# High-Precision Tungsten Isotopic Analysis by Multicollection Negative Thermal Ionization Mass Spectrometry Based on Simultaneous Measurement of W and O-18/O-16 Isotope Ratios for Accurate Fractionation Correction

Anne Trinquier, Mathieu Touboul, Richard J. Walker

► To cite this version:

Anne Trinquier, Mathieu Touboul, Richard J. Walker. High-Precision Tungsten Isotopic Analysis by Multicollection Negative Thermal Ionization Mass Spectrometry Based on Simultaneous Measurement of W and O-18/O-16 Isotope Ratios for Accurate Fractionation Correction. Analytical Chemistry, 2016, 88 (3), pp.1542-1546. 10.1021/acs.analchem.5b04006. hal-02331369

HAL Id: hal-02331369

https://univ-lyon1.hal.science/hal-02331369v1

Submitted on 13 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# High-Precision Tungsten Isotopic Analysis by Multicollection Negative Thermal Ionization Mass Spectrometry based on Simultaneous Measurement of W and ¹⁸O/¹⁶O Isotope Ratios for Accurate Fractionation Correction.

Anne Trinquier¹,*, Mathieu Touboul²,† and Richard J. Walker²

¹Thermo Fisher Scientific Bremen, Germany, ²Dept. of Geology, University of Maryland, College Park, MD, USA

**ABSTRACT:** Determination of the ¹⁸²W/¹⁸⁴W ratio to a precision of ± 5 ppm (2σ) is desirable for constraining the timing of core-formation and other early planetary differentiation processes. However, WO₃⁻ analysis by negative thermal ionization mass spectrometry (N-TIMS) normally results in a residual correlation between instrumental mass fractionation corrected ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W ratios that is attributed to mass-dependent variability of O isotopes over the course of an analysis and between different analyses. A second order correction using the ratio ¹⁸³W/¹⁸⁴W relies on the assumption that this ratio is constant in nature. This may prove invalid, as has already been realized for other isotope systems. The present study utilizes simultaneous monitoring of ¹⁸O/¹⁶O ratios and W isotope ratios, in order to correct oxide interferences on a per integration basis, and thus avoid the need for a double normalization of W isotopes. After normalizing W isotope ratios to a pair of W isotopes, following the exponential law, no residual W-O isotope correlation is observed. However, there is a non-ideal mass bias residual correlation between ¹⁸²W/¹⁹W and ¹⁸³W/¹⁹W with time. Without double normalization of W isotopes, and based on 3-4 duplicate analyses, the external reproducibility per session of ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W normalized to ¹⁸⁶W/¹⁸³W is 5-6 ppm (2σ, 1-3 µg loads). The combined uncertainty per session is less than 4 ppm on ¹⁸³W/¹⁸⁴W and less than 6 ppm on ¹⁸²W/¹⁸⁴W (2σ<sub>m</sub>) for loads of between 3000 and 50 ng.

The lithophile-siderophile ¹⁸²Hf-¹⁸²W decay pair is well-suited for constraining the timing of planetary core-formation events by coupling the isotopic ingrowth of ¹⁸²W from the decay of ¹⁸²Hf (T₁/₂ = 8.9 Myr) and elemental Hf/W ratios¹⁻¹³. In addition, it has been proposed that the presence of late-accreted extraterrestrial and/or core-derived materials in mantle materials can be detected through high-precision analysis of W isotopes⁶. Towards this end, high-precision W isotope analysis of ¹⁸²W/¹⁸⁴W, with long-term reproducibility of < 5 ppm (relative standard deviation, 2σ), has been achieved by N-TIMS analysis¹⁻³,⁶,¹³. However, a residual correlation has been observed between mass fractionation corrected ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W ratios that has been attributed to mass dependent variability of O isotopes between runs⁶. In order to obtain 5 ppm reproducibility (2σ) on ¹⁸²W/¹⁸⁴W, a double normalization scheme to ¹⁸³W/¹⁴W has previously been applied. However, the residual correlation precludes high precision measurement of ¹⁸³W/¹⁸⁴W and, as such, the method does not permit verification of whether or not this ratio is invariant in nature. This is potentially problematic when analyzing cosmochemical materials for which ¹⁸³W/¹⁸⁴W has been documented to vary⁸. Natural variability in this ratio would introduce an analytical bias, as previously observed when using double normalization protocols in the analysis of other isotopic systems¹⁴.

