Atomic Absorption Spectrometry

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Chapter 3
Atomic Absorption Spectrometry
3.1 Introduction
3.2 Principle
3.2.1 The relationship between atoms in the
ground state and atoms in an exited state
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? Atomic Absorption Spectrometry e.g.
? Boltzmans equation
3.2.2 The relationship between Atomic Absorption
and Atomic concentration
? Calculus measurement
? Volvss peak value absorption theory Fig3.2-1
3.3 Instruments
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3.3.1 Primary radiation sources
? The hollow-cathode lamp(HCL) Fig3.3-1
? Other radiation sources
3.3.2 Source of free atoms
? The flame atomizer
?. Constitution and working principle Fig 3.3-2
?. The flame Fig3.3-3
? The electro-thermal atomizer Fig3.3-4
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3.3.3 Optical diapersive systems Fig3.3-5
Fig3.3-6
3.3.4 Detectors
3.4 Interferences and its elimination method
3.5 Quantitative analytic method
3.5.1 Standard curve method
3.5.2 Standard additon method Fig3.5-1
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3.6 The sensitivity (S) and detection limit of
AAS analysis
? Sensitivity (S)
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? Detection limit (D)
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In contrast to atomic emission spectrometry
(AES), atomic absorption spectrometry (AAS) in
terms of analytical method is a recent technique,
having been described by Walsh in 1955.
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The first atomic absorption observations were
of the Fraunhofer lines in the solar spectrum and
were made in 1802 by Wollaston.
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Optical transitions used for AAS are between
the ground state, or near the ground state, and
the first excited levels (resonance levels).
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The population of the various levels is
described by the Boltzmann law.
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Since the absorption of the radiation is
proportional to the number of atoms in the ground
state, this explains why AAS is an efficient
method.
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In the case of Al, the most commonly-used
lines are Al I 309.28, Al I 308.22 nm, i.e., a
transition between the 3p and 3d subshells
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and Al I 396.15 nm and Al I 394.40 nm, i.e., a
transition between the 3p and 4s subshells.
Fig.3.2
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Boltzmans equation
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Lamberts law
The relationship between line integrated
absorption and the number of atoms in the ground
state
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Peak absorption coefficient is
In practical analytical work, lamberts law is
A K'LC
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The whole process is
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Characteristics
? Little working current
? Low pressure sealing
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e.g.
? Electrodeless discharge lamp ( EDL)
? Electrion HCl
? The diode lasers
? The continuum source
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The use of organic solvents increases not only
the efficiency of nebulization but also the
temperature of the flame.
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An alternative to the injection of wet
aerosols with flame-AAS is the introduction of
volatile species.
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Hydride generation is the most commonly-used
system elements such as As, Bi, Ge, Pb, Sb, Se,
and Sn.
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Hydrides are usually obtained by s\adding
sodium borohydride to acidified solutions.
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This atomizer makes use of an electrically
heated refractory material on which the sample is
deposited.
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A transient formation of free atoms is then
obtained.
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Both single- and double-beam systems are
used. Since beam splitting is performed with a
double-beam system,
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the signal to noise ratio is degraded compared
with a single-beam system.
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Monochromators make use of the
photomultiplier tube whereas the
echellegrating-based polychromator utilizes a new
solid-state detector
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which incorporates a set of photodiodes. About 60
photodiodes are used for both primary and
secondary lines of the most important elements.
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Chemical interference
Elimination
? Add a buffer
? Use a hotter flame such as the N2O-acetylene
flame
? Add chemical modifiers
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Spectral interference
Elimination
? Change HCL
? Decrease slit and light current
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Background subtraction
Elimination
? The deuterium lamp method
? The Zeeman effect method
? The Smith-Hieftje method