ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 19
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ANALYTICAL CHEMISTRY CHEM 3811
CHAPTER 19
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciencesClayton state university
CHAPTER 19
SPECTROPHOTOMETRY
COMPONENTS OF THE SPECTROPHOTOMETERPoPLightsourcemonochromator( selector) samplereadout detectorbAbsorption (UV-Vis)
COMPONENTS OF THE SPECTROPHOTOMETERLightsource monochromator( selector) samplereadout detectorAbsorption (IR)
COMPONENTS OF THE SPECTROPHOTOMETERSource& sample monochromator( selector)readout detectorEmission - Sample is an integral portion of the source
- Used to produce the EM radiation that will be measured
COMPONENTS OF THE SPECTROPHOTOMETERSource selectorsample monochromator( selector)readout detectorFluorescence
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
LIGHT SOURCEMercury 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
LIGHT SOURCEAlso for IR Region
- NiChrome wire (750 nm to 20000 nm)
- ZrO2 (400 nm to 20000 nm)
LIGHT SOURCELaser
- Produce specific spectral lines- Used when high intensity line source is required
Can be used forUV Visible FTIR
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
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
MONOCHROMATOR
SAMPLE CELLFused silica Cells (SiO2)- Transmits visible and UV radiation
Plastic and Glass Cells- Only good for visible wavelengths
NaCl and KBr Crystals- IR wavelengths
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)
PHOTODIODE ARRAY SPECTROPHOTOMETER- Records the entire spectrum (all wavelengths) at once
- Makes use of a polychromator
- The polychromator disperses light into component wavelengths
ANALYSIS OF A MIXTURE- Occurs when there is more than one absorbing species
Absorbance at a given = sum of absorbances from all species
AT = 1b1c1 + 2b2c2 + 3b3c3 + .
For the same sample cellb1 = b2 = b3 = b
AT = b(1c1 + 2c2 + 3c3 + .)
LUMINESCENCE- Includes any EM emission
Emission Intensity (I)
I = kPoc
k is a proportionality constantPo is the incident radiant powerc is the concentration of emitting species
- Only holds for low concentrations
LUMINESCENCEFluorescence- 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
LUMINESCENCEPhotoluminescence (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
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
ipa = anodic peak current ipc = cathodic peak currentModulated AbsorbanceAm = -log(I/Io)SPECTROELECTROCHEMISTRY
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