1. Atomic absorption spectroscopy is a quantitative analytical technique used to determine the concentration of metals and some nonmetals in solutions. It works by measuring the absorption of light by ground state atoms at their characteristic resonance wavelengths. 2. The technique involves atomizing the sample using a flame or electrothermal heating and passing the gaseous atoms through a beam of resonance wavelength light from a hollow cathode lamp. The amount of light absorbed is proportional to the number of atoms in the ground state. 3. Interferences can occur from spectral overlap, molecular absorption, light scattering, chemical interactions that form non-volatile compounds, and physical properties affecting atomization efficiency. Various methods such as changing operating parameters, adding chemical modifiers,

1. 1

2. Atomic Absorption Spectroscopy
（AAS）
Introduction
Elementary Theory
Instrumentation
Interferences
Experimental preliminaries
Applications
2

3. Introduction
Atomic absorption spectroscopy is
a quantitative method of analysis
that is applicable to many metals
and a few nonmetals.
3
I ntroduction

4. AAS
The technique was introduced in 1955 by Walsh
in Australia (A.Walsh, Spectrochim. Acta, 1955, 7,
108)
The application of atomic
absorption spectra to
chemical analysis
The first commercial atomic
absorption spectrometer was
introduced in 1959
Alan Walsh 1916-1998 4
I ntroduction

5. AAS
An atomic absorption spectrophotometer
consists of a light source, a sample compartment
and a detector.
Sample
Compartment
Detector
Light Source
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I ntroduction

6. 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. 6
I ntroduction

7. AAS
the gaseous metal atoms
specific resonance wavelength
extend of absorption
the extend of absorption vs the number of
ground state atoms present in the flame.
7
I ntroduction

8. Elementary Theory
Characters of the atomic absorption spectrum
Characteristic wavelength
Δ E = E1 – E0 = hc /
E1 - excited state
E0 – ground state
h – Planck’s constant
c – velocity of light
- wavelength 8
Elementary Theory

9. Characters of the atomic absorption spectrum
Profile of the absorption line
K0 - m m absor pt i on
axi al
coef f i ci ent
Δ - hal f w dt h
i
0 - cent r al wavel engt h
9
Elementary Theory

10. Characters of the atomic absorption spectrum
Nat ur al br oadening
Determined by the lifetime of the excited state and
Heisenberg’s uncertainty principle（10- 5 nm）
Doppler Br oadening （10-3 nm）
Results from the rapid motion of atoms as they emit or
absorb radiation
Collisional Br oadening
collisions between atoms and molecules in the gas
phase lead to deactivation of the excited state and thus
broadening the spectral lines
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Elementary Theory

11. Characters of the atomic absorption spectrum
Doppler Br oadening （10-3 nm）
Results from the rapid motion of atoms as they
emit or absorb radiation
11
Elementary Theory

12. The relationship between absorbance and
the concentration of atoms
Beer’s law
It = I0νe -Kνl
A = log （ I0ν/ It）= 0.4343 K l
It - intensity of the transmitted light
Io – intensity of the incident light signal
l – the path length through the flame (cm)
12
Elementary Theory

13. The relationship between absorbance and
the concentration of atoms
Integrated absorption
K d =( e2/mc) N0
K = the absorption coefficient at the frequency
e = the electronic charge
m = the mass of an electron
c = the velocity of light
f = the oscillator strength of the absorbing line
N0 = the number of metal atoms per milliliter able to
absorb the radiation
13
Elementary Theory

14. The relationship between absorbance and
the concentration of atoms
K d =( e2/mc) N0
The measurement of the integrated
absorption coefficient should furnish an
ideal method of quantitative analysis
14
Elementary Theory

15. The relationship between absorbance and
the concentration of atoms
The line width of an atomic spectral line is about
0.002 nm.
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.
15
Elementary Theory

16. The relationship between absorbance and
the concentration of atoms
This difficulty was overcome by Walsh,
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 centred
on the absorption frequency.
16
Elementary Theory

17. The relationship between absorbance and
the 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.
17
Elementary Theory

18. The relationship between absorbance and
the concentration of atoms
2
2(v v0)
Kv K 0 log ln 2
v0
2 ln 2 .e 2
K0 fNov
D mc
A = 0.4343 K0 l = K1N0v A = KC
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Elementary Theory

19. Instrumentation
Line
source Atomization Monochromator Detector
Nebulizer Read-out
Schematic diagram of a flame spectrophotomer
19
I nstrumentation

