Chapter 18 Spectroscopy

Interaction of Light and Matter Light = photons waves Oscillating electric and magnetic fields Frequency ( ) - number of full oscillations per second (sec -1 ) or Hertz As light travels thru space – distance from crest to crest is the wavelength  cm) The relations between the two is  c

Properties Speed of light c = x 10 8 m/sec in vacuum Light will slow down in other media v x = c/n n is the refractive index of the media Energy of light E = h h is Planck’s constant x J. sec

Electromagnetic Spectrum

The Interactions

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

The basic measuring device

Transmittance The fraction of the of light that gets to the detector. T = P/P o P o 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(P o /P) = - logT Beer’s Law (Beer Lambert Law) A =  bC A is the abosorbance - unitless b is the path length - usually in cm C is the concentration - usually in M  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

Sample Holder

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.

Example

Example (ferrozine 3 Fe(II))

Calibration

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