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

Abstract

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, β‐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 (Pobs) 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.