Item talk:Q155521

Effects of nitrate and water on the oxygen isotopic analysis of barium sulfate precipitated from water samples

BaSO₄ precipitated from mixed salt solutions by common techniques for SOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-1 isotopic analysis may contain quantities of H₂O and NOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-2 that introduce errors in O isotope measurements. Experiments with synthetic solutions indicate that δ¹⁸O values of CO produced by decomposition of precipitated BaSO₄ in a carbon reactor may be either too low or too high, depending on the relative concentrations of SOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-3 and NOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-4 and the δ¹⁸O values of the H₂O, NOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-5, and SOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-6. Typical δ¹⁸O errors are of the order of 0.5 to 1‰ in many sample types, and can be larger in samples containing atmospheric NOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-7, which can cause similar errors in δ¹⁷O and Δ¹⁷O. These errors can be reduced by (1) ion chromatographic separation of SOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-8 from NOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-9, (2) increasing the salinity of the solutions before precipitating BaSO₄ to minimize incorporation of H₂O, (3) heating BaSO₄ under vacuum to remove H₂O, (4) preparing isotopic reference materials as aqueous samples to mimic the conditions of the samples, and (5) adjusting measured δ¹⁸O values based on amounts and isotopic compositions of coexisting H₂O and NOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-10. These procedures are demonstrated for SOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-11 isotopic reference materials, synthetic solutions with isotopically known reagents, atmospheric deposition from Shenandoah National Park, Virginia, USA, and sulfate salt deposits from the Atacama Desert, Chile, and Mojave Desert, California, USA. These results have implications for the calibration and use of O isotope data in studies of SOurn:x-wiley:09514198:media:RCM3832:tex2gif-stack-12 sources and reaction mechanisms.