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 SO4S isotopic analysis may contain quantities of H2O and NO3 that introduce errors in O isotope measurements. Experiments with synthetic solutions indicate that d 18 O 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 ofSO4S and NO3 S and the d 18 O values ofthe H2O, NO3 S , andSO4 2S . Typical d 18 O errors are oftheorder of 0.5 to 1% in many sample types, and can be larger in samples containing atmospheric NO3 S , which can cause similar errors in d 17 O and D 17 O. These errors can be reduced by (1) ion chromatographic separation of SO4S from NO3 , (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 d 18 O values based on amounts and isotopic compositions of coexisting H2O and NO3 . These procedures are demonstrated for SO4 2S 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 SO4S sources and reaction mechanisms. Published in 2008 by John Wiley & Sons, Ltd.