The isotopic composition and atomic weight of molybdenum

International Journal of Mass Spectrometry and Zon Processes 130 (1994) 65-12 0168-l 176/94/%07.000 1994 - Elsevier Science Publishers B.V. All rights reserved

The isotopic composition molybdenum

65

and atomic weight of

Qi-Lu*, Akimasa Masuda Department of Chemistry, University of Electra-communications Tokyo 182, Japan

1-S-1, Chofugaoka, Chofushi,

(Received 14 May 1993; accepted 29 May 1993) Abstract The isotopic composition of molybdenum in a shelf reagent standard (MoOJ, 99.99%), 13 molybdenites, and three iron meteorites has been determined with a thermal ionization mass spectrometer. No especially-evident variations of molybdenum isotopes among these samples have been found. The mass fractionation of molybdenum occurring in isotopes during the measurements is discussed and our preferred natural abundances of molybdenum isotopes are: ‘*MO, 14.7287 f 0.0010; g4Mo, 9.2118 f 0.0006; g5Mo, 15.8935 f 0.0011; g6Mo, 16.6731 f 0.0011; “MO, 9.5692 & 0.0007; “MO, 24.2289 f 0.0017; ‘O”Mo, 9.6950 f 0.0007. Thus, the atomic weight of molybdenum is 95.9415 f 0.0001 (the value currently accepted by IUPAC [IUPAC Inorganic Chemistry Division, Pure Appl. Chem., 63 (1991) 9911 is 95.94 k 0.01). The uncertainties correspond to the 95% confidence limit calculated from all the terrestrial and meteoritic samples. Key wordr: Molybdenum; Isotopic composition; Atomic weight; Mass spectrometry

Introduction Recent studies indicate that some elements are anomalous in their isotopic relative abundances in meteorites compared to samples found on earth. Therefore, it is necessary to study isotopic compositions of elements as precisely as possible in inter-stellar chemistry. Molybdenum is a very interesting element because it has seven stable isotopes e*Mo, 94M~, 95Mo, 96M~, 97Mo, 98Mo and ‘O”Mo) whose abundances can be affected by nuclear effects to individually varying degrees. 97Mo can be related to the decay product of 97T~ (T,,* = 4 x lo6 years, electron capture), 95Mo, 97Mo, 98Mo and “‘MO can be produced by the spontaneous fission of uranium and plutonium.

Also, in ref. 2, the synthesis of these isotopes involve three processes (r-, s-, and p-process). As a result of these considerations, the measurement of the isotopic composition of molybdenum in various samples has become important in the past 40 years [3-131. We have developed a thermal ionization mass spectrometric method [13] and a chemical separation method [14] for precise isotopic measurement of molybdenum in terrestrial and meteoritic samples. In this paper, the new values of molybdenum isotopic abundances and the atomic weight are proposed. Experimental Chemistry

*Correspondingauthor. Permanent address:The University of Inner Mongolia, Huhhot, China. SSDZ 0168-l 176(93)03900-7

A tlow chart of the total analytical procedures

66

Q. Lu and A. Masudallnt. J. Mass Spectrom. Ion Processes I30 (1994) 65-72

for separation and purification of MO from molybdenites and iron meteorites is described in Fig. 1. The chemical reagents and materials used are described in our previous works [13,14]. A brief description of the chemical separation method follows.

Iron meteorite

Two to five grammes of iron meteorites (Accuna, Gibeon and Landes) are washed by 6 M HCl, acetone and distilled water, and then dissolved in a 120ml Teflon digestion vessel with 5 ml of 6 M HCL by using a microwave oven (MDS-81D, CEM Corp., Matthews, NC) as described in Table 1. The meteoritic samples are treated several times with 5ml of 6 M HCL until the residue is completely dissolved. Subsequently, the resulting solution is mixed with 15 ml of 0.75 M bis(2-ethylhexyl)hydrogen phosphate in cyclohexane in a 50ml separatory funnel. After it has been shaken for 5min, the aqueous phase is removed when separation of the immiscible phases is complete, and the organic phase is washed with three 15ml portions of 5 M HC104, and then with two 15 ml portions of 10M HNOs. For back-extraction of MO, 5 ml of 10 M HNO, in 3% H202 is added to the organic phase and the mixture is shaken for about 5min. The aqueous phase is transferred into a Teflon beaker, and the back-extraction process is repeated. The aqueous phase is then evaporated to dryness at about 70°C and the residue is dissolved in 1 M HF/O.Ol M HCl for further purification by means of anion exchange as follows. Before loading the MO extracted from the samples onto a pretreated 0.5ml anion exchange column (AG 1 X-8, 200 mesh), the anion exchanger is washed successively with concentrated HCl, water, concentrated ammonia solution, water, concentrated HNOs and finally 6 M HCl. Three milliliters of 0.01 M HCl/l .OM HF is used to equilibrate the anion exchange resin by washing it. Then 1 ml of 6 M HCl is added to remove any Zr present. The MO-containing fraction is sub-

Fig. 1. Flow chart of the isotopic analytical procedure for molybdenum isotopes in molybdenites and iron meteorites.

sequently collected by passing 2.0ml of 7 M HNOs through the column. An inductively coupled plasma mass spectrometer (PlasmaQuad, VG Elemental, Winsford, Cheshire, UK) was used to observe whether or not any Zr and/or Ru remains in the solution prepared from the meteorites. Molybdenite

An accurately weighed quantity (0.3 to 0.5 g) of homogenized molybdenite powder (> 150 mesh) with 10 g of 7 M HN03 and 3 g of concentrated H2S04 are added into a 120ml Teflon digestion vessel. The container is then placed in a microwave oven and treated according to the heating sequence given in Table 1. After cooling in a refrigerator the container is opened and the solution of molybdenites is ready for chemical separation. The chemical separation methods for molybdenite are similar to those applied for iron meteorite samples. Mass spectrometry

Isotopic abundances of MO are measured with a thremal ionization mass spectrometer (VG Sector

67

Q. Lu and A. Masudajlnt. J. Mass Spectrom. Ion Processes 130 (1994) 65-72

Table 1 Decomposition

of molybdenites

and iron meteorites

with a microwave

oven

Sample Iron meteorite

Molybdenite Quantity Acids

0.3-0.5 g 14M HNOs,

Microwave oven sequence Heating time (min) Power (%)a

1 3 20

aThe full power of the microwave

5g + cont.

2 5 0

3 3 30

HzS04,

5g

4 5 0

5 3 40

2-5g 6M HCL,

IOml

1 20 2

2 20 4

4 10 10

3 20 5

5 5 15

oven is 730 k 70 W.

54-30). A triple filament technique is used for the molybdenum isotopic analyses. The molybdenum is loaded with 2 ~1 of a saturated aqueous solution of boric acid and 1~1 of 1 M HNOs on an outgassed side filament of five-fold zone-refined Re metal. Generally, 20 pg of molybdenum is sufticient to yield a fairly stable 96Mo ion intensity of 2 x lo-l2 A for about 5 h. The determination of MO isotopic ratios is carried out at this relatively weaker ion beam intensity without deterioration in accuracy, because the natural abundances of the seven stable MO isotopes are similar to each other, i.e., about lo-24%. To avoid the collector bias, a single Faraday collector is employed in the measurement. Data acquisition is made by the peak jumping model. The ion beam for each mass is measured with 5 s integration time and 2 s magnet setting time. During the isotopic analyses, special attention is paid to the emission of Zrf and Ru+ ions from the filament and the samples, because 92Zr, 94Zr, 98Zr, 96R~, 98R~, and “‘Ru can give rise to isobaric interferences to 92Mo, 94Mo 96Mo, 98Mo, 98Mo, and “‘MO respectively. Also, particular attention is directed to the possible effects of MO+ and cluster ions (such as KzF etc.) and MO+ released from filament material impurities. Careful inspections are performed by a Daly ion counting detector before and after a run of sample measurement. The results of such measurements indicated that such isotopic interferences can be neglected.

Results

Mass fractionation The mass fractionation of molybdenum isotopes during evaporation has been monitored carefully for several cases. Figure 2 shows the variations of isotopic ratio 94Mo/98Mo over 7 h in three different cases. In cases (A) and (C), a high purity MO ribbon (99.99%) is welded onto the side filament and onto the central filament respectively, while in case (B) the solution of molybdenum purified from

0.888

P

0.384

\

0.382

6

is

5 Time, Hous

7.5

Fig. 2. Typical MO isotopic fractionation curves, error bars are 217. (A), MO metal ribbon welded onto side Re filament; (B), purified MO compound loaded onto side filament; (C), MO metal ribbon welded onto central Re filament.

