Balint Sawyer, Schwartz Morgan, Fowler Drew N, Linnekogel Stella, Clemons Sáde Cromratie, Burkemper Laura K
ORISE Participant, U.S. EPA Atlantic Coastal Environmental Sciences Division, Narragansett, Rhode Island, United States.
U.S. EPA Atlantic Coastal Environmental Sciences Division, Narragansett, Rhode Island, United States.
Rapid Commun Mass Spectrom. 2024 Sep 15;38(17):e9837. doi: 10.1002/rcm.9837.
In stable isotope mass spectrometry, isotope delta values are normalized to internationally recognized reference scales using a combination of certified and in-house isotope reference materials. Numerous techniques exist for performing this normalization, but these methodologies need to be experimentally assessed to compare their effect on reproducibility of isotope results.
We tested normalization methods by the number of reference materials used, their matrix, their isotope range, and whether normalization required extrapolating beyond the isotope range. We analyzed eight commercially available isotope reference materials on a ThermoFinnigan Delta-V isotope ratio mass spectrometer (IRMS) and an Elementar VisION IRMS for nitrogen and carbon isotope composition via solid combustion with an elemental analyzer and computed every possible isotope normalization (n = 612). Additionally, we assessed how sample matrix affected linearity effects on both instruments using five in-house reference materials.
Normalizations exhibited the best performance when the reference materials spanning an isotope range of at least 20‰ were matrix matched with the samples and did not require extrapolation beyond the calibration curve. When these conditions were not met, the number of reference materials used had a significant effect on accuracy, with normalizations composed of two reference materials exhibiting particularly inconsistent performance at isotope ranges below 20‰. Linearity effects were found to exceed instrument precision by two orders of magnitude irrespective of matrix type and were not predicted by working gas diagnostics.
Interlaboratory comparability of isotope results is improved when operators of elemental analyzer isotope ratio mass spectrometry (EAIRMS) systems select reference materials spanning an isotope range of at least 20‰. Additionally, using three or more isotopic reference materials, avoiding extrapolation beyond the range of the normalization curve, and matching the matrix of the reference materials to the samples improve normalizations.
在稳定同位素质谱分析中，使用经认证的和内部同位素参考物质的组合，将同位素δ值归一化到国际认可的参考尺度。存在多种执行这种归一化的技术，但这些方法需要进行实验评估，以比较它们对同位素结果重现性的影响。
我们根据所使用的参考物质数量、其基质、其同位素范围以及归一化是否需要在校准曲线范围之外进行外推来测试归一化方法。我们在ThermoFinnigan Delta-V同位素比率质谱仪（IRMS）和Elementar VisION IRMS上分析了八种市售同位素参考物质，通过元素分析仪进行固体燃烧来测定氮和碳同位素组成，并计算了每一种可能的同位素归一化（n = 612）。此外，我们使用五种内部参考物质评估了样品基质对两台仪器线性效应的影响。
当跨越至少20‰同位素范围的参考物质与样品的基质相匹配且不需要在校准曲线范围之外进行外推时，归一化表现出最佳性能。当这些条件不满足时，所使用的参考物质数量对准确性有显著影响，由两种参考物质组成的归一化在低于20‰的同位素范围内表现出特别不一致的性能。无论基质类型如何，线性效应被发现超过仪器精度两个数量级，并且工作气体诊断无法预测。
当元素分析仪同位素比率质谱仪（EAIRMS）系统的操作人员选择跨越至少20‰同位素范围的参考物质时，同位素结果的实验室间可比性得到改善。此外，使用三种或更多种同位素参考物质、避免在校准曲线范围之外进行外推以及使参考物质的基质与样品相匹配可改善归一化。
