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


  • This article is a U.S. Government work and is in the public domain in the U.S.A.


BaSO4 precipitated from mixed salt solutions by common techniques for SOmath image isotopic analysis may contain quantities of H2O and NOmath image 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 SOmath image and NOmath image and the δ18O values of the H2O, NOmath image, and SOmath image. Typical δ18O errors are of the order of 0.5 to 1‰ in many sample types, and can be larger in samples containing atmospheric NOmath image, which can cause similar errors in δ17O and Δ17O. These errors can be reduced by (1) ion chromatographic separation of SOmath image from NOmath image, (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 NOmath image. These procedures are demonstrated for SOmath image 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 SOmath image sources and reaction mechanisms. Published in 2008 by John Wiley & Sons, Ltd.