Comparative Analysis of Partial Structures of a Peat Humic and Fulvic Acid Using One‐ and Two‐Dimensional Nuclear Magnetic Resonance Spectroscopy

Abstract

Nuclear magnetic resonance (NMR) resonance integrals obtained from one-dimensional NMR spectra provide semiquantitative contents of humic constituents with limited resolution in structural detail. When supplemented by connectivity information available from homo- and heteronuclear two-dimensional NMR spectra a more reliable assignment of humic substructures becomes available. This is demonstrated with a comparative one- and two-dimensional NMR analysis of a fulvic and a humic acid obtained from Eriophorum peat. An example of a detailed analysis of the proton chemical shift region normally attributed to carbohydrates shows substantial contributions from amino acids, amino and desoxy sugars, and highly oxidized aliphatic chains of intermediate length. The very good resolution of structural detail by a combined analysis of all NMR spectra shows that the effect of the fractionation procedure on the composition and chemical structure of humic materials is very significant. The comparison of the partial structures comprising humic acid (HA) and fulvic acid (FA) of the peat humic materials studied indicates that FA is diagenetically downstream of HA, favoring the biopolymer degradation (BD) model of humification.

Coupling NMR to NOM

“…The first application of liquid-state 2D NMR spectroscopy to NOM can be traced back to the work of Buddrus and co-workers [87] followed by a long period in which liquid-state 2D NMR methods were not used in the characterization of NOM. However, in the last few years this has started to change, and the advantages of multi-dimensional NMR in the characterization of NOM have become apparent [81,82,86,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107]. The 2D NMR experiments used in the majority of the studies used in the characterization of NOM can be broken down based on through bond interactions (homonuclear and heteronuclear), through space interactions, and diffusion (not reviewed here).…”

“…In the analysis of NOM samples COSY and TOCSY based methods have been used by a large number of investigators [81,82,92,93,95,98,99,100,101,103,104,105,106,107]. While both techniques have their place, they appear to be most useful when used in combination.…”

“…A combination of COSY-45, Long-range COSY, TOCSY, and DQ-COSY (to resolve peaks near the diagonal that may not be resolved by the other techniques) can be envisioned for a thorough mapping of the proton coupled network. The closest work to this combination of homonuclear correlation peaks that has appeared in the literature to date is that of Hertkorn et al [95], in which both COSY and TOCSY spectra of two NOM samples (a fulvic acid and a humic acid) were analyzed in great detail and compared. From this work it was found that some cross peaks apparent in the TOCSY spectrum were not apparent in the COSY spectrum, and viceversa, for both samples analyzed.…”

Nuclear Magnetic Resonance Analysis of Natural Organic Matter

“…Similarly, improvements in NMR hardware, stability, and sensitivity have kept pace in both solid -state and solution -state NMR. Thus, it is not surprising that in the last decade the application of multidimensional NMR has grown exponentially (Simpson et al, 1997(Simpson et al, , 2001a(Simpson et al, ,c, 2002a(Simpson et al, , 2003aHaiber et al, 1999Haiber et al, , 2001aFan et al, 2000 ;Kingery et al, 2000 ;Knicker, 2000a ;Simpson, 2001 ;Hertkorn et al, 2002bHertkorn et al, , 2006Cook et al, 2003 ;Kaiser et al, 2003 ;Kim et al, 2003 ;Cardoza et al, 2004 ;Mao and Schmidt -Rohr, 2006 ). In 1989, the fi rst applications of multidimensional NMR were applied to humic substances (Buddrus et al, 1989 ).…”

“…During 2002, Simpson et al summarized a range of NMR data to explain the major structural components in soil NOM (Simpson et al, 2002a ), offered an in -depth study of diffusion based techniques (Simpson, 2002 ), and compared some basic 2D experiments (Simpson et al, 2002b ). In the same year, Hertkorn utilized 2D NMR to understand the utilization of aquatic humics by microorganisms (Hertkorn et al, 2002a ) and studied peat humic and fulvic acids (Hertkorn et al, 2002b ). The year 2003 saw the introduction of 3D NMR to study NOM (Simpson et al, 2003a ) and studies of a Laurentian fulvic acid (Cook et al, 2003 ).…”