Abstract
Radiocarbon ((14)C) 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 (C(ex)) is added during sample processing and subsequent isolation of CSRA samples. Here, we evaluate the quantity and radiocarbon signature of C(ex) added from two sources: preparative capillary gas chromatography (PCGC, C(PCGC)) and chemical preparation of CSRA of black carbon samples (C(chemistry)). 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 C(chemistry+PCGC) agree, both in magnitude and radiocarbon value (1.1 +/- 0.5 microg of C, fraction modern = 0.2). Half of the C(ex) 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 (14)C 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 C(ex) added during sampling processing, and to avoid isolating samples < or = 5 microg of carbon.
Highlights
Once corrected for graphitization and combustion, the mass of carbon graphitized in Compound specific radiocarbon analysis (CSRA) samples (Creported) originate from four sources: Creported)compound of interest isolated from the sample (Csample)+Cderivative+Cchemistry+CPCGC
The mass of carbon in the compound of interest isolated from the sample (Csample), the mass of derivative carbon (Cderivative), the
Half of the Cex was added during PCGC isolation and half was added during the chemical oxidation and derivatization
Summary
Extraneous or nonspecific background carbon (Cex) is added during sample processing and subsequent isolation of CSRA samples. 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). 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 e5 μg of carbon. Preparation of CSRA samples requires two sets of laboratory protocols, sample isolation, and 14C analysis, each of which introduce extraneous or nonspecific background carbon (Cex). We assess the mass and radiocarbon signatures of Cex specific to the chemical oxidation of organic matter for quantifying black carbon using PCGC. Materials Processed modern vanillin synthetic vanillin grass char hexane soot Solvents and Materials methanol dichloromethane biphenyl-2,2′-dicarboxylic acid TMS-diazomethane DB-XLB use GC process standard GC process standard method process standard method process standard solvent solvent internal standard derivatization agent GC column source
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