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SOIL CHEMISTRY

On the Use of Hydrofluoric Acid Pretreatment of Soils for Phosphorus-31 Nuclear Magnetic Resonance Analyses

Soil and Land Systems, School of Earth and Environmental Sciences, Univ. of Adelaide, South Australia 5064, Australia

^* Corresponding author (warwick.dougherty{at}dpi.nsw.gov.au).

Solid-state ³¹P nuclear magnetic resonance (NMR) has the potential to be used for investigating soil P forms without the need for extractions or pretreatment. Solid-state spectra typically suffer from poor resolution and low observability, however, in part due to interferences by paramagnetic species present in soil. Hydrofluoric acid treatment is routinely used in ¹³C and ¹⁵N NMR analysis of soil organic matter to remove these paramagnetic species and improve NMR spectra. We evaluated the use of HF pretreatment to improve ³¹P NMR spectra of four pasture soils. Hydrofluoric acid treatment of soils for 24 h resulted in almost complete removal of inorganic P (>90%), but also resulted in the loss of organic P (up to 49%). Mass-balance calculations revealed that much of the organic P removed was hydrolyzed. In contrast, most model organic P compounds were found to be resistant to acid-mediated hydrolysis. Little (<4%) of the P contained in phytic acid, ß-D-glucose phosphate, or DNA was hydrolyzed by HF in 24 h, although 29% of P in adenosine triphosphate was hydrolyzed. The observability of P (P_obs) by solid-state ³¹P NMR increased on HF treatment, but was still generally poor. Attempts to analyze the HF extracts by solution ³¹P NMR were unsuccessful due to the presence of high concentrations of paramagnetics. Comparison of solution ³¹P NMR spectra of NaOH–EDTA extracts of the soils before and after HF treatment indicated that specific organic P compounds, in particular inositol phosphates, were removed by HF. In this regard, HF treatment may have a role in the separation or fractionation of different organic P forms.

Abbreviations: CP, cross-polarization • DP, direct polarization • NMR, nuclear magnetic resonance • P_i, inorganic phosphorus • P_o, organic phosphorus • P_obs, percentage of sample phosphorus observable by spin counting using ³¹P nuclear magnetic resonance • SSB, spinning side bands