FFC Relaxometry of Poly-Dispersed Samples: some Special Aspects

Talk by  Stanislav Sykora and Ester Maria Vasini, Extra Byte, Castano Primo, Italy

Presented at 11th FFC-NMR Relaxometry Conference, Pisa (Italy), June 5-7, 2019.

DOWNLOAD the talk slides: PDF This page DOI: 10.3247/SL7Nmr19.003 Stan's Library | Stan's HUB

Please, cite this online document as:
Sykora S., Vasini E.M., FFC Relaxometry of Poly-Dispersed Samples: some Special Aspects,
Talk at 11th FFC-NMR Relaxometry Conference, Pisa (Italy), June 5-7, 2019.
DOI: 10.3247/SL7Nmr19.003.


Field cycling NMR relaxometry is often used to measure the characterize solid and semi-solid samples which can be at least approximately characterized by a single longitudinal relaxation time T1. The focus in these cases is on the field dependence of the T1, i.e., just the apparent dispersion curve.
However, many system of great importance both in life and materials sciences are not mono-exponential, either because they are chemically or morphologically heterogeneous or because, even though apparently homogeneous, they exhibit signal components relaxing at different rates.
When measuring such systems on instruments with fixed magnetic fields, one normally collects particularly large amounts of data (for example, hundreds of tau values in experiments like IR and SR) in order to characterize in great detail the empirical relaxation curves, and then attempts (with well known mathematical difficulties) to extract from those data the desired quantitative distributions of the relaxation times within the sample.
Carrying out the same procedure on a field cycling instrument, with the goal of acquiring the relaxation time distributions at different field strength, while highly desirable, encounters various obstacles. Some are just annoying but otherwise relatively straightforward. In particular, this regards the fact that data acquisition times need to be much longer than under the simplified mono-exponential assumption. Other would be problems are more subtle and are related to the fact that the various sample 'components'
    (i) have different relaxation rates and therefore exhibit different attenuation factors during the FC sequences. This leads to apparent distortions of the relaxion times distributions.
    (ii) they also exhibit different field dependencies of their relaxation rates and therefore their relaxation times distributions vary with magnetic field in a complex way which defies a simple analysis of the results.
In this presentation we focus in particular on point (i) trying to set up a theoretical and computational framework within which the distortions can be understood and possibly even corrected. Point (ii) will be discussed, but only in a qualitative way (work in progress).
[1] E. M. Vasini, Stanislav Sykora, Nuclear Magnetization Evolution During the Switching Time in Field Cycling NMR, Poster presented at 14th MRPM, Gainesville (FL, USA), February 18-22, 2018. DOI 10.3247/SL7Nmr.001.

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