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\begin{document}

\title{The absolute \selectlanguage{greek}δ \selectlanguage{english}18 O value for SLAP with respect to VSMOW reveals a much
lower value.\selectlanguage{english}}



\author[1]{Anita Aerts-Bijma}%
\author[1]{Albert C. van Buuren}%
\author[1]{Dipayan Paul}%
\author[1]{Harro Meijer}%
\affil[1]{Rijksuniversiteit Groningen Energy and Sustainability Research Institute Groningen}%


\vspace{-1em}



  
  \date{May 22, 2024}


\begingroup
\let\center\flushleft
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\endgroup





\selectlanguage{english}
\begin{abstract}
\textbf{RATIONALE}: SLAP is one of the two calibration materials for the
isotopic water scale. By consensus the established \selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value is -55.5%0, although there are firm
indications that \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O \textsubscript{SLAP} is
significantly more negative. The real \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O
\textsubscript{SLAP} value as such does not influence the isotopic water
scale, however knowledge of the size of isotopic scale contraction in
stable isotope measurements is vital for second order isotopes. In this
study quantification of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O
\textsubscript{SLAP} with respect to \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O
\textsubscript{VSMOW} is described. \textbf{METHODS:} SLAP-like water
was quantitatively mixed with highly \textsuperscript{18}O enriched
water to mimic VSMOW. The \textsuperscript{18}O concentration was
determined using an electron ionization quadrupole mass spectrometer.
The isotopic composition of the SLAP-like and VSMOW-like waters were
measured with an optical spectrometer, alongside real VSMOW and SLAP.
\textbf{RESULTS:} This study resulted in a much more depleted \selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value for SLAP than expected. The averaged outcome
of 7 independent experiments is \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O
\textsubscript{SLAP} -56.33 \selectlanguage{ngerman}± 0.03\selectlanguage{english}%0. There is a large discrepancy
between the actual isotopic measurements of even the most carefully
operating groups and the true \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value.
\textbf{CONCLUSIONS:} Although this finding as such does not influence
the use of the VSMOW-SLAP scale, it raises the intriguing question what
we actually measure with our instruments, and why even a fully corrected
measurement can be so far off. Our result has consequences for issues
like the transfer of \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O from and to the VPDB
scale, various fractionation factors, and the \selectlanguage{greek}\emph{Δ}
\selectlanguage{english}\textsuperscript{17}O. The absolute \textsuperscript{18}O abundance for
SLAP was determined at 1.88798 (43) x 10 \textsuperscript{-3} based on
the absolute \textsuperscript{18}O abundance of VSMOW and the presented
\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O \textsubscript{SLAP} in this paper.%
\end{abstract}%



\sloppy


The absolute \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value for SLAP with respect to VSMOW
reveals a much lower value.

\emph{Anita Th. Aerts-Bijma} , Albert C. van Buuren, Dipayan Paul, Harro
A.J. Meijer

Centre for Isotope Research (CIO), Energy and Sustainability Research
Institute Groningen, University of Groningen, Groningen, The Netherlands

Corresponding author: a.t.aerts-bijma@rug.nl

Abstract

\textbf{RATIONALE} : SLAP is one of the two calibration materials for
the isotopic water scale. By consensus the established\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value is -55.5%0, although there are firm
indications that \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} is
significantly more negative. The real\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} value as such does not
influence the isotopic water scale, however knowledge of the size of
isotopic scale contraction in stable isotope measurements is vital for
second order isotopes. In this study quantification of\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} with respect to\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{VSMOW} is described.

\textbf{METHODS:} ~SLAP-like water was quantitatively mixed with
highly\textsuperscript{18}O enriched water to mimic VSMOW.
The\textsuperscript{18}O concentration was determined using an electron
ionization quadrupole mass spectrometer. The isotopic composition of the
SLAP-like and VSMOW-like waters were measured with an optical
spectrometer, alongside real VSMOW and SLAP.

\textbf{RESULTS:} ~This study resulted in a much more depleted\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value for SLAP than expected. The averaged outcome
of 7 independent experiments is\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} -56.33 \selectlanguage{ngerman}± 0.03\selectlanguage{english}%0. There is a
large discrepancy between the actual isotopic measurements of even the
most carefully operating groups and the true\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value.

\textbf{CONCLUSIONS:} ~Although this finding as such does not influence
the use of the VSMOW-SLAP scale, it raises the intriguing question what
we actually measure with our instruments, and why even a fully corrected
measurement can be so far off. Our result has consequences for issues
like the transfer of\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O from and to the VPDB
scale, various fractionation factors, and the\selectlanguage{greek}\emph{Δ}
\selectlanguage{english}\textsuperscript{17}O. The absolute \textsuperscript{18}O abundance for
SLAP was determined at 1.88798 (43) x 10\textsuperscript{-3} based on
the absolute \textsuperscript{18}O abundance of VSMOW and the
presented\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} in this
paper.

Key Words

Water isotopic scale, SLAP, VSMOW, \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O,
calibration, IRMS, optical spectroscopy, electron ionization quadrupole
mass spectrometer (EI-QMS), \textsuperscript{18}O abundance,
VSMOW-CO\textsubscript{2}, VPDB-CO\textsubscript{2}

\textbf{1. INTRODUCTION}

The stable isotope scale of water has been successfully established and
maintained by the two primary reference waters: VSMOW and SLAP. In
principle, only one reference material per isotope and per medium would
be needed to define the isotopic scale, but two-point calibration leads
to a dramatic improvement in inter-laboratory comparisons, due to
various and variable scale contraction processes occurring in each
measurement process.

In 1976 during a consultants' meeting on stable isotopes at the IAEA in
Vienna, \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O measurements of SLAP from 45
laboratories were evaluated. The data showed a rather large spread with
measurements ranging between was from -54.53%0 to -56.5%0. The
averaged\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value for SLAP was -55.49%0, and
the standard deviation was 0.55%0 (2 data points were considered as
outliers (-49.2 and -53.92%0)). During this meeting, it was agreed
that\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O SLAP would be established at the
consensus value of -55.5%0 (Gonfiantini\textsuperscript{2,3}).

For deuterium, the other stable isotope of water, it is possible to
(re)produce the primary reference waters based on gravimetric mixtures
of isotopically pure waters. In this way, the absolute deuterium
abundances of VSMOW and SLAP has been precisely determined by several
authors (Hagemann\textsuperscript{4}, De Wit\textsuperscript{5},
Tse\textsuperscript{6}).

A similar experiment for oxygen is much harder, as
pure\textsuperscript{18}O and \textsuperscript{16}O waters are not
available. Only (Baertschi\textsuperscript{7}) has performed a very
extensive experiment, resulting in the absolute \textsuperscript{18}O
abundance of VSMOW, with a relative precision of 0.2%0.

In this study, we take the next step, namely determination of
the\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O of SLAP with respect to VSMOW. Instead
of determining the absolute abundance of SLAP, we focus on the relative
difference in \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O between VSMOW and SLAP,
which we aim to achieve with much higher precision ([?] 0.05%0). In this
way, we achieve a more accurate S.I. traceable result for the VSMOW-SLAP
scale.

