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 (δ C) composition and radiocarbon ( C) 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), δ C values (-11.9 to -26.5‰) and labile carbon content. We determined C/ C using accelerator mass spectrometry and δ C 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/ C and δ C 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 δ C 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.