The present study on a Thermo Scientific™ TRITON Plus™ is aimed at monitoring the isotopic variability of ¹⁸O/¹⁶O during W isotope analysis to apply oxide interference correction on a per integration basis, in order to determine ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W isotope ratios to high-precision

without double normalization of W isotopes. This is combined with propagation of the uncertainty on the ¹⁸³W/¹⁸⁴W ratio into the ¹⁸²W/¹⁸⁴W uncertainty when normalizing to ¹⁸³W for instrumental mass bias correction.

## EXPERIMENTAL SECTION

Analyses were conducted over a period of 9 months. SPEX CertiPrep™ tungsten standard loads of 3 to 0.05 µg were loaded onto double filament assemblies made from previously outgassed zone refined Re filaments¹. Lanthanum and Gado-linium were subsequently added as electron emitters⁶. The analytical protocol and results are summarized in Table S-1. Tungsten loads were analyzed for 60 to 240 min, depending on the amounts of W loaded. The analyses typically consisted of alternations of twenty 33 s integrations and one 120 s baseline. Oxygen was bled in the source (P<sub>O₂</sub> ~ 1-2 x 10<sup>-7</sup> mbar) to enhance oxide production. Modifying the weight proportion of La-Gd additives with respect to the W amount in the smaller loads (50-200ng) did not noticeably influence the length of analyses nor did it affect the Re/W ratio, nor the ¹⁸O/¹⁶O composition.

¹⁸²,¹⁸³,¹⁸⁴,¹⁸⁶W¹⁶O³, ¹⁸⁶W¹⁸O¹⁶O₂ and ¹⁸⁵,¹⁸⁷Re¹⁶O₃ species were measured in static multiple collection mode. ¹⁸O/¹⁶O ratios were inferred from the ¹⁸⁶W¹⁸O¹⁶O₂/¹⁸⁶W¹⁶O₃ ratios measured on a per integration basis and were averaged by intervals of 5 min (typically 10 integrations). A 10¹³ ohm resistor amplifier with 10 times higher signal-to-noise (Johnson) ratio compared to 10¹¹ ohm resistor amplifiers was used for amplifying
the low ion beam intensities on ¹⁸⁶W¹⁸O¹⁶O₂, typically < 100 fA. The gain calibration of the 10¹³ ohm resistor current amplifier was performed using the JNdi-1 neodymium reference standard in positive mode¹⁵. Typical external reproducibility of the gains on 10¹³ ohm resistor current amplifiers over a year was ≤ 100 ppm (2σ, 95% confidence level), and the amplifier gains were measured to < 50 ppm precision (relative standard error, 2σm). The relative uncertainty on ¹⁸⁰O/¹⁶⁰O using integrations of 5 min intervals was typically 200 ppm, which is significantly larger than the gain calibration uncertainty of 50 ppm, and reflects ¹⁸⁰O/¹⁶⁰O variability over a timescale of a few min. Consequently, the uncertainty associated with the gain calibration was found to be negligible and has not been propagated in the uncertainty assessment.

Rhenium ion beams were monitored with high ohmic current amplifiers (10¹² or 10¹³ ohm) for ion beams less than 500 fA, and 10¹¹ ohm current amplifiers, otherwise.

The cups collecting W oxide species were sequentially connected to the same set of 10¹¹ ohm amplifiers, in order to average out the stochastic calibration biases of the amplifiers and limit the propagation of the uncertainty of the gain calibration procedure to less than 1 ppm.

## DATA REDUCTION, RESULTS AND DISCUSSION

W and Re species, monitored simultaneously with the ¹⁸O/¹⁶O ratio were corrected for oxide interference on a per integration basis. Then, 3 exponential normalization schemes have been evaluated, including normalization to ¹⁸⁶W/¹⁸⁴W (N64) or ¹⁸⁶W/¹⁸³W (N63) with ¹⁸⁴W or ¹⁸³W as denominators. Results are summarized in Table S-1. It is shown that, in normal running conditions, there is no residual W-O isotope correlation after oxide interference and instrumental mass bias correction. However, non-ideal mass fractionation is evidenced by residual linear correlations between corrected W isotope ratios over time (Figs 1-5). Two strategies are evaluated to improve reproducibility: (1) a double normalization scheme of W isotopes (2) averaging 3-4 duplicates. Finally, recognizing the body of evidence supporting that the ¹⁸³W relative abundance is not invariant in nature and propagating the ¹⁸³W abundance uncertainty into the uncertainty on the ¹⁸²W relative abundance, it is found that averaging 3-4 duplicates is the most robust approach to optimize reproducibility and accuracy on both ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W ratios.