We quantify the difference in \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O between
VSMOW and SLAP by gravimetrical mixing of a SLAP-like water with
highly\textsuperscript{18}O enriched water to mimic VSMOW and compare
this with real VSMOW.

Although the real \selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP}value as such, does not
influence the use of the VSMOW-SLAP scale, the various measurements from
Gonfiantini\textsuperscript{3} from -54.53 to -56.5%0, and from
Verkouteren and Klinedinst\textsuperscript{1}, Barkan and
Luz\textsuperscript{8}, pointed out by Kaiser\textsuperscript{9}, from
-55.11 to -56.18%0, raise the intriguing question what the actual value
is. This real value can play an important role in understanding IRMS
issues, such as scale contraction caused by memory effects.
Understanding such IRMS side-effects is essential to work with a
well-maintained instrument and for correcting measurements accordingly.
Ideally, isotopic measurements from mass spectrometers and optical
spectroscopic instruments, should be very close to their actual values.
This is especially important if the isotopic values for different
materials have to be compared, for example \selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O in carbonates, or in atmospheric
CO\textsubscript{2}, in relation to that of water. Furthermore, recent
years have seen more complex, 'second order' isotope work, like
exploiting the very small differences in behavior between
\textsuperscript{17}O and \textsuperscript{18}O (expressed as
\textsuperscript{17}O excess, \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{17}O)
(Hofmann\textsuperscript{10}, Landais\textsuperscript{11}) and the
deviation from stochastic distribution of the rare isotopes in molecules
('clumped isotopes') (Eiler\textsuperscript{12},
Bernasconi\textsuperscript{13}). Also in these fields, understanding
(and correcting for) instrument-related isotope effects is crucial.

\textbf{2. EXPERIMENTAL SETUP AND METHOD}

Our experiments were aimed at quantifying the difference in\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O between VSMOW and SLAP by producing a surrogate
VSMOW by gravimetrical mixing of a SLAP-like water with
highly\textsuperscript{18}O enriched water, and compare this surrogate
with real VSMOW. To this end, several instruments and waters and
procedures were used, which are described in the next section.

2.1 Water portions.

For these experiments, a large batch (20 liter) of Antarctic water was
made available to us by the isotope hydrology laboratory of the IAEA in
Vienna. Its \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value was even slightly more
negative than that of SLAP. Portions of 1 liter of this batch were mixed
with demineralized Groningen tap water to mimic SLAP. Such large amounts
of water were needed to reach the accuracy goal of [?] 0.05%0 in the final
result for SLAP, because of gravimetric/weighing and sample handling
precision limitations. Obviously, such quantities of the real SLAP were
out of the question.

The reference waters SLAP and VSMOW (ampoules with 1 mL), for the
isotopic measurements, were provided by the IAEA Terrestrial Environment
Laboratory in Seibersdorf. In order to avoid additional uncertainty
contributions, the IAEA provided us with the real VSMOW and SLAP and not
their replacements VSMOW2 and SLAP2. The real VSMOW and SLAP were used
for isotopic comparison measurements with the SLAP-like and VSMOW-like
waters that were produced in the experiments.

For this study 6 highly \textsuperscript{18}O enriched water portions
were obtained from two manufacturers: three from Cortec (CortecNet,
Voisins le Bretonneux, France, specification \textsuperscript{18}O
\textgreater{} 99\%) and three from Rotem (Rotem industries Ltd., Arava,
Israel, specification \textsuperscript{18}O \textgreater{} 98\%). All
six water portions were from different production batches.

Furthermore, one virtually pure \textsuperscript{2}H\textsubscript{2}O
water (10 times 1 mL ampoules) was obtained from
Sigma-Aldrich\textsuperscript{2}H [?] 99.96\% (certificate of analysis
specified 99.978\%, determined via NMR analysis).

The SLAP-like product of mixing Antarctic water and Groningen
demineralized tap water, with the same \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O as
SLAP, will be referred to as SLAP-replicate-Oxygen henceforth in the
manuscript, and in short SLAP-rep-O.
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP-rep-O} is [?] -55.5%0 on
the VSMOW-SLAP scale. Similarly, VSMOW-rep-O refers to a VSMOW-like
water in\textsuperscript{18}O,
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{VSMOW-rep-O} is [?] 0%0. The
other produced replicates are VSMOW-rep-D
(\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{2}H\textsubscript{VSMOW-rep-D} [?] 0%0) and
VSMOW-rep-OD (\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H\textsubscript{VSMOW-rep-OD}
[?] 0%0,\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O\textsubscript{VSMOW-rep-OD} [?] 0%0).
So, the last replicate matches VSMOW in both water isotopes.

2.2 Instruments.

Accurate determination of the \textsuperscript{18}O concentration of the
highly enriched water was key to our efforts: to achieve an accuracy of
[?] 0.05%0 in the \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value for SLAP,
the\textsuperscript{18}O concentration of the highly enriched
H\textsubscript{2}\textsuperscript{18}O water had to be determined at [?]
\selectlanguage{ngerman}± 0.1\%. We were able to reach this precision and accuracy by performing
detailed mass scans using a quadrupole mass spectrometer (QMS) equipped
with an electron ionization (EI) ion source (Extorr XT100, Extorr Inc.,
USA), in combination with a bespoke spectral fitting program. The
measurements were carried out at an electron energy of 70 eV. For the
uncertainty in our signal we use the standard deviation of the
instrument's background signal-to-noise at m/z 5, as no peak is expected
at m/z 5, which was around 2 x 10\textsuperscript{-9} Pa. The total
integrated signal of m/z 1 to 41 was approximately 2 x
10\textsuperscript{-4} Pa. The base peak signal at m/z 20,
{[}H\textsubscript{2}\textsuperscript{18}O{]}\textsuperscript{+}, was
almost 1.3 x 10\textsuperscript{-4} Pa.

All water samples were analyzed using a LGR Liquid Water Isotope
Analyzer (LGR LWIA 912-0050), which is an off-axis integrated cavity
output spectrometer, to determine the triple-stable isotope
composition:\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O, \selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{17}O and\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H. Typically sample
measurements are bracketed with local references as well as
international references, details of which has been presented later in
the manuscript.

The portion of H\textsubscript{2}\textsuperscript{18}O water
(approximately 125 mg) was weighed on a Sartorius BP210 D (210 g,
readability 0.01 mg) analytical balance. The SLAP-like water used for
mixing (approximately 1000 g) was weighed on a precision balance from
Kern 572 (4210 g, readability 0.01 g).

To verify the NMR specification of the supplier
of\textsuperscript{2}H\textsubscript{2}O (and check in general that
sample handling of such highly enriched waters had a negligible
influence on the abundances), the \textsuperscript{1}H abundance
of\textsuperscript{2}H\textsubscript{2}O was analyzed using a NMR
(Bruker Avance NEO 600 MHz).

