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

Rationale: Preparation of BaSO4 from samples of atmospheric rain, snow, and aerosols has been used for delta O-18 and delta S-34 analyses. In the present study, we investigated the effect of various sample pretreatments on delta O-18 and delta S-34 values determined from a Na2SO4 reagent solution and samples of atmospheric precipitation to improve assay time and cost efficiency.Methods: BaSO4 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 delta O-18, BaSO4 prepared from the samples was treated with diethylenetriaminepentaacetic acid (DTPA) and SO42- and separated chromatographically. The values of delta O-18 and delta S-34 were measured using a continuous-flow elemental analyzer coupled to an isotope-ratio mass spectrometer.Results: The delta S-34 values in BaSO4 precipitated from Na2SO4 solution determined by methods (a)-(c) were consistent within precisions of +/- 0.5 parts per thousand. The delta O-18 values of methods (a) and (b) were consistent within +/- 0.2 parts per thousand, whereas the delta O-18 values of methods (c) and (d) increased with increasing HCl concentrations. Similar results were obtained from samples of atmospheric precipitations. The delta O-18 values from DTPA-treated BaSO4 were consistent with those obtained by chromatographic separation within +/- 0.5 parts per thousand.Conclusions: We found no significant differences in the effects of various pretreatments (acidification, heating, concentration, and drying) on delta O-18 and delta S-34 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 delta O-18 analysis.

Automated summary

In this study, the effect of various sample pretreatments on the delta O-18 and delta S-34 values in sulfate from atmospheric precipitation and aerosols was investigated. The results showed that acidification, heating, concentration, and drying did not significantly affect the delta O-18 and delta S-34 values if HCl was not added before heating and BaSO4 was treated with DTPA for the delta O-18 analysis.