Atomic absorption spectroscopy is a common technique for detecting metals and metalloids in samples. It works by vaporizing the sample into free atoms that can absorb light at specific wavelengths. The instrument works by using a light source, atomizer, monochromator, and detector. Samples are vaporized in a flame or graphite furnace and the amount of light absorbed is measured to determine concentration. It can analyze over 60 elements with high selectivity and is used for applications like water analysis and determining metal levels in biological samples.
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Atomic absorption is a very common for detecting
metals and metalloids in a samples.
It is very reliable and simple to use.
It can analyse over 62 elements.
It also measure the concentration of metals in the
sample.
INTRODUCTION
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The first atomic absorption spectrometer was built
by the CSIRO (Commonwealth Scientific and
Industrial Research Organisation) scientist Alan
Walsh in 1954.
HISTORY
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The technique uses basically the principle that free
atoms generated in an atomizer can absorb
radiation at specific frequency.
AAS quantifies the absorption of ground state
atoms in the gaseous state.
The atoms absorb ultraviolet or visible light and
make transition to higher electronic energy levels.
The analyte concentration is determined from the
amount of absorption.
PRINCIPLE
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Concentration measurement are usually from a
working curve after calibrating the instrument with
standards of known concentration.
Atomic absorption is a very common technique for
detecting metals and metalloids in environmental
samples.
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HCL is the most common radiation source in
AAS.
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, argon).
Each element has its own unique lamp
which must be used for that analysis.
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A small amount of the metal or salt of the element
for which the source is to be used is sealed inside a
quartz bulb.
This bulb is placed inside a small, self-contained RF
generator or “driver”. When power is applied to the
driver, an RF field is created.
The coupled energy will vaporize and excite the
atom inside the bulb causing them to emit their
characteristic spectrum.
ELECTRODELESS
DISCHARGE LAMP
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They are typically much more intense and, in some
cases, more sensitive than comparable HCL. Hence
better precision and lower detection limits where an
analysis is intensity limited.
EDL are available for a wide variety of elements,
including Sb, As, Bi, Cd, Cs, Ge, Pb, Hg, P, K, Rb, Se,
Te, Th, Sn and Zn.
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There is no nebulization, etc. The sample is
introduced as a drop (usually 10-50 uL)
The furnace goes through several steps:
a- Drying (usually just above 110 deg. C.)
b- Ashing (up to 1000 deg. C)
c- Atomization (Up to 2000-3000 C)
d- Cleanout (up to 3500 C or so). Waste is blown
out with a blast of Ar.
ELECTROTHERMAL
EVAPORATOR
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Samples are reacted in an external system with a
reducing agent, usually NaBH4.
Gaseous reaction products(volatile hydrides) are
then carried to a sampling cell in the light path of
the AA spectrometer.
To dissociate the hydride gas into free atoms, the
sample cell must be heated.
The cell is either heated by an air-acetylene flame
or by electricity.
HYDRIDE GENERATION
TECHNIQUE
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Flame is used to atomize the sample.
Sample when heated is broken into its atoms.
High temperature of flame causes excitation.
Electrons of the atomized sample are promoted to
higher orbitals, by absorbing certain amount of
energy.
FLAME ATOMIZER
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Graphite furnace atomic absorption spectrometry
(GFAAS) (also known as Electro thermal Atomic
Absorption spectrometry (ETAAS)) is a type
of spectrometry that uses a graphite-coated furnace
to vaporize the sample.
Instead of employing the high temperature of a
flame to bring about the production of atoms from
the sample and it is non-flame methods involving
electrically heated graphite tubes or rods.
ELECTROTHERMAL
ATOMIZERS
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• Aqueous samples should be acidified (typically with nitric
acid, HNO3) to a pH of 2.0 or less. Discoloration in a
sample may indicate that metals are present in the
sample. For example, a greenish color may indicate a
high nickel content, or a bluish color may indicate a high
copper content. A good rule to follow is to analyze clear
(relatively dilute) samples first, and then analyze colored
(relatively concentrated) samples. It may be necessary to
dilute highly colored samples before they are analyzed.
