Improved peak identification in 31P-NMR spectra of environmental samples with a standardized method and peak library

The technique of 31P-NMR spectroscopy has done more to advance the knowledge of organic P forms in environmental samples than any other method. Early 31P-NMR work limited identification to peaks that were clearly separated, such as orthophosphate and pyrophosphate, grouping the remaining peaks into broad categories such as orthophosphate monoesters and orthophosphate diesters. Advances in 31P-NMR methodology for environmental samples now produce clearer spectra, providing the potential to identify more peaks. However, there is at present no standard method for peak identification, and no library of chemical shifts of P forms analyzed under standardized, easily replicated conditions. Various research groups have conducted spiking experiments and have developed their own peak libraries. However, because the chemical shifts of P forms are affected by sample conditions such as pH and salt concentration, it can be difficult to use the work of one lab group to identify P forms in samples analyzed by another lab group under different conditions. For this paper, more than 50 P compounds were analyzed under standardized conditions that can easily be repeated by other research groups. These compounds include phosphonates, orthophosphate monoesters, orthophosphate diesters, polyphosphates, pyrophosphate, and a compound with an N–P bond. The chemical shifts of P forms analyzed for this P compound library were compared to those identified elsewhere, if available. In addition, recommendations are given for standardized spiking experiments to improve peak identification.