The Hydrogen nucleus is composed of a single proton. It has positive charge, and a property known as nuclear spin. The combination produces a magnetic moment (m). In the bulk of a sample, hydrogen atoms are randomly oriented.

The magnetic moment of hydrogen nuclei causes them to behave like tiny compass needles. When placed in a magnetic field the orientation of the hydrogen nuclei change so that their magnetic moments are aligned either with OR against the magnetic field (B₀). A slight excess align with the applied magnetic field (B₀) creating a net magnetization (M_Z) in the sample. The magnetic moments actually precess about the direction of the magnetic field, like a spinning top, at a frequency dependent on the strength of the magnetic field.

The Resonance Experiment

Since the external magnetic field (provided by a permanent magnet) is much greater than the sample's net magnetization, it is not possible to detect the sample's small magnetization in this state. The sample magnetization must be separated from the external magnetic field by applying a secondary magnetic field perpendicular (at a 90° angle) to the external field. A Radio Frequency (RF) pulse applied along the +X axis instantaneously creates an secondary magnetic field that causes the magnetic moments to tip away from the equilibrium state.

The length of the pulse determines how far. A pulse length that inverts the magnetization is called a 180 degree pulse. The combined precession of many spins generates a small but detectable excess oscillating magnetic field in the X-Y plane perpendicular to the external magnetic field. These oscillations induce an alternating voltage (the NMR signal) in a detection coil of the minispec electronics.

The Relaxation Process

The hydrogen nuclei (protons) are now in an excited state. After the pulse the protons in the sample exchange energy with each other and with their surroundings. The latter causes them to relax back to their equilibrium state (either aligned mainly with or against the magnetic field). After the pulse, the signal is detectable for milliseconds to seconds. The NMR signal decays because of relaxation. The decaying signal is called the FID (free induction decay). In solid matter, oscillations are heavily damped, and the signal decays relatively quickly. In a liquid, however, the surroundings are more mobile, thus causing less damping and a slower decay of the signal.  The FID signal shape can therefore be used to distinguish between solid and liquid components of a sample.