Accurate Determination of the Absolute Isotopic Composition and

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Accurate determination of the absolute isotopic composition and atomic weight of molybdenum by MC-ICP-MS with a fully calibrated strategy Panshu Song, Jun Wang, Tongxiang Ren, Tao Zhou, Yuanjing Zhou, and Song Wang Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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Analytical Chemistry

Accurate determination of the absolute isotopic composition and atomic weight of molybdenum by MC-ICP-MS with a fully calibrated strategy Panshu Song, Jun Wang∗, Tongxiang Ren, Tao Zhou, Yuanjing Zhou, Song Wang National Institute of Metrology China, Beijing 100029, P. R. China ∗ To whom correspondence may be addressed (E-mail: [email protected]; Phone: 86-10-84251244. Fax: 86-10-642716939)

ABSTRACT: A fully calibrated strategy has been investigated for the first time for the accurate determination of absolute isotopic composition and atomic weight of molybdenum using multiple-collector inductively coupled plasma mass spectrometry. The correction for instrumental mass bias was performed using synthetic isotope mixtures, which were gravimetrically prepared with all of the seven high purity and isotopically enriched molybdenum isotope materials together. Six natural molybdenum materials, including molybdenum standard solution NIST SRM 3134, were accurately measured and yielded the absolute isotopic composition (in at. %, k=1) of 92Mo-14.690(18), 94Mo-9.173(6), 95Mo-15.865(5), 96Mo-16.666(3), 97Mo-9.588(4), 98Mo-24.307(16), and 100Mo9.711(13). These isotopic data enable an atomic weight Ar (Mo) of 95.9466(34) (k=2) to be calculated, which is slightly lower than the current standard atomic weight 95.95(1) and with a much improved uncertainty. The associated uncertainties were evaluated according to the Guide to Expression of Uncertainty in Measurement of ISO/BIPM and Monte Carlo simulation to ensure that all sources of uncertainty were fully accounted for. A particular characteristic of the proposed new approach is that mass bias correction factor K for each isotope ratio of molybdenum can be achieved via fully experimental determination without using the traditional semi-empirical correction mathematical models. In addition, the relationship between mass of isotope and bias per mass unit β was investigated based on the thorough measurement data.

Molybdenum has seven naturally occurring isotopes, including 92 Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, and 100Mo. As a redoxsensitive trace metal, it is of considerable geochemical and geological importance. The investigation of natural variations in the isotopic composition of Mo is shown to be a powerful proxy for indicating paleo-oxidation state of the oceans and deciphering genesis of sulphide ore deposits.1-3 Molybdenum is also applied as an isotopic tracer in biochemistry and environmental studies, where it is employed to understand metabolic kinetics of molybdenum in life as well as to locate sources of pollutants in the nature.4-6 Improvements in mass spectrometry such as the advent of Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) have facilitated the recent determinations of molybdenum isotopic compositions.7-10 Compared with Thermal Ionization Mass Spectrometry (TIMS), MCICP-MS provides higher sensitivity and comparable or even greater precision for isotope ratio measurements. Besides, it offers particular advantage for the acquisition of accurate and precise isotope ratios for elements with high first ionization potentials, such as Mo (7.1eV). Nevertheless, the use of MCICP-MS also leads to larger instrumental mass bias effect, which greatly affect the accurate measurements of isotope ratios. The correction for mass bias effect during MC-ICP-MS measurement for molybdenum has been previously reported using doping with an external element (Zr, Ru, Pd)11-13 or double spike technique.14-15 However, these doping elements may behave differently with molybdenum in both plasma and inter-

