Open Access

Sm-Nd isotopic analysis of mixed standard solutions by multi-collector inductively coupled plasma mass spectrometry: evaluations on isobaric interference correction of Nd isotopic composition and external calibration of Sm/Nd ratio

  • Jong-Sik Ryu1,
  • Min Seok Choi1,
  • Youn-Joong Jeong1 and
  • Chang-sik Cheong1Email author
Journal of Analytical Science and Technology20134:8

DOI: 10.1186/2093-3371-4-8

Received: 12 March 2013

Accepted: 12 March 2013

Published: 18 April 2013

Abstract

Background

The Sm-Nd isotope system has long been used to provide information on the age and geochemical evolution of terrestrial rocks and extraterrestrial objects. Traditional thermal ionization mass spectrometry requires a refined chemical separation of Sm and Nd. Here, we present multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) Sm-Nd isotopic results for a series of mixed standard solutions with different Sm/Nd ratios to test the validity of isobaric interference corrections of Nd isotopic composition and external calibration of Sm/Nd inter-elemental ratio.

Findings

Reliable 143Nd/144Nd and 145Nd/144Nd ratios of the mixed solutions were obtained by using the exponential law and selected Sm isotopic compositions. The Sm/Nd ratios of the mixed solutions corrected by the standard bracketing method were consistent with the gravimetric values mostly within 1% difference.

Conclusions

This study provides a simple and high-throughput technique that can simultaneously measure Nd isotopic composition and Sm/Nd ratio without chemical separation between Sm and Nd.

Keywords

Sm-Nd MC-ICP-MS Isobaric interference Isotopic composition Inter-elemental ratio

Introduction

Sm and Nd are rare earth elements presenting in only small amounts in most rock-forming minerals. Sm and Nd each have seven naturally occurring isotopes (144Sm, 147Sm, 148Sm, 149Sm, 150Sm, 152Sm, 154Sm; 142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, 150Nd). One isotope of Sm (147Sm) decays by α-emission to one isotope of Nd (143Nd) with a half-life of 106 Ga (Lugmair & Marti 1978; Begemann et al. 2001). The Sm-Nd decay system has been efficiently used for determining the timing of major events occurred during the chemical evolution of planets and probing into the earth's interior (DePaolo 1988). The application of this system demands highly accurate Sm-Nd isotope data because its half-life is very long and natural variations in Sm/Nd inter-elemental ratio are typically quite limited.

Although the traditional thermal ionization mass spectrometry (TIMS) is still regarded as the benchmark technique for Sm-Nd isotopic measurement (Chu et al. 2009; Harvey & Baxter 2009; Ali & Srinivasan 2011), more recent multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) has also become a routine technique with high sample throughput and comparable precision to TIMS (Walder et al. 1993; Vance & Thirlwall 2002; Yang et al. 2010; Yang et al. 2011). The classic Sm-Nd isotope analysis requires a separation of two elements from the sample matrix by refined chemical procedures. Recently, however, it was reported that the 143Nd/144Nd ratio of geological samples could be measured accurately by MC-ICP-MS without Sm and Nd separation (Yang et al. 2010). This study further evaluates the validity of Nd isotopic and Sm/Nd elemental ratio measurements for a series of Sm + Nd mixed standard solutions by MC-ICP-MS technique, and revisited various sets of reported Sm isotopic composition.

Instrumentation

The Sm-Nd isotopic analysis of this study was conducted by using a Neptune MC-ICP-MS installed at the Korea Basic Science Institute (KBSI) in Ochang. This double focusing high-resolution ICP-MS 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 electron multiplier. The Faraday collectors were statically set to simultaneously detect the required isotopes: 140Ce (L4), 142Nd (L3), 143Sm (L2), 144(Sm + Nd) (L1), 145Nd (axial), 146Nd (H1), 147Sm (H2), 148(Sm + Nd) (H3), and 149Sm (H4). The sensitivity on 145Nd was typically around 5 V/Nd ppm (1011Ω resistors) in a low-resolution mode. Details of the other operational parameters are summarized in Table 1.
Table 1

