extinction coefficient is wave length specific. explain!?

Question

extinction coefficient is wave length specific. explain!

Answer 1

First remember remember the Beer-Lambert relation:

A = ebc

where A is the absorbance , e is the extinction coefficient, b is the path-length of light and c is the concentration of the solution.

All absorption/emission spectra of molecular substances are complex curves that depend on the structure of the molecule.

Absorption of light by molecules depends on what chemical moieties are present. We would expect this phenomenon to occur as predicted by quantum mechanics. Different chemical parts will absorb different wavelengths of light, so conjugated C-C double bonds will absorb one energy of light, while a C-N triple bond will absorb at another.

Think about a typical UV absorption (or IR for that matter). The strength of the signal is different at each wavelength.

Lets say the rule did not hold. Then all spectra of all molecules would be identical flat lines and we would not be able to tell what chemicals were present from spectral analysis.

This is why in an analytical experiment, you usually want to choose a wavelength with a fairly good e for the species you are analyzing for, especially if you are doing trace analysis.