face owing to the difference in elemental specialties.16-18 The presently-accepted standard atomic weight of molybdenum of Ar - 95.95(1) was released by CIAAW in 2015,19 based on Mayer and Wieser’s study using MC-ICP-MS in 2014.14 The absolute isotopic composition of NIST SRM 3134 was determined by a double spike technique with isotopically enriched isotopes 92Mo and 98Mo in their study. For mass bias correction, only one pair of molybdenum isotope ratio was calibrated with isotope mixtures, other isotope ratios were calculated by extrapolation from the single calibrated ratio by assuming mass-dependent isotope fractionation, and the bias per mass unit β was designated as a constant value for different molybdenum isotope ratios. However, to date, the details of the mass bias behavior during MC-ICP-MS measurement are still incompletely characterized. Therefore, it may be classified as partially calibrated because only two enriched isotopes are used to establish the calibration factors of all molybdenum isotopes. In recent years, the use of gravimetrically prepared isotope mixtures to calibrate instrumental mass bias has been considered as an authority calibration approach for the determination of absolute isotopic compositions.20-24 Most of previous studies used partially calibrated methods and traditional semiempirical correction mathematical models to establish the calibration factors of all molybdenum isotopes.18 In 2015, Malinovsky published a MC-ICP-MS measurement of absolute molybdenum isotope amount ratios using calibration with six synthetic isotope mixtures, which were prepared by mixing two isotopically enriched materials 95Mo and XMo (X= 92, 94,

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96, 97, 98, and 100) respectively.25 This approach avoided any assumption on mass-dependent isotope fractions in MC-ICPMS, inherent to the method of double spike. However, in order to obtain mass bias correction factors for all isotope ratios, six synthetic mixtures had to be successively measured one by one, which may induce measurement uncertainty caused by drift of the instrument and fluctuation of the mass discrimination. Up to now, it is very rare that mass bias correction factor K for each isotope ratio of element are obtained via fully experimental determination. In order to improve measurement uncertainty as well as enhance trueness of the result, the applicability of the traditional semi-empirical correction models for MC-ICP-MS also need to be verified through reliable experimental determination and rigorous assessment. In this work, we report the first fully calibrated strategy mass spectrometry for the measurement of absolute isotopic composition and atomic weight of molybdenum using MCICP-MS. All of the seven isotopically enriched molybdenum materials 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, and 100Mo, were provided together for gravimetrically preparing eight synthetic isotope mixtures with isotope ratios of R(XMo/95Mo) approximating 0.05-15. Therefore, the reference values for all molybdenum isotope ratios in these mixtures were able to be acquired directly according to the gravimetric data and isotopic abundances of isotopically enriched molybdenum. The correction factors of mass bias effect for all molybdenum isotope ratios were simultaneously determined with one synthetic mixture. This approach allowed avoiding the need to make any assumption of correction models for mass bias in MC-ICP-MS used everywhere. Through this approach, the absolute isotopic compositions and atomic weight for six natural molybdenum materials, including NIST SRM 3134 standard solution used as anchor point for the molybdenum delta scales were determined accurately. The measurement uncertainties were evaluated by using both GUM principle and Monte Carlo simulation to ensure that all sources of uncertainty were fully accounted for. In addition, based on the fully calibrated strategy and experimental determination data of the synthetic mixtures, relationship between the bias per mass unit β and isotope masses was illustrated.

EXPERIMENTAL SECTION

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ence materials GBW(E) 082428-GBW(E) 082431 including 69 elements were developed by National Institute of Metrology, China. Six natural molybdenum materials were investigated in this work, including: (1) Mo reference solution NIST SRM 3134 (10000 µg·g-1, Lot 891307), (2) Mo plasma standard solution JMC 38719 from Alfa Aesar- A Johnson Matthey Company (1000 µg·g-1, Lot 635979), (3) High purity MoO3 powder from STREM Chemicals Inc. (99.99%, Lot 23380800), (4) Single-element standard solution of Mo GBW(E) 080218 produced by National Institute of Metrology, China (100 µg·g1 , Lot 14061), (5) High purity MoO3 powder from Alfa AesarA Johnson Matthey Company (99.998%, Lot 24038), (6) High purity Mo metal from Sigma-Aldrich Co. LLC. (99.99%, Lot MKBL5672V) Purification of isotopically enriched Mo materials. Elemental impurities in the seven isotopically enriched molybdenum materials were determined by multi-element screen analysis of 68 elements using a High Resolution-Inductively Coupled Plasma Mass Spectrometry (Element II, Thermo Fisher Scientific, Germany). Typically, 1000 µg·g-1 enriched Mo solution was used for HR-ICP-MS measurement to ensure that all potential impurities were in detected range. External calibration was conducted for all elements, multi-element certified reference materials were diluted to 1, 5, 10 µg·kg-1 and used. To resolve the interferences in ICP-MS, the isotope with least affected by spectral interferences for each element was selected to test. High resolution was chosen to determine K, Ge, As, and Se, and medium resolution for other elements analysis. The results showed that the impurity contents of feed enriched Mo materials were 1140 µg·g-1 for 97Mo, and 100 µg·g-1 for 94Mo, respectively. The impurity contents of other feed enriched molybdenum materials were within this range. Therefore, all isotopically enriched molybdenum materials except 94Mo were purified by a vacuum sublimation method following the procedures described in our previous work.26 Firstly, feed enriched molybdenum materials in the form of metal powder were oxidized in a muffle furnace at about 600 º C for 8 h to form MoO3 powder. Then, the sublimation of MoO3 took place in a specially designed quartz casing at a temperature of 625-650 ºC and a pressure of 10−5 mbar for 2 h, in which MoO3 powder was placed in a quartz pan (heating center) and condensed at a chilled quartz tube mounted approx. 6 cm away from the quartz pan. Owing to the difference between vapor pressures of the individual elements, metallic and non-metallic impurities as well as gaseous traces were eliminated from the feed enriched molybdenum materials during these vacuum sublimation process. The product was white needle-like crystals. A maximum of 120mg feed enriched molybdenum materials could be purified in each purification process. After purification, the mass fractions of 68 elements in enriched molybdenum materials were determined by HR-ICPMS as described above. Dominant impurities, such as Ba, Cu, Fe, and Si, in enriched molybdenum materials were determined individually by external calibration method using

Reagents and materials. Nitric and hydrochloric acids were purified in-house prior to use by sub-boiling distillation of analytical-reagent grade raw materials (Beijing institute of chemical reagents, China). High purity ammonia hydroxide (≥99.99%) was purchased from Sigma-Aldrich Chemical Co. (USA). Ultrapure water (18 MΩ·cm) was obtained from a Milli-Q system (Millipore Corp., USA). All lab ware including quartz vessels and disposable plastics were cleaned by concentrated purified nitric acids and ultrapure water prior to use. Seven isotopically enriched molybdenum materials (enrichment degrees of 92.0%-99.4%) in the form of MoO3 powder for 92Mo, and Mo metal powder for 94Mo, 95Mo, 96Mo, 97 Mo, 98Mo and 100Mo, were purchased from Oak Ridge National Laboratory (USA). Four multi-element certified referTable 1. Purity of isotopically enriched Mo materials for preparing primary solutions PS-92Mo, PS-94Mo, PS-95Mo, PS-96Mo, PS97 Mo, PS-98Mo, and PS-100Mo Sample Purity (%) a

PS-92Mo 99.99(1) a

PS-94Mo 99.99(1)

PS-95Mo 99.99(1)

PS-96Mo 99.99(1)

PS-97Mo 99.98(1)

PS-98Mo 99.99(1)

PS-100Mo 99.99(1)

The combined standard uncertainties uc are given in parentheses.

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corresponding single element certified reference material to achieve low uncertainty. As summarized in Table 1, the purity of enriched 92Mo, 95Mo, 96Mo, 98Mo and 100Mo materials could achieve 99.99% after vacuum sublimation process. However, for enriched 97Mo, which had the least purity of feed material, the purity could only reach to 99.98% after purification. Dominant impurities Ba and Si in purified enriched 97Mo were determined individually. Isobaric elements Zr and Ru were not detected in all purified isotopically enriched molybdenum materials. Preparation of primary solutions for isotopically enriched molybdenum. Each of the purified isotopically enriched MoO3 materials (92Mo, 95Mo, 96Mo, 97Mo, 98Mo, 100Mo) and enriched 94Mo metal powder was filled into a small quartz vessel, which was weighed prior to use. The weighing process was operated on a Mettler-Toledo UMX2 balance with a maximum load of 5.1 g and sensitivity of 0.1 µg. To dissolve isotopically enriched molybdenum in the form of MoO3, 1.5 mL of water and 2.8 mL of high-purity NH3·OH were added into the vessel. After the complete dissolution of the sample, the vessel was placed on a hot plate and kept there to evaporate the solution to near dryness. Then, the sample was re-dissolved in a certain amount of 8 M HNO3 and quantitatively transferred to volumetric flask. Any remaining droplets of Mo-containing solution were collected into the same volumetric flask by rinsing the vessels with 10 mL of water. Table 2. Masses of isotopically enriched molybdenum materials for preparing primary solutions Sample 92 Mo 94 Mo 95 Mo 96 Mo 97 Mo 98 Mo 100 Mo a