2.3 Procedure

2.3.1 Approach 1

All the steps taken to prepare the various water-replicate samples
leading to the precise determination of\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} with respect to\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O\textsubscript{VSMOW} is illustrated in Figure 1.
The flow diagram illustrates the mixing steps from Antarctic water via a
SLAP-replicate to the two VSMOW-replicates created by adding
well-characterized H\textsubscript{2}\textsuperscript{18}O (left-hand
side), and with an extra step in which also the\textsuperscript{2}H-side
is modified (right-hand side). The most critical part of the process
entails the characterization of the highly enriched
\textsuperscript{18}O-water that is added. Important other, but more
standard determinations, are the initial creation of the SLAP-rep-O
water, as well as several additional determinations (such as the
determination of the \textsuperscript{17}O and \textsuperscript{2}H
content of the \textsuperscript{18}O-water, and the optical measurements
of the isotopic differences between the created SLAP-rep-O and SLAP, and
between the VSMOW-rep-O (or VSMOW-rep-OD) and VSMOW. The steps indicated
on the right-hand side of Figure 1 is further described in 2.3.2.

\textless{}Figure 1\textgreater{}

\selectlanguage{greek}\emph{Figure 1. Flow diagram and description of the used abbreviations
in the process of quantification of the
δ\textsuperscript{18}O\textsubscript{SLAP} value with respect to
δ\textsuperscript{18}O\textsubscript{VSMOW}. δ\textsuperscript{2}H and
δ\textsuperscript{18}O in this figure are expressed on the VSMOW-SLAP
scale.}\selectlanguage{english}

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 \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O of SLAP. As
the Antarctic water was isotopically ''lighter'' than SLAP, we had to
add approximately 18 g of demineralized Groningen tap water
(\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{2}H = -43.5%0, \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O =
-6.5%0) 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 \textsuperscript{18}O 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
enriched\textsuperscript{18}O-water that is added to the SLAP-O
replicate. This\textsuperscript{18}O 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 \textsuperscript{18}O
water in the QMS and we investigated the influence of several ionization
processes in the ion source of the QMS on this \textsuperscript{18}O
determination. At the end, we performed a validation of our QMS method
by diluting a H\textsubscript{2}\textsuperscript{18}O water portion with
1\% and 2\% H\textsubscript{2}\textsuperscript{16}O. 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 \textsuperscript{18}O water (background pressure was 1.5 x
10\textsuperscript{-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
enriched\textsuperscript{18}O.

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 \textsuperscript{18}O
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 \selectlanguage{greek}µ\selectlanguage{english}l water was injected and a scan of m/z 1 to 41
was performed. The measurement pressure was at 2.5 x
10\textsuperscript{-3} Pa. The QMS exclusively measured highly
enriched\textsuperscript{18}O 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{]}\textsuperscript{+}, {[}H\textsubscript{2}{]}\textsuperscript{+},
{[}O{]}\textsuperscript{+}, {[}OH{]}\textsuperscript{+},
{[}H\textsubscript{2}O{]}\textsuperscript{+},
{[}H\textsubscript{3}O{]}\textsuperscript{+} and
{[}O\textsubscript{2}{]}\textsuperscript{+} 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
enriched\textsuperscript{18}O 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
enriched\textsuperscript{18}O water (water portion D from Cortec) with
the various isotopologues and fragments for this range of m/z values.

\textless{}Figure 2\textgreater{}

\emph{Figure 2. A typical QMS spectrum of a highly
enriched\textsuperscript{18}O 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 \textsuperscript{18}O concentration, either
because of the interference of other species, or because of the very low
signal. Oxygen ({[}\textsuperscript{16}O{]}\textsuperscript{+}) from air
interferes with oxygen ({[}\textsuperscript{16}O{]}\textsuperscript{+})
from H\textsubscript{2}\textsuperscript{16}O (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
({[}\textsuperscript{14}N{]}\textsuperscript{+} 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
{[}\textsuperscript{17}O{]}\textsuperscript{+} and
{[}\textsuperscript{18}O{]}\textsuperscript{+} 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 \textsuperscript{18}O enriched water (Figure 2) a
very small signal from the recombined ion
{[}\textsuperscript{18}O\textsubscript{2}{]}\textsuperscript{+} is
visible at m/z 36. Approximately 1.5\% of the spectrum is in the form of
{[}O{]}\textsuperscript{+} (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
{[}\textsuperscript{18}O{]}\textsuperscript{+} ions, recombines to
{[}\textsuperscript{18}O\textsubscript{2}{]}\textsuperscript{+}. This
signal at m/z 36 doesn't significantly influence the
fitted\textsuperscript{18}O value and was therefore neglected in our
fitting program.

At m/z 1 and 2 signals from
{[}\textsuperscript{1}H\textsuperscript{+}{]} and
{[}\textsuperscript{2}H\textsuperscript{+}{]}/{[}H\textsubscript{2}\textsuperscript{+}{]}
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 \textsuperscript{18}O concentration. The fitting program
was written in R. The output of this R program, the fit parameters, were
besides the abundance of \textsuperscript{18}O, the size of the
fractions {[}H\textsubscript{2}O{]}\textsuperscript{+},
{[}OH{]}\textsuperscript{+}, {[}O{]}\textsuperscript{+} and thus the
size of the complementary fraction
{[}H\textsubscript{3}O{]}\textsuperscript{+} as well. Next to the
signals m/z 17-21, the abundances of \textsuperscript{17}O
and\textsuperscript{2}H were also input parameters for the fitting
program. The abundances of \textsuperscript{17}O and
\textsuperscript{2}H of the highly enriched \textsuperscript{18}O water
were separately determined to reduce the number of fitting parameters,
which is necessary as\textsuperscript{17}O and \textsuperscript{18}O in
the fit are in fact quite correlated. Determination by dilution and
comparison with reference waters is adequate in these two cases, as
neither\textsuperscript{2}H nor \textsuperscript{17}O abundances are
very critical in the fitting process. This is due to the fact that both
abundances are low anyway: \textsuperscript{2}H because it is in the
natural range, and \textsuperscript{17}O because we deal with highly
enriched \textsuperscript{18}O waters. Because of that, there is only
room for [?] 1\% \textsuperscript{17}O, and determination of
the\textsuperscript{17}O abundance with a relative precision of 5\% is
already more than adequate. Such precision is well-achievable using
dilution.

For determining \textsuperscript{17}O, the enriched waters were diluted
and measured alongside references IAEA 607, 608 and 609
(Faghihi\textsuperscript{15} and CIO laboratory standards, using the
LGR-LWIA. For determining \textsuperscript{2}H 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 (Hageman\textsuperscript{4} for \textsuperscript{2}H
and Li\textsuperscript{14} for \textsuperscript{17}O). The results of
the \textsuperscript{18}O, \textsuperscript{2}H and\textsuperscript{17}O
abundances corresponding to all the highly enriched
\textsuperscript{18}O water portions are presented in Table 3 (results
section), along with their uncertainties.

SLAP-rep-O was mixed with highly \textsuperscript{18}O 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
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O value close to VSMOW). We added a known
quantity of highly enriched \textsuperscript{18}O water needed to shift
the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O to 0%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. H\textsubscript{2}\textsuperscript{18}O 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
H\textsubscript{2}\textsuperscript{18}O water is significantly larger
than that of H\textsubscript{2}\textsuperscript{16}O (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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
value for SLAP with respect to VSMOW. As we took care that both the
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O 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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O difference between the officially
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values (VMSOW-SLAP scale) and the 'true' isotopic
difference did not play a role. The calculation of the resulting
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value for SLAP is straightforward, and has been
performed with the help of a validated spreadsheet
(Faghihi\textsuperscript{16}).

