A titanium(iii) chloride based approach for accurate measurement of sulfate oxygen isotope composition

Abstract

Accurate determination of sulfate oxygen isotope composition (δ¹⁸O–SO₄²⁻) is critical for constraining its sources and transformation pathways, yet the accuracy is often compromised by nitrate, which commonly co-precipitates with sulfate during sample preparation. Here we propose a simplified and efficient approach to obtain high-purity BaSO₄ through nitrate reduction using titanium(III) chloride (TiCl₃), thereby eliminating the impurity interference in the measurement of δ¹⁸O–SO₄²⁻. Experiments using nitrate-doped sulfate solutions and natural samples indicate that complete nitrate removal was achieved when the Ti³⁺/NO₃⁻ mass ratio was 20 ± 1.4. The δ¹⁸O–SO₄²⁻ values of treated K₂SO₄–KNO₃ mixtures were consistent with those of pure K₂SO₄ solution, suggesting that the reduction reaction did not affect δ¹⁸O–SO₄²⁻ measurements. Moreover, the δ¹⁸O–SO₄²⁻ values remained stable with increasing Ti³⁺ mass, excluding the possible alteration of the oxygen-isotope composition by complexation. The fact that δ¹⁸O values remained consistent for natural samples regardless of dilution with ultrapure water further suggests that no significant oxygen isotope exchange occurred between sulfate and water. Overall, the consistency of sulfate δ¹⁸O values between treated and un-treated solutions, as well as natural samples, demonstrated that the proposed method effectively removed nitrate from sulfate solutions and yielded accurate sulfate δ¹⁸O values. This advancement provides a streamlined and reliable approach for sulfate O isotope analysis, with broad applicability to similar environmental and geochemical studies.