# High-precision measurements of Mo isotopes by N-TIMS

Elsa Yobregat, Caroline Fitoussi, Eric Pili, Mathieu Touboul

► To cite this version:

Elsa Yobregat, Caroline Fitoussi, Eric Pili, Mathieu Touboul. High-precision measurements of Mo isotopes by N-TIMS. International Journal of Mass Spectrometry, 2022, 476, pp.116846. 10.1016/j.ijms.2022.116846. hal-03863622

HAL Id: hal-03863622

https://hal.science/hal-03863622v1

Submitted on 25 Jul 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
1 **High-precision measurements of Mo isotopes by N-TIMS**

2 **Elsa Yobregat¹, Caroline Fitoussi¹*, Eric Pili², Mathieu Touboul¹**

3

4 ¹Laboratoire de Géologie de Lyon (LGL-TPE), CNRS UMR 5276, Univ. Lyon, UCBL, ENS de Lyon, 46 allée

5 d'Italie, 69364 Lyon Cedex 7, France

6 ²CEA, DAM, DIF, F-91297 Arpajon, France

7 *Corresponding author: elsa.yobregat@ens-lyon.fr

8 **Keywords: Mo isotopes, N-TIMS**

9 **Abstract**

10 We have developed a new high-precision analytical method for measuring molybdenum (Mo) isotopes by
11 negative thermal ionization mass spectrometry (N-TIMS). This method represents an improvement over
12 existing methods, thanks to a new chemical separation method that yields a better purification of the Mo
13 fraction. In addition, we have used a multi-dynamic method for data collection with three different magnet
14 settings to avoid being sensitive to drifts in Faraday cup efficiencies. Our method resulted in a more reliable
15 thermal emission of MoO₃⁻ for a given run and we have obtained Mo isotope ratios measured on a series of
16 26 standards with the following reproducibilities (2 SD): ± 6.1, ± 5.8, ± 2.9, and ± 9.6 ppm for ⁹⁴Mo/⁹⁶Mo,
17 ⁹⁵Mo/⁹⁶Mo, ⁹⁷Mo/⁹⁶Mo, and ¹⁰⁰Mo/⁹⁶Mo, respectively. This represents an improvement by a factor of 2 to 3
18 compared with previously published methods, using N-TIMS or MC-ICPMS. This new analytical method
19 will have applications in the field of cosmochemistry (nucleosynthetic anomalies), environmental
20 geochemistry or nuclear forensics (mass-independent fractionation).

21 **1 Introduction**

22 Mo isotopes have become a widely used tracer in geochemistry and cosmochemistry over the past twenty
23 years thanks to their chemical and nuclear properties. First, Mo is a trace element present at the ppm level in
24 terrestrial rocks, making its isotope measurements accessible to mass spectrometry. Second, Mo is
25 characterized by three oxidation states in natural conditions (0, + 4, + 6). Since redox state changes are often
26 linked to isotope fractionation, sizeable mass-dependent fractionations have been observed in magmatic and
27 sedimentary materials. On this basis, Mo isotopes have been used to probe the variations in oxidation state of
28 the oceans and to investigate the geochemical cycle of this element (e.g. Kendall et al. [1]). From a
29 cosmochemical viewpoint, Mo is an element which is produced in stars by three distinct nucleosynthetic
30 processes, the p-, s-, and r-processes. As a consequence, significant nucleosynthetic anomalies have been
31 identified in planetary materials [2–4], reflecting variable contributions of different types of stars in the
32 composition of the Solar System. More recently, the association of Mo with U in nuclear materials has led to
33 the identification of mass-dependent Mo isotope fractionation in the field of nuclear forensics [5–8]. These
34 applications have created increasing interest in measuring Mo isotopes to higher precision with the goal of
35

36

37

38

39

40

41

42

43

44

reaching a few ppm level suitable for investigating mass-independent fractionation. In recent studies, it was
shown that it is possible to measure precisely Mo isotopes using a negative thermal ionization mass
spectrometric technique [9,10] or by MC-ICPMS [4]. Although these methods are perfectly viable and
robust, we have attempted to further improve the precision of existing techniques. This required significant
modifications of the chemical separation of Mo and a new optimization of the various measurement
conditions (namely, activator to sample ratio, filament configuration and filament geometry). In addition, we
have attempted to extend the type of matrices (including U-rich samples) that could be analyzed with our
analytical method. These new measurement conditions have led to a significant improvement in the final
precision in term of internal errors and external reproducibility compared with previously published
techniques.

45

## 2 Sample preparation

46

### 2.1 Sample dissolution

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

All acids used in this study were high purity, prepared by sub-boiling-distillation in Teflon. About two grams of standard rock powders BCR-1, JB-1a and JB-2 and about 20 mg of BCR-2 were dissolved in a closed (60 mL) Savillex Teflon beaker using 20 mL of a 5:1 mixture of concentrated HF-HNO₃ on a hot plate at 180°C for a week. Samples were then gently dried down at 120°C. The resulting fluorides were progressively broken down via subsequent, repeated addition of 0.1 mL of 30% H₂O₂ (Optima™) and concentrated HNO₃ (10 mL) on a hot plate at 120°C. Samples were dried down between each treatment. The samples were then converted to chloride form by repeated additions of 6M HCl followed by evaporation. When there were still residual fluorides, we added 20 mL of 5M HF, put closed Teflon beakers on a hot plate at 120°C for 12 hours, allowing additional fluorides to precipitate. The samples were then allowed to cool down and were then subjected to 30 minutes of ultrasonification. The samples were centrifuged and the supernatants were removed and saved for the ion exchange chromatography procedure. The remaining fluorides (~1 to 2 mL of precipitates) were rinsed with 5 mL of 5M HF. The closed Teflon beakers were put on a hotplate at 120°C for 2 hours, then in an ultrasonic bath for 30 minutes prior to a second centrifugation. The 5 mL supernatant resulting from this step was combined with the supernatant obtained after the first extraction. This step was repeated one more time. The sample solutions saved for ion exchange were dried down and the residues were dissolved in 10 mL of 0.6M HCl. As BCR-2 did not require fluorides centrifugation, it was used to ensure that this step does not produce mass-independent isotope fractionation. Two separate BCR-2 samples were dissolved, the first one was treated in 5M HF to remove fluorides, whereas for the second one, the fluorides produced during dissolution were directly and fully converted to chlorides in concentrated HCl.

67

### 2.2 Ion exchange chromatography

68

69

While numerous chemical separation methods for separating Mo from various matrices have been proposed in the literature (e.g. Nagai and Yokoyama [11]; Migeon et al. [5]), a new method appropriate for high
70

71

72

73

precision measurements by N-TIMS was developed in this study. Our goal was to obtain a high yield and
high purity Mo separate with special attention to elements that are potential isobaric interferences during
mass spectrometry or elements that have similar chemical properties and may limit Mo emission from the
filament.

74

75

76

77

78

79

The first step (Table 1a) used a 15 mL cation exchange column filled with AG50W-X8 Biorad™ resin (200-400 mesh) in a 0.6M HCl medium in order to separate Mo from most of the sample matrix. In this medium, Mo is not retained on the column while most of the major elements are fixed. Mo was eluted with 30 mL of 0.6M HCl. At this stage the Mo fraction is not pure and still contains other elements that were also eluted in 0.6M HCl such as Ti, Nb, Ta, W or Ru. The columns are then cleaned up with 6M HCl and utilized several times. The Mo fractions were dried down on a hot plate at 130°C.

80

81

82

83

84

85

86

87

88

The second step (Table 1b, Fig. S1) uses an anion exchange column (2 mL AG1X8, 200-400 mesh from Eichrom™), to further purify Mo from potentially interfering elements and from remaining major elements. The sample residues were dissolved in 2 mL of 1M HF + 0.02% H₂O₂ and loaded onto the column. The resin bed was successively rinsed with 10 mL of 1M HF + 0.02% H₂O₂, 8 mL of 9M HCl + 0.05M HF, 16 mL of 9M HCl + 1M HF to elute transition metals and high field strength elements such as Ru, Zr, W, Ru, Ti. A final rinsing step with 10 mL of 0.05M HNO₃ + 0.03M HF was specifically added to remove efficiently any remaining uranium after the first cation column. Molybdenum was then eluted using 10 mL of 6M HNO₃ + 3M HF and Mo cuts were dried down on a hot plate at 130°C. For large silicate samples (> 1 g), this step must be repeated to ensure complete removal of most abundant elements, such as Ti.

