Measurement of multiply substituted isotopologues ('clumped isotopes') of CO_2 using a 5 kV compact isotope ratio mass spectrometer: Performance, reference frame, and carbonate paleothermometry

摘要

RATIONALE: 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. METHODS: We summarize the sample preparation techniques, mass spectrometry, and data processing involved in establishing an absolute reference frame for the measurement of Δ_(47) in CO_2 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. RESULTS: 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 d~(18)O and d~(13)C values, water d~(18)O and d~2H values via equilibration, and d~(13)C and d~(15)N values of sedimentary organic material) in different reference frames. Repeated measurements of CO_2 from carbonate standards yielded external precisions comparable with those from other laboratories and adequate for paleothermometry to a precision of near or below ±5 oC. 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. CONCLUSIONS: 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.