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Lecture

ENCH 213 Lecture Notes - Monochrome, Ion, Pyrrolidine

5 pages90 viewsFall 2013

Department
Chemistry
Course Code
ENCH 213
Professor
Diane Beauchemin

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Flame atomic spectroscopy
oxidant acts as nebulizer gas as well
spray chamber contains obstructions (baffles, impact bead) to
either block big droplets or break them small ones
flame not efficient at exciting ground-state atoms
used mainly in AAS
A = Ebc, where
b is path length, E is a constant, c is concentration
long thin flame: increase path length, therefore increasing
absorption
Flame atomizer
combustion of a fuel/oxidant mixture
temperature varied by changing fuel/oxidant ratio (i.e. their flow rates)
“rich” flame = relatively rich in fuel
excess carbon reduces formation of MO, MOH
“lean” flame = excess oxidant
hotter than “rich” flame
organic solvent in sample may act as additional oxidant or fuel
depending on the solvent
A rich flame in atomic absorption spectroscopy would
a) decrease the concentration of metal oxides in the flames.
b) be hotter than a lean flame.
c) be preferred for refractory elements.
Flame atomizer
flame chose according to: analyte and the nature of matrix
Flame selection
air/acetylene
lower T
favors neutral atoms
path length = 10 cm
most common
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N2O/acetylene
higher T
may favor excited atoms
path length = 5 cm
watch out for explosion!
Possible problems
background emission from flame
C2, CH and OH emission bands
+ CN with N2O
may interfere with analyte signal
spectroscopic interference
overlap (total or partial) of analyte signal with that due to other elements, molecules or
flame
background correction to subtract the flame’s contribution
select other analyte wavelength, if interference from elements or molecules
Other possible problems
vaporization or chemical interference
formation of refractory compound (stable, hard to break into atoms) with SO42- and PO43-
Ca2+ (aq) + PO43- (aq) → Ca3(PO4)2
suppression of analyte signal
ex: suppression of Mg signal by Al
Antidotes to vaporization or chemical interference
increase residence time
look higher in the flame (equivalent of increasing residence time)
use a hotter flame (N2O...)
add a high concentration of a releasing agent (La3+ or Sr2+) to tie up the interferent
Ca2+ + PO43- + La3+ → LaPO4+ + Ca2+
use protective chelation to prevent the interfering anion from reacting with analyte
Ca2+ + EDTA + PO43- → Ca(EDTA) + PO43-
EDTA: protective agent
Ca(EDTA) is more stable in sltn than Ca3(PO4)2, however easily decomposes in flame
Yet other possible problems
increase in T required to break MO into M
increase in T may also favor excitation and/or ionization of M
not desirable for AAS
compromise required
ionization interference for easily ionized elements (Na, K): M → M+ + e-
add an ionization suppressor (100-1000 mg/L Cs to sample and standards) to get neutral Na
and K atoms (for AAS)
even more easily ionized than analyte
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