Chapter 18

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# Chapter 18 - PowerPoint PPT Presentation

Chapter 18. Spectroscopy. Interaction of Light and Matter. Light = photons waves Oscillating electric and magnetic fields Frequency ( n ) - number of full oscillations per second (sec -1 ) or Hertz

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## Chapter 18

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### Chapter 18

Spectroscopy

Interaction of Light and Matter
• Light = photons waves
• Oscillating electric and magnetic fields
• Frequency (n) - number of full oscillations per second (sec-1) or Hertz
• As light travels thru space – distance from crest to crest is the wavelength l (cm)
• The relations between the two is
• nl = c
Properties
• Speed of light
• c = 2.998 x 108 m/sec in vacuum
• Light will slow down in other media
• vx = c/n
• n is the refractive index of the media
• Energy of light
• E = hn
• h is Planck’s constant 6.626 x 10-34 J.sec
Measuring the Interaction
• The energy per second per unit area of a light beam.
• When a sample is in the light beam some energy is absorbed and less energy gets past the sample
Transmittance
• The fraction of the of light that gets to the detector.
• T = P/Po
• Po is the power that would arrive at the detector without a sample in the light path
• Percent T (%T) is just T * 100
Absorbance
• A = log(Po/P) = - logT
• Beer’s Law (Beer Lambert Law)
• A = ebC
• A is the abosorbance - unitless
• b is the path length - usually in cm
• C is the concentration - usually in M
• e is a measure of the compounds ability to absorb light – depends on chemical structure and will have units that are _______________ ??
Light Source
• Can be a simple lamp
• Mercury Discharge tube.
• Sun
Wavelength Selection
• Filter
• Prism
• Grating

The latter two break white light into a spectrum of color.

• Laser could serve as both source and wavelength selector.
Detection
• Phototube.
• Converts light energy into an electric current
• Photodiode array
• Eye
Method Development
• “Colored” compounds can be directly analyzed.
• “Colored” complexes can be made.
• Many metal analyses can be developed using color forming ligand. Colorimetry. Very mature field with good selectivity and good sensitivity.
Other Types of Spectroscopy
• Infrared - Bond vibrations
• UV – higher energy than visible
• Fluorescence – reemission of a photon at a different wavelength.
Homework
• Chapter 18
• 1 ,2, 4, 7, 10 and 16