Absolute isotope ratios defining isotope scales used in isotope ratio mass spectrometers and optical isotope instruments.

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 (R ). Therefore, R requires accurate determination. However, the literature contains a large number of R values, and selection of different R may lead to inconsistency in reporting and recalculating stable isotope results.

METHODS

We reviewed R used in the proprietary software provided by the manufacturers of stable isotope instruments commonly employed for analyses of stable HCNOS compositions. We compared the R values and assessed the potential implications of using different R and the normalization versus tank working gas standard for consistency in reporting stable isotope results.

RESULTS

Different R values are used by different manufacturers of stable isotope analytical instruments. For R( H/ H) two different but very similar values are used, 0.00015575 and 0.00015576; for R( C/ C) three different values are used, 0.0111802, 0.0112372 and 0.01118028; and for R( N/ N) two values, 0.0036782 and 0.0036765, are used. All manufacturers are using the same value for R( O/ O) , 0.00200520, but three different values for R( O/ O) , 0.002067200, 0.00208835 and 0.002088349. For R( S/ S) four different R are used, 0.0441509, 0.0441626, 0.044162589 and 0.0441520597.

CONCLUSIONS

The use of different R values may lead to differences in the isotope delta values obtained if the normalization versus working standard gas is applied. For the range of R 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 R values and recommendations for the best estimates are highly desirable to ensure worldwide consistency in stable isotope data reporting.