Rationale: The isotope delta is calculated from the isotope ratio of a sample and the absolute isotope ratio of the zero reference point defining each stable isotope international scale (Rstd ). Therefore, Rstd requires accurate determination. However, the literature contains a large number of Rstd values, and selection of different Rstd may lead to inconsistency in reporting and recalculating stable isotope results.
Methods: We reviewed Rstd used in the proprietary software provided by the manufacturers of stable isotope instruments commonly employed for analyses of stable HCNOS compositions. We compared the Rstd values and assessed the potential implications of using different Rstd and the normalization versus tank working gas standard for consistency in reporting stable isotope results.
Results: Different Rstd values are used by different manufacturers of stable isotope analytical instruments. For R(2 H/1 H)VSMOW two different but very similar values are used, 0.00015575 and 0.00015576; for R(13 C/12 C)VPDB three different values are used, 0.0111802, 0.0112372 and 0.01118028; and for R(15 N/14 N)Air-N2 two values, 0.0036782 and 0.0036765, are used. All manufacturers are using the same value for R(18 O/16 O)VSMOW , 0.00200520, but three different values for R(18 O/16 O)VPDB , 0.002067200, 0.00208835 and 0.002088349. For R(34 S/32 S)VCDT four different Rstd are used, 0.0441509, 0.0441626, 0.044162589 and 0.0441520597.
Conclusions: The use of different Rstd values may lead to differences in the isotope delta values obtained if the normalization versus working standard gas is applied. For the range of Rstd used in proprietary software, the potential differences are lowest for oxygen (< 0.002 ‰) and nitrogen (< 0.001 ‰), and highest for carbon (0.107 to 0.112 ‰) and sulfur (0.023 ‰). Evaluation of the existing Rstd values and recommendations for the best estimates are highly desirable to ensure worldwide consistency in stable isotope data reporting.
© 2020 John Wiley & Sons, Ltd.