Enriched isotopes (mg) 273.9707(9)a 71.4782(6) 348.2640(9) 99.0528(6) 128.0008(6) 358.2113(9) 355.0863(9)

Solution (g) 47.13328(6) 13.41615(2) 58.06827(6) 16.52851(2) 21.89385(6) 59.74326(6) 59.27729(6)

pared aqua regia into the vessel. Then, the vessel was placed on the hot plate to evaporate the solution to near dryness. High-purity NH3·OH was added to re-dissolve the sample for ensuring that the final existing form of enriched 94Mo in primary solution was in good agreement with other enriched molybdenum materials. The final weight of the primary solution was accurately measured by using a Mettler-Toledo XP205 balance with a maximum load of 220 g and a sensitivity of 0.01 mg. The blank sample was prepared in parallel under the same operations to involve any potential contamination during the preparation process of primary solution. Climatic conditions such as temperature, pressure and relative humidity were recorded constantly during the weighing procedure and applied to the air buoyancy correction for all weighing data. Masses for preparing each primary solution are summarized in Table 2. Preparation of synthetic isotope mixtures. Primary solutions of seven isotopically enriched Mo were filled into purified dry plastic bottles (5 mL, with a sucker), respectively. For gravimetrically preparing synthetic isotope mixtures, the desired amount of each primary solution was accurately weighed on the XP205 balance and mixed together in a volumetric flask. Air buoyancy correction was carried out for all weighing results. Eight synthetic isotope mixtures with molybdenum isotope ratios close to natural molybdenum material were prepared carefully. The concentrations and weighting data of the enriched molybdenum primary solutions used for preparing synthetic mixtures are listed in Table 3-4. Table 3. Concentrations of molybdenum in primary solutions and dilution (PS-95Modilution) of enriched 95Mo primary solution Primary solutions PS-92Mo PS-94Mo PS-95Mo PS-95Modilution PS-96Mo PS-97Mo PS-98Mo PS-100Mo


Isotopically enriched 94Mo metal powder was dissolved by adding 1.0 mL of water and 1.5 mL solutions of freshly pre-

a


Table 4. Masses of primary solutions and diluted primary solution for producing synthetic mixtures 1-8 Mass of primary solutions and diluted primary solution (g) Mix-1 Mix-2 Mix-3 Mix-4 Mix-5 Mix-6 PS-92Mo 0.35707(1) a 0.38106(1) 0.53487(1) 0.39286(1) 0.53723(1) 0.57099(1) PS-94Mo 0.11589(1) 0.14470(1) 0.19437(1) 0.18258(1) 0.19867(1) 0.21125(1) PS-95Mo 3.60844(1) 1.25315(1) 0.62710(1) 0.39661(1) 0.24090(1) PS-95Modilution 0.41552(1) PS-96Mo 0.29450(1) 0.36351(1) 0.49898(1) 0.42700(1) 0.51848(1) 0.55538(1) PS-97Mo 0.17210(1) 0.18572(1) 0.27933(1) 0.25327(1) 0.26995(1) 0.28604(1) PS-98Mo 0.48536(1) 0.54483(1) 0.74729(1) 0.61008(1) 0.75835(1) 0.79987(1) PS-100Mo 0.16414(1) 0.18131(1) 0.25766(1) 0.24429(1) 0.24893(1) 0.25482(1) a

