Tuesday, March 24, 2009

Spectrophotometer Simulation

The experiment models the emission of photons by a lamp, the absorption of those photons by a solute in a sample solution, and the detection of the photons by a photomultiplier tube. The photons are represented by little balls (a particulate matter model rather than a wave model). The simulation permits the simultaneous use of two different wavelengths.

The user may specify several operating conditions for the spectrophotometer.

  • Sample
    • Blank
        The molar absorptivity is zero for both wavelengths.
    • Unknown 1
        The wavelength and molar absorptivity are selected by the applet (the molar absorptivity is not shown to the user). The second wavelength is not used.
    • Unknown 2
        Same as Unknown 1
    • User Entered
        The user may specify both wavelengths and both molar absorptivities
  • Wavelength
      The wavelength of the photon in nanometers may be specified. The color of the "photon" is selected to loosely correspond with the wavelength.
  • Molar Absorptivity (L mole-1 cm-1)
  • Concentration (millimole L-1)
  • Cell Path Length (cm)
  • Intensity (photons/sec)
      The intensity is the number of photons per second emitted from the source.
  • To use the applet

    1. Select the desired sample.
    2. Start the simulation by clicking on the green button in the lower right corner of the applet.
    3. The intensity of photons reaching the detector may be measured using the timer in the lower left corner of the applet.

    Operational Notes and Tips

  • The first wavelength and molar absorptivity (left-most boxes) are always active. The wavelength must always lie in the range of 380 to 780 nm. The second wavelength can be disabled by entering a wavelength of 0.
  • When measuring an intensity after changing the operating conditions (e.g., concentration, cell path length, source intensity, or molar absorptivity), all sufficient time for the existing photons to cross the screen before starting or reseting the time.
  • On slow computers the animation of the photons can become jerky and photons may be released from the lamp in waves rather than randomly; this behavior can be especially pronounced when two different wavelengths and a high intensity of photons are used. These problems can be compounded by an operating system such as Windows 95 that is sluggish in switching tasks. To remedy or alleviate this problem, try
    • Operating the simulation with a lower intensity
    • Closing all other applications, thereby freeing more CPU time for the simulation.
    • Reboot the system (especially for Windows 95/98) to eliminate all unnecessary background processes.
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