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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 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 PoPo P Light source monochromator (λ selector) samplereadout detector b Absorption (UV-Vis)
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COMPONENTS OF THE SPECTROPHOTOMETER Light source 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 Source λ 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 (D 2 ) Arc Lamp - D 2 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) - ZrO 2 (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 (SiO 2 ) - 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 λ = sum of absorbances from all species A T = ε 1 b 1 c 1 + ε 2 b 2 c 2 + ε 3 b 3 c 3 + …. For the same sample cell b 1 = b 2 = b 3 = b A T = b(ε 1 c 1 + ε 2 c 2 + ε 3 c 3 + ….)
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LUMINESCENCE - Includes any EM emission Emission Intensity (I) I = kP o c k is a proportionality constant P o 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 Generally - Positive scan (oxidation) - absorbance decreases - Negative scan (reduction) - absorbance increases
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i pa = anodic peak current i pc = cathodic peak current Modulated Absorbance A m = -log(I/I o ) SPECTROELECTROCHEMISTRY
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