in
solution or directly in solid samples

3.  When a solution containing metallic species in
introduced into a flame, the vp of the metallic
species is obtained.
Energy
Aq sample molecules atoms Excited atoms*
Desirable in AAS
Desirable in AES

4.  When light of a particular wavelength is
allowed to pass through the flame (having
atoms of sample), Part of the light will be
absorbed . The absorption will be
proportional to the density of the atoms ion
the flame.
 In AAS the amount of light absorbed in
determined

5.  At , υ the total amount of light absorbed = (πe2/mc)*Nf
 Where, e = charge on electron.
 m = mass of electron.
 c = speed of light.
 N = total number of atoms that can absorb frequency ν
in the light path.
 f = the oscillator strength or ability for each atom to
absorb at frequency ν.
 As ,e, m, c are constant equation becomes
 Total amount of light absorbed = constant * N * f

6. Sodium Na11

9.  Absorption of a photon is accompanied by
the excitation of an electron from a lower-
energy atomic orbital to an orbital of higher
energy. Not all possible transitions between
atomic orbitals are allowed. For sodium the
only allowed transitions are those in which
there is a change of ±1 in the orbital
quantum number (l);
 thus transitions from s → p orbitals are
allowed, and transitions from s → d orbitals
are forbidden.

10.  The spectrum consists of a small number of
discrete absorption lines corresponding to
transitions between the ground state (the 3s
atomic orbital) and the 3p and 4p atomic
orbitals.
 Absorption from excited states, such as the
3p → 4s and the 3p → 3d transitions included
in Figure are too weak to detect.
 Because an excited state's lifetime is short

12. Atomic absorption spectroscopy is based on the
same principle as the flame test used in qualitative
analysis.

13. Light Source
Hollow-cathode lamp (HCL): The
cathode contains the element that is is
to be analysed.

14.  Hollow-cathode lamps are a type of discharge lamp that
produce narrow emission from atomic species. They get
their name from the cup-shaped cathode, which is made
from the element of interest. The electric discharge
ionizes rare gas atoms, which are accelerated into the
cathode and sputter metal atoms into the gas phase.
Collisions with gas atoms or electrons excite the metal
atoms to higher energy levels, which decay to lower levels
by emitting light.
 Windows -- Two window materials, quartz and borosilicate
glass, are available on standard types. Special window
materials have been developed, such as MgF2, and LiF.
 Gas Fills -- Two gas fills, argon and neon, are available on
standard types. In general, neon will give the highest
spectral output and lamps containing it are recommended

15. The internal electrodeless lamp or induction light
is a gas discharge lamp in which the power
required to generate light is transferred from
outside the lamp envelope to the gas inside via an
electric or magnetic field, in contrast with a
typical gas discharge lamp that uses internal
electrodes connected to the power supply by
conductors that pass through the lamp envelope.
There are two advantages to elimination of the
internal electrodes:
1) Extended lamp life
2) higher efficiency
Used for volatile elements such as As, Cd, Pb

16.  In case of emission measurements, there will
always be a positive error since emission from
flame is an additive value to the actual sample
emission. It is therefore obvious that we should get
rid of this interference from emission in flames.
Excluding the emission signal from flames can easily be
done by an addition of a chopper to the instrumental
design. The chopper is a motor driven device that has open
and solid (mirrors in some cases)

17.  Light from the source is modulated electronically
or chopped mechanically by rotating chopper. This
helps isolate and remove sample cell emissions
from light emitted by the source. The specific
wavelength isolated by monochromator is led to
the detector and the electrical signal generated is
proportional to the elemental concentration in the
sample

18.  Light beam from source is split into two beams by
the chopper. One beam passes directly through the
flame and the other beam passes round the flame.
Detector response represents the ratio of sample
and reference beams. Fluctuations in light intensity
are eliminated electronically to get greater
reliability of results.

19. Sample introduction:
Atomizers1) FLAME ATOMIZEER
2) NON-FLAME ATOMIZER

20. Burner system
The burner assembly comprises of nebulizer to reduce the
liquid sample to a fine aerosol, a spray chamber and a burner
head which is used to generate a flame to produce atoms of
the same elements that are present in the sample.
1) TOTAL CONSUMPTION BURNER
2) PREMIXED BURNER
Burner system
TOTAL CONSUMPTION BURNER

22.  The sample is nebulized and mixed with the fuel
and oxidant prior to introduction into the flame,
with the use of a series of baffles.
 the sample is drawn from the sample container via
the vacuum created by the rushing fuel and oxidant
(aspiration). A drain line is required in this design
in order to remove sample solution droplets that
do not make it all the way to the flame. The fuel,
oxidant and sample all meet at the base of the
flame and is premixed prior to entering the flame.

23.  With the graphite atomizer, a discreet volume
of sample solution is vaporized
and wastage is virtually eliminated.