Pressure baseline correction and high-precision CO2 clumped-isotope (∆47) measurements in bellows and micro-volume modes.

RATIONALE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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).