Pressure baseline correction and high-precision CO2 clumped-isotope (Δ47) measurements in bellows and micro-volume modes

RATIONALE CO2 clumped-isotope measurements (tracking enrichment of 16O13C18O, reported as Delta(47) values, on CO2 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 Delta(47) measurements. METHODS We measured Delta(47) values on CO2 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 (Delta(47) vs. delta(47)) established the effectiveness of this correction. RESULTS We have determined that drift in instrument calibration that compromises Delta(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 Delta 47 measurements and reduces the dependence of delta(47) values on delta(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 Delta 47 values, dramatically improves the external precision of Delta 47 measurements to near instrumental analytical uncertainty (68 ppm (1s) in bellows mode; 1012 ppm in micro-volume mode). Copyright (C) 2012 John Wiley & Sons, Ltd.