Anal. Chem. 1997, 69, 1135-1139
Measurement of the Atomic Weight of Ruthenium by Negative Thermal Ionization Mass Spectrometry Min Huang* and Akimasa Masuda
Department of Chemistry, The University of Electro-Communications, Chofugaoka 1-5-1, Chofu, Tokyo 182, Japan
Methods for ruthenium isotope measurement by both positive and negative ion thermal ionization mass spectrometry (PTI-MS and NTI-MS, respectively) are studied in this work. We found that Ru isotopes 100 and 104 are subjected to isobaric interferences from the silica gel used to improve sensitivity in PTI-MS. Measurement of ruthenium isotopes by NTI-MS is discussed here. The method is proved to be sensitive and free from the isobaric interferences. With precise isotope ratios of several ruthenium reagents measured by NTIMS, we calculated the atomic weight of ruthenium to be 101.064 98 ( 0.000 16 (2σ), which is much more accurate than the data of 101.07 ( 0.02 currently suggested by IUPAC. Atomic weights and isotopic compositions are essential data in science and technology. With technical and instrumental development of measurement, the Commission on Atomic Weights and Isotope Abundances (CAWIA) of IUPAC evaluates newly reported data every two years and recommends up-to-date values. Traditional chemical measurements of atomic weights suffer from many hazards due to unwanted chemical effects. To get precise results, current standard atomic weights are mostly based on mass spectrometry. Needless to say, it is more important to measure isotope abundance ratios accurately rather than nuclidic masses, because the uncertainties of nuclidic masses are often less than 10-7 and do not significantly affect the accuracies of atomic weights. Ru is an interesting element involved in several typical nuclear reactions. The atomic weight of Ru published in 1969 was based on the works1-3 from 1950s. No further work was reported until Devillers et al.4 measured its isotopic abundance ratios and yielded 101.068 ( 0.013 with the most recent nuclidic mass data in 1978. CAWIA recommended a new value 101.07 ( 0.02 in place of 101.07 ( 0.03 in 1983. However, the isotopic abundances still remain uncertain, especially for 96Ru and 98Ru. The relative deviation of abundance of 98Ru is as large as 3.2%. As a result, more precise measurement of the isotopic ratios is required for improvement of the atomic weight data. It is difficult, however, to achieve the goal by conventional thermal ionization mass spectrometry, i.e., positive ion thermal ionization mass spectrometry (PTI-MS), because of the high ionization potential of ruthenium. Devillers et al.4 and (1) Friedman, L.; Irsa, A. P. J. Am. Chem. Soc. 1953, 75, 5741-5743. (2) Baldock, R. Oak Ridge Natl. Lab. Rep. 1954, 1719-1722. (3) White, F. A.; Collins, T. L., Jr.; Rourke, F. M. Phys. Rev. 1956, 101, 17861791. (4) Devillers, C.; Lecomte, T.; Lucas, M.; Hagemann, R. Adv. Mass Spectrom. 1978, 7A, 553-564. S0003-2700(96)00648-8 CCC: $14.00
© 1997 American Chemical Society
Poths et al.5 improved the sensitivity by adding boric acid and silica gel. However, some interferences from impurities of silica gel or its related chemical formation could hardly be avoided. Since the 1980s, negative ion thermal ionization (NTI) MS has become a powerful technique for isotopic measurements of nonmetals and metals with very high ionization potentials.6-10 Theoretically, ruthenium should produce intense beams of negatively charged oxide ions under proper conditions. In our previous work,11 Ru isotopic ratios were measured in the form of RuO3by NTI-MS. In this work, a large sample size without silica gel enhancer is used to get a strong ion beam for isotopic measurement by PTI-MS. The result is compared with that of silica gel-enhanced PTI-MS in order to carefully investigate the interferences. Further, a method of precisely measuring Ru isotope ratios by NTIMS is explored and the results are used to calculate the atomic weight of Ru.
EXPERIMENTAL SECTION Instrumentation. The mass spectrometer used in this study was a VG Sector 54 which was briefly introduced elsewhere.11 In PTI-MS, both multicollector and single-collector measurements are widely used. A multicollector array has the advantage when the signal varies or fluctuates with time, while a single collector eliminates error due to differential responses of individual detectors. Since the signal of NTI-MS is very stable, a single Faraday detector was used in this work. Reagents. Five ruthenium reagents were used as terrestrial samples for this study: (1) ruthenium chloride (Aldrich Chemical Co., a 1 mg mL-1 solution was prepared by dissolving it in 5% HCl medium; (2) Aldrich ruthenium atomic absorption standard solution (1000 µg mL-1 Ru in 5% HCl); (3) ruthenium(III) chloride (Tanaka Rare-metal Co., 1 mg mL-1 in 5% HCl); (4) ruthenium red (Merck, >99%, dissolved to prepare a 1 mg mL-1 Ru solution in 5% HCl medium); (5) ruthenium(III) chloride in aqueous solution (Johnson Matthey, assay 19.91%, diluted to 1 mg mL-1 Ru in 5% HCl). (5) Poths, H.; Schmitt-Strecker, S.; Begemann, F. Geochim. Cosmochim. Acta 1987, 51, 1143-1149. (6) Heumann, K. G.; Schindlmeier, W.; Zeininger, H.; Schmidt, M. Fresenius Z. Anal. Chem. 1985, 320, 457-462. (7) Volkening, J.; Walczyk, T.; Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1991, 105, 147-159. (8) Walczyk, T.; Hebeda, E. H.; Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1994, 130, 237-246. (9) Rokop, D. J.; Schroeder, N. C.; Wolfsberg, K. Anal. Chem. 1990, 62, 12711274. (10) Creaser, R. A.; Papanastassion, D. A.; Wasserburg, D. J. Geochim. Cosmochim. Acta 1991, 55, 397-401. (11) Huang, M.; Liu, Y.; Masuda, A. Anal. Chem. 1996, 68, 841-844.
Analytical Chemistry, Vol. 69, No. 6, March 15, 1997 1135
Table 1. Comparison of Ru Isotope Ratios by PTI-MS Combined with and without the Silica Gel Technique (Normalization to 101Ru/96Ru ) 3.078 33) 98/96
99/96
100/96
102/96
104/96
this work
0.337 238 0.337 256 0.337 297 0.337 326 0.337 246 0.337 016
2.301 978 2.301 247 2.302 006 2.302 325 2.301 659 2.302 050
2.273 405 2.273 134 2.273 277 2.273 388 2.273 060 2.272 990
5.694 137 5.694 149 5.695 125 5.695 050 5.696 003 5.695 006
3.359 627 3.360 466 3.361 215 3.361 476 3.362 670 3.360 946
mean
0.337 230
2.301 878
2.273 209
5.694 912
3.361 070
Pothsa
0.337 232
2.301 846
2.273 683
5.694 858
3.362 582
Devillersb
0.337 3
2.302 9
2.280 6
5.690 6
3.361 6
a
Data from Table 1 in ref 5. b Data from Table 4 in ref 4.
The preparation of other reagents, including Ba(NO3)2 solution and hydroiodic acid, was described in our previous work.11 PTI Mass Spectrometry. Ru isotopic composition measurement by PTI-MS was carried out under high filament temperature, usually over 1800 °C, due to its high ionization potential and low ionization efficiency. At such high temperature, the molybdenum in the Re filament is also expected to be ionized and causes severe interference with the Ru measurement. In this work, zone-refined rhenium foil (0.03 × 0.001 mm, 99.999%, H. Cross Co.) was used and special attention was paid to remove Mo from the filament as much as possible prior to sample loading. With a degassing process under 5 A current for 3-4 h, the filament was found satisfactory for the measurement. In practice, the time for degassing was decided by the resistance change of the filament. The resistance would increase noticeably just a few minutes before the filament broke. Stopping of heating with proper timing would make the degassing treatment quite efficient. A 10-15 µg sample of Ru was loaded and dried under heating current of 0.8 A. A single filament was used in this study. The filament current was increased to 3.7 A (∼1800 °C) within 60 min. Then, the current was increased continuously to optimize the signal and focusing parameters. Usually, a 2 × 10-12 A set for intensity of 102Ru was used for the measurement. NTI Mass Spectrometry. A single platinum filament system was found to produce an ion beam intensive enough for measurement, with the assistance of HI and Ba(NO3)2. Details of the procedures were given in the previous publication.11 A set intensity [(2-4 × 10-12 A] for 102RuO3- could be reached when the heating current was over 2.7 A. RESULTS AND DISCUSSION PTI-MS. With the first ionization potential of 7.7 eV, ruthenium is one of the elements that is difficult to measure by PTIMS. Silica gel enhancement combined with a boric acid technique efficiently improved the sensitivity. In a work similar to that of Devillers et al.,4 Poths et al.5 found that there were isobaric interferences on mass 104, as well as 100 which was already reported by Devillers et al. The interferences were identified as 40Ca28Si16O and 88Sr16O species. A separation by cation exchange 2 was employed to remove traces of Ca and Sr in the samples and subsequently eliminate the interferences. However, the removal of Ca and Sr from the silica gel proved to be difficult. 1136 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
Figure 1. Plot of difference in relative isotopic ratios between Poths et al.5 and this work. Difference is defined as [(Poths) - (this work)]/ (this work) × 104.
