δ(¹⁸O/¹⁶O) Determinations in Water Using Inductively Coupled Plasma–Tandem Mass Spectrometry

A fully validated measurement approach for the determination of oxygen isotope ratios in water using inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) is described for the first time. Deuterium was used as a reaction gas to mass-shift the oxygen isotopes to the OD₃⁺ product ion, allowing for the removal of polyatomic ions (¹⁶O¹H⁺ and ¹⁶O¹H₂⁺) generated in the plasma that interfere on the less abundant oxygen stable isotopes (¹⁷O and ¹⁸O). The developed methodology was validated for δ_VSMOW-SLAP(¹⁸O/¹⁶O) determinations using four internationally recognized IAEA reference materials, as well as 11 in-house laboratory isotope reference materials that were characterized by isotope ratio mass spectrometry and cavity ring-down spectroscopy. δ_VSMOW-SLAP(¹⁸O/¹⁶O) values determined by ICP-MS/MS and the reference methods overlapped within the uncertainties and showed no significant difference for water materials with a low matrix load. Elevated levels of sodium, chloride, and silicon in the matrix lead to isotope ratio shifts of up to 12‰ in both the positive and negative directions. Additionally, comparisons with indicative values from the reference materials demonstrated that δ_VSMOW-SLAP(¹⁷O/¹⁶O) determinations are also possible using the ICP-MS/MS approach. While the uncertainty of the developed approach (median uncertainty <i>u</i>_c(δ_VSMOW-SLAP(¹⁸O/¹⁶O)) = 0.70‰) is not currently able to match that of existing methodologies, it is envisioned that oxygen isotope ratio determinations can be performed alongside multielemental analysis by a wider community for applications where lower uncertainties are not required.