ATTEMPTS TO IMPROVE SOLID STATE 13C NMR SPECTRA OF WHOLE MINERAL SOILS

Abstract

This study describes a number of different preparation techniques for recording solid state ¹³C NMR spectra of whole mineral soils. Removal of paramagnetic Fe³⁺ improves the quality of ¹³C NMR spectra of whole soils and of particle size fractions. The C:Fe ratio appears to be an important indicator for obtaining satisfactory ¹³C NMR spectra of whole soils and fractions separated from them. If the C:Fe ratio is >> 1, the quality of the spectrum will be good; if the ratio is > 1, a reasonable spectrum will be obtained, but if the ratio is < 1, the spectrum will be poor. Organic-matter-rich soil and particle size fractions separated by a flotation technique produce well-defined ¹³C NMR spectra, typical of humic materials. Reduction of C-enriched fractions with sodium dithionite and stannous chloride improves the spectral resolution. The data presented herein show that satisfactory solid state ¹³C NMR spectra can be run on untreated soil particle size fractions, non-magnetic portions of whole soils, and fractions enriched in soil organic matter by flotation, especially after chemical reduction. Key words: ¹³C NMR spectroscopy, paramagnetic mineral separation