Figure 2. Behaviors of ion beam intensity (×10-12 A) variation in PTI-MS and NTI-MS.
To verify the possible interferences, a large sample size (1015 µg), other than the silica gel technique, was employed for Ru isotopic ratio measurement in this work, though the Ru concentration in a real sample is extremely low. Table 1 gives our results as well as that reported by Poths et al.5 with the silica gel technique. All of the data were normalized to 101Ru/96Ru ) 3.078 33. The difference in relative isotopic ratios between Poths et al. and this work is plotted in Figure 1. The mean ratios of our results are very close to theirs, except those of 100Ru/96Ru and 104Ru/96Ru. The ratio discrepancies observed imply that the interferences due to 40Ca28Si16O2 and 88Sr16O ionic species were not completely eliminated in their work. Although very careful chemical separation of Ca and Sr from the samples was carried out and the interferences were reduced compared with the work by Devillers et al., it is not easy to completely remove Ca and Sr from silica gel solid particles. When the silica gel technique is employed, special attention should be paid to such interferences. NTI-MS. As described in our previous work,11 a strong RuO3ion beam was observed with the help of Ba(NO3)2 as an ion enhancer and HI as a reducer to prevent Ru loss in oxide before the ionization process. Oxygen correction was made on the basis of careful investigation and accurate measurement of the oxygen isotope composition. A detailed discussion was given elsewhere.11
Table 2. Mass Fractionation and Data Normalization (101Ru/96Ru ) 3.078 33)
samplea TANA RRED RUCL JOHN AASS av
‰ per mass unit discriminationb
98Ru/96Ru
99Ru/96Ru
100Ru/96Ru
101Ru/96Ru
102Ru/96Ru
104Ru/96Ru
1.80 ( 0.15 2.24 ( 0.25 2.01 ( 0.50 1.90 ( 0.48 1.40 ( 0.19
0.337 256 ( 0.000 064 0.337 317 ( 0.000 034 0.337 126 ( 0.000 050 0.337 218 ( 0.000 081 0.337 135 ( 0.000 024
2.302 00 ( 0.000 09 2.301 73 ( 0.000 08 2.302 03 ( 0.000 09 2.302 22 ( 0.000 12 2.302 34 ( 0.000 06
2.273 17 ( 0.000 11 2.273 19 ( 0.000 11 2.273 24 ( 0.000 11 2.273 41 ( 0.000 10 2.273 44 ( 0.000 08
3.078 33 3.078 33 3.078 33 3.078 33 3.078 33
5.692 97 ( 0.000 26 5.693 05 ( 0.000 43 5.694 02 ( 0.000 24 5.693 43 ( 0.000 35 5.692 90 ( 0.000 25
3.359 65 ( 0.000 20 3.360 32 ( 0.000 25 3.360 25 ( 0.000 18 3.359 39 ( 0.000 24 3.360 45 ( 0.000 18
0.337 210 ( 0.000 081 2.302 06 ( 0.000 23 2.273 29 ( 0.000 12
5.693 27 ( 0.000 46 3.360 01 ( 0.000 46
a TANA, ruthenium chloride, Tanaka Rare-metal Co., eight-run measurements; RRED, ruthenium red, Merck, five-run measurement; RUCL, ruthenium chloride, Aldrich Chemical Co., seven-run measurements; JOHN, ruthenium(III) chloride, Johnson Matthey, six-run measurements; AASS, ruthenium atomic absorption standard solution, Aldrich Chemical Co., three-run measurements. b ‰ deviations per mass unit were calculated from (Rm - Rn)/5Rn; here Rn is the value of the ratio, 101Ru/96Ru ) 3.078 33, used for normalization and Rm is the measured value of the ratio.
Figure 3. Relative variation of ruthenium ratios upon fractionation: (O) ln(99Ru/96Ru) ) f ln(101Ru/96Ru); (b) ln(102Ru/96Ru) ) f ln(101Ru/ 96Ru).
