Identification of Phytate in Phosphorus-31 Nuclear Magnetic Resonance Spectra: The Need for Spiking
2007
Smernik, Ronald J. | Dougherty, Warwick J.
Phosphorus-31 (³¹P) nuclear magnetic resonance (NMR) spectroscopy of sodium hydroxide–ethylenediaminetetra-acetic acid (NaOH-EDTA) extracts has recently become a widely used technique for the characterization of soil P. This technique has seemingly enabled easy identification and quantification of phytate (myo-inositol hexakisphosphate), a compound long believed to constitute a major proportion of organic P. Phytate is usually identified by its characteristic pattern of four resonances in the ratio 1:2:2:1. We report that the ³¹P NMR spectra of the NaOH-EDTA extracts of four Australian pasture soils contain a set of resonances that bear a striking resemblance to the phytate resonances but that are shown not to be phytate though careful addition (spiking) of pure phytate. The spiking experiments identify a much smaller set of resonances as being phytate. Quantification of these resonances shows that phytate comprises <5% of organic P and <3% of total P in these soils. We also show that the ³¹P chemical shift of phytate resonances is very sensitive to pH and ionic strength. These results highlight the potential for misassignment of resonances in ³¹P NMR spectra of NaOH-EDTA extracts of soil and the possibility that phytate concentrations may be overestimated using this technique and show the value of spiking as a definitive form of identification of ³¹P NMR resonances.
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