The extraction of physical information from any experiment relies on two unavoidable and intimately connected essential aspects: the experiment itself and data processing [1]. The former pertains to both the hardware specifications and the experimental protocol that defines the measurement quality, whereas the latter is only meaningful when supported by a validated physical model or, alternatively, by carefully constrained model-free approaches.

In the context of frequency-domain NMR relaxometry, the fast field-cycling (FFC) technique [2,3] offers clear and well-established advantages over other methodologies, such as rotating-frame relaxometry [4]. A key strength of FFC lies in the exceptionally broad range of Larmor frequencies that can be accessed through controlled cycling of the static magnetic field B0​. Depending on the instrumental implementation, this approach enables the exploration of Larmor frequency windows spanning several orders of magnitude in the laboratory-frame. Moreover, both the fastest and slowest relaxation processes that can be probed also depend on the associated instrumentation.

In this contribution, we briefly discuss an illustrative example in which a single FFC NMR experiment enables the simultaneous estimation of the elastic bending modulus of liposome membranes and the diffusion dynamics of their constituent lipid molecules. This case study highlights the critical interplay between the experimental design, the model-based data analysis and the pertinence of the used fitting engine. We conclude by briefly commenting on the broader implications of these considerations for FFC NMR relaxometry studies in porous media.


References

[1]- M. B. Marzola Coronel, L. Montes, E. A. Pilotta and E. Anoardo, Dynamic range and noise considerations in pattern search optimal fitting of experimental data: a case study using field-cycling NMR relaxometry of liposomal formulations, Journal of Physics D: Applied Physics, 2025, 58, 355305.
[2]- R. Kimmich and E. Anoardo, Field-cycling NMR Relaxometry, Progress in NMR Spectroscopy, 2004, 44, 257-320. 
[3]- New developments in NMR: Field-cycling NMR Relaxometry. Instrumentation, Model Theories and Applications, R. Kimmich Ed., RSC, Croydon, 2019.
[4]- K. Tolkkinen, S. E. Mailhiot, A. Selent, O. Mankinen, et al., SPICY: a method for single scan rotating frame relaxometry, PCCP 2023, 25, 13164