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>