1. Instrumental Chemistry
Chapter 6
Introduction to Spectroscopic Methods

2. Introduction
Absorbance can be defined as the base-ten logarithm of the reciprocal of the transmittance :
A = log 1/T = -log I/Io = -log P/Po

3. Reflection and scattering losses are significant

4. An experimental absorbance that closely approximates the true absorbance is then obtained with the equation
A = log Psolvent/Psolution » log Po/P

5. Beer’s Law
The fraction of the energy, or the intensity, of radiation absorbed in a thin layer of material depends on the absorbing substance and on the frequency of the incident radiation, and is proportional to the thickness of the layer.

6. A parallel beam of monochromatic radiation with power Po strikes the block perpendicular to a surface after passing through a length b of the material , which contains n absorbing particles , the beam’s power is decreased to P as a result of absorption.

7. Chemical Deviations:
Chemical deviations from Beer’s law are caused by shifts in the position of a chemical or physical equilibrium involving the absorbing species. A common example of this behavior is found with acid/base indicators. Deviations arising from chemical factors can only be observed when concentrations are changed.

8. Polychromatic radiation:
When radiation consists of two wavelengths, l and l1, and assuming that Beer’s law applies at each of these individually the absorbance at l is given by
log ( Po/P ) = A = ebc
Þ Po/P = 10ebc

9. Useful Websites Dealing With Spectroscopy
Chemical Abstracts Service:
Chemical Center Home Page:
Journal of Chemistry and Spectroscopy:
The Analytical Chemistry Springboard at Umea U.
Spectrum Chromatography: