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

Abstract

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 18O/16O 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 18O, 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 18O/16O 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 18O. We subsequently evaluated the consequences of Ar interferences for the quantification of O isotope fractionation in terms of isotope enrichment factors, ϵ O, and 18O kinetic isotope effects (18O 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 18O 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 18O/16O ratio measurement by CF-IRMS. Graphical abstract: [Figure not available: see fulltext.].