Fig. 1 The schematics of TC/EA-IRMS configuration.
(A) Autosampler, (B) TC/EA reactor, (C) Gas trap and GC column, (D) Con
Flo IV, (E) IRMS, (F) pyrolysis chromatograph
2.2.3. Sample encapsulation
procedure
Except for IAEA-601 and -602, all samples and silver capsules were
heated at 220˚C in an oven for more than 4 h before being encapsulated
into the silver capsule to remove adsorbed water molecules. Additives
such as fluorine compounds and graphite or additives homogenized with
the samples in advance were dried under the same conditions. Immediately
after retrieval from the oven, each sample was weighed to obtain an
oxygen mass of 100 µg. The measured weight was ranged to within ±5% of
the intended weight (100 μg of oxygen) in order to ensure quick
weighing. The samples and additives were sealed by folding the silver
capsule while pushing out air, and the capsules were again kept dry at
220°C for at least 4 h before being placed in an autosampler.
Considering the melting point of benzoic acid (122°C), IAEA-601 and -602
were not heated prior to, during, or following the weighing and sealing
procedures. Upon completion of the TC/EA-IRMS, the silver capsules were
promptly transferred to an autosampler. Prior to initiating the
measurement, the autosampler underwent an overnight purge using He gas
to eliminate any remaining gas.
2.2.4 experimental runs
Several recipes for the sample – additive mixture and mixing procedures
were compared to identify the most promising method. This can be divided
into four series of measurements (Table 2).
Run1: Analysis for comparing the reactivities of the
sample-PTFE mixtures with different mixing procedures and different C
sources, graphite, or Ni/C. Three different procedures were tested. (1)
Sample–PTFE–graphite enclosed in a silver capsule without
homogenization. (2) Sample–PTFE–Ni/C was enclosed in a silver capsule
without homogenization. (3) Sample–PTFE–Ni/C mixture was homogenized
using an agate mortar and pestle before weighing and encapsulation. For
these three recipes, the F/O and C/O molar ratios were >2.3
and >1.5, respectively.
Run2: Analysis of reactivities with different F sources. Six
inorganic fluorides (LiF, NaF, KF, CaF2,
BaF2, and AlF3) and graphite powder were
added and homogenized before weighing. The F/O and C/O molar ratios were
set to 2 and 1.5, respectively.
Run3: Analysis of reactivities with different F/O molar ratios.
NaF was added to the samples at F/O molar ratios of 2, 3, 4, and 6, and
homogenized with graphite or Ni/C before encapsulation.
Run4: Analysis for confirming O yields and
δ18O values using recipes that showed high reactivity
in Run1–3. NBS28, SWy-1, and other montmorillonite samples (TKN-01, 29,
31) were analyzed using (1) NaF and graphite with F/O and C/O molar
ratios of 6 and 1.5, respectively, and (2) NaF and Ni/C with F/O and C/O
molar ratios of 2 and 1.5, respectively.
Table 2. Mixing recipe of each analytical run