Modeling the Measurement: Δ 47 , Corrections, and Absolute Ratios for Reference Materials

Clumped isotope studies on CO 2 , Δ 47 , that is the excess in the isotopologue containing both 13 C and 18 O 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, 17 O excess, and shifts in absolute ratios for primary reference materials and parameters used for 17 O 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 45 R and 46 R linearity can skew Δ 47 results. That is 0.04‰/V and −0.04‰/V linearity on 45 R and 46 R respectively would also cause an apparent shift in the parameters used for 17 O corrections. Measurements were made on CO 2 (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 Δ 47 determination, as did apparent shifts in isotopologue abundances under different conditions. This may (partially) account for discrepancies seen in Δ 47 ‐temperature calibrations curves between laboratories. We also developed an easy way to precisely calculate the δ 13 C and δ 18 O that works well in spreadsheets without the need for multiple iterations.