High-Precision Determination of Carbon Isotope Composition and Concentration of Dissolved Inorganic Carbon in Seawater

Determination of the isotopic composition and concentration of dissolved inorganic carbon (DIC) in sea water requires not only high measurement accuracy, but also the development of unified approaches to data standardization and material collection. In this work, we studied the effect of sampling methods (“into containers” and “into vials with acid”) with and without the addition of a toxin) on the results of determining the values of δ¹³С(DIC) and the concentration of DIC in sea water. The analytical protocol is described in detail, based on many years of experience in the sample collection, measurement and standardization of data obtained for a large number of water samples of the Arctic seas. According to the above protocol, the values of δ¹³С(DIC) and [DIC] can be determined with an error of less than 0.05‰ (1σ) and 4.5 rel % respectively. It has been shown that sampling “in a vial with acid” with their storage for 4 months is accompanied by significant contamination by atmospheric carbon dioxide with an underestimation of δ¹³С(DIC) values by an average of 0.3–0.8‰ and an overestimation of [DIC] values by an average of two times. The absence of a toxin that stops biological activity does not lead to significant shifts in the DIC concentration, but it strongly affects the δ¹³С(DIC) values, which become underestimated by an average of 1‰. Using sampling “in a container” and toxin addition, along with following other recommendations, the samples retain the isotope and concentration parameters of DIC throughout the year.