Modeling the Measurement: Δ47, Corrections, and Absolute Ratios for Reference Materials

Abstract

AbstractClumped isotope studies on CO2, Δ47, that is the excess in the isotopologue containing both 13C and 18O at mass 47, can be very useful since they can give temperature estimates independent of the bulk isotopic composition. The measurement itself can be affected by a number of items. Here we develop a data processing model to examine the effects different interferences might have on the final calculated value. It incorporates known issues, for example, pressure baseline, 17O excess, and shifts in absolute ratios for primary reference materials and parameters used for 17O correction. We also included linearity effects as well as differences in isotopologue absolute abundances at a given m/z. What normally would be considered acceptable mass spectrometer 45R and 46R linearity can skew Δ47 results. That is 0.04‰/V and −0.04‰/V linearity on 45R and 46R respectively would also cause an apparent shift in the parameters used for 17O corrections. Measurements were made on CO2(g) equilibrated with water, and we were able to match up the effects seen with model results. Linearity and small differences in amplitude between sample and working reference gas affected Δ47 determination, as did apparent shifts in isotopologue abundances under different conditions. This may (partially) account for discrepancies seen in Δ47‐temperature calibrations curves between laboratories. We also developed an easy way to precisely calculate the δ13C and δ18O that works well in spreadsheets without the need for multiple iterations.