89

90

91

92

93

94

95

The third step (Table 1c), using a second anion-exchange column (1 mL AG1-X8, 200-400 mesh from Eichrom™), was specifically designed to remove any remaining Nb and Sn in the Mo cut. Indeed, before the addition of this column, it was observed that residual Nb and Sn were still present after the first separation steps and could hinder optimal emissivity of Mo ions in the ion source during N-TIMS measurements. The Mo residues were dissolved in 1 mL of 1M HF + 0.02% H₂O₂ for loading onto the columns. Nb was eluted with 6M HCl + 0.1M HF and Mo was then collected with 20 mL of 0.6M HCl, whereas Sn remained fixed on the resin bed [12].

96

97

98

99

100

101

102

The fourth step (Table 1d), using a cation exchange column filled with 0.5 mL of AG50W-X8 200-400 mesh (from Biorad™), was designed as a final purification step prior to loading the samples on Re filaments for TIMS analysis. Moreover, this step ensured further removal of U, which is necessary as this method is also designed for investigating U-rich samples. The samples were loaded in 0.6M HCl so that U was efficiently retained while Mo did not stick to the column and was directly collected. Any remaining Mo was rinsed from the column with 2.5 mL of 0.6M HCl. After evaporation, the Mo cut was repeatedly dissolved in 60 µl of aqua regia and dried down, in order to remove organics.

103

104

After chromatographic separation, a small aliquot (~ 0.5%) of the sample is separated for analysis using an ICP-MS to assess elemental purity and to determine the chemical yield at the end of the chemical procedure.

The yield of this 4-step ion exchange procedure is typically ~ 70%. No significant impurity of potentially interfering elements, such as Zr, Nb, Ru have been detected. Typical Mo blanks obtained with this analytical

105

106
method were below 1 ng (measured by isotope dilution on a Neptune Plus), which is well below the amount
of Mo used for a single isotope analysis by N-TIMS (~ 1 to 2 µg). Blanks and yields are similar to other
published methods.

110

111 **3 Mass Spectrometry**

112 **3.1 Instrumentation**

113 The Mo isotopes were measured by N-TIMS on a Thermo Fisher Scientific Triton Plus™ at the Laboratoire
de Géologie de Lyon. The separated molybdenum was taken up in a drop of 6M HCl and loaded on an ultra-
114 pure zone refined rhenium filament that was degassed for 60 minutes with a current of 4.2 A. In order to
115 optimize the measurements, we began with the identification of the best emission conditions for
116 molybdenum as a triple oxide. We tested different configurations for the emission of MoO₃⁻ ions based on
117 three parameters: (i) filament dimensions, (ii) filament geometry, (iii) amount of activator added to the Mo
118 load. For each test run, 1 µg of Mo was loaded on the filament and the quality of the MoO₃⁻ ions emission
119 was assessed by integrating the intensity of the ⁹⁸MoO₃⁻ signal for each test.
120

121 The width and thickness of rhenium filaments used for the emission of MoO₃⁻ ions could have an influence
122 on the ionization efficiency and the Mo speciation during analysis. Thus, we tested two types of filaments,
123 hereafter referred to as *Thin* filament (0.02489 × 0.7493 mm) and *Thick* filament (0.03987 × 0.6985 mm).
124 Our filaments have slightly different dimensions compared with those used in two recent published studies
125 (Nagai and Yokoyama [9]: 0.0305 × 0.705 mm, and Worsham *et al.* [10]: 0.030 × 0.760 mm).

126 Two filament assemblies were tested for Mo emission. When using *Simple* filaments, the same filament was
127 used for both evaporation and ionization of the sample while in the *Double* filament assembly, the Mo was
128 evaporated from a filament facing a second one heated at a higher temperature to enhance ionization. This
129 second configuration is thought to enable a good control of the evaporation while optimizing the ionization.
130 Both filament geometries were used in previous studies.

131 An activator consisting of a solution of lanthanum nitrate (La(NO₃)₃) was added to the Mo fraction on the
132 filament in order to enhance ionization. Based on previous studies [9,10], we used this solution, which serves
133 as an electron emitter and enhances the MoO₃⁻ emission by lowering the Re work function. As Nagai and
134 Yokoyama [9] pointed out that the La/Mo ratio has a strong influence on Mo ion emission, we tested ratios
135 from 1 to 15. The configuration of choice should have an optimal emission for a wide range of La/Mo ratios,
136 as the mass of the Mo load is difficult to determine with a high precision.

137 The effect of the various filament configurations on the emission is shown in Fig S1 (Supplementary data).
138 For both *Simple* and *Double* filament configurations, the *Thin* filaments have an emission efficiency lower
139 by a factor of 2 to 6 compared with the *Thick* filaments. The integrated signal for ⁹⁸MoO₃⁻ is lower than 1000
140 V (as measured on a Faraday cup equipped with a 10¹¹ Ω resistor) for *Thin Simple* filaments (Fig. 1a,
141 Supplementary data) and a better emission is observed for the *Thin Double* filaments (Fig. 1b,
Supplementary data) with an integrated signal reaching 3000 V. However, the optimal La/Mo ratio has a narrow range between 6 and 8. For the *Thick* filaments, both *Simple* and *Double* configurations produced a good MoO₃⁻ emission (Fig. 1c-S1d, Supplementary data). The *Thick Simple* filaments (Fig. 1c, Supplementary data) seem very efficient to ionize Mo, but the La/Mo ratio needed to produce such efficient emission also shows a narrow range (4 to 6). As an additional test, we attempted to modify the order of loading of the activator and Mo on the filament. We tested several ways of mixing Mo and the La activator. We found that the best Mo emission corresponded to filaments on which Mo was first evaporated before adding La activator (Fig. 1, Supplementary data). When La was mixed with Mo prior to loading on the filament, or before and after Mo loading, the emission was deteriorated. The *Thick Double* filaments produced a stable and high MoO₃⁻ signal for a larger range of La/Mo ratios (from 4 to 12) and this was considered as the optimal conditions (Fig. 1, Supplementary data). We also tested the addition of activator on the ionization filament, following the work of Worsham *et al.* [10]. Although this could produce very efficient ionization, the plateau of optimal La/Mo ratio was not as large as when the addition of the activator was limited to the evaporation filament (Fig. 1, Supplementary data). Therefore, the configuration chosen for best Mo emission was the *Thick Double* Re filament with a La/Mo ratio equal to ~ 9.

## 3.2 Data acquisition protocol

Previous studies of Mo isotopes by N-TIMS used a static single-line acquisition mode, where all isotopes were measured simultaneously on seven Faraday cups [9,10]. This method has the advantage of a short acquisition time. However, Faraday cups efficiencies drift with time, while cups slowly deteriorate, which could dramatically reduce the precision on the data. This effect has already been observed with other isotope systems such as Nd (e.g. Caro *et al.* [13]), Sr (e.g. Hans [14]), or Ba (e.g. Yobregat *et al.* [15]). In static configuration mode, datasets obtained over several months cannot be directly compared with each other. To compensate for the temporal drift in Faraday cups efficiencies, we used a multidynamic acquisition mode that consisted in switching the beams from one Faraday cup to another during analysis such that the isotopes of interest and the isotopes of the normalization ratio were measured on the same cups.

All measurements were performed using a 3-line acquisition scheme. The Faraday cup assembly was successively centered at masses 144, 145 and 146 amu, corresponding to ⁹⁶MoO₃⁻, ⁹⁷MoO₃⁻ and ⁹⁸MoO₃⁻ respectively (Table 2). First, this cup configuration first allowed multidynamic data reduction for all Mo isotope ratios, except for ⁹²Mo/⁹⁶Mo ratio, when a normalization to ⁹⁸Mo/⁹⁶Mo was applied. Second, this cup configuration was ideally suited for estimating the O isotope composition via the ¹⁰⁰Mo¹⁸O¹⁶O¹⁶O signal measured on cup H4 associated with a fixed (non-rotating) 10¹² Ω resistor. Third, this cup configuration allowed simultaneous collection of Mo¹⁶O₃⁻ at all Mo isotope masses, as well as ZrO₃⁻, NbO₃⁻, and RuO₃⁻ species for monitoring interferences on Mo trioxides. The interfering Zr, Nb, and Ru trioxides never reached detectable level and can be neglected.

The evaporation filament was heated to a current of 1.8 A, while the ionization filament was heated to a current of 3.45 A, where a typical emission for mass 146 (⁹⁸Mo¹⁶O₃) was 1.8V (with a 10¹¹ Ω resistor). The
obtained signal was stable over at least 8 hours, during which 1200 – 2400 isotope ratios were collected using 8 s integration time for each magnet setting (i.e. line) in blocks of 20 cycles. After each block of data collection, the amplifiers were electronically rotated relative to the Faraday cup detectors, thereby cancelling out any inter-amplifier biases. A 30 s baseline was also measured for each Faraday cup/amplifier pair by beam deflection before every block. The ion beam was refocused every 2 blocks using the automated focus capability of the Triton and peaks were centered every 4 blocks.

