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.