Characterization of heterogeneities and domains in aquatic and sedimentary organic matter by 1H spin diffusion: Potential for elucidating the formation mechanisms

Abstract

Although the information on domains and heterogeneities of natural organic matter (NOM) can provide insights into its formation mechanisms, the appropriate solid‐state NMR technique for measuring them is still lacking. The traditional technique requires mobility differences in NOM whereas NOM components are primarily rigid. We introduced a new 1H spin diffusion technique, 1H‐13C two‐dimensional heteronuclear correlation (2D HETCOR) NMR with 1H spin diffusion, for characterization of domains and heterogeneities in aquatic and sedimentary organic matter. It was achieved by collecting a series of 2D HETCOR spectra with a variable mixing time, tm, and monitoring the transfer of magnetization from one component to another. The rate of magnetization transfer provided the information on domains and heterogeneities because the magnetization of small domains or heterogeneities equilibrated faster than that of larger ones. Three samples, International Humic Substances Society (IHSS) Suwannee River NOM, IHSS Suwannee River humic acid (HA), and a sedimentary HA, were used. Two model polymers, a random copolymer poly(styrene‐n‐butyl methacrylate) with 0.6‐nm heterogeneity and a block copolymer polystyrene‐b‐poly(methyl methacrylate) with a domain size of 5 nm, were included for calibration. Within tm < 100 µs, half equilibration was reached for all three NOM samples and poly(styrene‐n‐butyl methacrylate), indicating that they were heterogeneous. In contrast, the spin diffusion of polystyrene‐b‐poly(methyl methacrylate) with 5‐nm domain was much slower. Unlike the traditional spin diffusion technique, this technique did not require the differential mobility in NOM and was suitable for investigating the domains and heterogeneities of NOM, which are mostly rigid.