Demineralization of marine and freshwater sediments for CP/MAS C-13 NMR analysis

A method was developed to demineralize sediment trap material and marine se diments containing labile organic matter (OM), in preparation for cross pol arization and magic angle spinning (CP/MAS) solid-state C-13 NMR analysis. Carbonate and silicate minerals were dissolved with HCl and a mixture of di lute HCl/HF, respectively. Demineralization kinetics were assessed for a ra nge of freshwater and marine sediments, as well as pure mineral and organic samples. For samples with a very low organic carbon (OC) concentration (< 1 wt.%) and samples containing a large fraction of acid-soluble OC, the org anic molecules solubilized during the dissolution of the mineral fraction w ere recovered by freeze-drying the supernatants following (i) removal of di ssolved calcium and residual HF by CaF2 precipitation, (ii) removal of the dissolved paramagnetic metals by sulfide precipitation, and (iii) desalting using ion-retardation chromatography. When applied to a wide range of fres hwater and marine particles, demineralization resulted in OC enrichment fac tors that varied between 2.2 and 20.8, with losses representing less than 2 0.5% of the initial OC content. X-ray diffraction and fluorescence analysis of the natural and demineralized samples showed that minerals and paramagn etic metals were effectively removed. While molecular fractionation might b e substantial when the mineral constituents were dissolved with HCl and HF, the small changes (< 11%) in the (C/N)(a) and (H/C)(a) ratios when the aci d-soluble organic matter was recovered suggest that the molecular compositi on of the organic fraction was not appreciably altered. In combination with CP/MAS C-13 NMR spectroscopy, this demineralization method allows comprehe nsive elucidation of the chemical structure of total OC, especially in samp les with very low OC concentrations and/or with a significant fraction of c hemically labile organic compounds. (C) 2001 Elsevier Science Ltd. All righ ts reserved.