## 3.3 Data reduction

### 3.3.1 Correction of isobaric interferences

Ionization of Mo as MoO₃⁻ mainly resulted in the production of Mo¹⁶O₃⁻ but also, in smaller relative abundances, of some isobaric species involving less abundant oxygen isotopes (Mo¹⁶O₂¹⁷O⁻, Mo¹⁶O¹⁷O₂⁻, Mo¹⁶O¹⁸O₂⁻, Mo¹⁶O¹⁷O¹⁸O⁻, Mo¹⁷O₃⁻, and Mo¹⁷O₂¹⁸O⁻). Other isobaric oxide species, such as Mo¹⁷O¹⁸O₂⁻ and Mo¹⁸O₃⁻, were also formed, but they generated no significant interferences. The complete list of Mo oxide isobars is given in Table 1.

The measured intensity at mass x (xI) can be corrected for signals derived from other isobaric Mo trioxides, adapting oxide correction equations developed earlier for W isotope measurement by N-TIMS [16]. At mass 140, there is no isobar species and no correction is necessary, therefore:

$$^{92}Mo^{16}O_3 = ^{140}I$$

where ⁹²Mo¹⁶O₃ corresponds to the signal contributed by this Mo trioxide to ¹⁴⁰I, the intensity measured at mass 140. For masses above 140, the interference correction can be done in a sequential manner as illustrated in the following equations for ⁹⁴Mo, ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo and ¹⁰⁰Mo:

$$^{94}Mo^{16}O_3 = ^{142}I - ^{92}Mo^{16}O_3 \times 3 \left[ \left( \frac{^{17}O}{^{16}O} \right)^2 + \frac{^{18}O}{^{16}O} \right] \quad (1)$$

$$^{95}Mo^{16}O_3 = ^{143}I - ^{94}Mo^{16}O_3 \times 3 \left( \frac{^{17}O}{^{16}O} \right) - ^{92}Mo^{16}O_3 \times \left[ 6 \left( \frac{^{17}O}{^{16}O} \right) \left( \frac{^{18}O}{^{16}O} \right) + \left( \frac{^{17}O}{^{16}O} \right)^3 \right] \quad (2)$$

$$^{96}Mo^{16}O_3 = ^{144}I - ^{95}Mo^{16}O_3 \times 3 \left( \frac{^{17}O}{^{16}O} \right) - ^{94}Mo^{16}O_3 \times 3 \left[ \left( \frac{^{17}O}{^{16}O} \right)^2 + \frac{^{18}O}{^{16}O} \right] - ^{92}Mo^{16}O_3 \\ \times 3 \left( \frac{^{18}O}{^{16}O} \right) \left[ \left( \frac{^{18}O}{^{16}O} \right) + \left( \frac{^{17}O}{^{16}O} \right)^2 \right] \quad (3)$$

$$^{97}Mo^{16}O_3 = ^{145}I - ^{96}Mo^{16}O_3 \times 3 \left( \frac{^{17}O}{^{16}O} \right) - ^{95}Mo^{16}O_3 \times 3 \left[ \left( \frac{^{17}O}{^{16}O} \right)^2 + \frac{^{18}O}{^{16}O} \right] - ^{94}Mo^{16}O_3 \\ \times \left[ 6 \left( \frac{^{17}O}{^{16}O} \right) \left( \frac{^{18}O}{^{16}O} \right) + \left( \frac{^{17}O}{^{16}O} \right)^3 \right] \quad (4)$$
$$ ^{98}\text{Mo}^{16}\text{O}_3 = ^{146}\text{I} - ^{97}\text{Mo}^{16}\text{O}_3 \times 3\left(\frac{^{17}\text{O}}{^{16}\text{O}}\right) - ^{96}\text{Mo}^{16}\text{O}_3 \times 3\left[\left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)^2 + \frac{^{18}\text{O}}{^{16}\text{O}}\right] - ^{95}\text{Mo}^{16}\text{O}_3 \\ \times \left[6\left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right) + \left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)^3\right] - ^{94}\text{Mo}^{16}\text{O}_3 \times 3\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right)\left[\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right) + \left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)^2\right] \quad (5) $$

$$ ^{100}\text{Mo}^{16}\text{O}_3 = ^{148}\text{I} - ^{98}\text{Mo}^{16}\text{O}_3 \times 3\left[\left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)^2 + \frac{^{18}\text{O}}{^{16}\text{O}}\right] - ^{97}\text{Mo}^{16}\text{O}_3 \times \left[6\left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right) + \left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)^3\right] \\ - ^{96}\text{Mo}^{16}\text{O}_3 \times 3\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right)\left[\left(\frac{^{18}\text{O}}{^{16}\text{O}}\right) + \left(\frac{^{17}\text{O}}{^{16}\text{O}}\right)^2\right] \quad (6) $$

198

199

200

201

202

203

The interference corrected isotope ratios are a function of the measured isotope ratios and the O isotope composition of Mo trioxides produced in the ion source. The most recent study regarding W isotopes [17] has shown that it is preferable to measure the O isotope composition in line, during the measurement, rather than using a predefined O isotope composition [16]. In the case of Mo isotopes, it is possible to determine the current ¹⁸O/¹⁶O ratio by measuring intensities at mass 150 and 148 and using the following equation:

$$ \frac{^{18}\text{O}}{^{16}\text{O}} = \frac{1}{3} \left[ \frac{^{100}\text{Mo}^{16}\text{O}^{16}\text{O}^{18}\text{O}}{^{100}\text{Mo}^{16}\text{O}^{16}\text{O}^{16}\text{O}} \right] \quad (7) $$

204

205

206

207

However, ¹⁰⁰Mo¹⁶O¹⁶O¹⁸O signals were very small (commonly, 2-5 mV). Therefore, the Faraday cup H4 was associated with a 10¹² Ω amplifier (Table 2) to estimate ¹⁸O/¹⁶O more precisely, similarly to the configuration used in Worsham et al. [10]. This 10¹² Ω amplifier was excluded from the amplifier rotation described above, and remained associated with the Faraday cup H4 throughout the whole analysis.

208

### 3.3.2 Correction of mass-dependent fractionation

209

210

211

212

213

214

215

216

Instrumental fractionation is the major limiting factor precluding accurate determination of isotope ratio by mass spectrometry. In thermal ionization ion sources, it is largely induced by preferential evaporation of light isotopes relative to heavier isotopes. As illustrated in Fig. S2 (Supplementary data), this process results in measured isotope ratios that display strongly correlated variations when plotted on binary logarithmic diagrams. Within each run, mass fractionation increases with time, resulting in a significant evolution of measured isotope ratios during the analysis. The effect of this mass-dependent fractionation can be corrected for a given isotope ratio (R<sup>xy</sup>) by normalizing to a second ratio (R<sup>zw</sup>) of two stable and non-radiogenic isotopes (z and w) using the exponential law [18]:

$$ R^{xy} = R^{xy}_{\text{measured}} \times \left( \frac{R^{zw}_{\text{true}}}{R^{zw}_{\text{measured}}} \right) \left( \frac{\ln\left(\frac{M_x}{M_y}\right)}{\ln\left(\frac{M_z}{M_w}\right)} \right) \quad (8) $$
where M corresponds to the atomic mass of the Mo trioxide species, Rxy is the mass fractionation corrected ratio, obtained via normalization to Rzw, and Rxy measured and Rzw measured are the measured isotope ratio.

All seven Mo isotopes are stable and non-radiogenic isotopes, and our 3-line acquisition scheme allows data reduction with several normalization ratios. Here, we report data normalized to ⁹⁸Mo/⁹⁶Mo = 1.453171 [19] to facilitate comparison with previous studies (e.g. Burkhardt et al. [3]). After mass-dependent fractionation correction relative to ⁹⁸Mo/⁹⁶Mo, correlations of isotope ratios and their variations with time disappear (Fig. S3, Supplementary data). Internal precisions on corrected ⁹²Mo/⁹⁶Mo, ⁹⁴Mo/⁹⁶Mo, ⁹⁵Mo/⁹⁶Mo, ⁹⁷Mo/⁹⁶Mo and ¹⁰⁰Mo/⁹⁶Mo ratios are typically reached after 1200 measurement cycles with a ~ 2 V ⁹⁸Mo beam (Table S1, Supplementary data).

