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Published byAnna McCarthy Modified over 9 years ago
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Essential Components of a UV-vis Spectrophotometer Monochromator Signal Processor Display Source Sample Transducer
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Continuum Line Sources - Continuous Source - Line Source - Pulsed
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Tungsten Xenon Arc Mercury Arc
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Optical Components (lens and mirror) *Appropriate optics can be critical!
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Monochromator - Filter - Monochromator: - Dispersion element (prism or grating) - Design Note the non-linear focal plane dispersion!
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Czerny – Turner Design for Monochromator Note the linear focal plane dispersion!
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Incident beam Reflected beam Normal to grating Normal to blaze i r Monochromatic light source Polychromatic light source
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Monochromator Figures of Merit Bandpass - The wavelength range that the monochromator transmits. Dispersion - The wavelength dispersing power, usually given as spectral range / slit width (nm/mm). Dispersion depends on the focal length, grating resolving power, and the grating order. For prismsFor gratings
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Resolution - The minimum bandpass of the spectrometer, usually determined by the aberrations of the optical system. Acceptance angle - A measure of light collecting ability, focal length / mirror diameter Blaze wavelength - The wavelength of maximum intensity in first order.
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Phototubes
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Photomultiplier Tube (PMT)
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Photodiode Array
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Charge-coupled Device
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Sample Compartment (Cuvettes)
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Single Beam
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Double Beam
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Singlet state: All electrons in the molecule are spin-paired lifetime: 10 -5 – 10 -8 s Triplet state: One set of electron spins is unpaired lifetime < 10 s
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Beers Law for a Multi-component Sample I I0I0 I I0I0
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I I0I0 I I0I0
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Concentration A Deviations from Beer’s Law Physical: a) Scattering b) Reflection f = [ (n 1 - n 2 ) / (n 1 + n 2 )] 2 = fraction reflected substance n = refractive index e.g. glass 1.5, air 1.0, water 1.3 c) Inhomogeneities d) Stray light 1. Scattering from grating (acting as a mirror causing scatter and reflection) 2. Overlapping orders 3. Diffraction at slits 4. Off-axis illumination 5. Scatter from interior 6. Dust
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Deviations from Beer’s Law Chemical: a) Equilibria involving chromophore e.g.Cr 2 O 7 2- + H 2 O 2 H + + 2 CrO 4 2- Absorptivities of Cr 2 O 7 2- and CrO 4 2- are quite different. b) Solute-Solvent interactions c) Solute-Solute interactions d) Fluorescence (gives positive deviation for %T negative deviation for Absorbance) Note: Non-zero intercept usually due to improper blank measurements or nonequivalent measurement conditions of blank and standards. Concentration A
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Photometric Titrations A > 0; T = P = 0 A + T P T > 0; A = P = 0 P > 0; T = A = 0 Vol titrant Absorbance
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A >> 0; T > 0 P = 0 A + T P Vol titrant Absorbance T >> 0; P > 0 A = 0 P >> 0; A > 0 T = 0
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