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Chapter 13

Chapter 13. An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry. Terms & Symbols. Beer’s Law. log P o /P = ebc = A The derivation of this law assumes That the incident radiation is monochromatic. The absorption occurs in a volume of uniform cross-section.

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Chapter 13

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  1. Chapter 13 An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry

  2. Terms & Symbols

  3. Beer’s Law log Po/P = ebc = A The derivation of this law assumes • That the incident radiation is monochromatic. • The absorption occurs in a volume of uniform cross-section. • The absorbing substances behave independently of each other in the absorbing process.

  4. Beer’s Law Limitations to Beer’s Law • Real Limitations to Beer’s Law • Apparent Chemical Deviations • Apparent Instrumental Deviations with Polychromatic Radiation • Instrumental Deviations in the Presence of Stray Radiation

  5. The Effects of Instrumental Noise on Spectrophotometric Analyses Types of Noise • Shot noise – This noise is generated by current flowing across a P-N junction and is a function of the bias current and the electron charge. The impulse of charge q depicted as a single shot event in the time domain can be Fourier transformed into the frequency domain as a wideband noise. • Thermal noise – In any object with electrical resistance the thermal fluctuations of the electrons in the object will generate noise. • White noise- The spectral density of thermal noise is flat with frequency. • Burst noise – Occurs in semiconductor devices, especially monolithic amplifiers and manifests as a noise crackle.

  6. The Effects of Instrumental Noise on Spectrophotometric Analyses Types of Noise • Avalanche noise – Occurs in Zener diodes are reversed biased P-N junctions at breakdown. This noise is considerably larger than shot noise, so if zeners have to be used as part of a bias circuit then they need to be RF decoupled. • Flicker noise – This noise occurs in almost all electronic devices at low frequencies. Flicker noise is usually defined by the corner frequency FL. Sources of Noise • Case I: sT = k1 • Case II: sT = k2(T2 + T) ½ • Case III: sT = k3T

  7. The Effects of Instrumental Noise on Spectrophotometric Analyses Effect of Slit Width on Absorbance Measurements

  8. Instrumentation Instrument Components • Sources • Wavelengths selectors • Sample containers • Radiation detectors • Signal processors and readout devices

  9. Instrumentation Sources- Light sources Deuterium and Hydrogen Lamps Tungsten Filament Lamps Tungsten Filament Lamps

  10. Instrumentation Types of Instruments • Single-beam • Double-beam in space • Double-beam in time • Multichannel

  11. Instrumentation Single-Beam Instruments

  12. Instrumentation Double-Beam Instruments

  13. Instrumentation Typical Instruments • Photometers • Visible Photometers • Probe-type Photometers • Ultraviolet Absorption Photometers • Spectrophotometers

  14. Instrumentation Most common spectrophotometer: Spectronic 20. • On/Off switch and zero transmission adjustment knob • Wavelength selector/Readout • Sample chamber • Blank adjustment knob • Absorbance/Transmittance scale

  15. Instrumentation • Visible Region

  16. Instrumentation • Single-Beam Instruments for the Ultraviolet/Visible Region

  17. Instrumentation • Single-Beam Computerized Spectrophotometers Inside of a single-beam spectrophotometer connected to a computer.

  18. Instrumentation • Double-Beam Instrumetents • Double-Dispersing Instruments • Diode Array Instruments

  19. References • http://www.anachem.umu.se/jumpstation.htm • http://userwww.service.emory.edu/~kmurray/mslist.html • http://www.anachem.umu.se/jumpstation.htm • http://userwww.service.emory.edu/~kmurray/mslist.html • http://www.anachem.umu.se/jumpstation.htm • http://www1.shimadzu.com/products/lab/spectro/uv3150.html • http://www.perten.com/product_range/diode_array/da_technology.html • http://repairfaq.ece.drexel.edu/sam/CORD/leot/course10_mod03/mod10-03.html • http://las.perkinelmer.com/catalog/Product.aspx?ProductID=L950 • http://www.olisweb.com/products/upgrades/ir983.php • http://imagers.gsfc.nasa.gov/ems/visible.html • http://biology.easternct.edu/courses/spectwenty.htm • http://www.cairnweb.com/systems/prod_lamp.html • http://www.odyseus.nildram.co.uk/RFIC_Theory_Files/Noise_Tutorial.pdf

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