Preparation and characterization of new sulfate reference materials for Δ17O analysis

Abstract

Atmospheric sulfate is widespread in the Earth system and has climate and environmental impacts. The oxygen isotope mass independent fractionation signal (i.e., Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O) 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 Δ¹⁷O(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 Δ¹⁷O 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 Δ¹⁷O value of (21.72 ± 0.15)‰ that is consistent with the accepted value. For the sulfate materials, we obtained average Δ¹⁷O values of (0.64 ± 0.06)‰, (2.09 ± 0.14)‰ and (6.48 ± 0.18)‰ (1σ) 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 Δ¹⁷O values of Sulf-A, Sulf-B and Sulf-C, they can serve as working standards for routine Δ¹⁷O(SO₄²⁻) analysis of samples of interest to improve data consistency.