The protons and neutrons that make up every atomic nucleus behave not like the tiny ping-pong-ball like structures taught in school, but more like gyroscopes that spin about their axes in random directions, generating their own minute magnetic fields. Felix Bloch and Edward Purcell demonstrated how manipulating and analysing the movement of these subatomic spinning tops could be useful in identifying the structure of molecules in solids and liquids.
Bloch and Purcell's discoveries were a major advance on the groundbreaking research by Isidor Isaac Rabi, who first established radio contact with the magnetic properties of atomic nuclei. When exposed to an external magnetic field, the normally randomly aligned gyroscope-like particles all line up in particular orientations. Rabi showed that radio waves can force these magnetic atomic nuclei to flip or change their orientation, releasing energy when they return back to their normal state – a phenomenon known as nuclear magnetic resonance. Magnetic nuclei from different chemical elements flip at different and unique combinations of magnetic field strength and radio frequency, so knowing the field strength and frequency at which these flips occur reveals the identity of the atoms.
Rabi's findings were in the artificial environment of atomic beams in a vacuum, but working independently on hydrogen atoms, Bloch and Purcell developed the technique so that it could be applied to liquids and solids. Their breakthrough provided researchers with the potential to place any solid or liquid in an electromagnetic field, expose it to radio waves, and identify the specific atoms that make up the chemical, without affecting it in any perceptible way. Such a degree of chemical analysis had been unattainable until then – a statement based on substance rather than spin.