Preparation and characterization of new sulfate reference materials for Δ17O analysis

Atmospheric sulfate is widespread in the Earth system and has climate and environmental impacts. The oxygen isotope mass independent fractionation signal (i.e., Delta O-17 = delta O-17 - 0.52 x delta O-18) of sulfate is a very useful metric to probe the specific formation pathways of atmospheric sulfate, and has implications for atmospheric oxidation chemistry. However, currently there is no O-17 enriched sulfate international reference material available for Delta O-17(SO42-) analysis and calibration. In this study, we introduced a new and cost-effective method that uses purified ozone reacting with sodium sulfite solution to produce O-17 enriched sulfate. In this way, we made three sodium sulfate materials enriched in O-17 with different magnitudes, and termed them Sulf-A, Sulf-B and Sulf-C. The Delta O-17 values of these materials were quantified using the pyrolysis method in quartz and platinum capsules. The pyrolysis system was tested by analyzing the nitrate international reference material USGS35 with a measured Delta O-17 value of (21.72 +/- 0.15)parts per thousand that is consistent with the accepted value. For the sulfate materials, we obtained average Delta O-17 values of (0.64 +/- 0.06)parts per thousand, (2.09 +/- 0.14)parts per thousand and (6.48 +/- 0.18)parts per thousand (1 sigma) for Sulf-A, Sulf-B and Sulf-C, respectively, when measured in platinum capsules. And these were adopted as their accepted values. Although additional measurements may be necessary to verify the accuracy of the Delta O-17 values of Sulf-A, Sulf-B and Sulf-C, they can serve as working standards for routine Delta O-17(SO42-) analysis of samples of interest to improve data consistency.

Automated summary

This study introduces a new and cost-effective method to produce O-17 enriched sulfate using purified ozone reacting with sodium sulfite solution. Three sodium sulfate materials enriched in O-17 were created and their Delta O-17 values were quantified. These materials can serve as working standards for routine Delta O-17(SO42-) analysis.