Automation of 13C/12C 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 CO2 trap and a continuous flow IRMS, which has made possible the high-throughput, automated measurements of C-13/C-12 ratios, and DOC concentrations for a wide range of aquatic samples. Precision of C-13/C-12 ratios increases exponentially with sample concentration, reaching 0.2 parts per thousand 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 mu g C with an isotopic signature varying from -20 to -12 parts per thousand during the lifetime of the combustion column. With appropriate blank correction procedures, accurate analyses (+/- 0.5 parts per thousand 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 delta C-13, our two laboratories analyzed a consensus reference material from a deep-ocean environment (CRM Batch 13 Lot # 05-13, Hansell 2013) and found delta C-13 values of -19.9 +/- 0.5 parts per thousand (n = 4) and -20.6 +/- 0.3 parts per thousand (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.