2.3.2 Approach 2

Highly enriched \textsuperscript{18}O water is not enriched in deuterium
(on the contrary, compared to water with natural abundances it is
depleted in deuterium). Therefore, after adding
H\textsubscript{2}\textsuperscript{18}O to SLAP-rep-O,
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{VSMOW-rep-O} is close to
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{VSMOW} but
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{2}H\textsubscript{VSMOW-rep-O} is still close
to\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H\textsubscript{SLAP}. In principle, this
does not matter for our experiment, as we only are interested in
the\textsuperscript{18}O 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
the\textsuperscript{18}O line in the LGR-LWIA (and thus its
determination of the \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O difference between
VSMOW-rep-O and VSMOW), in addition an extra step in the process was
introduced. Before SLAP-rep-O was mixed with
H\textsubscript{2}\textsuperscript{18}O,
pure\textsuperscript{2}H\textsubscript{2}O was added to mimic VSMOW in
deuterium (called VSMOW-rep-D, \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H [?] 0%0).
Subsequently VSMOW-rep-D and highly enriched \textsuperscript{18}O water
were mixed to get VSMOW-rep-OD (\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H [?] 0%0
and\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O [?] 0%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
added\textsuperscript{2}H\textsubscript{2}O to mimic VSMOW in deuterium
and therefore very precise quantification of
the\textsuperscript{2}H\textsubscript{2}O content was key. Determination
of\textsuperscript{2}H abundance of the
enriched\textsuperscript{2}H\textsubscript{2}O water by QMS, however,
was not as straightforward as determination of \textsuperscript{18}O
abundance of the enriched H\textsubscript{2}\textsuperscript{18}O. This
may be caused by the more complex spectrum for
\textsuperscript{2}H\textsubscript{2}O. The
\textsuperscript{2}H\textsubscript{2}O 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 enriched\textsuperscript{2}H 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 \textsuperscript{18}O 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 \textsuperscript{2}H\textsubscript{2}O 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 \textsuperscript{2}H\textsubscript{2}O 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 \textsuperscript{2}H\textsubscript{2}O water. Therefore,
we used a similar m/z signal range as was used for
the\textsuperscript{18}O determination.

In conclusion, this discrepancy of measured (and
fitted)\textsuperscript{2}H abundance (of about 99.7\%) and real
(specified)\textsuperscript{2}H 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
\textsuperscript{2}H 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.

\textless{}figure 3\textgreater{}

\emph{Figure 3. A typical QMS spectrum
of\textsuperscript{2}H\textsubscript{2}O m/z 1 to 40. The small inserts
show the logarithmic plots of m/z 0 to 10 (top) and 14 to 24 (bottom).}

In analogy to the mixing of enriched \textsuperscript{18}O water and the
replicate for SLAP, we added the amount
of\textsuperscript{2}H\textsubscript{2}O that was calculated to achieve
a \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H [?] 0%0 to
SLAP-rep-O.\textsuperscript{2}H\textsubscript{2}O was weighed in a glass
vial on an analytical balance (approximately 75 mg was weighed),
SLAP-rep-O was weighed on a precision balance (1 L) in a 1 L brown Duran
bottle. This vial was submerged in the 1 L flask with SLAP-rep-O. The
resulting mixture VSMOW-rep-D was stirred for at least 48 hrs. All
weights were corrected for the buoyancy effect.

In this second approach, the product VSMOW-rep-D (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H
[?] 0%0, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O, still SLAP-like, [?] -55.5%0) was the basis
of the mixing process with highly enriched \textsuperscript{18}O water.

The adding of highly enriched \textsuperscript{18}O water needed to
arrive at \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O [?] 0%0 and the mixing of those two fluids
was the same as described before.

Characterization of the isotopic delta values of VSMOW-rep-D was
performed by the LGR-LWIA by direct comparison with SLAP for
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O analysis and by direct comparison with VSMOW for
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H analysis (on the VSMOW-SLAP scale). Subsequently,
the produced VSMOW-rep-OD was measured by direct comparison with
original VSMOW water, for both isotopes.

For the calculation of the best \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value for SLAP
with the help of a validated spreadsheet (Faghihi\textsuperscript{16},
the \textsuperscript{17}O and \textsuperscript{18}O abundances of the
enriched \textsuperscript{2}H\textsubscript{2}O water had to be
characterized as well. In analogy to the determination
of\textsuperscript{2}H and \textsuperscript{17}O abundances in
enriched\textsuperscript{18}O water,
\textsuperscript{2}H\textsubscript{2}O was diluted first with
demineralized Groningen tap water (1:7). Carbon dioxide with a known
isotopic signature was equilibrated with this diluted
\textsuperscript{2}H\textsubscript{2}O at 25 \selectlanguage{ngerman}ºC for 48 hours (procedure
described in Meijer\textsuperscript{17}. CO\textsubscript{2}was
extracted and \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O was measured on a dual
inlet IRMS Mass spectrometer (a VG (now Isoprime) SIRA10). IAEA607 with
approximately the same \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O signal as the
diluted \textsuperscript{2}H\textsubscript{2}O water and some other
local CIO references were identically treated and were used for
normalization. \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{17}O was determined via a
method described in Elsig and Leuenberger\textsuperscript{18}.
The\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{13}C from the initial equilibration gas is
known, and deduced from the deviation in \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{13}C
of the CO\textsubscript{2} gas after equilibration and before
equilibration with the known \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{13}C,\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{17}O could be determined. IAEA607 and the same local
CIO references as for \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O analysis, were used
for normalization. The \textsuperscript{17}O and\textsuperscript{18}O
abundances of the\textsuperscript{2}H\textsubscript{2}O water are
presented in the results section, along with their standard deviations
of three repetitions.

The difference in stable isotopes measurements and the calculated stable
isotope values by the validated spreadsheet
(Faghini\textsuperscript{16}) translates directly into a best\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value for SLAP with respect to VSMOW. In addition,
the second approach has the beneficial side-effect that additionally a
best \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H for SLAP could be determined.
\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H\textsubscript{VSMOW-rep-D} was initially
calculated from the actually added (buoyancy-corrected) weight and
isotopic abundances of the \textsuperscript{2}H\textsubscript{2}O water
and the weight and isotopic delta values of SLAP-rep-O (on the
VSMOW-SLAP scale). Subsequently\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{2}H\textsubscript{VSMOW-rep-D} was measured alongside
VSMOW. The difference between this measurement and the calculated value
translates directly into a best\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H value for
SLAP with respect to VSMOW.

We took care that the differences between the replicates and the genuine
VSMOW and SLAP were small, so the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{2}H difference between the officially
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H values
(VMSOW-SLAP scale) and the 'true' isotopic difference, or in other
words, possible scale contractions, did not play a role.

