Fig. 8 . Observed δ18O values vs. previously
published ones.
For the samples used the FO6 recipe, the oxygen yields of NBS28 were
98.2 – 99.3% and that of SWy-1 were 90.0 – 91.6%. NBS28 showed
higher oxygen yields, which may be due to the longer CO peak with slight
tailing rather than a higher reactivity than that of SWy-1. For the
samples used the NiC_FO2 recipe, the oxygen yields of both NBS28 and
SWy-1 are approximately 110%. These anomalous yields cannot be
explained by the oxygen released from residues of standard samples.
Thus, an additional source of oxygen was suspected (e.g., the oxidation
of Ni metal in Ni/C during storage).
The samples with NaF at F/O = 2 in Run2 (Fig.3) showed a large deviation
of δ18O values (16 – 22 ‰), while the FO6 and the
NiC_FO2 recipes in Run4 resulted in the smaller deviation (0.29 and
0.23 ‰, respectively). SWy-1 tended to have a higher precision than
NBS28 in both recipes. This is probably due to the smaller particle size
of SWy-1 (< 2 μm) than NBS28. In addition, the NBS28_FO6
samples were analyzed in the first sequence among the other NaF-bearing
samples in the series of measurement runs (Fig. 6), which could be
related to the lower precision. By using a continuous-flow TC/EA-IRMS
method and KF as the fluorine source, 0.33 ‰ of 1σ for NBS28 and 0.29 ‰
for NBS27 biotite were reported 11. The precision in
this study was comparable to the above report, while the precisions of
the conventional methods (dual-inlet system, BrF5 or
CO2 laser fluorination) were at most 0.2‰25.
Although it was reported that δ18O values for NBS28
varied from 8.8 to 10.0‰ 25, 9.6‰ has been widely
accepted. This study showed a discrepancy of +1.2‰ for the FO6 recipe
and +0.4‰ for the NiC_FO2 recipe. The similar TC/EA-IRMS method
reported 10.23‰ for NBS28 11, showing a similar trend
to this study. For SWy-1, the reported values were very limited compared
with those of NBS28. A laser fluorination method provided 18.6 ± 0.23‰4, while a conventional BrF5fluorination method showed 19.2‰ 26. Compared to
these, the present study obtained lighter values; 17.8‰ for the FO6
recipe and 16.7‰ for the NiC_FO2 recipe.
3.5 Potentials and further improvements on the
methodology
Although this study used at least 100 mg of each sample and additive in
the process of homogenization in an agate mortar before encapsulation,
one measurement is possible with about 200 µg of sample in a silver
capsule. It is important to design pretreatment procedures for
homogenizing tiny volumes of material. The measurement time was
approximately 20–30 min for one silicate sample and 10 min for one
non-silicate standard. Therefore, it has the potential to measure more
than 50 unknown samples within 24 h. This is a major advantage of our
proposed method. The conventional BrF5 fluorination
method requires several milligrams of the sample and approximately 12 h
for fluorination.
We also confirmed that hydrogen isotope ratios could be measured using
the same instrument and configuration (data not shown). However,
hydrogen isotope ratio measurement requires approximately 20 μg of
hydrogen, and in the case of smectite or mica, it is equivalent to
approximately 4–5 mg of the sample to be loaded. Therefore, the
simultaneous measurement of hydrogen and oxygen isotope ratios for one
capsule is difficult; however, by preparing samples for hydrogen and
oxygen analyses, respectively, it is possible to measure both in a
series of runs.
Despite the improvement in the oxygen yields from silicates in this
study, problems persist. First, at the beginning of the series of
samples containing fluorine, excessive amounts of oxygen yield occurred.
This is probably due to the recapture of oxygen derived from standard
samples’ residue that were pyrolyzed in the preceding sequence. Because
the CO peak is recorded only once per sample in the continuous-flow IRMS
system, the entire sample may be wasted without eliminating the effect
of the preceding sample residue. Currently, a stable analysis can only
be performed by checking the pyrolysis chromatograph and arbitrarily
rejecting samples with abnormal behavior. Therefore, it is necessary to
optimize the loading method to prevent the waste of samples. Next, the
accuracy of the δ18O value needs to be examined with a
wider variety of silicate minerals. However, there are few international
standards for silicate minerals, which may hinder this verification.
Inter-laboratory comparisons using multiple methods for silicate
minerals other than NBS28, which are the most frequently reported, are
needed. Because the main focus of this study was to find a promising
recipe for separating oxygen from silicates, normalization based on
international water standards such as VSMOW2 and SLAP2 were not
achieved. To eliminate instrumental bias, VSMOW-SLAP scaling should be
addressed in future studies. Third, optimization of a recipe with as
little fluoride additive as possible may be required. In this study, the
higher the F/O ratio, the better the reactivity with silicate. Although
the IRMS detector did not show any fluoride contamination, the negative
effects of the fluoride gas produced on the instrument cannot be
completely ruled out. Regardless of whether it is in a continuous-flow
system or a dual-inlet system, the optimization of the recipe will be
performed in the future. The recipe produced in this study is
anticipated to exhibit a comparable outcome for TC/EA-IRMS in the
dual-inlet system.