18 O Isotope Labeling Combined with 31 P Nuclear Magnetic Resonance Spectroscopy for Accurate Quantification of Hydrolyzable Phosphorus Species in Environmental Samples.

³¹ P nuclear magnetic resonance (NMR) spectra can be biased due to the hydrolysis of labile P species during sample treatment and NMR analysis. This paper offers an approach to circumvent this problem by performing sample preparation and analysis in ¹⁸ O-enriched medium. Heavy ¹⁸ O isotope atoms were introduced into the resulting artificial hydrolysis products. The NMR signal of ¹⁸ O-labeled P was shifted upfield relative to the unlabeled P nuclei in natural metabolites. This isotope shift enabled an immediate differentiation of artificial hydrolysis products from natural metabolites. Moreover, the hydrolysis products could be accurately quantified. Our data suggest that the extent to which artificial hydrolysis alters NMR spectra varies among different types of environmental samples. For instance, 72-84% of the detected monoesters in the organic soils of this study were actually artificially hydrolyzed diesters. By contrast, artificial hydrolysis products in the mineral soils used for this study accounted for less than 6% of the total monoesters. Polyphosphate was also hydrolyzed to yield ¹⁸ O-labeled products in algal biomass.