Atomic Fluorescence Spectroscopy (AFS)

Introduction to AFS
Instrumentation of AFS

1. Atomic Fluorescence Spectroscopy
Dr. SajjadUllah
Institute of Chemical Sciences,
University of Peshawar

2. Atomic Fluorescence Spectroscopy
•SameapparatusasforAAS
•Radiativesourceisgenerally
Pulsedandthedetectoristuned
Torespondonlytoradiationthatoscillates
atthepulsefrequency
•I
F=KI
0C(lowDLwhenusinghighIsource)
(both line & continuous
High intensity sources)
P
M
T
line Source (low resolution monochromator)
Continuous source (high resolution monochromator)

3. Atomic Fluorescence occurs when neutral atomic
Species emit radiation after being excited by a line
or continuous source.
Photoexcitation is the same as in AAS but we
measure the emitted (rather than absorbed)
radiation in AFS
AFS

4. Four Major Categories of AF

5. A = log P
o/P = k C
Radiation Sources for AFS
So Source Intensity, I, must be very high within this
narrow absorptive bands.
Why?
Sufficient radiation to permit accurate measurement at the detector
can be achieved only if I of the EMR is high.
Important Considerations!!!
Atomic Spectral lines are very narrow (0.001-0.01 nm)
This requirement is not that critical for molecular electronic
spectroscopy as Molecule absorb in broader range and even broad
EMR band (= band pass of monochromator) can be absorbed.

6. Line Sources
Hollow Cathode Lamp
High Intensity HCL
Electrodeless DischageLamp (EDL): More common
LASER
Continuous Source:
Xenon-arc lamp
Radiation Sources for AFS

7. LASER (Light Amplification by Simulated Emission of Radiation)
Cr(III) dope Al
2O
3
Excitation of Cr(III) ions the ruby rod by radiation from
The flash lamp
Metastable
state
Population inversion occurs when more Cr ions are in the excited (
2
E) state
than in the ground state
Laserare devices that emit high-intensity coherent
(in-phase) radiation over a narrow (0.001-0.01 nm)
bandwidth

8. DC Argon Plasma
It relies on application of less than a
kilowatt of a DC between two carbon
anodes and a tungsten cathode.
The high Temperature in a DC
plasma can excite atomic/ionic
species
More and intense line than in Flames

9. Xenon short-arc lamp
Continuous source used for AAS
λ-range 200-700 nm
Requires a monochromatorfor λ
selection
Advantage?
Disadvantage?
Xegas
Electric arc between two electrode causes
excitation of Xefilled in a quartz tube at
high pressure and Xeatoms/ions upon
de-excitation give continuous spectrum
http://www.enlitechnology.com/show/xe-lamp-light-generation-mechanism.htm

10. Luminescence of Solids, Editor D.R. Vij

11. Photobiology: The Science of Life and Light, Editor:Lars OlofBjörn

12. Line Sources (AAS)
Hollow Cathode Lamp (HCL)
•MultielementHCL
•Demountable HCL
•High Intensity HCL
Electrodeless Discharge Lamp (EDL)
Temperature Gradient Lamp

13. Hollow Cathode Lamp
(Most popular)
Quartzwindow
Pyrexbody
Anode
Cathode

14. Filler Gas
(Ne or Ar)
at 1-2 Torr
W, Zr, Ni
Made up of
element of
interest or its
alloy
100-200 V
(1-25 mA)
Ar+
Ar
+
ions strike the cathode to cause Sputtering.
Why Low P?
Sputtering?
Disadvantage?
Multi-element HCL (2-7 elements)/Suffer
Composition change/More volatile
elements distills first and more, Caution!
Demountable HCL. Replaceable cathode,
but time/effort involved.
1 torr = 0.0013158 atm

15. HOW Hollow Cathode Lamp works?
a tungsten anode and a
cylindrical cathode
neon or argon at a pressure of 1
to 5 torr
The cathode is constructed of
the metal whose spectrum is
desired or served to support a
layer of that metal
Ionizetheinertgasatapotentialof~300V
Generateacurrentof~5to15mAasions
andelectronsmigratetotheelectrodes.
Thegaseouscationsacquireenoughkineticenergytodislodgesomeofthe
metalatomsfromthecathodesurfaceandproduceanatomiccloud.
Aportionofsputteredmetalatomsisinexcitedstatesandthusemitstheir
characteristicradiationastheyreturntothegroundsate
Eventually,themetalatomsdiffusebacktothecathodesurfaceortotheglass
wallsofthetubeandarere-deposited

