Abstract
We have investigated several building stone materials, including minerals and rocks, using continuous flow hyperpolarized xenon (CF-HP) NMR spectroscopy to probe the surface composition and porosity. Chemical shift and line width values are consistent with petrographic information. Rare upfield shifts were measured and attributed to the presence of transition metal cations on the surface. The evolution of freshly cleaved rocks exposed to the atmosphere was also characterized. The CF-HP 129Xe NMR technique is non-destructive and it could complement currently used techniques, like porosimetry and microscopy, providing additional information on the chemical nature of the rock surface and its evolution.
Highlights
The development of optically pumped xenon nuclear magnetic resonance (NMR) that exploits the diffusion of highly nuclear spin-polarized xenon gas on the surface or through samples [1,2]initiated completely new fields of application for the established 129Xe NMR: from hyperpolarized biosensors [3] to lung imaging [4], from combustion studies [5] to the characterization of single crystal inner surfaces [6,7,8] and polymer morphology [9]
Samples were shaped as regular rectangular cuboids (50 × 6 × 6 mm, see experimental section for more details) in order to fit in the NMR detection coil
We presented continuous flow HP 129Xe NMR spectra of several minerals and rocks
Summary
The development of optically pumped xenon nuclear magnetic resonance (NMR) that exploits the diffusion of highly nuclear spin-polarized xenon gas on the surface or through samples [1,2]. The interaction of xenon atoms with a material produces a chemical shift (δ) relative to the Larmor frequency of the free dilute gas. This observable change can be related to properties of the material including porosity and composition. Since neither Equation (2) or Equation (3) take into account the Xe-Xe interactions, in given temperature conditions CF-HP 129Xe NMR chemical shifts are univocally determined only by the surface chemistry and pore geometry of the material, whether the system is best described as microporous or mesoporous, and whether the shift is positive or negative.
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