Isotopic (δ13C, Δ14C) analysis of organic acids in marine samples using wet chemical oxidation

Susan Q. Lang, Gretchen L. Früh-Green, Stefano M. Bernasconi, Lukas Wacker

Limnol. Oceanogr. Methods 11:161-175 (2013) | DOI: 10.4319/lom.2013.11.161

ABSTRACT: We present a method for the isolation and off-line isotope analysis of formate and acetate in marine samples. Organic acids are separated by high performance liquid chromatography and collected in glass Exetainer® screwcapped vials that have been prespiked with an oxidant and flushed with helium. The vials are subsequently heated to convert the organic compounds to CO2 for radiocarbon and δ13C analysis. Small aliquots are sampled for measurement of δ13C by isotope ratio mass spectrometry, whereas the majority of the CO2 is saved for radiocarbon analysis by accelerator mass spectrometry using a gas ion source. Accurate δ13C and radiocarbon values were obtained for formate and acetate standards spiked into deep seawater and saline Milli-Q water at concentrations of 25 to 1000 μM C. The process blank associated with the isolation of formate for radiocarbon analysis was ~1.5 μg C and stable over time. Accurate results could be obtained for marine samples with only 25 μM formate. The radiocarbon analysis of acetate showed significantly higher and more variable extraneous carbon contributions, particularly for samples spiked into seawater. Potential improvements that may make the method appropriate for the radiocarbon analysis of acetate in seawater are discussed. The blanks associated with the wet chemical oxidation were assessed independently and found to be small and consistent (<0.3 μg C), potentially making this approach feasible for a broader set of compounds separated by HPLC for radiocarbon analysis.