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 = 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 = h
h is Planck’s constant 6.626 x 10-34 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/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 = 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 (ferrozine3Fe(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