Figure 4. Relative variation of ruthenium ratios upon fractionation: (O) ln(100Ru/96Ru) ) f ln(101Ru/96Ru); (b) ln(104Ru/96Ru) ) f ln(101Ru/ 96Ru).
Less isobaric interference was considered to be one of the advantages of NTI-MS. In the case of ruthenium measurement by PTI-MS, Zr, Mo, Pd, and other isobaric species such as 40Ca28Si16O2 and 88Sr16O are significant interfering sources; Mo emitted from the Re filament is an especially difficult problem at high filament temperature. In our study, the 98Mo contribution to 98Ru, the isotope most greatly interferred with by Mo, was 0.2% or even higher when the temperature was over 1900 °C, although special care was taken to remove Mo from the filament as much as possible during the degassing process. The interfering Mo species might come from the inner part of the filament because Mo on the surface could be removed when it was degassed. In NTI-MS, Zr, Pd, and Rh were found to form negative oxide ions with great difficulty. No detectable signals of MO3- (M ) Zr, Pd, Rh) were observed. MoO3- interference was hardly observed and the contribution of 98Mo to 98Ru was found to be less than 0.02%, if any. In this case, the Mo species is interpreted to come from the sample instead of the inner part of the filament, because the filament temperature was relatively low, usually lower than 900 °C. The Mo interference due to the impurity in the sample could be minimized by purifying the sample before it was loaded. The method may be more useful to accurately measure trace technetium isotopic compositions because interferences from Mo and Ru occur in PTI-MS. In NTI-MS, however, the most abundant ion form of Tc is TcO4-, while those of Ru and Mo are RuO3and MoO3-, respectively. Neither RuO4- nor MoO4- was found in our study. Another significant characteristic of NTI-MS is that the signal is very stable and can be sustained for a long time. The behavior of ion beam intensity with time in NTI-MS is given in Figure 2.
The ion beam emission was so stable that the intensity was almost the same even after 26 h (40 blocks) measurement without any adjustment of heating current of the filament. In contrast, in PTIMS the signal of Ru is relatively weak and decays very quickly, which leads to poor precision of measurement, especially when a single detector is employed. At optimized temperature for Ru measurement by PTI-MS, ∼1950 °C in our work, the intensity decreased nearly 40% in 0.5 h (in Figure 2). After one block measurement, ∼40 min, the filament temperature had to be raised to compensate for the intensity lost. Mass Fractionation and Ru Isotope Ratios. The ionization temperatures reported in NTI-MS were below 900 °C, much lower than that employed in PTI-MS. It is necessary to explore mass fractionation behavior in NTI-MS. However, few works published discussed it in detail until now. The results of mass fractionation of NTI-MS (in ‰ per mass units) are listed in Table 2. Generally speaking, a mass fractionation effect is more obvious in a “cold” atmosphere. However, the value for RUCL in our system is 2.01 ( 0.05, lower than the 3.8 ( 0.7 of PTI-MS in which the filament temperature was over 1900 °C. There may be two reasons for that. First, the ion detected in NTI-MS is RuO3- while that in PTI-MS is Ru+. It is already known that mass fractionation is obviously less in the highmass range than that in low-mass range. Second, with filament temperature increasing, heavier isotope enrichment may occur. In the work of Devillers et al.,4 the discrimination of per mass unit (‰ per mass unit) varies from 0.26 to 6.5 with a mean of 2.4 ( 2.0 after recalculation of their results in order to directly compare with our data. In Table 3 of their paper,4 the mean of ‰ Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
1137
0.187 ( 0.002
0.186 212 ( 0.000 044
0.186 201 ( 0.000 014 0.186 222 ( 0.000 034 0.186 230 ( 0.000 078 0.186 178 ( 0.000 038 0.186 230 ( 0.000 038
104
Table 4. Nuclidic Masses of Ruthenium12 nuclide
nuclidic
96Ru
95.907 596(9) 97.905 287(7) 98.9059 371(28) 99.9042 175(28) 100.905 580 8(31) 101.904 347 5(31) 103.905 422(6)
98Ru 99Ru 100Ru 101Ru 102Ru
0.316 ( 0.002
