Tulane University, Department of Earth and Environmental Science, 6823 St. Charles Avenue, New Orleans, LA, USA.
Rapid Commun Mass Spectrom. 2013 Aug 30;27(16):1847-57. doi: 10.1002/rcm.6634.
The burgeoning field of 'clumped isotope' paleothermometry, which has broad applications in geosciences, depends almost entirely on measurements made on two types of mass spectrometer with the same ion source design. Demonstration that these measurements can be carried out on a retrofitted mass spectrometer with a different type of ion source is important to the growing community of geoscientists considering employing this technique.
We summarize the sample preparation techniques, mass spectrometry, and data processing involved in establishing an absolute reference frame for the measurement of Δ47 in CO2 from carbonate minerals using a prototype 5 kV mass spectrometer. The prototype differs from previously marketed mass spectrometers only in the presence of six extra Faraday cups designed specifically to measure ion beams of masses 44-49. We employ pressure baseline corrections on the small mass 47 signal.
The system was fully capable of establishing absolute reference frames for clumped isotope carbonate paleothermometry, despite alternative use of the system for other types of measurements (carbonate δ(18)O and δ(13)C values, water δ(18)O and δ(2)H values via equilibration, and δ(13) C and δ(15)N values of sedimentary organic material) in different reference frames. Repeated measurements of CO2 from carbonate standards yielded external precisions comparable with those from other laboratories and adequate for paleothermometry to a precision of near or below ±5 °C. The accuracy was demonstrated by reasonable temperatures being obtained for different types of repeatedly measured carbonates. Measurements of the pressure baseline with every sample increased analysis times by 20 min, but decreased the stabilization time of the reference frame by days to weeks, while improving precision.
Simple modifications to existing instrumentation will allow more laboratories to perform clumped isotope measurements. The prototype mass spectrometer used herein provides precision comparable with all published data. We show that, where negatively charged species are associated with large ion beams, making pressure baseline corrections with every measurement is increasingly important no matter what instrument is used.
“凝聚同位素”古温标领域正在蓬勃发展，该领域在地球科学中有广泛的应用，几乎完全依赖于具有相同离子源设计的两种类型质谱仪进行的测量。证明可以在具有不同类型离子源的改装质谱仪上进行这些测量，对于考虑采用该技术的越来越多的地球科学家群体来说是很重要的。
我们总结了使用原型 5 kV 质谱仪为碳酸盐矿物中的 CO2 测量 Δ47 建立绝对参考框架所涉及的样品制备技术、质谱分析和数据处理。该原型与之前销售的质谱仪的唯一区别在于存在六个额外的法拉第杯，这些法拉第杯专门用于测量质量为 44-49 的离子束。我们对小质量 47 信号进行压力基线校正。
尽管系统用于不同参考框架下的其他类型测量（碳酸盐 δ(18)O 和 δ(13)C 值、水 δ(18)O 和 δ(2)H 值通过平衡以及沉积有机物质的 δ(13)C 和 δ(15)N 值），但该系统仍完全能够为凝聚同位素碳酸盐古温标建立绝对参考框架。对碳酸盐标准品 CO2 的重复测量得出的外部精密度与其他实验室相当，足以达到近 ±5 °C 或更高的古温标精度。通过对不同类型的重复测量碳酸盐获得合理的温度来证明准确性。每次测量都测量压力基线会将分析时间增加 20 分钟，但会将参考框架的稳定时间减少几天到几周，同时提高精度。
对现有仪器进行简单修改将允许更多的实验室进行凝聚同位素测量。本文中使用的原型质谱仪提供的精度可与所有已发表的数据相媲美。我们表明，在带负电荷的物质与大离子束相关的情况下，无论使用何种仪器，每次测量都进行压力基线校正变得越来越重要。
