Fluorination of Silver Phosphate in Nickel Reaction Vessels for Triple Oxygen Isotope Analysis

Increasingly, more laboratories are measuring the triple oxygen isotope values of phosphate. Standardization is key to having good interlaboratory results. Here, we present the triple oxygen isotope values of a suite of standards using fluorination and compare them with previously reported triple oxygen isotope values.

Silver phosphate (~2-3 mg) was fluorinated in nickel reaction vessels, and the resulting O₂ was analyzed on a Thermo MAT 253 for the paired δ¹⁷O-δ¹⁸O value (Δ'¹⁷O). Although the δ¹⁸O and δ¹⁷O values were variable, the fluorination reaction progresses along a slope of ~0.528, indicating that the Δ'¹⁷O is constant when calculated using a λ_ref = 0.528. The ¹⁸O/¹⁶O ratio was obtained using a TC/EA, and the δ¹⁷O value was corrected based on the measured Δ'¹⁷O value from fluorination and the δ¹⁸O values obtained on the TC/EA.

We provide δ¹⁷O, δ¹⁸O, and Δ'¹⁷O values for the following silver phosphate standards: NBS120c, USGS80, USGS81, AGLox, and B2207. Two new standards are also reported and available for distribution. To report our values on the VSMOW2-SLAP2 scale, we measured common silicate standards: San Carlos Olivine, NBS28, NMQ, and SKFS that have been calibrated to VSMOW2-SLAP2. Our reported Δ'¹⁷O error for silver phosphate is 10 per meg, calculated using the average long-term standard deviation of our two most replicated standards.

We report the δ¹⁷O, δ¹⁸O, and Δ'¹⁷O values for a large suite of silver phosphate standards and present two new standards that can be used for interlaboratory calibration. Our results are reported on the VSMOW2-SLAP2 scale by measuring silicates on the same instrument with the same reference gas. The fluorination method in nickel reaction vessels is easily adapted to other laboratories that have fluorination lines. Only 2-3 mg of silver phosphate is needed for Δ'¹⁷O measurements when paired with δ¹⁸O values using the TC-EA technique.