A new solution ³¹P NMR sample preparation scheme for marine sediments

A new approach for the preparation of marine sediment samples for solution 31P nuclear magnetic resonance spectroscopy (³¹P NMR) has been developed and tested. This approach addresses important aspects associated with sample pretreatments for marine sediments, including the effects of sample pretreatment on sedimentary P composition. The method increases the signals of low abundance P species in³¹P NMR spectra by quantitatively and precisely removing up to 80% of inorganic P (orthophosphate) from sediment samples while causing minimal alteration of the chemical structure of organic P compounds. This method uses a reductive step to solubilize P bound to iron oxyhydroxides, followed by a low pH digestion to extract P from authigenic and biogenic apatite, as well as P bound to calcium carbonate. These P forms combined represent the most abundant inorganic P reservoir in marine sediments. The sample residue is then extracted in an alkaline solvent, 0.25 M NaOH with 0.05 M Na2EDTA, and processed for³¹P NMR spectroscopy. The method was tested on natural marine sediment samples from different localities with high inorganic P content (>85% molybdate reactive P), and allowed for the detection of orthophosphate monoesters and pyrophosphate in samples for which only an orthophosphate signal could be resolved with an NaOH-EDTA extraction alone. This new approach will allow the use of³¹P NMR on samples for which low organic P concentrations previously hindered the use of this tool, and will help answer longstanding question regarding the fate of organic P in marine sediments.