Established and advanced solid-state NMR spectroscopy for a better understanding of the structure and function of natural organic matter in soils, water and sediments

Abstract

In geobiogeochemistry, 1-dimensional solid-state NMR spectroscopy has been proven as a powerful tool for the characterization of natural organic matter (NOM) in soils, sediments and waters. However, compared to solution NMR spectroscopy, this technique suffers from broad resonance lines and low resolution, which could be overcome by the use of 2-dimenstional solid-state NMR pulse sequences. Until recently, this approach has been unfeasible as a routine tool in geochemistry, mainly because of the low NMR sensitivity of the respective samples. Alternative to the use of higher magnetic fields, considerable signal enhancements can be achieved with the new developments in the field of dynamic nuclear polarization (DNP) solid-state NMR spectroscopy. Here, the improvement is achieved by a microwave-driven transfer of polarization from a paramagnetic centre to nuclear spins. Application of DNP to magic angle spinning (MAS) spectra of biological systems (frozen solutions) showed enhancements by factors of 40 to 50 (Hall et al., 1997). Applying this technique for the first time on NOM, lower but still promising enhancement factors were obtained which allowed the successful acquisition of 2D solid-state NMR spectra of such samples. In the present contribution, the possibilities and limitation of established solid-state NMR techniques in NOM research are discussed and first results obtained with solid-state DNP NMR are presented. Those first data demonstrate the great potential of the latter and that this approach opens new doors for a better understanding of biochemical processes in soils, sediments and water.