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8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
http://slidepdf.com/reader/full/metal-analysis-by-flame-and-plasma-atomic-spectroscopy 1/24
Metal Analysis by Flame and
Plasma Atomic SpectroscopyFlame
A. Atomization
1. Types of Atomization Processes
a.) Nebulizersb. Electrothermal atomization
2. Line Width
3. Effect of Temperature
B. InterferencesC. Sample Preparation
Plasma Emission Spectroscopy
8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
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Continuous Atomizers
• Used in AA
and DCP
(direct
currentplasma)
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Discrete Atomizers
• Sample is atomized all at once, allowing for
better detection limits
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Line Width
Which is wider, atoms or molecules spectra?
Factors contributing to line width:
1.) Uncertainty PrincipleLifetimes of excited states are only a finite
amount of time. There are uncertainties in
transition time.Called natural line width. Overall 10-4 A
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Doppler Broadening
Detector
Detector
l longer
l shorter
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Pressure Broadening
• Arises from collisions between analyte and other
atoms or ions in heated media, result in small
changes in g.s. energy and hence a spread in
wavelength
• In high pressure Hg and Xe lamps pressure
broadening is so extensive that continuous
radiation is produced.
8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
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Temperature
• Extremely important to have
it consistent in order to get
consistent amount of
atomization
8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
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Spray
Solution of
Analyte
Solid/ gas
aerosol
Gaseousmolecules
Atoms
Atomic
ions
Excited
molecules
Excited
atoms
Excited
ions
Nebulization
Desolvation
Volatilization
Dissociation
Ionization
hn molecular
hn atomic
hn atomic
8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
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Why does a reproducible
temperature matter so much?
1. Temperature effects population size of
ground and excited states
2. 10K change in temperature for Na results
in a 4% change in excited state
population
3. Emission spectroscopy more sensitive to
small temperature changes in flame than
are absorption and fluorescence
because they are based on excited state
populations
8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
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Flames and Atomization
• Flame temperature
and position are
critical for achieving
reproducibleatomization
1o combustion region: Not at
thermal equilibrium, blue due to
C2CH, & other radicals, not used
for analytical spectroscopy
Outer zone: Atoms from inner
core are converted to stable
molecular oxides, cooler
Interconal region: Pretty
narrow in stoichiometric
flames, rich in free atoms,
widely used for analyticalspectroscopy
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Gases used
Common Fuels Common
oxidants
Natural gas Air
H2 O2
Acetylene Nitrous Oxide
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• Why is the burner the shape
that it is?
• Sensitivity of metal with
burner height varies by metal,see transparency
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Electrothermal atomizers a.k.a.
graphite furnaces
• Sample is introduced
into graphite tube,
solvent evaporated, &
then heated rapidly to2000-3000K w/high
current
• Sample residence
time up to 1 second
• Detection limit 10-10 –
10-13g/Sample
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Disadvantages of Graphite
Furnaces
• 5-10% precision vs.
1% for flame or
plasma
• Slow/sample• Linear range <2
orders of magnitude
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Interferences
Spectral Interferences: Arises when
absorption or emission of other species in
solution lies very close to the same
wavelength
1. Can result from combustion products of
flame fuel or oxidant
2. Molecular oxides from sample itself
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Ways to correct for matrix
interferences
1. Two line correction method: Pick linevery close to analyte spectral line butthat analyte does not absorb at, subtract
2 lines2. Background correction: Subtract
continuous source (such as D2) fromsample
3. Zeeman correction: Magnetic fieldapplied produces plane polarized light,light goes through polarizer only when
sample is introduced
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Chemical Interferences
Result from chemical
processes occurring
during atomization
that alter theabsorption
characteristics of the
analyte: Easier to
correct for thenspectral interferences!
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1. Anion Interference
• Most common: Anions will reactw/analyte to produce a species oflow volatility ex. PO43- or SO42-.This will significantly reduce Ca (or
other metal’s) absorption bymaking Ca3(PO4)2 and CaSO4
• Cation interference also possible
• Can be minimized w/ releasingagents or protective agents which
react preferentially with interferingspecies, for ex. Sr will reactpreferentially w/PO43- making itpossible to determine Ca
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Protective Agents
• Form stable but
volatile complexes w/
analyte
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2. Ions in Flames
• Can be minimized w/ ionization supressor
which produces an excess of ions so
L’Chatlier’s principle is employed
• M === M+ + e-
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3. Formation of Stable Compounds
• Some analytes are
atomized with
difficulty, i.e. Hg or Pb
For these you mustuse a hotter flame or
a fuel rich flame
Cool flame
Hot flame
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Sample Preparation for Metal
Analysis
• Dissolved metals: What
passes through a 0.45um
membrane filter of an
unacidified sample
• Suspended metals: What
is retained on a 0.45um
membrane filter of an
unacidified sample
• Total metals: Sum ofdissolved and suspended
metals
• Acid extractable metals: [
] of metals in solution
after treatment ofunfiltered sample with hot
acid
8/12/2019 Metal Analysis by Flame and Plasma Atomic Spectroscopy
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Sample Handling for Metal Analysis
1. Filter immediately
2. Preserve with acid to
pH = 2-3
3. Can store up to 6months
4. Containers: teflon >
polypropylene >linear polyethylene >
glass
{ Avoid glass for trace
levels}
• Detergent wash, tap
water rinse, soak in acid,rinse with metal free
water
• Avoid paints, rubber,
paper, and metal objects
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Sample Extraction
• Sample is digested in
concentrated HCl, HNO3,
H2SO4, etc. by boiling tolowest volume before
precipitation, cover w/
watch glass to avoid
spattering
• Purpose of acid digestion
is to oxidize the organic
materials in sample and
dissolve all the metals
• Continuous atomizers
require samples to be in
solution but discrete
atomizers do not
• Organic solutions willaffect outcome, increase
sensitivity b/c less
surface tension resulting
in finer drop size