High-precision determination of 18O/16O ratios of silver phosphate by EA-pyrolysis-IRMS continuous flow technique
Résumé
A high-precision, and rapid on-line method for oxygen isotope analysis of silver phosphate is presented. The technique uses high-temperature elemental analyzer (EA)-pyrolysis interfaced in continuous flow (CF) mode to an isotopic ratio mass spectrometer (IRMS). Calibration curves were generated by synthesizing silver phosphate with a 13‰ spread in d18O values. Calibration materials were obtained by reacting dissolved potassium dihydrogen phosphate (KH2PO4) with water samples of various oxygen isotope compositions at 373 K. Validity of the method was tested by comparing the on-line results with those obtained by classical off-line sample preparation and dual inlet isotope measurement. In addition, silver phosphate precipitates were prepared from a collection of biogenic apatites with known d18O values ranging from 12.8 to 29.9‰(V–SMOW). Reproducibility of ±0.2‰was obtained by the EA-Py-CF-IRMS method for sample sizes in the range 400–500 μg. Both natural and synthetic samples are remarkably well correlated with conventional 18O/16O determinations. Silver phosphate is a very stable material and easy to degas and, thus, could be considered as a good candidate to become a reference material for the determination of 18O/16O ratios of phosphate by high-temperature pyrolysis.