A rapid throughput technique to isolate pyrogenic carbon by hydrogen pyrolysis for stable isotope and radiocarbon analysis

Rationale Rapid, reliable isolation of pyrogenic carbon (PyC; also known as char, soot, black carbon, or biochar) for the determination of stable carbon isotope (delta C-13) composition and radiocarbon (C-14) dating is needed across multiple fields of research in geoscience, environmental science and archaeology. Many current techniques do not provide reliable isolation from contaminating organics and/or are relatively time-consuming. Hydrogen pyrolysis (HyPy) does provide reliable isolation of PyC, but the current methodology is time consuming. Methods We explored the potential for subjecting multiple samples to HyPy analysis by placing up to nine individual samples in custom-designed borosilicate sample vessels in a single reactor run. We tested for cross-contamination between samples in the same run using materials with highly divergent radiocarbon activities (0.04-116.3 pMC), delta C-13 values (-11.9 to -26.5 parts per thousand) and labile carbon content. We determined C-14/C-13 using accelerator mass spectrometry and delta C-13 values using an elemental analyser coupled to a continuous flow isotope ratio mass spectrometer. Results Very small but measurable transfer between samples of highly divergent isotope composition was detectable. For samples having a similar composition, this cross-contamination is considered negligible with respect to measurement uncertainty. For samples having divergent composition, we found that placing a sample vessel loaded with silica mesh adsorbent between samples eliminated measurable cross-contamination in all cases for both C-14/C-13 and delta C-13 values. Conclusions It is possible to subject up to seven samples to HyPy in the same reactor run for the determination of radiocarbon content and delta C-13 value without diminishing the precision or accuracy of the results. This approach enables an increase in sample throughput of 300-600%. HyPy process background values are consistently lower than the nominal laboratory process background for quartz tube combustion in the NERC Radiocarbon Laboratory, indicating that HyPy may also be advantageous as a relatively 'clean' radiocarbon pre-treatment method.