atomic absorption spectroscopy
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IIntroductionntroduction
Elementary TheoryElementary Theory
InstrumentationInstrumentation
InterferencesInterferences
Experimental preliminariesExperimental preliminaries
AApplicationspplications
Atomic Absorption SpectroscopyAtomic Absorption Spectroscopy((AASAAS))
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Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals.
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The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108)
Alan Walsh 1916-1998
The application of atomic absorption spectra to chemical analysis
AASAAS
The first commercial atomic absorption spectrometer was introduced in 1959
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AASAAS
DetectorDetector
SampleSampleCompartmentCompartment
Light SourceLight Source
An atomic absorption spectrophotometer An atomic absorption spectrophotometer consists of a light source, a sample compartment consists of a light source, a sample compartment and a detector.and a detector.
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AAS
A much larger number of the gaseous metal atoms will normally remain in the ground state.
These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength.
If light of the resonance wavelength is passed through a flame containing the atoms in question, then part of the light will be absorbed.
The extend of absorption will be proportional to the number of ground state atoms present in the flame.
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the gaseous metal atoms
specific resonance wavelength
the extend of absorption vs the number of ground state atoms present in the flame.
extend of absorption
AAS
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Characteristic wavelength
Characters of the atomic absorption spectrum
ΔΔ EE = = EE11 –– EE00 = = hchc / /
E1 - excited state
E0 – ground state
h – Planck’s constant
c – velocity of light
- wavelength
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K0 - maximal absorption coeffi cient
Δ - hal f width
0 -central wavelength
Characters of the atomic absorption spectrum
Profile of the absorption line
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Natural bNatural broadeningroadeningDDeterminedetermined by the lifetime of the excited stateby the lifetime of the excited state andandHeisenbergHeisenberg’’s uncertainty principles uncertainty principle((1010--5 5 nmnm))
Doppler BroadeningDoppler Broadening(10-3 nm)Results from the rapid motion of atoms as they emit or absorb radiation
CollisionalCollisional BroadeningBroadeningcollisions between atoms and molecules in the gas phase lead to deactivation of the excited state and thus broadening the spectral lines
Characters of the atomic absorption spectrum
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Doppler BroadeningDoppler Broadening(10-3 nm)Results from the rapid motion of atoms as they emit or absorb radiation
Characters of the atomic absorption spectrum
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I t = I0ν e -Kν l
The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
A = log ( II00νν / / I t)= 0.4343 K l
Beer’s law
I t - intensity of the transmitted light
Io – intensity of the incident light signal
l – the path length through the flame (cm)
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K d=(e2/mc)N0
Integrated absorption
The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
K =the absorption coefficient at the frequency e = the electronic chargem = the mass of an electronc = the velocity of lightf = the oscillator strength of the absorbing lineN0 = the number of metal atoms per milliliter able to
absorb the radiation
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The measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis
The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
K d=(e2/mc)N0
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The line width of an atomic spectral line is about 0.002 nm.
The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
To measure the absorption coefficient of a line would require a spectrometer with a resolving power of 500 000.
The absolute measurement of the absorption coefficient of an atomic spectral line is extremely difficult.
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This difficulty was overcome by
The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
who used a source of sharp emission lines with a much smaller half-width than the absorption line. and the radiation frequency of which is centredon the absorption frequency.
Walsh,
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The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
In this way, the absorption coefficient at the centre of the line, K0 , may be measured instead of measuring the integrated absorption.
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2
2ln)(2
log0
00
v
vvKKv
ov
D
fNmc
eK
2.2ln20
A = 0.4343 K0 l = K1N0v A = KCA = KC
The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms
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InstrumentationInstrumentation
Line source Monochromator Detector
Read-outNebulizer
Schematic diagram of a flame spectrophotomer
Atomization
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Resonance line sources
•Provide the sharp emission lines with a much smaller half-width than the absorption line
Emit the specific resonance lines of the atoms in question
• Intensity
•Purity
•Background
•Stability
•Life-time
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Hollow cathode lamp Hollow cathode lamp (HCL)(HCL)
Cathode--- in the form of a cylinder, made of the element being studied in the flame
Anode---tungsten
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A hollow cathode lamp for Aluminum (Al)A hollow cathode lamp for Aluminum (Al)
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SpectrAASpectrAA -- AASAAS
motorizedMirror
HCL
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Sample atomization techniquesSample atomization techniques
Flame atomization
Electrothermal atomization
Hydride atomization
Cold-Vapor atomization
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Processes occurring during atomization
Flame atomization
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Nebulizer - burner
A typical premix burnerA typical premix burner
Flame atomization
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Nebuliser - burner
To convert the test solution to gaseous atoms
Nebuliserto produce a mist or aerosol of the test solution
Burner head
The flame path is about 10 –12 cm
Vaporising chamber
Fine mist is mixed with the fuel gas and the carrier gas.
