Table 2. Comparison of the distribution of 4 main Oxygen-bearing fragmentation ions in QMS source for natural water and18O enriched water (water portion A, Rotem). The total concentration of those 4 ions is considered as 1 (so 100%). For every ion, the averaged part of this total fraction is displayed. Between brackets the standard deviation of the repetitions (n) is displayed. Cortec water is even more enriched than Rotem water and shows slightly different fragmentation, an example is in the supplementary Table 5.
Our experiments also show a small isotope effect in the ionization efficiency between water with natural abundances and water with enriched18O. Although hard to determine due to the uncertainty in the amount of water injected, natural water seems to ionize ≈ 6% better than the enriched 18O water. As a result of this difference in ionization yield, the 16O fraction of the highly enriched 18O water is overrepresented.
As said, this ≈ 6% difference in ionization efficiency carries a relatively large uncertainty. Alternatively, we can use the results from the previously described 1 and 2% dilution experiment. There, the best fit between expected and determined abundance differences leads to an ionization efficiency from highly enriched 18O water compared to water with natural abundances of only 0.97 ± 0.03. This value would lead to a maximum deviation in the end result of δ18O for SLAP of - 0.02 ± 0.02‰. Therefore, we decided to neglect the possible slight difference in ionization yield in our fitting process.
In Table 3 the results of the 18O abundances are shown for the 6 highly enriched 18O water portions (A until F) from two different suppliers, Rotem and Cortec, measured with the QMS. Water portion D was measured twice, the latter being after 4 months of the first measurement set. As can be seen in Table 3, its18O abundance had decreased slightly, but significantly, after puncturing the septum in the closing cap of the vial. All highly enriched 18O waters matched the specification of the suppliers. Table 3 also provides17O and 2H abundances of these highly 18O enriched water portions, as determined via dilution.
<Table 3>