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

Methods

We reviewed R_std 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_std values and assessed the potential implications of using different R_std and the normalization versus tank working gas standard for consistency in reporting stable isotope results.

Results

Different R_std values are used by different manufacturers of stable isotope analytical instruments. For R(²H/¹H)_VSMOW two different but very similar values are used, 0.00015575 and 0.00015576; for R(¹³C/¹²C)_VPDB three different values are used, 0.0111802, 0.0112372 and 0.01118028; and for R(¹⁵N/¹⁴N)_Air-N2 two values, 0.0036782 and 0.0036765, are used. All manufacturers are using the same value for R(¹⁸O/¹⁶O)_VSMOW, 0.00200520, but three different values for R(¹⁸O/¹⁶O)_VPDB, 0.002067200, 0.00208835 and 0.002088349. For R(³⁴S/³²S)_VCDT four different R_std are used, 0.0441509, 0.0441626, 0.044162589 and 0.0441520597.

Conclusions

The use of different R_std 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_std 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_std values and recommendations for the best estimates are highly desirable to ensure worldwide consistency in stable isotope data reporting.

     

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