AS deals with e transfer transition of valence electron between electronic states

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AS deals with e transfer transition of valence
electron between electronic states
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Atomic absorption spectrometry (AAS)
Atomic spectrometry
Atomic fluorescence spectrometry (AFS)
Atomic emission spectrometry (AES)
???????????UV??
h?
source lamp or heated solid
sample holder
h?
h?
wavelength selector
Photoelectric transducer
I
Signal processor and readout
AAS
sample holder
h?
h?
wavelength selector
Photoelectric transducer
I
Signal processor and readout
AFS
h?
source lamp or laser
source and sample holder
h?
h?
wavelength selector
Photoelectric transducer
I
Signal processor and readout
AES
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AAS
Aabsorbance Ttransmittance Cconc. eabsorpivity
bpath length
I0
I
???????????
Light source ?P0??????? (P0?????)
FL k'F0C FL ? C ? F0
h?
h?
????????????,???????
???????????ground state,????????,?????????????????
(?????)
Ej
Nj/Ni Pje-?Ei/kT/Pi
Ei
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Energy level diagrams for (a) atomic sodium and
(b) magnesium Electronic state Term symbol
C1S2, 2S2, 2P2 term symbol ? 3P0lt3P1 lt3P2 lt1D2
lt1S0
????????
2S1/2
2P1/2 589.6 nm
Na?ground state?
??
2P3/2 589.0 nm
??,(?????)
Mg
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Linewidth (??????)
I
?? (?linewidth)
h/2
h
?

1. ?Na (5890Å) ??n 210-5 nm

1. Uncertainty effect (natural broadening)
2. Doppler broadening
3. Pressure broadening

• T 2500K
• (?????????,??????????)
• ??D kvT/vM ??D 0.045 nm

Atomic linewidth
?????,?line?? (collisional)

1. T 2500K flame/N2 ??L 0.035 nm

????line??, ?P??, T???? ?HCL 0.002 nm
(?HCL????????,???????) (Hollow Cathode Lamp)
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???????????, ?1. ???line???. 2. ???????
Temperature effect on the atomic spectra
Boltzmann equation
Nj/N0 gj/g0 exp(?E/RT)
AA????atoms?ground state, AES????,?excited
states atoms or ions ?.
Spectral line intensity
???excited??,???? (???????) ?conc.???,conc.
?????excited??,?intensity???,????????? ????
Iem
?Iem ? C (????????)
?
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??conc. ?,??? ???????????
????
h?
Detector
emission
????? (conc.?)
???? (a 0.0)
A/b
nif/b cm-2s
atom cell (?????????) ?????

1. AES
2. AAS

I
line
continuous
Line source
I/I0
I ' /I0'
?
Spectrometer???? 1-2 nm
?????????line source,??continuous source
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Slope 1/2
Slope 0
0
0
log aL
log aL
Slope 1
Slope 1
log ni
log ni
??????
??????
?????????ground state (T????????????)
Flame ??
atomizer
Furnace ??
??? ??? ??? (?????sample) 2600 ? ?? Spark (??)
(?????) ??1700 - 3150 ?
Sample?????
??????
2600-3000K
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??????????

• Light source (line)
• Atomization (???)
• Atom spectrum molecular spectra
• (??????????????, ???????)

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A
Sample?????????
???????? (Ag)n solution (Pb)n PbX (Au)n
vaporization
desolvation
Precesed spray
Aerosol transport
spray
Nebulization
????????? (??????continuous)
???
Sample
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Excited state
vapor



h?
h?
h?
Equilibration of vaporized species
Molecular vapor
Ionized atomic vapor e-
Atomic vapor
nebulization
vaporization
MX-(aq)
??
MX
M X
M e-
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Hollow cathode lamp
??1-5 torr?Ne?Ar
Ar e- ? Ar e2- ?????Ar,?Ar?????,???? M(s)
? M(g) ? h? M(s)
Ar
Emission????
EDLelectroless discharge lamp for volatile
element As, Se, Fe, Ge, Hg ??????????,????emissio
n
????????????, ??EDL??????????,??radicla
frequency,?source???? (??????radical
frequency,???)
MX
r. f. (radical frequency)
continuous flame, plasma
Atom cell
noncontinuous flame, plasma
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Hollow Cathode Lamp
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Electrodeless Discharge Lamp (EDL)