[Mo] (µmol g-1) 41.184(4) a 56.636(6) 41.908 (4) 8.3952 (8) 41.636(4) 40.338(4) 41.095(4) 40.499(4)

Mix-7 0.61181(1) 0.22547(1)

Mix-8 0.78011(1) 0.29327(1)

0.21710(1) 0.59172(1) 0.30505(1) 0.85095(1) 0.27259(1)

0.18633(1) 0.73516(1) 0.37742(1) 1.07123(1) 0.36226(1)


Molybdenum isotope ratio measurements. The isotope ratios of isotopically enriched molybdenum primary solutions, synthetic isotope mixtures, and natural molybdenum materials were all measured by an Isoprobe MC-ICP-MS with hexapole collision cell (GV Instrument, UK). Typical operating conditions for the instrument are detailed in Table 5. Nine Faraday cups were utilized to collect the seven isotopes of molybdenum along with 90Zr+ and 99Ru+ simultaneously. This allowed for the monitoring of potential isobaric interferences

from 92Zr+ on 92Mo+, 94Zr+ on 94Mo+, 96Zr+ on 96Mo+, 98Ru+ on Mo+, and 100Ru+ on 100Mo+, but no evidence of isobaric interferences were ever observed for all samples. The measurements were performed in static mode with low mass resolution (～450). Amplifier gain calibration and optimization of the MC-ICP-MS were performed daily. Typically, the signal intensity of 98Mo+ in 400 ng·g-1 natural molybdenum solution could reach 1.5 V. A single measurement was run in 2 blocks of 10 cycles. After each measure98



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ment, the nebulizer and spray chamber were rinsed with 5% HNO3 until the signal intensity had dropped to the background level to eliminate the memory effect from previous samples. Then, the signal of procedure blank was recorded and subtracted from the measured signals of the sample. For testing the stability of MC-ICP-MS, Mo isotope ratios in SRM3134 were monitored over the whole measurements. The relative standard deviation of the average ratios of 92Mo/95Mo, 97 Mo/95Mo, and 100Mo/95Mo were 0.006%, 0.008%, and 0.01%, respectively. The possible instrumental drift was corrected using standard-sample bracketing, which was done by the measurement of samples and standard solution NIST SRM 3134 alternatively. Table 5. MC-ICP-MS operating conditions for Mo isotope ratio measurements Parameters RF power Ar cooling gas flow rate Intermediate gas Nebulization gas Collision gas Sample cones Skimmer cones Sample uptake rate Mass resolution Cup configuration

1310 W 13.2 L min-1 1.05 L min-1 0.66 L min-1 Ar 2.4 mL min-1 Ni sampler cone Ni skimmer cone 0.2 mL min-1 450 90 L3 Zr 92 L2 Mo AX 94Mo 95 H1 Mo 96 H2 Mo 97 H3 Mo 98 H4 Mo 99 H5 Ru H6 100Mo

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Calibration of instrumental mass bias during MC-ICP-MS measurements implied finding an accurate value of correction factor Kij for each isotope ratio observed. Eq. (1) was applied to calculate the correction factor Kij.

(

(1)

(

in which Rij(ref) represents the reference value of each isotope ratio, Rij(meas) represents the measured value of each isotope ratio. The eight gravimetrically prepared synthetic mixtures with reference molybdenum isotope ratios were served as primary measurement standards for obtaining the Kij values. Reference isotope ratios of the mixtures Rij(ref) was defined by the following equation: !("#$

/(

'( ∑') &( *+,

(2)

'(

∑') &( -.*+,

*+,0 10

2 /(

*+,3 13

2 /(

*+,4 14

2 /(

*+,5 15

jth enriched isotope. n and /(

*+,

0.9746828(20) 0.0070766(5) 0.005391169(12) 0.0069359(8) 0.9201818(16) 0.0520497(9) 0.0022830(10) 0.0056443(8) 0.965328(6) 0.001811(4) 0.0019525(20) 0.00898406(4) 0.002084(3) 0.0018970(22) 0.004650524(16) 0.0011831(12) 0.0009221(7) 0.001965110(3) PS-100Mo 0.0003199(7) 0.0002161(4) 0.0003881281(4) a The combined standard uncertainties uc are given in parentheses.

