Correction of spike contribution for strontium isotopic measurement by thermal ionization mass spectrometry: a test for spike-standard mixed solutions
© Cheong et al.; licensee Springer. 2014
Received: 17 January 2014
Accepted: 12 February 2014
Published: 23 April 2014
The Rb-Sr isotope system has long been used for radiometric dating and petrogenetic investigation. The concentrations of Rb and Sr could be precisely measured by isotope dilution thermal ionization mass spectrometry combined with chemical purification of these elements. For the simultaneous measurement of Sr isotopic composition and isotope dilution, the contribution from the added spike should be carefully corrected.
Reliable 87Sr/86Sr ratios of the spike-standard mixed solutions were obtained using the new mass bias factor calculated on the basis of measured Sr isotopic ratios. This correction yielded reasonable 87Sr/86Sr ratios for overspiked standard solutions whose spike fractions reaching 25 wt.%.
The correction procedure described in this study shows that the simultaneous measurement of Sr isotopic composition and isotope dilution is available for the case that the spike proportion in the sample-spike mixture is significantly high (84Sr/86Sr > 3.7). The principle of this correction protocol can also be applied to other isotope systems such as Sm-Nd and Lu-Hf pairs.
KeywordsRb-Sr Thermal ionization mass spectrometry Isotope dilution Spike contribution
Rubidium-87 decays to stable 87Sr by emission of β− particle with a decay constant of 1.420 (±0.010) × 10−11 years−1 (Steiger and Jäger ) which corresponds to a half-life of 48.8 × 109 years. It is notable that this International Union of Geological Sciences-accepted decay constant is being considered to be high by 1% to 2% as summarized in Begemann et al. (). Rubidium and strontium belong to the groups IA and IIA, respectively. They are easily fractionated from each other through geological processes such as partial melting and igneous and metamorphic mineral growth. Therefore, the 87Rb-87Sr isotope system has long been used for radiometric dating and petrogenetic investigation (Faure and Mensing ).
The traditional thermal ionization mass spectrometry (TIMS) is still regarded as the benchmark technique to measure Rb-Sr isotope ratios. By TIMS, the inter-elemental ratio cannot be directly measured but is obtained through isotope dilution (Moore et al. ), a method of analyzing chemical substances by the addition of isotopically enriched spike to the sample. Considering the inhomogeneous distribution of Rb and Sr in most geological samples, simultaneous measurement of Sr isotopic composition and isotope dilution is essential. In this case, inevitable contribution from the spike should be carefully corrected.
This study evaluates the effectiveness of correction protocol to measure 87Sr/86Sr ratio in a series of mixture of 84Sr-enriched spike and standard solution. It has been tested whether a reasonable 87Sr/86Sr ratio could be obtained for overspiked standard solutions of which spike fractions reach 45 wt.%.
Availability and requirements
Preparation of spike and standard solution
The 84Sr-enriched isotope spike and standard solutions were prepared at the Korea Basic Science Institute (KBSI) in Ochang. In the following, all errors are quoted on the basis of 2 σ standard error, unless stated otherwise.
The spike, SrCO3 form, was provided by the Oak Ridge National Laboratory (batch number = 236,201, order number = 65-0056). It was dissolved in hot 10% HNO3 and diluted to about 15 μg/g solution with 5% HNO3. The AnApureTM ICP standard of 1,002 (±10) mg/L Sr (lot no. AEP-130-726) was diluted to 21.480 μg/g solution in 2% HNO3 (the specific gravity of 2% HNO3 = 1.028 g/mL) and then was admixed with the spike solution. Ten mixed solutions were prepared with mixing proportions (spike/standard) ranging from about 1:1.2 to 1:70 in weight.
The Rb-Sr isotopic analysis was conducted by using a Phoenix (Isotopx) TIMS, Cheshire, CW, UK, installed at the KBSI. This instrument is equipped with eight movable Faraday collectors and one fixed axial channel where the ion beam intensities can be measured with either a Faraday collector or an ion counting Daly detector.
In this study, all required isotopes were simultaneously detected on Faraday collectors. The sensitivity on 88Sr was typically around 3 V (1011 Ω resistors). The strontium isotope measurements of the standard and spike-standard mixed solutions were performed with a multi-dynamic mode (Lenz and Wendt ) in which the Faraday collectors were set to simultaneously detect 84Sr (axial), 86Sr (H2), 87Sr (H3), and 88Sr (H4) for the first static run, and 84Sr (L2), 86Sr (H1), 87Sr (H2), and 88Sr (H3) for the second run. One measurement consists of 5 blocks of 12 cycles with an integration time of 10 s. The mass bias was exponentially normalized to 86Sr/88Sr = 0.1194. The mean multi-dynamic 87Sr/86Sr acquired for NBS987 was 0.710252 ± 0.000007 (n = 7) during the course of this study.
The isotopic composition of 84Sr-enriched spike was measured with a multi-static mode (100 cycles, 5 blocks, integration time = 10 s) in which the Faraday collectors were set to detect 84Sr (axial), 86Sr (H2), 87Sr (H3), and 88Sr (H4). In this measurement, the instrumental mass fractionation was not normalized.
Results and discussion
Five measurements for the 84Sr spike yielded an average 84Sr/88Sr of 561.4 ± 0.9, 86Sr/88Sr of 0.7661 ± 0.0010, and 87Sr/88Sr of 0.1819 ± 0.0030, respectively. Therefore, the 84Sr enrichment and atomic weight of the spike are calculated to be 83.9242 and 99.65 at.%, respectively. Although the instrumental mass fractionation was not corrected during the TIMS measurement, this enrichment value is well matched with the suggested value (99.64 ± 0.01%, Tracy ). The exponentially normalized multi-dynamic measurement of the Sr standard yielded an average 87Sr/86Sr ratio of 0.707630 ± 0.000009 (n = 5).
Because the measured 84/86 ratio used in the above calculation has an uncertainty due to incorrect normalization to 86/88 = 0.1194, the 86t/86s should be recalculated after determination of the new mass bias factor (see below).
The newly corrected (84/86) ratios of the ten mixed solutions yielded a mean Sr concentration of 15.271 ± 0.031 μg/g for the 84Sr-enriched spike prepared in this study.
Finally, the (87/86)c(new) is used for the calculation of the (87/86)s: because (it is noted that 86t/86s can be recalculated on the basis of newly corrected 84/86).
Correction results for the spike-standard mixed solutions
2 σ SE
This study was supported by the KBSI grant (G34200).
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