**Oxide interference correction.** The oxygen isotopic composition was determined by an iterative process. In a first step, the O composition from Nier¹⁶ was used as reference and ¹⁸O/¹⁶O ratios of all integrations were inferred from the ¹⁸⁶W¹⁸O¹⁶O₂/¹⁸⁶W¹⁶O₃ ratios. Corresponding ¹⁷O/¹⁶O ratios were inferred using ¹⁸O/¹⁶O ratios and the terrestrial fractionation line. In a second step, the obtained ¹⁸O/¹⁶O and ¹⁷O/¹⁶O mean compositions of each analysis were used as new references, until the numerical difference between reference and derived values was within the analytical uncertainty. After correction of the oxide interferences on a per integration basis (typically 33 s each), the W species were normalized to ¹⁸⁶W/¹⁸⁴W = 0.92767 or ¹⁸⁶W/¹⁸³W = 1.98594 (ref. 17).

**Residual correlation with O isotopes?** When considering mass bias corrected W isotope ratios determined in small W loads (50-200 ng) with Re/W > 0.3, higher than average W isotope ratios are associated with higher than average O isotope ratios. This is accompanied by a correlation between W isotope ratios and the Re/W variation (Fig. S-1) and with a

covariation between O isotope ratios and Re/W ratios. This could result from differences in mass fractionation behavior for W and Re oxide species, perhaps resulting from differences in the Re and W to O complexing behavior at elevated Re/W ratios, leading to inaccurate estimates of both W and O isotope abundances, which are all inferred from W oxide species after Re oxide interference correction. On one 50 ng load, the mean Re/W ratio is 4, implying that the contribution of the interfering ¹⁸⁷Re¹⁷O¹⁶O₂ ion beam is twice as large as the interfered ¹⁸⁶W¹⁸O¹⁶O₂ species. Thus, for W loads < 200 ng, alternative loading strategies should be sought in order to minimize the uncertainty associated with large Re oxide correction, including using non Re filaments². As a result, only the W ratios for analyses characterized by Re/W < 0.3 are considered in the following sections. The other values, in *italic* form in Table S-1, are not included in the statistics calculation.

**Internal precision (Table S-1).** For 1 to 3 µg loads, the mean internal precision (2σm, n=45) on the oxide interference and mass fractionation corrected ratios is 6 ppm for ¹⁸²W/¹⁸⁴W~N64~, 5 ppm for ¹⁸²W/¹⁸³W~N63~, 4 ppm for ¹⁸²W/¹⁸⁴W~N63~, 5 ppm for ¹⁸³W/¹⁸⁴W~N64~ and 3 ppm for ¹⁸³W/¹⁸⁴W~N63~.

For 200 ng loads, the mean internal precision (2σm, n=3) on the oxide interference and mass fractionation corrected ratios is 10 ppm for ¹⁸²W/¹⁸⁴W~N64~ and ¹⁸²W/¹⁸³W~N63~, 8 ppm for ¹⁸²W/¹⁸⁴W~N63~, 9 ppm for ¹⁸³W/¹⁸⁴W~N64~ and 6 ppm for ¹⁸³W/¹⁸⁴W~N63~.

For 50 ng loads, the mean internal precision (2σm, n=2) on the oxide interference and mass fractionation corrected ratios is 9 ppm for ¹⁸²W/¹⁸⁴W~N64~, 10 ppm for ¹⁸²W/¹⁸³W~N63~, 7 ppm for ¹⁸²W/¹⁸⁴W~N63~, 9 ppm for ¹⁸³W/¹⁸⁴W~N64~ and 6 ppm for ¹⁸³W/¹⁸⁴W~N63~.

**Residual correlation between W isotopes corrected for instrumental mass bias.** The external reproducibility on ¹⁸²W and ¹⁸³W relative abundances over 9 months on 1-3 µg loads ranges from 10-11 ppm (normalization to ¹⁸⁶W/¹⁸³W) to 17-18 ppm (normalization to ¹⁸⁶W/¹⁸⁴W) (2σ, n=45, no outlier), which is significantly less than the 30-40 and 60-70 ppm (n=39) previously observed⁶.

