Precise determination of sulfur isotopic ratio in aqueous solutions by inductively coupled plasma mass spectrometry

Analytical procedures for complete separation of SO₄ ^2- in aqueous solutions and precise sulfur isotopic ratio determination using inductively coupled plasma double focusing sector field mass spectrometry (ICP-SMS) operated at low resolution (m/Δw = 300) were developed. A platinum guard electrode and an Aridus sample delivering apparatus were combined to reduce ¹⁶O¹⁶O⁺ interferences at m/z 32 and to enhance analytical sensitivity. A precision of better than 0.1% for successive measurements of seawater and IAPSO was achieved with 2-5 μl aqueous solutions in less than a minute of measurement time. The sulfate concentrations measured in rains, seawater and pore waters agree well with those obtained by ion chromatography. The downcore δ³⁴S variations in porewater at Site 1202 are greater than 50%, highlighting the effects of sulfate reduction. The sulfur isotopic compositions in rains and seawater samples reflect sensitively the source and oxidation/reduction conditions.