Towards an online ramped oxidation approach for thermal dissection and serial radiocarbon measurement of complex organic matter

Radiocarbon (¹⁴C) measurements provide a powerful tool to deconvolute sources and dynamics of organic matter in the environment. However, interpretation of conventional bulk-level ¹⁴C data is challenging due to the myriad components comprising organic matter in soils and sediments. Thermally ramped oxidation provides one approach for overcoming this limitation, and involves subjecting a sample to gradually increasing temperatures, serially oxidizing the OC to CO₂. Collected over prescribed temperature ranges ('thermal fractions'), this CO₂ is then analyzed for ¹⁴C content using accelerator mass spectrometry (AMS). While effective, current ramped oxidation methods are mostly 'offline', involving manual collection and subsequent AMS analysis of evolved CO₂, hindering sample throughput and reproducibility.

Here, we introduce a compact, online ramped oxidation (ORO) setup in which CO₂ from discrete thermal fractions is directly collected and measured for ¹⁴C by AMS equipped with a gas ion source. The setup comprises two modules: (i) an ORO unit with two sequential furnaces - the first, ramped from room temperature to 900 °C, holds the sample; the second, maintained at 900 °C, includes a catalyst ensuring complete oxidation to CO₂; and (ii) a dual-trap interface (DTI) collection unit with two parallel molecular sieve traps alternately collecting and releasing CO₂ from a given fraction for direct injection into the AMS.

Preliminary results indicate reproducible data, evident in both thermograms and F¹⁴C results. Analysis of natural reference samples reveals that measured ¹⁴C values and their associated uncertainties align with those reported in the literature using conventional “off-line” ramped oxidation methods, affirming the utility of the new ORO-DTI-AMS setup.

Our goal is to apply this new method for comprehensive investigation of a range of natural samples, with a particular focus on the improved understanding of the fate of OC held in permafrost soils in the context of on-going climate and carbon cycle change in high latitude ecosystems.