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Quantification of Extraneous Carbon during Compound Specific Radiocarbon Analysis of Black Carbon

2009; American Chemical Society; Volume: 81; Issue: 24 Linguagem: Inglês

10.1021/ac901922s

1520-6882

Lori A. Ziolkowski, Ellen R. M. Druffel,

Geology and Paleoclimatology Research

Radiocarbon (14C) is a radioactive isotope that is useful for determining the age and cycling of carbon-based materials in the Earth system. Compound specific radiocarbon analysis (CSRA) provides powerful insight into the turnover of individual components that make up the carbon cycle. Extraneous or nonspecific background carbon (Cex) is added during sample processing and subsequent isolation of CSRA samples. Here, we evaluate the quantity and radiocarbon signature of Cex added from two sources: preparative capillary gas chromatography (PCGC, CPCGC) and chemical preparation of CSRA of black carbon samples (Cchemistry). We evaluated the blank directly using process blanks and indirectly by quantifying the difference in the isotopic composition between processed and unprocessed samples for a range of sample sizes. The direct and indirect assessment of Cchemistry+PCGC agree, both in magnitude and radiocarbon value (1.1 ± 0.5 μg of C, fraction modern = 0.2). Half of the Cex is introduced before PCGC isolation, likely from coeluting compounds in solvents used in the extraction method. The magnitude of propagated uncertainties of CSRA samples are a function of sample size and collection duration. Small samples collected for a brief amount of time have a smaller propagated 14C uncertainty than larger samples collected for a longer period of time. CSRA users are cautioned to consider the magnitude of uncertainty they require for their system of interest, to frequently evaluate the magnitude of Cex added during sampling processing, and to avoid isolating samples ≤5 μg of carbon.