Page 66 - Tandem Techniques

Page 47

The wavelength of the electromagnetic radiation employed in NMR spectroscopy ranges from 3 m to 15
m which extends into the radio wave region.

So far, all the spectroscopic techniques mentioned are based on the measurement of the adsorption of
electromagnetic radiation similar in nature to that of visible light, but having different wavelengths and
frequencies. Mass spectrometry, however, is the exception and this technique should not, on a rational
basis, be termed a spectroscopic technique. Nevertheless, partly because mass spectra have some
similarities to other types of spectra, and partly for convenience, it has been included with other forms
of spectroscopy in the group of analytical methods known as spectroscopic techniques.

All spectroscopic techniques (with the exception of mass spectrometry) depend on the adsorption of
waves similar to those of visible light. Electro-magnetic waves differ only in frequency and wavelength.
They all travel at the same speed in a vacuum (that of light) and thus the product of the frequency and
wavelength, for any given light wave, is a constant and equal to the speed of light. Physically, an
electromagnetic wave could be considered to consist of a sinusoidal electric field, acting at right angles
and in phase with a sinusoidal magnetic field. It is the electric vector of the electromagnetic wave form
that provides the energy that is measured by spectroscopic instruments. The electric and magnetic fields
interact with matter in ways that are unique to the chemical nature of the substance through which they
pass. Consequently, the magnitude and nature of this interaction can provide a basis for identifying the
chemical structure of the interacting substance.

Electromagnetic radiation of different frequencies interact with different parts of a molecule to differing
extents. Thus each of the spectroscopic techniques that operate over different ranges of frequency can
provide information that is unique to some specific part of the molecule of which the interacting
material is composed. Of course, radiation of some wavelengths may not interact at all with the medium
through which it is