Interlaboratory test for stable carbon isotope analysis of dissolved inorganic carbon in geothermal fluids

Stable carbon isotope ratios have many applications in natural sciences. In the first worldwide interlaboratory proficiency test, the discrepancies in measured δ¹³ C_DIC values of natural waters were up to σ = ±3‰. Therefore, we continued the investigation on the analytical data quality assurance of individual laboratories and internal consistency among laboratories worldwide.

We designed and performed an interlaboratory comparison exercise for δ¹³ C analyses of ten water and two solid samples (Na₂ CO₃ , 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 δ¹³ C values were measured using dual inlet isotope ratio mass spectrometry (DI-IRMS) or continuous flow (CF)-IRMS.

The δ¹³ C values of solid Na₂ CO₃ and its aqueous solution were -5.06 ± 0.21‰ and 5.32 ± 0.24‰, respectively, while the δ¹³ C value of solid CaCO₃ was -4.49 ± 0.93‰. Similarly, the lake water has a consistent value (2.45 ± 0.19‰). The δ¹³ C values of geothermal water have a wide dispersion among individual laboratory measurements and among those of different laboratories; however, a trend exists in the δ¹³ C values measured at the three sampling points of each well.

The δ¹³ C values of solid Na₂ CO₃ 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 δ¹³ C values of solid CaCO₃ is associated with its lower chemical affinity than that of Na₂ CO₃ . The poor reproducibility in the δ¹³ C 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 H₃ PO₄ acid reaction).