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

Alysha Coppola

doi:10.2458/azu_js_rc.55.16303

Abstract

Extraneous carbon (Cex) 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 Δ14C values of Cex 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 Δ14C value of Cex 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 Cex as the difference between processed and unprocessed (bulk) Δ14C values in a variety of modern and 14C-free or “dead” BC standards. Considering a suite of hypothetical marine sedimentary samples of various sizes and Δ14C values and BC Ring Trial standards, we compare both methods of corrections and find agreement between samples that are >50 µg C. Because Cex can profoundly influence the measured Δ14C value of compound specific samples, we strongly advocate the use of multiple types of process standards that match the sample size to assess Cex and investigate corrections throughout extensive sample processing.

Highlights

Black carbon (BC) is produced from the incomplete combustion of fossil fuels and biomass, ubiquitous in the atmosphere, sediments, soils, and water, and influences a wide range of biogeochemical processes (Schmidt and Noack 2000; Watson et al 2005)
Extraneous carbon (Cex) 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)
Using a mass balance approach from the difference between the total Cchemistry+preparative capillary gas chromatograph (PCGC) determined from the process blank and CPCGC using the direct blank, we calculate that the Cchemistry in 2012 was 1.3 ± 0.8 μg C min–1 per 50 1-μL injections

Summary

Introduction

Black carbon (BC) is produced from the incomplete combustion of fossil fuels and biomass, ubiquitous in the atmosphere, sediments, soils, and water, and influences a wide range of biogeochemical processes (Schmidt and Noack 2000; Watson et al 2005). The turnover times of BC within these pools are determined by partially oxidizing aromatic BC in environmental matrices using the benzene polycarboxylic acid (BPCA) method to form marker compounds, benzene polycarboxylic acids (BPCAs). These environmental matrices contain non-BC organic matter, fine siliceous dust and heavy metals in a heterogeneous mixture, which can complicate the processing of a BC sample. Extensive processing adds extraneous (Cex) carbon, thereby influencing the size and 14C value of the BC sample (Santos et al 2007; Ziolkowski and Druffel 2009).

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