### 3.3.2.1 Static data reduction

Static isotopes data (Table S1, Supplementary data) are corrected for mass-dependent fractionation using simultaneously measured ⁹⁸Mo/⁹⁶Mo ratio, i.e. measured within the same line of the data acquisition scheme (these lines are shown in Table 2), according to:

$$R_{\text{static line 1}}^{xy} = R_{\text{measured on line 1}}^{xy} \times \left( \frac{R_{\text{true}}^{98/96}}{R_{\text{measured on line 1}}^{98/96}} \right) \left( \frac{\ln\left(\frac{M_x}{M_y}\right)}{\ln\left(\frac{M_{98}}{M_{96}}\right)} \right) \quad (9)$$

$$\text{with } R_{\text{measured on line 1}}^{xy} = \frac{\left( \frac{^{x}\text{Mo}^{16}\text{O}_3}{^{y}\text{Mo}^{16}\text{O}_3} \right)_{\text{line 1}}}{\left( \frac{^{x}\text{Mo}^{16}\text{O}_3}{^{y}\text{Mo}^{16}\text{O}_3} \right)_{\text{line 1}}}$$

The advantage of this method is that the isotope ratio used for mass fractionation correction is measured at the same time as the isotope ratios of interest, which ensures that there is no bias due to the temporal drift in mass fractionation. However, static data reduction does not account for biases due to the deterioration of Faraday cups accompanying ageing. This is an additional potential source of inaccuracy for isotope measurements and requires corrections at the ± 5 ppm precision level, as previously demonstrated for Nd [13,20,21], W [16] and Ba [15] isotopes. The same principle is used with the multistatic data reduction (see next section), except that the same operation is repeated several times.

### 3.3.2.2 Multistatic data reduction

In this data reduction mode, the Faraday cup configuration is offset by one mass unit after each integration time, using altogether three different configurations as shown in Table 2. By averaging the measurements over several configurations, the effect of cup deterioration can be limited using a multi-static data reduction. Multistatic Mo isotope ratios are calculated by averaging static isotope ratios obtained for each of the lines of the data acquisition scheme (Table S1, Supplementary data). Hence,

$$R_{\text{multistatic}}^{xy} = \frac{\sum_i R_{\text{static line i}}^{xy}}{n_i} \qquad (10)$$

with nᵢ equal to the number of lines involved in the multistatic calculation.
### 3.3.2.3 Multidynamic data reduction

The effect of cup deterioration is best minimized by multi-dynamic data reduction, which allows mathematical elimination of relative variations of cup efficiencies. In multi-dynamic calculations, measured ratios are corrected for mass fractionation using ⁹⁸Mo/⁹⁶Mo ratios measured on the same Faraday cups. All the isotope ratios could be determined with a multidynamic acquisition except for ⁹²Mo/⁹⁶Mo. The dynamic Mo isotope ratios are calculated as follows:

$$R_{\text{dynamic}}^{94/96} = R_{\text{measured on line 3}}^{94/96} \times \left(\frac{M_{94}}{M_{96}}\right) - \left(\frac{\ln\left(\frac{R_{\text{measured on line 2}}^{98/96}}{R_{\text{true}}^{98/96}}\right)}{\ln\left(\frac{M_{98}}{M_{96}}\right)}\right) \quad (11)$$

$$R_{\text{dynamic}}^{95/96} = R_{\text{measured on line 3}}^{95/96} \times \left(\frac{M_{95}}{M_{96}}\right) - \left(\frac{\ln\left(\frac{R_{\text{measured on line 2}}^{98/97} \times R_{\text{measured on line 1}}^{97/96}}{R_{\text{true}}^{98/96}}\right)}{\ln\left(\frac{M_{98}}{M_{96}}\right)}\right) \quad (12)$$

$$R_{\text{dynamic 1}}^{97/96} = \sqrt{R_{\text{measured on line 1}}^{97/96} \times R_{\text{measured on line 2}}^{97/96}} \times \left(\frac{M_{97}}{M_{96}}\right) - \left(\frac{\ln\left(\frac{R_{\text{measured on line 2}}^{98/96}}{R_{\text{true}}^{98/96}}\right)}{\ln\left(\frac{M_{98}}{M_{96}}\right)}\right) \quad (13)$$

$$R_{\text{dynamic 2}}^{97/96} = \sqrt{R_{\text{measured on line 1}}^{97/96} \times R_{\text{measured on line 3}}^{97/96}} \times \left(\frac{M_{97}}{M_{96}}\right) - \left(\frac{\ln\left(\frac{R_{\text{measured on line 1}}^{98/96}}{R_{\text{true}}^{98/96}}\right)}{\ln\left(\frac{M_{98}}{M_{96}}\right)}\right) \quad (14)$$

$$R_{\text{dynamic}}^{100/96} = R_{\text{measured on line 2}}^{100/96} \times \left(\frac{M_{100}}{M_{96}}\right) - \left(\frac{\ln\left(\frac{R_{\text{measured on line 2}}^{98/96} \times R_{\text{measured on line 3}}^{98/96}}{R_{\text{true}}^{98/96}}\right)}{\ln\left(\frac{M_{98}}{M_{96}}\right)}\right) \quad (15)$$

## 4 Results

The static, multistatic, and multidynamic Mo isotope ratios, calculated after correction from oxide interferences and mass-dependent fractionation with the method described above, are reported in Table S1 (Supplementary data). These Mo ratios are given for repeated analyses of (i) the NIST 3134 standard solution (ii) the NIST 3134 standard solution after it was purified using our chemistry procedure, (iii) a synthetic solution enriched in ⁹⁷Mo and ¹⁰⁰Mo and (iv) several terrestrial standard rock powders (BCR-1, BCR-2, JB-1a, and JB-2). The most accurate and precise results, presented in Table 4, were obtained using multidynamic acquisition scheme when compared to static or multistatic modes of acquisition.
The Mo isotope composition of the NIST 3134 solution processed through chemistry is significantly different from the composition of NIST 3134 solution directly deposited onto the filament (Fig. 2). This could reflect either that our chemical procedure induced mass-independent fractionation of Mo isotopes (such as described in Fujii et al. [22]), or that the original NIST 3134 solution was not totally free of impurities while affecting the isotope measurement. In such case, these impurities were actually cleaned by being processed through chemistry. To solve this conundrum, the NIST 3134 solution was processed twice through the chemical procedure. Within uncertainty, the standard solution processed twice has the same Mo isotope composition as the standard solution processed only once (Fig. 2). This indicates no mass-independent fractionation related to the chemical procedure. Rather, this suggests the presence of impurities within the NIST 3134 solution, which generated unexpected isobaric interferences. The NIST 3134 standard was checked for impurities using an ICPMS and nothing was detected above background. The impurities are possibly organic molecules. These interferences only generate isotope differences at the 10 ppm level, which probably explains why they are very difficult to detect, even with an ICPMS. It is difficult to predict whether these impurities could also affect MC-ICPMS analyses. In particular, one should note that the comparison of interferences between TIMS and ICPMS is difficult since (i) the ionization efficiency and the mechanisms of ion production (including molecules) are very different for TIMS and ICPMS (ii) the mass range where Mo isotopes are measured by ICPMS is 92-100 amu while it is 140-150 amu by N-TIMS. From the measurements of the purified NIST 3134 standard solution, we can therefore infer its Mo isotope composition: ⁹⁴Mo/⁹⁶Mo = 0.552569 ± 0.000003, ⁹⁵Mo/⁹⁶Mo = 0.953266 ± 0.000006, ⁹⁷Mo/⁹⁶Mo = 0.573976 ± 0.000002, and ¹⁰⁰Mo/⁹⁶Mo = 0.581522 ± 0.000004 (2 SD, n = 16), as averaged from repeated measurements over a period of 6 months. Our new analytical method for measuring Mo isotopes by N-TIMS yields long-term 2 SD external reproducibilities of ± 6.1 ppm on ⁹⁴Mo/⁹⁶Mo ratio, ± 6.6 ppm on ⁹⁵Mo/⁹⁶Mo ratio, ± 4.2 ppm on ⁹⁷Mo/⁹⁶Mo ratio, and ± 7.6 ppm on ¹⁰⁰Mo/⁹⁶Mo ratio. As shown in Fig. 2, our method gives significantly lower uncertainties compared with recently published methods, either using N-TIMS [9, 10] or MC-ICPMS [4]. It should be mentioned that measurements by MC-ICPMS require less material, which can be a crucial advantage in the case of low level samples (in Mo) and/or very precious samples (and therefore available in low amounts).

