ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 20

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 20
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 20 ATOMIC SPECTROSCOPY
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ATOMIC SPECTROSCOPY
- Used for elemental analysis - Deals with the
absorption and emission of radiation by atoms -
Deals with free atoms - Line spectra are
observed - Can be used for both qualitative and
quantitative analysis
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ATOMIC SPECTROSCOPY
- Atomic spectra have narrow lines ( 10-4
nm) Two Major effects That Cause Line
Broadening (yield linewidths of 10-3 to 10-2
nm) Doppler Broadening - Species may move
towards or away from detector - Result in doppler
shift and broadening of spectral lines Pressure
Broadening - Species of interest may collide with
other species and exchange energy - Increase in
temperature results in greater effect
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ATOMIC SPECTROSCOPY
- Liquid sample is sucked - Sample passes
through a plastic tube into a flame - Flame
breaks molecules into atoms (atomization) -
Monochromator selects wavelength that reaches the
detector - The concentration of elements is
measured by emission or absorption radiation -
Concentrations are measured at the ppm level
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ATOMIC SPECTROSCOPY
Atomization - The process of breaking analyte
into gaseous atoms
P
Po
Light source
monochromator (? selector)
readout
detector
Flame
Sample
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ATOMIC SPECTROSCOPY
Source - Line source is required to reduce
interference from other elements Hollow Cathode
Lamp (HC) - Produces emission lines specific for
the element used to construct the cathode -
Cathode is made from the element of interest -
Cathode must conduct current
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ATOMIC SPECTROSCOPY
Electrodeless Discharge Lamp - A salt of the
metal of interest is sealed in a quartz tube
along with an inert gas - A radio frequency (RF)
field excites the inert gas - Excited gas
ionizes metal - Light intensity is about 100
times greater than that of HC - Less stable than
HC
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ATOMIC EMISSION SPECTROSCOPY
- Does not require light source - Excited atoms
in the flame emit light that reaches the
detector (luminescence) Techniques Based on
Excitation Source - Flame Photometry - Furnace
(Electrical Excitation) - Inductively Coupled
Plasma
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ATOMIC EMISSION SPECTROSCOPY
Qualitative Analysis - Techniques rely on
specific emission lines
Element Hg Cu Ag Zn K
Emission Line (?) 2537 3248 3281 3345 3447
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ATOMIC EMISSION SPECTROSCOPY
Quantitative Analysis - Techniques rely on
intensity of emission lines I kPoc k is a
proportionality constant Po is the incident
radiant power c is the concentration of emitting
species
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ATOMIC EMISSION SPECTROSCOPY
Flame Photometry - For liquids and gases - Most
flame spectrometers use premix burner (sample,
fuel, and oxidant are mixed before reaching the
flamw) - Flame decomposes sample into metal
atoms (M) - Oxides (MO) and hydroxides (MOH) may
also form
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ATOMIC EMISSION SPECTROSCOPY
Flame Photometry - Flame may be rich (rich in
fuel) or lean - Rich flame reduces MO and MOH
formation (excess carbon reduces MO and MOH to
M) - Lean flame has excess oxidant and is
hotter - Good for Groups 1A and 2A elements
(easier to ionize)
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ATOMIC EMISSION SPECTROSCOPY
Furnace (Electrical Excitation) - For liquids
and solids - More sensitive than flame - Lower
detection limits than flame ( 100 times) -
Requires less sample than flame - Graphite
furnace is highly sensitive - Operates at a
maximum temperature of 2550 oC
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ATOMIC EMISSION SPECTROSCOPY
Inductively Coupled Plasma (ICP) - Makes use of
plasma (partially ionized gas) - Similar to
flame photometry but reaches much higher
temperatures (greater than 10000 K) - More
sensitive - A radio frequency (RF) is used to
excite an inert gas (Ar) - Excited gas ionizes
the sample
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ATOMIC ABSORPTION SPECTROSCOPY (AAS)
- Atoms absorb light from the source -
Unabsorbed light reaches the detector -
Quantitative analysis is based on the
absorption of light by free atoms - Makes use of
Beers Law
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ATOMIC ABSORPTION SPECTROSCOPY (AAS)
Drawback Flame Photometry - Most atoms remain in
