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Essential Components of a UV-vis Spectrophotometer

Sample. Transducer. Monochromator. Source. Signal Processor. Display. Essential Components of a UV-vis Spectrophotometer. Sources Continuous Source Line Source Pulsed. Continuum. Line. Mercury Arc. Xenon Arc. Tungsten. Optical Components (lens and mirror).

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Essential Components of a UV-vis Spectrophotometer

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  1. Sample Transducer Monochromator Source Signal Processor Display Essential Components of a UV-vis Spectrophotometer

  2. Sources • Continuous Source • Line Source • Pulsed Continuum Line

  3. Mercury Arc Xenon Arc Tungsten

  4. Optical Components (lens and mirror) *Appropriate optics can be critical!

  5. Note the non-linear focal plane dispersion! • Monochromator • Filter • Monochromator: • Dispersion element (prism or grating) • Design

  6. Czerny – Turner Design for Monochromator Note the linear focal plane dispersion!

  7. Normal to grating Normal to blaze Reflected beam   Incident beam r i Monochromatic light source Polychromatic light source

  8. 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 prisms For gratings

  9. 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.

  10. Phototubes

  11. Photomultiplier Tube (PMT)

  12. Photodiode Array

  13. Charge-coupled Device

  14. Sample Compartment (Cuvettes)

  15. Single Beam

  16. Double Beam

  17. Singlet state: All electrons in the molecule are spin-paired lifetime: 10-5 – 10-8 sTriplet state: One set of electron spins is unpaired lifetime < 10 s

  18. I0 I Beers Law for a Multi-component Sample I0 I

  19. I0 I Beers Law for a Multi-component Sample I0 I

  20. A Concentration Deviations from Beer’s Law Physical: a) Scattering b) Reflection f = [ (n1 - n2) / (n1 + n2)] 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

  21. A Concentration Deviations from Beer’s Law Chemical: a) Equilibria involving chromophore e.g. Cr2O72- + H2O  2 H+ + 2 CrO42- Absorptivities of Cr2O72- and CrO42- 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.

  22. A + T P Photometric Titrations A > 0; T = P = 0 Absorbance T > 0; A = P = 0 Vol titrant P > 0; T = A = 0

  23. A + T P A >> 0; T > 0 P = 0 Absorbance Vol titrant T >> 0; P > 0 A = 0 P >> 0; A > 0 T = 0

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