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Metal Analysis by Flame and Plasma Atomic Spectroscopy

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Metal Analysis by Flame and
Plasma Atomic Spectroscopy
Flame
A. Atomization
1. Types of Atomization Processes
a.) Nebulizers
b. Electrothermal atomization
2. Line Width
3. Effect of Temperature
B. Interferences
C. Sample Preparation
Plasma Emission Spectroscopy
Continuous Atomizers
• Used in AA
and DCP
(direct
current
plasma)
Discrete Atomizers
• Sample is atomized all at once, allowing for
better detection limits
Line Width
Which is wider, atoms or molecules spectra?
Factors contributing to line width:
1.) Uncertainty Principle
Lifetimes of excited states are only a finite
amount of time. There are uncertainties in
transition time.
Called natural line width. Overall 10-4A
Doppler Broadening
Detector
l longer
Detector
l shorter
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.
Temperature
• Extremely important to have
it consistent in order to get
consistent amount of
atomization
Solution of
Analyte
Nebulization
Spray
Solid/ gas
aerosol
Gaseous
molecules
Desolvation
Volatilization
Excited
molecules
hn molecular
Excited
atoms
hn atomic
Excited
ions
hn atomic
Dissociation
Atoms
Ionization
Atomic
ions
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
Flames and Atomization
• Flame temperature
and position are
critical for achieving
reproducible
atomization
Interconal region: Pretty
narrow in stoichiometric
flames, rich in free atoms,
widely used for analytical
spectroscopy
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
Gases used
Common Fuels
Common
oxidants
Natural gas
Air
H2
O2
Acetylene
Nitrous Oxide
• Why is the burner the shape
that it is?
• Sensitivity of metal with
burner height varies by metal,
see transparency
Electrothermal atomizers a.k.a.
graphite furnaces
• Sample is introduced
into graphite tube,
solvent evaporated, &
then heated rapidly to
2000-3000K w/high
current
• Sample residence
time up to 1 second
• Detection limit 10-10 –
10-13g/Sample
Disadvantages of Graphite
Furnaces
• 5-10% precision vs.
1% for flame or
plasma
• Slow/sample
• Linear range <2
orders of magnitude
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
Ways to correct for matrix
interferences
1. Two line correction method: Pick line
very close to analyte spectral line but
that analyte does not absorb at, subtract
2 lines
2. Background correction: Subtract
continuous source (such as D2) from
sample
3. Zeeman correction: Magnetic field
applied produces plane polarized light,
light goes through polarizer only when
sample is introduced
Chemical Interferences
Result from chemical
processes occurring
during atomization
that alter the
absorption
characteristics of the
analyte: Easier to
correct for then
spectral interferences!
1. Anion Interference
• Most common: Anions will react
w/analyte to produce a species of
low volatility ex. PO43- or SO42-.
This will significantly reduce Ca (or
other metal’s) absorption by
making Ca3(PO4)2 and CaSO4
• Cation interference also possible
• Can be minimized w/ releasing
agents or protective agents which
react preferentially with interfering
species, for ex. Sr will react
preferentially w/PO43- making it
possible to determine Ca
Protective Agents
• Form stable but
volatile complexes w/
analyte
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-
3. Formation of Stable Compounds
• Some analytes are
atomized with
difficulty, i.e. Hg or Pb
For these you must
use a hotter flame or
a fuel rich flame
Cool flame
Hot flame
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 of
dissolved and suspended
metals
• Acid extractable metals: [
] of metals in solution
after treatment of
unfiltered sample with hot
acid
Sample Handling for Metal Analysis
1. Filter immediately
2. Preserve with acid to
pH = 2-3
3. Can store up to 6
months
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
Sample Extraction
• Sample is digested in
concentrated HCl, HNO3,
H2SO4, etc. by boiling to
lowest 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 will
affect outcome, increase
sensitivity b/c less
surface tension resulting
in finer drop size
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