Page 101 - Tandem Techniques

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the intensity of the light falling onto the photomultiplier changes. By the use of a phase sensitive
amplifier, and electrical feed-back, the current though the modulator is automatically adjusted until no
AC component appears on the photomultiplier output. The feed-back current is then taken as a measure
of the optical rotation of the sample.

Nuclear Magnetic Resonance Spectroscopy

In order to understand the technique of nuclear magnetic resonance (NMR), it is necessary to consider
the magnetic properties of atomic nuclei. The nucleus of an atom spins and thus, if the charge is not
symmetrically placed on the atom, the spinning charge will constitute a circular current which will
produce an associated magnetic field like a small bar magnet. Not all nuclei possess an asymmetric
charge but among those that do are the hydrogen and the 13C nuclei, which are the nuclei of major
importance for NMR spectroscopy.

Figure 2.24
A Diagram of a Precessing Nucleus
If the spinning nucleus, with its associated magnetic field, is placed in a strong external magnetic field
the external field will act upon the spinning nucleus to try to change its spinning axis to be in line with
the magnetic field. Now according to Newton's law, when a force acts upon a spinning body to change
its axis of rotation then, to conserve the angular momentum, the spinning body will precess. The
precessing nucleus is depicted diagramatically in Figure 2.24.