2 /(

*+,7 17

2 /(

could be calculated by Eq.

1 /(

9 : ;

*+,

(3)

∑'(

')

? are the atomic masses of isotopes i and j, respectively. According to the fully calibrated strategy and experimental determination data of eight synthetic mixtures, the relationship between β and the average mass of molybdenum isotope pairs in MC-ICP-MS was directly displayed for the first time (Figure 1). The observations suggest that the bias per mass unit β is not a constant value for all molybdenum isotope pairs in MC-ICP-MS, which might be the main reason for the difference between the isotopic compositions of NIST SRM 3134 determined in this work and that reported in the aforementioned study by means of double spike technique.14 Additionally, no obvious linear trend over the scale range of isotope masses is found in this illustration. How to confirm correction factors K of different isotope ratios in the partially calibrated MC-ICP-MS method is still worth for further study.

Figure 1. Variation in bias per mass unit (β) against the corresponding average mass of Mo isotope pairs. β was calculated using the synthetic mixtures according to Eq. (7) Uncertainty evaluation. In this study the relative contributions of individual uncertainty components were all evaluated according to the Guide to Expression of Uncertainty in Measurement of ISO/BIPM32,33 and Monte Carlo simulation34 as well. The measurement uncertainty of absolute isotopic com-



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position of each natural molybdenum sample included uncertainty of purity analysis of each isotopically enriched material, random errors and potential systematic errors in weighing operations, measurement precisions of isotope ratios of isotopically enriched materials, natural sample and calibration mixtures, as well as uncertainty of atomic mass of each isotope. The uncertainties resulted from the mathematical iterative

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process for calculate the isotopic abundances of the isotopically enriched materials were also took into account. For the uncertainty assessment of the average isotopic composition and average atomic weight of six natural molybdenum samples, besides the measurement uncertainties mentioned above, the standard deviation of the six samples due to variations between samples were also included.

Table 8. Calibrated isotope ratios of six natural Mo materials and the δ98Mo/95Mo values calculated by comparing with NIST SRM 3134

a

δ98Mo/95Mo, ‰

Natural Mo

92/95

94/95

96/95

97/95

98/95

100/95

SRM 3134

0.92599(16) a

0.57822(4)

1.05052(19)

0.60430(15)

1.5321(4)

0.61202(16)

JMC 38719

0.92559(16)

0.57818(4)

1.05062(19)

0.60450(15)

1.5326(4)

0.61242(16)

0.33(9)

STREM MoO3

0.92589(16)

0.57825(4)

1.05042(19)

0.60430(15)

1.5320(4)

0.61192(16)

-0.06(9)

GBW(E)080218

0.92699(16)

0.57841(4)

1.05042(19)

0.60390(15)

1.5305(4)

0.61102(16)

-1.04(9)

Alfa MoO3

0.92649(16)

0.57830(4)

1.05022(19)

0.60410(15)

1.5312(4)

0.61142(16)

-0.59(9)

Aldrich Mo

0.92459(16)

0.57789(4)

1.05102(19)

0.60500(15)

1.5347(4)

0.61382(16)

1.70(9)


Table 9. The absolute isotopic compositions and atomic weight (Ar) of six natural Mo materials f (94Mo)

f (95Mo)

f (96Mo)

f (97Mo)

f (98Mo)

f (100Mo)

Ar

SRM 3134

a

0.146910(26)

0.091736(9)

0.158652(13)

0.166666(25)

0.095874(20)

0.24306(5)

0.097098(25)

95.94642(19)

JMC 38719

0.146829(26)

0.091719(9)

0.158633(13)

0.166662(25)

0.095894(20)

0.24311(5)

0.097150(25)

95.94713(19)

STREM MoO3

0.146903(26)

0.091746(9)

0.158661(13)

0.166660(25)

0.095879(20)

0.24306(5)

0.097088(25)

95.94638(19)

GBW(E)080218

0.147114(26)

0.091794(9)

0.158700(13)

0.166701(25)

0.095839(20)