The accuracy of our method was tested by analyzing a synthetic solution artificially enriched in ⁹⁷Mo and ¹⁰⁰Mo isotopes. The solution was prepared gravimetrically by mixing a double spike solution with the NIST 3134 solution to create ~20 ppm and ~30 ppm anomalies on ⁹⁷Mo/⁹⁶Mo and ¹⁰⁰Mo/⁹⁶Mo ratios respectively. The ⁹⁷Mo and ¹⁰⁰Mo individual spikes, purchased from Oak Ridge National Laboratory, have a purity of 94.2% and 97.2% and are mixed in proportion of 42.57% and 57.43% respectively. This double spike solution was carefully calibrated by Migeon et al. [5] for their Mo mass dependent fractionation study. After adding the double spike to the NIST 3134 Mo standard, the mixed solution was passed through the whole chemical purification procedure. The repeated analysis of the mixed solution yielded anomalies of 21.3 ± 3.6 ppm and 31.9 ± 5.8 ppm on ⁹⁷Mo/⁹⁶Mo and ¹⁰⁰Mo/⁹⁶Mo ratios respectively (Table 5) which are in excellent
agreement with the predicted values: µ<sup>97</sup>Mo = 19.8 ppm and µ<sup>100</sup>Mo = 27.8 ppm (see Table 4 caption for µ definition).

The Mo isotope compositions of BCR-1, BCR-2 (with and without precipitation), JB-1a, and JB-2 are all similar within uncertainties to the composition of the purified NIST 3134 solution (Table 4, Fig. 3). This result validates our chemical separation, and shows that it is applicable for processing large samples of silicate rocks. It also further highlights the accuracy and reproducibility of our new Mo isotope measurement method by N-TIMS for natural samples.

## 5 Conclusions

The new analytical method we developed in this study allows the precise analysis of Mo isotopes by negative thermal ionization mass spectrometry with an improved performance compared to previous works. Thanks to a more comprehensive chemical separation that resulted in more stable and interference-free Mo signals, we have obtained an improvement of uncertainties by a factor of 2 to 3 for all isotope ratios compared with previous methods, except for <sup>92</sup>Mo and <sup>97</sup>Mo. In contrast with previous N-TIMS technique, we have used a multidynamic mode for Mo isotope measurements. Our results show that the NIST 3134 solution commonly used as a standard is not perfectly devoid of interfering elements, at the level of precision we obtained. A mass-independent fractionation effect of our purification procedure can be ruled out as the NIST 3134 solution purified one more time did not yield any difference compared to the one purified only once. We therefore draw attention on the fact that high-precision Mo isotope determination by N-TIMS should use a purified NIST 3134 solution (or any other working standard free of impurities) to avoid isobaric interferences hampering the measurements. The validity of our technique has been further demonstrated by the analyses of standard reference materials of silicate rocks yielding uncertainties similar to those of standard solutions. This improvement should enable the detection of small Mo isotope anomalies (nucleosynthetic anomalies, mass-independent fractionation or nuclear reactions) with applications on a wide range of sample matrices in a large variety of domains such as cosmochemistry, early Earth studies, environmental chemistry and nuclear forensics.

**Credit authorship contribution statement** Conceptualization and methodology, E.Y., C.F., M.T.; Sample acquisition, C.F., M.T., E.P.; Formal analysis, E.Y., M.T.; Writing - original draft, C.F., M.T.; Writing - review & editing, E.Y., C.F., M.T., E.P.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
330 Acknowledgments

331 This work was done in the framework of the CEA-ENS Lyon collaboration CAJ_17-48/C29096 led by
332 Caroline Fitoussi and Eric Pili. We thank Amélie Hubert for fruitful discussion and kindly sharing the BCR-
333 2 reference material. We thank Labex LIO for financial contribution to the Triton Plus instrument. Mathieu
334 Touboul was supported by the Programme National de Planétologie (PNP) of CNRS/INSU, co-funded by
335 CNES.

336 References

[1] B. Kendall, T.W. Dahl, A.D. Anbar, The stable isotope geochemistry of molybdenum, Rev. Mineral. Geochemistry. 82 (2017) 683-732. https://doi.org/10.2138/rmg.2017.82.16.

[2] N. Dauphas, B. Marty, L. Reisberg, Molybdenum Evidence for Inherited Planetary Scale Isotope Heterogeneity of the Protosolar Nebula, Astrophys. J. 565 (2002) 640-644. https://doi.org/10.1086/324597.

[3] C. Burkhardt, T. Kleine, F. Oberli, A. Pack, B. Bourdon, R. Wieler, Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth, Earth Planet. Sci. Lett. 312 (2011) 390-400. https://doi.org/10.1016/j.epsl.2011.10.010.

[4] G. Budde, C. Burkhardt, T. Kleine, Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth, Nat. Astron. 3 (2019) 736-741. https://doi.org/10.1038/s41550-019-0779-y.

[5] V. Migeon, B. Bourdon, E. Pili, C. Fitoussi, An enhanced method for molybdenum separation and isotopic determination in uranium-rich materials and geological samples, J. Anal. At. Spectrom. 30 (2015) 1988-1996. https://doi.org/10.1039/c5ja00106d.

[6] V. Migeon, B. Bourdon, E. Pili, C. Fitoussi, Molybdenum isotope fractionation during acid leaching of a granitic uranium ore, Geochim. Cosmochim. Acta. 231 (2018) 30-49. https://doi.org/10.1016/j.gca.2018.04.004.

[7] V. Migeon, C. Fitoussi, E. Pili, B. Bourdon, Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: Towards a new nuclear forensic tool, Geochim. Cosmochim. Acta. 279 (2020) 238-257. https://doi.org/10.1016/j.gca.2020.03.046.

[8] J.M. Rolison, M. Druce, Q.R. Shollenberger, T.M. Kayzar-Boggs, R.E. Lindvall, J. Wimpenny, Molybdenum isotope compositions of uranium ore concentrates by double spike MC-ICP-MS, Appl. Geochemistry. 103 (2019) 97-105. https://doi.org/10.1016/j.apgeochem.2019.03.001.

[9] Y. Nagai, T. Yokoyama, Molybdenum isotopic analysis by negative thermal ionization mass spectrometry (N-TIMS): Effects on oxygen isotopic composition, J. Anal. At. Spectrom. 31 (2016) 948-960. https://doi.org/10.1039/c5ja00381d.

[10] E.A. Worsham, R.J. Walker, K.R. Bermingham, High-precision molybdenum isotope analysis by negative thermal ionization mass spectrometry, Int. J. Mass Spectrom. 407 (2016) 51-61.
365 https://doi.org/10.1016/j.ijms.2016.06.005.

366 [11] Y. Nagai, T. Yokoyama, Chemical Separation of Mo and W from Terrestrial and Extraterrestrial Samples via Anion Exchange Chromatography, Anal. Chem. 86 (2014) 4856–4863.
367
368 https://doi.org/10.1021/ac404223t.

369 [12] X. Wang, C. Fitoussi, B. Bourdon, Q. Amet, A new method of Sn purification and isotopic determination with a double-spike technique for geological and cosmochemical samples, J. Anal. At. Spectrom. 32 (2017) 1009–1019. https://doi.org/10.1039/C7JA00031F.

372 [13] G. Caro, B. Bourdon, J.L. Birck, S. Moorbath, High-precision 142Nd/144Nd measurements in terrestrial rocks: Constraints on the early differentiation of the Earth's mantle, Geochim. Cosmochim. Acta. 70 (2006) 164–191. https://doi.org/10.1016/j.gca.2005.08.015.

375 [14] U. Hans, T. Kleine, B. Bourdon, Rb-Sr chronology of volatile depletion in differentiated protoplanets: BABI, ADOR, and ALL revisited, Earth Planet. Sci. Lett. 374 (2013) 204-214.

377 [15] E. Yobregat, C. Fitoussi, B. Bourdon, A new method for TIMS high precision analysis of Ba and Sr isotopes for cosmochemical studies, J. Anal. At. Spectrom. 32 (2017) 1388–1399.
378
379 https://doi.org/10.1039/C7JA00012J.

380 [16] M. Touboul, R.J. Walker, High precision tungsten isotope measurement by thermal ionization mass spectrometry, Int. J. Mass Spectrom. 309 (2012) 109–117. https://doi.org/10.1016/j.ijms.2011.08.033.

382 [17] G.J. Archer, A. Mundl, R.J. Walker, E.A. Worsham, K.R. Bermingham, High-precision analysis of 182W/184W and 183W/184W by negative thermal ionization mass spectrometry: Per-integration oxide corrections using measured 18O/16O, Int. J. Mass Spectrom. 414 (2017) 80–86.
383
384
385 https://doi.org/10.1016/j.ijms.2017.01.002.

386 [18] W.A. Russell, D.A. Papanastassiou, T.A. Tombrello, Ca isotope fractionation on the Earth and other solar system materials, Geochim. Cosmochim. Acta. 42 (1978) 1075–1090.
387
388
389 https://doi.org/10.1016/0016-7037(78)90105-9.