• Cutaway of electrodeless discharge lamp

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Why do we need line source in AAS
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
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1. Phy. int. viscosity, surface tension, Temp.
2. Chem. int. continuous band, blackbody
   emission
3. Spectral int. ???????? (??????OH, CN, CO)

interferences
???????????
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??standard solution?AA????????
Signal
y mx b
Ex. (Pd)n
? Xylene 150 ppm
? 241 ppm
?? Xylene 225 ppm
? 441 ppm
Conc.
Standard (H2O) ?standard?????,??sample??
Nano-Pd????, ??standard????????
accuracy?
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Atomizer 2600-3000K
Sample cell??
Sample cell?5 cm
?? C2H2/air, C2H2/NO2
??????
??? pre-mixer burner
??? laminar flow flame (??) ??noise??
A ebc bsample cell ???
?????? molecular

1. Continuous band
2. Blackbody emission

?????????sample????
Sample???? (???,???????)
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???????viscosity, surface tension, Temp.
??????Ca
?????,??????
?????????
.OH
CaO (MO)
Ca
MX
??????????
????Al,??Ca-O-Al.??CaO??????Ca, ??????
???????,???????C2H2/NO2?? (??C2H2/air?,???C2H2/NO2
)
??????,?La3,???Ca-O-Al?Ca,??La-O-Al,??????
???????????? ????PO43-
Ti, Al, Zr, W, P refractory element MO???????,???
???????, ??C2H2/N2O ?????????emission.??absorption
????,??????,???????
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Sensitivity1 Abs???? (1????0.0044) ?Flame???c
haracteristic conc.' ETA???characteristic
mass'
Detection limit
Noise?,?????
0.0044
S/N 3
A
B
????sensitivity,??noise??????????
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• What are the limitations of flame atomic
  absorption spectrometry?
• You can only analyse one element at a time, why?
• The sensitivity of the method is limited to ppm
  levels (mg l-1), why?

Need a different hollow cathode lamp for each
element-lock
-Only 10 of the sample reaches the flame by the
nebulization process. -The sample only spends
0.001 s in the flame due to the high gas
velocities 200 cm/s
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• Walsh/Australia , Alkemade/Holland
• 1st commercial AAS
• 1965 H2O/C2H2 flame,
• 1970 ETA-AAS (Graphite furnace AAS)
• 1971 Zeeman-background correction
• 1975 ICP-AES
• 1977 Constant temperature graphite
  furnace
• 1978 Platform atomization
• 1983 Smith-Hieftje background correction
• 1984 STPE (Stabilized temp. platform
  furnace)
• 1990 Horizontally heated graphite furnace

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(???)
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• Atomization occurs in an electrically heated
  graphite tube
• The graphite tube is flushed with an inert gas
  (Ar) to prevent the formation of (non-absorbing)
  metal oxides

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• 0-20 mL samples (solution or slurries) or mgs of
  solids (direct solids analysis)
• matrix modifier use is common (Used to
  stabilize analyte or induce volatility of
  matrix)
• Samples are generally acidified (minimize loss by
  adsorption to sample container HNO3 typical and
  HCl is discouraged because of volatility of metal
  halides and stability of MCl(g).)
• For solution samples
• 30-45 s gently drying
• 30-60s thermal pretreatment
• 5 s (max ramp ca. 2k K/s)
• 10 s clean cycle (remove any remaining matrix)

Furnace Programming
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???????????????????
Temp. program
T (?) Ramp time (sec) Hold time (sec) Gas flow (mL/min)
1.?? Drying 150 10 45 150
2.?? Ashing 550 10 35 150 ????????? (???????????????)
3.??? Atomization 1900 0 (???????) 5 0 (????,???2 mL)
4.?? Clearing 2600 (???????) 1 4 150
5.?? cooling 20 1 5 150
???????????????
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n nebulization s solvation v vaporization a
atomization
eT enßsßvßa
???????

• What is the problem with adding the whole sample
  to the tube?