The external reproducibility on W ratios measured in 200 ng loads is 23 ppm on ¹⁸³W/¹⁸⁴W~N64~, 26 ppm on ¹⁸²W/¹⁸⁴W~N64~, 15 ppm on ¹⁸³W/¹⁸⁴W~N63~, 18 ppm on ¹⁸²W/¹⁸³W~N63~ and 10 ppm on ¹⁸²W/¹⁸⁴W~N63~ (2σ, n=3, Table S-1).

The external reproducibility on W ratios measured in 50 ng loads is 10 ppm on ¹⁸³W/¹⁸⁴W~N64~, 13 ppm on ¹⁸²W/¹⁸⁴W~N64~, 6 ppm on ¹⁸³W/¹⁸⁴W~N63~, 4 ppm on ¹⁸²W/¹⁸³W~N63~ and 7 ppm on ¹⁸²W/¹⁸⁴W~N63~ (2σ, n=2, Table S-1).

Residual correlations are resolvable between ¹⁸²W/¹⁸³W~N64/i~ and ¹⁸⁴W/¹⁸³W~N64/i~ ratios for 1 µg analyses that span over 9 months (Figs. 1-3). The most marked residual correlation is observed with the normalization to ¹⁸⁶W/¹⁸⁴W, which is the normalization ratio most far removed from the masses of interest.

Notably, the residual correlations between ¹⁸²W/¹⁸³W~N64/i~ and ¹⁸⁴W/¹⁸³W~N64/i~ ratios are not accompanied by W-O isotope correlations, nor correlations between W isotope and Re/W ratios, nor correlations between W isotope ratios and instrumental mass bias. Instead, there seems to be a monotonous drift over time of the W corrected isotope ratios (Fig.4) that could point towards non-ideal mass fractionation behavior. It cannot be
excluded that the electron deflection magnet in the source
induces small changes in the ion optics, as such non-ideal
mass bias effects, at the ppm level of reproducibility, have
previously been reported in positive mode analysis¹⁸. Howev-
er, based on the present set of data only, it is not clear why
there would be a monotonous drift with time. On the other
hand, it cannot be ruled out that slight cup aging induces non
linearity of W and O isotope acquisition and subsequent mass-
dependent fractionation of W isotopes as previously observed
on other isotope systems¹⁴. Besides, it is assumed but not veri-
fied that oxidation of W by the La -Gd additives at the surface
of the filament and oxidation by addition of oxygen through
the bleed valve follow the same mass fractionation laws. Rec-
ognizing that the variability of O isotope composition with
time plays a pivotal role in the instrumental mass bias correc-
tion, the assumption of a linear mass fractionation between O
isotopes needs further validation through direct analysis of
¹⁷O/¹⁶O ratios.

The regression slopes and associated uncertainties have
been determined for 1-3 µg loads using ISOPLOT 4 (devel-
oped by Dr Ken Ludwig of the Berkeley Geochronology Cen-
ter). One 1σ outlier in all three data reduction schemes has
been rejected from the data reduction (italic in Table S-1).

<figure><img src="image_2.png" /><figcaption>Figure 1. ¹⁸³W/¹⁸⁴W N6/4 -¹⁸²W/¹⁸⁴W N6/4 diagram after normalization to ¹⁸⁶W/¹⁸⁴W. 1-3 µg analyses are reported. Slope = 1.96 ± 0.27. MSWD=1.2</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2. ¹⁸⁴W/¹⁸³W N6/3 -¹⁸²W/¹⁸³W N6/3 diagram after normalization to ¹⁸⁶W/¹⁸³W. 1-3 µg analyses are reported. Slope = 0.55 ± 0.20. MSWD=2.0</figcaption></figure>

<figure><img src="image_4.png" /></figure>

Figure 3. ¹⁸²W/¹⁸⁴W N6/3 -¹⁸³W/¹⁸⁴W N6/3 diagram after normalization to ¹⁸⁶W/¹⁸³W. 1-3 µg analyses are reported. Slope = 1.09 ± 0.45. MSWD=2.9

<figure><img src="image_5.png" /><figcaption>Figure 4. ¹⁸²W/¹⁸⁴W N6/4 versus time for 1 µg analyses.</figcaption></figure>

When reduced on a per session basis (May 2014/November 2014/January 2015), the external reproducibility (2σ) on W ratios measured on 1-3 µg loads ranges from 4 to 15 ppm (normalization to ¹⁸⁶W/¹⁸⁴W) and 3 to 9 ppm (normalization to ¹⁸⁶W/¹⁸³W).