68

Q. Lu and A. Masudallnt.

Table 2 Ionizing temperatures and currents of filaments Case” C.F.b temp. (“C)

S.F.C current (A)

Filament loaded

Chemical state

A B C

3.5 0 0

Side Side Central

Metal MO Compound MO Metal MO

1900* 1750 1900

aSee Fig. 2 bCentral filament. ‘Side filament. *The current through the filament was about 4.2A.

molybdenite is loaded onto the side filament and evaporated to dryness. A dual wavelength optical pyrometer was used to measure the ionizing temperature of the central filament. The conditions of the measurement are given in Table 2. As for Fig. 2 (cf. Table 2), it is worthwhile to note the following: (1) when passing lower electric currents (compared to case (C)) through the side filament mounted with welded MO metal (case (A)) or passing no electric current through the side filament loaded with MO compound (case (B)), the observed change of “Mo/~~Mo ratio is smaller than case (C) where the MO metal was welded onto the central ionizing filament; (2) the extent of the change of isotopic ratio under consideration is similar in cases (A) and (B); (3) the observed isotopic ratio value is notably different in cases (A) and (B). Although it is difficult to give a quantitative interpretation of the fractionation phenomena observed, it is noted that the vaporization process of the sample seems to govern the variation of the observed isotopic ratios, because the mass fractionation of molybdenum isotopes depends strongly on the conditions of sample filament (single and triple) and on the chemical state of the molybdenum loaded. In cases (A) and (B), MO samples were loaded onto side filaments in quite different chemical states (pure metal and part of a compound) and the isotopic ratios of molybdenum decreased very slowly during the period of measurement, although the operating currents through the side filaments were 3.5A (about 1750°C) and OA for cases (A)

J. Mass Spectrom.

Ion Processes I30 (1994) 65-72

and (B), respectively. It is considered that the difference in chemical state between the metal and the compound results in the disagreement in the unnormalized values. Case (A) has the same MO sample as case (C), but the isotopic ratio decreased relatively rapidly with measurement time in case (C). The thermal vaporization and/or decomposition of metallic MO and compound MO (MoX) on the filament and the ionization of the gaseous atoms and molecules can be expressed as follows: Ma(s) [MoX(s)l -+ MO(g) [MoX(g)l Ma(g)

WXM + Mo%d+ e-

(1)

(2)

In reaction (1), normally the lighter isotopic species evaporates preferentially with consequent enrichment in heavy isotopes in the solid phase and the light/heavy isotopic ratios decrease steadily during the analysis. Very detailed discussion of some typical isotopic fractionation effects was presented by Moore and Heald [ 151.According to their findings, atomic vaporization generally enriches more lighter isotopes than molecular vaporization, so the difference between cases (A) and (B) can be explained in terms of the different chemical states of the samples. Compared to case (A), case (C) decreased very quickly, which might be attributed to the relatively high evaporation temperature because the MO sample of case (C) was loaded onto the central filament (ionizing filament). Under the high temperature, case (C) quickly undergoes a complete isotopic mixing of all residual sample and becomes more and more enriched in the heavier MO isotopes with the exhaustion of the sample. Here, it must be emphasized that there is little doubt that the remarkable isotopic differences in Fig. 2 can be assigned intrinsically not to the source of the samples, because the normalized data shown in Table 3 indicate that the relative deviation between the samples is less than 1 part per ten thousand. Normalization

To minimise the measurement errors resulting from the shift of the abundance ratios of molybdenum isotopes during the evaporation of MO

Q. Lu and A. Masuda/Int. J. Mass Spectrom. Ion Processes 130 (1994) 65-72

69

Table 3 Isotopic ratios of MO in MOST and iron meteorite? MO ratio

Source

92198 Aldrich (MOO,) Marz (metal) Daehwa mine (Korea) Hokuto (Japan) Matasvaara (Finland) Preissac (Canada) 471 (China) Aumanellur (India) Kataberget (Sweden) Onganja-2 (Namibia) Climax (USA) Brejui mine (Brazil) Mulgme (Australia) 110 (China) Daito (Japan) Ambalavayal (India) Bethlehem (Canada) Columbia (British) Accunab Gibeonb Landesb “95% confidence bIron meteorite.

