de Carvalho Carolina F M, Lehmann Moritz F, Pati Sarah G
Department of Environmental Sciences, University of Basel, Basel, Switzerland.
Rapid Commun Mass Spectrom. 2024 Jan 15;38(1):e9652. 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.
氧的稳定同位素分析是识别环境中氧消耗过程的一种有价值的工具；然而，缺乏用于氧同位素分析的参考材料。因此，通常采用来自环境空气的氧进行单点校准，这可能导致相当大的测量不确定性。我们的目标是开发一种简单的多点同位素比率校准方法，并确定与单点校准相关的氧的δ¹⁸O和δ¹⁷O值的测量误差。
我们利用从δ¹⁸O值为-56‰至+95‰、δ¹⁷O值为-30‰至+46‰的源水中提取的菠菜类囊体通过光合作用产生氧气。选择光合作用是因为这个过程不会引起同位素分馏，所以产生的氧气的氧同位素组成将与源水相同。通过气相色谱-同位素比率质谱联用（GC/IRMS）测量产生的氧气的δ¹⁸O和δ¹⁷O值，采用常见的单点校准。
产生的氧气的δ¹⁸O或δ¹⁷O值与相应源水的δ¹⁸O或δ¹⁷O值之间的线性回归分别得到斜率为0.99±0.01和0.92±0.10。在测试的δ范围内，单点校准因此分别为δ¹⁸O和δ¹⁷O引入了最大误差0.8‰和3.3‰。铁消耗氧气过程中氧气的三重氧同位素测量在未进行δ尺度校正时得到的δ¹⁷O-δ¹⁸O关系（λ）为0.49±0.01，略低于质量依赖型氧同位素分馏的预期值。
在氧同位素分析期间，使用来自环境空气的氧进行单点校准时，在δ¹⁸O尺度上不会引入显著偏差。然而，氧气形成和消耗实验均表明存在δ¹⁷O尺度压缩。因此，在未确定δ¹⁷O尺度校正因子的情况下，通过GC/IRMS进行单点校准无法准确测量δ¹⁷O值，例如可采用此处所述的氧气形成实验来确定校正因子。
