Compositional and beam-size-dependent effects on pressure baseline in clumped isotope mass spectrometry

Rationale 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. Methods 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 Delta(47) over a range of delta(47) (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. Results The results of our beam size experiments showed that a direct measurement of the baseline consistently introduced variability to measurements of the Delta(47) 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. Conclusions 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.