Measurement of multiply substituted isotopologues (’clumped isotopes’) of CO2 using a 5 kV compact isotope ratio mass spectrometer: Performance, reference frame, and carbonate paleothermometry
Brad E. Rosenheim, Jianwu Tang and Alvaro Fernandez
Tulane University, Department of Earth and Environmental Science, 6823 St. Charles Avenue, New Orleans, LA, USA
The field of ‘clumped isotope’ paleothermometry is a fast-growing field which has broad applications across the field of geosciences. Measurements of ‘clumped isotopes’ particularly CO2 evolved from mineral carbonates provide a paleothermometer capable of measuring the temperature of formation of the carbonate that is independent of the oxygen isotope composition of the formation waters. Measurements had previously all been made on two types of mass spectrometer that used the same ion source design and ion optics. This paper highlights the use of an Isoprime dual Inlet (e.g. iso DUAL INLET) with 5 kV compact isotope ratio mass spectrometer (IRMS) (e.g. isoprime precisION) from Elementar for accurate measurement of ‘clumped isotopes’. This benchtop set up has a much smaller footprint than previously used instruments, saving on precious laboratory space without compromising on precision of results.
The IRMS was modified to use 6 extra faraday cups designed to measure the ion beam masses from 44 – 49. The findings indicated that clumped isotopic scrambling (re-ordering of 13C-18O bonds) within the source of the mass spectrometer from carbonates was similar across different measurement platforms. This allows the mass spectrometry community to implement inter-laboratory comparisons and highlights that any differences in temperature reconstruction are more likely to derive from sample pretreatments than from the mass spectrometer reference frame. The precision of repeated CO2 measurements from carbonate yields was comparable to other laboratories; with paleothermometry to a precision of ±5 °C. Therefore, the isoprime precisION IRMS system allows a much wider range of laboratories to perform precise and accurate clumped isotope measurements that are comparable to published data. The iso DUAL INLET technology is also a valuable tool for high precision pure gases such as CO2, O2, H2 and SO2; and can be further enhanced with the iso MULTI PREP inlet device for carbonate materials and water samples.