389 [19] Q. Lu, A. Masuda, The isotopic composition and atomic weight of molybdenum, Int. J. Mass Spectrom. Ion Process. 130 (1994) 65–72. https://doi.org/10.1016/0168-1176(93)03900-7.

391 [20] M. Boyet, R.W. Carlson, A highly depleted moon or a non-magma ocean origin for the lunar crust?, Earth Planet. Sci. Lett. 262 (2007) 505–516. https://doi.org/10.1016/j.epsl.2007.08.009.

393 [21] A.S.G. Roth, B. Bourdon, S.J. Mojzsis, M. Touboul, P. Sprung, M. Guitreau, J. Blichert-Toft, Inherited <sup>142</sup>Nd anomalies in Eoarchean protoliths, Earth Planet. Sci. Lett. 361 (2013) 50–57.
394
395
https://doi.org/10.1016/j.epsl.2012.11.023.

396 [22] T. Fujii, F. Moynier, P. Telouk, F. Albarède, Mass-Independent Isotope Fractionation of Molybdenum and Ruthenium and the Origin of Isotopic Anomalies in Murchison, Astrophys. J. 647 (2006) 1506–1516. https://doi.org/10.1086/505459.
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Illustration of the emission tests with various La/Mo ratios and various filament configurations (Single or Double filament and Thin or Thick Re filaments). Blue diamonds correspond to filaments on which Mo was first evaporated before adding La activator.</figcaption></figure>

401

402

403
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Mo isotope ratios obtained by repeated analysis of the Mo standard solution NIST 3134 (blue symbols) and the standard solution NIST 3134 after passing once through our chemistry protocol (dark red symbols) and after passing twice through our chemistry protocol (open red symbols). All Mo isotope ratios shown here are corrected using multidynamic data reduction. Error bars show the ± 2 SE of individual measurement. The light blue and blue area indicate the ± 2 SD and ± 2 SE external reproducibility of the 26 measurements of the Mo standard solution performed over the year of this study. The light red and red area indicate the ± 2 SD and ± 2 SE external reproducibility for the 16 measurements of the Mo standard solution after purification using our chemistry protocol. Dotted lines indicate the average isotope ratios of the Mo standard solution before and after purification.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Mo isotopic data for the terrestrial standard rock powders, BCR-1 and BCR-2. Dynamic isotope ratios are shown here as µ*x*Mo, which corresponds to the deviation, in part per million (ppm), of the measured ratio of *x*Mo over ⁹⁶Mo, relative to the average ¹Mo/⁹⁶Mo of repeated analysis of the purified NIST standard (n = 16). Error bars show the ± 2 SE of individual measurements. The light red and red area indicate the ± 2 SD and ± 2 SE external reproducibilities for the 16 measurements of the purified Mo standard solution performed over the course of this study. Gray and dark gray area indicates ± 2 SD external reproducibility reached by previous studies of Mo isotopes by MC-ICPMS [4] and N-TIMS [10], respectively.</figcaption></figure>
<table><caption>**Table 1** Protocol for molybdenum purification</caption><thead><tr><th colspan="3">a. Cation exchange separation procedure: 15 mL AG50W-X8 200-400 mesh</th></tr></thead><tbody><tr><td rowspan="4">Cleaning</td><td>6M HCl</td><td>15 mL</td></tr><tr><td>Milli-Q H<sub>2</sub>O</td><td>15 mL</td></tr><tr><td>6M HCl</td><td>15 mL</td></tr><tr><td>Milli-Q H<sub>2</sub>O</td><td>15 mL</td></tr><tr><td>Conditioning</td><td>0.6M HCl</td><td>2 x 15 mL</td></tr><tr><td>Loading</td><td>0.6M HCl</td><td>10 mL</td></tr><tr><td>Mo Elution</td><td>0.6M HCl</td><td>2 x 15 mL</td></tr></tbody><thead><tr><th colspan="3">b. Anion exchange separation procedure: 2 mL AG1-X8 200-400 mesh</th></tr></thead><tbody><tr><td rowspan="2">Cleaning</td><td>6M HNO<sub>3</sub> + 3M HF</td><td>10 mL</td></tr><tr><td>Milli-Q H<sub>2</sub>O</td><td>10 mL</td></tr><tr><td>Conditioning</td><td>1M HF + 0.02 % H<sub>2</sub>O<sub>2</sub></td><td>10 mL</td></tr><tr><td>Loading</td><td>1M HF + 0.02 % H<sub>2</sub>O<sub>2</sub></td><td>2 mL</td></tr><tr><td>Elution 1: Ru, Cr, Fe, Mn</td><td>1M HF + 0.02 % H<sub>2</sub>O<sub>2</sub></td><td>10 mL</td></tr><tr><td>Elution 2: Zr, Mg, Al, Ca, Ti, V, Mn, Fe, Ru</td><td>9M HCl + 0.05M HF</td><td>8 mL</td></tr><tr><td>Elution 3: W, Ti, Ru, Zr, Mg</td><td>9M HCl + 1M HF</td><td>16 mL</td></tr><tr><td>Elution 4: U</td><td>0.05M HNO<sub>3</sub> + 0.03M HF</td><td>10 mL</td></tr><tr><td>Elution 5: Mo</td><td>6M HNO<sub>3</sub> + 3M HF</td><td>10 mL</td></tr></tbody><thead><tr><th colspan="3">c. Second anion exchange separation procedure: 1 mL AG1-X8 200-400 mesh</th></tr></thead><tbody><tr><td rowspan="2">Cleaning</td><td>6M HNO<sub>3</sub> + 3M HF</td><td>4 mL</td></tr><tr><td>Milli-Q H<sub>2</sub>O</td><td>4 mL</td></tr><tr><td>Conditioning</td><td>1M HF + 0.02 % H<sub>2</sub>O<sub>2</sub></td><td>5 mL</td></tr><tr><td>Loading</td><td>1M HF + 0.02 % H<sub>2</sub>O<sub>2</sub></td><td>1 mL</td></tr><tr><td>Elution 1: Nb</td><td>6M HCl + 0.1M HF</td><td>7 mL</td></tr><tr><td>Elution 2: Mo</td><td>0.6M HCl</td><td>20 mL</td></tr></tbody><thead><tr><th colspan="3">d. Second cation exchange separation procedure: 1 mL AG50W-X8 200-400 mesh</th></tr></thead><tbody><tr><td rowspan="4">Cleaning</td><td>6M HCl</td><td>5 mL</td></tr><tr><td>Milli-Q H<sub>2</sub>O</td><td>5 mL</td></tr><tr><td>6M HCl</td><td>5 mL</td></tr><tr><td>Milli-Q H<sub>2</sub>O</td><td>5 mL</td></tr><tr><td>Conditioning</td><td>0.6M HCl</td><td>10 mL</td></tr><tr><td>Loading</td><td>0.6M HCl</td><td>0.5 mL</td></tr><tr><td>Mo elution</td><td>0.6M HCl</td><td>2.5 mL</td></tr></tbody></table>

