* The expected 18O abundance of the diluted
mixtures is based on the exact weights of the highly enriched water and
with water with natural 18O abundance.
Further investigations into the reliability of our QMS-based abundance
determination involved the possibility that ionization processes in the
QMS source such as ion yield, and ion distribution might be dependent on
the specific oxygen isotope. Water samples in the ion source of the QMS
mainly ionize to [H]+, [O]+,
[OH]+,
[H2O]+ and
[H3O]+ ions. The distribution of
the oxygen-bearing fragmentation ions in natural water and in highly
enriched 18O water has therefore been compared. The
observations are shown in Table 2, and notable differences in
fragmentation pattern between enriched and natural water are visible,
especially for the fragmentation ion [O]+: it is
more preferred in natural 16O water than in18O water, the difference is more than 60% (relative)
/ 1% (absolute). In the fitting program, described in section 2.3,
these differences are taken into account.
<Table 2>