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Understanding Spectroscopy: The Interaction of Light and Matter

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This chapter explores the fundamental principles of spectroscopy, where light, as photons and waves, interacts with matter. Key concepts include frequency, wavelength, and the speed of light, with discussions on how light's energy is measured and absorbed by samples. Learn about concepts such as transmittance, absorbance, and Beer-Lambert Law, which relates absorbance to concentration and path length. Additionally, various light sources, measurement techniques, and different types of spectroscopy like infrared and fluorescence are introduced. This foundational knowledge is crucial for applications in analytical chemistry.

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Understanding Spectroscopy: The Interaction of Light and Matter

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Presentation Transcript

  1. Chapter 18 Spectroscopy

  2. 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

  3. 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

  4. Electromagnetic Spectrum

  5. The Interactions

  6. Measuring the Interaction • Irradiance, P • 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

  7. The basic measuring device

  8. 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

  9. 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 _______________ ??

  10. Light Source • Can be a simple lamp • Mercury Discharge tube. • Sun

  11. 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.

  12. Detection • Phototube. • Converts light energy into an electric current • Photodiode array • Eye

  13. Sample Holder

  14. 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.

  15. Example

  16. Example (ferrozine3Fe(II))

  17. Calibration

  18. Other Types of Spectroscopy • Infrared - Bond vibrations • UV – higher energy than visible • Fluorescence – reemission of a photon at a different wavelength.

  19. Homework • Chapter 18 • 1 ,2, 4, 7, 10 and 16

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