Extraneous Carbon Assessments in Radiocarbon Measurements of Black Carbon in Environmental Matrices

Abstract

ABSTRACT. 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 chromatographic and cation columns. We use 2 methods to evaluate the size and  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  14 C value of C ex directly from the process blank, generated by 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)  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  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  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.

Grass Material as a Modern Process Standard for 14C Analysis of n-Alkanes

“…The chemical extraction procedure (prior to PCGC) was not evaluated (apart from processing a blank for GC/MS analysis, see Results and Discussion) since it is a relatively simple procedure that does not require derivatization and thus it is unlikely to introduce as much C ex compared to PCGC isolation and solvent removal. It should be noted that evaluating the chemical extraction becomes relevant if there are co-eluting compounds in the reagents and solvents used in the extraction procedure and/or extensive chemical pre-treatments are used (Ziolkowski and Druffel 2009;Coppola et al 2013). Our results showed that our extraction methods do not contribute co-eluting compounds (see Results and Discussion).…”

“…We can also use the uncorrected F 14 C values of C 29 and C 31 n-alkanes isolated from the grass to estimate C ex derived from the entire sample procedure (chemical extraction + PCGC + solvent removal), assuming that the grass nalkanes have the same 14 C content of the bulk grass (our initial assumption) and thus any deviation represents C ex ( 14 C-free) added during the entire sample procedure (assuming addition of modern C ex during PCGC isolation is relatively insignificant; Druffel et al 2010;Kramer et al 2010;Coppola et al 2013). Our estimates show that ~ 0.91 ± 0.46 to 1.3 ± 0.65 µg C per minute, per 50 (1µl) injections is derived from the entire sample preparation procedure versus 0.75 ± 0.38 derived from PCGC isolation and solvent removal only (Table 3).…”

“…We used the amount of modern C ex estimated for solvent removal as the amount of modern C ex for PCGC isolation. Nevertheless the modern component of C ex derived from PCGC processing is generally less significant than the 14 Cfree component Kramer et al 2010;Coppola et al 2013). In addition, the modern F 14 C value of our grass material makes the evaluation of 14 C-free C ex relatively more relevant.…”

“…We estimated the amount of C ex as the mass of extraneous carbon needed to correct the F 14 C values of the processed standard materials to within 1σ of their bulk F 14 C value. We express C ex derived from PCGC isolation and solvent removal in µg C per minute, per 50 (1 µl) injections for consistency with published literature (Ziolkowski and Druffel 2009;Coppola et al 2013) although C ex values are unique to each laboratory and procedure. In the case of solvent removal, C ex is expressed as µg C (Table 1).…”

“…In the isolation of black carbon (BC), Ziolkowski and Druffel (2009) used commercially available modern and 14 C-free vanillin to determine carbon addition during PCGC isolation and BC reference materials from the BC Ring Trial (modern grass char, 14 C-free hexane soot; Hammes et al 2007) and blanks (no sample added), to evaluate the chemical and PCGC isolation steps combined. Coppola et al (2013) used a similar approach to assess C ex during isolation of BC using reference materials from the BC Ring Trial (modern grass char, wood char, and 14 C-free hexane soot) in addition to NIST Standard Reference Material urban dust aerosol (SRM 1649a), marine sediment (SRM 1941b) added to wood char, and US Geological Survey Green River Shale. Tao et al (2015) used solvents-only through the entire sample preparation procedure and solvents spiked with compounds of 14 C-free and of modern 14 C content after PCGC isolation as process standards.…”