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

[摘要] The primary objective of the research reported here hasbeen the development of a hybrid reference material (RM) to serve as a testof accuracy for elemental carbon (EC) isotopic (¹⁴C) speciationmeasurements. Such measurements are vital for the quantitative apportionmentof fossil and biomass sources of "soot" (EC), the tracer of fire that hasprofound effects on health, atmospheric visibility, and climate. Previousstudies of ¹⁴C-EC measurement quality, carried out with NIST SRM 1649a(Urban Dust), showed a range of results, but since the "truth" was not known forthis natural matrix RM, one had to rely on isotopic-chemical consistencyevidence (¹⁴C in PAH, EC) of measurement validity (Currie et al.,2002). Components of the new Hybrid RM (DiesApple), however, have known ¹⁴C andEC composition, and they are nearly orthogonal (isotopically andchemically). NIST SRM 2975 (Forklift Diesel Soot) has little or no ¹⁴C, and its majorcompositional component is EC; SRM 1515 (Apple Leaves) has the ¹⁴C content ofbiomass-C, and it has little or no EC. Thus, the Hybrid RM can serve as anabsolute 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 forconservation of its dominant soot fraction throughout the isolationprocedure.The secondary objective was to employ the Hybrid RM for the comparativeevaluation of the thermal optical kinetic (TOK) and thermal opticaltransmission (TOT) methods for the isolation of EC for micro-molar carbonaccelerator mass spectrometry (AMS). As part of this process, the relativelynew TOK method was subjected to a critical evaluation and significantdevelopment. Key findings of our study are: (1) both methods exhibitedbiomass-C "leakage"; for TOT, the EC fraction isolated for AMS containedabout 8% of the original biomass-C; for TOK, the refractory carbon (RC)isolated contained about 3% of the original biomass-C.; (2) the initialisothermal oxidation stage of the TOK method substantially reduced thetransfer of artifact char to the RC fraction, improving isolationcapabilities; (3) the Hybrid RM was not equal to the sum of its parts, withmatrix interactions inducing premature loss of EC which, however, could bequantified and minimized; (4) the three-stage TOK method provided a superiorcapability for carbonate quantification at the sub-micromolar level, with"reagent-free" removal of carbonate-C from EC – essential for low-levelEC-¹⁴C AMS.

[学科分类] 大气科学