Analytical Chemistry

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Analytical Chemistry
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Chapter 22Atomic Spectroscopy
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Types of Atomic Spectroscopy
1) Atomic Absorption (AA)
2) Atomic Emission (AE)
3) Atomic Fluorescence (AF)
Overhead
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Atomic Absorption Spectroscopy
Essentially replace cuvet used in UV/Vis with an
alternative sample cell
We will investigate two variations of this method
1) Flame Atomic Absorption (Flame-AA)
2) Graphite Furnace Atomic Absorption (GF-AA)
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Linewidth Problem
Beers Law is only true if the linewidth of the
radiation being measured is much narrower than
the linewidth of the absorbing sample.
Chalkboard
Atomic linewidths 10-2 A
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Hollow-Cathode Lamp (HCL)
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Simple AA Instrument
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Flame Regions
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Flame Atomizers
Laminar Flow Burner
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Electrothermal Atomizers
Graphite Furnace
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Matrix Modifiers
Analyte can be lossed during charring step!
Add chemical to either make matrix more volatile
or the analyte less volatile
Matrix Modifier
Example Nickel Nitrate

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Flame vs. Graphite Furnace
1) Flames yield higher precision (1-2)
2) Flame setup less time consuming
3) Flame nebulizers are inefficient (5 sample
analyzed)
4) Graphite furance 100 to 10,000 times more
sensitive than flame
5) Flames can only analyze liquids
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Background Correction
Must distinguish analyte signal from background
interference.
1) Beam chopping (corrects for emission)
2) Deuterium lamp (corrects for scattering)
3) Smith-Hieftje (corrects for scattering)
4) Zeeman (corrects for scattering)
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Atomic Emission Spectroscopy
Flame can also be used for emission spectroscopy
Temperature variations significantly impact
emission
Why?
Boltzmann Distribution (pp. 21-22)
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Inductively Coupled Plasma (ICP)
RF field (27 MHz) accellerates electrons that
then collide with the Ar to produce more
electrons and transfer thermal energy.
Temperatures up to 10,000 K may be reached!
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ICP Nebulizer
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ICP Torch Temperatures
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Sequential ICP
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Simultaneous ICP
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ICP Replacing Flame and GF
1) Atomization more complete in an ICP
2) Residence time longer than a flame
3) Oxide and hydroxide formation is minimized
4) Low background radiation from torch
5) Temperature is more uniform in an ICP
6) Low background radiation from torch
7) Calibration curves linear over 5 orders of
magnitude
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Spectral Interference
Can have situations when analytes signal
overlaps with that of another species (atomic or
molecular) in the sample.
Absorption or Emission
Best thing to do is use a different absorption or
emission peak.
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Chemical Interference
Results in decreased atomization
Add releasing agent to free analyte from
interference (e.g., La3)
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Ionization Interference

Ionized metals have different absorption spectra
than neutral species!
Add ionization suppressor (e.g., Cs)

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Standard Additions
Add known quantities of analyte to the unknown
and create calibration curve

Your sample is now the solvent!

Section 5-3, p. 101 of Harris
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Sensitivity
Detection limit smallest amount qualitatively
detectable
S/N 3
Standard deviation of blank sample times 3
Quantitation limit smallest amount
quantitatively detectable
S/N gt 5
Standard deviation of blank sample times 5 or more