Figure 2. A typical QMS spectrum of a highly enriched18O water from m/z 1-40. The insert plot shows the
partial pressure (Pa, plotted on a logarithmic scale) with respect to
m/z 14 to 24.
In the m/z range 16-24, several signals could not be used for our
fitting analysis of the 18O concentration, either
because of the interference of other species, or because of the very low
signal. Oxygen ([16O]+) from air
interferes with oxygen ([16O]+)
from H216O (m/z 16). Injecting water
without air is virtually impossible, and small leakages are always
present as well. The origin of this interference is air is clear from
its correlation with m/z 14
([14N]+ from air). Therefore m/z
16 was disregarded from the fit. This interference is small, and hence
the consequential interferences on m/z 17 and m/z 18 from air due to
[17O]+ and
[18O]+ are orders of magnitude
smaller and therefore negligible. Additionally, the very minor signals
arising from the various clumped isotopocule ions on m/z 22-24 (see
Table 5 in supplement) are too small to be of use.
In the spectrum of an 18O enriched water (Figure 2) a
very small signal from the recombined ion
[18O2]+ is
visible at m/z 36. Approximately 1.5% of the spectrum is in the form of
[O]+ (all three different oxygen isotopes
together), (see Table 5 in the supplement). Including the signal at m/z
36 in the spectral fit showed that about 7% of the
[18O]+ ions, recombines to
[18O2]+. This
signal at m/z 36 doesn’t significantly influence the fitted18O value and was therefore neglected in our fitting
program.
At m/z 1 and 2 signals from
[1H+] and
[2H+]/[H2+]
are visible in the spectrum (Figure 2). As these hydrogen fragments do
not contain oxygen, they were not included in the fitting program.
Five m/z values in the range 17-21 could be used for a successful fit,
yielding the 18O concentration. The fitting program
was written in R. The output of this R program, the fit parameters, were
besides the abundance of 18O, the size of the
fractions [H2O]+,
[OH]+, [O]+ and thus the
size of the complementary fraction
[H3O]+ as well. Next to the
signals m/z 17-21, the abundances of 17O and2H were also input parameters for the fitting program.
The abundances of 17O and 2H of the
highly enriched 18O water were separately determined
to reduce the number of fitting parameters, which is necessary as17O and 18O in the fit are in fact
quite correlated. Determination by dilution and comparison with
reference waters is adequate in these two cases, as neither2H nor 17O abundances are very
critical in the fitting process. This is due to the fact that both
abundances are low anyway: 2H because it is in the
natural range, and 17O because we deal with highly
enriched 18O waters. Because of that, there is only
room for ≤ 1% 17O, and determination of the17O abundance with a relative precision of 5% is
already more than adequate. Such precision is well-achievable using
dilution.
For determining 17O, the enriched waters were diluted
and measured alongside references IAEA 607, 608 and 609
(Faghihi15 and CIO laboratory standards, using the
LGR-LWIA. For determining 2H concentration, the
diluted enriched waters were measured alongside CIO laboratory standards
using the LGR-LWIA as well. In both cases we calculated the abundances
from our isotope delta-measurements using the literature values for the
abundances in VSMOW (Hageman4 for 2H
and Li14 for 17O). The results of
the 18O, 2H and17O abundances corresponding to all the highly
enriched 18O water portions are presented in Table 3
(results section), along with their uncertainties.
SLAP-rep-O was mixed with highly 18O enriched water to
mimic VSMOW, and referred to as VSMOW-rep-O (analogous to SLAP-rep-O,
VSMOW-rep-O refers to water with an isotopic δ18O value close to VSMOW). We added a known quantity
of highly enriched 18O water needed to shift the
δ18O to 0‰ when measured against VSMOW. SLAP-rep-O was
weighed on a precision balance (readability 0.01 g) in a 1 L Duran brown
glass flask. H218O was weighed on an
analytical balance (readability 0.01 mg) in a small glass vial. This
vial was submerged in the 1 L flask with SLAP-rep-O. To ensure complete
mixing, the resulting mixture, VSMOW-rep-O, was stirred for at least 48
hrs. Accurate determination of the weights of the mixing water portions
is extremely critical in the whole calculation chain, therefore weights
are also corrected for buoyancy effects, as the density of
H218O water is significantly larger
than that of H216O (1.11 g/mL instead
of 1 g/mL). The weighing was performed as fast as possible to keep
evaporation of water to a minimum.
Stable isotope measurements were performed using the LGR-LWIA. The
replicates were measured alongside the real VSMOW and SLAP, such that
scale contraction issues played no role (see below).
