On the isolation of elemental carbon (EC) for micro-molar <sup>14</sup>C accelerator mass spectrometry: development of a hybrid reference material for <sup>14</sup>C-EC accuracy assurance, and a critical evaluation of the thermal optical kinetic (TOK) EC isolation procedure

Abstract

Abstract. The primary objective of the research reported here has been the development of a hybrid reference material (RM) to serve as a test of accuracy for elemental carbon (EC) isotopic ( 14 C) speciation measurements. Such measurements are vital for the quantitative apportionment of fossil and biomass sources of "soot" (EC), the tracer of fire that has profound effects on health, atmospheric visibility, and climate. Previous studies of 14 C-EC measurement quality, carried out with NIST SRM 1649a (Urban Dust), showed a range of results, but since the "truth" was not known for this natural matrix RM, one had to rely on isotopic-chemical consistency evidence ( 14 C in PAH, EC) of measurement validity . Components of the new Hybrid RM (DiesApple), however, have known 14 C and EC composition, and they are nearly orthogonal (isotopically and chemically). NIST SRM 2975 (Forklift Diesel Soot) has little or no 14 C, and its major compositional component is EC; SRM 1515 (Apple Leaves) has the 14 C content of biomass-C, and it has little or no EC. Thus, the Hybrid RM can serve as an absolute isotopic test for the absence of EC-mimicking pyrolysis-C (char) from SRM 1515 in the EC isolate of the Hybrid RM, as well as a test for conservation of its dominant soot fraction throughout the isolation procedure.The secondary objective was to employ the Hybrid RM for the comparative evaluation of the thermal optical kinetic (TOK) and thermal optical transmission (TOT) methods for the isolation of EC for micro-molar carbon accelerator mass spectrometry (AMS). As part of this process, the relatively new TOK method was subjected to a critical evaluation and significant development. Key findings of our study are: (1) both methods exhibited biomass-C "leakage"; for TOT, the Correspondence to: L. A. Currie (lloyd.currie@nist.gov) EC fraction isolated for AMS contained about 8% of the original biomass-C; for TOK, the refractory carbon (RC) isolated contained about 3% of the original biomass-C.; (2) the initial isothermal oxidation stage of the TOK method substantially reduced the transfer of artifact char to the RC fraction, improving isolation capabilities; (3) the Hybrid RM was not equal to the sum of its parts, with matrix interactions inducing premature loss of EC which, however, could be quantified and minimized; (4) the three-stage TOK method provided a superior capability for carbonate quantification at the submicromolar level, with "reagent-free" removal of carbonate-C from EC -essential for low-level EC-14 C AMS.

Estimating contributions from biomass burning, fossil fuel combustion, and biogenic carbon to carbonaceous aerosols in the Valley of Chamonix: a dual approach based on radiocarbon and levoglucosan

“…Our graphite measurements of large samples are in agreement with the values reported in the literature for SRM 1649 and SRM 1649a (Currie et al, 1984(Currie et al, , 2002Szidat et al, 2004; Table 1. Analyses of mixtures of SRM 2975 and SRM 1515 standards in the form of solid graphite targets of large samples (roughly 1 mgC).…”

“…Measurements are made with solid target (graphitization, roughly 1 mgC). (Szidat et al, 2004) 0.507-0.61, depending on the sample preparation (Currie et al, 2002;Wise and Watters, 2007) 0.517/0.572 (simple/double combustion) (Heal et al, 2011) Solid measurement SRM 1649: 0.61 ± 0.04 (Currie et al, 1984) Wise and Watters, 2007;Heal et al, 2011). The F 14 C value for the online gas measurements is 0.505, with a SD of 0.028, N = 7, and a σ er of 0.010, whereas the determined F 14 C for the solid measurements is 0.532 with a SD of 0.004, N = 4, and a σ er of 0.002.…”

Understanding terrestrial organic carbon export : a time-series approach

“…Additionally, petroleum geochemists have long coupled thermal analysis methods with isotope ratio measurements in order to investigate the origins and maturity of thermogenic hydrocarbons, leading to the development of techniques such as pyGCisotope ratio mass spectrometry (IRMS; Galimov, 1988;Berner and Faber, 1996;Cramer, 2004 Lopez-Capel et al, 2006, 2008. Furthermore, Szidat et al (2004) and Currie and Kessler (2005) successfully separated and determined the radiocarbon ( 14 C) content of organic and elemental ("black") carbon fractions in aerosols using a stepped-temperature approach, confirming the possibility that thermal-isotope techniques can be used in tandem with radiocarbon analysis.…”

“…Such analyses separate compounds within a bulk sample based on various metrics of lability -that is, susceptibility to remineralization by chemical hydrolysis (Helfrich et al, 2007), uv light (Follett et al, 2014), heat (Szidat et al, 2004;Currie and Kessler, 2005;Rosenheim et al, 2008), microbial respiration (Beaupré et al, 2016) Like bulk measurements, serial oxidation techniques benefit from the fact that all carbon contained within a sample is analyzed, and results therefore reflect the entire complex OC mixture.…”

Towards On-Line 14C Analysis of Carbonaceous Aerosol Fractions

“…So far, the methods used to fully separate OC and EC for 14 C-based source apportionment have relied on rather time-consuming techniques, thus allowing only manual and off-line 14 C measurements (Szidat et al 2004b). A thermo-optical, "split-point"-based method for EC isolation and subsequent 14 C analysis was proposed by Currie and Kessler (2005), and presented minimized, but not totally suppressed, charringinduced EC contamination. A new separation method using a thermo-optical OC/EC analyzer in pure oxygen is currently under development to circumvent that charring issue and achieve in a single run the physical separation of OC and EC and their direct recovery as CO 2 for later off-line 14 C measurements.…”