The Application of ¹H HR-MAS NMR Spectroscopy for the Study of Structures and Associations of Organic Components at the Solid−Aqueous Interface of a Whole Soil

Abstract

High resolution-magic angle spinning nuclear magnetic resonance (HR-MAS NMR) allows the application of solution-state NMR experiments to samples that are not fully soluble and contain solids. Only the species in contact with the solvent system employed become NMR observable. In this study utilizing D2O as the solvent system we show it is possible to examine the structures at the solid-aqueous interface of a whole soil. Combining one- and two-dimensional HR-MAS NMR allowed, for the first time, the identification of fatty acids, aliphatic esters, and ethers/ alcohols as prominent species at the solid-aqueous interface of the soil with signals from sugars and amino acids also apparent. Few, if any signals from aromatic protons were observed when the soil was swollen in aqueous media, although these signals are observed in extracts from the same soil and when the soil is swollen with a more penetrating solvent(DMSO-d6)which is known to disassociate hydrogen bonds. These findings indicate that the soil aromatic moieties are protected in hydrophobic regions which are not water accessible. Furthermore, when the soil was amended with a herbicide (trifluralin), direct observations of interactions between the protons on a xenobiotic and the surrounding soil matrix were possible for the first time. HR-MAS promises to be a method that can be widely applicable to a range of complex environmental samples without the need for extraction, pretreatment, or purification.

Effect of phosphate on phenanthrene sorption in soils

“…1. It is clear that aliphatic moieties (0.5-2.8 ppm regions) and a combination of sugars, amino acids, aromatic methoxyl and CH 2 units adjacent to ether and ester groups (3-5.5 ppm regions) were the dominant fractions in the soil extracts [24], while the strong signal at 4.79 can be attributed to H 2 O in the soil samples. Distinct functional group compositions occurred in the two soils and significantly higher contents of the alkyl chains (at 1.9-2.8 ppm regions) were observed in the black soil.…”

HR-MAS NMR Spectroscopy in Material Science

“…The ability of HR-MAS to identify different motional regimes has proven powerful in the elucidation of the chemistry at water/soil interfaces. [74][75][76][77][78][79] These NMR studies show that at this interface fatty acids, aliphatic esters and alcohols are the prominent species, and that aromatic functional groups are protected by hydrophobic regions and are not directly accessible to the penetrating water, [74] while surface polymethylene groups may control the sorption properties of organo-clay complexes. [76,80] These types of studies have been extended to three-dimensional (3D) HMQC-TOCSY to further increase the resolution of the highly overlapping spectra from humic materials.…”

Sorption interaction of phenanthrene with soil and sediment of different particle sizes and in various CaCl2 solutions

“…1 H NMR spectra of the supernatants were obtained (Model Avance-DPX-500MHz, Bruker Analytic GmbH, Germany) under the following experimental conditions: the probe diameter was 5 mm, the recycle delay time was 1 s, the sweep width was 8012 Hz (16 ppm), the line broadening was 3.00 Hz, the scan number was 1000, and the detecting temperature was 295 K. The water gate sequence was used to depress the water peak (on water resonance: 4.704-4.706 ppm). The spectra were approximately divided into the following chemical shift regions: hydrocarbon chain (0-3.0 ppm), polar group such as sugar, amino acid, aromatic methoxyl, and methylene unit adjacent to ether and ester (3.0-6.5 ppm), and aromatic moiety (6.5-8.5 ppm) [34]. The spectra of dissolved organic matter are presented in Fig.…”