Absolute isotopic abundance ratios and the accuracy of Δ47 measurements
Absolute isotopic abundance ratios and the accuracy of Δ47 measurements
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DOI：
10.1016/j.chemgeo.2016.08.014
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发表时间：
2016-11-28
期刊：
影响因子：
3.9
通讯作者：
Affek, H. P.
中科院分区：
文献类型：
--
作者：
Daeron, M.;Blamart, D.;Affek, H. P.
成块同位素测量旨在量化给定样品中同位素物群体的一些统计特性，这需要参考材料（如VSMOW或VPDB）中绝对同位素丰度比的先验知识。在CO2的情况下，问题进一步复杂化，因为需要定义一个质量相关的分馏定律，将氧的三种稳定同位素联系起来。因此，从原始质谱数据到质量-47异常的转换（Delta（47））依赖于四个外部参数：VPDB的（C-13/C-12）比率、VSMOW（或VPDB-CO2）的（O-17/O-16）和（O-18/O-16）比率以及三重氧同位素线（lambda）的斜率。在这里，我们调查的影响，这些同位素参数施加对测量的Delta（47）值，使用（a）对应于7个月的测量的真实世界的数据;（B）基于随机生成的数据的简单模拟;（c）水平衡的CO2样品之间的精确比较和碳酸盐之间的标准认为共享准相同的Delta（47）值;和（d）重新处理的两个碳酸盐校准数据集与不同的斜率的Delta（47）与T。我们证明，使用不同的同位素参数集通常会产生系统的偏移高达0.04%的测量Delta（47）值，即使在以下建立的标准化程序。更重要的是，即使使用一组同位素参数，也会在测量的Delta（47）值中产生实验室内和实验室间的差异，如果这些参数中的一些不准确，并且取决于用于转换为Dennis等人（2011）绝对标度的标准品的同位素组成，这些误差应该与Delta C-13或Delta O-18强烈相关，或两者都更弱。基于对预期显示准相同Delta（47）值的样品的测量，例如具有不同碳和氧同位素组成的25摄氏度水平衡CO2，或高温标准ETH-1和ETH-2，我们得出结论，所有早期聚集同位素研究中使用的传统同位素参数集产生大量，由样品和标准品的相对本体同位素组成控制的系统误差。这些误差很可能是造成目前实验室间差异的关键因素之一，但不能轻易解释不同小组获得的碳酸盐校准斜率的冲突。相比之下，Brand等人（2010年）的同位素参数似乎可以产生准确的Delta（47）值，而与散装同位素组成无关。基于这些发现，我们提供了建议，旨在尽量减少错误的同位素参数的选择。(C)2016爱思唯尔B. V.保留所有权利。
Clumped isotope measurements aim to quantify some statistical properties of the isotopologue population in a given sample, which requires prior knowledge of the absolute isotopic abundance ratios in reference materials such as VSMOW or VPDB. In the case of CO2, matters are further complicated by the need to define a mass-dependent fractionation law linking the three stable isotopes of oxygen. Conversion from raw mass spectrometric data to mass-47 anomalies (Delta(47)) thus relies on four external parameters: the (C-13/C-12) ratio of VPDB, the (O-17/O-16) and (O-18/O-16) ratios of VSMOW (or VPDB-CO2), and the slope of the triple oxygen isotope line (lambda). Here we investigate the influence that these isotopic parameters exert on measured Delta(47) values, using (a) real-world data corresponding to seven months of measurements; (b) simple simulations based on randomly generated data; (c) precise comparisons between water-equilibrated CO2 samples and between carbonate standards believed to share quasi-identical Delta(47) values; and (d) reprocessing of two carbonate calibration data sets with different slopes of Delta(47) versus T.We demonstrate that the use of different sets of isotopic parameters generally produces systematic offsets as large as 0.04% in measured Delta(47) values, even after following the established standardization procedures. What's more, even using a single set of isotopic parameters can produce intra-and inter-laboratory discrepancies in measured Delta(47) values, if some of these parameters are inaccurate, and depending on the isotopic compositions of the standards used for conversion to the absolute scale of Dennis et al. (2011), these errors should correlate strongly with either delta C-13 or delta O-18, or more weakly with both. Based on measurements of samples expected to display quasi-identical Delta(47) values, such as 25 degrees C water-equilibrated CO2 with different carbon and oxygen isotope compositions, or high-temperature standards ETH-1 and ETH-2, we conclude that the traditional set of isotopic parameters used in all early clumped isotope studies produces large, systematic errors controlled by the relative bulk isotopic compositions of samples and standards. These errors are likely to be one of the key factors responsible for current inter-laboratory discrepancies, but cannot easily explain the conflicting carbonate calibration slopes obtained by different groups. By contrast, the isotopic parameters of Brand et al. (2010) appear to yield accurate Delta(47) values regardless of bulk isotopic composition. Based on these findings, we offer recommendations aiming to minimize errors related to the choice of isotopic parameters. (C) 2016 Elsevier B.V. All rights reserved.