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

Abstract

Preparation of BaSO4 from samples of atmospheric rain, snow, and aerosols has been used for δ18 O and δ34 S analyses. In the present study, we investigated the effect of various sample pretreatments on δ18 O and δ34 S values determined from a Na2 SO4 reagent solution and samples of atmospheric precipitation to improve assay time and cost efficiency. BaSO4 was prepared from a Na2 SO4 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 δ18 O, BaSO4 prepared from the samples was treated with diethylenetriaminepentaacetic acid (DTPA) and SO4 2- and separated chromatographically. The values of δ18 O and δ34 S were measured using a continuous-flow elemental analyzer coupled to an isotope-ratio mass spectrometer. The δ34 S values in BaSO4 precipitated from Na2 SO4 solution determined by methods (a)-(c) were consistent within precisions of ±0.5‰. The δ18 O values of methods (a) and (b) were consistent within ±0.2‰, whereas the δ18 O values of methods (c) and (d) increased with increasing HCl concentrations. Similar results were obtained from samples of atmospheric precipitations. The δ18 O values from DTPA-treated BaSO4 were consistent with those obtained by chromatographic separation within ±0.5‰. We found no significant differences in the effects of various pretreatments (acidification, heating, concentration, and drying) on δ18 O and δ34 S 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 δ18 O analysis.