The Department of The Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, USA.
Rapid Commun Mass Spectrom. 2012 Dec 30;26(24):2837-53. doi: 10.1002/rcm.6436.
CO(2) 'clumped-isotope' measurements (tracking enrichment of (16)O(13)C(18)O, reported as ∆(47) values, on CO(2) derived from carbonate minerals or the atmosphere) are becoming central to a wide range of geochemical investigations. We present a novel approach to address problems with instrument stability, external precision, and the analysis of small samples that have hampered the advancement of Δ(47) measurements.
We measured Δ(47) values on CO(2) gases introduced via dual inlet to an isotope ratio mass spectrometer. We developed a method for determining the 'pressure baseline' and integrating a correction to ion beam intensity measurements during analysis. We then tested this approach for both bellows and micro-volume modes of sample introduction. Heated gas and equilibrated gas lines (Δ(47) vs. δ(47)) established the effectiveness of this correction.
We have determined that drift in instrument calibration that compromises Δ(47) measurements results from a shift in the baseline signal on sensitive collectors (m/z 47, 48, and 49) that occurs when gas is admitted to the ion source. Applying a 'pressure baseline' (PBL) correction significantly stabilizes ∆(47) measurements and reduces the dependence of ∆(47) values on δ(47) values by up to an order of magnitude.
PBL-corrected heated gas and equilibrated gas calibrations in bellows and micro-volume modes are nearly identical and stable through time. Introduction of the PBL correction, a revision to the absolute reference frame approach to determining Δ(47) values, dramatically improves the external precision of Δ(47) measurements to near instrumental analytical uncertainty (6-8 ppm (1σ) in bellows mode; 10-12 ppm in micro-volume mode).
CO2“凝聚同位素”测量（追踪碳酸盐矿物或大气中衍生的 CO2 中 16O13C18O 的富集，以 Δ47 值表示）在广泛的地球化学研究中变得至关重要。我们提出了一种新方法来解决仪器稳定性、外部精度以及小样本分析等问题，这些问题一直阻碍着 Δ47 测量的发展。
我们通过双入口将 CO2 气体引入同位素质谱仪来测量 Δ47 值。我们开发了一种确定“压力基线”并在分析过程中整合离子束强度测量校正的方法。然后，我们在波纹管和微体积模式下对这种方法进行了测试。加热气体和平衡气体线（Δ47 与 δ47）证明了这种校正的有效性。
我们已经确定，仪器校准漂移会影响 Δ47 测量，这是由于当气体进入离子源时，敏感收集器（m/z 47、48 和 49）上的基线信号发生偏移。应用“压力基线”（PBL）校正可显著稳定 Δ47 测量，并使 Δ47 值对 δ47 值的依赖性降低一个数量级。
波纹管和微体积模式下的 PBL 校正加热气体和平衡气体校准几乎相同，且随时间保持稳定。引入 PBL 校正，即对确定 Δ47 值的绝对参考框架方法进行修订，可显著提高 Δ47 测量的外部精度，使其接近仪器分析不确定性（波纹管模式下为 6-8 ppm（1σ）；微体积模式下为 10-12 ppm）。
