Verma, M. P., Van Geldern, R., Carvalho, M. C., Grassa, F., Delgado Huertas, A., Monvoisin, G., Carrizo, D. (2020). Interlaboratory test for stable carbon isotope analysis of dissolved inorganic carbon in geothermal fluids. Rapid Communications in Mass Spectrometry 34, 9 DOI: 10.1002/rcm.8685

Rationale Stable carbon isotope ratios have many applications in natural sciences. In the first worldwide interlaboratory proficiency test, the discrepancies in measured delta C-13(DIC) values of natural waters were up to sigma = +/- 3 parts per thousand. Therefore, we continued the investigation on the analytical data quality assurance of individual laboratories and internal consistency among laboratories worldwide.

Methods We designed and performed an interlaboratory comparison exercise for delta C-13 analyses of ten water and two solid samples (Na2CO3, CaCO3), including two synthetic samples prepared by dissolving the carbonates individually. Three laboratories analyzed an additional sample set to assess solution stability, at least one month after the first set analysis period. The delta C-13 values were measured using dual inlet isotope ratio mass spectrometry (DI-IRMS) or continuous flow (CF)-IRMS.

Results The delta C-13 values of solid Na2CO3 and its aqueous solution were -5.06 +/- 0.21 parts per thousand and 5.32 +/- 0.24 parts per thousand, respectively, while the delta C-13 value of solid CaCO3 was -4.49 +/- 0.93 parts per thousand. Similarly, the lake water has a consistent value (2.45 +/- 0.19 parts per thousand). The delta C-13 values of geothermal water have a wide dispersion among individual laboratory measurements and among those of different laboratories; however, a trend exists in the delta C-13 values measured at the three sampling points of each well.

Conclusions The delta C-13 values of solid Na2CO3 and its solution, and lake water (i.e. DIC concentration samples >100 mg/L carbon) are consistent among all the participating laboratories. The dispersion in the delta C-13 values of solid CaCO3 is associated with its lower chemical affinity than that of Na2CO3. The poor reproducibility in the delta C-13 values of geothermal fluids, collected at three points of a geothermal well, despite overall consistent trends regarding their collection points suggests inadequate sample handling (atmospheric CO2 exchange) and/or inappropriate analytical approaches (incomplete H3PO4 acid reaction).