A Titanium(III) Chloride Based Approach for Accurate Measurement of Sulfate Oxygen Isotope composition

Abstract

Accurate determination of sulfate oxygen isotope composition (δ18O-SO2- 4) was critical for constraining its sources and transformation pathways, yet the accuracy was often compromised by nitrate that commonly co-precipitates with sulfate during sample preparation. Here we proposed a simplified and efficient approach to obtain high-purity BaSO4 through nitrate reduction using titanium (III) chloride (TiCl3), thereby eliminating the impurity disturbance for the measurements of δ18O-SO2- 4. Experiments using nitrate-doped sulfate solutions and natural samples indicate that complete nitrate removal was achieved when the Ti3+/NO-3 mass ratio was ≥ 20. The δ18O-SO2- 4 values of treated K2SO4 - KNO3 mixtures were consistent with those of pure K2SO4 solution, suggesting that the reduction reaction did not affect δ18O-SO2- 4 measurements. Moreover, the δ18O-SO2- 4values remained stable with the increase in the amount of Ti3+, excluding the possible alteration of complexation to the oxygen-isotope composition. Consistent δ¹⁸O values between natural sample treated with and without dilution by ultrapure water further indicated negligible oxygen isotope exchange between sulfate and water. Overall, the consistency sulfate δ¹⁸O values between treated and un-treated solution, as well as natural samples, demonstrated that the proposed method effectively removed nitrate from sulfate solutions and could yield accurate sulfate δ18O values. This advancement provides a streamlined and reliable approach for sulfate O isotope analysis, with broad applicability to environmental and geochemical studies.