Low Resolution Nuclear Magnetic Resonance

When a sample containing magnetic nuclei such as hydrogen protons is placed in a static magnetic field, B0, the nuclei align with the field and a bulk nuclear magnetisation develops. This process occurs exponentially in a characteristic time T1( the spin lattice relaxation time such that

where M(t) is the magnetisation at time t and M0 is the equilibrium magnetisation. In almost all food systems commonly encountered Tj values are of order 1.0 s. If the magnetisation is disturbed from the equilibrium direction, it precesses about the static magnetic field at the Larmor frequency given by

where y is the magnetogyric ratio of the nuclei in question. For hydrogen protons, y/27t = 42.57 Mhz/T. A suitable stimulus to initiate precession is a resonant radio frequency pulse called a 90° pulse applied via an excitation coil around the sample. The precessing magnetisation induces a transient response in the coil which is detected and demodulated and known as the free induction decay (FID) signal. The initial intensity of the signal, I0, is proportional to the number of nuclei in the sample.

Since the individual nuclear magnetic moments experience not only the applied field but also local magnetic fields, B](K. due to their molecular environment they precess at the local frequency

Variations in Bloc cause different nuclei to precess at different rates and the initial coherence of the nuclear magnetisation is lost. With the loss of coherence (dephasing) goes the loss of observed signal. For all but the very fastest decays which usually exhibit complex decay functions, this generally occurs exponentially in a characteristic time T2, the spin-spin relaxation time so that

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