SOLID-STATE FOURIER TRANSFORM INFRARED AND 31P NUCLEAR MAGNETIC RESONANCE SPECTRAL FEATURES OF PHOSPHATE COMPOUNDS
Solid-state spectroscopic techniques, including Fourier transform infrared (FT-IR) and solid-state 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies, are powerful tools for evaluating metal speciation and transformation mechanisms of P compounds in soil, animal manure, and other environment samples. These spectroscopic studies generally rely on the reference spectra of model compounds. However, such reference information is limited, especially for metal phytate compounds, which are an important component of many samples (e.g., 60-80% of soil organic P). In this study, we used solid-state FT-IR and 31P MAS NMR to examine metal phytates and their orthophosphate counterparts, as well as several pyrophosphates and polyphosphates. Comparisons of the FT-IR spectra of metal orthophosphates and phytates demonstrated distinct characteristics and differences. The triplet-peak feature in the 790- to 900-cm−1 range could be used to distinguish metal phytates from other P compounds that contained one or no peak in the same range. FT-IR spectral features centered at 1100 cm−1 (i.e., broad or splitting multiple peaks) could distinguish among different metal phytate compounds. We observed that the solid-state 31P NMR peaks of phytate compounds were broad, whereas those of inorganic P compounds were much sharper. We propose that the same attention should be paid to spinning sidebands as on the main peaks, because the features of spinning sidebands were unique for some P compounds. Finally, our observations indicated that wetting samples could change both intensity and position of chemical shifts and spinning sidebands, thus keeping samples dry is necessary to obtain repeatable and high-quality solid-state 31P MAS NMR spectra. The solid-state FT-IR and 31P NMR reference spectra established in this work may help in basic and applied environmental soil P studies by the two advanced spectroscopic technologies.