Electromagnetic radiation is a self-propagating wave with an electric component and a magnetic component. These 2 components oscillate at right angles to each other and are in phase with each other.

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CH 103 SPECTROPHOTOMETRY THE ELECTROMAGNETIC
SPECTRUM

• Electromagnetic radiation is a self-propagating
  wave with an electric component and a magnetic
  component. These 2 components oscillate at right
  angles to each other and are in phase with each
  other.
• Electromagnetic radiation travels at the speed of
  light. In fact, the light in this room is
  electromagnetic radiation.

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THE ELECTROMAGNETIC SPECTRUM

• The wavelength (?, the length of 1 cycle in
  meters) times the frequency (?, the number of
  cycles per second) equals the speed of light (c,
  a constant that equals 3.0 x 108 meters/second).
  That is,
• c ?? 3.0 x 108 meters/second
• If ? increases, then ? must so that c remains
  constant.
• If ? decreases, then ? must so that c remains
  constant.

decrease
increase
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THE ELECTROMAGNETIC SPECTRUM

• Electromagnetic radiation is also a stream of
  energy packets called photons.
• The energy of a single photon (E, in joules)
  equals Plancks constant (h, 6.626 x 10-34 joule
  second) times the frequency (?, the number of
  cycles per second). That is,
• E h? hc/?
• If the frequency (?) increases, the energy (E) .
• If the wavelength (?) decreases, the energy (E)
  .

increases
increases
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THE ELECTROMAGNETIC SPECTRUM
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THE ELECTROMAGNETIC SPECTRUM

• The ultraviolet (UV) region of the
  electromagnetic spectrum includes all wavelengths
  from 10 nanometers (nm) to 380 nm. The
  vacuum-ultraviolet region goes from 10 nm to 200
  nm because air absorbs strongly at these
  wavelengths so instruments must be operated under
  a vacuum in this region. The near-ultraviolet
  region goes from 200 nm to 380 nm.
• The visible (Vis) region goes from 380 nm to 780
  nm and can be seen by the human eye.
• The infrared (IR) region goes from 0.78
  micrometers (µm) or 780 nm to 300 µm. However,
  the near-infrared (0.8 µm to 2.5 µm) and the
  NaCl-infrared regions (2.5 µm to 16 µm) are the
  most commonly used by analytical chemists.

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THE ABSORPTION OF ELECTROMAGNETIC RADIATION BY
MOLECULES

• Humans see color when an object transmits or
  reflects visible light.
• More specifically, an object may absorb specific
  wavelengths of electromagnetic radiation. The
  unabsorbed wavelengths from the visible region
  are transmitted and seen as color.
• For example, leaves are green because the pigment
  chlorophyll absorbs violet, blue, and red light.
• Why is my car blue?
• Its blue because it absorbs yellow.

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THE ABSORPTION OF ELECTROMAGNETIC RADIATION BY
MOLECULES

• There are 3 ways that a molecule can absorb
  electromagnetic radiation. All 3 ways raise the
  molecule to a higher internal energy level. All
  these changes in energy are quantized that is,
  they occur at discrete levels.
• Rotational Transitions The molecule rotates
  around various axes. Rotational transitions
  require the least amount of energy. Purely
  rotational transitions can occur in the
  far-infrared and microwave regions.
• Vibrational Transitions Atoms or groups of atoms
  within a molecule vibrate relative to each other.
  Vibrational transitions require an intermediate
  amount of energy and typically begin to occur in
  the mid-infrared and far-infrared regions.
  Therefore, as energy is increased (or wavelength
  is decreased) vibrational transitions occur in
  addition to rotational transitions.
• Electronic Transitions An electron within a
  molecule is typically promoted from its ground
  state to an excited state. Electronic
  transitions require the most amount of energy and
  typically begin to occur in the visible and
  ultraviolet regions. Therefore, as energy is
  increased (or wavelength is decreased) electronic
  transitions occur in addition to vibrational and
  rotational transitions.

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THE ABSORPTION OF ELECTROMAGNETIC RADIATION BY
MOLECULES
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SCHEMATIC OF A SPECTROPHOTOMETER

• The most common light source for the visible
  region spectrophotometry is a tungsten filament
  incandescent lamp. A tungsten lamp emits useful
  light from approximately 325 nm to 3,000 nm.
• A monochromator uses a prism or a diffraction
  grating to separate polychromatic (many
  wavelengths) light into monochromatic (single
  wavelength) light.
• A cell or cuvette is used to hold the sample
  during analysis.
• The detector uses a phototube or a
  photomultiplier tube to convert light into an
  electrical signal that is sent to a recorder or
  computer.

