CHARACTERIZATION OF NITROGEN IN GEOLOGICAL ORGANIC MATTER SAMPLES USING ADVANCED SOLID-STATE NMR SPECTROSCOPY

An understanding of organic N forms in organic matter can provide important insights into diagenesis. Nitrogen as a limiting growth factor is closely related to biological productivity in both terrestrial and aquatic systems. In this study, advanced solid-state NMR techniques were employed to investigate N forms in complex organic matter. These techniques include improved ¹⁵N cross polarization/magic angle spinning (CP/MAS), ¹⁵N CP/MAS with dipolar dephasing to select nonprotonated nitrogen, and saturation pulse-induced dipolar exchange with recoupling (SPIDER) to detect carbons directly bonded to nitrogen. Two samples, a fulvic acid sample collected from the Pony Lake in Antarctica and a gilsonite sample from Bonanza, Utah, were studied. For the Pony Lake fulvic acid, the SPIDER spectrum exhibited a peak at 157 ppm assigned to a carbon bonded to at least two nitrogens; a five-membered hydantoin ring, which is found in the purine metabolite, allantoin, provides a better match than a six-membered dihydrouracil ring. Detection of pyrrolic N in gilsonite provides the evidence that gilsonite could originate from porphyrins. The advanced solid-state NMR techniques lead to much clearer identification of organic N structures than were achievable in the past.