2.4 Final uncertainty calculation.

To calculate the combined uncertainty for each single experiment, a
Monte Carlo simulation was performed for the full experimental process.
For all different sources in the total process, from weighing waters,
to\textsuperscript{18}O abundance measurements by QMS, until isotopic
measurements with the LGR-LWIA, the uncertainties were determined or
estimated.

To ascertain the contribution of uncertainties in the weighing process,
a flask was weighed multiple times in order to determine the
reproducibility of weighing. This procedure revealed that the spread in
the weighing of the same flask multiple times, was within 5 times the
uncertainty specified by the manufacturers, and thus multiplied by a
factor of 5. So, for weights measured on the precision balance the
accuracy was estimated at \selectlanguage{ngerman}± 0.05 g and for the analytical balance the
accuracy was estimated at ± 0.05 mg. As a part of the quality control
measures we have adopted in our laboratory, all balances, including the
ones used in this work, are frequently calibrated.

As a cautious estimate, the uncertainties for the
QMS\textsuperscript{18}O abundances of the enriched
\textsuperscript{18}O waters were chosen to be the standard deviations
of the repetitional measurements. In Table 3 this is displayed for every
enriched\textsuperscript{18}O water. The \textsuperscript{2}H
and\textsuperscript{17}O abundances are determined via dilution. The
isotopic measurements of the diluted \textsuperscript{18}O waters were
performed on two different measurement days, and performed nine times
per measurement day. From the weighted average of the total number of
analyses and twice the standard error of the mean,
the\textsuperscript{2}H and \textsuperscript{17}O abundances were
deduced with their 2\selectlanguage{greek}σ \selectlanguage{english}uncertainty.

The isotopic measurements for SLAP, VSMOW and their replicates were
measured on the LGR-LWIA. Per measurement day every replicate was
injected 60 times, and VSMOW and SLAP were injected 90 times. The
difference in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O (\selectlanguage{greek}Δ δ\selectlanguage{english}\textsuperscript{18}O) between
the replicate and its ``parent'' (so SLAP for SLAP-replicate, and VSMOW
for VSMOW-replicate) was averaged per measurement day. The error in the
mean in the parent-replicate \selectlanguage{greek}Δ δ\selectlanguage{english}\textsuperscript{18}O was calculated
(typically better than 0.03%0) and was the basis for the weights for
calculating the weighted average for every \selectlanguage{greek}Δ δ\selectlanguage{english}\textsuperscript{18}O
parent-replicate on multiple (typically 3) measurement days.

For the Monte Carlo simulation, a calculation was programmed in R. All
calculations steps were performed 10,000 times with all the parameters
and their uncertainties (assumed to belong to a normal distribution) as
described above. This Monte Carlo simulation gives the uncertainty for
product VSMOW-rep-O. A quadratic sum from the Monte Carlo uncertainty
and the standard error in the mean of the isotopic measurements for
product VSMOW-rep-O yielded the combined uncertainty per experiment. The
Monte Carlo simulation was performed for the full calculation process
for each experiment. The combined uncertainties per experiment are shown
in the supplementary material, Table 4, and in the graph, Figure 6.

The three main uncertainty components in this combined uncertainty are
the weight and \textsuperscript{18}O concentration determination of the
enriched \textsuperscript{18}O water and the\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O measurement of SLAP-rep-O.

Despite one extra step in the second approach, the uncertainty in the
final result is the same. In the first approach the \selectlanguage{greek}Δ\selectlanguage{english}\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O between SLAP-rep-O and SLAP and VSMOW-rep-O and
VSMOW leads to a best \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value for SLAP with
respect to VSMOW. In the second approach \selectlanguage{greek}Δ\selectlanguage{english}\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O
between VSMOW-rep-D and SLAP and VSMOW-rep-OD and VSMOW leads to a best
\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value for SLAP with respect to VSMOW.

All uncertainty sources are considered to be random errors, only causing
variability in the end result. In addition, there are two sources of
systematic error. The first would be a biased QMS~\textsuperscript{18}O
measurement method. It is unlikely, but still possible, that we
systematically measure an~\textsuperscript{18}O abundance that is~too
low. If this would be the case, the final end result for~\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value for SLAP would be more negative. In the next
section, we describe a number of tests we performed to scrutinize our
QMS-based abundance measurements.

The other source of systematic uncertainty is the~\textsuperscript{18}R
value for (V)SMOW and its uncertainty as reported by
Baertschi\textsuperscript{7}:~\textsuperscript{18}R = (2005.20 \selectlanguage{ngerman}± 0.45) x
10\textsuperscript{-6}. Changing this value by one standard deviation up
would lead to a 0.013\selectlanguage{english}%0 less negative delta value for SLAP.

As the total number of experiments is rather small (seven), the standard
error in the mean of the averaged results for\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value for SLAP for seven experiments, is increased
by multiplying with a Student's T-distribution factor.

\textbf{3. RESULTS}

The \textsuperscript{18}O characterization of our 6 different highly
enriched \textsuperscript{18}O waters were carried out by measuring QMS
spectra and fitting those spectra. As described in section 2.3.1, m/z
signals 17-21 from the QMS spectra were used for this spectral fitting
method. In Figure 4 an integrated true measured QMS signal from m/z
17-21 and the fitted signal from the bespoke program are compared, their
very small residuals (in the order of 10 ppm) are shown on top panel of
the figure. These residuals show that the fitted signals are in
excellent agreement with the measured signals. The displayed error bars
are the standard deviations of 14 measurements.

\textless{}Figure 4\textgreater{}

\emph{Figure 4. Relative abundance of m/z 17-21, together with the
results from the fit from one injection with \textsuperscript{18}O
water. On this scale, the small error bars (around
10\textsuperscript{-4}) are not visible. On top, residuals from the true
signal with respect to the fitted signal of QMS (m/z 17-21) are shown.
The error bars are the standard deviations from 14 separate injections.}

As a proof-of-method the enriched
H\textsubscript{2}\textsuperscript{18}O water was diluted
with~(approximately) 1 and 2\% water with \textsuperscript{18}O at
natural abundance. The expected differences in \textsuperscript{18}O
abundances between these dilutions and the not diluted enriched water
based on weights, are shown in Table 1. The measured
(fitted)\textsuperscript{18}O abundances and the expected
\textsuperscript{18}O abundances are within 0.03\% of each other. We
concluded that real (small) differences in abundances are correctly
measured.

\textless{}Table 1\textgreater{}

\emph{Table 1. QMS measurements of \textsuperscript{18}O abundances of
highly enriched H\textsubscript{2}\textsuperscript{18}O and 1 and 2\%
diluted H\textsubscript{2}\textsuperscript{18}O with water with
natural\textsuperscript{18}O abundance.}

\emph{* The expected \textsuperscript{18}O abundance of the diluted
mixtures is based on the exact weights of the highly enriched water and
with water with natural \textsuperscript{18}O 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{]}\textsuperscript{+}, {[}O{]}\textsuperscript{+},
{[}OH{]}\textsuperscript{+},
{[}H\textsubscript{2}O{]}\textsuperscript{+} and
{[}H\textsubscript{3}O{]}\textsuperscript{+} ions. The distribution of
the oxygen-bearing fragmentation ions in natural water and in highly
enriched \textsuperscript{18}O 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{]}\textsuperscript{+}: it is
more preferred in natural \textsuperscript{16}O water than
in\textsuperscript{18}O 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.

