Venturelli Ryan A, Rosenheim Brad E
College of Marine Science, University of South Florida, 140 7th Avenue South, St Petersburg, FL, 33701, USA.
Rapid Commun Mass Spectrom. 2019 Jan 15;33(1):140-148. doi: 10.1002/rcm.8303.
The analysis of carbonate samples for the application of clumped isotopes to paleoclimate reconstruction necessitates smaller beam intensities. However, there is a relationship between beam intensity and pressure-dependent baseline (PBL), and therefore between beam intensity and the correction for PBL. Here we explain the relationship between PBL and beam intensity to develop a better correction protocol and an improved understanding of clumped isotope mass spectrometry.
We describe a beam size experiment using our Isoprime isotope ratio mass spectrometer in which samples of the carbonate standard IAEA-C1 were analyzed at 30, 50, and 70 nA to establish an optimal protocol and a new method to correct for PBL using the theoretical constraint of invariable Δ over a range of δ (bulk isotope composition) values. We also explore the effects of both over- and under-correction of PBL on equilibrated and heated gas samples to understand the effect of mis-correction of PBL.
The results of our beam size experiments showed that a direct measurement of the baseline consistently introduced variability to measurements of the Δ of heated gases, equilibrated gases, and carbonate standards. These results necessitated a new protocol to account for PBL in our system. Our new approach flattens the reference frame line slope to 0 and, importantly, reduces the variability of data points about the heated gas line. We also describe, for the first time, an empirically derived description of the compositional effect of PBL.
A seemingly small change in our isotope ratio mass spectrometer resulted in a better understanding of PBL, for which we have developed an empirically based correction protocol to apply. Our new protocol has the potential to reduce analytical time for laboratories measuring PBL, and supports the need for carbonate mineral-based clumped isotope standards.
对碳酸盐样品进行团簇同位素分析以用于古气候重建，需要较小的束流强度。然而，束流强度与压力相关基线（PBL）之间存在关系，因此束流强度与PBL校正之间也存在关系。在此，我们解释PBL与束流强度之间的关系，以制定更好的校正方案，并增进对团簇同位素质谱法的理解。
我们描述了一项使用Isoprime同位素比率质谱仪进行的束斑尺寸实验，其中对碳酸盐标准品IAEA-C1的样品在30、50和70 nA下进行分析，以建立最佳方案和一种利用在一系列δ（整体同位素组成）值上Δ不变的理论约束来校正PBL的新方法。我们还探讨了PBL校正过度和不足对平衡气体样品和加热气体样品的影响，以了解PBL校正错误的影响。
我们的束斑尺寸实验结果表明，直接测量基线会持续给加热气体、平衡气体和碳酸盐标准品的Δ测量带来变异性。这些结果需要我们制定一个新方案来处理系统中的PBL。我们的新方法将参考框架线斜率平坦化为0，重要的是，减少了数据点围绕加热气体线的变异性。我们还首次描述了基于经验得出的PBL组成效应描述。
我们的同位素比率质谱仪看似微小的变化带来了对PBL更好的理解，为此我们开发了一种基于经验的校正方案来应用。我们的新方案有可能减少测量PBL的实验室的分析时间，并支持对基于碳酸盐矿物的团簇同位素标准品的需求。