<table><caption>**Table 2** Faraday cup configuration used for multi-dynamic data acquisition</caption><thead><tr><th>Cups</th><th>L4</th><th>L3</th><th>L2</th><th>L1</th><th>C</th><th>H1</th><th>H2</th><th>H3</th><th>H4</th></tr><tr><th>Amplifiers</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>11</sup> Ω</th><th>10<sup>12</sup> Ω</th></tr></thead><tbody><tr><td>Line 1</td><td><sup>92</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>94</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>95</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>96</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>97</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>98</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>99</sup>Ru<sup>16</sup>O<sub>3</sub></td><td><sup>100</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>100</sup>Mo<sup>18</sup>O<sub>16</sub>O<sup>16</sup>O</td></tr><tr><td>Line 2</td><td><sup>93</sup>Nb<sup>16</sup>O<sub>3</sub></td><td><sup>95</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>96</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>97</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>98</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>99</sup>Ru<sup>16</sup>O<sub>3</sub></td><td><sup>100</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>101</sup>Ru<sup>16</sup>O<sub>3</sub></td><td></td></tr><tr><td>Line 3</td><td><sup>91</sup>Zr<sup>16</sup>O<sub>3</sub></td><td><sup>93</sup>Nb<sup>16</sup>O<sub>3</sub></td><td><sup>94</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>95</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>96</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>97</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>98</sup>Mo<sup>16</sup>O<sub>3</sub></td><td><sup>99</sup>Ru<sup>16</sup>O<sub>3</sub></td><td><sup>101</sup>Ru<sup>16</sup>O<sub>3</sub></td></tr></tbody></table>
<table><caption>**Table 3** Isobars of MoO₃ used in the oxygen isotope correction*</caption><thead><tr><th>Mass</th><th>140</th><th>141</th><th>142</th><th>143</th><th>144</th><th>145</th></tr></thead><tbody><tr><th>Species</th><td><sup>92</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td><td></td><td><sup>94</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td><td><sup>95</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td><td><sup>96</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td><td><sup>97</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td></tr><tr><th>O interferences</th><td></td><td><sup>92</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td><sup>92</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>92</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>92</sup>Mo<sup>16</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>92</sup>Mo<sup>17</sup>O<sup>18</sup>O<sup>18</sup>O</td></tr><tr><th></th><td></td><td></td><td><sup>92</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td><sup>92</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>92</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>18</sup>O</td><td></td></tr><tr><th></th><td></td><td></td><td></td><td><sup>94</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td><sup>94</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>94</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>17</sup>O</td></tr><tr><th></th><td></td><td></td><td></td><td></td><td><sup>94</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td><sup>94</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>18</sup>O</td></tr><tr><th></th><td></td><td></td><td></td><td></td><td></td><td><sup>95</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td></tr><tr><th></th><td></td><td></td><td></td><td></td><td></td><td><sup>95</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td></tr><tr><th></th><td></td><td></td><td></td><td></td><td></td><td><sup>96</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td></tr><tr><th>Mass</th><th>146</th><th>147</th><th>148</th><th>149</th><th>150</th><th></th></tr><tr><th>Species</th><td><sup>98</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td><td></td><td><sup>100</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>16</sup>O</td><td></td><td></td><td></td></tr><tr><th>O interferences</th><td><sup>92</sup>Mo<sup>18</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>94</sup>Mo<sup>17</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>94</sup>Mo<sup>18</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>95</sup>Mo<sup>18</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>96</sup>Mo<sup>18</sup>O<sup>18</sup>O<sup>18</sup>O</td><td></td></tr><tr><th></th><td><sup>94</sup>Mo<sup>16</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>95</sup>Mo<sup>16</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>95</sup>Mo<sup>17</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>96</sup>Mo<sup>17</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>97</sup>Mo<sup>17</sup>O<sup>18</sup>O<sup>18</sup>O</td><td></td></tr><tr><th></th><td><sup>94</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>95</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>96</sup>Mo<sup>16</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>97</sup>Mo<sup>16</sup>O<sup>18</sup>O<sup>18</sup>O</td><td><sup>98</sup>Mo<sup>16</sup>O<sup>18</sup>O<sup>18</sup>O</td><td></td></tr><tr><th></th><td><sup>95</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>96</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>96</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>97</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>98</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>18</sup>O</td><td></td></tr><tr><th></th><td><sup>95</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>96</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>97</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>98</sup>Mo<sup>17</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>100</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>17</sup>O</td><td></td></tr><tr><th></th><td><sup>96</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>97</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>97</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>98</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>18</sup>O</td><td><sup>100</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td></td></tr><tr><th></th><td><sup>96</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td><sup>97</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td><sup>98</sup>Mo<sup>16</sup>O<sup>17</sup>O<sup>17</sup>O</td><td><sup>100</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td></td><td></td></tr><tr><th></th><td><sup>97</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td><sup>98</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td><sup>98</sup>Mo<sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td></td><td></td><td></td></tr></tbody></table>

*See text. The isobars that only contain <sup>16</sup>O are those that can be used for deriving the Mo isotope compositions. Some isobars (marked in grey) have a very low abundance and their interferences can be neglected. Other isobaric molecules are present in significant amount and requires interference correction.

