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

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.