Larger droplets of liquid fall out from the gas stream and discharged to waste.
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Fuel and oxidant
flame
Air – acetylene
Air- propane
Air- hydrogen
Nitrous oxide – acetylene
Auxiliary oxidant
Fuel
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Common fuels and oxidants used in flame spectroscopy
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Disadvantages of flame atomization
Only 5 – 15 % of the nebulized sample reaches the flame
A minimum sample volume of 0.5 – 1.0 mLis needed to give a reliable reading
Samples which are viscous require dilution with a solvent
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Graphite furnace technique
Electrothermal atomization
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Plateau Graphite Plateau Graphite TubeTube
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Graphite furnace technique
Process
drying ashing atomization
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Graphite furnace technique
Advantages
Small sample sizes ( as low as 0.5 uL)
Very little or no sample preparation is needed
Sensitivity is enhanced
( 10 -10 –10-13 g , 100- 1000 folds)
Direct analysis of solid samples
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Graphite furnace technique
Disadvantages
Background absorption effects
Analyte may be lost at the ashing stage
The sample may not be completely atomized
The precision was poor than the flame method
(5% -10% vs 1% )
The analytical range is relatively narrow
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Cold vapour technique
Hg2+ + Sn2+ = Hg + Sn (IV)
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Hydride generation methods
For arsenic (As), antimony (Te) and selenium (Se)
As (V) AsH3As0
(gas) + H2
NaBH4
(sol)
heat
in flame[H+]
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•Diffraction grating
Monochromator
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Detector
•Photomultiplier
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Read-out system
•meter
•chart recorder
• digital display
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Atomic absorption spectrophotometer
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Interferences
Spectral interferences
Chemical interferences
Physical interferences
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Spectral interferences
• Spectral overlap
(+, positive analytical error)
Cu 324.754 nm, Eu 324.753 nm
Al 308.215 nm , V 308.211nm,Al 309.27 nm
Avoid the interference by observing the aluminum line at 309.27 nm
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Spectral interferences
•non-absorption line
•molecular absorption (+)
Combustion products (the fuel and oxidant mixture)
Correct by making absorption measurements while a blank is aspirated into the flame
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Spectral interferences
• light scatter (+)
Metal oxide particles with diameters greater than the wavelength of light
When sample contains organic species or when organic solvents are used to dissolve the sample, incomplete combustion of the organic matrix leaves carbonaceous particles that are capable of scattering light
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Spectral interferences
• light scatter (+)
The interference can be avoided by variation in analytical variables, such as flame temperature and fuel-to –oxidant ratio
Standard addition method
Zeeman background correction
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Chemical interferences
----- Formation of compound of low volatility
Increase in flame temperature
Use of releasing agents (La 3+ )
Separation
Ca 2+ , PO43- Mg2+, Al3+
Use of protective agents (EDTA)
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Chemical interferences
----- Ionization
Adding an excess of an ionization suppressant (K)
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Physical interferences
•Viscosity
•Density
•Surface tension
•volatility
Matrix matching
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Experimental Preliminaries
Preparation of sample solutions
Optimization of the operating conditions
• resonance line
•slit width
•current of HCL•atomization condition
Calibration curve procedure
Experimental P reliminariesExperimental P reliminaries
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The standard addition technique
Experimental P reliminariesExperimental P reliminaries
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Sensitivity and detection limit
Sensitivity
• the concentration of an aqueous solution of the elements which absorbs 1% of the incident resonance radiation
• the concentration which gives an absorbance of 0.0044
Experimental P reliminariesExperimental P reliminaries
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Detection limit
Sensitivity and detection limit
• the lowest concentration of an analytethat can be distinguished with reasonable confidence from a field blank
D = c × 3σ / A
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Sensitivity and detection limit (ng/ mL)
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Advantages and disadvantages
High sensitivity
[10-10g (flame), 10-14g (non-flame)]Good accuracy
(Relative error 0.1 ~ 0.5 % )
High selectivity
Widely used
A resonance line source is required for each element to be determined