Implications of normalizing to ¹⁸³W/¹⁸⁴W or ¹⁸⁶W/¹⁸³W. A few ppm to ε level (part per 10,000) isotopic heterogeneity on the ¹⁸³W/¹⁸⁴W ratio could result from nucleosynthetic variability in cosmochemical materials⁸. The contribution of an isotopic variability on the ¹⁸³W/¹⁸⁴W ratio into the ¹⁸²W/¹⁸⁴W through normalization schemes has to be evaluated.

In the following, the effects of a potential isotopic variability
of the ¹⁸³W relative abundance is evaluated on 1-3 µg load
analyses, with the subsequent uncertainty propagation to
¹⁸²W/¹⁸³W and ¹⁸²W/¹⁸⁴W, when using normalization schemes
to ¹⁸⁴W/¹⁸³W and ¹⁸⁶W/¹⁸³W.

The reproducibility on ¹⁸³W/¹⁸⁴W<sub>N6/4</sub> by session ranges from 4 to 14 ppm (2σ, Table S-1). When applying a linear normalization to ¹⁸³W/¹⁸⁴W (similarly to ref.6), using the slope defined in the ¹⁸³W/¹⁸⁴W<sub>N6/4</sub> - ¹⁸²W/¹⁸⁴W<sub>N6/4</sub> diagram (Fig.2), one would obtain a reproducibility of 7 to 10 ppm per session on ¹⁸²W/¹⁸⁴W<sub>N6/4</sub> (2σ, Table S-1). The uncertainty on the ¹⁸³W/¹⁸⁴W<sub>N6/4</sub> ratio per session of 5-6 ppm (defined as the larger between the mean 2σ<sub>m</sub> of the runs and the 2σ of the replicate values divided by square root of the number of replicates), has been propagated into the 2σ<sub>m</sub> uncertainty per session of the ¹⁸²W/¹⁸⁴W<sub>N6/4</sub> ratios (2σ<sub>m</sub> = 9 to 11 ppm).

The reproducibility on ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> by session ranges from 3 to 9 ppm (2σ, Table S-1). The uncertainty on ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> (≤ 4 ppm), defined as the larger between the mean 2σ<sub>m</sub> of all analyses and the 2σ of all replicates divided by square root of the number of replicates, has to be propagated. An anomaly of 4 ppm on ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> (i.e an anomaly of 6 ppm on the abundance of ¹⁸³W), by virtue of normalizing to ¹⁸⁶W/¹⁸³W using the exponential law, propagates into an anomaly of -8 ppm on ¹⁸²W/¹⁸³W<sub>N6/3</sub>, and -4 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/3</sub>. The combined uncertainty on ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> and ¹⁸²W/¹⁸³W<sub>N6/3</sub> thus becomes ≤ 6 and ≤ 10 ppm (2σ<sub>m</sub>) respectively. If, in addition, one linear correction to ¹⁸³W/¹⁸⁴W (or ¹⁸⁴W/¹⁸³W) is applied, using the regression lines defined in Figs. 1-3, the combined uncertainty reduces to ≤ 8 ppm on ¹⁸²W/¹⁸³W<sub>N6/3</sub> or ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> after propagation of the ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> uncertainty through these double normalization schemes.

**Averaging into means of 3-4 duplicate analyses.** Here we
evaluate an approach based on averaging three to four dupli-
cate analyses to improve reproducibility on both ¹⁸³W and ¹⁸²W relative abundances.

For 1-3 µg loads, this yields a 2σ external reproducibility per session of 6-8 ppm on ¹⁸³W/¹⁸⁴W<sub>N6/4</sub> and 9-11 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/4</sub>, ≤ 5 ppm on ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> and ≤ 6 ppm on ¹⁸²W/¹⁸³W<sub>N6/3</sub> and ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> (the session of November, with solely 2 runs, is not included).

In the following, the uncertainty represents the larger be-
tween the mean 2σ<sub>m</sub> of all analyses and the 2σ of all replicates
divided by square root of the number of replicates. For
¹⁸²W/¹⁹W<sub>N6/3</sub> (where i=183 or 184), the 2σ<sub>m</sub> uncertainty also
includes the propagation of the 2σ<sub>m</sub> uncertainty on
¹⁸⁴W/¹⁸³W<sub>N6/3</sub>.

