* 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>