Speciation of trace amounts of organic carbon with NMR: the example of plant silica particles
Résumé
Determination of the C speciation in natural samples usually involves invasive sample preparation aiming at extracting the organic carbon from its matrix, i.e. a process prone to substantial losses and modification on the nature of the organic compounds. Nuclear magnetic resonance (NMR) is widely used to characterize the nature of natural organic matter, but suffers from an inherent lack of sensitivity, rendering this technique inoperative when the C concentration drops into the single digit % range. Recent advances in the Dynamic Nuclear Polarization (DNP) technique allow however to reconsider this sensitivity issue. Using the example of plant silica particles which occlude C amounts in the sub-percent range, we show that 13C DNP NMR has an improved sensitivity, able to detect 13C spin systems in the 10 ppm range. Binding sites of these organic compounds on the silica matrix are identified with the DNP technique applied to 29Si NMR.