New Ag₃PO₄ comparison material for stable oxygen isotope analysis

Rationale: A silver phosphate reference material (Ag₃PO₄) for the measurement of stable oxygen isotope compositions is much needed; however, it is not available from the authorities distributing reference materials. This study aims to fill this gap by calibrating a new Ag₃PO₄ stable isotope comparison material produced by the University of Natural Resources and Life Sciences (BOKU). Methods: Aliquots of Ag₃PO₄ were distributed to four laboratories who frequently measure the δ¹⁸O value in Ag₃PO₄; the University of Natural Resources and Life Sciences (BOKU), the University of Western Australia (UWA), the University of Helsinki (UH), and the Helmholtz Centre for Environmental Research (UFZ). The instruments used to perform the measurements were high-temperature conversion elemental analysers coupled with continuous flow isotope ratio mass spectrometers. The working gas δ¹⁸O value was set to 0‰ and the normalization was done by a three-point linear regression using the reference materials IAEA-601, IAEA-602, and NBS127. Results: The mean δ¹⁸O value of the new BOKU Ag₃PO₄ comparison material on the VSMOW-SLAP scale is 13.71‰ and the combined uncertainty is estimated as ±0.34‰. This estimated uncertainty is within the range typical for comparison materials of phosphates and sulphates. Consistent results from the different laboratories probably derived from similar instrumentation, and use of the same reference materials and normalization procedure. The matrix effect of the different reference materials used in this study was deemed negligible. Conclusions: The BOKU Ag₃PO₄ can be used as an alternative comparison material for stable oxygen isotope analysis and is available for stable isotope research laboratories to facilitate calibration.