0.315 520 ( 0.000 036
0.170 ( 0.001
0.170 600 ( 0.000 010 0.125 985 ( 0.000 012
0.126 ( 0.001 0.127 ( 0.001
0.127 579 ( 0.000 024
0.0552 ( 0.0005 IUPAC
0.0186 88 ( 0.000 010 0.055 419 ( 0.000 004 av
0.0188 ( 0.0005
0.315 525 ( 0.000 064 0.315 518 ( 0.000 052 0.315 537 ( 0.000 055 0.315 530 ( 0.000 049 0.315 490 ( 0.000 032 0.170 606 ( 0.000 030 0.170 602 ( 0.000 026 0.170 592 ( 0.000 016 0.170 602 ( 0.000 026 0.170 596 ( 0.000 018 0.125 981 ( 0.000 048 0.125 981 ( 0.000 022 0.125 979 ( 0.000 018 0.125 993 ( 0.000 044 0.125 990 ( 0.000 024 0.127 576 ( 0.000 035 0.127 563 ( 0.000 044 0.127 576 ( 0.000 034 0.127 589 ( 0.000 058 0.127 592 ( 0.000 048 0.055 422 ( 0.000 010 0.055 420 ( 0.000 008 0.055 417 ( 0.000 006 0.055 420 ( 0.000 016 0.055 418 ( 0.000 006 TANA RRED RUCL JOHN AASS
0.018 691 ( 0.000 008 0.018 694 ( 0.000 006 0.0186 82 ( 0.000 004 0.0186 89 ( 0.000 012 0.0186 83 ( 0.000 004
100 98 96 sample
Table 3. Ru Isotope Abundances (the Error is 2σ)
99
abundances, %
101
102
104Ru
1138 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997
mass u u u u u u u
Table 5. Atomic Weight of Ruthenium and Its Uncertainty atomic weighta
sample
a
TANA RRED RUCL JOHN AASS
101.064 93 ( 0.000 41 101.065 05 ( 0.000 32 101.065 06 ( 0.000 08 101.064 87 ( 0.000 45 101.065 01 ( 0.000 30
av
101.064 98 ( 0.000 16
IUPAC12 Devillers4
101.07 ( 0.02 101.068 ( 0.013
The uncertainties given here are 2σ.
per mass unit of sample I to sample XI was 1.3 ( 0.8. When the average of heating current of the filament was raised from 4.6(sample I to X) to 5.9 A (sample XI to XV), the ‰ per mass unit was 5.5 ( 0.9. They suggested that heavier isotope enrichment must be taken into account besides simple mass fractionation. Our data in Table 2 show that the variation of ‰ per mass unit discrimination in NTI-MS is less than that in PTI-MS. Figures 3 and 4 show the correlations of ratios of 99Ru/96Ru, 102Ru/96Ru, 100Ru/96Ru, and 104Ru/96Ru against 101Ru/96Ru, respectively. The data are with oxygen correction only but without mass fractionation correction. The power law, which is widely used in PTI-MS, is considered here. Mass fractionation shown in Figures 3 and 4 follows the power law very closely. Therefore, our results were corrected by the power law. In the work of Poths et al.,5 no interferences to the Ru isotopes by Mo were found, because no measurable Mo+ ions were produced at a filament temperature below 1500 °C. The normalization value, 101Ru/96Ru ) 3.078 33, the measured mean by Poths et al., was used for mass discrimination correction in this work. The isotope ratios of five chemical reagents are listed in Table 2. The results are the average of several run measurements. Each run includes 20 blocks, totally 200 cycles. The precisions in the table are quoted as 2σ. The results again reveal that isobaric interferences with 100Ru and 104Ru occurred in the work with the silica gel technique. Isotope Abundances and Atomic Weight of Ru. Table 3 lists the ruthenium isotopic abundances of the five chemical reagents. Based on the abundances and nuclidic mass data given in Table 4, the atomic weight of ruthenium was calculated and is listed in Table 5. The calculations of uncertainties of abundances and atomic weights were not performed by direct application of error (12) IUPAC, Atomic Weight of the Elements 1991. Pure Appl. Chem. 1992, 64, 1519.
propagation on the results of mass spectrometric measurement. For each set of data of a single run, the isotope abundances and the values of atomic weight were calculated for each run separately. Therefore, the uncertainties of the atomic weight given in the table are the uncertainties calculated out of several independent atomic weight determinations. With accuracy improvement of Ru isotope ratio measurements, the atomic weight of ruthenium was found to be 101.064 98 ( 0.000 16, much better
than the value of 101.07 ( 0.02 recommended by IUPAC in 1983 based on the work of Devillers et al. published in 1978.4 Received for review July 2, 1996. Accepted December 13, 1996.X AC960648N X
Abstract published in Advance ACS Abstracts, February 1, 1997.
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