Aihara, Y.

Price, W. S.

Willis, S. A.

English

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The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application   Towards accurate diffusion measurements of slowly diffusing species  W.S. Price1*, S.A. Willis1, Y. Aihara2  1Nanoscale Organisation and Dynamics Group, Western Sydney University, Penrith, NSW, Australia 2Samsung R&D Institute Japan, Osaka, Japan *w.price@westernsydney.edu.au  NMR-based diffusion measurements are now extremely common place and are generally seen as the method of choice for measuring self-diffusion due to their non-invasive nature and general applicability including no specific concentration requirements, minimal sample preparation and speed. Further, as motion is measured over a specific timescale the NMR diffusion measurements can also report on geometries within which diffusion is occurring [1-3]. Most contemporary NMR spectrometers are now able to generate modest pulsed magnetic field gradients (e.g., gradient amplitudes ≤ 0.5 T m-1) suitable for performing measurements on low viscosity/rapidly diffusing samples (e.g., D > 1 × 10-11 m2s-1) using predominantly 1H as the probe nucleus with reasonably long relaxation times. The nature of the diffusion to be measured is normally a priori clear cut (e.g., isotropic or anisotropic, Gaussian or restricted). Such measurements are generally straightforward and typically make only modest demands on spectrometer performance and the gradient generation hardware. However, many samples of interest are by nature more viscous or even solid samples (e.g., solid phase electrolytes) with less initial certainty regarding the nature of the diffusion to be measured. Further, the probe nucleus may be a less favourable isotope than 1H with an inherently much shorter relaxation time. Such measurements require special high current amplifiers and special NMR probes capable of generating high gradient pulses. Some commercial equipment is now capable of generating pulses in excess of 20 T m-1. Performing measurements with such pulses can push a spectrometer to its performance limits such that it becomes difficult to separate artefact from reality and some aspects have long been recognised (e.g., ref. [4]). In fact, even the calibration of such equipment is not straightforward [5, 6]. This presentation details the complications of high gradient diffusion measurements, methods for obviating the problems and our progress in developing a protocol that will allow definition of a usable range of measurement variables (e.g., maximum gradient strength that allows reproducible results). Such a protocol would facilitate the comparison of results between different machines.  References [1] J. Kärger, H. Pfeifer, W. Heink: Principles and applications of self-diffusion measurements by nuclear magnetic resonance, Adv. Magn. Reson., 12, 1-89 (1988). [2] W.S. Price: NMR Studies of Translational Motion, Cambridge University Press, Cambridge (2009). [3] P.T. Callaghan, Translational dynamics & magnetic resonance, 1st ed., Oxford University Press, Oxford (2011). [4] W.S. Price, K. Hayamizu, H. Ide, Y. Arata: Strategies for diagnosing and alleviating artifactual attenuation associated with large gradient pulses in PGSE NMR diffusion measurements, J. Magn. Reson., 139, 205-212 (1999). [5] N.N. Yadav, A.M. Torres, W.S. Price: An improved approach to calibrating high magnetic field gradients for pulsed field gradient experiments, J. Magn. Reson., 194, (2008) 25-28. [6] K.R. Harris, B. Ganbold, W.S. Price: Viscous calibration liquids for self-diffusion measurements, J. Chem. Eng. Data, 60, 3506-3517 (2015).  This work is licensed under a Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/1

Towards accurate diffusion measurements of slowly diffusing species

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Towards accurate diffusion measurements of slowly diffusing species

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