chapter 4 atomic absorption spectroscopy
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TRANSCRIPT
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CChapter 4 hapter 4 Atomic Absorption Atomic Absorption Spectroscopy
4.1. Theoretical Concepts 4.2. Atomic Absorption Instrumentation 4.3. Graphite Furnace Atomic
Absorption 4.4. Control of Analytical Interferences
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4.1. Theoretical Concepts
1. The Atomic Absorption Process
2. Theoretical Concepts
3. Quantitative Analysis
4. Characteristic Concentration
5. Detection Limits
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1. The Atomic Absorption Process
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Fig 4-3Grotrian diagrams for NaFig4-2 Grotrian diagrams for K
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2. Theoretical Concepts
Integral Formula of AAS⑴
Fig 4-4 Typical Shape of A Atomic Absorption line
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Integral Absorption FormulaBy atomic theory
0
2
fNdvk mce
v
Dv vkdvk
02ln0 21
02ln2
0
2
fNk mce
vD
By Line Shape Function ( the Nature distribute )
The Peak Absorption Coefficient k0 is:
(4.1)
(4.2)
(4.3)
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⑵ Peak Absorption Theoryby Lambert-Beer’s Law:
lkvv
veII 0
dveIdvIP lkv
v
v
vv
0
0
0
dvIPv
v
0
00
(4.4)
(4.5)
(4.6)
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dvI
dveI
PP
vv
lvkvvA
0
00
0
0lglg
lkdvI
dvIe
dvI
dveIA v
v
vv
lk
vv
lkvv
v
0
00
00
00
00
4343.0lglg0
002ln2 2
4343.0 KNflNA mce
vD
The Absorbance is:
When va>> ve, then kv≈k0,
(4.7)
(4.8)
(4.9)
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3. Quantitative Analysis
KCA (4.10)
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4. Characteristic Concentration
The ‘‘characteristic concentration’’ ( sometimes called ‘‘sensitivity’’) is a convention for defining the magnitude of the absorbance signal which will be produced by a given concentration of analyte. For flame atomic absorption, this term is expressed as the concentration of an element in milligrams per liter ( mg/L ) required to produce a 1% absorption ( 0.0044 absorbance ) signal.
(4.11)
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5. Detection Limit
Having obtained the data, make the calculation as follows:1. Average the two blank readings taken immediately before
and after each standard and subtract from the standard reading.
2. Calculate the mean and standard deviation for the set of corrected high-standard readings. Do the same for the set of corrected low standard readings.
3. If the ratio of the means does not correspond to the ratio of the concentration prepared to within statistical error, reject the data.
4. If the data pass the ratio-of-the-means test, calculate theconcentration detection limit as follows:
(4.12)
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4.2. Atomic Absorption Instrumentation
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1. Photometers for AAS
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A New Type Photometer for AAS
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2. line source (Hollow Cathode Lamp )
Fig 4-13
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Hollow Cathode Lamp Emission Process
Fig 4-14
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3. Atomizer for AAS (Pre-Mix Burner System)
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Fig 4-19 Tree Type Burner Head for different Type Flame
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4. Control of Analytical Interferences
Ionization Interference
Matrix Interference
Chemical Interference
Background Interference
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Ionization Interference
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Matrix Interference
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Chemical Interference
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The Method Of Standard Additions
No.Sample
Added
ml
Standard
Added
ml
Concentr.of standard
mg/L
Last
Concentriation mg/L
1
Vx
0
Cs
Vx
VL
2 VsVx+CsVs
VL
3 2VsVx+2CsVs
VL
4 3VsVx+3CsVs
VL
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Background Interference
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AD2=Ab,
AHCL= Aa+ Ab
Aa = AHCL- AD2
(4.13)
(4.14)
(4.15)
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4.3. Graphite Furnace Atomic Absorption
1. Graphite furnace atomizer components
2. The Graphite Furnace Power Supply and
Programmer
3. Quantitative analysis GFAAS 4. Effect of Matrix on Height and Area
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1. Graphite furnace atomizer components
The Graphite Furnace Atomizer
A basic graphite furnace atomizer is comprised of the following components:
graphite tube
electrical contacts
enclosed water cooled housing
inert purge gas controls
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THGA graphite tube
Fig 4-27
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2. The Graphite Furnace
Power Supply and
Programmer
A Graphite Furnace Temperature Program
1) Drying
2) Pyrolysis
3) Cool Down(optional)
4) Atomization
5) Clean Out
6) Cool Down
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