Isotopic ( 13 C) analysis of dissolved organic carbon in stream water using an elemental analyzer coupled to a stable isotope ratio mass spectrometer

A technique for measurement of the stable isotope composition of dissolved organic carbon (DOC) in stream water, using an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS), is described. Stream water samples were concentrated by rotary evaporation, acidified to remove dissolved inorganic carbon (DIC), and dried in silver cups prior to analysis. Precision was evaluated with standards (alanine and humic acid), and with stream water samples with varying 13 C enrichment. Standards and samples were also prepared in sealed quartz tubes for high‐temperature combustion (HTC) and analyzed by dual inlet for comparison. The δ 13 C values of natural abundance standards and samples measured by the two techniques differed by ≤0.3‰. Isotopically enriched stream water samples analyzed using the EA and HTC methods had comparable δ 13 C and atom % values (differed by ≤6%). Expensive quartz vessels and the extensive time required for sample lyophilization, combustion and cryogenic separation of combustion products before isotopic analysis for the HTC method are eliminated with the EA method. Overall, the EA method is simpler, faster, and more economical than the HTC method, and it can be used for 13 C‐enriched as well as natural abundance samples.