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Background: 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. 87 86 Findings: Reliable Sr/ Sr ratios of the spike-standard mixed solutions were obtained using the new mass bias factor 87 86 calculated on the basis of measured Sr isotopic ratios. This correction yielded reasonable Sr/ Sr ratios for overspiked standard solutions whose spike fractions reaching 25 wt.%. Conclusions: 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 84 86 is significantly high ( Sr/ Sr > 3.7). The principle of this correction protocol can also be applied to other isotope systems such as Sm-Nd and Lu-Hf pairs. Keywords: Rb-Sr; Thermal ionization mass spectrometry; Isotope dilution; Spike contribution Findings The traditional thermal ionization mass spectrometry Introduction (TIMS) is still regarded as the benchmark technique to 87 − Rubidium-87 decays to stable Sr by emission of β par- measure Rb-Sr isotope ratios. By TIMS, the inter-elemental −11 −1 ticle with a decay constant of 1.420 (±0.010) × 10 years ratio cannot be directly measured but is obtained through (Steiger and Jäger 1977) which corresponds to a half-life of isotope dilution (Moore et al. 1973), a method of analyzing 48.8 × 10 years. It is notable that this International Union chemical substances by the addition of isotopically enriched of Geological Sciences-accepted decay constant is being spike to the sample. Considering the inhomogeneous considered to be high by 1% to 2% as summarized in Bege- distribution of Rb and Sr in most geological samples, mann et al. (2001). Rubidium and strontium belong to the simultaneous measurement of Sr isotopic composition groups IA and IIA, respectively. They are easily fractionated and isotope dilution is essential. In this case, inevitable from each other through geological processes such as contribution from the spike should be carefully partial melting and igneous and metamorphic mineral corrected. 87 87 growth. Therefore, the Rb- Sr isotope system has This study evaluates the effectiveness of correction 87 86 long been used for radiometric dating and petrogenetic protocol to measure Sr/ Sr ratio in a series of mixture investigation (Faure and Mensing 2005). of Sr-enriched spike and standard solution. It has been 87 86 tested whether a reasonable Sr/ Sr ratio could be ob- tained for overspiked standard solutions of which spike * Correspondence: email@example.com fractions reach 45 wt.%. Division of Earth and Environmental Sciences, Korea Basic Science Institute, Ochang, Chungbuk 363-883, Republic of Korea Full list of author information is available at the end of the article © 2014 Cheong et al.; licensee Springer. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Cheong et al. Journal of Analytical Science and Technology 2014, 5:26 Page 2 of 4 http://www.jast-journal.com/content/5/1/26 Availability and requirements corrected during the TIMS measurement, this enrichment Preparation of spike and standard solution value is well matched with the suggested value (99.64 ± The Sr-enriched isotope spike and standard solutions 0.01%, Tracy 1999). The exponentially normalized multi- were prepared at the Korea Basic Science Institute (KBSI) dynamic measurement of the Sr standard yielded an aver- 87 86 in Ochang. In the following, all errors are quoted on the age Sr/ Sr ratio of 0.707630 ± 0.000009 (n =5). basis of 2 σ standard error, unless stated otherwise. The Sr isotope ratios measured for the spike-standard 86 88 The spike, SrCO form, was provided by the Oak Ridge mixed solutions have been normalized to Sr/ Sr = National Laboratory (batch number = 236,201, order 0.1194. This mass bias correction is not valid because 86 88 number = 65-0056). It was dissolved in hot 10% HNO the Sr/ Sr of the added spike was significantly shifted and diluted to about 15 μg/g solution with 5% HNO . from the natural value through the enrichment processes TM The AnApure ICP standard of 1,002 (±10) mg/L Sr from 0.1194 to 0.7661. The first step for the correction (lot no. AEP-130-726) was diluted to 21.480 μg/g solu- of spike contribution is to determine a new mass bias tion in 2% HNO (the specific gravity of 2% HNO = factor (α ) from the measured Sr isotope ratios. In the 3 3 new 1.028 g/mL) and then was admixed with the spike so- following, ‘s’ denotes the normal Sr standard, ‘t’ denotes lution. Ten mixed solutions were prepared with mixing the Sr spike, and ‘m’ and ‘c’ represent