The mixing process started with the characterization of the individual 1
liter batches of SLAP-rep-O water, by direct comparison with SLAP. We
then measured the produced VSMOW-rep-O by direct comparison with
original VSMOW water. The difference between this measurement and the
calculated value translates directly into a best δ18O
value for SLAP with respect to VSMOW. As we took care that both the
δ18O differences SLAP-rep-O vs SLAP and VSMOW-rep-O vs
VSMOW are small, their differences could be determined precisely. As
these differences between the replicates and the genuine VSMOW and SLAP
are small, the δ18O difference between the officially
δ18O values (VMSOW-SLAP scale) and the ’true’ isotopic
difference did not play a role. The calculation of the resulting
δ18O value for SLAP is straightforward, and has been
performed with the help of a validated spreadsheet
(Faghihi16).
2.3.2 Approach 2
Highly enriched 18O water is not enriched in deuterium
(on the contrary, compared to water with natural abundances it is
depleted in deuterium). Therefore, after adding
H218O to SLAP-rep-O, δ18OVSMOW-rep-O is close to δ18OVSMOW but δ2HVSMOW-rep-O is still close toδ 2HSLAP. In principle, this
does not matter for our experiment, as we only are interested in the18O side. However, to exclude the possibility that
this large difference in deuterium content between our VSMOW-rep-O and
the real VSMOW would influence the absorption of the18O line in the LGR-LWIA (and thus its determination
of the δ 18O difference between VSMOW-rep-O and
VSMOW), in addition an extra step in the process was introduced. Before
SLAP-rep-O was mixed with H218O, pure2H2O was added to mimic VSMOW in
deuterium (called VSMOW-rep-D, δ 2H ≈ 0‰).
Subsequently VSMOW-rep-D and highly enriched 18O water
were mixed to get VSMOW-rep-OD (δ 2H ≈ 0‰ andδ 18O ≈ 0‰). This second approach is shown on
the righthand side of Figure 1. It rules out spectroscopic biases in the
measurements, but otherwise is not different from the process described
in 2.3.1.
We started with the same Antarctic water as described before and added
Groningen tap water to produce SLAP-rep-O. Then we added2H2O to mimic VSMOW in deuterium and
therefore very precise quantification of the2H2O content was key. Determination of2H abundance of the enriched2H2O water by QMS, however, was not as
straightforward as determination of 18O abundance of
the enriched H218O. This may be caused
by the more complex spectrum for 2H2O.
The 2H2O spectrum, Figure 3,
illustrates that m/z peaks 17,19 and 21 are about two orders of
magnitude smaller than the adjacent m/z peaks 18 and 20. In the
supplementary material, Table 6 shows a highly enriched2H water with the various isotopologues and fragments
for this range of m/z values.
Peak tailing and leading of the larger peaks makes it difficult to
integrate the smaller peaks. These alternating small and large peaks,
are not present in the 18O spectrum (Figure 2, see
logarithmic insert plot). Another possible explanation is the common
knowledge that in high vacuum stainless steel tubes, there is always
outgassing of hydrogen. If this is the case in the QMS, H-exchange will
affect deuterium abundance measurements with the QMS, especially for
these nearly pure 2H2O waters. This
outgassing of hydrogen will obviously not affect the determination of
oxygen isotope abundances.
Furthermore, the m/z 1 the signal was much larger than we expected from
a nearly pure 2H2O water (m/z 1 is
approximately 1% of m/z 2, see Figure 3, top insert plot), an
observation that worried us initially. But after personal communication
with the manufacturer of the QMS, Extorr, we learned that this was
probably a source pressure related artifact. Working at higher pressures
can cause scattering. If the QMS is not tuned for these low m/z values,
a fraction of the scattered ions passes through the mass filter below
0.5. The actual m/z 1 is therefore not resolved well. This fact made
signals m/z 1 and 2 useless for obtaining the deuterium concentration of
the almost pure 2H2O water. Therefore,
we used a similar m/z signal range as was used for the18O determination.
In conclusion, this discrepancy of measured (and fitted)2H abundance (of about 99.7%) and real (specified)2H abundance (99.98%) must be attributed to reasons
mentioned above: the more complex spectrum and the continuous outgassing
of hydrogen in vacuum stainless tubes. To verify the specification of
the supplier we performed NMR analysis for accurate 2H
concentration analysis of the highly enriched deuterated water, which
corroborated the specified value, and also excluded the possibility that
sample handling of these highly enriched waters would lead to dilution
due to admixture of water (vapour) from the surroundings.
<figure 3>