0.607926 0.607868 0.607929 0.607907 0.607950 0.607907 0.607872 0.607872 0.607850 0.607896 0.607935 0.607905 0.607881 0.607967 0.607931 0.607984 0.607907 0.607880 0.607839 0.607732 0.607905

95198 f 13 & 22 f 15 f 15 f 14 f 20 f 11 f 15 f 25 f 20 + 15 zt 22 i 32 f 31 +z 18 + 19 f 23 f 20 f 43 f 37 f 46

limit, all data have been normalized

0.655964 0.655949 0.655967 0.655977 0.655981 0.655972 0.655999 0.655972 0.655964 0.655969 0.655995 0 655965 0.655964 0.656052 0.655985 0.655962 0.655964 0.655958 0.656006 0.655894 0.655992

yT = (xMIXT)’

f f f f f f f f f + f f zt f f f f f f f f

to 94Mo/98Mo

samples (which might be called a “variable systematic error”) is extremely important for the determination of natural isotopic abundance ratios and the atomic weight of molybdenum. Exponential law has been known to be the best mathematical method to correct the mass fractionation for molybdenum [13], which is written as yM/

96198

(3)

where X and Y denote the isotope ratios of masses MI/A43 and h4JM3, and subscripts T and M indicate an assumed “standard” ratio and a measured value, respectively. The value a in Eq. (3) depends on the differences in mass between M, and M2 and between M2 and M3. In the case under consideration (Fig. 3) where MI, M2, and M3 are 92Mo (or 97Mo), 94Mo and 98M~, u is evaluated to be In (92Mo/98Mo)/1n (94Mo/98Mo) and In (97Mo/98Mo)/ In (94Mo/98Mo). The data for case (C) (Fig. 2) presented in Fig. 3 demonstrate that the fractionations of 92Mo/98Mo and 97Mo/

13 15 2 10 9 12 7 11 18 13 9 9 22 23 15 12 13 12 37 26 26

97/98

0.688 146 0.688134 0.688136 0.688153 0.688132 0.688149 0.688140 0.688145 0.688137 0.688118 0.688163 0.688130 0.688164 0.688155 0.688162 0.688143 0.688152 0.688 165 0.688126 0.688151 0.688146

f 11 f 12 f 11 f IO f 10 f 12 f 7 + 12 i 18 f 15 & 10 + 1 f 19 f 20 & 16 f 11 f 13 f 11 f 37 f 29 & 34

0.394947 0.394951 0.394951 0.394951 0.394943 0.394953 0 394943 0.394950 0.394949 0.394954 0.394940 0.394948 0.394917 0.394956 0.394961 0.394935 0.394945 0.394942 0.394999 0 394927 0.394971

100/98 f f f f f f f f f + f f zt f f f * f f f f

7 3 4 6 6 8 4 6 10 8 6 8 10 I5 9 8 9 7 15 16 22

0.400129 0.400114 0.400125 0.400122 0.400145 0.400128 0.400128 0.400146 0.400147 0.400160 0.400131 0.400136 0.400112 0.400241 0.400181 0.400135 0.400146 0.400129 0.400143 0.400157 0.400104

f f f f f f f f f f i f f zt f f f f f f f

12 17 7 9 8

I1 6 8 15 12 8 27 14 33 10 9 IO 9 32 19 30

= 0.3802.

98Mo against 94Mo/98Mo can be accounted for strictly by the exponential law, although the extent 0.625

0.3975 D

0.620

0.3970

0.615

0.3965

% 7 0.610

0.3960

P

8 a

0.605

0.3955

2 s

% 8 0.600

0.3950

E ’

8

,m 0.595

0.3945 0.3940 T

0.376

0.376 0.380 0.362 0.384 Observed uMo/OWo

0.36t

i

0.3935

Fig. 3. Fractionanon data for 92Mo/98Mo vs. 94Mo/98Mo and 97Mo/98M~ vs. 94Mo/g8Mo, in case (C) of Fig. 2. (MO metal central filament). Twenty-nine data points are shown each data point LSthe average of seven ratios. All 29 pomts were taken in one sample run and the ratios were not corrected for mass fractionation. -, drawn according to the exponential law.

