Preservation methods for the isotopic composition of dissolved carbon species in non‐ideal conditions

<jats:sec> <jats:title>Rationale</jats:title> <jats:p> The stable carbon isotope compositions of dissolved inorganic carbon ( <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DIC</jats:sub> ) and dissolved organic carbon ( <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DOC</jats:sub> ) are readily affected by post‐sampling microbial activity if not adequately preserved. Existing preservation methods require rapid chilling, analysis, and/or toxic chemicals, all challenging to use safely in the field and during remote field seasons. Therefore, a preservation method that is safe but also effective over a range of storage times is needed when sampling waters at very remote sites. </jats:p> </jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p> Two samples, with different dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations, were filtered with a 0.2‐μm filter and preserved with six different methods, mercuric chloride, copper sulfate, phosphoric acid, benzalkonium chloride, zinc chloride, hydrochloric acid, and a filter‐only control. These samples were held at 4°C, 22°C, or 35°C. Regular measurement of the DIC and DOC <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C values were made over the following 60 days for <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DIC</jats:sub> and 66 days for <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DOC.</jats:sub> </jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p> Over the course of the experiment, mercuric chloride, copper sulfate, zinc chloride, and benzalkonium chloride resulted in <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DIC</jats:sub> fractionation at both 4°C and 22°C. Only filtering to 0.2 μm at the time of collection, with or without acidification with phosphoric acid, resulted in minimal isotopic fractionation at both 4°C and 22°C and over the entirety of the experiment. For <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DOC</jats:sub> values, only filtering to 0.2 μm minimized fractionation for both bulk and vial storage over 66 days at 22°C. </jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p> Filtering to 0.2 μm at the time of collection is more effective than the use of toxic chemicals for measuring <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DIC</jats:sub> and <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DOC</jats:sub> values. Phosphoric acid is as effective as only filtering for <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DIC</jats:sub> and may be ideal depending on sampling considerations. These results demonstrate not only that water samples can be preserved for <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DIC</jats:sub> and <jats:italic>δ</jats:italic> <jats:sup>13</jats:sup> C <jats:sub>DOC</jats:sub> analysis for long periods, but that preservation is best accomplished with non‐toxic or low‐toxicity methods. </jats:p> </jats:sec>