Purification of BaSO4 precipitate contaminated with organic matter for oxygen isotope measurements (δ18O and Δ17O)

RATIONALE: Sulfate precipitates are often contaminated with nitrates and organic materials (OM), which reduce the precision and accuracy of measurements of delta O-18 and Delta O-17 values in the sulfate. Although nitrates can be effectively removed using diethylenetriaminepentaacetic acid solution, removing OM from the precipitates is often difficult. One effective approach is to heat powdered precipitates to high temperatures. In this study, the effect of this procedure on the delta O-18 and Delta O-17 values of BaSO4 precipitate was fully examined. METHODS: OM-contaminated BaSO4 precipitates and O-18-and O-17-labeled purified BaSO4 precipitates were loaded into alumina and gold crucibles and heated at 450 degrees C, 600 degrees C and 800 degrees C for 2 h. The nitrogen and carbon contents in the initial and the final BaSO4 were measured using an elemental analyzer. The values of delta O-18 and Delta O-17 were measured using a temperature conversion/elemental analyzer coupled with an isotope ratio mass spectrometer, and a CO2 laser system coupled with an isotopic ratio mass spectrometer. RESULTS: OM was effectively (88 degrees 17%) removed from the BaSO4 precipitates by this treatment, and heating at 800 degrees C had the highest removal efficiency (98%). The differences in delta 18O and Delta O-17 values between the final and initial BaSO4 precipitates was -0.6% to 0.3%(average of -0.1%) and -0.24% to 0.10%(average of -0.02%), respectively. Significant positive relationships between the delta O-18 and Delta O-17 values of the initial BaSO4 precipitate and those of the high-T-treated aliquots were found, with slopes having mean values of 0.96 +/- 0.06 and 1.04 +/- 0.01, respectively. CONCLUSIONS: The result demonstrates high removal efficiency for OM in BaSO4 precipitates and no significant differences in the oxygen isotopic compositions between high-T treated BaSO4 and initial BaSO4. This study indicates that the modified high-T treatment (800 degrees C, 2 h) is an effective method for purifying BaSO4 precipitated from geological and environmental samples with a high OM content for delta O-18 and Delta O-17 measurements. Copyright (C) 2016 John Wiley & Sons, Ltd.