70

Q. Luand A. MasudalInt.J. Mass Spectrom. IonProcesses 130(1994)65-72

Table 4 Atom percent and atomic weight of molybdenuma Mass

92 94 95 96 97 98 100

Normalized value of g4Mo/gsMo 0.3865b

0.38334'

0.3802

14.9578 f0.0012 9.2824 f0.0011 15.9539 f0.00002 16.6730 f 0.0006 9.5331 f 0.0001 24.0478 f 0.0022 9.5521 f 0.0006

14.8528 f0.0013 9.2502 f 0.0011 15.9204 f 0.00002 16.6732 f0.0006 9.5496f0.0001 24.1306 f0.0023 9.6178 f 0.0006

14.7287 f 9.2118 f 15.8935 f 16.6731 f 9.5692 i 24.2289 f 9.6950 f

95.92066 f 0.00009

95.93020f0.0009

95.94154 * 0.00009

0.38334d 0.0010 0.0006 0.0011 0.0011 0.0007 0.0017 0.0007

14.84 f 0.04 9.25 f 0.02 15.92 f 0.04 16.68 f 0.04 9.55 f 0.02 24.13 f0.06 9.63 f 0.02 95.94 f 0.01

‘The uncertainties of mean values of this work are 95% confidential limits estimated for the samples listed in Table 3. bObtained from case (A) of Ftg. 2. ’ Calculated from IUPAC data. dReference 1.

of change of 94Mo/98Mo comes to about 3% (for further discussion, see ref. 13). In our previous work [13], 94Mo/98Mo was recognized as an appropriate pair of isotopes to be used for normalization to correct for the mass fractionation when comparing MO isotopic compositions in various samples and the standard, because this pair has a relatively large mass difference and can minimise the errors arising from instrumental fractionation factors. However, it seems that to determine the most appropriate value of 94Mo/98Mo for eliminating the known systematic bias from the measurements of the observed ratios still remains to be studied. Previous investigators [6-91 usually employed a value of 1.540 for 92Mo/‘ooMo to eliminate fractionation effects in making a comparison with former works. It is noted that the 92Mo/‘ooMo ratio of 1.540 was assumed arbitrarily. In our previous work, a grand mean value for 94Mo/98Mo of 0.3802 was chosen as a normalizing value, which resulted from MO compound (MOO,, 99.999%) used in conjunction with the triple filament technique. This value is based on 10 mass spectrometric analyses, each of which involves about 150 cycles; the data were collected from the beginning of evaporation of the sample to complete exhaustion of the sample. It is difficult to certify whether or not the ionization efficiency was the same throughout the run.

The corresponding value of 92Mo/‘ooMo obtained in this work was 1.519325, about 1.3% lower than that of Wetherill [7]. In a detailed discussion about the isotopic fractionation model for a multiple filament thermal ion source, the relation between the directly observed isotopic ratios and the ratios normalized by theory demonstrated that the observed isotopic ratios for samples loaded in various chemical states and the theoretically normalized ratios will be about the same as when the sample is about 60% consumed [15]. It was also suggested that the fractionation curve of a higher molecular weight species is closer to a normalized isotopic value than that of lighter species. These ideas lead us to the conclusion that the grand mean value of 0.3802 obtained for 94Mo/98M~ in case (B) of Fig. 2 can be considered to represent the qualified isotopic ratio. Isotopic composition of A40

The MO isotope data from analyses of a molybdenum reagent (Aldrich Moos, 99.999%), molybdenites and iron meteorites by use of the thermal mass spectrometry are given in Table 3. All observed data have been corrected against a 94Mo/98Mo ratio of 0.3802 to minimise the instrumental effects. The calculated total mean values of MO atomic percentages are shown in Table 4; the

Q. Lu and A. Masudallnt.

92

J. Mass Spectrom.

94 95 96 97 98

Ion Processes

130 (1994) 6S-72

100

Mo Isotopes

Fig. 4. The comparison of isotopic abundance ratios of molybdenum accepted by IUPAC and the values obtained in this work normalized by different normalizing values: (a), normalizing value of 94Mo/g8Mo = 0.3865; (b), normalizing value of g4Mo/ g8Mo calculated from IUPAC data; (c), normalizing value of g4Mo/g8M~ = 0.3802. The isotopic ratios are represented in parts per ten thousand for (b) and parts per thousand for (a) and (c).