the measured and proportions (spike/standard) ranging from about 1:1.2 corrected values, respectively. The (86t/86s) can be to 1:70 in weight. calculated using (84/86)m, (84/86)s, and (84/86)t: 84 84 s t 84 84 þ 84 Instrumental analysis s t 86 86 s s ¼ ¼ The Rb-Sr isotopic analysis was conducted by using a 86 86 þ 86 s t 1 þ 84 86 84 Phoenix (Isotopx) TIMS, Cheshire, CW, UK, installed t 86 s 86 86 t at the KBSI. This instrument is equipped with eight 1 þ movable Faraday collectors and one fixed axial channel where the ion beam intensities can be measured with either 84 84 86 m 86 s a Faraday collector or an ion counting Daly detector. ¼ ; 84 84 86 − In this study, all required isotopes were simultaneously 86 t 86 m detected on Faraday collectors. The sensitivity on Sr was 88 t t 86 typically around 3 V (10 Ω resistors). The strontium iso- 88 86 tope measurements of the standard and spike-standard mixed solutions were performed with a multi-dynamic Because the measured 84/86 ratio used in the above cal- mode (Lenz and Wendt 1976) in which the Faraday col- culation has an uncertainty due to incorrect normalization 84 86 lectors were set to simultaneously detect Sr (axial), Sr to 86/88 = 0.1194, the 86t/86s should be recalculated 87 88 (H2), Sr (H3), and Sr (H4) for the first static run, after determination of the new mass bias factor (see 84 86 87 88 and Sr (L2), Sr (H1), Sr (H2), and Sr (H3) for below). the second run. One measurement consists of 5 blocks The (86/88)s can be reduced by using (86/88)m, (88t/ of 12 cycles with an integration time of 10 s. The mass 88s), and (86/88)t: 86 88 bias was exponentially normalized to Sr/ Sr = 87 86 86 88 86 0.1194. The mean multi-dynamic Sr/ Sr acquired 88 88 88 s s t ¼ ; for NBS987 was 0.710252 ± 0.000007 (n =7) during the 1 þ course of this study. 86 86 88 86 88 The isotopic composition of Sr-enriched spike was t t ¼ 1 þ − measured with a multi-static mode (100 cycles, 5 blocks, 88 88 88 88 88 s s s m t integration time = 10 s) in which the Faraday collectors 84 86 87 The mass fractionation can now be accurately cor- were set to detect Sr (axial), Sr (H2), Sr (H3), rected using the calculated (86/88)s.The exponential and Sr (H4).Inthismeasurement,the instrumental law (Russel et al. 1978) yields a mass bias factor before mass fractionation was not normalized. (α )and after(α ) the spike correction as the old new following: Results and discussion Five measurements for the Sr spike yielded an aver- 84 88 86 88 0:1194 0:1194 age Sr/ Sr of 561.4 ± 0.9, Sr/ Sr of 0.7661 ± 0.0010, ln ln 86 86 ðÞ ðÞ 87 88 88 88 m s and Sr/ Sr of 0.1819 ± 0.0030, respectively. Therefore, a ¼ ; a ¼ ; old new 84 M M 86 86 the Sr enrichment and atomic weight of the spike are ln ln M M 88 88 calculated to be 83.9242 and 99.65 at.%, respectively. Although the instrumental mass fractionation was not where ‘M’ denotes the mass of the isotope. Cheong et al. Journal of Analytical Science and Technology 2014, 5:26 Page 3 of 4 http://www.jast-journal.com/content/5/1/26 Table 1 Correction results for the spike-standard mixed solutions 86/88 % SE 84/86 % SE 84/86 87/86 % SE 87/86 2 σ SE Spike wt.% m m c m s Mix 1 0.12090 0.0068 8.4583 0.0050 8.5427 0.705574 0.0011 0.707582 0.000016 45.1 Mix 2 0.12034 0.0089 3.6918 0.0094 3.7076 0.706777 0.0009 0.707634 0.000013 25.9 Mix 3 0.12030 0.0048 2.4858 0.0051 2.4929 0.707068 0.0010 0.707640 0.000014 18.9 Mix 4 0.12032 0.0049 1.9667 0.0050 1.9711 0.707165 0.0009 0.707615 0.000013 15.5 Mix 5 0.11982 0.0179 1.5658 0.0187 1.5686 0.707292 0.0010 0.707644 0.000014 12.6 Mix 6 0.12016 0.0016 1.2043 0.0022 1.2059 0.707349 0.0010 0.707618 0.000014 9.9 Mix 7 0.11983 0.0034 0.7787 0.0030 0.7794 0.707469 0.0008 0.707638 0.000011 6.5 Mix 8 0.11976 0.0143 0.4234 0.0147 0.4236 0.707568 0.0010 0.707653 0.000014 3.4 Mix 9 0.12023 0.0185 0.2831 0.0103 0.2832 0.707593 0.0010 0.707646 0.000014 2.1 Mix 10 0.11983 0.0145 0.2071 0.0173 0.2072 0.707575 0.0010 0.707610 0.000014 1.4 Average 0.707633 SD 0.000015 2σ SE 0.000010 The result of mix 1 is excluded. 