Purification of fire derived markers for μg scale isotope analysis (δ13C, Δ14C) using high performance liquid chromatography (HPLC)

Black carbon (BC) is the residue of incomplete biomass combustion. It is ubiquitous in nature and, due to its relative persistence, is an important factor in Earth's slow-cycling carbon pool. This resistant nature makes pure BC one of the most used materials for 14C dating to elucidate its formation date or residence time in the environment. However, most BC samples cannot be physically separated from their matrices, precluding accurate 14C values. Here we present a method for radiocarbon dating of the oxidation products of BC, benzene polycarboxylic acids, thereby circumventing interference from extraneous carbon. Individual compounds were isolated using high performance liquid chromatography (HPLC) and converted to CO2 via wet chemical oxidation for 13C and 14C isotope analysis. A detailed assessment was performed to identify and quantify sources of extraneous carbon contamination using two process standards with distinct isotopic signatures. The average blank was 1.6 ± 0.7 μg C and had an average radiocarbon content of 0.90 ± 0.50 F14C. We successfully analyzed the 14C content of individual benzene polycarboxylic acids with a sample size as small as 20-30 μg C after correcting for the presence of the average blank. The combination of δ13C and F14C analysis helps interpret the results and enables monitoring of extraneous carbon contribution in a fast and cost efficient way. Such a molecular approach to radiocarbon dating of BC residues enables the expansion of isotopic BC studies to samples that have either been too small or strongly affected by non-fire derived carbon.