\textless{}Table 2\textgreater{}

\emph{Table 2. Comparison of the distribution of 4 main Oxygen-bearing
fragmentation ions in QMS source for natural water
and\textsuperscript{18}O 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
enriched\textsuperscript{18}O. Although hard to determine due to the
uncertainty in the amount of water injected, natural water seems to
ionize [?] 6\% better than the enriched \textsuperscript{18}O water. As a
result of this difference in ionization yield, the \textsuperscript{16}O
fraction of the highly enriched \textsuperscript{18}O 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 \textsuperscript{18}O water
compared to water with natural abundances of only 0.97 \selectlanguage{ngerman}± 0.03. This
value would lead to a maximum deviation in the end result of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O for SLAP of - 0.02 \selectlanguage{ngerman}± 0.02\selectlanguage{english}%0. Therefore, we decided
to neglect the possible slight difference in ionization yield in our
fitting process.

In Table 3 the results of the \textsuperscript{18}O abundances are shown
for the 6 highly enriched \textsuperscript{18}O 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,
its\textsuperscript{18}O abundance had decreased slightly, but
significantly, after puncturing the septum in the closing cap of the
vial. All highly enriched \textsuperscript{18}O waters matched the
specification of the suppliers. Table 3 also
provides\textsuperscript{17}O and \textsuperscript{2}H abundances of
these highly \textsuperscript{18}O enriched water portions, as
determined via dilution.

\textless{}Table 3\textgreater{}

\emph{Table 3. \textsuperscript{18}O, \textsuperscript{2}H
and\textsuperscript{17}O abundances of 6 highly
enriched\textsuperscript{18}O water portions from Rotem (specified as
\textgreater{} 98\%) and Cortec (\textgreater{}
99\%).\textsuperscript{18}O abundances are measured by QMS, between
brackets the standard deviation of the repetitions (n) is
displayed.\textsuperscript{2}H and \textsuperscript{17}O abundances are
determined via dilution, and measured with our LGR-LWIA, on two
different measurement days, and with 9 repetitions per measurement day.}

\emph{* \textsuperscript{2}H and \textsuperscript{17}O abundances of
water portion D' was not remeasured} .

Following the determination of the \textsuperscript{18}O content of the
enriched waters, mixing the enriched water with the SLAP replicate to
produce SLAP-rep-O was performed. The first approach (only mixing with
H\textsubscript{2}\textsuperscript{18}O) was independently performed 4
times, so with 4 different \textsuperscript{18}O water portions (two
from Rotem and two from Cortec, \textsuperscript{18}O water portion A
until D). The second approach (mixing first
with\textsuperscript{2}H\textsubscript{2}\textsuperscript{16}O and
subsequently with H\textsubscript{2}\textsuperscript{18}O) was performed
once with the same \textsuperscript{18}O water as used in experiment 4
(\textsuperscript{18}O water portion D). As this portion was opened 4
months before using it the second time, the \textsuperscript{18}O
concentration was re-measured by QMS (now D'). The second approach was
independently performed with the two remaining \textsuperscript{18}O
waters as well (1 from Rotem and 1 from Cortec, \textsuperscript{18}O
water portions E and F).

An illustration, presented in Figure 5, shows the step-by-step procedure
adapted, as described in the sections before, to establish the best
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value for SLAP using measurements performed on
the LGR-LWIA.

\textless{}Figure 5\textgreater{}

\selectlanguage{greek}\emph{Figure 5. A full calculation scheme showing the steps involved in
one of the seven independent determination of the δ\textsuperscript{18}O
of SLAP.}\selectlanguage{english}

For the second approach (also mixing
with\textsuperscript{2}H\textsubscript{2}O) it was necessary to verify
the\textsuperscript{2}H concentration specification of the supplier. The
determination of the \textsuperscript{2}H abundance
of\textsuperscript{2}H\textsubscript{2}O water by QMS was not as
straightforward as the determination of \textsuperscript{18}O by QMS
appeared to be, which has been explained in section 2.3.2. The result of
the QMS fitted \textsuperscript{2}H measurement was nearly 0.3\% lower
than the specification of the supplier. Therefore, we analyzed this
sample using NMR and the results matched the supplier's specified value.
The specified \textsuperscript{2}H abundance of the almost
pure\textsuperscript{2}H\textsubscript{2}O water is 0.99978, the
measured (via dilution and CO\textsubscript{2} equilibration) and
calculated\textsuperscript{17}O and \textsuperscript{18}O abundances are
0.000808 (3), 0.005928 (6) respectively.

Between brackets the standard deviation of the three repetitions
for\textsuperscript{17}O and \textsuperscript{18}O abundances are shown.

With enriched water portions A until D the first approach is used (only
mixing with enriched \textsuperscript{18}O water). The
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} results with the
combined uncertainties as described before are shown in Figure 6 (black
solid circles).

The \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} results with the
combined uncertainties using the second approach are shown in Figure 6
as well (black open squares). The
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} results are also
presented in the supplementary material, Table 4. Significant difference
between the two approaches was not obvious, and hence all results for
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} were averaged. The
overall weighted mean of all data points is
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} = -56.33 \selectlanguage{ngerman}± 0.02\selectlanguage{english}%0.
Taking the Student's T-factor into account, the final outcome is
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O\textsubscript{SLAP} = -56.33 \selectlanguage{ngerman}± 0.03\selectlanguage{english}%0. This
final uncertainty does not include the two systematic effects mentioned
in section 2.4. These will be later discussed further in this section.

The second approach allowed determination of
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{2}H for SLAP as well. The
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{2}H\textsubscript{SLAP} results are shown in
the supplementary material, Table 4. The overall weighted mean of the
three experiments is \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{2}H\textsubscript{SLAP} =
-430.3 \selectlanguage{ngerman}± 0.3\selectlanguage{english}%0 (again including the Student's T-factor).

\textless{}Figure 6\textgreater{}

\selectlanguage{greek}\emph{Figure 6. Results of 7 experiments performed using 6 different
highly-enriched \textsuperscript{18}O water portions (A -- F in table
3), for determining δ \textsuperscript{18}O SLAP. The black circles
represent the results using the first approach, where VSMOW was mimicked
in δ \textsuperscript{18}O only. The black open squares represent the
results obtained using the second approach where VSMOW was mimicked in
both isotopes. The overall weighted mean of all data points is -56.33 ±
0.03%0 (including the Student T-factor). The error bars reflect the
combined uncertainty calculated using a Monte Carlo approach, taking
into account all individual error sources in the abundances, in the
isotopic measurements and in the weights.}\selectlanguage{english}

\textbf{4. DISCUSSION}

As explained in the introduction, the consensus values for SLAP with
respect to VSMOW were established in 1976. The established
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H value was based on the absolute abundance
measurements, however, the same for \selectlanguage{greek}δ \selectlanguage{english}\textsuperscript{18}O was lacking
and thus the mean \selectlanguage{greek}δ \selectlanguage{english}\textsuperscript{18}O value, -55.5%0, of an
interlaboratory calibration exercise performed at that time was chosen
by consensus. Among the representatives of the several participating
laboratories, there was already a discussion that possible memory
effects would contract the scale, so probably a more negative\selectlanguage{greek}\emph{δ}
\selectlanguage{english}\textsuperscript{18}O value would have been more appropriate.

