Environmental NMR: Solid-state Methods

Abstract

Solid-state NMR is widely used to characterize organic matter (OM), including litter inputs, peat, composts, and mineral soil, and to understand the fate of environmental contaminants. The combination of high-power decoupling, magic-angle spinning (MAS), and cross polarization (CP) to enhance sensitivity has greatly facilitated applications to 13C, with lesser use of 15N and 31P. Focusing on 13C, this article summarizes the development of the field and presents basic concepts of solid-state NMR, pulse sequences commonly used [direct polarization (DP), CP, dipolar dephasing (DD), and total sideband suppression (TOSS)], sample preparation and hydrofluoric acid pretreatment, general approaches to spectral acquisition and FID processing, removal of background signal, and data analysis including correction for spinning side bands (SSBs) and interpretation through a molecular mixing model of representative biopolymers. As CP NMR is inherently nonquantitative, and nuclei may be undectectable even with DP, techniques to improve quantitative reliability are discussed. More complex experiments can reveal spatially heterogeneous domains by generating subspectra of 13C associated with protons with different relaxation times, and more recent developments allow spectral editing and two-dimensional (2-D) applications such as heteronuclear correlation (HETCOR). Applications of solid-state 15N NMR are much more challenging, because of its low sensitivity and low natural abundance. Similarly, despite 100% natural abundance and high sensitivity, solid-state 31P NMR is also limited by the small chemical shift range of phosphate minerals, and peak broadening and large higher order sidebands because of their close association with paramagnetics and large chemical shift anisotropy (CSA). However, studies with these nuclei can still provide much insight into OM cycling and the fate of fertilizers and contaminants in the environment.