Managing argon interference during measurements of O-18/O-16 ratios in O-2 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 O-18/O-16 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, delta O-18, could be overestimated by more than 6 parts per thousand because of interferences from argon in air. Here, we systematically evaluated the extent of such Ar interferences on O-18/O-16 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 delta O-18. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors, epsilon(o), and O-18 kinetic isotope effects (O-18 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 delta O-18 measurements and experimental variability. Ar interferences can become more relevant and bias epsilon(o) values by more than 2 parts per thousand in aqueous samples where fractional O-2 conversion exceeds 90%. Practically, however, such samples would typically contain less than 25 mu M of O-2 at ambient temperature, an amount that is close to the method detection limit of O-18/O-16 ratio measurement by CF-IRMS.