Preparation and characterization of new sulfate reference materials for Δ 17 O analysis

Atmospheric sulfate is widespread in the Earth system and has climate and environmental impacts. The oxygen isotope mass independent fractionation signal ( i.e. , Δ 17 O = δ 17 O − 0.52 × δ 18 O) 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 Δ 17 O(SO 4 2− ) 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 Δ 17 O 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 Δ 17 O value of (21.72 ± 0.15)‰ that is consistent with the accepted value. For the sulfate materials, we obtained average Δ 17 O values of (0.64 ± 0.06)‰, (2.09 ± 0.14)‰ and (6.48 ± 0.18)‰ (1 σ ) 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 Δ 17 O values of Sulf-A, Sulf-B and Sulf-C, they can serve as working standards for routine Δ 17 O(SO 4 2− ) analysis of samples of interest to improve data consistency.