• After the instrument has warmed up and been calibrated,
a small aliquot (usually less than 100 microliters (µL) and
typically 20 µL) is placed, either manually or through an
automated sampler, into the opening in the graphite tube.
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WORKING
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The graphite furnace is an electrothermal atomizer
system that can produce temperatures as high as
3,000°C. The heated graphite furnace provides the
thermal energy to break chemical bonds within the
sample and produce free ground-state atoms.
Ground-state atoms then are capable of absorbing
energy, in the form of light, and are elevated to an
excited state.
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WORKING
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Sample holder:
Graphite tube: Samples are placed directly in the
graphite furnace which is then electrically heated.
Beam of light passes through the tube
GFAAS
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Three stages:
1. drying of sample
2. ashing of organic matter (to burn off organic
species that would
interfere with the elemental analysis.
3. vaporization of analyte atoms
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Greater sensitivity and detection limits (hundred- or
thousand fold improvements in the detection limit
compared with flame AAS) than other methods.
Direct analysis of some types of liquid samples.
Some solid sample do not require prior dissolution.
Low spectral interference.
Very small sample size (as low as 0.5µL).
ADVANTAGES OF GFAAS
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Expensive.
low precision.
low sample throughput.
requires high level of operator skill
DISADVANTAGES OF GFAAS
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GFAA has been used primarily for analysis of low
concentrations of metals in samples of water. The
more sophisticated GFAAs have a number of lamps
and therefore are capable of simultaneous and
automatic determinations for more than one element.
for the quantification of beryllium in blood and serum.
APPLICATIONS OF GFAAS
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The intensity of the light is fairly low, so a
photomultiplier tube (PMT) is used to boost the
signal intensity
A detector (a special type of transducer) is used to
generate voltage from the impingement of electrons
generated by the photomultiplier tube
DETECTOR
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INTERFERENCES &
CONTROL MEASURES
NON SPECTRAL
Matrix
Method of
Standard
Additions
Chemical
add an excess
of another
element or
compound
which
will form a
thermally
stable
compound
with the
interferent
using a
hotter
flame.
Ionization
adding an
excess of
an
element
which
is very
easily
ionized
SPECTRAL
Background
Absorption
Continuum
Source
Background
Correction
Zeeman
Background
Correction
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This may be caused by direct overlap of the analytical
line with the absorption line of the matrix element.
HOW TO OVERCOME ?
By choosing an alternate analytical wavelength
By removing the interfering element from the
sample.
SPECTRAL INTERFERENCE
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Formation of compound of low volatility
Decrease in calcium absorbance is observed with increasing
concentration of sulfate or phosphate.
HOW TO OVERCOME
By increasing flame temperature
Use of releasing agents (La 3+ )
Cations react with the interferent releasing the analyte
Use of protective agents:
They form stable but volatile compounds with analyte.
CHEMICAL INTERFERENCE
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Ionization of ground state gaseous atom with in a flame
will reduce extent of absorption in AAS.
M ↔ M+ + e
HOW TO MINIMIZE:
Low temperature of the flame
Addition of an excess of ionization suppressant e.g. the
alkali metals (K, Na, Rb, and Cs)
IONIZATION INTERFERENCE
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Advantages
1. High selectivity and sensitivity
2. Fast and simple working
3. Doesn’t need metals separation
4. Specific because the atom of a particular element can only
absorb radiation of their own characteristic wavelength
Disadvantages
1. Analysis doesn’t simultaneous
2. Can’t used for elements that give rise to oxides in flames
3. Limit types of cathode lamp (expensive).
ADVANTAGES AND
DIADVANTAGES
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Quantitative analysis.
Qualitative analysis.
Simultaneous multicomponent analysis.
Determination of metallic element in biological
materials.
Determination of metallic element in food industry.
Determination of Ca, Mg, Na, K in blood.
APPLICATIONS
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