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

Abstract

Rapid, reliable isolation of Pyrogenic Carbon (PyC; char, soot, black carbon; biochar) for determination of stable carbon isotope (δ13 C) composition and radiocarbon (14 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 to employ. Hydrogen pyrolysis (HyPy) does provide reliable isolation of PyC but the current methodology is time consuming. 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 to 116.3 pMC), δ13 C values (-11.9 to -26.5‰) and labile carbon content. We determined 14 C/13 C by accelerator mass spectrometry and δ13 C values by elemental analyser coupled to continuous flow isotope ratio mass spectrometer. Very small but measurable transfer between samples of highly divergent isotope composition was detectable. Where samples are of broadly similar composition, this cross contamination is considered negligible with respect to measurement uncertainty. Where samples are of divergent composition, it was found that placing a sample vessel loaded with silica mesh adsorbent between samples eliminated measurable cross-contamination in all cases for both 14 C/13 C and δ13 C values. It is possible to subject up to seven samples to HyPy in the same reactor run for determination of radiocarbon content and δ13 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 pretreatment method.