Figure 1. Flow diagram and description of the used abbreviations in the process of quantification of the δ18OSLAP value with respect to δ18OVSMOW. δ2H and δ18O in this figure are expressed on the VSMOW-SLAP scale.
For every experiment, we started with Antarctic water and made a fresh portion of SLAP-rep-O. After measuring the isotopic values of Antarctic water, we calculated how much demineralized Groningen tap water should be added, in order to mimic δ 18O of SLAP. As the Antarctic water was isotopically ”lighter” than SLAP, we had to add approximately 18 g of demineralized Groningen tap water (δ2H = -43.5‰, δ 18O = -6.5‰) to 1 liter of this Antarctic water. In total, we produced 7 portions of SLAP-rep-O, which were individually measured on the LGR-LWIA along with aliquots of SLAP.
The next step in the flow diagram shows the mixing of SLAP-rep-O with highly enriched 18O water in order to obtain VSMOW-rep-O. As said before, the most critical part of the whole process is the characterization of the highly enriched18O-water that is added to the SLAP-O replicate. This18O characterization is done by fitting a QMS spectrum of the enriched water. The steps we took for a careful determination are described in this section. We did our utmost to avoid memory effects from natural and highly enriched 18O water in the QMS and we investigated the influence of several ionization processes in the ion source of the QMS on this 18O determination. At the end, we performed a validation of our QMS method by diluting a H218O water portion with 1% and 2% H216O. The results of this validation by comparing the expected abundances based on weights with the measured abundances and the influence from several ionization processes are described in the Results section.
To get rid of possible memory effects, the ion source of the QMS was pumped for more than 48 hours at high vacuum, before measuring highly enriched 18O water (background pressure was 1.5 x 10-6 Pa). The mass spectrum with this “clean” source was considered as a background signal and was subtracted from the spectrum of the enriched water. For this background signal, it hardly made any difference if the previous injection, before the 48 hours of pumping, was a water with natural abundances or a water with enriched18O.
Water is very “sticky” and adsorbs to the walls of the injector, dead volumes and the ion source of the QMS. Therefore, the analyzing QMS setup needs to be saturated with highly enriched 18O water in order to reduce memory effects. Thus, more than 20 sequential identical sample injections were required to reach an equilibrium state. For every injection, 25 µl water was injected and a scan of m/z 1 to 41 was performed. The measurement pressure was at 2.5 x 10-3 Pa. The QMS exclusively measured highly enriched18O water for several months in a row.
Water molecules in the ion source of the QMS ionize, break and recombine to produce a combination of peaks corresponding to [H]+, [H2]+, [O]+, [OH]+, [H2O]+, [H3O]+ and [O2]+ ions. All of these ions contain the two different H-isotopes and three different O-isotopes. In Figure 2, a typical QMS spectrum of a highly enriched18O water is shown. All the main Oxygen-bearing fragments together produce ion signals from m/z 16 to 24. In the supplementary material, Table 5 shows a highly enriched18O water (water portion D from Cortec) with the various isotopologues and fragments for this range of m/z values.
<Figure 2>