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

Rationale The stable carbon isotope compositions of dissolved inorganic carbon (delta C-13(DIC)) and dissolved organic carbon (delta C-13(DOC)) 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. Methods Two samples, with different dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations, were filtered with a 0.2-mu m 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 degrees C, 22 degrees C, or 35 degrees C. Regular measurement of the DIC and DOC delta C-13 values were made over the following 60 days for delta(13)C(DIC)and 66 days for delta C-13(DOC.) Results Over the course of the experiment, mercuric chloride, copper sulfate, zinc chloride, and benzalkonium chloride resulted in delta(13)C(DIC)fractionation at both 4 degrees C and 22 degrees C. Only filtering to 0.2 mu m at the time of collection, with or without acidification with phosphoric acid, resulted in minimal isotopic fractionation at both 4 degrees C and 22 degrees C and over the entirety of the experiment. For delta(13)C(DOC)values, only filtering to 0.2 mu m minimized fractionation for both bulk and vial storage over 66 days at 22 degrees C. Conclusions Filtering to 0.2 mu m at the time of collection is more effective than the use of toxic chemicals for measuring delta(13)C(DIC)and delta(13)C(DOC)values. Phosphoric acid is as effective as only filtering for delta(13)C(DIC)and may be ideal depending on sampling considerations. These results demonstrate not only that water samples can be preserved for delta(13)C(DIC)and delta(13)C(DOC)analysis for long periods, but that preservation is best accomplished with non-toxic or low-toxicity methods.