Managing argon interference during measurements of 18O/16O ratios in O2 by continuous-flow isotope ratio mass spectrometry

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 18 O/ 16 O ratios of O 2 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, δ 18 O, could be overestimated by more than 6 ‱ because of interferences from argon in air. Here, we systematically evaluated the extent of such Ar interferences on 18 O/ 16 O ratios of O 2 for measurements by gas chromatography/IRMS and GasBench/IRMS and propose simple instrumental modifications for improved Ar and O 2 separation as well as post-measurement correction procedures for obtaining accurate δ 18 O. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors, ϵ O , and 18 O kinetic isotope effects ( 18 O KIEs) in samples where O 2 is consumed and Ar: O 2 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 δ 18 O measurements and experimental variability. Ar interferences can become more relevant and bias ϵ O values by more than 2 ‱ in aqueous samples where fractional O 2 conversion exceeds 90%. Practically, however, such samples would typically contain less than 25 μ M of O 2 at ambient temperature, an amount that is close to the method detection limit of 18 O/ 16 O ratio measurement by CF-IRMS. Graphical abstract