Selective diffusion spectroscopy using excitation sculpting

Abstract

Diffusion spectroscopy NMR provides a sensitive and fast way of determining diffusion coefficient. The coefficient is measured by fitting attenuation of resonance intensities to the Stejskal-Tanner equation, but, because it is an exponential equation, this fitting is quite sensitive to experimental artefacts. Intense resonances in NMR spectra, such as solvent signals, are a particular problem because small fractions of intensity of the intense resonances can significantly change the intensities of minor resonances and distort their calculated diffusion coefficients. This problem can be overcome by selective excitation of only the minor resonances, and the method of choice for this is excitation sculpting. This paper shows that the conventional excitation sculpting pulse sequence can be used directly for DOSY spectroscopy with minimal modification. The sequence suppresses intense resonances very effectively, allowing the reproducible measurement of diffusion coefficients of minor solutes in a solvent containing five separate resonances. It is sensitive and robust to convection. Excitation sculpting will allow the application of diffusion spectroscopy to samples which include multiple intense resonances, such as polymeric solvents. Copyright © 2015 John Wiley & Sons, Ltd.

Selective excitation enables encoding and measurement of multiple diffusion parameters in a single experiment

“…A key difference compared to previous selective diffusion reports is the encoding of information into individual resonances. This is in contrast to the method described by Howe (Howe (2017)), who demonstrated diffusion measurements by selective excitation of four resonances. Since every resonance was exposed to the same experiment parameters, all four resonances could be simultaneously excited resulting in a net gain in time per scan compared to the experiment described here.…”

“…There are two additional benefits to integrating selective elements to the experiment: first is the elimination of dominating, uninteresting signals (typically solvent) to improve sensitivity to minor components. This has been demonstrated for protein diffusion in water using a selective version of the stimulated echo experiment (Yao et al (2014)) and a selective version of the spin echo experiment for measuring minor components in a mixture (Howe (2017)). The second benefit, noted by both Yao and co-workers (Yao et al (2014)) and Howe (Howe (2017)), is to reduce spectral congestion and thereby improve DOSY data analysis, an active field of research in extracting diffusion coefficients from overlapping signals (Aguilar et al (2010); Colbourne et al (2011);Foroozandeh et al (2018); Lin et al (2020)).…”

Selective excitation enables encoding and measurement of multiple diffusion parameters in a single experiment

“…There are two additional benefits to integrating selective elements to the experiment: first is the elimination of dominating, uninteresting signals (typically solvent) to improve sensitivity to minor components. This has been demonstrated for protein diffusion in water using a selective version of the stimulated echo experiment (Yao et al, 2014) and a selective version of the spin echo experiment for measuring minor components in a mixture (Howe, 2017). The second benefit, noted by Yao et al (2014) and Howe (2017), is to reduce spectral congestion and, thereby, improve diffusion ordered spectroscopy (DOSY) data analysis, an active field of research in extracting diffusion coefficients from overlapping signals (Aguilar et al, 2010;Colbourne et al, 2011;Foroozandeh et al, 2018;Lin et al, 2020).…”

19F NMR matrix‐assisted DOSY: a versatile tool for differentiating fluorinated species in mixtures

“…Overlapping signals are very common in 1 H NMR spectra, particularly of mixtures, and by extension in 1 H DOSY. There are many strategies that can be used to avoid signal overlap, such as suppression of multiplet structure (pure shift methods) [26][27][28] , dispersing signals in multiple spectral dimensions [29][30][31][32] , using selective pulses [33] , or using other nuclei such as 13 C [34,35] , 29 Si [36] , 31 P [37,38] , 19 F [39][40][41] 6 Li [42] or 7 Li [43][44][45][46] for DOSY experiments. In DOSY, signals from different compounds are separated according to their diffusion coefficients, which depend on compound size and shape.…”

“…Overlapping signals are very common in 1 H NMR spectra, particularly of mixtures, and by extension in 1 H DOSY. There are many strategies that can be used to avoid signal overlap, such as suppression of multiplet structure (pure shift methods) , dispersing signals in multiple spectral dimensions , using selective pulses , or using other nuclei such as 13 C , 29 Si , 31 P , 19 F 6 Li or 7 Li for DOSY experiments.…”