Applicability of solid-state 13C CP/MAS NMR analysis in Spodosols: chemical removal of magnetic materials

Abstract

The presence of magnetic species, Fe in particular, decreases the effectiveness of nuclear magnetic resonance (NMR) analysis of soil organic matter (SOM), especially in Spodosols. This study was conducted to evaluate the utility of dithionite, stannous chloride, and hydrofluoric acid (HF) extraction for Fe removal and solid-state 13C NMR spectral enhancement in forested Spodosols. Extraction with 2% HF resulted in the greatest removal of Fe (45%–99%), while 4% (w/v) dithionite extraction was the least effective in removing Fe from the same soils (8%–54%). The 2% HF treatment also removed the largest proportion of total C (12%–67%) and total N (19%–66%). In five of the 10 mineral horizon soils studied, the C/N ratios changed more than 25% after HF treatment, suggesting that some alteration of bulk chemical composition occurred. The treatment had little effect on low-Fe samples because the quality of NMR spectra was similar between untreated and chemically treated organic horizon (Oa) soil samples. For soils high in Fe, as is typical in Spodosol mineral horizons, the 2% HF and the 0.05 M SnCl 2 treatment showed some spectral improvement but the results were inconsistent. 13C NMR spectra for soil HF extracts indicated that carbohydrate and carboxyl C groups were preferentially removed in the extraction. Some quantitative changes of C distribution among various structures were observed after HF treatment, possibly indicating the alteration of organic matter during extraction. Several factors may contribute to the differences in the determination of various C groups by 13C NMR spectroscopy before and after chemical treatment, including poor signal-to-noise ( S/ N) ratios, interactions between paramagnetic ions and different C moieties, preferential removal of some C groups by different chemicals, sideband interference, mass loss, and perhaps other factors. Based on our results, it seems premature to recommend routine chemical pretreatment as a means of improving 13C NMR spectral quality.