Improving the accuracy of δ18O and δ17O values of O2 measured by continuous‐flow isotope‐ratio mass spectrometry with a multipoint isotope‐ratio calibration

Abstract

RationaleStable isotope analysis of O2 is a valuable tool to identify O2‐consuming processes in the environment; however, reference materials for O2 isotope analysis are lacking. Consequently, a one‐point calibration with O2 from ambient air is often applied, which can lead to substantial measurement uncertainties. Our goals were to develop a simple multipoint isotope‐ratio calibration approach and to determine measurement errors of δ18O and δ17O values of O2 associated with a one‐point calibration.MethodsWe produced O2 photosynthetically with extracted spinach thylakoids from source waters with δ18O values of −56‰ to +95‰ and δ17O values of −30‰ to +46‰. Photosynthesis was chosen because this process does not cause isotopic fractionation, so that the O isotopic composition of the produced O2 will be identical to that of the source water. The δ18O and δ17O values of the produced O2 were measured by gas chromatography coupled with isotope‐ratio mass spectrometry (GC/IRMS), applying a common one‐point calibration.ResultsLinear regressions between δ18O or δ17O values of the produced O2 and those of the corresponding source waters resulted in slopes of 0.99 ± 0.01 and 0.92 ± 0.10, respectively. In the tested δ range, a one‐point calibration thus introduced maximum errors of 0.8‰ and 3.3‰ for δ18O and δ17O, respectively. Triple oxygen isotopic measurements of O2 during consumption by Fe2+ resulted in a δ18O–δ17O relationship (λ) of 0.49 ± 0.01 without δ scale correction, slightly lower than expected for mass‐dependent O isotopic fractionation.ConclusionsNo significant bias is introduced on the δ18O scale when applying a one‐point calibration with O2 from ambient air during O2 isotope analysis. Both O2 formation and consumption experiments, however, indicate a δ17O scale compression. Consequently, δ17O values cannot be measured accurately by GC/IRMS with a one‐point calibration without determining the δ17O scale correction factor, e.g. with the O2 formation experiments described here.