For 1-3 µg loads, the mean of 3-4 duplicate analyses yields
a 2σ<sub>m</sub> uncertainty per session of 3-6 ppm on ¹⁸³W/¹⁸⁴W<sub>N6/4</sub> and
4-8 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/4</sub> (Table S-1). Alternatively, the mean
of 3-4 duplicate analyses yields a 2σ<sub>m</sub> uncertainty per session
of 2-4 ppm on ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> and 8-9 ppm on ¹⁸²W/¹⁸³W<sub>N6/3</sub>,
and 5-6 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> (Table S-1).

For 200 ng loads, the 2σ<sub>m</sub> uncertainty is 13-15 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/4</sub> and ¹⁸³W/¹⁸⁴W<sub>N6/4</sub>, 9 ppm on ¹⁸⁴W/¹⁸³W<sub>N6/3</sub>, 10 ppm on ¹⁸²W/¹⁸³W<sub>N6/3</sub> and 6 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> (Table S-1).

For 50 ng loads, the 2σ<sub>m</sub> uncertainty is 9 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/4</sub> and 7 ppm on ¹⁸³W/¹⁸⁴W<sub>N6/4</sub>, 5 ppm on ¹⁸⁴W/¹⁸³W<sub>N6/3</sub>, 3 ppm on ¹⁸²W/¹⁸³W<sub>N6/3</sub> and 5 ppm on ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> (Table S-1).

Due to the limited amount of usable runs (n=3 for 200 ng and n=2 for 50 ng loads), however, more robust statistical information would require additional analyses.

The mean ¹²⁸W/¹³⁸W<sub>N6/1</sub> and ¹⁸⁴W/¹³⁸W<sub>N6/1</sub> of all analyses are
indistinguishable within respective uncertainties, irrespective
of the W amounts loaded.

<figure><img src="image_6.png" /><figcaption>Figure 5. ¹⁸³W/¹⁸⁴W<sub>N6/3</sub> - ¹⁸²W/¹⁸⁴W<sub>N6/3</sub> diagram after oxide correction and normalization to ¹⁸⁶W/¹⁸³W. Duplicates of 3-4 analyses of 1-3 µg analyses are reported. 2σ<sub>m</sub> per session < 5-6 ppm.</figcaption></figure>

The 2σ reproducibility of 5-6 ppm on ¹²⁸W/¹⁸⁴W<sub>N6/3</sub> is simi-
lar to that obtained with double normalization to W isotopes⁶.
Although derived by averaging 3-4 duplicate analyses, and
thus requiring more data, the present analytical protocol offers
the advantage of also measuring ¹⁸⁴W/¹⁸³W to high precision
(2σ ≤ 5 ppm). This represents a 5 to 8-fold improvement in
terms of reproducibility with respect to a protocol that does
not doubly normalize. In addition, recognizing that ¹⁸³W rela-
tive abundance varies in nature, there is a requirement for
propagating the uncertainty on the ¹⁸³W relative abundance in
any normalizing scheme involving ¹⁸³W (exponential and line-
ar), especially in previous studies that use double normaliza-
tion schemes⁶. Thus, the approach consisting in averaging 3-4
duplicate analyses has been found to be the most reproducible

for determination of ¹²⁸W relative abundance, while allowing
high precision determination of ¹⁸³W/¹⁸⁴W ratios.

CONCLUSION

By correcting oxide interference on a per integration basis, and
provided Re/W < 0.3, the presented technique offers a W iso-
tope analytical approach, in which there is no residual correla-
tion between O isotopes and instrumental mass bias corrected
¹²⁸W/W and ¹⁸³W/W ratios. There are, however, correlations
between instrumental mass bias corrected ¹²⁸W/W and
¹⁸³W/W ratios over 9 months, which can be mitigated by aver-
aging data over shorter sessions. The external reproducibility
on means of 3-4 duplicates of ¹²⁸,¹²⁸W/¹⁸⁴W<sub>N6/3</sub> is 5-6 ppm per
session of analysis (2σ). The 2σ<sub>m</sub> combined uncertainty on
¹²⁸W/¹⁸⁴W<sub>N6/3</sub> (including propagation of the 2σ<sub>m</sub> uncertainty on
¹⁸³W/¹⁸⁴W) is 5-6 ppm. This provides an analytical approach
well-suited for meeting the requirements for high-precision W
studies, such as investigations into the chronology of the late
impact history on Earth. To characterize O isotope fractiona-
tion and oxide interferences correction further, the presented
technique allows for including the in-run measurement of
¹⁷O/¹⁶O ratios, thus alleviating the need for providing better
correction schemes for O-isotope fractionation correction.

