Experimental method for calibrating δ13C in dissolved inorganic carbon (DIC) in water

In the river basin, the sequestration of atmospheric carbon into organic matter by   terrestrial plants, and the subsequent bacterial oxidation in the soil produce elevated pCO₂, playing a crucial role in the carbon cycle. This intense biogeochemical activity contributes to the formation of dissolved inorganic carbon (DIC) in waters through the weathering of silicates and carbonates, and particulate inorganic carbon (PIC) via mechanical erosion of rocks. Riverine degassing occurs due to the high CO₂ concentrations compared to the atmosphere, while the oxidation of organic matter and photosynthesis regulate the dynamics of DIC and PIC in the waters, influencing carbonate precipitation. This interplay highlights the rivers’ relevance in the global carbon cycle, evidencing the interaction between atmospheric and geological processes¹. Regarding δ¹³C DIC analysis, the sampling methods involve various steps such as precipitation, filtration, and the use of highly toxic products like mercury dichloride (HgCl₂)². The use of the Headspace method with prepared vials taken to the field facilitates sampling and the obtained results. However, it is also necessary to assert whether the values obtained in the aqueous matrix can be calibrated with values obtained from solid matrix standards with acid reaction, as generally, the standards for δ¹³C are analyzed. Therefore, we propose a way to ensure the values obtained in the analyses, through a δ¹³C calibration curve using a standard that is water-soluble so that the matrix effect of sampling is not limiting for the calibration of δ¹³C DIC. This allows for a calculation related to the carbonate concentration present in the sample based on the calibration curve. In the development of the method, the LSVEC (IAEA) lithium carbonate (LiCO₃) standard was chosen due to its water solubility with a KPS of 2.5 x 10 ^-2 mol L^-1. For each calibration curve, which was performed in triplicate with an n=5, increments of 0.010 mol L^-1, for each point on the curve, 0.200 mL of the corresponding solution was added using a syringe with a needle, in a pre-prepared 12 mL vial with 1 mL of 100% phosphoric acid and a flow of helium gas was conducted in the sealed vial to ensure an inert atmosphere free of atmospheric CO₂. Subsequently, a 72-hour equilibration period was followed for complete reaction and gas exchange. For carbon isotopic analyses, mass spectrometry with a gas source (Thermo Scientific™ Gas Bench Autosampler + Delta V Advantage Thermo Scientific™ Spectrometer). As for the results, the calibration curve was expressed by the amplification of a peak in volts (v). The obtained values were 6.350 v, 11.720 v, 19.410 v, 24.780 v, 31.003 v for the concentrations of 0.014 M, 0.024 M, 0.034 M, 0.044 M, and 0.054 M of LiCO₃, respectively. It was possible to determine the equation of the line as y = 1641.2x - 2.1362, with a determination coefficient (R²) of 0.9971. δ¹³C values were observed throughout the study having a δ13C result of –47.86 ± 0.75‰ which coincides with the reference value of LSVEC δ¹³C of -46.56±0.11 ‰. This preliminary result indicates that LSVEC can be used for δ¹³C DIC calibration, new studies will be conducted to validate the method of utilizing LSVEC as a standard for δ¹³C DIC, and subsequent quantification of carbonates in samples.

REFERENCES

1. Aucour, A.-M., et al., 1999. Use of ¹³C to trace origin and cycling of inorganic carbon in the Rhône river system. Chem. Geol. 159(1-4), 87-105.

2. Spötl, C., 2005. A robust and fast method of sampling and analysis of δ13C of dissolved inorganic carbon in ground waters. Isot. Environ. Health Stud. 41(3), 217-221.