Sample pretreatment effects on isotopic compositions of oxygen and sulfur in BaSO4 derived from atmospheric sulfate

Abstract

RationalePreparation of BaSO4 from samples of atmospheric rain, snow, and aerosols has been used for δ18O and δ34S analyses. In the present study, we investigated the effect of various sample pretreatments on δ18O and δ34S values determined from a Na2SO4 reagent solution and samples of atmospheric precipitation to improve assay time and cost efficiency.MethodsBaSO4 was prepared from a Na2SO4 solution by (a) evaporative concentration, (b) evaporation to dryness, (c) evaporation to dryness after adding HCl, and (d) evaporation to concentration after adding HCl, followed by cooling and then precipitation using a BaCl2 solution. To analyze the atmospheric precipitation samples for δ18O, BaSO4 prepared from the samples was treated with diethylenetriaminepentaacetic acid (DTPA) and SO42− and separated chromatographically. The values of δ18O and δ34S were measured using a continuous‐flow elemental analyzer coupled to an isotope‐ratio mass spectrometer.ResultsThe δ34S values in BaSO4 precipitated from Na2SO4 solution determined by methods (a)–(c) were consistent within precisions of ±0.5‰. The δ18O values of methods (a) and (b) were consistent within ±0.2‰, whereas the δ18O values of methods (c) and (d) increased with increasing HCl concentrations. Similar results were obtained from samples of atmospheric precipitations. The δ18O values from DTPA‐treated BaSO4 were consistent with those obtained by chromatographic separation within ±0.5‰.ConclusionsWe found no significant differences in the effects of various pretreatments (acidification, heating, concentration, and drying) on δ18O and δ34S values in sulfate from samples of atmospheric precipitation and aerosols extracted as BaSO4 if HCl was not added to the sample before heating and BaSO4 was treated with DTPA for the δ18O analysis.