87 86 84 86 Figure 1 Plots of corrected Sr/ Sr as a function of Sr/ Sr and weight fractions of the spike. Solid and dashed lines represent an 87 86 average Sr/ Sr and 2 σ standard deviation of the unspiked standard data, respectively. Error bars are based on within-run 2 σ standard error. Cheong et al. Journal of Analytical Science and Technology 2014, 5:26 Page 4 of 4 http://www.jast-journal.com/content/5/1/26 The newly corrected (84/86) ratio used for isotope Acknowledgements This study was supported by the KBSI grant (G34200). dilution is yielded as the following: Author details old 84 84 M 86 Division of Earth and Environmental Sciences, Korea Basic Science Institute, ¼ 2 Ochang, Chungbuk 363-883, Republic of Korea. Department of Earth System 86 86 M instrumentally measured cðÞ old Sciences, Yonsei University, Seoul 120-749, Republic of Korea. new Received: 17 January 2014 Accepted: 12 February 2014 84 84 M 86 86 M cðÞ new instrumentally measured References Begemann F, Ludwig KR, Lugmair GW, Min K, Nyquist LE, Patchett PJ, Renne PR, The newly corrected (84/86) ratios of the ten mixed Shin CY, Villa IM, Walker RJ (2001) Call for an improved set of decay solutions yielded a mean Sr concentration of 15.271 ± constants for geochronological use. Geochim Cosmochim Acta 65:111–121 0.031 μg/g for the Sr-enriched spike prepared in this Cheong CS, Kwon ST (2010) Calibration of Sm-Nd mixed spike by Teflon powder method. J Anal Sci Tech 1:30–36 study. Faure G, Mensing TM (2005) Isotopes: Principles and applications, 3rd edn. John Also, the (87/86)c can be newly calculated using mea- Wiley & Sons, Hoboken sured Sr isotope ratios as the following. Lenz H, Wendt I (1976) Use of a double collector for high-precision isotope ratio measurements in geochronology. Adv Mass Spectrom 7A:565–568 ∝ Moore LJ, Moody JR, Barnes IL, Gramlich JW, Murphy TJ, Paulsen PJ, Shields WR new 87 87 M (1973) Trace determination of rubidium and strontium in silicate glass 86 86 M standard reference materials. Anal Chem 45:2384–2387 cðÞ new instrumentally measured Russel WA, Papanastassiou DA, Tombrello TA (1978) Ca isotope fractionation on the earth and other solar system materials. Geochim Cosmochim Acta 42:1075–1090 Actually; Steiger RH, Jäger E (1977) Subcommission on geochronology: convention on the instrumentally measured use of decay constants in geo- and cosmochronology. Earth Planet Sci Lett old 87 M 86 36:359–362 Tracy JG (1999) High-purity enrichment of Sr. Nucl Instrum Methods Phys Res 86 M cðÞ old Sect A 438:1–6 Finally, the (87/86)c(new) is used for the calculation of doi:10.1186/s40543-014-0026-1 Cite this article as: Cheong et al.: Correction of spike contribution for the (87/86)s: strontium isotopic measurement by thermal ionization mass 86 86 spectrometry: a test for spike-standard mixed solutions. Journal of 87 87 t 87 t 87 ¼ 1 þ − because 86 s 86 cðÞ new 86 86 t 86 86 m s s Analytical Science and Technology 2014 5:26. 87 86 87 87 t 87 s t þ ðÞ ðÞ 87 þ87 86 86 86 86 86 s t s s s s t ¼ ¼ ¼ (it is noted 86 86 86 þ86 t t s t 1þ 1þ 86 86 s s that 86 /86 can be recalculated on the basis of newly t s corrected 84/86). Table 1 summarizes the spike correction results for the ten spike-standard mixed solutions. The corrected 87 86 Sr/ Sr ratios of the mixed solutions are displayed in Figure 1 with the measured values of the unspiked standard. As shown in Figure 1, the protocol described in this study successfully corrected the spike contribu- tion in the case that the spike fraction in the mixture 84 86 reaches 25 wt.% ( Sr/ Sr > 3.7). It is not clear mixture 87 86 why the corrected Sr/ Sr ratio for mix 1 solution (spike fraction = 45.1 wt.%) deviates from the unspiked 87 86 standard value. The corrected Sr/ Sr ratios of nine Submit your manuscript to a mixed solutions (mix 2 to mix 10) yield an indistinguishable journal and beneﬁ t from: average of 0.707633 ± 0.000010 from the unspiked value (0.707630 ± 0.000009). There is no trend between the cor- 7 Convenient online submission 87 86 84 86 rected Sr/ Sr and the spike fraction (or Sr/ Sr )in mixture 7 Rigorous peer review therange from 1.4to25.9wt.%. It is notedthatthe principle 7 Immediate publication on acceptance 7 Open access: articles freely available online of this correction protocol can also be applied to other 7 High visibility within the ﬁ eld isotope systems such as Sm-Nd and Lu-Hf pairs, as briefly 7 Retaining the copyright to your article interpreted in the study of Cheong and Kwon (2010). Competing interests Submit your next manuscript at 7 springeropen.com The authors declare that they have no competing interests.
"Journal of Analytical Science and Technology" – Springer Journals
Published: Dec 1, 2014
Keywords: Analytical Chemistry; Characterization and Evaluation of Materials; Monitoring/Environmental Analysis
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