Structural characterization of Humic Materials Using 13C Nuclear Magnetic Resonance Techniques: A Comparison of Solution and Solid-State Methods

Catherine F M Clewett ,Todd M Alam ,Eric Osantowski ,Michael J Pullin

doi:10.4172/2380-2391.1000121

Catherine F M Clewett , Todd M Alam + Show 2 more

Open Access

https://doi.org/10.4172/2380-2391.1000121

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Abstract

The analysis of carbon type distribution and chemical structure of natural organic matter (NOM) by 13C NMR spectroscopy is important for understanding its origins and reactivity. While prior work has used solution-state NMR, solid-state NMR has the potential to provide this information with less instrument time and sample processing, while providing an array of advanced filtering techniques. Quantitative and qualitative analyses with 13C solid-state magic angle spinning (MAS) NMR techniques are described for three commercially available samples and one fulvic acid sample isolated from the Rio Grande in New Mexico. This study demonstrates the utility of solid-state 13C NMR for aquatic NOM structural characterization by determining the % carbon type for three standard natural organic materials and comparing these results to the existing solution-state 13C NMR determinations. The solid-state 13C MAS NMR results are used to determine % carbon distribution, estimates of elemental composition (%C, %H, %(O+N)), aromatic fraction (fa), nonprotonated aromatic fraction (faN), an estimate of aromatic cluster size, and ratio of sp2 to sp3 carbons. Additionally, Gaussian deconvolution is used for a more detailed analysis of carbon type than frequency band integration techniques. The solid-state 13C NMR results of these analyses indicate the chemical composition of Rio Grande fulvic acid has higher aromatic fraction and nonprotonated aromatic fraction (fa =0.32, faN = 0.21) and a lower sp2/sp3 fraction and cluster size (sp2/sp3 = 0.66, C = 6) compared to Suwannee River fulvic acid (fa = 0.18, faN = 0.10, sp2/sp3 = 0.82, C = 8).

Highlights

Natural organic matter (NOM) is a complex and variable mixture of organic molecules that is a ubiquitous component of natural waters
The echo sequence used in the solid-state 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) removed the baseline roll typically encountered in solution-state spectra, so our conclusion was that the lower fraction determined in the solid-state NMR experiments was probably more accurate, in agreement with Thorn’s conclusions [13]
According to our NMR analysis results, the Rio Grande fulvic acid showed a higher proportion of acetal aromatic carbons, had a higher aromatic fraction and a higher nonprotonated aromatic fraction than Suwannee River fulvic acid

Summary

Introduction

Natural organic matter (NOM) is a complex and variable mixture of organic molecules that is a ubiquitous component of natural waters. The reactivity of NOM in the environment is affected by its chemical composition and structure. Both the amount of carboxylate functional groups and the fraction of carbon present in aromatic structures have been shown to affect NOM behavior in environmental systems [4,5,6,7]. While solution-state 13C NMR is a very useful tool in determining the structure of NOM based on the characteristic isotropic chemical-shift ranges of the various carbon types, the chemical shift alone is not always enough to identify or quantify specific functional groups. While filtering experiments based on spin-echo and polarization transfer techniques could be used to distinguish among overlapping carbon types [13], long acquisition times and baseline distortions have limited their use [9]

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