Theory

Introduction

Atoms consist of a nucleus and at least one electron, both of which have an electric charge. Some nuclei also have an associated spin. The combination of charge and spin gives rise to a magnetic field so a spinning nucleus can be seen to act like a tiny magnet (the same principles are used in reverse to make electric motors). When these nuclei are placed in a strong magnetic field they all line up in one of two directions, either with the field or against the field. It requires more energy for these to oppose the magnetic field than to be in line with it, as can be experienced by holding two magnets in opposite and similar directions. This leads to two possible energy levels that the nuclei can occupy.

Example of energy level splitting due to an applied magnetic field

Resonance

If a nucleus is given exactly the right amount of energy, it can be promoted from the lower energy (ground state) to the higher energy (excited state). This nucleus will then drop from the higher energy to lower energy by emitting exactly the same amount of energy as heat. When a nucleus undergoes this cycle of excitation and relaxation it is said to be 'in resonance', hence nuclear magnetic resonance.

Absorption

The amount of energy required to bring a nucleus into resonance depends on the isotope and the strength of the magnetic field. The stronger the applied field, the larger the energy difference between the two states. This means that higher frequency, hence higher energy, radiation is needed for resonance. For the magnetic fields used in typical spectrometers this energy corresponds to electromagnetic radiation in the radio frequency range.