Aerts-Bijma Anita Th, van Buuren Albert C, Paul Dipayan, Meijer Harro A J
Centre for Isotope Research (CIO), Energy and Sustainability Research Institute Groningen, University of Groningen, Groningen, The Netherlands.
Rapid Commun Mass Spectrom. 2024 Mar 30;38(6):e9678. doi: 10.1002/rcm.9678.
SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ O value is -55.5‰, although several expert laboratories measure significantly more negative δ O values. The real δ O value as such does not influence the isotopic water scale; however, knowledge of the size of isotopic scale contraction in stable isotope measurements is vital for second-order isotopes. This study describes the quantification of δ O with respect to δ O .
SLAP-like water was quantitatively mixed with highly O-enriched water to mimic VSMOW. The O concentration was determined using an electron ionization quadrupole mass spectrometer. The isotopic composition of the SLAP-like and VSMOW-like waters was measured using an optical spectrometer, alongside original VSMOW and SLAP.
This study resulted in a much more negative δ O value for SLAP than expected. The averaged outcome of seven independent experiments is δ O = -56.33 ± 0.03‰. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating isotope laboratories and the true δ O value.
Although this finding as such does not influence the use of the VSMOW-SLAP scale, it raises the intriguing question of what we actually measure with our instruments and why even a fully corrected measurement can be so far off. Our result has consequences for issues like the transfer of δ O from and to the VPDB scale, various fractionation factors, and Δ O. The absolute O abundance for SLAP was calculated as (1887.98 ± 0.43) × 10 based on the absolute O abundance of VSMOW and the presented δ O in this paper.
SLAP是同位素水标度的两种校准材料之一。经协商确定的δO值为-55.5‰，尽管一些专业实验室测得的δO值明显更负。实际的δO值本身并不影响同位素水标度；然而，了解稳定同位素测量中同位素标度收缩的大小对于二级同位素至关重要。本研究描述了相对于δO的δO定量。
将类似SLAP的水与高度富集18O的水定量混合以模拟VSMOW。使用电子电离四极杆质谱仪测定18O浓度。使用光学光谱仪测量类似SLAP和类似VSMOW的水的同位素组成，同时测量原始的VSMOW和SLAP。
本研究得出SLAP的δO值比预期的更负。七个独立实验的平均结果是δ18O = -56.33±0.03‰。即使是操作最仔细的同位素实验室的实际同位素测量值与真实的δO值之间也存在很大差异。
尽管这一发现本身并不影响VSMOW-SLAP标度的使用，但它提出了一个有趣的问题，即我们用仪器实际测量的是什么，以及为什么即使是完全校正后的测量值也可能偏差如此之大。我们的结果对诸如δ18O从VPDB标度的转换以及向VPDB标度的转换、各种分馏因子和Δ17O等问题都有影响。根据VSMOW的绝对18O丰度和本文给出的δ18O，计算出SLAP的绝对18O丰度为(1887.98±0.43)×10-6。
