EXTRANEOUS CARBON ASSESSMENTS IN RADIOCARBON MEASUREMENTS OF BLACK CARBON IN ENVIRONMENTAL MATRICES

摘要

Extraneous carbon (C_(ex)) added during chemical processing and isolation of black carbon (BC) in environmental matrices was quantified to assess its impact on compound specific radiocarbon analysis (CSRA). Extraneous carbon is added during the multiple steps of BC extraction, such as incomplete removal of solvents, and carbon bleed from the gas chro-matographic and cation columns. We use 2 methods to evaluate the size and Δ~(14)C values of C_(ex) in BC in ocean sediments that require additional pretreatment using a cation column with the benzene polycarboxylic acid (BPCA) method. First, the direct method evaluates the size and Δ~(14)C value of C_(ex) directly from the process blank, generatedby processing initially empty vials through the entire method identically to the treatment of a sample. Second, the indirect method quantifies C_(ex) as the difference between processed and unprocessed (bulk) Δ~(14)C values in a variety of modern and ~(14)C-free or "dead" BC standards. Considering a suite of hypothetical marine sedimentary samples of various sizes and Δ~(14)C values and BC Ring Trial standards, we compare both methods of corrections and find agreement between samples that are >50 μg C. Because C_(ex) can profoundly influence the measured Δ~(14)C value of compound specific samples, we strongly advocate the use of multiple types of process standards that match the sample size to assess C_(ex) and investigate corrections throughout extensive sample processing.