Fig. 2. The m /z 28, 29 and 30 traces during
analysis of NBS28 (left column) and SWy-1 (right column). (A, B) The
samples enveloped with PTFE in silver capsule. (C, D) Use of Ni/C
instead of graphite, (E, F) Use of Ni/C and the homogenization before
encapsulation. The intensity of reference gas of m /z 29
was set approximately 6000 mV for each measurement.
3.2. Influence of F source
(Run2)
The reactivities of Qz and SWy-1 were compared for the six fluorinated
compounds (Fig. 3). NaF and KF were relatively superior in terms of
reactivity for both Qz and SWy-1, followed by LiF and PTFE (Run1), while
CaF2, BaF2, and AlF3tended to show significantly lower reactivity. For SWy-1, KF tended to
form a shoulder on the right side of the CO peak, whereas NaF tended to
decrease the CO peak more smoothly toward the background. Using NaF, the
oxygen yields were 54–64% for Qz and 82-91% for SWy-1, showing
improvement in both reactivity and oxygen yield compared to measurements
with PTFE. However, a longer time was required for the CO peak to return
to background levels compared with the non-silicate standards. For both
Qz and SWy-1, the oxygen yields tended to converge to nearly 100% by
manually extending the integration width of the CO peak (approximately
900–1000 s from the peak start). After this convergence,
δ18O varied from 16 to 22‰ for SWy-1 (n = 5), while
the reported value is 18.6‰ 4. The main reason for the
large variation in yields and isotope ratio values may be the poor
reproducibility of the end positions owing to the long tailing of the
peak.
In addition, the workability of fluorides during weighing and
encapsulation was examined. KF showed a significant weight gain during
weighing due to its high hygroscopicity (6.9 wt% within 5 min and 13.2
wt% within 10 min after retrieving from the drying oven). For other
fluorides, no discernible weight changes were observed within 60 min.
Therefore, NaF is the most suitable fluorine source in terms of both
reactivity and workability.