Overestimation of orthophosphate monoesters in lake sediment by solution 31 P-NMR analysis

Abstract

Solution ³¹P nuclear magnetic resonance spectroscopy (³¹P-NMR) is a useful method for analyzing organic phosphorus (P_o). Unfortunately, the extraction conditions, which are highly alkaline and require long extraction times, make this analysis less effective. In this research, according to the lability of orthophosphate monoesters (mono-P_o) and orthophosphate diesters (diesters-P_o), we verified the hypothesized overestimation of mono-P_o in lake sediment using solution ³¹P-NMR. We set three scenes to redistribute the mono-P_o and diesters-P_o. Six components, including eight mono-P_o species, were detected in the NaOH–EDTA extracts of sediment samples using ³¹P-NMR. The results showed that mono-P_o (212.7 mg kg⁻¹) was the dominant P_o in the surface sediment. In the three scenes, mono-P_o decreased from 212.7 to 112.0 mg kg⁻¹, and diesters-P_o increased from 31.9 to 132.7 mg kg⁻¹. The ratio of mono-P_o to diesters-P_o increased from 6.7 to 0.8. Therefore, we deduced that the concentration of mono-P_o was overestimated, while that of diesters-P_o was underestimated, in most research because of the high pH and long extraction process. Diesters-P_o might be an important labile P source during the P "exhausted" period.