Instrumental setting and operational parameters

RF forward power

1200 W

RF reflected power

< 2 W

Cooling gas

15 L/min

Auxiliary gas

0.75 - 0.80 L/min

Sample gas

0.985 - 0.990 L/min

Extraction

−2 kV

Focus

−0.654 kV

Acceleration voltage

10 kV

Interface cones

Nickel

Spray chamber

Quartz dual cyclonic

Nebulizer

ESI PFA MicroFlow

Sample uptake rate

100 μL/min

Instrumental resolution

ca. 400

Mass analyzer pressure

2.9 × 10-9 mbar

Measurement of Nd standard solutions

The basic performance of KBSI Neptune was tested by using the JNdi standard solution with Nd concentration of 100 μg/L. The mass bias was exponentially normalized to 146Nd/144Nd = 0.7219. One measurement consists of 9 blocks of 10 cycles with an integration time of 4.194 s. The average 143Nd/144Nd ratio was 0.512100 ± 0.000004 (n = 10, 2σ S.E.), in reasonable agreement with the recommended value (0.512115 ± 0.000007) (Tanaka et al. 2000).

The in-house Nd standard solution of 100 μg/L was prepared from the AccuTrace™ Nd Reference Standard (lot no. B9035110, plasma emission standard). It yielded an average 143Nd/144Nd ratio of 0.512204 ± 0.000005 (n = 9, 2σ S.E.) with the same analytical design as above (9 blocks of 10 cycles with an integration time of 4.194 s). Two diluted in-house Nd solutions of 50 and 10 μg/L also yielded comparable results of 143Nd/144Nd = 0.512212 ± 0.000006 (n = 5, 2σ S.E.) and 0.512207 ± 0.000004 (n = 5, 2σ S.E.), respectively.

Isobaric interference correction

The contribution of 144Sm imposed on the 144Nd peak should be carefully corrected for accurate determination of 143Nd/144Nd ratio. The first step of correction in this study was to calculate the Sm mass bias factor, β(Sm), for which the exponential law (Russel et al. 1978) was applied as the following.
β Sm = ln 147 Sm / 149 Sm true / 147 Sm / 149 Sm measured ÷ ln M 147 / M 149 ,
(1)

where M denotes the mass of the isotope.

The exponential law also yields an equation for (144Sm/149Sm)measured:
144 Sm / 149 Sm measured = 144 Sm / 149 Sm true × M 149 / M 144 β Sm
(2)
The 144Sm intensity was calculated as multiplying Eq (2) by measured 149Sm intensity. Then, 144Nd intensity was calculated by subtracting the 144Sm intensity from the intensity on mass 144 using the equation.
144 Nd measured = 144 Sm + Nd measured [ 149 Sm measured × 144 Sm / 149 Sm true × M 149 / M 144 β Sm ]
(3)

Finally, the 144Sm-corrected 143Nd/144Nd ratio was exponentially normalized to 146Nd/144Nd = 0.7219.

The Sm isotope ratios (147Sm/149Sm and 144Sm/149Sm) have not been uniformly reported. The following sets of reported Sm isotopic composition were evaluated in this study.
147 Sm / 149 Sm = 1.0868 Yang et al. 2010 , 1.0851 Wasserburg et al. 1981 , 1.06119 MCFarlane & McCulloch 2007 , 1.0847 Berglund & Wieser 2011
(4)
144 Sm / 149 Sm = 0.22332 Yang et al. 2010 , 0.22249 Wasserburg et al. 1981 , 0.2103 McFarlane & McCulloch 2007 , 0.22214 Berglund & Wieser 2011 ,
(5)
To evaluate these Sm isotopic compositions, the in-house Nd standard solution of 100 μg/L was doped with AccuTraceTM Sm Reference Standard (lot no. B8085072, plasma emission standard), in which the concentration ratio of Sm to Nd was 0.2041. As depicted in Figure 1, analytical results (7 measurements, 1 block of 20 cycles with an integration time of 4.194 s) of this solution were different according to the employed Sm isotopic compositions (Yang et al. 2010; Wasserburg et al. 1981; McFarlane & McCulloch 2007; Berglund & Wieser 2011). Sm isotopic compositions reported in (Yang et al. 2010; Wasserburg et al. 1981; McFarlane & McCulloch 2007) yielded the 143Nd/144Nd ratios comparable with the unspiked value, which were 0.512190 ± 0.000014 (2σ S.E.), 0.512195 ± 0.000014 (2σ S.E.), and 0.512194 ± 0.000014 (2σ S.E.), respectively.
https://static-content.springer.com/image/art%3A10.1186%2F2093-3371-4-8/MediaObjects/40543_2013_Article_8_Fig1_HTML.jpg
Figure 1