In later years, thanks to improvements to both equipment and analysis
procedures such as correction for cross-contamination
(Meijer\textsuperscript{19}, laboratories indeed determined more
negative values for SLAP. In our laboratory, we typically find values
around \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O = -55.85%0 by IRMS
(CO\textsubscript{2}-H\textsubscript{2}O equilibration) and, more
recently to our surprise, \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O = -55.7%0 using
the LGR-LWIA. We expected that by having a well-maintained IRMS and
using the appropriate corrections, our results for SLAP would be close
to the real values.

However, Kaiser (2009) already suggested a re-analysis of the data of an
intercomparison exercise of 7 expert laboratories described in
Verkouteren and Klinedinst (2004), resulting in a much more
negative\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value for SLAP, i.e., -56.1 \selectlanguage{ngerman}±
0.2\selectlanguage{english}%0. On the other hand, the \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value of
-55.11%0 for SLAP measured by Barkan and Luz\textsuperscript{8}, is
puzzling. The method Barkan and Luz used was also based on the isotopic
exchange equilibration between H\textsubscript{2}O and
CO\textsubscript{2} in sealed ampoules, but followed by a fluorination
of water using CoF\textsubscript{3} to produce O\textsubscript{2}.
Although this approach is different from the standard equilibration
method, results should be identical as long as the fluorination is
complete. However, their approach consistently points towards less
negative values of -55.11%0 (Hillaire-Marcel\textsuperscript{22}).

For a robust locking of the second anchor of the VSMOW scale we
performed the work described in this paper. The reliability of our
method of quantitative \textsuperscript{18}O abundance determination
of\textsuperscript{18}O water using Quadrupole Mass Spectrometry is
crucial for our results. Taking various effects such as fragmentation
difference of H\textsubscript{2}\textsuperscript{16}O and
H\textsubscript{2}\textsuperscript{18}O into account, and by validating
the method with a dilution series, and considering the excellent
agreement of the fitted QMS signals and the measured ones, we are
confident that the method is reliable.

A systematic deviation of our \textsuperscript{18}O abundance result of
0.1\% higher/lower values would lead to a more/less negative result for
SLAP of 0.05%0. However, such a deviation is highly unlikely: It is good
to realize that, as we use very highly-enriched \textsuperscript{18}O
water (batches of 98\% and 99\% \textsuperscript{18}O), in fact we do
not measure this high \textsuperscript{18}O abundance, but rather
quantify the remaining part of \textsuperscript{16}O exactly by QMS.
Since there is only room for 1 to 2\% \textsuperscript{16}O, it is in
fact this amount that has to be measured with an accuracy of [?] 0.1\%,
which is not a high relative accuracy. Furthermore, if we would still
suffer from some systematic deviation, one can expect this deviation to
be larger for the water portions with 2\% \textsuperscript{16}O
remaining (the Rotem waters) than those with 1\% (Cortec). We see no
such effect in our results (Figure 6 and in the supplementary material
Table 4). The portion of \textsuperscript{17}O only plays a minor role
in the \textsuperscript{18}O/\textsuperscript{16}O ratio, and this
abundance can be determined using a dilution method.

The uncertainty in \textsuperscript{18}R\textsubscript{VSMOW} leads to a
systematic uncertainty in our final answer of \selectlanguage{ngerman}± 0.013\selectlanguage{english}%0, small compared
to our final uncertainty.

So, the result of this study is \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O -56.33 \selectlanguage{ngerman}±
0.03\selectlanguage{english}%0, thus a very negative \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O value for
SLAP, which was an unanticipated finding.

The implication of this is that apparently complete understanding of all
IRMS effects (not to mention those in optical spectroscopy) is still
lacking. Measuring cross-contamination-effects
(Meijer\textsuperscript{19}) obviously is not enough for correcting the
isotope measurement such that the measured delta values are very close
to the real delta values.

One of the issues emerging from this lack of complete understanding of
all IRMS effects, relates specifically to second order measurements such
as \textsuperscript{17}O excess (\selectlanguage{greek}Δ \selectlanguage{english}\textsuperscript{17}O) in water. For
these measurements, in which the small deviation of the measured
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{17}O from the natural relation between
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{17}O is determined (Meijer
and Li\textsuperscript{20}, Aron\textsuperscript{21}), the question
raises, how well these very small deviations (around 0.02%0 or less) can
be defined, if there are such large discrepancies between measured
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and real \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values. The
assumption that \textsuperscript{17}O and \textsuperscript{18}O will
fully obey mass dependent fractionation in the ion source of the IRMS
may not be completely true. To put it another way: if the measured scale
for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O is already so much contracted, who can
guarantee that the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{17}O scale contracts exactly
according to the equilibrium relation between \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{17}O and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O.

Also clumped isotope measurements, which determine the minute deviations
from stochastic distribution of the delta values for multiply
substituted isotopologues, can probably not rely on the fully
mass-dependent scale contraction of their machines. Also, here, full
understanding of IRMS effects is key.

The oxygen isotope compositions are typically reported on the VSMOW
scale, not only for water samples, but also for other types of samples,
such as oxides and silicates. The VPDB scale is mostly used for
reporting the stable isotope (carbon and oxygen) results of carbonate
minerals and also for oxygen isotope measurements in atmospheric
CO\textsubscript{2}. These two coexisting stable isotope scales for
reporting \textsuperscript{18}O/\textsuperscript{16}O ratios or
\selectlanguage{greek}\emph{δ}\selectlanguage{english}\textsuperscript{18}O values, can be converted into each other
(Hillaire-Marcel\textsuperscript{22}). For both scales an extra
conversion step to CO\textsubscript{2} is necessary, because the
measurand in the IRMS is CO\textsubscript{2}. This extra reaction step
is for the VSMOW-CO\textsubscript{2} scale, water equilibration of VSMOW
with CO\textsubscript{2} under standard conditions (first described by
Epstein and Mayeda\textsuperscript{23}, Meijer\textsuperscript{17} and
for the VPDB-CO\textsubscript{2} scale, acidification of IAEA-603
(formerly NBS-19) with phosphoric acid (McCrea\textsuperscript{24},
Meijer\textsuperscript{17}, Hillaire-Marcel\textsuperscript{22}. The
difference between VSMOW-CO\textsubscript{2} and
VPDB-CO\textsubscript{2} on the two \selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O
scales is 0.28-0.29%0 (Hillaire-Marcel\textsuperscript{22}). In our
laboratory, we have the habit to realize the two scales (water and
carbonate) independently and use this scale difference as a quality
check. When using two-point calibration scales, the result of a more
negative \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value for SLAP, (the second anchor of
the VSMOW scale), could give potential discrepancies in the transfer
of\selectlanguage{greek}\emph{δ} \selectlanguage{english}\textsuperscript{18}O from and to the VPDB scale. Considering
the fact that the water equilibration reaction is more robust and easier
to control (and therefore more reliable and accurate) than the
carbonate-acid reaction, we propose the
VSMOW-CO\textsubscript{2}\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O scale be defined as the
primary \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O scale. The definition of the
VPDB-CO\textsubscript{2} scale could then simply be expressed in terms
of the VSMOW-CO\textsubscript{2} scale. Final decisions about these
isotopic scales are under the auspices of the commission on isotopic
abundances and atomic weights (CIAAW).

