Solid-State Carbon-13 Nuclear Magnetic Resonance of Humic Acids at High Magnetic Field Strengths

Abstract

Use of solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has become commonplace in studies of humic substances in soils and sediments, but when modern high-field spectrometers are employed, care must be taken to ensure that the data obtained accurately reflect the chemical composition of these complex materials in environmental systems. In an effort to evaluate the quality of solid-state 13C NMR spectra obtained with modern high-field spectrometers, we conducted a series of experiments to examine spectra of various humic acids taken under a variety of conditions. We evaluate conditions for obtaining semiquantitative cross polarization magic angle spinning (CPMAS) 13C NMR spectra of humic acids at high magnetic field and spinning frequency. We examine the cross polarization (CP) dynamics under both traditional and ramp CP conditions on Cedar Creek humic acid. Fitted equilibrium intensities from these CP dynamic studies compare to within 3.4% of the intensities determined from a Bloch decay spectrum of the same sample. With a 1-ms contact time, ramp CP and traditional CP spectra were acquired on this sample and were found to compare to within 5.4% of the Bloch decay spectrum; however, the signal-to-noise ratio per hour of data acquisition was found to double under ramp CP conditions. These results demonstrate the power of applying modern solid-state NMR techniques at high magnetic field strengths. With these techniques, high-quality, semiquantitative spectra can be quickly produced, allowing the application of solid-state NMR techniques to more environmentally relevant samples, especially those where the quantity is limited.

Optimum liquid density in separation of the physically uncomplexed organic matter in Arctic soils

“…Finally, solid-state 13 C cross polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy is a non-destructive spectroscopy technique widely used to characterize solid SOM from different ecosystems at various stages of decomposition (Skjemstad et al, 1997;Preston, 2001;Simpson and Preston, 2008). The solid-state 13 C CPMAS NMR technique provides a large spectrum of the entire SOM qualities (Schnitzer, 1991;Dai et al, 2001;Dria et al, 2002;Simpson and Preston, 2008), which is suitable when very little is known about SOM. Several functional C groups of SOM are identified by solid-state 13 C NMR spectroscopy.…”

Dynamics of Cross Polarization in Solid State Nuclear Magnetic Resonance Experiments of Amorphous and Heterogeneous Natural Organic Substances

“…3 However, it has been shown that the monotonic CP model fails when it is applied for studying cross polarization either in polycrystalline materials 17 or in amorphous natural organic matter. 18,19 In fact, Taylor et al 17 reported that the best fit for variable contact time (VCT) experiments done on crystalline organic systems is a nonmonotonic model, in which two different cross polarization mechanisms are introduced. The faster CP mechanism is governed by the H-C internuclear distance, whereas the slower one is mediated by fast local segmental motions.…”

“…17 Proton-tocarbon distances and local segmental motions were also used by Abelmann et al 19 to interpret the CP behavior of humic substances from two different coals. Conversely, Dria et al 18 attributed the non-monotonic CP behavior of heterogeneous and amorphous NOM extracted from different sources only to different local dynamics experienced by the molecular groups resonating at the same chemical shift values.…”

“…In addition, two other different mathematical models were applied by Dria et al 18 and Abelmann et al 19 to study the CP dynamics of NOM. Dria et al 18 used a model which was empirically modified from the classical monoexponential one in order to account for the poor data fitting obtained by the application of the classical model.…”

“…In addition, two other different mathematical models were applied by Dria et al 18 and Abelmann et al 19 to study the CP dynamics of NOM. Dria et al 18 used a model which was empirically modified from the classical monoexponential one in order to account for the poor data fitting obtained by the application of the classical model. Nuclear magnetic resonance (NMR) experiments on carbon-13 in the solid state were done with cross polarization (CP) and magic angle spinning (MAS) in order to overcome the low NMR sensitivity of 13 C and the chemical shift anisotropy, respectively.…”