| 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 (14C) 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 14C-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 (14C in PAH, EC) of measurement validity (Currie et al.,
2002). Components of the new Hybrid RM (DiesApple), however, have known 14C and
EC composition, and they are nearly orthogonal (isotopically and
chemically). NIST SRM 2975 (Forklift Diesel Soot) has little or no 14C, and its major
compositional component is EC; SRM 1515 (Apple Leaves) has the 14C 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 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 sub-micromolar level, with
"reagent-free" removal of carbonate-C from EC – essential for low-level
EC-14C AMS. |