Methodologies for Extraction of Dissolved Inorganic Carbon for Stable Carbon Isotope Studies: Evaluation and Alternatives

摘要

The gas evolution and the strontium carbonate precipitation techniques to extract dissolved inorganic carbon (DIC) for stable carbon isotope analysis were investigated. Theoretical considerations, involving thermodynamic calculations and computer simulation pointed out several possible sources of error in the stable carbon isotope measurements of the DIC and demonstrated the need for experimental evaluation of the magnitude of the error. An alternative analytical technique, equilibration with outgassed vapor phase, is proposed. The experimental studies revealed that delta (13)C of the DIC extracted from a 0.01 molar NaHCO3 solution by both techniques agreed within 0.1 per mil with the delta (13)C of the solid NaHCO3. An approximate linear relationship was found between log molality of sulfate and the increase in delta (13)C of the DIC extracted by the precipitation technique. Similar trend was observed in the gas evolution technique; however, less in magnitude. The percentages of recovered DIC decreased linearly with log molality of sulfate in the precipitation technique and were independent of sulfate concentration in the gas evolution technique. Both the precipitation and gas evolution technique were found to be satisfactory for extraction of DIC from different kinds of natural water for stable carbon isotope analysis, provided appropriate precautions are observed in handling the samples. Laboratory tests indicated that sampling devices and laboratory preparation may alter the carbon isotope ratio of the samples.