Oxygen is one of the elements widely considered to be “unmeasurable” by ambient condition plasma-based mass spectrometric techniques due to its high background, low ionisation potential, and multitude of interferences on each isotope. As part of the Unconventional Research Funding call from Land Steiermark, Austria, we began to take the first steps by developing a method for the determination of oxygen isotope ratios in water using inductively coupled plasma – tandem mass spectrometry (ICP-MS/MS). As the technique utilizes a mass filter before the reaction cell, only interferences with the same mass-to-charge ratio as the target isotope of oxygen need to be considered. Nitrous oxide, oxygen, hydrogen, ammonia, and helium cell gases were investigated to mitigate interferences from hydrides of oxygen and nitrogen, as well as doubly-charged interferences from argon and sulphur. Additionally, matrix suppression effects were investigated in order to be able to correct for differences in matrix salt loads.

Initial isotope ratio measurements were conducted using purified water standards spiked gravimetrically with 18O-labelled water, the ratios of which agreed well with the calculated isotope ratios. (bias: Δ = +1.2 ‰, combined uncertainty: u_c = ±2.3 ‰ (k = 1)) Obtained combined uncertainties (>95% contribution from instrument precision) were significantly higher than for routinely applied methods such as isotope ratio mass spectrometry (IRMS) or cavity ring down spectroscopy (CRDS). The methodology was then further developed using water reference materials certified for δ¹⁸O/¹⁶O, where the intermediate precision (n = 6) was reduced to 0.7-0.9 ‰ (k = 1). Although the intermediate precision obtained using the ICP-MS/MS approach currently cannot compete with existing methods for determinations of oxygen isotope ratios, this study acts as a promising proof-of-concept for further developments in this field, where knowledge gained could be carried forward to future applications using plasma-based mass spectrometry approaches. In particular, the simultaneous determination of δ¹⁸O/¹⁶O ratios and elemental content, for example through laser ablation ICP-MS, opens up numerous possibilities.