16. Hollow Cathode Lamp (Cont’d)
High potential, and thus high currents lead to
greater intensities (Operators’ control) as more
sputtering occurs, BUT also leads to self-absorption
and resonant broading.
Self-absorption or Self-reversal:the greater currents
produce an increased number of unexcitedatoms in
the cloud. The unexcited atoms, in turn, are capable
of absorbing the radiation emitted by the excited
ones.Thisself-absorption leads to lowered
intensities, particular at the center of the emission
band
ResonanatBroading: Pressure broadening caused by
collision between identical atoms

17. Improvement…….High Intensity HCL
Most direct method of obtaining improved lamps
for the emission of more intense atomic resonance
lines is to separate the two functions involving the
production and excitation of atomic vapor
Boosted discharge hollow-cathode lamp (BDHCL)
or High Intensity HCL is introduced as an AFS*
excitation source by Sullivan and Walsh.
It has received a great deal of attention and a
number of modifications to this type of source have
been conducted.
*rarely used for AAS

19. Photomultiplier Detector
•The type is commonly used especially for low
radiant powers.
•The detector consists of a photoemissive cathode
(coated with cesium oxide) coupled with a series
of electron-multiplying dynode stages.
•The primary electrons ejected from the photo-
cathode are accelerated by an electric field so as to
strike onto the first dynode and then the e emitted
from 1
st
dynodes are directed onto the 2
nd
dynodes
and so on.
•Amplification = n
d
where d is the number of dynodes and n is the
no of electrode emitted per dynode. Usually 10
6
to10
7
e are emitted per photon

20. Photomultiplier Detector

21. Analysis with AFS
QuantitaiveAnalys: Working-curve method (or standard addition technique (if curves show linearity). Around 58
elements can be assayed.

22. Interferences in Atomic Spectroscopy
1-Chemical Interferences
2-Ionization interferences
3-Spectral interferences
4-Matrix interferences

23. Chemical Interferences
Chemical reaction in cell (removal of atoms)
Formation of compounds of low volatility
Calcium analysis in the presence of Sulfate (CaO.SO
3) or
phosphate(CaO.P
2O
5)
Formations of refractory oxides (Al
2O
3, Fe
2O
3) of unusual stability
in flames
Formation of carbides or cyanogen(CN)
2in furnaces
Solution!
Higher temperature or deceases O
2 concentration (fuel rich
flame), e.g., changing air-C
2H
2flame to N
2O-C
2H
2flame
Releasing agents: Cations (e.gLaCl
3 used for Ca, Mg, Sr) that
react preferentially with the interference ions.
(Ca
2+
(analyte)+ PO
4
3-
(interfering)+ La
3+
LaPO
4+ Ca
2
(free) )
Protection agents: form stable but volatile species with the
analytes (i.e. EDTA)

24. Ionization Interferences
Atom ionization (usually due to high T)
M ↔ M
+
+ e (group IA and IIA have low IE)

25. Spectral lines of ions occur at different λthan atomic lines
-decrease in Atomic signal
Ionization decreases at high concentration
-competition B/W atom for available E
Solutions
Use low temperature flame (air-propane)
Add large amount of easily ionizing element
(500-5000 µg/mL of Li, Na, K) element
Use high concentration of analyte

27. Matrix interferences
These are caused by the physical nature of the matrix enhancing
or depressing sensitivity
Example: Viscosity difference b/w sample and standards
(different nebulzation/aspiration/atomization)
Solutions!
Standard addition technique
Matching the matrix of sample
with that of standards
Solvent extraction or so to
isolate the analyte

28. Spectral Interferences
Overlapping (spectra of analyteand another atomic/molecular
species)
Background (non-specific) Interferences :
-Spectral interferences resulting from emission of EMR
from elements in cell
-Scattering or absorption by sample matrix or polyatomic
species
Positive error (analyteand matrix absorb the same λ)
Negative error: Interfering species emits same λas used for AA
measurement
Tb/Mg = 285.2 nm
Cr/Os = 290.0 nm
Ca/Ge = 422.7 nm
Examples:

29. Spectral Interferences…
Solutions
Chemical Separation prior to the assay
Modulation of the detector
Background correction
Modulationofthedetector:
Thedetectoristunedtothefrequencyofoscillationofthe
EMRsource,itdoesn’trespondtothesteady-stateemissions
fromthecell.Therefore,interferenceowingtoemissionfrom
thecelliseliminated

30. Background Correction
Background (non-specific) Interferences:
-Spectral interferences resulting from emission of EMR
from elements in cell
-absorption by polyatomic species or scattering within the
cell
-More severe in furnaces than flames
-Greater at shorter λs (more scattering)

31. Continuum-Source Correction
Background Correction
A
HCL= A
analyte+ A
background
A
D2= A
background

32. Continuum-Source Correction