The 143 Nd/ 144 Nd ratios of a Sm-doped Nd standard solution with 100 μg/L Nd and Sm/Nd of 0.2041. The isobaric interference was corrected using reported Sm isotopic compositions (9] Yang et al. 2010; 13] Wasserburg et al. 1981; 14 ] McFarlane & McCulloch 2007; 15 ] Berglund & Wieser 2011). Solid and dashed lines respectively represent an average measured 143Nd/144Nd ratio and 2σ S.D. of the unspiked Nd standard solution.

Sm isotopic compositions were iteratively solved to minimize the residual sum of squares between corrected 143Nd/144Nd ratio of Sm-doped Nd standard solution and the unspiked value using the Excel Solver. The result suggests that the ratios of 147Sm to 149Sm and 144Sm to 149Sm would be 1.0844 and 0.22233, respectively. Because these values are the closest to the recommended values in (Wasserburg et al. 1981), we hereafter use 1.0851 and 0.22249 as the (147Sm/149Sm)true and (144Sm/149Sm)true ratios for correcting mass fractionation of Sm and calculating its isobaric contribution to 144Nd.

The 143Nd/144Nd ratios of the Sm-doped Nd standard solutions with similar Sm/Nd ratio of around 0.20 but different Nd concentrations (200 and 50 μg/L) were 0.512195 ± 0.000022 and 0.512208 ± 0.000038, respectively, corroborating the validity of this correction design (Table 2). Furthermore, the corrected 145Nd/144Nd ratios of the mixed solutions were well consistent with a constant value of 0.348417 obtained by TIMS (Wasserburg et al. 1981). The correlation between β(Sm) and β(Nd) values indicates that the two factors are not identical and are roughly positively correlated with each other (β(Sm) = 0.516 × β(Nd)-0.997) (Figure 2).
Table 2

Nd isotope ratios of the Sm-doped Nd standard solutions with the Sm/Nd ratio of 0.2

 

Gravimetric Sm/Nd

143Nd/ 144Nd

2σ S.E.

145Nd/ 144Nd

2σ S.E.

n

100 μg/L Nd

0.2041

0.512195

0.000011

0.348415

0.000005

140

200 μg/L Nd

0.2022

0.512195

0.000022

0.348419

0.000010

20

50 μg/L Nd

0.2036

0.512208

0.000038

0.348420

0.000031

20

https://static-content.springer.com/image/art%3A10.1186%2F2093-3371-4-8/MediaObjects/40543_2013_Article_8_Fig2_HTML.jpg
Figure 2

The correlation between the Sm and Nd mass bias factors (β(Sm) and β(Nd)) measured for Sm-doped Nd standard solutions with the Sm/Nd ratio of 0.2041.

https://static-content.springer.com/image/art%3A10.1186%2F2093-3371-4-8/MediaObjects/40543_2013_Article_8_Fig3_HTML.jpg
Figure 3

Comparisons of the measured and corrected 147 Sm/ 145 Nd ratios with gravimetric values.

