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 13 C, with lesser use of 15 N and 31 P. Focusing on 13 C, 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 13 C 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 15 N NMR are much more challenging, because of its low sensitivity and low natural abundance. Similarly, despite 100% natural abundance and high sensitivity, solid-state 31 P 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. Introduction Principles of NMRThe basis of NMR spectroscopy is that only certain atomic nuclei have a nuclear spin I, and thus a magnetic moment μ, which interacts with a static magnetic field. These nuclei include 1 H, 13 C, 15 N, and 31 P (Table 1 1 ). In an external magnetic field B 0 , which is represented as a vector in the z direction, such nuclei split into 2I + 1 energy levels, with the splitting proportional to the field strength B 0 and γ , a constant for each nucleus. Nuclei with I = 1 / 2 , such as 1 H and 13 C, therefore, split into two populations. The very slight surplus of nuclei in the lower energy level, as determined by the Boltzmann equation, results at equilibrium in a macroscopic sample magnetization M 0 (Figure 1a). In fact, each nuclear spin is not statically aligned, but precessing around B 0 at the Larmor frequency ω, which corresponds to the energy gap ΔE:Rather than the laboratory (static) frame of reference, Figure 1 uses a reference frame rotating at the Larmor frequency, which simplifies the picture. In NMR, transitions between the energy levels are induced by a very bri...

Charcoal in Organic Horizon and Surface Mineral Soil in a Boreal Forest Fire Chronosequence of Western Quebec: Stocks, Depth Distribution, Chemical Properties and a Synthesis of Related Studies

“…The oldest sample (2,355 ysf) has the highest proportion of aryl C (71%) and aromaticity (0.79). The CP spectra underestimate aromatic C without attached hydrogens; this effect is more important for samples high in O and di-Oalkyl C including carbohydrate but becomes less important as the proportion of aromatic C increases (Baldock and Smernik, 2002;Czimczik et al, 2002;Ascough et al, 2008;McBeath et al, 2011;Preston, 2014;Preston et al, 2014a,b). Our spectra indicate a trend to higher aromaticity with increasing ysf (not shown), but obviously, the low sample size, especially at higher ysf and the qualitative aspect of the NMR areas does not allow us to establish statistical inference.…”

“…Thus, rather than using a vertical boundary, a baseline for the di-O-alkyl region was sketched in to allow a more accurate estimate of this region. Relative areas were determined by copying the spectra, and cutting and weighing the spectral regions, an old technique that is still useful for spectra with low signal-to-noise ratios (Preston, 2014;Preston et al, 2014a). Aromaticity was calculated as the ratio of intensities 110-165 ppm/0-165 ppm.…”

“…Relative areas and aromaticity were treated qualitatively to show trends, as CP efficiency varies for different C structures and environments. The VACP technique helps to overcome this, but compared to quantitative direct-polarization (DP) spectra, CP spectra underestimate aromatic C without attached hydrogens although as samples become almost completely aromatic the effect is diminished (Preston, 2014). Figure 2).…”

Soil and seedling response to dehydrated septic tank sludge versus forest floor additions at a disturbed site

“…The relative abundance of phenolic C was also highest in the mature sludge, which was reflected in a F o r R e v i e w O n l y 14 higher degree aromaticity (aromatic + phenolic C) compared to the other two materials. Peaks at 148ppm for all three materials correspond to C3 in guaiacyl units of condensed and hydrolysable tannins and lignins (Preston 1996). Carbonyl C, the most oxidized form of carbon, was highest for the forest floor material (Table 4).…”

“…The small peak at 30 ppm observed for the forest floor corresponds to the methylene groups in long chain aliphatic compounds (Kögel-Knabner 1997;Bartoszek et al 2008) found in waxes and cutins, polyesters of roots and bark, condensed tannins and sidechains of proteins (Preston 1996). The small peaks in the 20 -30 ppm region for the sludges are associated to C-CH 3 moieties (Keeler and Maciel 2000), and the sharper peak at 22 ppm likely corresponds to terminal methyl groups of alkyl chains and to acetyl methyl groups in hemicelluloses (Keeler and Maciel 2000).…”