Storage and hydrolysis of seawater samples for inorganic carbon isotope analysis

Preservation of seawater samples was tested for total inorganic carbon (ΣCO₂), stable carbon isotopes (δ¹³C), and radiocarbon (¹⁴C) applications using foil bags and storage by refrigeration and freezing. The aim was to preserve representative samples with minimal storage effects but without using toxic methods such as mercuric chloride poisoning. Hydrolysis of samples to CO₂ was based on existing methods. Results of IAEA-C2 standard used with deionized water stored in the foil bags showed complete reaction yields, ¹⁴C results within 2σ of the consensus value, and δ¹³C that were internally consistent, indicating that there were no procedural effects associated with the foil bags. ¹⁴C results were statistically indistinguishable across the storage times, for frozen and refrigerated seawater samples from a coastal site, Elie Ness, Fife, UK. The
scatter of ΣCO₂ concentrations and δ¹³C was within scatter observed in other studies for lake- and seawater samples preserved by acidification or using mercuric chloride. However, both ΣCO₂ and δ¹³C were less variable for frozen samples compared with refrigerated samples. The foil bags are lighter, safer to transport, and similar in cost to glass bottles and allow sample collection in the field and transfer to the hydrolysis vessel without exposure of the sample to atmosphere. Storage of seawater samples in the foil bags was considered a reliable, alternative method to poisoning for ΣCO₂, δ¹³C, and ¹⁴C, and freezing the samples is recommended for storage beyond a week.