Oxygen isotope studies of phosphite oxidation: Purification and analysis of reactants and products by high‐temperature conversion elemental analyzer/isotope ratio mass spectrometry

Rationale Increased attention has been recently focused on the origin and reactions of reduced‐P oxyanions such as phosphite [PO 3 (III)] in terrestrial and biological systems. We present new methods for studying O‐isotopic reactions between PO 3 (III) and other oxygen sources during oxidation of PO 3 (III) to PO 4 (V). Methods Na 2 (HPO 3 )·5H 2 O, used as a PO 3 (III) source, contains structural water due to its hygroscopic nature; thus, we developed a method for determining the δ 18 O value of PO 3 (III) after the removal of structural water. Next, we tested two techniques for purifying PO 4 (V) from aqueous PO 3 (III)/PO 4 (V) mixtures: (1) precipitation of PO 4 (V) as ammonium phosphomolybdate (APM); and (2) precipitation of PO 4 (V) as magnesium ammonium phosphate (MAP). The O‐isotope compositions, 18 O: 16 O (δ 18 O values), of Na 2 (HPO 3 ) and Ag 3 PO 4 were analyzed by TC/EA/IRMS. Results Structural water was removed from Na 2 (HPO 3 )·5H 2 O after drying at 100 °C under vacuum and the δ 18 O value of PO 3 (III) was obtained. The δ 18 O values of PO 4 (V), which was extracted from 18 O‐labeled PO 3 (III)/PO 4 (V) mixtures by APM and MAP precipitations, were not altered by the precipitation process. This result confirms that PO 3 (III) is not converted into PO 4 (V) by oxidation or hydrolysis under either strong acidic/oxidizing (APM) or alkaline (MAP) conditions for up to a 24‐h period. Conclusions We conclude that both APM and MAP precipitation are reliable and effective methods for the separation and purification of PO 4 (V) from aqueous PO 3 (III)/PO 4 (V) mixtures. The methods described here will permit the study of the isotopic evolution of various pathways of geochemical as well as biological PO 3 (III) oxidation. Copyright © 2015 John Wiley & Sons, Ltd.