2. Introduction
Atomic absorption spectrometry (AAS) is an analytical
technique that measures the concentrations of
elements.
Atomic absorption is so sensitive that it 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.
5. INSTRUMENTATION
Sharp-line radiation source (usually a hollow-cathode
lamp)
A solution nebulizer and burner (an electrically heated
furnace)
Mono-chromator
Photomultiplier
Recording system
6. SHARP-LINE SOURCES
The absorption line width for the ground state atoms
may be from 0.001-0.01nm. So the light sourced must
emit radiation of a line width less than that of the
absorption line width of the element being
determined.
This is achieved by vapour discharge lamps for certain
easily excited elements, e.g. sodium and potassium.
7. HOLLOW CATHODE LAMP
When a current flows between the anode and cathode
in this lamps, metal atoms are sputtered from the
cathode cup, and collisions occur with the filler gas.
A number of metal atoms become excited and give off
their characteristic radiation.
8. SAMPLE VAPORIZATION BY FLAME
The production of an homogeneous atomic vapour
from a sample is achieved by aspirating a solution into
a flame or evaporating small volumes in an electrically
heated tube furnace or from the surface of a carbon
rod. In all cases, the thermal energy supplied must :
(a) Evaporate the solvent
(b) Dissociate the remaining solids into their
constituent atoms without causing appreciable
ionization.
9. Monochromator
Some elements have a single emission line (principal
line). But several elements have more than one
emission line (secondary line).
Hence it is necessary to isolate required absorption
line from a radiation source by a grating
monochromator.
10. DETECTOR AND READOUT DEVICE
The intensity of radiation absorbed by elements, in the
UV or visible region(190-900nm) can be detected
using photometric detector like photomultiplier tube.
The readout device is capable of displaying the
absorption spectrum as well as the absorbance at a
specified wavelength.
11.
12. INTERFERENCES IN ATOMIC
ABSORPTION SPECTROSCOPY
Interferences in atomic absorption measurements can
arise from spectral chemical and physical sources.
SPECIAL INTERFACE resulting from the overlap of
absorption lines is rare because of the simplicity and
the sharpness of the lines.
However , broadband by molecular species can lead to
significant background interface.
Correction for this may be made by matrix matching
of samples and standards , or by use of a standard
addition method.
13. Advantages
Solutions, slurries and solid samples can be analysed.
Much more efficient atomization
Greater sensitivity
Smaller quantities of sample (typically 5 – 50 μL)
Provides a reducing environment for easily oxidized
14. DISADVANTAGES
Expensive
Low precision
Low sample throughput
Requires high level of operator skill
Sample must be in solution or at least volatile
Individual source lamps required for each element
15. Applications
Clinical analysis. Analysing metals in biological
fluids such as blood and urine.
Environmental analysis. Monitoring our
environment – e.g., finding out the levels of various
elements in rivers, seawater, drinking water, air, petrol
and drinks such as wine, beer and fruit drinks.
Pharmaceuticals. In some pharmaceutical
manufacturing processes, minute quantities of a
catalyst used in the process (usually a metal) are
sometimes present in the final product. By using AAS
the amount of catalyst present can be determined.
16. Industry. Many raw materials are examined and AAS
is widely used to check that the major elements are
present and that toxic impurities are lower than
specified – eg in concrete, where calcium is a major
constituent, the lead level should be low because it is
toxic.
Mining. By using AAS the amount of metals such as
gold in rocks can be determined to see whether it is
worth mining the rocks to extract the gold.
