Improving the accuracy of δ O and δ O values of O measured by continuous-flow isotope-ratio mass spectrometry with a multipoint isotope-ratio calibration.

Rationale: Stable isotope analysis of O is a valuable tool to identify O -consuming processes in the environment; however, reference materials for O isotope analysis are lacking. Consequently, a one-point calibration with O 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 δ O and δ O values of O associated with a one-point calibration.

Methods: We produced O photosynthetically with extracted spinach thylakoids from source waters with δ O values of -56‰ to +95‰ and δ O 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 O will be identical to that of the source water. The δ O and δ O values of the produced O were measured by gas chromatography coupled with isotope-ratio mass spectrometry (GC/IRMS), applying a common one-point calibration.

Results: Linear regressions between δ O or δ O values of the produced O 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 δ O and δ O, respectively. Triple oxygen isotopic measurements of O during consumption by Fe resulted in a δ O-δ O relationship (λ) of 0.49 ± 0.01 without δ scale correction, slightly lower than expected for mass-dependent O isotopic fractionation.

Conclusions: No significant bias is introduced on the δ O scale when applying a one-point calibration with O from ambient air during O isotope analysis. Both O formation and consumption experiments, however, indicate a δ O scale compression. Consequently, δ O values cannot be measured accurately by GC/IRMS with a one-point calibration without determining the δ O scale correction factor, e.g. with the O formation experiments described here.