[论文解读] Pros and cons of the technique of processing IRMS data as desired-delta values: uncertainty and comparability in results, a case study for determining carbon and oxygen isotopic abundance ratios as CO_2^+

B. P. Datta|arXiv (Cornell University)|Jan 5, 2011

Atmospheric and Environmental Gas Dynamics被引用 3

一句话总结

本文批判性地评估了将IRMS数据转换为'目标δ值'（例如δ¹³C、δ¹⁸O）用于CO₂⁺的常见做法，表明此类转换会引入显著的不确定性，并降低样本间的可比性。研究证明，绝对同位素丰度比值比差值（δ）值更准确，且使用辅助标准进行尺度转换无法完全消除对工作参考（W）的依赖，从而使得文献中广泛使用的标准转换公式失效。

ABSTRACT

In isotope ratio mass spectrometry (IRMS), any sample (S) measurement is performed as a relative-difference ((S/W)di) from a working-lab-reference (W), but the result is evaluated relative to a recommended-standard (D): (S/D)di. It is thus assumed that different source specific results ((S1/D)di, (S2/D)di) would represent their sources (S1, S2), and be accurately intercomparable. However, the assumption has never been checked. In this manuscript we carry out this task by considering a system as CO2+-IRMS. We present a model for a priori predicting output-uncertainty. Our study shows that scale-conversion, even with the aid of auxiliary-reference-standard(s) Ai(s), cannot make (S/D)di free from W; and the ((S/W)di,(A1/W)di,(A2/W)di) To (S/D)di conversion-formula normally used in the literature is invalid. Besides, the latter-relation has been worked out, which leads to e.g., fJ([(S/W)dJCO2pmp%],[(A1/W)dJCO2pmp%],[(A2/W)dJCO2pmp%]) = ((S/D)dJCO2pm4.5p%); whereas FJ([(S/W)dJCO2pmp%],[(A1/W)dJCO2pmp%]) = ((S/D)dJCO2pm1.2p%). That is, contrary to the general belief (Nature 1978, 271, 534), the scale-conversion by employing one than two Ai-standards should ensure (S/D)di to be more accurate. However, a more valuable finding is that the transformation of any d-estimate into its absolute value helps improve accuracy, or any reverse-process enhances uncertainty. Thus, equally accurate though the absolute-estimates of isotopic-CO2 and constituent-elemental-isotopic abundance-ratios could be, in contradistinction any differential-estimate is shown to be less accurate. Further, for S and D to be similar, any absolute estimate is shown to turn out nearly absolute accurate but any (S/D)d value as really absurd. That is, estimated source specific absolute values, rather than corresponding differential results, should really represent their sources, and/ or be closely intercomparable.

研究动机与目标

• 评估将IRMS数据转换为'目标δ值'（例如δ¹³C、δ¹⁸O）用于CO₂⁺同位素分析的有效性。
• 研究使用辅助参考标准（Ai）进行尺度转换是否能消除δ值计算中对工作实验室参考（W）的依赖。
• 评估IRMS数据处理中差值（δ）值与绝对同位素丰度比值的准确性与可比性。
• 检验(S/D)di值在不同来源之间是否可比，且独立于工作参考（W）的假设。

提出的方法

• 建立数学模型，预测在δ值转换框架下IRMS数据处理中的输出不确定性。
• 推导并分析从(S/W)di、(A1/W)di、(A2/W)di到(S/D)di的转换公式的固有缺陷。
• 以CO₂⁺为例，通过模拟同位素丰度比值，评估尺度转换对不确定性和可比性的影响。
• 比较使用一个与两个辅助标准（Ai）时(S/D)di值的不确定性，揭示标准假设中的不一致之处。
• 引入变换函数fJ与FJ，量化单辅助标准与双辅助标准转换之间不确定性差异。
• 使用图示说明与详细附录澄清符号并验证推导出的关系。

实验结果

研究问题

• RQ1 将IRMS数据转换为'目标δ值'(S/D)di是否真正消除了对工作实验室参考(W)的依赖？
• RQ2 使用辅助标准(Ai)进行尺度转换是否真能令(S/D)di值独立于W，如普遍假设的那样？
• RQ3 使用一个与两个辅助标准导出的(S/D)di值之间，不确定性的定量差异是多少？
• RQ4 为何在使用相同参考标准的情况下，(S/D)di值仍表现出不一致的准确性？
• RQ5 在IRMS数据处理中，绝对同位素丰度比值是否比差值（δ）值更准确？

主要发现

• 从(S/W)di、(A1/W)di、(A2/W)di到(S/D)di的标准转换公式在数学上无效，并引入系统性误差。
• 使用两个辅助标准(Ai)并不能确保(S/D)di的不确定性低于仅使用一个标准的情况；事实上，不确定性可能显著增加（例如，从1.2‰增至4.5‰）。
• 将δ估计值转换为其绝对值可提高准确性，而反向过程（将绝对值转换为δ值）则增加不确定性。
• 即使使用辅助标准，(S/D)di值仍依赖于工作参考(W)，与独立性的假设相矛盾。
• 绝对同位素丰度比值始终比相应的(S/D)di值更准确，当S与D相近时，后者被证明是“极为荒谬”的。
• 本研究证明，由于残留的W依赖性和不确定性的膨胀，(S/D)di值在不同来源之间不可靠地可比。