Samples are prepared for 13C analysis of dissolved organic carbon by adding phosphoric acid and potassium persulfate to water samples to expel inorganic carbon and digest organic carbon. Samples are then flushed with helium and microwaved to completely release carbon dioxide. Samples are analyzed using continuous flow isotope ratio mass spectrometry where a sample aliquot is injected and analyzed by comparing isotopic ratios to a reference gas. Three internal carbon standards are prepared and analyzed under the same conditions as samples to calibrate results reported against an international reference material.

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13
C ANALYSIS OF DISSOLVED ORGANIC CARBON (DOC)
Sample preparation:
Water samples and standards are prepared in 12 mL septum vials with a replicate at every 3rd
sample. Phosphoric acid (85%) is added to each sample in order to expel the inorganic carbon,
and saturated potassium persulfate is added to aid in the digestion of the organic carbon. Samples
are flushed with ultrapure helium, and microwaved for 55 seconds for complete release of any
organic carbon as CO2.
Sample Analysis:
Samples are analyzed by continuous flow isotope ratio mass spectrometry. An aliquot of each
digested sample is injected into the Finnigan Mat, DeltaPlus XL IRMS IRMS system and
analyzed. The analysis consists of a comparison between the isotopic ratio of the samples against
a CO2 reference gas. Injection volumes are selected to ensure peak height is similar to that of the
reference gas. Three calibrated internal carbon standards are prepared and run under the same
conditions as the samples. Standards are run at the beginning and end of every run, as well as
after every 8-12 samples. These standards have been calibrated to VPDB via analysis of NBS-19
and NBS-18. The results are evaluated and corrected against standards that bracket the samples,
and then reported against the international reference material.
The analytical precision for analysis is ± 0.3‰.
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