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THE SPECTRONIC 20D SPECTROPHOTOMETER
The controls.
Loading a sample.
THE HACH DR2010 SPECTROPHOTOMETER
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TRANSMITTANCE AND PERCENT TRANSMITTANCE

• A sample in a cell or cuvette during
  spectrophotometric analysis.
• Po the power of monochromatic light entering
  the sample.
• P the power of monochromatic light leaving the
  sample.
• a the absorptivity constant, which depends on
  the wavelength and the nature of the absorbing
  compound.
• b the path length through the absorbing
  compound.
• c the concentration of absorbing compound in
  the cuvette.

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ABSORBANCE

• The Beer-Bouguer-Lambert law, more commonly
  called Beers law, is
• A abc

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SELECTING ?max FOR SOLUTIONS WITH 1 ABSORBING
COMPOUND

• This is the absorption spectrum for Co(H2O)63.
  The wavelength at the absorbance maximum is
  called ?max. What is the ?max for Co(H2O)63?
• 510 nm.
• Why does ?max give the most sensitive
  measurement?
• It gives the largest response per mole of analyte.

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MIXTURES OF 2 ABSORBING COMPOUNDS

• Where is ?max for substance x? Where is ?max for
  substance y?
• The total absorbance (Atotal) at a given
  wavelength equals the sum of the absorbances for
  all compounds at this wavelength. That is,
• at ?1 Atotal Ax?1 Ay?1 ax?1bcx ay?1bcy
• at ?2 Atotal Ax?2 Ay?2 ax?2bcx ay?2bcy

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CASE STUDY UV/Vis SPECTROSCOPY AND THE FOX RIVER
MYSTERY

• In 1988 over 30,000 fish died suddenly and
  unexpectedly in the Fox River at Oshkosh,
  Wisconsin. Such fish kills are often caused by
  a lack of dissolved oxygen (O2), or a release of
  pesticides, organic compounds, chlorine (Cl2), or
  heavy metals into the environment. However, none
  of these caused the Fox River fish kill.

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CASE STUDY UV/Vis SPECTROSCOPY AND THE FOX RIVER
MYSTERY

• Finally, it was suggested that carbon monoxide
  (CO) gas from outboard motor exhaust at a testing
  facility might be causing this fish kill.
  Normally, O2 weakly bonds to the iron (Fe) atom
  in fish hemoglobin during respiration. However,
  CO tightly bonds to this Fe atom and as a result
  stops respiration.

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CASE STUDY UV/Vis SPECTROSCOPY AND THE FOX RIVER
MYSTERY

• The hypothesis that CO was causing this fish kill
  was tested by UV/Vis spectroscopy.
• In review, UV/Vis spectroscopy measures the
  absorption of electromagnetic radiation caused by
  electronic transitions within atoms and
  molecules. Different atoms and molecules will
  have different UV/Vis spectra.

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TODAYS EXPERIMENT

• Work in groups of 4.
• Every student does 1 quantitative analysis.
• Every student in each group gets a different
  color (blue, purple, red, or yellow) of crepe
  paper.
• Each student selects ?max from an absorption
  spectrum of their crepe paper.
• Each student makes 4 standard solutions by
  extracting different amounts of dye from their
  crepe paper.
• Each student uses these solutions and Beers law
  to make a calibration curve.
• Each student uses this calibration curve to
  measure the concentration of an unknown solution
  that was made from their assigned color of crepe
  paper.
• Every group does 1 qualitative analysis.
• Each group analyses the absorption spectrum from
  another unknown solution that was made by
  extracting the dye from 2 different colors of
  crepe paper. They will determine which 2 colors
  were used to make this solution.

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SOURCES

• Aquanic. 2006. Fishkill. Available
  http//aquanic.org/images/photos/ill-in/fishkill.j
  pg accessed 2 September 2006.
• Beck, J. 2006. Unit 3 Spectrophotometry.
  Available http//iws.ccccd.edu/jbeck/Spectrophoto
  metryweb/Page.html accessed 2 October 2006.
• Christian, G.D. 1986. Analytical Chemistry, 3rd
  ed. New York, NY John Wiley Sons, Inc.
• Harris, D.C. 1999. Quantitative Chemical
  Analysis, 5th ed. New York, NY W.H. Freeman
  Company.
• Raven, P.H., R.F. Evert, H. Curtis. 1981. Biology
  of Plants, 3rd ed. New York, NY Worth
  Publishers, Inc.
• Spencer, J.N., G.M. Bodner, L.H. Rickard. 2006.
  Chemistry Structure and Dynamics, 3rd ed. New
  York, NY John Wiley Sons, Inc.
• Wikipedia. 2006. ImageLight-wave.png. Available
  http//en.wikipedia.org/wiki/ImageLight-wave.png
  accessed 2 September 2006.