the unexcited state Furnace (Electrical
Excitation) - Most atoms remain in the unexcited
state Inductively Coupled Plasma (ICP) - Problem
of atoms remaining in the unexcited state is
minimal
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ATOMIC ABSORPTION SPECTROSCOPY (AAS)
Compared to Emission Advantages - Less dependent
on temperature - Fewer interferences - Better
sensitivity Disadvantage - Quantitative analysis
only - Only used for metals since most nonmetals
form oxides
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EEFECT OF TEMPERATURE
- More atoms are excited as temperature
increases - However, most are still in the atomic
state
number
Minimum energy for ionization
T1
T2
T3
T1 lt T2 lt T3
Energy
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EEFECT OF TEMPERATURE
- For a molecule with two energy levels Eo and
E - Ground state energy level Eo - Excited
state energy level E E - Eo ?E - At atom
(or molecule) may exist in more than one state at
a given energy level - Number of states is
referred to as degeneracies
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EEFECT OF TEMPERATURE
Degeneracy at Eo go Degeneracy at E g
E, g
Emission
Absorption
?E
Eo, go
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EEFECT OF TEMPERATURE
Boltzmann Distribution - Describes relative
populations of different states at thermal
equilibrium
- N/No is the relative population at
equilibrium - T is he temperature (K) - k is the
Boltzmanns constant (1.381 x 10-23 J/K)
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EEFECT OF TEMPERATURE
The Excited State Population - Increase in
temperature has very little effect on the ground
state population (though an increase in
population occurs) - Has no noticeable effect on
the signal in atomic absorption - Increase in
temperature increases the excited state
population (however small) - Rise in emission
intensity is observed
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EEFECT OF TEMPERATURE
Atomic Absorption - Not sensitive to temperature
variation Atomic Emission - Sensitive to
temperature variation ICP is mostly used for
emission
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BACKGROUND CORRECTION
- Backgorund emission or absorption should be
accounted for Two Common Approaches D2
Correction - Light from source and D2 lamp pass
through sample alternately - D2 output is not
very good at wavelengths greater than 350
nm Zeeman Correction - Atomic vapor is exposed
to a strong magnetic field - Splitting of the
atoms electronic energy level occurs - Background
absorption can then be directly measured
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INTERFERENCE
- Result of change in signal when analyte
concentration is unchanged Spectral
Interference - Overlap of analyte signal by other
signals from other species or flame or furnace -
Commonly caused by stable oxides Chemical
Interference - Chemical reactions of other
species with analyte - Caused by substances that
decrease the extent atomization of analyte -
Minimized by high flame temperatures
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INTERFERENCE
Ionization Interference - Ionization decreases
the concentration of neutral atoms - Prevalent in
analysis of metals with low ionization
energies (alkali metals) - Ionization suppressor
may be added to decrease the ionization of
analyte (CsCl is used for K analysis) - The
method of standard addition eliminates
interference - Known amounts of analyte are added
to unknown - Standard addition curve is plotted
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INDUCTIVELY COUPLED PLASMA-MASS
SPECTROMETRY (ICP-MS)
- Very sensitive and good for trace analysis -
Plasma produces analyte ions - Ions are directed
to a mass spectrometer - Ions are separated on
the basis of their mass-to-charge ratio - A very
sensitive detector measures ions - Very low
detection limits
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INDUCTIVELY COUPLED PLASMA-MASS
SPECTROMETRY (ICP-MS)
Drawback Isobaric Interference - Cannot
distinguish ions of similar mass-to-charge
ratio - HCl and H2SO4 create isobaric
interferences so are avoided - 138Ba2
interferes with 69Ga
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SUMMARY
Flame Absorption - Low cost - Different lamp
required for each element - Poor
sensitivity Furnace Absorption - High cost -
Different lamp required for each element - High
background signals - High sensitivity
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SUMMARY
Inductively Coupled Plasma Emission - High cost -
No lamp required - Low background signals - Low
interference - Moderate sensitivity Inductively
Coupled Plasma-Mass Spectrometry - Very high
cost - No lamp required - Least background
signals - Least interference - Very high
sensitivity