Manufacture, Detection, and Laboratory Evaluation of the Use of Oxygen-18 Labeled Phosphate as An Environmental Tracer.

The need for a safe, reliable tracer for phosphorus (P) has increased with growing concerns over excessive inputs of P into water bodies. There is only one stable isotope of P, and as a result researchers have looked to the oxygen isotope composition of phosphate (d¹⁸O_p) in order to infer the sources of P to water bodies as P exists in the environment primarily as phosphate.  The first goal of this research was to develop a simple procedure for the synthesis of highly enriched oxygen-18 labeled phosphate (OLP). The second goal was to develop procedures to detect OLP in biological samples using electrospray ionization mass spectrometry (ESI-MS). ESI-MS allows the characterization of different OLP species (synthesized OLP can have 0-4 oxygen-18 atoms), which is not possible using previously described detection methods. The final goal of this project was to determine the behavior of OLP in soil systems, including the rate of biological delabeling of OLP as it is utilized by organisms and any fractionation among the various OLP species. Previous OLP synthesis has relied upon manufacture in a water-free environment, but in this study OLP was synthesized by hydrolyzing POCl₃ with water containing 97 atom % ¹⁸O. The synthesized OLP composition was 5.6% P¹⁸O₄, 48.5% P¹⁸O₃¹⁶O, 36.3% P¹⁸O₂¹⁶O₂, 8.7% P¹⁸O¹⁶O₃, and 1.0% P¹⁶O₄, as determined by ESI-MS, which corresponds to 62.2% of the phosphate oxygen atoms as oxygen-18. These results show that a simple procedure can be used to produce highly enriched OLP.  Both sterilized and non-sterilized samples of various soils were incubated with OLP for a period of 50 d to determine a delabeling rate.  The incubated non-sterilized soil samples had an OLP half life that ranged from 14-24 d depending on soil type, while there was no delabeling of OLP in the sterilized soils.   These results indicate that delabeling is solely the result of biological utilization of OLP.  A sorption experiment was carried out with OLP in a soil slurry to examine the possibility of differential sorption to soil between the OLP species. Results of this experiment showed that substituting an oxygen-16 atom with an oxygen-18 atom in phosphate resulted in a 1.8% increase in the amount of that species sorbed to the soil surface across several soil types. The observed fractionation has implications for interpretation of natural abundance d¹⁸O values in soils, sediments, and natural waters.