Atomic Fluorescence
Spectroscopy
Background


First significant research by Wineforder and
Vickers in 1964 as an analytical technique
Used for element analysis
– Example: Trace elements in ground water

Has not found wide spread success because
there does not seem to be a distinct
advantage over established methods, i.e.
atomic absorbance
What is Atomic
Fluorescence?


“Atomic fluorescence spectroscopy (AFS)
is the optical emission from gas-phase
atoms that have been excited to higher
energy levels by absorption of radiation.”
“AFS is useful to study the electronic
structure of atoms and to make quantitative
measurements of sample concentrations.”
Why use AFS?


it is a quantitative technique for
determination of a large number of elements
used mostly in analysis of metals in
biological samples, agricultural samples,
water, and industrial oils
Jablonski Diagram
S2
Intersystem
crossing
S1
h
h
fluorescence
T1
phosphorescence
S0
Instrumentation

Virginia Tech website maintained by Professor Brian Tissue Department of Chemistry
General process for AFS

nebulization - converts the sample solution into a mist made
up of tiny liquid droplets

atomization - flow of gas carries sample into heated region
where sample molecules are broken into free atoms
– desolvation - the solvent is evaporated to
produce a solid molecular aerosal
– dissociation - molecules dissociate to produce
an atomic gas
– The atoms dissociate to produce ions and
electrons
General Process continued





Excitation due to light source
Fluorescence of sample
This fluorescence can be selected for certain
wavelengths by a monochromator
Then the detector reads the emission and
amplifies the signal
Then the readout device relays the data
Problems with Technique



Precision and accuracy are highly
dependent on the atomization step
Light source
molecules, atoms, and ions are all in heated
medium thus producing three different
atomic emission spectra
Problems continued

Line broadening occurs due to the
uncertainty principle
– limit to measurement of exact lifetime and
frequency, or exact position and momentum

Temperature
– increases the efficiency and the total number of
atoms in the vapor
– but also increases line broadening since the
atomic particles move faster.
– increases the total amount of ions in the gas and
thus changes the concentration of the unionized
atom
Interferences




If the matrix emission overlaps or lies too close to
the emission of the sample, problems occur
(decrease in resolution)
This type of matrix effect is rare in hollow cathode
sources since the intensity is so low
Oxides exhibit broad band absorptions and can
scatter radiation thus interfering with signal
detection
If the sample contains organic solvents, scattering
occurs due to the carbonaceous particles left from
the organic matrix
Interferences continued
Detection Limits


Are similar to those for Atomic Absorption
and Atomic Emission
Varies for different elements
*
Detection Limits (ng/mL) for Selected Elements
†
Element AAS, Flame AAS, Electrothermal AES, Flame AES, ICP AFS, Flame
Al
30
0.005
5
2
5
As
100
0.02
0.0005
40
100
Ca
1
0.02
0.1
0.02
0.001
Cd
1
0.0001
800
2
0.01
Cr
3
0.01
4
0.3
4
Cu
2
0.002
10
0.1
1
Fe
5
0.005
30
0.3
8
Hg
500
0.1
0.0004
1
20
Mg
0.1
0.00002
5
0.05
1
Mn
2
0.0002
5
0.06
2
Mo
30
0.005
100
0.2
60
Na
2
0.0002
0.1
0.2
—
Ni
5
0.02
20
0.4
3
Pb
10
0.002
100
2
10
Sn
20
0.1
300
30
50
V
20
0.1
10
0.2
70
Zn
2
0.00005
0.0005
2
0.02
*
-3
-3
Nanogram/milliliter = 10 g/mL = 10 ppm.
†
AAS = atomic absorption spectroscopy; AES = atomic emission spectroscopy; AFS = atomic
fluorescence spectroscopy; ICP = inductively coupled plasma.
Final conclusion


This technique offers some advantages for
some elements while other atomic
spectroscopy techniques may be better for
other elements
Future work on light sources and atomizers
will increase the analytical uses of this
technique