We further analyzed a series of Sm-doped Nd standard solutions of 100 μg/L Nd with different Sm/Nd ratios (Sm/Nd = ca. 0.1, 0.3, 0.4, and 0.5) to test the validity of correction scheme described above. The results show that the correction protocol is reasonable (Table 3). However, there is a systematic increasing trend in the corrected 143Nd/144Nd ratio with increasing Sm/Nd ratios, implying that the mass bias may not be perfectly corrected by the exponential law for high-Sm/Nd (> 0.5) samples.
Table 3

Sm-Nd isotopic data of the Sm-doped Nd standard solutions of 100 μg/L Nd

143Nd/ 144Nd

2σ S.E.

n

147Sm/ 145Nd

Nd (μg/L)

   

Gravimetric

Measured

Corrected

Gravimetric

Calculated

0.512185

0.000022

20

0.1805

0.1934

0.1818

100

101

0.512191

0.000026

20

0.5441

0.5796

0.5457

100

103

0.512214

0.000028

20

0.7275

0.7744

0.7302

100

103

0.512223

0.000026

20

0.8978

0.9641

0.9091

101

103

Calibration of Sm/Nd ratio

The instrumental mass bias on isotope measurements could be corrected by using either double-spike or standard bracketing methods. The latter method consists in interpolating the mass bias of an unknown sample between the biases inferred from two standard runs, one preceding and one following the sample analysis (Albarede & Beard 2004).

In order to correct the inter-elemental mass bias on the measurement, this study considered the Sm-doped Nd standard solution with the Sm/Nd of 0.2 as the bracketing standard and various mixed solutions with different Sm/Nd values (Sm/Nd = ca. 0.1, 0.3, 0.4, and 0.5) as unknown samples. During the measurements, the average correlation factor in the instrumental mass bias inferred from the standard runs was 0.943 ± 0.002 (n = 7, 2σ S.E.), which yielded consistent Sm/Nd ratios with gravimetric Sm/Nd values mostly within 1% difference (Table 3; Figure 3). This result indicates that the mass bias in the measurement of the Sm/Nd ratio can be reasonably corrected by the standard bracketing method.

The Nd concentrations of the mixed solutions were calculated based on the intensity of the bracketing standard of 100-101 μg/L. Calculated Nd concentrations were consistent with gravimetric values within 3% difference (Table 3).

Conclusions

We evaluated the capability of a Neptune MC-ICP-MS to obtain accurate Nd isotopic composition and Sm/Nd elemental ratio using a series of Sm + Nd mixed standard solutions with different Sm/Nd ratios. The isobaric interference correction using the exponential law and selected Sm isotopic composition yielded accurate 143Nd/144Nd and 145Nd/144Nd ratios for the mixed solutions, although there is a systematic increasing trend in the corrected 143Nd/144Nd ratios with increasing Sm/Nd ratios. The Sm/Nd ratios of the Sm-doped Nd standard solutions could be reliably calibrated by the standard bracketing method mostly within 1% difference from the gravimetric values. These results indicate that accurate Nd isotopic composition and Sm/Nd ratio can be simultaneously measured by a simple and high-throughput technique without chemical separation of Sm and Nd.

Declarations

Acknowledgements

This study was supported by the KBSI grants (G33200 and C33710).

