Combining benzalkonium chloride addition with filtration to inhibit dissolved inorganic carbon alteration during the preservation of seawater in radiocarbon analysis

Abstract. Benzalkonium chloride (BAC) addition has shown great promise as a disinfectant for measuring δ¹³C and ¹⁴C of dissolved inorganic carbon (DIC) in freshwater samples. However, it was reported that the effectiveness of BAC to prevent DIC change was reduced for the use of seawater samples. The present study aimed to evaluate the effectiveness of adding BAC as a disinfectant in carbon isotopic analyses of DIC in seawater samples. We compared the efficacy of BAC addition, filtration (0.22 μm PTFE or 0.2–0.45 μm PES filters), and a combination of BAC addition and filtration in preventing DIC alterations caused by biological activity. The combined procedure was effective in preserving seawater, although this assessment was based on results from a single seawater sample. The ¹⁴C concentration of samples treated with both BAC addition and filtration exhibited minimal changes, ranging from 0.2–0.4 pMC over 41 weeks, despite the addition of sugar included to increase biological activity several-fold. Although the complete elimination of biological effects may be challenging with the combined method, the observed changes remained within practical limits. Concerns about CO₂ contamination during sample filtration were also addressed and found to be negligible. These results suggest that combining filtration and BAC addition is an effective method for suppressing biological DIC alterations in ¹⁴C analysis, even in seawater samples.