You are also adding a matrix that will form a
burnt ashy material that could block the light
path and interfere with absorption measurements.
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em
?????
1. flame5 mL/min
4 µL/sec (5en, ?????)
10 L/min (gas)
100 L/min 1.6 L/sec (????)
Expand 10?
??
5 mL/min
4 µL sample/1.6L gas 2.510-6
2. Furnace (????)
5 mL/2 mL 2.510-3
???????
??1000?
?????
Atom density 1000 (?atom density????100?) Molecul
ar density 1000 (??? density????100?) ?matrix
effect ???? How to reduce or eliminate the matrix
effect for furnace AAS is the most important task
for accurate determination of real samples.
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Advantages of the graphite furnace All the
sample is present in the atom cell and this has
three advantages Long residence time compared
with flame Small samples can be analysed
Much higher sensitivity is achieved
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In FAAS, a key consideration is the height above
the burner that the analyte absorption is
measured at (burner positions are adjustable)!
Temperature value (adjusted using different
fuel/oxidant ratios) and consistency are
important.
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????? ()
???????
??????
????????
???????,??????,????????
??????,???????
t (?)
(??????, ???power??0???????????)
0.50.40.30.20.10
????
Absorbance
????
t (?)
1500 2000 2400 2700
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AA??? STPE ???

1. Fast heating rate
2. Gas stop (???????)
3. Pyrolytically Coated platform
4. Matrix, modifier ??

absorbance
Gas flow rate (mL/min)
????????,??????????
?????? (??CH4?????)
??????????,??????? (??????????????)
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??????????,??????? (??????????????)
Atomization mechanism

1. Reduction of solid oxide on graphite surface
   (?????????)

½ M(g)
?MO(s) C(s)
M(l)
M(g)
??????,????????????
M Co, Cr, Cu, Fe, Mo, Ni, Pb, Sn, V
?GFAAS 2 HNO3
(?MO???????????) Flame AAS
HCl
?standard???????. ???Si?????,??MX. ???????????

1. Thermal decomposition of solid oxide

MO(s)
M(g) ½ O2
M Al, Cd, Zn
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???????? ?CH4??? (pyrolytically-coated
graphite) ?????? ( Lvov platform)? ????????????tu
be??
?????? (??????)
Tube temperature
??
From wall
absorbance
From platform
t (??) ?
?????profile???????? ???????????????
(???????????)
?????sample??delay.?sample???????,??????????, ????
???,????
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Tube temperature
Volatillization from the tube wall
absorbance
?????????2?????????,??MO(s)
t (??) ?
Matrix modification reduce or eliminate
volatilization and vapor-phase interferences
?????????????
????
Pb
2200?
1805?
1600?
1400?
??????????? (????ashing??)
??AA??????, matrix modifier?????
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B (Al3 Mg)

• Matrix modifier ???
• ?????????
• ?????????

A (Al3)
Mg??Mg(NO3)2,??????
Absorbance
0.5 0.40.30.20.10
A,B??????
?0.3 M HNO3
2 µg Pd
Time (s)
Al3 Al3 Mg(NO3)2
????
4 µg Mo
20 µg Ni
????,???????
Pure Sb standard
1.4 M HNO3
????,??Al-Mg???couple???? (????,?ppb??)
Thermal pretreatment temperature
Matrix modifier (NH4)NO3
Ni(NO3)2 Mg(NO3)2
Mo(NO3)2
Pd(NO3)2 (???) ?? Pb,
Cd?Pd2???,????????
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????????
????? ()
100 80 60 40
???????
???? (?)
???????500 ??????
600 700 800 900 1000 1100
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Spectral Interferences in AAS

• Example Determination of Ba in the presence of
  Ca
• Both Ca and Ba atomize simultaneously
• Ca (g) oxidant ----gt CaOH (g)
• CaOH (g) exhibits broad band molecular absorption
• The observed absorbance is in error due to the
  non-atomic signal coming from CaOH (g)

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• 4. Interferences
• phy. Interference
• chemical interference
• spectral interference
• 5. Effective background correction
• AAS ???????
• Continuous background emission
• Band type molecular absorption
• (??????)

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Self-reversal
?0
Low HCL Current (Background Analyte) Signal
High HCL Current Background Signal
?1
?2
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Standard additions often used with GFAAS