Fig. 5. The m /z 28 traces during analysis of quartz and SWy-1 with the combination of Ni/C and NaF. Red arrows represent the positions of CO peak ends.
3.4. Oxygen yields and δ18O values using a promising recipe (Run4)
The study from Run1 to Run3 revealed two promising recipes: (1) a recipe using graphite and NaF with F/O = 6 (hereafter, this recipe will be referred to as ”FO6”), and (2) one using Ni/C and NaF with F/O = 2 (hereafter, ”NiC_FO2”). Using these two recipes, a series of measurements were performed for NBS28, SWy-1, TKN-01, -29, and -31 samples (Run4, 74 samples in total, including 36 standards). The oxygen yields monitored in this series of measurements are shown in Fig. 6. The oxygen yields shown here were calculated based on the CO peak area normalized to the oxygen mass and assuming that the average oxygen yields of IAEA-601 and -602 were 92.8% 24.
First, the oxygen yields of NBS18 calcite were about 60%, indicating that about 2/3 of the oxygen was converted to CO gas by the reaction CaCO3 + Cexcess → CaO + 2CO↑, and the remaining 1/3 became residue as CaO (melting point 2,572°C). For the NaF-bearing samples, the oxygen yields of the first samples in a sequence of five or six consecutive measurements tended to be anomalously high. Especially in the first sequence (NBS28_FO6, run order #16–20), the first sample showed 190% of yield and then gradually declined to approximately 92.8%. The chromatographs of the first sequence were shown in Fig. 7. In the subsequent sequences, only the first sample showed a significantly higher yield, whereas the yields of the second and subsequent samples tended to stabilize at approximately 90–100% for the sample using FO6 and approximately 110% for the samples using NiC_FO2. The high yield of the first sample in each sequence was probably derived from oxygen released from the residue of the standards analyzed in the just preceding sequence. NBS18 is expected to leave approximate 40% of the oxygen in the reactor. IAEA-601 and 602 were reported to have an approximate 93% of oxygen yield in thermal decomposition13,24 , and 7% may remain in the residue. Fluorine bearing samples just after the measurement of standards also show longer tailings. The wider peaks for the earlier samples in the same sequence (Fig. 7). Except for the first sequence of fluorine bearing samples (run order #16–20), tailings were rarely observed in the second or subsequent samples in each sequence. It was ruled out that abnormal yields could have been caused by the order in which the weigh-ins were done after the drying oven was removed. This is because, regardless of the weighing procedure’s order, the latter portion of the measurement series, which contained the FO6 and NiC_FO2 recipes in the same sequence (run order #48–53, #57–62, #66–71) likewise demonstrated the same trend. The oxygen yields of the samples with NiC_FO2 were higher than those of the samples with FO6, suggesting the possible oxidation of Ni in Ni/C during storage.