Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/23/2007
Publication Date: 7/19/2007
Citation: He, Z., Honeycutt, C.W., Xing, B., Mcdowell, R.W., Pellechia, P.J., Zhang, T. 2007. Solid State FT-IR and (31)P NMR Spectral Features of Phosphate Compounds. Soil Science. 172:501-515 Interpretive Summary: Phosphorus is an essential element for plant growth. However, phosphorus can also degrade surface water quality. Understanding phosphorus chemistry in soil is therefore important for maintaining crop production while sustaining good water quality. In this study, we evaluated analytical techniques for identifying particular phosphorus compounds and found them to be very useful. The report also serves to provide other scientists with specific details for ensuring appropriate use and interpretation of these methods. This research is expected to contribute to better understanding of phosphorus chemistry, thereby promoting environmentally sound phosphorus management practices.
Technical Abstract: 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 the environment. Studies of environmental samples by these techniques rely on the reference spectra of model compounds. However, such 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 pyro- and polyphosphates. Comparisons of the FT-IR spectra of metal orthophosphates and phytates demonstrated distinct characteristics and differences. The triplet-peak feature in the 790-900 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 while 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, since 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.