Submitted to: Proceedings of American Chemical Society National Meeting
Publication Type: Proceedings
Publication Acceptance Date: May 12, 2009
Publication Date: August 16, 2009
Citation: Heighton, L., Schmidt, W.F. 2009. Raman spectra of organic (myo-inositol hexakis phosphate) and inorganic P sepctra show pH dependence. Proceedings of American Chemical Society National Meeting.
Interpretive Summary: Just like baking powder (monobasic carbonate) is not interchangeable with baking soda (dibasic carbonate) in cookie recipes, the forms of phosphate (both in inorganic and organic forms) inherently matter in soil nutrient properties and in environmental runoff of forms of phosphate (P). Raman spectroscopy spectrum easily differentiates which forms of inorganic P (monobasic, dibasic or tribasic) are present. Only the first two are present in soils, and the ratio depends upon the acidity of the soil (pH). A major fraction of P in agricultural soils often is organic. Phytate has six phosphate groups chemically attached to a sugar molecule called inositol, and each of the six P groups has 2 acidic sites. Thus, the chemical phytate has 12, not three, chemical forms that can be present, and about half of them can be found in the typical soils pH range of from 5 to pH 7.5. We demonstrate that it is impossible to know and to measure the forms of P in soils without knowing the pH. Commonly used procedures which extract form of P often radically change the pH to extract P, which of course preclude knowing the forms that had been present before the extraction. Raman spectroscopy enables the identification of the forms of P in soils or runoff in situ without generating chemical waste and without perturbing soil structure or water characteristics. At normal soil pH values, raman spectroscopy can be used to simultaneously measure organic and inorganic P levels. The present technology however is sensitivity limited: P levels in samples need to be pre-concentrated by a factor of 10 to be detected, and levels of P in soils need to have about ten times more P than is normally present in soils. The length of time organic and inorganic P remain in soils most probably is very different. Simple, more rapid, more direct procedures will enable better nutrient and environmental management of P.
Understanding phosphorous fate and transport is in part limited by technical difficulties and/or access to expensive equipment associated with differentiating ortho-phosphate (P) from organic phosphate in complex environmental samples. Myo-inositol hexakis phosphate (IHP) is the most prevalent form of organic P in soil and manure. Recent research has shown phytate (anionic salt of IHP) to form pH dependent complexes with cationic mineral components (iron and copper) of soil. Vibrational spectroscopy provides an analytical tool capable of measuring small changes in the molecular environment. Raman spectroscopy was used to monitor pH dependent frequency shifts in P and IHP. Solutions of OP (600 mM) and IHP (100 mM), normalized for phosphate concentration were pH adjusted with 0.5 pH unit increments over a pH range of 3-10.5. Solutions were made in triplicate. Raman spectra were collected from 100-2500 cm-1 using a Horbia Jobin Yvon Raman spectrophotometer with a CCD and a 633 nm helium-neon laser. Significant spectral frequency shifts and changes in spectral intensity were observed as a function of pH for both IHP and P solutions, allowing for the differentiation of IHP and P at several pH points.