Identical treatment of sample and references, the frequent use of
international reference materials and clear guidelines about how to
express the results on the international scale(s) is key to provide
normalized interlaboratory-comparable stable isotope measurements. This
study does not affect those measurements; the VMSOW-SLAP scale can be
taken as is. However, knowing the absolute ratios and/or abundances of
all scale determining references would give us clear insight how large
the scale contraction processes really are. In fields where
VSMOW-SLAP-scaled \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values are converted into
absolute abundances and vice versa, our new \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value
for SLAP does matter. An example of such a field is energy expenditure
measurements using doubly-labelled water, in which the used enriched
reference waters will change their delta value
(Faghihi\textsuperscript{15}).

\textbf{5. CONCLUSIONS AND RECOMMENDATIONS}

With this work, the VSMOW-SLAP scale has in fact become metrologically
traceable to the System International (SI) units for both isotopes: the
combination of the absolute isotope \textsuperscript{18}O abundance for
VSMOW (Baertschi\textsuperscript{7} and our present result for SLAP with
respect to VSMOW leads to the absolute \textsuperscript{18}O abundance
for SLAP of 1.88798 (43) x 10\textsuperscript{-3}.
For\textsuperscript{2}H, this traceability has long been accomplished.
In this work, however, we also produce a new and probably more accurate
value for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H of SLAP with respect to VSMOW of -430.3
\selectlanguage{ngerman}± 0.3\selectlanguage{english}%0. This value is significantly lower than the values by
Hagemann\textsuperscript{4}, de Wit\textsuperscript{5} and
Tse\textsuperscript{6}. However, like in the \textsuperscript{18}O case,
the value for \textsuperscript{2}R for VSMOW influences the value we
obtain for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H of SLAP. In case we would use
the\textsuperscript{2}R\textsubscript{VSMOW} value reported by de Wit
(155.95 x 10\textsuperscript{-6}) in combination with
the\textsuperscript{2}R\textsubscript{SLAP} value of Hagemann (89.02 x
10\textsuperscript{-6}), the difference would translate into -429.2%0,
whereas our value for SLAP would change into -429.8%0. This ''friction of
values'' calls for a new gravimetric mixing experiment, now making use
of the better and easier optical measurements of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H
of water, combined with NMR determination of the purity of
the\textsuperscript{2}H and \textsuperscript{1}H waters. We plan to
perform such an experiment in the near future. When that is successful,
both the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{2}H and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O isotope scales
would become SI-traceable. That would be a first.

Best estimates for the absolute \textsuperscript{13}C abundance so far,
for the VPDB-scale have been determined by
Malinovsky\textsuperscript{25} (further work is in progress).
The\textsuperscript{18}O-side of this carbonate scale is much more
complicated, due to the fractionating process that is at the basis.
Furthermore, there still is no consensus on scale normalization, let
alone on the absolute \textsuperscript{13}C and \textsuperscript{18}O
abundances of such materials. For \textsuperscript{18}O, coupling
the\textsuperscript{18}O VPDB scale to VSMOW-SLAP using the
CO\textsubscript{2} - H\textsubscript{2}O equilibration process is
probably a more fruitful route towards pinpointing
this\textsuperscript{18}O VPDB-scale to SI units, certainly for
non-carbonate materials such as atmospheric CO\textsubscript{2}.

Acknowledgements

We acknowledge the valuable personal communication with Tyler Coplen
(USGS) and Manfred Groening (IAEA) about the history of the assigned
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O value for SLAP. We are grateful to the IAEA
Terrestrial Environment Laboratory in Seibersdorf for providing ampoules
of 1 mL of the original VSMOW and SLAP. We also want to thank Len
Wassenaar, from the IAEA Isotope Hydrology laboratory in Vienna for
providing the Antarctic water that formed the basis of this study. We
would like to express special thanks to Johan Kemmink (RuG) who
performed the NMR analysis of the \textsuperscript{2}H\textsubscript{2}O
water.

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  measurements.~\emph{J. Anal. At. Spectrom.} ~2019;34:147--159. DOI:
  10.1039/c8ja00281a
\end{enumerate}

Table 1\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& \textsuperscript{18}O Abundance measured & Difference in
\textsuperscript{18}O measured & Expected \textsuperscript{18}O
abundance\textsuperscript{*} & Expected difference in
\textsuperscript{18}O\textsuperscript{*}\tabularnewline
\midrule
\endhead
\textsuperscript{18}O water & 0.9800 & & &\tabularnewline
1\% diluted & 0.9675 & 0.0125 & 0.9678 & 0.0122\tabularnewline
2\% diluted & 0.9580 & 0.0220 & 0.9577 & 0.0223\tabularnewline
\bottomrule
\end{longtable}

Table 2\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Fragmentation ion fraction & n &
f{[}H\textsubscript{2}O{]}\textsuperscript{+} &
f{[}OH{]}\textsuperscript{+} & f{[}O{]}\textsuperscript{+} &
f{[}H\textsubscript{3}O{]}\textsuperscript{+}\tabularnewline
\midrule
\endhead
Natural water Average (stdev) & 20 & 0.76050 (24) & 0.19340 (19) &
0.02572 (8) & 0.02038 (31)\tabularnewline
\textsuperscript{18}O enriched Average (stdev) & 14 & 0.76674 (36) &
0.19805 (33) & 0.01505 (11) & 0.02016 (50)\tabularnewline
\bottomrule
\end{longtable}

Table 3\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Water portion & Brand \textsuperscript{18}O water &
\textsuperscript{18}O abundance (Stdev) & n & \textsuperscript{2}H
abundance (Stdev) & \textsuperscript{17}O abundance (Stdev) &
Remarks\tabularnewline
\midrule
\endhead
A & Rotem & 0.9799 (7) & 18 & 0.000017 (9) & 0.0047 (2) &\tabularnewline
B & Cortec & 0.9917 (1) & 8 & 0.000027 (6) & 0.0012 (2) &\tabularnewline
C & Rotem & 0.9832 (1) & 4 & 0.000026 (3) & 0.0095 (3) &\tabularnewline
D & Cortec & 0.9939 (3) & 7 & 0.000062 (3) & 0.0011 (3) &\tabularnewline
D' & Cortec & 0.9907 (4) & 5 & * & * & Water portion D, 4 months after
opening\tabularnewline
E & Rotem & 0.9818 (2) & 8 & 0.000032 (5) & 0.0074 (\textless{}1)
&\tabularnewline
F & Cortec & 0.9917 (2) & 8 & 0.000051 (2) & 0.0013 (\textless{}1)
&\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
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