426

427

428

429

430

431

432

433

<table><caption>**Table 4** Multidynamic Mo isotopic data for repeated analysis of the NIST 3134 standard solution, the NIST 3134 standard solution after it was purified via our chemistry protocol, and terrestrial standard rock powders (BCR-1, BCR-2). Isotope ratios are shown here as µ<sup>x</sup>Mo, which corresponds to the deviation, in part per million (ppm), of the measured ratio of <sup>x</sup>Mo over <sup>96</sup>Mo, relative to the average <sup>1</sup>Mo/<sup>96</sup>Mo for repeated analysis of the purified NIST standard (n = 16)</caption><thead><tr><th></th><th>µ<sup>94</sup>Mo</th><th>µ<sup>95</sup>Mo</th><th>µ<sup>97</sup>Mo</th><th>µ<sup>100</sup>Mo</th></tr></thead><tbody><tr><th>Mo NIST-1</th><td>23.2 ± 4.4</td><td>7.4 ± 3.8</td><td>-1.7 ± 2.3</td><td>25.7 ± 4.2</td></tr><tr><th>Mo NIST-2</th><td>20.8 ± 4.6</td><td>2.9 ± 3.9</td><td>0.8 ± 2.6</td><td>23.9 ± 4.4</td></tr><tr><th>Mo NIST-3</th><td>23.1 ± 4.6</td><td>1.8 ± 3.8</td><td>-2.3 ± 2.3</td><td>22.3 ± 4.3</td></tr><tr><th>Mo NIST-4</th><td>22.6 ± 3.5</td><td>3.0 ± 3.0</td><td>-0.6 ± 1.9</td><td>24.0 ± 3.3</td></tr><tr><th>Mo NIST-5</th><td>19.4 ± 3.7</td><td>6.6 ± 3.1</td><td>0.4 ± 2.0</td><td>21.7 ± 3.5</td></tr><tr><th>Mo NIST-6</th><td>24.8 ± 4.2</td><td>9.6 ± 3.3</td><td>-2.2 ± 2.1</td><td>26.1 ± 3.8</td></tr><tr><th>Mo NIST-7</th><td>15.7 ± 3.8</td><td>3.8 ± 3.2</td><td>-2.3 ± 2.1</td><td>18.5 ± 3.6</td></tr><tr><th>Mo NIST-8</th><td>21.4 ± 3.9</td><td>3.1 ± 3.3</td><td>-0.8 ± 2.2</td><td>22.9 ± 3.7</td></tr><tr><th>Mo NIST-9</th><td>16.3 ± 4.9</td><td>6.1 ± 3.6</td><td>-0.1 ± 2.3</td><td>15.2 ± 4.1</td></tr><tr><th>Mo NIST-10</th><td>21.2 ± 4.5</td><td>5.8 ± 3.9</td><td>0.1 ± 2.6</td><td>21.1 ± 4.5</td></tr><tr><th>Mo NIST-11</th><td>17.2 ± 4.6</td><td>9.7 ± 3.9</td><td>-3.0 ± 2.5</td><td>16.5 ± 4.5</td></tr><tr><th>Mo NIST-12</th><td>15.5 ± 6.3</td><td>7.3 ± 5.0</td><td>-1.9 ± 3.5</td><td>17.6 ± 5.9</td></tr><tr><th>Mo NIST-13</th><td>25.5 ± 6.9</td><td>3.1 ± 6.1</td><td>-2.2 ± 3.9</td><td>19.7 ± 6.7</td></tr><tr><th>Mo NIST-14</th><td>17.2 ± 4.6</td><td>9.7 ± 3.9</td><td>-3.0 ± 2.5</td><td>16.5 ± 4.5</td></tr><tr><th>Mo NIST-15</th><td>17.6 ± 4.1</td><td>6.8 ± 3.3</td><td>-1.2 ± 2.2</td><td>18.8 ± 3.8</td></tr><tr><th>Mo NIST-16</th><td>14.2 ± 6.9</td><td>0.1 ± 5.5</td><td>-0.3 ± 3.4</td><td>25.2 ± 6.5</td></tr></tbody></table>
<table><thead><tr><th></th><th>μ<sup>94</sup>Mo</th><th>μ<sup>95</sup>Mo</th><th>μ<sup>97</sup>Mo</th><th>μ<sup>100</sup>Mo</th></tr></thead><tbody><tr><td>Mo NIST-17</td><td>23.4 ± 4.7</td><td>6.0 ± 3.9</td><td>-1.5 ± 2.4</td><td>23.9 ± 4.7</td></tr><tr><td>Mo NIST-18</td><td>27.9 ± 4.8</td><td>9.9 ± 4.2</td><td>1.2 ± 2.6</td><td>20.4 ± 4.5</td></tr><tr><td>Mo NIST-19</td><td>19.2 ± 5.1</td><td>3.2 ± 4.3</td><td>-2.3 ± 2.7</td><td>29.3 ± 5.3</td></tr><tr><td>Mo NIST-20</td><td>23.5 ± 5.3</td><td>8.2 ± 4.5</td><td>-2.3 ± 2.8</td><td>33.1 ± 5.2</td></tr><tr><td>Mo NIST-21</td><td>28.1 ± 6.3</td><td>7.3 ± 5.3</td><td>-3.9 ± 3.4</td><td>31.5 ± 5.7</td></tr><tr><td>Mo NIST-22</td><td>19.0 ± 4.6</td><td>6.3 ± 4.1</td><td>-2.9 ± 2.5</td><td>22.9 ± 4.3</td></tr><tr><td>Mo NIST-23</td><td>24.0 ± 6.2</td><td>4.1 ± 5.1</td><td>-4.6 ± 3.2</td><td>19.9 ± 5.7</td></tr><tr><td>Mo NIST-24</td><td>27.9 ± 4.5</td><td>7.7 ± 4.1</td><td>-1.5 ± 2.6</td><td>30.3 ± 4.2</td></tr><tr><td>Mo NIST-25</td><td>28.2 ± 5.3</td><td>11.2 ± 4.6</td><td>-2.2 ± 2.8</td><td>20.9 ± 5.1</td></tr><tr><td>Mo NIST-26</td><td>27.2 ± 4.1</td><td>9.9 ± 3.6</td><td>-3.1 ± 2.2</td><td>28.9 ± 5.6</td></tr><tr><td><strong>NIST average ± 2σ SD (n = 26)</strong></td><td><strong>21.7 ± 6.1</strong></td><td><strong>6.2 ± 5.8</strong></td><td><strong>-1.7 ± 2.9</strong></td><td><strong>23.0 ± 9.6</strong></td></tr><tr><td>Mo NIST purified once-1</td><td>-1.5 ± 3.8</td><td>-0.6 ± 3.2</td><td>-4.1 ± 2.0</td><td>-5.6 ± 3.6</td></tr><tr><td>Mo NIST purified once-2</td><td>1.6 ± 4.1</td><td>-0.9 ± 3.7</td><td>-2.0 ± 2.2</td><td>-1.2 ± 3.9</td></tr><tr><td>Mo NIST purified once-3</td><td>-3.6 ± 4.8</td><td>5.2 ± 4.2</td><td>-2.5 ± 2.7</td><td>2.7 ± 4.9</td></tr><tr><td>Mo NIST purified once-4</td><td>-2.3 ± 4.1</td><td>-0.1 ± 3.4</td><td>-2.9 ± 2.1</td><td>-0.6 ± 3.8</td></tr><tr><td>Mo NIST purified once-5</td><td>3.5 ± 4.4</td><td>1.8 ± 3.8</td><td>1.2 ± 2.4</td><td>-4.9 ± 4.1</td></tr><tr><td>Mo NIST purified once-6</td><td>-2.3 ± 8.5</td><td>5.0 ± 4.8</td><td>0.8 ± 3.0</td><td>5.1 ± 5.4</td></tr><tr><td>Mo NIST purified once-7</td><td>-0.2 ± 4.2</td><td>-0.7 ± 3.5</td><td>0.7 ± 2.2</td><td>-3.5 ± 4.0</td></tr><tr><td>Mo NIST purified once-8</td><td>5.6 ± 4.9</td><td>-4.2 ± 4.3</td><td>-0.5 ± 2.8</td><td>0.5 ± 4.6</td></tr><tr><td>Mo NIST purified once-9</td><td>0.2 ± 4.4</td><td>4.2 ± 3.8</td><td>0.1 ± 2.4</td><td>-2.0 ± 4.5</td></tr><tr><td>Mo NIST purified once-10</td><td>5.8 ± 4.2</td><td>3.2 ± 3.3</td><td>2.3 ± 2.1</td><td>-2.0 ± 3.8</td></tr><tr><td>Mo NIST purified once-11</td><td>1.3 ± 3.7</td><td>-1.2 ± 3.2</td><td>0.2 ± 2.1</td><td>-3.2 ± 3.6</td></tr><tr><td>Mo NIST purified twice-1</td><td>-0.3 ± 6.1</td><td>-0.1 ± 4.9</td><td>-0.7 ± 3.3</td><td>5.5 ± 5.7</td></tr><tr><td>Mo NIST purified twice-2</td><td>-0.1 ± 6.3</td><td>-2.7 ± 5.3</td><td>2.3 ± 3.4</td><td>5.3 ± 6.3</td></tr><tr><td>Mo NIST purified twice-3</td><td>-3.1 ± 4.7</td><td>1.1 ± 4.0</td><td>3.8 ± 2.4</td><td>3.5 ± 4.5</td></tr><tr><td>Mo NIST purified twice-4</td><td>0.3 ± 4.6</td><td>-6.5 ± 4.2</td><td>-0.3 ± 2.5</td><td>-3.5 ± 4.5</td></tr><tr><td>Mo NIST purified twice-5</td><td>-4.8 ± 4.3</td><td>-3.3 ± 3.5</td><td>1.5 ± 2.4</td><td>3.9 ± 3.9</td></tr><tr><td><strong>Purified NIST average ± 2σ SD (n = 16)</strong></td><td><strong>0.0 ± 8.6</strong></td><td><strong>0.0 ± 6.6</strong></td><td><strong>0.0 ± 4.2</strong></td><td><strong>0.0 ± 7.6</strong></td></tr><tr><td>BCR-2 (with precipitation)</td><td>-0.4 ± 5.9</td><td>-4.3 ± 4.9</td><td>-5.0 ± 3.5</td><td>0.6 ± 5.7</td></tr><tr><td>BCR-2 (without precipitation)</td><td>1.2 ± 3.7</td><td>-2.6 ± 3.2</td><td>-3.4 ± 2.0</td><td>7.0 ± 3.6</td></tr><tr><td>BCR-2 (without precipitation)</td><td>4.2 ± 4.1</td><td>2.3 ± 3.4</td><td>-4.9 ± 2.3</td><td>-0.4 ± 3.7</td></tr><tr><td>BCR-2 (without precipitation)</td><td>5.6 ± 3.9</td><td>1.0 ± 3.1</td><td>-3.0 ± 2.1</td><td>8.7 ± 3.9</td></tr><tr><td><strong>BCR-2 average ± 2σ SD (n = 4)</strong></td><td><strong>2.3 ± 4.6</strong></td><td><strong>-1.0 ± 6.0</strong></td><td><strong>-4.0 ± 2.3</strong></td><td><strong>4.4 ± 10.4</strong></td></tr><tr><td>BCR-1</td><td>-2.0 ± 4.2</td><td>-3.6 ± 3.5</td><td>4.6 ± 2.2</td><td>3.0 ± 4.0</td></tr><tr><td><strong>BCR average ± 2σ SD (n = 5)</strong></td><td><strong>1.7 ± 6.3</strong></td><td><strong>-1.4 ± 5.9</strong></td><td><strong>-2.3 ± 8.0</strong></td><td><strong>3.8 ± 7.9</strong></td></tr></tbody></table>
<table><caption>**Table 5** Multidynamic Mo isotopic data for repeated analysis of the synthetic solution enriched in <sup>97</sup>Mo and <sup>100</sup>Mo.</caption><thead><tr><th></th><th>μ<sup>94</sup>Mo</th><th>μ<sup>95</sup>Mo</th><th>μ<sup>97</sup>Mo</th><th>μ<sup>100</sup>Mo</th></tr></thead><tbody><tr><td>Mix NIST + DS - 1</td><td>-0.2 ± 4.2</td><td>2.9 ± 3.5</td><td>19.9 ± 2.0</td><td>34.3 ± 3.9</td></tr><tr><td>Mix NIST + DS - 2</td><td>8.9 ± 4.0</td><td>0.8 ± 3.5</td><td>20.8 ± 1.7</td><td>35.7 ± 3.9</td></tr><tr><td>Mix NIST + DS - 3</td><td>2.2 ± 3.0</td><td>0.0 ± 2.5</td><td>21.6 ± 1.2</td><td>31.9 ± 2.8</td></tr><tr><td>Mix NIST + DS - 4</td><td>-0.8 ± 3.1</td><td>-3.3 ± 2.7</td><td>24.6 ± 1.4</td><td>31.1 ± 3.0</td></tr><tr><td>Mix NIST + DS - 5</td><td>-1.8 ± 3.0</td><td>-1.2 ± 2.5</td><td>21.4 ± 1.3</td><td>27.5 ± 2.8</td></tr><tr><td>Mix NIST + DS - 6</td><td>1.6 ± 3.2</td><td>2.2 ± 2.6</td><td>19.6 ± 1.5</td><td>30.6 ± 3.0</td></tr><tr><td>Average Mix NIST + DS (± 2 SD)</td><td>1.7 ± 7.7</td><td>0.2 ± 4.6</td><td>21.3 ± 3.6</td><td>31.9 ± 5.8</td></tr><tr><td>Predicted values</td><td>0.0</td><td>0.1</td><td>19.7</td><td>27.8</td></tr></tbody></table>
<figure><img src="image_5.png" /></figure>