UV- ANA1 Spectroscopie Atomique et moléculaire

EC- SPECTRO
Spectroscopie Atomique et moléculaire
Atomic absorption and
emission in a flame
ICP
Atomisation in a flame
Introduction
Atomic spectroscopy
Light : UV/visible
Transitions : between atomic electronic levels
Ex : sodium Na (Z=11)
Electron configuration in the ground state:
1s22s22p63s1
Electron configuration in the 1st excited state :
1s22s22p63p1
Excited state
absorption
Emission
ΔE= E1–E0= hc/ λ
Ground state
Ephotons and l caracteristic  identification and quantification
Atomisation in a flame
Introduction
atomisation
Cu, Cu2+, Cu2O
Mixture to
analyse
Cu
Gazeous atoms
I. The flame
1. Description
Combustion reaction : highly exothermic
Flame
enveloppe
Flame cone
Burner
head
4 Reaction with air of partial combustion products
3 Above the flame cone, partial combustion products
formed
2 In the flame cone combustion begins
1 Heating of gas
4
3
2
1
Température
 the height of the burner in the instrument must be optimized
I. The flame
2. Flame temperature
fuel
oxidant
Temperature
(K)
methane
Air
Oxygen
2115
3015
propane
Air
Oxygen
2200
3125
butane
Air
Oxygen
2175
3175
acetylene
Air
Oxygen
Nitrous oxide
2600
3400
3175
I. The flame
2. Flame temperature
Influence of the nature of fuel
Influence of the nature of the oxidant
Influence of flow
CH4(g) + 2 O2(g) = CO2(g) + 2 H2O(g)
2 volumes of dioxygen for one of methane
C2H2(g) + 5/2O2(g) = 2 CO2(g) + H2O(g)
2,5 volumes of dioxygen for 1 of acetylene
I. The flame
2. Flame temperature
If excess of fuel : reducing flame
If excess of oxidant : oxydizing flame
 Flows must be monitored and regulated
II. From ions in solution to atoms in flame
1. Nebulisation
Principle of pneumatic nebulizer
Only 5 –15 % of the nebulised sample
reaches the flame
II. From ions in solution to atoms in flame
1. Nebulisation
Source : Vogel’s
II. From ions in solution to atoms in flame
2. Atomisation process
Flame
Solution to
analyse
M+
solvated and
A- solvated
nebulisation
M+ solvated and
A- solvated
MA solide
desolvating
fusion
Each stage includes the risk of
interference because phase transfer
can be different in calibration
standards and samples.
Other ways to atomize :
Use of a plasma or oven
MA liquide
Vaporization
MA gaz
Atomization
(+ ionization)
M gaz +A gaz
II. From ions in solution to atoms in flame
3. Population
of electron energy levels in atoms
Transitions between the ground state E0 and the most
favorable excited state Er
Boltzmann
 (Er  E 0 ) 

exp 

n0 g0
k
T
B


nr
gr
n0 is the ground state population
nr is the excited state population
II. From ions in solution to atoms in flame
3. Population of electron energy levels in atoms
élément
l en nm
E en
eV
nr/n0
gr/g0
PLASMA
2000 K
2500 K
3000 K
7000K
Na
589
2,104
2
9,9.10-6
1,14.10-4
5,83.10-4
6,1.10-2
Ag
382,07
3,778
2
6,03.10-10
4,84.10-8
6,97.10-6
9,1.10-3
Cu
324,75
3,817
2
4,82.10-10
4,04.10-8
6,65.10-7
3,5.10-3
Pb
283,31
4,375
3
2,83.10-11
4,55.10-9
1,34.10-7
2,1.10-3
Zn
213,86
5,795
3
7,45.10-15
6,22.10-12
5,5.10-10
2,0.10-4
Many more atoms are in the ground state than in excited states so
absorption has a higher sensitivity compared to emission
Flame emission is only used for alkaline and alkaline earth elements
(but ICP for many more)
III. Flame emission Spectroscopy
Determination of alkaline and alkaline earth elements
Atom
excited in
the flame
Transition towards the ground state and
emission of a photon of resonance wavelength
(characteristic of each atom)
III. Flame emission Spectroscopy
Flame photometer :
l
III. Flame emission Spectroscopy
Application : alkaline and alkaline earth elements
Fuel : propane or butane Oxidant : air
Moderate flame temperature :
- alkaline and alkaline earth elements easily atomized
- excited levels easily reached
- if too high temperature ionization can occur (Signal decreases)
IV. (Flame) Atomic absorption spectroscopy (AAS)
Many more determinations possible
Based on light absorption phenomenon:
Excited state
Absorption of a photon
of the resonance
wavelength
Incident
radiation
(Io)
Ground state
Transmitted
radiation (It)
Atoms absorb a part of the
incident light
The ground state is far more populated than any excited state so absorption
technics are more sensitive than emission ones.