20. Resonance line sources
Emit the specific resonance lines of the atoms in
question
• Provide the sharp emission lines with a much
smaller half-width than the absorption line
• Intensity
• Purity
• Background
• Stability
• Life-time
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I nstrumentation

21. Hollow cathode lamp (HCL)
Cathode--- in the form of a cylinder, made
of the element being studied in the flame
Anode---tungsten
21
I nstrumentation

22. A hollow cathode lamp for Aluminum (Al)
22
I nstrumentation

23. SpectrAA - AAS
HCL
motorized
Mirror
23
I nstrumentation

24. 24
I nstrumentation

25. Sample atomization techniques
Flame atomization
Electrothermal atomization
Hydride atomization
Cold-Vapor atomization
25
I nstrumentation

26. Flame atomization
Processes occurring during atomization
26
I nstrumentation

27. Flame atomization
Nebulizer - burner
A typical premix burner
27
I nstrumentation

28. Nebuliser - burner
To convert the test solution to gaseous atoms
Nebuliser
to produce a mist or aerosol of the test solution
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.
Burner head
The flame path is about 10 –12 cm 28
I nstrumentation

29. Fuel and oxidant
Auxiliary
flame oxidant
Air- propane Fuel
Air- hydrogen
 Air – acetylene
 Nitrous oxide – acetylene
29
I nstrumentation

30. Common fuels and oxidants used in flame
spectroscopy
30
I nstrumentation

31. Disadvantages of flame atomization
Only 5 – 15 % of the nebulized sample
reaches the flame
A minimum sample volume of 0.5 – 1.0 mL
is needed to give a reliable reading
Samples which are viscous require dilution
with a solvent
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I nstrumentation

32. Electrothermal atomization
Graphite furnace technique
32
I nstrumentation

33. Plateau Graphite
Tube
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34. Graphite furnace technique
Process
drying ashing atomization
34
I nstrumentation

35. 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
35
I nstrumentation

36. 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
(less than two orders of magnitude) 36
I nstrumentation

37. Cold vapour technique
Hg2+ + Sn2+ = Hg + Sn (IV)
37
I nstrumentation

38. Hydride generation methods
For arsenic (As), antimony (Te) and selenium (Se)
NaBH4 heat
As (V) AsH3 As0(gas) + H2
[H+] in flame
(sol)
38
I nstrumentation

39. 39
I nstrumentation

40. Monochromator
• Diffraction grating
40
I nstrumentation

41. Detector
• Photomultiplier
41
I nstrumentation

42. Read-out system
• meter
• chart recorder
• digital display
42
I nstrumentation

43. Atomic absorption spectrophotometer
43
I nstrumentation

44. Interferences
Spectral interferences
Chemical interferences
Physical interferences
44
I nterferences

45. 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
45
I nterferences

46. 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
46
I nterferences

47. Spectral interferences
• light scatter （+）
Spectral interferences
• light scatter （+）
The interference can be avoided by
Metal oxide particles with diameters
variation greater than the wavelength of light
in analytical variables, such
as flame temperature and fuel-to –
When sample contains organic species
or when organic solvents are used to
oxidant ratio the of the organic matrix
dissolve
combustion
sample, incomplete
leaves carbonaceous particles that are
Standard addition method
capable of scattering light
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I nterferences
Zeeman background correction
48
I nterferences

48. Chemical interferences
----- Formation of compound of low volatility
Ca 2+ ， PO43- Mg2+, Al3+
Increase in flame temperature

Use of releasing agents (La 3+ )

Use of protective agents (EDTA)

 Separation
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I nterferences

49. Chemical interferences
----- Ionization
Adding an excess of an ionization
 suppressant (K)
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I nterferences

50. Physical interferences
• Viscosity
• Density
• Surface tension
• volatility
Matrix matching

51
I nterferences

51. Experimental Preliminaries
Preparation of sample solutions
Optimization of the operating conditions
• resonance line
• slit width
• current of HCL
• atomization condition
Calibration curve procedure
52
Experimental Preliminaries

52. The standard addition technique
53
Experimental Preliminaries

53. 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
54
Experimental Preliminaries

54. Sensitivity and detection limit
Detection limit
• the lowest concentration of an analyte
that can be distinguished with reasonable
confidence from a field blank
D = c × 3σ / A
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55. Sensitivity and detection limit (ng/mL)
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56. 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
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