AAS, working, principle, application

1. Working and Application of AAS
(Atomic Absorption Spectrometer)
Presented By:
Anuradha Verma
Research Scholar

2. Atomic absorption spectrometry
Analytical technique that measures the concentrations of
elements.
Sensitive, can measure down to parts per billion of a gram
(μg dm–3) in a sample.
The technique makes use of the wavelengths of light
specifically absorbed by an element. They correspond to the
energies needed to promote electrons from one energy level
to another, higher energy level

3. Principle
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 Pb, a lamp containing Pb emits light
from excited lead atoms that produce the right mix of
wavelengths to be absorbed by any lead atoms from
the sample.

4. Atomic absorption spectroscopy is based on the same principle as the flame test used in
qualitative analysis
When an alkali metal salt or a calcium, strontium or barium salt is heated strongly in the Bunsen
flame, a characteristic flame colour is observed.
In the flame, the ions are reduced to gaseous metal atoms.
The high temperature of the flame excites a valence electron to a higher-energy
orbital. The atom then emits energy in the form of (visible) light as
the electron falls back into the lower
energy orbital (ground state).

5. The ground state atom absorbs light of the
same characteristic wavelengths as it emits
when returning from the excited state to the
ground state.
The intensity of the absorbed light is
proportional to the concentration of the
element in the flame.
Quantitative
Analysis

6. How it Works
In AAS, the sample is atomised – converted into ground state free atoms in the
vapour state
Beam of electromagnetic radiation emitted from excited lead atoms is passed
through the vaporised sample.
Some of the radiation is absorbed by the atoms in the sample.
The greater the number of atoms in the vapour, the more radiation is absorbed. The
amount of light absorbed is proportional to the number of atoms.
A calibration curve is constructed by running several samples of known
concentration under the same conditions as the unknown.
The amount the standard absorbs is compared with the calibration curve and this
enables the calculation of the lead concentration in the unknown sample.

7. Schematic diagram of an atomic absorption
spectrometer

8. Components of AAS
• light source
• a sample cell to produce gaseous atoms
• Means of measuring the specific light
absorbed.

9. Light Source
“ourĐe of light is a ͚hollow Đathode laŵp͛.
Contains a W anode and a cylindrical hollow cathode made of the element to be
determined.
These are sealed in a glass tube filled with an inert gas – e.g. Ne or Ar– at a pressure of
between 1 Nm–2 and 5 Nm–2.

10. The ionisation of some gas atoms occurs by applying a potential
difference of about 300–400 V between the anode and the
cathode.
These gaseous ions bombard 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.

11. Optical System and Detector
Monochromator:
Select the specific wavelength of light (spectral line) 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.
Detector:
The light selected by the monochromator is directed onto a
detector that is typically a photomultiplier tube. This
produces an electrical signal proportional to the light
intensity

12. A photomultiplier measures the intensity of
the incident light and generates an electrical
signal proportional to the intensity.

13. Atomisation of the sample
Two systems are commonly used to produce
atoms from the sample.
Aspiration involves sucking a solution of the sample
into a flame.
Electrothermal atomisation : where a drop of
sample is placed into a graphite tube that is then
heated electrically.

14. Flame aspiration
Ethyne/air (flame temperature of 2200–2400 °C)
Ethyne/N2O (flame temperature of 2600– 2800 °C)
A flexible capillary tube connects the solution to the nebuliser. At
the tip of the Đapillary, the solutioŶ is ͚nebulised͛ – i.e. broken
into small drops.
The larger drops fall out and drain off while smaller ones
vaporise in the flame.
Only about 1% of the sample is nebulised.

15. Sample preparation
Chemical form of the element is usually
unimportant. This is because atomization
converts the sample into free atoms irrespective
of its initial state.
The sample is weighed and made into a solution
by suitable dilution.

16. Background absorption
It is possible that other atoms or molecules apart from those of the element being
determined will absorb or scatter some radiation from the light source.
Includes unvaporised solvent droplets, or compounds of the matrix (chemical species,
such as anions, that tend to accompany the metals being analysed) that are not removed
completely.
This means that there is a background absorption as well as that of the sample.
One way of measuring and correcting this
background absorption is to use two light sources, one
of which is the hollow cathode lamp appropriate to
the element being measured. The second light source
is a deuterium lamp.

17. Calibration
A calibration curve is used to determine the unknown
concentration of an element
e.g. lead – in a solution.
The instrument is calibrated using several solutions of known
concentrations.
A calibration curve is produced which is continually rescaled as
more concentrated solutions are used – the more
concentrated solutions absorb more radiation up to a
certain absorbance.
The calibration curve shows the concentration against the
amount of radiation absorbed
The sample solution is fed into the instrument and the
unknown concentration of the element – eg lead – is then
displayed on the calibration curve

18. Application
Determination of Trace Elements in Rice Products
Samples were dried for 12 hours at 85 oC. The samples were cooled and portions of
approximately 0.25 g were accurately weighed and transferred to microwave
digestion vessels.
4 ml nitric acid was added to each vessel and left uncovered for 1 hour. A further 4 ml
nitric acid was added to each vessel, the vessels sealed and samples digested in a high
pressure closed microwave digestion system, by ramping over 20 minutes to 170 oC.
Samples were left to cool before being made up to 250 ml with deionized water for
cadmium analysis. Duplicate samples were prepared for the analysis of copper, lead,
Manganese and zinc (from the same sample) with the digests made up
to 50 ml with deionized water.

20. Analysis of Zn and B in fertilizer
• Dissolve the mg amount of fertilizer in say in
100mL of water and analyse for Zn, B etc.

21. ANALYSIS OF COPPER IN VITAMINS
VITAMIN:
Place a mulit-vitamin tablet in a 125mL Erlenmeyer flask and add 25 mL of 6
M HCL.
PlaĐe this oŶ a hot plate aŶd ďriŶg it to a ďoil. “et the hot plate oŶ ͞High.͟
As soon as the mixture begins to boil, reduce the heat to a setting of about 4
on the hot plate and continue to heat it for 15 additional minutes.
ADD ADDITIONAL HCl if necessary to keep the mixture wet and the volume at
About 25 mL.
Filter the resultant mixture through filter paper into a 25 mL volumetric flask.
Dilute to the mark with distilled water and thoroughly mix.

22. Presence of chromium in sea water
Linked to skin disorder dermatitis.
• A sample of sea water was analyzed using AA
spectroscopy, along with six standard solutions.
• Can use calibration curve to find the
concentration of the chromium in the sea water.

23. Application in Various Fields
Clinical analysis
Analysing metals in biological
fluids such as blood and urine.
Environmental analysis
finding out the levels of various
elements in rivers, seawater,
drinking water, air, petrol and
drinks such as wine, beer and
fruit drinks
Pharmaceuticals
minute quantities of a catalyst
used in the process (usually a
metal) are sometimes present in
the final product
Industry
check major elements present,
toxic impurities are lower than
specified
Mining
metals such as gold in rocks can
be determined to see whether it
is worth mining the rocks to
extract the gold

24. Advantages
• inexpensive (equipment, day-to-day running)
• high sample throughput
• easy to use
• high precision

25. Disadvantages of Flame Atomic Absorption
Spectroscopy
• only solutions can be analysed
• relatively large sample quantities required (1 –
2 mL)
• less sensitivity (compared to graphite furnace)
• problems with refractory elements

26. Thank You