Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > The Characteristics of UV Absorption > Page 14

The different regions can provide absorption spectra for identification when compared with the spectra from a standard, but the two regions provide very different information with respect to structure elucidation.

The different regions can provide absorption spectra for identification when compared with the spectra from a standard, but the two regions provide very different information with respect to structure elucidation.

 

 

The Characteristics of UV Absorption

 

A substance adsorbs radiant energy when the radiation energy is equivalent to that required to increase the potential energy of the material by one or more increments. In the case of UV light the changes will be electronic in nature, e.g., raising the electronic state of an atom from its ground state (the state where all the electrons of the species are in their most stable orbitals) to one where at least one electron occupies a higher energy orbital than in the ground state.

 

A substance may have a number of excited states and, taking the sodium atom as an example, consider two photons of light, one where l=330 nm and the other where l=589 nm each striking a sodium atom and being adsorbed. This will raise the ground level energy (Ao) to energy levels of (A2) and A1) respectively.

 

Thus, as the photon energy is inversely proportional to the wavelength, (equation 3), then

 

Ao + photon589nm = A1

 

Ao + photon330nm = A2

 

where A1 < A2