values corrected by other possible normalizing values are also listed in Table 4 for comparison. Within the limits of precision of the present work, it can be seen (Table 3) that no especially evident variation of molybdenum isotopic composition is observed in these samples. Needless to say, the resultant “measured” isotopic abundance values depend on the value taken for the normalizing pair (94Mo/9*Mo ratio in the present case). Figure 4 demonstrates the effect of the normalizing value assumed for 94Mo/ 98Mo. If the value 0.3865, obtained for 94Mo/98Mo on the basis of measurement for metallic MO (Case (A) in Fig. 2), is taken as a standard, the set of resultant values should show a relative deviation (Fig. 4(a)) in comparison with the values resulting from the normalization against the IUPAC ratio for 94Mo/98Mo of 0.3833. However, if the value, 0.3802, obtained as an average for the compound molybdenum (Case (B) in Fig. 2) is chosen as a standard, the set of resultant isotopic values should show a relative deviation (Fig. 4(c)) in com-

71

parison with the values resulting from the IUPAC ratio of 0.3833. Note that the isotopic ratios are represented in parts per thousand for Fig. 4(a) and Fig. 4(c). Evidently, our relative isotopic abundances are in disagreement with the values accepted by IUPAC. The numerical values currently accepted by IUPAC for individual isotopes turns out to show the relative aberrations (Fig. 4(b)) to our values when normalized against 0.3833. (The aberration is represented here in parts per ten thousand.) It is noted that the data obtained over 20 years ago was with a 0.1% error in measurement and the isobaric interferences of Zr+ emitted from the ionizing filament on masses 92 and 94 escaped the notice of these workers. As compared with the data of our present work, the previous works strongly suggest that an increased instrumental fractionation effect with the mass number of molybdenum, Zr isobaric interferences (on masses 92 and 94 of molybdenum), and the measurement error are responsible for the discrepancies of MO isotopes from 92 to 100. Thus, based on the coherence of MO isotopic abundance ratios from terrestrial and meteoritic samples, the reliable data of molybdenum isotopic abundances in percent (calculated according to the masses of molybdenum isotopes) are proposed as: 92Mo, 14.7287 f 0.0010; 94Mo, 9.2118 f 0.0006; 95Mo, 15.8935 f 0.0011; 96Mo, 16.6731 f 0.0011; 97M~, 9.5692 f 0.0007 98Mo, 24.2289 f 0.0017 ‘O”Mo, 9.6950 f 0.0007. The preferred atomic weight of molybdenum is 95.9415 f 0.0001. References I 2 3 4 5 6 7 8 9

IUPAC Inorganic Chemistry Division, Pure Appl. Chem., 63 (1991) 991. E.M. Burbidge, G.R. Burbidge, W.A. Fowler and F. Hoyle, Rev. Mod. Phys., 29 (1957) 547. D. Williams and P. Yuster, Phys. Rev., 69 (1946) 556. G.E. Valley, Phys. Rev., 57 (1940) 1058. V.R. Murthy, J. Geophys. Res., 67 (1962) 905. V.R. Murthy, Geochim. Cosmochim. Acta, 27 (1963) 1171. G.W. Wetherill, J. Geophys. Res., 69 (1964) 4403. E.A.C. Crouch and T.A. Tuplin, Nature, 202 (1964) 1282. C.M. Stevens, Int. J. Mass Spectrom. Ion Phys., 8 (1972) 251.

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10 L.J. Moore, L.A. Machlan, W.R. Shields and E.L. Gamer, Anal. Chem., 46 (1974) 1082. 11 S. Becker and H.J. Dretze, Isotipenpraxis, 18 (1981) 70. 12 M. Koppe and K.G. Heumann, Fresenius Z. Anal. Chem., 331 (1988) 118.

J. Mass Spectrom. Ion Processes 130 (1994) 65-72

13 Qi-Lu and A. Masuda, J. Am. Sot. Mass Spectrom., 3 (1992) 10. 14 Qi-Lu and A. Masuda, Analyst, 117 (1992) 869. 15 L.J. Moore and E.F. Heald, Adv. Mass Spectrom., 7A (1978) 448.