Improving the accuracy of δ 18 O and δ 17 O values of O 2 measured by continuous-flow isotope-ratio mass spectrometry with a multipoint isotope-ratio calibration

• Published in: Rapid communications in mass spectrometry 2024-01, Vol.38 (1), p.e9652
• Main Authors: de Carvalho, Carolina F M, Lehmann, Moritz F, Pati, Sarah G
• Format: Article
• Language: English
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.9652

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. 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. 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. 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.