0.24288(5)

0.096968(25)

95.94453(19)

Alfa MoO3

0.147023(26)

0.091769(9)

0.158688(13)

0.166656(25)

0.095863(20)

0.24298(5)

0.097024(25)

95.94539(19)

Aldrich Mo

0.146598(26)

0.091626(9)

0.158554(13)

0.166643(25)

0.095926(20)

0.24333(5)

0.097324(25)

95.94947(19)

Average

0.14690(18)

0.09173(6)

0.15865(5)

0.16666(3)

0.09588(4)

0.24307(16)

0.09711(13)

95.9466(17)

Natural Mo

a

f (92Mo)


The individual parameters and their contributions to the uncertainty budget of the results were calculated using the GUM Workbench program.35 With regard to assessment of uncertainty propagation using Monte Carlo simulation, from corresponding normal distributions, 100000 groups of independent input values were randomly generated, and 100000 samples of Kij (output) values were obtained as a result. Hence the standard uncertainties could be calculated accordingly as the standard deviations. The uncertainty budget for atomic weight of molybdenum evaluated by Monte Carlo simulation was essentially in agreement with that evaluated by GUM principle. Compared with the previous research,25 the contributions of uncertainties resulted from isotopically enriched materials were effectively reduced by the use of high purity enriched isotope materials. As an absolute measurement method, the fully calibrated strategy mass spectrometry proposed in this study has the property of an unbroken chain of calibrations all with stated uncertainties, whereby ensures a clearly route to achieve SI unit traceability for isotope ratio measurement.

CONCLUSIONS All seven isotopically enriched molybdenum isotope materials 92 Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, and 100Mo were together used for gravimetrically preparing the synthetic isotope mixtures to successfully achieve a fully calibrated strategy. Consequently, the absolute isotopic compositions of six different

natural molybdenum materials including NIST SRM 3134 were accurately measured. It is worth noting that the enriched isotope materials were further purified and tested to well fit the requirement of the purity in this approach. A rigorous assessment of the measurement uncertainty was performed by GUM principle and Monte Carlo simulation as well, and had given a coherent result of the uncertainty budget for the isotopic compositions and atomic weight of molybdenum. Additionally, based on the fully calibrated strategy and experimental determination data of the synthetic mixtures, relationship between the bias per mass unit β and isotope mass was illustrated for the first time, which demonstrated β was not a constant value for all molybdenum isotope pairs in MC-ICPMS measurement. Since the partially calibrated MC-ICP-MS techniques still play an important role in isotope analysis, we propose that the relationship of correction factors of different isotope ratios need to be further explored. The new atomic weight of molybdenum derived from the six natural materials is Ar (Mo) = 95.9466(34) (k=2). The uncertainty of the atomic weight reported in this paper is much improved compared with that of the currently published IUPAC value of 95.95(1).19 As to be a proposed “best measurement”, the “zero-delta” reference NIST SRM 3134 yielded the absolute isotopic composition (in at. %, k=1) of 92Mo14.6910(26), 94Mo-9.1736(9), 95Mo-15.8652(13), 96Mo-


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16.6666(25), 97Mo-9.5874(20), 98Mo-24.306(5) and 100Mo9.7098(25). Delta notation δ98/95Mo values relative to NIST SRM 3134 were determined for the other five natural molybdenum materials. Among of them, JMC Mo plasma standard solution, STREM high purity MoO3 powder, and the high purity MoO3 powder from Alfa Aesar showed smaller δ98/95Mo isotopic variations than other natural materials as 0.33±0.09‰, 0.06±0.09‰ and -0.59±0.09‰, respectively. These natural molybdenum materials have the potential to be the isotope reference materials for molybdenum isotope measurements.

AUTHOR INFORMATION Corresponding Author * Jun Wang. Phone: 86-10-84251244. Fax: 86-10-642716939. E-mail: [email protected]

ACKNOWLEDGMENT This study was financially supported by the National Key Technology Research and Development Program (No. 2013BAK10B04) and National key research and development plan (No. 2017YFF0205801).

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Accurate Determination of the Absolute Isotopic Composition and

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