Automation of 13 C/ 12 C ratio measurement for freshwater and seawater DOC using high temperature combustion

We provide a detailed description of the hyphenation of an Aurora 1030C high temperature catalytic conversion DOC analyzer, a GD‐100 CO 2 trap and a continuous flow IRMS, which has made possible the high‐throughput, automated measurements of 13 C/ 12 C ratios, and DOC concentrations for a wide range of aquatic samples. Precision of 13 C/ 12 C ratios increases exponentially with sample concentration, reaching 0.2‰ or better for high concentration samples (>5 mg L ‐1 ), comparable to that obtained in a conventional elemental analyzer‐IRMS setup. The system blank contribution is the limiting factor in obtaining maximal performance; optimal system blanks values are on the order of 0.2 µg C with an isotopic signature varying from ‐20 to ‐12‰ during the lifetime of the combustion column. With appropriate blank correction procedures, accurate analyses (±0.5‰ or better) can be obtained on concentrations as low as 0.5 mg DOC L ‐1 , representing the lower limit typically observed in aquatic systems. Sample matrix does not affect reproducibility or accuracy; this method is amenable to both freshwater and seawater samples. Although no certified DOC standards exist for δ 13 C, our two laboratories analyzed a consensus reference material from a deep‐ocean environment (CRM Batch 13 Lot # 05‐13, Hansell 2013) and found δ 13 C values of ‐19.9 ± 0.5‰ (n = 4) and ‐20.6 ± 0.3‰ (n = 3), which corroborates previously reported values for similar samples (Bouillon et al. 2006; Lang et al. 2007; Panetta et al. 2008) and is consistent with its marine origin.