Uncertainties in the oxygen isotopic composition of barium sulfate induced by coprecipitation of nitrate

Coprecipitation of nitrate and sulfate by barium has probably resulted in significant error in numerous studies dealing with the oxygen isotopic composition of natural sulfates using chemical/thermal conversion of BaSO4 and analysis by isotope ratio mass spectrometry. In solutions where NO3-/SO42- molar ratios are above 2 the amount of nitrate coprecipitated with BaSO4 reaches a maximum of approximately 7% and decreases roughly linearly as the molar ratio decreases. The fraction of coprecipitated nitrate appears to increase with decreasing pH and is also affected by the nature of the cations in the precipitating solution. The size of the oxygen isotope artifact in sulfate depends both on the amount of coprecipitated nitrate and the delta O-18 and Delta O-17 values of the nitrate, both of which can be highly variable. The oxygen isotopic composition of sulfate extracted from atmospheric aerosols or rain waters are probably severely biased because photochemical nitrate is usually also present and it is highly enriched in O-18 (delta O-18 similar to 50-90 parts per thousand) and has a large mass-independent isotopic composition (Delta O-17 similar to 20-32 parts per thousand). The sulfate delta O-18 error can be 2-5 parts per thousand with Delta O-17 artifacts reaching as high as 4.0 parts per thousand. Copyright (c) 2008 John Wiley & Sons, Ltd.