chapter 18

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