Authors’ Affiliations

(1)
Division of Earth and Environmental Sciences, Korea Basic Science Institute

References

  1. Albarede F, Beard B: Analytical methods for Non-traditional isotopes. Rev Mineral Geochem 2004, 55: 113–152. 10.2138/gsrmg.55.1.113View ArticleGoogle Scholar
  2. Ali A, Srinivasan G: Precise thermal ionization mass spectrometric measurements of 142 Nd/ 144 Nd and 143 Nd/ 144 Nd isotopic ratios of Nd separated from geological standards by chromatographic methods. Int J Mass Spectrom 2011, 299: 27–34. 10.1016/j.ijms.2010.09.014View ArticleGoogle Scholar
  3. Begemann F, Ludwig KR, Lugmair GW, Min K, Nyquist LE, Patchett PJ, Renne PR, Shin C-Y, Villa IM, Walker RJ: Call for an improved set of decay constants for geochronological use. Geochim Cosmochim Acta 2001, 65: 111–121. 10.1016/S0016-7037(00)00512-3View ArticleGoogle Scholar
  4. Berglund M, Wieser ME: Isotopic compositions of the elements 2009 (IUPAC Technical Report). Pure Appl Chem 2011, 83: 397–410. 10.1351/PAC-REP-10-06-02View ArticleGoogle Scholar
  5. Chu Z, Chen F, Yang Y, Guo J: Precise determination of Sm, Nd concentrations and Nd isotopic compositions at the nanogram level in geological samples by thermal ionization mass spectrometry. J Anal At Spectrom 2009, 24: 1534–1544. 10.1039/b904047aView ArticleGoogle Scholar
  6. DePaolo DJ: Neodymium isotope geochemistry. Berlin, Heidelberg: Springer-Verlag; 1988.View ArticleGoogle Scholar
  7. Harvey J, Baxter EF: An improved method for TIMS high precision neodymium isotope analysis of very small aliquots (1–10 ng). Chem Geol 2009, 258: 251–257. 10.1016/j.chemgeo.2008.10.024View ArticleGoogle Scholar
  8. Lugmair GW, Marti K: Lunar initial 143 Nd/ 144 Nd: Differential evolution of the lunar crust and mantle. Earth Planet Sci Lett 1978, 39: 349–357. 10.1016/0012-821X(78)90021-3View ArticleGoogle Scholar
  9. McFarlane CRM, McCulloch MT: Coupling of in-situ Sm-Nd systematics and U-Pb dating of monazite and allanite with applications to crustal evolution studies. Chem Geol 2007, 245: 45–60. 10.1016/j.chemgeo.2007.07.020View ArticleGoogle Scholar
  10. Russel WA, Papanastassiou DA, Tombrello TA: Ca isotope fractionation on the earth and other solar system materials. Geochim Cosmochim Acta 1978, 42: 1075–1090. 10.1016/0016-7037(78)90105-9View ArticleGoogle Scholar
  11. Tanaka T, Togashi S, Kamioka H, Amakawa H, Kagami H, Hamamoto T, Yuhara M, Orihashi Y, Yoneda S, Shimizu H, Kunimaru T, Takahashi K, Yanagi T, Nakano T, Fujimaki H, Shinjo R, Asahara Y, Tanimizu M, Dragusanu C: JNdi-1: a new neodymium isotopic reference in consistency with LaJolla neodymium. Chem Geol 2000, 168: 279–281. 10.1016/S0009-2541(00)00198-4View ArticleGoogle Scholar
  12. Vance D, Thirlwall M: An assessment of mass discrimination in MC-ICPMS using Nd isotopes. Chem Geol 2002, 185: 227–240. 10.1016/S0009-2541(01)00402-8View ArticleGoogle Scholar
  13. Walder A, Platzner I, Freedman PA: Isotope ratio measurement of lead, neodymium and neodymium-samarium mixtures, hafnium and hafnium-lutetium mixtures with a double focusing multiple collector inductively coupled plasma mass spectrometer. J Anal At Spectrom 1993, 8: 19–23. 10.1039/ja9930800019View ArticleGoogle Scholar
  14. Wasserburg GJ, Jacobsen SB, DePaolo DJ, McCulloch MT, Wen T: Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim Cosmochim Acta 1981, 45: 2311–2323. 10.1016/0016-7037(81)90085-5View ArticleGoogle Scholar
  15. Yang Y, Wu F, Xie L, Zhang Y: High-precision measurements of the 143 Nd/ 144 Nd isotope ratio in certified reference materials without Nd and Sm separation by multiple collector inductively coupled plasma mass spectrometry. Anal Lett 2010, 43: 142–150.View ArticleGoogle Scholar
  16. Yang Y-H, Chu Z-Y, Wu F-Y, Xie L-W, Yang J-H: Precise and accurate determination of Sm, Nd concentrations and Nd isotopic compositions in geological samples by MC-ICP-MS. J Anal At Spectrom 2011, 26: 1237–1244. 10.1039/c1ja00001bView ArticleGoogle Scholar

Copyright

© Ryu et al.; licensee Springer. 2013

This article is published under license to BioMed Central Ltd. 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.