Figure 5. A full calculation scheme showing the steps involved in one of the seven independent determination of the δ18O of SLAP.
For the second approach (also mixing with2H2O) it was necessary to verify the2H concentration specification of the supplier. The determination of the 2H abundance of2H2O water by QMS was not as straightforward as the determination of 18O by QMS appeared to be, which has been explained in section 2.3.2. The result of the QMS fitted 2H measurement was nearly 0.3% lower than the specification of the supplier. Therefore, we analyzed this sample using NMR and the results matched the supplier’s specified value. The specified 2H abundance of the almost pure2H2O water is 0.99978, the measured (via dilution and CO2 equilibration) and calculated17O and 18O abundances are 0.000808 (3), 0.005928 (6) respectively.
Between brackets the standard deviation of the three repetitions for17O and 18O abundances are shown.
With enriched water portions A until D the first approach is used (only mixing with enriched 18O water). The δ18OSLAP results with the combined uncertainties as described before are shown in Figure 6 (black solid circles).
The δ 18OSLAP results with the combined uncertainties using the second approach are shown in Figure 6 as well (black open squares). The δ18OSLAP results are also presented in the supplementary material, Table 4. Significant difference between the two approaches was not obvious, and hence all results for δ18OSLAP were averaged. The overall weighted mean of all data points is δ18OSLAP = -56.33 ± 0.02‰. Taking the Student’s T-factor into account, the final outcome is δ18OSLAP = -56.33 ± 0.03‰. This final uncertainty does not include the two systematic effects mentioned in section 2.4. These will be later discussed further in this section.
The second approach allowed determination of δ2H for SLAP as well. The δ2HSLAP results are shown in the supplementary material, Table 4. The overall weighted mean of the three experiments is δ 2HSLAP = -430.3 ± 0.3‰ (again including the Student’s T-factor).
<Figure 6>