Fast Field Cycling Relaxometry (FFCR) is a NMR technique used to determine the longitudinal relaxation time (T1) over a range of magnetic fields spanning about 6 decades, from about 10e-6 up to ~1 Tesla.

• The boundaries of this range are not well defined: the lower limit is set by the local fields (more or less motionally averaged); the upper limit is chiefly determined by technical choices and compromises.
• This enormous range should be compared with the 0.1 T-20T interval currently covered by standard NMR supercon magnets and variable-field electromagnets.However, studies of T1 dispersion curve with an array of standard magnets is impractical, and the usefulness of T1( is limited by subtle differences relative to T1 and technical problems at high B1 fields (overheating of the sample, phase shifts of the transmitter during long pulses).

On the other hand, FFC relaxometry requires a specialized system, which does not compete with the sensitivity and resolution of most NMR spectrometer.

Since the very beginning of Nuclear Magnetic Resonance there had been great interest in relaxation phenomena, including their field dependence.
Starting in the fifties, methods for acquiring T1 dispersion profiles (plots of longitudinal relaxation time as a function of field intensity at a constant temperature) had been used.

In such methods a high magnetic field (polarization field) is applied to pre-polarize the sample in order to boost signal intensity; thereafter, the sample is allowed to relax in a second field (relaxation field) which can be set to any desired value, including zero. As the shortest measurable T1 is directly depending on the field switching time, it became soon evident that faster and more flexible electronic methods for switching the magnetic field would have to be applied. This implied the development of low-inductance, air-coil magnets and power supplies capable of switching the field electronically to any desired value in a matter of milliseconds while, at the same time, maintaining the high field stability and homogeneity required by NMR.

This approach, known as Fast Field Cycling, has been tried with success in several laboratories (S.H.Koenig and Brown at IBM, F.Noack and at the University of Stuttgart).
The few pioneering groups who had access to this type of instrumentation exploited the tight link between NMR relaxation phenomena and molecular dynamics to explore at least three possible application fields: the hydration of paramagnetic metal ions and organometallic complexes, the dynamics of liquid crystals, and the dynamics of proteins.

The wealth of information obtained from these studies, together with the awareness that many more application fields remain totally unexplored, led to the recent dramatic increase of interest in FFC NMR relaxometry.
These early studies have confirmed the theoretical work of many distinguished physicists, linking NMR relaxation phenomena to specific stochastic aspects of molecular dynamics.

The main information expected from the relaxation dispersion curves concerns motions characterized by temperature-activated frequencies.
In the example of the figure we show the Arrhenian plot of the relaxation rate (1/T1) expected in the presence of two motions with Lorentzian spectral densities:

• a "collective" motion with attempt frequency natt = 109 Hz, activation energy Ea = 6 kJ/mol, which modulates 20% of the rigid lattice line width
• a "diffusional" motion with natt = 1012 Hz and Ea = 30 kJ/mol.

In practice, FFCR is a convenient and, sometimes, unique method to follow, over extended temperature intervals, the dynamics of "coupled" systems such as liquid crystals, polymers, biomolecules, viscous fluids and glasses, solid electrolytes and, in general, solids with "slow" or correlated motions.

The figure shows a simulation of the spin-lattice relaxation rate (R1) as a function of temperature (T) (BPP model). Curves with red refer to the frequency range covered by FCNMR spectrometer. The blue area refers to the higher frequencies covered by the conventional High resolution NMR spectrometer.

The calculated quantity shown in the Figure is proportional to the R1 (relaxation rate) under the assumptions that:

• only dipolar interactions are present
• the isotropic BPP (Bloembergen-Purcell-Pound) formula may be used and
• the molecular dynamics is dominated by two processes of the Arrhenius type with some reasonable parameters.

The simulation illustrates the fact that relaxation data taken at low fields are more sensitive to molecular dynamics models than those measured at high fields.

Moreover, an FFC profile measured at a fixed sample temperature represents a vertical cut in the 2D surface. Repeating the measurement at different temperatures, the whole surface is sampled.
On the opposite, on traditional instruments one samples only one of the lines corresponding to a constant frequency.