Managing argon interference during measurements of (18)O/(16)O ratios in O(2) by continuous-flow isotope ratio mass spectrometry

ABSTRACT: Monitoring changes in stable oxygen isotope ratios in molecular oxygen allows for studying many fundamental processes in bio(geo)chemistry and environmental sciences. While the measurement of [Formula: see text] O/[Formula: see text] O ratios of [Formula: see text] in gaseous samples can be carried out conveniently and from extracting moderately small aqueous samples for analyses by continuous-flow isotope ratio mass spectrometry (CF-IRMS), oxygen isotope signatures, [Formula: see text] O, could be overestimated by more than 6[Formula: see text] because of interferences from argon in air. Here, we systematically evaluated the extent of such Ar interferences on [Formula: see text] O/[Formula: see text] O ratios of [Formula: see text] for measurements by gas chromatography/IRMS and GasBench/IRMS and propose simple instrumental modifications for improved Ar and [Formula: see text] separation as well as post-measurement correction procedures for obtaining accurate [Formula: see text] O. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors, [Formula: see text] , and [Formula: see text] O kinetic isotope effects ([Formula: see text] O KIEs) in samples where [Formula: see text] is consumed and Ar:[Formula: see text] ratios increase steadily and substantially over the course of a reaction. We show that the extent of O isotope fractionation is overestimated only slightly and that this effect is typically smaller than uncertainties originating from the precision of [Formula: see text] O measurements and experimental variability. Ar interferences can become more relevant and bias [Formula: see text] values by more than [Formula: see text] in aqueous samples where fractional [Formula: see text] conversion exceeds 90%. Practically, however, such samples would typically contain less than 25 [Formula: see text] M of [Formula: see text] at ambient temperature, an amount that is close to the method detection limit of [Formula: see text] O/[Formula: see text] O ratio measurement by CF-IRMS. GRAPHICAL ABSTRACT: [Image: see text] SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00216-022-04184-3.