ASSOCIATED CONTENT

Supporting Information

Table S-1 : W isotope analytical protocol and normalized data.

Fig. S-1 : ¹²⁸W/¹⁸⁴W<sub>N6/4</sub> – Re/W diagram.

AUTHOR INFORMATION

Corresponding Author

* Anne Trinquier. Thermo Fisher Scientific Bremen, Germany.
anne.trinquier@thermofisher.com. FAX +49 421 5493 396.

Present Address

†Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de
Lyon, France.

Author Contributions

All authors have given approval to the final version of the manu-
script.

Notes

A.T. is employed at Thermo Fisher Scientific. M.T and R.J.W declare no competing financial interest.

ACKNOWLEDGMENTS

The authors would like to acknowledge Peter Huelstede, Peter Komander and Hartmut Bars for technical assistance and the reviewers for constructive reviews which improved the quality of the paper. M.T. and R.J.W. acknowledge NSF EAR-CSEDI grant # 1265169.

REFERENCES

(1) Harper, C.L. JR.; Jacobsen, S.B. *Geochim. Cosmochim. Acta.* **1996**, *60* (7), 1131-1153.

(2) Quitté, G.; Birck, J.L.; Capmas, F.; Allègre, C.J. *Geostand-ard. Newslett.* **2002**, *26* (2), 149-160.

(3) Touboul, M.; Kleine, T.; Bourdon, B.; Palme, H.; Wieler, R. *Nature* **2007**, *450*, 1206-1209.

(4) Qin L.; Dauphas N.; Wadhwa M.; Masarik J.; Janney P.E. *Earth. Planet. Sci. Lett.* **2008**, *273*, 94-104.

(5) Iizuka, T.; Nakai, S; Sahoo, Y. V.; Takamasa, A.; Hirata, T.; Maruyama, S. *Earth. Planet. Sci. Lett.* **2010**, *291*, 189-200.
(6) Touboul, M.; Walker, R. J. *Int. J. Mass Spectrom.* **2011**, *309*, 109-117.

(7) Willbold, M.; Elliott, T.; Moorbath, S. *Nature* **2011**, *477*, 195-198.

(8) Burkhardt, C.; Kleine, T.; Dauphas, N.; Wieler, R. *ApJ* **2012**, *753*, L6-L11.

(9) Schulz T.; Upadhyay D.; Münker C.; Mezger K. *Geochim. Cosmochim. Acta* **2012**, *85*, 200-212.

(10) Holst, J.C.; Olsen, M.B.; Paton, C.; Nagashima, K.; Schiller, M.; Wielandt, D., Larsen, K.K.; Connelly, J.N.; Jørgensen, J.K.; Krot, A. N.; Nordlund, Å.; Bizzarro, M. *PNAS* **2013**, *110* (22) 8819-8823.

(11) Jacobsen, S. B.; Yu, G. *Meteorit. Planet. Sci.* **2015**, *50* (4), 555-567.

(12) Kruijer T.; Kleine T.; Fischer-Gödde M.; Sprung P. *Nature* **2015**, *520*, 534-537.

(13) Touboul, M.; Puchtel, I. S.; Walker, R. J. *Nature* **2015**, *520*, 530-533.

(14) Trinquier, A.; Birck, J.L.; Allegre, C.J. *JAAS* **2007**, *23* (12), 1565-1574.

(15) Trinquier, A.; Komander, P. *J. Radioanal. Nucl. Chem.* **2015**, *305* (3) 1-6.

(16) Nier, A.O. *Phys. Rev.* **1950**, *77*, 789-793.

(17) Völkening, J.; Köppe, M.; Heumann, K.G. *Int. J. Mass Spectrom. Ion Processes* **1991**, *107*, 361-368.

(18) Caro, G.; Bourdon, B., Birck, J.-L.; Moorbath, S. *Geochim. Cosmochim. Acta.* **2006**, *70*, 164-191.

Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem, etc., that is discussed in the manuscript. Consult the journal's Instructions for Authors for TOC graphic specifications.

Insert Table of Contents artwork here

<figure><img src="image_7.png" /></figure>