AAS is an analytical technique used to determine the concentration of metal atoms/ions in a sample. Metals make up around 75% of the earth’s chemical elements. In some cases, metal content in a material is desirable, but metals can also be contaminants (poisons). 

1. By
Dr. Neetu Soni, SHUATS,
Prayagraj, UP.

2.  Alan Walsh, 1950’s introduced the atomic absorption spectroscopy. Walsh
decided to measure absorption, not emission.
AAS is an analytical technique used to determine the concentration of metal
atoms/ions in a sample. Metals make up around 75% of the earth’s chemical
elements. In some cases, metal content in a material is desirable, but metals
can also be contaminants (poisons).
Atomic absorption spectroscopy (AAS) is based upon the principle that free
atoms in the ground state can absorb light of a certain wavelength. Absorption
for each element is specific, no other elements absorb this wavelength.

3. Atomic absorption spectroscopy has proved itself to be the most
powerful instrumental technique for the quantitative determination of
trace metals in liquids.
The method provides a total metal content of the sample and is almost
independent of the molecular form of the metal in the liquid for example
one can determine the sodium content of water sample and in most of
the cases it does not matter in what molecular form the sodium exits.
Atomic absorption spectroscopy does not demand sample preparation.
A disadvantage of the method is that only one element can be
determined at a time, change in light source and a change of analytical
wavelength are necessary to determine a second element.

4. Principle
The absorption of energy by ground state atoms in the gaseous state forms the
Atomic absorption spectroscopy.
When a solution containing metallic species is introduced into a flame the vapor
of metallic species will be obtained.
Sum of the metal atoms may be raised to an energy level sufficiently high to emit
the characteristic radiation of the metal- a phenomenon that is utilized in the familiar
technique of emission flame photometry. But a large percentage of the metal atoms
will remain in non- emitting ground state. These ground state atoms of the particular
element are receptive of light radiation of their own specific resonance wavelength.
 Thus when a light wavelength is allowed to pass through a flame having atoms of
metallic species, part of the light will be absorb and the absorption will be
proportional to the density the of the atoms in the flame. Thus the atomic
absorption spectroscopy, one determines the amount of light absorbed.

6. Disadvantages of Atomic absorption spectroscopy are as follows
 Separate lamp for each element to be determined is required
 This technique cannot be used very successful for emission of element like Al,
Ti, W, Mo, si, etc because these element give rise to the oxide in the flame.
 In aqueous solution, the predominant anion effects the signal to a negotiable
degree.

7. INSTRUMENTATION
Radiation sources: The radiation source for Atomic absorption
spectrophotometer should emit stable, intense radiation of the element to be
determined, usually a resonance line of the element.
Preferably the resonance spectral lines should be narrow as compared with the
width of the absorption lines to be measured. These lines should not be interfered
from other spectral lines which are not resolved by spectrophotometer. There
should be no general background or other extraneous lines a emitting within the
band pass of the monochromator. The problem of using such narrow spectral lines
has been solved by adopting a Hollow Cathode Lamp as radiation source.

8. Instrumentation used to carry out atomic absorption
spectrophotometry requires a source of light that
matches the narrow bands of light that a particular
atom absorbs (a hollow cathode lamp), a flame or
graphite furnace to heat the sample, a monochromator
to select the wavelength of light, and a photodetector.

9. Hollow cathode lamps (HCL) are discharge
lamps designed for use in Atomic Absorption
(AA) instruments. They consist of a cathode
made from the element of interest, an anode
and an inert filler gas contained in a glass
envelope.

11. Electrodeless discharge lamp
EDLs operate due to free electrons in the fill that are
accelerated by the MW field energy. They collide with
the gas atoms and ionize them to release more
electrons .

13. Atomizer
Flme atomizer :Total consumption burner and premix burner
Non flame atomizer: Carbon atomiser and L‘vov Plateform
Background corrector:
variety of background phenomena can interfere with the detector
and influence the results. Known broadly as background
absorption, issues like radiation scattering and molecular
absorption can result in an incorrect measurement of the element
content in a sample. Background correction is used to distinguish
background absorption from elemental absorption, thus returning
more accurate results.

14. Deuterium Background Correction
Two separate lamps are used to determine levels of background
absorption; a hollow cathode lamp (HCL) and a deuterium lamp. When
the HCL is on and the deuterium lamp off, the total absorbance is
measured. When the reverse is performed (deuterium on, HCL off), just
the background absorption is recorded. The background absorption is
then automatically subtracted from the total absorption to give the correct
atomic result.
Deuterium background correction does have its limitations, yet it remains
the most frequently used and inexpensive method.

15. This correction technique is used mainly in graphite furnace atomic
absorption. It is advantageous because it corrects high background
levels and it requires only one light source, eliminating the issue of
alignment.
The Zeeman technique uses an alternating magnetic field to split the
absorption line into three components. Like in the Deuterium
method, total absorption is measured with the magnetic field off and
background absorption with it on. The Zeeman technique is
considered more accurate because it can detect any kind of
background absorption. It is however much more expensive and
there can be a loss in sensitivity.

16. Monochromator
A monochromator is an optical device that transmits a
narrow band of wavelengths of light or other radiation
from a wider range of wavelengths. The atoms in the
AAS instrument accept the energy of excitation and
emit radiation.
A desired band of lines can be isolated with a
monochromator by passing a narrow band. The spectra
through a monochromator can be shown by a curve.

17. Detector
A detector can convert light coming from a
monochromator to a simplified electrical signal.
Generally, we used a photomultiplier tube as a
detector in the atomic absorption spectroscopy
instrument. A detector can be tuned to respond by a
specific wavelength or frequency.

18. The recorder can receive electrical signals from the
detector to convert them into a readable response. In
atomic absorption spectroscopy instrumentation, today we
used a computer system with suitable software for
recoding signals coming from the detector.
Recorder

21. Application of atomic absorption spectroscopy
Today, the atomic absorption spectroscopy technique is the most powerful tool
in analytical chemistry, forensic science environmental analysis, and food
industries. It is popular for analysts due to several advantages.
The most important advantage is the speed of analysis. It can analyze various
samples within a day.
Secondly, it is possible to determine all elements at trace concentration.
Thirdly, it is not always essential to separate the element before analysis
because AAS can be used to determine one element in presence of another.
The atomic absorption spectroscopy principle or instrumentation can be used
to analyze sixty-seven metals and several nonmetals such
as phosphorus and boron.