Transcript Advanced Spectroscopy

Advanced Spectroscopy
2. UV-VIS Spectroscopy
Revision
1. What are the wavelength ranges for the ultraviolet and
visible regions of the spectrum?
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UV: 200-400 nm (actually < 200 far UV)
Visible: 400-800 nm
Revision
2. What molecular or structural features give rise to
absorption of ultraviolet/visible (UV/VIS) radiation in
organic species? Give an example of an organic
compound that would not absorb UV/VIS radiation.
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multiple covalent bonds
unbonded electrons (N, O, Cl)
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hexane
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Revision
3. What molecular or structural features give rise to
absorption of ultraviolet/visible (UV/VIS) radiation in
ionic species? Give an example of an ionic compound
that would not absorb UV/VIS radiation.
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valence electrons
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NaCl
Revision
Analyte
Region
Solvent
Cell
copper sulfate
VIS
water
plastic
copper sulfate
UV/VIS
water
quartz
methylbenzene
UV
hexane
quartz
yellow NP dye
VIS
hexane
glass
Absorbing species - organic
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all species absorb < 200 nm
not a practical area for measurement (need vacuum)
near UV is 200-400 nm
N2 should absorb but doesn’t
conjugation increases absorbance and shifts  to higher values
atoms with non-bonded electrons attached to conjugated system add
to this
need a lot to get into the visible region
HO3S
N N
N(CH3)2
Absorbing species - inorganic
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some simple metal ions absorb weakly in the ultraviolet
or visible region eg Cu2+ and Ni2+
Exercise 2.1
 Why would weak absorption by a chemical species, eg
Cu2+, make it not useful for quantitative analysis?
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require a very high concentration to get 0.1-1 absorbance
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polyatomic ions, such as permanganate and dichromate much stronger absorbance
a combination of factors: multiple bonds and nonbonded electrons
complexes of metal ions and ligands are needed for
intense absorption
ligands are known as colour-forming reagents.
Cells
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cell and solvent should not absorb more than 0.2 at
wavelengths of interest
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quartz – UV/VIS
plastic – VIS (aqueous)
glass – VIS (organic)
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Solvents
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solvent cutoff  – above which where the solvent absorbs little
can be used for measurements
Exercise 2.2
 dimethylbenzene (250-300 nm)
 hexane, dichloroethane or trichloroethane
 sodium benzoate (250-320 nm)
 water
 aspirin (280-320 nm)
 acetonitrile, methanol or ethanol
Spoectrograde solvents
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designed for use in UV spectroscopy
not necessarily more pure than AR grade
guaranteed not to have absorbing impurities
 eg AR grade hexane might be 99.9% pure, but the
impurity could absorb (benzene)
 spectrograde hexane might only be 99% pure, but the
impurity is non-absorbing (heptane)
Radiation sources
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two required:
 a deuterium discharge lamp for the UV
 a tungsten filament globe for the visible
output of the tungsten filament is dependent on the
applied voltage
power supply contains a voltage regulator to ensure a
constant value
at the changeover  (around 350 nm) difference in
intensity of the two lamps dealt
Monochromators
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prisms require very high quality calibration and optics
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diffraction gratings considerably less expensive and
optically more efficient
Detectors - Photomultiplier tubes
output
+ve
electrode
dynode
electrons
light sensitive
–ve electrode
photon
• limited operating life due to
breakdown of the
photocathode
• cannot be exposed to the
sunlight or bright room
lighting
• a large semi-permanent dark
current results
Detectors – diode array
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bank of joined semiconductors
multi-channel instrument
diode is responsible for detecting a small portion of the
spectrum (1-2 nm per diode)
all diodes operate at the one time
fixed resolution determined by number of diodes
not a problem for broad peaks