ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 19 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of...
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Transcript of ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 19 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of...
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ANALYTICAL CHEMISTRY CHEM 3811
CHAPTER 19
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
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CHAPTER 19
SPECTROPHOTOMETRY
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COMPONENTS OF THE SPECTROPHOTOMETER
Po PLightsource
monochromator
(λ selector) sample readout detector
b
Absorption (UV-Vis)
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COMPONENTS OF THE SPECTROPHOTOMETER
Lightsource
monochromator
(λ selector) sample readout detector
Absorption (IR)
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COMPONENTS OF THE SPECTROPHOTOMETER
Source& sample
monochromator
(λ selector)
readout detector
Emission
- 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
Sourceλ selector
sample
monochromator
(λ selector)
readout detector
Fluorescence
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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 forUV
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)
ExamplesPhototube (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 λ = sum of absorbances from all species
AT = ε1b1c1 + ε2b2c2 + ε3b3c3 + ….
For the same sample cellb1 = b2 = b3 = b
AT = b(ε1c1 + ε2c2 + ε3c3 + ….)
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LUMINESCENCE
- Includes any EM emission
Emission Intensity (I)
I = kPoc
k is a proportionality constantk is a proportionality constantPPoo is the incident radiant power is the incident radiant power
c is the concentration of emitting speciesc is the concentration of emitting species
- Only holds for low concentrations- 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
Generally- Positive scan (oxidation) - absorbance decreases
- Negative scan (reduction) - absorbance increases
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-2.5
-1.5
-0.5
0.5
1.5
2.5
-0.4-0.200.20.40.60.8Volts vs Ag/AgCl
Curre
nt (M
illia
mps
)
Epa
Epc
ipa
ipc
0
200
400
600
800
1000
0 100 200 300 400 500 600
Time (Seconds)In
ten
sity
Io
I
ipa = anodic peak current ipc = cathodic peak current
Modulated Absorbance
Am = -log(I/Io)
SPECTROELECTROCHEMISTRY