ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 19

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 19
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 19 SPECTROPHOTOMETRY
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COMPONENTS OF THE SPECTROPHOTOMETER
Absorption (UV-Vis)
b
Po
P
Light source
monochromator (? selector)
sample
readout
detector
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COMPONENTS OF THE SPECTROPHOTOMETER
Absorption (IR)
Light source
monochromator (? selector)
readout
detector
sample
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COMPONENTS OF THE SPECTROPHOTOMETER
Emission
Source sample
monochromator (? selector)
readout
detector
- Sample is an integral portion of the source -
Used to produce the EM radiation that will be
measured
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COMPONENTS OF THE SPECTROPHOTOMETER
Fluorescence
Source ? selector sample
monochromator (? selector)
readout
detector
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LIGHT SOURCE
- A steady power supply is essential to provide
constant signal Tungsten Filament Lamp - Glows
at a temperature near 3000 K - Produces radiation
at wavelengths from 320 to 2500 nm - Visible and
near IR regions Dueterium (D2) Arc Lamp - D2
molecules are electrically dissociated - Produces
radiation at wavelengths from 200 to 400 nm - UV
region
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LIGHT SOURCE
Mercury and Xenon Arc Lamps - Electric discharge
lamps - Produce radiation at wavelengths from 200
to 800 nm - UV and Visible regions Silicon
Carbide (SiC) Rod - Also called globar -
Electrically heated to about 1500 K - Produces
radiation at wavelengths from 1200 to 40000 nm -
IR region
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LIGHT SOURCE
Also for IR Region - NiChrome wire (750 nm to
20000 nm) - ZrO2 (400 nm to 20000 nm)
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LIGHT SOURCE
Laser - Produce specific spectral lines - Used
when high intensity line source is required Can
be used for UV Visible FTIR
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MONOCHROMATOR
- Optical device - Disperses a beam of light into
its component wavelengths - Allows only a narrow
band of wavelengths to pass - Blocks all other
wavelengths Components - Two slits (entrance
and exit) - Concave Mirrors - Grating
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MONOCHROMATOR
- Mirror collimates light (parallel rays) -
Gating disperses light - Prisms were formerly
used to disperse light - Light coming through
entrance slit is polychromatic - Light out of
exit slit is monochromatic Dispersive
Spectrophotometers - Spectrophotometers that use
monochromators to select wavelengths
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MONOCHROMATOR
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SAMPLE CELL
Fused silica Cells (SiO2) - Transmits visible and
UV radiation Plastic and Glass Cells - Only
good for visible wavelengths NaCl and KBr
Crystals - IR wavelengths
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DETECTOR
- Produces an electric signal proportional to the
radiation intensity - Signal is amplified and
made available for direct display - A sensitivity
control amplifies the signal - May be controlled
manually or by a microprocessor (the use of
dynodes) Examples Phototube (UV) Photomultiplier
tube (UV-Vis) Thermocouple (IR) Thermister (IR)
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PHOTODIODE ARRAY SPECTROPHOTOMETER
- Records the entire spectrum (all wavelengths)
at once - Makes use of a polychromator - The
polychromator disperses light into component
wavelengths
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ANALYSIS OF A MIXTURE
- Occurs when there is more than one absorbing
species Absorbance at a given ? sum of
absorbances from all species AT e1b1c1
e2b2c2 e3b3c3 . For the same sample cell b1
b2 b3 b AT b(e1c1 e2c2 e3c3 .)
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LUMINESCENCE
- Includes any EM emission Emission Intensity
(I) I kPoc k is a proportionality constant Po
is the incident radiant power c is the
concentration of emitting species - Only holds
for low concentrations
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LUMINESCENCE
Fluorescence - Emission of light from a molecule
in an excited state - Initial absorbance followed
by emission Phosphorescence - Similar to
fluorescence - There is a delay (from seconds to
hours) before emission
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LUMINESCENCE
Photoluminescence (PL) - EM absorption and
re-radiation - Period between absorption and
emission is usually very short Chemiluminescence
(CL) - Emission of light as a result of a
chemical reaction Electrochemiluminescence
(ECL) - Emission produced during electrochemical
reactions - Luminescence as a result of
electrochemically generated species
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SPECTROELECTROCHEMISTRY
- Spectral and electrochemical signals are
simultaneously obtained - Change in absorbance
is concurrent with the electrochemistry Generall
y - Positive scan (oxidation) - absorbance
decreases - Negative scan (reduction) -
absorbance increases
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SPECTROELECTROCHEMISTRY
ipa anodic peak current ipc cathodic peak
current
Modulated Absorbance Am -log(I/Io)