IV. (Flame) Atomic absorption spectroscopy (AAS)
Elements studied with atomic absorption spectroscopy
He
H
Li
Be
B
C
N
O
F
Ne
Na Mg
Al
Si
P
S
Cl
Ar
K
Ca Sc Ti
Rb
Sr
Y
V
Zr Nb Mo Tc Ru Rh Pd Ag Cd
Cs Ba La Hf Ta
Fr
Ra Ac
Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
W Re Os Ir
Pt Au Hg
In
Sn Sb Te
Tl
Pb
Bi
I
Xe
Po At Rn
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa
U
Np Pu Am Cm Bk
Elements studied with AAS
Elements that cannot be studied with AAS
Cf
Es Fm Md No
Lr
IV. (Flame) Atomic absorption spectroscopy (AAS)
1.Spectrophotometer
Selection of a specific
resonance wavelength of the
element quantified
Burner
NNebulisation
Sample
solution
Absorbance :
A=log(I0/I)
IV. (Flame) Atomic absorption spectroscopy (AAS)
1. Spectrophotometer
Fuel-oxidant couples :
- mainly acetylene – air
- sometimes acetylene-nitrous oxide (N2O)
Price = 14000 €
IV. (Flame) Atomic absorption spectroscopy (AAS)
1. The spectrophotometer
Continuous source
Resonance line source : hollow
cathode lamp
l
l
l0
Slit width of the
monochromator
IV. (Flame) Atomic absorption spectroscopy (AAS)
2.Hollow cathode lamps
Hollow cathode lamps
The cathode contains the element of interest
IV. (Flame) Atomic absorption spectroscopy (AAS)
2.Hollow cathode lamp
Running principle
A high voltage applied across the cathode and the
anode ionizes the filling gas
The gas ions are accelerated toward the
cathode, and upon impact, they expel the
cathode material as gas atoms M
the collisions between Ar+ and M releases energy
used to excite M.
The desexcitation of M emits a photon of
wavelength characteristic of M
IV. (Flame) Atomic absorption spectroscopy (AAS)
3. Variant : GF AAS Graphite Furnace AAS or ET AAS electrothermal AAS
 Limits of flame atomization :
The atoms life time (10-3 - 10-2 secondes) is
superior to the time atoms stay in the analysis
area of the flame ( 10-5 à 10-4 s)
Avantage of oven atomization :
Higher residence time
Senstivity x 1000
Injection
Atomization
IV. (Flame) Atomic absorption spectroscopy (AAS)
3. Variant : GF AAS Graphite Furnace AAS or ET AAS electrothermal AAS
Temperature program
1- Drying ( 100°C)
2- Ashing ( 400°C)
3- Atomization (2000°C)
Pyrolysis (cleaning) (2300°C)
Avantages :
Inconvénients :
Absorbance peak
IV. (Flame) Atomic absorption spectroscopy (AAS)
3. Variant : GF AAS Graphite Furnace AAS or ET AAS electrothermal AAS
FAA
GFAAS
Fastness
200 samples/h
30 samples/h
Detection limits
0,001-0,02 ppm
0,002-0,01 ppb
High sensitivity (*100*1000)
Sample type
Solution+suspension
same + powder (very
little or no sample
preparation)
V. Interferences
Real C ≠ measured C
Which types ?
• Physical interferences
• Chemical interferences : chemical reaction in the flame
of the analysed atom
Mflame = M*flame
2 possibilities :
Reaction
Ionization
 formation of a refractory compound
MO, MOH, …
M+ + e ionisation
• Spectral interferences
Many informations about analysis conditions are found in
handbooks or equipment manufacturers
V. Interferences
1. Physical interferences
A
Cu, Mg
0.01
Aspiration
0.1
1.0
Signal
+++
+
Solution aqueuse H3PO4
H3PO4 (moL.L-1)
low viscosity
high viscosity
V. Interferences
1. Physical interferences
Physical interferences : Effect of viscosity and surface tension of solvent
A
MeOH
H2SO4
H2SO4 or
MeOH (%)
Aspiration
Size of
droplets
Signal
+++
MeOH (h=10-3 Pa.s )
( g = 22,6 mN/m)
(20°C)
+
Eau
(h=0,6.10-3 Pa.s )
( g = 72 mN/m)
V. Interferences
1. Physical interferences
Adding an organic solvent modifies :
-viscosity
-Surface tension (drop size)
-flame temperature
Ways to suppress physical interferences :
-Dilute samples
-Samples and standards at the same temperature
-Prepare samples and standards in the same mixture of
solvent and/or acid (matrix like or matrix matched).
