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

The presence of magnetic species, Fe in particular, decreases the effective ness of nuclear magnetic resonance (NMR) analysis of soil organic matter (S OM), especially in Spodosols. This study was conducted to evaluate the util ity of dithionite, stannous chloride, and hydrofluoric acid (HF) extraction for Fe removal and solid-state C-13 NMR spectral enhancement in forested S podosols. Extraction with 2% HF resulted in the greatest removal of Fe (45% -99%), while 4% (w/v) dithionite extraction was the least effective in remo ving 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 t he 10 mineral horizon soils studied, the C/N ratios changed more than 25% a fter HF treatment, suggesting that some alteration of bulk chemical composi tion occurred. The treatment had little effect on low-Fe samples because th e quality of NMR spectra was similar between untreated and chemically treat ed organic horizon (Oa) soil samples. For soils high in Fe, as is typical i n Spodosol mineral horizons, the 2% HF and the 0.05 M SnCl2 treatment showe d some spectral improvement but the results were inconsistent. C-13 NMR spectra for soil HF extracts indicated that carbohydrate and carbo xyl C groups were preferentially removed in the extraction. Some quantitati ve changes of C distribution among various structures were observed after K F treatment, possibly indicating the alteration of organic matter during ex traction. Several factors may contribute to the differences in the determin ation of various C groups by C-13 NMR spectroscopy before and after chemica l treatment, including poor signal-to-noise (S/N) ratios, interactions betw een paramagnetic ions and different C moieties, preferential removal of som e C groups by different chemicals, sideband interference, mass loss, and pe rhaps other factors. Based on our results, it seems premature to recommend routine chemical pretreatment as a means of improving C-13 NMR spectral qua lity. (C) 1999 Elsevier Science B.V. All rights reserved.