Page 68 - Tandem Techniques

Page 49

This problem is particularly difficult when liquid chromatography is employed as the separation
technique, but not such a problem with gas chromatography, as many gases are transparent to light of a
wavelength significantly below 180 nm. However, there are other practical difficulties that are met
when working in this wavelength range. The walls of the vessel or device in which the UV light is
generated may also be opaque to light of such wavelengths and transmission can be very poor. In
practice, the lower wavelength limit for tandem techniques is at present about 150 nm and most UV
spectroscopic measurements are carried out at wavelengths between 180 nm and 400 nm. The energy
carried by an electromagnetic wave is not continuous, but propagated in finite parcels called quanta.
The relationship between the radiation energy and wavelength is given by:

Where (E) is the energy of the radiation,
(l) is the wavelength of the radiation,

(c) is the velocity of light,
and (h) is Planks constant.

Radiation is only adsorbed by a substance when the energy of the radiation (as given by equation (2.2))
corresponds to the that needed to increase the potential energy of the substance by one or more
increments. The transfer of energy is achieved by the interaction of its electric vector with the
substance. Adsorption of UV/visible radiation changes the electronic state of a molecule and can, for
example, raise an electron from the ground state to one of its excited states. The electronic ground state
is one in which all of the electrons of the species are in their most stable orbitals. An electronic excited
state is one in which at least one of the electrons occupies a orbital of higher energy than that of the
ground state.

It would appear that if the radiation passing through a substance was programmed with respect to
wavelength, and the transmitted light monitored by an appropriate sensor, then the curve resulting from
the