A low-cost, high-throughput measurement for stable carbon isotope analysis of dissolved organic carbon in fresh water using wet chemical oxidation

Isotope ratio mass spectrometer (IRMS) techniques can be used to analyze the ¹³C of dissolved organic carbon (DOC) in freshwater samples. The IRMS is usually interfaced with an elemental analyzer (EA) or a wet chemical oxidation (WCO) total organic carbon analyzer. However, the EA-IRMS technique typically requires water samples to be evaporated or freeze-dried, which requires much water and preparation time. Meanwhile, WCO-IRMS requires an expensive total organic carbon (TOC) analyzer for isotopic measurements and the instrument limits the flexibility of the analyses. Here, we propose a new method for analyzing the ¹³C of DOC in freshwater samples, using WCO. The analyses are performed using a GasBench II head space sampler (Thermo Electron) on-line with an IRMS. This method allows multi-sample processing, which includes the removal of dissolved inorganic carbon (DIC) and oxidation of DOC to CO₂ on a dry heating block system. These procedures are conducted in glass vials, which are then placed in an auto sampler and analyzed on the GasBench II connected to the IRMS. It takes 20 min to remove both the DIC and WCO in the glass vials for multiple samples and 10 min to measure the carbon isotope in a sample. The main advantages of this method are the small sample volume requirement (10 μgC in a water sample), rapid analyses (~60 samples/d), and small initial cost if the laboratory already has a GasBench head space sampler. To investigate the performance of our method, we applied it to a solution of standard materials including some amino-acids, hydrogen phthalate, humic acids, and fulvic acids whose δ¹³C values were determined by EA-IRMS in advance. A good correlation was obtained between the δ¹³C values with the EA-IRMS measurement and with our method, using 6-ml standard solutions with a concentration of 2 mg-C/L (12 μgC). In our presentation, we will also show the results of DO¹³C measurements in natural fresh water, including soil water, groundwater, and stream water in a forest catchment.