V. Interferences
2. Chemical interferences
a/ Formation of refractory compounds
Influence of PO43- content on calcium determination ([Ca] = cst)
A
PO43-
flame
Ca3(PO4)2
Ca (Cl)2
CaO (réfractory)
Ca
V. Interferences
2. Chemical interferences
Formation of refractory compounds
A
Influence of Aluminium content on
magnesium absorbance ([Mg] = constant)
[Al]
flame
Mg +Al+ 2O2
MgAl2O4 (refractory)
V. Interferences
2. Chemical interferences
a/ Formation of refractory compounds
Solutions :
-Add a releasing agent as Sr for Mg determination :
MgAl2O4+Sr
SrAl2O4+ Mg
A (l )
Mg
Al (10ppm) Sr (50 ppm)
Sans Al et Sr
Al (10 ppm)
[Mg] en ppm
0,2
0,4
0,6
-Modify the matrix: adding of HCl as chloride salts are easier to
dissociate.
-Modify the flame temperature : mixture N2O / acetylene…ICP…
V. Interferences
2. Chemical interferences
b/ Ionisation
Emission of baryum as a function of potassium content
I
(1) Ba
(2) K
Ba2++2eK++e[K] in mg/L
1000
2000
3000
Solution : use of an ionisation buffer (CsCl)
I
Baryum calibration curves
With K (10ppm) and Cs (1000 ppm)
With CsCl (1000 ppm)
with K (10 ppm)
Ba alone
[Ba]
V. Interferences
2. Chemical interferences
c/ standard addition method
Some interferences are difficult to overcome even with the
previous techniques : matrix effect
How to fix the problem ?
- if the matrix is known, etalons should be prepared in this
matrix (calibration curve procedure)
-if the matrix is unknown : standard addition technique
Principle : add a same known quantity of sample solution in a
range of étalon solutions
 same matrix
V. Interferences
2. Chemical interferences
c/ standard addition method
If the signal S is in the linear range then S = K(cava/V +civi/V)
S
extrapolation
Confidence interval
Vi
x0  
V
cava/V=x
V. Interferences
2. Chemical interferences
c/ standard addition method
Interest:
- Check if there are matric effects or not
- Correct most interferences
Same slope= no
matrix effect
A
Spikes in 1% HCl (case of no
matrix effects)
External calibration in 1%
HCl
Spikes in 1% HCl (case of
matrix effects)
Different slope: matrix
effects
Cadded
V. Interferences
2. Chemical interferences
c/ standard addition method
Comparison
calibration curve method
 fast: 1 calibration for several samples
sample
 Good precision
 Low accuracy
Standard addition method
 time consuming :1 calibration per
 low precision
 high accuracy
V. Interferences
3. Spectral interferences
Spectral interferences or background effect: spectral overlap of the
analysis line with a molecular band (broad)
Spectre d’émission et d’absorption de
CaOH
Spectres d’absorption de KBr et KI
Ex determination of Ba when Ca is present: in the flame Ca(OH) formed
absorbs in a molecular band that contains lBa
V. Interferences
3. Spectral interferences
Element
absorption
Background
absorption
wavelength
- The absorption(or emission) lines characteristic of the
elements are thin
- The absorption bands of molecular compounds are wide
The background effect is corrected by the instrument
- Use of a continuous source (Deuterium correction)
- Correction by Zeeman effect
39
V. Interferences
3. Spectral interferences
Deuterium correction
V. Interferences
3. Spectral interferences
Correction using Zeeman effect
s0
s-
s+
Without
magnetic field:
A(s0) =AA+BG
With magnetic
field: A(s0) =BG
Evolution of energy levels
With the magnetic field,
AA occurs at wavelengths
outside the slit of the
monochromator
VI. Elementary analysis using ICP
1. Excitation by plasma source
Plasma : cloud of highly ionised gas, composed of ions(1 à 2%) , electrons and
neutral atoms
VI. Elementary analysis using ICP
2. Associated technics
ICP-OES : Inductive Coupling Plasma Optical Emission Spectroscopy
+ Simultaneous analysis of several elements
Complete atomisation (few chemical interferences)
High velocity
- Investment + consumables (15L argon/min)
Spectral interferences (multicharged excited cations present)
ICP–MS : ICP coupled to a mass spectrometer
+ Simultaneous multielement analysis
Excellent detection limits
- Maintenance MS and price
ICP-MS :
Introduction
of samples
Plasma torch
Detection limits for different determination methods :
FAA
GFAAS
ICP-OES
ICP-MS
As
150
0.05
2
0.0006
Cd
0.8
0.002
0.1
0.00009
Pb
15
0.05
1
0.00004
W
1500
-
1
0.005
Zr
450
-
0.5
0.0003
P
75000
130
4
0.1
S
-
-
10
28
Data from PERKIN-ELMER in ppb
The cost is function of the detection limit :
Around 14 000€ for FAA
25 000 € for GFAAS
50 000 € for ICP-AES
120 000 € for ICP-MS
Comparison of sensitivities
ICP-HRMS
ICP-MS
ICP-AES
GFAAS
FAAS
Analyse Traces
1 ppq
1 ppt
Analyse Majeurs
1 ppb
1 ppm
1,000 ppm
100%